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

Abstract

Abstract A series of CeO2 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 WO3 species was loaded on the above four kinds of CeO2 supports with various morphologies by impregnation method (respectively noted as four catalysts including 5%WO3/CeO2-hollowsphere, 5%WO3/CeO2-cube, 5%WO3/CeO2-rod, and 5%WO3/CeO2-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), N2 physical sorption, temperature programmed desorption of NH3 (NH3-TPD). The distinct activities of tungsten loaded on CeO2 with different morphologies were observed. 5%WO3/CeO2-mesopore especially exhibited abroad temperature window of high NO conversion (>95%, 200–420 °C), and an excellent N2 yield (>90%, 200–450 °C). Thereafter, NH3-SCR reaction was systematically studied the changes of surface acidity, active site status, and nitrite/nitrate adsorption behaviors. The surface acidity of CeO2 and 5%WO3/CeO2 were distinguishable. Lewis acid sites were commonly originated from CeO2, but WO3 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%WO3/CeO2 could be more gaseous NO2 and asymmetric vibration bridging nitrate species, while symmetric vibration bridging and monodentate nitrates over CeO2 could be readily generated. The adsorption of NO, NO2 and NH3 on molecular facet were investigated by DFT method, which clearly showed that the crystal plane of (110) is more reactive to NO and NH3 gases with respect to the crystal plane of (111).