M-H bond strength

The most striking vertical trend concerns the metal-hydride bond strengths, as summarized in Table 4.55. We define the first M—H bond strength (A mh(1)) for each species as the energy needed to break the first M—H bond ... 

Figure 7.6 Experimental volcano curve for H2 evolution on metals. M-H bond strength from overpotential data. Adapted from Ref [28].

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

Fig. 7.114. A plot of exchange current density for hydrogen evolution reaction vs. M-H bonding strength. (Reprinted hom S. U. M. Khan, Some Fundamental Aspects of Electrode Processes," in Modern Aspects of Electrochemistry, Vol. 15, R. E. White, J. O M. Bockris, and B. E. Conway, eds., Plenum 1983, p. 339.)...

How are these potential energy curves constructed That is not a question to be answered in detail in this book. However, let it be said that one needs knowledge of the quantum mechanics of chemical bond formation to do it. Owing partly to the woik by Anderson (1990), there is software that enables one (in hours, not days) to calculate the potential energy quantities needed in particular for the M—H bond strengths at... 

The amount of adsorbed hydrogen decreases in the presence of halide ions [395, 396]. This is due to a decrease in the M-H adsorption energy induced by ion-specific adsorption with partial charge transfer. The decrease in M-H bond strength results in an increase of overpotential. The effect is lower for Cl and higher for I -. However two joint effects are operative one due to electronic modifications, and the other one of an electrostatic nature related to a change in the local electric potential... 

Results concerning other interstitial compounds are sparse. The hydrogen overpotential on non-stoichiometric TiNx increases with the N content [434]. These materials are thus less active than pure Ti. The Tafel slope is about 120 mV independent of N content. Decrease in M-H bond strength with N content has been suggested. Hydrogen penetration has also been observed. 

Metallurgical methods are usually needed to prepare intermetallics [535]. Therefore, an extension of the surface area cannot be expected. If there is an increase in activity this can realistically be attributed to electrocatalytic effects. Since in most cases the Tafel slope is not lower but actually higher than for Ni [53], the activity enhancement has to be related to a change in the adsorptive properties of the surface so that both the M - H surface bond strength and the degree of coverage with H are different. For instance, in the case of TiFe, a lower M - H bond strength and a lower... 

Thus, precursors of (RPCP)Ir and Rh(PR3)2Cl gave excellent catalysts, in contrast with the converse pair, (RPCP)Rh and Ir(PR3)2Cl. Calculations (DFT) by Krogh-Jespersen[21] indicated that this surprising relationship was related to the respective M-H bond strengths of the fragments. Addition of H-H to the model... 

The extreme longevity of radicals MR3 and MfNR (Table III) must be mainly due to stcric hindrance to dimerization (44, 46). The low values of the M—H bond strengths [ D(M—H) C, 104 Si, 81 Ge, 73 Sn, 70 kcal mole-1] (110) do not favor H abstraction from the C—H bonds of the solvent, but the shorter lifetimes of the SnfNRjH and Ge(NR()3 radicals in hexane compared to SnR3 and GeR3 in benzene may reflect the ease of H abstraction from the two solvents. A third factor, for the tris(alkyl) radicals is the low probability of disproportionation, such as that shown for a carbon-centered radical,... 

The influence of the R and X substituents on the stretching modes of the M—H bond in IR spectra of the series R3 X MH (M = Si, Ge, Sn) has been investigated in some detail. The form of these vibrations is highly characteristic. Therefore, their frequency v(M—H) and intensity A(M—H), which characterize the M—H bond strength and polarity respectively, depend only on the electronic effects of the R and X substituents. If the R substituents at the M have only inductive influence upon v and A, the correlations given in equations 1 and 2 are observed35 (for M = Si and Ge). 

Fig. 16. Volcano plot oT Trasatti for logi o values plotted against M—H bond strength (compare with Fig. 2). (From Ref. 148.)...

This particular volcano plot is indeed nearly symmetric and demonstrates that Pt, Rh, Ir, and Re have the optimal M—H bond strength for maximum activity (exchange current density) for the hydrogen evolution/hydrogen oxidation reaction (HER/HOR). 

With careful experimentation one can get around these problems, however, and independent of whether the Tafel or the Heyrovsky equation obtains in a particular case, one may expect the volcano relationship to apply when we plot the exchange current density vs. the M-H bond strength for a variety of metals. This is indeed what is observed (see Fig. 5.40) at intermediate values of the M-H bond strength, the evolution of H2 is clearly the most effectively catalyzed. 

This situation is typical of metal radicals, the relatively low M-H bond strengths prohibit direct attack on H2 for thermodynamic reasons. Kinetic studies75 showed that oxidative addition of H2 obeyed an overall third-order rate law d[PJ/dr /c "Cr (CO)3Cp ]2[H2] with All1 0 kcal/mol and AS = -47 cal/(molK). The proposed mechanism for the third-order reaction is shown in Equation 10.57. 

FIGURE 1.6 Logarithm of exchange current density for the hydrogen evolution reaction as a function of the M-H bonding strength. 

MGaH4 series appears to be invariant whereas that for MBH4 series varies almost linearly with the size of M" . Also the width of the bands (in particular the valence band) varies nearly linearly with the size of M" . The bandwidth narrowing from LiBH4 to CSBH4 reflects the enhanced M-B and M-H bond strength along the series. ... 

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