978-1-4244-2120-6/08/$25.00 ©IEEE. Abstract— 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. I. INTRODUCTION AND BACKGROUND ARBON NANOTUBES (CNTs) and the more structurally complex carbon nanofibres (CNFs) have a wide range of applications, including potential uses in heterogeneous catalysis. Carbonaceous materials in catalysis are most commonly associated with catalyst deactivation, however highly ordered carbon structures have been shown to exhibit high activity and selectivity in a number of reactions The characterisation of such ordered carbonaceous materials represents a significant challenge with traditional optical spectroscopies and 13 C NMR spectroscopy. CNTs have been investigated using THz-TDS in a number of previous studies. 1-5 Previous work on CNTs 2 used a combination of the Drude-Lorentz (DL) model with an effective medium approximation (EMA) to explain the differences between pristine and hydrogen-functionalised CNTs as being due to a difference in the number of available charge carriers between the two samples. In this investigation we extend this treatment to CNFs that have been heat treated at different temperatures (PS vs. LHT vs. HHT samples) and interpret their different catalytic activities in terms of the availability of charge carriers as probed using THz-TDS. II. RESULTS AND DISCUSSION Figure 1 displays the frequency dependent absorption coefficient (A) and real refractive index (B) for the HHT, LHT and PS samples. Both the LHT and HHT samples show a change in both the absorption coefficient and real refractive index when compared to the PS sample, with a marked increase observed between the LHT and HHT samples. A progressively increasing gradient in the absorption coefficient is observed when comparing the PS to the LHT to the HHT sample. Similarly the refractive index of the HHT sample is markedly higher across the frequency range. The change in refractive index between the PS and LHT sample is less pronounced, but the LHT sample still appears to have a higher refractive index, especially above 1 THz. Further understanding of the structural and electronic changes observed across the samples was made possible by modeling their dielectric response using an EMA and DL model as previously reported. 2 Two EMAs were used, the Maxwell-Garnett (MG) model and the Bruggemann (BR) Edward P.J. Parrott a,b , J. Axel Zeitler a , James McGregor a , Shu-Pei Oei c , Husnu Emrah Unalan c , Swee-Ching Tan c , William I. Milne c , Jean-Phillipe Tessonnier d , Robert Schlögl d , and Lynn F. Gladden a a Department of Chemical Engineering, University of Cambridge, Pembroke St., Cambridge CB2 3RA, U.K. b Cavendish Laboratory, University of Cambridge, J. J. Thomson Ave., Cambridge CB3 0HE, U.K. c Centre for Advanced Photonics and Electronics, University of Cambridge, J. J. Thomson Ave., Cambridge CB3 0FA, U.K. d Fritz-Haber Institut der Max-Plank-Gessellschaft, Berlin D-14195, Germany Understanding the catalytic activity of heat treated carbon nanofibres: Investigation of their dielectric properties at THz frequencies C Figure 1: A) absorption coefficient and B) real refractive index for the PS, LHT and HHT samples along with the best fit MG and BR model calculations.