Design of a promoter to enhance the stability of catalysts for hydrocarbon reactions 1

ACKNOWLEDGEMENTS Firstly, I would like to express my sincere appreciation to my supervisor, Dr. Mark Saeys, for his encouragement, insight, support and guidance throughout the course of this research project. He has been an invaluable help by providing technical guidance and support pertaining to my research work. I also wish to extend my sincere gratitude to Dr. Armando Borgna from ICES for his supervision during my experimental studies in ICES. I am also thankful to Dr. Chen Luwei and other colleagues in ICES for their helpful discussions on my experimental studies. I would sincerely like to thank my group members such as Sun Wenjie, Tan Kong Fei, Hong Won Keon, Chua Yong Ping Gavin, Fan Xuexiang, Zhuo Mingkun, Su Mingjuan, Ravi Kumar Tiwari, Shangguan Wangzuo and Dianna Otalvaro for their help, support and encouragement throughout my research work. Finally, special thanks to my dear husband Ye Ming, for being there to support me as I pursue my doctorate degree. I am extremely grateful for his love, patience and especially his understanding, which have enabled my doctorate journey to be meaningful and successful. I TABLE OF CONTENTS Acknowledgements···········································································································I Table of contents············································································································· II Summary ······················································································································· VI Symbols and abbreviations ························································································· VIII List of tables ·················································································································· XI List of figures·············································································································· XIII Publications ············································································································· XVIII Chapter Introduction ····································································································· Chapter First Principles Based Design of Metal Catalysts ············································ 2.1 Introduction ·········································································································· 2.2 Models of Catalytic Surface ················································································· 2.3 Hydrogenation of Olefins and Aromatics ····························································· 2.4 Ammonia Synthesis···························································································· 15 2.5 Steam Reforming ······························································································· 19 2.6 Selective Catalytic Oxidation ············································································· 21 2.7 References ········································································································· 24 Chapter Computational Methods ··············································································· 27 3.1 Quantum Chemistry: Theory and Methods ························································ 27 3.1.1 Fundamentals······························································································ 27 3.1.2 Density Functional Theory (DFT) ······························································ 28 3.1.3 Exchange-Correlation Functional ······························································· 30 3.1.4 Plane-Wave Basis Sets ··············································································· 32 II 3.1.5 Pseudopotentials ························································································ 32 3.1.6 The Vienna Ab Initio Simulation Package·················································· 33 3.1.7 Nudged Elastic Band Method (NEB)·························································· 36 3.2 Computational Methodology ············································································· 38 3.3 References ········································································································· 39 Chapter First Principles Based Design of Ni Catalysts with Improved Coking Resistance ···································································································· 41 4.1 Introduction ······································································································· 41 4.2 Computational Methods ···················································································· 48 4.3 Thermodynamic Diagram for Chemisorbed Carbon on Ni Catalyst ·················· 50 4.3.1 Chemisorption of On-surface Carbon on Ni(111) ······································· 