Bound CO Molecules Research Articles

Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles.

Catalytic routes for upgrading CO2 to CO and hydrocarbons have been studied for decades, and yet the mechanistic details and structure-function relationships that control catalytic performance have remained unresolved. This study elucidates the elementary steps that mediate these reactions and examines them within the context of the established mechanism for CO hydrogenation to resolve the persistent discrepancies and to demonstrate inextricable links between CO2 and CO hydrogenation on dispersed Ru nanoparticles (6-12 nm mean diameter, 573 K). The formation of CH4 from both CO2-H2 and CO-H2 reactants requires the cleavage of strong C≡O bonds in chemisorbed CO, formed as an intermediate in both reactions, via hydrogen-assisted activation pathways. The C═O bonds in CO2 are cleaved via direct interactions with exposed Ru atoms in elementary steps that are shown to be facile by fast isotopic scrambling of C16O2-C18O2-H2 mixtures. Such CO2 activation steps form bound CO molecules and O atoms; the latter are removed via H-addition steps to form H2O. The kinetic hurdles in forming CH4 from CO2 do not reflect the inertness of C═O bonds in CO2 but instead reflect the intermediate formation of CO molecules, which contain stronger C≡O bonds than CO2 and are present at near-saturation coverages during CO2 and CO hydrogenation catalysis. The conclusions presented herein are informed by a combination of spectroscopic, isotopic, and kinetic measurements coupled with the use of analysis methods that account for strong rate inhibition by chemisorbed CO. Such methods enable the assessment of intrinsic reaction rates and are essential to accurately determine the effects of nanoparticle structure and composition on reactivity and selectivity for CO2-H2 reactions.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC5H4)CH2]3P(NC2H4)3N} with Fe(CO)5 resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)5 complex di-μ-carbonyl-tricarbon-yl[2,8,9-tris-(pyridin-2-yl-meth-yl)-2,5,8,9-tetra-aza-1-phosphabi-cyclo-[3.3.3]undeca-ne]ironnickel, [FeNi(C24H30N7P)(CO)5]. Characterization of the complex by 1H and 31P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)5 reveals three terminally bound CO mol-ecules on Fe0, two bridging CO mol-ecules between Ni0 and Fe0, and TPAP coordinated to Ni0. The Ni-Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)5 is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol-ecules are linked by C-H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π-π [inter-centroid distance = 3.2739 (5) Å] and C-H⋯π inter-actions present. The layers are linked by further C-H⋯π inter-actions, forming a supra-molecular framework.

How does the Environment Influence a Given Cation? A Systematic Investigation of [Co(CO)5 ]+ in Gas Phase, Solution, and Solid State.

IR spectroscopic studies of the gaseous metal carbonyl cations [Co(CO)5 ]+ ⋅mCO (m=1-4) indicated that the weakly bound CO molecules in a second coordination sphere perturb the structure of [Co(CO)5 ]+ causing the CO stretching frequencies ν(CO) to become noticeably redshifted. In this work, we aimed to establish the relationship between such gas phase IR spectra and those recorded in condensed phases, either as a solid salt or as a solution in the weakly basic solvent o-difluorobenzene. For this purpose, a series of [Co(CO)5 ]+ [WCA]- salts (WCA=weakly coordinating anion), with the counterions varying between more coordinating (WCA=F-Al(ORF )3 , (RF O)3 Al-F-Al(F)(ORF )2 ; RF =C(CF3 )3 ) and almost non-coordinating (WCA=Al(ORF )4 , F{Al(ORF )3 }2 ), were synthesized and characterized by vibrational spectroscopy as well as X-ray structure analysis. The experimental spectra differ considerably from that of the undisturbed gaseous [Co(CO)5 ]+ ion, as the structural deformation of [Co(CO)5 ]+ requires very little energy. Together with previously reported data, the perturbed condensed phase [Co(CO)5 ]+ ions were analyzed and compared with the gaseous [Co(CO)5 ]+ ⋅mCO ions. DFT calculations were performed on simply adapted [Co(CO)5 ]+ structures to allow the assignment of all the ν(CO) modes and a qualitative interpretation of structural deformations by external influences as a function of the environment (ligands, solvation, crystal packing). The analysis showed that especially the degenerate E' mode νe and the averaged asymmetric equatorial CO stretch e , which originates from a split of the E' mode, are a function of the interaction with the environment. Whereas for the more coordinating counterions e values of 2112-2120 cm-1 were obtained, for the less coordinating counterions e values of up to 2133 cm-1 were found, which is very close to that of gaseous [Co(CO)5 ]+ ⋅4CO, with a e value of 2135 cm-1 . This indicates the possibility of approximating the gas phase conditions in the condensed phases with the [Co(CO)5 ]+ ion probably being the prototypical probe molecule for investigating the strengths of interactions in different environments.

