Probing the properties of the (1 1 1) and (1 0 0) surfaces of LaB6 through infrared spectroscopy of adsorbed CO

The adsorption of carbon monoxide on the LaB₆(1 0 0) and LaB₆(1 1 1) surfaces was studied experimentally with the techniques of reflection absorption infrared spectroscopy and X-ray photoelectron spectroscopy. The interaction of CO with the two surfaces was also studied with density functional theory. Both surfaces adsorb CO molecularly at low temperatures but in markedly different forms. On the LaB₆(1 1 1) surface CO initially adsorbs at 90 K in a form that yields a CO stretching mode at 1502-1512 cm⁻¹. With gentle annealing to 120 K, the CO switches to a bonding environment characterized by multiple CO stretch values from 1980 to 2080 cm⁻¹, assigned to one, two, or three CO molecules terminally bonded to the B atoms of a triangular B₃ unit at the (1 1 1) surface. In contrast, on the LaB₆(1 0 0) surface only a single CO stretch is observed at 2094 cm⁻¹, which is assigned to an atop CO molecule bonded to a La atom. The maximum intensity of the CO stretch vibration on the (1 0 0) surface is higher than on the (1 1 1) surface by a factor of 5. This difference is related to the different orientations of the CO molecules on the two surfaces and to reduced screening of the CO dynamic dipole moment on the (1 0 0) surface, where the bonding occurs further from the surface plane. On LaB₆(1 0 0), XPS measurements indicate that CO dissociates on the surface at temperatures above 400 K.     

Coadsorption of water and CO molecules on Ru(0 0 1) at high CO coverages: comparisons with a Ru(0 0 1) electrode surface

The coadsorption of carbon monoxide (CO) and water molecules on a Ru(0 0 1) surface has been studied by infrared spectroscopy, LEED and STM. At high CO coverage phases, a 2×2-(2CO+D₂O) structure was observed on both UHV and electrode surfaces. Electrode potential dependent structures from CO and water adlayers on an electrode surface were reproduced on a UHV surface by controlling molecular orientations of the first layer and second over-layer water molecules. At lower CO coverages, a CO band center showed coverage dependent shift down to 1444 cm⁻¹ due to an electron transfer from a lone pair of a water molecule to CO 2π^*.     

RAIRS studies of CO adsorption on Pd/CeO2−x(1 1 1)/Pt(1 1 1)

Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on CeO_2−x-supported Pd nanoparticles at room temperature. The results show that when CeO_2−x is initially grown on Pt(1 1 1), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto CeO_2−x/Pt(1 1 1), most of the Pd grows directly on top of the CeO_2−x(1 1 1). RAIR spectra of CO adsorption on 1 ML Pd/CeO_2−x/Pt(1 1 1) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/CeO_2−x/Pt(1 1 1) spectra show vibrational bands consistent with the presence of Pd(1 1 1) and (1 0 0) faces, suggesting the growth of Pd nanostructures with well defined facets.     

Structure of the neopentane monolayer adsorbed on MgO(0 0 1): experiments and calculations

The monolayer of C(CH₃)₄ molecules adsorbed upon a highly homogeneous MgO powder surface has been studied by means of neutron diffraction techniques at the temperature 210 K. The structure of the monolayer presents a strong commensurability c(2 × 4) with the MgO(0 0 1) surface. The results of potential calculations support the proposed structure.     

Hydrogen adsorption on Mo1−xRex(110) (x=0–0.25) surfaces

The effects of adsorbed H on the Mo_1−xRe_x(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.     

Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability α_v of an adsorbate mode and of the dielectric screening ? due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability α_v associated with the internal C-O stretch mode (ν_C-O) of on-top CO is lowered by all coadsorbates. The dielectric screening ? within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ? is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of α_v and dielectric screening ?, as well as the involved line shifts of ν_C-O and ν_Ru-CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character.     

The structure of ammonia molecules on MgO(100) surfaces from monolayer to bulk condensation

The structure of ND₃ molecules adsorbed on MgO (100) surfaces has been studied by neutron diffraction within the 10–80 K temperature range and at 0.7, 1 and 2.3 monolayer coverage. The neutron spectra suggest that the monolayer presents a short range order with a hcp packing of ammonia molecules, a coherence length of 25 ± 2 Å and a nearest neighbour distance of 3.61 ± 0.04 Å The molecular spacing remains the same between 10 and 80K that we interpret as small higher order commensurate islands. Above one monolayer coverage, bulk crystallites form on top of the first monolayer.     

