CO2 Conversion Processes Research Articles

Consecutive Reduction of Five Carbon Dioxide Molecules by Gas-Phase Niobium Carbide Cluster Anions Nb3C4-: Unusual Mechanism for Enhanced Reactivity by the Carbon Ligands.

Studying the cleavage of the C═O bond during CO2 activation at room temperature is highly significant for comprehending the CO2 conversion processes. Herein, mass spectrometry experiments and density functional theory calculations indicate that the niobium carbide anions Nb3C4- can continuously convert five CO2 molecules to CO under thermal collision conditions, while the other clusters with less carbon ligands Nb3C1-3- reduce fewer CO2 molecules. Size-dependent reactivity of Nb3C1-4- cluster anions toward CO2 is observed. Interestingly, the carbon atoms in Nb3C4- not only act as highly active adsorption sites for CO2 but also serve as electron donors to reduce CO2. The stored electrons are released through a carbon-carbon coupling process. Our findings on the role of carbon ligands in enhancing transition metal carbide reactivity can offer new insights for designing active sites on catalysts with both high activity and selectivity.

Experimental and first-principles insights into an enhanced performance of Ru-doped copper phosphate electrocatalyst during oxygen evolution reaction

The oxygen evolution reaction (OER) is a vital half-reaction in many applications, such as the electrochemical H2O splitting, CO2, and N2 conversion processes. The OER involves a four-electron transfer and is a kinetically sluggish reaction that requires additional potential to drive. To enhance the electrochemical performance of the above-mentioned applications, highly efficient, corrosion-resistant, earth-abundant, and eco-friendly electrocatalysts are required. Here, we report a highly porous, minimally Ru-doped copper phosphate electrocatalyst obtained through co-precipitation. The optimized electrocatalyst (5% Ru-doped copper phosphate) exhibits a low overpotential of 340 mV to achieve 10 mA cm−2 compared to copper-based materials, and it remains stable over 20 h. The high performance is attributed to a high electrochemically effective surface area (ECSA) of 30.25 cm2, facilitating effective ion transportation at the electrode/electrolyte interface and excellent electrical conductivity. This result is supported by density functional theory calculations, which demonstrate that ruthenium enhances the electrochemical properties by increasing electronic conductivity, reducing the theoretical overpotential, and influencing the rate-determining step of the oxygen evolution reaction. Herein, the electrocatalyst is attractive for commercialization due to its utilization of minimal ruthenium in earth-abundant electrocatalysts, which offer competitive performance.

Advancing CO2 to CO conversion: Detailed impact analysis of atmospheric molecules in an ultrashort pulse laser filament plasma reactor

AbstractCapturing carbon dioxide (CO2) and transforming it into valuable fuels offers a dual benefit: the potential to reduce atmospheric CO2 levels and decrease our dependency on fossil fuels. Plasma‐assisted CO2 to carbon monoxide (CO) conversion stands out within the various CO2 recycling methods, with research primarily emphasizing its energy efficiency and conversion efficacy. However, investigations of CO2 conversion under real air conditions are relatively scarce, and the effects of the other atmospheric molecules on the CO2 conversion process require further exploration. We have induced plasma chemical reactions by generating ultrashort pulse laser filaments inside a sealed‐off cavity with variable air pressures. Simultaneously, we applied mid‐infrared laser absorption spectroscopy to monitor the time evolution of the reaction products. The peak CO2‐to‐CO conversion ratio was achieved at an air pressure of 8000 Pa, which resulted in a CO concentration of 82 ppm. The experimental results suggested that nitrogen (N2) plays a promoting role in CO2‐to‐CO conversion, while the presence of oxygen (O2) seems to hinder the process. The hydroxyl radical (OH) arising from water molecules (H2O) limits the accumulation of CO at lower air pressures. However, at higher air pressures, the reduced OH radical concentration shows negligible impact on the CO2‐to‐CO conversion ratios. In addition, the study revealed that atmospheric plasmas produce a high concentration of hydrogen cyanide (HCN), which is directly proportional to the levels of ambient humidity. This research contributes to the development of strategies to mitigate the production of harmful gases like HCN in CO2 conversion, thereby promoting the ecofriendly conversion of atmospheric CO2.

