Monolayer CO Research Articles

A theoretical study of CO adsorption on Cu(211) surface with coverage effects.

CO adsorption on the Cu(211) surface was investigated using energy, geometry and vibrational data, which were produced through periodic DFT computations. Adsorption on the (111) terrace, as well as the previously reported top and bridge sites of the step-edges, was studied at 0.25, 0.33, 0.50, 0.66, 0.75, and 1.00 monolayer (ML) CO coverage. Results showed that up to and including 0.50 ML, on-top or bridge adsorption is preferred on the step-edges. When 0.50 ML is exceeded, top-bridge alternating patterns become feasible on the step edges along with possible shifts towards the terrace. Several feasible patterns were identified at 0.66, 0.75, and 1.00 ML. Like step-edge adsorptions, alternating patterns on the terrace sites were found feasible at higher coverages. For all the studied cases, highest adsorption energies were found for the step-edge positions. In general, coordination number had a stronger effect than coverage on the calculated properties.

On the infrared fluorescence of monolayer 13CO:NaCl(100)

Computations are presented to describe and analyze the high levels of infrared laser induced vibrational excitation of a monolayer of absorbed (13)CO on a NaCl(100) surface. Extending the vibrational site-to-site surface hopping technique of Corcelli and Tully, kinetic Monte Carlo computations are used to incorporate single-quantum vibrational pooling and depooling of the (13)CO by phonon excitation to allow up to the n = 45 vibrational state under different lasing conditions. Previously unpredicted pooling peaks at n > 16 are calculated and, under the highest fluence conditions, pooling up to the n = 32 state is found in the calculation. These results lead to the prediction of a secondary local maximum in the dispersed fluorescence of monolayer CO:NaCl(100) under sufficiently high fluence excitation conditions. At times on the order of ms, we recover similar behavior for both high and low fluence results. The calculations confirm that, for situations where the Debye frequency limited n domain restriction approximately holds, the vibrational state population deviates from a Boltzmann population linearly in n, a result that we have derived earlier theoretically for a domain of n restricted to one-phonon transfers. This theoretically understood term, linear in n, dominates the Boltzmann term and is responsible for the inversion of the population of vibrational states, Pn.

CV or not CV: Development of Electrode Diagnostics for High-Temperature Polymer Electrolyte Fuel Cells

In order to achieve reliable data regarding high temperature polymer electrolyte fuel cell degradation (HT-PEFC) a novel diagnostic technique for electrochemically active surface area (ECSA) determination was developed. In situ cyclovoltammetric monolayer CO oxidation charge measurements were combined with real-time CO2 exhaust gas analysis. Different evaluating methods were developed to overcome the problem of side reactions during CO stripping. Furthermore, a calibration curve for absolute ECSA determination via CO stripping was established. These methods were additionally successfully extended and implemented to locally resolved ECSA measurements. In summary, this paper introduces novel fundamental HT-PEFC diagnostics for improved understanding of degradation phenomena.

Structural Dynamics at Monolayer–Liquid Interfaces Probed by 2D IR Spectroscopy

Monolayers functionalized with tricarbonyl- 1,10-phenanthroline rhenium chloride (RePhen(CO)3Cl) are studied in the presence of a variety of polar organic solvents (chloroform, tetrahydrofuran, dimethylformamide, acetoni- trile), hexadecane, and water. The headgroup, RePhen- (CO)3Cl, is soluble in the bulk polar organic solvents but not in hexadecane or water. The surface structural dynamics are studied using IR absorption, ultrafast two-dimensional infrared (2D IR) vibrational echoes, and heterodyne-detected transient gratings (HDTG). The headgroup is also studied in the bulk solvents. Immersion in solvent changes the environment of the surface bound RePhen(CO)3Cl as shown by a solvent- dependent symmetric CO stretch absorption frequency and line shape. The 2D IR spectroscopy is sensitive to spectral diffusion, which reports on the structural time dependence. In the absence of solvent (bare surface), the spectral diffusion takes place on a 37 ± 4 ps time scale, whereas in the presence of THF, MeCN, or CHCl3 it occurs on the 25 ± 6 ps time scale. However, in DMF, the spectral diffusion time constant is 15 ± 3 ps. In hexadecane the spectral diffusion slows to 77 ± 15 ps, slower than the bare surface. In D2O, the monolayer CO stretch is split into two bands, indicating distinct local structures, and the spectral diffusion of main band is again slower than the bare surface. HDTG spectroscopy is used to investigate the vibrational lifetime dynamics.

