A non-thermal lattice gas model for a dimer-trimer reaction on a catalytic surface: A computer simulation study

The kinetics of an irreversible dimer-trimer reaction of the type 3A₂ + 2B₃ → 6AB have been studied using a non-thermal (precursor mechanism) model on a square as well as on a hexagonal lattice surface by Monte Carlo simulation. When the range of the precursors (A atoms) is increased, the model gives production rates (reactive window widths) that are quite large as compared with those for thermal (Langmuir-Hanshelwood mechanism) model. The phase diagrams qualitatively resemble with the standard ZGB model except that the continuous transition point is eliminated when the range of the precursors is extended up to the third nearest neighbourhood. The diffusion of A atoms on the surface as well as their desorption from the surface with a certain probability is also considered to see their effects on the reaction mechanism.     

Lifted reconstruction as a feedback mechanism in the oscillating CO oxidation on Pt nanofacets: Microscopic evidences

Global and local oscillations in the CO oxidation reaction have been visualized in situ on the apex of a [1 0 0]-oriented Pt field emitter tip used as a well-defined model for catalytically active, nm-sized particles by Field Emission (FEM) and Lithium Field Desorption (Li-FDM) Microscopes. For the first time experimental evidence is provided that the reconstruction feedback mechanism of the self-maintained oscillations for the [1 0 0] and [1 1 0] orientations, which is well established on macroscopic single crystals, is also valid in the heterogeneous, nm-sized system with its different crystallographic orientations which are coupled by surface diffusion.     

Oxidation of CO on Fe2O3 model surfaces

Using first principles calculations based on a gradient corrected density functional formalism we show that Fe₂O₃ nano-particles with (1 0 0) and (0 0 0 1) surface orientations can oxidize CO to form CO₂ with or without the presence of O₂. However, depending on the surface orientation, the oxidation occurs through differing sequences. On the (1 0 0) surface, in the absence of O₂, two CO molecules are required for one CO oxidation in a concerted reaction while on a oxygen terminated (0 0 0 1) surface, a single CO molecule itself, without the aid of a second CO, can react with the lattice oxygen atoms to form CO₂. In the presence of O₂, the O vacancies created by an initial oxidation through lattice oxygen act as the favored sites for O₂ adsorption which can subsequently oxidize the incoming CO. Detailed reaction paths and the corresponding energetics for the proposed mechanisms are also studied.     

Effect of the Eley-Rideal step on catalytic oxidation of CO under periodic external pressure

The effect of periodic variation of external pressure on the catalytic oxidation of CO is simulated for a modified Ziff-Gulari-Barshad (ZGB) model including the Eley-Rideal (ER) step. The external CO partial pressure is periodically varied between its values in reactive and CO poisoned states. The results show different amplitudes of such perturbation significantly influence the CO₂ production rate. Latter also changes with the period of such oscillations, especially for perturbations with larger amplitudes. However, for a given period there is a critical value of the amplitude above which, the catalyst surface becomes irreversibly poisoned.     

Monte Carlo simulations of catalytic CO oxidation on fractal surfaces of dimension between two and three

We present a method for generating fractal surfaces of dimension between two and three. By using the method, five fractal surfaces with dimension 2.262, 2.402, 2.524, 2.631, and 2.771 are created. For each of these surfaces, the reaction of carbon monoxide and oxygen is simulated by using a Monte Carlo method based on the ZGB model [Phys. Rev. Lett. 24 (1986) 2553]. The results show that the catalytic CO oxidation proceeds more efficiently on a surface with higher fractal dimension. It is also found that as the fractal dimension of the surface becomes higher, the first-order kinetic phase transition point (y₂) is shifted to a higher partial pressure of CO. This implies that poisoning of the catalyst surface due to CO segregation sets in at a higher CO partial pressure for surfaces with more complexity.     

