CO Chemisorption Research Articles

Design of alkali metal oxide adsorbent for direct air capture: Identification of physicochemical adsorption and analysis of regeneration mechanism

Direct air capture (DAC) represents an advanced negative carbon emission technology, with the key being high-performance CO2 adsorbents. First, this work carefully identifies CO2 physisorption and chemisorption by CaO/HcATP (CaO loaded on acid-modified attapulgite) as DAC adsorbent. The chemisorption of amorphous "CaO" plays a crucial role in both the adsorption capacity and rate, with contributions of 66.8 % and 50.85 %, respectively. The adsorption capacity of CaO/HcATP is only 212.4 ± 25.7 µmol/g via the simple CO2 physisorption and improved by 426.7 µmol/g owning to the chemisorption of amorphous CaO. Second, the concentration of silanol groups on CaO/HcATP plays a pivotal role in the adsorption process. The concentration of silanol groups decreases to 3.85 OH/nm2 after undergoing 30 cycles of adsorption-desorption. Then it increases to 9.54 OH/nm2 by adsorbing the moisture in the air, resulting in a recovered adsorption capacity of 90.7 %. Furthermore, the pseudo-first-order adsorption kinetics model effectively predicted the experimental results. Finally, the dual loop of CO2 capture and regeneration is summarized using the CaO/HcATP as DAC adsorbent. The amorphous "CaO" interacts with the surface silanol of HcATP, synergistically capturing CO2 in the form of "CaO···CO2", which reduces desorption energy consumption. The wetting property of HcATP contributes to the regeneration of CaO/HcATP. This work contributes to establishing fundamental principles for designing cost-effective DAC adsorbents.

The effect of humidity on the enhanced CO2 adsorption of amine-functionalized microporous activated carbon.

Cost-effective and environmentally friendly sorbents were created for capturing CO2 by incorporating monoethanolamine (MEA) and tetraethylenepentamine (TEPA) onto a microporous activated carbon (AC) material. The application of a KOH reagent enhanced the surface area and pore volume of the carbon material. The BET, SEM, EDX, and FTIR techniques were employed to analyze the structural and surface properties of the developed samples. Raw AC possessed the highest surface area and largest micropore volume equal to 786 m2/g and 0.33 cm3/g, respectively. The amine impregnation increased the nitrogen content of the carbon material, but it also significantly reduced the BET surface area and total pore volume, which are primarily responsible for physically adsorbing CO2 towards ACs. The CO2 adsorption performance of the raw and impregnated ACs was experimentally evaluated using thermogravimetric analysis (TGA) at different adsorption temperatures (25 and 50 °C) and CO2 concentrations (10 and 90 vol.%). The findings demonstrated that the raw AC exhibited the highest capacity for CO2 adsorption. Specifically, at a temperature of 25 °C and pressure of 1 bar (10 vol.% CO2, N2 balance), the raw AC achieved an uptake of 1.2 mmol/g, which was 60.3% and 79.3% higher compared to the CO2 uptake of MEA-AC (0.5 mmol/g) and TEPA-AC (0.3 mmol/g), respectively. It is surprising that the combined uptake of CO2 + H2O increased by 12.5 and 23.4 wt.% (equivalent to 7 and 13 mmol/g) for MEA-AC and TEPA-AC, respectively, when humid flue gas was taken into account under the conditions of 50 °C, with 10 vol.% CO2, 4 vol.% H2O, and N2 balance. These results indicate that the presence of H2O facilitates the chemisorption of CO2 by the novel and highly promising carbon-based sorbent prepared in this study, leading to an increased capacity for adsorbing CO2 under water vapor containing flue gases.

Tuning the Electronic Properties of CumAgn Bimetallic Clusters for Enhanced CO2 Activation.

