Oxygen vacancy engineering of cerium oxides for carbon dioxide capture and reduction

Defects Tune the Strong Metal-Support Interactions in Copper Supported on Defected Titanium Dioxide Catalysts for CO2 Reduction

A highly active and stable Cu-based catalyst for CO₂ to CO conversion was demonstrated by creating a strong metal-support interaction (SMSI) between Cu active sites and the TiO₂-coated dendritic fibrous nano-silica (DFNS/TiO₂) support. The DFNS/TiO₂-Cu10 catalyst showed excellent catalytic performance with a CO productivity of 5350 mmol g^-1 h^-1 (i.e., 53,506 mmol g_Cu^-1 h^-1), surpassing that of almost all copper-based thermal catalysts, with 99.8% selectivity toward CO. Even after 200 h of reaction, the catalyst remained active. Moderate initial agglomeration and high dispersion of nanoparticles (NPs) due to SMSI made the catalysts stable. Electron energy loss spectroscopy confirmed the strong interactions between copper NPs and the TiO₂ surface, supported by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. The H₂-temperature programmed reduction (TPR) study showed α, β, and γ H₂-TPR signals, further confirming the presence of SMSI between Cu and TiO₂. In situ Raman and UV-vis diffuse reflectance spectroscopy studies provided insights into the role of oxygen vacancies and Ti³⁺ centers, which were produced by hydrogen, then consumed by CO₂, and then again regenerated by hydrogen. These continuous defect generation-regeneration processes during the progress of the reaction allowed long-term high catalytic activity and stability. The in situ studies and oxygen storage complete capacity indicated the key role of oxygen vacancies during catalysis. The in situ time-resolved Fourier transform infrared study provided an understanding of the formation of various reaction intermediates and their conversion to products with reaction time. Based on these observations, we have proposed a CO₂ reduction mechanism, which follows a redox pathway assisted by hydrogen.

Microwave-assisted synthesis of metal-organic chalcogenolate assemblies as electrocatalysts for syngas production

Today, many essential industrial processes depend on syngas. Due to a high energy demand and overall cost as well as a dependence on natural gas as its precursor, alternative routes to produce this valuable mixture of hydrogen and carbon monoxide are urgently needed. Electrochemical syngas production via two competing processes, namely carbon dioxide (CO₂) reduction and hydrogen (H₂) evolution, is a promising method. Often, noble metal catalysts such as gold or silver are used, but those metals are costly and have limited availability. Here, we show that metal-organic chalcogenolate assemblies (MOCHAs) combine several properties of successful electrocatalysts. We report a scalable microwave-assisted synthesis method for highly crystalline MOCHAs ([AgXPh] _∞: X = Se, S) with high yields. The morphology, crystallinity, chemical and structural stability are thoroughly studied. We investigate tuneable syngas production via electrocatalytic CO₂ reduction and find the MOCHAs show a maximum Faraday efficiency (FE) of 55 and 45% for the production of carbon monoxide and hydrogen, respectively.

Partially sintered copper‒ceria as excellent catalyst for the high-temperature reverse water gas shift reaction

For high-temperature catalytic reaction, it is of significant importance and challenge to construct stable active sites in catalysts. Herein, we report the construction of sufficient and stable copper clusters in the copper‒ceria catalyst with high Cu loading (15 wt.%) for the high-temperature reverse water gas shift (RWGS) reaction. Under very harsh working conditions, the ceria nanorods suffered a partial sintering, on which the 2D and 3D copper clusters were formed. This partially sintered catalyst exhibits unmatched activity and excellent durability at high temperature. The interaction between the copper and ceria ensures the copper clusters stably anchored on the surface of ceria. Abundant in situ generated and consumed surface oxygen vacancies form synergistic effect with adjacent copper clusters to promote the reaction process. This work investigates the structure-function relation of the catalyst with sintered and inhomogeneous structure and explores the potential application of the sintered catalyst in C1 chemistry.

