Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Characterization and activation

Nickel-calcium phosphate/hydroxyapatite catalysts exhibited high activity and selectivity in the partial oxidation of methane (POM). Characterizations have been performed in order to understand the characteristics of these catalysts. TEM showed that very fine, needle-shaped nickel particles of a few nanometers were present in the sample after the reaction. TPR showed the presence of three nickel species in the fresh (before-reaction) sample, confirming that the nickel in the calcium hydroxyapatite and phosphate structures comes out as fine particles and is reduced during the reaction. The crystalline phases observed were calcium phosphate, calcium hydroxyapatite, NiO, and metallic Ni. As the Ca/PO₄ ratio increased, the calcium hydroxyapatite phase tended to increase over the calcium phosphate phase. As the Ni/PO₄ ratio increased, the amount of NiO increased in the fresh samples and the amount of metallic Ni also increased in the used (after-reaction) catalysts with the decrease of the NiO. The catalytically active component is certainly the metallic nickel that is produced under reducing environment during the reaction, but its surface is totally reoxidized at 673 K and loses the activity completely. The catalyst could be activated by the reactants only, without the H₂ pretreatment. The first activation could be achieved at 923 K or below, and the subsequent activation at 723 K.