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Oscillations occuring in the catalytic decomposition of N 2 O into the elements over Cu-ZSM-5 were studied at 698 K in integral and gradientless reactors. Transient experiments by using step changes in the nitrous oxide concentration were employed to determine the amounts of adsorbed species in different phases of the oscillations. The experimental results can be modelled by a scheme of five reactions. The proposed reaction mechanism includes the dynamic interaction of two adsorbed species, atomic oxygen and NO 3 , and explains the crucial role of nitric oxide, which is formed in a slow side reaction. The results suggest that monovalent copper is the active site for the decomposition of N 2 O, and that this species is involved in a redox cycle.

We report a comparative study of unsteady behavior of CO oxidation over silica-supported platinum and Pt(100). Experiments were performed in a reaction cell allowing the first direct comparison between both catalyst types under the same conditions at pressures between 10 and 100 mTorr. The reaction is examined with mass spectrometry, time-resolved FT-IR spectroscopy, and ellipsomicroscopy for surface imaging (EMSI). For the supported catalyst, our measurements concentrated on the reaction rate as a function of control parameters pressure and temperature and time-resolved FT-IR, EMSI was used to follow spatio-temporal pattern formation on the Pt(100) single crystal. The parameter space (partial pressures of the reactants, catalyst temperature, pumping speed of the reactor) for unsteady behavior of the CO oxidation is much wider for the supported catalyst. In certain parameter ranges, spatio-temporal pattern formation governs the unsteady behavior of the single crystal, while for other parameters, very regular oscillations with no pattern formation are observed. Time-resolved FTIR spectroscopy was used to follow the CO coverages under reaction conditions, which stayed below 15% of the maximum coverage

This work qualitatively and quantitatively compares spatiotemporal patterns in homogeneous and heterogeneous models of a fixed catalytic bed for reactions with oscillatory kinetics. Pattern selection is based on the oscillatory or bistable nature of the kinetics and on the nature of fronts. The heterogenous model can exhibit local bistability while the homogeneous model does not admit this property: when flow terms are accounted for both models can be approximated by a cell-model suggesting a simple conversion between the parameters of both models. The parameter-conversion approximation (1/Pe,eff=1/Pe+1/β, where β is the external transport parameter) can predict quite well the dynamics of the heterogenous model with a pseudohomogeneous one when β≫Pe, but it fails when both parameters are comparable. The dynamic behavior is sensitive to the boundary conditions applied. Several qualitative differences between the two models are pointed out.

We present Monte Carlo simulations of phase separation during kinetic oscillations in catalytic reactions accompanied by adsorbate-induced surface restructuring. As an example, we analyze the NO–H 2 reaction on Pt(1 0 0). The lattice-gas model employed to describe surface restructuring and the reaction steps takes into account substrate–substrate, substrate–adsorbate and adsorbate–adsorbate lateral interactions. Using a reduced generic mechanism of the reaction, we show the type of spatio-temporal patterns which might be observed on the nm scale.

We present the first experiments that use the phase response method to determine components of the adjoint eigenvector (of the Jacobian matrix of the linearized system) of an oscillating reaction system. The Briggs−Rauscher reaction was studied near a supercritical Hopf bifurcation. Phase response curves for I- and Mn2+ have been determined, and from them corresponding components of the adjoint eigenvector have been deduced. The relative magnitudes and difference in arguments of these components agree reasonably well with those from a reduced model of the Briggs−Rauscher reaction, whereas agreement with results from quenching experiments is mixed.

We report results (numerical and experimental) indicating control of observed oscillatory dynamics in an electrochemical system under the influence of superimposed sinusoidal forcing. By varying the frequency and amplitude of the external forcing (control parameters) not only were the chaotic dynamics converted to periodic states (controlling chaos via entrainment), but the period-1 (P1) behavior could also be transformed to oscillatory dynamics of higher periodicity (for example: P2). Therefore for appropriate values of control constants and nature of the unperturbed (periodic and/or chaotic) dynamics, a final state of either lower (chaos→P1) or higher complexity (P1→P2) can be attained. Since no prior information regarding the system dynamics is required for implementation of the forcing control it is thus relevant for application to real systems.

Results are presented from experiments on the oxidation of ethanol at a Pt electrode in acid solution. The experiments were conducted under conditions of constant current. The results show that the oxidation process exhibits a bistable response in which two different branches of steady-state potential values exist under the same conditions. Measurements also reveal that the process exhibits excitability: a relatively small change or perturbation causes the potential to undergo a large and long excursion before it returns to its steady-state value. Oscillations are also examined with a focus on the correlation between characteristics of the oscillations and the length of time the system is allowed to relax. Bistability is discussed in terms of a model previously used to describe multistability in the oxidation of methanol. Comparisons are made between the nonlinear behavior observed in the oxidation of ethanol and that observed in the oxidation of methanol.

