Coadsorption Of CO Research Articles

Reorganization of adsorbed films by coadsorbing species

The coadsorption of CO and butane on a Pt(533) stepped surface has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The adsorption of butane on Pt(533) with CO preadsorbed on step-atop sites reveals that butane can force CO to tilt with a minimum angle of 42° away from the surface normal and displace CO from step-atop to step-bridge sites. The energy required for this tilting should be less than 20.5 kJ/mol. The coverage at which the compressed butane phase occurred was found to be the same at which this phase occurs on bare Pt(533). Together with the observed tilting and displacement of CO, this suggests that at low coverages butane adsorbs on the terraces, rotated 60° away from the step edge. The second monolayer phase then consists of tilted butane molecules having two hydrogen atoms in direct contact with the surface, situated near the step edge. The presence of butane also results in a downward shift of the CO stretch frequency, caused by electron donation in the 2π* antibonding CO orbital. When butane is preadsorbed at a submonolayer coverage exposure to CO leads to displacement of butane into a compressed phase and even into a multilayer phase. This effect becomes smaller as the initial butane coverage is increased to the multilayer regime.

CO interaction with co-adsorbed C 2H 4 on Cu(111) as revealed by friction with the conduction electrons

Simultaneous measurements of the DC resistance and IR reflectivity changes of thin epitaxially grown copper (111) films have been performed during co-adsorption of CO with C 2H 4. The linear dependence of these two quantities, which has previously been observed for CO on Cu(111) films, also holds for this coadsorbed system. Importantly, we have shown that the linearity factor is independent of the adsorbate type, as predicted by the scattering model of the conduction electrons. Co-adsorption of CO with C 2H 4 results in markedly more pronounced changes in IR reflectance and in resistance; these changes are higher than the sum of the individual changes of each species. The results are interpreted as a consequence of covalent through-metal bonding of the adsorbates via the lowest unoccupied molecular orbitals.

A density functional theory study of CO and atomic oxygen chemisorption on Pt(111)

Ab initio total energy calculations within a density functional theory framework have been performed for CO and atomic oxygen chemisorbed on the Pt(111) surface. Optimised geometries and chemisorption energies for CO and O on four high-symmetry sites, namely the top, bridge, fcc hollow and hcp hollow sites, are presented, the coverage in all cases being 0.25 ML. The differences in CO adsorption energies between these sites are found to be small, suggesting that the potential energy surface for CO diffusion across Pt(111) is relatively flat. The 5σ and 2π molecular orbitals of CO are found to contribute to bonding with the metal. Some mixing of the 4σ and 1π molecular orbitals with metal states is also observed. For atomic oxygen, the most stable adsorption site is found to be the fcc hollow site, followed in decreasing order of stability by the hcp hollow and bridge sites, with the top site being the least stable. The differences in chemisorption energies between sites for oxygen are larger than in the case of CO, suggesting a higher barrier to diffusion for atomic oxygen. The co-adsorption of CO and O has also been investigated. Calculated chemisorption energies for CO on an O/fcc-precovered surface show that of the available chemisorption sites, the top site at the oxygen atom's next-nearest neighbour surface metal atom is the most stable, with the other four sites calculated being at least 0.29 eV less stable. The trend of CO site stability in the coadsorption system is explained in terms of a ‘bonding competition’ model.

Temperature effects on the coadsorption of K–CO on Ni(100)

High-resolution electron energy-loss spectra of the coadsorption of potassium and CO on the Ni(100) surface show the role of temperature in driving chemical species along the surface towards the formation of ordered islands made of alkali atoms and CO species. At low potassium precoverages, the formation of K–CO islands occurs by heating the sample to 420 K. On the contrary, for a potassium coverage near 0.38 ML, K–CO patches of ordered phases grow spontaneously even at 220 K. The interpretation of the present data relies essentially on two literature works on the K–CO/Ni(100) system, one of them presenting calorimetric measurements and the other reoprting the theoretical temperature behaviour of the chemical species on the nickel surface.