Advanced process integration and machine learning-based optimization to enhance techno-economic-environmental performance of CO2 capture and conversion to methanol

Developing economically sustainable CO2 capture and conversion processes is essential to realize carbon neutrality. This study proposed an integrated process for CO2 capture and conversion-to-methanol (CCTM) and applied machine learning-based optimization to enhance techno-economic-environmental performance. After validating CO2 capture and CO2-to-methanol sections, an advanced CCTM design was developed and compared with conventional one regarding techno-economic-environmental performance across various operating scenarios. The advanced CCTM exhibited significant improvements in energy consumption (14.73–16.30%), production cost (0.81–1.28%), and net CO2 reduction (3.13–3.38%) owing to efficiently reusing waste heat, off-gas, and water resources. The one-at-a-time sensitivity analysis revealed roles of each variable and nonlinear variable-performance tendencies among operating variables in the advanced CCTM process. Subsequently, a well-developed deep neural network (DNN) model precisely formulated the relationship between key variables and performances. The DNN-based optimization provided optimum operating conditions within a minute, resulting in an 8.21 $/tMeOH (∼0.81%) reduction in production cost compared to base case of CCTM. Notably, the total CO2 capture rate of 92.53% at an optimal condition highlighted the significant contribution of advanced CCTM to carbon neutrality. The findings provide a viable reference for the effective and sustainable design and operation of an integrated CCTM process.

Two-stage conversion of CO2 to methanol and dimethyl ether using CuO–ZnO–Al2O3/protonated Y-type zeolite catalysts

The CuO-ZnO-Al2O3/protonated Y-type zeolite (CZA/HYZ) catalysts were prepared via co-precipitation method and employed in a two-stage CO2 conversion process using a dual fixed–bed column at different temperatures and 50 bar. Catalytic components of CZA/HYZ were copper species that have been identified with XPS. Remarkably, the fine structures of CZA/HYZ catalysts metal atoms were confirmed with XANES and EXAFS spectra. Molecular configurations of catalytic species in CZA/HYZ catalyst simulated from the XANES/EXAFS spectra-analyzed fine structural parameters were schematically displayed. The optimal catalytic performances of CZA/HYZ (CH3OH conversion=78.0%, CH3OCH3/HCOOH selectivity=91.7%/8.3%, CH3OCH3/HCOOH yield=71.5%/6.5%) were achieved at 250 ºC. The Arrhenius equation and a pseudo-first-order/second-order model were used to evaluate the activation energies and rate constants of CH3OH and CH3OCH3 formations at various catalytic temperatures. The low activation energies (2.720 and 1.160 kJ mol−1) and Gibbs energies (3.26 and −40.00 kJ mol−1) of CH3OH and CH3OCH3 formations at 250 ºC, demonstrated that their spontaneities were remarkably improved via CZA/HYZ, respectively. The total income from a 10–ton per day (10–TPD) for a petrochemical refinery plant waste gas utility process was USD $43,557d−1, as well a payback of 3.23 years, based on cost evaluation. The proposed process provides a continuous two-step process for manufacturing high–value-added chemicals using industrial CO2 emitted and syngas.

SO2-Tolerant Electrocatalytic Reduction of CO2 from Simulated Industrial Flue Gas

