Comparative density functional study of methanol decomposition on Cu4 and Co4 clusters.

A density functional theory study of the decomposition of methanol on Cu(4) and Co(4) clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H(2) and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu(4) cluster, methanol dehydrogenation through hydroxymethyl (CH(2)OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co(4) cluster, the dehydrogenation pathway through methoxy (CH(3)O) and formaldehyde (CH(2)O) is slightly more favorable. Each of these pathways results in formation of CO and H(2). The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H(2) and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd(4) and Pd(8) clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot.