| Catalysts play a vital role in technologies for chemical industry and environmental remediation. To overcome the resource (energy) shortage and environmental pollution, the catalysts must be innovated not only to be active at mild reaction conditions but also to be selective. Fundamental understandings of structure-property relation of catalysts are needed to rationally design the structure of highly active and selective catalyst. Single crystals-based materials have been extensively used as model catalysts to study structure-property relation of catalysts due to their simple surface structures. However, there exist the so-called "materials gap" and "pressure gap" between single crystals-based model catalytic systems and corresponding powder catalytic systems, so the lessons learned from single crystal and single crystal thin film model catalysts can not simply extend to powder catalysts. Nanocrystals with uniform morphology and surface structures can act as a novel type of model catalysts, meanwhile, they can be studied under the same conditions as the working powder catalysts. Therefore, they can bridge the gap between single crystal model catalysts and working powder catalysts.Based on the above ideas, we have studied the structure-property relations of CeO2-based catalysts, including CeO2, Pt/CeO2, Cu/CeO2and CuO-CeO2/multi-walled carbon nanotubes (MWCNT) catalysts, employing well-defined materials, particularly CeO2nanocrystals with different morphologies. The acquired major conclusions are as the following:(1) CuO-CeO2/MWCNT catalysts were prepared and demonstrated to be very active and selective in CO preferential oxidation (PROX). The activity and selectivity of CuO-CeO2/MWCNT are much higher than CuO-CeO2supported on Al2O3, SiO2and SBA. The surface structure of MWCNT, including defects and electronic structure, strongly affects the CuO-CeO2interaction. The oxides are preferentially anchored on the defective sites. The surface functional groups of MWCNT can improve the dispersion of the oxides. They can also reduce the Cu2+to Cu+which are the active sites of PROX reaction. However, too much surface functional groups will lead to undesirable reduction of CeO2that affects the activity. The CuO-CeO2/MWCNT catalysts experience deactivation at high temperatures due to the reduction of CuOx; however, the deactivated catalysts at high temperatures do not exhibit serious activity loss when they are used at low temperatures again, suggesting that the reduced CuOx at high temperatures can be re-oxidized at low temperatures under the PROX reaction condition.(2) CeO2nanocrystals exposing different morphologies and crystal planes were synthesized and used as supports to prepare Pt/CeO2and Cu/CeO2catalysts. Strong morphology effects of CeO2support were observed from the catalyst preparation process to the catalytic performance. For Pt/CeO2catalysts, the impregnated Pt precursor interacts more strongly with CeO2rods and cubes than with CeO2octahedra, and its reduction/decomposition is easier on CeO2octahedra than on CeO2rods and cubes. Pt/CeO2-octahedra catalyst contains the largest fraction of metallic Pt while Pt/CeO2-cubes catalyst contains the largest fraction of Pt2+species. The reducibility of pure CeO2and CeO2in Pt/CeO2catalysts follows the order of CeO2-rods> CeO2-cubes> CeO2-octahedra, and the promotion effect of Pt on the reducibility of CeO2is stronger in Pt/CeO2-rods and Pt/CeO2-cubes than in Pt/CeO2-octahedra. The Pt0-CeO2ensemble is more active than the Pt2+-CeO2ensemble in the catalysis of CO oxidation in excess O2. H2-assisted CO oxidation catalyzed by Pt/CeO2catalysts was observed in CO-PROX reaction and the Pt2+species and CeO2with a large concentration of oxygen vacancies constitute the active structure for this reaction. In Cu/CeO2catalysts, copper exists mainly as isolated Cu2+which insert into the surface crystal lattice of CeO2at CuO loadings of0.025%-0.05wt%; with the increase of copper loadings, Cu2+converted to CuO clusters and then CuO particles. The interaction of Cu2+with CeO2nanocrystals with different morphologies follows the order of rods/{110}> cubes/{100}> octahedra/{111}. Whereas Cu+on the CeO2cubes/{100} is most stable. The Cu(II) sites at the CuO-CeO2interface are facilely to be reduced to Cu(I) that strongly chemsorb CO, but there are not simple relations between the catalytic activity of CuO/CeO2catalysts and their capacity of CO chemisorption. Highly dispersed CuO species and Cu2+-O-Ce species were identified as the active sites for CO oxidation and water-gas-shift reaction, respectively.(3) The morphology-dependent interplay among the reduction behaviors, oxygen vacancies and hydroxyl reactivities of oxides were successfully demonstrated employed CeO2nanocrystals with different morphologies. Isothermal H2reduction simultaneously reduces CeO2nanocrystals, creates oxygen vacancies and produces hydroxyl groups on CeO2nanocrystals. The morphology of CeO2nanocrystals strongly affects the reduction process and the resultant local oxygen vacancy. The reduction process mainly occurs on the surfaces of CeO2cubes and rods enclosed by the {110} and {100} crystal planes but in the subsurface/bulk of CeO2octahedra enclosed by the {111} crystal planes, thus the resultant oxygen vacancies are majorly located on the surfaces of CeO2cubes and rods but in the subsurface/bulk of CeO2octahedra. The reactivity of hydroxyl groups on CeO2nanocrystals depends on the local oxygen vacancy concentration on their surfaces, in which the water formation pathway is preferred at low local oxygen vacancy concentrations whereas the hydrogen formation pathway becomes gradually favorable with the increasing local oxygen vacancy concentration. A novel CO-catalyzed low temperature H2production from hydroxyl groups was observed via the COOH surface intermediate on CeO2rods and cubes but not on octahedra. Oxygen vacancies and proper O-Ce-O geometry on CeO2surfaces are required for such a reaction mechanism, and CeO2{110} and {100} fulfill these requirements but CeO2{111} does not. |
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