Adsorbed CO Molecules Research Articles

Enhanced performance on CO2 adsorption and release induced by structural transition that occurred in TBAB·26H2O hydrates

The applications of ionic clathrate hydrates have greatly improved the efficiency and the conditions required for hydrate-based CO2 capture, but high energy input for hydrate growth and complicated treatment of hydrate slurry still hinder their commercial use. Here we chose TBAB·26H2O hydrate particles to adsorb CO2 molecules instead of TBAB solutions below 2 MPa and release them at ambient pressure. Results showed that the TBAB·26H2O hydrate could adsorb CO2 without induction time and enhance the gas storage capacity by structural transition, especially under high pressure. By using in situ Raman, CO2 molecules were found to fill the empty cages in TBAB·26H2O hydrate first, the formed nCO2·TBAB·26H2O hydrate then converted to nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates at 2 MPa. Macroscopic measurements revealed that around 20 volume of CO2 in standard state could be adsorbed by 1 volume of TBAB·26H2O hydrate sample at 1 MPa, but this volume ratio could reach 67 v/v at 2 MPa where structural change was thought to take place. The pressurized CO2 trapped in hydrate phase was assumed to destroy the structure of TBAB·26H2O hydrate easily, and force the water molecules to form a structure that more compatible with CO2 molecules. This may explain why nCO2·TBAB·26H2O hydrates barely grow from TBAB solutions when pressurized CO2 is injected. In the CO2 release process, the nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates quickly transformed back to nCO2·TBAB·26H2O hydrates and 60–80% of the captured CO2 could be released. Combing with their excellent gas selectivity, TBAB·26H2O hydrate particles would be an ideal material for hydrate-based CO2 capture.

Orbital mechanism of upright CO activation on Fe(100)

The knowledge of bond activation forms a cornerstone for modern chemistry, wherein symmetry rules of electronic activation lie in the heart of bond activation. However, the question as to how a chemical bond is activated remains elusive. By taking CO activated on Fe(100), herein, we have resolved the long‐standing fundamental question; we have found that excitations in the adsorbate feature the bond activation. We essentially have discovered contrasting electronic processes in respective σ and π electron systems of the adsorbed CO molecule. The σ electron system is involved in reversible hidden excitations/deexcitations between two occupied σ orbitals, whereas the π electron system is subject to irreversible π to π* excitations dispersed along the d‐band region, which is coupled to the rotational 2π electron couplings depending on the strength of molecule‐metal interactions. The σ excitations pertain to the Pauli repulsion mediated quantum nature with energy and entropy marked by the two energy levels, whereas the π to π* excitations fall into a new category of electronic excitations contributing to energy and entropy exchanges in a wide and continuous d‐band region. The findings that the internal states of the adsorbate are excited and that fundamental connections between the frontier orbitals and low‐lying orbitals are established as the molecule comes to the surface may open up new channels to realize more efficient bond activation and renew our thinking on probing the quantum mechanical nature of bond activation at surfaces with further possible impact on manipulation of orbital activation in femtochemistry and attochemistry.

CO adsorption on Fe-doped vacancy-defected CNTs – A DFT study

The adsorption of CO molecules on Fe-doped single-vacancy-defected carbon nanotubes (Fe-SV-CNTs) was computationally investigated. Results show that the interaction of Fe-SV-CNTs with CO, although is slightly weaker than that of Fe-doped intrinsic CNTs, is much stronger than that of intrinsic CNTs and single-vacancy-defected CNTs (SV-CNTs). After CO adsorption, the electrical conductivity of Fe-SV-CNTs has a much more significant change than that of other types of CNTs. Considering that doping transition metals on the defects of SV-CNTs could be achieved much more easily than on the lattices of intrinsic CNTs, Fe-SV-CNTs would be more practical as CO gas sensing material.

The Role of Cu in Adsorption of O2 and CO Molecules on the Pt12Cu Cluster

Using density functional theory method, we have studied O2 and CO molecules adsorption on the Pt13 and Pt12Cu clusters, respectively. Analysis of the adsorption energies, O–O bond length, and natural bond orbital (NBO) charge indicates that the copper doping plays an important role in the enhanced reduction of O2 molecule in low temperature fuel cells (LTFCs). On the other hand, contrary to the adsorption of O2 molecule, the calculated adsorption energies, NBO charge and molecular orbitals show that the copper doping significantly weakens the adsorption of the CO molecule, indicating that the Cu-doped Pt catalyst is resistant to CO poisoning in the LTFCs. Our studies provide an important clue to understand the catalytic mechanism of platinum copper alloy catalysts in the LTFCs.

