Palladium catalysts dispersion

J. P. Boitiaux, J. Cosyns, and S. Vasudevan, Hydrogenation of highly unsaturated hydrocarbons over highly dispersed palladium catalyst Part I. Behaviour of small metal particles, Appl. Catal. 6,41-51 (1983). 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Carbon supported powdered palladium catalysts have been widely used in the chemical industry. In addition to activity and selectivity of those catalysts, the recovery rate of the incorporated precious metal has a major impact on the economic performance of the catalyst. In this study, the effects of catalyst age, oxidation state of the incorporated metal and temperature treatment on the palladium leaching resistance as well as on activity and dispersion of carbon supported palladium catalysts were investigated. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Several supports were studied, including so-called poly-alumazane, which is prepared by subsequent treatment of silanol rich silica with aluminum trichloride and ammonia. With the resulting support palladium catalysts with very high dispersion were obtained. 

Figure 5 presents the formation of butane obtained on dispersed palladium catalysts from the adsorption of several sulfur compounds thiophene, thiophane, dibutylsulfide, butanethiol, dibutyldisulfide. Twogroups can be observed (1) thiols and disulfides that undergo a rather low-level dissociation in butane and (2) thiophene, thiophane, and dibutylsulfide, which are dissociated to a much larger extent. The dissociation of dibutylsulfide produces a quantity of butane twice that produced from thiophene and thiophane, indicating that the two bonds (sulfur — butyl radical) are broken altogether. 

This approach has even been used to prepare highly dispersed palladium catalysts supported on low surface area aluminas. 39 EXAFS analyses of the various stages in the interaction of Pd(acac)2 with alumina has shown that the initial reaction involves an octahedral aluminum vacant site. On heating the resulting ionic species was converted to a supported crystalline oxide which was then reduced to the Pd/Al203 catalyst. " ... 

The palladium catalyst supported on aluminosilicate is prepared by exchanging the surface protons of aluminosilicate with palladium-ammine complex cations, followed by washing with water, thermal decomposition, and reduction with hydrogen. This reduction easily transforms the exchanged palladiumammine complex cations into metallic palladium particles which are fine spheres and homogeneously dispersed through a cloud of the fine particles of aluminosilicate. 

However, earlier findings [247] showed that very highly dispersed palladium catalysts had very low activity for VA formation. This was attributed to the very small particles being inaccessible to the ethene feed due to their being completely embedded within the acetic acid/acetate liquid layer (ethene has very low solubility in acetic acid). Vinyl acetate formation may therefore be restricted to the larger Pd-Au alloy particles accessible to gaseous ethene. This... 

Indeed, the aged 1-500 and SGI-500 catalysts kept, respectively, 60 % and 69 % of their initial metallic dispersion values, whereas 86 % of the initial metallic dispersion was kept on the aged SG-500 catalyst (Table 1). In order to investigate the metal-support interaction suspected to be responsible of this improved thermal stability, XPS measurements were performed on the supported palladium catalysts. 

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