(83aj) Effect of Surface-to-Volume Ratio in Microplasma Reactors

Non-thermal plasma generated at atmospheric pressure can provide highly energetic electrons which is necessary to decompose molecules. Performing the plasma process in a microreactor can lead to precise control of residence time and extreme quenching conditions, enabling control over the reactants to selectively produce desirable products [1]. Inelastic collisions with processing gas can excite molecules and atoms giving radicals, electronically and vibrationally excited species. Extremely small volume and large surface area of microplasma reactors can be an efficient tool to study plasma-surface interactions. As the surface-to-volume ratio increases, presumably wall collisions will be important and these collisions will affect the rate of decomposition by changing the main pathways by which radicals and activated species are lost in the discharges. High surface-to-volume ratio can be attained in a microplasma reactor by splitting the flow into multiple microchannels so that maintaining the reactor volume the same. Figure 1(a) shows the schematic diagram of the approach. If the same electric field and power are applied to the microreactors, useful information on effect of surface on the decomposition mechanism could be obtained. CO₂ decomposition in plasma reactors have generated considerable interest because of its high energy efficiency [2]. In this study, we discuss the effect of glass surface on the decomposition of CO₂ at atmospheric pressure in glass type microplasma reactor experimentally and by chemical kinetic modeling. We simulate the barrier discharge with CO₂ gas. Modelling employs a well mixed plasma reactor model as available in Chemkin 4.1 [3]. An electron energy balance equates the time rate of change of electron's swarm internal energy to the net flow of electron enthalpy into and out of the reactor, accounting for net chemical production rates, surface losses, collisional losses and power deposition from the externally applied electromagnetic field. Reaction rate coefficients of the electron induced reactions was determined by 0-D solution of the Boltzmann equation using BOLSIG+ [4]. The relationship between reaction rate coefficient and average electron energy was described by fitting the Boltzmann solution results to a three parameters Arrhenius fit. Ionization, vibrational and electronic excitation processes were thought to play an important role in CO₂ plasma. Homogenous and heterogeneous reactions of the species were added to the Chemkin model. In Figure 1(b), calculated energy loss coefficients of important electron impact processes in CO₂ discharge are shown. These results imply that energy spent on dissociative electronic excitations (Electronic1&2) and ionization processes increase with increasing electron energy in the range of 1eV and 8eV. Comparison of the results of one channel microplasma reactor with multiple channels will be discussed to provide information on the surface processes. References [1] T. Nozaki, A. Hattori, K. Okazaki., Catal. Today, 98, (2004), 607. [2] V.D. Rusanov, A.A. Fridman, G.V. Sholin, Sov. Phys. Usp., 24, 447. [3] R. J. Kee, F. M. Rupley, J. A. Miller, M. E. Coltrin, J. F. Grcar, E. Meeks,H. K. Moffat, A. E. Lutz, G. Dixon-Lewis, M. D. Smooke, J. Warnatz, G. H. Evans,R. S. Larson, R. E. Mitchell, L. R. Petzold, W. C. Reynolds, M. Caracotsios, W. E. Stewart, P. Glarborg, C. Wang, C. L. McLellan, O. Adigun, W. G. Houf, C. P. Chou, S. F. Miller, P. Ho, P. Young, D. J. Young, CHEMKIN Release 4.0, Reaction Design, San Diego, CA (2004). [4] G.J.M. Hagelaar, L.C. Pitchford, Plasma Sources Sci. Technol., 14, (2005), 722-733.