Bioprotection Can Be Tuned with a Proper Protein/Saccharide Ratio: The Case of Solid Amorphous Matrices.

Saccharides, and in particular trehalose, are well known for their high efficiency in protecting biostructures against adverse environmental conditions. The protein dynamics is known to be highly inhibited in a low-water trehalose host medium, the inhibition being markedly dependent on the amount of residual water. Besides hydration, the protein/sugar ratio is expected to affect the properties of saccharide amorphous matrices. In this work, we report an infrared spectroscopy study in dry amorphous matrices of various sugars (the disaccharides trehalose, maltose, sucrose, and lactose, and the trisaccharide raffinose) containing myoglobin, at different protein/sugar ratios. We analyze the stretching band of the bound CO molecule and the water association band. Such bands have already been successfully exploited for the simultaneous study of thermal evolution of a matrix and embedded protein. The results show a high dependence of protein and matrix signals on the protein/sugar ratio, the system behavior evolving from situations where (i) the protein slaves the matrix to (ii) protein ↔ matrix coupling/uncoupling, then to (iii) the matrix slaving the protein, with increasing sugar concentration. This supports a mutual protein ↔ matrix structural and dynamic influence in low hydrated systems, indicating that the protein/solvent master and slave paradigm does not strictly hold, but the mutual relationship depends on the relative concentrations. Furthermore, for each sugar, an optimal protein/sugar concentration ratio can be identified, which maximizes the protein preservation; under such a condition, the water content is minimal.

CO Depletion: A Microscopic Perspective

Abstract In regions where stars form, variations in density and temperature can cause gas to freeze out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences, indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of AV is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also more mobile, and this could increase the reactivity within interstellar ices.

Solvation Effects on OH Adsorbates on Stepped Pt Surfaces

Density functional theory calculations were applied to OH formations on stepped Pt electrodes of Pt[n(111) × (111)] (n = 3, 4, and 6) for examining solvation effects on the OH adsorbates. Results indicated that OH adsorbates at terrace sites are slightly destabilized by water molecules adsorbed at step sites forming 1-dimensional water chains whereas OH adsorbates at step sites are significantly destabilized by water molecules adsorbed at terrace sites forming 2-dimensional honeycomb structures. On stepped Pt surfaces with narrow terrace widths, water molecules cannot exist at terrace sites, and therefore, the solvation effects on OH adsorbates at step sites disappear. Hence, OH adsorbates are formed at step sites at a low potential region, ca. 0.3 V (standard hydrogen electrode (SHE)). When high-coverage CO adsorbates are present on the stepped Pt surfaces, water molecules cannot exist at the terrace sites either because strongly bound CO molecules exclude the water molecules. In such conditions, OH form...

