METAL-METAL INTERACTION (MMI) ON SUPPORTED IRIDIUM-PLATINUM, RHODIUM-PLATINUM BIMETALLIC CATALYSTS (HETEROGENOLYSIS, REFORMING, ALLOY).

摘要

The interaction between metals in bimetallic catalysts, Ir-Au, Ir-Pt, and Rh-Pt, has been investigated. A strong metal-metal interaction (SMMI) takes place on Ir-Pt and Rh-Pt bimetallic catalysts and a weak metal-metal interaction occurs on Ir-Au catalysts. The main consequence of strong metal-metal interaction is a strong suppression of catalytic activity and a synergistic promotion of the chemisorption capacity of hydrogen. The catalytic consequences of the bimetallic interaction are interpretated as a combination of a geometric (ensemble) effect and of a localized electronic (ligand) effect.;Critical reaction sites (CRS) for a C(,2)-unit mode of reaction consists of at least two surface atoms. Reactions of ethane and n-butane through C(,2)-unit mode are characterized as demanding reactions which are a sensitive probe of surface structure. The activity depression as a function of composition variation for these reactions ranges from 1,300 to 35,700 times (ratio of active and inactive pure metal rates) on strong metal-metal interaction catalysts (Ir-Pt, Rh-Pt) where Pt is the inactive component. The CRS for the iso-unit mode of reaction requires at least one surface atom. Reaction of isobutane and neopentane on this kind of CRS is less demanding and exhibits an activity depression ranging from 19 to 60 times. For hydrogenolysis reaction, the order of activity is Rh > Ir Pt.;The surface structure of Ir-Au catalysts is such that iridium is highly dispersed on the support and is surrounded (or covered) by patches (or particles) of gold. The interaction is limited to the perimeter of the patches.;The support effect of Davison silica, Cab.O.Sil, alumina, and titania on Rh and Pt interaction is large, a factor of two orders of magnitude for ethane hydrogenolysis. The author believes that support plays an important role in catalytic activity also. (Abstract shortened with permission of author.).;The catalytic influence of this interaction was studied on two Ir-Au series, two Ir-Pt series, and four Rh-Pt series of catalysts. The Ir-Au and Ir-Pt highly dispersed catalysts were obtained by impregnating appropriate amounts of solution on a high surface area silica support. Titania, Cab.O.Sil, and alumina supported Rh-Pt catalysts were also prepared in order to study the effect of support. All catalysts were characterized by hydrogen and carbon monoxide chemisorption. In addition, several hydrocarbon reactions were studied, including ethane hydrogenolysis and reactions of n-butane, isobutane, and neopentane. The product distribution has been analyzed and a method for calculating the amount of secondary reactions of isobutane and neopentane has been developed. The metal dispersion on the silica supports are very high, ranging from 40% to 100%. The crystallite (bimetallic clusters) size is in the range of 0.5 nm to 3.0 nm.