Times Higher CO2 Research Articles

Dynamics of the Boundary Layer in Pulsed CO2 Electrolysis

AbstractElectrochemical reduction of CO2 poses a vast potential to contribute to a defossilized industry. Despite tremendous developments within the field, mass transport limitations, carbonate salt formation, and electrode degradation mechanisms still hamper the process performance. One promising approach to tweak CO2 electrolysis beyond today's limitations is pulsed electrolysis with potential cycling between an operating and a regeneration mode. Here, we rigorously model the boundary layer at a silver electrode in pulsed operation to get profound insights into the dynamic reorganization of the electrode microenvironment. In our simulation, pulsed electrolysis leads to a significant improvement of up to six times higher CO current density and 20 times higher cathodic energy efficiency when pulsing between −1.85 and −1.05 V vs SHE compared to constant potential operation. We found that elevated reactant availability in pulsed electrolysis originates from alternating replenishment of CO2 by diffusion and not from pH‐induced carbonate and bicarbonate conversion. Moreover, pulsed electrolysis substantially promotes carbonate removal from the electrode by up to 83 % compared to constant potential operation, thus reducing the risk of salt formation. Therefore, this model lays the groundwork for an accurate simulation of the dynamic boundary layer modulation, which can provide insights into manifold electrochemical conversions.

Dynamics of the Boundary Layer in Pulsed CO2 Electrolysis.

Electrochemical reduction of CO2 poses a vast potential to contribute to a defossilized industry. Despite tremendous developments within the field, mass transport limitations, carbonate salt formation, and electrode degradation mechanisms still hamper the process performance. One promising approach to tweak CO2 electrolysis beyond today's limitations is pulsed electrolysis with potential cycling between an operating and a regeneration mode. Here, we rigorously model the boundary layer at a silver electrode in pulsed operation to get profound insights into the dynamic reorganization of the electrode microenvironment. In our simulation, pulsed electrolysis leads to a significant improvement of up to six times higher CO current density and 20 times higher cathodic energy efficiency when pulsing between -1.85 and -1.05 V vs SHE compared to constant potential operation. We found that elevated reactant availability in pulsed electrolysis originates from alternating replenishment of CO2 by diffusion and not from pH-induced carbonate and bicarbonate conversion. Moreover, pulsed electrolysis substantially promotes carbonate removal from the electrode by up to 83 % compared to constant potential operation, thus reducing the risk of salt formation. Therefore, this model lays the groundwork for an accurate simulation of the dynamic boundary layer modulation, which can provide insights into manifold electrochemical conversions.

Enhanced chemical looping CO2 conversion activity and thermal stability of perovskite LaCo1-xAlxO3 by Al substitution.

The reverse water-gas shift chemical looping (RWGS-CL) process that utilizes redox reactions of metal oxides is promising for converting CO2 to CO at low temperatures. Metal oxides with perovskite structures, particularly, perovskite LaCoO3 are promising frameworks for designing RWGS-CL materials as they can often release oxygen atoms topotactically to form oxygen vacancies. In this study, solid solutions of perovskite LaCo1-xAlxO3 (0 ≤ x ≤ 1), which exhibited high CO production capability and thermal stability under the RWGS-CL process, were developed. Al-substituted LaCo0.5Al0.5O3 (x = 0.5) exhibited a 4.1 times higher CO production rate (2.97 × 10-4 CO mol g-1 min-1) than that of LaCoO3 (x = 0; 0.73 × 10-4 CO mol g-1 min-1). Diffuse reflectance infrared Fourier transform spectroscopy studies suggested that an increase in CO2 adsorption sites produced by the coexistence of Al and Co was responsible for the enhancement of CO production rate. Furthermore, LaCo0.5Al0.5O3 maintained its perovskite structure during the RWGS-CL process at 500 °C without significant decomposition, whereas LaCoO3 decomposed into La2O3 and Co0. In situ X-ray diffraction study revealed that the high thermal stability was attributed to the suppression of phase transition into a brownmillerite structure with ordered oxygen vacancies. These findings provide a critical design approach for the industrial application of perovskite oxides in the RWGS-CL processes.

