Model For CO Oxidation Research Articles

Response of a model of CO oxidation with CO desorption and diffusion to a periodic external CO pressure

We present a study of the dynamical behavior of a Ziff–Gulari–Barshad model with CO desorption and lateral diffusion. Depending on the values of the desorption and diffusion parameters, the system presents a discontinuous phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve between these phases. Inclusion of the diffusion term produces a significant increase in the CO 2 production rate. We further applied a square-wave periodic pressure variation of the partial CO pressure with parameters that can be tuned to modify the catalytic activity. Contrary to the diffusion-free case, this driven system does not present a further enhancement of the catalytic activity, beyond the increase induced by the diffusion under constant CO pressure.

Microscopic model of CO oxidation on Pt(1 1 1)

A new microscopic model, based on DFT/LDA modeling, is suggested for the Langmuir–Hinshelwood reaction of catalytic CO oxidation in coadsorbed O–CO layers on Pt(1 1 1). It has been found that only the oxygen atoms occupying threefold hollow sites of hcp type are chemically active. The potential barrier for the oxidation reaction significantly decreases due to changes in the adlayer oxygen states in the proximity to CO. The oxygen electronic density distribution is affected by approaching CO molecule which alters the oxygen position. Height of the barrier is estimated as 1.15 eV, which may be attributed to the upper limit of activation energy for the net reaction process.

Atomistic lattice-gas modeling of CO oxidation on Pd(100): Temperature-programed spectroscopy and steady-state behavior

We have developed an atomistic lattice-gas model for the catalytic oxidation of CO on single-crystal Pd(100) surfaces under ultrahigh vacuum conditions. This model necessarily incorporates an detailed description of adlayer ordering and adsorption-desorption kinetics both for CO on Pd(100), and for oxygen on Pd(100). Relevant energetic parameters are determined by comparing model predictions with experiment, together with some guidance from density functional theory calculations. The latter also facilitates description of the interaction and reaction of adsorbed CO and oxygen. Kinetic Monte Carlo simulations of this reaction model are performed to predict temperature-programmed reaction spectra, as well as steady-state bifurcation behavior.

Canard explosion and internal signal stochastic bi-resonance in the CO oxidation on platinum surface

Canard explosion is a kind of complex temporal behavior and is ubiquitous in excitable systems. It is associated with an abrupt change of amplitude and period of an oscillatory trajectory in a very narrow interval of a control parameter. We have analyzed in the present paper the behavior of canard explosion that is near a supercritical Hopf bifurcation and its response to white noise in a temporal model of CO oxidation on platinum surface. We have found that the presence of canard explosion gives rise to internal signal stochastic bi-resonance, thus demonstrating a novel functional feature of noise: selective amplifying signals with different periods.

Dynamic Phase Transition, Enhanced Reaction Rate, and Decay of Metastable States in a Model of CO Oxidation with CO Desorption

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Canard Explosion and Coherent Biresonance in the Rate Oscillation of CO Oxidation on Platinum Surface

The relationship between canard explosion and coherent biresonance is analyzed by numerically investigating a temporal dynamical model of CO oxidation on Pt surface. Canard explosion, manifesting itself by a dramatic change in the amplitude and period of a periodic orbit within a very narrow interval of a control parameter, is the result of multiple time scales in a dynamical system and is common in excitable systems. Coherent biresonance, namely, two peaks on the signal-to-noise ratio (SNR) curve when varying noise intensity, is a novel phenomenon of coherent resonance which is well-known in excitable systems. When the control parameter is varied from a stable fixed point, crossing the supercritical Hopf bifurcation, one of the peaks that corresponds to relatively larger noise intensity, keeps a constant height and position, while the other becomes higher and moves to lower noise level. When we consider the case in which two control parameters are perturbed by independent noise simultaneously, an interesting picture of one valley between two ridges appears on the 3D surface of SNR.

Insights into the redox properties of ceria-based oxides and their implications in catalysis

In the present work, the redox properties of ceria and related materials will be considered with a focus on these effects that are relevant in catalysis. Particularly, after briefly pointing out the relevant redox chemistry and characterization of ceria in classical TWC application, we will describe how the reduction/oxidation characteristics of ceria are of help in other applications, like the development of advanced ceria–zirconia based catalyst for auto exhaust treatment, and the elimination of soot from diesel engine exhaust. In the first example, different catalyst formulations based on ceria and zirconia are examined to see how catalytic activity is related with composition and a simple redox mechanism model for CO oxidation is also derived. A similar approach is used to characterize the role of ceria in the combustion of soot.

