Kinetics of Hydrogenation

The purpose of this section is to provide an overview of the principal kinetic features of the hydrogenation of ethene and of propene, as providing a framework (or at least part of one) within which discussion of mechanisms must be conducted. Their reactions with hydrogen (and with deuterium) are quite comparable the addition of the methyl group leads to somewhat higher reactivity, due to weaker chemisorption as might be predicted from its lower heat of hydrogenation (Table 7.1). Relative rates for other alkenes will be considered later. The problem of deactivation by carbon deposition has already been mentioned, but quantitative 

Alkene hydrogenation is considerably exothermic (Table 7.1), and maintaining isothermal conditions in a catalyst bed is, except at lowest conversions, not a 

TABLE 13. Kinetic Parameters for the Hydrogenation of Ethene on Other Metals  

Activation energies are very frequently between 30 and 50 kJ mol and for different metals in comparable physical form it is sometimes said that the same value suffices for all. Orders in hydrogen are always positive and very often are unity, while orders in ethene are usually either zero or sometimes negative. So general is this behaviour that attention is naturally drawn to the few exceptions. With platinum, orders of reaction are essentially independent of the physical form and the support, but are temperature-dependent (Table 7.4), the order in 

TABLE lA. Temperature-Dependence of Orders of Reaction for Ethene Hydrogenation on a Platinum Catalyst (0.04%Pt/Cab-O-Sil)  

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Reider G A, Hdfer U and Heinz T F 1991 Desorption-kinetics of hydrogen from the Si(111)7 7 surface J. Chem. Phys. 94 4080-3... 

The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62) was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (62a). 

Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. 

Gdowski GE, Farr JA, Madix RJ. 1983. Reactive scattering of small molecules from platinum crystal surfaces D2CO, CH3OH, HCOOH and the nonanomalous kinetics of hydrogen atom recombination. Surf Sci 127 541. 

Biegel, C. M. Gould, J. M., Kinetics of hydrogen ion diffusion across phospholipid vesicle membranes, Biochemistry 20, 3474-3479 (1981). 

The kinetics of hydrogen peroxide reduction catalyzed by Prussian blue has been investigated [12, 113]. In neutral media the reaction scheme of H202 reduction has been found to be the following ... 

Hayes, D., Grieser, F., and Furlong, D.N., Kinetics of hydrogen production form illuminated CdS/Pt/Na2S03 and ZnS/Pt/Na2S03 dispersions, /. Chem. Soc. Faraday Trans., 86, 3637, 1990. 

One approach to promoting the kinetics of hydrogen transfer to bound carbon monoxide is based on maximizing the difference in polarity of the carbon (eg. 6+) and hydrogen (eg. 6-) involved (Jt). This strategy leads naturally to a bimolecular approach, based upon MCO and M H. The additional degree of freedom which follows from employing two different transition metals is noteworthy as an alternative to cluster activation or catalysis. 

Sn—Sn bond formation can be achieved by indirect electrolysis considering the relative ease of SnH-bond activation. Tributylin hydride is a known H atom donor. It is attacked by radicals like Mn(CO)3P(OPh)3]2, electrogenerated from the anion Mn(CO)3P(OPh)3]2p. The kinetics of hydrogen transfer and coupling of Ph3Sn and Mn(CO)3P(OPh)3]2 was studied188. 

Kinetic and mechanistic studies by Casey et al. provided further insight into the mechanistic details of the hydrogenation of ketones and aldehydes, using a more soluble analogue of Shvd s catalyst (with p-tolyl groups instead of two of the Ph groups) [72]. The kinetics of hydrogenation of benzaldehyde by the Ru complex shown in Eq. (43) were first order in aldehyde and first order in the Ru complex the... 

The kinetics of hydrogenation transfer is covered by the use of an exchange superoperator assuming a pseudo first-order reaction. Thereby, competing hydrogenations of the substrate to more than one product can also be accommodated. In addition, the consequences of relaxation effects or NOEs can be included into the simulations if desired. Furthermore, it is possible to simulate the consequences of different types of pulse sequences, such as PH-INEPT or INEPT+, which have previously been developed for the transfer of polarization from the parahydrogen-derived protons to heteronuclei such as 13C or 15N. The... 

