Kinetics and mechanism of methanol decomposition over zinc oxide

The kinetics of CH3OH decomposition over ZnO was studied in the temperature ranges: 453 to 5130K, and 563 to 6130K. In the first range, CH3OH decomposed to H2 and CH2O, while concurrent decomposition of CH3OH and CH2O to H2, CO, and CO2 occurred in the second range. Production of CO2 was believed to be a side reaction resulting from the decomposition of a formate species. The effect of P(CH3OH) on the initial rate of production of H2, R(H2), and (CO + CO2), R/sub c/, is given by the equation: [P/sub m//R/sub i/]/sup 1/2/ = a + bP/sub/m/. No inhibition was observed for H2, CO, or CO2. At 5000K, CH3OH decomposed about 10 times faster than CH2O. Comparison of the initial rates of decomposition of CH3OH, CH3OD, and CD3OD in the low and high temperature ranges indicated that R/sub i/(CH3OH) = R/sub i/(CH3OD) > R/sub i/(CD3OD). The proposed mechanism assumes that the rate of the low temperature CH3OH decomposition is determined by that of the step: CH3O* + H* → CH2O/sub (g)/ + H/sub 2(g)/ + 2*, while the high temperature decomposition is governed by the step: CHO* + H* → H2/sub 2(g)/ + 2*. The low temperature decomposition has the apparent activation energies: E/sub app.)/ (CH3OH) = E/sub app./(CH3OD) = 89.0 kJ mol-1, and E/sub app./(CD3OD) = 95.7 kJ mol-1. The high temperature decomposition gave a break in the Arrhenius plot of ln R/sub c/ versus l/T at approx. 5900K. This was attributed to the decrease in the equilibrium constants, appearing in the rate law, with increasing temperature. Below 5900K, the apparent activation energies are: E/sub app./(CH3OH) = E/sub app./(CH3OD) = 127.1 kJ mol-1, and E/sub app./(CD3OD) = 175.4 kJ mol-1. 21 figures, 4 tables