Gas phase vs. liquid phase: monitoring H2 and CO adsorption phenomena on Pt/Al2O3 by IR spectroscopy

The adsorption phenomena occurring at the surface of a highly-dispersed Pt/Al2O3 catalyst for hydrogenation reactions were thoroughly investigated in the gas-phase by transmission IR spectroscopy and in the liquid-phase by ATR-IR spectroscopy. The reduction of Pt/Al2O3 in H-2 with the formation of Pt-hydrides and adsorption of CO were used as case studies for the comparison of the two different environments under otherwise similar experimental conditions. We found that compared to gas-phase, the selected solvent (cyclohexane) greatly affects the reducibility of Pt. Incomplete reduction under the adopted liquid-phase conditions was demonstrated by the formation of carbonate species at very low CO coverage. Moreover, cyclohexane competes with the adsorbates (H-2 as well as CO) for the occupancy of the available surface sites, thus affecting the shape, position and relative intensity of the infrared signals corresponding to the surface Pt-H and Pt-CO species and leading to an overall lower H and/or CO coverage. Our spectroscopic data show also that the presence of the solvent is crucial in maintaining the Pt nanoparticles in partially H-solvated state even in a H-2-free environment. This evidence might have important implications with respect to the catalyst reactivity and stability.