Solution-phase arrested-precipitation synthesis of nanoparticles
Solution-phase arrested-precipitation reactions offer precise control over nanoparticle synthesis by decomposing precursors in the presence of coordinating ligands, which restrict particle growth The high surface area of nanoparticles leads to increased surface energy and a tendency to aggregate, but ligand binding helps minimize this aggregation Typically, long-chain organic ligands functionalized with amines, phosphonic acids, carboxylic acids, or alkanethiols are used, with chain length adjustable to influence particle growth and spacing These ligands, having high boiling points, enable synthesis at elevated temperatures for well-crystallized products By modifying the polarity of ligands on nanoparticle surfaces, dispersion in various solvents is achievable Additionally, different morphologies can be obtained from the same composition based on the binding affinity of ligands on specific facets, with growth being faster on unpassivated facets.
La Mer’s model effectively explains the growth of nanoparticles in solution, as illustrated in Figure 1.2 The process begins with the rapid injection of precursors into a hot coordinating solvent, leading to thermal decomposition and the formation of monomers As the concentration of monomers surpasses the nucleation threshold, nuclei begin to form Once this threshold is reached, new nuclei formation halts, and existing nuclei grow by incorporating remaining monomers As the monomer concentration decreases further, particle growth continues through Ostwald ripening, where larger particles consume smaller ones with higher surface energy The rate of nucleation and growth of nanoparticles can be controlled by adjusting temperature, time, and surfactant levels.
Nu cle ati o n Inje ctio n
Gr ow th Fr om Sol u tion
Figure 1.1 Illustration of La Mer’s model for the nucleation and growth of colloidal nanocrystals
Transition metal phosphides
Binary transition metal phosphides are multifunctional materials utilized in energy storage, catalysis, magnetism, and optics They have gained significant attention in catalysis for applications such as the hydrogen evolution reaction, oxygen evolution reaction, hydrodeoxygenation, and hydrodesulfurization These materials can exist in various stoichiometries and structures, with metal-rich phases displaying metallic characteristics and phosphorus-rich phases exhibiting covalent properties X-ray absorption studies have shown that the electronic structure of metal-rich phosphides resembles that of pure metals, leading to varying properties for the same metal in different compositions For example, Ni2P serves as an effective hydrotreating catalyst, while Ni5P4 is suitable for lithium intercalation in battery applications Among iron-phosphorus phases, FeP2 acts as a small bandgap semiconductor, whereas Fe3P is a ferromagnet with a high transition temperature of 12 K Given their promising nanoscale properties, significant efforts have been dedicated to synthesizing transition metal phosphides at the nanoscale.
1.2.2 Synthesis of binary transition metal phosphides
Historically, solvothermal methods have been utilized for the preparation of transition metal phosphides, involving heating above the solvent's boiling point in a closed vessel Researchers, including Qian and colleagues, have successfully synthesized various metal phosphides using metal salts combined with sodium phosphide and white phosphorus (P4) as phosphorus sources Additionally, in 2008, Gillan and his team reported the synthesis of phosphorus-rich transition metal phosphides using metal chlorides and yellow phosphorus as precursors However, materials produced through solvothermal methods often exhibit significant polydispersity and particle aggregation, complicating the study of their size-dependent properties.
The temperature programmed reduction (TPR) method is utilized in the synthesis of transition metal phosphides, starting with the incorporation of precursor materials onto a support, such as silica, through incipient wetness impregnation In this process, precursors like Ni(NO3)2.6H2O and NH4H2PO4 are dissolved in water and added to the support in portions, ensuring the volume matches the pore volume of the support to facilitate capillary action Excess solution leads to slower diffusion processes After drying and calcination in air to remove the solvent, the resulting oxide material undergoes reduction via the TPR method to achieve the desired phase.
The Temperature Programmed Reduction (TPR) method involves passing a hydrogen (H2) gas mixed with an inert gas, such as nitrogen (N2) or argon (Ar), over a sample at varying temperatures to monitor the reduction process by measuring H2 consumption Increased H2 consumption signifies that the reduction is occurring Following the reduction, the sample is treated with a 1 mol% O2/He mixture to prevent pyrophoric reactions when exposed to air However, this technique has drawbacks, including the inhomogeneous nature of the particles and the influence of support interactions on particle sizes.
Solution-phase arrested-precipitation reactions allow for the controlled synthesis of binary transition metal phosphides, facilitating the study of how size, shape, and composition influence catalytic activity Table 1.1 showcases various transition metal phosphides that can be synthesized through these solution phase methods.
Table 1.1 Binary phases of transition metal phosphides reported via colloidal routes
The Brock group established a synthesis protocol for MnP nanoparticles, utilizing the Mn(0) complex (Mn2(CO)10) as the metal precursor and P(Si(Me3)3 as the phosphorous source.
The size of nanoparticles can be controlled by adjusting the reaction temperature and duration; for instance, a reaction at 220 °C for 24 hours yields particles measuring 5.11±0.48 nm, while a reaction at 250 °C for 18 hours followed by 220 °C for an additional 18 hours produces 6.67±0.33 nm nanoparticles The Brock group explored alternative phosphorus sources to the costly and highly reactive P(Si(Me3)3), discovering that alkyl phosphines, such as trioctylphosphine (TOP), can effectively provide phosphorus for manganese phosphide (MnP) formation Although high temperatures around 300 °C are necessary to cleave the P-C bond in TOP, its lower reactivity, reduced toxicity, and affordability make it a suitable phosphorus source for synthesizing metal phosphide nanoparticles.
Hyeon and colleagues successfully synthesized one-dimensional metal phosphide nanoparticles, including MnP, by injecting a metal-trioctylphosphine (M-TOP) complex into a heated surfactant solution They employed a syringe pump to ensure a steady and consistent flow rate of the metal precursor, resulting in uniform nanorods that could not be achieved through rapid injection methods.
Mn2(CO)10 served as an effective precursor for nanorod preparation, utilizing TOPO as both surfactant and solvent at a temperature of 330 °C The dimensions of the resulting nanorods varied, measuring 8×16 nm at an injection rate of 10 mL/h and 6×22 nm at 20 mL/h In contrast, using Mn(acac)2 as the metal precursor resulted in no nanocrystal formation, highlighting the critical role of the precursor in synthesizing phosphide nanoparticles Additionally, the reaction conducted with octyl ether demonstrated further implications for the synthesis process.
Spherical MnO nanoparticles were synthesized using PtP2 50 and oleylamine, highlighting the importance of the solvent system in determining the final product Analysis through PXRD and HRTEM indicated that the growth direction of the nanorods for each phase is perpendicular to the MnP structure.
(002) series of planes The rod formation was explained, due to the differences in binding affinity of ligands to different crystal facets during the growth process
The Brock group successfully synthesized FeP using Fe(acac)3 and (trimethylsilyl)phosphine as precursors, with trioctylphosphine as the solvent at temperatures ranging from 240 to 320 °C Their research also explored the influence of various coordinating solvents, including dodecylamine, myristic acid, and hexylphosphonic acid, on the final product The particles capped with dodecylamine and myristic acid exhibited similar sizes of approximately 5 nm, with FeP being the sole crystalline product However, when hexylphosphonic acid was used, no final product could be isolated.
The Brock group conducted a systematic study on the preparation of various phases of iron phosphide nanoparticles In this research, spherical iron nanoparticles were transformed into phosphide phases through a reaction with TOP, utilizing oleylamine as a stabilizing ligand and octadecene as a solvent Two primary strategies were employed: (i) injecting TOP into metal nanoparticles at 200 °C and subsequently heating to 350-385 °C, and (ii) cannulating nanoparticles into pre-heated TOP at 350-370 °C By varying heating temperatures, reaction times, and reactant concentrations, optimal conditions for obtaining phase-pure FexPy materials were identified Fe2P was formed at shorter reaction times with a rod-like morphology, while longer times and higher temperatures led to the incomplete transformation of Fe2P into spherical FeP nanoparticles Notably, the cannulation method successfully produced phase-pure FeP samples devoid of ferromagnetic impurities, suggesting it effectively avoids the formation of Fe2P intermediates that hinder complete conversion.
Fe2P or Fe2P to FeP did not appear to be happening topotactically
Hyeon and coworkers also studied the Fe-P system using their continuous injection approach
Fe2P and FeP nanorods were synthesized through the injection of an Fe-TOP complex, created from the reaction of Fe(CO)5 and TOP, at different temperatures—300 °C for Fe2P and 360 °C for FeP—in the presence of TOPO The Hyeon group found that using oleylamine instead of TOPO resulted in Fe2P as the final phase at 360 °C, attributing this to the phosphorus-rich environment provided by TOPO In relation to Co-P phases, the Robinson group produced Co2P hyperbranched nanocrystals by reacting Co(oleate)2 with trioctylphosphine oxide (TOPO) at 350 °C, identifying TOPO as both the coordinating solvent and phosphorus source Additionally, they reported that varying surfactant concentration allowed for the tuning of hyperbranched structures' morphology, ranging from six-arm symmetric stars to sheaflike forms.
In a separate study by the Robinson group structural evolution and diffusion during the transformation of different Co-P phases were studied 49 First, Co nanoparticle were prepared at
At 180 °C, Co2(CO)8 was used as a precursor to synthesize particles that were subsequently transformed into phosphide phases under varying conditions The Co2P phase was achieved by reacting for 1 hour at 300 °C with 3 mL of TOP, resulting in approximately 70% of the particles being hollow with an average size of 12.8 nm In contrast, the CoP phase was produced by conducting the reaction at 350 °C for 2.5 hours.
The study by the Robinson group indicates that during the transition from Co to Co2P, phosphorus (P) initially diffuses into the cobalt (Co) lattice, creating an amorphous Co-P layer, with hollow particles averaging 16.2 nm in size When Co is predominant in the core, P diffuses inward more rapidly than Co diffuses outward However, as Co2P becomes the dominant phase, the outward diffusion of Co accelerates, leading to the formation of hollow particles as a result of the Kirkendall effect.
Hydrodesulfurization
Stringent regulations on sulfur content in diesel fuel are driving the need for innovative methods to eliminate sulfur impurities in fossil fuel processing The sulfur oxides (SOx) released during fossil fuel combustion contribute to acid rain, while sulfur residues can impair the effectiveness of automotive catalytic converters, leading to increased emissions of unburned hydrocarbons and toxic oxides In the USA, the challenge of sulfur has escalated due to a transition from traditional Saudi Arabian oil, which contains approximately 1.8 wt% sulfur, to Canadian oil sands, where sulfur content is around 5 wt% This shift is illustrated in Table 1.2, which compares import volumes from these sources over different time periods.
Hydrodesulfurization (HDS) is the primary industrial method for removing sulfur from crude oil, utilizing a catalytic hydro-treating process that operates under high temperature and pressure due to the significant activation barrier for desulfurization Sulfur compounds in crude oil are classified into four categories based on their reactivity during HDS: (i) alkyl benzothiophenes, (ii) dibenzothiophenes and alkyl dibenzothiophenes lacking alkyl groups at the 4- and 6- positions, (iii) alkyl dibenzothiophenes with one alkyl group at either the 4- or 6- position, and (iv) alkyl groups present at both the 4- and 6- positions The compounds with alkyl groups at both positions are termed refractory compounds, which are challenging to desulfurize due to steric and electronic hindrances.
Table 1.2 Amount of crude oil imported to USA from two different sources (data source: US energy information administration)
The hydrodesulfurization process can occur via two distinct pathways, as illustrated in Scheme 1.1 for a dibenzothiophene (DBT) molecule, which belongs to group (ii) In the direct desulfurization (DDS) pathway, the sulfur atom is directly eliminated from the molecule through the cleavage of the carbon-sulfur (C-S) bond, resulting in the sulfur atom binding directly to the catalyst surface.
In the hydrogenation (HYD) pathway, the initial step involves the hydrogenation of aromatic rings, followed by the cleavage of the C-S bond The progression of this pathway is influenced by the steric bulk of sulfur-containing molecules and the characteristics of the catalyst used For instance, smaller molecules such as thiophene readily follow the DDS pathway, while larger, bulkier molecules like 4,6-DMDBT face challenges due to steric hindrance that protects the carbon structure.
Crude oil sources In May 2000
S bond, undergo the HYD pathway After the hydrogenation of the aromatic rings, the molecule loses planarity and the sulfur is then able to bind to the catalyst surface 67, 68
1.3.1 Conventional industrial HDS catalyst: Ni or Co promoted MoS 2
The currently used industrial catalyst for hydrodesulfurization is MoS2, promoted either with
The use of Co or Ni as promoters is believed to enhance the catalytic activity of Mo atoms by altering their electronic properties However, Mo atoms are primarily located on edge planes, which restricts the density of active sites for catalysis Over the past thirty years, extensive research has led to a twofold increase in the activity of conventional catalyst systems Nevertheless, it remains uncertain whether these catalysts can achieve the low sulfur levels required by effectively removing the most refractory thiophenes Consequently, alternative catalysts are being explored for the hydrodesulfurization (HDS) process.
