Bifunctional metal-free KAUST Catalysis Center 1 (KCC-1) as highly active catalyst for syngas production via methane partial oxidation

Abstract

The metal-free KAUST Catalysis Center 1 (KCC-1) was synthesized through microemulsion method with microwave assistance and was assessed for methane partial oxidation (MPO) under various operating conditions. The electronic spin resonance spectroscopy, pyridine-probed infrared spectroscopy, and temperature-programmed desorption of oxygen measurement indicated that the concentration of BrØnsted acid sites and oxygen vacancies in KCC-1 were at least 2-fold higher than its counterparts, which benefited MPO activity via the promotion of adsorption and dissociation of gaseous reactants. The principal species detected by post-reaction X-ray photoelectron spectroscopy (XPS) was surface-adsorbed oxygen species; its relative percentages among all oxygen species reduced in the order spent KCC-1 (77.1%) > spent MCM-41 (Mobil Composition of Matter number 41; 41.4%) > spent SiO2 (−). The catalytic performance followed the same trend, suggesting that the surface-adsorbed oxygen species was the key factor for MPO process. Additionally, the carbon deposition rate increased in the order SiO2 (16.8 mol/gcat/s) > MCM-41 (11.7 mol/gcat/s) > KCC-1 (7.7 mol/gcat/s), consistent with the results of post-reaction Raman measurements. By coupling the in situ Fourier-transform infrared and XPS results, it is suggested that the high concentration of oxygen vacancies in KCC-1 contributed to activate the CH4 molecules on acid sites via different O∗-assisted kinetically relevant C–H bond activation mechanism for combustion-reforming pathway; meanwhile it provided an excellent adsorption-desorption cycle of O2− species to inhibit the carbon deposition, thus creating a bifunctional reaction mechanism in MPO reaction.