Lateral displacement by transient mobility in chemisorption of CO on Pt(997)

We investigated adsorbed states of CO on Pt(997) at 11 K using infrared reflection absorption spectroscopy. At 11 K, thermal migration is suppressed and thus the initial chemisorption at terrace sites and step sites is controlled by the transient mobility of the adsorbing molecule. The initial occupation ratio between atop CO on the terrace and atop CO at the step is directly determined to be 3.6:1. With a simple isotropic migration model, we estimated the mean lateral displacement from the first impact point to the initial chemisorption site to be 6.8 A. We also discuss the origin of transient mobility of CO on metal surfaces.     

The dissimilar twins - a comparative, site-selective in situ study of CO adsorption and desorption on Pt(3 2 2) and Pt(3 5 5)

The adsorption and desorption of CO on stepped Pt(3 2 2) = Pt(S)-[5(1 1 1) × (1 0 0)] and Pt(3 5 5) = Pt(S)-[5(1 1 1) × (1 1 1)] were investigated using in situ high-resolution X-ray photoelectron spectroscopy at BESSY II, which allows to clearly distinguish between different step and terrace adsorption sites. For the two surfaces, with the same nominal terrace width of five atomic rows, but different step orientation, significant differences are observed. While for Pt(3 5 5) CO adsorption at steps only occurs at on-top sites, on Pt(3 2 2) both step on-top and bridge sites are occupied, albeit with a significantly lower coverage (0.07 vs. 0.13 ML at 200 K). On both surfaces terrace sites are only occupied when the step sites are almost saturated confirming the enhanced binding energy at step sites. CO adsorbed at the (1 1 1) steps on Pt(3 5 5) is more strongly bound than on the (1 0 0) steps on Pt(3 2 2), which is attributed to the different electronic and geometric structure of the steps. The relative occupation of terrace and step sites at a given coverage remains the same between 120 and 290 K on Pt(3 5 5) K, but shows major changes on Pt(3 2 2), between step on-top and bridge sites as well as terrace on-top and bridge sites. On Pt(3 5 5) a smaller CO terrace coverage is found (0.36 vs. 0.40 ML on Pt(3 2 2) at 200 K), mainly due to the lower occupation of terrace bridge sites. For Pt(3 2 2), an ordered adsorbate phase is deduced from a c(4 × 2)-like LEED pattern, which indicates adsorbate order beyond the extension of a single terrace. A model for this structure is proposed.     

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm⁻¹ for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

The influence of crystallographic steps on coadsorption: CO---O/Ni(119)

On low index nickel surfaces, repulsive interactions between atomic oxygen and CO drive the phase separation of these species into oxygen-rich and CO-rich islands. Because these adsorbates interact differently with crystallographic steps, the size and the structure of these islands are modified on stepped surfaces. We have monitored coadsorption-induced changes in the distribution of CO with IRRAS, observing six different CO stretching bands which are assigned to distinct local chemisorption environments. When oxygen fully saturates sites along the step edge, the steps are completely blocked from CO adsorption and virtually all the CO population on the terraces shifts from atop to bridge sites. This terrace site shift is similarly accomplished by atomic oxygen chemisorbed at terrace sites. From these coadsorption-induced changes in CO site distributions, constrained by the 10 Å terrace width, we conclude the through-metal O---CO interaction responsible for this CO site shift must be operative over a range of 5 Å. At θ_o = 0.18 ML, when oxygen occupies, but does not fully saturate the step edge, a new CO adsorption site is created, with a characteristic frequency of 1750 cm⁻¹. This new site is assigned to CO bonded to kinks along the step edge based upon its low intensity ( geometric kink density), enhanced binding strength and sensitivity to oxygen coverage. At higher oxygen coverages, compression of the CO adlayer is observed, with CO shifting to asymmetric bridge sites. As saturation coverage is approached, CO occupies weakly bound sites in close proximity ( 3 Å) to O adatoms, with high characteristic frequencies of 2100 cm⁻¹.     

Frequency shifts in the spectra of molecules adsorbed on metals,with emphasis on the infrared spectrum of adsorbed CO

Two processes are considered which cause the frequency of the band maximum in the infrared absorption spectrum of molecules adsorbed on metals to shift with increasing coverage. The first arises from the coverage-dependent modification of the local field acting on an adsorbed molecule while the second originates from vibrational coupling arising from a through-metal interaction between molecules adsorbed on different metal atoms. Infrared absorption strengths are discussed and the method used heretofore to obtain the extinction coefficients of adsorbed molecules is questioned. In particular, a factor of 2 which results from the constructive superposition of the incident and reflected electric fields is shown to be often ignored. We show that dipolar (through-space) coupling is too weak to account for the magnitudes of the shifts recently observed for CO on single-crystal Pt and Pd as coverage progressed from low to high values. Vibrational coupling is also shown to account for the disparity in the intensities of the two bands observed when ¹²CO and ¹³CO are coadsorbed on supported metals. Similar experiments on single-crystal surfaces are suggested to help determine to what extent the observed spectral frequency shifts are derived from adsorbate—adsorbate coupling as opposed to other mechanisms.     

