Synthesis And Catalytic Performance Of CeO₂-based Nanomaterials

Nanomaterials receive a great deal attention due to the unique optical, electronic, magnetic and catalytic performances, which are determined by their sizes, surface structures and interparticle interactions and mainly are represented as surface effect, volume effect and quantum tunneling effects. The application of nanomaterials in catalysis is well-known as a most potential research field since all these effects have great influence on the catalytic performance of nanomaterials.Herein, cerium dioxide nanomaterials with different morphologies and similar specific area were prepared successfully, including:nanoparticles, nanowires, nanorods, and nanospheres. Based on the systematic studies of their fine structures, redox properties and catalytic performances, it can be easily found that the catalytic performance of CeO2 nanomaterials is morphology-dependent, which is defined by the nature of crystal plane. The CeO2 nanoparticles mainly exposed the stable {111} plane on the surface, whereas the rod-shaped nanostructures preferentially expose the reactive{110} and{100} planes, giving higher oxygen storage capacity and catalytic activity for CO oxidation. Although both the CeO2 nanorods and the CeO2 nanowires predominantly expose the reactive{110} and{100} planes, the CeO2 nanowires favor to expose a large proportion of{100} planes on the surface, resulting in a much higher activity for CO oxidation than the nanorods.In order to get a further understanding of the morphology effects of CeO2 nanomaterials, the impact of hydrothermal conditions on the shape and catalytic performance of CeO2 was investigated. Moreover, the strategy of the nanowires evolution was proposed. The results show that although the catalytic performance of CeO2 greatly depends on the nature and the proportion of the surface-exposed facets, it is the combined results of both the specific area and the facets effect. For CeO2 nanospheres, the size of spheres is a function of the concentration of urea with a negative relationship. Accordingly, it can be inferred that the urea in this process hold a dual function. The bubbles produced by the hydrolysis of urea act as a structure directing agent, while the OH- and CO32- ions are the precipitator.CeO2 is usually treated as a support or an additive to achieve an enhanced performance. Therefore, the CeO2 nanoparticles were loaded by Au and the size effect of Au particle was studied in detail. Because the Au particle size has intensive influences on the adsorption of reactants and the redox properties of Au/CeO2, the catalytic activity deceased with increased Au particle size while the surface properties of CeO2 keeping unchanged. As a result of the further analysis of the relationship between the Au particle size and the TOF, it can be concluded that the most effective Au atoms for different reactions located at different parts of an Au nanoparticle. For the CO oxidation, the corner atoms with low coordination show best performance, while the step atoms do for WGSR.The Au/CoCeOx catalyst shows a favorable CO oxidation activity by applying CeO2 as an additive, which promoted the dispersion of CoOx and developed the specific area of Au/CoCeOx. The chemical state of Au plays an important role in the catalysis process. It was found that the conversion of CO increased with increasing amount of Au+ in the catalyst, and thus the cationic gold (Au+) was considered the main active species. Addition of water vapor to the feed gas promoted CO conversion to some degree, but the catalytic performance decreased rapidly due to the disproportionation of Au+ to Au3+ and Au0 particularly for the catalyst that contained relatively large amounts of Au+ species.