The urgent demand for efficient CO2 reduction technologies has driven enormous studies into the enhancement of advanced catalysts. Here, we investigate the electronic properties and CO2 adsorption properties of CumAgn bimetallic clusters, particularly Cu4Ag1, Cu1Ag4, Cu3Ag2, and Cu2Ag3, using generalized gradient approximation (GGA)/density functional theory (DFT). Our results show that the atomic arrangement within these clusters drastically affects their stability, charge transfer, and catalytic performance. The Cu4Ag1 bimetallic cluster emerges as the most stable structure, revealing superior charge transfer and effective chemisorption of CO2, which promotes effective activation of the CO2 molecule. In contrast, the Cu1Ag4 bimetallic cluster, in spite of comparable adsorption energy, indicates insignificant charge transfer, resulting in less pronounced CO2 activation. The Cu3Ag2 and Cu2Ag3 bimetallic clusters also display high adsorption energies with remarkable charge transfer mechanisms, emphasizing the crucial role of metal composition in tuning catalytic characteristics. This thorough examination provides constructive insights into the design of bimetallic clusters for boosted CO2 reduction. These findings could pave the way for the development of cost-effective and efficient catalysts for industrial CO2 reduction, contributing to global efforts in carbon management and climate change mitigation.

Shape-dependent CO chemisorption on Pt13 nanocluster deposited on reduced TiO2(110)

This article delves into the impact of nanoparticle shape on CO chemisorption and the reactivity of Pt13 nanocatalysts supported on reduced TiO2(110). Distinct reactivity in carbon monoxide adsorption is observed among nanoparticles, all composed of 13 platinum atoms but varying in shape. The calculated formation and CO adsorption energies are correlated to the electronic properties of the system and the oxidation states of the Pt atoms involved. Through an analysis of band shifting during the deposition of Pt clusters onto the oxide, and a comparison of the valence band maximum with the measured oxidation potential for the CO to CO2 reaction, we make predictions about the system's oxidation capability in this reaction. Our findings suggest that Pt13 clusters with cuboid, double triangle DT2 and octahedral Oh shapes, supported on the surface, are particularly advantageous for catalyzing the conversion of CO to CO2. Within these geometries, several configurations for CO adsorption are evaluated, focusing on the ratio between anionic and cationic Pt sites. This ratio appears to govern the activity for CO oxidation, aligning with recent experimental reports.

Selective Hydrogenolysis Conversion of Polyethylene into Alkyl Oil Over Iridium-based Catalyst.

Plastic not only brings convenience but also places a great burden on the environment. Utilizing plastic as a low-cost feed-stock for producing valuable chemicals and fuels is one of the most attractive directions. Among the huge types of plastics, polyolefins (PO), especially polyethylene (PE), were the most abundant type and the most difficult to upgrade. Hydrocracking and hydrogenolysis operate at relatively low reaction temperatures which show promising applications. Herein, Iridium-based catalysts were developed and proved to be effective in PE hydrogenolysis under relatively mild conditions. Catalysts were characterized by TEM, HRTEM, SEM, HAADF-STEM, XPS, CO chemisorption and H2 chemisorption etc. The Ir catalysts showed similar reactivity but better selectivity for liquid products than Ru under similar conditions. A highest 92.7 % percent of liquid products could be obtained under 250 °C, 3 MPa of H2 in 8 hours with Ir/γ-Al2O3 catalyst. The support could also affect the performance, including Lewis acid amount, surface areas, and morphology. And we suppose Iridium catalysts could serve as another choice for plastic hydrogenolysis under mild conditions.

Ni/Al2O3 promoted by CeO2, CeO2-La2O3, and CeO2-ZrO2 supported on cordierite monoliths for methane steam reforming

