自動車排ガス浄化用高機能Fe骨格置換ベータゼオライトの合成と評価

摘要

近年の世界各国におけるディーゼル自動車排出ガス規制の強化に伴い、排出ガス中の有害成分である窒素酸化物（NOx)、粒子状物質（particulate matter：PM)、炭化水素、一酸化炭素のより効率的な除去方法が求められている。これらのうち、酸性雨や光化学スモッグの原因物質として知られるNOxの除去方法の一つとして、アンモニア（尿素）を還元剤とする選択還元（NH_3-Selective Catalytic Reduction ： NH_3-SCR)が採用されている。高温、湿潤の過酷な使用条件に耐えうるSCR触媒として、FeやCuなどの活性金属を担持したゼォライトが有望である。%We tried to develop a novel catalyst with improved low-temperature activity in the selective catalytic reduction of NO_x with ammonia (NH_3-SCR). Highly dispersed active metal (Fe) in zeolite beta framework was the key concept to achieve the goal. To make Fe atoms be disperse well, isomorphously substituted Fe-BEA zeolites were hydrothermally synthesized. One possible drawback of this strategy was that they could result in low crystallinity due to narrower synthetic conditions than those of aluminosilicate beta zeolite. Therefore, fluoride was first used to ensure the synthesis of Fe-BEA zeolites. These Fe-BEA zeolites, especially Al-free Fe-BEA zeolite, showed performance in the NH_3-SCR superior to that of conventional Fe-loaded beta zeolites. We also succeeded in synthesizing highly crystalline Fe-BEA zeolite without using fluoride by replacing TEAOH with an excess amount of NaOH. With the results of characterization techniques e.g. UV-Vis and EPR, we showed that the isolated and tetrahedrally coordinated Fe~(3+) species is the one most relevant for NH_3-SCR activity.