Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel

Authors:  Janina Okal, Mirosław Zawadzki

Abstract:
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 1.5 nm with the metal loading from 0.5% to 4.5 wt.%. Airaging treatment caused severe agglomeration ofthe Ru phase and formation ofthe 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 (21–27 nm) as compared to that formed during aging process (21–40 nm), which leads to higher activity. However, stable catalytic activity was observed only for aged catalysts. The specific reaction rate (mol gRu⁻¹ s⁻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/ZnAll₂O₄ catalysts were in range of 120–129 kJ/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. 

Keywords:
Methane combustion
Ruthenium
Zinc aluminate
RuO₂ agglomeration
Deactivation

Published in: Applied catalysis A: General  (Volume 453, Pages 1-402, 26 February 2013)

Publisher: Elsevier

ISSN Information: 0926-860X

Catalytic combustion of methane over ruthenium supported on zinc aluminate spinel

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