Dynamics of dissociative adsorption of hydrogen on a CO-precovered Ru(0001) surface: a comparison of theoretical and experimental results

We have studied hydrogen dissociation on a CO-precovered Ru(0001) surface, by means of six-dimensional (6D) quasi-classical and quantum dynamics. The 6D potential energy surface has been built by applying a modified Shepard interpolation method to a set of density functional theory (DFT) data, for a coverage of 1/3 monolayer CO. We compared our theoretical results to the experimental ones obtained by Ueta et al. [ChemPhysChem, 2008, 9, 2372]. In order to do so, we have simulated the supersonic molecular beam used in the experiments by taking into account the energy distribution and rovibrational states population in the molecular beam. We find that both the energy and rovibrational states distributions of the molecular beam influence the reactivity, with the largest effect being caused by the energy distribution. However, a significant discrepancy between theory and experiment persists. We argue that this discrepancy could be due to the RPBE functional used in the DFT calculations and/or the neglect of CO-motion in the calculations.

Effects of composition on structure and activity of PtRu/C catalysts

A series of carbon supported PtRu bimetallic catalysts with varying Pt:Ru ratio were prepared and characterised using ex situ and in situ XRD, in situ EXAFS at 0 V vs. RHE, ex situ XPS and monolayer CO stripping voltammetry. Although the catalysts were found to be well mixed/alloyed, with no evidence of unalloyed Ru (oxides) present, the surfaces of the electrocatalyst nanoparticles were found to be enriched with Pt compared to the nominal bulk composition. The methanol oxidation activities of the catalysts were determined in 1.0 mol dm(-3) H2SO4. In agreement with published studies of polycrystalline bulk PtRu alloys the catalyst with a 0.6 surface fraction of Pt was found to give the best methanol oxidation activity at 30 degrees C. However, at 80 degrees C a greater surface fraction of Ru could be tolerated, with some activity at low current densities found for a Pt surface fraction as low as 0.2. The results support the conclusion that a limited amount of methanol dehydrogenation occurs at Ru sites or Ru dominated surface ensembles at 80 degrees C.

Density Functional Theory Study of CO and Hydrogen Co-adsorption on the Fe(111) Surface

Spin-polarized density functional theory calculations have been carried out to characterize CO and H2 co-adsorption on the Fe(111) surface. At 0.5 monolayer (ML) CO coverage, the most-stable adsorption configurations are close to those of individually adsorbed CO and H, and the interaction of adsorbed CO and H does not lead to any hydrogenated product. As the ratio of H2/CO increases, the most-table co-adsorption site of CO changes from the shallow-hollow site (sh) to the top site (t), while H maintains approximately at the top-shallow bridge site (tsb) or the quasi-fourfold site (qff). At 1 ML CO coverage, a similar trend is found where CO moves up to the first-layer Fe surface, while H prefers to diffuse into the subsurface when the H2/CO ratio increases. At very high H2/CO ratios, the most-stable configuration of CO has adsorption at the top site (t). In addition, formyl (CHO) is formed at H2/CO ratios of 1/2 and 1/1, deriving from hydrogenation of adsorbed CO in the bridge-like site (brl). The interaction nature of CO−H is also analyzed by the DOS analysis.

Density functional study of CO on Rh(111)

We present detailed density functional calculations for CO on Rh(111). At low coverage, the applied semilocal functionals clearly favor CO adsorption in the hollow site. This is in disagreement with experimental studies which all point towards atop adsorption at low coverage. The experimental assignment is confirmed by theoretical calculations of the vibrational frequencies and core level shifts at various coverages, ranging from $1∕9\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}3∕4$ $[(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}3\mathrm{C}\mathrm{O}]$ monolayer CO. For atop adsorption the calculated vibrational frequencies and the Rh surface core level shifts are indeed found to agree very well with experiment. To understand these controversial results, a molecular $\mathrm{GGA}+\mathrm{U}$ method is applied, which allows one to shift the CO $2\ensuremath{\pi}*$ orbital towards the vacuum level. This reduces the binding energy in the hollow site and brings the theoretical site preference in agreement with experiment. It is investigated how this molecular $\mathrm{GGA}+\mathrm{U}$ method influences the vibrational properties and the surface core level shifts. Furthermore, details on the molecular $\mathrm{GGA}+\mathrm{U}$ method are presented.

