The promotion of cobalt mordenite by palladium for the lean CH_4-SCR of NO_x in moist streams

摘要

The promotional effect of palladium on the catalytic performance of cobalt ion exchanged mordenite (Co-HMOR) for the lean NO_x reduction with methane in a wet reaction stream was evaluated. The addition of 0.1-0.15 wt.% Pd to Co-HMOR samples containing high Co~(2+) levels lead to a material that can convert 60% NO to N_2 and 60% CH_4 to CO_2 at 500 deg C in a reaction mixture containing 1000 ppm NO, 2700 ppm CH_4, 6% o_2, and 8% H_2O, at GHSV=30 000 h~(-1). NO reduction over Pd/Co-HMOR increases with Co loading up to about 90% cobalt exchange. The increase of Pd loading to Co-HMOR favors methane combustion instead of NO reduction. Experiments of NO oxidation to NO_2 in the presence of 2% water vapor show that the addition of Pd to Co under-exchanged samples produces much less NO_2 than was found under the same conditions on cobalt over-exchanged samples loaded with small amounts of Pd. On the other hand, almost total NO reduction to nitrogen was obtained between 500 and 550 deg C in the oxygen free feed (CH_4+NO+H_2O). The nature of the active species was investigated by XPS and TPR. Results from XPS reveal that cobalt is uniformly distributed as Co~(2+) cations at exchange positions.