Development of new exhaust gas purification catalyst for a co-generation system

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The activity of aluminas for the selective reduction of NO with methane in excess oxygen (CH_4-SCR-NO) was greatly dependent upon their chemical compositions. The selective catalytic reduction of NO proceeds by the successive reactions of(1)NO+1/2O_2→NO_2 and (2)NO_2+HC (hydrocarbon)→ N_2+CO_2+H2_O. In addition to the reactions (1)and (2),undesirable reaction (3)HC+O_2→CO_2+H_2O occurs. In particular, the presence of TiO_2 in some aluminas seems to affect their activity, and so the role of TiO_2 was examined in CH4-SCR-NO and related reactions using pure alumina (ALOA) and TiO_2-doped ALOA. As a result, it was found that the addition of TiO_2 to ALOA promoted the reaction (1)and inhibited effectively the reaction (3). These results will give a good explanation for higher activity of TiO_2-doped ALOA.Carbonaceous deposit produced from C_2H_4 effectively reduced NO to N_2 in the presence of O_2. The presence of NO greatly promoted the conversion of ethene to carbonaceous deposit, which was revealed to contain nitrogen atoms with the N/C atomic ratio of unity. This N-containing carbonaceous deposit reacted with O_2 or suitably a mixture of NO and O_2 to produce N_2. On the other hand, the presence of O_2 greatly suppressed the conversion of C_2H_4 to carbonaceous deposit.The arrangement of an Al_2O_3 catalyst bed and a Co/Al_2O_3 catalyst bed in a row has been shown to be very effective to expand the active temperature region for the reduction of NO. The reverse arrangement or the physical mixture of two catalysts did not widen it. Effectiveness of the dual beds system has been discussed from the viewpoint of oxidation activity of the catalysts.