High CO2 Selectivity Research Articles

Molecular Engineering of Cation Solvation Structure for Highly Selective Carbon Dioxide Electroreduction

AbstractBalancing the activation of H2O is crucial for highly selective CO2 electroreduction (CO2RR), as the protonation steps of CO2RR require fast H2O dissociation kinetics, while suppressing hydrogen evolution (HER) demands slow H2O reduction. We herein proposed one molecular engineering strategy to regulate the H2O activation using aprotic organic small molecules with high Gutmann donor number as a solvation shell regulator. These organic molecules occupy the first solvation shell of K+ and accumulate in the electrical double layer, decreasing the H2O density at the interface and the relative content of proton suppliers (free and coordinated H2O), suppressing the HER. The adsorbed H2O was stabilized via the second sphere effect and its dissociation was promoted by weakening the O−H bond, which accelerates the subsequent *CO2 protonation kinetics and reduces the energy barrier. In the model electrolyte containing 5 M dimethyl sulfoxide (DMSO) as an additive (KCl‐DMSO‐5), the highest CO selectivity over Ag foil increased to 99.2 %, with FECO higher than 90.0 % within −0.75 to −1.15 V (vs. RHE). This molecular engineering strategy for cation solvation shell can be extended to other metal electrodes, such as Zn and Sn, and organic molecules like N,N‐dimethylformamide.

Low-temperature catalytic performance improvement of Ru/TiO2{001} for o-dichlorobenzene oxidation

Chlorinated aromatics hydrocarbons were harmful to ecology and human health, whose catalytic combustion encounters problem of catalyst deactivation by chlorine poisoning. To simultaneously improve the catalytic activity, CO2 selectivity and stability of catalyst, the present study used the loading of Ru active component on the anatase TiO2 which preferentially exposes highly active {001} crystalline facets. Take o-dichlorobenzene (o-DCB) oxidation as a surrogate reaction, Ru/TiO2{001} showed lower T90% (temperatures with 90% conversion), higher CO2 selectivity and better stability than Pt/TiO2{001}, Pd/TiO2{001} and Ru/commercial TiO2 (Ru/TiO2-C). The excellent results could be attributed to the strong metal support interaction between Ru species and TiO2{001}, with more oxygen vacancies and higher surface oxygen concentration, more Lewis acid sites, enhanced low-temperature reducibility, more surface Ti3+ species, more surface Ru4+ species, and highly dispersed Ru species, favoring the excellent low temperature catalytic activity and CO2 selectivity of Ru/TiO2{001}. In addition, the less Cl species on the used 1.5% Ru/TiO2{001} indicated their effectively removal, which is the core issue for the deep oxidation of o-DCB. Furthermore, the in situ FTIR spectroscopy gave the detail oxidation route of o-DCB over 1.5% Ru/TiO2{001}, that the co-adsorption of o-DCB and O2 on Ru/TiO2{001} surface were calculated based on the density functional theory calculations. The present study gave a new insight into the new pathway for the design of promising catalysts for the treatment of CAHs.

Pyrrolic N-doped carbon catalysts for highly efficient electrocatalytic reduction of CO2 with superior CO selectivity over a wide potential window

The electrocatalytic reduction of CO2 to CO is an attractive approach for converting the greenhouse gas to value-added chemicals. However, current catalysts still suffer from low catalytic reactivity and low CO selectivity. Here, we report the synthesis of a series of N-doped carbon catalysts by thermal annealing of the mixture of urea and carbon in argon under varied temperatures, enabling the rational modulation of N species in carbon. The as-prepared NC-650 catalyst with pyrrolic N as the dominant N species can preferentially reduce CO2 to CO with almost 100% CO selectivity over a broad potential range (−0.45 to −1.05 V), coupled with remarkable long-term stability for over 300 h. DFT calculations reveal that the pyrrolic N site is unsaturated (valence state), which favors the formation of the N–COOH unit geometrically and electronically via proton-coupled electron transfer while inhibit hydrogen liberation, resulting in high CO2RR reactivity and CO selectivity of the NC-650 catalyst.

