Enhanced Two-Dimensional Dispersion of Group V Metal Oxides on Silica

The catalytic performance of supported metal oxides is often controlled by their two- or three-dimensional dispersion. Silica, one of the popular inert supports, triggers the undesired formation of three-dimensional nanoparticles at significantly lower loadings than other conventional supports like Al2O3, TiO2, Nb2O5, or ZrO2. This observation has been ascribed to the lower reactivity of surface SiOH groups toward the precursor, compared to other metal hydroxyl groups on different supports. In this contribution, we show that by promoting amorphous silica with low amounts of sodium, the surface density of two-dimensional metal oxide species can be significantly enhanced to the same level as all other oxide supports previously reported in the literature. This effect is demonstrated for the case of supported vanadia using a variety of spectroscopic techniques (i.e., Raman, diffuse reflectance UV-vis, and V-51-MAS NMR), as well as a catalytic activity study for the oxidative dehydrogenation of propane (ODHP), a structure-sensitive probe reaction. The propane consumption rate was found to increase linearly with the vanadium surface density while the propylene selectivity was not affected until a monolayer coverage of ca. 9 vanadia per nm(2) was surpassed. The method is also applicable to other group V metals (i.e., Nb- and Ta-oxide), opening new perspectives for supported metal oxides.