The electrochemical reduction of CO2 using copper-based electrocatalysts offers a route to produce high-value multicarbon (C2+) products from renewable electricity (Nat. Catal. 4, 952-958 (2021); Nature 614, 262-269 (2023)). To date, the efficient electrocatalytic conversion of CO2 to multicarbon products has only been possible when using impurity-free CO2 sources, such as from direct air capture. The generation of such high-grade CO2 streams is expensive, accounting for almost half of the total energy required for both capture and electroreduction processes (Nat. Catal. 4, 952-958 (2021)). Conversely, capturing CO2 from point sources, such as industrial flue gas, is more efficient due to the higher concentration of CO2 in the feed. However, trace amounts of sulfur dioxide (10 ~ 400 ppm SO2) inherently present in these streams will significantly degrade the CO2 conversion process. All previous attempts to convert CO2 with SO2 present in the feed have resulted in immediate catalyst poisoning and an irreversible loss of CO2 conversion activity.In this study, we designed a modified catalyst layer to react a stream of dilute CO2 containing 400 ppm SO2 to multicarbon products with high stability and performance metrics that match or exceed those achieved with pure CO2 streams. Driven by density function theory and COMSOL simulations, we designed an ionomer:copper:polytetrafluoroethylene (PTFE) (ICP) electrode that features both hydrophobic and highly-charged hydrophilic domains to limit water adsorption and promote CO2 over SO2 transport near the electrochemically active sites. This deactivates the SO2 poisoning mechanism, thus enabling stable and efficient CO2 conversion (see figure). Our approach achieved a sustained C2+ Faradaic efficiency (FE) of 50% for the initial 160 hours at 100 mA cm-2.In order to improve the overall C2+ current efficiency (jC2+) towards industrial scales, we applied our strategy in high-surface-area copper electrodes. We achieved CO2 conversion in the presence of 400 ppm SO2 with a C2+ FE of 76% at a current density of 700 mA cm-2, surpassing what can be achieved in existing integrated CO2 capture-electrolysis systems that use pure CO2. Overall, our approach provides a fully 140-fold increase in performance (FEC2+ × jC2+) compared to the best prior CO2 conversion systems with added SO2 (Nat. Nanotechnol. doi: 10.1038/s41565-022-01286-y (2023); J. Am. Chem. Soc. 141, 9902-9909 (2019)). These findings represent an important advancement in the field of CO2 conversion and highlight the potential of our strategy for industrial-scale applications. Figure 1

(Invited) Stabilization and Activation of Copper(I)-Oxide-Semiconducting Interfaces for Photoelectrochemical Reduction of Carbon Dioxide

Electroreduction of carbon dioxide to simple organic fuels and chemicals is a topic of growing scientific and technological interest. The reaction provides means for both reducing emissions of CO2 into atmosphere and storing renewable energy. The presentation will address low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. Among important issues are choice of the catalytic or semiconducting materials, their morphology and operating conditions including temperature, solvent, electrolyte, pH etc. There is a need to improve the reaction dynamics and selectivity toward specific products. In practical electrolysis cells, the CO2-reduction (at cathode) is accompanied by water oxidation (at anode or photoanode).Recently, we have concentrated on the development of hybrid materials by utilizing combination of metal oxide semiconductors thus capable of effective photoelectrochemical reduction of carbon dioxide. For example, the combination of conducting polymers, or titanium (IV) oxide, and copper (I) oxide has been considered 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. Among important issue is intentional stabilization, activation, and functionalization of the mixed-metal-oxide-based photoelectrochemcal interface toward better long-term performance and selectivity production of small organic molecules (C1-C4) and other chemicals. In this respect, ultra-thin films of conducting polymers (simple or polyoxometallate-derivatized) and supramolecular complexes (with nitrogen containing ligands and certain transition metal sites), sub-monolayers of metals (Cu, Au), networks of noble metal (Au, Ag) nanoparticles or layers of robust bacterial biofilms have been considered. The photobiocathode utilizing robust biofilms have also been demonstrated to stabilize copper(I) oxide surfaces and to induce the system’s activity toward reduction of carbon dioxide under illuminations with visible light.In the presentation, special attention will be paid to the mechanistic aspects of electroreduction of carbon dioxide, fabrication and characterization of highly selective and durable semiconductor photoelectrode materials and to the importance of the reaction conditions.