Modified NiFe-MOF with defects induced by -NH2 and -SH for enhanced adsorption and photocatalytic reduction of CO2

In this study, a Ni-Fe metal-organic framework modified by –NH2 and –SH was synthesized using a simple hydrothermal process for enhancing photocatalytic CO2 reduction. As expected, the N2S-NiFe (NH2-BDC/H4DSBDC, 2:1) displayed excellent photocatalytic CO2 reduction activity and high CO selectivity (12,412.23 μmol g−1 h−1) under visible light irradiation (λ ≥ 420 nm), which is 5 times that of NiFe-MOF. Notably, the excellent photocatalytic performance of N2S-NiFe can benefit from the rich defect trap site caused by the introduction of amino and sulfhydryl groups, accelerating charge transfer and promoting space charge separation, as supported by the photo-electrochemical properties. To better understand the CO2 adsorption mechanism, density functional theory calculations were performed, which revealed that the Niov site has more negative adsorption energy compared with the Feov site. This study provides a simple strategy to establish efficient photocatalysts for CO2 reduction through the modification of organic ligands.

MXene‐Regulated Metal‐Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

AbstractElectrocatalytic CO2 reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti3C2Tx MXene‐regulated Ag−ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag−ZnO/Ti3C2Tx catalyst achieves an outstanding CO2 conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm−2 at −0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up‐shifted d‐band center relative to Fermi level within MXene‐regulated Ag−ZnO interfaces contributes the high selectivity of CO. The CO2 conversion is highly correlated with the dominated linear‐bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal‐oxide interfaces with the regulation of MXene for high‐performance electrocatalysis beyond CO2 reduction.

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion.

Electrocatalytic CO2 reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti3 C2 Tx MXene-regulated Ag-ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag-ZnO/Ti3 C2 Tx catalyst achieves an outstanding CO2 conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm-2 at -0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag-ZnO interfaces contributes the high selectivity of CO. The CO2 conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO2 reduction.

Porosity engineering of biochar‐based single‐atom Ni catalysts to promote CO2 electroreduction over a broad potential window

AbstractDeveloping electrocatalysts for CO2 electroreduction with high activity and superior selectivity is extremely important and desirable from both academic and industrial perspectives. However, owing to competition with hydrogen evolution, highly efficient CO2 reduction is mostly achieved with high CO selectivity in a narrow potential range, which is incompatible with a large cell voltage required for industrial‐level CO2 reduction. Herein, we report an effective strategy to regulate CO2 reduction performances of single‐atom Ni electrocatalysts over a broad potential window by engineering their pore structures (micropores, mesopores, or hierarchical pores with both micropores and mesopores). It is revealed that hierarchically pores can significantly promote CO2 reduction efficiency of single‐atom Ni electrocatalysts. The hierarchically porous electrocatalyst achieves a maximum CO Faradaic efficiency (FECO) of 97.4% at −1.2 V (vs. RHE) and shows high FECO of >85% over a broad potential window from −0.7 to −1.7 V, much superior to electrocatalysts with other pore structures. More impressively, turnover frequency of the hierarchically porous electrocatalyst increases rapidly with increasing the applied potential and reaches 50,067 h−1 at −1.7 V. Such CO2 electroreduction promotion could be attributed to a synergistic effect of micropores for enhancing CO2 adsorption and mesopores for facilitating rapid release of product bubbles, which significantly improves CO2 reduction and suppresses hydrogen evolution.

