Research Overview

Our research focuses on the critical role that surface electron dynamics and interfacial charge transfer have on the selectivity and efficiency of catalytic energy conversion processes. Toward this goal my group has developed femtosecond soft x-ray reflection-absorption spectroscopy as a surface sensitive probe of ultrafast electron dynamics. We also study the closely related chemical reaction kinetics on catalytic surfaces. Research accomplishments and goals in three key areas related to this program are described below.

Ultrafast X-Ray Spectroscopy of Surfaces

We have recently demonstrated that x-ray absorption spectroscopy can be made surface specific by measuring near grazing angle reflectance using a tabletop high harmonic generation light source. Pump-probe transient reflectivity using this femtosecond light source is now enabling observation of ultrafast surface carrier dynamics with element, oxidation state, and spin state resolution. This gives us the ability to track the state of photoexcited charge carriers at a catalyst surface in real time with state-specific precision.

Much more is currently known about molecular photophysics and photochemical reaction dynamics compared to surface photochemistry due to the inability to probe surfaces selectively with sensitivity to oxidation state, spin state, carrier thermalization, lattice distortions, and charge trapping at defect states. Femtosecond soft x-ray reflectivity developed here is now enabling such studies with the goal of advancing the field of surface chemical physics. Various applications of this work are supported by the by the Department of Energy and the Air Force Office of Scientific Research.

Of additional interest is the development of soft x-ray sum frequency generation spectroscopy as a probe of electronic structure and charge carrier dynamics at buried interfaces.  The goal of this work is to extend traditional SFG spectroscopy from the infrared to the soft x-ray spectral region in order to serve as a powerful probe of electronic structure and element specific carrier dynamics at active electrochemical interfaces.

Photoelectrochemical Catalysis

The ability to convert sunlight and electricity to chemical energy is at the heart of energy conversion and storage.  We are developing new materials capable of selective CO2 reduction as well as efficient water oxidation.  Our efforts focus on identifying the relationship between the electronic structure of mixed metal and metal oxide catalysts and their catalytic performance.  These efforts rely on in-house ultrafast soft x-ray spectroscopy to probe the electronic structure of active surfaces and make real-time observations of carrier excitation, thermalization, separation, and injection to better understand surface photochemical reactions.

We have recently developed an earth-abundant copper-iron oxide catalyst showing high selectivity for CO2 reduction to acetate (i.e. a C–C bond coupling reaction). Only a handful of heterogeneous catalysts are capable of selective C–C bond coupling from CO2. This catalyst performs this selective chemistry with exceptionally high energy efficiency using a mixture of two earth-abundant metal oxides. The fundamental chemistry involved here is currently under investigation, and NSF recently funded a project to study the mechanism of C-C bond coupling from CO2 using sum frequency generation vibrational spectroscopy. This is a collaborative project with DFT theorist, Aravind Asthagiri from the Department of Chemical and Biomolecular Engineering at Ohio State University.

Nanoparticle Catalysis

Designing catalyst surfaces to achieve high selectivity is important for the efficient processing of petroleum products.  Perhaps more importantly, selective catalysis is critical for developing renewable routes for synthesis of numerous commercially important chemicals.  We are actively working to develop a more complete mechanistic understanding of surface chemical reactions that will inform the design of new efficient catalysts for renewable energy conversion and green chemical synthesis.

We have elucidated a surprising new mechanism for selective partial hydrogenation chemistry on bifunctional platinum/CeO2 and platinum/TiO2 catalysts. On bifunctional surfaces consisting of a metallic phase and an oxide phase, the interface between these two catalyst phases is commonly considered to be the “active site” for selective catalysis. However, using a series of nanopatterned surfaces, we have demonstrated that the oxide phase strongly influences selectivity of the metal catalyst for >50 nm across the actual interface. This surprisingly long length scale reveals a reaction mechanism where selectivity depends on the surface diffusion of active reaction intermediates across the phase boundary. This unexpected result significantly changes the current picture of reaction kinetics on bifunctional surfaces and provides a new set of design parameters for optimizing selectivity in these catalyst systems. We have recently received an NSF Major Research Instrumentation grant for the acquisition of a Near-Ambient Pressure X-Ray Photoelectron Spectrometer, which promises to greatly facilitate this long-term research direction.