Surface chemistry of carbon: activation of molecular oxygen

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Abstract
Three different industrial carbon blacks were characterized in their surface chemistry with respect to their relative abilities to activate molecular oxygen. A variety of techniques was used including x-ray diffraction, gasification experiments, electron spectroscopy as XPS and UPS, helium ion scattering spectroscopy, thermal desorption spectroscopy and the catalytic oxidation of aqueous SO2 as a “chemical probe” for activated oxygen. The activation properties for molecular oxygen at high temperatures were probed by analysing the gasification characteristics in 5 vol % oxygen-inert gas mixtures; at low temperatures the activity in the oxidation of SO2 to sulfuric acid with molecular oxygen was taken as indicator. For this reaction the carbons had to be activated by ammonia treatment at elevated temperatures. Thermal desorption spectroscopy and valence band photoemission provided data for the identification of reaction intermediates in the activation process of oxygen. The important role of surface chemical anisotropy caused by vast differences in bulk crystal structure is pointed out. The results are discussed within a model of oxygen activation which assumes two types of surface sites differing in their π electron density. Sites, rich in π electrons are graphitic and can activate oxygen to the O2- 2 species which is short-lived and can be accumulated in the carbon pores. Further activation involves dissociation of the peroxo group into O- species which finally react with the second sites with the formation of covalent carbon-oxygen bonds. The relative abundance of the low temperature reactive O2- 2 species and the high temperature active O- species is on as-received carbons, low for the peroxo species, and can be enhanced considerably by modifying the abundance of graphitic surface patches on carbon black and by generating suitable porosity.