Bioinspired Design of a Giant [Mn86] Nanocage‐Based Metal‐Organic Framework with Specific CO2 Binding Pockets for Highly Selective CO2 Separation

AbstractAdsorption‐based removal of carbon dioxide (CO2) from gas mixtures has demonstrated great potential for solving energy security and environmental sustainability challenges. However, due to similar physicochemical properties between CO2 and other gases as well as the co‐adsorption behavior, the selectivity of CO2 is severely limited in currently reported CO2‐selective sorbents. To address the challenge, we create a bioinspired design strategy and report a robust, microporous metal–organic framework (MOF) with unprecedented [Mn86] nanocages. Attributed to the existence of unique enzyme‐like confined pockets, strong coordination interactions and dipole‐dipole interactions are generated for CO2 molecules, resulting in only CO2 molecules fitting in the pocket while other gas molecules are prohibited. Thus, this MOF can selectively remove CO2 from various gas mixtures and show record‐high selectivities of CO2/CH4 and CO2/N2 mixtures. Highly efficient CO2/C2H2, CO2/CH4, and CO2/N2 separations are achieved, as verified by experimental breakthrough tests. This work paves a new avenue for the fabrication of adsorbents with high CO2 selectivity and provides important guidance for designing highly effective adsorbents for gas separation.

Bioinspired Design of a Giant [Mn86 ] Nanocage-Based Metal-Organic Framework with Specific CO2 Binding Pockets for Highly Selective CO2 Separation.

Adsorption-based removal of carbon dioxide (CO2 ) from gas mixtures has demonstrated great potential for solving energy security and environmental sustainability challenges. However, due to similar physicochemical properties between CO2 and other gases as well as the co-adsorption behavior, the selectivity of CO2 is severely limited in currently reported CO2 -selective sorbents. To address the challenge, we create a bioinspired design strategy and report a robust, microporous metal-organic framework (MOF) with unprecedented [Mn86 ] nanocages. Attributed to the existence of unique enzyme-like confined pockets, strong coordination interactions and dipole-dipole interactions are generated for CO2 molecules, resulting in only CO2 molecules fitting in the pocket while other gas molecules are prohibited. Thus, this MOF can selectively remove CO2 from various gas mixtures and show record-high selectivities of CO2 /CH4 and CO2 /N2 mixtures. Highly efficient CO2 /C2 H2 , CO2 /CH4 , and CO2 /N2 separations are achieved, as verified by experimental breakthrough tests. This work paves a new avenue for the fabrication of adsorbents with high CO2 selectivity and provides important guidance for designing highly effective adsorbents for gas separation.

Controlled synthesis of bimetallic Nb-Fe composite oxides with high thermal stability for catalytic degradation of binary VOCs pollutants

A string of niobium-iron composite oxides with varying Nb/Fe ratios and calcination temperatures were produced and utilized to remove mixed contaminants (500 ppmv chlorobenzene and 500 ppmv toluene in air), and the structure/texture of catalytic materials were systematically characterized. The findings of the structure-performance study revealed that the physicochemical qualities of Nb2O5-Fe2O3 were greatly enhanced than pure metal oxide. The presence of iron oxide hindered the crystallization of niobium oxide, which aided in the uniform dispersion of niobium-iron components into each other. Forming FeNbO4 at Nb/Fe molar ratio of 1/2 effectively improved the thermal stability of catalyst, withstood short-time thermal shock below 850 °C. The formation of niobium-iron composite oxides improved the specific surface area, optimized the pore size distribution, increased surface acid centers, enhanced reduction/oxidation cycle, and promoted metal-metal interaction for the Nb-Fe-O catalysts. At 320 °C, the catalytic degradation efficiency of 1Nb2Fe-500 could approach 90% for chlorobenzene and 85% for toluene, and it could sustain this efficiency for at least 80 h without evident deactivation, with a high CO2 selectivity of 86%. Furthermore, the Nb-Fe-O catalysts met the environmentally benign and low-cost requirements for environmental catalysis.

Biomass waste fern leaves as a material for a sustainable method of activated carbon production for CO2 capture

In this work, we report the use of activated carbon synthesized from a sustainable material - fern leaves - as a sorbent for carbon dioxide capture applications. The resource-friendly technology for activated carbon production was applied and described. The activated carbons were prepared by chemical and physical activation and carbonization at the same time at the temperature range of 500–900 °C. This method reduces energy consumption and resources. KOH and CO2 were used as activating agents. The evaluation of the CO2 adsorption ability of the activated carbon was supported by different methods including: elemental analysis using X-ray fluorescence spectroscopy, ash content, surface area and porosity measurements, Raman spectroscopy, X-ray spectroscopy and scanning electron microscopy. Results indicated that the optimum temperature of the synthesis was 700 °C. The highest achieved adsorption of CO2 was equal to 6.77 mmol/g and 3.58 mmol/g at 0 °C and 25 °C, respectively. The activated carbons synthesized from fern leaves showed high CO2 adsorption and selectivity. Moreover, the abundance and low cost of fern leaves make them very promising carbon sources for CO2 sorbents production.

