Simultaneous sulfidation of Mo and Co oxides supported on Au(111).

Here we present the results of a study carried out to investigate the simultaneous sulfidation of Co and Mo oxide nanoparticles on Au(111) as a synthesis strategy to prepare a model catalyst for hydrodesulfurization (HDS). We make use of scanning tunneling microscopy and X-ray photoelectron spectroscopy to track the changes in morphology and chemistry during the synthesis of a mixed Mo and Co oxide precursor and the sulfidation thereafter, to the respective sulfides. We investigated the effects of temperature and the duration of sulfidation on the completeness of the sulfidation process. Our study shows that the formation of MoS2 with the CoMoS edge (the desired model catalyst) is not affected by the time or the temperature of sulfidation. However, the yield of the Co-promoted MoS2 slabs is limited by the formation of large clusters due to the spreading of Mo and Co oxide phases upon sulfidation. Complete sulfidation of the mixed oxide precursor to Co-promoted MoS2 can be accelerated by increasing the sulfidation temperature to 730 K due to the thermally activated nature of Mo oxide sulfidation. Thus, we demonstrate that using a mixed Mo and Co oxide precursor as a starting point for the Co-promoted MoS2 phase for fundamental catalytic studies is a viable strategy.

In situ studies of NO reduction by H2 over Pt using surface X-ray diffraction and transmission electron microscopy.

In situ surface X-ray diffraction and transmission electron microscopy at 1 bar show massive material transport of platinum during high-temperature NO reduction with H2. A Pt(110) single-crystal surface shows a wide variety of surface reconstructions and extensive faceting of the surface. Pt nanoparticles change their morphology depending on the gas composition: They are faceted in hydrogen-rich environments, but are more spherical in NO-rich environments, indicating the formation of vicinal surfaces. We conclude that high coverage of NO combined with sufficient mobility of platinum surface atoms is the driving force for the formation of steps on both flat surfaces and nanoparticles. Since the steps that are introduced provide strongly coordinating adsorption sites with potential catalytic benefits, this may be of direct practical relevance for the performance of catalytic nanoparticles under high-pressure conditions.

Hydrodesulfurization of methanethiol over Co-promoted MoS2 model catalysts.

The process of hydrodesulfurization is one of the most important heterogeneous catalytic reactions in industry as it helps with reducing global SOx emissions by selectively removing the sulfur contaminants from commercial fuel. In this work, we successfully combine high-pressure scanning tunneling microscopy and reaction modeling using density functional theory to observe the hydrodesulfurization of methanethiol (CH3SH) on the Co-substituted S edges of a Co-promoted MoS2 model catalyst in situ at near-industrial conditions and investigate the plausible reaction pathways. The active sites on the Co-substituted S edges show a time-varying atomic structure influenced by the hydrodesulfurization reaction rate. The involvement of the edge Co site allows for the C-S bond scission to occur at appreciable rates, and is the critical step in the hydrodesulfurization of CH3SH. The atomic structures of the S-edge active sites from our reaction models match excellently with those observed in situ in the experiments.

A theoretical study of H(2) dissociation on (sq.rt(3) x sq.rt(3))R30 degrees CO/Ru(0001).

We have studied the influence of preadsorbed CO on the dissociative adsorption of H(2) on Ru(0001) with density functional theory calculations. For a coverage of 1/3 ML CO, we investigated different possible reaction paths for hydrogen dissociation using nudged elastic band and adaptive nudged elastic band calculations. One reaction path was studied in detail through an energy decomposition and molecular orbital type of analysis. The minimum barrier for H(2) dissociation is found to be 0.29 eV. At the barrier the H-H bond is hardly stretched. Behind this barrier a molecular chemisorption minimum is present. Next, the molecule overcomes a second barrier, with a second local chemisorption minimum behind it. To finally dissociate to chemisorbed atoms, the molecule has to overcome a third barrier. To move along the reaction path from reactants to products, the hydrogen molecule needs to rotate, and to significantly change its center-of-mass position. The procedure of mapping out reaction paths for H(2) reacting on low-index surfaces of bare metals (computing two-dimensional elbow plots for fixed impact high-symmetry sites and H(2) orientations parallel to the surface) does not work for H(2)+CO/Ru. The first barrier in the path is recovered, but the features of the subsequent stretch to the dissociative chemisorption minimum are not captured, because the molecule is not allowed to change its center-of-mass position or to rotate. The dissociative chemisorption of H(2) on CO/Ru(0001) is endoergic, in contrast to the case of H(2) on bare Ru(0001). The zero-point energy corrected energies of molecularly and dissociatively chemisorbed H(2) are very close, suggesting that it may be possible to detect molecularly chemisorbed H(2) on (sq.rt(3) x sq.rt(3))R30 degrees CO/Ru(0001). The presence of CO on the surface increases the barrier height to dissociation compared with bare Ru(0001). Based on an energy decomposition and molecular orbital analysis we attribute the increase in the barrier height mainly to an occupied-occupied interaction between the bonding H(2) sigma(g) orbital and the (surface-hybridized) CO 1pi orbitals, i.e., to site blocking. There is a small repulsive contribution to the barrier from the interaction between the H(2) molecule and the Ru part of the CO covered Ru surface, but it is smaller than one might expect based on the calculations of H(2) interacting with a clean Ru surface, and on calculations of H(2) interacting with the CO overlayer only. Actually, the analysis suggests that the Ru surface as a subsystem is (slightly) more reactive for the reaction path studied with CO preadsorbed on it than without it. Thus, the results indicate that the influence of CO on H(2) dissociation on Ru is not only a simple site-blocking effect, the electronic structure of the underlying Ru is changed.

