Surface density metric, supported metal oxides

Supported Metal Oxides and the Surface Density Metric... 

Many synthetic routes for preparing transition metal oxide catalysts produce a supported metal oxide structure consisting of an active metal oxide phase (the surface oxide) dispersed on a second, high surface area oxide (the support oxide) [1-3]. A key metric in characterizing SMOs is surface density. International Union of Pure and Applied Chemistry (lUPAC) defines surface density as mass per unit area [4]. For supported metal oxides, this is vaguely interpreted as the amount of supported metal oxide active phase per surface area of the underlying oxide support. This broad definition allows considerable latitude in whether total or exposed surface oxide content is considered and whether the surface area is of the uncovered support or final catalyst. Furthermore, absence of standardized methods to measure these parameters introduces additional variability into the determination of surface density. 

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. 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. Inconsistencies in these values come about because of (1) differences in their definitions, (2) difficulties in compatibilizing data from different characterization techniques, and (3) the use of support surface area instead of the overall composite SMO. These inconsistencies can make structural comparison of the same SMO composition, such as WO /ZrOj, difficult across different research groups. Calculated properly, however, the surface density metric provides the most simple and useful basis for understanding the relationship between surface nanostructure and catalytic and surface properties. 

Surface density (labeled as ps f) is a calculated value that incorporates both surface oxide loading and specific surface area into a single metric. Surface density is, by nature, an averaged quantity and does not account for nonuniform surface oxide distribution. It does not contain structural information on surface oxide dispersion analogous to metal dispersion in supported metal catalysts (e.g., flat sheets vs. hemispherical islands of metal) [89-92],... 

---