metal-CO Bond Research Articles

Interaction of NO with CO on Pd(100): ordered coadsorption structures and explosive reaction

An ordered mixed structure of c(3 × 2) is formed for a (NO + CO) coadsorption layer. The c(3 × 2) islands are considered to consist of equimolar NO and CO. The local fractional coverage (θNO + θCO) in the domain is estimated to be 0.33. Explosive production of CO2 takes place in the c(3 × 2) islands. The vacancy requirement model is considered to be valid for the autocatalytic reaction. Since the reaction is not accompanied with any substrate reconstruction, the autocatalytic behaviour is attributed only to the formation of mixed islands. The desorption of N2 follows, however, the second-order kinetics on Pd(100). As a result of the competition between NO and CO for the surface electrons, the CO-metal bond is weakened by the coadsorbed NO, which influences the explosive reaction. On the other hand, strengthening of the NO-metal bond is observed. When NO is in excess of CO, a p(3 × 2) structure coexists with the c(3 × 2) structure. The local coverage in the p(3 × 2) islands is estimated to be 0.33. In this coverage region, another path for the CO2 production is available.

Effects of relativity on the NiCO, PdCO, and PtCO bonding mechanism: a constrained space orbital variation analysis of density functional results

Using a density functional based constrained space orbital variation technique, we have analyzed the importance of relativistic effects on the various mechanisms which affect the metal-CO bond in Ni, Pd, and Pt monocarbonyls. The bonding is dominated by the π back-donation mechanism which, for Pd and Pt, is considerably reinforced by relativistic effects. The weaker PdCO bonding is rationalized by reduced back-donation. The combined effect of relativistic bond length contraction and of the atomic d orbital energies determines the trend in the metal-CO bond strengths. This trend is different from the one found for the coordinatively saturated M(CO) 4 tetracarbonyls.

Progress in the study of electrocatalytic reactions of organic species

New results in the electrocatalysis of organic reactions based in the use of spectroscopic methods are presented. The methods and systems discussed are: (a) thermal desorption spectroscopy applied to the characterization of the metal—CO bond on Pt and PtRu alloys; (b) in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) used to establish the composition of adsorbed ethanol at platinum; and (c) DEMS applied to discriminte between bulk and adsorbate reactions during the oxidation of formic acid catalysed by adsorbed Pb at platinum. The possibility of a reaction pathway highlighting the role of water is suggested.

Theoretical study of the monocarbonyls of first-row transition metal atoms

The results of density functional calculations on the most stable high-spin and low-spin states of MCO are given, where M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The ground states are found to be high spin for M=Sc, Ti, V, Cr, and Cu (2S+1=4, 5, 6, 7, and 2, respectively) and low spin for M=Mn, Fe, Co, and Ni (2S+1=4, 3, 2, and 1, respectively). From Sc to Cu, the M–CO binding energies with respect to ground state products are estimated to be 9, 16, 26, 13, −14, 14, 30, 54, and 19 kcal/mol. Where comparison with experiment is possible, the estimates are apparently too large by about 6 kcal/mol (FeCO), 13 kcal/mol (NiCO), and 12 kcal/mol (CuCO). The high-spin state MCO complexes with metal to the right of vanadium in the Periodic Table all have bent equilibrium geometries; all others are found to be linear. The calculated CO harmonic stretch frequencies generally overestimate the observed values, but follow a similar trend. The CO bond lengths, CO stretch frequencies, and metal–CO bond strengths all correlate well with the extent of π back donation. However, these correlations hold only within either the group of all high-spin states, or the group of all low-spin states. Thus, there are no simple trends in the calculated properties of ground state MCO complexes.

Theoretical study of the link between the infrared linewidth of the C–O stretching vibration and the chemisorbed CO–metal bond

The hopping integrals βam between the molecule and metal atomic orbitals are involved in both the description of the CO–metal chemisorption bond and the calculation of the infrared linewidth L of the C–O stretching vibration (related to an electron–hole pair process). Therefore, some information about the chemisorption bond may be extracted from the experimental values of L. The kinetic energy of the C and O nuclei may be transferred to the electrons using two channels: the 5σ orbital or the (1π, 2π) orbital pair. The βam integrals are assumed to be proportional to k0, the parameter ‘‘describing the chemisorption bond.’’ When k0 is fitted to the experimental values of L, we find k0=0.19 for CO/Pt and k0=0.23 for CO/Ni. In both of these cases, the CO–metal bond and the electron–hole pair process involve only the 5σ orbital.

Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface

AbstractThe interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg2+ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas‐phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the CO molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ωe, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.

