Activation Of CO Molecules Research Articles

Polyvinyl pyrrolidone-coordinated ultrathin bismuth oxybromide nanosheets for boosting photoreduction of carbon dioxide via ligand-to-metal charge transfer

Photoreduction of CO2 to useful ingredients remains a great challenge due to the high energy barrier of CO2 activation and poor product selectivity. Herein, Polyvinyl pyrrolidone (PVP) coordinated BiOBr was synthesized by a facile chemical precipitation method at room temperature. The CO2 photoreduction behaviors of PVP coordinated BiOBr were evaluated with H2O without sacrificial agent under the simulated sunlight. The evolution rates of CO and CH4 are 263.2 µmol g−1h−1 and 3.3 µmol g−1h−1, which are 8 times and 2 times higher than those of pure BiOBr respectively. Furthermore, the coordination of PVP on BiOBr surface enhances greatly the selectivity of product CO, which is close to 100%. Loading PVP onto BiOBr could not only induce and stabilize the oxygen vacancy, but also increase the charge density of BiOBr via the ligand to metal charge transfer (LMCT), which could be beneficial to the adsorption and activation of CO2 molecule. The photoreduction mechanism of CO2 for PVP coordinated BiOBr was proposed based on the improved charge density of BiOBr by the experimental results and Density functional theory (DFT) calculations. This finding provides a new pathway to boost the conversion efficiency and selectivity for the activation of CO2 photoreduction and new molecule insights into the role of PVP in photocatalysis.

Synergy of ferroelectric polarization and oxygen vacancy to promote CO2 photoreduction

Solar-light driven CO2 reduction into value-added chemicals and fuels emerges as a significant approach for CO2 conversion. However, inefficient electron-hole separation and the complex multi-electrons transfer processes hamper the efficiency of CO2 photoreduction. Herein, we prepare ferroelectric Bi3TiNbO9 nanosheets and employ corona poling to strengthen their ferroelectric polarization to facilitate the bulk charge separation within Bi3TiNbO9 nanosheets. Furthermore, surface oxygen vacancies are introduced to extend the photo-absorption of the synthesized materials and also to promote the adsorption and activation of CO2 molecules on the catalysts’ surface. More importantly, the oxygen vacancies exert a pinning effect on ferroelectric domains that enables Bi3TiNbO9 nanosheets to maintain superb ferroelectric polarization, tackling above-mentioned key challenges in photocatalytic CO2 reduction. This work highlights the importance of ferroelectric properties and controlled surface defect engineering, and emphasizes the key roles of tuning bulk and surface properties in enhancing the CO2 photoreduction performance.

A Ni nanoparticles encapsulated in N-doped carbon catalyst for efficient electroreduction CO2: Identification of active sites for adsorption and activation of CO2 molecules

Electrochemical CO2 reduction (ECR) offers a promising strategy to achieve global carbon balance and mitigate the global climate. However, it is still a challenge to fabricate excellent catalyst and illustrate the active site for adsorption and activation of CO2 molecules. Herein, we develop a reduced graphene oxide-supported N-doped carbon-encapsulated nickel (Ni@N-C/rGO) catalyst for ECR to CO. The optimal Ni@N-C/rGO(4,4′-bipy) catalyst exhibits high catalytic activity, selectivity and stability with a current density of 20 mA cm−2 and Faradaic efficiency of 88% toward CO at −0.97 V vs. RHE for 10 h. The well-designed poisoning and control experiments deduce that the catalytic activity originates from the N-doped carbon layer coated on Ni particles. Density functional theory calculations suggest that pyrrolic-N is the optimal active site for the adsorption and activation of CO2 molecules, and the spontaneous charge transfer from Ni 3d electrons to the π orbitals of the N-C skeleton promotes the easy desorption of *CO from the active sites, thereby improving the ECR activity.

An Extrinsic Faradaic Layer on CuSn for High-Performance Electrocatalytic CO 2 Reduction

An Extrinsic Faradaic Layer on CuSn for High-Performance Electrocatalytic CO ₂ Reduction

CO2‐Induced 2D Ni‐BDC Metal–Organic Frameworks with Enhanced Photocatalytic CO2 Reduction Activity

