Effect of water on the performance of Pd-ZSM-5 catalysts for the combustion of methane

Palladium-based catalysts were prepared using impregnation （I） and ion-exchange method （E） with ZSM-5 as support. Pd-ZSM-5（I） and Pd-ZSM-5（E） catalysts presented the high activity for the combustion of methane. The order of activity was consistent with Brcnsted acidity of the catalysts： Pd-ZSM-5（I）〉Pd-ZSM-5（E）. It was shown by FT-IR that methane was adsorbed on the acidic bridging hydroxyl groups of ZSM-5-supported Pd catalysts. The effect of water on the activity of Pd-ZSM-5 was investigated. The inhibition effect of water on the conversion of methane was observed. However, water promoted the stability of Pd-ZSM-5 obviously during extended time periods. XPS measurement showed that Pd/Si ratio near the surface of Pd-ZSM-5（E） decreased more pronouncedly with time in dry stream than that of Pd-ZSM-5（I）, this is attributed to the dispersion of Pd into the micropores. The addition of water, however, retarded Pd dispersion. And high partial pressure of methane reduced this effect of water vapor. The decrease in activity during the stability test can be explained on the basis of the reduction of Pd/Si ratio.

Catalytic Combustion of Methane over High Copper-Loading ZSM-5 Catalysts

Two series of Cu/ZSM-5 catalysts, loading from 5 to 20 wt% CuO, were prepared by the deposition-precipitation and impregnation methods, respectively. The catalysts prepared by the impreg- nation method showed better catalytic performances than those prepared by the deposition-precipitation method and the increase of copper loading favored methane conversion. 20Cu（I）/ZSM-5 had the highest activity with T90% of 746 K, and for 20Cu（D）/ZSM-5, T90% was as high as 804 K. The characterization of X-ray diffraction （XRD）, temperature-programmed reduction （TPR）, temperature-programmed desorption （TPD）, and X-ray photoelectron spectroscopy （XPS） revealed that the dispersion of copper species could be improved by using the deposition-precipitation method instead of the impregnation method, but the fraction of surface CuO, corresponding to active sites for methane oxidation, was larger on 20Cu（I）/ZSM-5 than 20Cu（D）/ZSM-5. The results of Pyridine-Fourier transform infrared spectrum （Py-FT-IR） showed that a majority of Lewis acidity and a minority of Brфnsted acidity were present on Cu/ZSM-5 catalysts. 20Cu（I）/ZSM-5 presented more Lewis acid sites. The number of Lewis acid sites changed significantly with preadsorption of oxygen. Adsorption of methane and oxygen on acid sites was observed. The properties of Cu/ZSM-5 catalysts were correlated with the activity for methane oxidation.