Subsurface oxygen formation on Pt(1 0 0): Experiments and modeling

Spatially resolved techniques, such as photoemission electron microscopy (PEEM) and ellipsomicroscopy for surface imaging (EMSI), have been particularly useful in the investigation of pattern formation during CO oxidation on platinum catalysts. One surprising result of these studies has been the discovery of subsurface oxygen on Pt(1 0 0). The formation of subsurface oxygen has been reported previously on Pt(1 0 0) during CO oxidation at low pressures (<1 × 10 −4 Torr). This communication reports the formation of subsurface oxygen at intermediate pressures (∼0.1 Torr). These observations show that subsurface oxygen plays a role in catalytic CO oxidation and pattern formation, which has implications for catalysts operated at higher pressures. New microkinetic models of CO oxidation that incorporate subsurface oxygen are discussed. Results of these models are qualitatively similar to experimental observations of subsurface oxygen, confirming the importance of this species in the reaction dynamics.

Quantum chemical modeling of CO oxidation by the active site of molybdenum CO dehydrogenase

The catalytic mechanism of molybdenum containing CO dehydrogenase has been studied using hybrid DFT methods with quite large chemical models. The recent high-resolution X-ray structure, showing the surprising presence of copper linked to molybdenum, was used as a starting point. A pathway was initially found with a low barrier for C-O bond formation and CO2 release. However, this pathway did not include the formation of any S-CO2 species, which had been suggested by experiments with an n-butylisocyanide inhibitor. When these SCO2 structures were studied they were found to lead to deep minima, making CO2 release much more difficult. A large effort was spent, including investigations of other spin states, varying the number of protons and electrons, adding water, etc., until a plausible pathway for S-C bond cleavage was found. In this pathway a water molecule is inserted in between molybdenum and the SCO2 group. Full catalytic cycles, including electron and proton transfers, are constructed both with and without S-C bond formation. When these pathways are extended to two full catalytic cycles it can be understood why the formation of the S-C bond actually makes catalysis faster, even though the individual step of CO2 release becomes much more difficult. These results agree well with experimental findings.

Modeling of relaxation oscillations in CO oxidation on metallic catalysts with consideration of reconstructive heterogeneity of the surface

This paper is devoted to the development of a low-dimensional kinetic model of CO oxidation on a metallic catalyst surface and theoretical study of arising nonlinear phenomena and relaxation oscillations of the reaction rate. Experimental results of studies on the adsorbed oxygen are presented and its significance in the CO oxidation mechanism on Ir catalyst is shown. We consider the conventional Langmuir–Hinshelwood mechanism of catalytic CO oxidation and take into account the possibility of a metallic surface modification during reaction due to the oxygen penetration into subsurface layers. We suggest that when the adsorbed oxygen concentration exceeds some critical value, a surface modification occurs and the reaction capability of adsorbed oxygen changes, so that the activation energy of the interaction between the adsorbed species sharply increases. Construction of bifurcation curves on the parametric plane ( P CO , P O 2 ) permits us to separate areas of CO and O 2 partial pressures, for which self-oscillations and/or multiplicity of steady states appear. This paper deals with parametric analysis of a new two-variable kinetic model of CO oxidation on Ir catalyst surface, but the approach developed is rather general and can be applied to studies of different catalytic reactions.

Response of a catalytic reaction to periodic variation of the CO pressure: IncreasedCO2production and dynamic phase transition

We present a kinetic Monte Carlo study of the dynamical response of a Ziff-Gulari-Barshad model for CO oxidation with CO desorption to periodic variation of the CO pressure. We use a square-wave periodic pressure variation with parameters that can be tuned to enhance the catalytic activity. We produce evidence that, below a critical value of the desorption rate, the driven system undergoes a dynamic phase transition between a CO2 productive phase and a nonproductive one at a critical value of the period and waveform of the pressure oscillation. At the dynamic phase transition the period-averaged CO2 production rate is significantly increased and can be used as a dynamic order parameter. We perform a finite-size scaling analysis that indicates the existence of power-law singularities for the order parameter and its fluctuations, yielding estimated critical exponent ratios beta/nu approximately 0.12 and gamma/nu approximately 1.77. These exponent ratios, together with theoretical symmetry arguments and numerical data for the fourth-order cumulant associated with the transition, give reasonable support for the hypothesis that the observed nonequilibrium dynamic phase transition is in the same universality class as the two-dimensional equilibrium Ising model.

