Persistent free radicals on carbon nanotubes and their catalytic effect on benzoyl peroxide decomposition

Abstract

Carbon nanotubes (CNTs) were found to exhibit a remarkable catalytic effect on the decomposition of benzoyl peroxide (BPO) in ethanol at room temperature. The catalytic effects of various carbonaceous materials were investigated by UV spectroscopy. The decomposition reaction for BPO/CNT/ethanol system was studied through gas chromatography-mass spectrometry, infrared spectroscopy and free radical trapping technique. Those results indicated the decomposition of BPO by CNTs occurred via a free-radical pathway involving benzoate radicals. The catalytic mechanism of CNTs was further investigated through electron spin resonance (ESR) spectroscopy and quantum chemical calculations. The ESR signals with same line shape and g-value were found in multi-walled CNTs, single-walled CNTs, and oxidation product of pyrenol. The persistent free radicals (PFRs) on CNTs were essentially aryloxy radicals ([ArO]•) with spin electrons dispersed on the aromatic plane. Polycyclic aromatic hydrocarbons with larger π-conjugated structures and more aryloxy radicals exhibited a stronger electron-transfer ability. Finally, a three-step pathway for CNTs to decompose BPO was proposed: (1) formation of an asymmetric peroxide (ArO-OOCC6H5) via free radical exchange; (2) conversion of ArO-OOCC6H5 into radicals ([ArO]• and C6H5COO•), or ions ([ArO]+C6H5COO−); (3) formation of benzoic acid by hydrogen or proton abstraction. This study offered tremendous insight into the electronic structures and catalytic mechanism of CNTs, and can facilitated their development for ROS production and scavenging.