Low CO Coverage Research Articles

Microscopic CO diffusion on a Pt(111) surface by time-resolved infrared spectroscopy.

A new technique, combining a pulsed supersonic molecular-beam source and time-resolved surface infrared spectroscopy, has been devised to measure the microscopic diffusion of molecules at surfaces. Following a rapid dose of a periodically stepped Pt(111) crystal face to a low total coverage of CO (\ensuremath{\sim}0.006 monolayer), the time evolution of CO molecules migrating from terrace to step sites was monitored with a temporal resolution as fast as 5 msec. When compared with a simple kinetic model, the data reveal statistical microscopic hopping rates for CO diffusion on the (111) plane over the $T=95\ensuremath{-}195$ K range investigated.

The adsorption and desorption of hydrogen and carbon monoxide on bimetallic RePt(111) surfaces

Bimetallic surfaces of rhenium on Pt(111) were prepared by vapor depositing rhenium on Pt(111). The adsorption and desorption behavior of CO and H 2 on bimetallic PtRe surfaces were studied using temperature programmed desorption, and compared to the behavior of CO and H 2 on the monometallic Pt(111) and Re(0001) counterparts. Depositing rhenium on a Pt(111) surface decreased the activation energy of desorption of hydrogen, and a surface composed of 0.37 monolayers of rhenium on Pt(111) showed an activation energy of hydrogen desorption that was 2.5 kcal mol less than the 19 kcal mol displayed by the monometallic Pt(111) surface. In contrast, the activation energy of desorption of CO from bimetallic surfaces depended very little on the bimetallic surface composition, and a value of 27 kcal mol was observed for low CO coverages. At saturation exposures of both H 2 and CO, maximum adsorption capacities were obtained for bimetallic surfaces. A surface composed of 0.2 monolayers of rhenium on Pt(111) adsorbed 20% more hydrogen than did Pt(111) alone, while a surface composed of 0.3 monolayers of rhenium on Pt(111) adsorbed 40% more CO than did Pt(111) alone. The results obtained show that surfaces exposing both rhenium and platinum atoms show adsorption/desorption behavior towards hydrogen and CO that is different than the behavior shown by either monometallic platinum or rhenium surfaces. Since the chemisorption behavior cannot be explained as a simple combination of the two metallic components of the surface, it is concluded that an electronic interaction between the two metals at the platinum-rhenium interface modifies the bonding of adsorbates at the mixed metal sites.

Laser-induced defects on a Ni(100) surface: Effects on adsorption and surface diffusion of CO

Pulsed-laser-induced desorption (LID), a method which has been used to measure surface diffusion, can cause changes in the morphology of the surface being studied. The object of this paper is to investigate the influence of laser-induced surface defects on the adsorption and diffusion of CO on Ni(100). The defects were produced by illuminating a Ni(100) surface with 73 MW/cm 2 of 1.06 microm radiation. Subsequent exposure of the surface to CO resulted in about 10% more adsorbed CO than without the laser-induced defects. The CO coverage on the laser pre-irradiated surface was further enhanced relative to the unirradiated surface by annealing the CO overlayer at 310 K. However, the degree of enhancement was dependent on the initial CO coverage. This increase in the local CO surface coverage was modelled by the presence of laser-induced surface defects which tightly bind the diffusing CO. From the coverage dependence of this phenomenon, we estimate that 2.4 ± 1% of the surface sites are tight binding defects. These defects have the greatest influence on LID diffusion coefficient measurements at low CO coverages where we estimate that the results may be in error by approximately a factor of 2 if one assumes an asymptotic limit equal to the initial coverage rather than experimentally determining the actual asymptotic coverage.

