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 NO_x 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.