High-field versus high-pressure: Weakly adsorbed CO species on Pt(1 1 1)

Adsorption of carbon monoxide on a nanosized apex facet of a [1 1 1]-oriented Pt field emitter tip was studied under high electrostatic field conditions (15–20 V/nm) by field ion appearance energy spectroscopy (FIAES). Weakly bound highly mobile CO species have been detected and their possible role in surface reactions such as CO oxidation on Pt was elucidated. The FIAES results are compared with those of vibrational sum frequency generation (SFG) spectroscopy of CO adsorption on a macroscopic Pt(1 1 1) single crystal, in a gas pressure range from UHV to atmospheric. SFG and FIAES are complementary in that SFG mainly provides information on strongly bound CO molecules, whereas FIAES nearly exclusively detects weakly bound species. Both high-field and high-pressure studies indicate the presence of a compressed CO adlayer including weakly bound species. The high-field mimics the high gas pressure, leading to similar high adsorbate coverage, which is required for the formation of weakly bound highly mobile CO species, which may be the active entities in CO oxidation.

Coupling between the Thermal Evolution of the Heme Pocket and the External Matrix Structure in Trehalose Coated Carboxymyoglobin

Proteins can assume a very large number of conformations (conformational substates), all concurring to its function. We present experimental evidence for the existence, in trehalose coated carboxymyoglobin, of a structured environment of the protein, tightly coupled to the heme pocket structure, as experienced by the bound CO molecule. This was evidenced by the strict correlation observed between the thermal evolution (300−20 K) of the CO stretching and of the water association bands in samples of carboxymyoglobin embedded in trehalose matrixes of different hydration. This observation put forward the coupling between the degrees of freedom of the matrix and those of the protein. In the driest sample, in which only tightly bound, nonremovable water molecules were present, temperature induced structural variations of both the heme pocket and the external matrix were small, even at room temperature. At variance, such variations were larger in two water richer samples in which their onset was already at ∼50 K...

The carbon monoxide derivative of human hemoglobin carrying the double mutation LeuB10→Tyr and HisE7→Gln on α and β chains probed by infrared spectroscopy

The fine structural properties of the distal heme pocket have been probed by infrared spectroscopy of ferrous carbon monoxy human hemoglobin mutants carrying the mutations LeuB10→Tyr and HisE7→Gln on the α, β, and both chains, respectively. The stretching frequency of iron-bound carbon monoxide occurs as a single broad band around 1943 cm −1 in both the α and the β mutated chains. Such a frequency value indicates that no direct hydrogen bonding exists between the bound CO molecule and the TyrB10 phenolic oxygen, at variance with other naturally occurring TyrB10, GlnE7 nonvertebrate hemoglobins. The rates of carbon monoxide release have been determined for the first time by a Fourier transform infrared spectroscopy stopped-flow technique that allowed us to single out the heterogeneity in the kinetics of CO release in the α and β chains for the mutated proteins and for native HbA. The rates of CO release are 15- to 20-fold faster for the mutated α or β chains with respect to the native ones consistent with the lack of distal stabilization on the iron-bound CO molecule. The present results demonstrate that residues in key topological positions (namely E7 and B10) for the distal steric control of the iron-bound ligand are not interchangeable among hemoglobins from different species.

Properties of Human Hemoglobins with Increased Polarity in the α- or β-Heme Pocket: CARBONMONOXY DERIVATIVES

The spectroscopic, conformational, and functional properties of mutant carbonmonoxy hemoglobins in which either the beta-globin Val67(E11) or the alpha-globin Val62(E11) is replaced by threonine have been investigated. The thermal evolution of the Soret absorption band and the stretching frequency of the bound CO were used to probe the stereodynamic properties of the heme pocket. The functional properties were investigated by kinetic measurements. The spectroscopic and functional data were related to the conformational properties through molecular analysis. The effects of this nonpolar-to-polar isosteric mutation are: (i) increase of heme pocket anharmonic motions, (ii) stabilization of the A0 conformer in the IR spectrum, (iii) increased CO dissociation rates. The spectroscopic data indicate that for the carbonmonoxy derivatives, the Val --> Thr mutation has a larger conformational effect on the beta-subunits than on the alpha-subunits. This is at variance with the deoxy derivatives where the conformational modification was larger in the heme pocket of the alpha-subunit (Cupane, A., Leone, M., Militello, V., Friedman, R. K., Koley, A. P., Vasquez, G. P., Brinigar, W. S., Karavitis, M., and Fronticelli, C. (1997) J. Biol. Chem. 272, 26271-26278). These effects are attributed to a different electrostatic interaction between Ogamma of Thr(E11) and the bound CO molecule. Molecular analysis indicates a more favorable interaction of the bound CO with Thr Ogamma in the beta-subunit heme pocket.