Fabrication of model Pt-ceria catalysts and an analysis of their performance for CO oxidation

Model Pt-ceria catalysts have been prepared by the evaporation of Pt onto ceria (CeO₂) films grown on Si(1 1 1) substrates. Photoelectron spectroscopy (XPS, UPS) data are used to characterise the surfaces and their adsorption characteristics, and CO oxidation has been used as a probe reaction to test the activity of the model catalysts.Pure ceria is catalytically-inactive under the test conditions employed, whereas the model Pt/ceria catalysts demonstrate high activity for CO oxidation. The model catalysts also reproduce many of the characteristics of their high-surface area analogues, including the possession of a characteristic light-off temperature, hysteresis in activity as a function of temperature and a negative-order dependence on the CO partial pressure.Many aspects of the behaviour of these catalysts are shown to be a direct result of the strong adsorption of CO. The sensitivity of the dispersed Pt towards oxidation is also experimentally-demonstrated and the importance of this phenomenon is discussed.     

A simple statistical model for V2O5 catalysis

In this work we propose a model to describe the selective oxidation of hydrocarbons at the surface of the V₂O₅ catalyst. The main ingredients of the model are the concentration of vanadium active sites, the surface and bulk diffusion rates of oxygen vacancies and the probability rate of a hydrocarbon reaction. The reactions take place at the free V₂O₅ (0 1 0) surface, and the diffusion of vacancies occur along the [0 1 0] (bulk) and [0 0 1] (surface) directions. The coupling between V₂O₅ and a given metal oxide support determines the concentration of the active vanadium sites, where the reactions can occur. Only the oxygen atoms, which are coordinated to three vanadium sites, take part of the oxidation process. In our calculations we employed two different approaches, single site and pair approximations, and some Monte Carlo simulations. We have found the dependence of the critical concentration of vacancies on the diffusion rates, probability of reaction, and fraction of active vanadium sites, for the catalyst to operate in an active steady state.     

Direct interactions between metal nanoparticles and support: STM studies of Pd on TiO2(1 1 0)

We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO₂(1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1-50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal-support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti³⁺ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.     

Direct comparison of the reactivity of the non-oxidic phase of Ru(0 0 0 1) and the RuO2 phase in the CO oxidation reaction

Applying in situ surface X-ray diffraction (SXRD) together with on-line mass spectrometry during the CO oxidation over Ru(0 0 0 1) allows a direct comparison of the reactivity of the non-oxidic state with that of the RuO₂(1 1 0) covered surface. This comparison reveals that the RuO₂(1 1 0) surface is a catalytically active phase at least as active as the non-oxidic phase. At high CO and O₂ pressures of 200 mbar and temperatures above 550 K, the CO oxidation reaction does not proceed isothermally on the RuO₂(1 1 0) surface. The released reaction heat leads rather to an increase of the sample temperature of up to 130 K accompanied by a self-acceleration of the CO oxidation reaction.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Studies of catalytic process of complete oxidation of methane by SSITKA method

This paper presents the results obtained by means of the steady state isotopic transient kinetic analysis for complete methane oxidation over the Pd(PdO)Al₂O₃ catalyst. The average surface life-time and surface concentration of methane and carbon dioxide were determined. It was found out that on the palladium catalyst there are adsorbed small amounts of methane (which does not take part in the process of oxidation) only at the temperature corresponding to the starting point of methane oxidation. Additionally, in the steady state of methane oxidation on the palladium catalyst there are present two different kinds of carbon dioxide: short- and long-resided on the catalyst surface. The average surface life-time of both kinds of carbon dioxide decreases with temperature. The surface concentration of long-resided carbon dioxide increases with temperature whereas the small maximum at about 380 °C is noticed for the surface concentration of short-resided carbon dioxide.     