50 4.3.2 Chemisorption of Subsurface Carbon on Ni(111) ······································· 51 4.3.3 Stability of Bulk Carbon in Ni Catalyst ······················································ 55 4.3.4 Distribution of Carbon Atoms between On-surface and Subsurface Sites ·· 56 4.3.5 Stability of Graphene Overlayer on Ni(111)··············································· 61 4.3.6 Chemisorption of Carbon at the Ni(211) steps············································ 63 4.4 Kinetics of Formation of Subsurface and Bulk Carbon, and of Graphene Islands on a Ni Catalyst·································································································· 65 4.4.1 Effect of the Unit Cell Size on the Diffusion Barrier of On-surface Carbon Atoms to the First Subsurface Layer··························································· 66 4.4.2 Effect of Coverage/Concentration on Kinetics of Carbon Diffusion··········· 67 4.4.3 Kinetics of the Formation of Graphene Islands from On-surface Carbon Atoms ········································································································ 77 4.5 Effect of Boron on the Stability of Carbon on a Ni Catalyst ······························ 83 4.5.1 Thermodynamic Diagram for Boron Chemisorption on Ni(111) ················ 83 4.5.2 Chemisorbed Boron at the Ni(211) Steps···················································· 88 4.6 Summary ······································································································· 90 4.7 References·········································································································· 92 III Chapter First Principles Study of the Effect of Carbon and Boron on the Activity of a Ni Catalyst ····························································································· 95 5.1 Introduction ······································································································· 95 5.2 Computational Methods ···················································································· 99 5.3 Effect of Subsurface Carbon and Boron on Methane Activation ····················· 102 5.3.1 Stability of subsurface carbon and boron·················································· 102 5.3.2 Clean Ni(111) Surface·············································································· 106 5.3.3 Ni(111) with Subsurface Carbon ······························································ 108 5.3.4 Ni(111) with Subsurface Boron································································ 114 5.4 Effect of Carbon and Boron on Methane Activation at Step Sites ··················· 115 5.4.1 Clean Ni(211) Surface·············································································· 116 5.4.2 Ni(211) Surface with Step Sites Blocked by Carbon ································ 117 5.4.3 Ni(211) Surface with Step Sites Blocked by Boron·································· 120 5.5 Summary ········································································································· 123 5.6 References ······································································································· 125 Chapter Effect of Boron on the Stability of Ni Catalysts during Steam Methane Reforming ··································································································· 127 6.1 Introduction ······································································································ 127 6.2 Catalyst Synthesis····························································································· 129 6.3 Catalyst Characterization ·················································································· 129 6.4 Catalyst Testing ································································································ 132 6.5 Results and Discussion······················································································ 133 6.5.1 Catalyst Characterization ·········································································· 133 6.5.2 Methane Steam Reforming········································································ 139 6.5 Summary ·········································································································· 147 6.5 References········································································································· 148 IV Chapter Conclusions and Future Suggestions ··························································· 151 V SUMMARY Deactivation by carbon deposition is a common challenge in many catalytic processes involving hydrocarbons, such as steam reforming of methane and heavier hydrocarbons over Ni-based catalysts. First principles Density Functional Theory (DFT) calculations were combined with experimental investigations to design Ni catalysts with improved stability. To