Development of New Methods of Studying Catalyst and Materials Surfaces with Ambient Pressure Photoelectron Spectroscopy

ConspectusThe surface of a catalyst is crucial for understanding the mechanisms of catalytic reactions at the molecular level and developing new catalysts with higher activity, selectivity, and durability. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is a technique studying the surface of a sample in the gas phase, mainly identifying chemical identity, analyzing oxidation state, and measuring surface composition.In the last decade, numerous photoelectron spectroscopic methods for fundamental studies of key topics in catalysis using AP-XPS have been developed. By tracking the evolution of the catalyst surface during catalyst preparation, AP-XPS can assist in identifying the parameters for preparing an expected catalyst structure. Additionally, it can uncover adsorbate coverage-induced surface restructuring by monitoring the photoemission features of key elements as the gas pressure increases. Surface phase transitions of a catalyst support, supported metal, or supported oxide nanoparticles and restructuring of supported single-atom sites may occur at temperatures lower than a catalysis temperature. AP-XPS can track these temperature-dependent phase transition or structural evolution under catalytic conditions. It also enables analysis of the electronic structure of the catalyst surface during catalysis by collecting valence band spectrum at a specific catalysis temperature. Moreover, it can detect stable intermediates formed at a temperature lower than the catalysis onset temperature and track their transformation to product molecules, providing significant insights in proposing a pathway closest to the actual but unknown one. Time-on-stream quantification of oxidation and reduction processes on catalyst surfaces allows for the study of kinetics of redox, including determinations of reaction order and activation barrier. One challenging task in accurately measuring catalytic reaction rates under kinetic control is measurement of the number of catalytic sites. AP-XPS is a valuable technique for this task, as it can qualitatively identify active sites and quantitatively measure the number of active sites under a specific catalytic condition. For photocatalytic and photoelectrocatalytic systems, AP-XPS helps elucidate charge transfer at the interface of a cocatalyst and semiconductor by identifying shifts in binding energy of a key element, shedding light on electron-hole separation. Photoelectron-induced excitation (PEIE) spectroscopy provides a unique capability for in situ measurement of gas products proximal to the catalyst surface within 0-0.1 mm during catalysis. It enables the on-site in situ identification of gas products and quantification of their partial pressures.The successful development of these methods highlights the unique capabilities of AP-XPS in addressing key topics in catalysis and uncovering crucial information about catalysts under reaction or catalytic conditions that other spectroscopy or microscopy techniques cannot. These advancements are expected to significantly benefit many fields in chemistry, chemical engineering, energy science, materials science, and environmental science. Applications of AP-XPS to study solid-liquid interfaces, especially at the electrode-electrolyte interface in electrochemical processes, are significant. These applications at solid-liquid interfaces include electrification-based chemical transformations, electrochemical CO₂ reduction, water electrolysis, electrochemical reduction of oxidants on the cathode and even oxidation of fuels in fuel cell process, and oxidation and reduction processes in batteries. Further development of instrumentation and spectral methods of AP-XPS will be beneficial to energy conversion, sustainable chemical transformation, and environmental remediation as well as materials design for quantum computing hardware.