Symmetrical distribution of P sites on thorn assembled Graphite carbon nitride microtube for photocatalytic conversion of CO2

Photocatalytic conversion of CO2 to form value-added fuels is a promising strategy to simultaneously mitigate climate changes and achieve carbon neutral. Graphite carbon nitride (g-C3N4) is one of the potential candidates for photocatalytic CO2 reduction, but limited by its low specifical surface area and absence of active sites. Here, we design a thorn grown tubular g-C3N4 (ThCN) with P sites by pyrolysis of supramolecular precursors prepared from melamine and hexachlorocyclo-triphosphonitrile (HCCP). ThCN offers an extraordinary photocatalytic activity in CO2 conversion with a yield of 442.2 μmol g−1 (170.1 μmol g−1 for CH4 and 272.1 μmol g−1 for CO) under AM 1.5G irradiation. The technique of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) enables the identification of a series of key intermediates involved in the production of CO and CH4 with high activity. Density functional theory (DFT) calculations reveal that ThCN with rich N-P-N sites reduce barrier energy from CO2 to COOH* and CHO* intermediate, which is beneficial to subsequent reduction process to produce CO and CH4. This work provides a significant insight into regulating molecule structure and morphology of g-C3N4 photocatalysts to promote the CO2 conversion process.

Revealing the CO2 Conversion at Electrode/Electrolyte Interfaces in Li-CO2 Batteries via Nanoscale Visualization Methods.

Lithium-carbon dioxide (Li-CO2 ) battery technology presents a promising opportunity for carbon capture and energy storage. Despite tremendous efforts in Li-CO2 batteries, the complex electrode/electrolyte/CO2 triple-phase interfacial processes remain poorly understood, in particular at the nanoscale. Here, using in situ atomic force microscopy and laser confocal microscopy-differential interference contrast microscopy, we directly observed the CO2 conversion processes in Li-CO2 batteries at the nanoscale, and further revealed a laser-tuned reaction pathway based on the real-time observations. During discharge, a bi-component composite, Li2 CO3 /C, deposits as micron-sized clusters through a 3D progressive growth model, followed by a 3D decomposition pathway during the subsequent recharge. When the cell operates under laser (λ=405 nm) irradiation, densely packed Li2 CO3 /C flakes deposit rapidly during discharge. Upon the recharge, they predominantly decompose at the interfaces of the flake and electrode, detaching themselves from the electrode and causing irreversible capacity degradation. In situ Raman shows that the laser promotes the formation of poorly soluble intermediates, Li2 C2 O4 , which in turn affects growth/decomposition pathways of Li2 CO3 /C and the cell performance. Our findings provide mechanistic insights into interfacial evolution in Li-CO2 batteries and the laser-tuned CO2 conversion reactions, which can inspire strategies of monitoring and controlling the multistep and multiphase interfacial reactions in advanced electrochemical devices.

Revealing the CO2 Conversion at Electrode/Electrolyte Interfaces in Li–CO2 Batteries via Nanoscale Visualization Methods

AbstractLithium–carbon dioxide (Li–CO2) battery technology presents a promising opportunity for carbon capture and energy storage. Despite tremendous efforts in Li–CO2 batteries, the complex electrode/electrolyte/CO2 triple‐phase interfacial processes remain poorly understood, in particular at the nanoscale. Here, using in situ atomic force microscopy and laser confocal microscopy‐differential interference contrast microscopy, we directly observed the CO2 conversion processes in Li–CO2 batteries at the nanoscale, and further revealed a laser‐tuned reaction pathway based on the real‐time observations. During discharge, a bi‐component composite, Li2CO3/C, deposits as micron‐sized clusters through a 3D progressive growth model, followed by a 3D decomposition pathway during the subsequent recharge. When the cell operates under laser (λ=405 nm) irradiation, densely packed Li2CO3/C flakes deposit rapidly during discharge. Upon the recharge, they predominantly decompose at the interfaces of the flake and electrode, detaching themselves from the electrode and causing irreversible capacity degradation. In situ Raman shows that the laser promotes the formation of poorly soluble intermediates, Li2C2O4, which in turn affects growth/decomposition pathways of Li2CO3/C and the cell performance. Our findings provide mechanistic insights into interfacial evolution in Li–CO2 batteries and the laser‐tuned CO2 conversion reactions, which can inspire strategies of monitoring and controlling the multistep and multiphase interfacial reactions in advanced electrochemical devices.

