Journal of Molecular Catalysis A: Chemical 219 (2004) 131–141 Direct observation of thermally activated NO adsorbate species on Au–TiO 2 : DRIFTS studies M.A. Debeila ∗ , N.J. Coville, M.S. Scurrell ∗ , G.R. Hearne Molecular Sciences Institute, School of Chemistry, Private Bag X3, University of the Witwatersrand, Johannesburg 2050, South Africa Received 16 March 2004; accepted 29 April 2004 Available online 9 June 2004 Abstract Adsorption of nitric oxide on Au–TiO 2 catalyst prepared by impregnation to incipient wetness was carried out using DRIFTS as a monitoring technique. Highly dispersed Au particles covering the TiO 2 support and blocking most sites (on TiO 2 ) for NO adsorption is indicated. The early spectra are dominated by M–ONO - (monodentate as well as bridging nitrito surface species) which disappear following an increase in NO pressure, presumably due to conversion to nitrate species which dominate the spectra after extended exposure to NO. Nitrate species decompose at high temperature to form NO 2 - species that coordinated to the surface as N-donor nitro complexes. Two characteristic bands due to ν(N–O) bond vibrations (1690–1696 and 1654 cm -1 ) of adsorbed NO at different sites were obtained on contact of the sample with NO. These bands are significantly red shifted relative to gas phase NO. The sites to which these NO adsorbates are bonded are populated first and represent the most stable sites for NO adsorbates on catalyst studied here at room temperature. The lower wavenumber band (1654 cm -1 ) desorbed and/or dissociated during NO adsorption and is tentatively assigned to NO adsorbed on interfacial sites involving both Au and TiO 2 support (Au–oxide interface) and/or Au sites in the vicinity of oxygen vacancy. The NO adsorbed state with absorption band at 1696–1690cm -1 , assigned to bridging sites, change from one adsorbed state to another at elevated temperatures, i.e. it decreased in intensity and red shifted to lower wavenumbers with concomitant development and growth of two bands at 1748–1754 and 1722–1730 cm -1 during progressive increase in the temperature of the system. Another pair of bands (2182–2178 and 2162 cm -1 ) developed at temperatures >100 ◦ C. These are due to thermally activated NO adsorbate states and are thermally stable. Reported data suggests that these NO adsorbates are bound on low valent and/or unstable high intrinsic energy edge Au sites. It is suggested that these thermally stable surface ‘Au=NO’ complexes are formed by altering the local atomic geometry to achieve higher stability, and once formed, they are irreversible. © 2004 Elsevier B.V. All rights reserved. Keywords: Au–TiO 2 ; NO; Adsorption; Thermally stable; DRIFTS 1. Introduction With the increase in global air pollution, efforts are continually being made to obtain more effective catalytic materials for use in various pollutant abatement processes. Various metals supported on a range of supports have been investigated for this process. Following a breakthrough in the use of gold as a heterogeneous catalyst [1], a large num- ber of reports demonstrating the high activity of supported gold catalysts emerged [2]. The extensively studied reaction is low temperature carbon monoxide oxidation [3–6]. One ∗ Corresponding authors. Tel.: +27-11-717-6761; fax: +27-11-717-6749. E-mail addresses: abel2000za@yahoo.com (M.A. Debeila), scurrell@aurum.wits.ac.za (M.S. Scurrell). application of supported gold catalysts which is currently be- ing investigated is the catalytic reduction of NO x to N 2 + O 2 with hydrocarbons [7–12] or CO [13–16] as reductants. Reduction of NO by CO in the absence of O 2 was reported to occur at temperatures below 100 ◦ C, yielding N 2 as the major product [16]. Gold supported on TiO 2 , ZnO, MgO and Al 2 O 3 also showed high activity for the reduction of NO with propylene at high temperatures (500 ◦ C) [12]. Ad- dition of Mn 2 O 3 to Au–Al 2 O 3 was shown to enhance NO 2 formation and improve the conversion of NO to N 2 [10]. This composite catalyst offers one of the best performances for NO conversion since it can maintain high conversion over a wide temperature range [10]. Cobalt promoted gold catalysts formulated for use in gasoline and diesel applica- tions showed a conversion window at temperatures between 220 and 350 ◦ C with maximum conversion at 290 ◦ C [17]. 1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2004.04.033