Structural effects during CO adsorption on Pt–bimetallic surfaces: I. The Pt(100) electrode

Synchrotron surface X-ray scattering (SXS) and rotating ring disk electrode (RRDE) measurements have been performed during the coadsorption of metal adatoms and carbon monoxide on the Pt(100) surface in an electrochemical environment. Whereas RRDE experiments give a macroscopic view of the coadsorption process, via the adsorption isotherms of deposited metal adatoms, the SXS measurements give insight into the structural changes that occur on the atomic level. The results show that both underpotentially deposited (UPD) Cu and Pb are displaced by CO from the Pt(100) surface, the driving force being the resulting negative shift in the surface free energy. When bromide is present in a c(2×2) structure on top of the Cu monolayer there is an activation barrier to the displacement effect. Coadsorption of CO then enhances the c(2×2) structure by displacing disordered Cu or Br that is present on the surface. Cycling the electrode potential, during which the Cu monolayer is desorbed, leads to complete blocking of the UPD process by CO. In contrast to these results, the reversible formation of a Bi c(2×2) UPD adlayer is unaffected by the presence of solution CO.

Carbonate formation by reacting CO 2 with an O 2 layer on Ag(110) studied by high resolution electron energy loss spectroscopy

The coadsorption and reaction of CO 2 with a layer of molecular oxygen preadsorbed on a Ag(110) surface have been studied with high resolution electron energy loss spectroscopy (HREELS). Clear evidence has been found for a direct reaction of CO 2 and O 2 to CO 3 at 100 K, resulting in a product coverage higher than attainable with atomic oxygen in the (2×1)O reconstruction. All internal modes of CO 3 have been detected, including the in-plane bend and the asymmetric stretch modes, which are degenerate at intermediate but split at saturation coverages. An intermediate species has been observed to be stable only in coadsorption with molecular oxygen, and has been tentatively assigned to a CO − 4 anion. The quantitative analysis of the isotopic shift observed with 13C 16O 2, 12C 18O 2 and 18O 2 is consistent with a planar adsorption geometry of a structurally unmodified CO 3 anion bound via the central carbon atom.

FTIR Study of the Low-Temperature Water–Gas Shift Reaction on Au/Fe2O3 and Au/TiO2 Catalysts

An FTIR and quadrupole mass spectroscopic study of the water–gas shift (WGS), the reverse WGS reactions, and the adsorption of the individual molecules involved has been carried out on Au/Fe2O3 and Au/TiO2 catalysts. The chemisorptions and the reactions on the two catalysts have been compared with the aim of gaining a better understanding of the role played by the two phases present in these catalysts and of the synergistic interplay between them in gold catalysts tested for a low-temperature water–gas shift reaction. Evidences are reported that H2 is dissociated already at room temperature on both the catalysts on gold sites, giving rise to hydrogen atoms that can react with adsorbed oxygen atoms or spillover on the supports where they can reduce the support surface sites. It is shown that CO is adsorbed molecularly on different surface sites, on the support cations, on Au0 sites exposed at the surface of small three-dimensional particles and also on Auδ− sites exposed at the surface of negatively charged clusters. The CO formed in the reverse WGS reaction appears chemisorbed only on the Au0 sites. The support sites and the Auδ− sites, where the CO appears as more strongly bonded, are present but not accessible to the CO formed by CO2 reduction, probably because these sites are covered by water. Water and OH groups are adsorbed on the supports, on gold sites, and at the interface between them. The effects of CO coadsorption on water dissociation and of H2 dissociation on CO2 reduction have been evidenced. The close similarity of the catalytic activity of the two examined samples indicates that the active sites for hydrogen dissociation and for water–CO reactive interactions are located at the surface of the metallic gold small particles where the reaction can take place by a red–ox regenerative mechanism.

