Controlling palladium particle size and dispersion as a function of loading by chemical vapour impregnation: an investigation using propane total oxidation as a model reaction†

A series of Pd/Al₂O₃ catalysts with metal weight loadings of 1.0 wt%, 2.5 wt%, and 5.0 wt% were synthesised by chemical vapour impregnation (CVI) and used for the total oxidation of propane. All the catalysts were highly active for propane total oxidation. Extensive characterisation showed essentially identical catalyst structural and chemical characteristics, with consistent nanoparticle size, dispersion, and metal oxidation state regardless of metal loading. The major difference between catalysts was the number of surface palladium sites which scaled directly with metal loading. Turnover frequency calculations showed that the intrinsic activity of each catalyst is the same, with conversion scaling with the number of active sites. The number of active sites was normalised experimentally with catalyst performance proving to be identical regardless of weight loading. This study shows that CVI is a technique that can produce active catalysts with high levels of control and consistency of active metal nanoparticles as a function of loading. The same level of control over dispersion and activity was not achieved when catalysts were prepared by conventional aqueous impregnation. The fundamental understanding of CVI is important for the design of highly active catalysts, which is exemplified for propane total oxidation, but has wider significance for other applications of supported metal catalysts.