Tunable Syngas Research Articles

Covalent Organic Framework Stabilized Single CoN4Cl2 Site Boosts Photocatalytic CO2 Reduction into Tunable Syngas

AbstractSolar carbon dioxide (CO2) reduction provides an attractive alternative to producing sustainable chemicals and fuel. However, the construction of a highly active photocatalyst was challenging because of the rapid charge recombination and sluggish surface CO2 reduction. Herein, a unique Co−N4Cl2 single site was fabricated by loading Co species into the 2,2′‐bipyridine and triazine‐containing covalent organic framework (COF) for CO2 conversion into syngas under visible light irradiation. The resulting champion catalyst TPy‐COF‐Co enabled a record‐high CO production rate of 426 mmol g−1 h−1, associated with the unprecedented turnover number (TON) and turnover frequency (TOF) of 2095 and 1607 h−1, respectively. The catalyst also exhibited favorable recycling performance and widely adjustable syngas production (CO/H2 ratio: 1.8 : 1–1 : 16). A systematical investigation including operando synchrotron X‐ray absorption fine structure (XAFS) spectroscopy, in situ attenuated total reflection surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS), and theoretical calculation indicated that the triazine‐based COF framework promoted the charge transfer towards the single Co−N4Cl2 sites that greatly promoted the CO2 activation by lowering the energy barrier of *COOH generation, facilitating the CO2 transformation. This work highlights the great potential of the molecular regulation of COF‐derived single‐atom catalysts to boost CO2 photoreduction efficiency.

Electrolytic conversion of CO2 to proportionally tunable syngas using nickel-nitrogen doped carbon materials derived from Chlorella sp.

Chlorella sp., a common type of algae found in global surface waters, is prone to causing "blooms" and can accumulate significant amounts of nutrients such as nitrogen, phosphorus, and carbon. While carbon materials derived from Chlorella sp. are widely used in the field of adsorption, there are relatively few reports focused on electrocatalysis. This study utilized Chlorella sp. as a raw material to synthesize functional carbon materials, and prepared nitrogen-doped (CN), nickel-doped (CNi), and nitrogen-nickel co-doped (CNNi) carbon-based catalysts with both metal and non-metal elements. The results showed that the CN, CNi and CNNi catalysts possessed certain electrocatalytic properties, among which the CNNi material had the best electrocatalytic activity towards CO2 reduction (CO2RR). In 1.0 M KHCO3, the CNNi catalyst exhibited an instantaneous current density of 15.4 mA/cm2 under the potential of -0.82 V (vs. RHE). In terms of product selectivity, the Faraday efficiency of CO on the CNNi electrode was up to 90 % when the electrode potential was -0.62 V (vs. RHE), while the selectivity of the CN and CNi catalysts for CO was only 59 % under the optimal conditions. The co-doping of nitrogen and nickel significantly enhanced the electrocatalytic activity and the selectivity towards CO of Chlorella-based carbon materials.

Tuning Sb Atoms on Cu‐Based Catalysts for Electrochemical Conversion of CO2 to Syngas

The electrochemical CO2RR into syngas provides an attractive way to produce raw materials for synthesizing various value‐added chemicals. However, a wide range of tunable syngas ratio of CO/H2 is hard to achieve during the electrochemical CO2 conversion process. Herein, we propose a facile electrodeposition method for the preparation of Sb modified Cu‐based catalyst with a high activity for the ratio‐tunable production of syngas. The Cu10Sb1 catalyst reaches a high FE of 71% for CO at a potential of ‐1.09 V vs. RHE with the tunable CO/H2 ratio of syngas from ~0.6 to ~7.8 at the applied potentials from ‐0.78 V to ‐1.09 V. Theoretical calculations reveal the tuning effect of Sb atoms on Cu‐based catalyst for the electrochemical conversion of CO2 to syngas, in which the formation of CO on Sb modified Cu‐based catalyst possesses low energy barrier in potential‐limiting steps during CO2RR. Meanwhile, the robust charge interaction between COOH* species and the surface of Sb modified Cu‐based catalyst enhances the activity of COOH* species, thus improving FE and current for conversion into syngas products. This work brings a new strategy for the synthesis of low‐cost syngas catalyst and the tuning effect of metal atom on Cu‐based catalyst for CO2RR.