To investigate how an oscillating reaction rate might influence the hydration of cementitious solids, a simple iterative mathematical model that includes recurrent passivation of the surface has been constructed. The algorithm generates a wide variety of hydration curves with a limited number of input parameters: i.e.. V0 (volume of unreacted solid after initial reaction), a, (degree of reaction upon initial wetting), x0 (fraction of particle surface passivated after initial wetting); running index: n (cycle number, with an arbitrary unit of time per cycle); constants: m (number of cycles with constant x, the relative rate of the hydration and passivation reactions), k (maximum linear rate of reaction front), and lambda (range 0-1, determines the general passivation behaviour, modelled by the algorithm x(new) = 4 lambda xold[1 - xold]). The hydration process is described by the sequence: (1) Delta V = k(1 - x)V^2/3, (2) Vnew = Vold - Delta V, (3) alpha = 1 - (V/V0) + alpha0.

Electrochemical oscillations of a new type, named oscillations C and D, appear, in addition to previously reported oscillations, named oscillations A and B, in the H 2 O 2 -reduction reaction on a Pt electrode in acidic solutions when small amounts of halide ions such as Cl − , Br − , and I − are added to the solution. Oscillation A appears in a narrow potential region of −0.25 to −0.32 V versus SCE where adsorbed hydrogen (upd-H) of a nearly full coverage is formed, and oscillation B appears in a region below ca. −0.35 V where hydrogen evolution occurs, whereas oscillations C and D appear in potential regions more positive than −0.30 V where halide ions are adsorbed on Pt. The detailed behavior of oscillations C and D is described and compared with that of oscillations A and B. Mathematical simulation has shown that the essential features of oscillations C (and D) can be understood by a reaction model which includes a kind of autocatalytic mechanism in which adsorbed halogen and OH promote dissociative adsorption of H 2 O 2 .

Self-sustained isothermal oscillations in the NO–H 2 reaction were found on the Ir(110) surface. The rates of NH 3 and N 2 formation oscillate exactly out of phase. The high activity state during the oscillations (high NO conversion) corresponds to a high rate of NH 3 formation, while in the low activity state a high rate of N 2 formation has been observed. The shape of the oscillations is very sensitive to changes in the H 2 /NO ratio or temperature. Build-up of a stable N species might play an important role in the mechanism of the oscillations.

The oscillatory reduction of S 2 O 2− 8 ions on the Au(110) electrode in dilute solutions of NaF at potentials positive to the known value of the potential of zero charge (PZC) has been studied. Experimental results based on cyclic voltammetry with rotating disk electrodes and potentiostatic current transients indicate that in the course of S 2 O 2− 8 reduction the surface state of the electrode is modified. It is proposed that the point of zero charge shifts to a more positive potential, and that the oscillatory S 2 O 2− 8 reduction is governed by the Frumkin effect.

The increasing difficulties in collecting information about dissipative systems that would be necessary and sufficient for desirable control over the manufacturing process are discussed. It is shown that the processes of mass transfer responsible for the solid-phase reversible chemical reactions in the MgO — Al2O3 — SiO2 system have an autocatalytic and oscillating nature. The possibility of providing in the heat-treatment process the requisite level of the evolution of the phase composition for obtaining materials with a specified combination of properties is discussed.

For investigating the Belousov-Zhabotinsky (BZ) reactions in free-fall experiments under microgravity, conducted at Japan Microgravity Center (JAMIC) in Hokkaido, the automated setup of batch-type, which was constructed in this work, had to be placed in the limited space and to have functions to prepare the reaction solution; it was also required to send it to the vessels after the necessary induction period and to collect image data via CCD cameras, including the process of integrating the capsule. These operation sequences of each device involved simple solid state relays. The initial start of the relay sequences was performed by a start command from the operation room of JAMIC. The propagation profile of the chemical patterns under microgravity of 10-5 g was collected as image data for ca. 9 s. They were not disturbed by the release of carbon dioxide, which often deforms the chemical patterns in the aging of BZ reactions.

An external periodic perturbation's effect is numerically discussed, when it acts on burst oscillation in a model for the Briggs-Rauscher reaction in a continuous stirred tank reactor. The numerical results reveal that perturbation generates chaotic, mixed, multiple periodic and single oscillations.

