Selective Reduction Of NO_x With C₃H₆ Over Supported Au Catalysts

Supported gold catalysts have been the hot spot in heterogeneous catalysis due to their exceptionally high activities at low temperatures.Such a unique feature of gold gives it the potential to solve the cold-start emission problem of engines.Therefore,it is desirable to examine the catalytic activity of supported gold catalysts for NO_x reduction.In this work,the selective catalytic reduction(SCR) of NO_x by C₃H₆ in the excess of oxygen was investigated over Au/MO_x/Al₂O₃(M:Ce,Fe) catalysts.The correlation between the structure of catalysts and their catalytic behaviors was studied.The main results presented in the dissertation have been summarized as follows:1.Au/CeO₂/Al₂O₃ catalysts were prepared by deposition-precipitation method and exhibited good low-temperatures activity in C₃H₆-SCR of NO in the excess of oxygen.The conversion of NO to N₂ was 46%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂ in He at 250℃and GHSV of 30,000 h^-1.Such a catalytic performance is superior to that of Pt/Al₂O₃,on which a significant amount of N₂O (about 30%) was formed under the same conditions.It was also found that adding 2%water vapour to the feed stream enhanced the NO conversion while the presence of 20 ppm SO₂ decreased NO conversion at low temperatures.It was interesting that in the simultaneous presence of 2%water vapour and 20 ppm SO₂,the conversion of NO to N₂ was increased and the temperature window was widened.The conversion of NO to N₂ reached 50%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂,2%H₂O, 20 ppm SO₂ in He at 300℃and GHSV of 30,000 h^-1.2.The Au/CeO₂/Al₂O₃ catalysts were characterized by different methods:(a) TPD results demonstrated that surface NO_y(y≥2) species were formed from co-adsorption of NO and O₂. The presence of SO₂ inhibited the formation of surface NO_y(y≥2) species,which results in the loss of catalytic activity at low temperatures.The introduction of water vapour and SO₂ created more acid sites and suppressed the combustion of C₃H₆.Therefore,the conversion of NO to N₂ was increased and the temperature window was widened.(b) The results of XRD, HRTEM,H₂-TPR and XPS revealed that gold existed in the form of metallic gold by calcination at 400℃and the size of gold nanoparticles was in the range of 3-5 nm.CeO₂ can not only provide the active oxygen for the reaction,but also stabilize the gold particles against sintering.There was strong interaction between Au and CeO₂,which facilitated the reduction of CeO₂.The synergistic effect between Au and CeO₂ was probably responsible for the superior catalytic performance of the Au/CeO₂/Al₂O₃.(c) In situ reflectance Fourier infrared transform spectroscopy(DRIFTS) results showed that NO_y(y≥2) species were observed on the surface of Au/CeO₂/Al₂O₃ after exposure to the gas mixture of NO and O₂.Formate and acetate species were presented after exposure to the gas mixture of C₃H₆ and O₂.Cyanide (-CN) and isocyanate(-NCO) species were observed as important intermediates in C₃H₆-SCR reaction.A reaction mechanism of the C₃H₆-SCR of NO over Au/CeO₂/Al₂O₃ catalyst has been proposed:NO_y and C_xH_yO_z species are formed due to the oxidation of NO and partial oxidation of C₃H₆ in the presence of O₂,and then C_xH_yO_z species react with NO_y species to generate C_aH_bO_cN_d intermediates including -CN and -NCO species,and then the intermediates react with NO₂ or(NO+O₂) to produce N₂,H₂O and CO₂/CO.3.Au/Fe₂O₃/Al₂O₃ catalysts were prepared by the homogeneous deposition-precipitation method and exhibited good low-temperature activity in C₃H₆-SCR of NO in the excess of oxygen.The conversion of NO to N₂ was 43%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂ in He at 300℃and GHSV of 30,000 h^-1. The addition of 2%water vapor to the feed stream enhanced slightly the NO conversion, while the presence of 20 ppm SO₂ decreased NO conversion at low temperatures.The catalysts were characterized by XRD,TEM,H₂-TPR and XPS techniques.The results indicated that that gold existed in the form of metallic gold by calcination at 400℃and the size of gold nanoparticles was in the range of 3-5 nm.There was strong interaction between Au and Fe₂O₃,which facilitated the reduction of Fe₂O₃.The synergistic effect between Au and Fe₂O₃ was probably responsible for the good catalytic performance of the Au/Fe₂O₃/Al₂O₃ catalyst at low temperatures.