Effect of Rh particle size on CO desorption from Rh/alumina model catalysts

The adsorption of CO on small Rh particles on oxidized Al(100) was studied using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). The desorption data for CO were obtained for two Rh Al 2O 3 samples as well as for Rh(111). The supported Rh model catalysts were prepared from thermal decomposition of [Rh(CO 2Cl] 2 on an oxidized Al(100) substrate. By varying the substrate temperature and the amount of deposited Rh, samples were prepared with average Rh particles sizes of 20 and 70 Å. TPD data from the 70 Å Rh particles were similar to that from Rh(111), with a major peak at 500 K and a shoulder at 400 K for a saturation CO exposure. TPD from the small particles was very different from Rh(111) and the larger particles, showing a single broad peak centered at 415 K. The data show a particle size effect for the desorption on CO from supported Rh. Redhead analysis of the data for first-order desorption gave an activation energy of 30 kcal/mol for all of the samples at low CO coverages. A more detailed analysis using peak widths and peak temperatures was also performed. These calculations gave a very low activation energy, 19.8 kcal/mol, and preexponential, 1×10 8 cm 2 s for the small particles. These very low values were interpreted to mean that multiple desorption states, mobile precursors, or coverage dependent activation energies were affecting the data from the small particles. Calculations of first-order desorption rates showed that desorption states different than those on Rh(111) are the dominant species on the small particles at high CO coverages.

Unexpected hydrogen induced displacement of chemisorbed CO from the Ni(100) surface

We report the first observation of hydrogen induced displacement of chemisorbed CO from the Ni (100) surface. This displacement is unexpected since the heat of adsorption for CO is 30 kcal/mol, about 7 kcal/mol larger than the 23 kcal/mol heat of adsorption for hydrogen. These displacement studies were performed in a UHV system equipped with Auger electron spectroscopy and facilties for temperature programmed desorption. Rates of displacement were measured by integrating CO temperature programmed desorption spectra for a series of displacement times. Hydrogen pressures in the 10−3 to 10−4 Torr range cause displacement of chemisorbed CO in the 290 to 330 K temperature range in a matter of minutes. After displacement, the surface contained only undisplaced CO and adsorbed hydrogen. No surface contamination was detected following CO temperature programmed desorption. The displacement reaction is clearly positive order in CO coverage. The CO coverage data suggests two first order reaction regions with a decrease in rate with decreasing coverage. Displacement is about half-order in hydrogen pressure. For low CO coverages (below 0.3 to 0.4 monolayer) and 309 K the displacement probability is around 5×10−7 per incident H2 molecule and has an activation energy of about 12±1 kcal/mol. At high coverages the displacement probability is about 1×10−6 per incident H2 molecule and the activation energy decreases to about 8±2 kcal/mol. No CO displacement was observed in the 270 to 330 K temperature range for pressures up to 10−3 Torr of He, Ne, CH4 or N2. The CO displacement rate is also insensitive to deuterium substitution. The thermal activation energies measured indicate that the metal surface is furnishing 8 and 12 kcal/mol for the displacement of high and low coverages of adsorbed CO, respectively. In order for adsorbed CO to be removed from the surface either the heat of desorption for a portion of the adsorbed CO must be decreased to match the thermal activation energies or direct energy transfer must be occurring during the displacement process. The current results do not allow us to distinguish between these two types of molecular mechanisms.

Vibrational phase relaxation at surfaces: The role of lateral interaction

We have studied the temperature and coverage dependence of the vibrational line shape of the C–O stretch vibration for CO on Ru(001). We find that the narrow IR line profile which is observed for the ordered structure of CO on Ru(001) is due to the strong dipole–dipole coupling and that for low CO coverage (or in the isotopic dilution limit), where the lateral interaction is effectively zero, the linewidth of the C–O stretch mode increases in accordance with theory. The implication of this result is briefly discussed.

