Operando Grazing Incidence XAS Study of Pd-Nanoparticle and -Aerogel Catalysts for the Electrochemical Reduction of CO2

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As global warming takes place at an unprecedented pace, it becomes increasingly important to develop negative emission (i.e., CO2-depleting) technologies to achieve the hoped-for net-zero target in 2050. The electrochemical CO2-reduction reaction (CO2RR) to carbon monoxide (CO) or formate is expected to be an economically viable approach to close the carbon cycle while reducing greenhouse gas emissions.[1] In this context, palladium (Pd) has been identified as an interesting CO2RR-catalyst owing to its ability to selectively produce formate vs. CO in the lower vs. higher overpotential regimes [i.e., at -0.1 to -0.4 vs. -0.5 to -0.9 V vs. the reversible hydrogen electrode (RHE), respectively].[2] To this day, the reasons for this potential-induced change in product selectivity remain poorly understood and a subject of open debate in the literature. In this regard, several publications have suggested that PdHx forms at the negative potentials at which the CO2RR takes place, and that this hydride acts as the active phase in the reduction of CO2 to formate.[3, 4] To elucidate this catalytic mechanism, we have investigated two different types of Pd catalysts: one consisting of dispersed Pd-nanoparticles supported on a carbon black, and a second one in the form of unsupported Pd nanoparticles tridimensionally interconnected into a network structure (i.e., a so called aerogel).[5] To track these materials’ potential-dependent PdHx-formation using X-ray absorption spectroscopy (XAS) at the Pd K-edge under CO2RR-conditions and link it to their partial current densities (pCDs) towards formate, we employed a newly designed spectroelectrochemical operando XAS flow cell. The latter enables spectral acquisition in a grazing incidence (GI) configuration allowing the use of thin layer electrodes (i.e., with a thickness < 1 μm) which in turn minimizes the accumulation of evolved gaseous bubbles along the CL-thickness, thus avoiding spectral artifacts related to the present of such bubbles. Moreover, the implementation of an ion-conductive membrane to separate the working- and counter-electrode compartments enables the accurate quantification of gaseous products via mass spectrometry and gas chromatography, as well as of liquid products by collecting aliquots of the electrolyte over time (which, in the case of formate, are subsequently analyzed through ion chromatography).Using this combination of spectroelectrochical and analytic techniques, we found that the two catalysts exhibit significantly distinct behaviors, as illustrated in Figure 1. Specifically, while for the C-supported Pd nanoparticles the stable pCD towards formate observed at -100 mV and -200mV vs. RHE is accompanied by a quick PdHx-formation (stabilizing at hydride stoichiometries of x ~ 0.6 vs. ~ 0.5, respectively), the unsupported Pd aerogel features a negligible formate- production capability at the same potentials, and the corresponding hydride phases only form gradually in the course of the potential holds and reach x-values of ~ 0.35 and x ~ 0.4, respectively. Moreover, the fact that for the C-supported Pd nanoparticles the higher pCD towards formate observed at -200 mV vs RHE corresponds to a hydride phase with a lower H-content compared that at -100 mV vs RHE indicates an indirect correlation between the formate- production rate and the nanoparticles H-content.In conclusion, these results provide valuable new insights into the important role of the time-dependent formation of PdHx on these materials’ CO2-to-formate selectivity. References Durst, J., et al., Electrochemical CO2 Reduction - A Critical View on Fundamentals, Materials and Applications. Chimia (Aarau), 2015. 69(12): p. 769-776.Diercks, J.S., et al., An Online Gas Chromatography Cell Setup for Accurate CO2-Electroreduction Product Quantification. Journal of The Electrochemical Society, 2021. 168(6).Min, X. and M.W. Kanan, Pd-catalyzed electrohydrogenation of carbon dioxide to formate: high mass activity at low overpotential and identification of the deactivation pathway. J Am Chem Soc, 2015. 137(14): p. 4701-8.Rahaman, M., A. Dutta, and P. Broekmann, Size-Dependent Activity of Palladium Nanoparticles: Efficient Conversion of CO2 into Formate at Low Overpotentials. ChemSusChem, 2017. 10(8): p. 1733-1741.Diercks, J.S., et al., Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions. ACS Catalysis, 2022: p. 10727-10741. Figure 1