As we struggle to meet world energy demand, efficient photo(electro)catalysts for CO2 reduction are at the center of chemists’ attention. Understanding of mechanisms of action of those catalysts is crucial for improving their \activity and designing future materials with better properties. Our work focuses on the bridge between homogeneous and heterogeneous catalysis taking advantages of both – tunability of molecular catalysts and robustness of heterogeneous structures. We are investigating how the electronic structure of molecular catalysts are affected by their incorporation in a conductive carbon support (CS). Specifically, we study CO2 reduction catalysts Co phthalocyanine (CoPc) and Re(bpy)(CO)3Cl using Co K-edge and Re L-edge X ray absorption spectroscopy (XAS) under applied electrochemical bias. Both of those catalysts have been shown to exhibit improved catalysis upon immobilization onto CS. Element specificity of XAS allow us to look at the electronic and structural changes that metal center undergoes under applied reductive potential, and extended X ray absorption fine structure (EXAFS) allow us to determine finer structural changes. We identified how the structure of Co and Re catalysts changes upon deposition onto CS, including both carbon nanotubes (CNT) and carbon fiber paper (CFP). Our analysis indicates that electrodeposition of the catalyst results in stronger interaction with CNT and CFP than simple spin coating deposition. In addition, the type of carbon support (CNT vs CFP) also affects the strength of interaction of molecular catalyst with the electrode. We also show how XAS can be used to determine if the catalyst and support are in strong electronic communication. Based on the changes in EXAFS under applied potential, we determined optimal catalyst loading and then XANES is used as a probe of oxidation state of metal center. Whether metal center is undergoing oxidation state changes is indicative of strong (no changes) or weak (it gets reduced) electronic communication with the electrode. We used this analysis and estimated which fraction the catalysts are strongly coupled to the conductive carbon support. This study shows how in situ electrochemical XAS could be used to evaluate the degree of interaction of molecular catalysts and conductive support it is deposited onto. It is also highlighting that not only molecular catalyst, but preparation method and type of support affect catalytic activity and selectivity.

Probing the degree of electronic coupling between molecular catalysts and conductive carbon support using X-ray absorption spectroscopy

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ACS Fall 2022

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