CO2 Selectivity Research Articles

Effectively regulating Ni distribution on CeO2 through formation of Ce1−xNixO2−y solid solution to enhance the selectivity in RWGS reaction

Nickel-based catalysts frequently encounter significant challenges in achieving both high CO2 conversion and superior CO selectivity during reverse water gas shift (RWGS) reactions. To address this issue, we developed a facile strategy to prepare highly dispersed Ni/CeO2 catalysts by forming Ce1−xNixO2−y solid solutions. Under conditions of 300–600 °C, 0.1 MPa, and a gas hourly space velocity (GHSV) of 40,000 mL gcat−1 h−1, the 5Ni/CeO2-A catalyst exhibited outstanding selectivity, approaching 100 %. The RWGS reaction rate reached 1332 μmolCO gNi−1 s−1 and maintained stable for 50 h almost without deactivation. The formation of Ce1−xNixO2−y solid solution uniformly disperses numerous Ni species in the CeO2 lattice, effectively inhibiting Ni aggregation. This highly dispersed Ni weakens CO adsorption and facilitates its direct desorption without excessive hydrogenation. Moreover, the catalyst remains well-dispersed state even after reduction. The formed Ce1−xNixO2−y solid solution generates abundant oxygen vacancies on the catalyst surface, enhancing H2 dissociation. The catalyst’s pore size distribution is also modified, creating stacked pores that favor reactant diffusion and expose more active centers. In-situ DRIFTS experiments demonstrate that Ce-OH plays a pivotal role in CO2 adsorption on the 5Ni/CeO2-A catalyst, while the stabilization of bicarbonate and formate intermediates by the highly dispersed Ni further promotes catalytic performance. This study introduces a novel preparation method and provides valuable insights for designing Ni-based catalysts with exceptional CO selectivity.

Electricity-driven selectivity in the photocatalytic oxidation of methane to carbon monoxide with liquid gallium-semiconductor composite

Metal co-catalysts in oxide semiconductors are essential to enhance photocatalytic reaction, but the selectivity is often low due to the conversion of the target molecules as more reactive in comparison with the reagents. Here, we explore the influence of electricity on the activity and selectivity of liquid gallium as a co-catalyst loaded in TiO2 and ZSM-5 supports the photocatalytic oxidation of methane. We first show that the polarization of the photocatalytic chips allows a more efficient wetting of liquid gallium into the semiconductor matrices, which increases the availability of the photogenerated charge carriers for reactions and improves the overall activity of the composites. More importantly, the oxide skin surrounding the spread liquid is found to be charged, resulting in a significant reduction of the CO oxidation. An increase of the CO selectivity from 10 % to 80 % upon polarization to 50 V, with a stable photocatalytic activity reaching 0.6 ± 0.01 mmol.g-1·h-1, makes the use of electricity quite appealing to tailor liquid metals in catalysis.

Investigating adsorption properties of CO2 and CH4 in subbituminous coals from Mamu and Nsukka formations: a molecular simulation approach

The study aimed to investigate the adsorption properties of methane (CH4) and carbon dioxide (CO2) in subbituminous coals from the Mamu and Nsukka formations, focusing on the CO2-Enhanced Coalbed Methane (ECBM) method. Proximate, ultimate, and FT-IR analyses determined the quality, age, and functional categories of these coals, confirming their subbituminous nature. Using molecular dynamics (MD) simulations, a unique amorphous subbituminous coal model was developed to study adsorption phenomena. Isosteric heat and adsorption isotherms for pure CO2 and CH4 were analyzed, alongside Grand Canonical Monte Carlo (GCMC) simulations to assess CO2 adsorption selectivity in a binary CO2 and CH4 mixture. Results showed that CO2 required more isosteric heat than CH4 in single-component scenarios and demonstrated stronger electrostatic interactions with heteroatom groups in the coal model, explaining its higher adsorption preference. In binary adsorption experiments, CO2 exhibited a higher affinity under specific conditions, particularly influenced by pressure variations. At lower pressures, CO2 selectivity decreased rapidly with increasing temperature, while at higher pressures, the influence of temperature diminished. These findings have established a theoretical and practical basis for optimizing CO2-ECBM extraction in Nigeria, highlighting the preferential adsorption of CO2 over CH4 in subbituminous coals from the Mamu and Nsukka formations under varying pressure and temperature conditions. Implementing CO2-ECBM extraction and storage in Nigeria could boost economic viability and help achieve net-zero goals, using insights from this study to guide policy development.Graphical

Engineering a Cu-Pd Paddle-Wheel Metal-Organic Framework for Selective CO2 Electroreduction.

