Abstract The surface chemistry of the Fischer-Tropsch catalytic reaction over Co has still several unknows. Here, we report an in-situ X-ray photoelectron spectroscopy study of Co $$\left(0001\right)$$ 0001 and Co( $$10\bar{1}4$$ 10 1 ¯ 4 ), and in-situ high energy surface X-ray diffraction of Co $$\left(0001\right),$$ 0001 , during the Fischer-Tropsch reaction at 0.15 bar - 1 bar and 406 K - 548 K in a H2/CO gas mixture. We find that these Co surfaces remain metallic under all conditions and that the coverage of chemisorbed species ranges from 0.4–1.7 monolayers depending on pressure and temperature. The adsorbates include CO on-top, C/-CxHy and various longer hydrocarbon molecules, indicating a rate-limiting direct CO dissociation pathway and that only hydrocarbon species participate in the chain growth. The accumulation of hydrocarbon species points to the termination step being rate-limiting also. Furthermore, we demonstrate that the intermediate surface species are highly dynamic, appearing and disappearing with time delays after rapid changes in the reactants’ composition.

We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain an atomistic picture of the catalyst's structure under in situ oxidizing and reducing conditions. The Cu/ZnO model catalysts were investigated at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. As the nanoparticles swell during oxidation, they maintain their epitaxy on basal ZnO (000 +/- 1) surfaces, whereas on the vicinal ZnO (10 (1) over bar4) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H-2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas the orientation of Cu NPs on vicinal ZnO was only partially restored. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No Cu-Zn bulk alloy formation is observed. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates.

A proof of concept is shown for the design of a high pressure heterogeneous catalysis reaction cell suitable for surface sensitive x-ray diffraction and x-ray reflectometry over planar samples using high energy synchrotron radiation in combination with mass spectrometry. This design enables measurements in a pressure range from several tens to hundreds of bars for surface investigations under realistic industrial conditions in heterogeneous catalysis or gaseous corrosion studies.

We investigated Cu/ZnO model catalysts for methanol synthesis to obtain an atomistic picture of activation and deactivation processes under in situ oxidizing and reducing conditions. We have investigated Cu nanoparticles with different shapes and aspect ratios grown epitaxially on basal and vicinal ZnO surfaces at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. During oxidation, they maintain their epitaxy on basal ZnO (000-1) surfaces, whereas on the vicinal ZnO (10-14) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas a random orientation is preferred for the (10-14) surface. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No proof for Cu-Zn alloy formation is found. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates, which is detrimental to the catalyst’s performance.

We report on differences in the magnetite (111) surface structure when prepared under oxidizing and reducing conditions. Both preparations were done under UHV conditions at elevated temperatures, but in one case the sample was cooled down while keeping it in an oxygen atmosphere. Scanning tunneling microscopy after each of the preparations showed a different apparent morphology, which is discussed to be an electronic effect and which is reflected in the necessity of using opposite bias tunneling voltages in order to obtain good images. Surface x-ray diffraction revealed that both preparations lead to Fe vacancies, leading to local O-terminations, the relative fraction of which depending on the preparation. The preparation under reducing conditions lead to a larger fraction of Fe-termination. The geometric structure of the two different terminations was found to be identical for both treatments, even though the surface and near-surface regions exhibit small compositional differences; after the oxidizing treatment they are iron deficient. Further evidence for the dependence of iron vs oxygen fractional surface terminations on preparation conditions comes from Fourier transform infrared reflection-absorption spectroscopy, which is used to study the adsorption of formic acid. These molecules dissociate and adsorb in chelating and bidentate bridging geometries on the Fe-terminated areas and the signal of typical infrared absorption bands is stronger after the preparation under reducing conditions, which results in a higher fraction of Fe-termination. The adsorption of formic acid induced an atomic roughening of the magnetite (111) surface which we conclude from the quantitative analysis of the crystal truncation rod data. The roughening process is initiated by atomic hydrogen, which results from the dissociation of formic acid after its adsorption on the surface. Atomic hydrogen adsorbs at surface oxygen and after recombination with another H this surface hydroxyl can form H2O, which may desorb from the surface, while iron ions diffuse into interstitial sites in the bulk.

