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 H-2. 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 degrees C, while Pd/TiO2-A delivered higher activity (82 vs 40%) at 225 degrees 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 N-2 and H2O. However, for the Pd/TiO2-A catalyst, bridging and monodentate nitrates were active NOx adsorbed species at 75 degrees C. The reaction pathway at 225 degrees 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 O-2 to form center dot O-2(-). All these results provided a new insight into the understanding of photothermal synergism in heterogeneous catalysis.

Automated summary

This study investigates the photothermal synergism over Pd/TiO2 catalysts with different rutile and anatase phases for the selective catalytic reduction of NO with H-2. The results show that the rutile phase catalyst exhibits better NOx conversion at lower temperatures, while the anatase phase catalyst is more active at higher temperatures. It is observed that light excitation has a more significant promotional effect on the rutile phase catalyst. The formation of nitrates and ammonia species on the catalyst surface is found to play an important role in the catalytic reaction. The valence band X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy results indicate that the photogenerated holes of both catalysts can activate NO and hydrogen.