Lanthanum modulated reaction pacemakers on a single catalytic nanoparticle.

Promoters are important in catalysis, but the atomistic details of their function and particularly their role in reaction instabilities such as kinetic phase transitions and oscillations are often unknown. Employing hydrogen oxidation as probe reaction, a Rh nanotip for mimicking a single Rh nanoparticle and field electron microscopy for in situ monitoring, we demonstrate a La-mediated local catalytic effect. The oscillatory mode of the reaction provides a tool for studying the interplay between different types of reaction pacemakers, i.e., specific local surface atomic configurations that initiate kinetic transitions. The presence of La shifts the bistable reaction states, changes the oscillation pattern and deactivates one of two pacemaker types for the La-free surface. The observed effects originate from the La-enhanced oxygen activation on the catalyst. The experimental observations are corroborated by micro-kinetic model simulations comprising a system of 25 coupled oscillators.

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction.

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO2) in H2 oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO2) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO2, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO2 support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Emergence of chaos in a compartmentalized catalytic reaction nanosystem.

In compartmentalized systems, chemical reactions may proceed in differing ways even in adjacent compartments. In compartmentalized nanosystems, the reaction behaviour may deviate from that observed on the macro- or mesoscale. In situ studies of processes in such nanosystems meet severe experimental challenges, often leaving the field to theoretical simulations. Here, a rhodium nanocrystal surface consisting of different nm-sized nanofacets is used as a model of a compartmentalized reaction nanosystem. Using field emission microscopy, different reaction modes are observed, including a transition to spatio-temporal chaos. The transitions between different modes are caused by variations of the hydrogen pressure modifying the strength of diffusive coupling between individual nanofacets. Microkinetic simulations, performed for a network of 52 coupled oscillators, reveal the origins of the different reaction modes. Since diffusive coupling is characteristic for many living and non-living compartmentalized systems, the current findings may be relevant for a wide class of reaction systems.

Pattern Formation in Catalytic H2 Oxidation on Rh: Zooming in by Correlative Microscopy.

Spatio-temporal nonuniformities in H2 oxidation on individual Rh(h k l) domains of a polycrystalline Rh foil were studied in the 10-6 mbar pressure range by photoemission electron microscopy (PEEM), X-ray photoemission electron microscopy (XPEEM), and low-energy electron microscopy (LEEM). The latter two were used for in situ correlative microscopy to zoom in with significantly higher lateral resolution, allowing detection of an unusual island-mediated oxygen front propagation during kinetic transitions. The origin of the island-mediated front propagation was rationalized by model calculations based on a hybrid approach of microkinetic modeling and Monte Carlo simulations.

Reaction Modes on a Single Catalytic Particle: Nanoscale Imaging and Micro-Kinetic Modeling.

The kinetic behavior of individual Rh(hkl) nanofacets coupled in a common reaction system was studied using the apex of a curved rhodium microcrystal (radius of 0.65 µm) as a model of a single catalytic particle and field electron microscopy for in situ imaging of catalytic hydrogen oxidation. Depending on the extent of interfacet coupling via hydrogen diffusion, different oscillating reaction modes were observed including highly unusual multifrequential oscillations: differently oriented nanofacets oscillated with differing frequencies despite their immediate neighborhood. The transitions between different modes were induced by variations in the particle temperature, causing local surface reconstructions, which create locally protruding atomic rows. These atomic rows modified the coupling strength between individual nanofacets and caused the transitions between different oscillating modes. Effects such as entrainment, frequency locking, and reconstruction-induced collapse of spatial coupling were observed. To reveal the origin of the different experimentally observed effects, microkinetic simulations were performed for a network of 105 coupled oscillators, modeling the individual nanofacets communicating via hydrogen surface diffusion. The calculated behavior of the oscillators, the local frequencies, and the varying degree of spatial synchronization describe the experimental observations well.

Single-Particle Catalysis: Revealing Intraparticle Pacemakers in Catalytic H2 Oxidation on Rh.

Self-sustained oscillations in H2 oxidation on a Rh nanotip mimicking a single catalytic nanoparticle were studied by in situ field emission microscopy (FEM). The observed spatio-temporal oscillations result from the coupling of subsurface oxide formation/depletion with reaction front propagation. An original sophisticated method for tracking kinetic transition points allowed the identification of local pacemakers, initiating kinetic transitions and the nucleation of reaction fronts, with much higher temporal resolution than conventional processing of FEM video files provides. The pacemakers turned out to be specific surface atomic configurations at the border between strongly corrugated Rh{973} regions and adjacent relatively flat terraces. These structural ensembles are crucial for reactivity: while the corrugated region allows sufficient oxygen incorporation under the Rh surface, the flat terrace provides sufficient hydrogen supply required for the kinetic transition, highlighting the importance of interfacet communication. The experimental observations are complemented by mean-field microkinetic modeling. The insights into the initiation and propagation of kinetic transitions on a single catalytic nanoparticle demonstrate how in situ monitoring of an ongoing reaction on individual nanofacets can single out active configurations, especially when combined with atomically resolving the nanoparticle surface by field ion microscopy (FIM).

A novel wireless sample temperature control system for field ion, field electron, and atom probe techniques.

A novel sample temperature control system for field ion microscopy (FIM), field electron microscopy (FEM), and atom probe techniques based on wireless data transmission was designed, built, and applied for FIM and FEM studies of surface reactions. The system solves the longstanding problem of the temperature control of micrometer- to nanometer-sized samples during the operation in field emission based techniques. The new system can also be used for other applications requiring the specimen to be under high electric potential (tens of kilovolts or even higher). The chosen case studies of nanocatalysis demonstrate the capabilities and superior performance of the new temperature control system.

