Methanol decomposition on oriented ZnO thin films

Abstract

We report temperature programmed decomposition spectra for CH 3OH adsorbed on C-axis oriented polycrystalline thin films of ZnO. For CH 3OH adsorbed at 300 K, decomposition occurs via two kinetically resolvable channels with different activation temperatures. The low temperature channel produces simultaneous H 2 and CH 2O desorption peaks at 585 K, and is assigned to the decomposition of surface methoxy species. The high temperature channel produces simultaneous H 2, CO and CO 2 peaks at 635 K, and is assigned to the decomposition of adsorbed formate species. The selectivity between the two channels is a function of initial CH 3OH coverage, with decomposition occurring primarily via the methoxy channel at low coverages and shifting to the formate channel at higher coverages. High initial coverage also delays decomposition of some of the methoxy species until 635 K. The product selectivity and coverage dependence indicate the importance of heterolytic H 2 adsorption sites, which are believed to be preferentially located on defected (0001) surfaces of ZnO.