AbstractThe rational design of 2D metal–organic framework (MOF) nanosheets with high active exposed surfaces and ultrathin thickness is a critical issue for CO2 reduction. How to fabricate 2D MOFs and simultaneously fulfill the strong adsorption and activation of CO2 molecules is a great challenge. Here, it is reported that CO2 molecules can help to fabricate single‐layer [Ni3(OH)2(1,4‐BDC)2‐(H2O)4]·2H2O, BDC = 1,4‐benzenedicarboxylate, (Ni‐BDC) MOF nanosheets with large sizes and uniform thickness by exchanging with the coordinated H2O molecules at Ni sites and further orientated chemically adsorption on it. CO2 molecules can not only help break the interlayer hydrogen bond of bulk materials realizing the successful transformation from 3D to 2D, but also contribute to expose the (200) planes with highly active in photocatalytic CO2‐to‐CO conversion, and a high production rate of 104 mmol g−1 h−1 and a high Faraday efficiency of 96.8% can be obtained. This strategy provides a novel prototypical model based on the strong interaction between metal active centers and reactive molecules to obtain 2D MOFs with high quality and activity.

Aspect ratio dependent photocatalytic enhancement of CsPbBr3 in CO2 reduction with two-dimensional metal organic framework as a cocatalyst

All-inorganic CsPbBr3 perovskites have been regarded as promising candidates for photocatalytic CO2 reduction, but the conversion efficiency is limited by serious charge recombination and poor surface reactivity. Herein, a two-dimensional Ni based metal-organic framework (NMF) has been employed as a cocatalyst, which not only acts as an electron extractor in enhancing the electron-hole separation, but also serves as a CO2 capturing agent in facilitating the adsorption and activation of CO2 molecules. As a result, 1.8, 2.5 and 4.1-fold enhancement in CO production has been realized for CsPbBr3 nanocubes, nanorods and nanowires, respectively, which is proved to be positively correlated with the aspect ratio of CsPbBr3. The aspect ratio dependent photocatalytic enhancement is attributed to the increased energy difference at the interface between the conduction band of CsPbBr3 and NMF with the elongation of CsPbBr3, which produces an enlarged driving force for promoted interfacial electron transfer and superior charge separation properties.

The effect of adsorbed oxygen species on carbon-resistance of Ni-Zr catalyst modified by Al and Mn for dry reforming of methane

Dry reforming of methane was investigated over Ni-Zr catalysts modified by Aluminum and Manganese. The catalysts were characterized by XRD, CO2-TPD, XPS, TGA, and Raman. Among all prepared catalysts, the 5Al-5 Mn (5 wt% Al and 5 wt% Mn) catalyst showed the highest CH4 and CO2 conversion at 700 °C DRM with low carbon deposition. The CO2-TPD results exhibited that the 5Al-5 Mn catalyst had the highest amounts of both total basic sites and medium-strength basic sites, which could promote the adsorption and activation of CO2 molecule during the DRM reaction, and further reduce the carbon deposition. The XRD results suggested that the addition of both Al and Mn led to smaller nickel particle size. Besides, the lower carbon deposition on 5Al-5Mn and 2.5Al-7.5Mn catalyst was derived from a higher content of surface adsorption oxygen species, which was verified by the O 1s results. While the lower number of basic sites, more strong basic sites and larger particle size on 5Mn and 5Al catalysts result in a higher amount of carbon deposition.

Role of metcar on the adsorption and activation of carbon dioxide: a DFT study.

Metallocarbohedrenes or metcars belong to one of the classes of stable nanoclusters having a specific stoichiometry. In spite of the available theoretical and experimental studies, the structure of pristine Ti8C12 metcar is still uncertain. We study the geometric structure of a titanium metcar, Ti8C12, together with its electronic properties and chemical activity towards adsorption and activation of CO2 molecule by means of density functional theory. Our results suggest that the CO2 molecule is strongly adsorbed and undergoes a significantly high degree of activation onto the Ti8C12 metcar. The migration of charge from titanium metcar to CO2 molecule attributes the high degree of activation of this molecule. In the infrared vibrational spectra for CO2 molecule adsorbed onto Ti8C12, we find a new signal which is absent in the corresponding spectra for gaseous CO2. In addition to adsorption energy, we also estimate the energy barrier for the dissociation of CO2 molecule to CO and O fragments on a Ti8C12 cluster. As a whole, this work reveals the ground state geometry of Ti8C12 metcar and highlights the role of this metcar in CO2 adsorption and activation, which are the key steps in designing potential catalysts for CO2 capture and its conversion to industrially valuable chemicals.

Ultrathin Single Crystalline MgO(111) Nanosheets**

AbstractSynthesizing high‐quality two‐dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long‐range single‐crystallinity and clean high‐energy surfaces are required. Reported here is the synthesis of single‐crystalline MgO(111) nanosheets by a two‐step process involving the formation of ultrathin Mg(OH)2 nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect‐rich surface displays terminal ‐OH groups, three‐coordinated O2− sites and low‐coordinated Mg2+ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red‐shifted CO and coupling of CO molecules into C2 species.

