Hydrogen proposed mechanism

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

According to the proposed mechanism for biological 0x1 dation of ethanol the hydrogen that is transferred to the coenzyme comes from C 1 of ethanol Therefore the dihydropyridme ring will bear no deuterium atoms when CD3CH2OH IS oxidized because all the deuterium atoms of the alcohol are attached to C 2... 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

DCHA is normally obtained in low yields as a coproduct of aniline hydrogenation. The proposed mechanism of secondary amine formation in either reductive amination of cyclohexanone or arene hydrogenation iHurninates specific steps (Fig. 1) on which catalyst, solvents, and additives moderating catalyst supports all have effects. 

Proposed mechanisms for the gaseous combustion processes which involve a series of chain reactions such as those for tire oxidation of hydrogen to form water vapour,... 

A proposed mechanism for the oxidation of propylene to acrolein is by a first step abstraction of an allylic hydrogen from an adsorbed propylene by an oxygen anion from the catalytic lattice to form an allylic intermediate ... 

The proposed mechanism includes the production of HCl from the pyro-hydrolysis of the metal chlorides. Similar reactions are likely for bromides and iodides. Fluorides however are relatively stable and would not be expected to hydrolyse. It was considered that this might account for the inability of fluorides to cause cracking. Hydrogen absorption by titanium alloys exposed to chloride salts at elevated temperatures has been detected and found to be proportional to the amount of moisture participating in the reaction. 

This yellow crystalline Pt(IV) complex decomposes slowly at 20° C with evolution of hydrogen and formation of tra r-Hl2SiPtI(PEt3)2. The proposed mechanism is shown in Eq. (63). 

Scheme 6 Proposed mechanism for catalytic hydrogenation of olefin with 5...

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

Scheme 67 The proposed mechanism for iron-catalyzed light-driven hydrogen generation from water...

Scheme 13 Model complexes for [FeFe]-hydrogenase 4 (right) and proposed mechanisms for electrocatalytic hydrogen generation (left) (a) EECC mechanism, (b) ECCE mechanism PTA = phosphatriazaadamantane [33]...

Recently, Moskaleva et al. have proposed a new mechanism based on electronic structure calculations." Earlier experimental studies by Kasdan et al. determined that methyne (HC) has a doublet ground state and with a doublet-quartet energy splitting (AEdq) of 71.5 + O.SkJ/mol." Moskaleva et al. noted that the initially proposed mechanism (for HCN and N(" S) atom formation) is therefore spin-forbidden, and they also proposed a more favorable and spin-allowed reaction on the doublet surface. This new route on the doublet energy surface proceeds through the formation of an NCN intermediate, with concomitant formation of (doublet) hydrogen atom. 

The fair agreement of expressions (2.67) and (2.71) with experimental data as well as agreement of independently obtained experimental data concerning kinetics of the change of a with the data on equilibrium enabled the author of paper [89] to conclude that the proposed mechanism of effect of hydrogen on electric conductivity of semiconductors can be one of active mechanisms. The heat of total reaction (2.63) calculated from the values found was about 4.6 kcal. 

According to the proposed mechanism, the hydrogenation of olefin by iridium catalyst should conform to the following rate expression,... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

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