Tailoring grain boundaries and doping on Cu-based electrocatalyst for efficient CO2 reduction reaction

Electrochemical conversion of CO2 on Cu-based catalysts offers an auspicious strategy for addressing the increasingly serious environmental issues. However, there is still a challenge in the regulation of selectivity for an efficient CO2 conversion process on Cu-based catalysts due to the complexity of the internal structure of catalysts. Herein, we step wisely control the intrinsic grain boundaries (GBs) and Sn doping in Cu substrate to optimize the CO2 reduction reaction (CO2RR) performance of catalyst, which also clearly clarify the contribution of above two factors towards CO2RR performance. As a result, the as-synthesized R-SnCu catalyst with both GBs and Sn doing exhibits the highest CO selectivity of 99 % at −0.8 V versus hydrogen electrode (vs. RHE). In-situ Raman spectroscopy was further used to provide critical evidence of the key intermediates during CO2RR. We find that compared to the catalysts containing only GBs or Sn doping, the R-SnCu catalyst controlled by both GBs and Sn doping exhibits a more pronounced adsorption preference for C-bound *COOH intermediate peaks related to CO generation. These findings provide new ideas for the effective design of high-performance Cu-based CO2RR catalysts.

Hydrogen-Tolerant La0.6Ca0.4Co0.2Fe0.8O3-d Oxygen Transport Membranes from Ultrasonic Spray Synthesis for Plasma-Assisted CO2 Conversion.

La0.6Ca0.4Co1-xFexO3-d in its various compositions has proven to be an excellent CO2-resistant oxygen transport membrane that can be used in plasma-assisted CO2 conversion. With the goal of incorporating green hydrogen into the CO2 conversion process, this work takes a step further by investigating the compatibility of La0.6Ca0.4Co1-xFexO3-d membranes with hydrogen fed into the plasma. This will enable plasma-assisted conversion of the carbon monoxide produced in the CO2 reduction process into green fuels, like methanol. This requires the La0.6Ca0.4Co1-xFexO3-d membranes to be tolerant towards reducing conditions of hydrogen. The hydrogen tolerance of La0.6Ca0.4Co1-xFexO3-d (x = 0.8) was studied in detail. A faster and resource-efficient route based on ultrasonic spray synthesis was developed to synthesise the La0.6Ca0.4Co0.2Fe0.8O3-d membranes. The La0.6Ca0.4Co0.2Fe0.8O3-d membrane developed using ultrasonic spray synthesis showed similar performance in terms of its oxygen permeation when compared with the ones synthesised with conventional techniques, such as co-precipitation, sol-gel, etc., despite using 30% less cobalt.

Fundamentals and Recent Progress in Magnetic Field Assisted CO2 Capture and Conversion.

CO2 capture and conversion technology are highly promising technologies that definitely play a part in the journey towards carbon neutrality. Releasing CO2 by mild stimulation and the development of high efficiency catalytic processes are urgently needed. The magnetic field, as a thermodynamic parameter independent of temperature and pressure, is vital in the enhancement of CO2 capture and conversion process. In this review, the recent progress of magnetic field-enhanced CO2 capture and conversion is comprehensively summarized. The theoretical fundamentals of magnetic field on CO2 adsorption, release and catalytic reduction process are discussed, including the magnetothermal, magnetohydrodynamic, spin selection, Lorentz forces, magnetoresistance and spin relaxation effects. Additionally, a thorough review of the current progress of the enhancement strategies of magnetic field coupled with a variety of fields (including thermal, electricity, and light) is summarized in the aspect of CO2 related process. Finally, the challenges and prospects associated with the utilization of magnetic field-assisted techniques in the construction of CO2 capture and conversion systems are proposed. This review offers a reference value for the future design of catalysts, mechanistic investigations, and practical implementation for magnetic field enhanced CO2 capture and conversion.

