Here we report an investigation of ultrathin tin oxide films on Pt3Sn(111) using low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (μ-LEED), scanning tunneling microscopy (STM), surface X-ray diffraction (SXRD), and high-resolution X-ray photoelectron spectroscopy (XPS). Oxidation at ∼390–410 °C produces triangular, two-dimensional oxide islands that nucleate rapidly and exhibit self-limited lateral growth, attributed to limited Sn diffusion from the subsurface of the crystal. μ-LEED shows that the initially formed (4×4) Sn oxide is subsequently converted to a more oxygen-rich (2×2n) “stripe” phase. At 630 °C, enhanced Sn mobility enables a closed (4×4) film. The (2×2n) phase is shown to consist of a (2×2) Sn lattice modulated by 1D stripe defects with spacings of n=4–6 atomic rows; LEED and SXRD measurements show diffraction features corresponding to this striped superstructure. The two oxides can be distinguished in XPS by their O 1s lineshapes: the (4×4) phase shows a clear doublet attributable to distinct O species, whereas the (2×2n) phase exhibits a broader envelope consistent with a distribution of O coordination environments. The Sn 3d5/2 spectra are similar for both phases, reflecting closely related Sn bonding motifs. The spectra are consistent with those of previous near-ambient-pressure XPS measurements, suggesting that the surface oxides forming under CO oxidation conditions are similar to those studied here.

Hydrodeoxygenation (HDO) is a catalytic process applied for the reduction of oxygen levels in hydrocarbons from bio-derived feedstock as part of the processing into renewable fuel. The MoS2-based hydrotreating catalysts, currently being applied for the HDO reaction, are exposed to a complex environment consisting of O-rich hydrocarbons and water, which adversely impacts the state and stability of the catalyst. Here, we analyze the structural and chemical state changes of MoS2 in HDO-relevant conditions using a combination of surface-sensitive techniques applied to a planar model system consisting of supported and structurally well-defined single-layer MoS2 nanoparticles supported on Au(111). As observed by scanning tunnelling microscopy and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), the exposure to mbar pressure of H2O at 600 K induces clear changes in both the shape and size of the MoS2 nanoparticles, explained by a preferential oxidation and etching of MoS2 edges. MoOx is observed on the surface due to the spatial separation of the oxide and etched sulfide phase. Interestingly, when H2/H2O gas mixtures are applied, the sulfur reduction and oxidation of MoS2 appear to be decoupled, indicating that the removal of edge S species is not a prerequisite for oxidation. Furthermore, the formed MoOx showed a preferred reduction of the oxide over the sulfide. Importantly, the atom-resolved imaging reveals that the progressive etching and phase segregation of MoS2 maintains access to pristine edge sites of the single-layer MoS2, explaining why HDO activity can be maintained even for highly oxidized catalysts.

In recent years, studies of surfaces at more realistic conditions has advanced significantly, leading to an increased understanding of surface dynamics under reaction conditions. The development has mainly been due to the development of new experimental techniques or new experimental approaches. Techniques such as High Pressure Scanning Tunneling/Force Microscopy, Ambient Pressure x-ray Photo emission Spectroscopy, Surface x-ray Diffraction, Polarization-Modulation InfraRed Reflection Absorption Spectroscopy and Planar Laser Induced Fluorescence at semi-realistic conditions has been used to study planar model catalysts or industrial materials under operating conditions. 2D-Surface Optical Reflectance has recently received attention as a useful experimental tool used in gaseous and liquid harsh conditions by providing complementary experimental information on planar model samples as well as being a powerful experimental tool on its own. The simplicity of the approach and the cost of the equipment makes it an attractive alternative and useful tool for surface science studies under reaction conditions. In this topical review, we review some recent studies that have been promoted by the technical development in optical components, image acquisition and computational image analysis.

The striped p x root 3 reconstruction of Au(111) is a textbook example of how electrode surfaces reorganise in response to an applied potential. Using in situ high-energy surface X-ray diffraction, we track the surface reconstruction in 0.1 M HClO4 electrolyte while the potential is cycled at both 5 mV s-1 and 2 mV s-1 between 0.06 V and 0.86 V versus RHE. Reciprocal-space maps, collected every similar to 10 s, show that the unit cell of the well-known herringbone reconstruction increases in length progressively as the potential is swept positively; the diffraction spots coalesce with the spot from the (111) surface and the reconstruction lifts completely above approximate to 0.7 V. The lifting and reformation dynamics of the surface reconstruction are seen to be relatively slow and continuous, when the potential is swept at 5 mV s-1 we observe the reconstruction lifting at more positive potentials than when swept at 2 mV s-1. Conversely the reforming of the reconstruction is also slow and is present at more positive potentials when the sweep rate is slower.

The preparation of ultra-thin PtSn-alloyed layers by molecular beam epitaxy was studied using low-energy electron microscopy (LEEM) and micro-diffraction (μ-LEED). Deposition at a sample temperature of 435 °C initially results in the formation of a Pt3Sn/Pt(111) layer showing a (2 × 2) reconstruction. With continued Sn deposition, a Pt2Sn/Pt(111) layer develops, showing a (3×3)R30° reconstruction. An ultra-thin tin oxide was formed from the (2 × 2) surface by exposure to molecular oxygen at temperatures of 500 °C and 590 °C, respectively. LEED shows the evolution of a new surface structure, which could be identified as an incommensurate rectangular 2.301.83.6 reconstruction with lattice parameters of a = (6.4 ± 0.1) Å and b = (8.6 ± 0.1) Å present in three domains rotated by 120° with respect to each other. This structure can be related to the zigzag reconstructions found for similar ultra-thin oxide systems. Contrarily, the (3×3)R30° structure showed no oxide formation even after extensive exposure to molecular oxygen. The usage of atomic oxygen, however, allows for oxidation of this surface and the growth of thicker oxides on both types of overlayers. At 500 °C this process is accompanied by substantial roughening of the surface.

