Shock-tube study of allene pyrolysis

Abstract

6291coverages; i.e., some metal ions protrude above adsorbed anions. The well-known fact that sulfides of many metals are active catalysts for a variety of reactions is generally ascribed to these protruding ions. In recent work with supported rhenium catalysts we found31 that sulfur "poisoning" induces a remarkable activity for hydrogenation and double-bond shift, illustrating the catalytic action of surface sulfides.The present results do not permit to discriminate between this geometric principle (corner atoms, protruding ions) and the "electronic" principle; Le., the positively charged Rh atoms (or ions) might have a higher intrinsic activity for hydroformylation than zerovalent Rh atoms. V. Conclusions1. Exposing a freshly reduced Rh/SiOz catalyst to a stream of highly diluted hydrogen sulfide in hydrogen, followed by reduction in hydrogen at 400 OC, results in a macroscopically uniform distribution of the sulfur atoms over the rhodium particles in the catalyst bed. The ability of the catalyst to strongly chemisorb carbon monoxide decreases linearly with the amount of sulfur dosed, up to a critical coverage with sulfur. Further dosing of sulfur does not affect C O chemisorption.2. Adsorbed sulfur selectively blocks the Rh surface for the bridging mode of chemisorbed CO.3. Adsorbed sulfur appears to leave adjacent Rh with a net positive charge; C O adsorbed on these atoms displays a highfrequency band in the I R region. 4. Low sulfur coverages, which reduce the capacity of chemisorbing C O by only a few percent, have a much stronger effect in reducing its ability to catalyze the hydrogenation of ethylene. This could indicate that the sites on the rhodium surface with highest hydrogenation activity also have the highest heat of adsorption for sulfur. 5 . The turnover frequency for hydroformylation is increased by sulfur at low coverages. This is tentatively rationalized by assuming that Rh atoms in corner positions are most active in hydroformylation, while the adsorption of sulfur is strongest on Freundlich sites, consisting of metal atom ensembles. It is also possible that surface reconstruction of rhodium covered with sulfur leads to additional protruding Rh ions.6. The promoting action of adsorbed sulfur is smaller than that of e.g. Zn ions reported previously. It is possible that in the latter case some chemical interaction between the metal ion and the oxygen end of coadsorbed CO enhances CO insertion into a metal-alkyl bond. Acknowledgment. A donation toward equipment by the Shell Co. Foundation and a research grant by the Monsanto Co. are gratefully acknowledged. Registry No. Rh, 7440-16-6; S, 7704-34-9; CO, 630-08-0; ethylene, 74-85-1.The high-temperature pyrolysis of allene was studied by analyzing reflected shock zone gas with time-of-flight (TOF) mass spectrometry. A 4.3% C3H4-Ne mixture yielded a carbon atom density of about 2.0 X lo1' atoms cm-3 over the temperature and pressure range of 1300-2000 K and 0.2-0.5 atm. Product and reactant profiles were obtained during observation times of 750 ps. T...

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