Adsorption of CO on Pd single crystal surfaces

Studies of CO adsorption on Pd(110), (210) and (311) surfaces as well as with a (111) plane with periodic step arrays were performed by means of LEED, contact potential and flash desorption measurements. Isosteric heats of adsorption were evaluated from adsorption isotherms. Earlier work with Pd(111) and Pd (100) surfaces is briefly reviewed, yielding the following general picture: The initial adsorption energies vary between 34 and $\text{40}\text{kcal}\text{mole}$ and close similarities exist for the dipole moments, the maximum densities of adsorbed particles and for the adsorption kinetics. At low and medium coverage the adsorbed particles are located at highly symmetrical adsorption sites, whereas saturation is characterized by the tendency for formation of close-packed layers.     

Flash desorption and equilibration of H2 and D2 on single crystal surfaces of platinum

The adsorption and flash desorption of hydrogen and the equilibration of H₂ and D₂ has been studied on the (110), (211), (111) and (100) planes of platinum. Desorption from Pt (211), a stepped surface composed of (111) and (100) ledges, yields a desorption spectrum which apparently is a composite of desorption from the individual ledges. Pt (110) is quite similar to the tungsten structural analog, W (211), in that both yield two-peak desorption spectra, and on both planes adsorption kinetics are dramatically different for filling of the two states. On all four planes adsorption kinetics are apparently proportional to (1 ? θ)², and estimates of the initial sticking probabilities show them to decrease in the order: (110) > (211) > (100) > (111). Equilibration activity follows approximately the same order [(110) > (211) > (111) > (100)] with a factor of ~ 5 difference between the most and least active planes; no extraordinary activity is observed for the stepped surface, Pt(211). Below ~ 570 K equilibration of H₂ and D₂ is activated by less than 2 kcal/mole with the magnitude dependent on the specific face, and above this temperature the reaction is nonactivated. The non-activated case apparently results from absorption followed by statistical mixing on the surface. Calculated rates for HD production per cm² based on this model are in excellent agreement with the experimental values for Pt(110) and Pt(211), and in somewhat poorer agreement in the case of Pt (111) and Pt (100). This latter is probably due to the greater inaccuracy in the values of the sticking coefficients on these planes.     

Elucidation of the nature of active oxygen in the reaction of low-temperature oxidation of CO on single crystal surfaces platinum and palladium

The temperature-programmed reaction (TPR) method, high-resolution electron energy loss spectroscopy (HREELS), and molecular beam method were used to elucidate the role surface reconstruction, subsurface oxygen (O_subs), and CO_ads concentration play in the low-temperature oxidation of CO on the Pt(100), Pt(410), Pd(111), and Pd(110) surfaces. The possibility of the formation of so-called hot oxygen atoms, which arise at the surface at the instant of dissociation of O_2ads molecules and can react with CO_ads at low temperatures (～150 K) to form CO₂, was examined. It was revealed that, when present in high concentration, CO_ads initiates the phase transition of the Pt(100)-(hex) reconstructed surface into the (1 × 1) non-reconstructed one and blocks fourfold hollow sites of oxygen adsorption (Pt₄-O_ads), thereby initiating the formation of weakly bound oxygen (Pt₂-O_ads), active in CO oxidation. For the Pt(410), Pd(111), and Pd(110) surfaces, the reactivity of O_ads with respect to CO was demonstrated to be dependent on the surface coverage of CO_ads. The ¹⁸O_ads isotope label was used to determine the nature of active oxygen reacting with CO at ～150–200 K. It was examined why a CO_ads layer produces a strong effect on the reactivity of atomic oxygen. The experimental results were confirmed by theoretical calculations based on the minimization of the Gibbs energy of the adsorption layer. According to these calculations, the CO_ads layer causes a decrease in the apparent activation energy E _act of the reaction due to changes in the type of coordination and in the energy of binding of O_ads atoms to the surface.     

NMR at single crystal surfaces

15 sites on 1 cm². To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin-polarized atomic beam of ⁶Li in the one case and ⁸Li in the other. The latter isotope is radioactive and undergoes a β-decay with a half-life of 0.84 s. Li adsorbed on the close-packed Ru(001) surface is investigated. The longitudinal relaxation time, T₁, is the main observable and is used to deduce the local electronic density of states [LDOS(E_F,r=0)] and Li diffusion barriers. The second experiment uses ⁶Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier transform NMR technique is demonstrated. This is done by observing the time-dependent flux of circularly polarized light emitted behind the foil after a 90° pulse has been employed at the surface. Electric field gradients and transverse relaxation times, T₂, are thus determined. A large difference between T₁ and T₂ is traced to the dimensionality of the system. Received: 21 March 1997/Accepted: 12 August 1997     

The thermal attenuation of elastic scattering of helium from copper single crystal surfaces

The dependence of diffraction peak intensities upon temperature for the scattering of helium from various copper surfaces: Cu (111), (100), (110), (113), (115) and (117), has been experimentally determined for two incident energies (21–63 meV) and a large range of incidence angles. For the close-packed faces (111) and (100), the data are consistent with a Debye-Waller formalism involving an effective surface mean square displacement <u²_eff〉. The quantitative fit with the data is good if anharmonic effects are properly taken into account. This result establishes that a generalized Debye-Waller formalism is, at least as a first approximation, relevant to the helium-surface diffraction. For the rougher surfaces the agreement remains good up to a threshold temperature above which the intensities are always lower than predicted by the model. It is proposed that this may be due to some kind of thermal roughening of the surface.     

