The effect of calcination temperature on the adsorption, of nitric oxide on Au-TiO2:: Drifts studies

Adsorption of NO on TiO2 and Au-TiO2 catalysts calcined at different temperatures has been studied using DRIFTS as a monitoring tool. It was found that NO adsorption on TiO2 is initially dominated by unidentate nitrite (1479, 1464 cm(-1) ), a small concentration of nitro species (1440, 1385, 1370 cm(-1)) which later were replaced by nitrate (1611, 1589 and 1480 cm(-1)) and which dominated the spectra after extended exposure of TiO2 to NO. Decomposition. of NO3- into NO2- at elevated temperatures is observed and nitrite bound to the surface to form nitrite (1407 cm(-1)), which dominates the spectra after cooing the system to 298 K. Coordinated NO on different V, sites (1906 and 1447 cm(-1)) detected in the early stages of NO adsorption and (1934 and 2005 cm(-1)) were also found after extended exposure of TiO2 to NO. For Au-TiO2, early spectra were also dominated by unidentate nitrite (1476 cm(-1)) for uncalcined Au-TiO2 (designated Dry-Au-T) and (1477 cm(-1)) for Au-TiO2 calcined at 973 K (designated Au-T973). In addition, bridging nitrite (1540 cm(-1)) is one of the dominant species seen on Dry-Au-TiO2. The nitrate surface species, which dominate the spectra after extended exposure, are virtually the same and behave in the same way under in situ thermal desorption experiments. However, nitrate formation on Au-TiO2 is relatively fast compared with the situation found for TiO2, Mechanisms for the formation of these species are discussed and compared with previously reported data. For Au-TiO2, two characteristic bands at 1685 and 1644 cm(-1) for Dry-Au-T and at 1682 and 1643 cm(-1) for Au-T973 were detected at room temperature and dominated the initial spectra. The sites containing these NO molecules were populated first. For Dry-Au-T, the higher wavenumber mode (1685 cm(-1)), assigned to bridging NO adsorbate, decreased in intensity during NO adsorption to produce a very weak feature and is assumed to contribute to the dissociation of NO on this catalyst. However, the lower wavenumber mode (1644 cm(-1)), assigned to NO adsorbed at the gold-oxide interface, shifts to lower wavenumber and acts as a precursor state for the activated NO states detected at 1750 and 1714 cm(-1) on raising the temperature to 473 K. In contrast, for Au-T973, the lower wavenumber mode (1643 cm(-1)) is associated with a unit which undergoes dissociation, while the high wavenumber mode (1682 cm(-1)) is assigned to a precursor state for the thermally activated NO states detected at 1744 and 1714 cm(-1) at 473 K. This trend is explained in terms of the changes in the degree of interaction between gold and the underlying TiO2 support brought about upon calcination. The results are discussed and correlated with previous observations on gold catalysts in an attempt to assess the impact of these thermally activated NO states on the decomposition and selective catalytic reduction of NO over gold catalysts. (c) 2005 Elsevier B.V. All rights reserved.