Christian Papp

RESEARCH, TEACHING, or OTHER INTERESTS

Physical and Theoretical Chemistry

Scopus Publications

Erratum to “Reactivity of graphene-supported Co clusters”
Natalie J. Waleska-Wellnhofer, Sophie Arzig, Fabian Düll, Udo Bauer, Phiona Bachmann, Johann Steinhauer, Christian Papp, and Thomas Risse
Elsevier BV



Engineering two-dimensional supramolecular self-assembly: The role of Cl atoms
Alisson Ceccatto, Gustavo Ramon Campi, Vanessa Carreño Diaz, Eidsa Brenda da Costa Ferreira, Natalie J. Waleska-Wellnhofer, Eva Marie Freiberger, Simon Jaekel, Christian Papp, Hans-Peter Steinrück, Duncan John Mowbray,et al.
Elsevier BV



Model Catalytic Studies on the Thermal Dehydrogenation of the Benzaldehyde/Cyclohexylmethanol LOHC System on Pt(111)
Valentin Schwaab, Felix Hemauer, Julien Steffen, Natalie J. Waleska‐Wellnhofer, Eva Marie Freiberger, Marius Steinmetz, Andreas Görling, Peter Wasserscheid, Hans‐Peter Steinrück, and Christian Papp
Wiley
AbstractWe investigated the dehydrogenation reaction and the thermal robustness of the liquid organic hydrogen carrier (LOHC) couple benzaldehyde/cyclohexylmethanol on a Pt(111) model catalyst in situ in synchrotron radiation photoelectron spectroscopy‐ and complementary temperature‐programmed desorption experiments. The system stores hydrogen in a cyclohexyl group and a primary alcohol functionality and achieves an attractive hydrogen storage capacity of 7.0 mass %. We observed a stepwise dehydrogenation mechanism, characterized by a low temperature dehydrogenation of the alcohol group at 235 K. However, stability limitations challenge the system's application as reversible hydrogen storage solution, as the resultant aldehyde was found to decompose during the dehydrogenation of its cyclohexyl group (between 250 and 350 K). A comparison of cyclohexylmethanol with the structurally related secondary alcohol (1‐cyclohexylethanol; 6.3 mass % hydrogen) revealed a parallel stepwise dehydrogenation pattern for both compounds, but a technically relevant superior thermal robustness of the latter, demonstrating the influence of the alcohol‐group's substitution degree on the dehydrogenation characteristics of alcohol‐functionalized LOHCs. Density functional theory calculations are in agreement with the experimentally observed stability trend.


Reactivity of graphene-supported Co clusters
Natalie J. Waleska-Wellnhofer, Sophie Arzig, Fabian Düll, Udo Bauer, Phiona Bachmann, Johann Steinhauer, Christian Papp, and Thomas Risse
Elsevier BV



Poisoning Resistance of Liquid GaPt Supported Catalytically Active Liquid Metal Solutions Model Systems
Christoph Wichmann, Michael Moritz, Haiko Wittkämper, Tzung-En Hsieh, Johannes Frisch, Marcus Bär, Hans-Peter Steinrück, and Christian Papp
American Chemical Society (ACS)



Supported Catalytically Active Liquid Metal Solutions: Liquid Metal Catalysis with Ternary Alloys, Enhancing Activity in Propane Dehydrogenation
Michael Moritz, Sven Maisel, Narayanan Raman, Haiko Wittkämper, Christoph Wichmann, Mathias Grabau, Deniz Kahraman, Julien Steffen, Nicola Taccardi, Andreas Görling,et al.
American Chemical Society (ACS)