Promotion of Ni-based catalysts supported on cordierite monoliths was investigated for methane steam reforming. First, cordierite was coated by ɣ-Al2O3, and then the catalyst components were deposited by successive wet impregnation, obtaining: NiAl/cordierite; NiCeAl/cordierite; NiCeLaAl/cordierite and NiCeZrAl/cordierite. The monolithic catalysts were characterized by adherence test, Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDX), N2 physisorption, Temperature-Programmed Reduction (TPR), CO chemisorption, as well as by in situ and operando Raman spectroscopy. The catalyst layer showed very good adherence and distribution of the components over the cordierite surface. The catalysts presented high activity between 600 and 800 °C at a H2O/CH4 molar ratio=1.5, except for NiCeAl/cordierite, which obtained lower methane conversion, associated with its lower nickel dispersion. The catalysts showed high stability under extreme conditions for carbon deposition (600 °C and H2O/CH4=1), over 90 h on stream. Raman spectroscopy revealed the presence of oxygen vacancies, which may be increased during ceria reduction, highlighting the potential of these catalysts to enhance the resistance to deactivation by carbon deposition in long-term tests.

Cascade CO2 Insertion in Carbanion Ionic Liquids Driven by Structure Rearrangement.

The CO2 chemisorption in state-of-the-art sorbents based on oxide/hydroxide/amine moieties is driven by strong chemical bonding formation in the carbonate/bicarbonate/carbamate products, which in turn leads to high energy input in sorbent regeneration. In addition, the CO2 uptake capacity was limited by the active sites' utilization efficiency, with each active site incorporating one CO2 molecule or less. In this work, a new concept and generation of sorbent was developed to achieve cascade insertion of multiple CO2 molecules by leveraging structure rearrangement as the driving force, leading to in situ generation of extra CO2-binding sites and significantly reduced energy input for CO2 release. The designed ionic liquids (ILs) containing carbanions with conjugated and asymmetric structure, deprotonated (methylsulfonyl)acetonitrile ([MSA]) anion, allowed the cascade insertion of two CO2 molecules via consecutive C-C and O-C bond formations. The proton transfer and structure rearrangement of the carboxylic acid intermediates played critical roles in stabilizing the first integrated CO2 and generating extra electron-rich oxygen sites for the insertion of the second CO2. The structure variation and reaction pathway were confirmed by operando spectroscopy, magnetic resonance spectroscopy (NMR), mass spectroscopy, and computational chemistry. The energy input in sorbent regeneration could be further reduced by harnessing the phase-changing behavior of the carbanion salts in ether solutions upon reacting with CO2, avoiding the energy consumption in heating the solvent. The fundamental insights obtained herein provide a promising approach to greatly improve the CO2 sorption performance via sophisticated molecular-scale structural engineering of the sorbents.

Highly Efficient and Reversible Carbon Dioxide Capture by Carbanion-Functionalized Ionic Liquids.

Due to the active unstable nature of carbon anions, it is challenging to develop carbanion-functionalized ionic liquids (ILs) for efficient and reversible carbon dioxide (CO2) capture. Here, a series of carbanion-based ILs with large conjugated structures were designed and a promising system was achieved through tuning the nucleophilicity of carbanions and screening the cation. The ideal carbanion-functionalized IL trihexyl(tetradecyl)phosphonium N,N-diethycyanoacetoamide ([P66614][DECA]) showed equimolar chemisorption of CO2 ( up to 0.98 mol CO2/mol IL) under ambient pressure and excellent absorption rate. What's more, the combined CO2 can be released easily, leading to excellent reversibility due to high stability of anion conjugated structures. More importantly, the presence of water had negligible effect on the absorption capacity, which makes it potential to be applied to the CO2 capture in industrial flue gas. The chemisorption mechanism of the carbanion and CO2 was confirmed by spectroscopic investigations and DFT calculations, where carboxylic acid product was formed through proton transfer after the carbanions reacted with CO2. Considering that high capacity, quick rate as well as excellent reversibility, these carbanion-functionalized ILs should certainly represent competitive candidates for further scale up and practical application in CO2 capture.

