Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite

Abstract

Calcium-hydroxyapatite and calcium-fluoroapatite loaded with different amounts of nickel were synthesized and characterized by several techniques including scanning electron microscopy, temperature programmed reduction (TPR), UV-visible-NIR and XPS spectroscopy. Three types of nickel species were detected in the two series of catalysts (i) Ni2+ ions exchanged with Ca2+ ions of the apatite framework. This Ni2+/Ca2+ exchange seems to be restricted to nickel loadings inferior to 1 wt% Ni, (ii) small particles of NiO exhibiting strong interactions with the carriers (for x > 1 wt% Ni) and (iii) large particles of NiO which appear at high loadings. The distribution of the nickel between these three hosting sites depends on the nature of the apatite. For instance the amount of exchanged Ni2+ ions in Ni(1)/CaHAp is twice more important than in Ni(1)/CaFAp. The Ni(x)/CaHAp and Ni(x)/CaFAp catalysts were tested in methane dry reforming with CO2. Methane conversion at 600 °C, increases with the nickel loading up to x = 4 where the activity is around the thermodynamic equilibrium (78%) and H2/CO ratio close to 1. These results were confirmed by the investigation of the catalysts activity versus the temperature. Carbon deposition on the catalysts was found to also increase with nickel loading but without provoking any significant decay of the activity after 4 h on stream. The encouraging results achieved were attributed to the synergy between the basic properties of the apatites, their aptitude to chemisorb CO2 and the catalytic features of the supported nickel.