Shape Effect Undermined by Surface Reconstruction: Ethanol Dehydrogenation over Shape-Controlled SrTiO3 Nanocrystals

To gain an in-depth understanding of the surface properties relevant for catalysis using ternary oxides, we report the acid-base pair reactivity of shape-controlled SrTiO3 (STO) nanocrystals for the dehydrogenation of ethanol, Cubes, truncated cubes, dodecahedra, and etched cubes of STO with varying ratios of (001) and (110) crystal facets were synthesized using a hydrothermal method. Low-energy ion scattering (LEIS) analysis revealed that the (001) surface on cubes of STO is enriched with SrO due to surface reconstruction, resulting in a high ratio of strong base sites. Chemical treatment with dilute nitric acid to form etched cubes of STO resulted in a surface enriched with Ti cations and strong acidity. Furthermore, the strength and distribution of surface acidic sites increase with the ratio of (110) facet from cubes to truncated cubes to dodecahedra for STO. Kinetic, isotopic, and spectroscopy methods show that the dehydrogenation of ethanol proceeds through the facile dissociation of the alcohol group, followed by the cleavage of the C-alpha-H bond, which is the rate-determining step. Co-feeding of various probe molecules during catalysis, such as NH3, 2,6-di-tert-butylpyridine, CO2, and SO2, reveals that a pair of Lewis acid site and basic surface oxygen atom is involved in the dehydrogenation reaction. The surface density of acid-base site pairs was measured using acetic acid as a probe molecule, allowing initial acetaldehyde formation turnover rates to be obtained. Comparison among various catalysts reveals no simple correlation between ethanol turnover rate and the percentage of either surface facet ((001) or (110)) of the STO nanocrystals. Instead, the reaction rate is found to increase with the strength of acid sites but reversely with the strength of base sites. The acid-base property is directly related to the surface composition as a result from different surface reconstruction behaviors of the shaped STO nanocrystals. The finding in this work underscores the importance of characterizing the top surface compositions and sites properties when assessing the catalytic performance of shape controlled complex oxides such as perovskites.