Surface Science 457 ( 2000 ) L413–L420 www.elsevier.nl/locate/susc Surface Science Letters Chemistry of thiophene on Mo(110), MoC x and MoS x surfaces: photoemission studies Jose ´ A. Rodriguez *, Joseph Dvorak, Tomas Jirsak Department of Chemistry, Brookhaven National Laboratory, Upton, New York, NY 11953, USA Received 11 January 2000; accepted for publication 14 March 2000 Abstract Synchrotron-based high resolution photoemission has been used to study the adsorption and decomposition of thiophene on clean Mo (110 ) and carbide- or sulfide-modified molybdenum ( MoC x and MoS x , respectively). Thiophene adsorbs molecularly on Mo(110) at 100 K. By 180 K, upon desorption of the thiophene multilayer, chemisorbed thiophene and its decomposition products (S and C x H y fragments) coexist on the Mo(110) surface. At 250 K, no CMS bonds are left. There is a continuous transformation of the C x H y species into atomic C at temperatures between 300 and 600 K. The chemistry of thiophene on clean and carbide-modified molybdenum is very similar. In contrast, MoS x is less reactive than Mo(110) or MoC x , even after introducing a large number of sulfur vacancies in the sulfide or after promoting it with nickel. Thiophene does not dissociate on MoS x or Ni/MoS x . The differences in chemical reactivity between MoS x and MoC x correlate well with changes in the electronic properties of Mo in these systems. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aromatics; Carbides; Molybdenum; Sulphides; Sulphur; Surface chemical reaction; Synchrotron radiation photoelectron spectroscopy 1. Introduction formed during the combustion of these fuels [5]. The most widely used catalyst in hydrodesulfuriza- tion ( HDS ) reactions consist of a mixture of cobalt In petroleum refineries, organosulfur molecules and molybdenum sulfide on a c-alumina support are removed from the oil by reaction with [2,3,6 ]. More stringent environmental legislation hydrogen to form H 2 S and hydrocarbons (hydro- stresses the need to develop a new generation of desulfurization process) [1–3]. This operation pre- HDS catalysts that lead to the ultimate goal of vents sulfur-containing molecules from reaching clean burning fuels [2,3,6 ]. and deactivating platinum reforming catalysts that Interstitial carbides of early transition metals are downstream in the refining of the oil [3,4]. In (groups 4–6) are produced by dissolving carbon addition, it improves the quality of gasoline-related into the metal lattices [7,8]. Early-transition metal products by reducing the amount of SO x pollutants carbides have attracted a lot of attention because they exhibit chemical and catalytic properties sim- * Corresponding author. Fax: +1-631-344-5815. E-mail address: rodrigez@bnl.gov (J.A. Rodriguez) ilar to those of group 8–10 metals [7,9]. Recently, 0039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0039-6028(00)00416-7