Selective modification of surface and bulk V5+/V4+ ratios and its effects on the catalytic performance of MoVTeO catalysts

Selective modification of surface and bulk V5+/V4+ ratios in MoVTeO catalysts has been achieved using nitric acid and saturated ammonia solutions as pH adjustors. The addition of nitric acid strongly suppressed V4+ in the bulk but encouraged it on the surface. Because the bulk-phase compositions of MoVTeO catalysts varied only slightly, the changes in the catalytic performance for selective propane oxidation to acrolein resulted mainly from the variation of the bulk and surface V5+/V4+ ratios. The high concentration of surface V5+ that was responsible for propane activation led to high selectivity of propylene and high conversion of propane. Increased bulk V4+ might result in oxygen vacancies, which promote the electron and oxygen transfer so as to increase the selectivity of deep oxidation products (COx). A low bulk V4+ concentration prefers selective oxidation products; the yield of acrolein was as high as 21.4%.