Scheme 1.1 Different HDS pathways for DBT molecule (direct desulfurization (DDS) vs hydrogenation (HYD) pathways)
1.3.2 Binary metal phosphides for HDS
Transition metal phosphides have gained significant attention as alternative catalysts for hydrodesulfurization (HDS) due to their isotropic properties, which can enhance the density of catalytic sites when optimized Research by Oyama and colleagues on dibenzothiophene (DBT) HDS catalytic activity revealed that the effectiveness of various metal phosphides supported on silica, prepared through conventional temperature program reduction, follows the trend: Fe2P < CoP < MoP < WP < Ni2P Consequently, Ni2P has been the focus of numerous studies aimed at evaluating the impact of preparation methods, support materials, and other factors to maximize HDS activity and stability.
Ni2P features a Fe2P-type hexagonal crystal structure comprising two distinct metal sites: Ni(1), which is tetrahedrally coordinated to four P atoms, and Ni(2), which is square pyramidal and coordinated to five P atoms The bulk Ni2P structure is formed by alternating atomic layers with stoichiometries of Ni3P and Ni3P2 along the (0001) direction Research suggests that surface Ni(2) sites promote the hydrogenation (HYD) pathway, whereas Ni(1) sites are more conducive to the dehydrogenative coupling (DDS) pathway.
Figure 1.2 Structure of MoS2 catalyst 67
Figure 1.1 Fe2P type Ni2P structure with square pyramidal M(2) and tetrahedral M(1) sites (Copyright from Elsevier
2010) 69 Figure 1.2 Structure of MoS2 catalyst 67
Figure 1.3 Fe2P type Ni2P structure with square pyramidal M(2) and tetrahedral M(1) sites (Copyright from Elsevier
2010) 69 Figure 1.4 Structure of MoS2 catalyst 67
1.3.3 Ternary nickel phosphide systems for HDS
To enhance the catalytic activity of Ni2P, researchers have investigated the effects of incorporating secondary metals such as Fe and Co Bussell and colleagues found that silica-supported Ni2-xCoxP exhibited a 34% increase in catalytic activity, particularly for the composition Ni1.92Co0.08P1.6, with optimal performance in nickel-rich variants In a similar study on Ni2-xFexP, the highest activity was recorded for Ni1.97Fe0.03P2.0 Mӧssbauer spectroscopy revealed that in Ni-rich Ni2-xFexP materials, nickel predominantly occupies M(2) sites, while iron tends to prefer M(1) sites as its concentration increases This higher activity in nickel-rich compositions is attributed to the surface enrichment of phosphorus, which results from the preferential segregation of nickel into M(2) sites, thereby reducing sulfur poisoning during the catalytic process.
Smith and colleagues investigated the hydrodesulfurization (HDS) catalytic activity of 4,6-dimethyldibenzothiophene (4,6-DMDBT) using Ni2-xCoxP materials They found that the material with a nominal composition of Ni2Co0.08P exhibited the highest catalytic activity, along with an increase in selectivity for the desired DDS product.
The Ni2Co0.08P catalyst exhibits a significant increase in DDS selectivity, rising from less than 3% for Ni2P to 49%, attributed to a higher density of acidic sites on its surface, as demonstrated by n-propylamine titration This abundance of acidic sites facilitates the isomerization of the 4,6-DMDBT molecule, favoring the DDS pathway.
Oyama and coworkers studied the 4,6-DMDBT HDS activity for TPR prepared Ni2-xFexP materials and observed a shift in the product selectivity towards the DDS pathway with increasing
Extended X-ray absorption fine structure (EXAFS) studies on three Ni2-xFexP compositions (x=0.5, 1, 1.5) indicate that iron (Fe) substitution for nickel (Ni) at M(2) sites predominantly occurs in compositions with x=1 and 1.5, aligning with Mӧssbauer studies that suggest substitution begins at x>0.6 This substitution at M(2) sites, crucial for the hydrogenation (HYD) pathway, shifts product selectivity towards the dehydrogenation-dehydrocyclization (DDS) pathway in Fe-rich compositions Additionally, infrared (IR) spectroscopic analysis of carbon monoxide (CO) adsorbed on the catalyst surface shows a decrease in bond frequency with increasing Fe content, suggesting a transfer of electron density from Fe to Ni atoms.
1.3.5 Solution-phase synthesis of Ni 2 P for HDS studies
In 2007, the Brock group introduced discrete Ni2P nanoparticles (~10 nm) synthesized through solution-phase methods as model catalysts for thiophene hydrodesulfurization (HDS) They discovered that the surface ligation chemistry of these nanoparticles significantly influenced their activity and stability under severe HDS conditions (500 K-650 K, 3 MPa) Subsequent research by the Brock group further assessed the size-dependent HDS catalytic activity of Ni2P nanoparticles, evaluating three distinct sizes for their effectiveness.
Ni2P nanocrystals of various sizes (~5 nm, ~10 nm, ~15 nm) were synthesized using colloidal methods and supported on silica via incipient wetness impregnation These particles exhibited low catalytic activity due to sintering under harsh hydrodesulfurization (HDS) conditions To mitigate sintering, a novel technique was developed to encapsulate the nanoparticles in a mesoporous silica matrix, enhancing their thermal and phase stability during HDS Recently, Jin and colleagues synthesized Ni2P nanoparticles in solution at 330 °C, utilizing triphenylphosphine as the phosphorus source, and combined metal and phosphorus precursors with an MCM-41 support from the beginning, resulting in in-situ supported Ni2P particles By adjusting the synthesis temperature, they successfully produced various phases, including Ni, Ni12P5, and Ni2P Their studies on dibenzothiophene (DBT) hydrodesulfurization revealed that the solution-phase synthesized materials demonstrated superior catalytic activity compared to those prepared via temperature-programmed reduction (TPR), attributed to improved particle dispersion and a lower phosphorus concentration on the catalyst surface.
Water splitting as a renewable energy source
The global population's rapid growth is projected to increase energy consumption from 17 TW in 2010 to 27 TW by 2040 Fossil fuels, known for their high energy density and ease of combustion, have been the most dependable energy source thus far However, the depletion of fossil fuels and environmental concerns, such as the greenhouse effect, have shifted global focus towards alternative energy sources Renewable options like solar and wind energy present viable alternatives, yet their effectiveness is influenced by time and weather conditions, highlighting the need for advanced energy storage technologies.
Molecular hydrogen is an ideal energy storage solution due to its high specific energy and the fact that it only emits water when combusted Therefore, developing affordable, clean, and scalable methods for hydrogen production is crucial Currently, steam-methane reforming is a common technique for hydrogen production, but it poses environmental challenges by emitting CO2 To address this energy crisis, water splitting has gained significant attention as a viable alternative This electrochemical process effectively separates water into molecular hydrogen at the cathode and oxygen at the anode.
The reaction for producing clean H2 fuel through water splitting has a free energy of 237.2 kJ/mol under standard conditions, requiring 1.23 V per electron transferred The oxygen evolution reaction (OER) at the anode is a significant bottleneck due to its sluggish kinetics, necessitating an efficient catalyst to lower the overpotential and enhance reaction rates Density functional theory studies indicate that Ru oxides are among the most effective materials for OER, making Ru-based catalysts highly researched However, their high cost and scarcity limit large-scale applications Consequently, recent advancements have shifted towards non-noble transition metal phosphides, which have shown promise as efficient, stable, and conductive catalysts for water splitting.
1.4.1 Binary and ternary metal phosphides as OER catalysts
The Schaak group first identified the impressive hydrogen evolution reaction (HER) catalytic activity of Ni2P in acidic solutions, attributing this to the exposure of (001) facets on nanoparticle surfaces Since then, transition metal phosphides have gained significant attention as electrocatalysts for water splitting In terms of oxygen evolution reaction (OER), cost-effective and Earth-abundant binary metal phosphides, particularly Ni2P and CoP, have demonstrated promising catalytic performance Studies by Du and colleagues revealed that different morphologies of Ni2P, such as nanowires and nanoparticles, exhibited overpotentials of approximately 400 mV and 500 mV, respectively Additionally, Li and coworkers reported a 400 mV overpotential for a CoP hollow polyhedron catalyst Recently, research has shifted towards ternary phosphide systems, which combine two metals from Ni, Fe, Co, and Mn, showing enhanced OER activity compared to binary counterparts, as indicated by reduced overpotentials needed to achieve a current density of 10 mA cm^-2.
Li et al synthesized CoMnP nanoparticles through colloidal methods, achieving an impressive overpotential of 0.33 V for OER catalytic activity, outperforming Co2P (0.37 V) and CoMnO2 (0.39 V) Similarly, Garcia et al developed various morphologies of CoFeP nanoparticles by converting oxide phases to phosphides, finding that the sea-urchin-like structure with a composition of Co1.08Fe0.92P exhibited the highest catalytic activity, marked by an overpotential of 0.37 V in 0.1M KOH, surpassing both commercial Ir catalysts and binary phases like Fe2P and Co2P Additionally, Read et al prepared metal phosphide films from commercially available metal foils, discovering that NiFeP demonstrated the highest OER catalytic activity with a lowest overpotential of 0.28 V, outperforming binary phases such as Ni2P (0.34 V) and Co2P (0.37 V).
Thesis statement
As regulations on sulfur content in diesel fuel tighten, the need for effective sulfur removal methods in fossil fuel processing has become critical Hydrodesulfurization (HDS) is the prevalent industrial process for eliminating sulfur compounds from crude oil, utilizing various catalysts, including Co-Mo, Ni-Mo, and Ni-W sulfides supported on silica or alumina While sulfided molybdenum-based materials are commonly used, their lamellar structure limits catalytic activity due to low active site density This raises concerns about their ability to meet future ultralow sulfur regulations In contrast, metal phosphides like Co2P, CoP, Ni2P, MoP, and WP have emerged as promising catalysts for HDS, offering an isotropic distribution of metal sites that enhances catalytic performance Notably, Ni2P supported on silica has demonstrated superior performance and resistance to sulfur poisoning, even outperforming sulfided molybdenum under optimized conditions.
Recent research has shifted focus from binary phosphides to ternary phosphides in hydro-treating, aiming to achieve synergistic effects The interaction between two metals within the ternary structure is anticipated to enhance material activity through structural or electronic effects, though the extent of this enhancement is highly dependent on the specific metals and their composition.
Recent catalytic studies on binary and ternary phosphides have primarily utilized the traditional temperature programmed reduction (TPR) method for material preparation, which involves depositing metal and phosphorus precursors onto a silica support followed by reduction to create supported phosphide catalysts However, this method often results in samples with varying particle sizes, complicating the analysis of how active site density and quality influence catalytic activity and mechanisms To address this issue, there is a need for synthetic methods that can produce phase-pure materials with narrow polydispersity Solution-phase arrested precipitation reactions offer excellent control over the size, shape, and composition of discrete nanoparticles, facilitating a more precise evaluation of size and composition effects.
To address the impending energy crisis and environmental challenges linked to fossil fuel combustion, the production of renewable energy through electrocatalytic water splitting is gaining significant attention This process necessitates an efficient catalyst to minimize the overpotential for the oxygen evolution reaction (OER) While ruthenium and iridium metal oxides are recognized as effective OER catalysts, their limited availability and high costs hinder their industrial application Strategies to mitigate these issues involve the development of catalysts made solely from Earth-abundant metals or the dilution of noble metals with more affordable yet active alternatives Recently, metal phosphides have shown promise as effective electrocatalysts for OER, although research has yet to thoroughly explore the performance of noble metal phosphides, particularly those containing Ru and Ir.
This dissertation research aimed to achieve two primary objectives: firstly, to develop synthetic methodologies for creating bimetallic ternary phosphide nanoparticles (Ni2-xMxP; M=Co, Ru) through solution-phase arrested-precipitation reactions, ensuring the production of phase-pure and nearly monodisperse nanoparticles Secondly, the study sought to assess the composition-dependent catalytic activity of these nanoparticles and investigate the influence of active site density on their catalytic performance.
To achieve the first goal, crystalline phase-pure Ni2-xCoxP and Ni2-xRuxP nanoparticles were synthesized across a wide composition range, with x values up to 1.7 for Co and 1 for Ru, ensuring narrow size distributions The study evaluated the structural and morphological changes resulting from compositional variations, identifying synthetic parameters that allow for precise control over the size and morphology of Ni2-xCoxP nanoparticles Detailed discussions on the synthesis and characterization of these ternary phosphide nanoparticles are presented in Chapters 3 and 4.
