Supported nanosized gold catalysi the influence of support morphology and reaction mechanism 2

Chapter Experimental Section 2.1 Catalyst Preparation The method of preparation strongly influences the particle size.1-4, which is believed to be one of the important factors that can influenc catalysts activity 5-8 For most of the reactions, only the catalysts with gold particles smaller than nm lead to high activity; this is especially true for the oxidation of carbon monoxide 9,10 There are two kinds of chemical preparation methods that are widely used in the preparation of gold nano particles supported on metal oxide support The first kind is termed as co-precipitation, in which the support and the gold precursor are formed at the same time Co-precipitation is the carrying down by a precipitate of substances normally soluble under the conditions employed.11 There are three main mechanisms of co-precipitation: inclusion, occlusion, and adsorption.12 An inclusion occurs when the impurity occupies a lattice site in the crystal structure of the carrier, resulting in a crystallographic defect; this can happen when the ionic radius and charge of the impurity are similar to those of the carrier An adsorbate is an impurity that is weakly bound (adsorbed) to the surface of the precipitate An occlusion occurs when an adsorbed impurity gets physically trapped inside the crystal as it grows Besides its applications in chemical analysis and in radiochemistry, coprecipitation is also "potentially important to many environmental issues closely related to water resources, including acid mine drainage, radionuclide migration in fouled waste repositories, metal contaminant transport at industrial and defense sites, metal concentrations in aquatic systems, and wastewater treatment technology" 13 Coprecipitation is also used as a method of magnetic nanoparticle synthesis.14 However - 28 - the co-precipitation method tends to be difficult to control and to reproduce Nucleation and growth can easily occur in the solution rather than on the carrier, which results in undesirably large metal particles (low dispersion) and an inhomogeneous metal distribution on the carrier, The second kind of methods includes impregnation, ion adsorption, depositionprecipitation and colloid-based methods In these methods the gold precursor is applied to the preformed support Among these methods, deposition-precipitation and colloid-based method were utilized in the preparation of gold nanoparticles supported on iron oxide support Deposition-precipitation is a modification of the precipitation methods in solution It consists of the conversion of a highly soluble metal precursor in another substance of lower solubility, which specifically precipitates onto a support and not in solution The conversion into the low soluble compound, and then into the precipitate, is usually achieved by raising the pH of the solution This can also be done by decreasing the pH, or by changing the valence state of the metal precursor through electrochemical reactions or by using a reducing agent, or by changing the concentration of a complexing agent The key point for a successful depositionprecipitation is, therefore, the gradual addition of the precipitating agent to avoid local rise of concentration above the solubility curve, which would cause a rapid nucleation of the precipitate in solution Deposition-precipitation is widely used in the preparation of nanosized gold catalysts, but G.C Bond mentioned in his book that the term deposition-precipitation employed here is not so accurate 15 It might be more suitable to be described as grafting or ion adsorption, because deposition-precipitation hints the forming of hydroxide or hydrated oxide for the deposition onto the surface of - 29 - the metal oxide support, while precipitation involves a nucleation by the support and all the active phase attached to the support Colloidal-based method has the advantage of small mean particle size and narrow size distribution under appropriate conditions, and the influence of the support is almost ignorable This method was first used in preparing nanosized gold particles by John Turkevich and his associates in 1950s 16,17 They used sodium citrate as the reduction agent for the AuCl4- ion Many other reducing agents have been used ever since for the reduction of gold related ions, including phosphorus, sodium thiocyanate, poly ethylene-imine, tetrakis phosphonium chloride and sodium borohydride The advantage of using the colloidal route for preparing supported gold catalysts lies in the way that condition of preparation can be manipulated to give particles having a narrow size distribution about the desired mean In this thesis Au/iron oxide catalysts were prepared by various methods, including coprecipitation (CP), deposition-precipitation (DP), and