CO adsorption on the CO-precovered Pt(111) surface characterized by density-functional theory

Ab initio density-functional investigations of the gradual adsorption of increasing amounts of CO on partially precovered Pt(111) surfaces are presented. Our calculations show that up to precoverages as high as 0.5 monolayer (ML) CO, the adsorption energy is minimally influenced by lateral interactions and that adsorption of additional CO molecules remains an unactivated process. The saturation coverage is estimated to be $\ensuremath{\sim}0.67$ ML. Beyond this limit the adsorption energies are strongly reduced and substantial barriers against further adsorption are built up. For the high-coverage limit, we have examined several adsorption geometries proposed in the literature as well as a novel configuration. Energetic considerations, the calculated adsorption geometries, and the analysis of the calculated frequency spectra all favor a model with the $c(\sqrt{3}\ifmmode\times\else\texttimes\fi{}3)$ periodicity as proposed on the basis of the experimental data.

Density functional study of the adsorption of CO on Fe( [formula omitted

Density functional studies for the clean Fe(1 1 0) surface and for the adsorption of CO on Fe(1 1 0) at a half and a quarter monolayer CO coverage are reported. For the clean surface, we find only a modest surface relaxation and a modest increase of the magnetic moment of the first surface layer. At low CO coverage, CO adsorbs on top of Fe. But at half coverage, the density functional calculations predict that an off symmetry position between the hollow and bridge site results in the largest binding energy. Comparison of the vibrational frequencies with experiment however suggest, that the experimentally observed situation corresponds to atop adsorption at both coverage. Possible reasons for the discrepancy are discussed.

Determination of the Platinum and Ruthenium Surface Areas in Platinum−Ruthenium Alloy Electrocatalysts by Underpotential Deposition of Copper. I. Unsupported Catalysts

Underpotential deposition (upd) of copper has been used to characterize platinum, ruthenium, and platinum−ruthenium high-surface-area unsupported (powder black) electrocatalysts. The surface areas thus obtained compare favorably with those determined by the more conventional electrochemical methods of monolayer CO and hydrogen oxidation. The differing adsorption energies for Cu on either Pt or Ru allow the peaks for upd copper deposited on alloy Pt−Ru to be resolved into their constituent components. Thus, in addition to the surface area, the surface composition of the Pt−Ru electrocatalyst can be determined. This approach distinguishes between bare ruthenium (i.e., metallic) and oxidized ruthenium sites as the upd copper does not deposit on the latter. The ruthenium surface area is found to remain high up to 0.45 V (vs RHE) and then to fall linearly with potential. Polarization at high potentials [1.45 V (vs RHE)] leaves a material in which metallic ruthenium cannot be recovered by electrochemical reduct...

Dissociation of CO on the Ru(1 1 [formula omitted] 0) surface

The dissociation of CO on the Ru(1 1 2 ̄ 0) surface was investigated by thermal desorption spectroscopy and high resolution electron energy loss spectroscopy (HREELS). Up to about 20% of a monolayer CO dissociates on the Ru(1 1 2 ̄ 0) surface at room temperature. Oxygen (68 meV) and carbon (42 meV) are identified as dissociation products by HREELS. At temperatures below 240 K the precursor of dissociation can be stabilized. In this state, CO is tilted and presumably bonded at a kind of a fourfold hollow site. During annealing, CO 2 formation is observed from CO reacting with oxygen from the dissociation.

Davydov and site symmetry splitting in polarized infrared spectra of ordered monolayers. A quantitative theoretical analysis of vibrational frequencies and integrated absorptions for CO 2 on NaCl(100)

In the polarized infrared (IR) spectra of ordered physisorbates on insulator single crystal surfaces we observed absorption multiplets, which were assigned to correlation field or Davydov splittings, in some cases combined with site symmetry splittings. In our analysis of the infrared absorptions, based on the classical theory of dynamic dipole coupling, a two-layer model is used. The electric field of the incident light at the surface is dependent on the refractive index of the substrate and the angles of incidence and polarization, as manifested in the Fresnel equations. Analytical expressions are presented for the vibrational frequencies and the s- and p-polarized integrated absorptions of the correlation field and site symmetry multiplets for unit cells with two molecules and the two-dimensional space group pg. Many adsorbates belong to this class. The expressions presented are useful in determining the orientational order of adsorbates. As an example the monolayer CO 2 on NaCl(100) is treated, and the calculated data are compared with the measured spectra.

