Res. Chem. Intermed. , Vol. 29, No. 6, pp. 553– 574 (2003) Ó VSP 2003. Also available online - www.vsppub.com Reaction pathways of carboxylic acids over TiO 2 single crystal surfaces: diketene formation from bromo-acetic acid D. J. TITHERIDGE, J. N. WILSON and H. IDRISS ¤ Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 23 May 2003; accepted 27 May 2003 Abstract —This work presents the rst investigation of a halo-carboxylic acid (Br-CH 2 COOH) over the surface of an oxide single crystal (the f 011 g -faceted TiO 2 (001) single crystal). A very rich chemistry is observed. This is broadly divided into three categories: elimination of HBr to make ketene, dimerisation of two molecules of ketene to 4-methyl-2-oxetanone and 1,3-cyclobutanedione, and further reaction of the latter to a mass spectrometer m=e 70 signal attributed to crotonaldehyde (formed by ring opening). Temperature programmed desorption (TPD) and Scanning Kinetic Spectroscopy (SKS) gave complementary results with SKS opening a simple way for investigating surface chemical reactions in UHV conditions with high surface coverage at still high temperatures. A successful modeling of SKS data was conducted providing the activation energies (E a ) for ketene desorption, with a reaction order n close to 1, for both CH 3 COOH (E a D 21:3 kcal/mol) and BrCH 2 COOH (E a D 17:2 kcal/mol). In order to further understand the surface reaction of BrCH 2 COOH semi-empirical PM3 computation of its adsorption and reaction on a Ti 8 O 29 H 26 cluster representing the (011) TiO 2 surface was conducted and compared to that of CH 3 COOH on the same cluster. Dissociative adsorptions of both the O H and C Br bonds are more stable than the non-dissociative adsorption modes. The di-coordinated species, TiOC(O)CH 2 Os , formed by the simultaneousdissociationof both C Br and O H bonds of BrCH 2 COOH appears the most plausible surface intermediate for the observed carbon coupling reactions. Keywords: TiO 2 (001); acetic acid; bromoacetic acid; scanning kinetic spectroscopy; ketene; diketene. INTRODUCTION Studying model reactions on the surfaces of oxide materials has provided a wealth of information and knowledge regarding the elementary steps of chemical reactions (see Refs [1, 2] and references therein), the reaction thermodynamics [3] and the ¤ To whom correspondence should be addressed. Fax: (64-9) 373-7422. E-mail: h.idriss@auckland.ac.nz