Studies on CO bonding to Rh clusters using an intermediate neglect of differential overlap theory to model heterogeneous catalytic reactions

The interaction of CO with rhodium clusters of different sizes is studied by means of Intermediate Neglect of Differential Orbital (INDO) calculations. Interactions of both σ and π type are well-known to contribute to the metal CO bond formation. The relative importance of the repulsive Rh5σCO interaction with the metal and the back-bonding to the π* CO empty orbital in the stabilization of the bond is analyzed as a function of the cluster size and the coordination geometry. It is found that the shift of charge in the larger metal clusters brings about a reduction of the Rh5σ CO repulsion, and this redistribution is important to change the optimal CO bonding geometry from bridge to on top. © 1992 John Wiley & Sons, Inc.

Spin trapping of radicals formed by sonolysis of some organometallic compounds

It is well established that ultrasound has a profound influence on the reactivity of many organometallic compounds [l, 21. In some cases products of remarkable catalytic activity are generated [3]. However, little is known about the mechanisms underlying these reactions. Based on product analyses, Suslick et al. [3-S] have concluded that metal carbonyls undergo non-radical cleavage of metal-CO bonds upon sonolysis. It was shown by Riesz and co-workers [6] that spin trapping provides a suitable method for detecting free radicals formed in ultrasoundinduced chemical reactions. The usefulness of this method for the detection of free radicals in sonochemical reactions of organometallics was demonstrated for a variety of organotin compounds [7]. SO far, however, spin trapping of sonolytically generated metal-centred radicals has failed. Here we wish to report spin trapping studies on some organometallic carbonyl compounds where, at least in one case, spin adducts of metal-centred radicals were obtained upon irradiation with ultrasound.

Theoretical analysis of the vibrational shifts of CO chemisorbed on Pd(100)

Vibrational modes of CO bridge bonded to the Pd(100) surface are analyzed by means of ab initio cluster wavefunctions. The dependence of the C-O vibrational mode on the height of the CO ligand from the metal surface has been examined. It is shown that the C-O stretch frequency, ω, initially decreases as CO approaches the surface; this is due to the metal to CO 2π ∗ back donation. However, when the metal-CO bond length becomes sufficiently short, the frequency increases. The stretching of the CO molecule against the surface “wall” leads to an increase in the frequency which over-compensates the decrease due to the π back donation. Vibrational ω shifts induced by an uniform electric field, F, have been determined. The computed tuning rate, dω dF = 1.72 × 10 −6 ( V cm ) , is in excellent agreement with the experimental value, 1.55 × 10 −6 cm −1 ( V , cm ) . The Stark tuning rate is not altered significantly by the electronic rearrangement induced by the field; the shifts in ω arise primarily from the field-dipole electrostatic interaction while the chemical effects are small. Although the consequences of the electronic rearrangements which are caused by the applied field F, are small for the change in ω, these chemical effects lead to large changes in the intensity of the infrared, IR, transition. There is a large “ḩemical” change in the dynamic dipole moment, e ∗, when a uniform electric field is applied which leads to about 20% change in the IR intensity; this is a major origin for the observed intensity change. However, we give evidence that these chemical changes may be within the CO unit and may not involve changes of the charge transfer or dative bonding between the metal and CO.

Effect of oxide promoters on the surface characteristics of carbon-supported Co and Ru catalysts

The effect on the surface structure of various oxide promoters (MgO, CeO x , VO x ) incorporated in carbon-supported Ru or Co catalysts was studied. Characterization of the different preparations was carried out by H 2 and CO chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. Information about the metallic dispersion and oxidation states was obtained. Both metals and oxide promoters are well dispersed within the porous structure of carbon, although they are not uniformly distributed across the catalyst pellets. The effect of the different precursors on the location and dispersion of the active phases in the carbon porous structure was also examined. The interaction of CO with catalyst surfaces was monitored by temperature-programmed desorption, a noticeable effect of the oxide promoters on the metal-CO bond being observed.

Carbon-supported Fe 3(CO) 12, Ru 3(CO) 12 and Os 3(CO) 12: Decomposition rates, CO adsorption properties and CO hydrogenation behavior

The thermal decomposition of the three dodecacarbonyl clusters, Fe 3(CO) 12, Ru 3(CO) 12 and Os 3(CO) 12, on a clean carbon surface has been monitored by DRIFTS (Diffuse Reflectance Fourier Transform Infrared Spectroscopy) at different temperatures under a He or H 2 atmosphere. First-order rate constants and activation energies were determined. Different intermediate species were observed, and the absence of oxygen species on the surface allowed the formation of zero-valent metal particles during decomposition. The subsequent highly dispersed, carbon-supported metal catalysts have been characterized by the calorimetric measurement of the heats of adsorption of CO and by their kinetic behavior in the CO hydrogenation reaction. The CO heats of adsorption and the activation energies for both decarbonylation and methanation showed the same trend among the clusters as the initial metal-CO bond strengths.

