Investigation of the structure and activity of VOx/CeO2/SiO2 catalysts for methanol oxidation to formaldehyde

The effect of ceria on the partial oxidation of methanol to formaldehyde over VOx/CeO2/SiO2 catalysts was investigated. A two-dimensional layer of ceria on silica was prepared by grafting cerium (IV) t-butoxide (Ce(OC4H9)4) onto high surface area, mesoporous silica, SBA-15, and then calcining the resulting product in air at 773 K. Ce surface concentrations obtained this way ranged from 0.2 to 0.9 Ce nm−2. Next, V was introduced by grafting VO(OiPr)3 onto CeO2/SiO2 in order to achieve a surface concentration of 0.6 V nm−2. XANES spectra indicate that all of the V is in the 5+ oxidation state and Raman spectra show that vanadia exist as pseudo-tetrahedra bonded to either silica or ceria. Data from Raman spectroscopy and temperature-programmed desorption of adsorbed methanol indicate that with increasing Ce surface density, most of the V becomes associated with the deposited ceria. The turnover frequency for methanol oxidation is nearly two orders of magnitude higher for VOx/CeO2/SiO2 than for VOx/SiO2, whereas the apparent activation energy and apparent first-order pre-exponential factor are 17 kcal/mol and 1.4 × 106 mol CH2O (mol V atm s)−1, respectively, for VOx/CeO2/SiO2 and 23 kcal/mol and 2.3 × 107 mol CH2O (mol V atm s)−1, respectively, for VOx/SiO2.