Surface-Structure Libraries: Multifrequential Oscillations in Catalytic Hydrogen Oxidation on Rhodium.

Multifrequential oscillating spatiotemporal patterns in the catalytic hydrogen oxidation on rhodium have been observed in situ in the 10-6 mbar pressure range using photoemission electron microscopy. The effect is manifested by periodic chemical waves, which travel over the polycrystalline Rh surface and change their oscillation frequency while crossing boundaries between different Rh(hkl) domains. Each crystallographically specific µm-sized Rh(hkl) domain exhibits an individual wave pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction, altogether creating a structure library. This unique reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as a feedback mechanism of kinetic oscillations. Formation of a network of subsurface oxygen as a result of colliding reaction fronts was observed in situ. Microkinetic model analysis was used to rationalize the observed effects and to reveal the relation between the barriers for surface oxidation and oscillation frequency. Structural limits of the oscillations, the existence range of oscillations, as well as the effect of varying hydrogen pressure are demonstrated.

Visualizing catalyst heterogeneity by a multifrequential oscillating reaction.

It is well documented that different surface structures of catalytically active metals may exhibit different catalytic properties. This is typically examined by comparing the catalytic activities and/or selectivities of various well-defined smooth and stepped/kinked single crystal surfaces. Here we report the direct observation of the heterogeneity of active polycrystalline surfaces under reaction conditions, which is manifested by multifrequential oscillations during hydrogen oxidation over rhodium, imaged in situ by photoemission electron microscopy. Each specific surface structure, i.e. the crystallographically different µm-sized domains of rhodium, exhibits an individual spiral pattern and oscillation frequency, despite the global diffusional coupling of the surface reaction. This reaction behavior is attributed to the ability of stepped surfaces of high-Miller-index domains to facilitate the formation of subsurface oxygen, serving as feedback mechanism of the observed oscillations. The current experimental findings, backed by microkinetic modeling, may open an alternative approach towards addressing the structure-sensitivity of heterogeneous surfaces.

Probing adsorption on a nanoscale: field desorption microspectroscopy.

Combining an energy analyzer with a field ion microscope equipped with a probe-hole which corresponds to just few atomic surface sites, spatially resolved energy analysis of ions field desorbed from the adsorbent surface is possible on a nm-scale. The experimentally measured values of the kinetic energy of field ions can be related (by means of a thermionic cycle) to the physically meaningful binding energy of corresponding adsorbed species. The development of the technique into a full serviceable micro-spectroscopy on a nanoscale allowed recent detection of the weakly adsorbed CO species on Pt(111) which are largely analogous to those adsorbed at high pressures and provided first results for the binding energy of Li adatoms in a coadsorption system, namely Li-O-W(112) for various lithium and oxygen coverages. In the present contribution, an overview of the experimental possibilities of the technique is given and recent results are discussed.

Local reaction kinetics by imaging.

In the present contribution we present an overview of our recent studies using the "kinetics by imaging" approach for CO oxidation on heterogeneous model systems. The method is based on the correlation of the PEEM image intensity with catalytic activity: scaled down to the µm-sized surface regions, such correlation allows simultaneous local kinetic measurements on differently oriented individual domains of a polycrystalline metal-foil, including the construction of local kinetic phase diagrams. This allows spatially- and component-resolved kinetic studies and, e.g., a direct comparison of inherent catalytic properties of Pt(hkl)- and Pd(hkl)-domains or supported µm-sized Pd-powder agglomerates, studies of the local catalytic ignition and the role of defects and grain boundaries in the local reaction kinetics.

High-field versus high-pressure: weakly adsorbed CO species on Pt(111).

Adsorption of carbon monoxide on a nanosized apex facet of a [111]-oriented Pt field emitter tip was studied under high electrostatic field conditions (15-20V/nm) by field ion appearance energy spectroscopy (FIAES). Weakly bound highly mobile CO species have been detected and their possible role in surface reactions such as CO oxidation on Pt was elucidated. The FIAES results are compared with those of vibrational sum frequency generation (SFG) spectroscopy of CO adsorption on a macroscopic Pt(111) single crystal, in a gas pressure range from UHV to atmospheric. SFG and FIAES are complementary in that SFG mainly provides information on strongly bound CO molecules, whereas FIAES nearly exclusively detects weakly bound species. Both high-field and high-pressure studies indicate the presence of a compressed CO adlayer including weakly bound species. The high-field mimics the high gas pressure, leading to similar high adsorbate coverage, which is required for the formation of weakly bound highly mobile CO species, which may be the active entities in CO oxidation.

Spatial desynchronization of glycolytic waves as revealed by Karhunen-Loeve analysis.

The dynamics of glycolytic waves in a yeast extract have been investigated in an open spatial reactor. At low protein contents in the extract, we find a transition from inwardly moving target patterns at the beginning of the experiment to outwardly moving spiral- or circular-shaped waves at later stages. These two phases are separated by a transition phase of more complex spatiotemporal dynamics. We have analyzed the pattern dynamics in these three intervals at different spatial scales by means of a Karhunen-Loeve (KL) decomposition. During the initial phase of the experiment, the observed patterns are sufficiently described by the two dominant KL modes independently of the spatial scale. However, during the last stage of the experiment, at least 6 KL modes are needed to account for the observed patterns at spatial scales larger than 3 mm, while for smaller scales, 2 KL modes are still sufficient. This indicates that in the course of the experiment, the local glycolytic oscillators become desynchronized at spatial scales larger than 3 mm. Possible reasons for the desynchronization of the glycolytic waves are discussed.