arXiv subjects

Florian Kraushofer

Publications and source records attributed to Florian Kraushofer.

At least 19 recordsLinked to original sources

arXiv2026

CO on a Rh/Fe3O4 single-atom catalyst: high-resolution infrared spectroscopy and near-ambient-pressure scanning tunnelling microscopy

Infrared reflection absorption spectroscopy (IRAS) offers a powerful route to bridging the materials and pressure gaps between surface science and powder catalysis. Using a newly developed IRAS setup optimised for dielectric single crystals, we investigate CO adsorption on the model single-atom catalyst Rh/Fe3O4(001). IRAS resolves three species: monocarbonyls at isolated, twofold-coordinated Rh adatoms, monocarbonyls at fivefold-coordinated Rh atoms embedded in the surface, and gem-dicarbonyls at isolated, twofold-coordinated Rh adatoms. Under ultra-high vacuum (UHV) conditions, RhCO monocarbonyl species at adatom sites dominate. Rh(CO)2 gem-dicarbonyl formation is kinetically hindered and occurs predominantly through CO-induced dissociation of Rh dimers rather than sequential adsorption of two CO molecules at an isolated, twofold Rh adatom. The sequential-adsorption pathway to Rh(CO)2 becomes accessible at millibar CO pressures as evidenced by near-ambient-pressure scanning tunnelling microscopy (NAP-STM). These findings link the UHV behaviour to that expected under realistic reaction conditions. Assignments of the vibrational frequencies to individual species rely on isotopic labelling, thermal treatments, and a review of previous SPM, XPS, and TPD data, supported by density functional theory (DFT)-based calculations. While theory provides qualitative insight, such as the instability of dicarbonyls on fivefold-coordinated Rh atoms, it does not yet reproduce experimental frequencies quantitatively and is sensitive to the computational parameters, highlighting the need for robust experimental benchmarks. The spectroscopic fingerprints established here provide a reliable foundation for identifying Rh coordination environments in oxide-supported single-atom catalysts.

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arXiv2026

Hydrogen Activation via Dihydride Formation on a Rh1/Fe3O4(001) Single-Atom Catalyst

Hydrogen activation is a key elementary step in catalytic hydrogenation. In heterogeneous catalysis, it usually proceeds through dissociative adsorption on metal nanoparticles followed by surface diffusion or spillover, whereas homogeneous catalysts activate H2 through dihydride or dihydrogen intermediates at a single metal center. Here, we show that isolated Rh adatoms supported on Fe3O4(001) activate hydrogen through formation of a stable dihydride species without atomic H spillover. Temperature-programmed desorption, X-ray photoelectron spectroscopy, and scanning tunneling microscopy collectively reveal strong (approximately 1 eV) hydrogen adsorption exclusively at isolated Rh1 sites, while isotope-exchange experiments further demonstrate that hydrogen remains localized. Density-functional theory based calculations indicate a barrierless conversion from molecular H2 to the dihydride, and random-phase approximation calculations further confirm the relative stability of the dihydride. Together, these results show that single-atom Rh sites cleave and bind H2 through a dihydride pathway analogous to homogeneous complexes, establishing a mechanistic bridge between homogeneous and heterogeneous catalysis.

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arXiv2025

Structural Optimization in Tensor LEED Using a Parameter Tree and $R$-Factor Gradients

Quantitative low-energy electron diffraction [LEED $I(V)$] is a powerful method for surface-structure determination, based on a direct comparison of experimentally observed $I(V)$ data with computations for a structure model. As the diffraction intensities $I$ are highly sensitive to subtle structural changes, local structure optimization is essential for assessing the validity of a structure model and finding the best-fit structure. The calculation of diffraction intensities is well established, but the large number of evaluations required for reliable structural optimization renders it computationally demanding. The computational effort is mitigated by the tensor-LEED approximation, which accelerates optimization by applying a perturbative treatment of small deviations from a reference structure. Nevertheless, optimization of complex structures is a tedious process. Here, the problem of surface-structure optimization is reformulated using a tree-based data structure, which helps to avoid redundant function evaluations. In the new tensor-LEED implementation presented in this work, intensities are computed on the fly, eliminating limitations of previous algorithms that are limited to precomputed values at a grid of search parameters. It also enables the use of state-of-the-art optimization algorithms. Implemented in \textsc{Python} with the JAX library, the method provides access to gradients of the $R$ factor and supports execution on graphics processing units (GPUs). Based on these developments, the computing time can be reduced by more than an order of magnitude.