Infrared spectroscopy of carbon monoxide and nitric oxide on palladium(111) in aqueous solution: unexpected adlayer structural differences between electrochemical and ultrahigh-vacuum interfaces

Abstract Coverage-dependent infrared reflection-absorption spectra (IRAS) measured for carbon monoxide and nitric oxide adlayers dosed onto Pd(111) in acidic aqueous solution are examined in comparison with IRAS and other adlayer structural data available on Pd(111) in ultrahigh vacuum (UHV). Unlike most other ordered Pt-group surfaces, the coverage-dependent spectral fingerprints for both electrochemical adlayers on Pd(111) are markedly different from their UHV-based counterparts. These disparities are magnified after allowance is made for the differences in surface potentials, and hence interfacial electrostatic fields, between the electrochemical and UHV-based systems. Specifically, at low dosed CO coverages ( θ CO ν CO) band is obtained at ca. 1830–1880 cm −1 , consistent with threefold-hollow coordination, which is replaced at higher coverages up to saturation ( θ CO =0.75) by a pair of bands at 1940–1960 and 1890–1920 cm −1 , suggestive of ‘bridging-like’ along with hollow CO binding. Unusually similar θ CO -dependent infrared spectra were obtained upon sequential partial electrooxidation of a saturated layer, indicating that any CO islands thereby forming are largely dissipated. While the infrared spectra are qualitatively similar to the behavior in UHV at low coverages, the electrochemical ν CO frequencies are significantly (30–40 cm −1 ) higher than expected, and the higher-frequency ν CO feature seen for θ CO >0.4 has no obvious counterpart at the Pd(111)/UHV interface. These behavioral differences can be partly, yet not entirely, rationalized in terms of water coadsorption. Dosed NO adlayers exhibit largely a single NO stretching ( ν NO ) band blueshifting by ca. 30 cm −1 up to ca. 1750 cm −1 at saturation, suggestive of largely atop NO coordination. The absence of the markedly lower-frequency ν NO bands seen at low dosages on Pd(111) in UHV is ascribed to partial segregation of NO and coadsorbed water, yielding higher local NO coverages. These unexpected ν CO and ν NO spectral properties at Pd(111) electrodes with respect to the corresponding UHV-based systems are also discussed in comparison with corresponding IRAS behavior for CO and NO adlayers on other low-index Pt-group surfaces. The coadsorption of CO and NO on Pd(111) is also briefly considered. The form of the ν CO / ν NO spectra indicate the presence of some CO segregation as well as CO/NO molecular intermixing.

Surface diffusion of Ni on Si(111) with coadsorption of Co

The effect of the coadsorption of Co and Ni on an Si(111) surface structure and on the diffusion of adsorbed atoms is investigated by low-energy electron diffraction and Auger electron spectroscopy. It is established that surface structures similar to those formed with the adsorption of Co alone are formed with the Ni and Co coadsorption on an Si(111) surface. It is found that the contribution of surface diffusion to the transport of Ni atoms is sharply higher on an Si(111) surface with submonolayer Co concentrations in the temperature range 500–750 °C than for a pure surface, where the main mechanism of Ni transport along the surface is diffusion of Ni atoms through the bulk of Si.

A Model Study of CO−CO Adsorbate Interaction on Si(100)-2×1

The CO−CO adsorbate interaction on Si(100)-2×1 has been investigated with ab initio molecular orbital and hybrid density functional theory calculations using cluster models of the surface. Different adsorption combinations for one and two CO molecules on single- and double-dimer cluster models, Si9H12 and Si15H16, respectively, are described. Our calculations indicate that the second CO molecule is physisorbed on the same surface Si dimer where the first CO molecule is chemisorbed. The chemisorption of the first CO molecule induces a change in the charge of the surface Si dimer atoms which inhibits further adsorbate−surface interaction. The dissociation energy of the physisorbed second CO molecule is less than 1 kcal/mol. Adsorption of the second CO molecule on the second Si dimer is energetically preferred over coadsorption of CO on the same Si dimer. The 2OC-normal.d14 structure is the most stable configuration, with the two CO molecules adsorbed diagonally across the two Si dimers. The dissociation energy of the chemisorbed second CO molecule in the 2OC-normal.d14 structure is 13.8 kcal/mol, about 4 kcal/mol less stable than the first adsorbed CO. Adsorption of two CO molecules in a bridge configuration indicates a weak surface−adsorbate and/or adsorbate−adsorbate interaction. The dissociation energy of the single chemisorbed CO in the bridged state is 5.0 kcal/mol while that of the second CO is 1.7 kcal/mol. A mixed configuration, i.e., OC-normal with OC-bridge, was found to be unstable based on the Si15H16 surface model.

