Oxidation state and symmetry of magnesia-supported Pd13O(x) nanocatalysts influence activation barriers of CO oxidation

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.     

Titanium-decorated graphene oxide for carbon monoxide capture and separation

We propose titanium-decorated graphene oxide (Ti-GO) as an ideal sorbent for carbon monoxide (CO) capture and separation from gas mixtures. Based on first-principles calculations, Ti-GO exhibits a large binding energy of ~70 kJ mol(-1) for CO molecules, while the binding energies for other gases, such as N(2), CO(2), and CH(4), are significantly smaller. The gas adsorption properties of Ti-GO are independent of the local GO structures once Ti atoms are anchored by the oxygen-containing groups on the GO surface. The strong interaction between CO molecule and Ti is a result of dative bonding, i.e., hybridization between an empty d orbital of Ti and an occupied p orbital of CO. Adsorption isotherms from grand canonical Monte Carlo simulations clearly demonstrate the strong selectivity of Ti-GO for CO adsorption in a mixture with other gas.     

Hydrogen-induced transition from dissociative to molecular chemisorption of CO on vanadium clusters

We report on the size-dependent interaction of carbon monoxide molecules with hydrogen covered vanadium clusters containing between 5 and 20 atoms. Structural information on these hydrogen covered vanadium clusters and their complexes with CO is obtained from infrared multiple photon dissociation spectroscopy, complemented with density functional theory calculations for the V5 to V9 cluster sizes. The non-dissociative or dissociative binding of CO on the metal clusters is detected by the presence or absence of the nu(CO) stretching band in the infrared spectra. It is found that the CO molecule dissociates on bare vanadium clusters, while it adsorbs intact on all saturated hydrogen covered V5-20+ clusters, with the distinctive exceptions of V5+, V9+, V11+, and V19+. We show that dissociative chemisorption is prevented when the potential binding sites of atomic C and O atoms are blocked by H atoms.     

Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium alloy catalysts.

The kinetics for the oxidation of carbon monoxide in the presence of excess oxygen over Pt-Rh alloy catalysts were studied by using the reversed-flow gas chromatography technique. Suitable mathematical analysis equations were derived by means of which the rate constants for the oxidation reaction of carbon monoxide, as well as for the adsorption and desorption of the reactant CO on the catalysts pure Pt, 75 atom% Pt+25 atom% Rh, 50 atom% Pt+50 atom% Rh, 25 atom% Pt+75 atom% Rh and pure Rh supported on SiO2 were determined. All the catalysts show a maximum rate constant for the production of CO2 at a characteristic temperature close to that found in the literature. The rate constants for the adsorption of CO increase generally with increasing temperature, while those for the desorption decrease with increasing temperature. From the variation of the rate constants with temperature activation energies for the oxidation reaction and adsorption of CO were determined, which are sensitive to the composition of the catalytic surface. The appearance of CO2 and carbon, when introducing pure CO into the column with the catalysts, verified a partial dissociative adsorption (e.g., disproportionation) of CO on the catalysts used. The latter indicates a mechanism for the CO oxidation through a partial dissociative adsorption of CO followed by the reaction of adsorbed CO molecules with adsorbed O atoms.     

Infrared spectroscopic and density functional theory studies on the CO dissociation by scandium and yttrium dimers

Reactions of laser-ablated scandium and yttrium atoms with dilute carbon monoxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. On the basis of the results of the isotopic substitution, the change of laser power and CO concentration and the comparison with density functional theory (DFT) calculations, the absorption at 1193.4 cm(-1) is assigned to the C-O stretching vibration of the Sc(2)[eta(2)(mu(2)-C,O)] molecule, which has a single bridging CO that is tilted to the side. This CO-activated molecule undergoes ultraviolet-visible photoinduced rearrangement to the CO-dissociated molecule, c-Sc(2)(mu-C)(mu-O). The cyclic c-Sc(2)(mu-C)(mu-O) molecule has a bridging carbon on one side of the Sc(2) unit and a bridging oxygen on the other. The analogous Y(2)[eta(2)(mu(2)-C,O)] molecule has not been observed, but the CO-dissociated c-Y(2)(mu-C)(muO) molecule has been observed in the Y + CO experiments. DFT calculations of the geometry structures, vibrational frequencies, and IR intensities strongly support the assignments. The CO activation mechanism has also been discussed. Our experimental and theoretical results schematically depict an activation process to CO dissociation.     

