Free CO Molecule Research Articles

Electrocatalytic Urea Synthesis via N2 Dimerization and Universal Descriptor.

Electrocatalytic urea synthesis through N2 + CO2 coreduction and C-N coupling is a promising and sustainable alternative to harsh industrial processes. Despite considerable efforts, limited progress has been made due to the challenges of breaking inert N≡N bonds for C-N coupling, competing side reactions, and the absence of theoretical principles guiding catalyst design. In this study, we propose a mechanism for highly electrocatalytic urea synthesis using two adsorbed N2 molecules and CO as nitrogen and carbon sources, respectively. This mechanism circumvents the challenging step of N≡N bond breaking and selective CO2 to CO reduction, as the free CO molecule inserts into dimerized *N2 and binds concurrently with two N atoms, forming a specific urea precursor *NNCONN* with both thermodynamic and kinetic feasibility. Through the proposed mechanism, Ti2@C4N3 and V2@C4N3 are identified as highly active catalysts for electrocatalytic urea formation, exhibiting low onset potentials of -0.741 and -0.738 V, respectively. Importantly, taking transition metal atoms anchored on porous graphite-like carbonitride (TM2@C4N3) as prototypes, we introduce a simple descriptor, namely, effective d electron number (Φ), to quantitatively describe the structure-activity relationships for urea formation. This descriptor incorporates inherent atomic properties of the catalyst, such as the number of d electrons, the electronegativity of the metal atoms, and the generalized electronegativity of the substrate atoms, making it potentially applicable to other urea catalysts. Our work advances the comprehension of mechanisms and provides a universal guiding principle for catalyst design in urea electrochemical synthesis.

Bubble Growth in Poly(methyl methacrylate) and Carbon Dioxide Mixture.

In this paper, we study bubble nucleation and growth in a poly(methyl methacrylate) and CO mixture by molecular dynamics simulations. It is known in the foaming industry that the bubble size has a more uniform distribution with a higher start-up pressure. The real physical reason remains unclear. In this work, we found that the free volume-rich polymer segments could adsorb many small-size bubbles in the region close to the polymer chain. The existence of these small bubbles limits the number of free CO molecules, which is helpful for bubble stabilization. Moreover, the free volume of polymer segments decreases with an increase of the start-up pressure. As a result, the size of the large bubbles becomes more uniform with a higher startup pressure.

Anisotropic Stark effect of carbon monoxide: emergent orbital cooperativity

ABSTRACT Applying external electric fields to molecules gives rise to spectral shifting and splitting, a phenomenon known as the Stark effect. However, a fundamental question of how electronic structures of molecules are modified by electric fields is still not well understood. By applying electric fields to a carbon monoxide molecule, herein we have successfully addressed the fundamental question at orbital scales and discovered that the Stark effect exhibits anisotropic characters depending on the direction of the electric fields with respect to the molecular axis. Based on the fact that applying electric fields along the molecular axis always preserved the orthogonality between the sigma and pi electrons, we found that orbital resemblance-based cooperativity can only operate within either the sigma system in which sigma electrons somehow prefer to resemble each other or the pi electron system in which the 1π electrons experience polarization-based self-resemblance. However, switching the electric field vertical to the molecular axis breaks down the orthogonality between the sigma system and pi electron systems, opening up electronic channels that allow σ electron systems to resemble π electrons. Such orbital cooperativity represents a new physical effect beyond the conventional Stark effect. Moreover, we have found that applying electric fields to the molecule would modify its molecular orbital diagram, depending on the directions of the electric fields; the electric field along the carbon-to-oxygen direction basically retains the MO diagram of the free CO molecule, with noticeable intra-orbital electron redistributions, whereas the oxygen-to-carbon electric field does create new states of molecular orbital contributions.

Convincing Synthesis of Atomically Thin, Single-Crystalline InVO4 Sheets toward Promoting Highly Selective and Efficient Solar Conversion of CO2 into CO.

Atomically thin, single-crystalline InVO4 sheets with the uniform thickness of ∼1.5 nm were convincingly synthesized, which was identified with strong, low-angle X-ray diffraction peaks. The InVO4 atomic layer corresponding to 3 unit cells along [110] orientation exhibits highly selective and efficient photocatalytic conversion of CO2 into CO in the presence of water vapor. Surface potential change measurement and liquid photoluminescence decay spectra confirm that the atomically ultrathin structure can shorten the transfer distance of charge carriers from the interior onto the surface and decrease the recombination in body. It thus allows more electrons to survive and accumulate on the surface, which is beneficial for activation and reduction of CO2. In addition, exclusively exposed {110} facet of the InVO4 atomic layer was found to bind the generating CO weakly, facilitating quick desorption from the catalyst surface to form free CO molecules, which provides an ideal platform to catalytically selective CO product.

Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

AbstractRecent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.

Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm.

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.

Effects of Azole Drugs on Conformational Dynamics and Energetics Associated with Diatomic Ligand Photorelease and Migration within Bacterial Flavohemoglobins

Bacterial flavohemoglobins (fHbs) are a widely distributed class of proteins which are though to protect microbes from nitrosative stress due to the immune response of a host organism via nitric oxide dioxygenase (NOD) activity. There has been interest lately in developing fHbs as targets for antibiotics due to their ubiquity in bacteria and the discovery of inhibition of NOD activity of fHbs by azole-based drugs which bind with high affinity (<12 μM). To determine how azole drugs affect fHbs we have used photoacoustic calorimetry (PAC) and transient absorption spectroscopy (TA) to calculate enthalpy and volume changes (ΔH and ΔV) and kinetic parameters for CO photorelease and rebinding to fHbs from Cupriavidus necator (FHP) and Staphylococcus aureus (HMPSa) in the presence and absence of the drugs ketoconazole and miconazole. We observe that binding of drugs to fHbs results in a more exothermic ΔH for CO photorelease and faster bimolecular rebinding of CO. We also use classical molecular dynamics (cMD) to characterize ligand migration pathways present in FHP. We have conducted cMD simulations of CO-FHP WT and FHP WT containing free CO molecules using a locally enhanced sampling (LES) algorithm. We have carried out a 10 ns simulation of CO-FHP WT and a 30 ns simulation of the LES-enabled CO:FHP WT. We observe dissociation of 18 of 20 LES-enhanced CO molecules from the distal heme pocket. The primary escape pathways are between the CD-loop and heme-propionate groups and through a pathway exiting the protein matrix near the N-terminal end of the A-helix. These pathways are close to the locations of ketoconazole and miconazole binding predicted by AutoDock Vina, suggesting that drug binding inhibits NOD activity by closing migration pathways for O2 and NO.

Theoretical insights into the CO dimerization and trimerization on Pt nanocluster

CO dimerizaiton and trimerization on icosahedral Pt55 cluster.

Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm(-1) on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm(-1) below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti(4+) sites. At a higher density of defects, adsorption on Ti(3+) sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti(4+) sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm(-1). These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.

Density functional theory study of the adsorption of CO on Wn (n= 16) clusters

CO molecules adsorbed on the Wn clusters are systematically investigated by using density functional theory at the B3LYP/LANL2DZ level.The result indicates that the ground state structures of WnCO clusters are generated when CO molecules are adsorbed on Wn clusters or anionic cluster. We find that among the molecular adsorption states exists mainly the form of end-on type geometry, and that the bridge site adsorption type geometry plays a supplementary role. On the face, the adsorption is a non-dissociative adsorption. The CO bond length increases 0.1200.123 nm in WnCO cluster (compared with 0.116 nm in free CO molecule), which demonstrates that the CO molecules are activated. The stability analysis shows that W3CO and W5CO clusters are more stable than other clusters; natural bond orbital (NBO) analysis indicate that the interaction between W atom and CO molecule is primarily contributed by hybridization of molecular orbits within CO and 6s, 5d, 6p and 6d orbits of W atoms.

Molecular geometry of OC⋅⋅⋅AgI determined by broadband rotational spectroscopy and ab initio calculations

Pure rotational spectra of the ground vibrational states of six isotopologues of OC···AgI have been measured by chirped-pulse Fourier transform microwave spectroscopy. The spectra are assigned to determine the rotational constant, B(0), centrifugal distortion constant, D(J), and nuclear quadrupole coupling constant of the iodine atom, χ(aa)(I). The complex is linear. Isotopic substitutions at the silver, carbon, and oxygen atoms allow bond lengths to be established by the r(0), r(s), and r(m)((1)) methods of structure determination. The length of the C-O bond, r(CO), in the r(0) geometry for OC···AgI is 0.008 Å shorter than that found in the free CO molecule. The length of the Ag-I bond, r(AgI), is 0.013 Å shorter than in free AgI. χ(aa)(I) is determined to be -769.84(22) MHz for OC···(107)AgI implying an ionic character of 0.66 for the metal halide bond. Attachment of carbon monoxide to the isolated AgI molecule results in an increase of the ionic character of AgI of 0.12. The molecular structure and spectroscopic parameters determined from the experimental data are presented alongside the results of calculations at the explicitly correlated coupled-cluster singles, doubles and perturbative triples level. Vibrational frequencies, the electric dipole moment, the nuclear quadrupole coupling constant, and the dissociation energy of the molecule have been calculated.

