Adsorption of Carbon Monoxide on Pd(111) and Palladium Model Catalysts

The adsorption of carbon monoxide on Pd(111) and SiO2/Si(100)-supported palladium particles, ranging in size from 1.5 to 9.5 nm, was investigated with ellipsometry. By calibrating the sensitivity of the ellipsometric parameter δΔ for the carbon monoxide coverage on Pd(111) with LEED, ellipsometry could be used for a quantitative analysis of the adsorption isotherms. The saturation coverage of carbon monoxide appeared to be ≈0.5 ML on all samples, with respect to the number of surface atoms and within the temperature (400–540 K) and pressure range (up to 1 Pa) studied. The initial isosteric heat of adsorption was 148±5 kJ/mol on all samples. The heat of adsorption decreased with an increasing CO coverage. The decrease of the heat of adsorption with an increasing CO coverage occurred in a similar way on all samples. From these results we conclude that there are no particle size effects with the adsorption of CO on SiO2/Si(100)-supported palladium particles. Since other studies on the adsorption of CO on palladium model catalysts do find a particle size effect, it is concluded that the particle size effects are dependent on the support or that the observed particle size effect is in fact a support effect.