Effect of the electronic structure of a substrate on the photoinduced behavior of adsorbed NO and CO molecules

Photoprocesses in systems produced by adsorption of NO and CO molecules on the Pt(111) and Ni(111) surfaces, as well as on the (111) surface of Pt-Ge alloy, is studied by the IR absorption spectroscopy, resonant multiphoton ionization, and UV photoelectron spectroscopy methods. The energy of photons varies between 2.3 and 6.4 eV. The character of the processes depends on the type of the metallic substrate. On the Pt(111) surface, NO molecules dissociate or are desorbed, depending on the degree of coverage. On the Ni(111) surface, the molecules only dissociate. Conversely, NO molecules adsorbed on the (111) surface of the Pt-Ge alloy are only desorbed from the surface. In the CO/Pt(111) and CO/Pt(111)-Ge systems, CO molecules adsorbed on on-top adsorption sites are desorbed under the action of the photons, while those occupying bridging adsorption sites change their properties insignificantly. A model of photoinduced processes is suggested. According to this model, the lifetime of a state excited by charge transfer between the valence band of the metal and the 2π-antibonding molecular orbital plays a decisive part in the occurrence of one or the other of these processes.     

Adsorption states of NO on the Pt(1 1 1) step surface

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N-O stretching modes at 1490 cm⁻¹, 1631 cm⁻¹ and 1700 cm⁻¹ at 0.2 ML. The 1490 cm⁻¹ and 1700 cm⁻¹ peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm⁻¹ peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm⁻¹. The interactions between NO species at different sites on Pt(9 9 7) are also discussed.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.     

Field enhancement at crystal steps: An intensity effect in vibrational spectra of adsorbed molecules

An enhancement of the electric field near a step in a single-crystal metal surface could give rise to an increase in the molar extinction coefficient for molecules adsorbed on the step. For CO adsorbed on a stepped surface. Pt(S)-[4(111)×(100)]. we calculate the extinction coefficient to be about 2.2 times greater for molecules adsorbed on the step than for molecules adsorbed on the middle of the terrace.     

Collision-induced migration of CO on Pt(9 9 7) surface

Lateral displacement of adsorbates induced by collisions with energy-controlled rare gas atoms was examined in an ultra high vacuum chamber using Fourier-transform infrared (FTIR) spectroscopy and a supersonic molecular beam apparatus. A stepped Pt(9 9 7) surface was exposed to CO molecules and subsequently to energy-controlled Ne or Ar atoms. There was no change in the CO stretching mode region of the FTIR spectrum of the Pt(9 9 7) surface after Ne atoms having an average translational energy of 0.23 eV were collided with it. However, when Ne atoms having an average translational energy of 0.56 eV were collided with the surface, the intensity of the peak assigned to the CO stretching mode at terrace sites decreased, while that at step sites increased with increasing the exposure to the Ne atoms. This is the demonstration of collision-induced migration, showing that CO molecules adsorbed at the terrace sites migrate laterally to the step sites upon collision with high-energy Ne atoms. In addition, the experimental results demonstrate the existence of an additional energy barrier for jumps across the steps. This investigation demonstrates an advantage of using a molecular beam for studying adsorbate migration.     

Adsorption and decomposition of NO on Pt (112)

Adsorption and decomposition of NO on Pt (1 1 2) have been studied by temperature programmed desorption (TPD), ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). NO adsorbs molecularly on Pt (1 1 2) at 95 K. About half amount of NO molecules adsorbs at the terrace sites and remaining half amount adsorbs at the step sites at a full monolayer coverage. Then about half of NO molecules adsorbed at step sites decomposes at around 483 K desorbing N₂, promptly.     

Site occupation of CO adsorbed on Ni(100) at high CO pressures

The adsorption of CO on Ni(100) has been investigated by infrared reflection-absorption spectroscopy in the temperature range of 85 to 300 K. At 300 K we achieved CO coverages up to 0.8 ML by applying CO pressures up to 1 mbar. The fraction of CO molecules adsorbed at terminal and bridge sites on Ni(100) at a fixed temperature depends strongly on the total CO coverage. The variation of the adsorption site occupancy with coverage is well reproduced by a lattice-gas model which takes into account the lateral interaction of the adsorbed molecules.     

Search for adsorption potential energy minima of NO on Pt(9 9 7) at 11 K

We observed four kinds of adsorbed NO molecules on Pt(9 9 7) at 11 K using infrared reflection absorption spectroscopy (IRAS). The peaks at 1690, 1484 and 1615 cm⁻¹ are assigned to the N-O stretching modes of the on-top site and the hollow site on the terrace and the bridge site at the step, respectively. The 1385 cm⁻¹ peak is observed below ∼70 K. We assign the 1385 cm⁻¹ peak to the hollow site of the (1 1 1) microfacet at the step or the lower-terrace hollow site nearest to the step. By heating, site-to-site hopping to the more stable site occurs and the relative stability of four adsorption sites can be determined.     