Hydrophilic versus hydrophobic coadsorption: Carbon monoxide and water on Rh(111) versus Pt(111)

Carbon monoxide is a major poison to anodic reactions in aqueous fuel cells. To ascertain the effects of a controlled aqueous environment on the adsorption of CO on electrocatalytically active metals, the coadsorption of CO and water on Rh(111) and Pt(111) surfaces at 100 K was studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). On Rh(111) low coverages of CO shift monolayer water desorption from 182 to 207 K, indicating net attractive COH 2O interactions. Water shifts the CO stretching frequency from 2020 to 1620 cm −1, suggesting a displacement of CO from atop to three-fold hollow sites. A site shift is corroborated by XPS data. Concurrent changes in water vibrational features suggest formation of a mixed phase in which CO and water occupy adjacent sites. It has been hypothesized that such an adsorption geometry would facilitate the normally slow electrooxidation of CO and of fuels such as methanol. On Pt(111), in contrast, all coverages of CO decrease the desorption temperature of water, indicating net repulsive COH 2O interactions. Water splits the single 2110 cm −1 CO stretch of low coverages of CO into two peaks corresponding to the atop and bridging species also seen for higher coverages of CO on the water-free surface. This result and the lack of a change in water vibrational features suggest that on Pt(111) the coadsorbates separate into incompressible islands containing only water and compressible, internally repulsive, patches containing only CO. If the disparate behavior of Rh(111) and Pt(111) also occurs in room temperature aqueous solutions, comparison of their electrooxidation activity in the presence of CO could provide a conclusive test of whether or not rational provision of adjacent CO and water binding sites is likely to be a productive strategy for the development of improved electrooxidation catalysts.

IRAS study of the adsorption of CO on Ni(111): Interrelation between various bonding modes of chemisorbed CO

The IRAS (infrared reflection absorption spectroscopy) method has been applied to the study of CO adsorption on Ni(111). Evidence has been found for the coexistence of two-fold and three-fold coordinated CO species at low CO coverages. A continuous coverage-dependent conversion from three-fold to two-fold bridged CO has been observed. Accurate correlation of the development of the IRAS spectra with CO coverage, with CO temperature programmed desorption behavior, and with LEED behavior has been established through studies of the frequency and lineshape of the spectral features. Evidence for inhomogeneous CO layers at the highest CO coverages was obtained. This work establishes baseline behavior for other IRAS studies of this system to be reported.

Binding and orientations of CO on Fe(110), (100), and (111): A surface structure effect from molecular orbital theory

Calculations based on the atom superposition and electron delocalization molecular orbital method show that at low coverage CO binds on the high-coordinate site in the perpendicular orientation on Fe(110), on the high-coordinate site in the lying-down orientation on Fe(100), and on the di-σ bridging site in the lying-down orientation on Fe(111). These findings confirm the earlier suggestions regarding CO binding site and orientation on the (110) and (100) surfaces. However, they do not support the previously proposed deep-hollow binding site for CO on the (111) surface. The differences for thee favored CO binding site and orientation on Fe(110), (100), and (111) surfaces are explained on the basis of surface-atom coordinations and atom-atom spacings. In the favored lying-down CO orientations on the (100) and (111) surfaces, 4σ and π donation interactions coupled with the familiar 5σ donation to the surfaces, and back-donation to the CO π ∗ orbitals are responsible for binding to the surfaces.

Amplitude anisotropy of the bending vibration of Co adsorbed on Ni(110)

The polar angle distribution of C 1s photoelectron intensity ( hv = 1253.6 eV) of CO adsorbed on Ni(110) is characterized by a strong forward scattering peak. This peak was measured at 120 K for low coverage CO and at 300 K for a range of coverages in both the [ 1 10 ] and [001] azimuths. The full width at half maximum of this peak was found to vary with temperature as well as with azimuth. The latter illustrates the amplitude anisotropy of the NiCO bending vibration. The vibrational potential exhibits a larger force constant in [ 1 10 ] than in [001] direction as expected.

Vibrational phase relaxation at surfaces: The role of lateral interaction

We have studied the temperature and coverage dependence of the vibrational lineshape of the C-O stretch vibration for CO on Ru(001). We find that the narrow IR-lineprofile which is observed for the ordered structure of CO on Ru(001) is due to the strong dipole-dipole coupling and that for low CO-coverage (or in the isotopic dilution limit), where the lateral interaction is effectively zero, the linewidth of the C-O stretch mode increases in accordance with theory. The implication of this result is briefly discussed.