Optimizing the binding energy between the intermediate and the active site is a key factor for tuning catalytic product selectivity and activity in the electrochemical carbon dioxide reduction reaction. Copper active sites are known to reduce CO2 to hydrocarbons and oxygenates, but suffer from poor product selectivity due to the moderate binding energies of several of the reaction intermediates. Here, we report an ion exchange strategy to construct Cu-Pd paddle wheel dimers within Cu-based metal-organic frameworks (MOFs), [Cu3-xPdx(BTC)2] (BTC=benzentricarboxylate), without altering the overall MOF structural properties. Compared to the pristine Cu MOF ([Cu3(BTC)2], HKUST-1), the Cu-Pd MOF shifts CO2 electroreduction products from diverse chemical species to selective CO generation. In situ X-ray absorption fine structure analysis of the catalyst oxidation state and local geometry, combined with theoretical calculations, reveal that the incorporation of Pd within the Cu-Pd paddle wheel node structure of the MOF promotes adsorption of the key intermediate COOH* at the Cu site. This permits CO-selective catalytic mechanisms and thus advances our understanding of the interplay between structure and activity toward electrochemical CO2 reduction using molecular catalysts.

Novel polyethylene glycol methyl ether substituted polysiloxane membrane materials with high CO2 permeability and selectivity

Membrane gas separation is a promising technology for CO2 capture. However, one of the key challenges is the development of stable-in-time and highly permeable materials with increased CO2 permselectivity. In this study, two series of novel polysiloxanes with linear (methyl oligoethyleneglycol allyl ether (PEG8)) and branched (vinyl-bis- [methyltriethyleneglycol] (B-PEG4)) oxygen-containing side substituents were synthesized. By crosslinking PEG-substituted polymethylsiloxanes with polydimethylsiloxane (PDMS), a series of CO2-highly selective membrane materials with a PDMS content of 6.5–50 wt% was obtained for the first time. The effect of the side chain geometry on the sorption and transport properties of these membranes for different gases (N2, O2, CO2, CH4, C2H6) was evaluated. The specific interactions between the PEG substituents and CO2 that contribute to the enhanced selectivity of these materials were identified. The introduction of both linear and branched PEG substituents leads to in an increase in the selectivity of diffusion, solubility, and CO2/gas permeability, compared to PDMS.The CO2/N2 and CO2/CH4 permselectivities and diffusion selectivities are increases, as well as the permeability coefficients of the membrane materials for all gases are decreases, as the PDMS content reduces from 50 % to 6.5 % wt. Within the PDMS concentrations range between 6.5 and 12.5 % wt., there was a sharp increase in diffusion and permeability coefficients of membrane materials. For this polysiloxanes with linear PEG substituents, an unexpectedly high diffusion selectivity for CO2 was observed. Such value determined the maximum CO2 permselectivity among the polysiloxanes studied. The most promising membrane material from these series was identified as PEG8-substituted polysiloxane with 12.5 wt% PDMS. It has a CO2 permeability coefficient of 1300 Barrer, CO2/N2 selectivity of 37 and CO2/CH4 selectivity of 10.

Study on the methanol aqueous phase reforming performances over Pt/CeO2 catalysts: The effect of CeO2 morphologies and oxygen vacancies