CO chemisorption on free gas phase metal clusters

Pulsed fast flow reactor techniques have been used to study the reactivity of CO toward clusters of many different transition metals; V, Fe, Co, Ni, Cu, Nb, Mo, Ru, Pd, W, Ir, and Pt as well as Al, for clusters containing up to 14 atoms. Clusters are produced by pulsed laser vaporization of metal substrates, injected into the reactor, formed into a molecular beam, and detected by photoionization time of flight mass spectrometry. Our results show that CO is readily chemisorbed on most transition metal clusters containing five or more metal atoms, and that the reactivity for larger clusters varies by a factor of 2 or 3, depending on both cluster size and metal type. Depending on the metal, certain atoms, dimers, trimers, and tetramers exhibit little evidence of reactivity toward CO. This observation is explained in terms of a competition between unimolecular decomposition and collisional stabilization, and leads to a prediction of the ordering of the metal–CO bond strengths.

Synchrotron radiation photoemission studies of CO chemisorption on Pt/Ta(110) and Ni/Ta(110)

The effect of overlayer structure and electronic density near the Fermi level on CO chemisorption is studied for Pt or Ni deposited on Ta(110) using angle-resolved synchrotron radiation photoemission spectroscopy. Pt films form two-dimensional pseudomorphic islands at submonolayer coverage, but undergo a structural change to a close-packed Pt(111)-like structure at ∼0.7 monolayer coverage. Absence of CO induced photoemission features at monolayer coverage suggests that the CO–Pt bond has been weakened and this prevents the retention of CO on a close-packed Pt monolayer at room temperature. Close-packed Pt films three or more layers thick readily chemisorb CO. At liquid-nitrogen temperature, CO is retained on Pt/Ta(110), and photoemission features, such as the CO 4σ shake-up satellite seen for weakly bonded CO on noble metals, appear for the CO phase chemisorbed on a Pt monolayer. This photoemission feature is absent for CO chemisorbed on thicker Pt films. Photoemission examination of the CO–metal bonding geometry suggests that the CO molecules lie close to the surface normal for both Pt monolayers and the thicker Pt films. From the photoemission data, we conclude that the CO–metal bond strength is reduced for CO–Pt/Ta(110). Comparable studies of CO on Ni/Ta(110) were not possible because the three-dimensional clustering of Ni produced a more complicated surface which had a number of inequivalent CO bonding sites.

Comment on ‘inverse photoemission from CO coadsorbed with K on Pt(111)’ by V. Dose, J. Rogozik, A.M. Bradshaw and K.C. Prince

A qualitative model of K-CO interaction on Pt(111) is proposed which permits a straight-forward explanation of the downward shift of the 2 π-derived orbital of CO measured by inverse photoemission. The essential point of the model is a weakened metal-CO bond due to coadsorbed K but an increased energy of adsorption of CO because of the extra electrostatic interaction between K( δ+) and CO( δ-). Direct evidence for this weakened metal-CO bond comes from a lower metal-CO stretch frequency at 350 cm -1 compared to 470 cm -1 for CO on Pt(111).

Comment on ‘inverse photoemission from CO coadsorbed with K on Pt(111)’ by V. Dose, J. Rogozik, A.M. Bradshaw and K.C. Prince

A qualitative model of K-CO interaction on Pt(111) is proposed which permits a straight-forward explanation of the downward shift of the 2 π -derived orbital of CO measured by inverse photoemission. The essential point of the model is a weakened metal-CO bond due to coadsorbed K but an increased energy of adsorption of CO because of the extra electrostatic interaction between K( δ +) and CO( δ −). Direct evidence for this weakened metal-CO bond comes from a lower metal-CO stretch frequency at 350 cm −1 compared to 470 cm −1 for CO on Pt(111).

Structure and bonding characteristics of bis(2,3-dicarnomethoxynorbornadiene)dicarbonylmolybdenum and related complexes

A crystal of bis(2,3-dicarbomethoxynorbornadiene)dicarbonylmolybdenum is in space group P2 1/ c, with a 11.0406(25), b 14.7281(29), c 15.3310(27), β 110.38(2)° and Z = 4. Very large out-of-plane torsional angles (27–41°) for the vinylic carbomethoxy groups are observed. The bonding properties therefore are analyzed by comparison of spectroscopic data with those of related complexes, i.e. L 2Mo(CO) 2 where L = norbornadiene, 2- p-tosylnorbornadiene, and 2-carbomethoxynorbornadiene. Instead of σ-bond, metal to CC π-coordinations are believed to exist in these complexes. The presence of electron-withdrawing groups on the NBD ligands has increased their chelation strength, but at the same time has weakened the metalCO bonds.

