Oscillations in the carbon monoxide oxidation on platinum surfaces observed by field electron microscopy

Городецкий, В. В.; Block, J.H.; Drachsel, W.; Ehsasi, M.

doi:10.1016/0169-4332(93)90312-y

Cited by 60 publications

(25 citation statements)

References 10 publications

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“…Using video-field electron microscopy they were able to make visible wave fronts in simple catalytic reactions like the CO oxidation [1]. The phenomena eventually resulted in selfsustained oscillations.…”

Section: Introductionmentioning

confidence: 94%

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

Bocarmé

Kruse

2011

Ultramicroscopy

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a b s t r a c tThe adsorption of NO and its reaction with H 2 over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10 À 3 Pa pressure range and at sample temperatures between 400 and 600 K. By varying the H 2 partial pressure while keeping the other control parameters constant, the NO +H 2 reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H 2 pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO + H 2 /Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO + seems to be the dominating imaging species under all conditions. At high relative H 2 pressures (the ''hydrogen-side''), H atoms seem to diffuse subsurface. This process is blocked at lower H 2 pressure (the ''NO-side'') due to NO ad and O ad accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO 2 + species in the mass spectra.

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Section: Introductionmentioning

confidence: 94%

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

Bocarmé

Kruse

2011

Ultramicroscopy

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“…10 Indeed, atomically sharp spatial structures have in fact been observed in recent field emission microscopy and field ion microscopy studies of surface reactions, 3 including CO-oxidation on Pt tips. 11 Furthermore, the mean-field reaction-diffusion equation formalism was recently modified to describe the evolution of sharp ''phase boundaries,'' directly relating their existence to attractive adspecies interactions. 12 However, in CO-oxidation on Pt͑100͒, the ''uphill diffusion'' of CO to growing 1ϫ1-CO islands, which occurs in a macroscopically uniform system, does not correspond to ''chemical diffusion,'' in the sense defined above.…”

Section: Introductionmentioning

confidence: 99%

Percolative diffusion of CO during CO oxidation on Pt(100)

Tammaro

Evans

1996

The Journal of Chemical Physics

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During CO-oxidation on Pt(100), CO diffuses in a "disordered environment" produced by a complex pattern of reconstructed and unreconstructed regions of the substrate. Macroscopic diffusion of CO is effectively only possible on percolating 1X1-regions of the substrate. We treat the spatio-temporal behavior observed in this reaction system accounting in the simple way for the percolative nature of CO-diffusion. This is done via incorporation into the reaction-diffusion equations of a suitable chemical diffusion coefficient, exploiting ideas from the theory of transport in disordered media. We use these equations to analyze the propagation of reactive, O-rich pulses into a CO-covered 1X1-background. Disciplines Mathematics | Physics CommentsThe following article appeared in Journal of Chemical Physics 104,9 (1996) During CO-oxidation on Pt͑100͒, CO diffuses in a ''disordered environment'' produced by a complex pattern of reconstructed and unreconstructed regions of the substrate. Macroscopic diffusion of CO is effectively only possible on percolating 1ϫ1-regions of the substrate. We treat the spatio-temporal behavior observed in this reaction system accounting in the simple way for the percolative nature of CO-diffusion. This is done via incorporation into the reaction-diffusion equations of a suitable chemical diffusion coefficient, exploiting ideas from the theory of transport in disordered media. We use these equations to analyze the propagation of reactive, O-rich pulses into a CO-covered 1ϫ1-background.

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confidence: 99%

“…Concentration patterns will hence be formed on extended single crystal surfaces, but also on monolithic polycrystalline material where the individual grains have typically much larger diameters [18]. However, other types of RD phenomena will develop on the small (≤ 10 nm) crystal planes of supported catalyst particles as modeled in experiments with field emission tips [19,20].(ii) Gas-phase coupling Varying reactivity will also affect the partial pressures of the reactants, even in a flow system. As outlined above, at low pressures these changes will propagate practically instantaneously and may offer a rather effective "global" coupling mechanism, which synchronizes different parts of a single crystal surface or even of separated samples [21].…”

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confidence: 99%

Non‐Linear Dynamics: Oscillatory Kinetics and Spatio‐Temporal Pattern Formation

Ertl

2008

Handbook of Heterogeneous Catalysis

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The rate of a catalytic reaction is governed by the underlying mechanism, i.e. the sequence of elementary reaction steps, and depends on the external control parameters such as the concentrations (= partial pressures) p j of educt and product species and temperature T, if transport processes are excluded in this context. More specifically, usually the adsorbed species are assumed to be randomly distributed over the surface, and their concentrations (= coverages) θ i are described within a mean-field approximation so that the reaction rate (= number of molecules r formed per unit time, dn r /dt t ) is given by dn r dt = f θ i , T ( ) (1) whereby the coverages θ i are in turn determined by a set of differential equations (modeling the individual steps of adsorption, desorption and surface reaction) of the typeProper treatment then requires that additional constraints given by heat and mass balance are taken into consideration.Under flow conditions with the external control parameters being kept fixed, usually the reaction proceeds at steady state with constant rate, i.e., dn r /dt = constant and dθ i /dt = 0. Due to the nonlinear character of the quoted differential equations, this has, however, not necessarily to be the case; the kinetics may -for certain ranges of parameters -become oscillatory or even irregular (chaotic). Such systems are typically far away from equilibrium, and (i) By far the most detailed insights into the underlying mechanisms and the general phenomenology of spatio-temporal self-organization were obtained in studies with well-defined single crystal surfaces by applying the arsenal of modern surface physical methods.(ii) Single crystal studies are usually conducted with bulk samples under low pressure conditions (≤ 10 -4 mbar)where the temperature changes associated with varying reaction rate are negligible. In addition, the mean-free path of gaseous molecules is comparable or even larger than the dimensions of the reaction vessel, which is operated as a continuous flow reactor. Hence, concentration gradients in the gas phase are practically instantaneously (≤ 10 -3 s)transmitted. Analysis of such experiments is thus appreciably simplified.(iii) Experiments with polycrystalline, i.e. "real", catalysts are usually conducted at elevated partial pressures (≥ 1 mbar) under which the heat released by the reaction may lead to noticeable temperature variations. The resulting thermokinetic effects may become dominating, so that even the nonuniformity of the surface chemistry is masked and quite novel phenomena may arise. Ideally, the reaction is conducted in a way, which may be described by a continuously stirred tank reactor (CSTR), i.e., perfect mixing ensures that the concentrations are everywhere identical.However, frequently a concentration profile exists along the reactor, which is hence rather of the plug-flow type. It is thus evident that heat and mass transfer limitations may play important roles with such systems.Generally, an extended system such as a single crystal surface, or even more ...

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Oscillations in the carbon monoxide oxidation on platinum surfaces observed by field electron microscopy

Cited by 60 publications

References 10 publications

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

Percolative diffusion of CO during CO oxidation on Pt(100)

Non‐Linear Dynamics: Oscillatory Kinetics and Spatio‐Temporal Pattern Formation

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