colloids-based methods, using HAuCl4 (sigma-aldrich) and Fe(NO3)3·9H2O (sigma-aldrich) as precursors In the case of the co-precipitation (CP) method, an aqueous mixture of the HAuCl4 and Fe(NO3)3 precursors was poured into an aqueous solution of Na2CO3 (0.25M) which was maintained at 70oC under vigorous stirring (500 rpm) The precipitate was washed, dried, and calcined in air at 110oC for 12 hrs This co-precipitation sample is coded AuCP In the deposition-precipitation method, Au nanoparticles were deposited on iron oxide support by keeping the pH value of the aqueous solution of HAuCl4 at pH = using 0.1M NaOH The Fe2O3 support was generated, prior to the DP process, from 1.0 M Fe(NO3)3 solution Excessive amount of 1.0M NaOH solution was added to the Fe(NO3)3 solution drop-wisely till all the iron ions in the - 30 - solution were deposited Then the mixed solution was thoroughly washed using DI water by centrifugation The slurry after centrifuge was dried in 110oC oven for 48 hours The above prepared sample was then calcined at 500oC for hour The asprepared iron oxide was mainly presented in -Fe2O3 phase, with small amount of γFe2O3 phase detectable by XRD This self-prepared iron oxide sample was used as the support for the AuDP catalyst (The deposition-precipitation sample is coded AuDP) Two other samples, AuCH and AuCM were prepared using colloid-based method with assistance of the ultrasound irradiation18 The support used for AuCH was commercial Fe2O3 (hematite, Sigma-Aldrich), while that for AuCM was commercial Fe3O4 (Magnetite, Sigma-Aldrech) In colloid-based method L-lysine was added as a capping agent, which has better control on gold particle size compared to conventional DP method used in literature HAuCl4 (1mM) was reduced by NaBH4 (0.1M) During the reduction period, colloid-based method was applied The nano-Au particles were deposited on iron oxide supports The slurry was dried at 70ºC after centrifuge four times using DI water As chloride ions is a poison to the catalytic reaction and may affect the activity of catalyst, the addition of capping agent and reduction agent and the followed washing procedure are able to remove almost of chlorine in the solution 2.2 X-ray Photoelectron Spectroscopy X-ray Photoelectron Spectroscopy (XPS) is widely used to investigate the chemical compositions and oxidation state of surfaces Its surface specificity, applicability to nearly all elements, and sensitivity to chemical state give XPS great potential for contributing to the understanding of a wide variety of catalyst problems.19 The X-rays penetrate far into the solid (1~10 m) but the mean free path for the escape of a 100- 31 - 1500 eV electron without energy loss is only 1-8 nm Secondary electron emission and inelastic losses account for much of the background in the spectrum, but the information carried in the spectral peaks applies to a thin surface layer because of the relatively short electron mean free path Thus, XPS is inherently a surface technique Surface analysis by XPS is accomplished by irradiating a sample with mono energetic soft X-rays (usually Mg K (1253.6 eV) or Al K (1486.6 eV)) under ultra-high vacuum (UHV) conditions, causing electrons to be emitted from the surface region by the photoelectric effect and analyzing the energies of the detected electrons The emitted photoelectrons have measured kinetic energies given by 20 KE h BE s (2.1) where h is the energy of the photon, BE is the binding energy of the atomic orbital from which the electron originates, and s is the spectrometer work function The binding engery which is theoretically equivalent to the ionization energy of the electron can also be regarded as the energy difference between the initial and final states after the photoelectron has left the atom Thus each element has a unique set of binding energies The binding energies of the core-level electrons are sufficiently affected by differences in the chemical potential and polarizability of the neighboring compounds This would cause a detectable photoelectron energy shift, which ranges from 0.1 up to 10 eV This kind of shift is termed as chemical shift Several types of peaks are observed in XPS spectra Photoelectron lines, Auger lines and shake-up lines are the mostly encountered feature in our research The dominant features in an XPS spectrum are photoelectron lines Most intense photoelectron lines are relatively symmetrical and have typically the narrowest FWHM of 1-2 eV Less intense photoelectron lines at higher binding energies are usually 1-4 eV wider than the lines - 32 - at lower binding energies All of the photoelectron lines of insulating solids are of the order of 0.5 eV wider than photoelectron lines of conductors Auger lines are observable in XPS spectra, due to relaxation of the excited ions left after photoemission The Auger electron