Efficient photoreduction of CO2 to tunable syngas by Zn-doped Co3O4: Tuning binding strength to versatile reaction intermediates simultaneously

Photochemical reduction of CO2 (CO2RR) to syngas is an attractive approach for carbon–neutral recycling. Manipulating binding strength to reactive species on catalyst surfaces is challenging but essential for achieving efficient syngas production with a desired CO/H2 ratio. Here, we demonstrate that the binding strength to key reaction intermediates (*CO2, *H and *CO) on the Co3O4 surface can be fine-tuned by incorporating varying amount of Zn atoms. Dedicated experiments and theoretical simulations unveil that Zn doping allows for simultaneous control of both CO2 adsorption/activation and the adsorption affinity of *CO on Zn-Co3O4 catalysts, resulting in a significant enhancement in CO2 to CO conversion. Moreover, Zn doping can alter the hydrophilicity of the catalyst surface, affecting the water adsorption behavior and adjusting the hydrogen evolution reaction (HER) activity. As a result, the CO/H2 ratio can be readily tuned via varying the Zn doping level. This study presents an effective and economical method approach to manipulate the electronic structure of catalyst surfaces for controlling the kinetics of CO2RR and HER reactions, providing valuable insight into the design of highly active transition metal oxides (TMOs) based photocatalysts for syngas production with a tailored CO/H2 ratio.

Covalent Organic Framework with Donor1‐Acceptor‐Donor2 Motifs Regulating Local Charge of Intercalated Single Cobalt Sites for Photocatalytic CO2 Reduction to Syngas

AbstractCovalent organic framework (COF) has attracted increasing interest in photocatalytic CO2 reduction, but it remains a challenge to achieve high conversion efficiency owing to the insufficient active site and fast charge recombination. Rationally optimizing the electronic structures of COF to regulate the local charge of active sites precisely is the key point to improving catalytic performance. Herein, intercalated single Co sites coordinated by imine‐N motifs have been designed by using trinuclear copper‐based imine‐COFs with distinct electronic moieties via a molecular engineering strategy. It is confirmed that the charge delivery property and local charge distribution of these heterometallic frameworks can be profoundly influenced by electronic structures. Among these featured structures with mixed‐state copper clusters, Co/Cu3‐TPA‐COF stands out for an exceptional photocatalytic CO2 reduction activity and tunable syngas (CO/H2) ratio by changing various bipyridines. Experimental and theoretical results indicate that interlayer Co‐imine N motifs on the donor1‐acceptor‐donor2 structures facilitate the formation of a highly separated electron‐hole state, which effectively induces the oriented electron transfer from dual electron donors to Co centers, achieving an enhanced CO2 activation and reduction. This work opens up an avenue for the design of high‐performance COF‐based catalysts for photocatalytic CO2 reduction.

Covalent Organic Framework with Donor1-Acceptor-Donor2 Motifs Regulating Local Charge of Intercalated Single Cobalt Sites for Photocatalytic CO2 Reduction to Syngas.

Covalent organic framework (COF) has attracted increasing interest in photocatalytic CO2 reduction, but it remains a challenge to achieve high conversion efficiency owing to the insufficient active site and fast charge recombination. Rationally optimizing the electronic structures of COF to regulate the local charge of active sites precisely is the key point to improving catalytic performance. Herein, intercalated single Co sites coordinated by imine-N motifs have been designed by using trinuclear copper-based imine-COFs with distinct electronic moieties via a molecular engineering strategy. It is confirmed that the charge delivery property and local charge distribution of these heterometallic frameworks can be profoundly influenced by electronic structures. Among these featured structures with mixed-state copper clusters, Co/Cu3-TPA-COF stands out for an exceptional photocatalytic CO2 reduction activity and tunable syngas (CO/H2) ratio by changing various bipyridines. Experimental and theoretical results indicate that interlayer Co-imine N motifs on the donor1-acceptor-donor2 structures facilitate the formation of a highly separated electron-hole state, which effectively induces the oriented electron transfer from dual electron donors to Co centers, achieving an enhanced CO2 activation and reduction. This work opens up an avenue for the design of high-performance COF-based catalysts for photocatalytic CO2 reduction.