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arXiv2024

ViPErLEED package I: Calculation of $I(V)$ curves and structural optimization

Low-energy electron diffraction (LEED) is a widely used technique in surface-science. Yet, it is rarely used to its full potential. The quantitative information about the surface structure, contained in the modulation of the intensities of the diffracted beams as a function of incident electron energy, LEED I(V), is underutilized. To acquire these data, minor adjustments would be required in most experimental setups, but existing analysis software is cumbersome to use. ViPErLEED (Vienna package for Erlangen LEED) lowers these barriers, introducing a combined solution for data acquisition, extraction, and computational analysis. These parts are discussed in three separate publications. Here, the focus is on the computational part of ViPErLEED, which performs automated LEED-I(V) calculations and structural optimization. Minimal user input is required, and the functionality is significantly enhanced compared to existing solutions. Computation is performed by embedding the Erlangen tensor-LEED package (TensErLEED). ViPErLEED manages parallelization, monitors convergence, and processes input and output. This makes LEED I(V) more accessible to new users while minimizing the potential for errors and the manual labor. Added functionality includes structure-dependent defaults, automatic detection of bulk and surface symmetries and their relationship, automated symmetry-preserving search procedures, adjustments to the TensErLEED code to handle larger systems, as well as parallelization and optimization. Modern file formats are used as input and output, and there is a direct interface to the Atomic Simulation Environment (ASE) package. The software is implemented primarily in Python (version >=3.7) and provided as an open-source package (GNU GPLv3 or later). A structure determination of the $α$-Fe2O3(1-102)-(1x1) surface is presented as an example for the application of the software.

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arXiv2024

Digging its own Site: Linear Coordination Stabilizes a Pt1/Fe2O3 Single-Atom Catalyst

Determining the local coordination of the active site is a pre-requisite for the reliable modeling of single-atom catalysts (SACs). Obtaining such information is difficult on powder-based systems, so much emphasis is placed on density functional theory-based computations based on idealized low-index surfaces of the support. In this work, we investigate how Pt atoms bind to the (1-102) facet of Fe2O3, a common support material in SAC. Using a combination of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and an extensive computational evolutionary search, we find that Pt atoms significantly reconfigure the support lattice to facilitate a pseudo-linear coordination to surface oxygen atoms. Despite breaking three surface Fe-O bonds, this geometry is favored by 0.84 eV over the best configuration involving an unperturbed support. We suggest that the linear O-Pt-O configuration is common in reactive Pt-based SAC systems because it balances thermal stability with the ability to adsorb reactants from the gas phase, and that extensive structural searches are likely necessary to determine realistic active site geometry in single-atom catalysis.

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arXiv2024

ViPErLEED package II: Spot tracking, extraction and processing of I(V) curves

As part of the ViPErLEED project (Vienna package for Erlangen LEED, low-energy electron diffraction), computer programs have been developed for facile and user-friendly data extraction from movies of LEED images. The programs make use of some concepts from astronomical image processing and analysis. As a first step, flat-field and dark-frame corrections reduce the effects of inhomogeneities of the camera and screen. In a second step, for identifying all diffraction maxima ("spots"), it is sufficient to manually mark and label a single spot or very few spots. Then the program can automatically identify all other spots and determine the distortions of the image. This forms the basis for automatic spot tracking (following the "beams" as they move across the LEED screen) and intensity measurement. Even for complex structures with hundreds to a few thousand diffraction beams, this step takes less than a minute. The package also includes a program for further processing of these I(V) curves (averaging of equivalent beams, manual and/or automatic selection, smoothing) as well as several utilities. The software is implemented as a set of plugins for the public-domain image processing program ImageJ and provided as an open-source package.

