A New Discrete Model for the Non-Isothermic Dynamics of the Exothermic CO-Oxidation on Palladium Supported Catalyst

Abstract

The oxidation of CO on a palladium support catalyst under normal pressure in a continuous flow 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 flow. Palladium particles with an average diameter of 4 nm were formed under these conditions. The conversion rate shows a dynamic behaviour with self-affine pattern of excursions to a smaller conversion rate on a time scale of some seconds. The influence of the flow rate upon the dynamics of the CO conversion pattern was studied. Increasing flow 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 flow rate and the distribution of particle size is introduced. The temperatures of the palladium particles have an important influence upon the velocities of reaction, oxidation and reduction. Arrhenius equations were used to describe the influence of temperature. To obtain self-affine 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.