We evaluated the hydrodesulfurization (HDS) activity of Ni2-xMxP nanoparticles (M=Co, Ru) and the oxygen evolution reaction (OER) activity of Ni2-xRuxP nanoparticles The study revealed that Ni-rich compositions of Ni2-xCoxP exhibited notable HDS activity, with IR spectroscopy indicating increased electron density on Ni sites in Co-rich materials, leading to a higher turnover frequency despite a decrease in the total number of active sites In terms of OER performance, ternary Ni2-xRuxP nanoparticles surpassed binary end products, achieving optimal overpotential of 0.34 V at 10 mAcm -2 in 1.0 M KOH at x=0.75 Additionally, preliminary data showed that the introduction of Ru in Ni-rich phases reduces HDS activity towards 4,6-DMDBT.
Chapter 6 will summarize the findings on the solution-phase synthesis of ternary phosphide nanoparticles, highlighting their effectiveness as model catalysts for hydrodesulfurization (HDS) and as electrocatalysts for oxygen evolution reaction (OER) The chapter will also emphasize the importance of two-metal interactions in enhancing the performance of these ternary phosphide nanocrystals Furthermore, it will outline potential future research directions inspired by these insights.
Transition metal phosphide nanoparticles were synthesized in inert conditions, and their characteristics, including phase, size, morphology, and composition, were analyzed alongside nanoparticles encapsulated in mesoporous silica This chapter details the synthesis materials and techniques, as well as the characterization methods employed, such as Powder X-Ray Diffraction (PXRD), Electron Microscopy (EM), Energy Dispersive Spectroscopy (EDS), Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), X-ray photoelectron spectroscopy (XPS), and assessments of surface area and porosity.
Materials
Nickel acetylacetonate (Ni(acac)2, 95% purity, Alfa Aesar), cobalt(II) acetylacetonate (Co(acac)2, 99% purity, Sigma-Aldrich), n-octyl ether (95% purity, TCI America), oleylamine (>50% purity, TCI America), tri-n-octylphosphine (TOP, 97% purity, STREM Chemicals), tetraethylorthosilicate (TEOS, ACROS), ruthenium chloride hydrate (40-43% metal, Pressure Chemicals), Nafion (5%, LQ-1105, Ion Power), sodium hydroxide (≥98% purity, Aldrich), chloroform (Fisher Scientific), hexanes (technical grade, Fisher), and methanol and ethanol (200 proof, Decon Laboratories) were all utilized as received in this study.
1 Portions of the text in this chapter were reprinted or adapted with permission from: Chem Mater 2015, 27, 4349-
Experimental techniques
The glove box technique creates an inert environment essential for handling air-sensitive chemicals and reactions, particularly in metal phosphide syntheses where TOP is utilized It operates under an argon atmosphere, with a photohelic pressure gauge ensuring positive pressure to prevent contamination A copper catalyst and molecular sieves are integrated into the system, allowing continuous argon flow to minimize oxygen and water exposure Over time, the catalyst may oxidize, necessitating periodic regeneration to uphold optimal performance.
493 purchased from Vacuum Atmospheres is used in this dissertation research
This dissertation research employs standard Schlenk line techniques for the solution phase synthesis of metal phosphides A Schlenk line features two manifolds: one connected to a vacuum pump via a cold trap for vacuum creation, and the other linked to an inert gas supply such as argon or nitrogen This setup facilitates an inert atmosphere, essential for conducting air-sensitive reactions.
Characterization techniques
PXRD, or Powder X-ray Diffraction, is a fundamental technique in solid-state chemistry used to characterize materials by revealing their structural details, including phase, composition, and crystallinity This method relies on the diffraction of X-ray radiation by atoms, producing a unique fingerprint pattern for each material With a wavelength of approximately 1 Å (10^-10 m), X-rays are comparable to the inter-atomic spacing within a lattice, allowing the lattice to function effectively as a diffraction grating for the radiation.
The generation of X-ray radiation involves a setup that includes an anode, a cathode, and an evacuated chamber The cathode, typically made of tungsten (W), produces electrons when heated by a filament current These electrons are then accelerated by a high voltage of 30-60 kV and collide with the anode, generating X-rays by exciting the core electrons in the metal target The X-ray radiation passes through a beryllium (Be) window, which is transparent to radiation due to its low atomic number The collision between the accelerated electrons and the anode converts kinetic energy into heat, necessitating a continuous water cooling system to manage the anode's temperature Some powder X-ray diffraction (PXRD) machines utilize a continuously rotating disk-shaped anode to enhance X-ray generation efficiency by reducing heat production.
Figure 2.1 Schematic diagram of an X-ray tube with main components (adapted from West) 105
The production of X-rays typically involves a copper (Cu) anode, where striking electrons ionize a 1s electron, creating a vacancy in the K shell Electrons from higher energy levels, such as 2p or 3p, transition to fill this vacancy, releasing energy in the form of X-ray radiation An X-ray spectrum consists of two main components: a broad spectrum, or white radiation, generated by the deceleration of electrons upon colliding with the anode, and discrete spectral lines representing monochromatic radiation These lines include Kα (2p → 1s) and Kβ (3p → 1s) transitions, with the Kα line being of higher intensity and primarily utilized in diffraction experiments The Kα line appears as a doublet, known as Kα1 (1.54051 Å) and Kα2, due to the two spin states of the 2p level.
Figure 2.2 Illustration of X-ray generation process with the Cu metal as the anode (adapted from
Monochromatic radiation is essential for X-ray experiments, and to eliminate Kβ radiation, a filter is employed This filter is usually made of a metal with one or two atomic numbers lower than that of the anode metal; for copper anodes, nickel is the most commonly used filter.
X-ray fluorescence can significantly affect the quality of powder X-ray diffraction (PXRD) spectra by causing interference due to the ejection of inner shell electrons in the sample, which leads to the emission of X-ray radiation as outer shell electrons fill the vacancies The extent of this interference is influenced by the anode material used and the specific material being analyzed For example, when using a copper anode (wavelength 1.5418 Å), cobalt and nickel exhibit different ionization potentials for their 1s electrons, resulting in ionization of cobalt but not nickel To reduce X-ray fluorescence interference, selecting the appropriate anode material based on the analyte is crucial Additionally, the wavelength of X-ray radiation emitted by each metal correlates with its atomic number, as described by Moseley’s law.
Table 2.1 Anode materials with different wavelengths of X-rays produced and the suitable filters to eliminate Kβ radiation (adapted from Pecharsky) 107
The orderly arrangement of atoms in a crystal resembles an optical grating, where the atomic spacing is comparable to the wavelength of X-rays This similarity allows for diffraction from atoms to be likened to optical diffraction Bragg's law effectively demonstrates the diffraction phenomena occurring within a crystal structure.
As shown in Figure 2.3, two waves a and b interact with two atoms present in two adjacent A and
In crystal planes, the distance between two planes is referred to as dhkl When an incident beam (a) strikes the first plane at an angle Ɵ, it is reflected at the same angle Meanwhile, a second beam (b) penetrates the first plane and is reflected by the second plane The difference in distance traveled by these two waves can be represented by the equation xyz = 2dsinƟ (2.2).
Bragg's law defines the relationship between the path length difference and the wavelength of radiation, indicating that constructive interference occurs when this difference is an integer multiple of the wavelength This specific angle, known as Bragg's angle, allows for the optimal conditions for constructive interference, while waves at different angles experience destructive interference The equation representing this principle is nλ = d sin(θ), where n is an integer (n = 1, 2, 3, ).
Figure 2.4 depicts X-ray diffraction occurring in finely powdered samples, where various orientations meet Bragg’s angle, resulting in constructive diffraction in multiple directions The diffracted radiation can be captured using photographic film, a CCD camera, or a movable detector.
The intensity of peaks in the PXRD pattern is crucial for understanding the structural properties of materials During the diffraction process, X-ray electric fields interact with atomic electrons, resulting in scattering in various directions This scattering is essential for analyzing material structures.
Figure 2.3 Illustration of Bragg's law (adapted from West) 105 sample
Diffraction from a powdered sample reveals that the intensity is a cumulative result of individual electron intensities, defined as the form factor (f) This form factor, which is dependent on the atomic number, decreases as the scattering angle increases Additionally, the form factor is directly related to the structure factor (F hkl), a crucial parameter that influences the overall peak intensity observed in a powder X-ray diffraction (PXRD) pattern.
Fhkl= ∑ f j jexp{-i2π(hxj+kyj+lzj) (2.4) fj = form factor for j th atom h,k,l = miller indices for the reflection xj,yj,zj= fractional coordinates for the j th atom
The PXRD pattern is essential for analyzing the growth direction and size of crystallites in a sample Broad peaks typically indicate smaller crystallite sizes in nanoparticles due to fewer crystalline planes, which limits destructive interference near the Bragg angle In anisotropic materials, the preferred growth direction results in narrower, more intense peaks, while those corresponding to shorter axes are broader, reflecting the number of diffracting planes The peak breadth can be utilized to estimate crystallite size (t) using the Scherrer equation, represented as t = kλ.
𝛽𝐶𝑜𝑠Ɵ (2.5) λ= wavelength of X-ray k= shape factor (typically 0.9 for spherical particles) β= Full width half maximum of the peak (FWHM) Ɵ=Bragg angle
In this dissertation research, powder X-ray diffraction (PXRD) data were collected on a
Rigaku RU 200B rotating anode diffractometer with Cu Kα radiation (0.154 nm) operated at 40kV,
X-ray diffraction analysis was conducted on Ni2-xCoxP samples using a Bruker D2 Phaser diffractometer with Cu Kα radiation at 30 kV and 150 mA, employing a step size of 0.02° For the RuxPy and Ni2-xRuxP samples, the same instrument was operated at 10 mA, with data collected over a 2Ɵ range of 30-80° on a zero background quartz holder with minimal grease Silicon served as an internal standard, and the resulting diffraction patterns were compared with powder diffraction files from the ICDD database for accurate phase identification The crystallite size was analyzed using Jade 5.0 software, applying the Scherrer equation for calculations.
Microscopy techniques are essential for magnifying materials, with resolution (δ) being influenced by the radiation wavelength (λ), as indicated in Equation 2.6 Optical microscopes, which utilize visible light, have a resolution limit of a few hundred nanometers, making them inadequate for imaging nanoscale samples The relationship is defined by the formula δ = 0.61λ.
𝜇𝑆𝑖𝑛𝛽 (2.6) μ= refractive index of the viewing medium β= semi-angle of collection of the magnifying lens λ= ℎ
V= accelerating voltage m0= mass of electron eV= kinetic energy
In 1925, de Broglie revealed the wave-like nature of electrons, leading to the development of electron microscopy, which enables imaging at the nanoscale The small wavelength of electrons allows for exceptional resolution in material characterization, with electron microscopes operating at 100 kV achieving a resolution of approximately 0.24 nm, compared to the 300 nm resolution of optical microscopes using green light (550 nm) In the field of nanotechnology, electron microscopy serves as a crucial tool for analyzing the size, morphology, composition, and crystallinity of materials Key processes involved when a sample is bombarded with electrons include transmission for transmission electron microscopy (TEM), backscattering for scanning electron microscopy (SEM), and X-ray generation for energy dispersive spectroscopy (EDS) This chapter will focus on the TEM and EDS techniques relevant to this dissertation research.
Elastically scattered electrons Inelastically scattered electrons
Figure 2.5 Different processes undergone by bombarded electrons interacting with a specimen
Co x P Nanoparticles
Introduction
Transition metal phosphides have gained recognition as effective catalysts across various applications, particularly highlighted in Chapter 1 While significant research has focused on binary phases, recent investigations have begun to uncover the potential benefits of ternary phases Notably, ternary phosphides with the formula Ni2-xCoxP exhibit enhanced hydrodesulfurization (HDS) activity compared to the best binary phase, Ni2P, especially at low cobalt concentrations (x ≤ 0.1) when synthesized via the TPR method Additionally, research by Zhang and colleagues has demonstrated that Ni2-xCoxP serves as a potent catalyst for hydrazine decomposition, showcasing its highest activity under specific conditions.
The Ni1.0Co1.0P1.5 composition shows potential as an active catalyst, yet the lack of synthetic methods for creating nanoparticles with independently adjustable composition and size hinders the development of a comprehensive functional model Discrete Ni2-xCoxP nanoparticles have been scarcely reported, with the only notable study being a 2014 synthesis of Co1.33Ni0.67P nanorods by Peng and colleagues, which demonstrated activity for hydrogen evolution reactions (HER) This highlights the critical need for a rational synthetic strategy that allows for the independent tuning of size, shape, and composition in ternary phosphide nanoparticles to enhance their catalytic performance.