Layer-by-layer enriching active Ni-N3C sites in nickel-nitrogen-carbon electrocatalysts for enhanced CO2-to-CO reduction

Transition metal and nitrogen co-doped carbons (M-N-C) have proven to be promising catalysts for CO2 electroreduction into CO because of the high activity and selectivity. Effective enrichment of the active transition metal coordinated nitrogen sites is desirable but is challenging for a practical volumetric productivity. Herein, we report four kinds of model electrocatalysts to unveil this issue, which include the NC structures with surface N-functionalities, Ni-N-C_I with one layer of surface Ni-N3C sites, NC@Ni-N-C_I with surface N-functionalities and underneath Ni-N3C sites as well as Ni-N-C_II with doubled surface Ni-N3C sites. The X-ray absorption spectroscopy indicates the coordination configuration of Ni-N3C. For NC catalysts, when N-doping level increased from 3.5 at% to 8.4 at%, the CO partial current density increased from below 0.1 mA/cm2 to 3 mA/cm2. Introducing one layer of Ni-N3C onto the NC structures leads to a 54 times higher CO partial current density than that of NC, in the meantime the FECO is 66 times higher. Furthermore, doubling the density of surface Ni-N3C sites by a layer-by-layer method doubles the CO partial current density (jCO), indicating its potential to achieve a high density of active coordinated sites and current densities.

Modulating the electronic structure of Au using a heterostructure for efficient electrochemical CO2 reduction

Electrochemical CO2 reduction reaction (CO2RR) is a promising method to convert CO2 into high value carbon products. For commercial applications, the energy efficiency of CO2RR needs enhancement. Designing superior electrocatalysts is important to increase energy efficiency. This paper reports that the catalytic properties of Au can be enhanced by metal-support interaction (MSI) through CeO2 and a multi-walled carbon nanotube (MWCNT). A heterostructure for MSI is constructed by matching work function using CeO2 and MWCNT to modify the electronic structure of Au via cascade electron transfer. Through MSI, the Au/CeO2/MWCNT heterostructure exhibits 4.2 times higher CO selectivity than Au/CB, and 1.6 times higher CO selectivity than Au/CeO2 at 200 mA cm−2. As a result, the energy efficiency of CO production from CO2RR can be significantly increased using Au/CeO2/MWCNT. These findings show the importance of MSI in designing high-performance catalysts for CO2RR.

In-plane graphene incorporated borocarbonitride: Directional utilization of disorder charge via micro π-conjugated heterointerface for photocatalytic CO2 reduction

Despite the potential of hexagonal boron carbon nitride (h-BCN) with a unique graphite-like structure in the field of photocatalysis, the serious photocarrier recombination caused by disordered charge transfer has limited its practical application. In this work, we propose a conceptual design of in-plane heterostructures consisting of graphene sheet and non-phase-separated borocarbonitride (Gsheet–BCN) with micro π-conjugated heterointerface for directional utilization of disorder charge. The obtained G150–BCN exhibits 10 times longer carrier decay lifetime (16.82 ns) and 3.7 times higher CO yield (51.14 μmol g−1 h−1) than pristine BCN. It was confirmed by systematical characterizations and theoretical calculations that the micro π-conjugated heterointerface enables the directional transport of photo-induced electrons from BCN to graphene domains, resulting in enhanced carrier separation and CO2 reduction efficiency. This work developed a unique synthetic route to the design and construction of heterojunction with proper interface and rational electronic structure for efficient separation of photoexcited charges.