develop a molecular level understanding of the coking mechanism on Ni catalysts, the stability of different forms of carbon that can exist on Ni catalyst and the kinetics of carbon diffusion were studied using first principles calculations. Extended graphene islands were found to be the most stable form of carbon on a Ni catalyst, with a carbon binding energy of –760 kJ/mol. However, the formation of graphene islands resembles a nucleation process and requires critical islands of about 15-20 carbon atoms. Step sites are the preferred adsorption sites for carbon atoms and can act as nucleation sites for the formation of graphene islands. On-surface carbon atoms are relatively unstable with binding energies of around –660 kJ/mol. Subsurface octahedral sites are also more stable than on-surface sites, and subsurface carbon is expected to build up easily under typical steam reforming reaction conditions. The presence of subsurface carbon significantly decreases the activity of Ni catalysts and the methane activation energy increases from 101 kJ/mol to 143 kJ/mol, when all the sites in the first subsurface layer are occupied by carbon atoms. VI Calculations indicate that boron atoms preferentially bind at the step sites and at octahedral sites just below the surface. Boron and carbon atoms hence show similar a relative binding preference, and boron is proposed to selectively block both step and subsurface sites. In addition, subsurface boron atoms were found to restructure the Ni(111) surface and lower the methane dissociation barrier from 101 kJ/mol to 64 kJ/mol. Hence, boron atoms are believed to enhance the catalyst stability and not reduce the catalyst activity. In order to validate our DFT predictions, Ni catalysts promoted with 0.5 wt% and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming. Experiments at 800 ºC and at a Gas Hourly Space Velocity (GHSV) of 330,000 cm3/hr·gcat demonstrate that promotion with 1.0 wt% boron not only reduces the activity loss from 21% to 6%, but also enhances the initial conversion from 56% to 61%. At a higher GHSV of 660,000 cm3/hr·gcat, 1.0 wt% boron reduces the activity loss from 70% to 30%. A Temperature Programmed Oxidation and Scanning Electron Microscopy study of the catalysts confirmed that boron assists in preventing carbon buildup. The theoretical predictions were experimentally validated, showing that boron promotion enhances the catalysts stability during steam reforming. VII SYMBOLS AND ABBREVIATIONS Symbols ψ ( x, R ) Wave function ρ (r ) ε xc Electronic density Exchange-correlation energy per particle of the uniform electron gas E Total energy of the system Eb Binding energy Eedge Edge energy Eee Electron-electron repulsion energy Egraphene Total energy per carbon atom for the graphene-covered surface Encl [ ρ0 (r )] Non-classical contribution in electron-electron repulsion energy Ene Nucleus-electron interaction energy E0 [ ρ0 (r )] Ground state energy E XC [ ρ (r )] Exchange-correlation functional J [ ρ0 (r )] Coulomb integral in electron-electron repulsion energy Intermediate states in NEB Ri S1 Spin T [ ρ (r )] Kinetic energy functional V ( x, R ) Potential energy Vext (r ) External potential VIII Abbreviations CPO Catalytic partial oxidation DFT Density functional theory FFT Fast fourier transformations FLAPW Full-potential linearised augmented plane-wave-method GGA Generalized gradient approximation HREELS High-resolution electron energy loss spectroscopy ICP-OES Inductively coupled plasma-optical emission spectrometry LDA Local density approximation MC Monte carlo MEP Minimum energy path NEB Nudged elastic band PDOS Projected density of states PES Potential energy surface PAW Projector-augmented-wave PES Potential energy surface PW91 Perdew-Wang 91 RPBE Revised Perdew-Burke-Ernzerhof RMM Residual minimization method SEM Scanning electron microscopy SHSV Gas hourly space velocity SPARG Sulfur passivated reforming TCD Thermal Conductivity Detector SR Steam reforming TPD Temperature Programmed Desorption TOF Turnover frequency TPO Temperature programmed oxidation UHV Ultra high vacuum VASP Vienna ab initio simulation package XRD X-Ray diffraction IX XPS X-ray photoelectron spectroscopy X LIST OF TABLES Table 4.1. Carbon binding energies for the on-surface hcp hollow sites, the octahedral sites for the first and second subsurface layer and for octahedral sites in the Ni bulk 54 Table 4.2 Influence of the unit cell size on the activation energy and reaction energy for carbon atom diffusion from the on-surface fcc hollow site to the octahedral site below. 67 Table 4.3 Carbon diffusion barriers (kJ/mol) as a function of the carbon concentrations in the first and the second subsurface layer. 75 Table 4.4. Boron binding energies for the on-surface hcp hollow sites, the octahedral sites for the first and second subsurface layer and for octahedral sites in the Ni bulk. 87 Table 4.5 Boron binding energies for different configurations of four boron atoms in a p(2x2) unit cell. 87 Table 5.1 Structure and binding energy for different surface structures with subsurface carbon and boron. 