Heats of adsorption and activation energies of surface processes measured by infrared spectroscopy

The temperature-programmed adsorption equilibrium of CO at 4 kPa on Pd/SiO2 and of CO2 at 101.3 kPa on γ-Ga2O3 was studied by infrared spectroscopy between 298 and 723 K. The relative coverage (θ) evolution of each adsorbed species as a function of the adsorption temperature was measured by normalizing the IR absorption bands. The Temkin's model was used to fit the experimental data of θ versus temperature of each surface species and to obtain the initial (θ = 0) and final (θ = 1) isobaric heats of adsorption of linear and bridged CO adsorbed on palladium, and of HCO3− (bicarbonate), b-CO32− (bidentate carbonate), br-CO32− (bridged carbonate) and CO2− (carboxylate) on gallium oxide. The reactive adsorption of CO (101.3 kPa) on gallium oxide was also studied. CO reacts with surface OH groups to give formate species from 448 K. These formate species are further decomposed to CO2(g) and Ga–H species from 573 K onwards. The whole process was modeled by considering the rate of reaction for each elementary step. Thus, the activation energy of the reactive adsorption of CO and of the formate decomposition on Ga2O3 was calculated. The calculated kinetic parameters are discussed and compared to those reported in the literature.

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