The co-adsorption of H 2 and CO on Ni(110)

The effect of co-adsorption of hydrogen and CO on Ni(110) has been examined with HREELS. In the mixed adlayer at low CO and hydrogen coverage, the spectra indicate that no strong interaction between H (a) and CO (a) occurs. At high hydrogen coverage the surface reconstructs and consequently the adsorption site for the CO is modified. The reconstruction and site modification is reversible and depends on the hydrogen coverage.

Spectroscopic detection of (111) facets on supported Pd crystallites: Site blocking by ethylidyne on Pd/Al 2O 3

Transmission IR spectroscopy has been used to study very low CO coverages on Pd/Al 2O 3 surfaces. It has been shown that on clean Pd surfaces, the first CO molecules adsorbed are adsorbed on (111) facets, as evidenced by the close agreement of CO vibrational frequency with those observed for 3-fold bridging and 2-fold bridging CO species on Pd(111). In distinction to this behavior, when the Pd surface is precovered with ethylidyne (CCH 3), a species thought to form only in 3-fold sites, the IR features for low coverages of CO no longer correspond to those for CO adsorption on Pd(111). A model is proposed in which CCH 3 effectively blocks all sites for CO adsorption on (111) facets on Pd crystallites. CO adsorption isotherms indicate that these (111) facets comprise 30 ± 10% of the total Pd crystallite surface.

Spectroscopic detection of (111) facets on supported Pd crystallites: site blocking by ethylidyne on Pd/Al 2O 2

Transmission IR spectroscopy has been used to study very low CO coverages on Pd/Al 2O 3 surfaces. It has been shown that on clean Pd surfaces, the first CO molecules adsorbed are adsorbed on (111) facets, as evidenced by the close agreement of CO vibrational frequency with those observed for 3-fold bridging and 2-fold bridging CO species on Pd(111). In distinction to this behavior, when the Pd surface is precovered with ethylidyne (≡CCH 3), a species thought to form only in 3-fold sites, the IR features for low coverages of CO no longer correspond to those for CO adsorption on Pd(111). A model is proposed in which ≡CCH 3 effectively blocks all sites for CO adsorption on (111) facets on Pd crystallites. CO adsorption isotherms indicate that these (111) facets comprise 30±10% of the total Pd crystallite surface.

The coadsorption of nitrogen with carbon monoxide and oxygen on the Ru(001) surface: Local chemical interactions in mixed overlayers

High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. Disordered oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of ordered oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.

Infrared reflectance absorbance spectroscopy of coadsorbed CO and H2O on Pt(111) surfaces

We have used polarization modulated infrared reflectance absorbance spectroscopy (IRRAS) to study the interactions between coadsorbed CO and H2O on Pt(111) surfaces at 150 K. Two types of behavior are observed, depending on CO coverage. For CO coverages greater than θ≊1/2, the vibrational band for linear CO species shifts from 2100 cm−1 for an H2O-free surface to 2095 cm−1 for H2O exposures up to 15 L. For larger H2O exposures a second, broader band appears at around 2080 cm−1. These results indicate that H2O adsorbs in clusters on top of a saturated CO adlayer, without strongly perturbing the CO molecules underneath. In contrast, for CO coverages less than θ≊1/4, adsorption of H2O at 150 K results in the disappearance of the linear CO vibrational band, with its subsequent reappearance upon desorbing the H2O layer. This indicates that for low CO coverages, coadsorbed H2O can displace CO out of its preferred linear coordination mode.

Infrared Absorption Spectra of CO Adsorbed on Cu-Ru(001)

ABSTRACTThe electronic properties and dispersion of copper layers on Ru(001) are investigated utilizing high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe. Adsorption of CO on an annealed, epitaxial copper monolayer reveals that copper is modified by the underlying substrate. Both the C-O stretching frequency at low CO coverage and its shift upon increase in coverage indicate stronger backdonation than for thick copper films which exhibit Cu(lll) behavior. At submonolayer coverages (0.25 < θCu < 1.0), copper forms islands on Ru(001) which behave similar to the copper monolayer. Rough copper films evaporated at 85K exhibit a drastically different behavior upon CO adsorption. Vibrational spectra allow us to distinguish 2-dimensional and 3-dimensional copper aggregates of random orientation as well as clusters of small size (1–3 atoms). Annealing of a θCu = 1.0 layer leads to 2- and 3-dimensional ordering at 250K and to complete surface wetting of the copper monolayer at 350K to 540K.