This chapter discusses a synthetic protocol developed to prepare different compositions of
Ni2-xCoxP nanoparticles (with x≤1.7) exhibit nearly monodisperse characteristics and allow for precise control over their composition Furthermore, specific synthetic methods have been identified to regulate the morphology and size of these nanoparticles, utilizing a Ni:Co ratio of 1:1 as a key example.
2 Portions of the text in this chapter were reprinted or adapted with permission from: Chem Mater 2015, 27, 4349-4357
At Wayne State University, I synthesized nanoparticles and conducted various characterizations, including PXRD, TEM, and EDS Additionally, X-ray photoelectron spectroscopy and Rietveld refinement were performed by Prof Mark E Bussell’s group at Western Washington University Elemental mapping and line scanning of ternary phosphide particles were carried out by Dr Yi Liu at the electron microscopy facility at Oregon State University.
Experimental
All materials used in the synthesis of Ni2-xCoxP nanoparticles are given in Chapter 2
3.2.1 Synthesis of Ni 2-x Co x P nanoparticles (0≤x≤2)
All reactions were carried out under argon atmosphere using standard Schlenk line techniques
In a typical synthesis, 2.0 mmol of nickel and cobalt precursors (Ni(acac)2 and Co(acac)2) were combined with 5.0 mL of oleylamine, 10.0 mL of octyl ether, and 4.0 mL of trioctylphosphine in a Schlenk flask The mixture was degassed at 110 °C for 20 minutes to eliminate moisture and oxygen, followed by argon purging for another 20 minutes The temperature was then raised to 230 °C and maintained for 90 minutes, after which it was increased to 350 °C An additional 6 mL of trioctylphosphine was injected as the temperature rose, and the mixture was heated for 4.5 hours at 350 °C The resulting black product was isolated through ethanol precipitation, then dispersed in chloroform, sonicated for 5-10 minutes, and reprecipitated with ethanol, repeating this sonication and precipitation process at least three times.
Results and Discussion
The synthesis of Ni2-xCoxP nanoparticles follows a refined protocol based on our previous work with phase-pure Ni2P nanoparticles Initially, Ni and Co acetylacetonate complexes are mixed with trioctylphosphine (TOP) in a solution of octyl ether and oleylamine (OA), and the mixture is heated to 230°C to form the Ni-Co-P alloy precursor particles These precursor particles are then converted into crystalline ternary phosphide by adding more TOP and heating to 350°C In our earlier Ni2P research, we established that a TOP:M:OA molar ratio of 10.0:2.0:8.0 yielded nearly monodisperse amorphous NixPy precursor particles, and we adjusted the final TOP:M ratio to 11.2 by injecting an additional 6.0 mL of TOP at 350°C.
Increase the Temperature to 350 0 C and inject 6 or 9 mL of TOP
Co(acac) 2 x mmol octyl ether 10 mL oleylamine 5 mL
Precipitate nanoparticles in ethanol, centrifuge and disperse in chloroform
Scheme 3.1 Reaction protocols for the synthesis of Ni2-xCoxP nanoparticles
3.3.1 Structure and morphological changes of ternary phosphide nanoparticles with composition
Ni2P and Co2P exhibit distinct crystal structures, with Ni2P adopting a hexagonal Fe2P type and Co2P following an orthorhombic structure Both compounds feature two metal sites, M(1) with a tetrahedral geometry and M(2) with a square pyramidal geometry, coordinated by phosphorus (P) The primary difference lies in the arrangement of rhombohedral subcells that contain the M(1) and M(2) sites Additionally, each P atom is coordinated in a tri-capped trigonal prismatic geometry formed by Ni atoms.
The products of various targeted compositions were analyzed through PXRD for phase identification, TEM for morphological assessment, and EDS for compositional analysis Figure 3.2 displays the PXRD patterns corresponding to the different targeted compositions.
The patterns in Figure 3.2 correspond to the hexagonal Ni2P structure in nickel-rich compositions, which remains stable up to a composition of Ni0.67Co1.33P, where a notable change in the pattern occurs In nickel-rich compositions, a broad peak near 55° 2Ɵ indicates overlapping reflections of the (300) and (211) planes due to the small size of the crystallites However, at the composition of Ni0.67Co1.33P, two distinct peaks emerge, indicating a shift away from each other.
The Co2P structure-type exhibits an orthorhombic configuration, while the Fe2P structure-type is hexagonal As cobalt (Co) content increases, a notable splitting occurs, which can be linked to the separation of the (300) and (211) reflections due to Co incorporation or a transition to the Co2P structure type This observation highlights the significant structural changes associated with varying levels of cobalt.
Inte ns ity (arb.un its) (312)(130)(201) (320) Co
The PXRD patterns for various compositions of Ni2-xCoxP reveal distinct features, with reference patterns for Co2P and Ni2P included for comparison Notably, sharp peaks marked with an asterisk (*) indicate an internal Si standard, while peaks denoted with a square (■) signify CoP impurities Compositions up to x=1.75 show clear evidence of the desired ternary phase, indicating the feasibility of preparing homogeneous materials without detectable crystalline impurities However, at the Co-rich composition of x=1.9, CoP appears as an impurity Additionally, attempts to achieve phase-pure Co2P result in a mixture of Co2P, CoP, and unidentified impurities, as illustrated in Figure 3.3.
In te n si ty (a rb u n its)
Figure 3.3 PXRD pattern of the reaction attempted to synthesize pure Co2P
Table 3.1 presents the target and actual metal ratios of Ni:Co, as evaluated through Energy Dispersive Spectroscopy (EDS), alongside the crystallite sizes determined by the Scherrer method applied to Powder X-ray Diffraction (PXRD) data Additionally, it includes particle sizes measured via Transmission Electron Microscopy (TEM) and the refined lattice parameters for various compositions of Ni2-xCoxP.
All the PXRD patterns were calibrated using silicon as an internal standard Crystallite sizes were calculated by application of the Scherrer equation to the most intense peak, around 41°,
Ni 2 P type refined Co 2 P type refined
Cobalt Fraction (Co/(Co+Ni))
The hexagonal unit cell parameters of the Ni2P structure were analyzed in relation to Co content, with crystallite sizes ranging from 8-11 nm and no clear size trends based on composition PXRD patterns were refined using the Rietveld method to determine lattice constants for phase-pure Ni2-xCoxP materials, with refined values and molecular volumes detailed in Table 3.1 For nominal compositions with x1) exhibit an amorphous structure, while the amorphous Ru2P phase can be crystallized through high-temperature annealing (450 ºC) These nanoparticles show effective oxygen evolution reaction (OER) catalytic activity, peaking at x=0.75 with an overpotential of 0.34 V in 1.0 M KOH Despite RuO2 being a leading catalyst, Ru2P phosphide shows lesser performance (0.56 V), indicating that material crystallinity and electronic effects at high Ru concentrations may influence catalytic activity The introduction of Ru modifies the redox characteristics of Ni metal, similar to observations in other bimetallic phosphides, which lowers the activation barrier for OER Current research aims to clarify site preferences in Ni2-xRuxP to better understand the observed activity trends.
In te n si ty (a rb u n it s)
Nu mbe r of P articl es
The PXRD pattern and TEM image of Ni2P particles, prepared for OER catalytic testing, reveal inhomogeneities in site distributions and the formation of Ru-rich phases through reductive annealing This process is essential for understanding the structural and electronic contributions to catalyst activity.
Ni2-xRuxP nanoparticles opens the door for the composition-dependent study of other processes, such as hydrotreating studies, which are presently underway
PROBING HYDRODESULFURIZATION CATALYTIC ACTIVITY OF
NI 2-X M X P (M=CO, RU) NANOPARTICLES ENCAPSULATED IN MESOPOROUS
Introduction
As the demand for low sulfur fuels rises, metal phosphides are gaining attention as effective catalysts in the hydrodesulfurization (HDS) process Recent studies have concentrated on bimetallic ternary phosphides, which enhance catalytic activity through the synergistic interaction of two metals Notably, the Bussell group has reported on ternary phosphides of Ni2P that incorporate small amounts of cobalt to improve performance.
Fe demonstrates superior HDS catalytic activity compared to binary counterparts when synthesized via the TPR method on silica However, the inhomogeneity of TPR-prepared materials complicates the understanding of how surface characteristics and particle size influence catalytic performance The Brock group investigated the size-dependent HDS catalytic activity of Ni2P nanoparticles produced through solution phase methods, revealing that these nanoparticles experience significant sintering when supported using the common incipient wetness technique under HDS conditions To address this issue, a novel approach was developed to encapsulate the nanoparticles in a mesoporous silica matrix, preserving their size and phase, which facilitated comprehensive size-dependent studies of HDS catalytic activity.
This study evaluates the hydrodesulfurization (HDS) catalytic activity of Ni-rich Ni2-xCoxP and Ni2-xRuxP compositions encapsulated in mesoporous silica The preparation of nanoparticles, their encapsulation in a silica matrix, and the characterization of the encapsulated materials regarding phase, morphology, and surface area were conducted at Wayne State University Further catalyst characterization and assessment of HDS catalytic activity were performed by Prof Mark E Bussell's group at Western Washington University.
3 Portions of the text in this chapter were reprinted or adapted with permission from: Surface Science 2016, 648, 126-135
Experimental
5.2.1 Preparation of Ni 2-x Co x P and Ni 2-x Ru x P nanoparticles
Nanoparticles were prepared as described in Chapter 3 and 4
5.2.2 Encapsulation of nanoparticles in mesoporous silica
Nanoparticles were initially dispersed in chloroform and added drop-wise to a 0.1 M CTAB aqueous solution, which was heated to 75-80 °C to facilitate chloroform evaporation and nanoparticle phase transfer The resulting dispersion was diluted to 150 mL with nanopure water, followed by the addition of 2.5 mL of 1 M NaOH and 4.6 mL of TEOS to initiate base-catalyzed polymerization This reaction was completed within 20-30 minutes, after which the silica particles were isolated through centrifugation The isolated product underwent three washes with water and methanol to eliminate residual NaOH and ethanol by-products, then was dried overnight in an active vacuum before being calcined in air.
To eliminate CTAB, the material was calcined at 425 °C for 2-3 hours Subsequently, the oxide formed during this process was transformed back into phosphide by treating it with PPh3 or NaH2PO2 in a furnace, utilizing a continuous flow of 5% H2/Ar at 400 °C.
Scheme 5.1 Protocol for the encapsulation of nanoparticles in mesoporous silica
The metal phosphide loadings (wt%) of the encapsulated samples were evaluated using ICP-
MS based on the total metal content (assuming a 2:1 stoichiometry of metal:P) by ICP-MS (Agilent
In this study, 2 mg samples were digested in 5 mL of concentrated nitric acid over a period of 3-4 days and subsequently diluted 48 times with nano pure water Calibration was performed using a series of external standards across the relevant concentration range The surface area and pore size distributions of the encapsulated silica samples were analyzed using a Micromeritics Tristar II surface area/porosimeter, with all samples degassed for 16 hours at 423 K under nitrogen flow prior to analysis Surface areas were calculated using the Brunauer-Emmett-Teller (BET) multimolecular adsorption method, while pore size distributions were determined by the Barrett-Joyner-Halenda (BJH) method Additionally, carbon monoxide (CO) pulsed chemisorption measurements were conducted using a Micromeritics Autochem 2950 instrument with a thermal conductivity detector, where approximately 0.1000 g of catalyst was degassed in a 60 mL/min argon flow for 30 minutes, followed by reduction in a 60 mL/min flow of 10.0 mol% H2/Ar at a heating rate of 10 K/h from room temperature.
Induce phase transfer of nanoparticles
Add x ml of TEOS at relevant pH
Isolate incorporated particles and wash with water and methanol
Heat at 400 ° C in Ar/H 2 with PPh 3
The catalyst samples were heated to 673 K and maintained at this temperature for one hour Following this, they were degassed with a helium flow of 60 mL/min at the same temperature for 30 minutes before conducting chemisorption measurements The CO chemisorption was assessed by introducing a 10.0 mol% CO/He mixture into the helium stream at 273 K, continuing the pulsing until the peak areas of three consecutive pulses stabilized, allowing for the calculation of the chemisorption capacity.
IR spectroscopic experiments were performed in an ultra-high vacuum (UHV) system with a base pressure of approximately 5 x 10^-9 Torr, featuring a high-pressure cell with CaF2 windows The setup includes a Mattson RS-1 FTIR spectrometer with a narrow-band MCT detector, allowing for transmission mode IR spectrum collection Catalyst samples, approximately 10 mg each, were prepared by pressing them into a nickel mesh and monitored for temperature using a chromel-alumel thermocouple After mounting, the samples underwent overnight evacuation to achieve a pressure of ≤ 10^-8 Torr In situ reduction was conducted in 100 Torr H2 for 30 minutes at temperatures of 475, 575, 650, and 700 K, followed by evacuation and heating to remove weakly bonded species.