From Atomic Scale Ordering to Mesoscale Spatial Patterns in Surface Reactions: A Heterogeneous Coupled Lattice-Gas Simulation Approach

A current challenge in the modeling of surface reaction-diffusion systems is to connect the length scales from a realistic atomistic treatment of local ordering of adsorbed reactants and of reaction kinetics to an "exact" description of mesoscale spatial pattern formation. We discuss a heterogeneous coupled lattice-gas approach which utilizes parallel kinetic Monte Carlo simulations of lattice-gas models to simultaneously determine the local reaction kinetics and diffusive transport properties at various macroscopic "points" distributed across the surface. These simulations are periodically coupled to reflect macroscopic mass transport via surface diffusion. We place particular emphasis on the key issue of correctly describing the associated chemical or collective diffusion flux. This method is general, but it is discussed here only in the context of simple models for the bistable CO oxidation reaction on surfaces, where macroscopic mass transport involves only diffusion of adsorbed CO. Detailed analysis is presented for a canonical model of CO oxidation which incorporates nearest-neighbor exclusion between adsorbed oxygen, leading to superlattice ordering of this reactant. In this model, interactions of adsorbed CO with other adspecies are ignored. This reduces CO surface transport from a many-particle to a single-particle problem, although one with the complexity of "ant-in-the-labyrinth"-type percolative diffusion (the labyrinth being formed by coadsorbed oxygen).

On one dimensional chemical diode and frequency generator constructed with an excitable surface reaction

The oxidation of carbon monoxide on a Pt(110) surface is considered as a medium for chemical information processing in which bits of information are represented by traveling pulses of high oxygen coverage. Using numerical simulations for a model of CO oxidation we demonstrate that in such system one dimensional chemical signal diode can be realized by setting a proper profile of temperature. We also show that a pulse splitting can occur on a temperature inhomogeneity. The phenomenon of pulse splitting can be used to construct one dimensional generator of a train of pulses with adjustable frequency.

Manipulation of surface reaction dynamics by global pressure and local temperature control: A model study

Specific catalyst design and external manipulation of surface reactions by controlling accessible physical or chemical parameters may be of great benefit for improving catalytic efficiencies and energetics, product yield, and selectivities in the field of heterogeneous catalysis. Studying a realistic spatiotemporal one-dimensional model for CO oxidation on Pt(110) we demonstrate the value and necessity of mathematical modeling and advanced numerical methods for directed external multiparameter control of surface reaction dynamics. At the model stage we show by means of optimal control techniques that species coverages can be adjusted to desired values, aperiodic oscillatory behavior for distinct coupled reaction sites can be synchronized, and overall reaction rates can be optimized by varying the surface temperature in space and time and the CO and O2 gas phase partial pressure with time. The control aims are formulated as objective functionals to be minimized which contain a suitable mathematical formulation for the deviation from the desired system behavior. The control functions pCO(t) (CO partial pressure), pO2(t) (O2 partial pressure), and T(x,t) (surface temperature distribution) are numerically computed by a specially tailored optimal control method based on a direct multiple shooting approach which is suitable to cope with the highly nonlinear unstable mode character of the CO oxidation model.

Experimentally Validated Model for CO Oxidation on PtRu/C in a Porous PEFC Electrode

Carbon monoxide oxidation in a porous polymer electrolyte fuel cell (PEFC) electrode with PtRu/C catalyst was studied with steady-state polarization curves and open-circuit decay measurements. The Tafel slope was about 210 mV/decade and the reaction order for CO was about 0.45 at 0.4 V vs. RHE. This experimental behavior is explained with a mathematical model with CO adsorbing on Pt and water adsorbing on Ru. Kinetic parameters are determined from a fitting of the model to both the steady-state and the transient measurements. A single rate-determining step cannot account for the polarization curves over the whole potential range. At mid-range potentials the oxidation step is rate-determining but at lower potentials the water adsorption might be rate-determining. The open-circuit decay measurements gave noise-free measurements and confirmed the accuracy of the steady-state measurements. The obtained model for CO oxidation where the coverage of on Pt is determined can be used together with a model for oxidation. © 2004 The Electrochemical Society. All rights reserved.

From atomic scale reactant ordering to mesoscale reaction front propagation: CO oxidation on Pd(100)

We utilize a heterogeneous coupled lattice-gas (HCLG) approach to connect the length scales from a realistic atomistic description of surface reactions to the associated mesoscale spatiotemporal behavior. This method is applied to describe reaction front structure in a model for CO oxidation on Pd(100) which incorporates complex ordering of CO and O adlayers, and a precise treatment of the chemical diffusion for interacting CO adlayers in an environment of coadsorbed O.