Interactions of CO + K on Ru(001): Structure and bonding

Recent studies of CO + K on Ru(001) revealed an anomalously low CO streaching frequency of 1460 cm −1 for low CO and K coverages. Hoffmann and de Paola proposed a side-on bound molecule with the CO molecular axis parallel to the metal surface: Weimer and Umbach suggested that CO is bound perpendicular to the surface, as on clean Ru(001), but that the CO is sp 2 hybridized in the presence of K(ads). The main objectives of the present work were to search for configurational changes of adsorbed CO on K + Ru(001) and to compare these results with CO + O(ad). We used the ESDIAD (electron stimulated desorption ion angular distribution) method in combination with LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The ESDIAD data for low coverages ( θ K × 0.1) of CO + K on Ru(001) show that the O + ESD signal from CO is suppressed by coadsorption with K; this can arise from a reorientation of the CO molecular axis, from perpendicular to inclined, or from increased reneutralization rates due to coadsorption with K. TDS provides evidence for strong interaction between CO and K, and a combination of LEED and ESD measurements suggest the formation of ordered species having K x (CO) 2 x stoichiometry.

Studies on self-sustained reaction-rate oscillations: I. Real-time surface infrared measurements during oscillatory oxidation of carbon monoxide on platinum

Time-resolved infrared absorption features in the 1800–2400 cm −1 region during a typical cycle in the oscillatory oxidation of CO over a platinum foil were obtained by Fourier transform infrared reflection absorption spectroscopy. Pretreatment of the foil in an oxidizing environment at high temperatures was found to be necessary to induce large-amplitude, stable oscillations. The oscillations are approximately square-wave in shape, with a high and a low reaction-rate branch. The level of chemisorbed CO in the high reaction-rate branch is typically below the noise level, while in the low reaction-rate branch substantial substantial surface coverages of CO can be observed. No evidence for CO bridge-bonded to the platinum substrate or chemisorbed in the presence of a subsurface Pt oxide could be found at any time during the oscillation cycle. Evidence is presented for the existence of CO islands in the low reaction-rate branch. It is also shown that the low reaction rate realized in this branch is not due to blocking of the surface by chemisorbed CO.

CO poisoning of [formula omitted]: Structural effects on the hydrogenolysis of methylcyclopropane

The hydrogenolysis of methylcyclopropane has been used as a probe reaction to investigate the structure sensitivity of the deactivation of a series of well-characterized PtAl2O3 catalysts by carbon monoxide.After a standardized reduction procedure, the catalysts were prepoisoned by pulse chemisorption of carbon monoxide at 298 K to varying levels of surface coverage, then the hydrogenolysis was carried out under differential conversion conditions at 273 K. Results were obtained in terms of reaction turnover frequency and selectivity as a function of percentage of metal exposed (Dh) and surface coverage of carbon monoxide. The general pattern of structure sensitivity for this reaction on PtAi2O3 was not altered substantially upon progressive poisoning, although the change in absolute magnitude of turnover frequency was larger for low-Dh catalysts than for high-Dh catalysts. Product selectivity between isobutane and n-butane was not affected. A comparison with prior results obtained for a similar series of PtSiO2 (Önal, I. and Butt, J. B., J. Chem. Soc. Faraday Trans, 178, 1887, 1982) is given. Comparison of prepoisoning from COAr pulses with that from COH2 pulses indicate differences for high-Dh catalysts. Thus there may be differences in apparent poisoning behavior as indicated by prepoisoning experiments compared to continuous introduction of poison with the reactants. Comparison of the mode of prepoisoning, pulse chemisorption vs thermal desorption from a monolayer, indicated generally similar results for the two methods. Some discrepancies appearing at low coverages of CO may be due to reordering of the adlayer in the thermal desorption treatment.