Selective CO2 to proportionally tunable syngas at lower onset potential enabled by bimetallic chalcogenides in polyoxometalate medium

Highly active noble metals and developed modified electrocatalysts are limited in terms of economy and kinetics for the electrocatalytic CO2 reduction reaction (ECO2RR), respectively. Herein, different bimetallic chalcogenides (CuInSe2, CuInS2, AgInS2) were found to have the ability to prepare proportionally tunable syngas in PV3Mo9 medium at an ultra-low onset potential of −0.1 V (vs. RHE). The ECO2RR system established by CuInSe2 and PV3Mo9 achieved the widest CO/H2 range (1.0–5.1), with a maximum jCO of −29.3 mA·cm−2. The PV3Mo9 medium had a positive effect on reducing the onset potential and improving the reaction kinetics. Theoretical calculations illustrated that the high activity of CuInSe2 in PV3Mo9 medium depended on the metal-like characteristic generated by the interaction of bimetallic orbitals and the mild regulation of *CO adsorption energy. This work demonstrates the broad application prospects of bimetallic chalcogenides in POM media in terms of ECO2RR activity and economic benefits.

Bipolaronic Motifs Induced Spatially Separated Catalytic Sites for Tunable Syngas Photosynthesis From CO2.

Photocatalytic reduction of CO2 into syngas is a promising way to tackle the energy and environmental challenges; however, it remains a challenge to achieve reaction decoupling of CO2 reduction and water splitting. Therefore, efficient production of syngas with a suitable CO/H2 ratio for Fischer-Tropsch synthesis can hardly be achieved. Herein, bipolaronic motifs including Co(II)-pyridine N motifs and Co(II)-imine N motifs are rationally designed into a crystalline imine-linked 1,10-phenanthroline-5,6-dione-based covalent organic framework (bp-Co-COF) with a triazine core. These featured structures with spatially separated active sites exhibit efficient photocatalytic performance toward CO2-to-syngas conversion with a suitable CO/H2 ratio (1:1-1:3). The bipolaronic motifs enable a highly separated electron-hole state, whereby the Co(II)-pyridine N motifs tend to be the active sites for CO2 activation and accelerate the hydrogenation to form *COOH intermediates; whilst, the Co(II)-imine N motifs increase surface hydrophilicity for H2 evolution. The photocatalytic reductions of CO2 and H2O thus decouple and proceed via a concerted way on the bipolaronic motifs of bp-Co-COF. The optimal bp-Co-COF photocatalyst achieves a high syngas evolution rate of 15.8mmol g-1 h-1 with CO/H2 ratio of 1:2, outperforming previously reported COF-based photocatalysts.

Integrating CO2 reduction, H2O oxidation and H2 evolution catalysts into a dual S-scheme artificial photosynthetic system for tunable syngas production

It remains a challenge to achieve overall photocatalytic conversion of CO2 with H2O into tunable syngas and O2. Herein, by inserting BiVO4 into the heterointerface of hollow core-shell CdS@T-COF S-scheme photocatalyst, an innovative CdS/BiVO4@T-COF dual S-scheme heterojunction is developed, which gives an almost stoichiometric evolution of syngas (453.6 μmol gcat−1 h−1) and O2 (230.5 μmol gcat−1 h−1). Moreover, the CO output and selectivity only decline by around 20% when pure CO2 is replaced by diluted CO2 (15%) atmosphere. The BiVO4 embedding not only augments the internal electric field and band bending, upgrading the separation efficiency of active carriers, but also alters the roles of the components involved, separating the occurrence of CO, O2 and H2 evolution on T-COF, BiVO4 and CdS in lower energy barriers. Besides, varying the T-COF thickness can modulate the availability of CO2, H2O and photon to different sites, thereby adjusting the CO/H2 ratio in a wide window. This work opens an avenue for customizing three-in-one artificial photosynthetic systems for coupling three half reactions.

