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...

The thermal decomposition of 1,2 butadiene has been studied behind reflected shock waves over the temperature and total pressure ranges of 1300-2000 K and 0.20-0.55 atm using mixtures of 3% and 4.3% 1,2 butadiene in Ne. The major products of the pyrolysis are C2H,, C4Hz, C2H4, CH, and CsH6. Toluene was observed as a minor product in a narrow temperature range of 1500-1700 K. In order to model successfully the product profiles which were obtained by time-of-flight mass spectrometry, it was necessary to include the isomerization reaction of 1,2 to 1,3 butadiene. A reaction mechanism consisting of 74 reaction steps and 28 species was formulated to model the time and temperature dependence of major products obtained during the course of decomposition. The importance of C3H3 in the formation of benzene is demonstrated.C3H3 similar to that in the allene pyrolysis experiments. The C -CH3 bond in 1,2 butadiene is the weakest bond [41. Almost equal amounts of benzene were detected over comparable temperature ranges. However, the respective benzene profiles displayed different shapes. The purpose of this study is to understand the differences in the rates of benzene formation from 1,2 butadiene and allene pyrolyses and to provide additional evidence for the role of C3H3. A previous study of Collin and Lossing [51 concentrated on the ionization and dissociation of 1,2 butadiene and the subsequent determination of the C3H3 heat of formation. The emphasis herein is on the identity of the reaction products, their temporal behavior, and the mechanism attendent to the thermal decomposition.

The high temperature pyrolysis of 1,3-butadiene has been investigated in the shock tube with two time-resolved diagnostic techniques: laser schlieren measurements of density gradient with 1, 2 , 4, and 5% C4Hs in Ar or Kr, 0.26 < Pa < 0.66 atm, over 1550-2200 K, and time-of-flight mass spectra for 3% C4H6-Ne, P, -0.4 atm, 1400-2000 K. When combined with a recent single-pulse shock tube product analysis covering 1050-2050 K, these measurements permit a complete modeling of major species in C4HI pyrolysis. Extrapolated density gradients and product analyses show initiation is dominated by C4H6 -2C2H,., significant falloff and Arrhenius curvature being seen in the derived rates. A restricted rotor. Gorin model RRKM fit to these rates with reasonable parameters generates

The thermal decomposition of acetylene has been studied in the temperature and pressure regimes of 1900-2500 K and 0.3-0.55 atm using a shock tube coupled to a time-of-flight mass spectrometer. A series of mixtures varying from 1.0-6.2% CzHz diluted in a Ne-Ar mixture yielded a carbon atom density range of 0.24-2.0 x 1017 atoms cm-3 in the reflected shock zone. Concentration profiles for CzH2, C4H2, and CsHz were constructed during typical observation times of 750 ps. CsH2 and trace amounts of C4H3 were found in relatively low concentrations at the high-temperature end of this study. A mechanism for acetylene pyrolysis is proposed, which successfully models this work and the results obtained by several other groups employing a variety of analytical techniques. Two values of the heat of formation for C,H(134 ? 2 and 127 t 1 kcal/mol) were employed in the modeling process; superior fits to the data were attained using the latter value. The initial step of acetylene decomposition involves competition between two channels. In mixtures (<200 ppm) where the acetylene concentrations are less than 2.18 x lo-' mol ~m -~, the decay is predominantly first order with respect to CZH,; in mixtures >200 ppm, the dominant initial step is second order. The rate constant for the second-order reaction is described by the equationBenzene concentrations predicted by the model are below the TOF detectability limit.C4H3 was observed in the 6.2% C,H2 mixture in accordance with the proposed mechanism.

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