Transmission FTIR spectra were recorded in the range of 4000-1000 cm -1 with 128 scans at a resolution of 2 cm -1 To remove the effects of gas phase CO, the sample spectrum was compared to a blank nickel mesh spectrum obtained with 1.0 or 5.0 Torr CO After pretreatment and degassing, the catalyst sample was cooled to room temperature under a pressure of approximately 1 x 10 -8 Torr, followed by the acquisition of a background IR spectrum Subsequently, IR spectra of adsorbed CO were gathered at 298 K with the catalyst exposed to 1.0 and 5.0 Torr CO, and then in ultra-high vacuum after evacuating the gas phase CO The final IR spectra presented were generated by subtracting the background spectrum.
Prior to CO exposure, the IR spectrum was obtained, followed by measurements of hydrogenation-dehydrogenation (HDS) catalysts, which were subsequently analyzed through X-ray diffraction and bulk carbon and sulfur analyses After completing the HDS experiment, the liquid feed was halted while maintaining H2 gas flow for one hour at the reaction temperature The reactor was then cooled to room temperature, depressurized, and purged with helium at 60 mL/min for 30 minutes The system was left open for at least three hours to allow gradual exposure of the catalyst to ambient air Bulk carbon and sulfur content in the HDS-tested catalyst samples was analyzed using a LECO SC-144DR Sulfur and Carbon Analyzer, where approximately 0.10 g of catalyst was combusted in an oxygen-rich environment at around 1625 K for three minutes, with the resulting CO2 and SO2 quantified via IR detection and reported as weight percent carbon and sulfur Additionally, about 0.05 g of the HDS-tested catalysts was utilized to obtain an X-ray diffraction pattern.
HDS activity measurements were conducted in a fixed-bed, continuous flow reactor at a total pressure of 3.0 MPa and temperatures ranging from 548 to 648 K The reactor feed comprised a decalin solution with 3000 ppm DBT (including 1000 ppm 4,6-DMDBT) and 500 ppm dodecane, the latter serving as an internal standard for gas chromatographic analysis The liquid feed was injected at a rate of 5.4 mL/h into a hydrogen flow of 100 mL/min, where it was vaporized before entering the reactor A catalyst (approximately 0.15 g, 16-20 mesh size) was mixed with quartz sand, filling a reactor tube of 1.1 cm diameter and 40 cm length The reactor temperature was monitored using a thermocouple in direct contact with the catalyst bed Encapsulated catalysts underwent pretreatment by heating from room temperature to 673 K over one hour in a 60 mL/min hydrogen flow, maintained at this temperature for two hours before cooling back to room temperature in continued hydrogen flow Both helium and hydrogen were purified through molecular sieves and oxygen removal traps prior to use.
Following pressurization of the reactor with H2 to 3 MPa, the catalyst was heated to 548 K in
The experiment involved a flow of 100 mL/min of hydrogen (H2) for 30 minutes, followed by the introduction of a liquid feed into the reactor After stabilizing the reactor for approximately 12 hours, effluent samples were collected at 30-minute intervals over a 2-hour period The catalyst temperature was then increased by 25 K, with a stabilization period of 3 hours before further sampling at the same intervals This process continued until sampling was completed at the maximum catalyst temperature of 648 K The liquid reactor effluent samples from the three types of hydrodesulfurization (HDS) activity measurements were subsequently analyzed off-line using a gas chromatograph (Agilent 6890N) equipped with an HP-5 column and a flame ionization detector.
In te n si ty (a rb u n its)
Results and Discussion
5.3.1 Evaluation of DBT HDS catalytic activity of Ni 2-x Co x P nanoparticles
5.3.1.1 Characteristics of the encapsulated Ni 2-x Co x P catalysts
Three targeted compositions of Ni2-xCoxP (x=0.08, 0.25, 0.5) were synthesized, exhibiting a Ni2P structure with nearly monodisperse particles, as illustrated in Figure 5.1 To ensure stability during hydrodesulfurization (HDS) testing, these particles were encapsulated in a mesoporous silica matrix Characterization of the encapsulated materials, shown in Figure 5.2, confirmed that all compositions maintained the Ni2P structure without any detectable impurities The crystallite size, calculated using the Scherrer equation, ranged from 11 to 13 nm, as detailed in Table 5.1.
Figure 5.1 PXRD patterns and TEM images of the as prepared Ni2-xCoxP nanoparticles for HDS catalytic testing Insets show particle size distributions
The TEM images of encapsulated materials reveal the dispersion of nanoparticles within a mesoporous silica matrix, with high-resolution TEM (HRTEM) images confirming their crystalline nature, showing lattice planes with a d-spacing of approximately 0.5 nm, corresponding to the (100) planes of the Ni2P structure The (001) plane, identified as the most active crystallite plane for hydrogen evolution reaction (HER) and hydrodesulfurization (HDS), is exposed on the surface of the Ni1.5Co0.5P particles Prior to catalytic activity testing, surface area and pore size distribution analyses were conducted, as illustrated by nitrogen physisorption isotherms The BET surface areas of the Ni2-xCoxP@mSiO2 nanocatalysts exceed 400 m²/g, indicating high porosity, with average pore diameters ranging from 3.4 to 5.3 nm The pore size is crucial, as excessively small pores may restrict molecule access to the catalyst surface, while overly large pores may fail to retain the catalyst within the silica matrix.
The catalyst loading (mass %) of these silica encapsulated samples were evaluated using ICP-
MS Metal phosphide loading for all compositions vary in the range 10-12 wt% of the silica sample (Table 5.1)
In te n si ty ( ar b u n it s)
Figure 5.2 PXRD patterns and TEM images of the encapsulated nanoparticles Insets show
HRTEM image for a representative particle for each composition with lattice fringes
Figure 5.3 HRTEM image of a Ni1.5Co0.5P nanoparticle showing lattice fringes corresponding to exposed (100) planes with 0.5 nm spacing and the {100}, {001} and {101} sets of facets
Figure 5.4 Nitrogen adsorption-desorption isotherms for the Ni2-xCoxP@mSiO2 nanocatalysts The inset depicts the pore size distribution calculated from the adsorption branch of the isotherm
Table 5.1 Physicochemical data for the Ni2-xCoxP@mSiO2 nanocatalysts
Carbon monoxide (CO) serves as an effective probe molecule to evaluate the accessibility of molecules to the catalyst surface, revealing that all compositions exhibit CO chemisorption capacities, which indicate some exposure of the catalyst surface, even with a mesoporous silica shell present around the nanoparticles These capacities can be utilized to quantify the number of active sites on the catalyst surface Notably, the CO chemisorption capacities showed a decreasing trend with increasing cobalt (Co) content, except for the Ni1.92Co0.08P@mSiO2 nanocatalyst This observation aligns with previous findings on O2 chemisorption capacities for TPR-prepared Ni2-xCoxP/SiO2 catalysts.
Table 5.2 Dibenzothiophene HDS catalytic data for the Ni2-xCoxP@mSiO2 nanocatalysts
BJH average pore diameter (nm)
Ni1.92Co0.08P@mSiO2 11.2 Ni1.89Co0.11P1.10 13 938 5.3
Ni1.75Co0.25P@mSiO2 9.80 Ni1.72Co0.28P1.06 11 1270 5.3
Ni1.50Co0.50P@mSiO2 11.6 Ni1.46Co0.54 P1.12 13 903 3.4
DBT HDS Activity (nmol DBT/gãs)
To assess the electronic characteristics of the active sites in the catalysts, infrared (IR) spectra of adsorbed carbon monoxide (CO) on the nanocatalysts were obtained at a temperature of 298 K and a pressure of 1.0 Torr, as illustrated in Figure 5.5.
The infrared (IR) spectrum of CO on the Ni2P@mSiO2 sample closely resembles that of the previously studied Ni2P/SiO2 catalyst at 8 K and a CO pressure of 5.0 Torr Notably, two distinct peaks are observed at 2097 cm⁻¹ and 2202 cm⁻¹, which correspond to CO that is terminally bonded to surface nickel and phosphorus sites, respectively.
A slight increase in the infrared spectrum around 1915 cm⁻¹ indicates the presence of CO bridge-bonded to Ni sites; however, this adsorption mode is inhibited on Ni2P due to surface phosphorus atoms disrupting adjacent nickel sites necessary for bridge-bonded CO formation The IR spectrum confirms that the Ni and P sites on encapsulated Ni2P nanoparticles are accessible to gas-phase molecules Upon evacuation of CO gas, the absorbance related to CO on phosphorus sites disappears, while the absorbance for CO terminally bonded to nickel sites remains.
The Ni1.50Co0.50P@mSiO2 nanocatalyst exhibits a notable shoulder absorbance between 2040-2060 cm -1, suggesting CO adsorption on surface cobalt sites When cobalt is incorporated into the Ni2P lattice to create Ni2-xCoxP, a shift in the νCO absorbance for Ni terminally bonded CO occurs, moving from 2097 cm -1 to 2089 cm -1 This shift is influenced by the cobalt content in the bimetallic phosphide phase, indicating that increased cobalt leads to enhanced electronic donation from cobalt to nickel Consequently, the increased electron density on nickel sites facilitates greater backbonding interactions.
The interaction between Ni and the CO molecule leads to a weakening of the CO bond Oyama and colleagues noted a similar trend in the νCO absorbances for various Ni2-xFexP/SiO2 catalysts, supporting this conclusion Additionally, shifts in XPS binding energies and XANES absorption edges for bulk Ni2-xCoxP materials indicate a slight transfer of electron density from Co to Ni in these bimetallic phosphides.
5.3.1.2 HDS catalytic activity and selectivity of Ni 2-x Co x P@mSiO 2 catalysts
The conversions of dibenzothiophene (DBT) in relation to reaction temperature are illustrated in Figure 5.7 for the Ni2-xCoxP@mSiO2 nanocatalysts within the temperature range of 548-648 K Notably, these nanocatalysts demonstrate measurable DBT conversions starting at 548 K, which supports previous findings regarding the accessibility of active sites on nanoparticle surfaces to reactant molecules Additionally, the DBT hydrodesulfurization (HDS) activities, calculated from DBT conversion to hydrocarbon products at 623 K, are detailed in Table [insert table number].
Figure 5.5 Infrared spectra of adsorbed CO on Ni2-xCoxP@mSiO2 nanocatalysts
5.2 The Ni1.92Coo0.08P@mSiO2 nanocatalyst was the most active (on a mass catalyst basis) of the different compositions investigated; in general, the DBT conversions and HDS activities decreased with increasing Co content At 623 K, the Ni1.92Co0.08P@mSiO2 was 31% more active for DBT HDS than Ni2P@mSiO2 when the activities are normalized on a mass of catalyst basis This result is consistent with a study of Ni2-xCoxP/SiO2 catalysts in which a Ni1.92Co0.08P/SiO2 catalyst prepared from a P-enriched precursor (P/(Ni+Co) = 0.80) was the most active catalyst for thiophene HDS, with an activity 34% higher than that of Ni2P/SiO2 catalyst prepared from a precursor having P/Ni = 0.80 71 A similar finding was observed for unsupported phases, as a nominal composition of Ni2 Co0.08P was found to have an activity 67% higher than that of Ni2P for the HDS of 4,6-DMDBT 72 In both studies, it was concluded that the small amount of Co in the bulk and supported Co0.08Ni2P resulted in surface enrichment in P; this enrichment may due to a restructuring of the surface of the metal phosphide particles that results in a higher proportion of Ni in M(2) sites than in Ni2P The Ni1.92Co0.08P@mSiO2 nanocatalyst had a significantly higher
The Ni2P@mSiO2 nanocatalysts exhibit a higher CO chemisorption capacity compared to Ni2-xCoxP@mSiO2, with values of 25 and 17 µmol/g, respectively, likely due to a surface restructuring that enhances the exposure of Ni sites on the nanoparticles As the cobalt content increases in the Ni2-xCoxP@mSiO2 nanocatalysts, both the hydrodesulfurization (HDS) activities and CO chemisorption capacities decline steadily, reflecting a similar trend seen in silica-supported Ni2-xCoxP/SiO2 catalysts synthesized via the TPR method.
The turnover frequencies (TOFs) for DBT HDS, derived from CO chemisorption capacities as a measure of active site densities, show an increasing trend with higher Co content in Ni2-xCoxP nanoparticles, as detailed in Table 5.2 and illustrated in Figure 5.7 Notably, the calculation assumes one CO molecule adsorbed per Ni site While HDS activities on a mass basis decline with increased Co content, the CO chemisorption capacities diminish at a faster rate, leading to a rise in TOFs The Ni1.50Co0.50P@mSiO2 nanocatalyst exhibits the highest TOFs at 0.0088 s -1, nearly double that of Ni2P@mSiO2, which has a TOF of 0.0052 s -1 Enhanced electron density on Ni sites, as indicated by IR studies, may promote critical HDS steps, including H2 dissociation and organosulfur molecule adsorption.
