Study On Cr-Ce Catalysts For Low-temperature Selective Catalytic Reduction Of NO_x

Our coal-dominated energy structure will not change in the future for a long time, but burning coal produces large amounts of nitrogen oxides NOx, brought serious environmental problems, such as acid rain, photochemical smog and the greenhouse effect and so on. China’s total NOx is higher and its management is more difficult, and the emission control is no time to delay. The national NOx emissions reached 20 million 780 thousand tons in 2014. Among many NOx treatment methods, ammonia selective catalytic reduction is the most promising technology.At present, the industrialization V2O5-WO3(MoO3)/TiO2 catalyst exists many problems, for example, it has a narrow and high active temperature window (300～400℃) and high ash and high total sulfur will reduce its activity and service life. An attractive option is to locate the SCR unit downstream of the desulfurizer and electrostatic precipitators where the most SO2 and ash have been removed but still a little quantity SO2 exists. So, develop low temperature SCR catalysts to avoid reheating process and also possess the capability of SO2-tolerance is necessary to match the layout.In this paper, the low temperature SCR performance and sulfur tolerance of Cr-C_eO_x composite oxide catalysts were investigated, and the catalytic activity and sulfur tolerance of Cr-C_eO_x composite oxide catalysts doped with transition metal additives were also investigated. The physical properties and structure of the catalysts were characterized by BET, XRD, TPR, TPD, FT-IR, SEM, and so on.The low temperature SCR properties and sulfur tolerance of series Cr-C_eO_x composite oxide catalysts were studied. It was found that an Cr(0.7)-C_eO_x-400 catalyst prepared by citric acid method showed the highest activity, giving almost 100%NOx conversion at 125-250℃ with a gas hourly space velocity of 36000 h-1. Poisoning of Cr-C_eO_x composite oxide catalyst by SO2 is irreversible. The system characterization showed that the catalyst has higher surface area with the larger molar ratio of Cr/(Cr+Ce). After Cr was added to Ce, the crystal structure of Ce could be reduced obviously.After the catalyst is poisoned, the catalyst surface formed ammonium sulfate and the active composition was also vitriolization, which resulted in rapid catalyst deactivation.The proper amount of acid is one of the reasons of high activity catalyst.The effect of doping additives on the sulfur resistance of Cr-C_eO_x composite oxide catalyst was studied. The results showed that the catalyst with the doping agent Co had better resistance to sulfur. By the additive composition optimization, The results showed that Co-Cr-Ce ternary composite oxide catalyst has the best sulfur tolerance, when the molar ratio of Co/(Cr+Ce) is 7.5%, particularly, the catalyst give almost 80%NOx conversion at 175℃ with a gas hourly space velocity of 60,000 h-1 and 150 ppm SO2, furthermore, The catalyst give almost 82.5% NOx conversion at 175℃ with a gas hourly space velocity of 10,000 h-1 and 100 ppm SO2.The system characterization showed that the proper amount of Co doped was beneficial to increase the surface area of the catalyst, reduce the reduction temperature of Cr6+ in the three component catalyst, improve the weak adsorption amount of NH3 and NO, and reduce the adsorption of SO2.