Single-crystal Pt is well known to demonstrate oscillatory and complex dynamic behaviour, for instance, during the catalytic oxidation of CO. This type of behaviour is also observed for presently investigated supported Pt catalyst, EUROPT-3. In the present case self-oscillations have been examined during CO oxidation using in situ FTIR, applied to both steady-state experiments and step response, transient isotopic labelling and concentration programming experiments. The goal of the present investigation was to establish the feedback mechanism that is essential in explaining complex dynamic behaviour and to apply periodic operation to suppress the self-oscillations. For low CO/O 2 ratios the reaction demonstrates so-called regime I kinetic behaviour with reaction orders in CO and O 2 being, respectively, one and zero. Platinum is slowly oxidised, thereby blocking sites for adsorption and subsequent reaction. At a critical point, corresponding to a degree of oxidation of the Pt of approximately 61%, regime II type behaviour is observed: the system exhibits multiplicity and self-oscillations. With progressing oxidation of Pt both the period and the amplitude of the oscillations increase. Although the existence of oxidised Pt seems to be correlated to the emergence of self-oscillations, quantitative analysis of the oxidation and reduction kinetics of Pt reveals that the dynamics of these reaction steps are at least two orders of magnitude too slow to act as feedback mechanism. A qualitative comparison of other known feedback mechanisms indicates that a phase transition mechanism can describe the observed self-oscillations. It is shown that forced concentration oscillations suppress the self-oscillations. Periodic reduction of the catalyst in CO is effective in keeping the amount of oxidised Pt low, and thereby prevents the system from entering regime II.

A new method applying a non-linear chemical system under conditions far from thermodynamic equilibrium in microvolume/microconcentration quantitative analysis is described. The chemical system used as a matrix is the Bray–Liebhafsky reaction in a non-equilibrium stationary state close to a bifurcation point. The method is based on monitoring the response of this system to perturbations by Cl–, Br–, I–, Mn2+, malonic acid and quercetin analyte solutions, which are followed potentiometrically either by an Ag+/S2– ion-sensitive or by a Pt electrode. A linear response of the potential shift versus the logarithm of the analyte concentrations is found in the following ranges: 1.3 × 10–6 mol dm–3 ≤ [Cl–] ≤ 1.6 × 10–4 mol dm–3, 1.0 × 10–6 mol dm–3 ≤ [Br–] ≤ 8.3 × 10–5 mol dm–3, 2.0 × 10–6 mol dm–3 ≤ [I–] ≤ 1.0 × 10–4 mol dm–3, 8.4 × 10–7 mol dm–3 ≤ [Mn2+] ≤ 8.3 × 10–5 mol dm–3, 3.8 × 10–7 mol dm–3 ≤ [malonic acid] ≤ 2.1 × 10–5 mol dm–3 and 1.5 × 10–8 mol dm–3 ≤ [quercetin] ≤ 3.7 × 10–5 mol dm–3. Under the investigated conditions an improved detection limit for all halides tested is obtained. Unknown concentrations of the analytes can be determined from a standard series of calibration curves to an accuracy within ±5%. In addition, the application of potentiometric measurements in microvolume/microconcentration quantitative analysis is diversified.

Discrete and continuous control strategies are applied to regulate the Belousov–Zhabotinsky–continuous stirred tank reactor (BZ–CSTR) chaotic chemical reaction onto the stabilized states. The orderly intrinsic states of the chaotic system, such as unstable periodic-1 orbit (UPO-1), unstable periodic-2 orbit (UPO-2) and unstable periodic-4 orbit (UPO-4) are obtained by the discrete nonlinear perturbation feedback method from within the whole chaotic region (NPF-WCR). The non-intrinsic orderly states, such as period-4, period-2, period-1 and steady state can be maintained by the continuous linear and nonlinear self-interacting feedback control strategy. The differences between the two methods are also discussed. An '‘extremum space control’' method is suggested.

An existing scaling model of a reaction–diffusion system is extended to a circular trajectory. The equations describe the evolution of a slow (X) and a fast (Y) concentration variable. The fast variable jumps between extreme values across a reaction interface as the rate parameter becomes very large. The model is reduced to one equation for the dynamics of the smooth (slow) variable while the Y-jumps occur at two interfaces spatially located on a ring. The equation is solved subject to 2π-periodicity conditions on the ring and continuity conditions at the interfaces. Both static (with zero velocity) and moving (with velocity v) wave solutions are found. An analogy is then drawn between our reaction–diffusion system and oscillating chemical reactions such as the Belousov–Zhabotinskii (BZ) reagent, confined to a torus-shaped container. A toroidal thin tube with a very small diameter could simulate the ring geometry. The conjectured waves capture the oscillations of the catalyst (ferroin), with the maxima and minima corresponding to the ferroin and ferriin, spatial domains in the doughnut, respectively. The non-stationary wave solutions predict a migration of those domains yielding swinging (back and forth) patterns along the ring. The azimuthal position of the interfaces exhibits temporal oscillations. Thus these simulations suggest interesting experiments on spatio–temporal patterns in excitable chemical media in annular reactors.

In this paper studies on the oscillation regularity of the classical B–Z reaction system, and the calorimetric curves of the reaction system measured at three temperatures, 25, 27 and 29°C are described. A new way is presented for studying the regularity properties of chemical oscillation phenomena from the viewpoint of reaction heat effects.

