Effect of oxygen addition, reaction temperature and thermal treatments on syngas production from biogas combined reforming using Rh/alumina catalysts

•0.5% Rh/Al2O3 catalysts showed high activity on biogas reforming.•Catalyst thermal pretreatments notably affect metallic dispersion.•Adding O2 to the feed enhances CH4 conversion and catalysts’ stability.•Combined oxy-CO2 reforming reduced carbon deposition on the catalysts.•A O2/CH4=0.12M ratio is adequate for combined reforming of biogas.