An ab initio investigation on (CO2)n and CO2(Ar)m clusters: geometries and IR spectra

An ab initio investigation on CO(2) homoclusters is done at MPWB1K6-31++G(2d) level of theory. Electrostatic guidelines are found to be useful for generating initial structures of (CO(2))(n) clusters. The ab initio minimum energy geometries of (CO(2))(n) with n=2-8 are T shaped, cyclic, trigonal pyramidal, tetragonal pyramidal, tetragonal bipyramidal, pentagonal bipyramidal, and pentagonal bipyramid with one CO(2) molecule attached to it. A test calculation on (CO(2))(20) cluster is also reported. The geometric parameters of the energetically most favored (CO(2))(n) clusters match quite well their experimental counterparts (wherever available) as well as those derived from molecular dynamics studies. The effect of clustering is quantified through the asymmetric C-O stretching frequency shift relative to the single CO(2) molecule. (CO(2))(n) clusters show an increasing blueshift from 1.8 to 9.6 cm(-1) on increasing number of CO(2) molecules from n=2 to 8. The energetics and geometries of CO(2)(Ar)(m) clusters have also been explored at the same level of theory. The geometries for m=1-6 show a predominant T type of the argon-CO(2) molecule interaction. Higher clusters with m=7-12 show that the argon atoms cluster around the oxygen atom after the saturation of the central carbon atom. The CO(2)(Ar)(m) clusters exhibit an increasing redshift in the C-O asymmetric stretch relative to CO(2) molecule of 0.7-5.6 cm(-1) with increasing number of argon atoms through m=1-8.     

First principles study of adsorption and dissociation of CO on W(111)

The adsorption and dissociation of carbon monoxide on the W(111) surface is studied with density functional theory. The CO molecule is found to adsorb in end-on configurations (alpha states) and inclined configurations (beta states). The dissociation of the most strongly bound beta state CO is found to have an activation energy of about 0.8 eV, which is lower than the energy required to desorb CO molecularly from the surface. The diffusion of CO and O on W(111) is predicted to be facile at room temperature, whereas C atoms are virtually immobile up to approximately 600 K, according to our calculations. Preadsorbed carbon atoms are shown to prevent the dissociation of CO by blocking the most strongly bound beta state adsorption site and by blocking the dissociation pathway. We predict that dissociation of CO on W(111) is a self-poisoning process.     

CO adsorption and CO and O coadsorption on Rh(111) studied by reflection absorption infrared spectroscopy and density functional theory

The adsorption of carbon monoxide on Rh(111) and on oxygen modified Rh(111) was investigated using thermal desorption spectroscopy, reflection absorption infrared spectroscopy (RAIRS), and density functional theory. The results show that CO adsorbs on Rh(111) in on top sites at low coverages. With increasing coverage hollow sites and bridge sites get occupied according to the RAIRS results. A new vibrational feature at high wave numbers was found in the on top region of the CO stretching frequency. This feature can be explained by a local high density CO structure where two CO molecules are adsorbed in the ( radical3x radical3)R30 degrees structure. The coadsorption of oxygen and carbon monoxide leads to a shift of the CO stretching frequency to higher wave numbers with increasing O to CO ratio. CO adsorption on a (2x1) oxygen layer is possible and RAIRS shows that the CO adsorbs in on top and most likely in bridge sites in this case.     

Density functional studies on the Pauson--Khand reaction.

The Pauson--Khand reaction represents a one-step Co(2)(CO)(8)-catalyzed synthesis of cyclopentenone through [2 + 2 + 1] assembly of one molecule each of alkene, alkyne, and carbon monoxide. Density functional studies (B3LYP/631LAN) on the reaction pathway of the Pauson--Khand (PK) reaction reported here for the first time provides valuable information on the structures and energetics of various intermediates and transition states. The PK reaction consists of olefin insertion, CO insertion, and reductive elimination steps. The olefin insertion step was found to be an irreversible step that determines the stereo- and regiochemistry of the overall reaction. The following steps are low activation energy processes and reversible. The bond-forming events occur only on one of the two metal atoms, while the second metal atom not only acts as an anchor that fixes the metal cluster to the organic substrate but also exerts electronic influences on the reaction at the first atom.     