Metabolic regulation of Shewanella oneidensis for microbial electrosynthesis: From extracellular to intracellular

Shewanella oneidensis MR-1 (S. oneidensis MR-1) has been shown to benefit from microbial electrosynthesis (MES) due to its exceptional electron transfer efficiency. In this study, genes involved in both extracellular electron uptake (EEU) and intracellular CO2 conversion processes were examined and regulated to enhance MES performance. The key genes identified for MES in the EEU process were mtrB, mtrC, mtrD, mtrE, omcA and cctA. Overexpression of these genes resulted in 1.5–2.1 times higher formate productivity than that of the wild-type strains (0.63 mmol/(L·μg protein)), as 0.94–1.61 mmol/(L·μg protein). In the intracellular CO2 conversion process, overexpression of the nadE, nadD, nadR, nadV, pncC and petC genes increased formate productivity 1.3-fold–3.4-fold. Moreover, overexpression of the formate dehydrogenase genes fdhA1, fdhB1 and fdhX1 in modified strains led to a 2.3-fold–3.1-fold increase in formate productivity compared to wild-type strains. The co-overexpression of cctA, fdhA1 and nadV in the mutant strain resulted in 5.59 times (3.50 mmol/(L·μg protein)) higher formate productivity than that of the wild-type strains. These findings revealed that electrons of MES derived from the electrode were utilized in the energy module for synthesizing ATP and NADH, followed by the synthesis of formate in formate dehydrogenase by the combinatorial effects of ATP, NADH, electrons and CO2. The results provide new insights into the mechanism of MES in S. oneidensis MR-1 and pave the way for genetic improvements that could facilitate the further application of MES.

Electrolyte engineering for efficient molten-carbonate electrolysis of CO2

Molten carbonate electrolysis cells (MCEC) are a promising electrochemical CO2 capture and conversion process. However, the CO2 absorption and energy efficiencies are limited by the sluggish CO2 absorption kinetics and high electrode overpotentials. Herein, we propose an electrolyte engineering strategy to improve the CO2 absorption rate and reduce overpotentials at both the anode and the cathode. When BO33− was added into molten Li2CO3-Na2CO3-K2CO3, the CO2 adsorption efficiency was improved from 3.3% to 60%, the overpotential was reduced by ∼ 567 mV at the cathode and by ∼ 238 mV at the anode under 200 mA cm−2. Accordingly, the energy efficiency reached 76.2 % at 650 °C. The improved CO2 absorption and energy efficiencies are thanks to the BO33− that changes the thermodynamic properties of the molten carbonate, i.e., the BO33−–CO32− complex reduces the energy barrier for the reduction of CO32− at the cathode and for the liberation of O2− that can be oxidized at a lower potential than CO32− at the anode. Therefore, the electrolyte engineering is an effective strategy for designing high-temperature CO2 electrolyzers with high CO2 absorption and energy efficiency.

Hierarchically structured SnO/NiCo2O4/CC catalysts with stability and superior faradaic efficiency for electrocatalytic CO2 reduction to formic acid

SnO-based catalysts are among the most common and promising catalysts for electrocatalytic CO2 conversion, while their feature of easy deactivation greatly hinders the industrial application. Here, we developed highly stable SnO/NiCo2O4/CC catalysts by constructing SnO/NiCo2O4 hierarchical structures with a large number of complex pores on carbon cloth (CC). Through a series of characterization, we found that the solid oxide pairs (Ni3+/Co3+) contained in the SnO/NiCo2O4/CC catalysts could protect the SnO from being reduced to Sn monomers during the electrocatalytic process, greatly enhancing the stability of the electrocatalysts. The SnO/NiCo2O4/CC catalyst remained constant in current density during the 30 h CO2 conversion process, and the loss resistance activity is more than 300% of the SnO/CC catalyst. In addition, the SnO/NiCo2O4/CC catalyst presented a high faradaic efficiency of 91.87% in the CO2 electroreduction process, much higher than most of the SnO-based catalysts. The DFT calculations and analysis show that the superior faradaic efficiency is mainly attributed to the heterostructure of SnO(0 0 1)/NiCo2O4 on CC plane, which significantly reduces the Gibbs free energy of producing the *OOCH intermediates. This study provides an effective strategy in designing SnO-based electrocatalysts with high performance for CO2 reduction.