An infrared study of ethene and CO coadsorption on Pt[111] and a Pt/SiO 2 catalyst: ambiguities in the interpretation of difference spectra

An infrared spectroscopic study shows that the sequential adsorption of ethene and then CO on a finely divided Pt/SiO 2 catalyst at 294 K displaces the π and di-σ ∗ C 2H 4 species that occur on non-[111] facets after the initial ethene adsorption, but leaves the ethylidyne species on [111] facets undiminished. Extra absorptions from ethylidyne are revealed by the removal of the other species.The principal absorptions of the ethylidyne species are affected in very different ways, particularly in relative intensities, as a result of the CO post-adsorption. On a single-crystal Pt[111] surface only the the ethylidyne species is present after adsorption at 360 K. This is fractionally removed by CO post-adsorption while intensity changes occur to the remaining ethylidyne absorptions that are analogous to those observed with the finely divided catalyst. It is shown that in each case the intensity changes in the ethylidyne spectra are visually exaggerated in the difference spectra. This occurrence is shown be due to the partial overlap of the analogous features in the parent spectra, leading to partial and selective cancellations in the difference spectra. The phenomenon, which is of general significance in difference spectroscopy, is explored systematically by means of computer simulations. The spectrum of CO itself, obtained after post-adsorption on either ethylidyne-covered sample, shows predominantly adsorption on linear (on-top) sites in contrast to the results of analogous earlier experiments involving Rh or Pd surfaces where the CO adsorbs on three-fold sites.

Adsorption sites in O and CO coadsorption phases on Rh(111) investigated by high-resolution core-level photoemission

High-resolution core-level spectroscopy is used in combination with low-energy electron diffraction (LEED) and photoelectron diffraction to identify the adsorption sites for three different coadsorbed phases consisting of ordered overlayers of oxygen coadsorbed with CO on the Rh(111) single-crystal surface. The three ordered overlayer structures, which may be denoted as 2O+CO/Rh(111), O+CO/Rh(111) and O+2CO/Rh(111), all show (2×2) LEED patterns. In the 2O+CO and O+CO phases the CO molecules are found to occupy only on-top sites while the O+2CO phase shows CO molecules in both on-top and three-fold hollow sites. In all cases the oxygen atoms are found in three-fold hollow sites. For the O+CO and O+2CO phases our results confirm previous determinations by LEED, while the 2O+CO phase has not been observed before on Rh(111). The core-level binding energies of the C 1s and O 1s core levels for both adsorbates are characteristic of the adsorption site and are very close to the binding energies found for the pure cases of only oxygen or CO adsorbed on Rh(111). In the coadsorption phases we find that the interaction between the adsorbates has only a minor influence on the core-level binding energies. For the O+2CO/Rh(111) coadsorption phase we find that a full CO coverage is not obtained; less than 80% of the unit cells contain two CO molecules, in line with previous findings.

Coverage grating template for the study of surface diffusion: K coadsorbed with CO on Re(001)

The coverage grating method as a template for coadsorbates is introduced for the study of surface diffusion. The effect of coadsorbed CO inside coverage troughs formed by laser induced thermal desorption (LITD) on potassium surface diffusion on Re(001) has been investigated using the coverage grating optical second harmonic diffraction method. Enhancement of the first-order diffraction peak at a certain CO coadsorption coverage, observed for the first time, demonstrates the very strong electronic interaction between these coadsorbates. The activation energy for K surface diffusion and the pre-exponential factor significantly increase with CO coverage. At initial potassium coverage of 1.0 ML, the activation energy for potassium diffusion increases from 5.0 kcal/mol on the clean rhenium surface to 15.0 kcal/mol in the presence of 0.065 ML CO, and the pre-exponential factor increases from 5.6×10 −3 to 2.0×10 2 cm 2/s. The activation energy doubles for potassium coverages of 0.9 ML and 0.8 ML as a result of the same CO coverage. TPD measurements indicate that strong attractive interactions exist between CO and K on Re(001). Coadsorbed CO and K stabilize each other while forming K x –CO surface complexes which slow down the potassium surface diffusion predominantly by site-blocking effect. This site-blocking effect increases as the number of potassium atoms ( x) interacting with a single CO coadsorbate increases at higher initial K coverages. The nature of this site-blocking is discussed.

