Electrochemical Conversion of CO₂ to Syngas with Palladium-Based Electrocatalysts

ConspectusElectrocatalytic reduction of CO₂ is a desirable method to produce valuable products from CO₂. One of the main research challenges for electrocatalysis that produces carbon-containing products from CO₂ is avoiding unwanted hydrogen production. Instead of totally eliminating hydrogen, our approach makes use of the readily available protons in aqueous electrolyte to coproduce CO and H₂, making synthesis gas (syngas) with a tunable CO:H₂ ratio; the resulting syngas can then be used as feedstock for existing thermocatalytic processes such as Fischer-Tropsch and methanol synthesis reactions. We discovered that palladium hydride (PdH), formed under electrocatalytic reaction conditions, is an effective electrocatalyst that enables this unique product distribution. We employed in situ synchrotron techniques to determine the formation of the PdH phase during electrocatalytic reduction of CO₂. We also performed density functional theory (DFT) calculations to determine the binding energies of key intermediates on PdH to identify descriptors to correlate experimentally observed trends in activity and selectivity.Since first reporting on the potential application of PdH to produce syngas, our research group has refined control over the syngas product selectivity, improved the activity, and reduced the loading of Pd in electrocatalysts. We achieved this by the following approaches: understanding the structure-function relationship with shape-controlled Pd nanoparticles, determining the cation and isotopic effects of electrolyte, alloying Pd with inexpensive secondary metals, supporting Pd on transition metal carbides and nitrides, and utilizing single atom Pd catalysts. At each step, we monitored the phase transition from Pd to PdH under reaction conditions with in situ synchrotron-based X-ray absorption and X-ray diffraction techniques by identifying the onset potential for the appearance of the characteristic Pd-Pd bond length and diffraction patterns associated with PdH formation. We also identified descriptors for syngas production on PdH, bimetallic PdH, and supported PdH catalysts by correlating DFT calculations of PdH stability in different catalytic systems as well as the effect of PdH formation on the binding strength of reaction intermediates. The research methodology established here is useful not only for continued optimization of Pd-based syngas-producing electrocatalysts but also for enhancing activity while reducing the loading of precious metals for other electrocatalytic applications. Moreover, we feel the advances in electrocatalytic syngas production described here represent a critical step toward sustainable CO₂ utilization that should inspire continued efforts.