K. Christmann

Interaction of hydrogen with solid surfaces

The present article focusses on the chemosorptive and physisorptive behavior of hydrogen interacting with solid surfaces. Depending on the electronic structure of the solid hydrogen will either physically interact and form a weak van der Waals type of bond, or the H2 molecule will dissociate and the H atoms will form a chemical bond with the surface atoms. In some cases the interaction is not just restricted to the surface but may lead to absorption or even hydride formation. Also, reversible or irreversible changes of the structure of the solid may occur as the hydrogen is interacting. This leads to relaxation, reconstruction and facetting phenomena. After some introductory remarks recent results on H2 physisorption and H chemisorption phenomena will be presented and compiled whereby emphasis is put on metal single crystal work. Particular attention will be paid to the energetics and kinetics of the adsorption process, and to the structural properties of hydrogen adsorbed layers. In this context, order-disorder transitions within chemisorbed phases will be considered as well as structural phase transformations involving the underlying solid surfaces (H-induced surface reconstruction). Thereafter a section will be devoted to theoretical model calculations describing the mechanisms of the H chemisorption and the bonding.

Adsorption of hydrogen on a Pt(111) surface

Abstract The H 2 /Pt(111) system has been studied with LEED, ELS, thermal desorption spectroscopy and contact potential measurements. At 150 K H 2 was found to adsorb with an initial sticking coefficient of about 0.1, yielding an atomic H:Pt ratio of about 0.8:1 at saturation. H 2 /D 2 exchange experiments gave evidence that adsorption is completely dissociative. No exrea LEED spots due to adsorbed hydrogen were observed, but the adsorbate was found to strongly damp the secondary Bragg maxima in the I / V spectrum of the specular beam. The primary Bragg maxima were slightly increased in intensity and shifted to somewhat lower energy. A new characteristic electron energy loss at −15.4 eV was recorded upon hydrogen adsorption. The thermal desorption spectra were characterized by a high temperature (β 2 -) state desorbing with second order kinetics below 400 K and a low temperature (β 2 -) state that fills up, in the main, after the first peak saturates. The β 2 -state is associated with an activation energy for desorption E ∗ of 9.5 kcal/mole. The decrease E ∗ with increasing coverage and the formation of the β 1 -state are interpreted in terms of a lateral interaction model. The anomalous structure in the thermal desorption spectra is attributed to domains of non-equilibrium configuration. The work function change Δϑ was found to have a small positive maximum (∼ 2 mV) at very low hydrogen doses (attributed to structural imperfections) and then to decrease continuously to a value of −230 mV at saturation. The variation of Δϑ with coverage is stronger than linear. The isosteric heats of adsorption as derived from adsorption isotherms recorded via Δϑ compared well with the results of the analysis of the thermal desorption spectra.

Chemisorption geometry of hydrogen on Ni/111/ - Order and disorder

The location of a half monolayer of ordered hydrogen adatoms on Ni(111) has been analyzed by Low‐Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS), and Work Function (Δφ) measurements. It is found that the hydrogen atoms are arranged in an overlayer of graphitic structure with a (2×2) unit cell with respect to the substrate unit cell. In the ordered regions, the hydrogen adatoms occupy both types of three fold hollow sites without a detectable difference in the Ni–H bond lengths between the two sites. The Ni–H bond length is found to be 1.84±0.06 A, corresponding to an overlayer‐substrate spacing of 1.15±0.1 A. The relation between this structure and its observed order–disorder phase diagram as a function of temperature and hydrogen coverage is discussed. The disorder is discussed in detail, and a novel ’’atomic band structure’’ interpretation is given.

Adsorption of hydrogen on Pd(100)

Abstract The energetic, kinetic and structural properties of hydrogen chemisorbed on a Pd(100) surface were studied by means of thermal desorption, work function and LEED measurements. Under the applied conditions no interference with bulk dissolution occurs and dissociative adsorption gives rise to a continuous increase of the work function by up to 0.20 eV. The dipole moment of the adsorbate complex is constant up to θ ≈ 0.9 and then increases until saturation at θ ≈ 1.35 (at 170 K) is reached. The formation of a second adsorbed state at high coverages manifests itself also by a low-temperature shoulder in the thermal desorption spectra and in the variation of the isosteric heat of adsorption, E ad , with coverage: E ad remains practically constant ( 24.5 kcal mole ) up to θ ≈ 0.9 and then decreases. The sticking coefficient is initially rather high ( s 0 ≈ 0.5) and varies with coverage in a way which can be described by a precursor-state model. The preexponential factor for desorption is about 10 −2 cm 2 atom −1 s −1 . Desorption follows second order kinetics only at very low coverages, at high θ it exhibits quasifirst order. This effect is attributed to the existence of lateral interactions between adsorbed hydrogen atoms which manifest themselves also in the appearance of a c(2 × 2) LEED pattern at low temperatures. The “extra” diffraction spots attain their maximum intensity at θ = 0.5, and a structural model is proposed whereafter in this phase the H atoms occupy next-nearest neighboring adsorption sites with local fourfold symmetry. Order-disorder transitions were followed by recording the intensity of the half-order spots as a function of temperature at various coverages. The resulting phase diagram exhibits a critical temperature T c = 260 K at θ = 0.5 and is slightly asymmetric with respect to this coverage. The data are analysed in terms of a lattice gas model and estimates for the pairwise interaction energies yield repulsion between nearest neighbors ( w 1 = 0.5 kcal mole ) and attraction between next-nearest neighbors ( w 2 = −0.3 kcal mole ). The additional operation of non-pair-wise interactions is made responsible for the asymmetric shape of the phase diagram. Whereas the adsorbed layer is obviously localized at T ⩽ 270 K , a detailed analysis of the adsorption entropy reveals that for T ⩾ 370 K a rather good description can be obtained with a model of delocalized two-dimensional translation.

