Influence of the calcination temperature on the activity of hydroxyapatite-supported palladium catalyst in the methane oxidation reaction

[Display omitted] •The influence of the calcination temperature on the properties of Pd/HAP catalysts is investigated.•The rise of Tc expands the HAP lattice and induces an evolution of Pd2+ structure from Td to D4h.•Two distinct reducible species are identified in Pd/HAP-1073; one of them manifests SMSI.•Methane oxidation TOF increases with Tc owing to the progressive increase in the strength of MSI.•The most active species consist of the surface PdO phase. In the present study, a series of four hydroxyapatite (HAP) supported palladium samples, with a Pd loading close to 0.5 %, obtained through their calcination at 773, 873, 973, or 1073 K has been investigated. These samples have been characterized using a wide battery of complementary techniques. From these studies, it was found that the rise of the calcination temperature induces a progressive dehydroxylation of the support and a structure evolution of the species containing Pd2+, from tetrahedral (Td) to square planar geometry (D4h). Moreover, this enhances markedly the metal-support interactions. For instance, at the highest temperature (1073 K), Pd particles were found encapsulated by a thin support layer. Consequently, two distinct reducible species have been identified; one of them manifests SMSI. This increase in the Pd-HAP interaction strength seems to (i) expand the HAP lattice, (ii) change the Pd2+ coordination from Td to D4h geometry, (iii) promote PdO reduction and (iv) suppress CO chemisorption. These entire properties do compensate the poor textural properties and benefit the efficiency and stability of the Pd active phase in methane oxidation reaction.