A detailed characterization study of Ni/CeO2 catalysts identifies Ni availability as the primary factor affecting CO2 methanation performance

The structure of a catalyst has a strong impact on its performance. Here we investigate the physicochemical properties of Ni/CeO2 in an attempt to draw structure–activity relationships for CO2 methanation. A combination of characterisation methods (X-ray diffraction (XRD), 4D Scanning Transmission Electron Microscopy (4D-STEM), CO chemisorption and oxygen storage capacity study etc.) clearly demonstrates the effect of Ni crystallite size, Ni availability (i.e. catalytically accessible Ni) and oxygen vacancies at the Ni-CeO2 interface in Ni/CeO2 during CO2 methanation. Among them, the role of exposed Ni active sites is highlighted, and two possible optimisation schemes i.e. changing the support calcination temperature and the final calcination atmosphere are proposed to obtain a better dispersal of Ni NPs (nanoparticles) on CeO2. Both modification methods do not affect the reaction route, and the activity differences of Ni/CeO2 can be explained by the various hydrogenation rate of formate species, as confirmed by in situ diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) measurements.

Impact of Inverse Manganese Promotion on Silica-Supported Cobalt Catalysts for Long-Chain Hydrocarbons via Fischer–Tropsch Synthesis

One of the challenges in Fischer–Tropsch synthesis (FTS) is the high reduction temperatures, which cause sintering and the formation of silicates. These lead to pore blockages and the coverage of active metals, particularly in conventional catalyst promotion. To address the challenge, this article investigates the effects of the preparation method, specifically the inverse promotion of SiO2-supported Co catalysts with manganese (Mn), and their reduction in H2 for FTS. The catalysts were prepared using stepwise incipient wetness impregnation of a cobalt nitrate precursor into a promoted silica support. The properties of the catalysts were characterized using XRD, XPS, TPR, and BET techniques. The structure–performance relationship of the inversely promoted catalysts in FTS was studied using a fixed-bed reactor to obtain the best performing catalysts for heavy hydrocarbons (C5+). XRD and XPS results indicated that Co3O4 is the dominant cobalt phase in oxidized catalysts. It was found that with increase in Mn loading, the reduction temperature increased in the following sequence 10%Co/SiO2 < 10%Co/0.25%Mn-SiO2 < 10%Co/0.5%Mn-SiO2 < 10%Co/3.0%Mn-SiO2. The catalyst with the lowest Mn loading, 10%Co/0.25%Mn-SiO2, exhibited higher C5+ selectivity, which can be attributed to less MSI and higher reducibility. This catalyst showed the lowest CH4 selectivity possibly due to lower H2 uptake and higher CO chemisorption.

Efficient CO2 Desorption Catalysts: from Material Design to Kinetics Analysis and Application Evaluation

Catalytic amine-solvent regeneration has been validated as an energy-saving strategy for CO2 chemisorption by boosting reaction kinetics under mild conditions. The upscale performance evaluation and long-term durability are indispensable steps for industrial application but have been scarcely reported thus far. Here, we report a ZrO2/Al2O3 pack catalyst that possesses strong metal oxide-support interactions, a porous structure, active and stable Zr–O–Al coordination, promoted proton transfer and a 40.7% decrease in the energy activation of carbamate decomposition, which significantly accelerates CO2 desorption kinetics. The upscale experiment and cost evaluation based on industrial flue gas revealed that the use of packing catalysts can reduce energy consumption by 27.56% and optimize the overall cost by 10.49%. The active sites present excellent stability in alkaline solvents. This work is the first to investigate the ability of high-technology readiness (technology readiness level at 6 (TRL 6)) for catalytic amine-solvent regeneration, providing valuable insights for potential applications involving efficient CO2 capture with catalyst assistance.

