Regulating C-C coupling in thermocatalytic and electrocatalytic CO x conversion based on surface science

Atomically Dispersed Metal Catalysts for the Conversion of CO2 into High-Value C2+ Chemicals

The conversion of carbon dioxide (CO2) into value-added chemicals with two or more carbons (C2+) is a promising strategy that cannot only mitigate anthropogenic CO2 emissions but also reduce the excessive dependence on fossil feedstocks. In recent years, atomically dispersed metal catalysts (ADCs), including single-atom catalysts (SACs), dual-atom catalysts (DACs), and single-cluster catalysts (SCCs), emerged as attractive candidates for CO2 fixation reactions due to their unique properties, such as the maximum utilization of active sites, tunable electronic structure, the efficient elucidation of catalytic mechanism, etc. This review provides an overview of significant progress in the synthesis and characterization of ADCs utilized in photocatalytic, electrocatalytic, and thermocatalytic conversion of CO2 toward high-value C2+ compounds. To provide insights for designing efficient ADCs toward the C2+ chemical synthesis originating from CO2, the key factors that influence the catalytic activity and selectivity are highlighted. Finally, the relevant challenges and opportunities are discussed to inspire new ideas for the generation of CO2-based C2+ products over ADCs.

Stabilizing Co2C with H2O and K promoter for CO2 hydrogenation to C2+ hydrocarbons

The decomposition of cobalt carbide (Co₂C) to metallic cobalt in CO₂ hydrogenation results in a notable drop in the selectivity of valued C₂₊ products, and the stabilization of Co₂C remains a grand challenge. Here, we report an in situ synthesized K-Co₂C catalyst, and the selectivity of C₂₊ hydrocarbons in CO₂ hydrogenation achieves 67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co₂C in the reaction, while the stabilization of Co₂C is dependent on the reaction atmosphere and the K promoter. During the carburization, the K promoter and H₂O jointly assist in the formation of surface C^* species via the carboxylate intermediate, while the adsorption of C^* on CoO is enhanced by the K promoter. The lifetime of the K-Co₂C is further prolonged from 35 hours to over 200 hours by co-feeding H₂O. This work provides a fundamental understanding toward the role of H₂O in Co₂C chemistry, as well as the potential of extending its application in other reactions.

Synergy between plasmonic and sites on gold nanoparticle-modified bismuth-rich bismuth oxybromide nanotubes for the efficient photocatalytic CC coupling synthesis of ethane

The photocatalytic reduction of carbon dioxide (CO₂) to fossil fuels has attracted widespread attention. However, obtaining the high value-added hydrocarbons, especially C₂₊ products, remains a considerable challenge. Herein, gold (Au) nanoparticle-modified bismuth-rich bismuth oxybromide Bi₁₂O₁₇Br₂ nanotube composites were designed and tested. Au nanoparticles act as electron traps and thermal electron donors that promote the efficient separation and migration of carriers to form the C₂₊ product. As a result, compared with the pure Bi₁₂O₁₇Br₂ nanotubes, Au@Bi₁₂O₁₇Br₂ composites can not only produce the carbon monoxide (CO) and methane (CH₄), but also covert CO₂ into ethane (C₂H₆). In this study, Au@Bi₁₂O₁₇Br₂-700 was used to obtain a C₂H₆ production rate of 29.26 μmol h^-1 g^-1. The selectivities during a 5-hour test reached 94.86% for hydrocarbons and 90.81% for C₂H₆. The proposed approach could be used to design high-performance photocatalysts to convert CO₂ into high value-added hydrocarbon products.

Stabilization of Exposed Metal Nanocrystals in High-Temperature Heterogeneous Catalysis

Colloidal metal nanocrystals with uniform sizes, shapes, compositions, and architectures are ideal building blocks for constructing heterogeneous catalysts with well-defined characteristics toward the investigation of accurate structure-property relationships and better understanding of catalytic mechanism. However, their applications in high-temperature heterogeneous catalysis are often restricted by the difficulty in maintaining the high metal dispersity and easy accessibility to active sites under harsh operating conditions. Here, a partial-oxide-coating strategy is proposed to stabilize metal nanocrystals against sintering and meanwhile enable an effective exposure of active sites. As a proof-of-concept, controlled partial silica coating of colloidally prepared Pd_0.82 Ni_0.18 nanocrystals with the size of 8 nm is demonstrated. This partially coated catalyst exhibits excellent activity, selectivity, and stability, outperforming its counterparts with fully coated and supported structures, in reverse water gas shift (RWGS) catalysis particularly at high operating temperatures. This study opens a new avenue for the exploration of colloidal metal nanocrystals in high-temperature heterogeneous catalysis.

Efficacious CO2 Adsorption and Activation on Ag Nanoparticles/CuO Mesoporous Nanosheets Heterostructure for CO2 Electroreduction to CO

It is an ongoing pursuit for researchers to precisely control the catalyst's surface for high-performance CO₂ electrochemical reduction (CO₂ER). In this work, CuO mesoporous nanosheets (CuO MNSs) with rough edges decorated by small Ag nanoparticles (Ag NPs) with a tunable amount of Ag were synthesized on a Cu foil at normal atmospheric temperature through two-step solution-phase reactions for CO₂ER to CO. In this special Ag NPs/CuO MNSs heterostructure, the mesoporous CuO NSs with rough edges favored gas infiltration, while decorated Ag NPs expanded the active sites for CO₂ molecule adsorption. Ag NPs endowed Ag NPs/CuO MNSs with good electrical conductivity and promoted the adsorbed CO₂ molecules to obtain electrons from the catalyst. Especially, the Ag-CuO interface stabilized the *COOH intermediate with strong bonding, which is important in boosting CO₂ER to CO. The optimal Ag_1.01%/CuO can catalyze CO₂ER to CO with a Faradaic efficiency of 91.2% and a partial current density of 10.5 mA cm^-2 at -0.7 V. Moreover, it exhibited prominent catalytic stability, retaining 97.8% of the initial current density and 97.6% of the original Faradaic efficiency for CO after 12 h of testing at -0.7 V. Notably, the Faradaic efficiency of CO on Ag_1.01%/CuO can retain over 80% in the potential area from -0.6 to -0.9 V, embodying its high selectivity for CO. This work develops precious metal/metal oxide heterostructures with a low precious metal loading for efficacious CO₂ER to CO and beyond.

Highlights and challenges in the selective reduction of carbon dioxide to methanol

Carbon dioxide (CO₂) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H⁺/6e^- reduction of CO₂ affords CH₃OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO₂ reduction to CH₃OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO₂ reduction to CH₃OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH₃OH synthesis.

Recent developments in catalyst pretreatment technologies for cobalt based Fisher–Tropsch synthesis

Stringent environmental regulations and energy insecurity necessitate the development of an integrated process to produce high-quality fuels from renewable resources and to reduce dependency on fossil fuels, in this case Fischer–Tropsch synthesis (FTS). The FT activity and selectivity are significantly influenced by the pretreatment of the catalyst. This article reviews traditional and developing processes for pretreatment of cobalt catalysts with reference to their application in FTS. The activation atmosphere, drying, calcination, reduction conditions and type of support are critical factors that govern the reducibility, dispersion and crystallite size of the active phase. Compared to traditional high temperature H2 activation, both hydrogenation–carbidisation–hydrogenation and reduction–oxidation–reduction pretreatment cycles result in improved metal dispersion and exhibit much higher FTS activity. Cobalt carbide (Co2C) formed by CO treatment has the potential to provide a simpler and more effective way of producing lower olefins, and higher alcohols directly from syngas. Syngas activation or direct synthesis of the metallic cobalt catalyst has the potential to remove the expensive H2 pretreatment procedure, and consequently simplify the pretreatment process, which would make it more economical and thus more attractive to industry.

Highly Selective Olefin Production from CO2 Hydrogenation on Iron Catalysts: A Subtle Synergy between Manganese and Sodium Additives

Mn and Na additives have been widely studied to improve the efficiency of CO₂ hydrogenation to valuable olefins on Fe catalysts, but their effects on the catalytic properties and mechanism are still under vigorous debate. This study shows that Fe-based catalysts with moderate Mn and Na contents are highly selective for CO₂ hydrogenation to olefins, together with low selectivities for both CO and CH₄ and much improved space-time olefin yields compared to state-of-the-art catalysts. Combined kinetic assessment and quasi in situ characterizations further unveil that the sole presence of Mn suppresses the activity of Fe catalysts because of the close contact between Fe and Mn, whereas the introduction of Na mediates the Fe-Mn interaction and provides strong basic sites. This subtle synergy between Na and Mn sheds light on the importance of the interplay of multiple additives that could bring an enabling strategy to improve catalytic activity and selectivity.

Electrocatalytic Deuteration of Halides with D2 O as the Deuterium Source over a Copper Nanowire Arrays Cathode

Precise deuterium incorporation with controllable deuterated sites is extremely desirable. Here, a facile and efficient electrocatalytic deuterodehalogenation of halides using D₂ O as the deuteration reagent and copper nanowire arrays (Cu NWAs) electrochemically formed in situ as the cathode was demonstrated. A cross-coupling of carbon and deuterium free radicals might be involved for this ipso-selective deuteration. This method exhibited excellent chemoselectivity and high compatibility with the easily reducible functional groups (C=C, C≡C, C=O, C=N, C≡N). The C-H to C-D transformations were achieved with high yields and deuterium ratios through a one-pot halogenation-deuterodehalogenation process. Efficient deuteration of less-active bromide substrates, specific deuterium incorporation into top-selling pharmaceuticals, and oxidant-free paired anodic synthesis of high-value chemicals with low energy input highlighted the potential practicality.

Photocatalytic CO2 conversion: What can we learn from conventional COx hydrogenation?

Solar-driven reduction of CO₂ into fuels/feedstocks is a promising strategy for addressing energy and CO₂ emission issues. Despite great research efforts, it still remains a grand challenge to achieve efficient and highly selective reduction of CO₂ owing to the large bond energy of CO₂ and the diversity of reduction products. In addition to the control of light harvesting and charge transfer like photocatalytic water splitting, the design of catalytically active sites is highly important to promote CO₂ reduction activity and selectivity (e.g., C-C coupling). In fact, we can learn a lot from conventional CO₂ hydrogenation and syngas conversion in terms of active site design. In this article, we demonstrate how to design catalytically active sites for efficient and highly selective photocatalytic reduction of CO₂ by sorting out the rules from the existing research on conventional CO_x hydrogenation, with a focus on enhancing C[double bond, length as m-dash]O activation and C-C coupling to form value-added products. This article aims to highlight the challenges in the field of photocatalytic CO₂ conversion and the connection of photocatalysis with conventional catalytic systems, providing the readers the opportunities to join the research.

Potential-tuned selective electrosynthesis of azoxy-, azo- and amino-aromatics over a CoP nanosheet cathode

Azoxy-, azo- and amino-aromatics are among the most widely used building blocks in materials science pharmaceuticals and synthetic chemistry, but their controllable and green synthesis has not yet been well established. Herein, a facile potential-tuned electrosynthesis of azoxy-, azo- and amino-aromatics via aqueous selective reduction of nitroarene feedstocks over a CoP nanosheet cathode is developed. A series of azoxy-, azo- and amino-compounds with excellent selectivity, good functional group tolerance and high yields are produced by applying different bias input. The synthetically significant and challenging asymmetric azoxy-aromatics can be controllably synthesized in moderate to good yields. The use of water as the hydrogen source makes this strategy remarkably fascinating and promising. In addition, deuterated aromatic amines with a high deuterium content can be readily obtained by using D2O. By pairing with anodic oxidation of aliphatic amines to nitriles, synthetically useful building blocks can be simultaneously produced in a CoP||Ni2P two-electrode electrolyzer. Only 1.25 V is required to achieve a current density of 20 mA cm-2, which is much lower than that of overall water splitting (1.70 V). The paired oxidation and reduction reactions can also be driven using a 1.5 V battery to synthesize nitrile and azoxybenzene with good yields and selectivity, further emphasizing the flexibility and controllability of our method. This work paves the way for a promising approach to the green synthesis of valuable chemicals through potential-controlled electrosynthesis.