Bifunctional mechanism for the selective catalytic reduction of NOx on Rh/sulfated titania

The selective catalytic reduction of NO by propene has been investigated on a series of Rh catalysts supported by sulfated titanias. The dispersion of Rh oxide was measured by FTIR, from the observation of NO adsorption, and the number of acid sites by gravimetry, from the observation of NH3 adsorption. The addition of sulfated titania to Rh/SiO2 as a mechanical mixture increased the rate of N2 formation, and a higher rate was observed when Rh was supported on sulfated titania. A quantitative analysis of the influence of the acidity of the support shows that the activity for N2 increases with the number of acid sites. An increase in the surface of Rh oxide also leads to an increase in rate up to a plateau, at which point the rate is limited by an acid-catalysed reaction. This behaviour is attributed to a bifunctional mechanism for the reaction. In the presence of 25 ppm of SO2 and 3% water, these catalysts reach 40% yield in N2 at a GHSV = 35,000 h−1 with no loss of properties.