832 Surface Science 189/190 (1987) 832-841 North-Holland, Amsterdam DECOMPOSITION OF METHANOL OVER Rh AND Ru N. KRUSE, G.-K. CHUAH *, G. ABEND, D.L. COCKE * and J.H. BLOCK Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg4-6, D-IO00 Berlin 33, Germany Received 6 April 1987; accepted for publication 14 May 1987 The reaction of methanol with stepped single crystal surfaces of Ru and Rh has been studied by pulsed field desorption mass spectrometry (PFDMS) in order to provide insight into the reaction mechanisms of the decomposition reaction. CH3OH dissociates easily on Ru and Rh and initially forms adsorbed OCH 3 at temperatures T= 300-700 K. At temperatures T> 460 K, on Ru(001) the surface coverage of COad is limited by thermal desorption during the reaction time t a. CH3OH readily decomposes at empty sites and, consequently, the amount of detected reaction intermediates is small. For T < 460 K high intensities of CH~ (OCH~-) are observed, indicating that the decomposition of adsorbed methoxy, OCH3, is inhibited by the product species, COad. Thus, here the thermal desorption of COad is the rate determining step. This inhibition can be suppressed by field desorption of COad. In this way we observe that the decomposition reaction is faster than the shortest measurable time, t R = 100 #s, even at T = 300 K. The influence of the electrical field on the decomposition reaction over Rh has been studied by applying a steady field, F R, during the reaction interval between the pulses. At F R = 4 V/nm the intensities of COH § COH~, and CH~- increase steeply. This increase is most pronounced for COH~- and coincides with a decrease of the CO + and H + intensities. Thus, the steady electrical field reduces the reaction rate and stabifizes the intermediate stages. 1. Introduction The interaction of methanol with noble metal single crystal surfaces has been the subject of intense study by means of various methods of surface physics [1-9]. one of the main interests concerns the reaction pathways which lead to the formation of CO and H 2. A methoxy species, OCH3, has been identified by means of UPS and EELS as a possible reaction intermediate [1,2,4-9]. In a recent study adsorbed oxygen has been found to promote the dissociative adsorption of methanol on Rh(lll) [10]. This paper reports on research which is aimed at elucidating further details of the decomposition reaction over stepped Ru and Rh surfaces with (001) orientation of the terraces. We employed PFDMS which has been shown to provide insight into the kinetics and mechanisms of surface reactions [11-13]. * On leave from Department of Chemistry, Texas A&M University, College Station, TX 77843, USA. 0039-6028/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)