Towards a Molecular Understanding of the Reactivity of Oxide Catalysts
Glen R. Jenness, Stavros Caratzoulas, Dionisios G. Vlachos, Raymond J. Gorte
Due to petroleum depletion and an increasing concentration of greenhouse gases, there is a considerable interest in the refinement of biomass as a renewable source of fuels and value added chemicals. Biomass derivatives are often oxygen rich, which results in a need for catalysts that can selectively remove the extraneous oxygen atoms. Metal oxides, such as γ-Al2O3, function as a solid Lewis acid and have been shown to be an effective and highly reusable catalyst for the selective removal of oxygen from biomass-derived compounds. γ-Al2O3 is one example of a solid Lewis acid with a well known dehydration chemistry. However, due to the underlaying crystal structure, this material presents a heterogeneous set of Lewis acid sites, each with an unknown amount of Lewis acidity. This presents a challenge in the characterization of this material, as the dehydration properties would change from site to site.
In order to solve this problem, we examined both the adsorption and dehydration properties of the (110) and (100) facets of γ-Al2O3 using density functional theory (DFT). We demonstrated a correlation between the conduction band properties of the γ-Al2O3 surface and the adsorption energies and dehydration barriers. This lead to the introduction of a novel descriptor of the Lewis acidity: the mean of the s-conduction band (E*s). The E*s was successful in a molecular understanding of the heterogeneous Lewis acidity of this material, and how it affected both the adsorption and dehydration properties. Furthermore, we were able to use this descriptor to understand how the surface hydration of the (110) surface affects the Lewis acid properties and the origin of the decrease in Lewis acidity as a result of hydration.1
In the current study, we expanded on this earlier work to include both TiO2 and ZrO2. Both oxides have been shown to display multiple crystal facets under reaction conditions, and as a result understanding the Lewis acidity of these materials is challenging. We are able to show a correlation between the E*s descriptor and the adsorption properties of the various facets of TiO2 and ZrO2, in agreement with our previous work. Comparison of the adsorption properties of CO and isopropyl alcohol (IPA) revealed that as the adsorbate increases in complexity, additional interactions with the surface are present, resulting in a decrease in the correlation. In order to probe these effects further, we applied several density-of-states (DOS) analysis methods, including the crystal orbital overlap population (COOP) and the crystal orbital Hamilton population (COHP). Through a careful analysis of these results, we are able to determine the effects of the Lewis acidity from a molecular standpoint. These findings are used to propose suitable catalytic materials for the selective reduction of oxygenated reactants.
(1) Jenness, G. R. et al. J. Phys. Chem. C 2014, 118, 12899–12907.