Norbert Memmel

Monitoring and modifying properties of metal surfaces by electronic surface states

Abstract The wave functions of electronic surface states are localized near the surface of solids. Therefore surface states are well suited to monitor the physical and chemical state of the surface. If surface states are (partially) occupied, they contribute to the charge density near the surface and to the total energy of the system. Thus they modify the properties of the surface. The influence is particularly pronounced for processes occurring far in front of the surface, where the charge density caused by “competing” bulk states is low or in cases where the total energy differences are only small. Examples from various fields of surface physics are given where metal surface states can be used to monitor and modify surface properties.

Growth of ultrathin iron films on Cu( 001): an ion-scattering spectroscopy study

Abstract Low-energy ion scattering has been applied to study the pseudomorphic growth of ultrathin fee iron films on a Cu(001) single-crystal surface. At all coverages — even as low as 0.1 monolayer (ML) — iron is found to about equal amounts in both the surface and the first subsurface layer, since part of the iron atoms are incorporated into the original copper surface. This is particular evident for the 0.1 ML film, which does not exhibit any iron defects, such as steps or adatoms. Instead copper surface defects become visible upon iron deposition. Near 2 ML the substrate is covered for the most part by a relatively smooth bilayer indicating the coalescence of the iron islands. Up to a film thickness of around 6 ML the surface defect density of the iron layers decreases with increasing coverage and raises again at larger coverages. Above ∽ 10 ML the pseudomorphic fee growth breaks down and domains of bcc iron with (110) orientation are formed.

Hydrogen Production by Methanol Steam Reforming on Copper Boosted by Zinc-Assisted Water Activation

For use of polymer electrolyte membrane fuel cells (PEMFC) in mobile power applications, an efficient source of CO-depleted hydrogen is needed. To avoid technical and safety problems of hydrogen handling, storage, and transport, methanol can be used as practical and abundant energy carrier for on-board H2 generation, as it has the advantage of a high energy density. Hydrogen generation from methanol can be performed by catalytic methanol steam reforming (MSR): CH3OH+H2O→CO2+3 H2. Methanol conversion must be carried out with very high CO2/H2 selectivity to avoid CO poisoning of the fuel-cell anode. A number of promising selective MSR catalysts are already available. Apart from advanced copper-based catalysts,1, 2 special attention is presently paid to the highly MSR-selective reduced state of Pd/ZnO,3 containing a particularily stable intermetallic PdZn (1:1) active phase.3, 4 Therefore, we recently studied related “inverse” near-surface PdZn intermetallic phases, showing that three-dimensional PdZn active site ensembles are equally important for selective dehydrogenation of methanol (thus avoiding CO) and for efficient water activation.5 For the less costly Cu/ZnO catalysts, originally designed for methanol synthesis, improvements towards a technical MSR application regarding sintering stability, pyrophoricity, and selectivity are still required. Empirical development of Cu/ZnO catalyst preparation and activation has aimed in a particularily large Cu0–ZnO contact.6 Nevertheless, it is very difficult to derive an unambiguous causality for the role of the contact on technical catalysts. It is known that zinc leads to an improvement in the desired properties, but a clear assignment of a predominant promotional effect (both from the theoretical and experimental side) is still missing. In the Cu/ZnO literature, seemingly incompatible model interpretations can be found, involving the “metallic copper model”,7 the “special site model”,8 the “morphology model”,7, 9 the “spillover model”,10 and last but not least the “Cu-Zn alloy model”.8, 11 Consequently, the Cu-ZnO(H) contact most likely constitutes a combination of promotional effects. The central aim of our study is to highlight the aspect of zinc-promoted water activation. This is achieved by using an ultrahigh-vacuum (UHV) “inverse” model catalyst approach, which, in contrast to investigations on real catalyst systems, allows the zinc segregation behavior and the changes in redox chemistry of both copper and zinc to be better followed. This provides a solid basis for directional promotion of microkinetic steps leading to enhanced CO2 selectivity.

Promotors, poisons and surfactants: Electronic effects of surface doping on metals

The interaction of adsorbates with metal surfaces is discussed. It is shown that the evanescent charge density produced by occupied sp derived surface states yields a considerable contribution to the Pauli repulsion experienced by adsorbate particles with a closed-shell electronic structure, e.g. rare-gases or molecules such as H2 or N2. For rare-gases this results in a reduction of the binding energy in the presence of occupied surface states, for molecules this gives rise to an additional contribution to the dissociation barrier. Suitable surface dopants are able to depopulate surface states and thereby to reduce the dissociation barrier. Such dopants can substantially promote catalytic reactions in which the dissociation from the gas phase or a physisorbed precursor is the rate limiting step. In contrast to closed-shell systems the bonding interaction for metal adsorbates on metal substrates is enhanced by occupied surface states. This leads to an extra diffusion barrier at steps, because the surface state amplitude drops to zero at upper step edges. The additional step-edge barrier, which is a kinetic hindrance for layer-by-layer growth, can be reduced by surface dopants depopulating the corresponding surface state. Such dopants promote layer-by-layer growth and act therefore as surfactants. It is concluded that the effect of promoters in catalysis and of surfactants in metal epitaxy is in part due to the same basic mechanism, namely the depopulation of surface states.