Characterization and catalytic activity of soft-templated NiO-CeO2 mixed oxides for CO and CO2 co-methanation

Nanosized NiO, CeO2 and NiO-CeO2 mixed oxides with different Ni/Ce molar ratios were prepared by the soft template method. All the samples were characterized by different techniques as to their chemical composition, structure, morphology and texture. On the catalysts submitted to the same reduction pretreatment adopted for the activity tests the surface basic properties and specific metal surface area were also determined. NiO and CeO2 nanocrystals of about 4 nm in size were obtained, regardless of the Ni/Ce molar ratio. The Raman and X-ray photoelectron spectroscopy results proved the formation of defective sites at the NiO-CeO2 interface, where Ni species are in strong interaction with the support. The microcalorimetric and Fourier transform infrared analyses of the reduced samples highlighted that, unlike metallic nickel, CeO2 is able to effectively adsorb CO2, forming carbonates and hydrogen carbonates. After reduction in H2 at 400 °C for 1 h, the catalytic performance was studied in the CO and CO2 co-methanation reaction. Catalytic tests were performed at atmospheric pressure and 300 °C, using CO/CO2/H2 molar compositions of 1/1/7 or 1/1/5, and space velocities equal to 72000 or 450000 cm3·h−1·gcat−1. Whereas CO was almost completely hydrogenated in any investigated experimental conditions, CO2 conversion was strongly affected by both the CO/CO2/H2 ratio and the space velocity. The faster and definitely preferred CO hydrogenation was explained in the light of the different mechanisms of CO and CO2 methanation. On a selected sample, the influence of the reaction temperature and of a higher number of space velocity values, as well as the stability, were also studied. Provided that the Ni content is optimized, the NiCe system investigated was very promising, being highly active for the COx co-methanation reaction in a wide range of operating conditions and stable (up to 50 h) also when submitted to thermal stress.

CO2 Reduction by Hydrogen Pre-Reduced Acceptor-Doped Ceria

The reactivity of H₂ pre-reduced acceptor-doped ceria materials Gd_0.10 Ce_0.90 O_2-δ (GDC10) and Sm_0.15 Ce_0.85 O_2-δ (SDC15) was tested with respect to the reduction of CO₂ to CO in the context of the reverse water-gas shift reaction. It was demonstrated that not only oxygen vacancies, but also dissolved hydrogen is a reactive species for the reduction of CO₂ . Dissolved hydrogen must be considered upon discussion of the mechanism of the reverse water-gas shift reaction on ceria-derived materials apart from oxygen vacancies and formates. The reduction of CO₂ is preceded by the formation of carbonate species of different thermal stability and reactivity. The stability of these carbonates was directly demonstrated by in situ infrared spectroscopy and revealed the largely reversible nature of CO₂ ad- and desorption. In comparison to pre-reduced samples, decreased carbonate coverage is obtained after oxidative treatments of GDC10 and SDC15. No significant effect of the sample treatment (O₂ oxidation or H₂ reduction) on the surface carbonate stability was noticed. Mono-dentate carbonates and carboxylates appear to be more easily formed on pre-reduced (i. e. defective) samples. Ce⁴⁺ reduction to Ce³⁺ (by H₂ ) and re-oxidation to Ce⁴⁺ (by CO₂ ) on GDC10/SDC15 were directly monitored by infrared spectroscopic analysis of a distinct, IR-active electronic transition of Ce³⁺ . These results show the complex interplay of oxygen vacancy/dissolved hydrogen reactivity and surface chemical aspects in acceptor-doped ceria materials.

Atomic layer confined vacancies for atomic-level insights into carbon dioxide electroreduction

The role of oxygen vacancies in carbon dioxide electroreduction remains somewhat unclear. Here we construct a model of oxygen vacancies confined in atomic layer, taking the synthetic oxygen-deficient cobalt oxide single-unit-cell layers as an example. Density functional theory calculations demonstrate the main defect is the oxygen(II) vacancy, while X-ray absorption fine structure spectroscopy reveals their distinct oxygen vacancy concentrations. Proton transfer is theoretically/experimentally demonstrated to be a rate-limiting step, while energy calculations unveil that the presence of oxygen(II) vacancies lower the rate-limiting activation barrier from 0.51 to 0.40 eV via stabilizing the formate anion radical intermediate, confirmed by the lowered onset potential from 0.81 to 0.78 V and decreased Tafel slope from 48 to 37 mV dec-1. Hence, vacancy-rich cobalt oxide single-unit-cell layers exhibit current densities of 2.7 mA cm-2 with ca. 85% formate selectivity during 40-h tests. This work establishes a clear atomic-level correlation between oxygen vacancies and carbon dioxide electroreduction.

