Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

The role of oxygen-permeable ionomer for polymer electrolyte fuel cells

In recent years, considerable research and development efforts are devoted to improving the performance of polymer electrolyte fuel cells. However, the power density and catalytic activities of these energy conversion devices are still far from being satisfactory for large-scale operation. Here we report performance enhancement via incorporation, in the cathode catalyst layers, of a ring-structured backbone matrix into ionomers. Electrochemical characterizations of single cells and microelectrodes reveal that high power density is obtained using an ionomer with high oxygen solubility. The high solubility allows oxygen to permeate the ionomer/catalyst interface and react with protons and electrons on the catalyst surfaces. Furthermore, characterizations of single cells and single-crystal surfaces reveal that the oxygen reduction reaction activity is enhanced owing to the mitigation of catalyst poisoning by sulfonate anion groups. Molecular dynamics simulations indicate that both the high permeation and poisoning mitigation are due to the suppression of densely layered folding of polymer backbones near the catalyst surfaces by the incorporated ring-structured matrix. These experimental and theoretical observations demonstrate that ionomer’s tailored molecular design promotes local oxygen transport and catalytic reactions.

Modeling phase formation on catalyst surfaces: Coke formation and suppression in hydrocarbon environments

We develop a simulation toolset employing density functional theory (DFT) in conjunction with grand canonical Monte Carlo (GCMC) to study coke formation on Fe-based catalysts during propane dehydrogenation (PDH). As expected, pure Fe surfaces develop stable graphitic coke structures and rapidly deactivate. We find that coke formation is markedly less favorable on FeC and FeS surfaces. Fe-Al alloys display varying degrees of coke resistance, depending on their composition, suggesting that they can be optimized for coke resistance under PDH conditions. Electronic structure analyses show that both electron-withdrawing effects (on FeC and FeS) and electron-donating effects (on Fe-Al alloys) destabilize adsorbed carbon. On the alloy surfaces, a geometric effect also isolates Fe sites and disrupts the formation of graphitic carbon networks. This work demonstrates the utility of GCMC for studying the formation of disordered phases on catalyst surfaces and provides insights for improving the coke resistance of Fe-based PDH catalysts.

Understanding Potential-dependent Competition Between Electrocatalytic Dinitrogen and Proton Reduction Reactions

One of the key challenges to practical electrochemical N2 reduction reaction (NRR) is to lower the overpotential and suppression of the side reaction known as the hydrogen evolution reaction (HER) during the NRR. The experimental NRR activity has been consistently shown to reach a maximum at early stage before reaching the mass-transfer limit and decreases with large overpotentials for many heterogeneous catalysts. Though the volcano-type current-potential relationship shown for NRR is unusual with limited reaction rates at higher overpotentials, the mechanistic origin has not been clearly explained, making the design principles for practical NRR still lacking. Herein, we investigate the potential-dependent reaction activity of NRR and HER using the constant electrode potential method and microkinetic modeling. It manifests the dominating proton adsorption over dinitrogen at small overpotentials leading to the inadequate reaction selectivity towards NRR at many metal catalyst surfaces. A clear potential-dependent competition between the N2 adsorption and *H formation is characterized by the degree of charge transfer in the adsorption process. It is also demonstrated that the larger charge transfer in *H formation is a general phenomenon applied to all heterogeneous catalyst surfaces considered here, that poses a fundamental challenge to realize practical electrochemical NRR. We suggest several strategies to overcome the latter challenges based on the present understandings.