The Norbornadiene/Quadricyclane Pair as Molecular Solar Thermal Energy Storage System: Surface Science Investigations
Felix Hemauer, Hans‐Peter Steinrück, and Christian Papp
Wiley
AbstractFor the transition to renewable energy sources, novel energy storage materials are more important than ever. This review addresses so‐called molecular solar thermal (MOST) systems, which appear very promising since they combine light harvesting and energy storing in one‐photon one‐molecule processes. The focus is on norbornadiene (NBD), a particularly interesting candidate, which is converted to the strained valence isomer quadricyclane (QC) upon irradiation. The stored energy can be released on demand. The energy‐releasing cycloreversion from QC to NBD can be initiated by a thermal, catalytic, or electrochemical trigger. The reversibility of the energy storage and release cycles determines the general practicality of a MOST system. In the search for derivatives, which enable large‐scale applications, fundamental surface science studies help to assess the feasibility of potential substituted NBD/QC couples. We include investigations under well‐defined ultra‐high vacuum (UHV) conditions as well as experiments in liquid phase. Next to the influence of the catalytically active surfaces on the isomerization between the two valence isomers, information on adsorption geometries, thermal stability limits, and reaction pathways of the respective molecules are discussed. Moreover, laboratory‐scaled test devices demonstrate the proof of concept in various areas of application.


h-BN in the making: The surface chemistry of borazine on Rh(111)
Eva Marie Freiberger, Fabian Düll, Phiona Bachmann, Johann Steinhauer, Federico J. Williams, Hans-Peter Steinrück, and Christian Papp
AIP Publishing
Borazine is a well-established precursor molecule for the growth of hexagonal boron nitride (h-BN) via chemical vapor deposition on metal substrates. To understand the formation of the h-BN/Rh(111) moiré from borazine on a molecular level, we investigated the low-temperature adsorption and thermally induced on-surface reaction of borazine on Rh(111) in situ using synchrotron radiation-based high-resolution x-ray photoelectron spectroscopy (XPS), temperature-programmed XPS, and near-edge x-ray absorption fine structure measurements. We find that borazine adsorbs mainly as an intact molecule and have identified a flat-lying adsorption geometry. Borazine multilayers are observed to desorb below 200 K. Starting at about 300 K, dehydrogenation of the remaining borazine and borazine fragments takes place, and disordered boron nitride starts to grow. Above 600 K, the formation of the h-BN sets in. Finally, at 1100 K, the conversion to h-BN is complete. The h-BN formed by deposition and post-annealing was compared to the h-BN grown by an established procedure, proving the successful preparation of the desired two-dimensional material.


Impact of Catalysis-Relevant Oxidation and Annealing Treatments on Nanostructured GaRh Alloys
Tzung-En Hsieh, Johannes Frisch, Regan G. Wilks, Christian Papp, and Marcus Bär
American Chemical Society (ACS)
In this study, we examine the surface-derived electronic and chemical structures of nanostructured GaRh alloys as a model system for supported catalytically active liquid metal solutions (SCALMS), a novel catalyst candidate for dehydrogenation reactions that are important for the petrochemical and hydrogen energy industry. It is reported that under ambient conditions, SCALMS tends to form a gallium oxide shell, which can be removed by an activation treatment at elevated temperatures and hydrogen flow to enhance the catalytic reactivity. We prepared a 7 at. % Rh containing the GaRh sample and interrogated the evolution of the surface chemical and electronic structure by photoelectron spectroscopy (complemented by scanning electron microscopy) upon performing surface oxidation and (activation treatment mimicking) annealing treatments in ultrahigh vacuum conditions. The initially pronounced Rh 4d and Fermi level-derived states in the valence band spectra disappear upon oxidation (due to formation of a GaOx shell) but reemerge upon annealing, especially for temperatures of 600 °C and above, i.e., when the GaOx shell is efficiently being removed and the Ga matrix is expected to be liquid. At the same temperature, new spectroscopic features at both the high and low binding energy sides of the Rh 3d5/2 spectra are observed, which we attribute to new GaRh species with depleted and enriched Rh contents, respectively. A liquefied and GaOx-free surface is also expected for GaRh SCALMS at reaction conditions, and thus the revealed high-temperature properties of the GaRh alloy provide insights about respective catalysts at work.


