Hex Phase Transition Research Articles

Damped oscillation in theNO+CO∕Pt(100)reaction system

Considering the influence of the surface defects formed in the surface restructuring phase transition in the CO+NO/Pt(100) reaction system, we propose a lattice gas model to investigate the damped oscillation in the high-temperature oscillatory regime by means of a Monte Carlo simulation. The simulation results show that the persistent oscillation can change into a damped one when the fraction of the defects increases. The production rate of CO2 is near to the maximum value when the oscillation is damped to the end. Furthermore, it is found, in the early stage of the oscillation, that the NO decomposition mainly occurs in the 1 x 1 phase and the hex phase is inactive for the reaction. However, as the reaction proceeds, defects are gradually formed in the 1 x 1 <==> hex phase transition, the hex phase becomes active and dominative for the NO decomposition, and then the oscillation becomes damping. The simulation results give an explanation for some previous experimental phenomena.

Simulation of oscillatory behaviour in the NO+H 2 reaction on a partially reconstructed Pt(100) surface

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.

Delay-induced chaos in catalytic surface reactions

Deterministic chaos related to a sequence of period-doubling bifurcations (the Feigenbaum transition) has been observed in the NO+CO and NO+H2 reactions on Pt(100). On a microscopic scale, these reactions are accompanied by the formation of 1×1 adsorbate islands due to the properties of the 1×1↔hex phase transition of Pt(100). A simple skeleton model is constructed that describes the behavior of a population of reacting islands which are globally coupled together via the gas phase. Investigations of this model show that the experimentally observed chaotic behavior can result from delays in the response of the reacting islands to partial pressure variations in the gas phase.

Synchronization in oscillatory surface reactions on single crystal surfaces

In oscillatory surface reactions on single crystal surfaces the partial pressure variations that accompany the oscillations in the reaction rate represent a global interaction between the local oscillators. The NO+CO reaction on Pt(100) exhibits both synchronized and unsynchronized oscillatory behavior depending on the substrate phase. Unsynchronized oscillations occur on the 1×1 phase but on the hex phase one finds synchronized oscillatory behavior. The mechanism leading to synchronization in the oscillations on Pt(100)-hex can be traced back to a critical dependence of the 1×1⇄hex phase transition on the partial pressures of NO and CO. At both ends of the temperature window for rate oscillations one finds well-defined transitions to a stationary reaction rate. These bifurcations are discussed in terms of a transition from synchronized to unsynchronized behavior. In particular, the occurrence of deterministic chaos in connection with a Feigenbaum scenario is interpreted as being due to such a transition.

Simulations of the NO+NH3 and NO+H2 reactions on Pt(100): Steady state and oscillatory kinetics

A model for the reactions of NO+NH3 and NO+H2 has been developed for simulating the reduction of NO on Pt(100) in the 10−6 mbar pressure range for temperatures between 300 and 700 K. The model consists of seven ordinary differential equations for describing the coverage changes of six adsorbed species as well as an equation for describing the 1×1⇄hex phase transformation. Simulations of the N2 and H2O reaction rates for both reaction mixtures reproduced the hysteresis effects and the existence range for kinetic oscillations, which were found in the experiments. In addition, the occurrence of the so-called ‘‘surface explosion’’ in both reaction systems is well described by the model. In contrast to the NO+CO reaction on Pt(100), where oscillations may also take place on a pure 1×1 substrate, the 1×1⇄hex phase transition occurs during oscillations for the NO+NH3 and NO+H2 reactions. The transitions between different adsorbate/substrate phases during one oscillatory cycle which are predicted by the model are in agreement with experimental observations made by photoemission electron microscopy (PEEM) for the NO+NH3 reaction. Using values for the constants which were taken from experiments, the model provided quantitative predictions of the absolute reaction rates as well as the relative rates of the competing reaction channels, e.g., N2 and NH3 production in the case of the NO+H2 reaction. The similar dynamical behavior observed in the NO+H2 and NO+NH3 reactions on Pt(100) is attributed to the insensitivity of NO reduction to the source of the hydrogen atoms.