The reactions of oxygen are not a necessary part of the explanation of the oscillations in the Bray–Liebhafsky reaction. However, oxygen can modify the shape of the oscillations and the range of concentrations where they are observed. We discuss the mechanism previously proposed and suggest that the chemical effects of oxygen result mainly from the oxidation of iodide by radical processes. The escape of oxygen has also a physical effect, the loss of iodine in the gas phase. The effects of light and stirring rate are related to the effects of oxygen.

Simultaneous calorimetric and potentiometric/thermometric measurements were performed on three uncatalyzed bromate oscillators (UBOs) showing distinct temporal structures. The following UBOs were chosen: a) the gallic acid (3,4,5-trihydroxy benzoic acid)—bromate; b) the 1-hydroxy-4-[(1-hydroxy-2-methylamino)ethyl] benzene (HME)—bromate; and c) the 1,4-cyclohexanedione (CHD)—bromate system. Their oscillating reactions were monitored by an isoperibolic batch calorimeter of the Calvet-type at 20°C and with a quasi-adiabatic Dewar setup at room temperature, and in addition by platinum/calomel potentiometry and thermometry. Shape of the calorimetric curves, number and duration of the oscillations and their frequencies varied considerably between these three UBOs. The underlying chemical reaction schemes are discussed in connection with the energetic background.

While various reactions in the inorganic subset of the oscillatory Briggs-Rauscher (BR) reaction were clarified in the recent years, the organic subset of the present mechanisms contains only one process: the iodination of malonic acid. Further organic reactions can play a role, however, if malonic (MA) and iodomalonic (IMA) acids can be oxidized in the BR reaction. As CO2 and CO should be products if such oxidations can take place, the main aim of this work was to learn whether these gases are produced in a significant amount in a BR system. In our BR experiments, a rather intense evolution of both gases was observed with an oscillatory and a nonoscillatory component. With the initial conditions applied here, one from every 6 carbon atoms was oxidized either to CO2 or to CO in the course of the BR reaction. The amount of CO2 was about 4 times higher than that of CO. Experiments are in progress to disclose the reactions which generate the measured gases and their role in the mechanism of the BR reaction.

Our recently developed mathematical model for the description of the NO+H 2 /Pt(100)-(1×1) reaction [J. Chem. Phys. 108 (1998) 3740] is extended to include the adsorbate-induced (1×1)⇔hex phase transition of a Pt(100) surface. Under certain pressure and temperature conditions, the phase transition is of critical importance for the kinetics of the reaction, since the two phases exhibit strongly different catalytic activity. The model successfully reproduces the experimentally observed non-linear phenomena: rate hysteresis during a heat–cool cycle and oscillatory behaviour, including periodic and chaotic oscillations. In agreement with experiment, rate oscillations occur in the vicinity of the hex⇒(1×1) phase transition as the hex-reconstructed surface is cooled down in a NO+H 2 atmosphere. At relatively high temperatures, oscillations take place on a largely hex-reconstructed surface. Decreasing the temperature causes an increase in the adsorbate coverage and, as a result, an increase in the fraction of the (1×1) phase. At relatively low temperatures, oscillations take place on a surface that is completely in the (1×1) structure. Thus, the simulation results show that the (1×1)⇔hex surface phase transition of Pt(100) is not essential for producing oscillatory behaviour, even if oscillations may proceed on a largely hex-reconstructed surface. The requirement of vacant sites for NO dissociation on Pt(100)-(1×1) is the most important step in the oscillation mechanism for the NO+H 2 /Pt(100) system.

Transient Turing-like spatial patterns were recently reported by Munster et al. when the components of the methylene blue oscillator are mixed with acrylamide and a polymerization initiator to form a polyacrylamide gel. We have qualitatively reproduced these results, but we demonstrate that, although the observed patterns have the appearance of Turing structures, they actually result from Rayleigh–Benard convection in a mixture of acrylamide, N,N′-methylene-bisacrylamide, triethanolamide, sulfide ions and ammonium peroxodisulfate during the polymerization and gelation process. Neither methylene blue nor sulfite is required for patterns to develop.

The influence of heavy water on the Bray−Liebhafsky (BL) oscillating reaction was investigated for varying amounts of D2O, at three different temperatures. Evolution of the system was monitored potentiometrically. Simultaneous recordings of gaseous oxygen over the reaction solution were also performed. In separate experiments the iodine concentration was monitored spectrophotometrically. Replacement of H2O by D2O progressively intensifies the reactions of oxidation of the iodine species, compared to the reactions of their reduction. As a result there is a critical ratio of D2O/H2O, after which the dynamics of the system is considerably altered. The same effect is observed at all temperatures, but it is more pronounced at lower temperatures. Two possible explanations for the progressively intensified oxidation are discussed: the reverse isotope effect and the selective energy transfer. It is established that the oxidation and reduction of the iodine species in the BL reaction do not proceed through the sa...

