Platinum-rhenium catalysts carbon monoxide

In considering the nature of platinum-rhenium catalysts, we begin with a comparison of the chemisorption properties of alumina-supported rhenium, platinum, and platinum-rhenium catalysts (40). Data on the chemisorption of carbon monoxide and hydrogen at room temperature are given in Table 4.4 for catalysts with platinum and/or rhenium contents in the range of interest for reforming applications. 

The data on the catalyst containing rhenium alone indicate signficant chemisorption of carbon monoxide, but no chemisorption of hydrogen. As expected, the platinum catalyst chemisorbs both carbon monoxide and hydrogen, and the values of CO/M and H/M are nearly equal. The platinum-rhenium catalyst exhibits a value of CO/M about twice as high as the value of H/M. This result approximates what one would expect if hydrogen chemisorbed on only the platinum component of the catalyst. While this chemisorption behavior is consistent with the possibility that the platinum and rhenium are present as two separate entities in the catalyst, they do not rule out the possibility that bimetallic clusters of platinum and rhenium are present. 

For catalysts that were simply dried in air at 110°C after impregnation of the alumina with H2PtClfe and Re207, it was concluded that a platinum-rhenium alloy formed on reduction. This conclusion was based on the observation that the presence of platinum accelerated the reduction of oxygen chemisorbed on the rhenium and on results showing that the frequencies of the infrared absorption bands of carbon monoxide adsorbed on platinum and rhenium sites in platinum-rhenium catalysts were different from those found with catalysts containing only platinum or rhenium. However, for catalysts calcined in air at 500°C prior to reduction in hydrogen, it was concluded that the platinum exhibited much less interaction with the rhenium (66,71). 

Studies by a group at the Shell Laboratories in Amsterdam (73) have been reported as evidence of interaction between platinum and rhenium in catalysts containing these two elements. These workers, on the basis of infrared spectroscopy studies of carbon monoxide chemisorbed on platinum-rhenium catalysts, and also of X-ray photoelectron spectroscopy measurements on the catalysts, concluded that platinum-rhenium bonds are present in the surface. 

In the infrared spectroscopy studies they observed that the stretching frequency of the chemisorbed carbon monoxide was higher on a Pt-Re/Si02 catalyst than on a Pt/Si02 catalyst. When they contacted a Re/Si02 catalyst with carbon monoxide, they observed only a weak band that rapidly disappeared and could not be restored on further exposure to carbon monoxide. They concluded that the carbon monoxide in this case dissociated, with resultant irreversible poisoning of the rhenium by carbon. Consequently, they attributed the infrared band observed for the platinum-rhenium catalyst to carbon monoxide chemisorbed on platinum atoms. It was concluded that the band was shifted in frequency from that observed for the platinum catalyst because of the interaction between platinum and rhenium. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

The data are expressed in terms of the quantities CO/M and H/M, which represent the number of adsorbed carbon monoxide molecules and hydrogen atoms, respectively, divided by the number of metal atoms (platinum and/or rhenium) in the catalyst. The values of CO/M represent the amounts of adsorbed carbon monoxide retained after the adsorption cell is evacuated at room temperature for 10 minutes to approximately 10 6 torr after completion... 

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