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Research on Chemical Intermediates

, Volume 29, Issue 6, pp 553–574 | Cite as

Reaction pathways of carboxylic acids over TiO2 single crystal surfaces: diketene formation from bromo-acetic acid

  • D. J. Titheridge
  • J. N. Wilson
  • H. Idriss
Article

Abstract

This work presents the first investigation of a halo-carboxylic acid (Br-CH2COOH) over the surface of an oxide single crystal (the {011}-faceted TiO2(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 (Ea) for ketene desorption, with a reaction order n close to 1, for both CH3COOH (Ea = 21.3 kcal/mol) and BrCH2COOH (Ea = 17.2 kcal/mol). In order to further understand the surface reaction of BrCH2COOH semi-empirical PM3 computation of its adsorption and reaction on a Ti8O29H26 cluster representing the (011) TiO2 surface was conducted and compared to that of CH3COOH 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)CH2Os, formed by the simultaneous dissociation of both C-Br and O-H bonds of BrCH2COOH appears the most plausible surface intermediate for the observed carbon coupling reactions.

TIO2 001 ACETIC ACID BROMOACETIC ACID SCANNING KINETIC SPECTROSCOPY KETENE DIKETENE. 

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© VSP 2003 2003

Authors and Affiliations

  • D. J. Titheridge
  • J. N. Wilson
  • H. Idriss

There are no affiliations available

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