Catalytic Decomposition of Perfluorocompounds
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Agency (EPA) and the U.S. Semiconductor Industry
Association (SIA) are
investigating inhibition methods of reducing PFC emission [2]. Semiconductor industries are trying to reduce their emission using several methods: process optimization, alternative chemicals, recovery and recycle, and decomposition. Decomposition methods such as combustion, chemical adsorption and so on are currently being studied. The process of catalytic decomposition is expected to be effective in operating at lower temperatures and in small spaces if catalysts of high activity and durability are developed. In this paper, the PFC decomposition activity and durability of several catalysts was investigated. EXPERIMENTAL Catalyst preparation Catalysts were prepared from commercial metal oxides having a diameter of 2-4 mm. These particles were pulverized and sieved to a range of 0.5-1 mm. Then, the PFC decomposition activity was investigated. Binary oxide catalysts were prepared from the powder of raw materials and several metal nitrates or metal precursors. The slurries were dried and calcined in air. The calcined catalysts were pulverized and sieved to a range of 0.5-1 mm and the activity was investigated.
59 Mat. Res. Soc. Symp. Proc. Vol. 497 01998 Materials Research Society
Measurement of activity
The experimental apparatus used to measure the PFC decomposition activity of
is shown
catalysts
C2F6
CF4
mp
on
in Fig. 1. PFC
Pm Gas
chromatograph
Dehumid Reactor decomposition activities were measured Catalyst by a conventional fixed bed flow reactor. A ' Electric gaseous mixture of PFC (C2F6, CF4, CHF3), furnace H20 and Air(bal.) was fed with a space Wet alkali Mist velocity (SV) of 1,000-3,000h'. The PFC scrubbers catchers concentration changed within a range of 0.5-3 vol%. The volume of H20 (water Fig.l Experimental apparatus to measure the PFC vapor) was 50 times as large as that of PFC. decomposition activities of catalysts. The concentration of PFC was measured with a gas chromatograph using either FID or TCD detectors and conversion rates were calculated from the concentration differences. Catalyst analys The X-ray diffraction patterns were measured with a Rigaku Denki RU-200 X-ray analyzer. Adsorption isotherms of nitrogen on catalysts were measured by a volumetric method using a Shimazu Accusorb-2100. The specific surface area was calculated using the B.E.T. equation from the nitrogen adsorption isotherms. RESULTS PFC decomposition reaction The change in the free energy (A G) of representative CF4 decomposition reactions was calculated from thermodynamic data at 1000K [3]. The thermal decomposition (Eq.(1)) and the combustion (Eq.(2)) are impractical because of the unfavorable thermodynamic equilibrium. On the other hand, it is thermodynamically possible to decompose CF4 in the presence of such compounds as H20, H2, CH4 and 02, because the AG of schemes (3), (4) and (5) show negative values. Though the methane additive reaction mostly proceeds in these reactions, the hydrolysis reaction is the most practical from the point
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