Reaction mechanism of H2-promoted selective catalytic reduction of NO with NH3 over Ag/Al2O3
Reaction mechanism of H2-promoted selective catalytic reduction of NO with NH3 over Ag/Al2O3
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DOI：
10.1021/jp0668100
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发表时间：
2007-01
影响因子：
3.7
通讯作者：
K. Shimizu;A. Satsuma
中科院分区：
文献类型：
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作者：
K. Shimizu;A. Satsuma
在Ag/Al 2 O3催化剂上，当反应气中加入1%的H2时，473 K下NH3-SCR还原NO的反应速率提高了630倍。通过动力学和光谱研究研究了这种戏剧性活性改善的机制原因。动力学研究表明，在673 K以下，H2的加入提高了NH3 + O2反应的速率，降低了NH3 + O2反应的活化能，表明H2的加入对NH3的氧化活化是有效的.原位紫外可见光谱结果表明，在H2-NH3-SCR反应过程中，Ag +离子和Agn δ+团簇（n ≤ 8）共存. H2-NH3-SCR反应气体混合物在423 K下暴露后的ESR谱表明，催化剂上形成了超氧阴离子，且超氧阴离子的量随H2浓度的增加而增加。反应过程中团簇的相对数量和稳态NO还原速率随H2浓度的增加而增加，且速率与团簇的相对数量有较好的相关性，表明团簇在H2-NH3-SCR反应中起着活性物种的作用. Ag n δ+团簇和H2还原Ag +形成的质子参与了分子氧还原活化为超氧阴离子的过程，超氧阴离子应是NH3活化为NHX（x ≤ 2）中间体的有效氧化剂。
The rate of the selective catalytic reduction of NO with ammonia (NH 3 -SCR) on Ag/Al 2 O 3 at 473 K increased by a factor of 630, when H 2 (1%) was added to the reaction gas mixture. The mechanistic cause of this dramatic activity improvement was investigated by kinetic and spectroscopic studies. Kinetic studies indicate that H 2 addition increases the rate of NH 3 reaction in NH 3 + O 2 below 673 K and decreases the activation energy for NH 3 + O 2 reaction, indicating that H 2 addition is effective for the oxidative activation of NH 3 . In situ UV-vis results under reaction conditions show that the Ag + ion and Ag n δ+ cluster (n ≤ 8) coexist during H 2 -assisted NH 3 -SCR reaction (H 2 -NH 3 -SCR). ESR spectra after exposing H 2 -NH 3 -SCR reaction gas mixture at 423 K show that the superoxide ion is formed on the catalyst, and its amount increased with H 2 concentration. The steady-state NO reduction rate and relative amount of the cluster during the reaction increased with H 2 concentration, and the rate correlates fairly well with the relative amount of the cluster, indicating that the cluster acts as active species in H 2 -NH 3 -SCR. It is concluded that Ag n δ+ cluster and protons formed by H 2 reduction of Ag + ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as effective oxidant for N-H activation of NH 3 to NH X (x ≤ 2) intermediate.