The temperature dependence of the interaction of NO + CO on Pt{1 0 0}

Infrared reflection absorption spectroscopy together with mass spectrometry has been used to investigate the interaction of NO and CO on Pt{1 0 0}, initially prepared in the reconstructed ‘hex’ phase, under ambient pressures of these gases, in the temperature range 300-500 K. The results allow the local and total coverages of adsorbed CO and NO to be related to the rate of reaction to produce gas phase CO₂, and provide insight into the species present on the surface during the so-called low temperature oscillatory reaction regime of this process. At temperatures below that at which NO dissociation occurs (approximately 390-400 K) adsorption is controlled by the non-reactive displacement of NO by CO and results in a CO-poisoned surface. Above 400 K when significant CO₂ production occurs, the NO coverage increases to produce a surface with NO and CO fully intermixed; the increase in NO coverage is attributed to the higher rate of NO arrival from the gas phase (with a partial pressure ratio of P_NO:P_CO>1) at free surface sites created by NO dissociation and subsequent reaction with CO. The competition between these two processes of non-reactive NO displacement by CO and reactive displacement of CO by NO is proposed to determine the parameter space of the low temperature oscillatory regime. Rapid equilibration between bridged and atop CO species leads to them appearing to exhibit identical reaction behaviour. Particularly at the lowest reaction temperatures (around 400 K), islands of pure CO may coexist on the surface but not participate in the reaction. Under conditions corresponding to the high temperature oscillatory regime, small quantities of absorbed CO, but no NO, are seen on the surface.     

On the difficulty for finding the orientational geometries of adsorbed monolayers: illustration with the CO/MgO system

With the example of the (4 × 2) commensurate phase of CO adsorbed on MgO(100) below 40 K, we illustrate the difficulty for determining the adsorption sites and the orientations of the admolecules. This system takes advantage of abundant experimental information issued from electron and helium diffraction patterns and infrared signals, and from a lot of theoretical calculations including ab initio approaches as well as semi-classical atom-atom methods. However, due to the specific arrangement of the molecules along the Mg troughs of the substrate, a lot of orientational configurations for the molecules in the unit cell correspond to very close adsorption energy values and no configuration perfectly fits the whole set of experimental data. We thus consider the features which can be at the origin of such a discrepancy.     

A RAIRS study of methoxy and ethoxy on oxidised Cu(100)

The adsorption and reaction of methanol and ethanol on a preoxidised Cu(100) surface was studied with Reflection-Absorption Infra-Red Spectroscopy (RAIRS). Both alcohols reacted with the modified surface well below room temperature, undergoing OH bond scission to form alkoxide species. The alkoxides were stable up to 340–360 K at which stage they desorbed as aldehydes. Vibrational assignments of the observed modes were made with reference to the RAIRS spectra of the alcohols and their deuterium substituted analogues on Cu(100) together with literature values for the gaseous, liquid and solid alcohols. Application of the metal surface selection rule to the alkoxide spectra indicate that in both cases the C---O bond lies perpendicular to the surface. The methoxy species is therefore assigned to an upright C_3v configuration. Oxygen precoverage is found to govern the amount of alkoxide formation which passes through a maximum at a precoverage of about 150 L.     

Reactivity of 3-hexyne on oxygen modified Ru(0 0 1) surfaces: Observation of oxametallacycles by RAIRS

The chemical behaviour of 3-hexyne on oxygen modified Ru(0 0 1) surfaces has been analysed under ultrahigh-vacuum, using reflection-absorption infrared spectroscopy (RAIRS). The effects of oxygen coverage, 3-hexyne exposure and adsorption temperature were studied. Two modified Ru(0 0 1) surfaces were prepared: Ru(0 0 1)-(2 × 2)-O and Ru(0 0 1)-(2 × 1)-O that correspond to oxygen coverages (θ_O) of 0.25 and 0.5 ML, respectively. The striking result is the direct bonding to an O atom when the modified surfaces are exposed to a very low dose (0.2 L) of 3-hexyne at low temperature (100 K). For θ_O = 0.25 ML, an unsaturated oxametallacycle [Ru-O-C(C₂H₅)C(C₂H₅)-Ru] is proposed, identified by RAIRS for the first time, through the νCC and νCO modes. Further decomposition at 110 K yields smaller oxygenated intermediates, such as acetyl [μ₃-η²(C,O)-CH₃CO], co-adsorbed with a small amount of carbon monoxide and non-dissociated species. The temperature at which a fraction of molecules undergoes complete C-C and C-H bond breaking is thus much lower than on clean Ru(0 0 1). The ultimate decomposition product observed by RAIRS at 220 K is methylidyne [CH]. Another key observation was that the adsorption temperature is not determinant of the reaction route, contrarily to what occurs on clean Ru(0 0 1): even when 3- hexyne strikes the surface at a rather high temperature (220 K), the multiple bond does not break completely. For θ_O = 0.5 ML, a saturated oxametallacycle [Ru-O-CH(C₂H₅)-CH(C₂H₅)-Ru] is also proposed at 100 K, identified by the ν_asO-C-C (at 1043 cm⁻¹) and ν_sO-C-C (at 897 cm⁻¹) modes, showing that some decomposition with C-H bond breaking occurs. For this oxygen coverage, the reaction temperatures are lower, and the intermediate surface species are less stable.     