Manipulating photogenerated electron flow in nickel single‐atom catalysts for photocatalytic CO2 reduction into tunable syngas

AbstractThe key to designing photocatalysts is to orient the migration of photogenerated electrons to the target active sites rather than dissipate at inert sites. Herein, we demonstrate that the doping of phosphorus (P) significantly enriches photogenerated electrons at Ni active sites and enhances the performance for CO2 reduction into syngas. During photocatalytic CO2 reduction, Ni single‐atom‐anchored P‐modulated carbon nitride showed an impressive syngas yield rate of 85 μmol gcat−1 h−1 and continuously adjustable CO/H2 ratios ranging from 5:1 to 1:2, which exceeded those of most of the reported carbon nitride‐based single‐atom catalysts. Mechanistic studies reveal that P doping improves the conductivity of catalysts, which promotes photogenerated electron transfer to the Ni active sites rather than dissipate randomly at low‐activity nonmetallic sites, facilitating the CO2‐to‐syngas photoreduction process.

Tunable Syngas Formation at Industrially Relevant Current Densities via CO2 Electroreduction and Hydrogen Evolution over Ni and Fe-derived Catalysts obtained via One-Step Pyrolysis of Polybenzoxazine Based Composites.

Simultaneous electroreduction of CO2 and H2O to syngas can provide a sustainable feed for established processes used to synthesize carbon-based chemicals. The synthesis of MOx/M-N-Cs (M = Ni, Fe) electrocatalysts reported via one-step pyrolysis that shows increased performance during syngas electrosynthesis at high current densities with adaptable H2/CO ratios, e.g., for the Fischer-Tropsch process. When embedded in gas diffusion electrodes (GDEs) with optimized hydrophobicity, the NiOx/Ni-N-C catalyst produces syngas (H2/CO = 0.67) at -200mA cm-2 while for the FeOx/Fe-N-C syngas production occurs at ≈-150mA cm-2. By tuning the electrocatalyst's microenvironment, stable operation for >3h at -200mA cm-2 is achieved with the NiOx/Ni-N-C GDE. Post-electrolysis characterization revealed that the restructuring of the catalyst via reduction of NiOx to metallic Ni NPs still enables stable operation of the electrode at -200mA cm-2, when embedded in an optimized microenvironment. The ionomer and additives used in the catalyst layer are important for the observed stable operation. Operando Raman measurements confirm the presence of NiOx during CO formation and indicate weak adsorption of CO on the catalyst surface.

Flexible control of Co/Zn-nitrogen coordination in ZIFs for electrochemical CO2 reduction to tunable syngas.

Bimetallic CoxZny-ZIF catalysts were synthesized with adjustable Co/Zn-nitrogen sites for the electrochemical CO2RR to tunable syngas, resulting in a H2/CO ratio ranging from 1 : 1 to 4 : 1. The excellent CO2 adsorption capacity and large specific surface areas were responsible for the high activity of all these CoxZny-ZIF materials. This work has the potential to drive advancements in both the MOF application sector and electrochemical CO2 reduction.

Photocatalytic Conversion of Diluted CO2 into Tunable Syngas via Modulating Transition Metal Hydroxides.

The conversion of diluted CO2 into tunable syngas via photocatalysis is critical for implementing CO2 reduction practically, although the efficiency remains low. Herein, we report the use of graphene-modified transition metal hydroxides, namely, NiXCo1-X-GR, for the conversion of diluted CO2 into syngas with adjustable CO/H2 ratios, utilizing Ru dyes as photosensitizers. The Ni(OH)2-GR cocatalyst can generate 12526 μmol g-1 h-1 of CO and 844 μmol g-1 h-1 of H2, while the Co(OH)2-GR sample presents a generation rate of 2953 μmol g-1 h-1 for CO and 10027 μmol g-1 h-1 for H2. Notably, by simply altering the addition amounts of nickel and cobalt in the transition metal composite, the CO/H2 ratios in syngas can be easily regulated from 18:1 to 1:4. Experimental characterization of composites and DFT calculations suggest that the differing adsorption affinities of CO2 and H2O over Ni(OH)2-GR and Co(OH)2-GR play a significant role in determining the selectivity of CO and H2 products, ultimately affecting the CO/H2 ratios in syngas. Overall, these findings demonstrate the potential of graphene-modified transition metal hydroxides as efficient photocatalysts for CO2 reduction and syngas production.