Activated g-C3N4 Photocatalyst with Defect Engineering for Efficient Reduction of CO2 in Water

The conversion and utilization of CO2 into hydrocarbon fuel using solar energy is an attractive way to reduce CO2 emissions. In the photocatalytic CO2 conversion process, a synergistic enhancement of both photoreaction and chemical reaction is quite important for promoting the conversion. Here, we present a facile method for preparing a defective and activated graphitic carbon nitride (g-C3N4) photocatalyst, which exhibits a superior CO2-to-CO production of 32.06 μmol g–1 without sacrificial agent or noble metal. This is about 9.1 times higher than that of the defect containing g-C3N4 (3.51 μmol g–1). The activated g-C3N4, in which the TiO2 species coexist, not only improved the CO production to 54.03 umol g–1 but also reduced CO2 into CH4 with 2.50 umol g–1 in 5 h. This is because the coexistence of TiO2 species in activated g-C3N4 promotes molecule and electron transfer, as observed through experimental characterization techniques. Therefore, this work provides a novel insight into the synthesis of multifunctional g-C3N4 for efficient photocatalytic CO2 reduction with H2O.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*−, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Silver-Doped Zeolitic Imidazolate Framework (Ag@ZIF-8): An Efficient Electrocatalyst for CO2 Conversion to Syngas

To enable the reuse of carbon dioxide (CO2), electrocatalytic reduction of CO2 (CO2RR) into syngas with a controllable H2/CO ratio is considered a cost-effective and intriguing approach. Here, a number of silver (Ag)-doped, zeolitic imidazole framework composites were prepared by a facile method. The outcomes demonstrate that CO2 electroreduction on Ag-doped ZIF-8 catalysts produces just CO and H2, without having any liquid fuel, resulting in a total faradaic efficiency approaching 100%. The most optimal Ag-Zn-ZIF-8 (10% Ag, 90% Zn) demonstrates good selectivity for syngas (CO and H2) that can be easily adjusted from 3:1 to 1:3 (H2/CO) by changing the applied voltage during the CO2 conversion process.

Electrified CO2 valorization driven by direct Joule heating of catalytic cellular substrates

The growing environmental concerns have driven catalytic CO2 valorization as a forward-looking solution to mitigate the carbon footprint of valuable chemical products. CO2 conversion processes into synthesis gas, such as CO2 reforming of methane (CRM) or reverse water–gas shift (RWGS), may have a strategic role for the future sustainable production of chemicals and energy carriers. However, fuel combustion to supply the heat of the associated endothermic reactions would result in unwanted CO2 emissions, frustrating the overall objective. Electrification of the endothermic processes may represent the technological solution to such an issue. Here we report a promising approach for the direct electrification of the CO2 reforming of methane (eCRM) and reverse water–gas shift (eRWGS) processes in washcoated structured reactors. We employ catalytically activated open-cell foams that provide optimal heat and mass transfer properties and serve as Joule heating substrates for the catalytic conversion of CO2 via reaction with methane or hydrogen. The proposed reactor system with Joule-heated Rh/Al2O3-coated foam exhibited excellent catalytic and electrical stability for more than 75 h, operating up to 800 °C and approaching equilibrium conversion at high space velocity, i.e., GHSV of 600 and 100 kNl/kgcat/h for eRWGS and eCRM, respectively. Such a reactor concept has potential to ensure remarkably low specific energy demand for CO2 valorization. Assuming an optimized process configuration approx. 0.7 kWh/Nm3CO2 is calculated for eRWGS. By replacing fuel combustion with Joule heating driven by renewable electricity, the electrified CO2 valorization processes provide an important approach for dealing with the intermittent nature of renewable sources by storing the energy in chemicals with a low carbon footprint.