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Various calcium hydroxyapatites [Calo~z(HP04)z( PO,)6 -,(OH), -,; z = 0, stoichiometric apatite (Ca/P = 1.67; atomic ratio); 0 < z 6 1, non-stoichiometric apatites ( 1.67 > Ca/P > 1.50)], together with the apatites of Ca/P = 1.72, were analysed by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy and their catalytic properties in the oxidation of methane at 973 K were examined in the presence and absence of tetrachloromethane as a gas-phase additive. The XRD patterns of each hydroxyapatite were the same. However, the nearestneighbour distances of the Ca-0 bond of stoichiometric and non-stoichiometric hydroxyapatites as estimated by EXAFS were 2.41,2.37,2.38 and 2.39 A for each catalyst of Ca/P = 1.72, 1.68, 1.64 and 1.58, respectively. In the oxidation of methane on each hydroxyapatite in the absence of CCl,, the conversion of methane was little influenced by the value of Ca/P but the selectivities to C2H6 and CO showed a maximum and minimum, respectively, at Ca/P = 1.68. On addition of CCl, into the feedstream, the selectivity to C 0 2 on Ca/P = 1.68 and 1.72 decreased as the time on-stream increased while sharp decreases in conversion were observed with time on-stream on non-stoichiometric hydroxyapatites of Ca/P = 1.64 and 1.53, respectively, without suppression of the selectivity to C 0 2 .

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