H processing of copper surfaces

Changes in the crystallographic structure of polycrystalline copper surface due to the atomic hydrogen treatment are described. Samples of polycrystalline OFHC copper plates were exposed to a flux of thermal hydrogen atoms with the density between 1×10²¹ atoms/m²s and 1×10²³ atoms/m²s. Hydrogen atoms partially accommodate and recombine in the potential well on the surface. During these processes copper atoms on the surface displace, which leads to a recrystallization of the samples. The effects were investigated by a scanning electron microscope (SEM).     

Polar- and azimuth-angle-dependent rotational and vibrational excitation of desorbing product CO2 in CO oxidation on palladium surfaces

Clear polar and azimuth angle dependencies were found in rotational and vibrational energies of product CO2 in CO oxidation on Pd surfaces. On Pd(110)-(1x1), with increases in polar angle, both energies decreased in the [001] direction but remained constant in [110]. On the Pd(110) with missing rows, both energies increased in [001] but decreased in [110], indicating that the transition state changes with the geometry of the substrate. On Pd(111), the rotational energy greatly increased, but the vibrational energy decreased. Such angular dependence of internal energy provides new dimensions in surface reaction dynamics.     

Nuclear acoustic resonance in single crystal copper over the temperature range 4–250 K

Nuclear acoustic resonance absorption signals have been observed in ⁶³Cu and ⁶⁵Cu in single crystal copper. The ⁶³Cu dispersion signal was also observed. The ⁶³Cu absorption signal was studied over the temperature range of 4–250 K. The results are in agreement with existing theories.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

Isotope exchange studies of the oxidation and reduction of SrTiO3 single crystal surfaces by water and hydrogen

Thermal desorption studies of chemisorbed D₂ and D₂O on a reduced SrTiO₃(111) surface reveal that D₂ causes the reduction of the crystal, whereas D₂O causes its oxidation. Thermal desorption of H₂¹⁸O indicates that there is a 15% exchange between the oxygen in the adsorbed water molecules and the lattice oxygen.     

Optical transmittance and microgravimetric studies of the oxidation of 〈100〉 single crystal films of copper

Thin single crystal copper films have been grown and oxidized on (100) faces of cleaved sodium chloride discs suspended from a vacuum ultramicrobalance. Optical transmittance measurements between 400–800 nm and electron microscopic investigations were also used to characterize the oxidation process. Polycrystalline copper films grown at room temperature are substantially the same as those grown previously on glass substrates. Single crystalline growth at 325 ° C on rock salt produces a characteristic transmittance curve due to the “island” nature of the films. These curves compare favorably with other previously published results. Single crystal copper films oxidized to CuO_0.67 at temperatures of 117–159°C in 100 Torr of oxygen for films less than 500 A thick. For films 378 to 1000 Å thick, compositions of CuO_0.52 to CuO_0.62 were obtained between 123–176°C. The oxidation to less than CuO_0.67 is attributed to the existence of islands in these films which are thicker than the average film thickness, and require higher temperatures or thinner films to permit oxidation to CuO_0.67 before the nucleation of CuO sets in.     

Influence of crystal orientation on copper oxidation failure

The influences of crystal orientation on copper oxidation were investigated. The results indicated that crystal orientation of copper substrate has a great effect on the growth rate, the morphology of oxide film and the extent of oxidation failure. Shear test showed the adhesion strength between Cu(1 1 0) and its oxide film was the highest, whereas, the adhesion strength between Cu(3 1 1) and its oxide film was the lowest. SEM observations revealed that the oxide film grown on Cu(3 1 1) delaminated from substrate seriously, while the oxide film grown on Cu(1 0 0) and Cu(1 1 0) did not reveal such a phenomenon. Cu(1 0 0) and Cu(1 1 0) exhibited thinner oxide thickness compared to those on Cu(3 1 1) and Cu(1 1 1). The activation energy for oxide growth on Cu(1 0 0) and Cu(1 1 0) was calculated to be the highest while that on Cu(3 1 1) was the lowest.     

Unoccupied surface resonances on aluminum single crystal surfaces

Surface resonances above the Fermi level of Aluminum have been observed using surface soft-X-ray absorption spectroscopy. These surface resonances occur at 4.3 eV above the L_2.3 absorption threshold on the Aluminum (100) surface and at 12.1 eV on the Aluminum (111) surface. Excited state and surface EXAFS aspects of the spectra are discussed.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

Work function of stepped tungsten single crystal surfaces

The work function of regularly stepped tungsten single crystal planes has been investigated as function of terrace width and step orientation. The terraces were always formed by (110) planes. One set of samples exhibited step edges running parallel to the [001] direction but different terrace widths. Another set with almost equal terrace width showed step edges of different directions with correspondingly different edge structures. The work function measurements were performed using thermionic emission in the temperature range 2000–2800 K. The work function decreases linearly with step density for a given step orientation. Different step orientations give rise to slightly different work function reductions. The results are interpreted by attributing additional dipole moments to edge atoms. Different edge structures lead to different dipole moments. These findings are consistent with Smoluchowski's “smoothing effect” of the electron charge distribution caused by the structural arrangement of surface atoms. The temperature coefficient of the work function also depends strongly on step density and step orientation.     

The application of photoemission microscopy to the study of heterogeneous reactions over metal single crystal surfaces

The development of UHV instrumentation for photoemission electron microscopy (PEEM) has opened up the possibility of investigating in situ the morphology of metal single crystal surfaces during simple catalytical reactions in the pressure range up to 10^–4 mbar. We have studied the Pt(100) surface during the transition from the high to low rate branches of the CO oxidation reaction. Steps, step bunches and grain boundaries have been identified. They play an important role in the decomposition of CO and the deposition of carbon and probably regulate the reactivity of the Pt surface.