Photothermal Synergism on Pd/TiO2 Catalysts with Varied TiO2 Crystalline Phases for NOx Removal via H2-SCR: A Transient DRIFTS Study

In this work, photothermal synergism over Pd/TiO2 catalysts with varying rutile and anatase phases was investigated for selective catalytic reduction of NO with H2. In the absence of light, two NOx reduction peaks were observed over Pd/TiO2 catalysts. Pd/TiO2-R exhibited better NOx conversion (65 vs 30%) at 75 °C, while Pd/TiO2-A delivered higher activity (82 vs 40%) at 225 °C. Light excitation exerted a more significant promotional effect over Pd/TiO2-R than over the Pd/TiO2-A catalyst. Diffuse reflectance infrared spectroscopy (DRIFTS) with transient experiments indicated that more nitrates and ammonia species formed on Pd/TiO2-R. For Pd/TiO2-R, bridging, mondentate, and bidentate nitrates served as active NOx adsorbed species at all the investigated reaction temperatures. Ammonia species that originated from the reduction of active NOx species were highly active intermediates, which can react with active NOx species to form N2 and H2O. However, for the Pd/TiO2-A catalyst, bridging and monodentate nitrates were active NOx adsorbed species at 75 °C. The reaction pathway at 225 °C facilitates the reaction between adsorbed NO2 and spiltover hydrogen. From the results of valence band X-ray photoelectron spectroscopy (VBXPS) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS), it could be deduced that the photogenerated holes of both Pd/TiO2 catalysts could activate NO and hydrogen. Moreover, the electrons in the conduction band of the Pd/TiO2-R catalyst had stronger reduction ability, which could combine with O2 to form •O2–. All these results provided a new insight into the understanding of photothermal synergism in heterogeneous catalysis.