Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers

Adsorption and reduction of NO2 over pitch-based ACFs, both as received and calcined at 1100 degreesC, were studied in a range of concentrations (NO2, 250-1000 ppm; O-2, 0-10%) and temperatures (30-70degreesC). Repeated adsorption after regeneration at 300 degreesC, temperature-programmed desorption (TPD) and diffuse reflectance fourier transformation infrared spectroscopy (DRIFTS) were also applied to analyze the adsorbed NO2 species. Pitch-based ACFs showed rapid NO production and adsorption at 30 degreesC which stayed at similar conversions until the rapid breakthrough of NO2. A higher reaction temperature of 70 degreesC decreased the ratio of NO2 adsorption to reduction in the stationary state and shortened the breakthrough time. Higher NO2 concentration increased the rates of both adsorption and reduction to shorten breakthrough time, whereas the presence of oxygen changed the NO2 profiles by enhancing the NO2 adsorption rate and decreasing both the rate and the capacity of reduction. It must be noted that 10% O-2 allowed still significant production of NO. The molar O/N ratio evolved from TPD decreased and converged to a constant value according to the NO2 adsorption time, showing that NO2 species adsorbed on the ACF changed from NO2 to NO3 along with the time of NO2 adsorption. Such a trend was confirmed by DRIFTS spectra of adsorbed NO2. These results suggest two kinds of NO2 adsorption sites. Site 1 adsorbs NO2 molecules strongly, transferring one oxygen to another adsorbed molecule on a similar site to form NO3 ad. Although oxygen in the gas phase oxidized adsorbed NO2 to some extent, especially in the initial stage, disproportionation is still dominant at 10% O-2. Such disproportionation produces gaseous NO, leaving NO3 on the surface. Site 2 adsorbs NO2 weakly. Saturation of both sites terminates the adsorption and reduction and results in the breakthrough of NO2. Adsorbed NO3 produces both NO and NO2 when heated, leaving one or two oxygen atoms on the surface, which are evolved as CO and CO2 at the same time, restoring a stationary ability for adsorption and reduction of NO2 through carbon loss. (C) 2002 Elsevier Science Ltd. All rights reserved.