Noble vanadium core-shell catalysts for methane oxidation to formaldehyde

Abstract

The recent development of hydraulic fracturing technology for collecting shale gases further stimulates to convert abundant methane to a more valuable chemical feedstock by less depending on petroleum resources. Nevertheless, direct catalytic conversion of methane is challenging, because of a stable tetrahedral geometry of methane with high bond dissociation energy. While avoiding the production of carbon dioxide by full oxidation, selective conversion of methane to formaldehyde is important for the purpose of versatile utilization of natural gas. As an efficient and thermally stable catalyst, new SiO2@V2O5@Al2O3 core@shell nanostructures are designed via hydrothermal synthesis and subsequent atomic layer deposition (ALD) method. In particular, the thickness of Al2O3 shells over SiO2@V2O5 cores can be controlled by the number of ALD cycles. Through catalytic methane oxidation in a flow reactor operated at 600 °C, SiO2@V2O5@Al2O3-(50) nanostructures after 50 cycles of ALD is proven to be the best catalyst with 26.5 % of conversion and 47.9 % of HCHO selectivity which is the highest methane conversion never achieved before from any vanadium based catalysts at 600 oC. With versatile characterizations with TEM, EDS, in situ XRD, Raman, H2-TPR, and diffuse reflectance UV-vis spectroscopy, we find that the Al2O3 shell of the SiO2@V2O5@Al2O3 core@shell catalyst provides new surfaces to create highly dispersed Td-monomeric species by interactions between the Al2O3 and V2O5 nanoparticles during the ALD process. It is revealed that the surface Al2O3 shell not only protects V2O5 nanoparticles against sintering at 600 oC but also anchors the new Td-monomeric vanadium species which is responsible for the high conversion in methane oxidation.