Methanation palladium zeolites

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

Okumura, K. and Niwa, M. (2002) Support effect of zeolite on the methane combustion activity of palladium. Catal. Surv. Japan, 5, 121-126. 

Nishizaka, Y. and Misono, M. (1994) Essential role of aridity in the catalytic reduction of nitrogen monoxide by methane in the presence of oxygen over palladium-loaded zeolites. Chem. Lett., 2237-2238. 

Ce-Y and Pr-Y zeolites were found to exhibit significant activity using propene as reductant and they show temperatures of maximum NO conversion within the range reported for the transition-metal-exchanged ZSM-5 and Y zeolites. In this way, Nishizaka and Misono (1993) have reported that, when methane was used as the reductant, the activity of two palladium loaded catalysts (i.e., Pd-H- and Pd-Ce-H-ZSM-5) was found to be comparable to the activity of transition-metal-exchanged ZSM-5. However, these catalysts were ineffective for the reduction of NO by propylene. On the other hand, the addition of alkaline-earth-metal ions (Mg, Ca, Sr, and Ba) enhanced the activity of the Ce-ZSM-5 catalysts, particularly at temperatures above 350°C (Yokoyama and Misono 1992, 1994a,b). 

Irrespective of the nature of the reaction intermediate, enolic type (11) or surface carbide (12), the dechne of the turnover number for the zeolites with higher Si/Al ratio can be explained as follows. For platinum (13) and palladium (14,15) loaded zeolites, support effects are known to exist. The higher the acidity (and the oxidizing power) of the zeolite, the higher will be the electron-deficient character of the supported metal. It also is well established now (16) that the average acidity of hydrogen zeohtes increases with the Si/Al ratio. This explains why the electron deficient character of ruthenium should increase with the Si/Al ratio of the zeolite, and a stronger interaction with adsorbed CO should be expected. Vannice (19,20) reported that the N value for CH4 formation decreases when the heat of adsorption for CO increases. All this explains why the tmnover number of the methanation reaction over ruthenium decreases when the Si/Al ratio of the zeolite support increases. 

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