Mesoporous Ni/Ce1−xNixO2−y heterostructure as an efficient catalyst for converting greenhouse gas to H2 and syngas

A heterostructure of highly dispersed Ni nanoparticles in pore channels of Ni–CeO₂ solid solution, having excellent thermo-stability, redox properties, and metal/support synergy, is identified as an efficient nanocatalyst for converting greenhouse gas into H₂ energy and syngas.

Nanoparticles-in-concavities as efficient nanocatalysts for carbon dioxide reforming of methane to hydrogen and syngas

The ceria concavity-loaded Ni nanoparticle catalysts can lead to more active sites and promote CO₂ dissociative activation and CO desorption, thus enhancing significantly the catalytic performances for methane dry reforming with CO₂.

Effect of Dopants on the Adsorption of Carbon Dioxide on Ceria Surfaces

High-surface-area nanosized CeO2 and M-doped CeO2 (M=Cu, La, Zr, and Mg) prepared by a surfactant-templated method were tested for CO2 adsorption. Cu, La, and Zr are doped into the lattice of CeO2, whereas Mg is dispersed on the CeO2 surface. The doping of Cu and La into CeO2 leads to an increase of the CO2 adsorption capacity, whereas the doping of Zr has little or no effect. The addition of Mg causes a decrease of the CO2 adsorption capacity at a low Mg content and a gradual increase at a higher content. The CO2 adsorption capacity follows the sequence Cu-CeO2 >La-CeO2 >Zr-CeO2 ≈CeO2 >Mg-CeO2 at low dopant contents, in line with the relative amount of defect sites in the samples. It is the defect sites on the surface, not in the bulk of CeO2, modified by the dopants that play the vital role in CO2 chemisorption. The role of surface oxygen vacancies is further supported by an in situ IR spectroscopic study of the surface chemistry during CO2 adsorption on the doped CeO2.

A urea-assisted template method to synthesize mesoporous N-doped CeO2 for CO2 capture

Novel mesoporous nitrogen doped CeO₂ has been prepared via a simple urea-assisted template method. The synthesized N-doped mesoporous CeO₂ exhibits significantly enhanced CO₂ adsorption capacity compared to undoped mesoporous CeO₂.

Aminosilanes grafted to basic alumina as CO2 adsorbents--role of grafting conditions on CO2 adsorption properties

Solid oxide-supported amine sorbents for CO2 capture are amongst the most rapidly developing classes of sorbent materials for CO2 capture. Herein, basic γ supports are used as hosts for amine sites through the grafting of 3-aminopropyltrimethoxysilane to the alumina surface under a variety of conditions, yielding the expected surface-grafted alkylamine groups, as demonstrated by FTIR spectroscopy and (29)Si and (13)C cross-polarization magic-angle spinning (CPMAS NMR) spectroscopy. Grafting amine sites on the surface in the presence of water leads to a high density of amine sites on the surface whereas simultaneously creating a unique type of aluminum species on the surface, as demonstrated by both 1D and 2D (27)Al MAS NMR spectroscopy. The thus prepared sorbents result in higher CO2 adsorption capacities and amine efficiencies compared to sorbents prepared in the absence of water or similar amine loading sorbents prepared using silica supports. In situ FTIR spectra of the sorbents exposed to CO2 at various pressures show no distinct difference in the nature of the adsorbed CO2 species on alumina- versus silica-supported amines, whereas water adsorption isotherms show that the improved performance of the amine-grafted alumina support is not a consequence of retained water on the more hydrophobic aminoalumina materials. The findings demonstrate that amine-grafted, basic alumina materials can be tuned to be more efficient than the corresponding silica-supported materials at comparable amine loadings, further demonstrating that the properties of amine sites can be tuned by controlling or adjusting the support surface properties.