Highly Selective, Defect-Induced Photocatalytic CO2 Reduction to Acetaldehyde by the Nb-Doped TiO2 Nanotube Array under Simulated Solar Illumination

The adsorption and activation of CO2 molecules on the surface of photocatalysts are critical steps to realize efficient solar energy-induced CO2 conversion to valuable chemicals. In this work, a defect engineering approach of a high-valence cation Nb-doping into TiO2 was developed, which effectively enhanced the adsorption and activation of CO2 molecules on the Nb-doped TiO2 surface. A highly ordered Nb-doped TiO2 nanotube array was prepared by anodization of the Ti-Nb alloy foil and subsequent annealing at 550 °C in air for 2 h for its crystallization. Our sample showed a superior photocatalytic CO2 reduction performance under simulated solar illumination. The main CO2 reduction product was a higher-energy compound of acetaldehyde, which could be easily transported and stored and used to produce various key chemicals as intermediates. The acetaldehyde production rate was over ∼500 μmol·g-1·h-1 with good stability for repeated long-time uses, and it also demonstrated a superior product selectivity to acetaldehyde of over 99%. Our work reveals that the Nb-doped TiO2 nanotube array could be a promising candidate with high efficiency and good product selectivity for the photocatalytic CO2 reduction with solar energy.

Highly Efficient and Selective CO2 Electro-Reduction to HCOOH on Sn Particle-Decorated Polymeric Carbon Nitride.

Electrochemical conversion of CO2 into liquid fuels by efficient and earth-abundant catalysts is of broad interest but remains a great challenge in renewable energy production and environmental remediation. Herein, a Sn particle-decorated polymeric carbon nitride (CN) electrocatalyst was successfully developed for efficient, durable, and highly selective CO2 reduction to formic acid. High-resolution X-ray photoelectron spectroscopy confirmed that the metallic Sn particles and CN matrix are bound by strong chemical interaction, rendering the composite catalyst a stable structure. More notably, the electronic structure of Sn was well tuned to be highly electron-rich due to the electron transfer from N atoms of CN to Sn atoms via metal-support interactions, which favored the adsorption and activation of CO2 molecules, promoted charge transport, and thus enhanced the electrochemical conversion of CO2 . The composite electrocatalyst demonstrated an excellent Faradaic efficiency of formic acid (FEHCOOH ) up to 96±2 % at the potential of -0.9 V vs. reversible hydrogen electrode, which remained at above 92 % during the electrochemical reaction of 10 h, indicating that the present Sn particle-decorated polymeric carbon nitride electrocatalyst is among the best in comparison with reported Sn-based electrocatalysts.

Nitrogen-Doped Carbon Cages Encapsulating CuZn Alloy for Enhanced CO2 Reduction.

CuZn alloy, regarded as the active sites, shows excellent catalytic activity for the reverse water gas shift reaction, whereas the incorporation of N atoms, especially pyridinic N, can greatly improve its catalytic properties because of the strong promotion capacity for adsorption and activation of CO2 molecules. Herein, the synthesis strategy involving Cu-doped Zn-based metal-organic frameworks is utilized to prepare CuZn alloy coated in an N-doped carbon shell. The excellent catalytic ability for CO2 transformation originates from the synergistic catalytic effect between CuZn alloy and pyridinic N. The strong adsorption and activation capacity for CO2 of pyridinic N is ascribed to the lone pair of electrons on the N atom and the high electron density in its vicinity.

Controlled Formation of Defective Shell on TiO2 (001) Facets for Enhanced Photocatalytic CO2 Reduction

AbstractDefective black TiO2 has attracted considerable attention as photocatalyst for highly efficient solar energy conversion. However, it still remains a challenge to understand the basic role of defective shell on the basis of crystal facet engineering, due to the difficulty in generating defects merely on a specific facet. Herein, we develop a novel strategy to precisely control the formation of defective shell only on {001} facets of TiO2 nanocrystals due to the anisotropic effect of the crystal facets, which enables the investigation of defective shell on a specific facet to be achieved. It is found that the defective shell, middle interface layer and crystalline core together constitute a homojunction (TiO2‐homojunction). The defective shell of TiO2‐homojunction realizes the effective spatial separation of photogenerated electrons and holes. Moreover, the presence of Ti3+ ion and oxygen vacancies in defective shell can induce high free electron concentration and serve as active sites to promote the adsorption and activation of CO2 molecules. As a result, the TiO2‐homojunction exhibits significant photocatalytic behavior for CO2 reduction in a gas‐solid system. More interestingly, the position of redox reaction is reversed due to the controlled formation of defective shell, where {001} facets act as reduction sites and {101} facets as oxidation sites during photocatalytic process.

