Morphology effect of diverse ceria with active tungsten species on NH3-SCR behaviors

Abstract A series of CeO 2 with diverse morphologies (hollowsphere, cube, and rod) was synthesized by hydrothermal method, while a worm-like mesoporous ceria was synthesized by evaporation-induced self-assembly (EISA) method. 5 (wt.)% of WO 3 species was loaded on the above four kinds of CeO 2 supports with various morphologies by impregnation method (respectively noted as four catalysts including 5%WO 3 /CeO 2 -hollowsphere, 5%WO 3 /CeO 2 -cube, 5%WO 3 /CeO 2 -rod, and 5%WO 3 /CeO 2 -mesopore). Thereafter, these solids were comparatively tested for the reaction of low-temperature selective catalytic reduction (SCR) of NO with ammonia. The physicochemical properties were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), N 2 physical sorption, temperature programmed desorption of NH 3 (NH 3 -TPD). The distinct activities of tungsten loaded on CeO 2 with different morphologies were observed. 5%WO 3 /CeO 2 -mesopore especially exhibited abroad temperature window of high NO conversion (>95%, 200–420 °C), and an excellent N 2 yield (>90%, 200–450 °C). Thereafter, NH 3 -SCR reaction was systematically studied the changes of surface acidity, active site status, and nitrite/nitrate adsorption behaviors. The surface acidity of CeO 2 and 5%WO 3 /CeO 2 were distinguishable. Lewis acid sites were commonly originated from CeO 2 , but WO 3 greatly enhanced the amount and strength of the Bronsted acid sites based on DRIFT study. In addition, the oxygen isotopic temperature programmed exchange experiment indicated that the introduction of tungsten inhibited the surface lattice oxygen mobility and the related oxidative activity, corresponding to poor oxygen exchange ability. Furthermore, the mechanisms were deduced by the DRIFT spectra, and density functional theory (DFT) calculations. NO adspecies of 5%WO 3 /CeO 2 could be more gaseous NO 2 and asymmetric vibration bridging nitrate species, while symmetric vibration bridging and monodentate nitrates over CeO 2 could be readily generated. The adsorption of NO, NO 2 and NH 3 on molecular facet were investigated by DFT method, which clearly showed that the crystal plane of (110) is more reactive to NO and NH 3 gases with respect to the crystal plane of (111).