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

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

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H2O+/H3O+) cause image formation of the O2-H2 reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO-H2 reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO+H2 reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.info:eu-repo/semantics/publishe

Nonlinear Dynamics of Reactive Nanosystems: Theory and Experiments

SCOPUS: ch.binfo:eu-repo/semantics/publishe

The role of fluctuations in bistability and oscillations during the H2 + O2 reaction on nanosized rhodium crystals

A combined experimental and theoretical study is presented of fluctuations observed by field ion microscopy in the catalytic reaction of water production on a rhodium tip. A stochastic approach is developed to provide a comprehensive understanding of the different phenomena observed in the experiment, including burst noise manifesting itself in a bistability regime, noisy oscillations, and nanopatterns with a cross-like oxidized zone separating the surface into four quadrants centered on the {111} facets. The study is based on a stochastic model numerically simulating the processes of adsorption, desorption, reaction, and transport. The surface diffusion of hydrogen is described as a percolation process dominated by large clusters corresponding to the four quadrants. The model reproduces the observed phenomena in the ranges of temperature, pressures, and electric field of the experiment.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Catalytic CO Hydrogenation: Mechanism and Kinetics from Chemical Transients at Low and Atmospheric Pressures

The hydrogenation of CO over Co model catalysts was studied using relaxation-type methods operating in situ either at atmospheric pressures or under surface science conditions. Emphasis was laid on providing information on the surface composition and on how it changes with time under catalytic reaction conditions. Using pressure forcing in chemical transient kinetics (CTK), the build-up of the steady-state was studied at 503 K and atmospheric pressure to demonstrate that the active catalyst surface is not metallic but covered with carbon, oxygen and hydrogen in excess of a monolayer equivalent. Both buildup and backward transients suggest CO to act as the "monomer" which probably inserts into an O-H bond to form the primary surface complex necessary for hydrocarbon and oxygenate formation. Repetitive electric field pulses (pulsed field desorption mass spectrometry, PFDMS) at low pressures have allowed the CO dissociation kinetics on a nano-sized Co 3D crystal ("tip") to be monitored in the millisecond time range. No evidence for the occurrence of the Boudouard reaction was obtained in either PFDMS or CTK. Adsorbed CH x (x = 1-3) species were detected in small amounts demonstrating that CO dissociation is fast compared to carbon hydrogenation. Adsorbed Co-subcarbonyl species, Co(CO)x were also detected by PFDMS and possibly mediate the necessary surface mobility during the initial restructuring of the catalyst. Surface carbon seems to inhibit Co-subcarbonyl formation. © Springer Science+Business Media, LLC 2008.SCOPUS: cp.jinfo:eu-repo/semantics/publishe

Complex oscillation patterns during the catalytic hydrogenation of NO2 over platinum nanosized crystals

We studied the catalytic reduction of NO2 with hydrogen on platinum model catalysts by means of field emission techniques. Atomic resolution field ion microscopy (FIM) was used to characterize Pt samples conditioned as tips. The catalytic reaction was investigated with video frequency resolution using field electron emission microscopy (FEM). Kinetic instabilities eventually merging into self-sustained periodic oscillations were observed. Data analyses revealed monomodal and bimodal oscillations as well as the coexistence of both mono-and bimodal regimes. Fourier transforms and interpeak intervals were used to characterize these different modes. We propose a mechanism that qualitatively explains the emergence of oscillations and the appearance of bimodal oscillations. © 2014 American Chemical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe