Catalytic Surface Chemistry Of Supported Au Nanocatalysts

As a representative system of nanocatalysis, supported Au nanocatalysts have been extensively studied in heterogeneous catalysis. Supported Au nanocatalysts have demonstrated excellent performance in a variety of important reactions, and accordingly, various reaction mechansims have been proposed to explain the fascinating Au catalysis. However, solid experimental evidence still lacks in the proposed reaction mechanisms, even for CO oxidation reaction.In this doctoral dissertation, the structure-activity relation and reaction mechanisms of supported Au nanocatalysts in CO oxidation and propylene epoxidation with H₂ and O₂ were comprehensively studied with the focus on the Au particle size effect and Au-support interaction. The main results are:1. Evolutions of various surface species on Au/CeO₂ catalysts with different gold particle sizes ranging from 1.7±0.6 to 3.7±0.9 nm during CO oxidation at room temperature were in-situ and time-resolved studied employing the time-resolved Operando-DRIFTS technique, and size-dependent reaction pathways and their contributions to the CO oxidation were successfully demonstrated. Chemisorbed CO（a）, carbonate, bicarbonate and formate species form on Au/CeO₂ catalysts upon CO adsorption, and their types, concentrations, and intrinsic oxidation reactivity to produce CO₂ vary with the size of supported Au particles. The intrinsic oxidation reactivity of CO（a） does not depend much on the Au particle size while the intrinsic decomposition reactivity of carbonate, bicarbonate and formate species strongly depend on the Au particle size and are facilitated over Au/CeO₂ catalysts with large Au particles. Therefore, the size effect of supported Au particles on CO（a） oxidation is to affect the specific density of surface adsorption sites on Au particles for CO（a）, and the size effect of supported Au particles on the decomposition of carbonate, bicarbonate, and formate species is to open their decomposition reaction pathways over Au/CeO₂ catalysts with large supported Au particles.2. CO and CO₂ chemisorption on uniform CeO₂ and TiO₂ nanocrystals with different morphologies were comprehensively studied with in-situ DRIFTS, and the formed surface species were observed to be morphology-dependent. CO or CO₂ chemisorbed at the metal sites and bidentate and bridged carbonates involving the O sites are sensitive to the surface composition and the local coordination environments of surface metal cations and oxygen anions and can be correlated well with the surface structures of facets exposed on oxide nanocrystals. Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO₂; and carbonate species formed by CO chemisorption can probe the different facets of TiO₂.3. Employing anatase TiO₂ nanocrystals predominantly enclosed the{001} facets, anatase TiO₂ nanocrystals predominantly enclosed the{100} facets, and P25 predominantly enclosed the{101} facets as the supports, the morphology effect of TiO₂ on the Au-TiO₂ interaction, structures and catalytic performances of Au/TiO₂ catalysts in C₃H₆ epoxidation with H₂ and O₂ were comprehensively studied. CO adsorption was used to probe the strong morphology-dependent interplay between the Au-TiO₂ interaction and the structure of Au/TiO₂ catalyst:the Au^δ- species is largest in Au/TiO₂-{001} due to the creation of surface oxygen vacancies of TiO₂-{001} upon Au loading whereas the fraction of Auδ+ species is largest in Au/TiO₂-{100} due to the preserved surface stoichiometric of TiO₂-{100} upon Au loading. Au/TiO₂-{100} with the largest fraction of Auδ+ species is most active but least selective toward hydrogen peroxide in H₂ oxidation reaction while Au/TiO₂-{001} with the largest fraction of Au^δ- species is most selective toward H₂O₂. In C₃H₆ epoxidation with O₂ and H₂ the ensemble consisting of intimately-contacting Au^δ-and Ti4+ on anatase TiO₂{001} and{101} facets with the weak adsorption ability is the active structure and Au/TiO₂-{001} catalyst containing the largest amount of active Au^δ--Ti⁴⁺ ensemble is most active. The structure of Au/TiO₂-{001} catalysts varies with the Au loading. An increase of the Au loading to 4.97% results in the increase of the active Au^δ--Ti^4- ensemble and subsequently the catalytic activity in the PO formation; however, a further increase of the Au loading to 9.96% results in the substabtial increase of the Auδ+ species and subsequently the decrease of the catalytic activity in the PO formation. In-situ DRIFTS results demonstrate an enhanced stability of PO adsorbed on 9.96%-Au/TiO₂-{001} catalyst that leads to the increased probability of the deep oxidation of adsorbed PO and subsequently the decreased selectivity toward PO.4. Effects of hydrogenation treatments on the structures and photocatalytic performances in water reduction of anatase TiO₂ nanocrystals with various morphologies were studied and strong morphology/facet-dependent effects were demonstrated. F¹⁺ color centers and Ti³⁺ species are created by the hydrogenation treatments of TiO₂ nanocrystals predominantly enclosed with the{001} facets （TiO₂-{001}） and mainly locate in the subsurface/bulk region; O₂- species, F¹⁺ color centers, Ti³⁺species and Ti⁴⁺-O. radical species are created in TiO₂ nanocrystals predominantly enclosed with the{100} facets （TiO₂-{100}） and locate from the surface to the subsurface/bulk region; and O₂- species, F1+ color centers and Ti³⁺ species are created in TiO₂ nanocrystals predominantly enclosed with the{101} facets （TiO₂-{101}） and locate from the surface to the subsurface/bulk region. The created defects enhance the light absorption/charge creation of TiO₂ nanocrystals but also the charge recombination probabilities, and hydrogenated TiO₂-{100} and TiO₂-{101} nanocrystals exhibit similar photocatalytic activity to the corresponding TiO₂ nanocrystals. However, an electric field was found to form between the stoichiometric surface and the defective subsurface for hydrogenated TiO₂-{001} nanocrystals. This facilitates the surface charge separation processes and leads to much higher photocatalytic activity of hydrogenated TiO₂-{001} nanocrystals than TiO₂-{001} nanocrystals.