Reduction of carbon monoxide to methane by hydrogen was investigated with a nonthermal plasma reactor in which Ni/alumina catalyst pellets was filled. The effect of reaction temperature, pressure and voltage on the conversion of CO was examined. It was found that the nonthermal plasma significantly enhanced the catalytic conversion of CO. The effect of the nonthermal plasma was especially remarkable at lower temperatures and pressures. At high temperatures, the catalyst itself exhibited very high catalytic activity for the conversion of CO. Since high pressure is unfavorable for creating electrical discharge plasma, the increase in pressure lowered the discharge power, thereby weakening the effect of the nonthermal plasma. With the nonthermal plasma alone, there was no conversion of CO. The reaction products identified by FTIR spectra were CH 4 , CO 2 and H 2 O. FTIR spectra also showed that CO was converted primarily into CH 4 with high selectivity above 90% at most experimental conditions.
Abstract:The purpose of this study is to explain how plasma improves the performance of selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with a hydrocarbon reducing agent. In the plasma-coupled SCR process, NO x reduction was performed with n-heptane as a reducing agent over Ag/γ-Al 2 O 3 as a catalyst. We found that the plasma decomposes n-heptane into several oxygen-containing products such as acetaldehyde, propionaldehyde and butyraldehyde, which are more reactive than the parent molecule n-heptane in the SCR process. Separate sets of experiments using acetaldehyde, propionaldehyde and butyraldehyde, one by one, as a reductant in the absence of plasma, have clearly shown that the presence of these partially oxidized compounds greatly enhanced the NO x conversion. The higher the discharge voltage, the more the amounts of such partially oxidized products. The oxidative species produced by the plasma easily converted NO into NO 2 , but the increase of the NO 2 fraction was found to decrease the NO x conversion. Consequently, it can be concluded that the main role of plasma in the SCR process is to produce partially oxidized compounds (aldehydes), having better reducing power. The catalyst-alone NO x removal efficiency with n-heptane at 250 • C was measured to be less than 8%, but it increased to 99% in the presence of acetaldehyde at the same temperature. The NO x removal efficiency with the aldehyde reducing agent was higher as the number of carbons in the aldehyde was more; for example, the NO x removal efficiencies at 200 • C with butyraldehyde, propionaldehyde and acetaldehyde were measured to be 83.5%, 58.0% and 61.5%, respectively, which were far above the value (3%) obtained with n-heptane.
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