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arXiv2023

Oxygen-Terminated (1x1) Reconstruction of Reduced Magnetite Fe$_3$O$_4$(111)

The (111) facet of magnetite (Fe$_3$O$_4$) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Fe$_{\rm oct2}$ termination in reducing conditions. All three structures change the coordination of iron in the kagome Fe$_{\rm oct1}$ layer to tetrahedral. With atomically-resolved microscopy techniques, we show that the termination that coexists with the Fe$_{\rm tet1}$ termination consists of tetrahedral iron capped by three-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.

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arXiv2023

Real-space investigation of polarons in hematite Fe2O3

In polarizable materials, electronic charge carriers interact with the surrounding ions, leading to quasiparticle behaviour. The resulting polarons play a central role in many materials properties including electrical transport, optical properties, surface reactivity and magnetoresistance, and polaron properties are typically investigated indirectly through such macroscopic characteristics. Here, noncontact atomic force microscopy (nc-AFM) is used to directly image polarons in Fe2O3 at the single quasiparticle limit. A combination of Kelvin probe force microscopy (KPFM) and kinetic Monte Carlo (KMC) simulations shows that Ti doping dramatically enhances the mobility of electron polarons, and density functional theory (DFT) calculations indicate that a metallic transition state is responsible for the enhancement. In contrast, hole polarons are significantly less mobile and their hopping is hampered further by the introduction of trapping centres.

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arXiv2023

Hematite $α-Fe_{2}O_{3}(0001)$ in top and side view: resolving long-standing controversies about its surface structure

Hematite $α-Fe_{2}O_{3}(0001)$ is the most-investigated iron oxide model system in photo and electrocatalytic research. The rich chemistry of Fe and O allows for many bulk and surface transformations, but their control is challenging. This has led to controversies regarding the structure of the topmost layers. This comprehensive study combines surface methods (nc-AFM, STM, LEED, and XPS) complemented by structural and chemical analysis of the near-surface bulk (HRTEM and EELS). The results show that a compact 2D layer constitutes the topmost surface of $α-Fe_{2}O_{3}(0001)$; it is locally corrugated due to the mismatch with the bulk. Assessing the influence of naturally-occurring impurities shows that these can force the formation of surface phases that are not stable on pure samples. Impurities can also cause the formation of ill-defined inclusions in the subsurface and modify the oxidation phase diagram of hematite. The results provide a significant step forward in determining the hematite surface structure that is crucial for accurately modeling catalytic reactions. Combining surface and cross-sectional imaging provided the full view that is essential for understanding the evolution of the near-surface region of oxide surfaces under oxidative conditions.

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arXiv2022

Single-Atom Catalysis: Insights from Model Systems

The field of single atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modelling SAC utilize periodic boundary conditions and low index facets of an idealized support. Thus the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modelling, i.e. comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh vacuum (UHV) environment. In this article, we review the status of the surface science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular the surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well as CeO2 and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low index surfaces of typical supports. In a perspective, we discuss the possibility for expanding such studies into other relevant supports.

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arXiv2022

Single Rh adatoms stabilized on α-Fe2O3(1-102) by co-adsorbed water

Oxide-supported single-atom catalysts are commonly modelled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Here, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and x-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on alpha-Fe2O3(1-102) in the presence of 2x10^(-8) mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 °C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms, and argue that their presence should be assumed when modelling SAC systems.

