SusChEM: CO2 Photo-Electrochemistry on Metal Oxides Surfaces Studied by Vibrational Sum Frequency Generation Spectroscopy and Density Functional Theory

Ohio State University, The, Columbus OH

Investigators

Abstract

Catalysts are chemical substances that provide lower-energy reaction pathways to increase the speed of a chemical reaction. Catalysts can be recycled many times during a chemical reaction. Some catalysts, called photo-electrocatalysts, are able to use light as an energy source to produce the reaction of interest. In this project, funded by the Chemical Catalysis Program of the Chemistry Division, Professors L. Robert Baker and Aravind Asthagiri are using a combination of an experimental technique called sum frequency generation (SFG) vibrational spectroscopy and computational modeling called density functional theory (DFT) to investigate carbon dioxide (CO2) reduction by a series of copper and copper/iron containing photo-electrocatalysts. SFG vibrational spectroscopy is able to show how molecules important in CO2 reduction interact with the catalyst surfaces during the reaction. DFT calculations are used to help explain the SFG data generated during the reaction. The combination of the two techniques leads to a complete picture of the surface reaction mechanism, allowing for better design of the catalyst and the photo-electrocatalysis process. The ability to photo-electrochemically reduce CO2 using sunlight has the potential to enable us to recycle carbon and stabilize the environmental impacts of CO2 emissions from automobiles and other sources. This research helps to address the challenge of making fuels in an environmentally friendly and economically viable way. These research activities are integrated with an outreach plan to improve student recruitment and retention in science, technology, engineering and mathematics (STEM) fields. Through the TEK8 program at the Ohio State University, undergraduate students and K-12 teachers gain hands-on experience in the investigators' research groups. The students then translate this experience to middle school students through age-appropriate learning modules designed to inspire K-12 students to pursue future STEM education. The challenge of understanding CO2 surface chemistry on a photo-electrocatalyst requires a combined knowledge of metal oxide surface chemistry as well as the semiconductor photo-physics that determine the localized atomic sites of photo-excited electrons responsible for driving the reductive chemistry. In this project funded by the Chemical Catalysis Program of the Chemistry Division, Professors L. Robert Baker and Aravind Asthagiri are using sum frequency generation (SFG) spectroscopy and complementary density functional theory (DFT) computational modeling to investigate CO2 activation and subsequent reduction on a series of CuO/CuFeO2 surfaces as a function of relative phase composition. CuO/CuFeO2 catalysts with varying amounts of CuO and CuFeO2 show tunable selectivity between acetate and formate production. The ability to probe the molecular intermediates associated with these reaction pathways, coupled with the ability to tune these relative rates, is providing a valuable case study for a thorough mechanistic investigation of the surface properties that mediate reaction kinetics leading to tunable selectivity for CO2 reduction. Complementary measurements using femtosecond soft x-ray spectroscopy show electron thermalization kinetics and site-specific charge localization with element specificity. Through information from the soft x-ray spectroscopy and DFT-derived band structure, the team obtains the reduction potential of site-localized photo-excited electrons in the oxide surface and compares these potentials to the calculated change in free energy of formation of elementary steps on these respective atomic sites. This combination of approaches bridges the fields of semiconductor photo-physics with surface chemistry and catalysis in order to provide a fundamental framework for understanding the selectivity of CO2 photo-electrochemistry on metal oxides surfaces. This research helps to address the challenge of environmentally friendly and economically stable fuel production and chemical synthesis from CO2 using earth-abundant metal oxide catalysts. These research activities are integrated with an outreach plan to improve student recruitment and retention in STEM fields. Through the TEK8 program at the Ohio State University, undergraduate students and K-12 teachers gain hands-on experience in the investigators' research groups. They then translate this experience to middle school students through age-appropriate learning modules designed to inspire K-12 students to pursue future STEM education.