Separate H2 and CO production from CH4‐CO2 cycling of Fe‐Ni

AbstractCH4‐CO2 chemical looping is proposed for separate H2 and CO production using nanostructured Fe‐Ni looping materials. The product streams are obtained by first feeding CH4, which decomposes to H2 and carbon. The latter acts as reductant for the subsequent CO2 feed, which together with Fe re‐oxidation yields CO. After 25 CH4‐CO2 cycles, 10Fe5Ni@Zr has a higher H2 space–time yield than 10Fe0Ni@Zr ( vs. ), a 2.6 times higher CO yield () and lower deactivation. This improvement has two reasons: (i) CH4 activation over Ni leading to cracking, (ii) product hydrogen causing deeper FeO reduction. Deactivation follows from accumulated carbon, non‐reactive for CO2. On Ni and Fe sites, carbon can be removed by lattice oxygen or CO2, yielding more CO compared to the theoretical value for Fe oxidation. However, carbon that migrates away from the metals requires oxygen for removal, which restores the activity of the Ni‐containing samples.

Synergetic modulation of surface alkali and oxygen vacancy over SrTiO3 for the CO2 photodissociation

Photochemical conversion of CO2 into solar fuels is one of the promising strategies to reducing the CO2 emission and developing a sustainable carbon economy. For the more efficient utilization of solar spectrum, several approaches were adopted to pursue the visible-light-driven SrTiO3. Herein, oxygen vacancy was introduced over the commercial SrTiO3 (SrTiO3−x ) via the NaBH4 thermal treatment, to extend the light absorption and promote the CO2 adsorption over SrTiO3. Due to the mid-gap states resulted from the oxygen deficiency, combined with the intrinsic energy level of SrTiO3, the SrTiO3−x catalyst exhibited excellent CO productivity (4.1 μmolˑg−1ˑh−1) and stability from the CO2 photodissociation under the visible-light irradiation (λ > 400 nm). Then, surface alkalization over SrTiO3−x (OH-SrTiO3−x ) was carried out to further enhance the CO2 adsorption/activation over the surface base sites and provide the OH ions as hole acceptor, the surface alkali OH connected with Sr site of SrTiO3 could also weaken the Sr–O bonding thus facilitate the regeneration of surface oxygen vacancy under the light illumination, thus resulting in 1.5 times higher CO productivity additionally. This study demonstrates that the synergetic modulation of alkali OH and oxygen vacancy over SrTiO3 could largely promote the CO2 photodissociation activity.

Achieving simultaneous Cu particles anchoring in meso-porous TiO2 nanofabrication for enhancing photo-catalytic CO2 reduction through rapid charge separation

A facile solvo-thermal approach was successfully employed to prepare titanium oxide (TiO2) nano-aggregates with simultaneous copper particles anchoring. The as-synthesized composite could convert CO2 into CH4 and CO products under simulated solar irradiation. The impact of copper loading amounts on the photo-reduction capability was evaluated. It was found proper amount of Cu loading could enhance the activity of CO2 photo-reduction. As a result, the optimal composite (TiO2-Cu-5%) consisting of TiO2 supported with 5% (mole ratio) Cu exhibits 2.2 times higher CH4 yield and 3 times higher CO yield compared with pure TiO2. Conduction band calculated from the band gap and valence X-ray photoelectron spectroscopy (XPS) indicated TiO2 nano-aggregates have suitable band edge alignment with respect to the CO2/CH4 and CO2/CO redox potential. Furthermore, with involving of Cu particles, an efficient separation of photo-generated charges was achieved on the basis of photocurrent response and photoluminescence spectra results, which contributed to the improved photo-catalytic performance. The present work suggested that the Cu-decorated TiO2 could serve as an efficient photo-catalyst for solar-driven CO2 photo-reduction.

Microstructured ZrO2 coating of iron oxide for enhanced CO2 conversion

This work provides a way to design oxygen carriers for chemical looping. Starting from Prussian blue microcubes, Zr-promoted Fe-based materials are developed for the conversion of CO2 into CO. Thermally induced oxidative decomposition of ZrO2-coated Prussian blue under the protection of the firstly formed ZrO2 shell leads to the formation of hollow structured materials.Detailed activity tests and in-situ characterization are used to investigate the relationship between material structure, stability and CO yield from CO2 conversion. Throughout one hundred H2-CO2 redox cycles, 10 %Fe2O3/ZrO2 exhibits a 27 times higher CO space-time yield (86mmol∙s-1∙kgFe2O3-1) than unpromoted, yet hollow structured Fe2O3. This superior performance derives from the dual role that ZrO2 plays: protecting the structure, thereby improving the iron oxide sintering resistance, and facilitating the reduction of iron oxide during redox cycles. The hollow structure and the interaction between Fe2O3 and ZrO2 are the main reasons for the enhanced redox activity.

