Recombination of H atoms on metals

Recombination of H atoms on metals (after Melin and Madix [90]) 

The efficiencies of various metals for the recombination of H atoms are given in Table 8. In all cases, the recombination is first-order and the activation energy is less than 5 kJ mole 1. The fact that the state of anneal of the metal can alter 7 by a factor of ten [14] suggests that the reaction may occur on minority sites, rather than on a well-defined surface characterised by the intrinsic properties of the metal. This is certainly the explanation for the wide range of values reported for palladium. The extent of diffusion of H atoms into the bulk is also a complicating factor, making 7 dependent on the time of exposure to H atoms for palladium, this effect can be so gross as to result in distortion of the sample. 

The values of 7 for aluminium, copper and perhaps silver seem high enough to be explicable in terms of the discussion of Sect. 2.3.2(b). 

The values of the accommodation coefficient ]3 for the recombination over silver and gold (see Table 6) are probably high enough to be compatible with reaction between mobile adatoms, but the low value for copper is not. Once again, we face the same dilemma posed by recombination over the Group IB metals. 

For the other metals on which fast chemisorption of H2 molecules occurs, the discussion of Sect. 2.3.2(a) for the Rideal—Eley mechanism applies. The measured values of 7 and first-order kinetics can be accounted for in terms of different values of /cr (and Kl l). Even if here, 

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