The dynamic behaviour of the NO-H 2 reaction over Rh(111) was studied using mass spectrometry and Auger electron spectroscopy in the 10 -7 -10 -5 mbar total pressure range for NO:H 2 ratios between 1:5 and 1:30. On cooling the sample in a flow of the gases, an increase in the rate of formation of N 2 occurred between 500 and 450 K, which indicated the presence of an autocatalytic step in the reaction sequence. A heat-cool cycle of the sample actually produced a large hysteresis in the formation of all products : N 2 , NH 3 and H 2 O. This type of behaviour was also observed under similar conditions over the Rh(533) surface, as were regular macroscopic rate oscillations. During these oscillations, N 2 production was out of phase with that of NH 3 and H 2 O. The Rh(533) surface consists of four-atom wide (111) terraces separated by (100) steps. The steps seem to play an important role in the synchronisation of the oscillations, as only irregular macroscopic rate oscillations could be observed over Rh(111) under these conditions. Furthermore, the introduction of microscopic defects on the Rh(111) surface by means of Ar + ion sputtering led to an increase in the synchronisation of the oscillations, these defects are presumably playing a similar role to the steps on the Rh(533) surface.

The oscillatory kinetics of CO oxidation over a (111)-oriented platinum crystal at an oxygen pressure of 10 −2 mbar is observed. This is surprising because it cannot be explained by the reconstruction model commonly utilised to describe oscillations on Pt(110) and Pt(100). Additionally, with a relatively new imaging technique (EMSI), island growth is observed on the surface, exhibiting a feature that has not been observed previously with PEEM (photoemission electron microscope). By combining these two experimental results, an explanation in terms of oxidation reduction cycles of the surface becomes plausible.

A new Monte Carlo simulation of CO oxidation over a non-isothermal supported catalyst is presented. It uses the Monte Carlo method applied to multiple 2D crystallites distributed on a flat support in combination with the partial differential energy balance to simulate the non-isothermal reaction on supported catalysts. The simulation predicts ignition and extinction transitions similar to those observed during a temperature programmed reaction experiment. However, no auto-oscillatory behavior is predicted when the surface is assumed to have constant activity. Stochastic fluctuations at each crystallite cannot be synchronized through the thermal coupling to form global oscillations. The model was extended to include a surface with variable activity. In this case an oxidation/reduction mechanism altering the oxygen sticking coefficient provides a bistable mechanism for auto-oscillations. This variable activity model predicts auto-oscillations and changes in oscillatory behavior with temperature and O 2 /CO ratio similar to those experimentally observed. Simulation results also show that thermal communications among crystallites affect the shape of global oscillations. Irregular and chaotic oscillations are predicted when there are differences in the crystallite size, activity, and distribution of the crystallites on the support.

The addition of bromide ions to the component solutions of the Briggs– Rauscher oscillating system produces a variety of phenomena, depending on the sequence of the addition and on the initial bromide concentration. If the addition is made some minutes after the mixing of H2O2 and acidic IO3− and before adding malonic acid and Mn2+ ions, the oscillations last for five or six cycles, then suddenly ceases. If the addition is made immediately after the mixing of H2O2 and acidic IO3− and before adding malonic acid and Mn2+ ions, the oscillations do not start at all. The addition of bromide ions to an actively oscillating BR reaction causes a rapid suppression of the oscillations. Our observations may be accounted for by a mechanism involving the IBr species. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 641–646, 1998

El desarrollo del trabajo práctico en la enseñanza de la química es fundamental porque garantiza la adquisición de saberes procedimentales característicos de esta ciencia, sin embargo estas actividades muchas veces se encuentran limitadas cuando no se cuenta con un laboratorio en las instituciones educativas. En este trabajo se presenta una propuesta didáctica que ataca esta problemática brindado la oportunidad de realizar prácticas de laboratorio sobre reacciones oscilantes a partir de una caja didáctica que desarrolla el concepto de auto-organización, dicha propuesta se enmarca en el aprendizaje activo como generador de participación de los estudiantes, su construcción se fundamenta en el análisis epistemológico de la auto-organización y el desarrollo conceptual de las reacciones oscilantes particularmente la reacción Briggs-Rauscher, convirtiéndola en una herramienta innovadora para el docente que facilita el desarrollo del trabajo práctico aún en la ausencia de un laboratorio. Abstract. Practice work about the teaching in the chemistry is important because is the way obtain procedural knowledge, characteristic in this science, but this activities sometimes has been restricted when is not in the school lab. In this way we show a didactic proposal for talk this problem, giving an opportunity doing practice in the lab about oscillating chemical reactions using a didactic box develops self-organization concept, taking active learning how a means of student participation, its structure based in epistemologic analysis of self-organization and conceptual developing of oscillating reactions, mainly Briggs-Rausher reaction, make it in an innovating tool for teacher in a practice work without lab. Maestría

The catalytic CO oxidation at Pt(110) is observed with RHEED. Due to differential pumping of the electron gun, rocking curves can be measured up to pressures in the 10− 3 mbar range. This allows, for the first time, structural investigations with atomic resolution under time-dependent reaction conditions. At very sharply defined values of CO and O pressures at 535 K, the intensities of the diffraction spots oscillate with a frequency of about 0.6 Hz. These oscillations are associated with a periodic lifting of the (1 × 2) missing-row phase into a disordered (1 × 1) phase, which takes place spatially homogeneously over the observed part of the surface. A multiple scattering scheme for RHEED intensity calculations is used to evaluate the rocking curves. The changes of the substrate structure and the variation of the coverages of several adsorption sites for CO and O are obtained. A simplified version of differential rate equations allows the time-dependent sticking, desorption and reaction of the adsorbates during the oscillations to be visualized and yields estimates of the relevant rate constants.

