This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermore, the role of trace amounts of oxygen during growth is discussed and related to graphene-induced surface reconstructions during cooling. Above all, this work demonstrates the potential of the method for in situ studies of surface dynamics on active metal catalysts.

Titanium Silicalite-1 (TS-1) shows an outstanding ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as byproduct. 1,2 Despite the industrial use of the TS-1/H2O2 system for the production of more than one million tons of propylene oxide per year, 3 the active site structure remains elusive, although it has been studied for almost 40 years by spectroscopic and computational methods. 4-10 TS-1 is a zeotype of MFI structure in which a small fraction of Si-atoms (1-2 %) are substituted by Ti, and its catalytic properties are generally attributed to isolated Ti(IV) sites. 1 Herein, we analyze a series of highly active and selective TS-1 propylene epoxidation catalysts. By UV-Vis and Raman spectroscopy, as well as electron microscopy, we show that Ti is well-dispersed in all samples, with formation of small TiOx clusters at high Ti-loadings. Most notably, irrespective of Ti-content, all samples show a characteristic solid-state 17 O NMR signature when contacted with H2 17 O2, indicating the formation of bridging peroxo species on dinuclear Tisites. Using DFT (density functional theory) calculations, we propose a mechanism of propylene epoxidation on a dinuclear site, in which the cooperativity between two titanium atoms enables a low-energy reaction pathway where the key oxygen-transfer transition state bears strong resemblance to that of olefin epoxidation by peracids.The active species in TS-1 are commonly proposed to be isolated Ti(IV) sites bearing peroxo 11 or hydroperoxo moieties, 12 although the involvement of terminal Ti-oxo and activated H2O2 on Ti(IV) has also been discussed (Fig. 1a,b). 7 In contrast, the only homogeneous Ti-based epoxidation catalysts able to efficiently utilize H2O2 as primary oxidant are dinuclear, such as the Berkessel-Katsuki epoxidation catalyst 1 (Fig. 1c). [14][15][16][17][18][19] While the structural characterization of molecular systems is well-established and has enabled the isolation of peroxo compounds, obtaining information on the structure of Ti-sites in TS-1 with molecular-level precision has proven more challenging.Recent work by some of us has shown that solid-state 17 O NMR spectroscopy is a powerful tool for understanding and assessing the reactivity of peroxo species. 20 Oxygen-17 is an NMRactive quadrupolar nucleus whose spectroscopic properties can be readily measured by solidstate NMR and computed by DFT. The NMR signature (chemical shift and quadrupolar coupling) is highly sensitive to the symmetry and electronic structure around the oxygen atoms. We thus reasoned that 17 O NMR spectroscopy would be a valuable tool to harness the signature of the active sites in TS-1 and thereby probe their structure. In this study, we investigate five TS-1 samples prepared in the BASF laboratories (Table 1). 21,22 Two of these samples have a Ti-content of 1.9 wt%, one of which was prepared on hundred-kg scale (sample 1), the other three samples have Ti-loadings of 1.5 wt%, 1.0 wt%, and 0.5 wt%. The five samples have surface areas between 4...

Copper is a widely studied catalyst material for the electrochemical conversion of carbon dioxide to valuable hydrocarbons. In particular, copper-based nanostructures expressing predominantly {100} facets have shown high selectivity toward ethylene formation, a desired reaction product. However, the stability of such tailored nanostructures under reaction conditions remains poorly understood. Here, using liquid cell transmission electron microscopy, we show the formation of cubic copper oxide particles from copper sulfate solutions during direct electrochemical synthesis and their subsequent morphological evolution in a carbon dioxide-saturated 0.1 M potassium bicarbonate solution under a reductive potential. Shape-selected synthesis of copper oxide cubes was achieved through: (1) the addition of chloride ions and (2) alternating the potentials within a narrow window where the deposited non-cubic particles dissolve, but cubic ones do not. Our results indicate that copper oxide cubes change their morphology rapidly under carbon dioxide electroreduction-relevant conditions, leading to an extensive restructuring of the working electrode surface.

Iridium oxide-based catalysts are uniquely active and stable in the oxygen evolution reaction. Theoretical work attributes their activity to oxyl or μ1-O species. Verifying this intermediate experimentally has, however, been challenging. In the present study, these challenges were overcome by combining theory with new experimental strategies. Ab initio molecular dynamics of the solid–liquid interface were used to predict spectroscopic features, whereas sample architecture, developed for surface-sensitive X-ray spectroscopy of electrocatalysts in confined liquid, was used to search for these species under realistic conditions. Through this approach, we have identified μ1-O species during oxygen evolution. Potentiodynamic X-ray absorption additionally shows that these μ1-O species are created by electrochemical oxidation currents in a deprotonation reaction.

During the electrochemical reduction of oxygen, platinum catalysts are often (partially) oxidized. While these platinum oxides are thought to play a crucial role in fuel cell degradation, their nature remains unclear. Here, we studied the electrochemical oxidation of Pt nanoparticles using in situ XPS. When the particles were sandwiched between a graphene sheet and a proton exchange membrane that is wetted from the back, a confined electrolyte layer was formed, allowing us to probe the electrocatalyst under wet conditions. We show that the surface oxide formed at the onset of Pt oxidation has a mixed Pt δ+ /Pt 2+ /Pt 4+ composition. The formation of this surface oxide is suppressed when a Br-containing membrane is chosen due to adsorption of Br on Pt. Time-resolved measurements show that oxidation is fast for nanoparticles: even bulk PtO 2 · n H 2 O growth occurs on the subminute time scale. The fast formation of Pt 4+ species in both surface and bulk oxide form suggests that Pt 4+ -oxides are likely formed (or reduced) even in the transient processes that dominate Pt electrode degradation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO x centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally Ir V species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ 1 -O and μ 1 -OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ 3 -O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ 1 -OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.

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