Effect of Cu loading and Si/Al ratio on the selective catalytic reduction of NO with C3H6 catalyzed over Cu/ZSM-5

Abstract

As increased the Cu content of CuZSM-5 having Si/Al of 11.75, the activity in the low temperature region (leq 350,^circ!C) for NO SCR by C3H6 and the maximum conversion of NO increased. In the case of CuZSM-5 having Si/Al of 22, the conversion of NO was increased with increase in Cu content in the low temperature region (leq 400,^circ!C). However, the maximum conversion of NO with excess-exchanged CuZSM-5 decreased. In comparison with two types of CuZSM-5 which has the same amounts of Cu but different Si/Al ratio, the temperature of maximum NO conversion with CuZSM-5 having Si/Al of 11.75 was lower by 50,^circ!C than that of 22. These results indicated that all the exchanged Cu ions do not have the same NO SCR activity and that the catalytic activity was affected by the degree of ion-exchange and Si/Al ratio. From the result of TPO experiment which was performed with CuZSM-5 after the reaction at 350,^circ!C for 30min, the formation of CO2 decreased with increase in Cu content. The maximum peak of CO2 formation during TPO was located at the temperature higher than the maximum conversion temperature. The formation of coke species, which are formed by C3H6 during SCR reaction, played an important role on deactivation process and was affected by Cu loading and Si/Al ratio. The temperature at which NO conversion reached the maximum was the same as that of C3H6 conversion to CO2. FT-IR results acquired after NH3 was adsorbed on CuZSM-5, the amount of NH3 species adsorbed was independent of Cu loading and larger for Si/Al of 11.75 than for that of 22. From the result of NO TPD over CuZSM-5 with the same amount of Cu, the adsorption capability for NO was higher for the catalyst having Si/Al of 11.75. This result supports that all the copper ions in CuZSM-5 do not show the same NO adsorption capability. To study the reducibility of Cu ions in CuZSM-5 having the same Si/Al ratio, the IR experiment was performed after CO adsorption following the treatment of NO/O2 at 350,^circ!C. The intensity of Cu^+-CO band increased with the amount of Cu ions. However, this intensity decreased in the case of over-exchanged CuZSM-5. The intensity of CO band for Si/Al of 11.75 was stronger than that of 22. During SCR reaction at 350,^circ!C, the mobility of copper ion was observed from the recessed sites to the main channel intersection depending on the C3H6/NO ratio. In the case of CuZSM-5 with Si/Al of 11.75, the band intensity of 2157cm^{-1} and 2137cm^{-1} increased with increasing Cu loading. However the catalyst with Si/Al of 22 did not show the band of 2157cm^{-1} when C3H6/NO is larger than 0.6.