General Remarks and Conclusions

However, the experimental evidence collected during recent years, concerning mostly the nickel-copper alloy systems, complicated this almost currently accepted interpretation of the alloy catalytic behavior (45). Chemisorptive and subsequent catalytic phenomena appeared to require a different approach for elucidation. The surface reactivity had to be treated as a localized quality of the atoms at the interface, influenced by their neighbors in the crystal lattice (78-80). A detailed general discussion of catalysis on alloys is beyond the scope of this review. In the monograph by Anderson (81) and in the review by Moss and Whalley (82), recently published, a broad survey of the catalytic reactivity of alloys may be found. 

The hydride phase may be present in a catalyst as a result of the method of its preparation (e.g. hydrogen pretreatment), or it may be formed during the course of a given reaction, when a metal catalyst is absorbing hydrogen (substrate—e.g. in H atom recombination product—e.g. in HCOOH decomposition). The spontaneous in situ transformation of a metal catalyst (at least in its superficial layer) into a hydride phase is to be expected particularly when the thermodynamic conditions are favorable. 

Moreover, a specially active hydrogen species present in a reaction mixture (e.g. atomic hydrogen, protons) (83) or forming during the surface reaction (37) can penetrate into a metal catalyst lattice and become 

There are, however, cases when the hydrogen pretreatment results in an enhanced catalytic activity of an alloy. The phenomenon may be explained also in connection with a metal-metal hydride transformation, namely as a post-hydride effect. 

In the particular case of nickel-copper alloys their hydrogen pretreatment may result in phase segregation (48), at least at the surface. The desegregated rich in nickel alloy can display its relatively high catalytic quality and even keep it down to a certain temperature (lower than in the case of nickel itself), which would be the critical temperature of a given Ni-Cu-H system. 

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