Low to high-pressure CO2 separation using amine-functionalized Mobile Composite Matter, isotherm modelling and heat of adsorption study

ABSTRACT Adsorptive CO2 separation is an effective process in carbon capture and sequestration. In this work, Benzylamine, Monoethanolamine, and Aminoethylethanolamine impregnated MCM-48 are used to enhance CO2 adsorption capacity. The materials are characterized using BET, XRD, TGA, FT-IR, FESEM and elemental analysis techniques. The CO2 uptakes are measured using high pressure sorption equipment in the temperature and pressure ranges of 25–80°C and 0–30 bar. The effect of amine loadings is pronounced in the chemisorption of CO2. The Sips isotherm model is better to correlate the experimental data than that of in Freundlich, Langmuir and Toth models. The isosteric heat of adsorption is calculated using Van’t Hoff plot in the temperature range of 25–60°C to understand the degree of heterogeneity and the surface energy distribution of the adsorbents. The CO2 adsorption capacity (3.33 mmol CO2/g) and amine efficiency (14.1 mmol CO2/g amine) of 30% AEEA impregnated MCM-48 is highest among the amine-functionalized MCM-48 adsorbents reported till date in the open literature. The MCM-48-30%AEEA shows 5.2 times higher CO2 adsorption capacity than that of in MCM-48 (0.637 mmol/g) at 25°C and 1 bar pressure. The MCM-48-30%AEEA is proposed as a promising adsorbent in low- to high-pressure post-combustion flue gas and natural gas purification.

김치 유래 복합 종균 적용이 김치의 발효과정에 미치는 영향

In this study, we compared the changes in the quality of kimchi fermented with a complex starter and a single starter. Lactic acid bacteria strains isolated from kimchi were used as starters, and fermention was performed at 10°C for 26 days. The rate of reduction in lactic acid bacteria was low, the initial pH was maintained at a constant level, and the dominance rate of the starter was high at 83% on the 15th day of fermentation with the complex starter, compared to fermentation with the control and single starters. Regarding the CO2 and free sugar content, which are factors affecting product quality and are influenced by lactic acid bacteria fermentation, 1.4 and 1.5 times higher CO2 and mannitol production, respectively, were observed during fermentation of free sugars with the complex starter than in fermentation with the single starter. These results show that the use of a complex starter can improve the distribution quality of kimchi as it results in an extended shelf life and enhanced sensory quality.

Phase junction-confined single-atom TiO2–Pt1–CeO2 for multiplying catalytic oxidation efficiency

The phase junction confinement within the TiO2–Pt1–CeO2 ensemble leads to 5 times higher CO oxidation efficiency under 300 K.

Manipulating Atomic Structures at the Au/TiO2 Interface for O2 Activation.

The metal/oxide interface has been extensively studied due to its importance for heterogeneous catalysis. However, the exact role of interfacial atomic structures in governing catalytic processes still remains elusive. Herein, we demonstrate how the manipulation of atomic structures at the Au/TiO2 interface significantly alters the interfacial electron distribution and prompts O2 activation. It is discovered that at the defect-free Au/TiO2 interface electrons transfer from Ti3+ species into Au nanoparticles (NPs) and further migrate into adsorbed perimeter O2 molecules (i.e., in the form of Au-O-O-Ti), facilitating O2 activation and leading to a ca. 34 times higher CO oxidation activity than that on the oxygen vacancy (Vo)-rich Au/TiO2 interface, at which electrons from Ti3+ species are trapped by interfacial Vo on TiO2 and hardly interact with perimeter O2 molecules. We further reveal that the calcination releases those trapped electrons from interfacial Vo to facilitate O2 activation. Collectively, our results establish an atomic-level description of the underlying mechanism regulating metal/oxide interfaces for the optimization of heterogeneous catalysis.