The physisorption of CO 2 and the stepwise chemisorption of SO 2 on the CsF(100) single crystal surface

The adsorption of CO 2 and SO 2 on the (100) surface of the extremely basic and hygroscopic insulator CsF, prepared in situ by cleaving under UHV, was studied for the first time using high-resolution polarization infrared surface spectroscopy (PIRS) and low energy electron diffraction (LEED). The polarized infrared spectra of the asymmetric stretching vibration ν 3 of the monolayer CO 2CsF(100), prepared at 80 K, show a correlation field doublet, those of the bending vibration ν 2 a crystal field doublet. The LEED pattern reveals a ( 2 × 2 ) R45° superstructure with two glide planes (symmetry pgg). Comparison of measured with computed spectra allows the determination of the adsorbate orientational ordering. The tilt angle of the molecules is 5°±5°; the intermolecular azimuth angle is about 170°. The adsorption behavior of SO 2 is completely different, showing three phases. At 101 K and 1.3×10 −9 mbar up to a dosage of ~7 L, phase I grows. Starting at 5–6 L, phase II replaces phase I, characterized by spectral lines identified as the symmetric and asymmetric OSO-stretching mode of the fluorosulfinate anion (SO 2F −). Exposure of more than 20 L results in the additional formation of phase III, which was found to be physisorbed SO 2. These results could be relevant to applications in the widespread catalysis of organic synthesis by CsF and in the effective adsorption of the pollutant SO 2 in smoke gases.

Structural and vibrational properties of carbon monoxide adlayers on the copper (001) surface

The structure and vibrational states of a prototype adsorbate-substrate system—carbon monoxide on the copper (001) surface—have been calculated from first principles within local density functional theory. Three CO coverages have been examined: θ=0 (bare surface), 0.5, and 1. These systems are represented by a well converged slab model within which all atomic degrees of freedom are treated on an equal footing. The computed structural relaxations and vibrational frequencies are generally in excellent quantitative agreement with the available experimental measurements. The full monolayer is found to be energetically favorable to the half monolayer plus free CO molecule. This indicates that the maximum stable coverage is greater than θ=0.5, in agreement with experiment. The vibrational analysis reveals that resonant coupling between adsorbate and substrate motions has a profound effect on the vibrational spectra, for example, the low-frequency, in-plane frustrated translational motion of the CO molecules mixes with long-wavelength copper phonons to form a broad resonance peak. This implies a finite lifetime which, for the half-monolayer system, is computed to be 3.0 ps, in excellent agreement with the measured value of 2.3±0.4 ps. For the full-monolayer system, the predicted lifetime is 0.7 ps; however this system is presently inaccessible to experiment. Resonant coupling is also found to affect the Rayleigh wave of the copper (001) surface. At half-monolayer CO coverage, this mode resonantly mixes with bulk copper phonons developing a finite lifetime, which is predicted to be 5.2 ps. To our knowledge, the lifetime of this mode has never been measured. For the fully covered surface, the Rayleigh wave does not form a resonance because the phonon coupling is forbidden by symmetry.

High-resolution PIRSS using a tunable diode laser: the multiple of the collective v2 bending vibration of the p(2 × 1) monolayer CO 2 adsorbed on NaCl(001)

Polarization infrared surface spectroscopy (PIRSS) has been used to investigate the multiplet of the collective v 2 bending vibration of the p(2 × 1) monolayer CO 2 adsorbed on NaCl(001) in the spectral range 650–665 cm −1 at temperatures between 10 and 90 K. Tunable diode laser spectroscopy with a resolution of at least 5 × 10 −3cm −1 appears to be a powerful method to study the lineshape of extremely narrow absorptions observed for highly ordered adsorbates on insulator single crystal surfaces. The experiments have been performed in transmission geometry with a lead salt laser diode. Four polarized absorptions are expected in the spectral range of the bending vibration of CO 2 adsorbed on NaCl(001) at monolayer coverage due to the combination of site symmetry and correlation field splitting. They have been observed and studied in detail using four different laser modes. The continuously tunable range of a single mode is limited to about 3 cm −1. All spectra have been recorded as a function of temperature. In the low-temperature limit, linewidths down to 0.04 cm −1 have been observed, the bands being clearly asymmetric. One of the observed quartet components shows an unexpected doublet splitting under the extremely high resolution employed.

High-resolution PIRSS using a tunable infrared diode laser: the Davydov doublet of the antiferroelectric (2 × 1) monolayer CO adsorbed on NaCl(001)

The temperature dependence of the Davydov doublet of the antiferroelectric (2 × 1) monolayer CONaCl(001) has been studied between 11 and 25 K by means of polarization infrared surface spectroscopy (PIRSS) using a tunable lead salt diode laser with a resolution better than 5 × 10 −3cm −1. At 11 K the linewidth of the symmetric vibrational exciton, polarized perpendicular to the surface, was determined to be 0.035 cm −1, the linewidth of the antisymmetric vibrational exciton, polarized parallel to the surface, to be 0.025 cm −1, the sharpest spectral line of an adsorbate observed. To obtain phase relaxation times T 2 in the low-temperature region, we fitted the parameters of the exchange dephasing model to the experimental data. A satisfactory fit for both the frequency and the linewidth is obtained at temperatures below 25 K. Above 25 K the antisymmetric vibrational exciton becomes extremely broad and finally disappears, and a successful fit was obtained only for the linewidth of the symmetric vibrational exciton. It is shown how the vibrational energy relaxation time T 1 may be obtained by varying the spectral power of the laser light.

