Identification of the active sites on CeO2-WO3 catalysts for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study

A series of CeO2-WO3 (WxCe) catalysts were prepared for the SCR of NOx with NH3. Among the three representative samples, the W1Ce catalyst exhibited great activity during SCR reaction and a large amount of N-2 from NH3 oxidation, whereas the W0.05Ce catalyst yielded more NO from NH3 oxidation than the W3Ce at high temperature. Three types of metal oxide species (isopolytungsten species, Ce-2(WO4)(3) compounds and crystalline WO3) were observed on the catalyst surfaces with increased WO3 loading. CeO2, rather than WO3 or Ce-2(WO4)(3), presented favorable reducibility behavior. Furthermore, CeO2 with intrinsic oxygen vacancies and unsaturated Wn+ cations of crystalline WO3 provided Lewis acid sites; meanwhile, the W-O-W or W=O modes of Ce-2(WO4)(3) provided Bronsted acid sites. Based on the Raman spectra, the W-O species of the [WO4] or [WO6] units could be the active sites. The results suggest a reaction mechanism consisting of two independent cycles, denoted as a redox cycle due to the excellent oxygen storage capability and reducibility of cubic fluorite CeO2 (for NH3 activation) and an acid site cycle, resulting from Bronsted acid sites formed on the W-O-W species of Ce-2(WO4)(3) (for NH3 adsorption). Surface nitrate species cannot be involved directly in SCR reaction with gaseous NH3; additionally, the adsorbed NO2 demonstrated activity over the W1Ce catalyst at 200 degrees C. (C) 2013 Elsevier B.V. All rights reserved.