A density functional study of carbon monoxide adsorption on small cationic, neutral, and anionic aluminum phosphide clusters

The chemisorption of CO on aluminum phosphide (AlP)n (n=1–12) clusters are studied with density functional theory. Among various possible CO adsorption sites, the on-top site is identified to be the most favorable chemisorption sit. The Al-top sites are the preferred one in the most cases for one CO adsorption in (AlP)n (n=3–12) clusters, irrespective of the charge state of the clusters. The Al–P bond lengths decrease generally as the size of the cluster increases. There is a slight increase in the mean Al–P bond lengths after CO adsorption on the lowest-energy sites of the most AlP clusters. In general, adsorption energies of CO are found to decrease with an increase in the cluster size for both neutral and ionic clusters. The adsorption energies of CO on the cationic (AlP)nCO+ (n=2–12) clusters are greater than those on the neutral and anionic complexes. The result shows that large adsorption energies of CO on neutral and ionic AlP clusters and large highest occupied and lowest unoccupied molecular-orbital gaps for (AlP)CO, (AlP)CO+ and (AlP)CO− make these species behaving like magic clusters. The CO vibrational frequencies νCO in all the cationic complexes are always larger than the corresponding quantities in the neutral and anionic clusters. All of them are smaller than those in the free CO molecule.

Low kinetic energy photoelectron diffractions for C 1s and O 1s electrons of free CO molecules in the EXAFS region

We have measured molecular-frame photoelectron angular distributions from carbon 1s and oxygen 1s levels of CO molecules up to a photoelectron kinetic energy (KE) of ∼150 eV. The backward-scattering intensities exhibited a strong modulation as a function of the kinetic energy of the photoelectrons, whereas the intensities for the forward-scattering gradually increased and then became nearly constant over KE ∼100 eV. Multiple scattering calculations with a muffin-tin potential qualitatively reproduced the experimental results. The present results may be considered as the observation of low-energy photoelectron diffraction patterns for gaseous free CO molecules, which are involved in modulations in extended X-ray absorption fine structure (EXAFS) spectra.

Infrared spectra of carbon monoxide–hydrogen sulfide van der Waals complexes in the C–O stretching region

The CO-stretching vibration–rotation spectra of CO–H 2S, CO–D 2S, and CO–HDS complexes have been studied in the 2150 cm −1 region using a supersonic slit-jet expansion and a tunable diode laser spectrometer. The spectra were analyzed with the help of very recent microwave pure rotational studies of the same complexes. Two bands were assigned for each of the symmetric hydrogen sulfide isotopes, corresponding to the two nuclear spin modifications, para and ortho. The band origins were blue shifted, relative to the free CO molecule, by about 3.8 cm −1 for CO–H 2S and 4.3 cm −1 for CO–D 2S. These are considerably smaller shifts than exhibited by the related CO–water complexes, indicating that the intermolecular forces in CO–H 2S are weaker and more isotropic.

Lewis acidity and basicity of cation-exchanged zeolites: QM/MM and density functional studies

The structure of Li +, Na + and K +-exchanged faujasite (FAU) zeolite is studied by means of cluster model and QM/MM approach at HF/6-31G ∗∗ level of calculations. When CO molecule interacts with cation, “hard” site present in the zeolite channel, the CO bond length decreases compared to the free CO molecule and this decrease in CO bond length depends on Lewis acid strength of the cation. The strength of Lewis acidity of the cations and basicity of framework oxygen atoms is investigated using density functional (DF) methods. We used local reactivity descriptors based on hard–soft acid–base concept as acidity and basicity parameters. The calculated ‘relative electrophilicity’ and ‘relative nucleophilicity’ values are found to be better descriptors of acidic and basic sites, respectively. From relative electrophilicity ( s k +/ s k −) value the Lewis acidity of the cation-exchanged faujasite zeolite is found to decrease in the order: [Li-FAU]>[Na-FAU]>[K-FAU], and from relative nucleophilicity ( s k −/ s k +) value of the framework oxygen atom, the Lewis basicity is found to increase in the order: [Li-FAU]<[Na-FAU]<[K-FAU].

Properties of CO adsorbed in ZSM5 zeolite: Density functional theory study using the embedding scheme based on electron density partitioning

The CO molecule is frequently used as a probe in studies of zeolites where it adsorbs on metal cations. Compared with the free CO molecule, the stretching frequency of CO adsorbed in a zeolite is blue-shifted. The magnitude of the shift depends on the cation. The theoretical studies by Ferrari et al. [J. Chem. Phys., 105, 4129 (1996)] show that the isolated cation does not provide a good model of the zeolite because the calculated shifts are significantly overestimated. In this work, the effects of the interactions between the Me+CO (Me=Li, Na, or K) complex and the zeolite framework on the properties of CO adsorbed on the cation site are investigated. The properties of the investigated complexes are studied using the embedded molecule approach applying the orbital-free effective embedding potential derived within the subsystem formulation of density functional theory. In order to identify the major microsopic effects affecting the properties of the bound probe molecule, a hierarchy of cluster models is used to represent the zeolite framework. For the largest cluster model applied, the calculated frequency shifts agree within few cm−1 with experimental data.

