| Heterogeneous catalytic reactions are generally so complex that the fundamental studies are challenging. Surface science studies of model catalysts with well-defined structures have been demonstrtaed as a successful strategy for the fundamental studies of complex heterogeneous catalysis. In this thesis, we prepared CeO2/Au(110) inverse model catalysts and studied the reaction mechanisms of low-temperature CO oxidation and WGS reaction; we also prepared Ga2O3-Pd/Al2O3 model catalysts with atomic layer deposition (ALD) technique and studied the structure-activity relation and reaction mechanism of selective hydrogenation of C2H2 in ecxess C2H4, and meanwhile studied the adsorption and reaction of Ga(CH3)3 on Pd(111) to understand the ALD process. The main results achievements are summarized as follows:(1) We prepared various surface species on CeO2/Au(110) inverse model catalysts by adsorption and co-adsorption of CO, O2, CO2 and water and studied the subsequent surface reactions to successfully establish the surface reaction network to produce CO2 by the oxidation of CO, including:CO oxidation to produce CO2 above 350 K by surface lattice oxygen via surface carbonate intermediates on CeO2, above 290 K by adsorbed molecular O2 on CeO2 via adsorbed surface carbonate intermediates, above 210 K by surface lattice oxygen via hydroxyl groups-assisted decomposition of surface carbonate intermediates on CeO2, above 200 K by hydroxyl groups via surface carboxylate or formate intermediates on CeO2, above 160 K by surface lattice oxygen via water-assisted decomposition of surface carbonate intermediates on CeO2; and the interfacial oxidation of CO adsorbed on the Au surface by hydroxyl groups on CeO2 to produce CO2 above 130 K.(2) We studied the adsorption and reaction of Ga(CH3)3 on Pd(111) and the effects of pre-adsorbed H and O adatoms. Ga(CH3)3 adsorbs dissociative ly on Pd(111) at 140 K to form a mixture of Ga(CH3)x (x=1-3) and CHX species, then Ga(CH3)x on clean Pd(111) follows a progressive Ga-C bond cleavage process between 275 and 325 K to eventually produce CH4 and H2. When the surface is pre-covered by H atoms, Ga(CH3) is the major surface intermediate and decomposes to produce CH4 and H2 at -315 K. When the surface is pre-covered by O atoms, Pd-O-Ga(CH3)2 is the likely surface intermediate and decomposes to produce CH4 and H2 at ～258K.(3) Ga2O3-Pd/Al2O3 model catalysts were prepared with ALD technique. It was found that Ga2O3 adlayers were preferentially deposited at edges and facets other than the {111} facets of Pd particles, leading to the transformation of the low coordinated edge sites of the {111} facet into the {111}-terrace like sites. The Pd(111) terraces are active in catalyze the selective hydrogenation of C2H2 in excess C2H4, the deposition of an appropriate amount of Ga2O3 on Pd particles much enhances the catalytic activity; meanwhile, the deposition of Ga2O3 adlayers could suppresses the formation of poisoning polymeric carbon deposits on other open facets that can migrate to and position the neighboring {111} facets, significantly enhancing the catalytic stability. These results successfully develop a novel concept to simultaneously promote the catalytic activity and stability of supported Pd catalysts in the selective hydrogenation reaction of acetylene to ethylene by selective deposition of Ga2O3 with ALD technique.In summary, we have fabricated model catalysts with well-defined structures with which the reaction mechanisms and structure-activity relations of several catalytic reactions have been understood. These results are of importance in the structural design of efficient catalysts for the relevant catalytic reactions. |
PDF Full Text Request