(617cj) The Effect of the Reaction Environment on the Surface Acidity of t-ZrO2 (101) Investigated By DFT

Zirconium oxide is an important ceramic material with a variety of commercial applications. This metal oxide has been successfully used as catalyst or catalyst support due to its thermal stability and acid-basic properties. In particular, the tetragonal phase of ZrO₂ has been repeatedly identified as the most catalytically active phase.[1] However, the assignment of specific catalytic performance is difficult without complete understanding of chemical properties of the catalysts at a molecular level.

In this study, the acidity of the most stable (101) facet of tetragonal zirconia (t-ZrO₂) was investigated using density functional theory (DFT) by means of ammonia adsorption. In order to cover a broader range of experimental conditions, different surface models were carefully selected to account for reductive or humid environments, i.e., stoichiometric terminations, oxygen vacancy defects, and hydroxylated surfaces were employed to characterize the acidity of the (101) facet of t-ZrO₂. Our results reveal that the acidity of t-ZrO₂ (101) is mainly due to the electron deficient metal atoms (Lewis acid sites) exhibiting an adsorption energy of ammonia (E_ads) up to 120 kJ/mol on a stoichiometric, vacancy-free surface. An oxygen vacancy site leads to a stronger NH₃-Zr interaction causing the adsorbate to dissociate and release up to 190 kJ/mol upon dissociative adsorption. In contrast, a humid environment leads to the formation of surface hydroxyl groups subsequent to the dissociation of water molecules on acid-base pairs; however, this only creates weak Brønsted acid sites with E_ads < 40 kJ/mol. Ammonia adsorption microcalorimetry and DRIFTS-pyridine desorption experiments are in very good agreement with our DFT calculations.[2] This study provides detailed information on the acidity of ZrO₂, which is a catalytically important property affecting conversion and selectivity of chemical reactions. Furthermore, the use of different model surfaces accounting for diverse environmental conditions is particularly relevant, since it provides insights on tuning the surface acidity by adjusting reaction conditions accordingly.

REFERENCES:

[1] K. Samson, M. Sliwa, R.P. Socha, K. Gora-Marek, D. Mucha, D. Rutkowska-Zbik, J.-F. Paul, M. Ruggiero-Mikolajczyk, R. Grabowski, J. Sloczynski, Influence of ZrO₂ Structure and Copper Electronic State on Activity of Cu/ZrO₂ Catalysts in Methanol Synthesis from CO2, ACS Catal. 4 (2014) 3730â??3741.

[2] J.M. Arce-Ramos, L.C. Grabow, B.E. Handy, M.G. Cárdenas-Galindo, Nature of Acid Sites in Silica-Supported Zirconium Oxide: A Combined Experimental and Periodic DFT Study, J. Phys. Chem. C. 119 (2015) 15150â??15159. doi:10.1021/acs.jpcc.5b02394.