Ethanol as a Binder to Fabricate a Highly‐Efficient Capsule‐Structured CuO−ZnO−Al2O3@HZSM‐5 Catalyst for Direct Production of Dimethyl Ether from Syngas

AbstractThis work reports a highly efficient capsule‐structured CuO−ZnO−Al2O3@HZSM‐5 (CZA@HZSM‐5‐EtOH) core‐shell catalyst for the direct conversion of syngas to dimethyl ether by a facile physical coating method with ethanol as a binder through coating micrometer‐sized HZSM‐5 shell on the prior‐shaped millimeter‐sized CZA core, it shows 2.9 times higher CO conversion with the 2.7 times higher turnover frequency and 9.2 times higher dimethyl ether space‐time yield of the CZA@HZSM‐5‐SS catalyst prepared by a similar process but with silica sol as a binder (315.5 vs 34.3 gDME kgcat−1 h−1). The relationship between the structure and performance was explored by a variety of characterization techniques including X‐ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X‐ray diffraction (XRD), ammonia temperature programmed desorption (NH3‐TPD), nitrogen adsorption‐desorption, H2‐temperature‐programmed reduction (H2‐TPR) and H2‐TPR after oxidation of the samples by N2O. CZA@HZSM‐5‐EtOH can be considered as a highly efficient and practical catalyst for dimethyl ether synthesis from syngas. This work presents a new avenue to design other bifunctional catalysts for the cascade reactions in which the raw materials can be converted into an intermediate over the core and then the as‐formed intermediate over the core can be subsequently converted into the final product.

Inhibiting Hydrogen Evolution using a Chloride Adlayer for Efficient Electrochemical CO2 Reduction on Zn Electrodes.

A challenge in electrochemical CO2 reduction is inhibiting hydrogen evolution reaction (HER). Herein, a bulk Zn electrode was chosen to reveal the effect of specifically adsorbed Cl- ions on HER during CO2 reduction. We found that in the Cl- ion-containing electrolyte, the Zn-Cl adlayer formed because the weakly solvated Cl- ions can strip the solvation shell and form a direct chemical bond with the metal Zn. As a result, the Zn-Cl bonds on the Zn electrode notably blocked the hydrogen evolution and significantly facilitated the electron transfer process via the Cl to the vacant orbitals of inert CO2, therefore stabilizing the CO2- radical and leading to a four times higher CO Faradaic efficiency for CO2 reduction in the KCl electrolyte than that in the KHCO3 electrolyte at a moderate potential of -1.2 V versus a reversible hydrogen electrode.

Effect of surface ligands on gold nanocatalysts for CO2 reduction.

Nanoparticle catalysts display optimal mass activity due to their high surface to volume ratio and tunable size and structure. However, control of nanoparticle size requires the presence of surface ligands, which significantly influence catalytic performance. In this work, we investigate the effect of dodecanethiol on the activity, selectivity, and stability of Au nanoparticles for electrochemical carbon dioxide reduction (CO2R). Results show that dodecanethiol on Au nanoparticles significantly enhances selectivity and stability with minimal loss in activity by acting as a CO2-permeable membrane, which blocks the deposition of metal ions that are otherwise responsible for rapid deactivation. Although dodecanethiol occupies 90% or more of the electrochemical active surface area, it has a negligible effect on the partial current density to CO, indicating that it specifically does not block the active sites responsible for CO2R. Further, by preventing trace ion deposition, dodecanethiol stabilizes CO production on Au nanoparticles under conditions where CO2R selectivity on polycrystalline Au rapidly decays to zero. Comparison with other surface ligands and nanoparticles shows that this effect is specific to both the chemical identity and the surface structure of the dodecanethiol monolayer. To demonstrate the potential of this catalyst, CO2R was performed in electrolyte prepared from ambient river water, and dodecanethiol-capped Au nanoparticles produce more than 100 times higher CO yield compared to clean polycrystalline Au at identical potential and similar current.