Elucidating the role of extended surface defects at Fe surfaces on CO adsorption and dissociation

The adsorption and dissociation of hydrocarbons on metallic surfaces during catalytic reactions in a steam reforming furnace often lead to the carburization of the catalysts and metallic surfaces involved. This process is greatly accelerated by the presence of intrinsic defects like vacancies and grain boundaries and is succeeded by surface to subsurface diffusion of C. We employ both density functional theory and reactive force field molecular dynamics simulations to investigate the effect of surface defects on CO dissociation rate directly related to metal dusting corrosion. We demonstrate that stable surface vacancy clusters with large binding energies accelerate the adsorption of CO molecules by decreasing the corresponding dissociation energies. In addition, we demonstrate that the appearance of multiple GBs at the surface leads to an enhancement of the CO dissociation rate. Furthermore, we demonstrate that the increase in surface roughness by emerging GBs leads to an increase in CO dissociation rate.

Influence of Support Acidity of Pt/Nb2O5 Catalysts on Selectivity of CO2 Hydrogenation

In solid acid catalysis, understanding the impact of support acidity on catalytic performance has remained a controversial issue. The selected catalytic systems often rely on mixing different substances to control the degree of acidity, which in turn, also modifies other parameters in the system, making it challenging to perform a definitive study. To specifically investigate the role of support acidity, we performed a systematic study employing Nb2O5 as the catalyst support, which acidity can be controlled by calcination. The catalytic behavior of the fabricated Pt/Nb2O5 catalysts was evaluated using CO2 hydrogenation to methanol (MeOH) and dimethyl ether (DME). An increase in the acidity of the support resulted in an improvement in the CO2 conversion owing to the strong interaction between the Pt and the catalyst support, but it was detrimental for the production of MeOH because of the unfavorable adsorption of CO2 molecules and the formation of carbon-containing species on the surface of the support with high acidity. DME selectivity was enhanced with an increase in catalyst acidity, confirming the role of solid acids for the production of DME from CO2 reduction. By controlling the calcination temperature of Nb2O5, tunable support acidity was obtained. CO2 conversion increased while the selectivity of methanol and dimethyl ether decreased with increasing support acidity.

A first-principles study on the interaction of CO molecules with VIII transition metals-embedded graphitic carbon nitride as an excellent candidate for CO sensor

We studied the adsorption behavior of CO molecules over graphitic carbon nitride (gCN) and VIII transition metals (TM)-embedded gCN systems (TM=Ni, Pd, and Pt atoms) using density functional theory. The results indicated that the Pt-embedded gCN is excellent candidate for adsorption of CO molecules with adsorption energy of −2.77 eV, which is much better than those of the other adsorbents. Furthermore, it was observed that the band gap energies of TM-embedded systems were less than that of pristine gCN and decoration of transition metal atoms leads to the formation of mid gap impurity states, resulting in increase of electrical conductivity. Additionally, the Lowdin charges displayed that upon adsorption of CO molecules, this molecule acts as an electron acceptor and gCN systems behave as an electron donor with electron transfer from d-orbitals of transition metal atoms to the states of CO molecule. The results of spin polarized band structure indicated that the pristine gCN, Ni and Pd-embedded systems are non-magnetic, whereas Pt-embedded gCN induces non-zero magnetic moment equal to 1.35 μB. Therefore, our results revealed that among the TM-embedded systems, Pt-embedded gCN is more effective than those of the other adsorbents in sensing and removing of this gas from the atmosphere.

Investigations of the temperature-dependent electron paramagnetic resonance spectra and local structures for a cobalt(II) porphyrin complex within a metal-organic framework.

The interaction between an adsorbed CO molecule and the unsaturated coordinated Co2+ center in the metal-organic framework (MOF) PCN-224 is investigated by analyzing the electron paramagnetic resonance (EPR) parameters (g factors and hyperfine structure constants) and the adsorption energies at various temperatures. Six- and five-coordinated octahedral models (four planar N with two and one axial CO molecules, respectively) are constructed to simulate the local structures of the Co2+ centers at different temperatures. Because of the Jahn-Teller effect of the Co2+ centers, the C2-Co-N4 and C-Co-N4 combinations undergo different tetragonal elongation distortions along the C4 axis, characterized by the relative elongation ΔZ and displacement ΔZ' of Co2+ at different temperatures. Given the agreement between the calculated and experimental EPR parameters, as well as the adsorption properties, the six- and five-coordinated models are regarded as suitable for low- and high-temperature systems, respectively. These studies may be helpful to understand the properties of similar MOFs with adsorbed molecules under the effect of ambient temperature.

