Theoretical Study For The Catalytic Activity Of Gold And Gold-Platinum Noble Metal Nanomaterials

In the 1980's, supported Au has been prognosticated by Hutchings having higher catalytic activity toward the hydrochlorination of acetylene. Subsequently, it was found by Haruta et al that Au can exhibit surprisingly high catalytic activity for CO oxidation as low as-70℃when it is highly dispersed on Fe2O3,Co3O4,NiO. These vital findings make Au nanoparticles attract sustained experimental and theoretical interest and change the traditional impression about its inert activity. Using supported gold nanoparticles as catalysts, many important reactions have been achieved so far, including the oxidation of CO, the direct synthesis of H2O2 from H2 and O2, the reduction of nitrogen oxides, selective hydrogenation of unsaturated hydrocarbon, the combustion of hydrocarbons and so on. In the past decade, many experimental and theoretical studies have been devoted to unveiling the origin of the catalytic activity of Au nanoparticles. The effects of preparation method, support oxides, oxidation state, size and morphology of Au particles have been investigated. However, there is no consensus on the crucial factors that govern the catalytic activity of Au nanoparticles. And our understanding for the mechanisms of CO oxidation and H2O2 formation from H2 and O2 are still far from complete.Pt and Pt-based particles, which have extensive application, are regarded as the most active catalysts with the highest selectivity among the chemical elements. It is worthy to note that they have potential application in proton exchange membrane fuel cell, such as direct methanol fuel cell. However, the Pt electrode can be easily poisoned by the byproduct CO and loss the catalytic activity. Moreover, the scarce world reserves of Pt make it much more expensive than the other noble metals, such as Au. Thus, the enhanced activity and most effective utilization of Pt catalyst are desired. Up to date, much effort has been devoted to develop the "less expensive and more efficient" Pt-based catalysts. Several groups have consistently observed the higher catalytic activity of Pt-Au bimetallic nanoparticles toward low temperature CO oxidation compared to the corresponding monometallic catalysts. Moreover, the cost of Au modified Pt catalysts is much lower than pure Pt catalysts. Therefore, Pt-Au bimetallic catalysts have attracted special interest in recent years. However, for the microstructures of Pt-Au bimetallic catalysts, some groups reported that Pt-Au NPs exhibit alloy properties while the others found that Au tends to migrate to the surface and cover the active Pt sites. The existing contrary results indicate that the atomic ordering in nano-scaled Pt-Au bimetallic particles, which importantly influences the catalytic activity of nanoparticles, is a complex issue and still not understood well. Therefore, it is worthy to theoretically ascertain the stable atomic ordering of Pt-Au NPs. Moreover, the origin for the improved catalytic activity of Pt-Au bimetallic particles remains unclear.Considering the issues concerned above, employing density functional theory we explored the mechanism of CO oxidation and H2O2 formation from H2 and O2 which are promoted by Au clusters, and investigated the quantum size effect and charge state effect on gold catalytic activity in this thesis. At the same time, we also studied the geometrical and electronic structures of Pt-Au bimetallic clusters, researched the mechanism of CO oxidation mediated by Pt-Au bimetallic clusters or slabs and elucidated the influence of Au addition for Pt catalytic activity.The major innovative conclusions in this thesis are listed as follows:1. A theoretical investigation of the formation of H2O2 from H2 and O2 over anionic gold clusters Aun-(n=1-4) were performed at the BPW91/LANL2DZ/6-311G(d,p) level. In all cases, the reactions proceed via two elementary steps:the initial H2 dissociation to form an OOH-containing intermediate and the subsequent isomerization of this intermediate into the product-like intermediate. Energetically, the reactions over Au2- and Au4- are significantly less demanding than the ones over Au-and Au3-. In particular, Au-is relatively less active to hydrogenate O2 because the barrier of the rate-determining step is as high as 40.60 kcal mol-1. The barriers for both the odd-and even-membered sequences slightly decrease with the cluster size. The present results shows that quantum size effects appear to play a less important role for the reactivity of anionic Au clusters, in contrast, the additional charge on even-numbered gold clusters seems to be a dominant factor for the high reactivity.2. By carrying out density functional theory calculations, we studied the CO oxidation promoted by cationic, neutral, and anionic Au trimers, which represent the prototypes of Au-cluster-based catalysts with different charge states. The reaction is explored along three possible pathways:one involves the reaction of the initial complexes between Au trimers and O2 with CO; another is related to O2 interacting with the complexes between Au trimers and CO; the third refers to a self-promoting mechanism, i.e., the second CO oxidation is promoted by a pre-adsorbed CO molecule. The theoretical results show that all three species may promote the reaction, as indicated by calculated low energy barriers and high exothermicities, supporting the fact that cationic, neutral, and anionic Au species were all observed to present catalytic activity toward CO oxidation. Along the reaction coordinates for all the reactions, Au-carbonate species are not found to be the necessary intermediates although they are calculated to be energetically very stable. In contrast, by performing atom-centered density matrix propagation molecular dynamics simulations, the formation of such highly stable species is attributed to the effective collision between Au-oxides and CO2 with the carbon atom of CO2 directly attacking the O atom in the oxides.3. The geometrical and electronic structures of the smallest AumPtn (m+n=4-6, 13) bimetallic clusters have been investigated by performing DFT calculations. It is found that Au and Pt atoms prefer to form the core-shell-like structure with Pt atoms assembling together forming the core while the Au atoms like to surround the Pt atoms forming the shell, and the evenly mixed clusters are structurally unstable. This is attributed to the distinct bonding nature of Pt-Pt, Pt-Au, and Au-Au bonds. The present studies have provided atomic-level insight into the geometrical and electronic structure of Au/Pt bimetallic clusters, which can offer assistance to some extent for understanding the microstructure of Au/Pt bimetallic nanomaterials.4. A theoretical exploration for the reactivity of PtmAun (m+n=4) clusters toward CO oxidation have been presented, aiming at understanding the improved catalytic activity of Pt-Au bimetallic catalysts. In all situations, the reaction proceeds according to the single-center mechanism except the Au4-involved reaction. The Pt sites in the bimetallic clusters are the active centers, while Au sites are formally spectators and their role is to avoid the excess adsorption of CO around the active centers. The activity of Pt active centers in the bimetallic clusters seems not to be dependent on its surroundings. Based on the calculated results, we show a picture of the ideal "less expensive and more effective" Pt-Au catalyst for CO oxidation, where Pt atoms (active center) are suitably spaced (stabilized) by Au atoms, which more weakly adsorb CO than Pt atoms and thus leave room for the coming O2 necessary to CO oxidation.5. The detailed mechanisms for the dissociation of H2O and the oxidation of s by OHads on Pt(111) and PtAu3(111) have been investigated in order to explicit the origin for the improved catalytic activity of Pt-Au bimetallic catalysts. The calculated results indicate that the active sites are still on Pt atoms in PtAu3(111) surface. The adsorption of H2O, CO and OH on PtAu3(111) show unapparent difference in comparison with that on Pt(111). However, PtAu3(111) shows enhanced activity toward the dissociation of H2O. We attribute this to the change of electronic structures of Pt in PtAu3(111) surface. The analysis of density of states (DOS) indicate the upshift of d band center of Pt in PtAu3(111), which make the activity of Pt improve. In the case of the COadS oxidation by OHadS, PtAu3(111) shows comparable activity. The present studies deepen the understanding for the improved catalytic activity of Pt-Au bimetallic catalysts.