The apolar distal histidine mutant (His69-->Val) of the homodimeric Scapharca hemoglobin is in an R-like conformation.

The effect of the apolar mutation of the distal histidine (His69-->Val) has been studied in the cooperative homodimeric hemoglobin from the mollusc Scapharca inaequivalvis. Absorption, circular dichroism, and resonance Raman spectroscopy point to a more symmetric heme structure of the deoxy derivative, which is indicative of an R-like conformation of the deoxy heme. Resonance Raman spectroscopy also brings out alterations in the geometry and interactions of the bound CO molecule. The iron-carbon stretching frequency is decreased by about 30 cm-1 with respect to the native protein, while the diatomic ligand stretching frequency is increased by about the same degree. Consistent with the structural changes, the ligand binding properties are significantly altered. In the mutant the overall rate and the affinity for CO binding are increased about 100-fold with respect to the native protein, and cooperativity is abolished. In addition, the amplitude and the rate of the geminate rebinding process increase significantly. This finding may be correlated to the longer average residence time of the photolyzed CO molecule within the heme pocket of the H69V mutant, as indicated by molecular dynamics simulations.

Weakly chemisorbed CO layer on Rh(100) as detected by reverse flash and field ion appearance energy measurements

Using the reverse flash (RF) method, the sticking coefficient and binding energy of CO molecules on Rh(100) are obtained for CO surface densities up to 1.2 × 10 15 cm −2. The presence of weakly bound CO molecules with a binding energy of 0.34 eV is detected at n > 1.0 × 10 15 cm −2. Mass-to-charge-resolved retarding potential analysis of CO + emitted from Rh(100) is performed and absolute values of CO + field ion appearance energy obtained. By applying a thermionic cycle field-dependent values of the binding energy of CO Rh(100) at high coverages are obtained. A comparison of field-free (RF) results and field ion appearance energy spectroscopy (FIAES) data reveals the weakly bound (field-stabilized) CO layer as the origin of CO + field ions emitted from Rh(100) for field ion imaging.

Ambiguities in adsorbate site assignment from vibrational frequencies. A TLEED structural study of (2 X 1)COPd(110)

The structure of the (2 × 1)COPd(110) surface phase has been determined by LEED intensity analysis. The CO molecule is found to be adsorbed in an atop site, tilted by 11° ± 4° with respect to the surface normal, with a CO bond length of 1.16 ± 0.04 Å. Interestingly, the CO vibrational frequency for this system (2003 cm −1) is virtually identical to the frequency observed for the (2 × 1)CONi(110) surface phase (1998 cm −1) which a previous LEED study has shown involves bridge bound CO molecules. The result indicates that care must be taken in assigning site symmetries on the basis of CO stretching frequencies alone.

CO oxidation over Au/ZrO 2 catalysts: Activity, deactivation behavior, and reaction mechanism

Gold catalysts supported on amorphous zirconia have been prepared by coprecipitation. These catalysts exhibit a high initial CO oxidation activity, which is due to a synergy between the zirconia and the supported gold particles. The activity decreases significantly over a time scale of ≈20 h. The deactivation behavior is only weakly influenced by pretreatments in O 2, CO, or CO 2. However, the long-time stability is found to depend strongly on the O 2/CO feed ratio. With stoichiometric reaction mixtures (O 2/CO = 0.5), the activity stays nearly constant over the time of observation. In the presence of excess oxygen (O 2/CO = 1 or 40, respectively), rapid deactivation is observed. From a careful monitoring of the carbon balance, and from thermal desorption/mass spectrometry experiments, a progressive covering of the surface by oxygenated carbon species is deduced. Diffuse reflectance FTIR experiments indicate the presence of carbonates, formate, and several types of differently bound CO molecules on the surface. Temperature-dependent experiments reveal that these species are involved in a series of interconnected equilibria, which are coupled to the relevant adsorption/de sorption processes. Oxidized and reduced states of the catalyst can be distinguished. The deactivated state in excess oxygen is characterized by a high concentration of surface formate. On the deactivated catalyst, oxydation of formate and of CO appears to be slow, possibly as a consequence of site blocking.

Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 112. Synthesis of rhenium–rhodium complexes; crystal structure of [ReRh{µ-σ,η5-C2B9H7(CH2C6H4Me-4)Me2}(CO)4(η-C5H4Me)

Treatment of [NEt4][Rh(CO)2(η5-C2B9H9Me2)] in CH2Cl2 with [Re(CC6H4Me-4)(CO)2(η-C5H4Me-4)]-[BCl4] affords the dimetal complex [ReRh{µ-σ,η5-C2B9H7(CH2C6H4Me-4)Me2}(CO)4(η-C5H4Me)]. The structure was established by X-ray diffraction. The Re–Rh bond [2.888(1)Å] is spanned by the C2B9H7(CH2C6H4Me-4)Me2 fragment. The open pentagonal face of the C2B9 cage is η5 co-ordinated to the rhodium, and there is an exopolyhedral B–Re [2.17(2)Å]σ bond involving a boron atom in the α site with respect to the carbon atoms in the [graphic omitted] ring; the boron atom in the β site [graphic omitted] carries the CH2C6H4Me-4 substituent. The rhenium and rhodium atoms are each ligated by two terminally bound CO molecules, and the rhenium is also co-ordinated by the η-C5H4Me group. The related complexes [ReRh{µ-σ,η5-C2B9H7(CH2C6H4Me-4)Me2}(CO)3L(η-C5H5)](L = CO or PMe2Ph) have also been prepared. In contrast with the aforementioned results the complex [Mn(CC6H4Me-4)(CO)2(η-C5H4Me)][BCl4] reacts with [NEt4][Rh(CO)(PPh3)(η5-C2B9H9Me2)] and [NEt4][Rh(cod)(η5-C2B9H9Me2)](cod = cycloocta-1,5-diene) to give the mononuclear rhodium species [Rh{σ,η5-H(C6H4Me-4)C2B9H9Me2}(CO)(PPh3)] and [Rh{σ,η5-CH(C6H4Me-4)C2B9H8Me2}(cod)], respectively. The NMR data (1H, 13C-{1H}, 11B-{1H}] for the new complexes are reported and discussed.

Two types of conformers with distinct Fe-C-O configuration in the ferrous CO complex of horseradish peroxidase. Resonance Raman and infarared spectroscopic studies with native and deuteroheme-substituted enzymes.

The presence of at least two types of conformers in the ferrous CO complex of horseradish peroxidase has been demonstrated with the use of native and deuteroheme-substituted enzymes. Type I conformers, predominant in acidic pH, exhibited both an Fe-CO stretching and an Fe-C-O bending Raman line together with an infrared C-O stretch band below 1920 em-1. On the other hand, type II conformers, dominant species in alkaline pH, showed only an Fe-CO stretching Raman line with the C-O stretch above 1930 cm-1. They were interconvertible either by the changes in pH or by the binding of benzhydroxamate, a substrate for the enzyme. The pKa value for the pH-dependent interconversion of CO complex of deuteroheme-substituted enzyme was 8.3. These findings were interpreted to mean that the bound CO molecule in type I conformers was more tilted over the heme-plane than that in type II conformers. A steric hindrance by the bound substrate or the protonated form of a distal amino acid residue, presumably of histidine, is considered to be the cause for the isomerization. By summarizing present and previous data on the vibrational frequencies of heme-carbonyl complexes, we found that there are inverse-linear relationships between the square of Fe-CO and that of C-O stretching frequencies, while squares of Fe-CO stretching and Fe-C-O bending frequencies were linearly correlated with each other. Also found is that the dissociation rate constant of CO molecule from heme-carbonyl complexes is a linear function of the Fe-CO stretching frequency. The significance of these results is discussed.

