Radius of curvature effect of V-MCM-41 probed by methanol oxidation

A series of V-MCM-41 samples was prepared with very long-range ordered structure, different pore sizes, and constant composition by applying a recently developed quantitative model [Y.H. Yang, S.Y. Lim, C. Wang, D. Harding, G.L. Haller, Micropor. Mesopor. Mater. 67 (2-3) (2004) 245-257]. Oxygen titration of catalysts pre-reduced with methanol vapor at the reaction temperature showed a constant active site density regardless of the pore diameter at lower temperature. A pore wall radius of curvature effect on the catalytic reaction existed when the methanol oxidation was used as a probe reaction over a 100-K temperature range (573-673 K). The effect was represented by a "volcano curve" in which the turnover frequency increases with increasing pore diameter to a maximum and then decreases. The effect of pore size on active site density at temperatures above the reaction temperature was observed, a result associated with the pore size effect on stability of SiOV units in the pore wall. Based on previous research, there were no surface vanadium compounds in our catalysts, a fact confirmed by Raman spectroscopy for both fresh and used catalysts. Several different characterization techniques, including nitrogen physisorption, X-ray diffraction, and ultraviolet-visible spectroscopy, were used to demonstrate that the structural stability of catalysts determines the activity and how vanadium incorporation into the framework controls the selectivity of reactions. Hydrogen temperature-programmed reduction experiments over the same series of V-MCM-41 catalysts demonstrated that reduction stability was influenced by the radius of curvature, and that there was a strong correlation between the reducibility and selectivity of formaldehyde and dimethyl ether.