Adsorption behavior and reaction properties of NO and CO on Rh(1 1 1)

Reactions between NO and CO on Rh(1 1 1) surfaces were investigated using infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. NO adsorbed on the fcc, atop, and hcp sites in that order, whereas CO adsorbed initially on the atop sites and then on the hollow (fcc + hcp) sites. The results of experiments with NO exposure on CO-preadsorbed Rh(1 1 1) surfaces indicated that the adsorption of NO on the hcp sites was inhibited by preadsorption of CO on the atop sites, and NO adsorption on the atop and fcc sites was inhibited by CO preadsorbed on each type of site, which indicates that NO and CO competitively adsorbed on Rh(1 1 1). From a Rh(1 1 1) surface with coadsorbed NO and CO, N₂ was produced from the dissociation of fcc-NO, and CO₂ was formed by the reaction of adsorbed CO with atomic oxygen from dissociated fcc-NO. The CO₂ production increased remarkably in the presence of hollow-CO. Coverage of fcc-NO and hollow-CO on Rh(1 1 1) depended on the composition ratio of the NO/CO gas mixture, and a gas mixture with NO/CO ? 1/2 was required for the co-existence of fcc-NO and hollow-CO at 273 K.     

RAIRS investigations on the orientation and intermolecular interactions of adenine on Cu(1 1 0)

The adsorption of the DNA base adenine (C₅N₅H₅) on Cu(1 1 0) has been investigated as a function of coverage and temperature using reflection absorption infrared spectroscopy. These data provide important information on the nature of the local adsorbed complex and the intermolecular lateral interactions that come into play at high coverages. The RAIR spectra are consistent with an adsorption geometry in which the molecular ring is substantially tilted from the surface plane and in which the two amino hydrogens are equidistant from the surface, thus rationalising the appearance of a very strong βNH₂ scissoring mode, along with the activity of in-plane vibrational modes and the observation of the symmetric ν_symNH₂ stretch, but not the asymmetric ν_asymNH₂ stretch. In addition, coordination to the metal surface is proposed to occur at the N(9) position with a possible additional interaction through the N(3) position, both of which are the favoured coordination points in the metal complexes of adenine. This is a strong interaction and leads to a highly stable adsorbed layer. Our data also provide the first direct evidence of hydrogen bonding in the adlayer as coverage is increased, attributed to interactions between the amino group of one molecule and the N(1) and N(7) positions of a neighbouring species. When adsorption is carried out at room temperature, a very heterogeneous adlayer is created in which a diversity of molecular aggregates co-exist. However, upon annealing, a more ordered hydrogen bonded adlayer is formed in which one type of hydrogen bonding assembly is preferred. Finally, we propose that the hydrogen bonded assemblies created at a surface probably involve bent hydrogen bonds which arise from a compromise between the strong molecule-metal interactions that orientate the molecule and weaker lateral hydrogen bonding interactions that dictate the two-dimensional architecture.     

Adsorption of CO on a pseudomorphic palladium monolayer on Mo(110)

Adsorption of CO on a Pd monolayer (ML) supported on Mo(110) has been studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). Three ordered CO substructures denoted as are observed with LEED. The binding energy of C0 on the 1.0 ML Pd/Mo(110) surface is reduced by 12 kcal/mol relative to the Pd(111) surface, consistent with previous results for supported palladium monolayers on other substrates. Two vibrational states of C0 are observed near 1950 and 2050 cm⁻¹, with the feature at the lower wavenumber having the smaller binding energy.     