Bromination of 2D materials
Eva Marie Freiberger, Julien Steffen, Natalie J Waleska-Wellnhofer, Felix Hemauer, Valentin Schwaab, Andreas Görling, Hans-Peter Steinrück, and Christian Papp
IOP Publishing
Abstract The adsorption, reaction and thermal stability of bromine on Rh(111)-supported hexagonal boron nitride (h-BN) and graphene were investigated. Synchrotron radiation-based high-resolution x-ray photoelectron spectroscopy (XPS) and temperature-programmed XPS allowed us to follow the adsorption process and the thermal evolution in situ on the molecular scale. On h-BN/Rh(111), bromine adsorbs exclusively in the pores of the nanomesh while we observe no such selectivity for graphene/Rh(111). Upon heating, bromine undergoes an on-surface reaction on h-BN to form polybromides (170–240 K), which subsequently decompose to bromide (240–640 K). The high thermal stability of Br/h-BN/Rh(111) suggests strong/covalent bonding. Bromine on graphene/Rh(111), on the other hand, reveals no distinct reactivity except for intercalation of small amounts of bromine underneath the 2D layer at high temperatures. In both cases, adsorption is reversible upon heating. Our experiments are supported by a comprehensive theoretical study. DFT calculations were used to describe the nature of the h-BN nanomesh and the graphene moiré in detail and to study the adsorption energetics and substrate interaction of bromine. In addition, the adsorption of bromine on h-BN/Rh(111) was simulated by molecular dynamics using a machine-learning force field.


Engineering large nanoporous networks with size and shape selected by appropriate precursors
Alisson Ceccatto, Eva Marie Freiberger, Natalie J. Waleska-Wellnhofer, Simon Jaekel, Duncan John Mowbray, Christian Papp, Hans-Peter Steinrück, and Abner de Siervo
Elsevier BV



Study on Growth of Tungsten Bronze Phase from Niobate Perovskite Ceramics in Controlled Atmosphere for Photoferroelectric Applications
Xi Shi, Yang Bai, Christoph Wichmann, Michael Moritz, Michel Kuhfuß, Christian Papp, and Neamul H Khansur
Wiley
AbstractRecent research has found that by introducing A‐site deficiency into Ba/Ni co‐doped (K,Na)NbO3 ABO3‐type perovskite, a beneficial interface for photoferroelectric applications is formed between the perovskite and tungsten bronze (TB) phases. To date, such an interface is formed only spontaneously, and the growth mechanism of the TB phase in the perovskite phase is unclear. This work investigates controlled interface formation using KNBNNO (K0.50Na0.44Ba0.04Ni0.02Nb0.98O2.98) annealed at different temperatures for different durations, and in various atmospheres. Structural, microstructural, and chemical analyses suggest that vacuum, N2, and O2 atmospheres promote the growth of the TB phase from the sample surface, of which the thickness increases with annealing temperature and duration. In contrast, annealing in air does not promote such growth due to lower evaporation of K and Na. Among all atmospheres, the growth starts the earliest, i.e., at 800 °C, in vacuum compared to that as late as 1000 °C in O2. The association of growth of the TB phase with the degree of alkali volatilization that is dependent on the atmosphere, and that with the resultant variation in diffusion rate, uncovers the formation mechanism of the beneficial interface that may also be applicable to other KNN‐based materials for advanced photoferroelectric applications.


