Kinetics and mechanism of methanol decomposition over zinc oxide

Abstract

The kinetics of CH 3OH decomposition over ZnO was studied in the temperature ranges 453–513 K and 563–613 K. In the first range, CH 3OH decomposed to H 2 and CH 2O, while in the second range CH 3OH decomposed to CH 2O, CO, CO 2, and H 2 with the CO and CO 2 probably formed through formaldehyde or formate intermediates. The initial rate of production of hydrogen, R H 2 , depended on the methanol pressure P m according to ( P m R H 2 ) 1 2 = a + bP m ; the initial rate of production of carbon monoxide plus carbon dioxide, R c, depended on P m according to ( P m R c ) 1 3 = c + dP m . Both R H 2 and R c were independent of hydrogen, carbon monoxide, and carbon dioxide pressures over the temperature and pressure ranges studied. At 500 K and a given pressure, CH 3OH decomposed more rapidly than CH 2O. In both the low- and high-temperature ranges the decomposition of CH 3OH, CH 3OD, and CD 3OD follow R CH 3OH = R CH 3OD > R CD 3OD . A mechanism accounting for these findings is presented. Principal features are CH 3O ∗ + H ∗ → H 2 + CH 2O ∗ followed by either CH 2O ∗ → CH 2O(g) + ∗ (the only step observed at low temperature) or CH 2O ∗ + ∗ → CHO ∗ + H ∗ → CO + H 2. The fraction of formaldehyde decomposing increases with T and is about 20% at 593 K.