Inhibition of methanol oxidation by water vapor - Effect on measured kinetics and relevance to the mechanism

The kinetics of methanol oxidation to formaldehyde was studied over an iron molybdenum oxide catalyst in a continuous flow reactor with external recycling at temperatures of 200-300 degrees C. The kinetics of the reaction were well described by a power law rate expression of the form r(2) = k (PCH3OHPO2PH2Oz)-P-x-P-y where x = 0.94 +/- 0.06, y = 0.10 +/- 0.05, and z = -0.45 +/- 0.07. The measured activation energy was 98 +/- 6 kJ/mol. When product inhibition by water vapor is not taken into account in such a power law kinetic rate expression, the apparent reaction orders in methanol and oxygen, x' and y', and the activation energy E' are all lower than their true values: x' = (1-delta)x, y' = (1-delta)y, and E' = (1-delta)E, where delta = -z/(1-z). Methanol chemisorbs dissociatively to form methoxy and hydroxyl groups, and the rate-determining step is the decomposition of the methoxy intermediate. Product inhibition occurs through kinetic coupling, whereby water vapor chemisorbs dissociatively to form hydroxyl groups, which serve to reduce the steady state concentration of methoxy groups on the catalyst surface by reacting with them to reform methanol. (C) 1996 Academic Press, Inc.