The States of Adsorbates and Absorbates

The equilibrium and the rate of spillover depend upon the nature of the bonding of the adsorbate at all stages from the donor to the donee. Little is known and the facts are complicated and grow increasingly so they cannot be detailed here but need some mention, at least for the simpler molecules most often used in experimental work on spillover. 

At low coverages the adatoms carry a small negative charge whereas nearer saturation the adsorbate is polarized positively. Insofar as species of both charges coexist on the metal surface they presumably form an array, with some degree of order, comprising paired, donor, polar states (— and +) which may correlate with polarized adsorbed states of hydrogen on a polar donee. 

Absorbed hydrogen dissolves as protons with a closely associated, somewhat delocalized electron, but among the metals of greatest interest as catalysts the process is exothermal only for palladium, some palladium-rich alloys, transitional elements, and alloys with i-electron/atom ratios ca. 5.6, and some 

Semi-conductors generally chemisorb hydrogen in a number of forms among which we may distinguish again molecular, polar and covalent states, e.g. hydrogen on zinc oxideand on chromia.  

The adsorption by insulators is small,as with hydrogen, and controlled by 

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