The adsorption of carbon monoxide on evaporated metal films

Abstract

Calorimetric measurements at 0 °C. have been made of the heats of adsorption of carbon monoxide on evaporated films of iron, cobalt, nickel, manganese, titanium, molybdenum, tungsten, tantalum, niobium, zirconium, platinum, palladium and rhodium. The variation of the heat of adsorption with coverage is considered in terms of a precursor state whose energy determines the form of the variation. For the adsorption on tungsten, the present results and heat values calculated from data derived from flash filament desorption studies show encouraging similarities, but comparison with results from field emission microscopy is disappointing. Theoretical calculations of heats of adsorption have been criticized with special reference to carbon monoxide adsorption. Attention is again drawn to the linear dependence of heat of adsorption on metallic radius. Saturation coverages of carbon monoxide and monolayer values for krypton before and after chemisorption are reported. In an attempt tb interpret these values, possible configurations of adsorbed carbon monoxide are considered for all the metals. Simple restrictions relating to (i) the form of bonding, namely linear or bridge, (ii) the M —C bond length and the M—C—M bond angle, (iii) the number of bonds in which any one metal atom can participate, (iv) the form of the potential energy of interaction energy between neighbours, and (v) the krypton monolayer concentrations as deduced in the preceding paper, are applied in arriving at maximum coverages for the various regular arrangements of admolecules which are conceivable. For titanium and zirconium, the experimental coverages are readily accounted for by these means, with the conclusion that carbon monoxide is held by a bridge bond with each metal surface atom able to participate in only one bridge. For the face-centred cubic metals, similar conclusions are compatible with the experimental results, although they are not so well founded as for the hexagonal, close packed metals. The difficulties of applying the discussion to the body-centred cubic metals are also considered. Arguments are presented which cast doubt on the view that the reduction in the adsorption of krypton after chemisorption is the result of sintering of the metal film owing to the liberation of the heat of adsorption. An alternative explanation in terms of a redistribution of sites for the adsorption of krypton due to the presence of the chemisorbed layer is examined and found to be inadequate in the simple form used.