| Popis: |
In an attempt to improve our knowledge of bimetallic catalysts usually used in reforming we studied three series of catalysts: Pt-Re, Pt-Ir and Ir-Os. For our Pt-Re catalysts, both metals are reduced to the metallic state and a catalytic effect of platinum on the reduction of rhenium oxide has been evidenced. The infrared spectra of carbon monoxide adsorbed on those catalysts have shown that platinum and rhenium interact together, they are probably alloyed. The catalytic activities of those Pt-Re, Pt-Ir and Ir-Os bimetallic catalysts were determined for various organic reactions. The effect of alloying is different for the various reactions, but the activity of the bimetallic catalysts for a given reaction is never the sum of the activities of the two metals separated. Some important synergistic effects were found. Surprisingly, a marked analogy between the curves activity for given reaction versus the composition of the catalysts has been found for the three series Pt-Re, Pt-Ir and Ir-Os. Some interpretations for those observations are discussed. The use of bimetallic catalysts has been a considerable improvement in catalytic reforming [1–8]. Pt-Re [1–4] and Pt-Ir [5–8] are among the most widely used in this area and the catalytic effects of adding metallic promoters to platinum are multiple. Those promoters act by i) changing the initial selectivity ii) decreasing the sintering rate iii) decreasing the coking rate during reaction of hydrocarbon and iv) maintaining the initial selectivity despite coke deposition and activity loss. However little is known on the reasons of those effects and many questions are still pending. For example, what is the degree of interaction between the two metals. Even the oxydation state of rhenium is still a matter of controversy. This is why we started a few years ago a study on bimetallic catalysts containing platinum, rhenium, iridium or osmium in collaboration with a group of research of the Institut de Recherches sur la Catalyse of Villeurbanne. The bimetallic catalysts have been characterized by their physico-chemical properties and their catalytic properties for various organic reactions. 1) The investigation of the reduction by hydrogen by means of i) differential thermal analysis ii) thermogravimetry and iii) volumetry 2) The study of the metallic phase after reduction by hydrogen-oxygen titration and IR study of CO chemisorbed 3) The study of the activity of the catalysts in the following reactions: -dehydrogenation of cyclohexane and 1,1,3-trimethyl-cyclohexane [9] -hydrogenation of benzene -epimerization of 1,2-dimethyl-cyclohexane [10] -hydrogenolysis of cyclopentane and butane [11,12] -benzene-deuterium exchange. |