Performance of mesoporous ceria supported Ru catalysts for oxidative degradation of aqueous solution of plastic monomer bisphenol A in a fixed bed flow reactor: Structure-activity insights, optimization of reaction conditions, kinetics, catalyst stability, and characterization

One of the important defects is the oxygen vacancy on metal oxide surfaces, and the defects function as the active sites in different catalytic reactions. Herein, Ru/CeO2-rod and Ru/CeO2-cube catalysts were prepared via the ESI method, and CeO2-rod and CeO2-cube supports were synthesized by hydrothermal processes. The activity of these catalysts was tested in a high-pressure fixed-bed reactor, and the oxidative degradation of industrial organic raffinate containing the plastic chemical bisphenol A (BPA) was compared with their oxygen vacancy concentration estimated after calcination. The Ru/CeO2-rod catalyst showed higher catalytic activity with 90 % BPA conversion at 453 K, 30 bar, 0.5 % Ru metal content, 15 O2 to BPA molar ratio, and 2 g−1.h−1 BPA feed weight hourly space velocity (WHSV). In contrast, 80 % BPA degradation was achieved using the Ru/CeO2-cube catalyst under similar reaction conditions. The higher oxygen vacancy concentration and the higher interfacial surface area between metal and the support of the CeO2-rod supported Ru catalyst triggered the higher BPA degradation at low oxidation conditions. Besides, the atomic ratio, Ce3+/Ce4+, is lower in the Ru/CeO2-rod catalyst than that of the Ru/CeO2-cube catalyst, indicating the higher Ce4+ available in the catalyst, leading to the higher degradation of BPA. Moreover, the Ru/CeO2-rod catalyst possesses {111} planes that boost the generation of energetic Ru to improve the pursuit of the catalyst. The Weisz-Prater modulus and Thiele modulus values indicated that the surface reaction is the rate limitation step and there is no diffusion limitation. TEM, HRTEM, XPS, Raman, H2-TPR, XRD, CO chemisorption and BET surface area analyzers were used to characterize the catalysts.

Competing Kinetic Consequences of CO2 on the Oxidative Degradation of Branched Poly(ethylenimine).

Amine-functionalized porous solid materials are effective sorbents for direct air capture (DAC) of CO2. However, they are prone to oxidative degradation in service, increasing the materials cost for widespread implementation. While the identification of oxidation products has given insights into degradation pathways, the roles of some species, like CO2 itself, remain unresolved, with conflicting information in the literature. Here, we investigate the impact of CO2 on the oxidative degradation of poly(ethylenimine)-alumina (PEI/Al2O3) sorbents under conditions encompassing a wide range of CO2-air mixture compositions and temperatures relevant to DAC conditions, thereby reconciling the conflicting data in the literature. Degradation profiles characterized by thermogravimetric analysis, in situ ATR-FTIR, and CO2 capacity measurements reveal nonmonotonic effects of CO2 concentrations and temperatures on oxidation kinetics. Specifically, 0.04% CO2 accelerates PEI/Al2O3 oxidation more at low temperatures (<90 °C) compared to 1% and 5% CO2, but this trend reverses at high temperatures (>90 °C). First-principles metadynamics, machine learning accelerated molecular dynamics simulations, and 1H relaxometry experiments show that chemisorbed CO2 acid-catalyzes critical oxidation reactions, while extensive CO2 uptake reduces PEI branch mobility, slowing radical propagation. These contrasting kinetic effects of CO2 explain the complex degradation profiles observed in this work and in prior literature. Collectively, this work highlights the importance of considering atmospheric components in the design of DAC sorbents and processes. Additionally, it identifies the unconstrained branch mobility and local acid environment as two of the major culprits in the oxidation of amine-based sorbents, suggesting potential strategies to mitigate sorbent degradation.

CoCorrole-Functionalized PCN-222 for Carbon Monoxide Selective Adsorption.

The high risk of CO poisoning justifies the need for indoor air quality control and warning systems based on the detection of low concentrations (ppm-ppb) of CO. Cobalt corrole complexes selectively bind CO vs. O2, CO2, N2, opening new fields of applications. By combining the CO chemisorption properties of cobalt corroles with the known sorption capacity of MOFs, we hope to obtain high performance sensing materials for CO detection. In addition, the exposed metal sites of MOFs lead to CO2 physisorption, allowing the co-detection of CO and CO2. In this work, PCN-222, a stable Zr-based MOF made from Ni(TCPP) with natural vacancies, has been used as a porous matrix for the grafting of electron-poor metallocorroles. The materials were characterized by powder XRD, SEM and optical microscopy, BET analyses and gas adsorption measurements at 298 K. No degradation of the crystalline structure of PCN-222 was observed. At 1 atm, the adsorbed CO(g) volumes measured for the best materials were 12.15 cm3 g-1 and 14.01 cm3 g-1 for CoCorr2@PCN-222 and CoCorr3@PCN-222 respectively, and both materials exhibited high CO chemisorption and selectivity against O2, N2, and CO2 at low pressure due to the highest energy of the chemisorption process vs physisorption.