Surface topographical changes on Pt(100) and Pt(111) in the NO + CO reaction: a reflection electron microscopy study

The structural modifications which are induced by the NO + CO reaction on Pt surfaces were studied using reflection electron microscopy (REM) to image atomic steps. Small single crystal spheres of Pt (diameter ≈ 0.3 mm) were prepared and then exposed in a UHV chamber to a flow of CO and NO at 10 −6 Torr. The sample temperature was cycled between 300 and 700 K over an exposure time of typically ∼ 1 h. On the (100) orientations, the reaction led to a strong roughening of the substrate and to the development of a domain structure. As was demonstrated in adsorption experiments with CO (NO) alone, the major contribution to the roughening originated from the 1 × 1 ⇄ hex phase transition of Pt(100). On the (111) facets the flat terraces were only weakly affected by the reaction, but the step edges were quite strongly attacked leading to a meandering of the initially straight step edges. The observations made on Pt(111) are consistent with a mechanistic picture in which the NO + CO reaction takes entirely place at step and kink sites.

Bifurcation analysis of the three-variable model for the NO+CO reaction on Pt surfaces

Under isothermal conditions at low pressure (10−6 mbar range), the NO+CO reaction exhibits oscillatory behavior on a Pt(100) surface. Based on the results of in situ low-energy electron diffraction (LEED) measurements which showed that the 1×1⇄hex phase transition is not essential for producing oscillations, a three-variable model of coupled differential equations was developed which instead relies solely on the autocatalysis provided by the stoichiometry of the individual reaction steps. This model has been analyzed with the help of bifurcation theory using realistic values for the constants in the equations. The results demonstrate that the model reproduces, quite well, the existence range for oscillations on Pt(100). Two oscillatory regions exist with a large one located above the stoichiometry ratio pNO: pCO=1 and a very small one which is found just below pNO: pCO=1. Only the former one has a counterpart in the experiment. At low temperature two isolated branches of the reaction exist which merge at higher T in a transcritical bifurcation thus creating a peculiar hysteresis loop in the shape of a mushroom. Bifurcation analysis has also been applied to investigate the role of the internal parameters, e.g., the role of the constants in the differential equations. The most critical constants were those which control the dissociation of NO and, therefore, are decisive whether ignition or extinction in the reaction takes place. The high degree of qualitative and quantitative agreement which could be achieved with the three-variable model demonstrates that the model provides a reasonable description of the experiments.

The NO + NH 3 reaction on Pt(100): steady state and oscillatory kinetics

The NO + NH 3 reaction was investigated on a Pt(100) surface in the 10 −6 mbar range using Video-LEED, work function measurements and measurements of the product partial pressures of N 2 and H 2O. Sustained kinetic oscillations, as observed in the N 2, H 2O and work function signals, were detected between 425 and 450 K for p NO = 1.1 × 10 −6 mbar and p NH 3 = 4.7 × 10 −6 mbar. The dependence of the oscillation period on temperature and on the p NH 3 / p NO ratio was determined. In situ LEED measurements demonstrated that oscillations in the reaction rate are coupled to the 1 × 1 ⇄ hex phase transition. Isotopic exchange experiments with 15NO and 14NH 3 showed that depending on the temperature and p NH 3 / p NO ratio, significant deviations from a random mixing of 15N and 14N on the surface occur. This is interpreted as indication for an attractive interaction between NO ad and NH xad ( x = 1–3).

Kinetic oscillations in the NO + CO reaction on Pt(100): Experiments and mathematical modeling

The reaction of NO and CO on Pt(100) exhibits two branches of steady state production of N2 and CO2 and the occurrence of kinetic oscillations. This system was studied under steady flow conditions in the 10−6 mbar total pressure range using low-energy electron diffraction-(LEED), work function measurement, and mass spectrometry for determination of the reaction rate. These studies revealed that kinetic oscillations can only be initiated from one of the two stable reaction branches. Two separate existence regions were detected in which the oscillations are always damped. Oscillations can be very reproducibly excited by slight decreases in temperature. The 1×1■hex phase transition of the surface structure was observed to take place only in one of the two regions of reaction rate oscillations. Its influence seems to be of minor relevance to the mechanism of oscillations as oscillations in one region occur on the surface that maintains a 1×1 structure. The experiments were modeled by a set of coupled differential equations based on knowledge about the elementary reaction steps. The model calculations reproduced the steady states of the reaction as well as the occurrence of kinetic oscillations in different ranges in excellent agreement with experimental observation. In the model, the phase transition also has no relevance for the oscillation mechanism. The occurrence of oscillations can be rationalized in terms of a periodic sequence of autocatalytic ‘‘surface explosions’’ and the restoration of an adsorbate-covered surface. The damping, experimentally observed, is attributed to insufficient spatial coupling between different regions of the surface.

