Application of Coordinatively Supported and Activated Palladium Nanoparticles to Enhance Electrocatalytic Reduction of Carbon Dioxide

There has been growing interest in the electrochemical reduction of carbon dioxide (CO₂), a potent greenhouse gas and a contributor to global climate change, and its conversion into useful carbon-based fuels or chemicals. Numerous homogeneous and heterogeneous catalytic systems have been proposed to induce the CO₂ reduction and, depending on the reaction conditions (applied potential, choice of buffer, its strength and pH, local CO₂ concentration or the catalyst used), various products that include carbon monoxide, oxalate, formate, carboxylic acids, formaldehyde, acetone or methanol, as well as such hydrocarbons as methane, ethane, and ethylene, are typically observed at different ratios. These reaction products are of potential importance to energy technology, food research, medical applications and fabrication of plastic materials.

            Given the fact that the CO₂ molecule is very stable, its electroreduction processes are characterized by large overpotentials, and they are not energy efficient. To produce highly efficient and selective electrocatalysts, the transition-metal-based molecular materials are often considered. Because reduction of CO₂ can effectively occur by hydrogenation, in the present work, we concentrate on such a model catalytic system as nanostructured metallic palladium capable of absorbing reactive hydrogen in addition to the ability to adsorb monoatomic hydrogen at the interface. We are going to demonstrate that palladium nanocenters can be generated within the coordination architecture of tridentate Schiff-base-ligands by electrodeposition from the supramolecular complex of palladium(II), [Pd(C₁₄H₁₂N₂O₃)Cl₂]₂∙MeOH. The resulting Pd nanoparticles (diameters, 5-10 nm) are stabilized and activated by nitrogen coordination sites, and the electrocatalytic system exhibits appreciable activity toward reduction carbon oxide (IV) in 0.1 mol dm^-3 KHCO₃.  The respective voltammetric peak currents are ca. three times larger than those observed at conventional palladium nanoparticles (diameter, ca. 10-20 nm) under analogous experimental conditions and at the same loading of palladium (100 μg cm^-2).  Despite that fact that the degree of agglomeration of nanostructured palladium is much lower when it has been generated within the macromolecular network, it is reasonable to expect that some specific interactions between nitrogen coordination centers and metallic Pd exist. The process of electrosorption of hydrogen at the Schiff-base-ligand supported palladium nanostructures seems to be more reversible (when investigated in KHCO₃) and dominated by the hydrogen absorption rather than the surface adsorption phenomena (characteristic of conventional Pd nanoparticles).

            We are also going to address the possibility of combining copper oxide nanostructures together with coordinatively supported Pd nanocenters to drive photoelectrocatalytic reduction of carbon dioxide.

Acknowledgement

            We appreciate collaboration with Adam Gorczynski, Maciej Kubicki, and Violetta Patroniak from Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland that has led to fabrication of functionalized palladium supramolecular coordination compound.