ArticleReforming reactions onPd/Al2O3 active sites in variously pretreated catalysts
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AbstractChlorine-freePd/γ-Al2O3 catalysts were studied in conversions ofn-hexane, 2,2-dimethylbutane and methylcyclopentane in excess hydrogen. All these reactions appeared as useful probes for diagnosing changes inPd/Al2O3 catalysts caused by their different pretreatments. The effect of high temperature reduction on catalytic properties ofPd/Al2O3 is twofold: it generates Lewis acidity in γ-Al2O3 and, at the same time, induces Pd-alumina interactions leading to the formation of PdAl species very active in isomerization. The use of two, similarly dispersed,Pd/Al2O3 catalysts of different metal loading allows to distinguish contributions of two types of active centers mentioned above. The reaction of 2,2-dimethylbutane conversion appeared the most convenient catalytic probe because it did not virtually suffer with time-on-stream, and, simultaneously, displayed largest variations with the catalyst pretreatment. Among various reaction pathways, alkane isomerization experiences most pronounced changes and, in effect, becomes the most sensitive reaction route in diagnosing transformations in supported palladium catalysts. Methylcyclopentane conversion at 260°C and below proceeds primarily on palladium surface. Acidic sites in γ-Al2O3 and PdAl species contribute only slightly to the overall activity, however, these centers generate a noticeable ring enlargement selectivity.

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