Development of a reactor for the in situ monitoring of 2D materials growth on liquid metal catalysts, using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy.

Liquid metal catalysts (LMCats) (e.g., molten copper) can provide a new mass-production method for two-dimensional materials (2DMs) (e.g., graphene) with significantly higher quality and speed and lower energy and material consumption. To reach such technological excellence, the physicochemical properties of LMCats and the growth mechanisms of 2DMs on LMCats should be investigated. Here, we report the development of a chemical vapor deposition (CVD) reactor which allows the investigation of ongoing chemical reactions on the surface of a molten metal at elevated temperatures and under reactive conditions. The surface of the molten metal is monitored simultaneously using synchrotron x-ray scattering, Raman spectroscopy, and optical microscopy, thereby providing complementary information about the atomic structure and chemical state of the surface. To enable in situ characterization on a molten substrate at high temperatures (e.g., ∼1370 K for copper), the optical and x-ray windows need to be protected from the evaporating LMCat, reaction products, and intense heat. This has been achieved by creating specific gas-flow patterns inside the reactor. The optimized design of the reactor has been achieved using multiphysics COMSOL simulations, which take into account the heat transfer, fluid dynamics, and transport of LMCat vapor inside the reactor. The setup has been successfully tested and is currently used to investigate the CVD growth of graphene on the surface of molten copper under pressures ranging from medium vacuum up to atmospheric pressure.

Dynamics of hydrogen dissociation on stepped platinum.

We have studied the reactivity of hydrogen on the Pt(211) stepped surface using supersonic molecular beam techniques. We observe an energy dependence that is indicative of indirect adsorption below 9 kJ mol(-1) and direct adsorption between 0 and 37 kJ mol(-1). Comparison of our results to predictions based on six-dimensional quantum dynamics calculations for Pt(211) [R. A. Olsen et al., J. Chem. Phys. 128, 194715 (2008)] yields reasonable agreement. Discrepancies between theory and our experiments at low kinetic energy strongly indicate that the wells in the used potential energy surface are too shallow. Discrepancies at high kinetic energy point toward neglect of degrees of freedom vital to capture the full dynamics.

Fast and reliable pre-approach for scanning probe microscopes based on tip-sample capacitance.

Within the last three decades Scanning Probe Microscopy has been developed to a powerful tool for measuring surfaces and their properties on an atomic scale such that users can be found nowadays not only in academia but also in industry. This development is still pushed further by researchers, who continuously exploit new possibilities of this technique, as well as companies that focus mainly on the usability. However, although imaging has become significantly easier, the time required for a safe approach (without unwanted tip-sample contact) can be very time consuming, especially if the microscope is not equipped or suited for the observation of the tip-sample distance with an additional optical microscope. Here we show that the measurement of the absolute tip-sample capacitance provides an ideal solution for a fast and reliable pre-approach. The absolute tip-sample capacitance shows a generic behavior as a function of the distance, even though we measured it on several completely different setups. Insight into this behavior is gained via an analytical and computational analysis, from which two additional advantages arise: the capacitance measurement can be applied for observing, analyzing, and fine-tuning of the approach motor, as well as for the determination of the (effective) tip radius. The latter provides important information about the sharpness of the measured tip and can be used not only to characterize new (freshly etched) tips but also for the determination of the degradation after a tip-sample contact/crash.

Supersonic molecular beam studies of dissociative adsorption of H2 on Ru(0001).

We examined reactivity of H(2) on Ru(0001) using molecular beam techniques and we compared our results to experimental results for similar systems. The dissociative adsorption of H(2) on Ru(0001) is similar to that on Pt(111) and Ni(111), although on ruthenium nonactivated adsorption is strongly suggested. However, we find no clear signature of a steering- or precursor-based mechanism that favors nonactivated reaction paths at low kinetic energy. In comparison to Pd(111) and Rh(111) our results indicate that a universal mechanism enhancing reactivity at low energy does not have a mass dependence. In addition, we have compared our results to predictions of reactivity for H(2) on Ru(0001) from six-dimensional dynamical calculations using two different generalized gradient approximation functionals. It leads us to conclude that the PW91 functional yields a more accurate value for the minimum energy path but does not impose enough corrugation in the potential. The revised-Perdew-Burke-Ernzerhof (RPBE) functional appears to behave slightly better at higher energies, but we find significant quantitative disagreement. We show that the difference is not due to different energy resolutions between experiment and theory. However, it may be due to a dependence of the reactivity on rotational state or on omission of relevant dimensions in the theoretical description.