Stationary and spatially oscillating temperature fronts separating regions with high and low temperatures (amplitudes up to 120 K) formed during the atmospheric oxidation of carbon monoxide on a Pd on alumina ring kept in a mixed reactor. The sharp fronts bounded either one or two low temperature regions. The transition from stationary to spatially oscillating fronts occurred at a feed CO/O2 concentration ratio close to 2.0 and was independent of the reactor temperature. The net rate of local heat generation on the ring was nonuniform leading to angular variation of the temperature of the ignited state. Front motions were affected by both the nonuniformity of the system and the interaction between the catalyst and the ambient gas. The two fronts bounding a low temperature region usually moved at different velocities and their velocities increased with increasing oxygen concentration. In most cases the two fronts did not move in phase, but sometimes they moved in tandem generating a back-and-forth pulse motion. The spatial amplitude of the oscillations decreased as the slope of the net heat generation activity became steeper. The spatial variation of the standard deviation of the temperature was helpful in assessing the dynamics of the oscillating fronts.

The oxidation of CO on a palladium support catalyst under normal pressure in a continuous¯ow reactor was observed. For experimental work an X type zeolite loaded with 0.5 weight % palladium was used as catalyst. The palladium salt was inserted into the zeolite by ion exchange and then reduced under hydrogen¯ow. Palladium particles with an average diameter of 4 nm were formed under these conditions. The conversion rate shows a dynamic behaviour with selfaf®ne pattern of excursions to a smaller conversion rate on a time scale of some seconds. The in¯uence of the¯ow rate upon the dynamics of the CO conversion pattern was studied. Increasing¯ow rate causes increasing of frequency of maximum excursions, increasing of smaller excursions, increasing of complexity of the pattern and decreasing of maximum conversion (base line).The system was simulated by a time and space discrete automaton. The model is a considerable extension of that described by Liauw et al. [7]. For example, in the extended model the temperature of palladium particles (non-isothermic conditions), the¯ow rate and the distribution of particle size is introduced. The temperatures of the palladium particles have an important in¯uence upon the velocities of reaction, oxidation and reduction. Arrhenius equations were used to describe the in¯uence of temperature. To obtain self-af®ne patterns, the coupling of oscillators with very different frequencies are necessary. These were obtained by different self-organised`operating temperatures' of the palladium particles in the active state. During the catalytic oxidation of CO, the heat production and the heat loss for each model palladium particle equilibrate very fast and a temporary maximum`operating temperature' occurs for each of them. This operating temperature of each model palladium particle depends upon the size of each particle.

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