The promotion of cobalt mordenite by palladium for the lean CH 4-SCR of NO x in moist streams

Abstract

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 °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 °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+ in fresh Pd/Co-HMOR samples but, Pd exists as Pd 0 and Pd 2+ 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 Co 2+ cations at exchange positions.