105 Table 5.2 Methyl and hydrogen binding energies (kJ/mol) for the clean Ni(111) surface, the Ni(111)-CSS surface with subsurface carbon, and the Ni(111)-BSS surface with subsurface boron. 110 Table 5.3 Transition state geometries and barriers for methane activation on Ni(111), Ni(111)-CSS and Ni(111)-BSS surfaces. 113 Table 5.4 Methyl and hydrogen binding energies (kJ/mol) for the Ni(211) surface, the Ni(211) surface with all step sites occupied by carbon, Ni(211)-Cstep,100%, by boron, Ni(211)Bstep,100%, and with half of the step sites occupied by boron, Ni(211)-Bstep,50%. 119 Table 5.5 Transition state geometries and barriers for methane activation on Ni(211), Ni(211)-Cstep,100%, Ni(211)-Bstep,50% and Ni(211)Bstep,100% surfaces. 121 Table 6.1 Bulk composition (ICP-OES), surface composition (XPS), particle size (XRD) and dispersion for calcined 15 wt % Ni/γ- 137 XI Al2O3 catalysts promoted with boron. XII LIST OF FIGURES Figure 2.1 Three approaches and examples for modeling chemisorption and reactivity on surfaces. (Left) cluster approach, maleic anhydride on Pd; (center) embedding scheme: ammonia adsorption in a zeolite cage; (right) periodic slab model: maleic anhydride adsorption on Pd(111). Figure 2.2 Representative kinetic Monte Carlo simulation snapshot for ethene hydrogenation over Pd. 13 Figure 2.3 Overview of the different reaction paths for benzene hydrogenation. The dominant reaction path is indicated in boldface. The hydrogenation activation energies for every step along the dominant reaction path are indicated. The energy values are given in kJ/mol. 13 Figure 2.4 Calculated turnover frequencies for ammonia synthesis as a function of the adsorption energy of nitrogen for various transition metals and alloys. 16 Figure 2.5 The calculaterd potential energy diagram for NH3 synthesis from N2 and H2 over Ru(0001) (dashed curve) and stepped Ru(0001) (solid curve). 16 Figure 2.6 Energies for the species on Ni(211) and Ni(111). All energies are relative to CH4 and H2O in the gas phase and calculated using the results for the individual species. 20 Figure 2.7 Conversion of n-butane as a function of time during steam reforming in a 3% n-butane-7% hydrogen-3% water in helium mixture at a space velocity of 1.2h-1. The dashed curve shows the n-butane conversion for the Ni and the solid curve is for the Au/Ni supported catalyst. 20 Figure 3.1 Typical flow-chart of VASP for the self consistent determination of the Kohn-Sham ground state. 35 XIII Figure 3.2 Schematic illustration of the nudged elastic band method. Starting from an initially guessed reaction path (dashed line) the chain converges to the nearest minimum path on the PES (full line). 37 Figure 4.1 Carbon binding energies for chemisorption at the four high symmetry sites of the Ni(111) surface as a function of coverage. The symbols indicate the calculated binding energies. 51 Figure 4.2 Binding energies for the on-surface hcp hollow sites (∆) and the octahedral sites of the first (□) and second (x) subsurface layer of the Ni(111) surface as a function of the coverage and the concentration. The symbols indicate calculated binding energies 54 Figure 4.3 Binding energies per carbon atom for selected configurations of four carbon atoms distributed over the onsurface and subsurface sites of a p(2x2) unit cell as a function of the carbon concentration in the first subsurface layer. The distribution between the first and second subsurface layer (●), between the on-surface fcc and the first subsurface layer (■) and between the on-surface hcp and the first subsurface layer (▲) are presented. 58 Figure 4.4 Energy diagram for different distributions of four carbon atoms over the on-surface hollow and the subsurface octahedral sites of a p(2x2) unit cell. Starting from four onsurface carbon atoms, the system lowers its total energy by filling subsurface octahedral sites. Solid lines indicate the thermodynamically preferred pathways. 59 Figure 4.5 Possible high symmetry adsorption modes for a graphene overlayer on a Ni(111) surface. Carbon atoms are located at (A) both the fcc and hcp threefold hollow sites; (B) atop and fcc threefold hollow sites; (C) atop and hcp threefold hollow sites; (D) two near atop sites. 62 XIV Figure 4.6 The geometry of carbon at fivefold coordinated site on Ni(211) surface. Carbon atoms are represented by smaller balls and Ni atoms are represented larger balls Left panel, 50% carbon step coverage and right panel, 100% carbon step coverage. 64 Figure 4.7 Calculation procedure for carbon diffusion barriers as a function of on-surface coverage and of subsurface carbon concentration. 70 Figure 4.8 Carbon diffusion barriers as a function of carbon concentration in the first subsurface layer for low (1/9 ML, ∆), average (4/9 ML, ●) and high (1.0 ML, ◇ ) surface coverages. 