Au-Catalyzed Energy Release in a Molecular Solar Thermal (MOST) System: A Combined Liquid-Phase and Surface Science Study
Roman Eschenbacher, Felix Hemauer, Evanie Franz, Andreas Leng, Valentin Schwaab, Natalie J. Waleska‐Wellnhofer, Eva Marie Freiberger, Lukas Fromm, Tao Xu, Andreas Görling,et al.
Wiley
AbstractMolecular solar thermal systems (MOSTs) are molecular systems based on couples of photoisomers (photoswitches), which combine solar energy conversion, storage, and release. In this work, we address the catalytically triggered energy release in the promising MOST couple phenylethylesternorbornadiene/quadricyclane (PENBD/PEQC) on a Au(111) surface in a combined liquid‐phase and surface science study. We investigated the system by photoelectrochemical infrared reflection absorption spectroscopy (PEC‐IRRAS) in the liquid phase, conventional IRRAS and synchrotron radiation photoelectron spectroscopy (SRPES) in ultra‐high vacuum (UHV). Au(111) is highly active towards catalytically triggered energy release. In the liquid phase, we did not observe any decomposition of the photoswitch, no deactivation of the catalyst within 100 conversion cycles and we could tune the energy release rate of the heterogeneously catalyzed process by applying an external potential. In UHV, submonolayers of PEQC on Au(111) are back‐converted to PENBD instantaneously, even at 110 K. Multilayers of PEQC are stable up to ~220 K. Above this temperature, the intrinsic mobility of the film is high enough that PEQC molecules come into direct contact with the Au(111) surface, which catalyzes the back‐conversion. We suggest that this process occurs via a singlet–triplet mechanism induced by electronic coupling between the PEQC molecules and the Au(111) surface.


General Synthesis of Alkyl Amines via Borrowing Hydrogen and Reductive Amination
Matthias Elfinger, Christof Bauer, Jörg Schmauch, Michael Moritz, Christoph Wichmann, Christian Papp, and Rhett Kempe
Wiley
AbstractAmines are a very important class of compounds and the selective synthesis of differently substituted primary, secondary and tertiary alkyl amines is challenging. Here we present the synthesis of primary, secondary, and tertiary alkyl amines from ammonia and alcohols, aldehydes, ketones and hydrogen by combining borrowing hydrogen or hydrogen autotransfer and reductive amination with hydrogen. The key is a nanostructured, bimetallic Co/Sc catalyst able to mediate both reactions or concepts efficiently. We observe a broad product scope, a very good functional group tolerance, upscaling is easily accomplished and our catalyst is reusable.


Author Correction: Isolated Rh atoms in dehydrogenation catalysis (Scientific Reports, (2023), 13, 1, (4458), 10.1038/s41598-023-31157-y)
Haiko Wittkämper, Rainer Hock, Matthias Weißer, Johannes Dallmann, Carola Vogel, Narayanan Raman, Nicola Taccardi, Marco Haumann, Peter Wasserscheid, Tzung-En Hsieh,et al.
Springer Science and Business Media LLC



Isolated Rh atoms in dehydrogenation catalysis
Haiko Wittkämper, Rainer Hock, Matthias Weißer, Johannes Dallmann, Carola Vogel, Narayanan Raman, Nicola Taccardi, Marco Haumann, Peter Wasserscheid, Tzung-En Hsieh,et al.
Springer Science and Business Media LLC
AbstractIsolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga9Rh2 phase and the hitherto not-studied Ga3Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems.


Bromine Adsorption and Thermal Stability on Rh(111): A Combined XPS, LEED and DFT Study
Eva Marie Freiberger, Julien Steffen, Natalie J. Waleska‐Wellnhofer, Anton Harrer, Felix Hemauer, Valentin Schwaab, Andreas Görling, Hans‐Peter Steinrück, and Christian Papp
Wiley
AbstractThis study addresses a fundamental question in surface science: the adsorption of halogens on metal surfaces. Using synchrotron radiation‐based high‐resolution X‐ray photoelectron spectroscopy (XPS), temperature‐programmed XPS, low‐energy electron diffraction (LEED) and density functional theory (DFT) calculations, we investigated the adsorption and thermal stability of bromine on Rh(111) in detail. The adsorption of elemental bromine on Rh(111) at 170 K was followed in situ by XPS in the Br 3d region, revealing two individual, coverage‐dependent species, which we assign to fcc hollow‐ and bridge‐bound atomic bromine. In addition, we find a significant shift in binding energy upon increasing coverage due to adsorbate‐adsorbate interactions. Subsequent heating shows a high thermal stability of bromine on Rh(111) up to above 1000 K, indicating strong covalent bonding. To complement the XPS data, LEED was used to study the long‐range order of bromine on Rh(111): we observe a (√3×√3)R30° structure for low coverages (≤0.33 ML) and a star‐shaped compression structure for higher coverages (0.33–0.43 ML). Combining LEED and DFT calculations, we were able to visualize bromine adsorption on Rh(111) in real space for varying coverages.