Screening of SFG signals from bridged CO on Ni(111) by the coexistence of linear CO

A spectroscopic study of adsorbed CO on Ni(111) was carried out using infrared-visible sum-frequency generation (SFG) and infrared reflection absorption spectroscopy (IRAS). An anomalous coverage-dependence of the SFG signal intensities of bridged and linear CO was found. While the SFG signal of bridged CO at 1920 cm −1 gained intensity with increasing CO coverage in the region between 0 and 0.4 ML (1 monolayer, ML: saturation coverage at 130 K), it weakened and disappeared when linear CO (2074 cm −1 ) was present on the surface above 0.47 ML. The IRA peak of bridged CO, however, did not disappear at the increased coverage suggesting that the weakening of the SFG signal of bridged CO is not caused by the decrease in the coverage but by the suppression of the SFG process of bridged CO by the coexistence of linear CO. The spectra of isotope mixtures of C 16O and C 18O clearly indicated that the weakening of SFG signals is not caused by the dipole–dipole coupling between the vibrational modes of bridged and linear CO. The anomalous coverage-dependence of the SFG signal was ascribed to the change of the Raman tensor by the coadsorption of linear CO.

FTIR study of the low-temperature adsorption and co-adsorption of CO and N 2 on NaY zeolite: evidence of simultaneous coordination of two molecules to one Na + site

Na + cations in S II positions in NaY zeolite are able, at 85 K, to coordinate two CO or N 2 molecules as well as to form mixed Na +(CO)(N 2) species. In all these complexes, the ligands behave as independent oscillators. The Na +( 15N 2) 2 species (IR band at 2255.5 cm −1) are typical at equilibrium pressures >30 Pa, whereas the respective linear complexes (band at 2258.5 cm −1) prevail at lower pressures. The dicarbonyls (band at 2163 cm −1) are the principal complexes observed at CO pressures higher than ca. 0.1 Pa and evacuation converts them to linear carbonyls (band at 2174 cm −1). The mixed Na +(CO)( 15 N 2 ) complexes [ ν(C–O) at 2167 cm −1 and ν( 15 N – 15 N ) at 2254 cm −1] are produced during CO and 15 N 2 co-adsorption and easily lose one 15 N 2 ligand, being thus converted to linear carbonyls.

Reaction channels for the catalytic oxidation of CO on Pt(111)

The catalytic oxidation of CO on O-precovered Pt(111) surfaces has been modeled via ab initio local-density-functional calculations. It is shown that at coverages of ${\ensuremath{\Theta}}_{\mathrm{O}}\ensuremath{\sim}0.5$ the coadsorption of CO stabilizes the chemisorbed molecular precursor of ${\mathrm{O}}_{2}$ over the dissociated atomic oxygen. The barrier for the reaction between the coadsorbed molecules is lowest if first the molecular ${\mathrm{O}}_{2}$ bond is broken. However, at this high coverage the barrier for the ${\mathrm{O}}_{2}+\mathrm{CO}$ reaction is higher than for ${\mathrm{O}}_{2}$ desorption. Therefore oxidation will be preceded by a partial desorption of ${\mathrm{O}}_{2}.$ At reduced coverage, the barrier for the oxidation reaction is strongly reduced. At the transition state the nascent O atoms are only bridge or top bonded and therefore quite reactive. After desorption of ${\mathrm{CO}}_{2},$ the adsorbed atomic O can react with a second CO molecule via a transition state similar to that for the molecular reaction.

Low-temperature CO and 15N2 adsorption and co-adsorption on alkali cation exchanged EMT zeolites: an FTIR study