CeO2-supported catalysts have been extensively studied for hydrogen production via aqueous phase reforming of methanol (APRM) although several issues remain unclear, such as the influence of different morphologies on catalytic performance and the role of their oxygen vacancies in enhancing hydrogen yield. In this study, CeO2 with three distinct morphologies were synthesized, and platinum was loaded onto these supports via a photoreduction method to produce highly-dispersed catalysts designated as Pt/CeO2-R (rod-shaped), Pt/CeO2-C (cubic) and Pt/CeO2-O (octahedral) respectively. A series of APRM experiments suggested that Pt/CeO2-R exhibited the highest hydrogen production capacity (146.2 mmol-H2/gcat.). This superior performance is likely to attribute to the highly-dispersed Pt on the rod-shaped CeO2 support, which provides a substantial number of active metal sites, promoting the efficient adsorption and activation of methanol throughout the process. The anchoring effect of platinum on support surface and redox properties of Pt species were also thoroughly investigated with various characterization techniques. Further analysis revealed a strong correlation between turnover frequency (TOF) and the presence of oxygen vacancies on Pt/CeO2. Notably, abundant oxygen vacancies on catalyst enhanced substrate adsorption and the rapid conversion of CO* intermediates adsorbed on platinum, which accelerated the water-gas shift (WGS) reaction through a faster redox pathway, leading to higher hydrogen production with low CO selectivity.

Prolonging Charge Carrier Lifetime via Intraband Defect Levels in S-Scheme Heterojunctions for Artificial Photosynthesis.

S-scheme heterostructure photocatalysts, distinguished by unique charge-transfer pathways and exceptional catalytic redox capabilities, have found widespread applications in addressing challenging chemical processes, including the photocatalytic reduction of CO2 with a high reaction barrier. Nevertheless, the influence of intraband defect levels within S-scheme heterojunctions on charge separation, carrier lifetime, and surface catalytic reactions has, for the most part, been overlooked. Herein, we develop a tunable defect-level-assisted strategy to construct an electron reservoir, effectively prolonging the lifetime of charge carriers through the rapid capture and gradual release of photoelectrons within WO3-x/In2S3 S-scheme heterojunctions, as authenticated by femtosecond transient absorption spectroscopy and theoretical simulations. The surface photoredox mechanism, unraveled by Gibbs free energy calculations, demonstrates that oxygen-vacancy-induced defect states in WO3-x/In2S3 heterojunctions unlock the rate-determining H2O oxidation into free oxygen molecules by forming metastable oxygen intermediates, contributing to the facilitation of H2O photooxidation. This distinct role, combined with the extended carrier lifetime, results in boosted CO2 photoreduction with nearly 100 % CO selectivity in the absence of any photosensitizer or scavenger. Our work sheds light on the role of controllable defect levels in governing charge transfer dynamics within S-scheme heterojunctions, thereby inspiring the development of more advanced photocatalysts for artificial photosynthesis.

Building ternary mixed matrix membranes with ionic liquid-functional COF fillers for efficient CO2 separation

Developing novel mixed matrix membranes (MMMs) with high CO2 permeability and selectivity, while overcoming the limitations of traditional MMMs, presents a significant challenge. Herein, we reported a simple and straightforward method to develop ternary mixed matrix membranes (TMMMs) featuring excellent matrix-filler compatibility for CO2 separation through the modification of the covalent organic framework (COF) using ionic liquid (IL) additives. A novel IL ([Bmim][Tf2N]) encapsulated COF filler (IL@TpTta-COF) was facilely prepared, and incorporated into the PIM-1 matrix to develop a series of TMMMs.The incorporation of ILs with high affinity towards CO2 within the TpTta-COF improves the molecular sieving effect of the functional filler, and helps to enhance polymer-filler interfacial compatibility, and mitigates particle agglomeration during the preparation of TMMMs. Notably, the resultant TMMMs demonstrate remarkable stability, and achieve an impressive CO2 permeability of 10,035 Barrer and an excellent selectivity of 30.2 for CO2/N2 mixtures, exceeding the Robeson upper bound in 2019. This research underscores the potential of integrating functional additives to produce novel COF-based MMMs with enhanced CO2 separation efficiency for industrial applications.

Synergistic effect between amino functional group and graphene oxide in selective photocatalytic CO2 reduction to formic acid

Carbon dioxide (CO2) reduction to useful possible chemicals is of great significance to energy supply, while formic acid is one of the most desirable product among the many other chemicals. In this study, a novel reduced graphene oxide (rGO) coupled doped TiO2 with NH2-MIL-125(Ti)(TiM) composite photocatalysts TiO2@rGO@TiM was designed by using a solvent-thermal method to facilitate the photocatalytic CO2 reduction to formic acid under the visible light. Benefiting from the special structure of TiO2@rGO@TiM, up to a rate of 113 µmol•g−1•h−1 of formic acid was produced in a photochemical process, which is four times greater than that of pure TiO2 and TiM without any sacrificial agent. The results of structure and photoelectrochemical characterization demonstrated that the enhanced production of formic acid was attributable to the synergistic effect generated by the incorporation of the amino functional group and reduced graphene oxide, resulting in the effective photo-induced carrier spatial segregation and transfer and improving the catalytic activity.