Poly(benzimidazobenzophenanthroline)-Ladder-Type Two-Dimensional Conjugated Covalent Organic Framework for Fast Proton Storage**
Mingchao Wang, Gang Wang, Chandrasekhar Naisa, Yubin Fu, Sai Manoj Gali, Silvia Paasch, Mao Wang, Haiko Wittkaemper, Christian Papp, Eike Brunner,et al.
Wiley
AbstractElectrochemical proton storage plays an essential role in designing next‐generation high‐rate energy storage devices, e.g., aqueous batteries. Two‐dimensional conjugated covalent organic frameworks (2D c‐COFs) are promising electrode materials, but their competitive proton and metal‐ion insertion mechanisms remain elusive, and proton storage in COFs is rarely explored. Here, we report a perinone‐based poly(benzimidazobenzophenanthroline) (BBL)‐ladder‐type 2D c‐COF for fast proton storage in both a mild aqueous Zn‐ion electrolyte and strong acid. We unveil that the discharged C−O− groups exhibit largely reduced basicity due to the considerable π‐delocalization in perinone, thus affording the 2D c‐COF a unique affinity for protons with fast kinetics. As a consequence, the 2D c‐COF electrode presents an outstanding rate capability of up to 200 A g−1 (over 2500 C), surpassing the state‐of‐the‐art conjugated polymers, COFs, and metal–organic frameworks. Our work reports the first example of pure proton storage among COFs and highlights the great potential of BBL‐ladder‐type 2D conjugated polymers in future energy devices.


Unraveling the Effect of Rh Isolation on Shallow d States of Gallium-Rhodium Alloys
Tzung-En Hsieh, Sven Maisel, Haiko Wittkämper, Johannes Frisch, Julien Steffen, Regan G. Wilks, Christian Papp, Andreas Görling, and Marcus Bär
American Chemical Society (ACS)



Nano-SMSI on Bimetallic FePt Clusters
Natalie J. Waleska-Wellnhofer, Eva Marie Freiberger, Felix Hemauer, Valentin Schwaab, and Christian Papp
American Chemical Society (ACS)



Surface science and liquid phase investigations of oxanorbornadiene/oxaquadricyclane ester derivatives as molecular solar thermal energy storage systems on Pt(111)
Felix Hemauer, Daniel Krappmann, Valentin Schwaab, Zarah Hussain, Eva Marie Freiberger, Natalie J. Waleska-Wellnhofer, Evanie Franz, Frank Hampel, Olaf Brummel, Jörg Libuda,et al.
AIP Publishing
The transition to renewable energy sources comes along with the search for new energy storage solutions. Molecular solar thermal systems directly harvest and store solar energy in a chemical manner. By a suitable molecular design, a higher overall efficiency can be achieved. In this study, we investigate the surface chemistry of oxa-norbornadiene/quadricyclane derivatives on a Pt(111) surface. Specifically, we focus on the energy storage and release properties of molecules that are substituted with ester moieties of different sizes. For our model catalytic approach, synchrotron radiation-based x-ray photoelectron spectroscopy measurements were conducted in ultra-high vacuum (UHV) and correlated with the catalytic behavior in the liquid phase monitored by photochemical infrared reflection absorption spectroscopy. The differences in their spectral appearance enabled us to unambiguously differentiate the energy-lean and energy-rich isomers and decomposition products. Next to qualitative information on the adsorption motifs, temperature-programmed experiments allowed for the observation of thermally induced reactions and the deduction of the related reaction pathways. We analyzed the selectivity of the cycloreversion reaction from the energy-rich quadricyclane derivative to its energy-lean norbornadiene isomer and competing processes, such as desorption and decomposition. For the 2,3-bis(methylester)-substitution, the cycloreversion reaction was found to occur between 310 and 340 K, while the thermal stability limit of the compounds was determined to be 380 K. The larger 2,3-bis(benzylester) derivatives have a lower apparent adsorption energy and a decomposition onset already at 135 K. In the liquid phase (in acetonitrile), we determined the rate constants for the cycloreversion reaction on Pt(111) to k = 5.3 × 10−4 s−1 for the 2,3-bis(methylester)-substitution and k = 6.3 × 10−4 s−1 for the 2,3-bis(benzylester) derivative. The selectivities were of >99% and 98% for the two molecules, respectively. The difference in the catalytic behavior of Pt(111) for both derivatives is less pronounced in the liquid phase than in UHV, which we attribute to the passivation of the Pt(111) surface by carbonaceous species under ambient conditions.


