Inhibition of catalytic oxidation of carbon/carbon composites by phosphorus

The inhibition effectiveness of thermally deposited phosphorus (P) compounds on the carbon oxidation catalyzed by potassium or calcium acetate has been investigated. The P deposit was formed by impregnating carbon/carbon composite samples with methanol solution of methyl-phosphoric acid or phosphorus oxychloride and heating at ca. 600 °C. An amorphous layer formed by a relatively large amount of P deposit functioned as a barrier for the access of the catalyst to the carbon surface even though it had almost no barrier effect for O₂ access. The catalytic effect of calcium was almost completely suppressed by such deposit, but the effect of potassium was only partially suppressed due to the superior wetting ability and mobility of potassium species. Small amounts of P deposit showed similar inhibition effects on non-catalyzed oxidation, while their effects on catalytic oxidation were not as good. Characterization of P-deposited carbon samples by XPS, XRD, SEM and TPD, as well as ab initio MO calculations, suggested that the inhibition effect mainly resulted from the formation of oxygen-containing P groups which may include metaphosphates, C-O-PO₃ groups and C-PO₃ groups. Those groups are suggested to act as a physical barrier against carbon/catalyst interfacial contact as well as to block the active carbon sites. The presence of bridge oxygen bonded to a carbon site and a P group appears to be a critical factor for maintaining the inhibition effect. Indeed, the loss of such oxygen or connecting bond seems to result in loss of inhibition.