Metallic MoO2‐Modified Graphitic Carbon Nitride Boosting Photocatalytic CO2Reduction via Schottky Junction

Directly using solar energy to realize photocatalytic reduction of CO2to hydrocarbon fuels is an effective tactics to solve the energy crisis and carbon emission. Although graphite carbon nitride (g‐C3N4) has been widely studied as a star photocatalyst for CO2reduction, the extremely fast charge recombination rate seriously limits its performance. Loading suitable co‐catalysts to construct an effective junction is considered an efficient way to solve this issue and promote photocatalytic performance. In this work, metallic molybdenum dioxide (MoO2) is dispersed on g‐C3N4nanosheets to construct a Schottky junction photocatalyst. The Schottky junction between MoO2and g‐C3N4induces efficient charge separation and transfer. As a result, the optimal MoO2/g‐C3N4Schottky junction photocatalyst exhibits a 15 times higher CH4yield and five times higher CO yield compared with pure g‐C3N4. This article provides a new route to construct a Schottky junction for boosting photocatalytic activity.

Capsule-Structured Copper-Zinc Catalyst for Highly Efficient Hydrogenation of Carbon Dioxide to Methanol.

To develop a new and efficient CO2 -to-methanol catalyst is of extreme significance but still remains a challenge. Herein, an innovative indirect two-step strategy is reported to synthesize a highly efficient capsule-structured copper-based CO2 -to-methanol catalyst (CZA-r@CZM). It consists of a structurally reconstructed millimeter-sized Cu/ZnO/Al2 O3 core (CZA-r) with intensified Cu-ZnO interactions, which is made by a facile hydrothermal treatment in an alkaline aqueous solution, and a Cu/ZnO/MgO (CZM) shell prepared by an ethylene glycol-assisted physical coating method. The CZA-r core displays 2.7 times higher CO2 hydrogenation activity with 2.0 times higher CO selectivity than the previously reported Cu/ZnO/Al2 O3 (CZA-p), whereas the CZM shell can efficiently catalyze hydrogenation of the as-formed CO from the CZA-r core to methanol as it passes through the shell. As a result, the developed capsule-structured CZA-r@CZM catalyst exhibits 2.4 times higher CO2 conversion with 1.8 times higher turnover frequency and 2.3-fold higher methanol space-time yield than the CZA-p catalyst (729.8 vs. 312.6 gMeOH kgcat -1 h-1 ). In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) experiments reveal that the CO2 hydrogenation reaction proceeds through a reverse water-gas shift reaction followed by a CO hydrogenation pathway via an *H3 CO intermediate. This work not only produces an efficient CO2 -to-methanol catalyst, but also opens a new avenue for designing superior catalysts for other consecutive transformations.

(Invited) Engineering Au/TiO2 Interface for Electron-Driven Heterogeneous Catalysis

Interfacial electronic properties of Metal/oxide have been extensively studied due to its importance for heterogeneous catalysis, however, to date, the exact role of interfacial atomic structures in governing catalytic processes still remains elusive. Herein, we demonstrate how the manipulation of atomic structures at the Au/TiO2 interface significantly alters the interfacial electron distribution and prompts O2 activation. It is discovered that at the defect-free Au/TiO2 interface, electrons transfer from Ti3+ species into Au nanoparticles (NPs) and further migrate into adsorbed perimeter O2 molecules (i.e., in the form of Au-O-O-Ti), facilitating O2 activation and leading to a 34 times higher CO oxidation activity than that on the oxygen vacancy (V o)-rich interface, at which electrons from Ti centers are trapped by interfacial V o ­on TiO2 and hardly interact with perimeter O2 molecules. Collectively, our results establish an atomic-level description of the underlying mechanism regulating the atomic structure at metal/oxide interfaces for optimizing heterogeneous catalysis.