Surface-Halogenation-Induced Atomic-Site Activation and Local Charge Separation for Superb CO2 Photoreduction.

Solar-energy-driven CO2 conversion into value-added chemical fuels holds great potential in renewable energy generation. However, the rapid recombination of charge carriers and deficient reactive sites, as two major obstacles, severely hampers the photocatalytic CO2 reduction activity. Herein, a desirable surface halogenation strategy to address the aforementioned concerns over a Sillén-related layer-structured photocatalyst Bi2 O2 (OH)(NO3 ) (BON) is demonstrated. The surface halogen ions that are anchored on the Bi atoms by replacing surface hydroxyls on the one hand facilitate the local charge separation, and, on the other hand, activate the hydroxyls that profoundly boost the adsorption of CO2 molecules and protons and facilitate the CO2 conversion process, as evidenced by experimental and theoretical results collectively. Among the three series of BON-X (X = Cl, Br, and I) catalysts, BON-Br shows the most substantially enhanced CO production rate (8.12 µmol g-1 h-1 ) without any sacrificial agents or cocatalysts, ≈73 times higher than that of pristine Bi2 O2 (OH)(NO3 ), also exceeding that of the state-of-the-art photocatalysts reported to date. This work presents a surface polarization protocol for engineering charge behavior and reactive sites to promote photocatalysis, which shows great promise to the future design of high-performance materials for clean energy production.

Adsorption and Interaction of CO and NO Molecules on Pure and Oxidized Surfaces of Al−Mo(110) Alloy

The coadsorption of carbon-monoxide- and nitric-oxide molecules and their interactions on surfaces of Al−Mo(110) alloy and the system resulting from its oxidation (Al−Mo−O) are investigated using an array of spectroscopic techniques (X-ray photoelectron, Auger electron, infrared, and thermal-desorption spectroscopies), low-energy electron diffraction, and work-function measurements in ultra-high vacuum. Al−Mo(110) alloy is fabricated by the thermal annealing (at 800 K) of an aluminum film several layers thick deposited onto the Mo(110) surface. Aluminum atoms diffuse into the substrate to yield a surface alloy of hexagonal structure characteristic of stoichiometric Al2Mo alloy. Unlike dissociative adsorption on both Mo(110) and Al(111) surfaces, CO and NO adsorb molecularly at the surface of Al−Mo(110) alloy. At 200 K, the adsorption of CO molecules on the Al−Mo(110) surface containing pre-adsorbed NO molecules has a dramatic effect on the latter by displacing them to higher-coordinated adsorption sites and at the same time causing their molecular axis to tilt toward the adsorbent surface plane. Heating this system to 320 K results in the reduction of nitric oxide by carbon monoxide to yield CO2 and surface nitrides. This can be a consequence of surface reconstruction that leads to the formation of additional adsorption/reaction sites at the Al/Mo interface and to changes in the substrate’s d-band filling as a result of alloying. The oxidation of CO by NO proceeds with a markedly higher efficiency at the surface of the Al−Mo−O system; this system results from oxidation of the Al−Mo(110) alloy by oxygen at 700 K and exposure up to 1500 L.

First-principles study of CO and OH adsorption on in-doped ZnO surfaces

We present a first-principles computational study of CO and OH adsorption on non-polar ZnO (101¯0) surfaces doped with indium. The calculations were performed using a model ZnO slab. The position of the In dopants was varied from deep bulk-like layers to the surface layers. It was established that the preferential location of the In atoms is at the surface by examining the dependence of the defect formation energy as well as the surface energy on In location. The adsorption sites on the surface of ZnO and the energy of adsorption of CO molecules and OH-species were determined in connection to In doping. It was found that OH has higher bonding energy to the surface than CO. The presence of In atoms at the surface of ZnO is favorable for CO adsorption, resulting in an elongation of the C-O bond and in charge transfer to the surface. The effect of CO and OH adsorption on the electronic and conduction properties of surfaces was assessed. We conclude that In-doped ZnO surfaces should present a higher electronic response upon adsorption of CO.

Photocatalytic reduction of carbon dioxide using graphene oxide wrapped TiO2 nanotubes

Photocatalytic conversion of carbon dioxide to valuable chemicals by using semiconductor materials is one of the major challenges in materials science. Higher efficiency and product yield can be achieved by designing of a suitable photocatalyst with retarded recombination of photogenerated electron-hole pair and sufficient potential for the reduction of CO2. In this work, unique composite architecture of graphene-oxide wrapped TiO2 nanotubes for the photocatalytic reduction of CO2 has been developed. The TiO2 nanotubes formed by electrochemical anodization are wrapped with graphene oxide/reduced graphene oxide (GO/rGO) layers which also form interconnecting bridges between the adjacent nanotubes. Such a unique nanostructure helped the separation of photogenerated electron-hole pairs with better charge transfer to perform the reduction of adsorbed CO2 molecules. The rGO/TiO2 multi-leg nanotubes (MLNTs) have shown the highest photocatalytic activity with maximum yield of CO c.a. 1348 μmol g−1 within the first 20 min, which stabilize to about 760 μmol g−1 after 2 h of UV-A irradiation. More importantly, the CO formation rate is about an order of magnitude higher than that from graphene supported TiO2 nanocrystals.

