The oxidation-reduction kinetics of palladium powder
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I.
INTRODUCTION
THE increased use of powders in a number of energyrelated processes has renewed interest in the kinetic behavior of finely divided matter under various conditions of application. Since the behavior of small particles is often dominated by surface rather than volume energy considerations, their apparent reactivities may be modified by c o n c o m i t a n t mass transport processes driven by curvature-related activity gradients. Sintering resulting from such mass transport has been discussed in association with decomposition or reduction reactions of finely divided solids, 1'2'3 and a consideration of the role of sintering in gas-solid interactions has been presented recently. 4 Palladium powders present a nearly ideal model for an experimental investigation in this area. Palladium oxide can be made thermodynamically unstable over a relatively wide experimental temperature range by reducing the ambient pressure to ordinary vacuum levels. Investigations on the oxidation of this and other metals in powder form appear to be totally absent. One investigation on the interaction of oxygen with submicrometer-sized palladium powders has been reported in an early brief publication. 5 It was proposed, on the basis of X-ray data, that the exposure of Pd powders to air at temperatures between 453 and 573 K does not result in the formation of PdO initially, but in some "preoxide" layer with definite X-ray diffraction peaks. Similar observations were made on Ag powders, except that in this case the "preoxide" diffraction peaks persisted to relatively high temperatures and disappeared only when "sintering" had taken place. However, neither the nature of this sintering process nor the method by which its existence had been established were elucidated in this early work. 5 In this paper we present the results of an investigation on the kinetics of the cyclic oxidation and reduction of submicrometer-sized palladium powders over the temperature range 848 to 923 K. II.
Engelhard Industries of New Jersey. A scanning electron micrograph of the powder, Figure 1, shows a high degree of agglomeration. The specific surface area of the powder was measured as 19.2 m2g -1 by a nitrogen adsorption (BET) method. The high surface area reflects the small size of the particles and their relatively loose state of agglomeration. To ensure that all oxidation-reduction experiments start with oxide-free powders, each 600 mg sample is heated initially in v a c u u m ( 1 0 -4 Pa) at 873 K until the weight is constant, typically three hours. Weight losses resulting from this treatment are < 1 pct. These experiments are referred to hereafter as initial reduction. Gravimetric measurements are made in a continuous recording Cahn-R1000 microbalance. Samples are placed in an alumina crucible which is suspended inside a quartz reaction tube by means of a Pt ribbon. The reaction tube is surrounded by elements of a Lindberg electric resistance furnace. Typically, the constant temperature zone is about an order of magnitude longer than the maximum
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