Directional Gateway to Metal Oxidation: 3D Chemical Mapping Unfolds Oxygen Diffusional Pathways in Rhodium Nanoparticles.

The interaction of oxygen with a reactive metal is ubiquitous, yet the precise atomic-level mechanisms and pathways leading to the formation of a surface oxide are not well-understood. We report oxygen atom distributions inside Rh single nanoparticles using atom probe microscopy (APM) and demonstrate that mainly facets of the ⟨022̅⟩ crystallographic directions act as oxygen-permeable gateways. The highly anisotropic spatial distribution of incorporated oxygen atoms is in agreement with video-field emission analyses according to which {113} facets of the ⟨022̅⟩ zones act as portals for subsurface diffusion. In addition to providing a more fundamental understanding of the precursor states to metal corrosion, in particular for the case of nanosized metal particles, our studies are also relevant for heterogeneous catalysis where catalytic activity and selectivity conform to reaction-induced structural changes of metal nanoparticles.

Reconstructing stochastic attractors from nanoscale experiments on a non-equilibrium reaction.

We studied the catalytic NO2(g) + H2(g)/Pt system on model platinum catalysts with nanoscale spatial resolution by means of field emission microscopy (FEM). While the surface of the catalyst is in a non-reactive state at low H2 partial pressure, bursts of activity are observed when increasing this parameter. These kinetic instabilities subsequently evolve towards self-sustained periodic oscillations for a wide range of pressures. Combining time series analyses and numerical simulations of a simple reaction model, we clarify how these observations fit in the traditional classification of dynamical systems. In particular, reconstructions of the probability density around oscillating trajectories show that the experimental system defines a crater-like structure in probability space. The experimental observations thus correspond to a noise-perturbed limit cycle emerging from a nanometric reactive system. This conclusion is further supported by comparison with stochastic simulations of the proposed chemical model. The obtained results and simulations pave the way towards a better understanding of reactive nanosystems.

Emergence of Chemical Oscillations from Nanosized Target Patterns.

This work investigates experimentally the mechanism by which chemical oscillations emerge in a nanometric system. We monitor the spatiotemporal dynamics of an oscillating reaction on the surface of a nanosized three-dimensional Pt model catalyst. Using high-resolution field emission techniques, we are able to show that the oscillations are generated by nanoscale chemical target patterns of much shorter characteristic time than the period with which the oscillations occur. Our observations are made for a specific reaction system-NO_{2} reduction with hydrogen-and represent the first experimental evidence for the presence of target patterns at the nanoscale. They can be seen as an experimental demonstration of reaction-diffusion mechanisms to hold at the nanoscale as they do at the macroscale. These results shed new light on the emergence of complexity through different time and length scales.

Fluctuating Dynamics of Nanoscale Chemical Oscillations: Theory and Experiments.

Chemical oscillations are observed in a variety of reactive systems, including biological cells, for the functionality of which they play a central role. However, at such scales, molecular fluctuations are expected to endanger the regularity of these behaviors. The question of the mechanism by which robust oscillations can nevertheless emerge is still open. In this work, we report on the experimental investigation of nanoscale chemical oscillations observed during the NO2 + H2 reaction on platinum, using field electron microscopy. We show that the correlation time and the variance of the period of oscillations are connected by a universal constraint, as predicted theoretically for systems subjected to a phenomenon called phase diffusion. These results open the way to a better understanding, modeling, and control of nanoscale oscillators.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO.

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Adsorption-induced restructuring and early stages of carbon-nanotube growth on Ni nanoparticles.