Role of defects in tuning the adsorption of CO over graphene-supported Co13 cluster

Using first-principles calculation, we studied the role of defects in tuning the adsorption of CO molecule over graphene-supported Co13 cluster, with a focus on the adsorption geometries and stabilities, electronic properties. It is shown that Co13-graphene interfaces exhibit covalent bonding characters with the most stable on C-vacancies support. The supports stabilize the adsorption of both intact and dissociative CO molecule over Co13/graphene composites, especially the vacancies and N-doped supports outperform the others. Moreover, intact CO molecule is energetically more favorable than its dissociative adsorbing over all the composites, and notably doping of B or P atom in support gives rise to quite similar effects. Overall, C-vacancies and atomic N-embedded in graphene supports can effectively improve CO adsorption on cobalt cluster, beneficial for CO conversion in Fischer–Tropsch synthesis.

CNx stabilized Ni-Ga nanoparticles for CO2 hydrogenation: Role of preparation methods

Abstract Carbon based framework are believed to be adsorbed large amount of hydrogen reversibly on its surface and this adsorbed active H-species can increase the rate of surface hydrogenation reactions. The basic nitrogen functionality if present can provides additional sites for adsorption of CO2 molecule. On this regard, CO2 hydrogenation has been investigated on multifunctional nitrogen doped carbon (CNx) consisting of Ni-Ga nanoparticles. Alloy nanoparticles (2–10 nm) of Ni3Ga and Ni3Ga5 has been identified from HR-TEM in combination of d value calculation form XRD. Whereas, mix Ni and Ni-Ga phase appears in case of higher loading catalyst i.e. NG-CNx-3. Under experimental conditions (2 MPa, 280 °C), the CO2 hydrogenation products are CH4, CO (major) and methanol. The activity of the synthesised catalysts strictly depends on the synthesis method and support. The catalyst consisting 14 and 9 (wt %) of Ni and Ga on CNx (NG-CNx-2) shows highest catalytic activity in terms of CO2 conversion. While NGBCNx-2 with 14 and 9 (wt %) of Ni and Ga on BCNx is found to be active towards methanol selectivity which may be due to the finely dispersed Ni-Ga alloy nanoparticles with size distribution between 2–5 nm.

Ligand-Mediated Reactivity in CO Oxidation of Niobium-Nickel Monoxide Carbonyl Complexes: The Crucial Roles of the Multiple Adsorption of CO Molecules.

The heteronuclear metal oxide complexes are of great significance in heterogeneous catalytic oxidation of CO. However, previous studies are mainly focused on the composition of metal oxide, charge state, the support and the active oxygen species, with little attention paid to adsorbed CO ligands. Herein, the ligand-mediated reactivity in CO oxidation of niobium-nickel monoxide carbonyl complexes has been successfully identified. The NbNiO(CO) n- ( n = 5-6) anions are determined to be O-bridged complexes. In contrast, the NbNiO(CO) n- ( n = 7-8) anions are characterized to be η2-CO2-tagged complexes. The crucial roles of the multiply adsorbed CO molecules that can facilitate not only the competitive binding with bridging oxygen atom to the transition metal centers but also the electron accumulation of transition metal atoms have been discovered. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over heteronuclear metal oxide under CO-rich feed condition.

A DFT study of CO adsorption on the pristine, defective, In-doped and Sb-doped graphene and the effect of applied electric field

The adsorptions of CO molecules on the pristine, defective, In-doped and Sb-doped graphene were investigated through the density functional theory (DFT) calculations. The stable geometries, electronic properties and charge transfers of the graphene based systems were calculated to study the interaction between the adsorbed CO molecule and the built material. It was found that only the weak physical adsorption took place when the CO was placed on the pristine graphene and the doped graphene. On the contrary, the defective graphene exhibited a high affinity to the CO molecule, acting as a potential sensing material to interact with CO through a strong chemisorption with the high adsorption energy of ~ 1.996 eV, which was little affected by the increased coverage effect of CO molecules. Moreover, this adsorption energy was calculated to be enhanced by ~ 62.6% and there was extra charge of 0.31 e transferred from the adsorbed CO molecule to the defective graphene under the negative electric field. Our research revealed that the applied electric field could be an effective method to improve the gas sensing performance of defective graphene.

