Surface roughness effects in the catalytic behavior of vanadia supported on SBA-15

SBA-15 is a template-synthesized mesoporous silica that has found extensive use as a model support for catalytic studies. Thorough structural analyses describe a dual micropore-mesopore structure with a broad distribution of micropore size that we alternatively describe as fractal surface roughness. SBA-15 materials with varying surface roughness were systematically prepared followed by grafting with sub-monolayer coverage of vanadium oxide (VO _x). VO_x-SBA-15 samples were characterized using nitrogen adsorption, UV-vis spectroscopy, and Raman spectroscopy and tested in the catalytic partial oxidation of methanol to formaldehyde as well as propane to propene. SBA-15 supports with smoother surfaces favor the formation of more polymeric vanadia species at the same surface density loading. Smooth surface catalysts result in a ∼20% lower selectivity of methanol to formaldehyde, and the apparent activation energy on smooth surfaces is ∼25 kJ/mol lower than on rough surfaces (75 versus 100 kJ/mol, respectively). In contrast to methanol, propane results show a 15% higher selectivity to propene on smooth surfaces. A model of silica hydroxyl distribution is proposed to explain the differences in vanadia speciation and resulting catalytic behavior. These results are significant for our understanding of the nature of vanadium species in partial oxidation catalysts and illustrate the importance of considering differences in support surface morphology in analyzing catalytic behavior.