The kinetics of CO oxidation over an alumina supported Cu catalyst are examined using successive oxidation and reduction cycles. Experiments were done at a temperature of 493 K with isotopically labelled gases in a tubular reactor. Surface species were monitored during transients in an FTIR flow cell. For the reoxidation of the catalyst a three-step mechanistic model is proposed. The kinetic constants are determined by mathematical modelling. The role of carbonates is found to be minor in the production of CO 2 in contrast to carbonyls which are shown to be the intermediates. Anet dissociation of the CO bond was observed during reduction caused by adsorption of CO 2 on a partly reduced catalyst under formation of carboxylates and its subsequent decomposition to CO, thereby leaving oxygen on the catalyst. A complete mechanistic scheme is presented which allows us to describe qualitatively and in part quantitatively the experimental results. This study shows that the use of forced oscillations is a powerful tool with strong elucidating abilities in mechanistic investigations in heterogeneous catalysis.

Self-sustained, regular oscillations were observed in NO + CH4+O2reaction over Pd/TiO2catalysts at specific temperatures and oxygen concentrations. The oscillatorybehavior was investigated combining NO reduction and methane combustion reaction studies with additional catalyst characterization experiments performed under controlled atmospheres. The catalyst wasprepared using a wet impregnation technique with Pd–acetate as the precursor for palladium. A fixed-bed, flow reactor system was used to perform the NO+CH4+O2and CH4+O2reaction experiments.Feed and product analyses were done on-line using gas chromatography–mass spectrometry, chemiluminescence, and wet chemistry techniques. Additional catalyst characterization was performed usingthermal gravimetric analysis and high-temperature, controlled-atmosphere X-ray diffraction techniques. Detailed analysis of the oscillatory behavior has indicated that oscillations in the product and reactant profiles are coupled with temperature oscillations in the catalyst. When combined withcontrolled-atmosphere characterization experiments, these results, which can be reproduced in both NO+CH4+O2and CH4+O2systems, suggested that the oscillations are the result of periodic phase change ofpalladium on the surface. These cyclic phase transformations, in turn, are the result of temperature variations that are caused by the varying levels of exothermicity of the two major reactions, namely NOreduction and CH4combustion, that are favored by the metallic and the oxidic sites, respectively.

The reaction of nitric oxide with carbon monoxide on a catalyst of 0.5 wt% Pt-3.4 wt% MoO3 supported on c~-A1203 has been investigated in different reactors at atmospheric pressure. A fixed-bed reactor with external recycle loop exhibited bistability and hysteresis as CO concentration or temperature were varied. In a tubular fixed-bed reactor non-isothermal periodic oscillations (ATm~x=40 K) were found in a specific pressure-concentration-temperature region (p-c-T region). The amplitude and period time of the oscillations changed smoothly inside the p-c-Tregion and died out suddenly at the borders of the region. The oscillations were of a relaxation type and lasted for more than 48 h. With fresh catalyst, a new quartz reactor and a specific pretreatment they were reproducible. It was shown that oscillations had kinetic origins and that the changes of the temperature synchronised the elementary oscillators. An autocatalytic model is proposed to explain the experimentally observed phenomena.

The standard free energy of formation of IO . and HIO 2 in water solution at 298K are estimated to be ΔG 0 10 .=120 kJ/mol and ΔG HIO2 =-75kJ/mol. Values of standard reduction potentials for half-reactions HIO 2 + 2 H + +2e - → HIO +H 2 O, IO . + H + +e - → HIO 3 - +3H + +2e → HIO 2 + H 2 O and HIO 2 +H + +e → IO . + H 2 O are calculated to be E° = 1.35 V, E° = 2.3 V, 0.92 V and E° = 0.4 V. Energetic requirements during the process of oxidation of iodine in acidic media are introduced as an important criteria in examination of the models of the Bray-Liebhafsky reaction mechanism.

