Scientific profile Christian Papp

My research is focused on the in situ analysis of surfaces and interfaces facilitating synchrotron radiation for

1. the fundamental understanding of surface processes,
2. materials characterization and synthesis,
3. the analysis of heterogeneous catalysis,
4. development of new in situ techniques.

In the following his main achievements and projects of the past years are summarized.

Fundamental insights in surface reactions

These studies, performed with in situ high-resolution X-ray photoelectron spectroscopy at third generation synchrotron sources, resulted in an unprecedented insight to surface reactions. Various basic concepts of analyzing surface reactions were used and a deep understanding of the processes ongoing on the surface were analyzed. The reactions studied range from the oxidation of sulfur and CO to C-H and C-C activation as well as C-C coupling on surfaces. The analysis allowed for determining various reaction intermediates, the reaction kinetics, kinetic isotope effects, activation energies and the rate determining step in the surface reactions. Also various spectroscopic details were evaluated with XPS such as the vibrational properties of adsorbates and reaction intermediates, adsorption sites, also “within” a molecule, were analyzed. For the analysis of surface reactions, new time resolved measurements methods were established. These studies are setting a standard for a thorough and complete molecular understanding of the surface reactions, with all reaction intermediates identified and the kinetics of the reaction determined.

See e.g.
C. Papp, H.-P. Steinrück Surf. Sci. Rep. 68 (2013) 446.
R. Streber, C. Papp et al. Angew. Chem. Int. Ed. 48 (2009) 9925.

Liquid metals / alloys

Liquid metals such as gallium and alloys of gallium with catalytically active metals are a novel topic from the fundamental side, since they allow for studying liquid metallic systems in an UHV environment. Additionally, they are of interest for heterogeneous catalysis as they have been shown to be effective catalysts with only a fraction of the often expensive transition metal content. These systems therefore promise new and fascinating insights to the liquid / gas interface, and they allow for studying the influence of low concentrations of the catalytically active metal in an inert matrix. Thus, typical effects such as ligand and ensemble effects as well as the changes in the electronic structure can be studied in a liquid system.

See e.g. Taccardi, Papp et al. Nat. Chem. 10 (2017) 862; Wittkämper, Papp et al. J. Chem. Phys. (2020) DOI: 10.1063/5.0021647

Liquid organic hydrogen carriers

Besides the fundamental understanding of the reaction of small molecules, the reactivity of larger molecules is investigated in the framework of the Cluster of Excellence ‘Engineering of Advanced Materials’ and a cooperation with BMW. Studying these particular systems is motivated by one of the grand present challenges of mankind, namely the storage of energy. Among several approaches, one potential solution is “chemical storage” of hydrogen using Liquid Organic Hydrogen Carrier (LOHC) materials. These substances are high boiling organic molecules, which can be reversibly hydrogenated and dehydrogenated using heterogeneous catalysts.
Despite the high relevance of such systems, the molecular level understanding of the catalytic dehydrogenation and hydrogenation of LOHCs is still at its infancy. This is partly due to the size of the molecules (forty and more atoms) which makes them a major challenge for surface science methods. Nevertheless, we demonstrated that by an in situ XPS study of such molecules on model catalysts, we can obtain detailed insights into the mechanisms of dehydrogenation and also of relevant side-reactions, at the molecular level.

See e.g.
C. Gleichweit C. Papp et al. ChemSusChem 6 (2013) 974.
C. Papp et al. Chem. Rec. 14 (2014) 879.
P. Preuster, C. Papp et al. Acc. Chem. Res. 50 (2017) 74.

 

Growth and Chemical Modification of Graphene

Graphene, i.e., a single layer of graphite, is a promising candidate for future carbon-based electronics. Its high potential for applications stems from the specific electronic structure of freestanding graphene, which shows a linear dispersion at the Dirac point, indicating the existence of relativistic quasiparticles. The growth of graphene on metal surfaces has received significant attention as a potential low energy route for its production: the required growth temperatures are significantly lower than on SiC, but nevertheless high-quality, large area graphene sheets can be formed. In our investigations we synthesized graphene on transition metal surfaces and thereafter characterized the system with angle resolved photoemission and x-ray photoemission. Besides pristine graphene we demonstrated the synthesis of doped graphene layers, with dopants such as nitrogen and boron, with various approaches. For all cases, we studied the band structure and thus the changes that occurred due to doping. In our studies, we observed coexisting structures of graphene on a nickel surface and different doping, i.e. n-or p- type doping, depending on the dopant geometry. As a further graphene-related topic we use monodisperse metal nanoclusters grown on a graphene Moire as a model system for catalysis. Here the aim is to understand carbon-supported heterogeneous catalysis, with the possibility to tune the cluster chemistry by tuning the graphene electronic structure.

See e.g.
J. Englert, C. Papp et al. Nat. Chem. 3 (2011) 279.
R. Koch, C. Papp et al. Phys. Rev. B 86 (2012) 75401.
K. Gotterbarm, C. Papp et al. J. Phys. Chem. C 118 (2014) 15934.
C. Papp Catal. Lett. 147 (2017)2683.

Near ambient pressure X-ray photoemission

A further in situ technique used in the group of C. Papp is near ambient pressure X-ray photoelectron spectroscopy (NAPXPS), a modern tool to study the surface of liquids and solid surfaces under ambient conditions. This technique allows insights in various areas, particularly in atmospheric, environment and catalysis sciences. NAPXPS adds important new information in the field of surfaces in the presence of gases and vapors, closing the gap between high pressure and ultra high vacuum conditions.
The systems studied range from model catalysis systems such as the CO oxidation on platinum, to of novel heterogeneous catalysts as Pt nanoparticles on and in titania nanotubes, ethanol steam reforming on Co ceria systems, bimetallic catalysts such as Pt/Ga and to liquid systems as in the case of the CO₂ capture reaction of functionalized ionic liquids.

See e.g.
I. Niedermayer, C. Papp et al. J. Am. Chem. Soc. 136 (2014) 436.
L. Ovari, C. Papp et al. J. Cat. 307 (2013) 132.
S. Krick Calderon, C. Papp et al. Angew. Chem. Int Ed. 56 (2017) 2594.
N. Taccardi, C. Papp et al. Nat. Chem. 9 (2017) 862.

Hard X-ray photoemission and angle resolved hard X-ray photoemission

The use of hard X-rays in photoemission opens the new opportunities in the field of materials characterization. The inherent surface sensitivity of soft X-rays paved the way to investigate surface properties of novel materials; nevertheless, information on the bulk properties such as the electronic structure cannot be obtained easily. C Papp was involved in the pioneering experiments that showed that hard x-rays with a significantly higher escape depth of the photoelectrons allow to directly study buried interfaces, omnipresent in electronic devices, as e.g, gating materials, or the bulk electronic structure, with this new approach.

See e.g.
A. Gray, C. Papp et al. Nat. Mat. 10 (2011) 759.
C. Papp et al. J. Appl. Phys. 112 (2012) 114501.

Soft X-ray standing waves

A second approach to study buried interfaces are soft X-ray standing wave experiments. This technique, based on the hard X-ray standing wave technique, now allows for investigating systems that have dimensions, relevant to industrial applications, i.e. nanometers. Typical systems studied were strongly correlated materials, layered systems for spintronic applications, etc.

See e.g.
S. H. Yang, C. Papp et al. Phys. Rev. B 84 (2011) 184410.
A. Gray, C. Papp et al. Phys. Rev. B 82 (2010) 205116.