Selective Catalytic Reduction Of NO Over Cu-Supported Ti-Based Mixed Oxides And Reaction Mechanism Studies

The traditional three-way catalysts can only convert NOx to N2efficiently under stoichiometric conditions with the air/fuel ratio of14.6approximately. Wheares, the diesel engines typically operate under lean-burnt conditions, and the presence of O2inhibits the deNOx efficiency of three-way catalysts significantly. Recently, the selective catalytic reduction of NOx by hydrocarbons (HC-SCR) is considered as a potential technology for the control of NOx pollution from diesel vehicles, and the exploitation of catalysts with high efficiency used for HC-SCR reacation has attracted a lot of attention. Though the metal oxide catalysts possess the excellent hydrothermal stability and the much lower cost compared with noble metal catalysts, the current efficiency of them is not satisfactory for commercial application. Herein, the novel Cu/Ti1-xCexO2-δ and Cu/Ti1-xZr2-δ mixed oxides were fabricated by impregnation-urea homogeneous coprecipitation method in the present study, and the catalytic activity of them in C3H6-SCR reaction was tested. A series of characterizations were carried out to investigate the changes of the structure and physical-chemical properties of catalysts caused by the Ce or Zr-doped, and the relationship between these changsed and the catalytic activity was discussed. Moreover, in situ FTIR experiments were used to clarify the behaviors of reaction intermediates and the reaction mechanisms over the catalysts.(1) The Cu/Ti1-xCexO2-δ (the molar ratio of Ce/(Ti+Ce) was denoted as x) mixed oxide catalysts with large surface areas (200-275m2/g) were synthesized by impregnation-urea homogeneous coprecipitation method. Compared with Cu/TiO2and Cu/CeO2catalysts, the partial substitution of Ti by Ce and the generation of Ti-O-Ce mixed bond induced more surface defects, and consequently enhanced the generation of surface Lewis acid sites and increased the ratio of Cu2+and adsorbed oxygen species on the catalyst surface. When the x value ratio was0.1-0.2, the NOx conversions of Cu/Ti1-xCexO2-δ mixed oxide catalysts were higher than that of Cu/TiO2catalyst. However, when the x value was higher than0.3, the yields of NO2were enhanced significantly and therefore declined the NOx conversions of catalysts. The catalyst with x value of0.1exhibited the best performance, which possessed the highest capacity towards NO and C3H6activation, and also endowed the nitrates and oxygenates the highest reactivity. More importantly, the reaction between the nitrates and oxygenates led to the generation of-NCO species, which were the key intermediates for the production of N2. The enhancement of the formation of-NCO species was the crucial reason for the high efficiency of the Cu/Ti0.9Ce0.1O2-δ catalyst.(2) The Cu/Ti1-xZrxO2-δ (the molar ratio of Zr/(Ti+Zr) was denoted as x,) mixed oxide catalysts were synthesized by impregnation-urea homogeneous coprecipitation method. The substitution of Ce by Zr led to the further enhancement of C3H6-SCR activity, and Cu/Ti0.7Zr0.3O2-δ catalyst exhibited the best deNOx efficiency. Moerover, the presenct of Zr inhibited the production of NO2significantly. The generation of Ti-O-Zr mixed bond led to the distortion of crystal structure and the imbalance of electron charge density. The generation of more defects provided more adsorption sites and reactive sites on the catalyst surface, and promoted the capacity of catalyst towards the NO adsorption and activation, which favored the improvement of C3H6-SCR performance.(3) In situ FTIR were used to investigate the C3H6-SCR reaction mechanism of Cu/Ti0.7Zr0.3O2-δ. Compared with Cu/TiO2and Ti0.7Zr0.3O2-δ catalysts, the capacity of Cu/Ti0.7Zr0.3O2-δ towards NO and C3H6activation as well as the generation of-NCO species was enhanced significantly. Moreover, the coexistence of the Cu, Ti and Zr elements led to the formation of a new intermediate of-CN species. The activation of C3H6to formate and acetate was the rate-determining step for the generation of-NCO and-CN species. The generation of N2over Cu/Ti0.7Zr0.3O2-δ catalyst was considered as the paralleled pathways:1) by the hydrolysis reaction of-NCO species, which produced NH3and further reacted with nitrates and/or NOx to generate N2;2) by the reaction between the-CN species and nitrates and/or NO2.(4) The SO2-poisoning on Cu/Ti0.7Zr0.3O2-δ catalyst was tested. The presence of SO2led to the decline of low-temperature activity of catalyst (150-250℃), but slightly increased the NO conversion at high temperatures (above275℃). The decrease of low-temperature activity of catalyst was caused by the competitive adsorption between NO and SO2and the suppression of C3H6activation caused by the catalyst sulfation. However, this inhibition effect was weakened by increasing the reaction temperature. When the temperature was above275℃, C3H6were activated to acetate efficiently and took participate in the generation of-NCO species. Moreover, the-CN species acted as the precursor and reacted with SO2and sulfates to produce-NCO species, which promoted the concentration of-NCO species on the catalyst surface and was considered as the crucial reason for the excellent SO2-resistance of catalyst.