Nickel Dynamics Switches the Selectivity of CO2 Hydrogenation

AbstractThe Reverse Water Gas‐Shift reaction (CO2+H2 CO+H2O) allows to balance syn‐gas under industrial conditions. Nickel has been suggested as a potential catalyst but the temperature required is too high, more than 800 °C, limiting practical implementation but when lowering the temperature methanation occurs. Simulations via Density Functional Theory on well‐defined surfaces have systematically failed to reproduce these experimental results. But under reaction conditions, Ni surfaces are not static and DFT models coupled to microkinetics show that low temperatures (high CO coverages) drive the generation of Ni adatoms that are the active sites for methanation. At higher temperatures, the adatom population decreases, and the selectivity towards CO increases. Thus the mechanism behind the selectivity switch is driven by the dynamics induced by reaction intermediates. Our work contributes to the inclusion of dynamic aspects of materials under reaction conditions in the understanding of complex catalytic behaviour.

Towards superior CO2RR catalysts: Deciphering the selectivity puzzle over dual-atom catalyst

The electrocatalytic CO2 reduction reaction (CO2RR) is one of the most important electrocatalytic reactions. Starting from a well-defined *CO intermediate, the CO2RR can bifurcate into two pathways, either forming a hydrogenation product by *CO bond hydrogenation or leading to CO desorption by *C bond cleavage. However, it is perplexing why many dual-atom catalysts (DACs) exhibit high CO selectivity in experiments, despite previous theoretical studies arguing that the *CO bond hydrogenation is thermodynamically more favorable than the *C bond breaking. Furthermore, the selectivity is contingent upon the potential and is perturbed by the hydrogen evolution reaction (HER), which is far from clear. Using ab initio molecular dynamics and a “slow-growth” sampling method to evaluate the potential-dependent kinetics, we uncover the selectivity origin of CO2RR to CO on a typical NC-based DAC (CuFe-N6-C). Importantly, the results show that at higher CO* coverage, CO* desorption kinetics are accelerated, while the competing *CO bond hydrogenation reaction is inhibited at varying potentials. Furthermore, the selectivity of the HER is observed to increase as the potential decreases. However, at higher CO* coverage, the energy barrier for the *C bond cleavage is lower than that for HER, suggesting that HER is suppressed on CuFe-N6-C. Our work unlocks a long-standing puzzle about the selectivity of important DAC catalysts for CO2RR and provides insights for more effective catalyst design.

CoZnyB/Nb2CTx(MXene) for cinnamaldehyde hydrogenation to cinnamyl alcohol: Effects of Zn

Amorphous cobalt boride (CoB) catalysts have superior CO selectivity for cinnamaldehyde (CAL) hydrogenation and low activity. This work prepared CoZnyB/Nb2CTx(MXene) catalysts with different Zn ratios (y=0, 0.025, 0.05, 0.075, and 0.1) via a wet reduction method, which were subsequently utilized in the selective hydrogenation of CAL to cinnamyl alcohol (COL). The CoZn0.05B/Nb2CTx catalyst exhibited superior low-temperature activity, with 91.4 % CAL conversion and a 66.3 % COL yield. SEM and XPS analyses further revealed that Zn enhanced the catalytic activity by generating smaller amorphous alloy particles and exposing more Co0 active sites. Furthermore, the NH3-TPD and adsorbed pyridine DRIFTS results showed that Zn modified the surface acidity of the CoZnyB/Nb2CTx catalyst by generating both Lewis acid sites and Brønsted acid sites, which can facilitate the adsorption of CAL. This work demonstrated that Zn can increase the number of exposed active sites and modify the surface acid properties of amorphous CoB catalysts.

