Methanethiol chemistry on TiO 2 -supported Ni clusters O. Ozturk b , J.B. Park a , T.J. Black a , J.A. Rodriguez c , J. Hrbek c , D.A. Chen a, * a Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, United States b Physics Department, Gebze Institute of Technology, Kocaeli 41400, Turkey c Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, United States article info Article history: Received 30 May 2008 Accepted for publication 21 July 2008 Available online 8 August 2008 Keywords: Low energy ion scattering (LEIS) Soft X-ray photoelectron spectroscopy Scanning tunneling microscopy Surface chemical reaction Thermal desorption Nickel Titanium oxide Clusters abstract The thermal decomposition of methanethiol on Ni clusters grown on TiO 2 (1 1 0) was studied by temper- ature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and low energy ion scatter- ing (LEIS). On all of the Ni surfaces investigated, methane and hydrogen were observed as gaseous products in the TPD experiments, and the only sulfur-containing species that desorbed from the surface was methanethiol itself at low temperatures. The two pathways for methanethiol reaction were hydrode- sulfurization to produce methane and nonselective decomposition, which leaves atomic carbon and sul- fur on the surface. From high resolution XPS studies, methyl thiolate was identified as the surface intermediate for reaction on TiO 2 and on all of the Ni surfaces investigated, similar to what is observed on single-crystal Ni surfaces. However, the binding sites for methyl thiolate on the 1 ML (monolayer) Ni clusters were different from those on the Ni clusters at coverages of 2.5 ML and higher, based on the S(2p) binding energies for methyl thiolate. No distinct changes in activity or selectivity were observed for the smaller Ni clusters grown at low coverage compared to the more film-like Ni surfaces other than what could be accounted for by changes in total surface area. Interactions between the Ni clusters and the TiO 2 support had two main effects on chemical activity. First, carbon was oxidized by oxygen from the TiO 2 lattice to produce CO at temperatures above 800 K. Second, annealing induced encapsulation of the Ni clusters by reduced TiO x and chemisorbed oxygen. At 800 K, the Ni clusters were totally encapsu- lated, resulting in a complete loss of methanethiol activity; partial encapsulation at 700 K caused a smal- ler decrease in activity accompanied by increased oxidation of carbon by lattice oxygen. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The chemistry of sulfur-containing molecules on transition me- tal surfaces has long been a topic of interest for understanding cat- alysts used in hydrodesulfurization reactions [1,2]. In particular, methanethiol has been well-studied as a probe molecule for hydrodesulfurization reactions on single-crystal Ni surfaces like (111) [3–5], (110) [6], and (100) [7–10], since Ni is an important component in desulfurization catalysts [11–17]. Methanethiol reactions have also been investigated on other transitions metal surfaces, including Mo(1 1 0) [18], Ru(0001) [19], Fe(110) [20], W(100) [21], and Pt(1 1 1) [22]. In almost all cases, methyl thiolate has been found to be the main intermediate formed from S–H bond scission at low temperatures. Upon heating the surfaces, methyl thiolate undergoes hydrodesulfurization to produce gaseous meth- ane and nonselective decomposition to produce hydrogen gas and atomic carbon on the surface; both reaction pathways result in atomic sulfur left on the surface. For the Ni surfaces, the selectivity for hydrodesulfurization is 80% [4,6], but this value drops to 50% on Mo(110) [18,23], 40% on Pt(111) [24] and 30% on W(0 0 1) [25]. On Pt(111), CH 2 S has been reported as a reaction intermediate in nonselective decomposition processes [22,24], and on Mo(1 1 0), gaseous methyl radicals have been observed in addition to methane [23]. In work reported here, the thermal chemistry of methanethiol is investigated on Ni clusters supported on TiO 2 (1 1 0) as a function of Ni cluster size, coverage and interactions with the titania support. A number of studies have shown that clusters can exhibit activity different from the bulk surfaces due to particle size effects [26–30], differences in active sites [31–33], and interactions between the clusters and the supports [34–38]. Titania is a support of particular interest since highly reducible metal oxides like titania are known to strongly interact with transition metal clusters [39–43]. In addi- tion, titania is one of the few bulk oxides that can be made semi- conducting for scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS) studies simply by heating in ultrahigh vacuum. In this pro- cess, oxygen preferentially desorbs from the crystal as O 2 , resulting in an n-type semiconductor. The chemistry of methanethiol has been studied by TPD and XPS on the supported Ni clusters with Ni coverages ranging from 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.07.032 * Corresponding author. Tel.: +803 777 1050; fax: +803 777 9521. E-mail address: chen@mail.chem.sc.edu (D.A. Chen). Surface Science 602 (2008) 3077–3088 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc