Astoichiometric components, surface

Hydrogen is the most important astoichiometric component. Even the effect of other added components can sometimes be interpreted in terms of governing the availability of surface hydrogen. This explains why adding a second (catalytically inactive) metal to platinum may have the same effect on the selectivity as surface hydrogen or nonmetallic additives 107) (see also Section II,B,5). 

Various astoichiometric components (hydrogen, carbon, and others, for example, silicium and aluminum) present may interact with localized and nearly free electrons to differing extents. According to the localized free electron interplay model of metal catalysts developed by Knor 163, 164) the ratio of the two types of electrons may influence the catalytic properties considerably. For example, a subsurface proton attracts nearly free electrons and thus uncovers some localized orbitals. Carbon may interact first with localized electrons 164). This may be one of the reasons why their effects are of opposite character. The collective efforts of catalytic and surface chemists are necessary to bring some clarity to the multitude of problems arising here. 

Added astoichiometric surface components (hydrogen and carbon, eventually other metals) these influence activity and selectivity. 

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