The non-linear model of Gruyters, Pasteur and King for oscillatory behaviour in the reduction of NO on Pt{100} is extended to include the formation of N 2 O. The oscillatory period and its sensitivity to temperature are again reproduced well by the model, but we now show that, also in agreement with experiment, the rates of formation of N 2 , NH 3 and H 2 O are in-phase with each other, whereas the formation rate of N 2 O is out-of-phase. Using only independent experimentally determined parameters, the oscillatory behaviour is found to be crucially dependent on the value taken for the reaction order, n , for (1 × 1) island growth. For example at T = 434 K, p NO = 1.1 × 10 −6 mbar and p H 2 = 2 × 10 −5 mbar, oscillatory behaviour was only observed for 4.08 ≤ n

Chemical reactions carried out under strongly non-equilibrium conditions can result in a variety of interesting effects such as oscillations, chemical waves, kinetic phase transitions, bistability, instabilities and chaos - all of these effects being mathematically due to the strong non-linearities in the kinetic rate equations describing these chemical processes. The present article reviews the various types of non-linear behaviour observed in several NO reduction reactions over Pt-group metals. A large part of this review deals with oscillations and spatiotemporal pattern formation in the reaction over Rh surfaces: the reaction dynamics of this system has been realized on the microscopic, mesoscopic and macroscopic scales using field emission microscopy, photoemission electron microscopy and macroscopic rate measurements respectively. The various mechanisms of this and the other reactions reviewed in this article are also considered.

In this paper we report a chemical oscillation catalyzed by [Ni(Me2[14]l,3-diene N4)]2+ (Me2[14]1,3-diene N4 denotes 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene) in BrO3-pyruvic acid-H2SO4 system. The domain of the existence of the oscillation waa obtained. The effect of initial concentration of the components on the oscillation were studied. The features of the oscillations were described in detail. We also examined the effects of Ag+, Hg2+, CCl4, free radical inhibitors, etc. on the oscillations.

The role of surface compounds in the oscillating anodic reactions on nanostructured electrodes has been examined by computer simulation of the oscillating anodic oxidation of formaldehyde on Ag nanostructures and the oscillating reaction design based on the autocatalytic reduction of Pd surface compounds by hypophosphite in Ag-Pd nanostructures. Experiments and computer simulations show that the surface compounds may be responsible for the complex non-linear phenomena in the nanostructured systems.

The oxidation of propene by nitrous oxide has been studied in detail on Pt/SiO2 of widely varying dispersion (Dh) at 160–350°C, using a tubular flow reactor under atmospheric pressure. The partial and total oxidations proceeded in parallel on two different active sites and the catalytic oscillation appeared only in the partial oxidation to form propanal (PA). For the total oxidation route, it was elucidated that the reaction was exact structure insensitive with an activation energy of 85 ± 5kJ/mol through all Dh's and that there were stable surface intermediates consisting of four different species each of which could produce CO2 and H2O by the reaction with N2O or produce propane by the consecutive reaction with H2. The turnover frequency, amplitude, period and selectivity of the oscillating PA were significantly varied depending on the percentage of platinum surface exposed, clearly indicating a strong structure-sensitive oscillation. As a possible trigger for the oscillations, the surface coverage fluctuation of weakly adsorbed propene was a reasonable choice because of its actual oscillatory behavior from θ=0.07 to 0.28 which was synchronized with the responses of the oscillatory reaction. The reaction pathway selectivity to form PA was found to be 89% for the catalyst with Dh = 80% indicating a better catalyst which can be designed by changing Dh.

The application of oscillating reactions to the analysis of real samples by the analyte pulse perturbation technique is reported. The oscillating reaction involving the H 2 O 2 –NaSCN–CuSO 4 system in an alkaline medium was used for the expeditious, convenient, automatic determination of vanillin, paracetamol and ascorbic acid in food and pharmaceutical samples. Six methods (two per analyte) were developed that allow the determination of 1.5–40 µmol of vanillin, 0.5–6 µmol of paracetamol and 0.5–5 µmol of ascorbic acid in real samples. The proposed methods are precise (RSD < 5%) and have a reasonable sample throughput (6–10 h - 1 ) for this type of reaction, which can be implemented with modular equipment available at any laboratory. The effect of influential variables and the selectivity of the determinations was studied in relation to the types of sample used. Determinations are virtually direct and require no removal of interfering species. The results were validated by comparison with standard methods.