71 Figure 4.9 Diffusion barrier as a function of surface coverage for a subsurface carbon concentration of 0%. 73 Figure 4.10 Relationship between diffusion barriers and reaction energies for carbon diffusion from on-surface fcc sites to octahedral sites in the first subsurface layer. 73 Figure 4.11 Model graphene structures used to determine the energy cost for creating small size graphene islands: (A) a single line structure; (B) a double line structure. The white circle indicate unsaturated carbon atoms of the graphene structure, termed edge atoms. 78 Figure 4.12 Model used to calculate the stability of small graphene islands on the Ni(111) surface. Black and grey circles indicate saturated, internal graphene atoms with a carbon binding energy of –760 kJ/mol, white circles indicate unsaturated edge atoms with a carbon binding energy of –635 kJ/mol. 79 Figure 4.13 The total energy of cluster as a function of the number of atoms in the cluster. 79 XV Figure 4.14 Proposed mechanism for carbon deposition on Ni-based catalysts. On-surface carbon atoms, C*, are formed by extensive dehydrogenation of hydrocarbon molecules, CmHn, or by CO disproportionation. The on-surface carbon atoms can (i) react with adsorb oxygen and hydrogen to form products, (ii) diffuse to the Ni bulk, or (iii) combine to form graphene islands. 82 Figure 4.15 Boron binding energies for chemisorption at the four high symmetry sites of the Ni(111) surface and at the octahedral sites of the first subsurface layer as a function of coverage. 85 Figure 4.16 Ni(111) surface with subsurface boron atoms after surface reconstruction (top (a) and side (b) view). There are two distinct top sites (T1 and T2) for the surface with subsurface boron. 85 Figure 4.17 Reconstruction for 100% boron coverage at the steps. 89 Figure 5.1 Top view of methyl adsorbed at (a) fcc-t site and (b) fcc-b sites on Ni(111) surface. 107 Figure 5.2 Ni(111) surface with subsurface carbon (top (a) and side (b) view) and with subsurface boron atoms after surface reconstruction (top (c) and side (d) view). The high symmetry adsorption sites are indicated in (e). There are two distinct top sites (T1 and T2) for the surface with subsurface boron. 109 Figure 5.3 Orbital scheme for C-H bond activation on transition metal surfaces. 112 Figure 5.4 Correlation between the methane activation barrier and the centre of the d-band projected on the surface Ni atoms relative to the Fermi level for the different surfaces considered. 112 XVI Figure 5.5 Adsorption sites on a stepped Ni(211) surface (a and b). Different step bridge site can be distinguished for 50% coverage of the steps (c). Step reconstruction for 100% boron coverage at the steps (d). 118 Figure 6.1 H2 TPD profiles for 15 wt% Ni/γ-Al2O3 catalysts, unpromoted and promoted with 1.0 wt% B. 135 Figure 6.2 Ni 2p and B 1s XPS spectra for calcined and reduced 15 wt% Ni/γ-Al2O3 catalysts with various amount of boron. The spectra are normalized against Al. 139 Figure 6.3 CH4 conversion (a and c) and normalized conversion (b and d) as a function of time on stream. Reaction conditions for Fig. a, b, c and d: T=800ºC, P = atm, CH4:H2O:N2=10:10:1, methane flowrate = 50 Nml/min, catalyst weight w = 20 mg (a and b) and 10 mg (c and d), and GHSV = 330,000 cm3/hr·gcat (a and b) and 660,000 cm3/hr·gcat (c and d). Fitted rate coefficients for a linear deactivation model (eq. 6.4) are given in b and d. Reaction conditions for Fig. e, see Fig. 6.4. 143 Figure 6.4 TPO profiles for 15 wt% Ni/ γ-Al2O3 with 0.0, 0.5 and 1.0 wt% boron after reaction. Reaction conditions: T=750ºC, P=1 atm, CH4:H2O:N2=1:1:1, methane flowrate = 50 Nml/min, catalyst weight = 50 mg, and GHSV = 180,000 cm3/hr·gcat. Amount of CO2 evolved: 1.70 mmol/gcat (unpromoted catalyst), 0.31 mmol/gcat (0.5 wt% B), and 0.34 mmol/gcat (1.0 wt% B). 144 Figure 6.5 SEM images of 15%Ni/γ-Al2O3 catalysts after steam reforming. (a) unpromoted; (b) 0.5 wt% B; (c) 1.0 wt% B. Reaction conditions: see Fig. 6.4. 146 XVII PUBLICATIONS 1. Jing Xu, Mark Saeys, “Improving the coking resistance of Ni-based catalysts by promotion with subsurface boron” Journal of Catalysis, 242 (2006), 217-226 2. Jing Xu, Mark Saeys, “First principles study of the coking resistance and the activity of a boron promoted Ni catalyst” Chemical Engineering Science, 62 (2007) 5039-5041 3. Jing Xu, Mark Saeys, “Coking mechanism and promoter design for Ni-based catalyst: a first principle study” International Journal of Nanoscience, Vol. 6, No.2 (2007) 131-135 4. Jing Xu, Mark Saeys, “First principles study of the stability and the formation kinetics of subsurface and bulk carbon on a Ni catalyst”, Journal of Physical Chemistry C, 112 (2008) 9679-9685 5. Jing Xu, Luwei Chen, Kong Fei Tan, Armando Borgna, Mark Saeys, “Effect of boron on the stability of Ni catalysts during steam methane reforming”, Journal of Catalysis, in press 6. Jing Xu, Mark Saeys “First principles study of the effect of carbon and boron on the activity of a Ni catalyst”, accepted 7. Kong Fei Tan, Jing Xu, Armando Borgna, Mark Saeys, “First principles based design of a Fisher-Tropsch synthesis catalyst with enhanced stability”, in preparation XVIII [...]