Inverse photoemission study of carbon monoxide bonding to transition metals

Abstract The unoccupied CO derived bands for the densely packed CO monolayer on Ni(110), Pd(llO), and Pt(110) have been investigated by inverse photoemission. In all three cases four CO derived bands can be observed in the region of the 2π ∗ and 5σ ∗ orbitals. For Ni and Pd the band dispersions show the signature of π bands. No evidence is found for unoccupied σ contributions, i.e. for 5σ donation. However, in Pt the band dispersions are significantly different and hint at a significant 5σ donation. The energy position of the CO derived manifold is rather similar for all three metals. These results on the electronic structure are consistent with thermodynamical and vibrational data reported in the literature for the three systems.

Adsorption site of oxygen on Pd(111)

Abstract The adsorption of oxygen at room temperature and saturation coverage is studied using scanning tunneling microscopy and low-energy ion scattering. Oxygen forms an ordered p(2×2) overlayer with the oxygen atoms residing in threefold hcp hollow sites.

From Oxide-Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

This Minireview summarizes the fundamental results of a comparative inverse‐model versus real‐model catalyst approach toward methanol steam reforming (MSR) on the highly CO2‐selective H2‐reduced states of supported Pd/ZnO, Pd/Ga2O3, and Pd/In2O3 catalysts. Our model approach was extended to the related Pd/GeO2 and Pd/SnO2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO2‐selectivity‐guiding structural and electronic effects on the molecular level, to establish a knowledge‐based approach for the optimization of CO2 selectivity. Regarding the inverse‐model catalysts, in situ X‐ray photoelectron spectroscopy (in situ XPS) studies on near‐surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd2Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallicoxide interface, unsupported bulk intermetallic compounds of PdxGay were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water‐activating role of the special Pd2Ga‐β‐Ga2O3 intermetallicoxide interaction. The inverse‐model studies were complemented by their related “real‐model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd5Ga2, PdIn, Pd3Sn2, and Pd2Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin‐film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetaloxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metalsupport interactions. To fully understand the bimetalsupport interactions in the supported systems, our studies were extended to the MeOH‐ and formaldehyde‐reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetaloxide contact” systems, a pronounced “bimetaloxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system‐specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted.

Compressed CO adsorption layers on the (110) surfaces of Ni, Pd, and Pt. An inverse photoemission study of the unoccupied electronic band structure

Abstract The unoccupied band structure of a CO monolayer on Ni(110), Pd(110), and Pt(110), respectively, has been investigated by inverse photoemission. For Ni and Pd the CO-derived band structures exhibit only marginal differences. The observed bands are interpreted in terms of 2π x - and 2π y -derived COCO bonding and antibonding bands. The level ordering in the center of the surface Brillouin zone, as obtained from a polarization analysis, revealed a similar splitting for the 2π x and the 2π y levels at normal incidence. This indicates a COCO interaction of approximately the same magnitude in the x as well as the y direction. Consequently, the CO molecules seem to be arranged in a nearly hexagonal structure at monolayer coverage which in turn is due to the COCO repulsion dominating over the CO-substrate interaction in the tightly packed monolayer. The observed level ordering of the 2π y bands is consistent with such a geometry. On Pt(110), a distinctly different electronic structure indicates a different bonding of the CO to the metal substrate, namely a reduced 2π backbonding and a more prominent role of the 5σ dative bonding. This may also give rise to a slightly different adsorption geometry.

An inverse photoemission study of CO adsorption on clean and potassium promoted Ru(001)

Inverse photoemission spectra from a CO covered Ru(001) surface reveal strong CO induced emission at 5 eV above the Fermi energy. The peak position of the CO induced feature is coverage dependent and exhibits shifts up to 1 eV. The energy position of this CO 2 π * derived state correlates well with measured CO adsorption energies. CO/K coadsorption exhibits a complex behaviour. In particular a new previously unobserved emission around 10 eV above E F is found whose nature is discussed in relation to the two existing models: linearly bonded CO with increased 2 π * occupation and sp 2 -hybridized bridge bonded CO.

Coadsorption of CO and H on Ni(110): evidence for a strong local CO-H interaction

Abstract We have investigated differently prepared coadsorbate layers of H and CO on Ni(110) by thermal desorption, low-energy electron diffraction, ultraviolet photoemission and inverse photoemission. In addition we have carried out cluster calculations in order to model the electronic structure of the mixed COH layer on this surface and to investigate effects of H coadsorption on bond lengths and the corresponding stretching vibrations. We find evidence for a strong local COH interaction. No global H-induced effects, such as d-band filling, need to be invoked for the interpretation of the photoemission spectra. Moreover, neither changes in the long-range order of the coadsorbate layer nor a surface reconstruction seem to profoundly affect the character of the COH interaction. This supports the assumption of a local interaction mechanism.