Dib e n z o th io p h e n e Co n v e rs io n (% )
Figure 5.6 Dibenzothiophene HDS conversion vs temperature for Ni2-xCoxP@mSiO2 nanocatalysts
Sulfur removal in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) occurs through two main pathways: direct desulfurization (DDS) and hydrogenation (HYD) Research utilizing a sulfided Co-Mo catalyst supported on Al2O3 has elucidated this reaction network The DDS pathway involves the hydrogenolysis of C-S bonds without affecting the aromatic rings, yielding biphenyl (BP) as the final product In contrast, the HYD pathway entails the hydrogenation of one or both aromatic rings before the C-S bond hydrogenolysis, resulting in cyclohexylbenzene (CHB) and bicyclohexane (BCH) as final products The selectivity of HDS products derived from DBT is influenced by the reaction temperature.
The Ni2P@mSiO2 and Ni1.5Co0.5P@mSiO2 nanocatalysts, illustrated in Figure 5.8 (a) and (b), demonstrate a consistent selectivity for BP as the major product across all compositions at 623 K, as detailed in Table 5.3 This finding aligns with previous studies on TPR-prepared Ni2P/SiO2 and Ni1.97Fe0.03P/SiO2 catalysts Notably, the Ni2P@SiO2 nanocatalyst maintained a BP selectivity of approximately 80% throughout the temperature range of 548-648 K, with only minor fluctuations in product selectivity observed.
DBT HDS Activity (nmol DBT/gãs)
Co fraction (Co/(Co+Ni))
The study examines the catalytic performance of Ni2-xCoxP@mSiO2 nanocatalysts in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) The results indicate that at 548 K, partially hydrogenated tetrahydrodibenzothiophene (4H-DBT) constitutes approximately 10% of the product yield, but this drops to zero at 648 K Conversely, the selectivity for cyclohexylbenzene (CHB) rises with increasing temperature, starting at 5% at 548 K and reaching 18% at 648 K Additionally, the fully hydrogenated product, BCH, consistently represents less than 3% of the total products across all tested temperatures.
Ni1.5Co0.5P@mSiO2 composition follow the same trend in product selectivity for all temperatures
As shown in Table 5.3 regardless of the amount of Co, all compositions consist of a similar proportion of products at 623 K
Scheme 5.2 Dibenzothiophene HDS reaction network
Table 5.3 Dibenzothiophene HDS product selectivities at 623 K for different Ni2-xCoxP@mSiO2 catalysts
Catalyst 4H-DBT BP CHB BCH
Conclusions
Research on Ni2-xCoxP nanoparticles encapsulated in mesoporous silica revealed their effectiveness as a model system for hydrodesulfurization (HDS) reactions involving bimetallic phosphides The Ni1.92Co0.08P@mSiO2 nanocatalyst exhibited the highest dibenzothiophene (DBT) HDS activity when assessed on a mass basis, aligning with findings from TPR-prepared silica-supported Ni2-xCoxP catalysts Notably, while the overall HDS activity diminished with increased cobalt content, the turnover frequencies (TOFs) demonstrated an opposite trend, highlighting the complex relationship between cobalt incorporation and catalytic performance.
4 ,6 -DM DB T H DS Pro d u ct Sel ec tivity (%) x()
The chemisorption capacity and activity of Ru Fraction (Ru/(Ru+Ni) are influenced by the higher turnover frequencies (TOFs) associated with increased electron density in Co-rich materials, stemming from charge density transfer from Co to Ni The selectivity of the DBT product in Ni2-xCoxP@mSiO2 nanocatalysts remains largely consistent with Co incorporation, with the direct desulfurization (DDS) pathway being the preferred route across all compositions.
Preliminary data on the hydrodesulfurization (HDS) of 4,6-DMDBT using Ni2-xRuxP@mSiO2 nanoparticles revealed a surprising decrease in catalytic activity with the introduction of noble metal Ru While 3-3′-DMCHB, produced via the hydrogenation (HYD) pathway, remained the predominant product across all compositions, the dependence of the direct desulfurization (DDS) pathway on composition was evident Further research is necessary to explore how composition influences the activity and selectivity of Ni2-xRuxP@mSiO2 catalysts Notably, the nanoparticles encapsulated in mesoporous silica demonstrated excellent phase stability, resistance to sintering, and protection against sulfur poisoning, making them ideal robust model systems for investigating the effects of particle size, shape, and composition on hydrodesulfurization processes.
Conclusions
The rising demand for low sulfur diesel fuels and the shift towards crude oil with higher impurities are prompting the need for innovative sulfur removal techniques in fossil fuel refining Hydrodesulfurization (HDS) remains the primary industrial method, utilizing Ni (Co) promoted sulfided molybdenum as the leading catalyst; however, its lamellar structure limits effectiveness Concurrently, the search for renewable energy solutions, particularly through electrocatalytic water splitting, has gained momentum due to concerns over fossil fuel depletion and greenhouse gas emissions This process requires efficient catalysts to minimize overpotential in the oxygen evolution reaction (OER) Recent research has shifted towards ternary metal phosphides as catalysts for both HDS and OER, leveraging the synergistic effects of metal interactions and allowing for the incorporation of abundant, low-cost metals to enhance catalytic activity To advance this field, new synthetic methodologies must be developed to create novel materials, focusing on the production of discrete, phase-pure, and nearly monodisperse nanoparticles to evaluate the influence of active site density on catalyst performance.
Solution-phase arrested precipitation reactions enable precise control over the size, shape, and composition of nanoparticles This research successfully synthesized nearly monodisperse ternary compositions, Ni2-xCoxP and Ni2-xRuxP, achieving sizes of 9-14 nm (S.D < 20%) and 5-10 nm (S.D < 15%), respectively, while maintaining excellent compositional accuracy Various synthetic parameters, such as metal:P ratio, temperature, and heating rate, were found to significantly influence the morphology (dense vs hollow), size (ranging from 7-25 nm), and composition of the nanoparticles in the NiCoP system The capability to finely tune nanoparticle size paves the way for conducting size-dependent catalytic studies.
The synthesis of ternary phosphide nanoparticles requires tailored approaches for different metal compositions In the Ni-Co-P system, both metal precursors are introduced simultaneously with TOP, while in the Ni-Ru-P system, Ru particles must be synthesized first before adding the Ni precursor and TOP The redox behavior of metal precursors is influenced by their ligand environment; for instance, Ni and Co acac precursors can be reduced with TOP present, whereas RuCl3 is resistant to reduction in the same conditions, preventing nanoparticle formation if TOP is used initially.
Catalytic studies revealed significant synergism in the Ni-Co-P and Ni-Ru-P systems The Ni1.92Co0.08P composition exhibited the highest dibenzothiophene hydrodesulfurization (DBT HDS) activity, while the Ni1.25Ru0.75P composition demonstrated superior oxygen evolution reaction (OER) activity with respect to overpotential The electronic interactions between the metals in these ternary phosphide nanoparticles influenced their redox properties, as indicated by infrared spectroscopic studies Notably, Ni sites in Co-rich phases showed increased electron density due to electron transfer from Co, enhancing the HDS process In the Ni-Ru-P system, the introduction of Ru facilitated the formation of an oxidized Ni species, which is believed to be the active species for OER, leading to improved catalytic performance Furthermore, the redox characteristics of the metals in the ternary phosphides were confirmed during the post-reduction phase of the Ni2-xRuxP materials, with the final phase being sensitive to the phosphorus source used Preliminary studies on the HDS activity of the Ni-Ru-P system suggest that the lower activity of Ru-incorporated materials may stem from the highly oxidizing nature of Ni in these compounds, contrasting with the reducing behavior of Ni in the Ni-Co-P system.
The identification of a sweet spot in the OER catalytic activity indicates that factors beyond the electronic interactions between two metals, such as site occupancy (tetrahedral versus square pyramidal), significantly influence catalytic performance.
Encapsulated materials serve as excellent model catalysts for investigating how active site density influences catalytic activity Their durability allows them to retain size and shape stability even in harsh hydrodesulfurization (HDS) conditions, making them suitable for systematic studies This enables researchers to explore the effects of particle size, shape, and composition, ultimately aiding in the development of more effective hydrotreating catalysts.
Prospectus
6.2.1 Size dependent HDS catalytic studies of ternary phosphide nanoparticles
We successfully tuned the particle size and ensured that the encapsulated systems remain robust under hydrodesulfurization (HDS) conditions Therefore, it is essential to conduct size-dependent HDS catalytic studies to evaluate how active site density influences catalytic performance Previous research has indicated that the site occupancy of metals (M(1) and M(2)) is size-dependent, highlighting the importance of investigating the effects of varying particle sizes on both HDS activity and selectivity.
6.2.2 Determine the site occupancy of different metals in ternary phase using X-ray absorption studies
The hexagonal Ni2P structure features two distinct metal sites: M(1), which is tetrahedral, and M(2), which is square pyramidal Previous studies using EXAFS and Mӧssbauer spectroscopy have shown that the occupancy of these metal sites is dependent on composition By examining the site occupancy in solution-phase prepared Ni-Co-P and Ni-Ru-P nanoparticles, we can gain a deeper understanding of how this occupancy influences their catalytic activity.
6.2.3 XPS studies to probe the surface nature of encapsulated nanoparticles
The surface characteristics play a crucial role in determining catalytic behavior, making it essential to analyze the surface nature of encapsulated materials using surface-sensitive techniques like X-ray Photoelectron Spectroscopy (XPS) This method allows for the examination of the surface state and elemental composition of materials encapsulated in mesoporous silica By comparing the XPS data of the encapsulated materials with that of the as-prepared particles, researchers can gain insights into the surface modifications that occur during the encapsulation process.
6.2.4 Catalytic properties of ternary phosphide nanoparticles for other applications
Hydrodeoxygenation (HDO) is a crucial catalytic process in biofuel production from biomass pyrolysis, effectively removing oxygen to enhance energy density, reduce viscosity, and improve volatility and thermal stability Transition metal phosphides serve as effective catalysts in HDO, with a study by the Bussell group highlighting Ru2P as the most efficient catalyst for furan HDO, achieving an impressive activity of 12,390 mmol furan/g.s, while Ni2P also demonstrates significant activity at 1,610 mmol furan/g.s Future research could explore the HDO performance of a ternary phase combining both Ru and Ni.
Ni metals in the phosphide lattice
According to Appendix A, we can obtain phase-pure crystalline RuxPy phases, which can be evaluated for hydrodeoxygenation (HDO) activity This allows for a comparison of their performance against materials prepared via temperature-programmed reduction (TPR).
Ternary phosphides, specifically Ni2-xCoxP, are promising candidates for photocatalytic water splitting, particularly in enhancing hydrogen evolution reactions (HER) Previous studies suggest that combining these phosphide nanoparticles with semiconducting materials to create hybrid composites can significantly improve energy conversion efficiency in photocatalytic HER applications.
APPENDIX A PREPARATION OF ENCAPSUALTED CRYSTALLINE Ru x P y
The Brock group previously demonstrated that crystalline Pd5P2 nanoparticles can be synthesized through the thermal treatment of amorphous PdxPy particles encapsulated in mesoporous silica Following a similar methodology, amorphous Ru:P~2 material was encapsulated in mesoporous silica and reduced under Ar/H2 at 450 °C, but it remained amorphous after one hour To achieve crystallization, the material was subsequently heated to 500 °C with PPh3, resulting in the formation of Ru2P phase peaks, though the product was not phase-pure Further heating to 550 °C without a P source yielded primarily crystalline products, along with impurity phases indexed to Ru or RuP These findings indicate that while crystalline Ru-P phases can be formed from encapsulated materials at elevated temperatures, achieving phase-pure Ru2P necessitates adjustments in the Ru:P ratio.
In this study, Ru nanoparticles were encapsulated in mesoporous silica and annealed at 550 °C with 1g of PPh3 To assess the impact of calcination, an additional sample of the same encapsulated material underwent calcination in air at 425 °C for 2 hours before the reduction process The results, illustrated in figure A2(a), highlight the characteristics of the directly reduced material.
The analysis of 1 g of PPh3 reveals peaks that correspond exclusively to the Ru2P phase, indicating that a phase-pure Ru2P can be successfully synthesized using this method The material, when calcined and reduced with 1 g of PPh3, demonstrates consistent results.
Ru metal as an impurity phase in the PXRD pattern in addition to the Ru2P phase (Figure A2(b))
To prepare encapsulated crystalline RuP phase, amorphous Ru:P~1 materials were encapsulated in silica and heat treated under reducing conditions The direct reduction with 1 g of PPh3 resulted in broad peaks indicative of the RuP phase, while the calcined material reduced under the same conditions exhibited distinct crystalline peaks corresponding to phase pure RuP These findings highlight that encapsulated materials can achieve phase pure Ru-P phases, emphasizing the significant impact of the calcination step on the resultant phase.