Fibrous Pb(II)‐Based Coordination Polymer Operable as a Photocatalyst and Electrocatalyst for High‐Rate, Selective CO2‐to‐Formate Conversion

AbstractA nonporous [Pb(tadt)]n (tadt = 1,3,4‐thiadiazole‐2,5‐dithiolate) coordination polymer, KGF‐9, with a 2D infinite (−Pb−S−)n structure has been previously reported as a precious‐metal‐free photocatalyst for selective CO2‐to‐formate conversion under visible light. In the present work, a microwave (MW)‐assisted solvothermal reaction is used to synthesize KGF‐9 with improved physicochemical properties and catalytic activity. Compared with KGF‐9 prepared by the previously reported methods, that prepared by the new synthesis route exhibited a greater specific surface area, greater crystallinity, and greater photoconductivity. These improved properties led to a drastic increase of the apparent quantum yield (AQY) for selective formate production, from 2.6 to 25% at 400 nm; this AQY represents a record‐high value among reported heterogeneous photocatalysts for CO2‐to‐formate conversion. Interestingly, the AQY for formate production is unchanged irrespective of the light intensity (0.04–14 mW cm−2), indicating little contribution of charge accumulation in the bulk during the reaction (i.e., indicating efficient charge transport to surface reactants). When composited with Ketjen Black, KGF‐9 enabled the electrochemical conversion of CO2 to formate in aqueous solution while maintaining a high selectivity. A high‐rate reduction of CO2 to formate with a total absolute current density of 200–300 mA cm−2 is achieved, with a Faradaic efficiency (FE) of >90%.

Encapsulation of an Au25 Nanocluster inside a Porphyrin Nanoring Enhances Singlet Oxygen Generation and Photo-Electrocatalytic CO2 Reduction.

The synthesis of molecular host-guest complexes with enhanced performance, relative to those of their components, is a central theme in supramolecular chemistry. Here we explore a host-guest system consisting of an atomically precise gold nanocluster bound inside a zinc porphyrin nanoring. UV/Vis absorption and fluorescence titrations with different sized nanorings revealed strong binding between a pyridinethiol-coated Au25 nanocluster and a nanoring consisting of six zinc porphyrin units, and complexation is confirmed by mass spectrometry. Formation of this assembly enhances the stability of the gold nanocluster. The host-guest complex also exhibits remarkable activity and selectivity for photochemical CO2 to CO conversion and singlet oxygen generation.

Encapsulation of an Au25 Nanocluster inside a Porphyrin Nanoring Enhances Singlet Oxygen Generation and Photo‐Electrocatalytic CO2 Reduction

AbstractThe synthesis of molecular host–guest complexes with enhanced performance, relative to those of their components, is a central theme in supramolecular chemistry. Here we explore a host‐guest system consisting of an atomically precise gold nanocluster bound inside a zinc porphyrin nanoring. UV/Vis absorption and fluorescence titrations with different sized nanorings revealed strong binding between a pyridinethiol‐coated Au25 nanocluster and a nanoring consisting of six zinc porphyrin units, and complexation is confirmed by mass spectrometry. Formation of this assembly enhances the stability of the gold nanocluster. The host‐guest complex also exhibits remarkable activity and selectivity for photochemical CO2 to CO conversion and singlet oxygen generation.

Effect of Calcination Temperature in Large-Aperture Medium-Entropy Oxide (FeCoCuZnNa)O on CO2 Hydrogenation for Light Olefins

The catalytic hydrogenation of carbon dioxide is not only a way to mitigate the greenhouse effect but also provides high-value chemicals. In this work, a medium-entropy oxide catalyst (FeCoCuZnNa)O was prepared by the sol–gel method for highly active and selective hydrogenation of CO2 to value-added hydrocarbons. When reacted at 290 °C, 2.5 MPa, and 2500 mL·gcat−1·h−1, the CO2 conversion and selectivity of olefin were affected by the calcination temperature of the catalyst, and the best performances were 39% and 41.3%. The large pore size and oxygen vacancies (Ov) formed by (FeCoCuZnNa)O promote the activation of CO2 and promote the C-C coupling reaction of Fe5C2 in a hydrogenation reaction. The promoted C-C coupling reaction was related to the surface enrichment of iron species. The presence of Ov also inhibited the excessive hydrogenation reaction, further improving the selectivity of light olefins. In addition, (FeCoCuZnNa)O did not show significant deactivation within 75 h, indicating that the catalyst has strong industrial potential.

