The significance of the order of impregnation on the activity of vanadia promoted palladium-alumina catalysts for propane total oxidation

Abstract

The increased activity of alumina-supported palladium catalysts promoted with vanadium oxide has been investigated. Three different vanadium promoted Pd/Al₂O₃ catalysts with the same composition but synthesized employing sequential and co-impregnation were tested for the total oxidation of propane. The order of impregnation was critical to produce high activity catalysts. Vanadium and palladium co-impregnation on the Al₂O₃ support led to the most active catalyst, whereas the step-wise impregnated catalysts show a catalytic performance similar to or slightly better than unpromoted palladium catalysts. The high activity of the co-impregnated catalysts is related to the particle size and oxidation state of the palladium particles; and to the redox properties of vanadium species. The most active catalyst presents relatively large palladium particles in combination with increased reducibility of vanadium species and a relatively high amount of V⁴⁺ within the bulk of the catalyst and on the surface. STEM shows that, compared to catalysts containing only Pd or V, co-addition of the Pd and V species drastically altered the particle size distribution and morphology of the PdO_x particles, and simultaneously caused the monolayer dispersion of the VO_x species to become much patchier in nature. It also showed that the microstructure of the catalysts was similar for the different orders of impregnation, but some differences between the morphology of PdO_x particles were observed.