H. Conrad

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 ∞ √pH2, 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 Ead 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 CO on Pd single crystal surfaces

Abstract 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 40 kcal 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.

Interactions between oxygen and carbon monoxide on a Pd(111) surface

Adsorption, mutual interactions and transient product formation between oxygen and carbon monoxide on a Pd(111) surface were studied by means of LEED, UPS and isothermal as well as temperature-programmed desorption spectrometry at temperatures between 200 and 550 K. CO adsorption at 200 K was found to cause the formation of additional periodic overlayer structures above θ = 0.5, apart from the already known surface structures at θ < 0.5. Saturation is reached with a hexagonal close-packed layer at θ = 0.66 (≜1 × 1015moleculescm2). Depending on the sequence of gas exposure and on the partial coverages of COad andOad, different stages of coadsorption (island formation, “compression” of the 2 × 2-Oad layer into a √3 × √3 R 30° structure, growth of a mixed phase with 2 × 1 periodicity etc.) may be distinguished, which influence the electronic properties and the reactivity of the adsorbates. At high surface concentrations CO2 formation proceeds with an appreciable rate already at 200 K, whereas at low coverages temperatures above 400 K are needed. The results demonstrate that no appropriate description of the kinetics of catalytic CO2 formation over the whole range of surface concentrations may be achieved in terms of simple rate laws involving coverage-independent rate constants (and activation energies) and mere surface concentrations of the reactants. There is on the other hand no indication for a reaction path involving the collision of a gaseous CO molecule with an adsorbed oxygen atom (Eley-Rideal mechanism), i.e. the reaction proceeds always with chemisorbed CO via Oad + COad → CO2 (Langmuir-Hinshelwood mechanism).

Interaction of NO and O2 with Pd(111) surfaces. I.

The adsorption of NO on Pd(111) was studied by means of LEED, UPS and thermal desorption measurements. Non-dissociative adsorption is characterized by additional maxima in the photomission spectra at 2.6, 9.2 and 14.6 eV below the Fermi level originating from chemisorption levels which are derived from the highest occupied molecular orbitais of NO. Thermal desorption takes place from three distinct states (α, β and γ) corresponding to binding energies of about 15, 17 and 31 kcal/mole, respectively, with about equal populations. The α-state is associated with a 2 × 2 LEED pattern and the β-state with a c4 × 2 structure, whereas the γ-state corresponds to disordered adsorption at low coverages. Plausible structure models are proposed for the ordered structures with θ = 0.75 for the α-state and θ = 0.5 for the β-state. The strong decrease of the adsorption energy is explained in terms of pronounced short-range repulsive interactions between neighbouring adsorbate molecules.

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NOad. The mean adsorption energy is about 25 kcalmole. Dissociation of adsorbed NO starts already at −120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N2 occurs around 600 °C with an activation energy of about 52 kcalmole. A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of Nad + Oad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H2. This yields an Nad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ −2, −8.8 and −14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. Nad as well as Oad causes a peak at −5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × √3R30° structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a √72 × √72R19° LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 A. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θo = θco = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with Oad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with COad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.

A LEED/UPS study on the interaction of oxygen with a Ni(111) surface

Abstract Exposure of a Ni(111) surface to oxygen leads at first to the formation of a chemisorbed overlayer which is characterized by a 2 × 2-superstructure and a maximum in the photoemission spectrum ( hv = 40.8 eV) centered at 5.6 eV below the Fermi level E F . The emission from the Ni d -states is nearly unaffected at this stage of interaction. After high oxygen exposures the epitaxial growth of NiO can be identified from the LEED pattern. The corresponding photoelectron spectrum is strongly altered and exhibits close agreement with the transition energies as calculated by Messmer et al. for a NiO 6 10- -cluster.

Photoemission spectra of adsorbed layers on Pd surfaces

Adsorption of CO, NO, H2 and O2 on (110) and polycrystalline Pd surfaces was studied by means of ultra-violet photoelectron spectroscopy (UPS) using Hel and Hell resonance radiation. Generally, a marked decrease of the emitted intensity from the energy range just below the Fermi level was observed after adsorption. Adsorption of CO causes the appearance of two peaks at 7.9 and 10.8 eV below Ef; after NO adsorption two maxima at 9.3 and 15.5 eV were observed. Absorption of oxygen leads to a peak at 5.8 eV. Attempts are made to correlate these additional structures with energy levels of the free molecules. The usefulness of the UPS technique in studying surface reactions is demonstrated with the thermal decomposition of NO as well as with the displacement of adsorbed NO by CO.

Coadsorption of hydrogen and carbon monoxide on a Pd (110) surface

Abstract The coadsorption of H 2 and CO on a Pd (110) single-crystal surface has been studied by means of LEED, flash desorption spectroscopy, and work function measurements. Carbon monoxide may displace preadsorbed hydrogen completely from the surface, whereas no displacement of adsorbed CO by hydrogen takes place. If the surface concentrations of adsorbed hydrogen and carbon monoxide are both equal to one-third of their maximum values, a mixed 1 × 3 structure is formed, where both species are in intimate contact. The mutual interaction must be rather weak since no variation of the desorption energy of H 2 was observed. The only desorbing particles were H 2 and CO without any detection of product formation. The contributions to the work function change from the individual components are additively superposed in the mixed adsorbate layers, indicating that the dipole moments are not mutually affected to any measurable extent.

Polynuclear metal carbonyl compounds and chemisorption of co on transition metal surfaces

Abstract Ultraviolet photoelectron spectra from Rh6(CO)16 and from CO adsorbed on a Pd(111) single crystal surface exhibit close similarities which become further evident from a comparison of vibrational and thermal properties. These results demonstrate the localized character of the chemisorption bond and justify cluster approximations for its theoretical description.