Direct observation of thermally activated NO adsorbate species on Au-TiO2:: DRIFTS studies

Adsorption of nitric oxide on Au-TiO2 catalyst prepared by impregnation to incipient wetness was carried out using DRIFTS as a monitoring technique. Highly dispersed Auz particles covering the TiO2 support and blocking most sites (on TiO2) 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 NO2- species that coordinated to the surface as N-donor nitro complexes. Two characteristic bands due to v(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 TiO2 support (Au-oxide interface) and/or Au sites in the vicinity of oxygen vacancy. The NO adsorbed state with absorption band at 1696-1690 cm(-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 degreesC. 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 An 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. (C) 2004 Elsevier B.V. All rights reserved.