Christian Weilach

Comparison of the reactivity of different Pd-O species in CO oxidation.

The reactivity of several Pd-O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd(5)O(4) surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO(x) suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s(-1). The Pd(5)O(4) surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10(-6) mbar CO at T > 500 K led only to partial reduction toward PdO(x) suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd(5)O(4) surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.

Mind the gap! Spectroscopy of catalytically active phases

Recent advances in model catalysis and in spectroscopic methods that can operate at ambient pressure now enable us to investigate a catalyst in its active state, i.e. while it is functioning. Laser spectroscopy, polarization-modulated infrared (IR) spectroscopy, and high pressure photoelectron spectroscopy allow monitoring of the transformation of catalysts from the ‘as-prepared’ to the ‘active-state’, which may involve pronounced changes in catalyst structure and composition. The ultimate goal is the characterization and control of the active sites. A number of case studies are presented for Pd-based catalysts, including both single crystal and nanoparticle model catalysts, which illustrate the complex and dynamic behavior of catalytically active surfaces, with emphasis on Pd hydrides, Pd−C phases, Pd surface oxides, and bimetallic surfaces. There is clearly a need to ‘mind the gap’ between investigations under ultrahigh vacuum (UHV) and those at ambient pressure, as well as to account for the inherent differences between supported nanoparticles and extended single crystal surfaces.

Growth of an Ultrathin Zirconia Film on Pt3Zr Examined by High-Resolution X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, Scanning Tunneling Microscopy, and Density Functional Theory

Ultrathin (∼3 Å) zirconium oxide films were grown on a single-crystalline Pt3Zr(0001) substrate by oxidation in 1 × 10–7 mbar of O2 at 673 K, followed by annealing at temperatures up to 1023 K. The ZrO2 films are intended to serve as model supports for reforming catalysts and fuel cell anodes. The atomic and electronic structure and composition of the ZrO2 films were determined by synchrotron-based high-resolution X-ray photoelectron spectroscopy (HR-XPS) (including depth profiling), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Oxidation mainly leads to ultrathin trilayer (O–Zr–O) films on the alloy; only a small area fraction (10–15%) is covered by ZrO2 clusters (thickness ∼0.5–10 nm). The amount of clusters decreases with increasing annealing temperature. Temperature-programmed desorption (TPD) of CO was utilized to confirm complete coverage of the Pt3Zr substrate by ZrO2, that is, formation of a closed oxide overlayer. Experiments and DFT calculations show that the core level shifts of Zr in the trilayer ZrO2 films are between those of metallic Zr and thick (bulklike) ZrO2. Therefore, the assignment of such XPS core level shifts to substoichiometric ZrOx is not necessarily correct, because these XPS signals may equally well arise from ultrathin ZrO2 films or metal/ZrO2 interfaces. Furthermore, our results indicate that the common approach of calculating core level shifts by DFT including final-state effects should be taken with care for thicker insulating films, clusters, and bulk insulators.