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 thermal decompositions of pyrazine, pyrimidine, and pyridine in shock waves have been investigated using the complementary techniques of laser-schlieren (LS) densitometry and time-of-flight (TOF) mass spectrometry (1600−2300 K, 150−350 Torr). A free radical chain reaction with initiation by ring C−H fission in the pyrolyses of all three azines is proposed. The measured C−H fission rates are compared and analyzed by RRKM theory. Barriers of 103 ± 2 kcal/mol for pyrazine, 98 ± 2 for pyrimidine, and 105 ± 2 for pyridine have been determined, supporting values lower than the barrier for C−H fission in benzene, 112 kcal/mol. The lower barriers for the azines are explained by the additional contributions of resonance structures of azyl radicals due to neighboring N−C interactions, which serve to further stabilize the azyl radicals. Detailed chain mechanisms are constructed to model the LS profiles and the TOF concentration profiles of the major products, hydrogen cyanide, acetylene, cyanoacetylene, and diacetylene. Of particular interest are the TOF observations and the mechanistic explanation of temperature dependent maxima seen in the formation of cyanoacetylene in the presence or absence of excess H2.

The unimolecular dissociation of the C3H4 isomers allene and propyne has been examined using two complementary shock-tube techniques:  laser schlieren (LS) and time-of-flight (TOF) mass spectrometry. The LS experiments cover 1800−2500 K and 70−650 Torr, in 1, 2, and 4% propyne/Kr and 1 and 2% allene/Kr, whereas the TOF results extend from 1770 and 2081 K in 3% allene or propyne in Ne. The possible channels for unimolecular dissociation in the C3H4 system of isomers are considered in detail, using new density functional theory calculations of the barriers for insertion of several C3H2 into H2 to evaluate the possibility of H2 elimination as a dissociation route. The dominant path clearly remains CH fission, from either isomer, as suggested in earlier work, although some small amount of H2 elimination may be possible from allene. Rate constants for the CH fission of both allene and propyne were obtained by the usual model-assisted extrapolation of LS profiles to zero time using an extensive mechanism constructed to be consistent with both the time variation of LS gradients and the TOF product profiles. This procedure then provides rate constants effectively independent of both the near-thermoneutral isomerization of the allene/propyne and of secondary chain reactions. Derived rate constants show a strong, persistent pressure dependence, i.e., a quite unexpected deviation (falloff) from second-order behavior. These rate constants are nearer first than second order even for T > 2000 K. They are also anomalously large; RRKM rates using literature barriers and routine energy-transfer parameters are almost an order of magnitude too slow. The two isomers show slightly differing rates, and falloff is slightly less in allene. It is suggested that isomerization is probably slow enough for this difference to be real. The anomalously large rates and falloff are both consistent with an unusually large low-pressure-limit rate in this system. Extensive isomerization of these C3H4 is possible for energies well below their CH fission barriers, and this can become hindered internal rotation in the activated molecule. On the C3H4 surface we identify three such accessible rotors. State densities for the molecule including these rotors are calculated using a previous general classical formulation. Insertion of these state densities into the RRKM model results in rates quite close to the measured magnitudes, and showing much of the observed falloff. The increase in the low-pressure rate is as much as a factor of 8; a necessary but nonetheless remarkable effect of anharmonicity on the unimolecular rate. This again demonstrates the importance of accessible isomerization and consequent hindered internal rotation on the rate of dissociation of unsaturated species.

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As shown in Figure 5, our present plot is rather similar to that of Baulch et al.15 Lin and Bauer6 obtained k6 = 2.8 X 1012 exp(99.6 kJ) cm6• moL1 2-s_1. In order to explain the negative activation energy, they proposed the following mechanism for the reaction: 0(3P) + CO('Here, C02*(3B2) and C02*('Sg+) represent vibrationally excited C02 in the 3B2 and '2g+ states, respectively. Further, they assumed that the rate-determining step is a transition between the 3B2 state and the * 8+ state and calculated that the crossing point between the 3B2 and * 8+ states is 31.4 kJ/mol below the 3B2 dissociation limit. On the other hand, Clyne and Thrush19 studied the reaction 0(3P) + )(' +) -* C02(%+) + hv (18) Warnatz,

Vibrational relaxation, incubation times, and unimolecular dissociation of C4H4O have been investigated over the extended temperature range 500−3000 K in 2−5% furan−krypton mixtures, 2% furan−neon mixtures, and in pure furan. The experiments were performed in shock waves using laser-schlieren (LS) densitometry and time-of-flight (TOF) mass spectrometry. At low temperatures and low pressures, only vibrational relaxation was observed using the LS technique. This relaxation is unexpectedly slow and shows a strong nonexponential time dependence. Unimolecular dissociation is observed in TOF experiments between 1300 and 1700 K in a pressure range of 175−250 Torr as well as LS experiments between 1700 and 3000 K for pressures between 100 and 600 Torr. The TOF experiments show that under the given conditions two molecular dissociation channels leading to C2H2 + CH2CO or to C3H4 + CO are dominant. The branching ratio between these channels has been determined between 1300 and 1700 K. At low temperatures, the molecular channel leading to C3H4 and CO is preferred, but a channel switching was observed around 1700 K. The domination of these molecular channels is consistent with the shape of the LS profiles, and these have been successfully modeled with just these two reactions. The overall unimolecular rate constant is in the falloff regime close to the low-pressure limit. By use of statistical reaction rate theory, the total unimolecular rate constant could be modeled over an extended temperature and pressure range using a value of 〈ΔE〉all = 50 cm-1 for the furan dissociation. In a small range of conditions at low pressures and high temperatures, both the vibrational relaxation and dissociation were resolved and incubation times estimated.

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