Evidence for two different adsorption sites of CO on Pt(111) from infrared reflection spectroscopy

The adsorption of CO on Pt(111) surfaces has been studied under clean conditions by a highly surface sensitive double-beam infrared reflection spectroscopy (IRS). In contrast to results of other authors two stretching vibrations of adsorbed CO rather than one are detected near 2100cm⁻¹ and 1870cm⁻¹. This is in agreement with recent findings in high-resolution electron energy loss spectroscopy (ELS). The results are discussed in terms of two adsorption sites: CO adsorbed in on-top positions and double coordinated on bridging sites, respectively. Furthermore, a precursor state and a preferential adsorption in islands at low coverage is taken into account.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Adsorption properties of CO on low-index Pt3Sn surfaces

The adsorption properties of CO on Pt₃Sn were investigated by utilizing quantum mechanical calculations. The (1 1 1), (1 1 0) and (0 0 1) surfaces of Pt₃Sn were generated with all possible bulk terminations, and on these terminations all types of active sites were determined. The adsorption energies and the geometries of the CO molecule at those sites were found. Those results were compared with the results obtained from the adsorption of CO on similar sites of Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) surfaces. The comparison reveals that adsorption of CO is stronger on Pt surfaces; this may be the reason why catalysts with Pt₃Sn phase do not suffer from CO posioning in experimental works. Aiming to understand the interactions between CO and the metal adsorption sites in detail, the local density of states (LDOS) profiles were produced for atop-Pt adsorption, both for the carbon end of CO for its adsorbed and free states, and for the Pt atom of the binding site. LDOS profiles of C of free and adsorbed CO and Pt for corresponding pure Pt surfaces, Pt(1 1 1), Pt(1 1 0) and Pt(0 0 1) were also obtained. The comparison of the LDOS profiles of Pt atoms of atop adsorption sites on the same faces of bare Pt₃Sn and Pt surfaces showed the effect of alloying with Sn on the electronic properties of Pt atoms. Comparison of LDOS profiles of the C end of CO in its free and atop adsorbed states on Pt₃Sn and LDOS of Pt on bare and CO adsorbed Pt₃Sn surface were used to clear out the electronic changes occurred on CO and Pt upon adsorption. The study showed that (i) inclusion of a Sn atom at the adsorption site structure causes dramatic decrease in stability which limits the number of possible CO adsorption sites on Pt₃Sn surface, (ii) the presence of Sn causes angles different from 180° for M-C-O orientation, (iii) the presence of Sn in the neighborhood of Pt on which CO is adsorbed causes superposition of the 5σ/1π derived-state peaks at the carbon end of CO and changes in adsorption energy of CO, (iv) Sn present beneath the adsorption site strengthens the CO adsorption, whereas neighboring Sn on the surface weakens it for all Pt₃Sn surfaces tested and (v) the most stable site for CO adsorption is the atop-Pt site of the mixed atom termination of Pt₃Sn(1 1 0).     

CO adsorption on Ni, Pd, Cu and Ag deposited on MgO, CaO, SrO and BaO: Density functional calculations

The adsorption properties of CO molecules adsorbed on Ni, Pd, Cu and Ag atoms deposited on O²⁻, F and F⁺ sites of MgO, CaO, SrO and BaO terrace surfaces have been studied by means of density functional calculations and embedded cluster model. The examined clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The adsorption properties of CO have been analyzed with reference to the basicity of the oxide support, bond order conservation energy, pairwise and non-pairwise additivity, associative adsorption, electrostatic potentials, and orbital interactions. CO adsorption on an oxide support is drastically enhanced when CO is adsorbed on a metal deposited on this support. A dramatic change is found, and explained, when one compares the CO binding energy to O²⁻ and F sites. The formation of a strong bond at the support-metal interface has a considerable consequence on the metal-CO binding energy. The binding of CO is dominated by the metal-CO pairwise additive term, and the non-additivity term increases with increasing the basicity of the support. While the classical contributions to the electrostatic interactions are quite similar for the deposited metals, they are quite dissimilar when going from defect-free to defect-containing surfaces. The adsorption properties correlate linearly with the basicity and energy gaps of the oxide support where the electrostatic potential generated by the oxide modifies the physical and chemical properties of the adsorbed metal and therefore its reactivity versus the CO adsorbate.     

Estimation of friction of a single chemisorbed molecule on a surface using incident atoms

Friction of a single molecule chemisorbed on a surface is a very fundamental property related to the dynamics of the diffusion of the molecule. We used a newly developed method, in which energy-controlled rare gas atoms collide with CO molecules on a stepped surface to displace CO molecules from initial terrace sites to final step sites. The friction coefficient of CO molecules on the surface was determined directly by comparing the experimentally measured increase in the number of CO molecules at step sites with classical molecular dynamics simulation results.