Structural and Catalytic Studies of Nanoparticle-Supported Metal Oxides
Supported metal oxides represent a large class of materials used in many catalytic processes. There are a number of well-established preparation methods that lead to some molecular structure control of the supported metal oxide in the form of isolated, polymerized, and crystalline species. Advances in metal oxide-forming chemistries have come about recently that provide new ability to form pure metal oxides and mixed metal oxides, with structural and compositional control at the nanometer-level. We are exploring several synthesis routes to NP-supported metal oxides. There is emerging evidence that suggests that nanocrystalline domains of the surface oxide can be active sites, based on studies of WOx/ZrO2 materials in methanol dehydration.
In addition, we have analyzed one overlooked aspect of supported metal oxides, the surface density metric. Confusion in SMO literature can arise because there is no generally accepted method for determining surface density. As the metric that characterizes the surface oxide of supported metal oxide catalysts, surface density allows one to consider the various structures of the surface oxide on a common scale, independent of total oxide content, preparation method, calcination treatment, and surface area of the support oxide. Some analogy can be made with supported metal catalysts and the metal dispersion metric.
Surface saturation and monolayer coverage are important threshold surface density values at which surface oxide crystals form and at which complete consumption of surface hydroxyl groups of the support oxide occurs, respectively. Differences in these values come about, though, because of (i) inconsistency in their definitions, (ii) difficulties in compatibilizing data from different characterization techniques, and (iii) and the use of support surface area instead of the overall composite SMO. These differences can make structural comparison of the same SMO composition difficult across different research groups. Calculated properly, however, surface density provides the most simple and useful basis for understanding the relationship between surface nanostructure and catalytic and surface properties.
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