Surface and bulk properties, catalytic activities and selectivities in methane oxidation on near-stoichiometric calcium hydroxyapatites

Abstract

Various calcium hydroxyapatites [Ca_10–z(HPO₄)_z(PO₄)_6–z(OH)_2–z; z= 0, stoichiometric apatite (Ca/P = 1.67; atomic ratio); 0 < z⩽ 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 nearest-neighbour distances of the Ca–O bond of stoichiometric and non-stoichiometric hydroxyapatites as estimated by EXAFS were 2.41,2.37, 2.38 and 2.39 Å 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 CCl₄, 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 CCl₄ into the feedstream, the selectivity to CO₂ 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 CO₂.