Monitoring the interaction of sulfur dioxide with a TiO 2 (110)surfaceat300Kbyscanningtunnelingmicroscopy N. Hartmann 1 , J. Biener 2 , R.J. Madix * Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA Received 25 May 2001; accepted for publication 7 December 2001 Abstract Scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were used to study the interaction of sulfur dioxide with the bulk-terminated (1  1) and the reconstructed (1  2)structureoftheTiO 2 (110)surfaceat300K.Duringexposurenewfeaturesappearfirstontheaddedrowsofthe reconstructed (1  2) surface structure. In general these new features are randomly distributed, i.e., no ordered ad- sorbate phase builds up. However, in some areas locally ordered linear structures can be observed along the (1  2) rowswithaperiodicitytwoorthreetimeshigherthanthesubstrate.Similartotheseobservations,randomlydistributed spotsalsoappearon single addedrowsofthe(1  2)surfacestructurethatarelocatedontopofthe(1  1)structureof theTiO 2 (110)surface.Incontrast,additionalfeaturesonthe(1  1)structureitselfcanbeseenonlyafterhigherSO 2 exposure.Here,anorderedadsorbatephasewitha(2  1)structureisformed.Thenewfeaturesofthis(2  1)structure are located on top of the (1  1) rows along the [001] direction, which previously have been shown to represent the fivefold coordinated Ti surface cations. Subsequent AE spectra display a single peak in the sulfur region at 151.3 eV, confirming the predominant formation of Ti–S bonds during SO 2 exposures.InLEEDadiffuse(1  2) structure with faint (1  2) half-order reflections can be seen, demonstrating the overall disorder on the surface. Point defects on the stoichiometric (1  1) surface structure as well as on the reconstructed (1  2) surface structure are unaffected by SO 2 exposure. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide; Sulphur dioxide; Scanning tunneling microscopy; Catalysis 1. Introduction Thestructureandpropertiesoftransition-metal oxides have been studied for many years because of their importance in a wide range of technolog- ical applications such as photoelectrolysis [1], semiconductor devices [2], gas sensors [3] and ca- talysis [4–9]. In heterogeneous catalysis, for in- stance, metal oxides serve as a support for metal catalysts [6–9] or as catalysts themselves [4,5]. During the last decade, both of these aspects of Surface Science 505 (2002) 81–92 www.elsevier.com/locate/susc * Corresponding author. E-mail addresses: nils.hartmann@uni-essen.de (N. Hart- mann), rjm@chemeng.stanford.edu (R.J. Madix). 1 Permanent address: Universit€ at Essen, Fachbereich Che- mie, D-45117 Essen, Germany. 2 Permanent address: Universit€ at Bayreuth, Bayerisches Geoinstitut, D-95440 Bayreuth. 0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0039-6028(02)01099-3