The effect of synthesis solution pH on the physicochemical properties of Co substituted MCM-41

Abstract

The adjustment of the initial synthesis solution pH affects the structure and stability of C16 Co-MCM-41. This pH effect was systematically investigated using N2 physisorption, TPR, in-situ FTIR, and X-ray absorption spectroscopy. Co-MCM-41 catalysts with the same pore size but greater porosity were synthesized with increasing pH from 10.5 to 12. The distribution of Co ions with respect to being on the pore wall or in the silica wall framework was changed by pH; higher pH produced Co ions mainly distributed just subsurface or in the interior of the silica wall. These pH effects significantly affected the reduction stability of the Co-MCM-41 sample similar to that of the pore radius of curvature effect (Lim et al, J. Phys. Chem. B, 109 (2005) 2285), resulting in stable and size controllable sub-nanometer Co clusters that are useful for catalyst design for specific reactions.