Understanding the Dielectric Properties of Heat-Treated Carbon Nanofibers at Terahertz Frequencies: a New Perspective on the Catalytic Activity of Structured Carbonaceous Materials

Terahertz time domain spectroscopy (THz-TDS) has been used to study the electrical and optical properties of a series of carbon nanofibers (CNFs) that have undergone different heat treatments. The high-temperature heat-treated (HHT) sample displayed increased absorption and real refractive indices across the range 0.3−3.5 THz when compared to the low-temperature heat-treated (LHT) and pyrolitically stripped (PS) samples. The experimental results were fitted by using a Drude−Lorentz model and an effective medium approximation to yield the electrical parameters of the sample such as the plasma frequency, phonon mode frequency, and oscillator strength. These parameters were used to rationalize the differences as being due to an increase in graphitic order in the HHT sample when compared to the LHT sample and to an even greater extent the PS sample. HHT, LHT, and PS CNFs can be used as catalysts for the oxidative dehydrogenation of ethylbenzene to styrene. They exhibit different catalytic yield and selectivity which can be correlated with their dielectric properties at terahertz frequencies. The results suggest that THz-TDS is a useful tool for characterizing the graphiticity of CNFs in terms of electon density and mobility which, in turn, correlate with the catalytic performance of these materials