Model Studies Of Gold Catalysis

The gold rush in heterogeneous catalysis has greatly inspired the fundamental studies of gold catalysis. The size-dependent reactivity of supported gold nanocatalysts has always been a hot issue, although the active site and the reaction mechanism are still not fully understood, the low-coordinated Au atoms and the size-related Au-support interface are commonly believed as the most important factors affecting the reactivity. In this thesis, stepped Au single crystal surfaces (Au(997) and Au(110)-(1×2)) having low-coordinated Au atoms at step sites, as well as Au/TiO2(110) surface have been fabricated as model catalysts. The NOx decomposition reactions, oxidation and hydrogenation reactions have been investigated on Au surfaces and the adsorption of small molecules, dissociation of H2O and oxidation of propene have been studied on Au/TiO2(110) surface. Furthermore, the adsorption of small molecules and the reactivity of Pt/TiO2interface on Pt/TiO2(110) surface have also been studied to make a comparison with that of Au/TiO2interface. The main results are summarized as follows:1) Low-temperature NO decomposition to form N2O on Au surfaces is structure sensitive.(NO)2dimer species is the active surface species for the NO decomposition. The decomposition activity of (NO)2dimer depends on its adsorption site and configuration. The thermodynamically more stable (NO)2dimer species exhibits a much larger activation energy for the decomposition reaction. This result illustrates the origination of the exceptional high activity at low reaction temperatures on Au surfaces. NO2chemisorbs molecularly and reversibly on both Au surfaces and the low-coordinated Au atoms enhance its adsorption. An amorphous physisorbed N2O4multilayer forms at large NO2exposures and subsequent heating causes its isomerization and transformation and further decomposition into NO(g) and NO2(g), forming O(a) on the surface. This reaction can be utilized to prepare O(a) on inert Au surfaces under UHV conditions.2) Different types of O(a) species are prepared on Au(997) surface by thermal decomposition of amorphous physisorbed N2O4multilayers, including O(a) on the (111) steps and O(a) adatoms and islands on the (111) terraces with their stability follows the order of O(a) on the (111) steps> O(a) adatoms on the (111) terraces> O(a) islands on the (111) terraces. Reactivity and interaction with small molecules of different types of O(a) species have been systematically investigated. O(a) species on Au(997) surface facilely react with NO to form chemisorbed NO(a), while do not react with NO2and there is only strong repulsive interaction between coadsorbed O(a) and NO2(a). O(a) species on Au(997) surface react with H2O to form surface hydroxyls(OH(a)) at105K and stabilize the adsorption of CO2. O(a) species on Au(997) surface facilely react with CO at105K to form CO2(a) and its desorption is obviously affected by the co-adsorbates. O(a) species on Au(997) surface react with HCOOH to form HCOO(a) and OH(a) at105K, with further oxidation to form H2O and CO2at elevated temperatures. The oxidation of HCOOH has higher reaction barrier on steps than that on terraces.3) Hydrogenation reactions of acetylene, ethylene and butadiene have been studied on Au(997) surface. Selectively hydrogenation of acetylene to ethylene and butadiene to butene on steps, as well as hydrogenation of butadiene to both butene and butane on terraces are observed on Au(997) surface. Collaborative DFT calculations show that the selectivity of catalytic reaction originates from the adsorption site dependence of both the adsorption energies of reactants, intermediates and products and the reaction barriers. These results provide us fundamental understanding of the catalytic mechanism of selective hydrogenation reaction of butadiene and acetylene.4) The adsorption of small molecules, dissociation of H2O and oxidation of propene have been investigated on Au/TiO2(110) surface with different Au coverage. Both adsorption of CO and propene are size-dependent, and enhanced adsorption is observed on Au particles in smaller size on Au/TiO2(110) surface with lower Au coverage. H2O dissociates on the interface of Au/TiO2(110) surface, forming H2at250K, and its dissociation does not depend on the particle size of Au. Oxidation of propene are influenced by both Au coverage and the amounts of co-adsorbates. Partial oxidation products at low propene coverage are obviously detected at a temperature lower than200K.5) The adsorption of small molecules and dissociation of H2O have also been studied on Pt/TiO2(110) surface with different Pt coverage. Desorption peak of CO, originating from adsorption on both Pt-TiO2interface and Pt particles, shifts to higher temperature as increasing Pt coverage, indicating the stronger adsorption of CO on Pt particles in larger size. Dissociation adsorption of H2on Pt particles with vicinal Pt atoms can be observed on Pt/TiO2(110) surfaces at higher Pt coverage. Decomposition of H2O to form H2are also observed on Pt/TiO2interface. We have investigated the surface structure, adsorption of small molecules and reactivity of Pt/TiO2(110) surfaces after annealing in vacuum at1000K and subsequent annealing in O2at700K. After annealing, only Pt/TiO2(110) surface with a Pt coverage of0.04ML retains the adsorption of CO on Pt/TiO2interface and the H2O dissociation reactivity; on Pt/TiO2(110) surface with a Pt coverage of0.41ML, adsorption of H2is quenched and the H2O dissociation and CO adsorption on Pt/TiO2interface are greatly decreased; on Pt/TiO2(110) surface with a Pt coverage of2.5ML, all of these adsorption and reactions are quenched. These results clearly demonstrate the strong metal-support interaction (SMSI) in Pt/TiO2and the SMSI on Pt/TiO2(110) surface is influenced by the particle size of Pt. SMSI on Pt/TiO2(110) surface can be effectively decreased when the Pt particles are smaller.In addition, we have investigated the photocatalytic reaction of atomic H and propene on rutile TiO2(110) surface. Formation of H(a)-Ti species and their photocatalyzed production of H2are detected on reduced TiO2(110) surface. Photo-induced desorption of propene and photocatalyzed partial oxidation of propene are observed on TiO2(110) surface. These results deepen our fundamental understanding of the photocatalytic reactions on TiO2surface.