Palladium loaded zeolite

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. 

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. 

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 800°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface. 

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. 

On Figure 11 is shown the successive reactions of the synthesis of methylisobutylketone. The use of a palladium loaded cation exchange resin or zeolite enables to realize this synthesis in one step in a trickle-bed reactor (UT) ... 

Hydrocracking is catalyzed by substances that promote cracking and hydrogenation together. In commercial use are Ni, Co, Cr, W, and V or their oxides, presulfided before use, on acid supports. Zeolites loaded with palladium also have been used. 

Loaded palladium is usually found as Pd(II) ions in zeolites. However, oxygen treatment leaves few palladium ions in the unusual Pd(III) form while in vacuo calcination forms Pd(I) ions. 

Jiang, Y.-X., Si, D., Chen, S.-P., and Sun. S.-G. 2006. Self-assembly film of zeolite y nanocrystals loading palladium on an au electrode for electrochemical applications. Electroanalysis 18, 1173-1178. 

Preferential oxidation catalysts usually consist of precious metals such as platinum, ruthenium, palladium, rhodium, gold and alloys of platinum with tin, ruthenium [164] or rhodium. Typical carrier materials are alumina and zeolites [164], such as zeolite A, mordenite and zeolite X. Other possible carriers are cobalt oxide, ceria, tin oxide, zirconia, titania and iron oxide [214]. A high precious metal loading usually improves catalyst performance [164]. 

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