Catalytic Decomposition Of N₂O In Nitric Acid Industrial Off-gas

N2O is a serious greenhouse gas.The emission of N2O from nitric acid industry exhaust is one of the main sources of industrial exhaust emissions.According to a low content of N20 in tail gas of nitric acid industry,and the presence of impurities such as NO,H2O,and O2,a series of rare earth metal La and alkaline earth metal Sr,modified Co3O4 catalysts were prepared by co-precipitation method,and the performance of the catalytic decomposition of N2O was evaluated.The prepared catalysts were characterized and analyzed,and their structure-activity relationships were discussed.Two different preparation methods were studied respectively:sol-gel method and co-precipitation method.The catalytic activity of the catalyst prepared by co-precipitation method was obviously higher than that of sol-gel method.The preparation process of the co-precipitation method was done using Na2CO3 as precipitant,550? as calcination temperature,and the calcining time was 4 h.The addition of rare earth La significantly enhanced the catalytic activity of Co3O4.The reaction conditions were 25000 h-1 for space velocity and 2000 ppm of N2O+N2 as feeding gas.When the molar ratio of La/Co was 0.1,the conversion rate of catalytic decomposition of N20 at 350? reached more than 95%.The addition of La inhibited the growth of the crystal grains of Co3O4 and significantly increased the specific surface area of the catalyst.It was also noted a decrease of the bond energy of Co-O and an increase of the conversion of Co3+ to the active site Co2+ which enhanced the redox ability of the catalyst.By adding the impurity gases of NO,H20,and O2 to the feeding gas,N20 can be completely catalytically decomposed at 450?.The addition of alkaline-earth metal Sr also improved the catalytic activity of C03O4.The addition of alkaline earth metal Sr improved the catalytic activity of Co3O4 of which the optimum molar ratio of Sr/Co is 0.3.Sr0.3Co can completely catalyze the decomposition of 2000 ppm N20+N2 for a reaction temperature of 400?.In the presence of impurity gas such as NO,H20,and O2,the complete catalytic decomposition temperature of NiO was 500 ?.The results of characterization showed that the mechanism of Sr addition is similar to that of La,which ended the growth of Co304 crystals.The decrease of the size of Co3O4 crystals leads to an increase of the specific surface area and the redox capacity of the catalyst.Hence,the number of active sites Co2+ increased to a certain amount.In addition,the catalyst showed good thermal stability.The catalyst was prepared in bigger powder and was molded by using it as a raw material.The results showed that the molding method had a significant limpact on the catalytic activity of the catalyst,and the extruded catalyst exhibited significantly better catalytic activity than the catalyst prepared by the coating method.When the reaction temperature was 425?,the conversion rate of 2000 ppm N20 that catalyzes the decomposition of 10000 h-1 air velocity was over 99%.The long cycle operation is performed at 450? for 72 hours.The conversion of the catalyst was always maintained at 100%.