Effect of the TiO2 crystal structure on the activity of TiO2-supported platinum catalysts for ammonia synthesis via the NO–CO–H2O reaction

Abstract

Selective formation of NH3, a potential hydrogen carrier for renewable energy applications, by means of the NO–CO–H2O reaction over TiO2-supported Pt catalysts was investigated as a way to recover NOx species emitted from combustion processes. All of the tested Pt/TiO2 catalysts showed almost complete conversion of NO to NH3 at temperatures >250 °C; but at temperatures <200 °C, the activity of Pt supported on anatase TiO2 was higher than the activities of Pt on rutile TiO2 or a mixture of anatase and rutile TiO2. Transmission electron microscopy revealed that the Pt particle size on anatase TiO2 was smaller than that on rutile TiO2. Temperature-programed reduction by hydrogen revealed that Pt interacted with the anatase TiO2 support but not with the rutile TiO2 support. Diffuse reflectance infrared Fourier transform spectroscopy and measurements of kinetic parameters revealed that the CO adsorption behavior and the activity of hydrogen supply from H2O depended on the support. Specifically, Pt supported on anatase TiO2 retained the adsorbed CO at temperatures up to 200 °C, and the reaction orders with respect to CO and H2O were −0.06 and 0.39, respectively. In contrast, Pt supported on rutile TiO2 desorbed CO below 200 °C, and the reaction orders with respect to CO and H2O were −0.57 and 0.03, respectively. The difference of the catalytic activity at low temperature between Pt/TiO2(A) and Pt/TiO2(R) is due to the difference of the reactivity of CO and H2O.