Behaviour of Rh supported on hydroxyapatite catalysts in partial oxidation and steam reforming of methane: On the role of the speciation of the Rh particles

Abstract

Rh/hydroxyapatite samples were prepared by impregnation and investigated for the partial oxidation (POM) and steam reforming (SRM) of methane. The catalysts were analysed by BET, XRD, DRS, XPS, H2-TPR, TEM, H2 chemisorption, CO2-TPD and NH3-TPD techniques. The characterisation results showed that, after calcination, Rh existed in three different forms in the samples: (i) large crystallites of Rh2O3 deposited on the surface of the catalysts, (ii) RhOx in small particles exhibiting strong interaction with the support and (iii) a phase of Rh2+ species which incorporated the hydroxyapatite framework.Operating in the POM and SRM processes the reduced Rh(x)/HAP catalysts resulted highly active and exhibited excellent stability at 973 K (for 30 h). This behaviour was explained by their high coke-resistance. The activity of the catalyst with the optimum loading (1%), in SRM, was compared with that of a commercial Rh/Al2O3 catalyst. The conversion and H2 and CO yields values achieved on the former were all close to those exhibited by the latter. This comparable behaviour was explained by suitable properties provided by the HAP support such as reducibility, lower surface acidity and larger pore sizes (ensuring a better diffusion of the reactants and the products during the SRM reaction). These properties seemed to compensate the low dispersion of the Rh active phase induced by its low specific surface area.