Heat of adsorption of carbon monoxide on a Pd/Al2O3 solid using in situ infrared spectroscopy at high temperatures

The adsorption of CO (1% CO/He mixture, 1 atm total pressure) on a reduced 1.4% Pd/Al2O3 solid is studied in the temperature range 300-800 K by in situ Fourier transform infrared (FTIR) spectroscopy using a suitably constructed small-internal-volume infrared cell. Two main IR bands are detected above and below 2000 cm(-1), ascribed to linear and bridged adsorbed CO species on palladium atoms, respectively. The change in IR band intensities with adsorption temperature is used to determine the evolution in the coverage theta of the various adsorbed species with this parameter. The observed curves are in good accord with an adsorption model assuming a linear decrease in the heats of adsorption in step with the increase in coverage. This allows determination of the heats of adsorption of each species at various coverages. For instance, at theta approximate to 0, the heats of adsorption are 92 and 168 kJ/mol for the linear and bridged CO species, respectively, These values decrease respectively to 54 and 92 kJ/mol at theta approximate to 1. As previously observed on platinum-containing solids, the disproportionation reaction 2CO --> C-ads + CO2 detected at high temperatures (a) limits the adsorption temperature and the partial pressures of CO, and (b) creates some changes in the surface of the palladium particles. The heats of adsorption of the adsorbed species determined according to the present experimental procedure are discussed in relation to data in the literature concerning various Pd-containing solids: monocrystals, model particles, and supported catalysts. Several experimental data, i.e., effects of aging of the solid, are presented and discussed in relation to the heats of adsorption of CO. (C) 1999 Academic Press.