The weakly bound complex NH3-CO: Observation and analysis of spectra in the infrared (C-O stretching) and millimetre wave (76–106 GHz) regions

The van der Waals complex NH3-CO has been studied in the infrared and millimetre wave regions, and analysed on the basis of the distinction between the ortho- and para- NH3-CO nuclear spin species. The spectrum and energy level pattern of ortho-NH3-CO are relatively simple, closely resembling those of the Ne-CO and Ar-CO complexes. The situation for para-NH3-CO is somewhat more complicated, with observed states having K values which differ by ±1 from that of the analogous orthoNH3-CO state. This indicates that the lowest energy orientation of para-NH3-CO in the complex lies in a II configuration. The present results correlate very well with the only previous study of NH3-CO, in the microwave region, and clarify the nature of the rotational series observed there. The infrared band centre of NH3-CO is only very slightly shifted (<0.2cm−1) from that of the free CO molecule, indicating that the CO vibration has little effect on the intermolecular forces.

Infrared spectrum of the CH3OH – CO complex in the C–O stretching region

The infrared spectrum of the weakly-bound molecular complex methanol - carbon monoxide has been observed, for the first time in the gas phase, by means of a parallel (Δ K = 0) band in the region of the carbon monoxide C–O stretching vibration. The band origin, 2154.5 cm–1, represents a blue shift of 11.2 cm–1 relative to the free CO molecule; its proximity to that of the H2O – CO complex points to a similarity of the hydrogen bonding in the two systems. The observed structure of the band could be well reproduced by a simulation based on the methyl internal rotation component of A-symmetry. A component of E-symmetry must also be present, with roughly equal intensity, but the two components were not individually resolved. The observed transitions showed evidence of predissociation broadening, leading to an estimate of t [Formula: see text] 0.5 ns for the lifetime of the upper (C–O stretching) state.PACS Nos.: 3320E, 3425, 3520P, 3640

Structural and vibrational properties of carbon monoxide adlayers on the copper (001) surface

The structure and vibrational states of a prototype adsorbate-substrate system—carbon monoxide on the copper (001) surface—have been calculated from first principles within local density functional theory. Three CO coverages have been examined: θ=0 (bare surface), 0.5, and 1. These systems are represented by a well converged slab model within which all atomic degrees of freedom are treated on an equal footing. The computed structural relaxations and vibrational frequencies are generally in excellent quantitative agreement with the available experimental measurements. The full monolayer is found to be energetically favorable to the half monolayer plus free CO molecule. This indicates that the maximum stable coverage is greater than θ=0.5, in agreement with experiment. The vibrational analysis reveals that resonant coupling between adsorbate and substrate motions has a profound effect on the vibrational spectra, for example, the low-frequency, in-plane frustrated translational motion of the CO molecules mixes with long-wavelength copper phonons to form a broad resonance peak. This implies a finite lifetime which, for the half-monolayer system, is computed to be 3.0 ps, in excellent agreement with the measured value of 2.3±0.4 ps. For the full-monolayer system, the predicted lifetime is 0.7 ps; however this system is presently inaccessible to experiment. Resonant coupling is also found to affect the Rayleigh wave of the copper (001) surface. At half-monolayer CO coverage, this mode resonantly mixes with bulk copper phonons developing a finite lifetime, which is predicted to be 5.2 ps. To our knowledge, the lifetime of this mode has never been measured. For the fully covered surface, the Rayleigh wave does not form a resonance because the phonon coupling is forbidden by symmetry.

Interaction of CO Molecules with Electron-Deficient Pt Atoms in Zeolites: A Density Functional Model Cluster Study

The coordination and electronic structure of monatomic Pt species in mordenite have been investigated by scalar−relativistic density functional model cluster calculations using CO molecules as a probe. It was found that anchoring a Pt−CO moiety by the protons of one or two acidic hydroxyl groups increases the CO stretching frequency compared to that of free Pt−CO, but leaves the frequency still smaller than that of a free CO molecule, in line with experiment. The results for various molecular model complexes support the hypothesis that the platinum species in mordenite are electron-deficient. An alternative model comprising “naked” protons interacting with Pt−CO moieties can be ruled out since the calculated CO frequency is too large. The dependence of the CO stretching frequency on the acidity of the Bronsted groups and on the electronic charge of the Pt species is discussed.