... energy for different surface structures with subsurface carbon and boron 10 5 Table 5.2 Methyl and hydrogen binding energies (kJ/mol) for the clean Ni (11 1) surface, the Ni (11 1)-CSS surface with subsurface carbon, and the Ni (11 1)-BSS surface with subsurface boron 11 0 Table 5.3 Transition state geometries and barriers for methane activation on Ni (11 1), Ni (11 1)-CSS and Ni (11 1)-BSS surfaces 11 3 Table 5.4... structure; (B) a double line structure The white circle indicate unsaturated carbon atoms of the graphene structure, termed edge atoms 78 Figure 4 .12 Model used to calculate the stability of small graphene islands on the Ni (11 1) surface Black and grey circles indicate saturated, internal graphene atoms with a carbon binding energy of –760 kJ/mol, white circles indicate unsaturated edge atoms with a carbon binding... Methyl and hydrogen binding energies (kJ/mol) for the Ni( 211 ) surface, the Ni( 211 ) surface with all step sites occupied by carbon, Ni( 211 )-Cstep ,10 0%, by boron, Ni( 211 )Bstep ,10 0%, and with half of the step sites occupied by boron, Ni( 211 )-Bstep,50% 11 9 Table 5.5 Transition state geometries and barriers for methane activation on Ni( 211 ), Ni( 211 )-Cstep ,10 0%, Ni( 211 )-Bstep,50% and Ni( 211 )Bstep ,10 0% surfaces... study of the stability and the formation kinetics of subsurface and bulk carbon on a Ni catalyst”, Journal of Physical Chemistry C, 11 2 (2008) 9679-9685 5 Jing Xu, Luwei Chen, Kong Fei Tan, Armando Borgna, Mark Saeys, “Effect of boron on the stability of Ni catalysts during steam methane reforming”, Journal of Catalysis, in press 6 Jing Xu, Mark Saeys “First principles study of the effect of carbon and... surface as a function of coverage The symbols indicate the calculated binding energies 51 Figure 4.2 Binding energies for the on-surface hcp hollow sites (∆) and the octahedral sites of the first (□) and second (x) subsurface layer of the Ni (11 1) surface as a function of the coverage and the concentration The symbols indicate calculated binding energies 54 Figure 4.3 Binding energies per carbon atom for. .. diffuse to the Ni bulk, or (iii) combine to form graphene islands 82 Figure 4 .15 Boron binding energies for chemisorption at the four high symmetry sites of the Ni (11 1) surface and at the octahedral sites of the first subsurface layer as a function of coverage 85 Figure 4 .16 Ni (11 1) surface with subsurface boron atoms after surface reconstruction (top (a) and side (b) view) There are two distinct top sites... adsorption sites are indicated in (e) There are two distinct top sites (T1 and T2) for the surface with subsurface boron 10 9 Figure 5.3 Orbital scheme for C-H bond activation on transition metal surfaces 11 2 Figure 5.4 Correlation between the methane activation barrier and the centre of the d-band projected on the surface Ni atoms relative to the Fermi level for the different surfaces considered 11 2... on a Ni (11 1) surface Carbon atoms are located at (A) both the fcc and hcp threefold hollow sites; (B) atop and fcc threefold hollow sites; (C) atop and hcp threefold hollow sites; (D) two near atop sites 62 XIV Figure 4.6 The geometry of carbon at fivefold coordinated site on Ni( 211 ) surface Carbon atoms are represented by smaller balls and Ni atoms are represented larger balls Left panel, 50% carbon... energies are relative to CH4 and H2O in the gas phase and calculated using the results for the individual species 20 Figure 2.7 Conversion of n-butane as a function of time during steam reforming in a 3% n-butane-7% hydrogen-3% water in helium mixture at a space velocity of 1. 2h -1 The dashed curve shows the n-butane conversion for the Ni and the solid curve is for the Au/Ni supported catalyst 20 Figure 3 .1. .. values are given in kJ/mol 13 Figure 2.4 Calculated turnover frequencies for ammonia synthesis as a function of the adsorption energy of nitrogen for various transition metals and alloys 16 Figure 2.5 The calculaterd potential energy diagram for NH3 synthesis from N2 and H2 over Ru(00 01) (dashed curve) and stepped Ru(00 01) (solid curve) 16 Figure 2.6 Energies for the species on Ni( 211 ) and Ni (11 1) All . 15 -20 carbon atoms. Step sites are the preferred adsorption sites for carbon atoms and can act as nucleation sites for the formation of graphene islands. On-surface carbon atoms are relatively. boron atoms were found to restructure the Ni (11 1) surface and lower the methane dissociation barrier from 10 1 kJ/mol to 64 kJ/mol. Hence, boron atoms are believed to enhance the catalyst stability. 78 Figure 4 .12 Model used to calculate the stability of small graphene islands on the Ni (11 1) surface. Black and grey circles indicate saturated, internal graphene atoms with a carbon binding