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

氧化铝在过量氧条件下与甲烷选择性还原NO （CH_4-SCR-NO）的活性很大程度上取决于其化学成分。NO的选择性催化还原是通过(1)NO+1/ 2o_2→NO_2和(2)NO_2+HC（烃）→N_2+CO_2+H2_O的连续反应进行的。除(1)和(2)反应外，还发生了不良反应(3)HC+O_2→CO_2+H_2O。特别是某些氧化铝中TiO_2的存在似乎会影响它们的活性，因此用纯氧化铝（ALOA）和掺TiO_2的ALOA考察了TiO_2在CH4-SCR-NO及相关反应中的作用。结果发现，TiO_2加入ALOA对反应(1)有促进作用，对反应(3)有有效抑制作用。这些结果将很好地解释tio_2掺杂ALOA活性较高的原因。由C_2H_4生成的碳质沉积物在O_2存在下有效地将NO还原为N_2。NO的存在极大地促进了乙烯向碳质沉积的转化，碳质沉积中含有N/C原子比为1的氮原子。这种含氮的碳质沉积物与O_2或适当的NO和O_2的混合物反应生成N_2。另一方面，O_2的存在极大地抑制了C_2H_4向碳质矿床的转化。Al_2O_3催化剂床与Co/Al_2O_3催化剂床的连续布置对扩大还原NO的活性温度范围非常有效。两种催化剂的反向排列或物理混合不会使其变宽。从催化剂的氧化活性角度讨论了双床体系的有效性。

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.