σ−π Contributions in metal-co bond: A theoretical study of RhCO and PdCO

Abstract The electronic structure of linear RhC0 and PdCO molecules has been studied by means of effective core potential calculations including configuration interaction. The study of the 4 Δ and 2 Σ + states of RhCO and the 3 Σ + and 1 Σ + states of PdCO has shown that their low-spin states are bound and their high-spin states are repulsive. The binding energies of RhCO and PdCO are 4.0 and 7.2 kcal/mol, for the equilibrium bond lengths of 3.87 and 4.03 au, respectively. The interaction between the diffuse 5s metal orbital and the carbon lone pair produces a repulsion in the α-space, as depicted by the negative region along the metal-carbon axis in the electron density difference contour maps. The bonding in low-spin states of RhCO and PdCO is due to the metal-CO π “backdonation”. The present investigation on RhCO and PdCO, preliminary results on ScCO, as well as published results on FeCO, NiCO, CuCO and PtCO allows us to make a general conjecture that a low occupancy of the outer s atomic orbital of the transition metal atom in the complex favours the bonding interaction with the CO moiety. The analogy with properties of LiCO and NaCO systems is pointed out.

Origin of lone pair binding energy shifts in photoemission from adsorbed molecules: CSOV analysis for the Cu 5CO cluster

The chemisorptive interaction of the CO/Cu(100) system is studied with a Cu 5(1,4)CO cluster model using the constrained space orbital variation (CSOV) analysis. This analysis has been used previously to examine the origin of the CO-metal bond and to show that the energetic importance of the metal to CO π donation is larger than that of the CO to metal σ donation. In the present work, we use it to determine the origin of the differential CO 5σ ionization potential shift observed in the photoemission spectra of CO/Cu(100). This shift is shown to be largely due to electrostatic, environmental potential effects. It does not indicate the nature of the CO-metal bond but does indicate the CO adsorption geometry.

Surface reactivity of platinum-nickel single crystal alloys: Carbon monoxide adsorption

Carbon monoxide adsorption has been studied on (111) faces of single-crystal alloys — Pt 10Ni 90, Pt 50Ni 50 and Pt 78Ni 22 — by vibrational EELS and TDS measurements. On the nickel rich sample, on which the outer layer is notably enriched in platinum with respect to the nominal bulk concentration, multi-bonded nickel sites and on-top platinum sites for CO chemisorption are evidenced. The multi-bonded sites are preferentially filled. In the platinum rich samples, which exhibit a quasi complete platinum surface layer, linearly bonded species are observed first; multi-bonded CO adspecies appear only at higher coverage. Whatever the considered alloy sample the metal-CO bond strengths are weaker than on pure metals, as demonstrated by the lower values of both the metal-CO stretching mode energy and the temperature of the thermodesorption mass-28 peak. That weakening is more pronounced on the Pt 50Ni 50 sample.

Bonding of CO to metal surfaces: A new interpretation

The interaction of CO with Na, Mg, and Al surfaces has been studied using the molecular-orbital cluster model. These metals are chosen since we wish to study the CO interaction with the metal valence $\mathrm{sp}$ electrons in the absence of $d$ electrons. Our conclusion is that there is substantial $\ensuremath{\sigma}$ repulsion and that the bonding arises primarily from the metal $\ensuremath{\pi}$ to CO $2{\ensuremath{\pi}}^{*}$ interaction and donation. This is based on the analysis of self-consistent-field calculations for linear $X\mathrm{CO}$ and two-layer ${X}_{4}(1,3)\mathrm{CO}$ clusters, where $X$ represents the metal atom. They are chosen to simulate normal adsorption at the on-top site with the C atom adjacent to the surface, a geometry known to be appropriate for certain transition-metal atoms. The potential curve for the interaction as a function of metal-C distance has an attractive or repulsive character directly related to the amount of adsorption-site $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ character in the bare (metal atoms only) cluster. A corresponding orbital analysis unambiguously shows that the free CO orbitals are essentially unchanged upon interaction with the metal; there is very little CO $5\ensuremath{\sigma}$ to metal donation. It also shows that the binding of CO to the metal is associated with a considerable change and donation of the metal $\ensuremath{\pi}$ electrons to CO $2{\ensuremath{\pi}}^{*}$, The $\ensuremath{\sigma}$ orbitals of primarily metal character hybridize and polarize away from CO in order to reduce the metal CO $5\ensuremath{\sigma}$ repulsion. This interpretation, which is different from the usual picture of the metal-CO bond, is supported by an extensive set of results. We also consider the effects of the interaction for the valence photoelectron peaks of chemisorbed CO and obtain a new interpretation for the significance of the shift of the $5\ensuremath{\sigma}$ ionization toward the $1\ensuremath{\pi}$. Predicted features of the CO ionization are compared to experimental photoemission spectra for CO/A1(111). Their agreement provides support for an adsorption geometry with CO normal to the surface.