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

Abstract

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 NO x reduction peaks were observed over Pd/TiO2 catalysts. Pd/TiO2-R exhibited better NO x 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 NO x adsorbed species at all the investigated reaction temperatures. Ammonia species that originated from the reduction of active NO x species were highly active intermediates, which can react with active NO x species to form N2 and H2O. However, for the Pd/TiO2-A catalyst, bridging and monodentate nitrates were active NO x 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.

TiO2 Coating on SiO2 Nanosphere Supported Pd Catalyst for the H2–SCR of NOx in the Presence of Oxygen

“…Compared to the two catalysts, the intensities of NH 3 and NH 4 + species over 0.5Pd/SiO 2 –TiO 2 (C) are obviously stronger than those over Pd/TiO 2 , which suggests that more reactive intermediates were formed on 0.5Pd/SiO 2 –TiO 2 (C). Previous study reported that NH 4 + was thought to be a significant intermediate, and is more reactive than NH 3 in H 2 –SCR, which can react with active NO x species to form N 2 and H 2 O. , Besides, the formation of NH 4 + can lead to a further activity window at a higher temperature when the reductant H 2 is completely consumed . Therefore, more NH 3 and NH 4 + reactive intermediates were formed on 0.5Pd/TiO 2 (S), which is one of the reasons for its superior activity even at high temperatures.…”

“…Compared to the two + was thought to be a significant intermediate, and is more reactive than NH 3 in H 2 −SCR, which can react with active NO x species to form N 2 and H 2 O. 59,60 Besides, the formation of NH 4 + can lead to a further activity window at a higher temperature when the reductant H 2 is completely consumed. 61 Therefore, more NH 3 and NH 4 + reactive intermediates were formed on 0.5Pd/TiO 2 (S), which is one of the reasons for its superior activity even at high temperatures.…”

“…Upon subsequently switching to NO + O 2 again, the characteristic peaks of NH 3 and NH 4 + vanished, and all peaks assigned to adsorbed NO x species reappeared. Compared to the two catalysts, the intensities of NH 3 and NH 4 + species over 0.5Pd/ 59,60 Besides, the formation of NH 4 + can lead to a further activity window at a higher temperature when the reductant H 2 is completely consumed. 61 Therefore, more NH 3 and NH 4 + reactive intermediates were formed on 0.5Pd/TiO 2 (S), which is one of the reasons for its superior activity even at high temperatures.…”