The low-temperature adsorption and co-adsorption of CO and 15N2 on a series of Me-EMT zeolites (Me=Li, Na, K, Rb and Cs) have been studied by FTIR spectroscopy. Adsorption of 15N2 on Na-EMT at 85 K results in the formation of Na+ (15N2)2 geminal species (IR band at 2255 cm-1), which are converted, at low equilibrium pressures, into Na+–15N2 linear complexes (IR band at 2258.5 cm-1). Both species are removed by evacuation. CO adsorption at 85 K results in the formation of dicarbonyls at high pressures (2164 cm-1), which are converted into linear carbonyls at low pressures (2175 cm-1). Mixed Na+(15N2)(CO) complexes are produced when 15N2 and CO are co-adsorbed. The latter easily lose one 15N2 ligand, thus being converted into linear carbonyls. On Li-EMT only Li+–15N2 species were detected, characterized by a band at 2263 cm-1, and only linear Li+ carbonyls were produced after CO adsorption (band at 2183 cm-1). In addition, a weak band at 2199 cm-1 (tentatively assigned to the Al3+–CO species) was detected. The spectra of 15N2 and CO adsorbed on K-EMT, Rb-EMT and Cs-EMT display some similarities. In all cases geminal and linear dinitrogen complexes are detected; however, the spectral differences between them are very small. Upon CO adsorption two principal bands (2160 and 2147 cm-1 for K-EMT; 2159 and 2151 cm-1 for Rb-EMT; and 2157 and 2144 cm-1 for Cs-EMT) and two weak features (2275 and 2120 cm-1 for K-EMT; 2261 and 2119 cm-1 for Rb-EMT; and 2242 and 2123 cm-1 for Cs-EMT) are detected. Analysis of the spectra allows assignment of the bands around 2120 cm-1 to O-bonded CO, the bands in the 2185–2155 cm-1 region to CO polarized by metal cations, and the band at 2151–2144 cm-1 to mixed Me+(OC)(CO) species. The reasons for the formation of different complexes are discussed.

FTIR spectroscopic evidence of formation of geminal dinitrogen species during the low-temperature N2 adsorption on NaY zeolites

Adsorption of N2 on NaY zeolites at 85 K and equilibrium pressures higher than 1 kPa results in the formation of geminal dinitrogen complexes characterized by an IR band at 2333.5 cm−1 (2255.4 cm−1 after adsorption of 15N2). With decreasing equilibrium pressure the complexes tend to loose one N2 ligand, thus forming linear species characterized by an IR band at 2336.8 cm−1 (2258.7 cm−1 after adsorption of 15N2). All species disappear completely after evacuation. Co-adsorption of N2 and CO revealed that the dinitrogen complexes are formed on Na+ cations. The changes in the concentrations of the linear and geminal N2 species with the changes in the equilibrium pressure are excellently described by equations of adsorption isotherms proposed earlier for mono- and di-carbonyls.

FTIR study of the interaction of CO and CO 2 with silica-supported PtSn alloy

The interaction between CO 2 and the silica-supported PtSn alloy gave rise to an infrared band at 2339 cm −1 which was assigned to the coordination of this molecule to the bimetallic phase. The coadsorption of CO and CO 2 indicates that both compete for similar coordination sites on the supported PtSn alloy. A CO singleton vibration frequency at ca. 2054 cm −1 was determined for both Pt/SiO 2 and PtSn/SiO 2 catalysts. A dilution effect caused by tin in the silica-supported PtSn alloy is proposed. The presence of isolated Pt sites in the particles of this alloy determine their unique capability to coordinate the CO 2 molecule in the active sites.

FTIR study of CO and NH3 co-adsorption on TiO2 (rutile)

FTIR spectroscopy has been used to study CO and ammonia adsorption and co-adsorption on rutile. Ammonia is adsorbed coordinatively, but with time, different dissociative ammonia forms appear. At room temperature CO is adsorbed forming one type of Ti4+–CO carbonyls alone (absorption band at 2191 cm−1) and is oxidized to bicarbonates. At 100 K CO occupies, at first, the sites detected at room temperature, whereas the increase in amount of adsorbed CO results in new adsorption forms: (i) CO adsorbed on titanium cations inert at room temperature (monitored by an intense band with a maximum at 2183 cm−1), (ii) CO adsorbed on hydroxyl groups (band at 2150 cm−1), and (iii) physically adsorbed CO (band at 2140 cm−1). Pre-adsorbed ammonia decreases the acidity of both, the Lewis acid sites and the surface Ti4+–OH groups. Low-temperature CO adsorption on rutile partly poisoned by ammonia allows to detect only the sites inert towards CO adsorption at room temperature. On the basis of the results obtained, the active sites for adsorption of ammonia and CO are discussed and comparison between the surface properties of anatase and rutile is made.