Interaction of hydrogen with Pt(111): The role of atomic steps

Abstract Previous work on the interaction of hydrogen with Pt(111) was extended to a stepped Pt(S)-9 (111) × (111) surface in order to elucidate the role of structural imperfections as “active sites” in surface reactions. Experiments were performed by means of LEED, AES, ELS, thermal desorption and work function techniques which also served for characterization of the properties of the clean surfaces. The saturation coverage with dissociatively adsorbed hydrogen at 120 K was found to be near unity. The LEED pattern showed the appearance of additional weak streaks which were interpreted as being caused by one-dimensional order of those hydrogen atoms which are located in the vicinity of the steps. At low coverages the adsorption energy was found to increase up to about 12 kcal/mole whereas the corresponding value of the flat surface is 9.5 kcal mole . Above θ ≈ 0.3 the Ead versus θ-curves were identical for both types of surfaces. Pronounced effects were observed with kinetic processes: The presence of steps increases the initial sticking coefficient by a factor of four to a value of 0.34, and the activity for the H2 — d2 exchange reaction is enhanced by an order of magnitude whereby however the apparent activation energy remains constant. With the stepped plane the work function at first increases up to 25 mV at θ = 0.25 and then decreases to a final value of −350 mV, whereas adsorption on an perfect (111) plane causes only a continuous decrease. A detailed inspection of the data leads to the proposal of a model in which two (slightly different) types of adsorbed hydrogen atoms are associated with the atomic steps. Although the variations of the metalhydrogen binding energies between sites at steps and at low index planes are only very small there may be significant implications on the kinetics of surface reactions.

Adsorption of CO on Pd(100)

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 A and a C–O bond length of 1.15±0.1 A, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short‐range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with c...

Evidence for ‘‘subsurface’’ hydrogen on Pd(110): An intermediate between chemisorbed and dissolved species

We report experimental evidence for the formation of a ‘‘subsurface’’ hydrogen species at 130 K on a Pd(110) single crystal surface. This is well separated by distinct activation energy barriers from species which are chemisorbed or which are dissolved in the Pd bulk, thereby confirming a model of H absorption in Pd in which a subsurface state acts as a reaction intermediate.

Model studies on bimetallic Cu/Ru catalysts: II. Adsorption of hydrogen

Abstract The chemisorption of hydrogen on pure and Cu-covered Ru(0001) surfaces was examined by means of low energy electron diffraction (LEED), work function (Δϑ) measurements, and thermal desorption spectroscopy (TDS). H 2 adsorbs dissociatively on the Ru(0001) surface with an initial sticking probability s 0 = 0.25 (± 0.1). Two chemisorption states, β 1 and β 2 with adsorption energies of 10.5 (± 2) and 16.5 (± 1) kcal/mole, respectively, are observed. The saturation density is approximately 1.3 × 10 15 atoms/cm 2 at T = 150 K, corresponding to a coverage θ H = 0.85 (± 0.15). The work function change depends in a complex manner on coverage and is strongly affected by surface impurities. Cu deposits on the Ru surface suppress the hydrogen adsorption capacity drastically. Whereas small amounts (~5% of a Cu monolayer) reduce the H saturation density of hydrogen by about 50% but have little influence on the heat of adsorption of hydrogen, E ad , higher Cu concentrations (0.1–0.8 Cu ml) give rise to a decrease of E ad by ~2–3 kcal/mole and cause a strong inhibition of the hydrogen chemisorption process. The results suggest that ensembles of up to 5–10 adjacent Ru atoms are involved in the hydrogen chemisorption bond whose concentration is steeply decreasing by addition of small amounts of copper. The conclusions are in agreement with those reached by Sinfelt et al. [ J. Catal. 42 , 227 (1976)] with Cu/Ru “bimetallic cluster” catalysts.

The vibrations and structure of pyridine chemisorbed on Ag(111): the occurrence of a compressional phase transformation

Abstract High-resolution electron energy loss and UV photoemission spectroscopies have been used to study the chemisorption of pyridine on clean Ag(111) at T ≈ 140 K . Pyridine is weakly chemisorbed and undergoes a compressional phase transfomation from a Π-bonded species to a more weakly bound, nitrogen-lone-pair bonded species The molecular orientations of both chemisorbed phases are determined.

Steady and nonsteady rates of reaction in a heterogeneously catalyzed reaction: Oxidation of CO on platinum, experiments and simulations

The rate of reaction for oxidation of CO over (210) and (111) single‐crystal surfaces of platinum has been studied as a function of reactant pressures (PO2,PCO) and sample temperature (T), both experimentally and by computer simulation. Experimental results on both surfaces show regions with a steady high rate of reaction followed by a nonsteady transition region and, at high CO pressures, a region with low reactivity caused by CO poisoning of the surface. At constant sample temperature, the transition region can be narrow and depends critically on the ratio of the gas phase concentration of reactants (PCO/PO2). The temperature dependences of the experimental data indicate that the critical ratio and the details for the occurrence of CO poisoning are strongly affected by surface processes such as adsorption, desorption, and diffusion ordering and reconstruction phenomena. A computer simulation model of the Langmuir–Hinshelwood surface reaction as developed by Ziff et al. was used for the simulation of the...