Liquid Organic Hydrogen Carriers: Model Catalytic Studies on the Thermal Dehydrogenation of 1-Cyclohexylethanol on Pt(111)
Valentin Schwaab, Felix Hemauer, Eva Marie Freiberger, Natalie J. Waleska-Wellnhofer, Hans-Peter Steinrück, and Christian Papp
American Chemical Society (ACS)



Surface Studies on the Energy Release of the MOST System 2-Carbethoxy-3-Phenyl-Norbornadiene/Quadricyclane (PENBD/PEQC) on Pt(111) and Ni(111)
Felix Hemauer, Valentin Schwaab, Eva Marie Freiberger, Natalie J. Waleska, Andreas Leng, Cornelius Weiß, Johann Steinhauer, Fabian Düll, Philipp Bachmann, Andreas Hirsch,et al.
Wiley
AbstractNovel energy‐storage solutions are necessary for the transition from fossil to renewable energy sources. Auspicious candidates are so‐called molecular solar thermal (MOST) systems. In our study, we investigate the surface chemistry of a derivatized norbornadiene/quadricyclane molecule pair. By using suitable push–pull substituents, a bathochromic shift of the absorption onset is achieved, allowing a greater overlap with the solar spectrum. Specifically, the adsorption and thermally induced reactions of 2‐carbethoxy‐3‐phenyl‐norbornadiene/quadricyclane are assessed on Pt(111) and Ni(111) as model catalyst surfaces by synchrotron radiation‐based X‐ray photoelectron spectroscopy (XPS). Comparison of the respective XP spectra enables the distinction of the energy‐rich molecule from its energy‐lean counterpart and allows qualitative information on the adsorption motifs to be derived. Monitoring the quantitative cycloreversion between 140 and 230 K spectroscopically demonstrates the release of the stored energy to be successfully triggered on Pt(111). Heating to above 300 K leads to fragmentation of the molecular framework. On Ni(111), no conversion of the energy‐rich compound takes place. The individual decomposition pathways of the two isomers begin at 160 and 180 K, respectively. Pronounced desorption of almost the entire surface coverage only occurs for the energy‐lean molecule on Ni(111) above 280 K; this suggests weakly bound species. The correlation between adsorption motif and desorption behavior is important for applications of MOST systems in heterogeneously catalyzed processes.


Preparation of geometrically highly controlled Ga particle arrays on quasi-planar nanostructured surfaces as a SCALMS model system
André Hofer, Nicola Taccardi, Michael Moritz, Christoph Wichmann, Sabine Hübner, Dominik Drobek, Matthias Engelhardt, Georg Papastavrou, Erdmann Spiecker, Christian Papp,et al.
Royal Society of Chemistry (RSC)
2D SCALMS model system preparation offering a high degree of geometric control of the Ga matrix particles by combination of nanostructured surfaces with independent adjustment of the substrates' surface chemistry in a thermal decomposition approach.


Temperature-dependent dielectric anomalies in powder aerosol deposited ferroelectric ceramic films
Udo Eckstein, Jörg Exner, Andreja Bencan Golob, Katarina Ziberna, Goran Drazic, Hana Ursic, Haiko Wittkämper, Christian Papp, Jaroslaw Kita, Ralf Moos,et al.
Elsevier BV