The adsorption of CO 2 and N 2O on the MgO(001) single crystal surface: a comparative PIRSS and LEED study

The adsorption of the isoelectronic molecules CO 2 and, for the first time, N 2O on the MgO(001) surface has been studied at 80 K using high-resolution polarization infrared surface spectroscopy (PIRSS) and low energy electron diffraction (LEED). High-quality surfaces were prepared by cleaving a single crystal in situ under UHV. At 80 K for the monolayer N 2OMgO(001) a (2√2 × √2)R45° superstructure with one glide plane parallel to the long side of the rectangular unit cell was found by LEED. The corresponding infrared spectra of the asymmetric stretching vibration v 3 are characterized by three sharp, polarization dependent absorptions. Two of them are attributed to a correlation field splitting in the ordered layer. Line B at 2217.0 cm −1 is assigned to the antisymmetric collective vibration of the monolayer. Its dynamical dipole moment is oriented parallel to the surface and perpendicular to the projection of the dynamical dipole of line A at 2229.6 cm −1. Therefore the symmetric collective vibration is attributed to line A, its dynamical dipole moment being inclined relative to the surface. The origin of the third absorption (C) at 2216.1 cm −1 is not yet understood. The same superstructure is observed for the monolayer CO 2MgO(001). The polarized infrared spectra in the v 3 spectral range are similar for both adsorbates, two lines being due to a correlation field splitting.

Vibrational Stark effect of CO on NaCl(100)

The effect of local electric field at the NaCl(100) surface on the vibrational spectrum of adsorbed (vs free) CO is calculated and compared with previous experiments. We consider the change in both the vibrational frequency ν and the integrated IR absorption S. The calculation is based on the known molecular properties of free CO and uses S at F=0 as calculated earlier by Poppe, Wijers, and van Silfhout [Phys. Rev. B 44, 7917 (1991)]. We show how to account for the position dependence of F along the molecular axis in calculating ν and S. This is important for the NaCl(100) surface since F varies strongly in the empty site where the CO adsorbs. The ratio of calculated to observed Δν in the limit of low coverage is 0.98±0.17; the ratio of calculated to observed S with monolayer CO coverage is 1.10±0.12. The calculated effects agree with the data for CO on NaCl(100). We also discuss how the vibrational Stark effect may apply to CO on MgO(100) and at cation sites in zeolites.

The monolayer CO adsorbed on MgO(100) detected by polarization infrared spectroscopy

The adsorption of carbon monoxide on the MgO(100) surface, prepared in situ by cleaving a single crystal in ultra high vacuum, has been studied at temperatures between 32 and 56 K using polarization Fourier transform infrared spectroscopy. Above 45 K a single IR absorption at 2150.5 cm −1 is detected with p-polarization only which indicates a dynamic CO dipole perpendicular to the surface. Below 45 K the spectra of the monolayer are completely different. Using p-polarization the main IR absorption is detected at 2151.2 cm −1. Two additional features are observed at 2137.2 and at 2132.5 cm −1, only the latter is also observed with s-polarization. The spectrum indicates an ordered monolayer with two energetically different adsorption sites per unit cell. A c(4 × 2) symmetry is inferred in concert with helium atom scattering [Gerlach et al., Surf. Sci. 331–333 (1995) 1490]; from LEED [Audibert et al., Surf. Sci. 273(1992) L467; Panella et al., J. Phys. Paris 4 (1994) 905] a (4 × 2) symmetry was derived. The p-polarized absorption at 2151.2 cm −1 is assigned to a dynamic CO dipole perpendicular to the surface, the doublet is attributed to the in-phase and the out-of-phase vibrational exciton of two tilted CO molecules at energetically equivalent sites. Their projections on the surface are oriented antiparallel, the dipole of the in-phase exciton at 2137.2 cm −1 is perpendicular to the surface and has no azimuthal component, the dipole of the out-of-phase exciton at 2132.5 cm −1 is parallel to the surface. A weak prevalence of one domain is inferred from the spectra.