We have used grazing incidence X-ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeOx films on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O2 and CO at 150ºC, attributed to a transformation between FeO bilayer and FeO2 trilayer phases. The trilayer phase is also reduced upon heating in CO+O2, consistent with a Mars-van-Krevelen type mechanism for CO oxidation. At higher temperatures, the monolayer film dewets the surface, resulting in a loss of the observed reducibility. A similar iron oxide film prepared on Au(111) shows little sign of reduction or oxidation under the same conditions. The results highlight the unique properties of monolayer FeO and the importance of the Pt support in this reaction. The study furthermore demonstrates the power of grazing-incidence XAFS for in situ studies of these model catalysts under realistic conditions.

Self-stabilized, heterometallic pair-sites can enable fine-tuning of catalytic functionality while also mitigating dynamic structural changes that degrade catalytic performance. This study demonstrates the development and characterization of trimetallic PtxCrxAg1-2x (x ≤ 0.1) alloys with active Pt–Cr pair-ensembles for non-oxidative ethanol dehydrogenation, leveraging predictions that favorable bonding stabilizes Pt–Cr pairs diluted in Ag. Operando X-ray absorption spectroscopy confirms the preferential formation and stability of Pt–Cr pairings dispersed throughout the Ag matrix, and ambient-pressure X-ray photoelectron spectroscopy shows that Pt–Cr sites have significant activity for ethanol dehydrogenation, while suppressing reaction processes that deactivate binary Pt–Ag and Cr–Ag alloys. This work demonstrates that stabilizing heterometallic pair sites within trimetallic alloys provides a new avenue for designing catalysts with discrete active sites that are durable and highly selective.

Platinum–tin catalysts have been used industrially for decades, but their structural evolution under reaction conditions and the causes of their improved activity relative to platinum are still not well understood. In this study, we investigate Sn/Pt(111) and Sn/Pt(223) surface alloys during oxygen exposure and catalytic CO oxidation using grazing incidence X-ray absorption spectroscopy (GI-XAS). The spectra were collected in fluorescence mode, and the XANES region was used for analysis. In O2, the Sn/Pt surface alloys segregate, and tin oxide similar to SnO2 forms. In CO and O2, Sn(II) oxide forms on the Sn/Pt surfaces. The spectra from the Sn(II) oxide qualitatively agree with the simulated spectra of previously studied surface oxides but not with SnO. The study provides insights into the behavior of well-defined Sn/Pt surfaces under realistic conditions and connects ambient-pressure catalysis with previous surface science research.

Traditional methods to study electrochemical (EC) processes, although successful, are based on current/voltage measurements, providing information about performances rather than offering a direct observation of chemical and structural changes occurring at the electrode surface. These processes are localized at the electrode-electrolyte interface, the structure of which is the main determinant of their behavior, but most surface sensitive experimental techniques are limited to ex situ conditions, owing to the need for an ultra-high vacuum environment. In this contribution, we report operando X-ray absorption spectroscopy in total external reflection geometry (Refle-XAFS) at P64 beamline (DESY, Hamburg), using a simple and versatile EC flow cell designed for multimodal surface sensitive studies with hard X-ray scattering and spectroscopy techniques. We show that the Refle-XAFS method can be used to study chemical surface changes of industrial alloys and model electrodes in harsh electrochemical environments, without being limited to thin film samples. The surface passive film development and breakdown of a corrosion-resistant Ni-Cr-Mo alloy and the electro-oxidation of polycrystalline gold (poly-Au), relevant for fundamental studies on water electrolysis, were investigated. Despite the strong attenuation of the beam by the electrolyte and the PEEK walls of the EC cell, nanoscale surface oxide films were detected using beam energies down to 8 keV. The passivity breakdown region of Ni alloy 59 in 1 M NaCl at pH 7 and pH 12 was identified, showing differences in the composition of the surface oxides during anodic polarization. The electro-oxidation of poly-Au in 0.05 M H2SO4 was observed, showing a progression from two-dimensional Au1+/3+ to three-dimensional thick Au3+ surface oxide/hydroxide during OER.

The cleaning process of the bimetallic Pt3Sn(111) surface has been studied by means of low-energy electron microscopy (LEEM), microspot low-energy electron diffraction (mu.-LEED), and X-ray photoemission electron microscopy (XPEEM). Different cleaning procedures, performed under ultra-high vacuum conditions (UHV), including sputtering with argon ions and repeated cycles of annealing up to 1500 K were investigated. In this work, we show that a clean Pt3Sn(111) surface of high structural quality with a sharp and brilliant (2 x 2) bulk reconstruction in LEED as well as a perfectly smooth surface with terraces of micron size can be achieved by sputtering, annealing at very high temperatures, followed by a subsequent slow (0.09 K/s) and careful cooling procedure. Additionally, we show the possibility of tailoring the Sn concentration in the topmost layers of Pt3Sn(111) as a function of annealing temperature and subsequent cooling rate. Structural changes of the surface are induced by Sn segregation combined with a surface order-disorder transition at 1340 K. Moreover, two new surface reconstructions depending on the cooling rate are reported.