Adsorption and surface reaction pathway of NH3 selective catalytic oxidation over different Cu-Ce-Zr catalysts

Abstract

The adsorption species and reaction mechanism for NH3 selective catalytic oxidation (NH3-SCO) over series of Cu-Ce-Zr catalysts prepared by citric acid sol–gel (SOL), homogeneous precipitation (HP) and incipient wetness impregnation (IW) methods were systematically investigated by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT) and Mass Spectroscopy (MS). The redox characteristics of catalysts were determined by the synergistic effect among each catalyst components. The reaction pathway on all of three catalysts applied on the NH3-SCO were investigated and compared through the formation of intermediate species and final products. The Cu-Ce-Zr-SOL catalyst with the best catalytic activity presented the excellent adsorption ability of NHx and NOx species. A large amount of intermediate species over Cu-Ce-Zr-SOL surface facilitated the reaction. On the other hand, the most of NH3 was located in Brønsted acid sites in the form of NH4+ over Cu-Ce-Zr-HP which was stable during the reaction. The larger CuO particle and a vast of B acid sites on Cu-Ce-Zr-IW showed the remarkably negative effect for the NH3 adsorption and activation. Moreover, less amounts of intermediates formed on Cu-Ce-Zr-HP and Cu-Ce-Zr-IW. That resulted in the catalytic performance of those two catalysts were inferior to Cu-Ce-Zr-SOL. However, the reaction pathway of all three catalysts was all following to iSCR route. The adsorbed NH3 was activated to form NHx and HNO intermediate species. With the increase of temperature, NHx species would be further oxidized to nitrate species, which could further react with NHx. It was also observed that the catalytic activity of surface oxygen species was higher than gaseous oxygen on Cu-Ce-Zr catalyst, and it had priority to react with NHx. The gaseous oxygen took part in the reaction and facilitated the formation of N2 as the temperature was relatively higher.