One-Step Synthesis of a High Entropy Oxide-Supported Rhodium Catalyst for Highly Selective CO Production in CO2 Hydrogenation.

High entropy oxide (HEO) has shown to be a new type of catalyst support with tunable composition-function properties for many chemical reactions. However, the preparation of a metal nanoparticle catalyst supported on a metal oxide support is time-consuming and takes multiple complicated steps. Herein, we used a one-step glycine-nitrate-based combustion method to synthesize highly dispersed rhodium nanoparticles on a high surface area HEO. This catalyst showed high selectivity to produce CO in CO2 hydrogenation with 80% higher activity compared to rhodium nanoparticle-based catalysts. We also studied the effect of different metal elements in HEO and demonstrated that high CO selectivity was achieved if one of the metals in the metal oxide support favored CO production. We identified that copper and zinc were responsible for the observed high CO selectivity due to their low *CO binding strength. During hydrogenation, a strong metal-support interaction was created through charge transfer and formed an encapsulated structure between rhodium nanoparticles and the HEO support to lower the *CO binding strength, which enabled high CO selectivity in the reaction. By combining different metal oxides into HEO as a catalyst support, high activity and high selectivity can be achieved at the same time in the CO2 hydrogenation reaction.

CO2 to Value-Added Chemicals: Synthesis and Performance of Mono- and Bimetallic Nickel–Cobalt Nanofiber Catalysts

In an epoch dominated by escalating concerns over climate change and looming energy crises, the imperative to design highly efficient catalysts that can facilitate the sequestration and transformation of carbon dioxide (CO2) into beneficial chemicals is paramount. This research presents the successful synthesis of nanofiber catalysts, incorporating monometallic nickel (Ni) and cobalt (Co) and their bimetallic blend, NiCo, via a facile electrospinning technique, with precise control over the Ni/Co molar ratios. Application of an array of advanced analytical methods, including SEM, TGA–DSC, FTIR-ATR, XRD, Raman, XRF, and ICP-MS, validated the effective integration and homogeneous distribution of active Ni/Co catalysts within the nanofibers. The catalytic performance of these mono- and bimetallic Ni/Co nanofiber catalysts was systematically examined under ambient pressure conditions for CO2 hydrogenation reactions. The bimetallic NiCo nanofiber catalysts, specifically with a Ni/Co molar ratio of 1:2, and thermally treated at 1050 °C, demonstrated a high CO selectivity (98.5%) and a marked increase in CO2 conversion rate—up to 16.7 times that of monometallic Ni nanofiber catalyst and 10.8 times that of the monometallic Co nanofiber catalyst. This significant enhancement in catalytic performance is attributed to the improved accessibility of active sites, minimized particle size, and the strong Ni–Co–C interactions within these nanofiber structures. These nanofiber catalysts offer a unique model system that illuminates the fundamental aspects of supported catalysis and accentuates its crucial role in addressing pressing environmental challenges.