Theoretically Designed Cu10Sn3-Cu-SnOx as Three-Component Electrocatalyst for Efficient and Tunable CO2 Reduction to Syngas.

Electrocatalytic transformation of CO2 to various syngas compositions is an exceedingly attractive approach to carbon-neutral recycling. Meanwhile, the achievement of selectivity, stability, and tunability of product ratios using single-component electrocatalysts is challenging. Herein, the theoretically-assisted design of the triple-component nanocomposite electrocatalyst Cu10Sn3-Cu-SnOx that addresses this challenge is presented. It is shown that Cu10Sn3 is a valuable electrocatalyst for suitable CO2 reduction to CO, SnO2 for CO2 reduction to formate at large overpotentials, and that the Cu-SnO2 interface facilitates H2 evolution. Accordingly, the interaction between the three functional components affords tunable CO/H2 ratios, from 1:2 to 2:1, of the produced syngas by controlling the applied potentials and relative contents of functional components. The syngas generation is selective (Faradaic efficiency, FE = 100%) at relatively lower cathodic potentials, whereas formate is the only liquid product detected at relatively higher cathodic potentials. The theoretically guided design approach therefore provides a new opportunity to boost the selectivity and stability of CO2 reduction to tunable syngas.

Efficient production of tunable syngas through multistage sorption-enhanced steam gasification of corncob

As “carbon neutrality” becomes one of the most critical topics in the world, interest in capturing carbon dioxide from industrial processes is growing. In this study, we proposed an efficient process for recycling biomass resources. Firstly, a bifunctional Ni/CaO catalyst based on Ce modification was developed by conducting corncob steam gasification experiments at 600 °C in a tube furnace. At Ce doping of 0.75 and Steam/Carbon = 7, the H2 concentration reached 87.65 ± 1.46 vol% and the H2 yield reached 40.04 ± 0.74 mmol/gbiomass. The cycle test verified that Ce0.75-Ni1/Ca7 had good cycle stability. Secondly, we designed multi-stage variable temperature steam gasification experiments. The experimental results showed that the CO yield in the second stage was 15 times higher than before the introduction of gasification carbon, and the total syngas yield was increased by 2.32 times. Furthermore, the H2/CO molar ratio could be controlled by modifying the addition of gasification carbon, which was advantageous for downstream product synthesis. Implement the synergistic capture and conversion of CO2 in an integrated process, while simultaneously producing high-value synthesis gas, fully utilizing the biomass.

Fe-Ni bimetallic composite N-doped carbon catalyst for electrocatalytic reduction of CO2 to produce efficient and controlled syngas

The reduction of CO2 to a tunable syngas by electrochemical catalysis for the industrial synthesis of hydrocarbons, alcohols, aldehydes, and other compounds can effectively realize carbon recycling. Herein, in this paper, non-precious metal FeNi precursors were synthesized by solution method, and then a series of FeNi-NC catalysts were obtained by one-step pyrolysis. The results show that on the prepared catalysts, the products of CO2 electroreduction are CO and H2, and the total Faraday efficiency is close to 100 %. Comparison of the prepared catalysts showed that the Fe3Ni7-NC 800 °C had the best selectivity for CO, with a maximum Faraday efficiency of 81.3 % for CO(FECO) generation at a potential of − 0.9 V (vs. RHE), and a current density of − 22.5 mA cm−2 at − 1.2 V. The catalysts had better durability. Moreover, the syngas ratio from 1:1 to 6:1 (CO/H2) can be easily adjusted by controlling the potential applied during the process over a wide potential range, and the resulting syngas in different ratios can be used for the production of chemical raw materials and fuels.

