The promotion of cobalt mordenite by palladium for the lean CH4-SCR of NOx in moist streams

The promotional effect of palladium on the catalytic performance of cobalt ion exchanged mordenite (Co-HMOR) for the lean NOx 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 Co2+ levels lead to a material that can convert 60% NO to N-2 and 60% CH4 to CO2 at 500 degreesC in a reaction mixture containing 1000 ppm NO2 2700 ppm CH4, 6% O-2, and 8% H2O, 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 NO2 in the presence of 2% water vapor show that the addition of Pd to Co under-exchanged samples produces much less NO2 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 550degreesC in the oxygen free feed (CH4 + NO + H2O). The nature of the active species was investigated by XPS and TPR. Results from XPS reveal that cobalt is uniformly distributed as Co2+ in fresh Pd/Co-HMOR samples but, Pd exists as Pd-0 and Pd2+ on the surface. The addition of Pd to Co-HMOR shifts the maxima in the temperature-programmed reduction profiles to lower temperatures and increases the reducibility of Co2+ cations at exchange positions. (C) 2002 Elsevier Science B.V. All rights reserved.