Deposition and Characterization of Highly Oriented Mg3(VO4)2 Thin-Film Catalysts: 3. Effect of Exposure to Reaction Conditions

Abstract The response of oriented and amorphous thin films of magnesium orthovanadate (Mg 3 (VO 4 ) 2 ) to reactive conditions typically encountered during oxidative dehydrogenation demonstrates that these films are excellent models for bulk Mg 3 (VO 4 ) 2 powder catalysts. Fully oxidized films show little change in surface composition or chemical state upon treatment in propane, oxygen, or propane/oxygen mixtures at 673 K. As the films become more oxygen deficient, changes in both composition and chemical state are observed at the film surfaces. Reductive treatments in pure propane generally result in at least partial reduction of surface vanadium from V 5+ to V 3+ , and enhancement of the vanadium surface concentration. Oxidative treatments in pure oxygen reverse these effects. Behavior is highly dependent on sample history. Initial oxidation treatments reduce the magnitude of the response of the film to subsequent reduction/oxidation cycles, while initial reduction treatments increase the magnitude of the response of the film to subsequent oxidation/reduction cycles. These effects are purely kinetic in nature and can be understood from consideration of the chemical behavior of oxygen vacancies in Mg 3 (VO 4 ) 2 . These observations suggest that under reaction conditions magnesium vanadate surface composition may be quite dynamic, and that such dynamic variations must be considered when developing active site models for these catalysts. Comparison of the response of amorphous and (021)-oriented films provides a first indication that reactions over these catalysts may be structure sensitive.