Sum frequency generation and density functional studies of CO-H interaction and hydrogen bulk dissolution on Pd(1 1 1)

CO-H interaction and H bulk dissolution on Pd(1 1 1) were studied by sum frequency generation (SFG) vibrational spectroscopy and density functional theory (DFT). The theoretical findings are particularly important to rationalize the experimentally observed mutual site blocking of CO and H and the effect of H dissolution on coadsorbate structures. Dissociative hydrogen adsorption on CO-precovered Pd(1 1 1) is impeded due to an activation barrier of ∼2.5 eV for a CO coverage of 0.75 ML, an effect which is maintained down to 0.33 ML CO. Preadsorbed hydrogen prevented CO adsorption at 100 K, while hydrogen was replaced from the surface by CO above 125 K. The temperature-dependent site blocking of hydrogen originates from the onset of hydrogen diffusion into the Pd bulk around 125 K, as shown by SFG and theoretical calculations using various approaches. When Pd(1 1 1) was exposed to 1:1 CO/H₂ mixtures at 100 K, on-top CO was absent in the SFG spectra although hydrogen occupies only threefold hollow sites on Pd(1 1 1). DFT attributes the absence of on-top CO to H atoms diffusing between hollow sites via bridge sites, thereby destabilizing neighboring on-top CO molecules. According to the calculations, the stretching frequency of bridge-bonded CO with a neighboring bridge-bonded hydrogen atom is redshifted by 16 cm⁻¹ when compared to bridging CO on the clean surface. Implications of the observed effects on hydrogenation reactions are discussed and compared to the C₂H₄-H coadsorption system.     

Coadsorption of CO and C6H6 on Co(0 0 0 1)

We have studied the influence of CO on the adsorption of benzene on the Co(0 0 0 1) surface using LEED, XPS, TDS and work function measurements. CO was found to reduce the benzene adsorption, but even at saturation CO exposure no complete blocking was observed. Thermal desorption of the coadsorbed layer featured CO and H₂ peaks indicating partial dehydrogenation of benzene and retaining of the CO bond. Ordered LEED structures were found with all coverages: Pre-adsorption of CO led to patterns already seen for pure carbon monoxide adsorption. Pre-adsorption of benzene showed the known structure of pure benzene also with small CO exposures, but higher CO exposures yielded a mixture of and patterns.     

The adsorption of CO on Ir(111) investigated with FT-IRAS

The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm⁻¹, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm⁻¹. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm⁻¹ for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm⁻¹ at a coverage of 0.5 ML.     

Characterisation of the interaction of glycine with Cu(1 0 0) and Cu(1 1 1)

Reflection-absorption infrared spectroscopy (RAIRS) has been used to characterise the interaction of standard and fully deuterated glycine with Cu(1 0 0) and Cu(1 1 1). RAIRS shows clearly that the surface interaction leads to formation of the adsorbed deprotonated glycinate (NH₂CH₂COO-) species, with some evidence for changes in orientation with coverage previously seen on Cu(1 1 0). Qualitative low energy electron diffraction observations were also conducted to characterise the long-range ordering, although effects of electron-beam-induced radiation damage limited the information obtained. Nevertheless, the results do suggest some subtle isotopic-mass-related structural variations. The results are discussed in the context of previously published scanning tunnelling microscopy and photoelectron diffraction measurements.     

Cluster and band structure ab initio calculations on the adsorption of CO on acid sites of the TiO2(110) surface

The interaction of CO with the cationic sites of the TiO₂(110) surface has been investigated with cluster models and band structure calculations. The analysis of Hartree-Fock and correlated wavefunctions has shown that CO adsorbs at a distance of 4.4–4.5 bohr and a binding energy of 0.7–0.8 eV at low coverage. The bond strength is determined by the CO polarization and the CO б-donation to the surface while the electrostatic attraction is almost exactly cancelled by the Pauli repulsion. The adsorption is accompanied by two important measurable features, a considerable blue shift of the CO vibrational frequency and an increase of the CO 5б ionization potential. Both these effects have been analyzed in detail. We found that the CO ω shift is largely due to a combination of the electrostatic Stark effect and of the repulsion occurring when the CO molecule stretches in the presence of the rigid surface. The change in the 1π–5б binding energies, on the other hand, has an entirely electrostatic origin. Neither the ω shift nor the 5б binding energy shift are determined by the б-donation mechanism. Nevertheless, the occurrence of a charge transfer from CO to the empty levels of the Ti centers is well documented by (a) the energy and dipole moment change associated to this mechanism, (b) the expectation value of a projection operator which measures the charge associated with a given orbital, and (c) the CO dynamic dipole moment. The same analyses also rule out the occurrence of a Ti-to-CO back donation.