Preparation Of Cu-Mn Modified Molecular Sieve Catalyst And Basic Research Of The SCR Catalyst To Remove NO_x From Diesel Engine Exhaust

Nitrogen oxides (NOx) from diesel engine exhaust, which is the major cause of dust-haze, acid rain and photochemical smog, have become one of the main sources of atmosphere pollution in our country. Selective catalytic reduction (SCR) of NOx is considered to be one of the most efficient technologies to eliminate NOX from diesel exhaust. But diesel engine exhaust contains high concentration of oxygen, water vapor and hydrocarbons, which make the three-way catalyst used for petroleum vehicles cannot effectively work. Zeolites receive more and more attention for their unique structure, good adsorption and high stability. So NH3-SCR catalysts based on zeolites are applied widely in researches. This paper exchanges Cu and Mn to ZSM-5 and SAPO-34 as SCR catalysts to remove NOX from diesel engine exhaust, studying the SCR activity and stability of the catalysts in the reaction.Firstly, synthesis parameters of the Cu-Mn/ZSM-5 are investigated. Results show that the Cu-Mn/ZSM-5 with Cu(CH3COO)2 and Mn(CH3COO)2 as metal precursor, ion-exchange as preparation method, Cu:Mn=3:2 shows the best NH3-SCR activity in removing NOx. Cu-Mn co-doping can significantly enhance deNOx activity of Cu/ZSM-5 at low temperature and keep high active stability at middle-high temperature. But Cu-Mn/ZSM-5 shows low hydrothermal stability, the NOx conversion after hydrothermal treatment at 750 ℃ for 24 h significantly decreases in the whole temperature region. It is observed that Cu-Mn/ZSM-5 obviously aggregate after hydrothermal treatment, no dealumination and structural damage take place, and the concentration of Cu2+ in octahedral coordination is reduced. The injection of 2000 ppm C3H6 obviously inhibits the catalytic activity of Cu-Mn/ZSM-5 in the whole temperature region, which indicates that Cu-Mn/ZSM-5 have low C3H6 tolerance. The Cu-Mn/ZSM-5 catalyst also aggregates after NH3-SCR with 2000 ppm C3H3, and the BET and cavity volume are also significantly decreased. The ration of Cu+/Cu2+ is increased after SCR which means more Cu ion is converted to copper species with lower oxidation site and the concentration of Cu2+ is decreased. The reduction of Cu2+ in the surface of Cu-Mn/ZSM-5 is the main reason of activity decline after hydrothermal treatment and NH3-SCR with 2000 ppm C3H6.Since Cu-Mn/ZSM-5 shows low hydrothermal stability and C3H6 tolerance, this paper chooses Cu-Mn co-doping SAPO-34 catalyst as the carrier which contains CHA framework structure. The result shows that the Cu-Mn/SAPO-34 with Cu(CH3COO)2 and Mn(CH3COO)2 as metal precursor, ion-exchange as preparation method, Cu/Mn=3:2 shows the best NH3-SCR activity. Cu-Mn co-doping can significantly improve the activity of Cu/SAPO-34 in the low temperature, revealing nearly 90% NOx conversion at 200℃, about 60% higher than Cu/SAPO-34. Cu-Mn/SAPO-34 also keeps high activity and stability at middle-high temperature. The doping of Cu and Mn maintains the morphology and crystal structure of SAPO-34, improves the Cu content and Cux+ amount (Cu+ and Cu2+), and thus improves the Cu/SAPO-34 catalytic activity at low temperature. Cu-Mn co-doping also improve the surface acidity of catalyst, so that the Cu-Mn/SAPO-34 can adopt of more NH3 and NO to generate reaction intermediates. Cu-Mn/SAPO-34 also shows high tolerance to the variation of temperature and hydrothermal stability. Hydrothermal treatment causes no obvious decline in deNOx activity of Cu-Mn/SAPO-34, which even increases after hydrothermal treatment in the temperature below 240 ℃. Cu-Mn/SAPO-34 keeps good crystallinity and morphology after hydrothermal treatment, with no aggregation and dealuminzation occurring. The increasing octahedral coordination of Cu2+ content is an important factor of catalyst activity increasing at low temperature after hydrothermal treatment. Cu-Mn/SAPO-34 also shows high hydrocarbon resistance, as the catalyst exhibits good dispersion and structure stability after SCR reaction. The BET of catalyst declines after reaction with no reduction in the pore volume, which means the CHA microporous structure can effectively hinder C3H6 migrate into the internal structure and protect the active sties, making Cu-Mn/SAPO-34 well tolerant to HC.