A kinetic study of catalytic methanol decomposition over nickel supported on silica

A kinetic study of catalytic methanol decomposition to carbon monoxide and hydrogen over nickel supported on silica has been carried out with a flow reaction system in the temperature range of 433-498 K. The active site probably consists of a pair of nickel atoms which can strongly adsorb hydrogen and carbon monoxide. The reaction pathway is assumed to be: (1) dissociative adsorption of methanol to methoxy groups and hydrogen adsorbed on nickel sites; (2) decomposition of the methoxy groups to adsorbed carbon monoxide and hydrogen; and (3) desorption of the surface carbon monoxide and hydrogen species. In the second process, which is the rate determining step, the surface hydrogen species promote the decomposition of the methoxy groups, but this contradicts previous studies of decomposition over a clean nickel surface on which hydrogen atoms in the methoxy groups probably transfer to free nickel sites. The kinetic analysis shows that almost all the active sites are occupied by adsorption species and the number of free nickel sites is very small under the chosen reaction conditions. Thus, the transfer of hydrogen to free sites will be a minor step, although this process should be energetically advantageous to the decomposition step in which the methoxy groups and the adsorbed hydrogen atoms interact.