Insight into the Selectivity of Isopropanol Conversion at Strontium Titanate (100) Surfaces: A Combination Kinetic and Spectroscopic Study

Abstract

This work aims to better understand the role of interfacial molecular structure that governs selectivity and activity in heterogeneous catalytic reactions. To address this, a comprehensive study of isopropyl alcohol conversion over an archetypal perovskite material, strontium titanate (SrTiO3 or STO), was performed with an array of techniques sensitive to orthogonal aspects of the ensuing chemistry. Cubic-shaped STO nanoparticles with only the (100) facet exposed were synthesized and used to study the ensemble kinetic conversion of isopropyl alcohol over the surfaces, which showed selectivity to form acetone, with minor propylene products appearing at elevated temperatures. These results in combination with inelastic neutron scattering measurements provide not only insight into the selectivity and overall activity of the catalysts but also low-frequency vibrational signatures of the adsorbed and reacted species. To complement these measurements, pristine thin films of STO (100) were synthesized and used in combination with vibrational sum frequency generation spectroscopy to extract the absolute molecular orientation of the adsorbed molecules at the interface. It was found that the isopropyl alcohol assumes an orientation where the −CH group points toward the STO surface; this prereaction geometry offers an obvious pathway to produce acetone by abstracting the α proton and, thus, provides a mechanistic explanation of selectivity at STO (100) surfaces. This insight opens up pathways to explore and modify surfaces to tune the activity/selectivity through a molecular level understanding of the reactions at the surface.