In te n si ty (a rb u n its) b a
The PXRD patterns of encapsulated amorphous Ru:P~2 materials were analyzed after being annealed in a reducing environment under various conditions The samples included those annealed at 450 °C for 1 hour, those annealed at 500 °C for 2 hours with 1 g of PPh3, and the products from the latter step that were further annealed at 550 °C for 2 hours without a phosphorus source.
The PXRD patterns of encapsulated Ru nanoparticles reveal significant findings from annealing in a reducing environment under various conditions Specifically, the nanoparticles were either directly reduced at 550 °C for 2 hours with 1 g of PPh3 or first calcined at 425 °C for 2 hours before reduction at the same temperature and duration Additionally, the encapsulated amorphous Ru:P~2 materials demonstrated distinct PXRD patterns when annealed at 450 °C for 1 hour, 500 °C for 2 hours with 1 g of PPh3, and further annealed at 550 °C for 2 hours without a phosphorus source.
In te n si ty (a rb u n its)
In te n si ty (a rb u n its)
The PXRD patterns of encapsulated Ru nanoparticles were analyzed after annealing in a reducing environment under various conditions Specifically, the first condition involved directly reducing the encapsulated material at 550 °C for 2 hours using 1 g of PPh3 The second condition included an initial calcination of the encapsulated material at 425 °C for 2 hours, followed by reduction at 550 °C for an additional 2 hours with the same amount of PPh3.
Int en s ity ( ar b un its )
In te nsi ty (a rb u ni ts)
Figure A3 PXRD patterns of the encapsulated amorphous Ru:P~1 nanoparticles annealed in reducing environment under different conditions (a) as encapsulated material directly reduced at
550 °C for 2 h with 1 g PPh3 (b) encapsulated material first calcined at 425 °C for 2 h and then reduced at 550 °C for 2 h with 1 g PPh3
Figure A3 PXRD patterns of the encapsulated amorphous Ru:P~1 nanoparticles annealed in reducing environment under different conditions (a) as encapsulated material directly reduced at
550 °C for 2 h with 1 g PPh3 (b) encapsulated material first calcined at 425 °C for 2 h and then reduced at 550 °C for 2 h with 1 g PPh3
APPENDIX B PERMISSION/LICENCE AGREEMENT FOR COPYRIGHT
ELSEVIER LICENSE TERMS AND CONDITIONS
This Agreement between Don Ruchira Liyanage ("You") and Elsevier ("Elsevier") consists of your license details and the terms and conditions provided by Elsevier and Copyright Clearance Center
Licensed Content Publication Surface Science
Licensed Content Title Probing hydrodesulfurization over bimetallic phosphides using monodisperse Ni 2-x M x P nanoparticles encapsulated in mesoporous silica
Licensed Content Author Samuel J Danforth,D Ruchira Liyanage,Asha Hitihami-
Mudiyanselage,Boris Ilic,Stephanie L Brock,Mark E Bussell Licensed Content Date June 2016
Type of Use reuse in a thesis/dissertation
Are you the author of this
Title of your thesis/dissertation
Synthesis and Characterization of Transition Metal Phosphide Nanoparticles for Catalytic Applications: Model Catalysts for Hydrodesulfurization and Electrocatalysts for the Oxygen Evolution Reaction
Estimated size (number of pages)
Requestor Location Don Ruchira Liyanage
4747 Anthony Wayne Dr DETROIT, MI 48201
PERMISSION/LICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE
This type of permission/license, instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order Please note the following:
Permission is granted for your request in both print and electronic formats, and translations.
If figures and/or tables were requested, they may be adapted or used in part.
Please print this page for your records and send a copy of it to your publisher/graduate school.
Appropriate credit for the requested material should be given as follows:
"Reprinted (adapted) with permission from (COMPLETE REFERENCE
CITATION) Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words.
One-time permission is granted only for the use specified in your request
No additional uses are granted (such as derivative works or other editions) For any other uses, please submit a new request.
Copyright © 2016 Copyright Clearance Center, Inc All Rights Reserved Privacy statement Terms and Conditions Comments? We would like to hear from you E-mail us at customercare@copyright.com
If you're a copyright.com user, you can login to RightsLink using your copyright.com credentials Already a
RightsLink user or want to learn more?
Composition, Size, and Morphology in Discrete Ni 2– x Co x P Nanoparticles
Danforth, Yi Liu, et al
Thank you for your order
This Agreement between Don Ruchira Liyanage ("You") and Elsevier ("Elsevier") consists of your license details and the terms and conditions provided by Elsevier and Copyright
Your confirmation email will contain your order number for future reference
Licensed Content Journal of Catalysis
Licensed Content Title Mửssbauer spectroscopy investigation and hydrodesulfurization properties of iron–nickel phosphide catalysts Licensed Content Author Amy F Gaudette,Autumn W Burns,John R Hayes,Mica C Smith,Richard H
Bowker,Takele Seda,Mark E Bussell
Type of Use reuse in a thesis/dissertation
Number of 1 figures/tables/illustrations
Are you the author of this No
Title: Mửssbauer spectroscopy investigation and hydrodesulfurization properties of iron–nickel phosphide catalysts
Smith,Richard H Bowker,Takele Seda,Mark E Bussell
Date: 25 May 2010 Copyright © 2010 Elsevier Inc All rights reserved.
Will you be translating? No
Title of your Synthesis and Characterization of Transition Metal Phosphide thesis/ dissertation Nanoparticles for Catalytic Applications: Model Catalysts for
Hydrodesulfurization and Electrocatalysts for the Oxygen Evolution Reaction
Requestor Location Don Ruchira Liyanage 4747 Anthony Wayne Dr
4747 Anthony Wayne Dr DETROIT, MI 48201 United States
Total Attn: Don Ruchira Liyanage
Copyright © 2016 Copyright Clearance Center, Inc All Rights Reserved Privacy statement Terms and
Conditions Comments? We would like to hear from you E-mail us at customercare@copyright.com
1 Alivisatos, A P Perspectives on the Physical Chemistry of Semiconductor Nanocrystals
2 Alivisatos, A P Semiconductor Clusters, Nanocrystals, and Quantum Dots Science 1996,
3 Leslie-Pelecky, D L.; Rieke, R D Magnetic Properties of Nanostructured Materials Chem Mater 1996, 8, 1770-1783
4 Eustis, S.; El-Sayed, M A Why Gold Nanoparticles Are More Precious Than Pretty Gold: Noble Metal Surface Plasmon Resonance and Its Enhancement of the Radiative and Nonradiative Properties of Nanocrystals of Different Shapes Chem Soc Rev 2006, 35, 209-217
5 Sun, Y.; Xia, Y Gold and Silver Nanoparticles: A Class of Chromophores with Colors Tunable in the Range from 400 to 750 nm Analyst 2003, 128, 686-691
6 Valden, M.; Lai, X.; Goodman, D W Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties Science 1998, 281, 1647-1650
7 Rao, C.; Kulkarni, G.; Thomas, P J.; Edwards, P P Size‐Dependent Chemistry: Properties of Nanocrystals Chem.-Eur J 2002, 8, 28-35
8 El-Sayed, M A Small Is Different: Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals Acc Chem Res 2004, 37, 326-333
9 Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M A Chemistry and Properties of Nanocrystals of Different Shapes Chem Rev 2005, 105, 1025-1102
10 Huynh, W U.; Dittmer, J J.; Alivisatos, A P Hybrid Nanorod-Polymer Solar Cells Science 2002, 295, 2425-2427
11 Michalet, X.; Pinaud, F F.; Bentolila, L A.; Tsay, J M.; Doose, S.; Li, J J.; Sundaresan, G.; Wu, A M.; Gambhir, S S.; Weiss, S Quantum Dots for Live Cells, in Vivo Imaging, and Diagnostics Science 2005, 307, 538-544
12 Talapin, D V.; Lee, J.-S.; Kovalenko, M V.; Shevchenko, E V Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications Chem Rev 2010, 110, 389-458
13 Brock, S L.; Perera, S C.; Stamm, K L Chemical Routes for Production of Transition- Metal Phosphides on the Nanoscale: Implications for Advanced Magnetic and Catalytic Materials Chem.–Eur J 2004, 10, 3364-3371
14 Brock, S L.; Senevirathne, K Recent Developments in Synthetic Approaches to Transition Metal Phosphide Nanoparticles for Magnetic and Catalytic Applications J.Solid State Chem
15 Muthuswamy, E.; Kharel, P R.; Lawes, G.; Brock, S L Control of Phase in Phosphide Nanoparticles Produced by Metal Nanoparticle Transformation: Fe2P and FeP ACS Nano 2009,
16 Muthuswamy, E.; Savithra, G H L.; Brock, S L Synthetic Levers Enabling Independent Control of Phase, Size, and Morphology in Nickel Phosphide Nanoparticles ACS Nano 2011, 5, 2402-2411
17 Senevirathne, K.; Tackett, R.; Kharel, P R.; Lawes, G.; Somaskandan, K.; Brock, S L Discrete, Dispersible Mnas Nanocrystals from Solution Methods: Phase Control on the Nanoscale and Magnetic Consequences ACS Nano 2009, 3, 1129-1138
18 Zhang, Y.; Regmi, R.; Liu, Y.; Lawes, G.; Brock, S L Phase-Coexistence and Thermal Hysteresis in Samples Comprising Adventitiously Doped MnAs Nanocrystals: Programming of Aggregate Properties in Magnetostructural Nanomaterials ACS Nano 2014, 8, 6814-6821
19 Hettiarachchi, M A.; Abdelhamid, E.; Nadgorny, B.; Brock, S L Synthesis of Colloidal MnSb Nanoparticles: Consequences of Size and Surface Characteristics on Magnetic Properties
20 Hitihami-Mudiyanselage, A.; Arachchige, M P.; Seda, T.; Lawes, G.; Brock, S L Synthesis and Characterization of Discrete FexNi2–XP Nanocrystals (0 < X < 2): Compositional Effects on Magnetic Properties Chem Mater 2015, 27, 6592-6600
21 Liyanage, D R.; Danforth, S J.; Liu, Y.; Bussell, M E.; Brock, S L Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2–XCoxP Nanoparticles Chem Mater 2015,
22 Li, D.; Arachchige, M P.; Kulikowski, B.; Lawes, G.; Seda, T.; Brock, S L Control of Composition and Size in Discrete CoxFe2-XP Nanoparticles: Consequences for Magnetic Properties Chem Mater 2016, 28, 3920-3927
23 Murray, C B.; Norris, D J.; Bawendi, M G Synthesis and Characterization of Nearly Monodisperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites J Am
24 Guzelian, A A.; Katari, J E B.; Kadavanich, A V.; Banin, U.; Hamad, K.; Juban, E.; Alivisatos, A P.; Wolters, R H.; Arnold, C C.; Heath, J R Synthesis of Size-Selected, Surface- Passivated InP Nanocrystals J Phys Chem 1996, 100, 7212-7219
25 Yu, W W.; Wang, Y A.; Peng, X Formation and Stability of Size-, Shape-, and Structure- Controlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals Chem Mater
26 Klimov, V I., Nanocrystal Quantum Dots CRC Press: 2010
27 Souza, D.; Pralong, V.; Jacobson, A.; Nazar, L A Reversible Solid-State Crystalline Transformation in a Metal Phosphide Induced by Redox Chemistry Science 2002, 296, 2012-
