Understanding heterogeneous catalysis via surface science studies
It is well known that about 85% of all industrial chemical processes are catalytic or, at least, contain one catalytic step. Among these, about 80% are heterogeneous processes (with the remainder being homogeneous and bio-catalysis). For 2005 it has been estimated that this created a turnover of around 900 billion $. The molecule-solid interaction is clearly governed by the exact atomic and electronic structure of the solid surface which often contains nanostructures (nanoparticles, interfaces, etc). Despite their importance, many catalytic processes are still not understood on a molecular level. The interaction of molecules with solid surfaces is, of course, also relevant for related fields such as materials science, power generation, sensors, microelectronic devices, protective and selfcleaning/ antibacterial coatings, etc.
In order to elucidate the mechanism of surface reactions, GR's group utilizes two approaches: (i) surface science studies of planar model catalysts grown in ultrahigh vacuum (UHV) and (ii) studies of industrial catalysts and of technological catalysts prepared under wellcontrolled conditions. Combining these two approaches enables one, on the one hand, to deduce atomic/molecular information and, on the other hand, to extend this knowledge to technological catalysts. Vice versa, the studies of industrial systems provide a feedback of how relevant the model studies are. A recent example of this approach is illustrated in the following Figure. Hydrogen generation by methanol steam reforming on PdZn catalysts could be understood based on the close interplay between studies on surface science model catalyst and technological catalysts. In both cases synchrotron-based in situ spectroscopy was applied, finally allowing to identify the PdZn phase being active in methanol steam reforming. It was found that for high H2/CO2 selectivity a PdZn multilayer (~4 layers) is required whereas a PdZn monolayer largely behaves like pure Pd (producing H2/CO despite 50% Zn on the surface). Furthermore, this study strongly relied on the cooperation with research groups at University Innsbruck, ETH Zürich, Swiss Light Source (SLS) and the Fritz Haber Institute Berlin.
In the model catalyst approach a variety of well-defined nanostructured surfaces is used, including single crystals, ultrathin films and oxide supported nanoparticles. Even a very simple catalyst such as Pd nanoparticles on an oxide support provides a complex functional system, because processes can occur on the metal, on the oxide and at the metal-oxide interface. This “multi-functionality” is a well-known but hardly controlled effect in catalysis. “Designing” model catalysts by deliberately changing their atomic structure, defect density and composition allows disentangling these effects. Comparison of results obtained from nanoparticles and single crystals allows determining functionalities and specific properties that require nanoscale structures. Particular emphasis is placed on characterizing functioning catalysts under “real world” conditions (e.g. atmospheric pressure, high temperature) because functional materials typically change properties depending on their environment. It is also important to note that many surface atomic structures deviate from simple truncations of the wellknown bulk structures, with significant effect on the physical-chemical properties.
The methods applied in the surface science approach include polarization-modulated IR spectroscopy (PM-IRAS), sum frequency generation (SFG) laser spectroscopy, X-ray photoemission spectroscopy (XPS), photoemission electron microscopy (PEEM), low energy electron diffraction (LEED), temperature programmed methods, and others. The group has developed new instrumentation such as an extended UHV surface and interface analysis system (sample preparation and transfer under UHV), coupled to a high pressure reactor fitted for in situ PM-IRAS and SFG spectroscopy.
Recently, this was extended by combining mass spectroscopy and PEEM microscopy, which enables one determine the global and local reaction kinetics of surface reactions on polycrystalline noble metals (foil), respectively. Imaging surface reactions by PEEM allows to investigate differently oriented crystalline grains (µm-size) simultaneously under identical reaction conditions (temperature, gas flux), giving access to specific activities, coupling effects between facets, oscillatory behavior etc. This approach has been successfully applied to Pt and Pd surfaces and will be extended to bimetallic PdZn in the near future.
GR's group is specifically recognized for ambient pressure studies of catalytic reactions on UHV-grown nanoparticles, an approach that has only been developed by very few groups worldwide. Topics of current and planned projects encompass the characterization of structure and functionality of monometallic and bimetallic noble metals (e.g. Pd, Au, Ni, Pd-Au, Pd-Zn) supported on various oxides (Al2O3, CeO2, ZrO2), with a focus on their interaction with gas phase molecules (e.g. CO, H2, O2, MeOH)
The group also performs related in situ studies on industrial-grade catalysts by FTIR, EXAFS, XPS, XRD, complemented by electron microscopy for structure characterization. The comparison with the UHV-grown model catalysts allows revealing the specifics of the respective catalytic materials.