甲烷催化还原NO_x中Co基/分子筛催化剂的助剂和载体优化

Optimization of Promoter and Support for Co-based/zeolites Catalysts in Catalytic Reduction of NO_x by CH_4

采用共浸渍法制备了系列Co基/分子筛催化剂,在固定床催化反应器中考察了不同催化剂在甲烷催化还原NO_x中的催化行为,优化了Co基/分子筛催化剂的助剂和载体,并通过催化表征手段阐明了其构效关系.结果表明,Fe和SAPO-34分别作为优化助剂和载体制备得到的Co-Fe/SAPO-34催化剂具有最高的催化活性,在450℃时,NO_x的最大转化率可以达到52.7%.CO_2对Co-Fe/分子筛催化剂活性无明显抑制作用,H_2O对催化剂的活性抑制是可逆的,SO_2对催化剂活性抑制作用明显.Co-Fe/SAPO-34催化剂表面钴物种以CoO和Co(OH)_2为主,Co-Fe/ZSM-5催化剂表面钴物种则以Co_3O_4和Co(OH)_2为主,Co-Fe/Beta催化剂则可能以CoO、Co Al2O_4和Co_3O_4为主.各Co-Fe/分子筛催化剂表面Fe^(2+)/Fe^(3+)含量比依次为Co-Fe/ZSM-5(3.98)>Co-Fe/SAPO-34(0.52)>Co-Fe/Beta(0.43).活性组分钴物种的形态和合适的Fe^(2+)/Fe^(3+)含量比对Co-Fe/分子筛催化剂上甲烷催化还原NO_x至关重要.Co-Fe/分子筛催化剂上CH_4-SCR反应机制为:NO在Br?nsted酸位上吸附氧化成NO^+,同时CH_4在Br?nsted酸位上吸附活化成—C=O和—COO等活性物种,生成的NO^+在Co2+和Fe^(3+)等催化活性位上转化成硝酸盐等中间产物,中间产物硝酸盐与活化物种(—C=O和—COO)反应生成N_2和CO_2.

Catalytic behavior of Co-based/zeolites catalysts was investigated in NOx reduction by CH4. Optimization of promoter and support was investigated by catalytic tests, and the relationship between catalytic activity and catalyst structure was illustrated by catalyst characterization. Co-Fe/SAPO-34 exhibited the highest activity among various Co-base/zeolites catalysts. The maximum conversion of NOx with 52.7% was obtained on Co-Fe/SAPO-34 at 450℃. The inhibition of activity of Fe/zeolites became severe in the presence of SO2, CO2, and H2O. CO2 exerted virtually no effect on the SCR activity of Co-Fe/zeolites. The inhibition of NOx conversion by H2O was reversible for Co-Fe/zeolites catalysts. Cobalt species were mainly present in CoO and Co（OH）2 states in Co-Fe/SAPO-34. Co3O4and Co（OH）2 were the main cobalt species of Co-Fe/ZSM-5, while CoO, CoAl2O4 and Co3O4 might be present in Co-Fe/Beta. The ratio of Fe2＋/Fe3＋ in the surface layer of Co-Fe/zeolites decreased in the order of Co-Fe/ZSM-5（3.98） 〉 Co-Fe/SAPO-34（0.52） 〉 Co-Fe/Beta（0.43）. The active states of cobalt species and suitable ratio of Fe2＋/Fe3＋ were important for the activity of Co-Fe/zeolites in CH4-SCR. CH4-SCR over Co-Fe/zeolite catalysts started with the adsorption of NO and CH4 on Brnsted acid sites of the zeolite to produce NO＋and carbon-containing species（-C=O and -COO） in the presence of oxygen, respectively. Subsequently, the important intermediates of nitrate species were generated from NO＋ at the active sites. Finally, nitrate species reacted with carbon-containing species to form N2 and CO2.