Catalytic oxidation of carbon-carbon composite aircraft brakes (5)

In addition to alkali-metals, alkaline-earth metals like calcium have also been reported to catalyse the oxidation of carbon. Calcium-catalysed carbon gasification was studied by Cazorla and co-authors by using high-purity carbon which was loaded with CaCO3 by means of eigher ion-exchange, aqueous impregnation or through sintering. Steady-state and step-response experiments were carried out at 800C initially in flowing helium and then in CO2. The authors labeled the reactant CO2 atoms by using ¹³CO₂. They observed that uncatalysed reaction produced equal amount of ¹³CO and ¹²CO, as theoretically expected. However, the catalysed reaction produced unusually higher amounts of ¹²CO, but this was not the case when the reaction temperature was further increased. From this, the authors suggested that a redox cycle for the catalysed reaction involving the formation of a higher oxide is to be excluded. The CaO/CaCO3 catalytic behaviour was in contrast to the behaviour of the alkali metals which accordingly to McKee and Chatterji and Carabiniero et al. from peroxides. In addition, in the work of Cazoria and co-authors, when the flowing gas was switched from ¹³CO₂ to helium, the desorption curves showed a continuous ¹³CO₂ and ¹³CO decay which was greater than that in the non-catalysed reaction, implying retention of these species by the catalyst-containing sample. However, CaCO3 can’t form at the temperatures and pressure used in the experiments, implying that retention of ¹³CO₂ occurs only at the calcium-carbon interface and around CaO particles. The higher amount of ¹²CO observed during the catalysed reaction was therefore only attributable to the decomposition of CaCO3 which was deemed the catalytically active species. The model proposed by the authors is thus summarized as:

2Ca¹³CO₃-C → CaO-C(O)+ 2¹³CO         (11)

2CaO + (OCO) → 2CaO-C + CO₂           (12)

Perhaps the most extensive investigations in recent years on the catalytic effect of potassium and calcium acetates on the oxidation of C-C composite aircraft brakes as well as graphite powder were conducted by Wu and Radovic. The catalyst-loading methods for the two carbon materials were different; for the C-C composite material, catalyst loading was performed by means of impregnation in an aqueous solution of 99% calcium or potassium acetate, while for graphite powder, loading was performed by physically mixing graphite with 3 weight% of acetate. The catalytic oxidation was analysed by comparing the reaction rates of the samples at different temperatures. The reactivity, R, of the samples, defined as the rate of weight loss, dw/dt divided by the initial weight, wo

was calculated from the release rates of CO and CO2 gaseous products. see fig.13. The authors observed potassium to have a much stronger ability to catalyse the oxidation reaction than calcium did. The catalytic effect of potassium was equally strong with the C-C composite material as well as with graphite powder. On the other hand, calcium acetate mixed with graphite powder showed only a limited catalytic effect. The burn-off profiles for potassium-catalysed reaction displayed a monotonic increase in the oxidation rate for both the C-C material and graphite. On the other hand the calcium-catalysed reaction showed a monotonic decrease in the case of the C-C material and an initial decrease followed by an increase in the oxidation rate in the case of graphite. The authors also reported that the potassium catalytic effectiveness varied with different amounts of loaded catalyst, but it was insensitive to the loading method. The calcium catalytic effectiveness was attributed to an optimal initial interface contact with the carbon surface; the low wettability of the calcium catalytic species for carbon and the subsequent loss of the contact during the later stages of oxidation led to a decrease in the catalytic activity of calcium. In the case of calcium there was therefore, dependency on both the amound of the catalyst loaded into the samples and the impregnation method. This feature was confirmed by further tests that showed that pre-treatment in argon enhanced the catalytic action with potassium, whereas the opposite effect was found with calcium. Both the potassium and calcium catalytically-active species were thought to undergo a redox cycle as proposed initially by McKee and Chatterji and were more effective than carbon alone to adsorb and dissociate the gaseous reactants. Optimal catalysis therefore relied on the effective transportation of oxygen from the catalyst to the carbon and this was reported to be dependent upon the optimal interfacial contact between the catalyst and carbon. In order to achieve optimal catalysis, good interfacial contact between the catalytically active species and carbon must be established and maintained throughout the oxidation reaction. Potassium acetate decomposed at lower temperatures than calcium acetate and this is thought to give potassium carbonate the ability to form better contact. Moreover, potassium salts and oxide have a lower melting point than those of calcium conferring potassium salts higher nobility and thus the ability to re-disperse throughout the oxidation reaction and maintain constant optimal interfacial contact. Calcium salts and oxides have lower mobility and do not re-disperse throughout the oxidation reaction. The ability of calcium to catalyse the oxidation reaction thus relies solely on good initial interfacial contact with carbon. As the oxidation proceeds, the calcium catalytically active species progressively loses contact with carbon and thus catalyst de-activation occurs. This is the pattern observed by the authors leading to the monotonic decrease of the reaction rate of the calcium-catalysed reaction. The results of SEM and XRD showed that the PAN fibers used in their study underwent oxidation before the matrix, in agreement with the observations of Carabiniero et al.