We investigated the orientation and morphology of Cu nanoparticles grown under ultrahigh-vacuum conditions on ZnO(0001), ZnO(000 (1) over bar), and ZnO(10 (1) over bar4) single crystal surfaces by scanning tunneling microscopy, high-energy grazing incidence X-ray diffraction, low-energy electron diffraction, and scanning electron microscopy. The (111) oriented Cu NPs on basal ZnO showed only small area fractions of high indexed Cu(225) and Cu(331) facets. Cu NPs grown on ZnO(10 (1) over bar4) show alignment of Cu [111] with the ZnO [0001] direction, which is at an angle of 24.8 degrees to the ZnO(10 (1) over bar4) surface normal. Because of this tilt, the NPs exhibit a shape with a larger fraction of high indexed facets such as (335), (221), (113), and (55 (1) over bar). In addition, the direct interaction of subsequent Cu(111) planes to the underlying substrate results in unequal amounts of ABCA and ACBA stacked NPs. Small NPs are found to interact strongly with the vicinal surface, giving rise to a surface corrugation with a multiple of the surface step distance. The high density of low-coordinated Cu surface atoms potentially increases the overall catalytic activity for methanol synthesis and CO2 hydrogenation reactions.

Water in confinement exhibits altered properties in molecular arrangement, bonding, and interaction with its neighboring environment, as compared to its bulk counterpart. In this work, periodically arranged D2O nano droplets of ∼1 nm size on top of a graphene/iridium moiré superstructure were investigated by Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) under ultrahigh vacuum conditions at ∼120 K. The IR bands of D2O clusters differ significantly from those observed for bulk D2O amorphous solid water or crystalline ice phases. Blue-shifted symmetric and asymmetric stretching bands with narrower band widths and modified band intensity ratios were observed, pointing to an enhanced internal order and a reduced nearest neighbor distance. Furthermore, two IR bands of "dangling" deuterium atoms were detected originating from threefold coordinated water molecules at the surface of the clusters and at their interface to the graphene layer. The latter arose only with the transition from the water clusters to an amorphous solid water layer. We propose that upon coalescence, opposing local dipoles trigger a hydrogen bond rearrangement at the interface. Our results represent a first step toward an atomistic understanding of water in confinement.

Magnetite (Fe3O4) is a very important and diverse transition metal oxide with applications in catalysis, data storage and biomedical imaging. In a recent study magnetite nanoparticles linked by oleic acid molecules show exceptional isotropic mechanical properties [1]. It was demonstrated by FT-IR spectroscopy that the oleic acid molecules remain stable on the particle surface until 350°C at which crosslinking of the organic phase starts and a formation of well defined (111) and (100) facets sets in. The adsorption mechanism and adsorption-induced near surface structural changes however still remain unknown. The interface between the oleic acid molecules and the distinct facets of the magnetite nanoparticles is essential for an understanding of the advanced mechanical properties. To give further insight on how these nanoparticles interact with organic molecules it is crucial to study the structure and adsorption behavior at magnetite single crystal surfaces. The focus is on the surface orientations (111) and (001) since they are also present on the nanoparticle facets. The (111) surface of magnetite has six possible bulk terminations (fig. 1), which can be divided into two oxygen-terminated (O1 and O2) and four ironterminated surfaces (tet1, oct1, tet2, oct2). The correct termination of the clean surface structure is still under debate. Previous LEED I-V studies favor the Fetet1 termination with 6 Å distance between neighboring iron atoms [2]. Recent studies on the water adsorption on magnetite (111) however support the Feoct2 termination [3]. We investigated the clean surface structure of magnetite (111) by STM. The surface presents flat terraces with a step height of 4.8 Å. This is exactly the length of repetition of the different terminations and indicates a homogeneous termination. Formic acid adsorption on the magnetite (111) surface was studied by XPS and FT-IR spectroscopy. We find that the molecule dissociates on the surface. Formic acid binds to the surface in bidentate and monodentate configuration which is different to the (001) surface, where only bidentate adsorption was observed [4]. Furthermore we show results on the adsorption of water and organic molecules like formic acid and oleic acid on magnetite (111) under UHV conditions obtained by FT infrared spectroscopy and X-ray photoemission spectroscopy. These findings are complemented by surface X-ray diffraction data obtained at Soleil Synchrotron.