Interaction mechanism of NO2 with carbon black:: effect of surface oxygen complexes

Adsorption of NO2 on carbon black has been examined for different NO2 concentrations (400 or 600 ppm) and temperatures (50 or 100degreesC). The influence of chemisorption of oxygen on the carbon surface at 300degreesC before exposure to NO2 has been reported. Temperature programmed desorption (TPD) was performed after NO2 adsorption to identify the adsorbed species by measuring the amounts of evolved gases NO, NO2, CO and CO2. During the exposure of the carbon black to NO2, both adsorption and reduction of NO2 to NO has been observed. From the oxygen and nitrogen balance data, it is shown that the NO2 adsorption step leads to the formation of both oxygen and nitrogen surface complexes such as -C(O) and -C(ONO2). Based on the TPD results, a desorption process has been proposed. Chemisorption of oxygen before exposure to NO2 leads to an increase, during the adsorption step, of both nitrogen adsorbed and NO evolved species. This behaviour has been attributed to already existing oxygen complexes resulting from O-2 chemisorption. In fact, NO2 adsorbs mainly on preexisting oxygen complexes. A relationship between the additional adsorbed NO2 during the adsorption step and the excess of CO2 released during the TPD below 500degreesC has been established. (C) 2004 Elsevier B.V. All rights reserved.