Carbon adsorption on various Ni surfaces is investigated as a function of coverage via a combination of first-principles simulations and field emission microscope experiments. It is found that carbon can be efficiently stored as subsurface carbides, but with different energetics on differently oriented surfaces depending on their compactness and density of adsorption sites. In the resulting morphological reshaping, {113} facets are predicted to grow at the expense of {111} and {100} facets, in excellent agreement with experimental observations. Moreover, at high coverage on the {113} surface the carbon adsorption energy passes through a maximum after which a structural crossover is realized such that carbon atoms tend to ascend to the surface to form one-dimensional chains (which are the precursors of graphitic nanostructures). This rationalizes the experimental observation of an incubation time between carbon storage and the beginning of catalytic growth, and provides insight into the early stages (nucleation mechanism) of carbon nanotubes on Ni nanoparticles.

Mechanism and kinetics of the catalytic CO-H2 reaction: An approach by chemical transients and surface relaxation spectroscopy.

The aim of this paper is to demonstrate the importance of providing time-resolved information in catalysis research. Two truly in situ methods will be presented and compared for their merits and drawbacks: chemical transient kinetics (CTK) and pulsed field desorption mass spectrometry (PFDMS). The presentation will be given by way of example choosing the syngas (CO/H2) reaction over cobalt-based catalysts as a catalytic process. Despite numerous efforts in the past, the mechanism of this reaction is still under debate. In CTK the reaction is studied on a metal-supported catalyst under flow conditions in a pressure range extending from atmospheric pressure down to 100 Pa. Sudden changes in the partial pressures of the reactants then allow following the relaxation to either steady-state conditions ("transients") or cleanoff ("back transients"). In PFDMS short field pulses of several volts per nanometer are applied to a model catalyst which resembles a single metal particle grain (a "tip"). These pulses intervene during the ongoing reaction under flow conditions at pressures ranging from 10(-1) to 10(-5) Pa and cause field desorption of adsorbed species. This method is particularly suited to detect reaction intermediates in a time-dependent manner since the repetition frequency of the pulses can be systematically varied. It is shown that both methods lead to complementary results. While CTK allows conclusions on the mechanism of CO hydrogenation by following the time-dependent formation of hydrocarbon species, PFDMS provides insight into the initial steps leading to adsorbed CxHy species. A quantitative assessment of the CTK data allows the demonstration that the catalyst under working conditions is in an oxidized rather than metallic state. The initial steps to oxidation are also traced by PFDMS. Most importantly, however, CTK results allow formulation of a reaction mechanism that is common for both hydrocarbon and oxygenate formation and is based on the occurrence of a formate-type species as the most abundant surface intermediate.

Kinetic instabilities during the NO(x) reduction with hydrogen on Pt crystals studied with field emission on the nanoscale.

This paper reviews field emission studies of kinetic instabilities occurring during the catalytic reduction of nitric oxide (NO) and nitrogen dioxide (NO(2)) by hydrogen on three-dimensional platinum crystals. Emphasis is placed on revealing that both field ion microscopy (FIM) and field electron microscopy (FEM) can image such instabilities under truly in situ reaction conditions with a lateral resolution on the nanoscale. In particular, oscillatory behavior with rapid ignition from a state of low to a state of high catalytic activity is demonstrated for both NO and NO(2) reduction. Results of a local chemical probing during FIM studies of the NO+H(2) reaction are also shown and provide clear evidence for the oscillatory behavior of water (detected as H(2)O(+) and H(3)O(+)) formation and for diffusion supply of NO into surface regions emptied during the stage of high catalytic activity. The rapid ignition ("surface explosion") of the catalytic cycle is discussed on the basis of an autocatalytic mechanism of the NO decomposition. On the (001) plane of the Pt crystal small island formation is seen to occur during the low-activity state of the catalytic cycle. Islands have a size equivalent to approximately 3 nm, move independently from each other, and do not merge when colliding. A tentative model is discussed associating islands with patches of hydroxyl groups. Very regular oscillatory behavior is demonstrated for the NO(2) reduction using FEM. Advantages as well as shortcomings of the FEM/FIM experimental approach are discussed and an outlook on future studies using local chemical probing will be given wherever appropriate. (c) 2002 American Institute of Physics.