Development of Nanostructured Hybrid Materials for Electrocatalytic and Photoelectrocatalytic 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 various products that include carbon monoxide, oxalate, formate, carboxylic acids, formaldehyde, acetone or methanol, in addition to various hydrocarbons at different ratios. Given the fact that the CO₂ molecule is very stable, its electroreduction processes are characterized by large overpotentials. To produce highly efficient and selective electrocatalysts, the transition-metal-based molecular materials are often considered. It is believed that, during electroreduction, the rate limiting step is the protonation of the adsorbed CO product to form the CHO adsorbate.

            Because reduction of CO₂ can effectively occur by hydrogenation, to optimize the conventional electrocatalytic approach, we propose 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. Under such conditions, the two-electron reduction of CO₂ typically to CO is favored. To produce highly dispersed and reactive nanoparticles, we generate them by electrodeposition from N-coordination complexes of palladium(II). The resulting metallic Pd nanoparticles, rather than Pd cationic species, are stabilized and activated by nitrogen coordination centers from the macromolecular matrix. In the present research, we demonstrate that stabilization and activation of highly dispersed nanostructured metallic Pd centers with a supramolecular architecture of “privileged ligands” results in the electrocatalytic enhancement of the CO₂ reduction. Reduction of carbon dioxide begins now at less negative potentials and is accompanied by significant enhancement of the CO₂-reduction current densities. Among important issues are specific interactions between nitrogen coordinating centers and metallic palladium sites at the electrocatalytic interface.

Another possibility to enhance electroreduction of carbon dioxide is to explore direct transformation of solar energy to chemical energy using transition metal oxide semiconductor materials. We show here that, by intentional and controlled combination of metal oxide semiconductors, we have been able to drive effectively photoelectrochemical reduction of carbon dioxide. The combination  of titanium (IV) oxide (TiO₂) and copper (I) oxide (Cu₂O) has been explored toward the reduction of carbon (IV) oxide (CO₂) before and after sunlight illumination. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from photoelectrochemical reduction of carbon dioxide mostly to methanol (CH₃OH), as demonstrated upon identification of final products using conventional and mass-spectrometry assisted gas chromatography. On mechanistic grounds, the role of TiO₂ seems to be not only stabilizing: the oxide is also expected to prevent the recombination of charge carriers.