Photocatalytic CO2 reduction to syngas using metallosalen covalent organic frameworks

Metallosalen-covalent organic frameworks have recently gained attention in photocatalysis. However, their use in CO2 photoreduction is yet to be reported. Moreover, facile preparation of metallosalen-covalent organic frameworks with good crystallinity remains considerably challenging. Herein, we report a series of metallosalen-covalent organic frameworks produced via a one-step synthesis strategy that does not require vacuum evacuation. Metallosalen-covalent organic frameworks possessing controllable coordination environments of mononuclear and binuclear metal sites are obtained and act as photocatalysts for tunable syngas production from CO2. Metallosalen-covalent organic frameworks obtained via one-step synthesis exhibit higher crystallinity and catalytic activities than those obtained from two-step synthesis. The optimal framework material containing cobalt and triazine achieves a syngas production rate of 19.7 mmol g−1 h−1 (11:8 H2/CO), outperforming previously reported porous crystalline materials. This study provides a facile strategy for producing metallosalen-covalent organic frameworks of high quality and can accelerate their exploration in various applications.

Efficient visible-light-driven CO2 reduction into tunable syngas by cobalt phthalocyanine modified flower-like ZnIn2S4 nanosheets

Photocatalytic carbon dioxide (CO2) reduction into syngas (the mixture of CO and H2) has been considered as an attractive method to solve the greenhouse effect and energy shortage. Polychalcogenides semiconductors such as ZnIn2S4 (ZIS) have been widely utilized in photocatalysis but are still hampered by limited photo-induced charge separation and poor product selectivity. Molecular catalysts for CO2 reduction possess superior selectivity for specific reduction products but suffer from low stability and recycling difficulty. Herein, we modified a molecular phthalocyanine (Pc) on flower-like ZIS nanosheets. This hybrid exhibited enhanced photocatalytic CO2 reduction and exhibited a tunable ratio of CO/H2 in syngas (0.01–4.65) under visible light irradiation. The enhanced photocatalytic activity of ZIS-Pc hybrids can be attributed to dimension-matched ultrathin heterojunction, which facilitated charge separation, broad light utilization, and recycling stability. The unique design holds great potential for other highly efficient photocatalytic reactions.

Axial Dual Atomic Sites Confined by Layer Stacking for Electroreduction of CO2 to Tunable Syngas.

Arranging atoms in an orderly manner at the atomic scale to create stable polyatomic structures is a very challenging task. In this study, we have developed three-dimensional confinement areas on the two-dimensional surface by creating regional defects. These areas are composed of vertically stacked graphene layers, where Ni and Fe atoms are anchored concentrically to form axial dual atomic sites in high yield. These sites can be used to produce tunable syngas through the electroreduction of CO2. Theoretical calculations indicate that the Ni sites vertically regulate the charge distribution of the adjacent Fe sites in the layer below, resulting in a lower d-band center. This, in turn, weakens the adsorption of the *CO intermediate and inhibits the production of H2 at the Fe site. Our research presents a novel approach for concentrated creation of dual atomic sites by building a confinement-selective surface.

ZnxZrO2+x-Based Photocatalyst with Dual Active Sites for Tunable Syngas Production from CO2 Photoreduction

Photocatalytic reduction of CO2 in H2O to CO and H2 for syngas production is an attractive option to solve global warming and generate a feedstock for the Fischer–Tropsch process. The syngas generated from CO2 photoreduction with a precise and controllable ratio of H2/CO remains a major challenge to date. In this work, CdS-sensitized nanoscale ZnxZrO2+x solid solutions with dual active sites were synthesized as photocatalysts for tunable syngas H2 and CO production from CO2 photoreduction. The staggered band energy alignments, electron paramagnetic resonance results, and the site of palladium/gold nanoparticles by photodeposition indicate that the charge transfer in the CdS/ZnxZrO2+x heterojunction mainly follows the conventional type II mechanism. Density functional theoretical calculation results reveal that Zn and Zr sites on ZnxZrO2+x are favorable for the adsorption of H2O and CO2 molecules and generation of H2 and CO, respectively. The H2/CO ratio can be controlled in the range from 0.9 to 25.0 by adjusting the content of Zn in ZnxZrO2+x. Moreover, syngas production can be further improved by the modification of Pd nanoparticles. For the first time, this work proposes a strategy to construct a photocatalyst with dual active sites for effectively controlling the H2/CO ratio in syngas synthesis through a fundamental understanding of adsorption and catalytic reaction sites in both H2 evolution and CO2 reduction reactions.