arXiv subjects

Zdenek Jakub

Publications and source records attributed to Zdenek Jakub.

10 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.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

arXiv2022

Role of Polarons in Single-Atom Catalysts: Case Study of Me1 [Au1, Pt1, and Rh1] on TiO2(110)

The local environment of metal-oxide supported single-atom catalysts plays a decisive role in the surface reactivity and related catalytic properties. The study of such systems is complicated by the presence of point defects on the surface, which are often associated with the localization of excess charge in the form of polarons. This can affect the stability, the electronic configuration, and the local geometry of the adsorbed adatoms. In this work, through the use of density functional theory and surface-sensitive experiments, we study the adsorption of Rh1, Pt1, and Au1 metals on the reduced TiO2(110) surface; a prototypical polaronic material. A systematic analysis of the adsorption configurations and oxidation states of the adsorbed metals reveals different types of couplings between adsorbates and polarons. As confirmed by scanning tunneling microscopy measurements, the favored Pt1 and Au1 adsorption at oxygen vacancy sites is associated with a strong electronic charge transfer from polaronic states to adatom orbitals, which results in a reduction of the adsorbed metal. In contrast, the Rh1 adatoms interact weakly with the excess charge, which leaves the polarons largely unaffected. Our results show that an accurate understanding of the properties of single-atom catalysts on oxide surfaces requires a careful account of the interplay between adatoms, vacancy sites, and polarons.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

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.

cond-mat.mtrl-sci↗

arXiv2018

Water Agglomerates on Fe3O4(001)

Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task, and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe3O4(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH-H2O bonds drive the formation of partially-dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially-dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature programmed desorption (TPD) reveals the coverage of stable structures, monochromatic x-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and non-contact Atomic Force Microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search.

physics.chem-ph↗

arXiv2016

A Multi-Technique Study of $CO_2$ Adsorption on $Fe_3$$O_4$ Magnetite

The adsorption of $CO_2$ on the $Fe_3$$O_4$(001)-($\sqrt{2}$ $\times$ $\sqrt{2}$)R45° surface was studied experimentally using temperature programmed desorption (TPD), electron spectroscopies (UPS and XPS), and scanning tunneling microscopy (STM). $CO_2$ binds most strongly at defects related to Fe2+ including antiphase domain boundaries in the surface reconstruction and above incorporated Fe interstitials. On the pristine surface, $CO_2$ adsorbs molecularly at fivefold-coordinated Fe3+ sites with a binding energy of 0.4 eV. Above a coverage of 4 molecules per ($\sqrt{2}$ $\times$ $\sqrt{2}$)R45° unit cell, further adsorption results in a compression of the first monolayer up to a density approaching that of a $CO_2$ ice layer. Surprisingly, desorption of the second monolayer occurs at a lower temperature ($\approx$ 84 K) than $CO_2$ multilayers ($\approx$ 88 K), suggestive of a metastable phase or diffusion-limited island growth. The paper also discusses design considerations for a vacuum system optimized to study the surface chemistry of metal oxide single crystals, including the calibration and characterisation of a molecular beam source for quantitative TPD measurements.

cond-mat.mtrl-sci↗