Mechanistic insights into C2 and C3 product generation using Ni3Al and Ni3Ga electrocatalysts for CO2 reduction

Using Light and Electrons to Bend Carbon Dioxide: Developing and Understanding Catalysts for CO2 Conversion to Fuels and Feedstocks

Our global society generates an unwieldy amount of CO₂ per unit time. Therefore, the capture of this greenhouse gas must involve a diverse set of strategies. One solution to this problem is the conversion of CO₂ into a more useful chemical species. Again, a multiplicity of syntheses and products will be necessary. No matter how elegant the chemistry is, commercial markets often have little use for a small set of compounds made in tremendous yield. Following this reasoning, the Bocarsly Research Group seeks to develop new electrochemical and photochemical processes that may be of utility in the conversion of CO₂ to organic compounds. We focus on investigating proton-coupled charge transfer mechanisms that produce both C1 and carbon-carbon bonded products (C2+).In early work, we considered the reduction of CO₂ to formate at electrocatalytic indium and tin electrodes. These studies demonstrated the key role of surface oxides in catalyzing the reduction of CO₂. This work generated efficient systems for the formation of formate and paved the way to studies using non-copper, intermetallic electrocatalysts for the generation of C2+ species. Most notable is the efficient formation of oxalate at an oxidized Cr₃Ga electrode. Oxalate has recently been suggested as a potential nonfossil, alternate organic feedstock.Separately, we have focused on the electrocatalytic effects of pyridine on the reduction of CO₂ in aqueous electrolyte. These studies demonstrated that electrodes that normally yield a low hydrogen overpotential (Pd and Pt) show suppressed H₂ evolution and strongly enhanced activity for CO₂ reduction in the presence of pyridinium. Methanol was observed to form in high Faradaic yield at low overpotential using this system. The 6-electron, 6-proton reduction of CO₂ in the presence of pyridinium was intriguing, and significant effort was placed on understanding the mechanism of this reaction both on metal electrodes and on semiconducting photocathodes. P-GaP electrodes were found to provide exceptional behavior for the formation of methanol using only light as the energy source.The pyridinium studies highlighted the role of protons in the overall reduction of CO₂, stimulating our interest in the chemistry of MnBr(bpy)(CO)₃ and related compounds. This complex was reported to electrochemically reduce CO₂ to CO. We saw these reports as an opportunity to study the detailed nature of the proton-coupled electron transfer (PCET) mechanism associated with CO₂ reduction. Our investigation of this system revealed the role of hydrogen-bonding in CO₂ reduction and pointed the way for the construction of a photochemical process for CO generation using a [(bpy)(CO)₃Mn(CN)Mn(bpy)(CO)₃]⁺ photocatalyst.Based on our studies to date, it appears likely that heterogeneous systems can be assembled to convert CO₂ into products that are "beyond C2 products." This may open up new practical chemistry in the area of fossil-based replacements for both synthesis and fuels. Systems with pragmatic efficiencies are close to reality. Electrochemical reactors using heterogeneous electrocatalysts show the stability and product selectivity needed to generate industrial opportunities. Continued growth of mechanistic understanding is expected to facilitate the chemical design of cogent systems for the taming of CO₂.

Recent Development of Electrocatalytic CO2 Reduction Application to Energy Conversion

Carbon dioxide (CO₂ ) emission has caused greenhouse gas pollution worldwide. Hence, strengthening CO₂ recycling is necessary. CO₂ electroreduction reaction (CRR) is recognized as a promising approach to utilize waste CO₂ . Electrocatalysts in the CRR process play a critical role in determining the selectivity and activity of CRR. Different types of electrocatalysts are introduced in this review: noble metals and their derived compounds, transition metals and their derived compounds, organic polymer, and carbon-based materials, as well as their major products, Faradaic efficiency, current density, and onset potential. Furthermore, this paper overviews the recent progress of the following two major applications of CRR according to the different energy conversion methods: electricity generation and formation of valuable carbonaceous products. Considering electricity generation devices, the electrochemical properties of metal-CO₂ batteries, including Li-CO₂ , Na-CO₂ , Al-CO₂ , and Zn-CO₂ batteries, are mainly summarized. Finally, different pathways of CO₂ electroreduction to carbon-based fuels is presented, and their reaction mechanisms are illustrated. This review provides a clear and innovative insight into the entire reaction process of CRR, guiding the new electrocatalysts design, state-of-the-art analysis technique application, and reaction system innovation.