Y. Z. He, W. G. Mallard, and W. Tsang, Kinetics of hydrogen and hydroxyl radical attack on phenol at high temperatures,/. Phys. Chem. 92,2196-2201 (1988). 

Hussey et al. (109) have recently reported measurements of the kinetics of hydrogenation in the liquid phase of a large number of cycloalkenes, both individually and competitively. The data permit the kind of analysis... 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

Fernandez and Sanchez [18] investigated the kinetics of hydrogen absorption and desorption by activated magnesium powder (several cycles of hydrogen absorp-tion/desorption at 375°C) using a volumetric technique. They pointed out that for-mation/decomposition of metal hydrides comprises a number of steps taking place in series transport to the surface, dissociation, H chemisorption, surface-... 

In kinetic studies, the abstraction of F from CF3CH2OH by ArN2+BF4 was shown to be an ionic process, whereas abstraction of an a-H atom from CH3CH2OH was determined to be a radical process. In other kinetic work, the expansion of cyclopentanones has been used to measure the kinetics of hydrogen-atom abstraction reactions for a range of hydrogen donors in benzene including (Me0)2P(0)H( 1.2 x... 

The kinetics of hydrogen abstraction by the CF3COC(C2F5)2 radical have been smdied by ESR with the degree of delocalization calculated using the MNDO/PM3 method. All aspects of the solution structure, stability, and chemistry of carbon-centred fluorine-containing free radicals have been discussed. ... 

The kinetics of hydrogenation of/ra 5-[IrCl(CO)(PPh3)2] in toluene and other organic solvents as well as that of the hydrogenation of trans-[IrCl(CO)(TPPMS)2] [78, 79] in water were studied in detail by Atwood and co-workers [80,81], The rate of both reactions could be described by an overall second-order rate law ... 

Knowledge of the variation of electron transfer rate with electrode potential is important for the understanding of electrochemical reactions. The first experiments in this area were prompted by the observation that nitrobenzenes and aromatic carbonyl compounds are reduced in acid solution with little competition from the hydrogen evolution process. This is the case even though the electrode potential is more negative than the value calculated for the reversible evolution of hydrogen in the same solution. The kinetics of hydrogen evolution have been examined in detail. 

Chen L, Lasia A (1991) Study of the kinetics of hydrogen evolution reaction on nickel-zinc Alloy electrodes. J Electrochem Soc 138 3321-3328... 

Berman A, Epstein M (2000) The kinetics of hydrogen production in the oxidation of liquid zinc with water vapor. Int J Hydrogen Energy 25 957-967... 

Figure 5. Kinetics of hydrogen consumption (a) and dependences of DHL content on the reaction time (b) (Pd/Zn-HPS)...

Lee, Y.-N., J. Shen, P. J. Klotz, S. E. Schwartz, and L. Newman, Kinetics of Hydrogen Peroxide-SulfurOV) Reaction in Rainwa-... 

Catalytic species Solvent Temp. range, °C. Kinetics of hydrogen activation Proposed mechanism of hydrogen splitting Refs. 

This table includes only those systems for which the kinetics of hydrogen activation have been established a number of other systems are referred to in the text. 

D Evelyn, M. P., Cohen, S. M., Rouchouze, E. and Yang, Y. L. Surface bonding and the near-first-order desorption kinetics of hydrogen from Ge(100)2x 1. Journal of Chemical Physics 98, 3560-3 (1993). 

Thermodynamics and Kinetics of Hydrogen Absorption in Rare Earth-Cobalt (R2C07 and RC03) and Rare Earth-Iron (RFes) Compounds... 

Smatlak, C. R., J. M. Gossett, and S. H. Zinder, Comparative kinetics of hydrogen utilization for reductive dechlorination of tetrachloroethene and methanogenesis in an anaerobic enrichment culture , Environ. Sci. Technol., 30, 2850-2858... 

Kinetics of hydrogenation and isomerisation of the n-butenes over various metal catalysts... 

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