Chemistry of di- and tri-metal complexes with bridging carbene or carbyne ligands. Part 42. Synthesis of the compounds [N(PPh3)2][Fe2W(µ-CO)(µ-NO)(µ3-CR)(CO)6(η-C5H5)](R = Me or C6H4Me-4) and the X-ray crystal structure of the tolylmethylidyne complex

The compounds [Fe2W(µ3-CR)(µ-CO)(CO)8(η-C5H5)](R = Me or C6H4Me-4) and [N(PPh3)2][NO2] react in tetrahydrofuran at room temperature to afford the salts [N(PPh3)2][Fe2W(µ-CO)(µ-NO)(µ3-CR)(CO)6(η-C5H5)]. An X-ray diffraction study was carried out on the species with R= C6H4Me-4, which crystallises with a molecule of tetrahydrofuran. The anion contains an essentially isosceles triangle of metal atoms [Fe–W 2.702(2) and 2.71 6(2), Fe–Fe 2.607(3)Å], capped by the CC6H4Me-4 group [µ3-C–Fe 1.969(13) and 2.008(10), µ3-C–W 2.055(11)Å]. Each iron atom carries three terminally bound CO molecules. The tungsten atom is bonded by the cyclopentadienyl ligand, and in addition is attached to a CO and an NO group. These carbonyl and nitrosyl groups asymmetrically bridge the two Fe–W edges of the metal triangle [W–C–0160.0(9), W–N–O 157.0(10)°]. Protonation of the salt [N (PPh3)2][Fe2W(µ-CO)(µ-NO)(µ3-CC6H4Me-4)(CO)6(η-C5H5)] with HBF4·Et2O affords thedi-irontungsten complex [Fe2W(µ-CC6H4Me-4)(µ3-NOH)(CO)7(η-C5H5)]. A structurally related species [Fe2W(µ-CC6H4Me-4)(µ3-NC6H4Me-4)(CO)7(η-C5H5)] is obtained by heating a mixture of [Fe2W(µ3-CC6H4Me-4)(µ-CO)(CO)8(µ-C5H5)] with McC6H4N3–4 in tetrahydrofuran. Spectroscopic data (i.r. and n.m.r.) for the new complexes are reported and discussed.

Effects of dispersion on the activity and selectivity of alumina-supported ruthenium catalysts for carbon monoxide hydrogenation

A study was performed to determine the extent to which metal dispersion affects the activity and selectivity of Ru Al 2O 3 catalysts used for CO hydrogenation. For dispersions below 0.7, the specific activity for synthesis of methane and C 2+ products decreases with increasing dispersion, but neither the probability for chain growth nor the olefin-to-paraffin ratio is affected. The decrease in activity over this range is ascribed to a decrease in the fraction of sites present on planar surfaces. For dispersions above 0.7, the specific activity for synthesis of all products decreases dramatically, and is accompanied by a slight decrease in the probability of chain growth and a rapid decrease in the olefin-to-paraffin ratio. These changes are attributed to changes in the electronic properties of the Ru microcrystallites with size and the occurrence of metal/support interactions. In situ infrared spectra reveal that only those sites that adsorb one linearly bound CO molecule per Ru atom are active for CO hydrogenation. Adsorption of two CO molecules per Ru site is also observed but these site are catalytically inactive.

An angle resolved photoemission determination of the coordination of CO on the ZnO(0001) surface

Angle resolved ultraviolet photoemission spectroscopy employing polarized He ii resonance radiation has been used to determine the bonding geometry of CO to ZnO(0001) at 80 K. Bound CO molecules are oriented perpendicular to the zinc-terminated surface forming a linear Zn–C–O complex. This result, along with earlier work on the ZnO(101̄0) surface, is explained by a concise and coherent model for CO chemisorption on ZnO surfaces. The role of defect sites for CO chemisorption is considered.