A rich catalog of C-C bonded species formed in CO2 reduction on a plasmonic photocatalyst

The understanding and rational design of heterogeneous catalysts for complex reactions, such as CO₂ reduction, requires knowledge of elementary steps and chemical species prevalent on the catalyst surface under operating conditions. Using in situ nanoscale surface-enhanced Raman scattering, we probe the surface of a Ag nanoparticle during plasmon-excitation-driven CO₂ reduction in water. Enabled by the high spatiotemporal resolution and surface sensitivity of our method, we detect a rich array of C₁-C₄ species formed on the photocatalytically active surface. The abundance of multi-carbon compounds, such as butanol, suggests the favorability of kinetically challenging C-C coupling on the photoexcited Ag surface. Another advance of this work is the use of isotope labeling in nanoscale probing, which allows confirmation that detected species are the intermediates and products of the catalytic reaction rather than spurious contaminants. The surface chemical knowledge made accessible by our approach will inform the modeling and engineering of catalysts.

Efficacious CO2 Photoconversion to C2 and C3 Hydrocarbons on Upright SnS-SnS2 Heterojunction Nanosheet Frameworks

In this work, SnS-SnS₂ heterostructured upright nanosheet frameworks are constructed on FTO substrates, which demonstrate promising photocatalytic performances for the conversion of CO₂ and water to C2 (acetaldehyde) and C3 (acetone) hydrocarbons without H₂ formation. With post annealing in designated atmospheres, the photocatalytic activity of the SnS-SnS₂ heterostructured nanosheet framework is critically enhanced by increasing the fraction of crystalline SnS in nanosheets through partial transformation of the SnS₂ matrix to SnS but not obviously influenced by improving the crystallinity of the SnS₂ matrix. DFT calculations indicate that transformed SnS possesses the CO₂ adsorption sites with significantly lower activation energy for the rate-determining step to drive efficient CO₂ conversion catalysis. The experimental results and DFT calculations suggest that the SnS-SnS₂ heterojunction nanosheet framework photocatalyst experiences Z-scheme charge transfer dynamic to allow the water oxidation and CO₂ reduction reactions occurring on the surfaces of SnS₂ and SnS, respectively. The Z-scheme SnS-SnS₂ heterostructured nanosheet framework photocatalyst exhibits not only efficient charge separation but also highly catalytic active sites to boost the photocatalytic activity for CO₂ conversion to C2 and C3 hydrocarbons.

Selective CO2 reduction towards a single upgraded product: a minireview on multi-elemental copper-free electrocatalysts

Electrocatalytic conversion of the greenhouse gas carbon dioxide to liquid fuels or upgraded chemicals is a critical strategy to mitigate anthropogenic climate change. To this end, we urgently need high-performance CO₂ reduction catalysts.

Au/Pb Interface Allows the Methane Formation Pathway in Carbon Dioxide Electroreduction

The electrochemical conversion of carbon dioxide (CO₂) to high-value chemicals is an attractive approach to create an artificial carbon cycle. Tuning the activity and product selectivity while maintaining long-term stability, however, remains a significant challenge. Here, we study a series of Au–Pb bimetallic electrocatalysts with different Au/Pb interfaces, generating carbon monoxide (CO), formic acid (HCOOH), and methane (CH₄) as CO₂ reduction products. The formation of CH₄ is significant because it has only been observed on very few Cu-free electrodes. The maximum CH₄ formation rate of 0.33 mA cm^–2 was achieved when the most Au/Pb interfaces were present. In situ Raman spectroelectrochemical studies confirmed the stability of the Pb native substoichiometric oxide under the reduction conditions on the Au–Pb catalyst, which seems to be a major contributor to CH₄ formation. Density functional theory simulations showed that without Au, the reaction would get stuck on the COOH intermediate, and without O, the reaction would not evolve further than the CHOH intermediate. In addition, they confirmed that the Au/Pb bimetallic interface (together with the subsurface oxygen in the model) possesses a moderate binding strength for the key intermediates, which is indeed necessary for the CH₄ pathway. Overall, this study demonstrates how bimetallic nanoparticles can be employed to overcome scaling relations in the CO₂ reduction reaction.