Research Activities

Our research activities focus on atomistic-level understanding of heterogeneously catalyzed reactions including mechanisms, kinetics and thermodynamics of elementary surface processes. We study surface reactions over well-defined single crystals and nanostructured supported model catalysts under ultra high vacuum conditions. The main focus of these studies lies on finding detailed correlations between the structural properties of nano-sized supported catalysts and activity and activity and selectivity of surface reactions. A variety of surface science methods - multi-molecular beam techniques (effusive and supersonic beams), single crystal adsorption calorimetry (SCAC), infrared reflection absorption spectroscopy (IRAS), scanning tunneling microscopy (STM) and polarization-modulation infrared reflection absorption spectroscopy  (PM-IRAS)  – is employed in these studies in order to address both kinetic and thermodynamic aspects of interaction of gas phase molecules with model well-defined surfaces.

Major Research Topics:

  •    Enantioselective reactions on model chirally modified surfaces.
  •    Selective hydrogenation of multi-unsaturated hydrocarbons over nanostructured model supported catalysts.
  •    CO2 hydrogenation to higher mass hydrocarbons: towards renewable energy conversion and storage.
  •    Energetics of surface processes by Single Crystal Adsorption Calorimetry.
  •    Water interaction with oxides

 

Enantioselective reactions on chirally modified surfaces

Imparting chirality to non-chiral metal surfaces by adsorption of chiral modifiers is a highly promising route to create effective heterogeneously catalyzed processes for production of enantiopure pharmaceuticals. A molecular-level understanding of enantioselective processes on chiral surfaces is an importance prerequisite for the rational design of new enantiospecific catalysts. In our studies, we are aiming at a fundamental level understanding of the structure of chirally modified surfaces, the bonding of the prochiral substrate on the chiral media and the details of the kinetics and dynamics of enantioselective surface reactions. A full mechanistic picture can be obtained if these aspects will be understood both on the extended single crystal surfaces, mimicking a local interaction of the modifier-substrate complexes with a metal, as well as on the small chirally modified nanoparticles that more accurately resemble the structural properties and high catalytic activity of practically relevant powdered supported catalyst. To achieve these atomistic insights, we apply a combination of ultrahigh vacuum (UHV) based methods for studying reaction kinetics and dynamics (multi-molecular beam techniques) and in-situ surface spectroscopic (IRASA) and microscopic (STM) tools on well-defined model surfaces consisting of metal nanoparticles supported on thin single crystalline oxide films. Complementary, the catalytic behaviour of these chirally modified model surfaces is investigated under ambient pressure conditions with enantiospecific detection of the reaction products that enables detailed atomistic insights into structure-reactivity relationships.

 

Selective hydrogenation of multi-unsaturated hydrocarbons over nanostructured model supported catalysts

Atomistic–level understanding of surface processes is a key prerequisite for rational design of new catalytic and functional materials. In our studies, we investigate mechanisms, kinetics and thermodynamics of heterogeneously catalyzed reactions and adsorption processes on nanostructured model supported catalysts to provide fundamental insights into the surface chemistry. By employing pulsed multi-molecular beam techniques, infrared reflection-absorption spectroscopy and synchrotron-based spectroscopies on well-defined nanostructured model surfaces (e.g. metallic nanoparticles supported over planar thin oxide films grown on metal single crystals), we study mechanistic details of complex multi-pathway surface reactions, such as hydrocarbon transformation in presence of hydrogen or selective hydrogenation of multi-unsaturated hydrocarbons. The ultimate goal of our research is obtaining detailed correlations between reactivity, selectivity and the particular structure of the catalytic surface.

Specifically, we investigate hydrogenation and isomerization of olefins and α,β-unsaturated ketones over Pd nanoparticles supported on well-defined model Fe3O4/Pt(111) oxide film and Pd(111). By varying structural properties and chemical composition (e.g. by depositing surface modifiers) of model catalysts and performing detailed kinetic investigations, we are aiming at an atomistic-level understanding of hydrocarbon conversions with hydrogen and revealing the role of specific surface sites, such as e.g. low-coordinated surface sites, in competing reaction pathways.

Recently, we have shown that selective hydrogenation of the C=O bond in acrolein to form an unsaturated alcohol is possible over Pd(111) with nearly 100 % selectivity. However, this process was found to require a very distinct modification of the Pd(111) surface with an overlayer of oxopropyl spectator species that are formed from acrolein during the initial stages of reaction and turn the metal surface selective towards propenol formation.

