COMPARATIVE ADSORPTION/DESORPTION STUDIES ON ZINC, ZINC OXIDE AND COPPER/ZINC OXIDE SURFACES (METHANOL DECOMPOSITION, METHOXY, FORMATE INTERMEDIATE, FILMS, SINGLE CRYSTAL)

Comparative adsorption/desorption studies on Zn, ZnO, and Cu/ZnO surfaces have been performed under ultra high vacuum conditions by temperature programmed desorption (TPD), temp- erature programmed decomposition, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS).; Hydrogen is adsorbed in its atomic state on Zn(0001) surfaces and its desorption obeys half-order kinetics with a desorption energy of 11.1 kcal/mole. Kinetic order studies strongly suggest that hydrogen island formation occurs on Zn(0001) surfaces. H(,2) uptake and XPS intensity measurements indicate that the initial oxidation process of Zn(0001) surfaces involves the formation and growth of multi-layer oxide islands. A Zn-O adsorption site on the perimeter of the island is proposed for hydrogen adsorption from which the H(,2) desorption energy is 18.0 kcal/mole. TPD spectra also show that hydrogen adsorption on the ZnO(000(')1) surface is irreversible (stable surface hydroxyl) below 400 K.; Thin ZnO films supported on Au foils were prepared using an RF plasma sputtering technique and have been used as a model ZnO catalyst surface. TPD data show that these ZnO films consist primarily of polar oriented Zn- and ZnO-like adsorption sites. CH(,3)OH decomposition on this model ZnO surface occurs via two kinetically resolvable channels at 585 and 635 K with different activation temperatures. The channels are assigned to the decomposition of surface methoxy species and surface formate species. The selectivity between the two channels is a function of initial CH(,3)OH coverage. Heterolytic H(,2) adsorption sites are believed to preferentially locate on (0001)-Zn surfaces. Coadsorption of H(,2)O has lower the decomposi- tion temperature of methoxyl intermediates, but H(,2)O poisoning or blocking effect have not been seen on ZnO surfaces. The results are independent of whether H(,2)O is adsorbed before or after CH(,3)OH. CH(,3)OH and H(,2)O adsorption occur on Zn and Zn-O pair site respectively. On Cu/ZnO surfaces (Cu:Zn = 40:60), an additional decomposition channel is observed at 415 K, presumably due the decomposition of a methoxy intermediate. The decrease in activation energy simply suggests that the cleavage of the C-H bond is via the hydride transfer to a neighboring Cu site. The active Cu/ZnO site consists of a Zn-O pair and a neighboring metallic Cu.