REACTIONS OF METHANOL ON RH(111) AND RH(111)-(2X2)O SURFACES - SPECTROSCOPIC IDENTIFICATION OF ADSORBED METHOXIDE AND ETA-1-FORMALDEHYDE

The reactions of methanol were examined on clean and oxygen-predosed Rh(lll) surfaces under ultra-high-vacuum conditions. Methoxy intermediates were formed at 140 K after methanol adsorption on the clean surface at 100 K. High-resolution electron energy loss spectroscopy (HREELS) indicated that these methoxides began to decompose to CO and H2 at 210 K. The addition of one-quarter monolayer of oxygen to the surface produced two principal effects on the reactions on the Rh(lll) surface. The first was the alteration of reaction and desorption kinetics via lateral interactions with other adsorbed species. This change was manifested in both the stabilization of methoxide to higher temperatures than observed on the clean surface and the isolation of an adsorbed formaldehyde intermediate produced by methoxide dehydrogenation. Formaldehyde was not isolated after a methanol dose on the clean surface. The oxygen also changed the desorption kinetics of CO and H2. The second role of oxygen was direct reaction with surface species. An example was the direct transfer of the hydroxyl hydrogen of methanol to the surface oxygen, which enhanced the formation of methoxy intermediates at low temperature. These hydroxyls reacted with atomic hydrogen and desorbed as water at 230 K. The results of this study are qualitatively similar to the reactions of oxygenates on other Group VIII metals, although the stabilities of adsorbates and the selectivities of their reactions vary among these metals. © 1990, American Chemical Society. All rights reserved.