In situ STM study of nanosized Ru and Os islands spontaneously deposited on Pt(1 1 1) and Au(1 1 1) electrodes

We provide an overview of structure and reactivity of selected bimetallic single crystal electrodes obtained by the method of spontaneous deposition. The surfaces that are described and compared are the following: Au(1 1 1)/Ru, Pt(1 1 1)/Ru and Pt(1 1 1)/Os. Detailed morphological information is presented and the significance of this work in current and further study of nanoisland covered surfaces in the catalytic and spectroscopic perspective is highlighted. All surfaces were investigated by in situ STM and by electroanalytical techniques. The results confirm our previous data that nanosized Ru islands are formed with specific and distinctive structural features, and that the Ru growth pattern is different for Au(1 1 1) and Pt(1 1 1). For Au(1 1 1), Ru is preferentially deposited on steps, while a random and relatively sparse distribution of Ru islands is observed on terraces. In contrast, for Ru deposited on Pt(1 1 1), a homogeneous deposition over all the Pt(1 1 1) surface was found. Os is also deposited homogeneously, and at a much higher rate than Ru, and even within a single deposition it forms a large proportion of multilayer islands. On Au(1 1 1), the Ru islands on both steps and terraces reach the saturation coverage within a short deposition time, and the Ru islands grow to multilayer heights and assume hexagonal shapes. On Pt(1 1 1), the Ru saturation coverage is reached relatively fast, but when a single deposition is applied, Ru nanoislands of mainly monoatomic height are formed, with the Ru coverage not exceeding 0.2 ML. For Ru deposits on Pt(1 1 1), we demonstrate that larger and multilayer islands obtained in two consecutive depositions can be reduced in size--both in height and width--by oxidizing the Ru islands and then by reducing them back to a metallic state. A clear increase in the Ru island dispersion is then obtained. However, methanol oxidation chronoamperometry shows that the surface with such a higher dispersion is less active to methanol oxidation than the initial surface. A preliminary interpretation of this effect is provided. Finally, we studied CO stripping reaction on Pt(1 1 1)/Ru, Au(1 1 1)/Ru and on Pt(1 1 1)/Os. We relate CO oxidation differences observed between Pt(1 1 1)/Ru and Pt(1 1 1)/Os to the difference in the oxophilicity of the two admetals. In turn, the difference in the CO stripping reaction on Pt(1 1 1)/Ru and Au(1 1 1)/Ru with respect to the Ru islands is linked to the effect of the substrate on the bond strength and/or adlayer structure of CO and OH_ads species.     

Ceria nanoformations in CO oxidation on Pt(1 1 1): Promotional effects and reversible redox behaviour

A well-defined CeO_x/Pt(1 1 1) model catalytic system has been fabricated using the self-assembling of Ce adatoms on a Pt(1 1 1) surface with a subsequent oxidation of the nucleating Ce submonolayer (0.3 ML). The resulting system of the “inverse supported catalyst” type consists of CeO_x nanoformations (2D islands of 5-15 nm size and ∼0.3 nm in height) more or less uniformly distributed over the Pt(1 1 1) surface. This CeO_x/Pt(1 1 1) system has been tested in the CO oxidation reaction where both the CO₂ production rate and the Ce oxidation state were monitored in situ. An enhanced reactivity and a remarkable shift of the bistable region of the reaction towards higher CO pressures were observed when compared to a clean Pt(1 1 1) surface. The CeO_x islands exhibit a pronounced redox behaviour that follows the hysteresis cycle of the reaction. The usefulness of such a type of the “inverse model catalyst” for studying the oxygen diffusion supply and the redox behaviour of ceria in the ceria-platinum catalysts is demonstrated.     

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.     

Mechanistic study of the CO oxidation reaction on the CuO (111) surface during chemical looping combustion