The influence of CO, CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO2 and H2O content on the performance of chlorinated NiCeO2 catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO2 increases with the increasing H2 concentration, is independent of CO2 concentration and decreases with increasing CO and H2O concentrations; the rate of CO2 methanation is weakly sensitive to H2 and CO2 concentrations and decreases with increasing CO and H2O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO2 hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO2 was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO2 (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO2-η-Al2O3/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.

High Coverage CO Adsorption on Fe6O6 Cluster Using GGA + U

The adsorption of CO molecules on Fe6O6 cluster was systematically studied at different coverage by GGA + U calculations and atomic thermodynamics. Starting from single CO molecule adsorption on Fe6O6 cluster, we have varied the concentration and configuration of CO molecules. It has been found that one surface iron atom of Fe6O6 cluster can coadsorb two CO molecules which can be explained well by the spatial effect. The phase diagrams show that twelve CO molecules binding on Fe6O6 cluster is favorable thermodynamically. It has been found that six CO molecules binding on Fe6O6 cluster is the saturation adsorption according to the stepwise adsorption energy, and the different adsorption states can coexist for two-CO molecules binding on Fe6O6 cluster at high temperature according to probability distribution plot. The adsorption mechanism of CO on Fe6O6 cluster was analyzed by the projected density of states and compared with Fe3O4 surfaces and other small iron oxide clusters.

Adsorption and Desorption Properties of CO2 on CeO2 Nanoparticles Prepared via Different Synthetic Routes

The reversible adsorption and desorption of CO2 was investigated in the temperature range between room temperature and 250°C on the CeO2 powders prepared through different synthetic routes. The adsorption of CO2 was evaluated with respect to the weight change in the thermogravimetric analysis. Each CeO2 powder was composed of well-crystallized nanometer-sized grains independent of the synthetic routes. The specific surface area varied with the synthetic route, while it was not only the parameter determining the CO2 adsorption. It was found that the crystallite size grown to some extent, or crystallinity, was advantageous for the CO2 adsorption. In addition, the CeO2 powders prepared from a Ce(III) source through the pyrolysis of the precursors containing carbonate groups tended to enhance the CO2 adsorption. The highest value of the CO2 adsorption was 0.051 g per 1 g of CeO2. The surface coverage for the sample, evaluated with {adsorbed CO2 molecules} × {adsorption cross section of CO2}/{specific surface area of the CeO2 sample determined with N2 adsorption}, reached 1.02, which corresponded to a situation that the surface was fully covered with CO2 molecules.

Stabilization of 2D graphene, functionalized graphene, and Ti2CO2 (MXene) in super-critical CO2: a molecular dynamics study.

The stabilization of nanoparticles is a main concern to produce an efficient nanofluid. Here, we carry out Molecular Dynamics (MD) modeling to evaluate the stabilization of 2D graphene, oxygen- and hydrogen-functionalized graphene, and Ti2CO2 MXene nano-sheets in carbon dioxide (CO2) liquid under supercritical conditions. The rheological properties (i.e., viscosity and self-diffusion) of the nanofluids with the corresponding nano-sheets are also calculated. The results show that pristine graphene aggregates in the liquid and the adsorbed CO2 molecules on the graphene surfaces could not provide enough repulsive forces to avoid their aggregation. The stabilization of graphene is improved by surface functionalization of graphene with oxygen and hydrogen functional groups. This is because, first, the presence of the corresponding surface groups can alter the electrostatic forces of the graphene nano-sheets and, second, a larger number of CO2 molecules attracted by the functionalized nano-sheets can provide additional repulsive forces between the nano-sheets. The results demonstrate that Ti2CO2 MXene nano-sheets aggregate severely although they are covered by oxygen surface terminations and attract more CO2 molecules than the pristine graphene. This is attributed to the strong interlayer coupling between the MXene nano-sheets so that the electrostatic repulsion of the MXene surfaces could not overcome the interlayer coupling. Based on our results, the stabilization of the corresponding 2D nano-sheets in CO2 liquid is in the order: oxygen-functionalized graphene > hydrogen-functionalized graphene > pristine graphene > Ti2CO2 MXene. The calculation of the rheological properties of the nanofluids with different nano-sheets demonstrates that immersing the nano-sheets in the CO2 liquid affords a superior viscosity enhancement as much as possible to be dispersed in the liquid. This study expands the potential applications of these 2D materials in producing SC-CO2 based nanofluids.