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Microplasma Reactors with Integrated Carbon Nanofibers and Tungsten Oxide Nanowires Electrodes

Anil Agiral1, K. Seshan2, Leon Lefferts2, and J.G.E. (Han) Gardeniers1. (1) Mesoscale Chemical Systems, MESA+ Institue for Nanotechnology, University of Twente, P.O. Box 217, Enschede, 7500 AE, Netherlands, (2) Catalytic Processes and Materials, MESA+ Institue for Nanotechnology, University of Twente, P.O. Box 217, Enschede, 7500 AE, Netherlands

Miniaturized plasma sources have generated considerable interest due to a number of important applications [1]. Performing the plasma process in a microreactor leads to precise control of residence time enabling control over the reactants to selectively produce desirable products [2]. In this work, we incorporated carbon nanofibers (CNFs) and tungsten oxide nanowires into a microplasma reactor to increase the reactivity and efficiency of the barrier discharge at atmospheric pressure. These nanostructures have remarkable field emission characteristics due to their high aspect ratio structures generating local field enhancement at the apex of nanoscale tips. Field electron emission and field ionization which occurs at the tips of nanostructures can supply free electrons and ions which can contribute to pre-breakdown current during initiation of discharge and can lower the breakdown voltage for microplasma reactors. CNFs and tungsten oxide nanowires were incorporated into a silicon chip electrode which was placed in a microreactor. Characteristics of dielectric barrier discharge with nanowires and carbon dioxide decomposition reactions with CNFs and their comparison with plane-to-plane electrodes without nanostructures were investigated. Silicon chips where nanofibers and nanowires were incorporated, were placed in glass microreactors. Microchannels, inlet and outlet holes were created by powder blasting and then thermal bonding to seal the channels. Chemical vapour deposition was employed to grow the nanostructures on silicon chip electrodes. Representation and the real picture of the device is shown in Fig.1(a) and Fig. 1 (b). Carbon dioxide conversion was tested with a quadrupole ion-trap mass spectrometer. Decrease in breakdown voltage during barrier discharge generation resulted in higher number of microdischarges and higher power deposition at the same measured electric field comparing with plane cathodes. Fig. 1(c) shows that conversion of carbon dioxide was enhanced using CNFs electrodes at the same measured voltage levels with plane electrodes due to increase in energy deposition and reactive microdischarges. To better understand the importance of electron impact reactions in carbon dioxide plasma generated in the microreactor, Boltzmann equation for electrons was solved using Bolsig+ code [3] and reaction rate coefficients as a function of average electron energy is calculated (Figure. 1(d)). Calculations show that increasing the electric field in microreactors results in the increase of the number of dissociative electronic excitation channels. References [1] K.H. Becker, K.H. Schoenbach, J.G. Eden, J.Phys.D:Appl.Phys., 39, (2006), 55. [2] T. Nozaki, A. Hattori, K. Okazaki., Catal. Today, 98, (2004), 607. [3] G.J.M. Hagelaar, L.C. Pitchford, Plasma Sources Sci. Technol., 14, (2005), 722-733.