The mechanism of kinetic oscillations in the NO + CO reaction on Pt(100)

Under constant flow conditions the reaction of NO and CO to the products N2 and CO2 on a Pt(100) surface exhibits multiple steady states and kinetic oscillations. Using Video LEED, measurement of the work function change and of the reaction rate the kinetic oscillations were investigateed in the 10−7 mbar range. Kinetic oscillations were found to occur in two separate temperature regions around 400 K. In the upper temperature range, the oscillations are coupled to the 1 × 1 ⇌ hex phase transition, while those in the lower temperature range proceed entirely on a 1 × 1 surface. Based on a mathematical model consisting of three coupled differential equations it could be shown that the 1 × 1 ⇌ hex phase transition is not essential for the oscillating mechanism. The main role in the mechanism can instead be ascribed to the autocatalytic behavior of the surface reaction between COad and NOad.

The mechanism of the “explosive” NO + CO reaction on Pt(100): experiments and mathematical modeling

Coadsorbed NO and CO on a Pt(100) surface react upon heating to form extremely narrow TPR product peaks of CO 2 and N 2 at ~ 400 K whose position and half-width are almost independent of the initial coverages. Video-LEED, TDS and work-function measurements were used to study the adsorption behavior of NO and the surface reaction between coadsorbed NO and CO. The LEED experiments demonstrated that at higher coverages the “explosive” formation of CO 2 and N 2 is not accompanied by the 1 × 1 → hex phase transition which is shifted up by ~ 50 K to higher temperature. This result rules out the phase transition as a possible explanation for the narrowness of the product peaks in TPR. A new model is proposed based on the vacant site requirement for NO dissociation in which the autocatalytic increase in the number of vacant sites during the surface reaction is responsible for the ‘surface explosion”. This simple vacancy model is modified by taking island formation into account in order to reproduce the coverage independent position and shape of the product peaks. Island formation occurs in the NO/CO-induced lifting of the hex reconstruction as a consequence of the higher adsorption energy of NO/CO on the 1 × 1 phase. Based on these mechanistic steps a mathematical model was developed consisting of five coupled differential equations whose solutions reproduce the experimental results quite accurately.

Interaction of NO and H 2 with Pt(100)

The catalytic reduction of NO by H 2 on Pt(100) exhibits sustained kinetic oscillations at low feed-gas pressures (total pressures less than 10 –3 Pa) and isothermal conditions. As part of a study of these phenomena work function change, low energy electron diffraction (LEED) and thermal-desorption spectroscopy (TDS) measurements have been made of the interaction of NO and H 2, separately and together, with the Pt(100) surface. Coadsorption of H 2 and NO at 170 K leads to the appearance of extremely narrow TDS product peaks of N 2 and H 2O at ∼~ 400 K. Temperatures cycles under conditions of constant H 2 and NO partial pressures revealed the existence of a hysteresis in the reaction rate that is associated with the 1 × 1 ⇌ hex phase transition. Besides H 2O one also observes NH 3 as a reaction product with the product ratios depending on the temperature and on the surface structure. A specific procedure is required for the initiation of kinetic oscillations. This involves first heating the sample to 550 K in an appropriate NO/H 2 mixture and then cooling into a temperature window between 410 and 450 K.

Reflection electron microscopy of the catalytic etching of Pt single-crystal spheres in CO + O 2

The influence of the catalytic oxidation of CO on structural changes of a Pt catalyst was studied with a Pt single-crystal sphere in the 10 −4 Torr range at 200 °C sample temperature. Reflection electron microscopy (REM) served to follow the structural changes which are induced by the catalytic reaction on the Pt(100) and Pt(111) facets. Using an intensity-enhanced Bragg reflection for imaging the surface, the diffraction contrast allows one to resolve atomic steps. Both surfaces are homogeneously roughened by the catalytic reaction, but the Pt(100) surface is much more strongly attacked than Pt(111). Small positive islands develop on an initially flat (100) terrace which are preferentially oriented in the directions of the unit cell of the hex reconstruction. The mechanism of the roughening process on Pt(100) is traced back to the adsorbate-induced 1 × 1 ⇌ hex phase transition which can strongly enhance the mass transport of Pt atoms under reaction conditions. Oxide formation or mass transport via volatile Pt compounds is considered to play a negligible role under the conditions applied here.