Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel

[Display omitted] ► Zinc aluminate spinel with high surface area is promising support for catalysts used in methane combustion. ► Ru/ZnAl2O4 catalysts exhibit good activity for methane combustion. ► The crystallite size of the RuO2 oxide plays important role in the combustion of methane. Activity of the Ru/ZnAl2O4 catalysts, H2-reduced or air-aged at 700°C, for the combustion of methane under O2-rich conditions was studied in this work. High surface area ZnAl2O4 spinel was synthesized by the unconventional co-precipitation method. Catalysts were prepared using Ru(NO)(NO3)3 as metal precursor and characterized by H2 chemisorption, O2 uptake, BET, XRD and TEM. The size of ruthenium particles in fresh catalysts varied from 1.1 to 1.5nm with the metal loading from 0.5% to 4.5wt.%. Air-aging treatment caused severe agglomeration of the Ru phase and formation of the well-crystallized RuO2 oxide. Under reaction conditions, highly dispersed Ru species were easily oxidized and RuO2 oxide was the active phase for methane combustion. The fresh catalysts were more effective than aged samples in terms of light-off temperature and temperature needed for the complete methane conversion. The mean crystallite size of the RuO2, formed during combustion reaction over fresh catalysts, depended on metal loading and was lower (21–27nm) as compared to that formed during aging process (21–40nm), which leads to higher activity. However, stable catalytic activity was observed only for aged catalysts. The specific reaction rate (μmolgRu−1s−1) for the fresh catalysts was found to decrease about 50% when the Ru loading increases from 0.5% to 4.5%. The apparent activation energies Eapp on Ru/ZnAl2O4 catalysts were in range of 120–129kJ/mol and did not depend on the metal loading and catalyst pre-treatment. Catalytic activity could be partly explained by the changes in the morphology and the crystallite size of the RuO2 phase and may suggest a structure sensitivity of CH4 combustion over ruthenium. Catalyst deactivation observed for the fresh catalysts, originate mainly from severe modifications of the Ru phase but not from changes in the ZnAl2O4 support structure.