Phase-transformable metal-organic polyhedra for membrane processing and switchable gas separation

The capability of materials to interconvert between different phases provides more possibilities for controlling materials’ properties without additional chemical modification. The study of state-changing microporous materials just emerged and mainly involves the liquefication or amorphization of solid adsorbents into liquid or glass phases by adding non-porous components or sacrificing their porosity. The material featuring reversible phases with maintained porosity is, however, still challenging. Here, we synthesize metal-organic polyhedra (MOPs) that interconvert between the liquid-glass-crystal phases. The modular synthetic approach is applied to integrate the core MOP cavity that provides permanent microporosity with tethered polymers that dictate the phase transition. We showcase the processability of this material by fabricating a gas separation membrane featuring tunable permeability and selectivity by switching the state. Compared to most conventional porous membranes, the liquid MOP membrane particularly shows the selectivity for CO2 over H2 with enhanced permeability.

Efficient Selective CO2 Composite Sorbent from Amino Acid Ionic Liquids and Silica Gel: 2H NMR Spectroscopy Provides Insight on the CO2-Binding Mechanism and in-Pore Microscopic Viscosity.

A promising CO2 sorbent based on the [Emim][Gly] (1-ethyl-3-methylimidazolium glycinate)/silica gel composite has been studied. CO2 sorption experiments have shown that the optimum loading of [Emim][Gly] ionic liquid in silica gel is 40 wt.%. The 2-step process CO2 binding mechanism in [Emim][Gly] has been proposed based on the results of sorption experiments, 2H NMR spectroscopy and ab-initio calculations. The impact of CO2 on the microscopic viscosity and the dynamical melting of ionic liquid has been thoroughly investigated. 2H NMR spectroscopy has revealed that CO2 strongly binds cation and anion in [Emim][Gly], forcing them to move in a correlated fashion.

The Effect of Electrolyte pH and Impurities on the Stability of Electrolytic Bicarbonate Conversion.

Electrolytic bicarbonate conversion holds the promise to integrate carbon capture directly with electrochemical conversion. Most research has focused on improving the faradaic efficiencies of the system, however, the stability of the system has not been thoroughly addressed. Here, we find that the bulk electrolyte pH has a large effect on the selectivity, where a higher pH results in a lower selectivity. However, the bulk electrolyte pH has no effect on the stability of the system. A decrease in CO selectivity of 30% was observed within the first three hours of operation in an optimized system with 3 M KHCO3 and gap between the membrane and electrode. Single-pass electrolyte experiments at various constant pH values (8.5, 9.0, 9.5, and 10.0), show that only at a pH of 10 the CO selectivity was stable during three hours, reaching a faradaic efficiency toward CO of only 18% as compared to an initial 55% at pH 8.5. Trace metal impurities present in the electrolyte were found to be the cause of the decrease in stability as these deposit on the electrode surface. By complexing the trace metal ions with ethylenediaminetetraacetic acid (EDTA), the metal deposition was avoided and a stable CO selectivity was obtained.

Molecular catalyst coordinatively bonded to organic semiconductors for selective light-driven CO2 reduction in water

The selective photoreduction of CO2 in aqueous media based on earth-abundant elements only, is today a challenging topic. Here we present the anchoring of discrete molecular catalysts on organic polymeric semiconductors via covalent bonding, generating molecular hybrid materials with well-defined active sites for CO2 photoreduction, exclusively to CO in purely aqueous media. The molecular catalysts are based on aryl substituted Co phthalocyanines that can be coordinated by dangling pyridyl attached to a polymeric covalent triazine framework that acts as a light absorber. This generates a molecular hybrid material that efficiently and selectively achieves the photoreduction of CO2 to CO in KHCO3 aqueous buffer, giving high yields in the range of 22 mmol g−1 (458 μmol g−1 h−1) and turnover numbers above 550 in 48 h, with no deactivation and no detectable H2. The electron transfer mechanism for the activation of the catalyst is proposed based on the combined results from time-resolved fluorescence spectroscopy, in situ spectroscopies and quantum chemical calculations.