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arXiv2022

Rapid oxygen exchange between hematite and water vapor

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline r-cut (1-102) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

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arXiv2022

CO oxidation by Pt2/Fe3O4: metastable dimer and support configurations facilitate lattice oxygen extraction

Heterogeneous catalysts based on sub-nanometer metal clusters often exhibit strongly size-dependent properties, and the addition or removal of a single atom can make all the difference. Identifying the most active species and deciphering the reaction mechanism is extremely difficult, however, because it is often not clear how the catalyst evolves in operando. Here, we utilize a combination of atomically resolved scanning probe microscopies, spectroscopic techniques, and density functional theory (DFT)-based calculations to study CO oxidation by a model Pt/Fe3O4(001) single-atom catalyst. We demonstrate that (PtCO)2 dimers, formed dynamically through the agglomeration of mobile Pt-carbonyl species, catalyse a reaction involving the oxide support to form CO2. Pt2 dimers produce one CO2 molecule before falling apart into two adatoms, releasing the second CO. Interestingly, Olattice extraction only becomes facile when both the Pt-dimer and the Fe3O4 support can access metastable configurations, suggesting that substantial, concerted rearrangements of both cluster and support must be considered for reactions occurring at elevated temperature.

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arXiv2021

Surface Reduction State Determines Stabilization and Incorporation of Rh on α-Fe2O3(1-102)

Iron oxides (FeOx) are among the most common support materials utilized in single atom catalysis. The support is nominally Fe2O3, but strongly reductive treatments are usually applied to activate the as-synthesized catalyst prior to use. Here, we study Rh adsorption and incorporation on the (1-102) surface of hematite (α-Fe2O3), which switches from a stoichiometric (1x1) termination to a reduced (2x1) reconstruction in reducing conditions. Rh atoms form clusters at room temperature on both surface terminations, but Rh atoms incorporate into the support lattice as isolated atoms upon annealing above 400 °C. Under mildly oxidizing conditions, the incorporation process is so strongly favoured that even large Rh clusters containing hundreds of atoms dissolve into the surface. Based on a combination of low energy ion scattering and scanning tunnelling microscopy data, as well as density functional theory, we conclude that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer.

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arXiv2021

Atomic-scale studies of Fe3O4(001) and TiO2(110) surfaces following immersion in CO2-acidified water

Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water, and studied its interaction with TiO2 and Fe3O4 surfaces. Here, we describe a simple approach to vary the pH through the partial pressure of CO2 (pCO2) in the surrounding vacuum chamber, and use this to study how these surfaces react to an acidic solution. The TiO2(110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe3O4(001)-(rt2 x rt2)R45 surface begins to dissolve at a pH 4.0-3.9 (pCO2 = 0.8-1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe3O4(001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced, and contains a significant accumulation of bicarbonate (HCO3-) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO3)2), which precipitate onto the surface when the water evaporates.

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arXiv2021

Ni modified Fe3O4(001) surface as a simple model system for understanding the Oxygen Evolution Reaction

Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4 (001) single crystal, using a combination of surface science techniques in ultra-high-vacuum such as low energy electron diffraction (LEED), x-rays photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an Fe:Ni composition ratio in the range 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized.

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arXiv2021

Unravelling CO adsorption on model single-atom catalysts

Understanding how the local environment of a single-atom catalyst affects stability and reactivity remains a significant challenge. We present an in-depth study of Cu1, Ag1, Au1, Ni1, Pd1, Pt1, Rh1, and Ir1 species on Fe3O4(001); a model support where all metals occupy the same 2-fold coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag1-CO) or weaken it (as for Ni1-CO), but CO-induced structural distortions reduce adsorption energies from those expected based on electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.

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arXiv2020

IrO2 Surface Complexions Identified Through Machine Learning and Surface Investigations

A Gaussian Approximation Potential (GAP) was trained using density-functional theory data to enable a global geometry optimization of low-index rutile IrO2 facets through simulated annealing. Ab initio thermodynamics identifies (101) and (111) (1x1)-terminations competitive with (110) in reducing environments. Experiments on single crystals find that (101) facets dominate, and exhibit the theoretically predicted (1x1) periodicity and X-ray photoelectron spectroscopy (XPS) core level shifts. The obtained structures are analogous to the complexions discussed in the context of ceramic battery materials.

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