(100) Preferentially Exposed Pt–Sn Nanosheets with Enhanced C–C Bond Scission as a High-Performance Catalyst for Ethanol Oxidation

Ethanol is a kind of green and sustainable liquid fuel with high energy density, which has a promising application prospect in liquid fuel cells. Improving the anti-poisoning ability and C–C bond cleavage ability of Pt-based catalysts is critical for achieving efficient ethanol oxidation reaction. Here, Pt–Sn nanosheets preferentially exposed with metastable (100) facets rather than thermodynamically stable (111) facets were synthesized through a simple hydrothermal method. Our experiment results demonstrated that the as-synthesized (100) faceted Pt–Sn nanosheets exhibited a good electrocatalytic performance, i.e., higher CO2 selectivity and better stability when compared with Pt–Sn nanoparticles with irregular morphology and commercial Pt/C. The electrochemical in situ infrared spectroscopy tests revealed that the Pt–Sn nanosheets alleviate the binding strength of reaction intermediates and further promote the electrooxidation of intermediates, thereby displaying a good CO2 selectivity at a lower potential and over a wide range of potentials. This work demonstrates the important roles of the facet effect and alloying effect in electrocatalytic properties of noble metal catalysts.

Unexpected high CO2 over C2H2 separation performance by high-silica CHA zeolite membranes

Energy-efficient CO2/C2H2 separation by membranes is critical for producing acetylene by partial combustion of natural gas. Separating CO2 and C2H2 by membranes is challenging because they have the same kinetic diameter and similar dimensions. Here we demonstrate for the first time high-silica chabazite (CHA) zeolite membranes with surprisingly high CO2 selectivity over C2H2. Both unary permeation and binary mixture permeation data demonstrated consistent high CO2 permselectivity >30. Equilibrium adsorption isotherms indicate only a slight adsorption selectivity for C2H2, implying that a high diffusion selectivity for CO2 is required for the effective separation. The dynamic uptake data, however, points to slight C2H2 diffusion selectivity in CHA zeolite crystals. Hypotheses are proposed to explain the inconsistency between crystal and membrane diffusion results.

The Facile Microwave-Assisted Coprecipitation Route to Obtain Polyoxoniobate (Na7(H3O)Nb6O19·14H2O) Nanorods Modified with Copper for CO2 Photoreduction

The CO2 reduction by solar means has been discussed as an alternative to emission abatement, a fundamental topic for sustainable, carbon-free production in the future. However, the choice of efficient systems, starting with the catalysts, is still a critical issue, especially due to the poor activity of available options. Polyoxometalates have been extensively studied as promising photocatalysts due to their semiconducting properties. Nevertheless, the synthetic conditions of polyoxoniobate are stringent due to the low reaction activity of Nb species, the lack of soluble precursors, and the narrow pH range. Unlike the literature, in the present study, we report a simple polyoxoniobate synthesis method. This synthesis method has some remarkable features, such as low processing time and temperature and good activity and selectivity in the CO2 photoreduction process. The results revealed an outstanding efficiency for the CO2 reduction reaction with a high selectivity of CO2 to CO conversion (92.5%). Furthermore, C2 compounds (e.g., acetate) were produced in the liquid phase of the reaction system. Our findings are significant for indicating the potential of polyoxoniobate for CO2 photoreduction, which opens a way to control competitive reactions with synthesis, leading to higher selectivity.

Study of CO2 permeation in Zn2+‐modified Al2O3‐carbonate membrane

AbstractMolten carbonate‐based membranes for CO2 capture have received attentions because of their high CO2 selectivity, potential energy‐saving capability and environmental friendliness. Zn2+‐modified Al2O3/carbonates membranes with the enhanced CO2 permeability have been developed in this work. Interfaces of LiAlO2 were formed on the surface of Al2O3 due to the carbonates incorporation. Microstructural and interfacial characterisation of the membrane revealed that the outermost LiAlO2 layer was due to the reactions between Li2CO3 and ZnAl2O4, resulting in the dissolution of ZnO in the molten carbonate. CO2 permeability of 0.5% ZnAl2O4/Al2O3/carbonates reached 9.12 × 10−12 mol.m−1s−1 Pa−1 at 700°C, higher than that of Al2O3/carbonates, because of the dissolved ZnO. With the increase of ZnAl2O4, CO2 permeability was decreased. The dissolved ZnO in the molten carbonates could enhance the ionic conductivity, whereas a higher amount of ZnO than its solubility will attenuate its effects on CO2 permeation.