Ultrathin SiC Nanosheets with High Reduction Potential for Improved CH4 Generation from Photocatalytic Reduction of CO2

AbstractExploitation of low‐cost and efficient photocatalysts for selective reduction of CO2 into fuels such as methane or methanol is highly desired in the field of solar‐to‐fuel conversion. Herein, ultrathin SiC nanosheets (NSs) with high crystallinity are prepared using two‐dimensional reduced graphene oxide as sacrificial template. The ultrathin feature of SiC NSs readily shortens the migration distance of photoexcited carriers, thereby reducing the recombination probability and providing more energetic electrons to participate in CO2 reduction. Consequently, both the yield (3.11 μmol h−1 g−1) and selectivity (90.6%) of CH4 over SiC NSs demonstrate dramatic improvements in comparison with that of micro‐size SiC (0.76 μmol h−1 g−1, 77.9%) and commercial TiO2 (1.46 μmol h−1 g−1, 61.0%). The initial absorption and activation of CO2 molecules on the surface of SiC NSs is confirmed by the in situ diffuse reflectance infrared Fourier transform spectroscopy. The SiC NSs can promote the deep reduction of carbon intermediate into CH4 thanks to the strong reduction potential of photoexcited electrons. This work not only proves ultrathin SiC NSs to be a promising photocatalyst for CO2 reduction, but also provides novel insights into deep photoreduction of CO2 to CH4.

Three-in-One Oxygen Vacancies: Whole Visible-Spectrum Absorption, Efficient Charge Separation, and Surface Site Activation for Robust CO2 Photoreduction.

A facile and controllable in situ reduction strategy is used to create surface oxygen vacancies (OVs) on Aurivillius-phase Sr2 Bi2 Nb2 TiO12 nanosheets, which were prepared by a mineralizer-assisted soft-chemical method. Introduction of OVs on the surface of Sr2 Bi2 Nb2 TiO12 extends photoresponse to cover the whole visible region and also tremendously promotes separation of photoinduced charge carriers. Adsorption and activation of CO2 molecules on the surface of the catalyst are greatly enhanced. In the gas-solid reaction system without co-catalysts or sacrificial agents, OVs-abundant Sr2 Bi2 Nb2 TiO12 nanosheets show outstanding CO2 photoreduction activity, producing CO with a rate of 17.11 μmol g-1 h-1 , about 58 times higher than that of the bulk counterpart, surpassing most previously reported state-of-the-art photocatalysts. Our study provides a three-in-one integrated solution to advance the performance of photocatalysts for solar-energy conversion and generation of renewable energy.

Three‐in‐One Oxygen Vacancies: Whole Visible‐Spectrum Absorption, Efficient Charge Separation, and Surface Site Activation for Robust CO2 Photoreduction

AbstractA facile and controllable in situ reduction strategy is used to create surface oxygen vacancies (OVs) on Aurivillius‐phase Sr2Bi2Nb2TiO12 nanosheets, which were prepared by a mineralizer‐assisted soft‐chemical method. Introduction of OVs on the surface of Sr2Bi2Nb2TiO12 extends photoresponse to cover the whole visible region and also tremendously promotes separation of photoinduced charge carriers. Adsorption and activation of CO2 molecules on the surface of the catalyst are greatly enhanced. In the gas‐solid reaction system without co‐catalysts or sacrificial agents, OVs‐abundant Sr2Bi2Nb2TiO12 nanosheets show outstanding CO2 photoreduction activity, producing CO with a rate of 17.11 μmol g−1 h−1, about 58 times higher than that of the bulk counterpart, surpassing most previously reported state‐of‐the‐art photocatalysts. Our study provides a three‐in‐one integrated solution to advance the performance of photocatalysts for solar‐energy conversion and generation of renewable energy.

