Selective Catalytic Reduction Of NO_x By Acetylene Over Mordenite-based Catalysts

The elimination of NO_x in presence of excess oxygen is a challenge for the people all over the world. Selective catalytic reduction of NO_x by hydrocarbons (HC-SCR) is a method of application prospect to remove nitric oxide from the exhaust gas of lean-burn engines. To improve the selectivity at low temperature in the selective catalytic reduction (SCR) of NO_x, the performance of the mordenite-based catalysts and the active reaction species were investigated for the SCR of NO_x by acetylene (C2H2-SCR). A possible mechanism was proposed to explain the catalytic performance of the mordenite-based catalysts in the C₂H₂-SCR. The main results obtained in our investigations are as follows:(1) NO conversion to N₂ over 0.1 g catalyst at 350℃in the C₂H₂-SCR was increased from 60 % to 70 % by 0.5 % molybdenum (accounted in MoO₃) incorporating into HMOR. Although the elimination of NO obtained in the C₂H₂-SCR is on the same level as those in C₃H₈- and C₃H₆-SCR reported in literature, it can be considered as a development due to the reductant in our case (800 ppm C₂H₂, 1600 ppm NO, 9.95 % O₂ in He) is only ca. 1/3 of that used in C₃H₈- and C₃H₆-SCR in carbon moles. In addition, the reaction temperature at which NO could be considerably removed by the reductant was 150 degrees lower compared to that of CH4-SCR reported in literature.(2) The promotional effect of molybdenum incorporated into HMOR on C₂H₂-SCR and the active reaction species were investigated by means of in situ Fourier transform infrared (FTIR) spectroscopy and NO_x-TPD techniques. It was suggested that bridging nitrate species formed on the HMOR, which was proved to be active nitric species towards reductant at higher temperature (300℃), can be considerably increased in population by molybdenum loading on the zeolite and consequently the activity of the catalyst for C₂H₂-SCR above 300℃was substantially increased. In the temperature range of 250-450℃, both nitrosonium ions (NO⁺) and bidentate nitrate species were fairly active towards acetylene to produce isocyanate species which could be easily hydrolysed to amide species. Based on a proportional relation between the activity of the catalysts for C₂H₂-SCR and the population of amide species thus formed on the catalysts in the temperature range of 250-450℃, we proposed that amide species is a crucial intermediate for the C₂H₂-SCR. (3) No effect of molybdenum on C₂H₂ adsorption could be observed by flow-adsorption techniques, therefor we proposed that the significant promotional effect of molybdenum on C₂H₂-SCR has no concern with the reductant adsorption and the reductant participates in C₂H₂-SCR reaction in gas phase. The proposal was proved by in situ Fourier transform infrared spectroscopy.