Fabricating self-templated and N-doped hierarchical porous carbon spheres via microfluidic strategy for enhanced CO2 capture

N-doped porous biochar is a promising material for CO2 capture due to its easy regeneration and high CO2 selectivity. In this study, a new strategy was proposed to design monodisperse porous carbon spheres by microfluidics system with chitosan in-situ grow ZIF-8, utilizing ZIF-8 as a self-sacrificing template and nitrogen dopant. The mesoporous volume and N content of carbon spheres can be adjusted through the ZIF-8 content in the microfluidic system and carbonization temperature, within 0.01 ∼ 0.48 cm3/g and 3.23 ∼ 19.25%, respectively. The optimum sample (CZ2-900) exhibited superior CO2/N2 selectivity at 40. Adsorption kinetics and diffusion resistance indicated that CO2 adsorption in CZ2-900 is controlled by the process of CO2 diffusion from the surface of the basic site to mesopores. This was the first attempt at using a microfluid chip to grow ZIF-8 in chitosan skeleton and pyrolysis approach to synthesize N-doped porous carbon sphere where both additional template agent and activator are non-used, revealing the potential of microfluidic technology for the precise synthesis of highly selective CO2 capture materials.

Chemical looping reforming of toluene as bio-oil model compound via NiFe2O4@SBA-15 for hydrogen-rich syngas production

Hydrogen-rich syngas was a clean energy and an important industrial material. Based on the decoupling strategy of biomass chemical looping gasification process, this paper proposed a strategy of metal oxides embedded into molecular sieves to prepare highly dispersed and nanosized oxygen carriers for producing hydrogen-rich syngas. NiO@SBA-15, Fe2O3@SBA-15, and NiFe2O4@SBA-15 were prepared by the impregnation method, and the reaction conditions on the chemical looping reforming of toluene were investigated. The results showed that NiFe2O4@SBA-15 had the highest toluene conversion rate of 93.4% and a relatively high CO selectivity rate of 80.7%. It was confirmed that the embedding strategy can effectively enhance the nanocrystallization and dispersion of metal oxides in oxygen carriers, which could effectively reduce sintering. The inverse spinel structure of NiFe2O4 made the oxygen carrier have more metal adsorption sites and a closer reaction distance, which were beneficial to the adsorption and reaction of the fuel. After testing, the optimum reaction temperature was 750 °C, and the optimum weight hourly space velocity was 1.168 h−1. In the 10 cycles of testing of 20 NiFe2O4@SBA-15, the average conversion rate of toluene was 95.34%, the moderate selectivity of CO in the gaseous product was 94.83%, the average H/C ratio was 1.97, which indicated that the cycle stability is good. It provided a reference for developing and designing future oxygen carriers of biomass chemical looping reforming.

Hydrogenation of CO2 to Valuable C2-C5 Hydrocarbons on Mn-Promoted High-Surface-Area Iron Catalysts

Mn-promoted bulk iron catalysts with a high specific surface area (82–211 m2·g−1) were synthesized by coprecipitation followed by drying under supercritical conditions. The catalysts were tested in the CO2 hydrogenation to valuable C2-C5 hydrocarbons. The Mn-promoted iron catalysts exhibited better textural properties than the bare Fe2O3 catalyst, allowing better dispersion of the active phase, easier reduction and carburization of iron oxides and, consequently, resulting in higher catalytic activity than the bare Fe2O3 catalyst. The best activity results were obtained by catalyst promotion with a very low amount of Mn (Mn/Fe atomic ratio of 0.05). Upon steady state conditions (T = 340 °C, total pressure of 20 bar and H2/CO2 = 3), this catalyst exhibited high CO2 conversion (44.2%) and selectivity to C2-C4 hydrocarbons (68%, olefin to paraffin ratio of 0.54), while the selectivity to C5+ hydrocarbons, CH4 and CO was about 3.2, 38.5 and 5%, respectively. A close correlation was found between catalyst textural properties and CO2 conversion. The most active MnFe-0.05 catalyst exhibited high stability during 72 h of reaction related to a low amount of soft coke formation and catalyst activation through the formation of the χ-Fe5C2 phase during the on-stream reaction.