Effect of metal elements doping on the CePO4 catalysts for selective catalytic reduction of NO with NH3.

• The M-doped CePO4 catalysts were prepared by the coprecipitation method. • The incorporation of M improved the NH 3 -SCR activity on the CePO 4. • The reaction mechanism followed the Eley-Rideal and Langmuir-Hinshelwood mechanism. A series of Metals doped M-CePO 4 (M =Zr, Co, Mo, Cu) catalysts were prepared via the coprecipitation method for selective catalytic reduction of NO with NH 3 (NH 3 -SCR). The results showed that the NO x conversion of M-CePO 4 were higher than that of CePO 4 , especially the Mo-CePO 4 catalysts exhibited the best low-temperature activity and N 2 selectivity, the Mo-CePO 4 catalysts can reach about 99% at 300 °C and excellent 70 h long stability with the coexistence of SO 2 and H 2 O. The effect of metal elements doping on physiochemical properties of catalysts were characterized by XRD, BET, Raman, TEM, XPS, H 2 -TPR and NH 3 -TPD. SEM and TEM analysis. The results showed that part of M ions doped into CePO4 lattice resulted in lattice shrinkage. More importantly, the MO x species could not only increased the relative content of Ce3+ and surface adsorption oxygen (O′) of M-CePO 4 catalysts, but also increased the specific surface area of the catalysts. It could provide more reaction sites for active gas to promote the activation of NH 3 and enhance oxidation–reduction performance. The in-situ DRIFTS results suggested that NH 3 were adsorbed on both Lewis and Bronsted acid sites which can easily react with NO x gas. The NO species were adsorbed on Mo-CePO 4 in the form of bidentate nitrate. The main reactants of Mo-CePO 4 catalysts were NH 3 linked to Lewis acid sites and bidentate nitrate species on the catalytic surface. The Eley–Rideal and Langmuir–Hinshelwood mechanisms coexisted in the SCR reaction route. According to the characterization results of the catalysts, the mechanism of Mo promoting the activity of NH 3 -SCR at low temperature was proposed (Fig. 13.). NH 3 binds to Lewis acid centers, and NH4+ adsorbs on Brønsted acid centers, monotone nitrates and bidentate nitrates. Studies had shown that NH 2 is an important medium in the reaction of NO (gas phase) to produce NH 2 NO and then decompose into N 2 and H 2 O. As shown in Fig. 12(a), the reaction of NH 3 -SCR on the catalyst follows the Eley-Rideal (E-R) mechanism. The NH 3 Lewis acid sites and NH4+ acid sites adsorbed on Brønsted react with NO (gas phase). As shown in Fig. 12(b), monodentate and bidentate nitrates reacted with adsorbed NH 3 species, following the Langmuir-Hinshelwood (L-H) mechanism [51]. The Mo6+/Mo5+ and Ce4+/Ce3+ redox cycles on the catalyst surface participate in the whole process of the catalytic reaction. At this stage, the electron transfer process promoted the conversion of oxygen vacancy and unsaturated chemical bond, thus enhancing the adsorption activation capacity of NO and NH 3 , promoting the decomposition of NO molecule, and improving the catalytic performance. The –NH 2 s species were intermediate adsorbed and reacted with the activated nitrate, eventually becoming N 2. In the process of NO reduction, NH 3 showed selectivity to the reaction intermediates, preferentially reacted with monotone nitrate and bidentate nitrate. [Display omitted] [ABSTRACT FROM AUTHOR]