possesses the kinetic energy equal to the difference between the energies of the initial ion and the doubly charged final ion Shake-up losses are final state effects which arise when the photoelectron imparts energy to another electron of the atom This electron ends up in a higher unoccupied state (shake-up) This results in a satellite peak a few eV higher in BE than the main peak During our XPS analysis, charging often occurs because our nano gold on semiconductor (CuO or TiO2) samples might lack of delocalized electrons to neutralize the positively charged macroscopic clusters created by the ejection of photoelectrons and/or Auger electrons As a result, a positive potential builds up at the surface of the sample, which retards the outgoing electrons This retardation appears in the spectrum as an additional positive shift in binding energies Referring to the BE of C1s (284.6 0.4 eV) contributed from the adventitious carbon is the commonly used procedure to solve the charging problem Any shift from this value is taken as a measure of the steady-state static charging Quantification of XPS spectra is quite straightforward The elemental composition of the interested sample determined as followed: N %X = (A /S ) / Σ x x i=1 (A /S ) x x (2.2) where X is the element, A the area under the peak of element X in the spectrum, and x S is the relative atomic sensitivity factor %X is the fractional atomic concentration x - 33 - In our research, the S data are provided by the instrument manufacture VG Scientific x Ltd 2.3 Secondary ion mass spectrometry (SIMS) Secondary ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions It is particularly useful in detecting hydrogen species on surfaces While only charged secondary ions emitted from the material surface through the sputtering process are used to analyze the chemical composition of the material, these represent a small fraction of the particles emitted from the sample There are two different modes of analytical SIMS application: static and dynamic SIMS In static SIMS, 21,22 the information on the composition of the uppermost monolayer is generated virtually without disturbing the composition and structure of the surface This is achieved by very low primary ion current densities For an ion current density of 10-9 A cm-2 the life-time of a monolayer is in the order of some hours Essentially static SIMS provides a mass spectrum of the surface The mix of elemental and cluster ions in the spectrum can generate a rich store of information regarding the chemistry of the surface layer Hence static SIMS is a technique of great potential for understanding the chemical behavior and structure of surfaces In dynamic SIMS, high primary ion-current densities up to some A cm-2 is applied, thus resulting in a short lifetime of a monolayer down to the 10 -3 sec range This fast surface erosion continuously moves the surface into the bulk material, thus supplying - 34 - information on the chemical composition of original subsurface layers of the bombarded sample The secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion ranges There are three basic analyzers available: sector, quadrupole, and time-of-flight The time of flight mass analyzer separates the ions in a field-free drift path according to their kinetic energy Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) uses a pulsed primary ion beam to desorb and ionize species from a sample surface The resulting secondary ions are accelerated into a mass spectrometer, where they are mass analyzed by measuring their time-of-flight from the sample surface to the detector There are three different modes of analysis in TOF-SIMS; 1) mass spectra are acquired to determine the elemental and molecular species on a surface; 2) images are acquired to visualize the distribution of individual species on the surface; and 3) depth profiles are used to determine the distribution of different chemical species as a function of depth from the surface 23 It is the only analyzer type able to detect all generated secondary ions simultaneously and is the standard analyzer for static SIMS instruments Static SIMS is the process involved in surface atomic monolayer analysis TOF-SIMS provides spectroscopy for characterization of chemical composition, imaging for determining the distribution of chemical species, and depth profiling In the spectroscopy and imaging modes, only the outermost (1-2) atomic layers of the sample are analyzed The actual desorption of material from the surface is caused by a "collision cascade" which is initiated by the primary ion impacting the sample surface The emitted secondary ions are extracted into the TOF analyzer by applying a high voltage potential between the sample surface and the mass analyzer - 35 - TOF-SIMS spectra are generated using a pulsed primary ion source (very short pulses of