Model Catalyst Studies Of CO Oxidation And Hydrogenation

Surface chemistry studies of single crystal model catalysts are a main strategy for the fundamental understanding of catalytic reaction mechanisms on solid surfaces. In this thesis, we have employed FeO(Ⅲ)/Pt(Ⅲ) inverse model catalyst and Co(0001) model catalyst to respectively study the reaction mechanism of CO oxidation at low temperatures and CO hydrogenation reactions. The main results are:(1) A polarization-modulated infrared reflection-absorption spectroscopy (PM-IRAS) system that can be operated at pressures from UHV to near ambient-pressure was set up and used to prelimilary study CO adsorption and oxidation on Pt(Ⅲ). The adsorption structure of CO on Pt(Ⅲ) was found to vary with the CO pressure; the apparent activation energy of CO(a)+O2 reaction on Pt(Ⅲ) was identified to be 57.4 kJ/mol by time-resolved PM-IRAS spectra; and the influences of temperature and reactant compositions on CO oxidation and surface species on Pt(Ⅲ) were established.(2) OH-participated CO oxidation mechansom at FeO-Pt interface of FeO(Ⅲ)/Pt(Ⅲ) inverse model catalyst was studied. Employing 18O-labelled FeO/Pt(Ⅲ) surface, we demonstrate that O in FeO does not participate the CO+O:reaction at FeO-Pt interface but does participate the CO+OH reaction at FeO-Pt interface, which provides solid experimental evidence for water-activated lattice oxygen of oxide support for CO oxidation. H adatoms on Pt(Ⅲ) surface of FeO(Ⅲ)/Pt(Ⅲ) inverse model catalyst were found to be capable of spillovering to FeO-Pt interface to enhance the OH concentration therein and subsequent CO2 production by CO oxidation with OH groups, the presence of H-bonding network on FeO(Ⅲ)/Pt(Ⅲ) inverse model catalyst facilitates such a H spillover process.(3) Via controlled preparation of isotope-labelled surface adsorbates on an FeO(Ⅲ)/Pt(Ⅲ) inverse model catalyst, we demonstrate that elementary surface reactions for CO oxidation with similar activation energies exist both on the Pt surface and at the FeO-Pt interface when surface hydroxyl groups and water are present. Water and hydroxyl groups can also enhance the dissociation probability of molecularly-adsorbed O2 to produce oxygen adatoms. The proton transfer from surface hydroxyl groups to adjacent oxygen adatoms connects the elementary surface reactions on the Pt surface and at the FeO-Pt interface to constitute a surface reaction network. These results for the first time provide a view of proton transfer-connected elementary surface reaction network for the unanimous understanding of low-temperature CO oxidation catalyzed by metal-oxide nanocatalysts and reveal the dual roles of surface hydroxyl groups to promote CO oxidation and bridge various surface reaction pathways.(4) CH:(a) species on Co(0001) was prepared by the thermal decomposition of CH2I2, and the reactivity were studied without and with the presence of H and C adatoms. CH:(a) is not stable on Co(0001) and decomposes into C and H at low coverages. The decomposition of CH:(a) is suppressed on H(a)-covered Co(0001), and CH2(a) can either hydrogenate to produce CH4 or self-couple to produce C2H4 and C3H6. The decomposition of CH:(a) is partly suppressed on C-covered Co(0001), some CH:(a) still decomposes while others self-couple to selectively produce C3H6. These results demonstrate that the surface chemistry of CH2(a) on Co(0001) sensitively depend on the co-existing surface species.(5) The surface chemistry of Co(0001) under FT synthesis condition was studied by synchrotron radiation photoelectron spectroscopy (SRPES) with an quasi in-situ high pressure and high temperature reactor. CO was found to facilely decompose at high pressures, and the resulting atomic C and O can be hydrogenated respectively to CHx species and water under high pressure H2. Molecular-adsorbed CO(a) on Co(0001) was also found to hydrogenate into HCO species under high pressure H2 that can decompose into CHx species at elevated temperatures. The Co(0001) surface is metallic with atomic C and alkyl fragments as the stable adsorbates under the FT synthesis condition. These results suggest both the coverages and hydrogenation activity of CO and atomic C should be considered when discussing the contributions of various surface reactions to FT synthesis.The above results from model catalyst studies deepen the fundamental understanding of CO oxidation at low temperatures and CO hydrogenation reactions.