Many‐body interactions of carbon monoxide cyclic oligomers: A computational study

Structural properties and energetics for the optimized carbon monoxide cyclic oligomers are analyzed at the correlated ab initio second‐order Møller–Plesset (MP2) and density functional methods (B3LYP and mPW1PW), using Dunning's cc‐pVXZ (X = D, T, Q) basis set, augmented with diffuse functions. Many‐body interactions of the stable carbon monoxide cyclic oligomers, (CO)₄ and (CO)₅ are computed at the MP2/aug‐cc‐pVTZ level. Contributions of two‐ to five‐body terms to each of these oligomers for their interaction energies, including corrections for basis set superposition error (BSSE) are investigated by using function counterpoise and its generalized version. It has been found that three‐body terms are attractive in nature and essential in order to describe the cooperative effects in the stable cyclic CO oligomers. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Reactions of gadolinium atoms and dimers with CO: formation of gadolinium carbonyls and photoconversion to CO activated molecules

Reactions of gadolinium atoms and dimers with carbon monoxide molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. Mononuclear Gd(CO)x (x = 1-3) and dinuclear Gd2(CO)x (x = 1, 2) gadolinium carbonyls formed spontaneously on annealing. The Gd(CO)x complexes are CO terminal-bonded carbonyls, whereas the Gd2CO and Gd2(CO)2 carbonyl complexes were characterized to have asymmetrically bridging and side-on-bonded CO, which are drastically activated with remarkably low C-O stretching frequencies. The cyclic Gd2(mu-C)(mu-O) and Gd3(mu-C)(mu-O) molecules in which the C-O triple bond is completely cleaved were also formed on annealing. The Gd2(CO)2 complex rearranged to the more stable c-Gd2(mu-O)(mu-CCO) isomer, which also has a four-membered ring structure with one CO being completely activated.     

CO在α-U(001)表面的吸附

采用密度泛函理论中的广义梯度近似,计算了CO在α-U(001)表面的吸附、解离和扩散.结果表明:CO分子以CU3OU2构型化学吸附在α-U(001)表面,吸附能为1.78-1.99eV;吸附后表层U原子向上迁移,伴随着褶皱的产生;CO分子与表面U原子的相互作用主要是U原子的电子向CO分子最低空轨道2π*转移,以及CO2π*/5σ/1π-U6d轨道间杂化而生成新的化学键;CO解离吸附较分子吸附在能量上更为有利,h1(C)+h2(O)和h1(C)+h1(O)(h:空位)解离态吸附能分别为2.71和3.08eV;近邻三重穴位之间C、O原子的扩散能垒分别为0.57和0.14eV,预示O原子较C原子更易在U(001)表面扩散迁移.     

Structures and vibrational frequencies of CO adlayers on Rh(111) surface

The adsorption of carbon monoxide on single-crystal transition metal surfaces has been the subject of numerous studies, because it has served as a model system for the adsorption of small molecules on transition metal surfaces, and its industrial importance is obvious in such areas as catalytic reaction. The bonding of carbon monoxide to rhodium is of special interest since this metal catalyzes the hydrogenation of CO to produce hydrocarbons in both heterogeneous and homogeneous media, and it …     

Dispersion-corrected DFT study on the carbon monoxide sensing by B<Subscript>2</Subscript>C nanotubes: effects of dopant and interferences

Electrical sensitivity of a boron carbon nanotube (B₂CNT) was examined toward carbon monoxide (CO) molecule by using dispersion-corrected density functional theory calculations. It was found that CO is weakly adsorbed on the tube, releasing energy of 3.5–4.1 kcal/mol, and electronic properties of the tube are not significantly changed. To overcome this problem, boron and carbon atoms of the tube were substituted by aluminum and silicon atoms, respectively. Although both Al and Si doping make the tube more reactive and sensitive to CO, Si doping seems to be a better strategy to manufacture CO chemical sensors due to the higher sensitivity without deformation of nanotube structure after adsorption procedure. Moreover, it was shown that some interference molecules such as H₂O, H₂S and NH₃ cannot significantly change the electronic properties of B₂CNT. Therefore, the Si-doped tube might convert the presence of CO molecules to electrical signal.     