Identification of K+-determined reaction pathway for facilitated kinetics of CO2 electroreduction

Cations such as K+ play a key part in the CO2 electroreduction reaction, but their role in the reaction mechanism is still in debate. Here, we use a highly symmetric Ni-N4 structure to selectively probe the mechanistic influence of K+ and identify its interaction with chemisorbed CO2−. Our electrochemical kinetics study finds a shift in the rate-determining step in the presence of K+. Spectral evidence of chemisorbed CO2− from in-situ X-ray absorption spectroscopy and in-situ Raman spectroscopy pinpoints the origin of this rate-determining step shift. Grand canonical potential kinetics simulations - consistent with experimental results - further complement these findings. We thereby identify a long proposed non-covalent interaction between K+ and chemisorbed CO2−. This interaction stabilizes chemisorbed CO2− and thus switches the rate-determining step from concerted proton electron transfer to independent proton transfer. Consequently, this rate-determining step shift lowers the reaction barrier by eliminating the contribution of the electron transfer step. This K+-determined reaction pathway enables a lower energy barrier for CO2 electroreduction reaction than the competing hydrogen evolution reaction, leading to an exclusive selectivity for CO2 electroreduction reaction.

Supramolecular Complexation-Enhanced CO2 Chemisorption in Amine-Derived Sorbents.

A supramolecular complexation approach is developed to improve the CO2 chemisorption performance of solvent-lean amine sorbents. Operando spectroscopy techniques reveal the formation of carbamic acid in the presence of a crown ether. The reaction pathway is confirmed by theoretical simulation, in which the crown ether acts as a proton acceptor and shuttle to drive the formation and stabilization of carbamic acid. Improved CO2 capacity and diminished energy consumption in sorbent regeneration are achieved.

The effect of calcination gas atmosphere on the structural organization of Ru/Ce0.75Zr0.25O2 catalysts for CO2 methanation

In this study, supported 5 wt% Ru/Ce0.75Zr0.25O2 catalysts were prepared by sorption-hydrolytic deposition technique, followed by calcination under different conditions, namely in reductive H2/N2 (Ru/CeZr_red) and oxidative air (Ru/CeZr_ox) atmospheres. A thorough characterization of the catalysts was carried out by XRD, CO chemisorption, HRTEM, Raman spectroscopy, XPS, and in situ DRIFTS. The findings showed that the choice of the calcination atmosphere has a great impact on the structural organization of the catalyst. Calcination in air results in the formation of the larger crystalline RuO2 particles, which tend to enlarge upon reduction under CO2 methanation conditions. The reductive treatment results in a much higher dispersion of supported Ru species and a more pronounced metal-support interaction (MSI). In the Ru/CeZr_red catalyst, a large amount of atomic scale Ru species was detected along with metallic Ru nanoparticles and clusters. However, it was found that the superiority of the Ru/CeZr_red catalyst in the Ru dispersion did not result in the significant advantage in the CO2 methanation activity. In situ DRIFTS results suggested that the MSI affects the ability of Ru species to adsorb and activate reagent molecules, in particular, enhances the adsorption strength of the intermediate CO species.

