Surface chemistry and reactivity of well-defined multilayered supported M 1O x/M 2O x/SiO 2 catalysts

Abstract

A series of group 5–7 transition metal oxides (TMOs) were supported on SiO 2 and surface-modified SiO 2 containing surface AlO x , ZrO x , and TiO x species. The surface reactivity of these silica supported oxides was chemically probed with CH 3OH-temperature-programmed surface reaction (TPSR) spectroscopy. The selectivity of the model supported MO x catalytic active sites on SiO 2 generally reflect the same product distribution as their corresponding bulk MO x counterparts toward dimethyl ether (DME), formaldehyde (HCHO) and CO 2 from surface acidic, redox, and basic sites, respectively. The reactivity of the surface MO x sites generally was suppressed by anchoring of the surface MO x species onto the SiO 2 support. The general surface chemistry trend followed the known inorganic chemistry of the corresponding bulk MO x TMOs. For the multilayered supported M 1O x /M 2O x /SiO 2, with M 1 representing the group 5–7 TMOs and M 2 representing Al, Zr or Ti, the selectivity of the catalytic active sites was generally comparable to that for the model-supported M 1O x /SiO 2 catalysts. The reactivity of the surface VO x , MoO x , and ReO x redox sites increased by one to four orders of magnitude with the introduction of the surface modifiers; however, the reactivity of the surface WO x acidic site was mildly suppressed by the presence of the surface modifiers. The reactivity of the basic CrO x site was only mildly perturbed by the surface modifiers. The reactivity trend of the catalytic TMOs sites was related to the electronegativity properties of the anchoring substrate cations (Si > Al > Zr ∼ Ti).