The classic Belousov-Zhabotinsky reaction was modified by adding phenol as a second organic substrate that kinetically competes with the malonic acid in the reduction of Ce4+ to Ce3+ and in the removal of molecular bromine of the reaction mixture. The oscillating reaction of two substrates exhibited burst firing and an oscillatory period of long duration. Analysis of experimental data shows an increasing of the bursting phenomenon, with a greater spiking in the burst firing and with a longer quiescent state, as a function of the initial phenol concentration increase. It was hypothesized that the bursting phenomenon can be explained introducing a redox cycle between the reduced phenolic species (hydroxyphenols) and the oxidized ones (quinones). The hypothesis was experimentally and numerically tested and from the results it is possible to conclude that the bursting phenomenon exhibited by the oscillating reaction of two substrates is mainly driven by a p-di-hydroxy-benzene/p-benzoquinone redox cycle. A reação de Belousov-Jabotinsky clássica foi modificada pela adição de fenol como um segundo substrato orgânico que compete cineticamente com o ácido malônico na redução de Ce4+ para Ce3+ e na remoção de bromo molecular da reação. A reação oscilante de dois substratos exibiu oscilações abruptas e período oscilatório de longa duração. A análise de dados experimentais mostra um aumento do fenômeno abrupto, com pico maior e estado quiescente mais longo, como função do aumento da concentração de fenol inicial. Hipotetizou-se que o fenômeno de oscilação abrupta pode ser explicado pela introdução de um ciclo redox entre as espécies fenólicas reduzidas (hidroxifenois) e as oxidadas onas (quinonas). A hipótese foi testada experimentalmente e numericamente e dos resultados concluiu-se que o fenômeno oscilatório abrupto exibido pela reação oscilante de dois substratos é impulsionado principalmente por um ciclo redox p-di-hidroxi-benzeno/p-benzoquinona.

A bipyridinium dication-substituted Bodipy fluorophore, with a terminal carboxylic acid function, provides two alternative stations for cucurbit[7]uril. Changing pH from basic to acidic results in shuttling of the cucurbit[7]uril from one station to another. In addition, this shuttling is accompanied by a change in the emissive properties of the Bodipy dye, which is only observed in the presence of cucurbit[7]uril. More striking, is a demonstration of autonomous shuttling of the pseudorotaxane system in an oscillating pH system.

The Briggs-Rauscher reaction is known as a nonlinear and non-equilibrium chemical oscillation reaction. The reaction solution is composed of malonic acid as organic substrate, hydrogen peroxide and iodate in sulphuric acid as oxidizing agent, and manganese (II) as metal catalyst. The calorimetric behaviour, in terms of the total heat Q and the heat evolving rate q for each chemical oscillation, was followed with the use of a heat exchange calorimeter of the batch type assembled by the authors. Simultaneously, the concentration of iodide ions produced as the intermediate species was measured as the potential difference E by the common potentiometric cell incorporated in the calorimeter and the released oxygen volume G was also observed by a simple flowmeter. The starting point of the peak on the curve of q against time t coincided with that in the curve of E against t. The switching concentration of iodide between radical and nonradical paths was calculated from literature values of rate constants. The heat evolving period of Q coincided with the period in the radical path. The curve of Q against t also coincided with that of G against t. The total volume of released oxygen was larger than that calculated from the stoichiometric reaction formula.

The first observations of Li-modified isothermal kinetic oscillations during CO oxidation on a Pt field emitter tip are reported. The oscillations are realized on the surface of a [111]-oriented Pt tip in the presence of a Li submonolayer coverage of about 2.5 × 1014cm−2 and are compared to the corresponding Li-free oscillations occurring on the same tip. In both cases the oscillatory process is visualized using the field electron emission (FEM) mode of the lithium field desorption microscope (Li-FDM). The origin of the FEM imaging contrast of coadsorbed LiCO and LiO layers on Pt is clarified. The influence of the coadsorbed Li submonolayer on the oscillating behavior of CO oxidation on Pt is discussed.

A novel chemical oscillating reaction in the KMnO 4 NH 2 CH 2 COOHH 3 PO 4 CSTR system in the presence and absence of Ag + has been described. The reaction kinetics in a closed Mn0 4 − NH 2 CH 2 COOHH 3 PO 4 system has been analyzed and a possible mechanism has been proposed. The catalytic effect of Ag + in the above reaction has also been studied.

Aluminium metal is dissolved in 3 M HNO3 with mercury(II) nitrate as a catalyst. Visible oscillations in gas evolution are observed for 2×10−4 M Hg(NO3)2 dissolving solutions. The oscillations are followed quantitatively by monitoring both solution temperature and the intensity of a UV-visible transmittance peak at 369 nm. This peak is attributed to mercury(I) species in solution.

CO/NO reactions have been studied over two catalysts, Pt/CeOx/Al2O3and Pt/Al2O3, under net reducing conditions. The Pt/Al2O3catalyst deactivated with time at 400°C whereas the Pt/CeOx/Al2O3catalyst showed oscillations in the reaction rate at this temperature. The oscillations had periods of 18 min. By using Fourier transform infrared spectroscopy, an attempt was made to reveal the species that inhibits the reaction over Pt. It is suggested that CeOxprovides dissociation centers for NO under conditions where NO dissociation is hindered by the inhibiting species on the Pt sites.

New reaction schemes showing photochemical instabilities in closed isothermal conditions are proposed. Bistability and oscillations are predicted for a system in which one reactant is in rapid dimer-monomer equilibrium. The necessary feedback originates from the light absorption law at high total absorption of a stirred solution. For the experimental observation of instabilities, the rates of thermal and photochemical reactions must be comparable. The necessary relations of the parameters are obtained from the presented equations.