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.

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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.

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Synchronization and spatiotemporal selforganization in the NO+CO reaction on Pt(100). I. Unsynchronized oscillations on the 1×1 substrate

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The NO+CO reaction, which has been shown to exhibit damped kinetic oscillations on a Pt(100) surface after initial excitation, has been subjected to periodic and random forcing of the temperature and of the CO partial pressure. The experiments were conducted in the 10−7 mbar range and measurements of the CO2 production rate and of the work function were used to follow the response of the system. The response behavior is characterized by strong resonance effects and by the absence of quasiperiodic oscillations. The system is highly sensitive to temperature modulation, but rather insensitive to modulation of pCO with the latter requiring an amplitude of more than 5% of pCO for producing sustained oscillations. Random forcing experiments demonstrate that the response of the system can be described as a bandpass filter since only frequencies close to the natural frequency of the system are amplified. The results of the experiments led to the conclusion that the damping effect is due to the absence of an efficient synchronization mode under isothermal conditions at low pressures.

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