银在甲烷选择性催化还原NOx反应中不同催化行为的研究

 采用XRD，TEM和UV－Vis方法研究了Ag－HZSM－5催化剂中活性组分银的价态和结构变化，并将这种变化与其在CH4选择性催化还原NOx反应中的活性和选择性相关联．结果表明，惰性气氛下高温处理使交换到分子筛阳离子位上的Ag＋自还原为Ag0，并在热的作用下聚集成纳米尺度的银颗粒Agn．纳米银颗粒Agn的形成提高了银催化剂在CH4选择性催化还原NOx反应中的活性，但它的长大又促进了CH4和O2的直接燃烧，使CH4选择性催化还原NOx反应的选择性降低．氧气气氛下高温预处理抑制了银颗粒的长大，并且氧气的氧化作用使银颗粒带有一定电荷，形成小的荷电纳米银粒子Agy＋x．荷电纳米银粒子的形成使催化剂的活性降低，但提高了CH4选择性催化还原NOx反应的选择性．

Behavior of Different Silver Species in the Selective Reductionof NOx by CH4 over Ag-HZSM-5 Catalyst

Different silver species formed over Ag HZSM 5 catalyst have been identified by XRD, UV Vis and TEM, and their catalytic performance was investigated through the reaction of the selective catalytic reduction (SCR) of NO x by CH 4. By the pretreatment in a He stream at high temperatures, Ag + can self reduce to Ag 0 and then Ag 0 can aggregate into nano sized silver particles. The catalytic activity of the silver catalyst is increased by the nano silver particles formed. Much larger silver particles, formed with increasing the temperature, can accelerate the direct combustion of CH 4 with O 2 and decrease the selectivity of SCR of NO x by CH 4. High temperature pretreatment in 30%O 2/He stream depresses the growing of large silver particles, and makes silver particles charged to form Ag y + x . The presence of Ag y + x leads the catalyst to show better selectivity but poor activity in the reaction of SCR of NO x by CH 4.