The surface structure and reactivity of black carbon

Abstract

Abstract Fourier transform infrared (FT-IR) spectroscopy has been the most definitive analytical tool in a comprehensive program on the structure and reactivity of black carbon (in the form of n -hexane soot). In combination with other techniques, it has revealed the soot structure, as produced by high temperature incomplete combustion, to be predominantly aromatic with a surface coverage by oxygen-containing functional groups of about 0.5. Particularly well suited to following net changes in surface groups, and gas phase reactant/product concentrations, FT-IR has been the key technique in determining the kinetics and mechanisms of some important heterogeneous reactions of black carbon with gas phase oxidant molecules. For example, the reaction of NO 2 /N 2 O 4 with soot follows a dual path mechanism, down to 2 p.p.m., which is reflected in the rate law: initial rate = (k 1 + k 2 [ soot ] 1 2 ) P NO2 . On the other hand, catalytic decomposition initiates the reaction with ozone, followed by the formation of surface carboxylic groups and gaseous CO 2 and H 2 O. The evidence suggests that dissociation of ozone yields a steady-state concentration of excited oxygen atom which is actually the oxidant. FTIR combined with chemical measurements has proven that a high solubility observed for carbon particles exposed to ozone has its origin in the hydrolysis of the surface carboxylics. Significant effects of simulated solar radiation on the reactions, especially in the soot/SO 2 /H 2 O/O 2 system, has been revealed by FTIR. Infrared will continue its central role in the examination of increasingly complex systems containing black carbon, particularly through its interface with ancillary techniques.