Infrared spectroscopic and density functional theory studies on the reactions of cadmium atoms with carbon monoxide in solid argon

Reactions of laser-ablated cadmium atoms with carbon monoxide molecules in solid argon have been investigated using matrix isolation infrared spectroscopy. On the basis of isotopic substitution, the absorption at 1858.2 cm(-1) is assigned to the C-O stretching of the CdCO molecule, which is formed during the sample deposition. Cadmium di- and tricarbonyls, Cd(CO)n (n = 2, 3), have not been observed under the same experimental conditions. Density functional theory calculations have been performed on the cadmium carbonyls Cd(CO)n (n = 1-3), which lend strong support to the experimental assignments of the infrared spectra. It is predicted that the CdCO molecule is a linear triplet molecule and its formation involves Cd 5s --> 5p promotion. This promotion increases the Cd-CO bonding by decreasing the sigma repulsion and increasing the Cd 5p orbital --> CO pi back-donation. The absence of cadmium di- and tricarbonyls, Cd(CO)n (n = 2, 3), has also been discussed in some detail.     

Atom-molecule interactions on transition metal surfaces: a DFT Study of CO and several atoms on Rh(100), Pd(100) and Ir(100).

Density functional theory (DFT) calculations have been performed to determine the interaction energy between a CO probe molecule and all atoms from the first three rows of the periodic table coadsorbed on Rh(100), Pd(100) and Ir(100) metal surfaces. Varying the coverage of CO or the coadsorbed atom proved to have a profound effect on the strength of the interaction energy. The general trend, however, is the same in all cases: the interaction energy becomes more repulsive when moving towards the right along a row of elements, and reaches a maximum somewhere in the middle of a row of elements. The absolute value of the interaction energy between an atom-CO pair ranges from about -0.40 eV (39 kJ mol(-1)) attraction to +0.70 eV (68 kJ mol(-1)) repulsion, depending on the coadsorbate, the metal and the coverage. The general trend in interaction energies seems to be a common characteristic for several transition metals.     

The interaction of formohydroxamic acid with carbon monoxide: FTIR matrix isolation and quantum chemistry studies

Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) and carbon monoxide have been recorded. The experimental results indicate formation of three isomeric complexes. In two complexes the NH or OH groups of formohydroxamic acid are attached to the carbon atom of the CO molecule and in the third complex the NH group interacts with the oxygen atom of CO. The formohydroxamic acid complex with two CO molecules is also trapped in the matrix. One of the two CO molecules interacts with the NH group and the second one with the OH group of HCONHOH, in both cases the site of interaction is the carbon atom of CO. Theoretical studies of the structure and spectral characteristics of the complexes were carried out on the DFT(B3LYP)/6-311++G(2d,2p) level. The calculated vibrational frequencies for the complexes present in the matrices are in good agreement with the experimental data. The calculations show also an additional potential energy minimum corresponding to the complex in which the OH group of formohydroxamic acid is attached to the oxygen atom of carbon monoxide.     

Density functional theory study of CHx (x=1-3) adsorption on clean and CO precovered Rh(111) surfaces

CH(x) (x=1-3) adsorptions on clean and CO precovered Rh(111) surfaces were studied by density functional theory calculations. It is found that CH(x) (x=1-3) radicals prefer threefold hollow sites on Rh(111) surfaces, and the bond strength between CH(x) and Rh(111) follows the order of CH(3)相似文献   

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

Synthesis,spectroscopic characterization,and crystal structure of the bimetallic complex [Ni2(mu-CO)(CO)2(mu-NH(PPh2)2)2

The dinuclear Ni(0) complex [Ni2(mu-CO)(CO)2(mu-dppa)2] (1; dppa = bis(diphenylphosphino)amine) was synthesized by two routes in good yield. Complex 1 has a triclinic crystal system and P1 space group, with a = 13.009(1) A, b = 13.063(2) A, c = 14.664(2) A, alpha = 79.91(1) degrees, beta = 79.96(1) degrees, gamma = 71.32(1) degrees, and Z = 2. The structure of this compound exhibits two mu-coordinated dppa ligands in a cis, cis arrangement. Nickel atoms are at a 2.5824(7) A distance. Theoretical calculations predict a 0.39 bond order between metal atoms. The cyclic voltammograms show two quasi-reversible redox pairs, which correspond to the successive oxidation of the metal centers. The dinuclear complex described absorbs carbon monoxide, yielding a mixture of nickel carbonyl compounds.