Surface potential of mixed adsorbates on metal films

The surface potential changes effected at the surface of sintered evaporated films of two metals, copper and nickel, by the chemisorption of pairs of gases at —183 °C have been measured. The sign and magnitude of the dipole moment of the chemisorbed bond (which is directly related to the surface potential change) are specific to the chemisorbed species and the induced polarization effects of neighbouring dipoles are small. Consequently, an approximate additivity rule has been used by which the surface potential change of an adsorbed layer comprising two different adsorbates can be calculated from the relative amounts and the individual dipole moments of each adsorbate. Significant departures from this rule indicate the occurrence of a surface reaction with the formation of a new surface complex, the nature of which depends on the order of addition of the individual gases at — 183 °C. By measuring the amount of, and analyzing the nature of, the products desorbed on raising the adsorbent to room temperature, followed by a measurement of the amount of one of the adsorbates re-adsorbed at — 183 °C, and simultaneously recording the surface potential changes, information about the structure of the first-formed complex, together with some details of its possible mode of decomposition, is obtained. The present method of detecting intermediaries at the adsorbent surface during the process of a simple heterogeneously-catalyzed process is complementary to the infra-red spectroscopic method originated by Eischens and his co-workers. The present paper represents a preliminary investigation of the potential value of the surface potential method and has been applied to the reactions of (i) carbon monoxide and hydrogen, (ii) carbon monoxide and oxygen, and (iii) hydrogen and oxygen. The results are limited to processes occurring at room temperature and to two metal adsorbents, copper and nickel, chosen because the former has a full d -band whereas the latter has available empty d -orbitals. Marked differences are found with reaction (i), but these are less marked, particularly when oxygen is chemisorbed first, in reactions (ii) and (iii). The limitations of the general approach are discussed and the disadvantages of the use of the space-charge limited-diode technique are noted.