Calcium-lead hydroxyapatites: Thermal and structural properties and the oxidation of methane

The oxidation of methane on calcium (HAp) and calcium-lead hydroxyapatites (PbHAp) has been studied in the presence and absence of tetrachloromethane (TCM) as a gas-phase additive, In the absence of TCM both the conversion and selectivities were strongly influenced by the structure and/or the lead content of the catalysts, particularly at 973 K. The addition of lead produced substantial increases the C2+ selectivities with concomitant decreases in that of the carbon oxides while the introduction of TCM suppressed the formation of CO2, apparently resulting from the formation of chlorapatites. Significant differences between the results observed at reaction temperatures of 873 and 973 K can be attributed to the structural transformation of the hydroxyapatites to the corresponding phosphates at the higher temperature, Extended X-ray absorption fine structure spectroscopy has been employed to provide further information on the relevant structural factors in the oxidation process. (C) 1998 Academic Press.