Cu-based oxides oxygen carriers and catalysts are found to exhibit attractive activity for CO oxidation, but the dispute with respect to the reaction mechanism of CO and O₂ on the CuO surface still remains. This work reports the kinetic study of CO oxidation on the CuO (111) surface by considering the adsorption, reaction and desorption processes based on density functional theory calculations with dispersion correction (DFT-D). The Eley–Rideal (ER) CO oxidation mechanism was found to be more feasible than the Mars-van-Krevelen (MvK) and Langmuir–Hinshelwood (LH) mechanisms, which is quite different from previous knowledge. The energy barrier of ER, LH, and MvK mechanisms are 0.557, 0.965, and 0.999 eV respectively at 0 K. The energy barrier of CO reaction with the adsorbed O species on the surface is as low as 0.106 eV, which is much more active in reacting with CO molecules than the lattice O of CuO (111) surface (0.999 eV). A comparison with the catalytic activity of the perfect Cu₂O (111) surface shows that the ER mechanism dictates both the perfect Cu₂O (111) and the CuO (111) surface activity for CO oxidation. The activity of the perfect Cu₂O (111) surface is higher than that of the perfect CuO (111) surface at elevated temperatures. A micro-kinetic model of CO oxidation on the perfect CuO (111) surface is established by providing the rate constants of elementary reaction steps in the Arrhenius form, which could be helpful for the modeling work of CO catalytic oxidation.     

Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions

Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.     

In-situ study of the catalytic oxidation of CO on a Pt(1 1 0) surface using ambient pressure X-ray photoelectron spectroscopy

CO and O₂ co-adsorption and the catalytic oxidation of CO on a Pt(1 1 0) surface under various pressures of CO and O₂ (up to 250 mTorr) are studied using ambient pressure X-ray photoelectron spectroscopy (APXPS) and mass spectrometry. There is no surface oxide formation on Pt under our reaction conditions. CO oxidation in this pressure (<500 mTorr), O₂ to CO ratio (<10), and temperature (150 °C) regime is consistent with the Langmuir-Hinshelwood reaction mechanism. Our findings provide in-situ surface chemical composition data of the catalytic oxidation of CO on Pt(1 1 0) at total pressures below 1 Torr.     

Oscillations, period doublings, and chaos in CO oxidation and catalytic mufflers

Early experimental observations of chaotic behavior arising via the period-doubling route for the CO catalytic oxidation both on Pt(110) and Ptgamma-Al(2)O(3) porous catalyst were reported more than 15 years ago. Recently, a detailed kinetic reaction scheme including over 20 reaction steps was proposed for the catalytic CO oxidation, NO(x) reduction, and hydrocarbon oxidation taking place in a three-way catalyst (TWC) converter, the most common reactor for detoxification of automobile exhaust gases. This reactor is typically operated with periodic variation of inlet oxygen concentration. For an unforced lumped model, we report results of the stoichiometric network analysis of a CO reaction subnetwork determining feedback loops, which cause the oscillations within certain regions of parameters in bifurcation diagrams constructed by numerical continuation techniques. For a forced system, numerical simulations of the CO oxidation reveal the existence of a period-doubling route to chaos. The dependence of the rotation number on the amplitude and period of forcing shows a typical bifurcation structure of Arnold tongues ordered according to Farey sequences, and positive Lyapunov exponents for sufficiently large forcing amplitudes indicate the presence of chaotic dynamics. Multiple periodic and aperiodic time courses of outlet concentrations were also found in simulations using the lumped model with the full TWC kinetics. Numerical solutions of the distributed model in two geometric coordinates with the CO oxidation subnetwork consisting of several tens of nonlinear partial differential equations show oscillations of the outlet reactor concentrations and, in the presence of forcing, multiple periodic and aperiodic oscillations. Spatiotemporal concentration patterns illustrate the complexity of processes within the reactor.     

Structural specificity in CO and H2 oxidation over single crystal copper surfaces

The reaction rates of CO and H₂ with preadsorbed oxygen on copper surfaces in the 〈110〉 zone presented by a cylindrical single crystal are found to display strong structural sensitivity. In both cases the highest reaction rates are observed for surface structures involving both steps and low index face terraces, and in particular surfaces vicinal to {311} and between either {111} or {100} are found to exhibit the highest rates of reaction. Possible mechanisms for the reaction are discussed, and the data are extrapolated to investigate the steady state oxidation of CO in a CO/O₂ gas mixture over copper surfaces in the 〈110〉 zone. The semi-empirical model describing the steady state oxidation reaction predicts that this reaction also exhibits marked crystallographic anisotropy at CO/O₂ partial pressures chosen to maximise the reaction rate.