Electrochemical CO 2 reduction (ECR) to value-added fuels and chemicals provides a ''clean'' and efficient way to mitigate energy shortages and to lower the global carbon footprint. The unique structures of two-dimensional (2D) nanosheets and their tunable electronic properties make these nanostructured materials intriguing in catalysis. Various 2D nanosheets are showing promise for CO 2 reduction, depending on the preferred reaction product (HCOOH, CO, CH 4 , CH 3 OH, or CH 3 COOH). In this review, we focus on recent progress that has been achieved in using these 2D materials for ECR. We highlight procedures available for tuning catalytic activities of 2D materials and describe the fundamentals and future challenges of CO 2 catalysis by 2D nanosheets.
Nitrogen fixation under ambient conditions remains a significant challenge. Here, we report nitrogen fixation by Ru single-atom electrocatalytic reduction at room temperature and pressure. In contrast to Ru nanoparticles, single Ru sites supported on N-doped porous carbon greatly promoted electroreduction of aqueous N 2 selectively to NH 3 , affording an NH 3 formation rate of 3.665 mg NH 3 h À1 mg À1Ru at À0.21 V versus the reversible hydrogen electrode. Importantly, the addition of ZrO 2 was found to significantly suppress the competitive hydrogen evolution reaction. An NH 3 faradic efficiency of about 21% was achieved at a low overpotential (0.17 V), surpassing many other reported catalysts. Experiments combined with density functional theory calculations showed that the Ru sites with oxygen vacancies were major active centers that permitted stabilization of *NNH, destabilization of *H, and enhanced N 2 adsorption. We envision that optimization of ZrO 2 loading could further facilitate electroreduction of N 2 at both high NH 3 synthesis rate and faradic efficiency.
Conspectus Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO2 reduction reaction (CO2RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO2. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO2 is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO2 reduction. Given the reaction barriers associated with multiple electron–proton reduction of CO2 to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O–), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduction occurs. Useful catalysts in this area are typically transition-metal complexes with organic ligands and electron transfer properties that utilize combinations of metal and ligand redox levels. As a way to limit the amount of catalyst, in device-like configurations, catalysts are added to the surfaces of conductive substrates by surface binding, in polymeric films, or on carbon electrode surfaces with molecular structures and electronic configurations related to catalysts in solution. Immobilized, homogeneous catalysts can suffer from performance losses and even decomposition during long-term CO2 reduction cycles, but they are amenable to detailed mechanistic investigations. In parallel efforts, heterogeneous nanocatalysts have been explored in detail with the development of facile synthetic procedures that can offer highly active catalytic surface areas. Their high activity and stability have attracted a significant level of investigation, including possible exploitation for large-scale applications. However, translation of catalytic reactivity to the surface creates a new reactivity environment and complicates the elucidation of mechanistic details and identification of the active site in exploring reaction pathways. Here, the results of previous studies based on transition-metal complex catalysts for CO2 electroreduction are summarized. Early studies showed that transition-metal complexes of Ru, Ir, Rh, and Os, with well-defined structures, are all capable of catalyzing...
within acceptable limits are needed. Among the many possible solutions, electrochemical CO 2 reduction (ECR) offers a potentially sustainable approach not only for depressing CO 2 concentration but also converting it into fuels and commodity chemicals. [2] Unfortunately, the CO chemical bond in CO 2 (≈806 kJ mol −1 ) is thermodynamically very stable and its conversion is an uphill energy process with a high activation barrier. Moreover, during electrochemical reduction of CO 2 , the hydrogen evolution reaction (HER) inevitably occurs as a competing reaction, which is a major stumbling block for CO 2 reduction especially in aqueous electrolytes. [3] From these scenarios, robust catalysts that can selectively reduce CO 2 in lieu of protons at high turnover frequency (TOF) and faradaic efficiency (FE) for CO 2 reduction are desired.Since Hori's pioneering study on electroreduction of CO 2 in the 1980s, [4] Cu, [5] Au, [6] Ag, [7] Zn, [8] Sn, [9] and Bi [10] among others, have been widely investigated for electrocatalysis of CO 2 reduction, due to their promising capability to convert CO 2 into valuable chemicals and fuels while the HER is largely suppressed. Earth-abundant first-row transition metals such as Fe, Co, and Ni, however, are highly active for HER and also easily Electrochemical reduction of carbon dioxide (CO 2 ) to fuels and value-added industrial chemicals is a promising strategy for keeping a healthy balance between energy supply and net carbon emissions. Here, the facile transformation of residual Ni particle catalysts in carbon nanotubes into thermally stable single Ni atoms with a possible NiN 3 moiety is reported, surrounded with a porous N-doped carbon sheath through a one-step nanoconfined pyrolysis strategy. These structural changes are confirmed by X-ray absorption fine structure analysis and density functional theory (DFT) calculations. The dispersed Ni single atoms facilitate highly efficient electrocatalytic CO 2 reduction at low overpotentials to yield CO, providing a CO faradaic efficiency exceeding 90%, turnover frequency approaching 12 000 h −1 , and metal mass activity reaching about 10 600 mA mg −1 , outperforming current state-of-the-art single atom catalysts for CO 2 reduction to CO. DFT calculations suggest that the Ni@N 3 (pyrrolic) site favors *COOH formation with lower free energy than Ni@N 4 , in addition to exothermic CO desorption, hence enhancing electrocatalytic CO 2 conversion. This finding provides a simple, scalable, and promising route for the preparation of low-cost, abundant, and highly active single atom catalysts, benefiting future practical CO 2 electrolysis.
A comparative study of experimental and theoretical combinatorial and high-throughput screening methods for the development of novel materials is presented. Both methods were applied to the development of new anode fuel cell alloy catalysts with improved CO tolerance. Combinatorial experimental electrocatalysis was performed on a 64-element electrode array. Sputter-deposited ternary thin-film electrocatalysts of composition PtRuM (M = Co,Ni,W) were screened in parallel for their methanol oxidation activity, and their individual geometric and specific chronoamperometric current density were monitored and evaluated against standard PtRu catalysts. Density functional theory calculations of a variety of model ternary PtRuM alloy catalysts yielded detailed adsorption energies and activation barriers. Feeding these thermodynamic and kinetic data into a simple micro kinetic model for the CO electro oxidation reaction, the relative activities of a number of PtRuM ternary alloys were calculated. The experimental and theoretical computational results reveal very similar trends in electrocatalytic activity as a function of alloy composition; they also point at similar ternary PtRuM alloys as candidates for improved anode catalysts for low-temperature fuel cells.
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