28 Sun, M.; Liu, H.; Qu, J.; Li, J Earth‐Rich Transition Metal Phosphide for Energy Conversion and Storage Adv Energy Mater 2016, 6, 1600087
29 Carenco, S.; Portehault, D.; Boissière, C.; Mézailles, N.; Sanchez, C Nanoscaled Metal Borides and Phosphides: Recent Developments and Perspectives Chem Rev 2013, 113, 7981-
30 Xie, R.; Battaglia, D.; Peng, X Colloidal InP Nanocrystals as Efficient Emitters Covering Blue to near-Infrared J Am Chem Soc 2007, 129, 15432-15433
31 Xiao, P.; Chen, W.; Wang, X A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution Adv Energy Mater 2015, 5, 1500985
32 Suzuki, N.; Horie, T.; Kitahara, G.; Murase, M.; Shinozaki, K.; Morimoto, Y Novel Noble- Metal-Free Electrocatalyst for Oxygen Evolution Reaction in Acidic and Alkaline Media Electrocatal 2016, 7, 115-120
33 Zhao, H Y.; Li, D.; Bui, P.; Oyama, S T Hydrodeoxygenation of Guaiacol as Model Compound for Pyrolysis Oil on Transition Metal Phosphide Hydroprocessing Catalysts Appl
34 Prins, R.; Bussell, M E Metal Phosphides: Preparation, Characterization and Catalytic Reactivity Catal Lett 2012, 142, 1413-1436
35 Blanchard, P E R.; Grosvenor, A P.; Cavell, R G.; Mar, A X-Ray Photoelectron and Absorption Spectroscopy of Metal-Rich Phosphides M2P and M3P (M = Cr−Ni) Chem Mater
36 Oyama, S T Novel Catalysts for Advanced Hydroprocessing: Transition Metal Phosphides J Catal 2003, 216, 343-352
37 Lu, Y.; Tu, J.-P.; Xiong, Q.-Q.; Xiang, J.-Y.; Mai, Y.-J.; Zhang, J.; Qiao, Y.-Q.; Wang, X.- L.; Gu, C.-D.; Mao, S X Controllable Synthesis of a Monophase Nickel Phosphide/Carbon (Ni5P4/C) Composite Electrode via Wet-Chemistry and a Solid-State Reaction for the Anode in Lithium Secondary Batteries Adv Funct Mater 2012, 22, 3927-3935
38 Su, H L.; Xie, Y.; Li, B.; Liu, X M.; Qian, Y T A Simple, Convenient, Mild Solvothermal Route to Nanocrystalline Cu3P and Ni2P Solid State Ionics 1999, 122, 157-160
39 Xie, Y.; Su, H L.; Qian, X F.; Liu, X M.; Qian, Y T A Mild One-Step Solvothermal Route to Metal Phosphides (Metal=Co, Ni, Cu) J.Solid State Chem 2000, 149, 88-91
40 Barry, B M.; Gillan, E G Low-Temperature Solvothermal Synthesis of Phosphorus-Rich Transition-Metal Phosphides Chem Mater 2008, 20, 2618-2620
41 Sawhill, S J.; Phillips, D C.; Bussell, M E Thiophene Hydrodesulfurization over Supported Nickel Phosphide Catalysts J Catal 2003, 215, 208-219
42 Molina, R.; Poncelet, G Α-Alumina-Supported Nickel Catalysts Prepared from Nickel Acetylacetonate: A TPR Study J Catal 1998, 173, 257-267
43 Reiche, M A.; Maciejewski, M.; Baiker, A Characterization by Temperature Programmed Reduction Catal Today 2000, 56, 347-355
44 Perera, S C.; Tsoi, G.; Wenger, L E.; Brock, S L Synthesis of MnP Nanocrystals by Treatment of Metal Carbonyl Complexes with Phosphines: A New, Versatile Route to Nanoscale Transition Metal Phosphides J.Am Chem Soc 2003, 125, 13960-13961
45 Park, J.; Koo, B.; Yoon, K Y.; Hwang, Y.; Kang, M.; Park, J.-G.; Hyeon, T Generalized Synthesis of Metal Phosphide Nanorods via Thermal Decomposition of Continuously Delivered Metal−Phosphine Complexes Using a Syringe Pump J.Am Chem Soc 2005, 127, 8433-8440
46 Perera, S C.; Fodor, P S.; Tsoi, G M.; Wenger, L E.; Brock, S L Application of De- Silylation Strategies to the Preparation of Transition Metal Pnictide Nanocrystals: The Case of FeP Chem Mater 2003, 15, 4034-4038
47 Park, J.; Koo, B.; Hwang, Y.; Bae, C.; An, K.; Park, J.-G.; Park, H M.; Hyeon, T Novel Synthesis of Magnetic Fe2P Nanorods from Thermal Decomposition of Continuously Delivered Precursors Using a Syringe Pump Angew Chem Int Ed 2004, 43, 2282-2285
48 Zhang, H.; Ha, D.-H.; Hovden, R.; Kourkoutis, L F.; Robinson, R D Controlled Synthesis of Uniform Cobalt Phosphide Hyperbranched Nanocrystals Using Tri-n-Octylphosphine Oxide as a Phosphorus Source Nano Lett 2011, 11, 188-197
49 Ha, D.-H.; Moreau, L M.; Bealing, C R.; Zhang, H.; Hennig, R G.; Robinson, R D The Structural Evolution and Diffusion During the Chemical Transformation from Cobalt to Cobalt Phosphide Nanoparticles J Mater Chem 2011, 21, 11498-11510
50 Henkes, A E.; Vasquez, Y.; Schaak, R E Converting Metals into Phosphides: A General Strategy for the Synthesis of Metal Phosphide Nanocrystals J.Am Chem Soc 2007, 129, 1896-
51 Yin, Y.; Rioux, R M.; Erdonmez, C K.; Hughes, S.; Somorjai, G A.; Alivisatos, A P Formation of Hollow Nanocrystals through the Nanoscale Kirkendall Effect Science 2004, 304, 711-714
52 Chiang, R.-K.; Chiang, R.-T Formation of Hollow Ni2P Nanoparticles Based on the Nanoscale Kirkendall Effect Inorg Chem 2006, 46, 369-371
53 Carenco, S.; Portehault, D.; Boissière, C.; Mézailles, N.; Sanchez, C 25 th Anniversary Article: Exploring Nanoscaled Matter from Speciation to Phase Diagrams: Metal Phosphide Nanoparticles as a Case of Study Adv, Mater 2014, 26, 371-390
54 Layan Savithra, G H.; Muthuswamy, E.; Bowker, R H.; Carrillo, B A.; Bussell, M E.; Brock, S L Rational Design of Nickel Phosphide Hydrodesulfurization Catalysts: Controlling Particle Size and Preventing Sintering Chem Mater 2013, 25, 825-833
55 Carenco, S.; Boissière, C.; Nicole, L.; Sanchez, C.; Le Floch, P.; Mézailles, N Controlled Design of Size-Tunable Monodisperse Nickel Nanoparticles Chem Mater 2010, 22, 1340-1349
56 Li, D.; Senevirathne, K.; Aquilina, L.; Brock, S L Effect of Synthetic Levers on Nickel Phosphide Nanoparticle Formation: Ni5P4 and NiP2 Inorg Chem 2015, 54, 7968-7975
57 Callejas, J F.; Read, C G.; Roske, C W.; Lewis, N S.; Schaak, R E Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction Chem Mater 2016, 28, 6017-6044
58 Fruchart, R.; Roger, A.; Senateur, J P Crystallographic and Magnetic Properties of Solid Solutions of the Phosphides M2P, M = Cr, Mn, Fe, Co, and Ni J Appl Phys 1969, 40, 1250-1257
59 Yoon, K Y.; Jang, Y.; Park, J.; Hwang, Y.; Koo, B.; Park, J.-G.; Hyeon, T Synthesis of Uniform-Sized Bimetallic Iron–Nickel Phosphide Nanorods J.Solid State Chem 2008, 181, 1609-
60 Ye, E.; Zhang, S.-Y.; Lim, S H.; Bosman, M.; Zhang, Z.; Win, K Y.; Han, M.-Y Ternary Cobalt–Iron Phosphide Nanocrystals with Controlled Compositions, Properties, and Morphologies from Nanorods and Nanorice to Split Nanostructures Chem.-Eur J 2011, 17, 5982-5988
61 Colson, A C.; Whitmire, K H Synthesis of Fe2–XMnxp Nanoparticles from Single-Source Molecular Precursors Chem Mater 2011, 23, 3731-3739
62 Mendoza-Garcia, A.; Zhu, H.; Yu, Y.; Li, Q.; Zhou, L.; Su, D.; Kramer, M J.; Sun, S Controlled Anisotropic Growth of Co-Fe-P from Co-Fe-O Nanoparticles Angew Chem 2015,
63 Li, D.; Baydoun, H.; Verani, C N.; Brock, S L Efficient Water Oxidation Using CoMnP Nanoparticles J Am Chem Soc 2016, 138, 4006-4009
64 Knudsen, K G.; Cooper, B H.; Topsứe, H Catalyst and Process Technologies for Ultra Low Sulfur Diesel Appl Catal., A 1999, 189, 205-215
65 Pawelec, B.; Navarro, R M.; Campos-Martin, J M.; Fierro, J L Towards near Zero-Sulfur Liquid Fuels: A Perspective Review Catal Sci Tech 2011, 1, 23-42
66 Topsứe, H.; Clausen, B S.; Massoth, F E., Hydrotreating Catalysis Springer: 1996
67 Gates, B C.; Topsứe, H Reactivities in Deep Catalytic Hydrodesulfurization: Challenges, Opportunities, and the Importance of 4-Methyldibenzothiophene and 4,6- Dimethyldibenzothiophene Polyhedron 1997, 16, 3213-3217
68 Shafi, R.; Hutchings, G J Hydrodesulfurization of Hindered Dibenzothiophenes: An Overview Catal Today 2000, 59, 423-442
69 Gaudette, A F.; Burns, A W.; Hayes, J R.; Smith, M C.; Bowker, R H.; Seda, T.; Bussell,
M E Mửssbauer Spectroscopy Investigation and Hydrodesulfurization Properties of Iron–Nickel Phosphide Catalysts J Catal 2010, 272, 18-27
70 Oyama, S T.; Lee, Y.-K The Active Site of Nickel Phosphide Catalysts for the Hydrodesulfurization of 4,6-DMDBT J Catal 2008, 258, 393-400
71 Burns, A W.; Gaudette, A F.; Bussell, M E Hydrodesulfurization Properties of Cobalt–Nickel Phosphide Catalysts: Ni-Rich Materials Are Highly Active J Catal 2008, 260, 262-269
72 Abu, I I.; Smith, K J The Effect of Cobalt Addition to Bulk MoP and Ni2P Catalysts for the Hydrodesulfurization of 4, 6-Dimethyldibenzothiophene J Catal 2006, 241, 356-366
73 Ted Oyama, S.; Zhao, H.; Freund, H.-J.; Asakura, K.; Włodarczyk, R.; Sierka, M Unprecedented Selectivity to the Direct Desulfurization (DDS) Pathway in a Highly Active FeNi Bimetallic Phosphide Catalyst J Catal 2012, 285, 1-5
74 Zhao, H.; Oyama, S T.; Freund, H.-J.; Włodarczyk, R.; Sierka, M Nature of Active Sites in Ni2P Hydrotreating Catalysts as Probed by Iron Substitution Appl Catal., B 2015, 164, 204-
75 Senevirathne, K.; Burns, A W.; Bussell, M E.; Brock, S L Synthesis and Characterization of Discrete Nickel Phosphide Nanoparticles: Effect of Surface Ligation Chemistry on Catalytic Hydrodesulfurization of Thiophene Adv Funct Mater 2007, 17, 3933-3939
76 Song, H.; Dai, M.; Song, H.-L.; Wan, X.; Xu, X.-W.; Jin, Z.-S A Solution-Phase Synthesis of Supported Ni2P Catalysts with High Activity for Hydrodesulfurization of Dibenzothiophene J Mol Catal A: Chem 2014, 385, 149-159
77 Kudo, A.; Miseki, Y Heterogeneous Photocatalyst Materials for Water Splitting Chem
78 Lewis, N S.; Nocera, D G Powering the Planet: Chemical Challenges in Solar Energy Utilization Proc Natl Acad Sci USA 2006, 103, 15729-15735
79 Fabbri, E.; Habereder, A.; Waltar, K.; Kotz, R.; Schmidt, T J Developments and Perspectives of Oxide-Based Catalysts for the Oxygen Evolution Reaction Catal Sci Tech 2014,
80 Man, I C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H A.; Martínez, J I.; Inoglu, N G.; Kitchin, J.; Jaramillo, T F.; Nứrskov, J K.; Rossmeisl, J Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces ChemCatChem 2011, 3, 1159-1165
81 Rossmeisl, J.; Qu, Z W.; Zhu, H.; Kroes, G J.; Nứrskov, J K Electrolysis of Water on Oxide Surfaces J Electroanal Chem 2007, 607, 83-89
82 Lee, Y.; Suntivich, J.; May, K J.; Perry, E E.; Shao-Horn, Y Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions J Phys Chem Lett 2012, 3, 399-404
83 Mamaca, N.; Mayousse, E.; Arrii-Clacens, S.; Napporn, T W.; Servat, K.; Guillet, N.; Kokoh, K B Electrochemical Activity of Ruthenium and Iridium Based Catalysts for Oxygen Evolution Reaction Appl Catal., B 2012, 111–112, 376-380
84 Reier, T.; Oezaslan, M.; Strasser, P Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials ACS Catal
85 Paoli, E A.; Masini, F.; Frydendal, R.; Deiana, D.; Schlaup, C.; Malizia, M.; Hansen, T W.; Horch, S.; Stephens, I E L.; Chorkendorff, I Oxygen Evolution on Well-Characterized Mass- Selected Ru and RuO2 Nanoparticles Chem Sci 2015, 6, 190-196