Understanding the catalytic activity of heat treated carbon nanofibres: Investigation of their dielectric properties at THz frequencies

Terahertz time-domain-spectroscopy (THz-TDS) has been used to study the electrical and optical properties of a series of carbon nanofibres (CNFs) that have undergone different heat treatments. The high temperature heat treated (HHT) sample exhibited increases in both absorption and real refractive index across the range 0.3-3.5 THz when compared to the low temperature heat treated (LHT) sample and pyrolitically stripped (PS) sample. The experimental results were fitted 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 rationalise the differences between the samples as being due to an increased order or graphicity in the HHT sample when compared to the LHT sample, and to an even greater extent when comparing the HHT sample to 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 activity which can be explained by their dielectric properties at THz frequencies. The results suggest that THz-TDS may become a useful tool for fundamental research into the role of electron mobility in catalyst performance.