Production of syngas at lower temperatures through microwave-enhanced dry reforming of methane

This study presents a novel microwave-assisted dry reforming of methane (MW-DRM) method, significantly enhancing syngas (H2+CO) production at a lower temperature of 400 °C. Compared to conventional heating dry reforming of methane (CH-DRM) processes, MW-DRM achieves higher conversion rates of CH4 (84.6%) and CO2 (85.7%) at a lower temperature of 500 °C, significantly reducing energy consumption. The CsRu/CeO2 catalyst exhibited excellent stability under microwave heating, maintaining high conversion rates and selectivity for 8 h. Detailed characterization using XRD, BET surface analysis, Raman spectroscopy, TEM, and CO chemisorption revealed insights into the structural changes of the catalyst, highlighting the selective impact of microwave heating on the catalyst structure. This study provides a promising approach for sustainable and cost-effective syngas and hydrogen production, aligning with greenhouse gas reduction goals.

The promotion of rare earth on Pt-SiO2-Al2O3 catalyst for NO oxidation in diesel exhaust

The effective oxidation of NO is the key to the catalytic treatment of diesel vehicle exhaust. Improving the NO oxidation ability is mainly to optimize the chemical state of the active site (Pt), followed by increasing the number of active centers. Considering that the atoms with electronegativity weaker than Pt can provide electrons to change the chemical state of Pt, rare earth elements with weak electronegativity are selected as additives to obtain Pt/M-SiO2-Al2O3 (M = Sc, Y, La, Ce, Pr, Nd, Sm, Eu) catalysts by co-impregnation method. Among them, the Pt/La-SiO2-Al2O3 catalyst has the best NO oxidation performance, compared with the Pt-SiO2-Al2O3, the NO conversion rate increased by 28 %. Due to the special outer electron arrangement of La, its electronegativity is the smallest in rare earth, which makes its outer electrons easy to transfer to Pt atoms, and has the strongest electron interaction. Thus, the highest content of Pt0 was detected in CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) and X-ray photoelectron spectroscopy (XPS). In addition, CO chemisorption and transmission electron microscopy (TEM) confirmed that the introduction of La in the catalyst can significantly improve the dispersion of precious metals. Notably, this simple approach of affecting electron interaction by introducing weakly electronegative elements provides a reference for the design of noble metal catalysts with different valence states to solve complex problems.

Carbon Dioxide Capture by Niobium Polyoxometalate Fragmentation.

High oxidation state metal cations in the form of oxides, oxoanions, or oxoperoxoanions have diverse roles in carbon dioxide removal (direct air capture and point source). Features include providing basic oxygens for chemisorption reactions, direct binding of carbonate, and catalyzing low-temperature CO2 release to regenerate capture media. Moreover, metal oxides and aqueous metal-oxo species are stable in harsh, point-source conditions. Here, we demonstrate aqueous niobium polyoxometalate (POM) carbon capture ability, specifically [Nb6O19]8-, Nb6. Upon exposure of aqueous Nb6 to CO2, Nb6 fragments and binds chemisorbed carbonate, evidenced by crystallization of Nb-carbonate POMs including [Nb22O53(CO3)16]28-and [Nb10O25(CO3)6]12-. While Rb/Cs+ counter cations yield crystal structures to understand the chemisorption processes, K+ counter cations enable higher capture efficiency (based on CO3/Nb ratio), determined by CHN analysis and thermogravimetry-mass spectrometry of the isolated solids. Sum frequency generation spectroscopy also showed higher carbon capture efficiency of the K-Nb6 solutions at the air-water interface, while small-angle X-ray scattering (SAXS) provided insights into the role of the alkalis in influencing these processes. Tetramethylammonium counter cations, like K+, demonstrate high efficiency of carbonate chemisorption at the interface, but SAXS and Raman of the bulk showed a predominance of a Nb24-POM (HxNb24O72, x ∼ 9) that does not bind carbonate. Control experiments show that carbonate detected at the interface is Nb-bound, and the Nb-carbonate species are stabilized by alkalis, demonstrating their supporting role in aqueous Nb-POM CO2 chemisorption. Of fundamental importance, this study presents rare examples of directing POM speciation with a gas, instead of liquid phase acid or base.