Electrocatalytic and Photoelectrochemical Reduction of Carbon Dioxide at Hierarchical Hybrid Films of Copper(I) Oxide Decorated with Tungsten(VI) Oxide Nanowires

Unique hybrid systems for electroreduction of CO2 under both conventional and visible-light-induced conditions are proposed and designed here by over-coating copper(I) oxide with tungsten(VI) oxide nanowires. When Cu2O and WO3 nanostructures are sequentially deposited on glassy carbon substrate, the resulting system has exhibited high electrocatalytic activity toward reduction of CO2 in phosphate buffer of pH = 6.1. By introducing WO3, the Cu-based system becomes more selective against the competitive hydrogen evolution and exhibits higher CO2-reduction currents relative to the performance of single components. During electroreduction, highly catalytic Cu sites are generated or intercalated within WO3 nanowires partially reduced to mixed-valence hydrogen-absorbing tungsten(VI,V) oxide bronzes, HxWO3, co-existing with sub-stoichiometric tungsten(VI,IV) oxides, WO3-y. Strong adsorption and activation of CO2 molecule has been demonstrated at the WO3-decorated Cu2O interface. Under photoelectrochemical conditions involving illumination with sun-light, the proposed hierarchical system of Cu2O semiconductor deposited onto the transparent fluorine-doped conducting glass electrode and decorated with WO3 nanowires is well-behaved and active toward CO2-reduction in the Na2SO4 neutral medium. In addition to the Cu2O stabilization effect, the heterojunction formed by p-type Cu2O and n-type WO3 semiconductors seems to facilitate charge distribution and separation at the photoelectrochemical interface.

Adsorption mechanism of CO molecule on Al(111) surface: periodic DFT investigation

The adsorption mechanism of the CO molecule on Al(111) surface has been investigated systematically at the atom-molecule level by the method of periodic density functional theory. The adsorption energies, adsorption structures, charge transfer, and density of states have been calculated in a wide range of coverage. It is found that the hcp-hollow site is the energetically favorable site. A significant positive correlation has been found between the adsorption energy (Eads) and coverage. The adsorbed CO molecules are almost perpendicular on the surface with the C atom facing the surface. There is an obvious charge transfer from Al atoms to the C atom; the Al atoms that have interaction with the C atom offer the most charge. The 4σ, 1π, and 5σ molecular orbitals of CO are found to contribute to bonding with the Al. The charges filling in the 2π molecular orbital contribute to C–O bond activation. In conclusion, the passivation of aluminum surface and the activation of CO molecule occur simultaneously in the adsorption of CO on Al surface.

(Invited) Role of Heterojunctions in Activation of CO2 Molecule

Photo-electro-reduction of CO2 is continuously gaining a considerable interest in view of possible production of carbon-based energy carriers. Given the periodic availability of solar light, of primary importance is to orient the CO2 conversion versus formation of easily storable fuels such as, e.g. methanol or ethanol. Thus, it is important to diversify the CO2 reduction products not only by the applied potential but by careful optimisation of the working system. A default approach, consists in using p-type semiconductors as a main component of the CO2-reduction cathodes. However, the intrinsic drawback of p-type semiconductor photo-cathodes lies in low electron density in the CB and, as generally observed, the lack of catalytic properties towards CO2 reduction. Thus, in an attempt to increase the conduction band electron population, we will present herein a build-up hybrid photo-electrodes including p-type - n-type semiconductors junctions. An additional advantage of creating such p-type – n-type semiconductor heterojunction is protection of the p-type component against photocorrosion induced by largely negative potentials that offers, at the same time, the possibility to enlarge the choice of employed p-type semiconductors to relatively unstable ones. The requirements for the n-type semiconductors are fulfilled by several metal oxides such as TiO2 or SrTiO3, or KTaO3. Apart the role of heterojunction in CO2 activation, we will also point out a particular aspect of using a model system involving Au NPs of different sizes deposited on a TiO2/Ti substrate by examining the electro-reduction of carbon monoxide. This reaction appears as being the crucial, kinetically difficult, step in the electro-reduction of CO2 to alcohols and hydrocarbons. Recognition and understanding of an individual process might already give a picture of a whole system.

Co nanoparticles anchoring three dimensional graphene lattice as bifunctional catalyst for low-temperature CO oxidation

Despite tremendous efforts, developing a series of heterogeneous catalysts for CO oxidation elimination with high activity at low cost remains a great challenge. Here, we report a bifunctional Co-based nanocrystals anchoring in the graphene oxide layer as a high-performance catalyst for CO oxidation. Although a single cobalt or graphene oxide catalysts alone have the lower catalytic activity, their combination exhibits an unexpected, surprisingly high CO activity that is further enhanced by a series of oxygen-containing functional groups on the surface of graphene oxide sheets. The Co/graphene oxide calcined at 300°C (Co/GO–300°C) exhibits the outstanding catalytic activity with T100=51°C but superior stability, producing a high-performance non-precious metal-based catalyst for CO oxidation elimination. Under a series of critical characterizations, the unusual catalytic activity of Co/GO–300°C sample is attributed to the synergistic effect between the oxygen-containing functional groups of graphene oxide sheets and Co species. Especially, the existence of Co3+ species could promote the adsorption and activation of CO molecules.