XPS study of the adsorption of SO2 and NH3 over supported tin dioxide catalysts used in de-NOx catalytic reaction

Alumina and titania supported tin dioxide catalysts presenting various Sn loadings were prepared by impregnation. The acidity and basicity of the samples were determined by adsorption of ammonia and sulfur dioxide, respectively, using adsorption microcalorimetry and X-ray photoelectron spectroscopy. The surface reactivity of the samples and the chemical composition of the adsorbed species were determined as well as the heats of adsorption of the probe molecules. The catalysts were tested in the selective catalytic reduction of NO by C2H4. Both calorimetry and XPS experiments have shown that the SnO2/TiO2 series of samples was markedly more acidic than the SnO2/Al2O3 series, as the N/(Ti+Sn) molar ratios were noticeably higher than the corresponding N(Al+Sn) ratios. The number of acidic sites seemed to increase with the tin content when tin dioxide was well dispersed on the support. They are of Lewis type. It was shown that sulfur dioxide adsorption led to the formation of three types of species: SO2, sulfites, and sulfates. The basicity of the SnO2/Al2O3 series of samples was weaker than that of the alumina support and passed through a minimum around 12 wt % Sn. On the contrary, as the acidity, the basicity of the SnTi series did not seem dependent on the Sn concentration. This can be correlated to the bad dispersion of SnO2 on TiO2. In the NO reduction by C2H4 reaction, the turnover frequency mainly depends on the Sn dispersion, and the Sn centers are very active even at low amounts.