Summary of some recent results on olefin hydrogenation over Pd nanoparticles (for more information see Ludwig et al, J. Catal., 2011, 284, 148-156 and Dostert et. al., J. Amer.Chem.Soc. 137 (2015) 13496–13502 )

 

CO2 hydrogenation to higher mass hydrocarbons: towards renewable energy conversion and storage

The activation of small molecules on oxide surfaces is an important elementary step in many catalytic reactions. Of particular interest is the potential use of CO2 as a sustainable and readily available feedstock. Efficient and inexpensive conversion of CO2 by partial hydrogenation into valuable chemicals, such as methanol or higher mass hydrocarbon compounds that can be used as liquid fuels, holds great potential to minimize the dependence of society on fossil fuels and simultaneously contribute to reducing CO2 emission into the atmosphere. Activation and initial steps of hydrogenation of the CO2 molecule are the most important and challenging steps in this process.

In our studies, the adsorption and chemical transformations of CO2 over model oxide materials is investigated by combination of molecular beams (effusive and supersonic), infrared reflection-absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) experiments under ultra-high vacuum conditions.

 

Energetics of Surface Processes by Single Crystal Adsorption Calorimetry

Establishing the correlation between the energetics of adsorbate-surface interaction and the structural properties of a catalyst is an important fundamental issue and an essential prerequisite for understanding the realistic catalytic processes. We recently developed and set up a new Single Crystal Adsorption Calorimetry (SCAC) apparatus based on the molecular beam techniques and operated under UHV conditions. This method is applied for quantitative measurements of the adsorption and reaction enthalpies of gaseous molecules on model supported catalysts. The major advantage of using SCAC consists in the ability to directly measure the interaction strength of gaseous molecules with the surface of interest. The traditionally used experimental techniques for probing the energetics of adsorption - temperature-programmed desorption (TPD) and equilibrium adsorption isotherm measurements - provide reliable results only for systems with fully reversible adsorption i.e. most of the catalytically-relevant processes, involving dissociation, reaction with co-adsorbates, clustering or diffusion into bulk cannot be probed by these methods correctly. In our experimental setup, we for the first time combined the direct measurement of adsorption and reaction energies by SCAC with the tools for preparation of model catalysts. This unique combination allows us to perform direct calorimetric measurements on surfaces with different levels of complexity ranging from the extended single crystal metal surfaces to the complex nano-structured supported catalysts, providing an experimental possibility to controllably vary certain structural features and correlate them with the adsorption strength of adsorbed species. Such data provide highly important benchmarks for theoretical calculations and are generally not available at the moment.

We apply SCAC to determine the adsorption heats of gas phase molecules (such as carbon monoxide and oxygen) and surface reaction (e.g. CO oxidation) on Pd nanoparticles supported on a well-define Fe3O4/Pt(111) film. Particularly, we systematically vary the Pd cluster size in the range of ~ 100 to 5000 Pd atoms to address the energetics of CO and O interaction with the nanoparticles of different dimensions. As a reference for interaction with an extended surface, the adsorption heats are also determined on Pd(111). First results showed strong and unexpected dependence of adsorption heats on the particle size, which can be traced back to size-dependent electronic properties of Pd nanoparticles. (Peter et. al., Angew. Chem. Int. Ed. 52 (2013) 5175-5179)

 

Water interaction with oxides

Finding detailed correlation between the energetics of adsorbate-surface interaction and the structural properties of a catalyst is an important fundamental issue and an essential prerequisite for understanding the realistic catalytic processes. Interaction of water with oxide surfaces is particularly important for many technological and environmental applications. In this study, we investigate interaction of water with model Fe3O4(111)/Pt(111) and Fe3O4(100)/Pt(100) surfaces by employing a newly developed single crystal adsorption calorimetry (SCAC) set up to determine the adsorption and dissociation heats of water on these oxides with high accuracy. Complementary, the evolution of water species and their chemical transformations on iron oxide surfaces are monitored by vibrational spectroscopy (IRAS). We show that water dissociates readily on iron oxide surfaces forming a variety of hydroxyl groups and hydroxyl-water complexes. The energetics of these processes depends strongly on the particular surface termination as well as on the surface coverage of species formed during water adsorption and dissociation. Particularly for low water coverages, we show that water dissociates readily on iron oxide surfaces forming a dimer-like hydroxyl-water complex and proved that the generally accepted model of water dissociation to two individual OH groups is incorrect. The combination of adsorption energy measurements with spectroscopic identification of surface species allows us to obtain deep atomistic insights into water interaction with oxides and provide important benchmarks for theoretical calculations. (Dementyev et. al., Angew. Chem. Int. Edit. 54 (2015) 13942-6)