Microkinetic Modeling for Water-Promoted CO Oxidation, Water−Gas Shift, and Preferential Oxidation of CO on Pt

A comprehensive surface reaction mechanism on Pt is presented that is capable of describing CO oxidation, H2 oxidation, water−gas shift (WGS), preferential oxidation (PROX) of CO, and the promoting role of H2O on CO oxidation reasonably well. This mechanism consists of a literature CO oxidation model, a surface reaction mechanism for H2 oxidation on Pt developed here, and coupling reactions between the CO and H2 chemistries included for the first time. Thermodynamic consistency, which is shown to be essential for WGS, is ensured in all steps of the entire mechanism. The CO−H2 coupling via the CO + OH reaction, which may involve direct CO2 formation, CO* + OH* ↔ CO2* + H*, as well as an indirect pathway via the carboxyl intermediate, is explored. It is shown that this coupling plays a significant role in capturing the promoting effect of H2O on the CO oxidation-temperature-programmed reaction experiments at low temperatures as well as the overall speed of the WGS and PROX reactions. With the parameters use...

Deactivation of nanosize gold supported on zirconia in CO oxidation

An Au/ZrO 2 catalyst containing nanosize gold was investigated with respect to its activity in CO oxidation. The high initial activity observed at temperatures down to 230 K was followed by a gradual decrease. Two main reasons for the catalyst deactivation were found: (i) adsorption of CO and its accumulation as carbonates, this deactivation being reversible and after heating the catalyst surface being restored by CO 2 evolution, (ii) agglomeration of Au particles, which causes irreversible however weak deactivation. IR, XPS and TPD data were provided in support of a probable model of CO oxidation and deactivation on Au/ZrO 2 catalyst.

On the mechanism of fast oxygen storage and release in ceria-zirconia model catalysts

CO oxidation over fresh and aged CexZr1−xO2 (0.15≤x≤1) mixed oxides was carried out under oscillating feed-stream composition by feeding the reactor with CO, He and O2 at a frequency of 0.2Hz. The activity of fresh, high surface area (HSA) catalysts of different compositions is similar and depends mainly on exposed surface redox centers; on the contrary, aged, low surface area (LSA) samples of ceria-zirconia show larger differences of activity with the composition. The sequence and the shape of the product peaks suggest that the kinetics of oxygen storage/release is influenced by surface phenomena that are not taken into account when the reaction is carried out under stationary condition. In particular, in HSA samples the CO2 peak splits into two components. The first component is observed when CO is pulsed over the catalyst, while the second component appears when O2 is pulsed. Temperature and composition of the catalyst determine the intensity ratio of the two peaks. The first peak increases with the amount of zirconia and is the result of direct reaction of CO with the oxygen from the oxide. The second contribution to CO2 production is highly influenced by surface-area and temperature and may be related to a more complex surface dynamic. In situ diffuse reflectance infrared (DRIFT) spectra show that chemisorbed carbonate-like species are involved as intermediates. On the basis of these results, a mechanism and a model for CO oxidation is proposed.

Oscillatory CO oxidation on Pd(1 0 0) studied with in situ scanning tunneling microscopy

We have studied the catalytic oxidation of CO on Pd(1 0 0) at atmospheric pressures, using a combined flow reactor and scanning tunneling microscope (STM). By switching from a CO-rich flow to an O 2-rich flow, and vice versa, we could reversibly oxidize and reduce the surface, as we have observed with the STM. Both during the initial oxidation and during the catalytic reaction on the oxide, the surface became rough. Coinciding with the formation of the palladium oxide there was a step up in the CO oxidation rate. Whereas the reaction rate on the metallic surface followed traditional Langmuir–Hinshelwood kinetics, the reaction rate on the oxide was proportional to the CO-pressure and did not depend on the O 2-pressure. We suggest that the high reaction rate on the oxide resulted from the low stability of the oxide, making it highly reactive, and that the reaction followed the Mars–Van Krevelen mechanism, in which the oxide is continually ‘consumed’ and reformed. The switching between the metallic surface and the oxide showed significant hysteresis in the CO-pressure for O 2-rich conditions. This resulted in a CO-pressure window where reaction was stable both on the metal and on the oxide. We have observed that the reaction switched spontaneously between these branches, leading to oscillations in the reaction rate. Our interpretation is in strong disagreement with well-established models for CO oxidation on Pt-group metal surfaces.