Effect of surface modification on multi-walled carbon nanotubes for catalytic oxidative dehydrogenation using CO2 as oxidant

Abstract

Multi-walled carbon nanotubes (MWCNTs) with surface modification are prepared by the introduction of different oxygenated groups and metal oxides. It is found that surface modification of MWCNTs can induce significant changes in Raman feature, redox capability and catalytic dehydrogenation performance of these MWCNT composites. Since oxygenated groups with lone electron pairs or carbon–oxygen double bond can form the conjugated structure with sp2-bonding surface carbon on MWCNTs and partially compensate the sp2 hybridized structure of their surface carbon, surface modification only with oxygenated groups for MWCNTs has a small influence of electronic structure state of surface carbon and show no obvious shift of Raman bands and similar high reduction temperature of MWCNTs except Raman intensity ratio and reductive peak area of MWCNTs. The catalytic dehydrogenation performance of these MWCNTs with surface modification is not determined by itself reactivity of surface carbon but the type of functional groups on surface carbon and the hydroxylation of MWCNTs, especially MWCNTs with artificial defects, are used to produce efficient active sites for dehydrogenation reaction to enhance their catalytic activities. Surface modification of CeO2 nanoparticles for MWCNTs can change characteristic Raman band and reduction capability of MWCNTs and CeO2 due to the presence of strong interaction and charge transfer between MWCNTs and CeO2. Simultaneously, π-electron deficient inner surface of MWCNTs can strongly interact with CeO2 to promote the further activation of CeO2. Therefore, CeO2-MWCNTs composites show the increased catalytic activities due to surface modification with CeO2 nanoparticles. Noticeably, CeO2 nanoparticles decorated on inner surface of MWCNT channels show higher catalytic efficiency than that on their outer surface.