Cu交换量对Cu-SAPO-34催化剂上NH_3选择性催化还原NO_x反应的影响

Effects of copper content in Cu-SAPO-34 on its catalytic performance in NH_3-SCR of NO_x

选用不同种类的铵盐和调变Cu2+离子交换时间制备不同铜交换量的Cu-SAPO-34催化剂。并考察了铜交换量对催化剂在氨选择性催化还原(NH3-SCR)NOx反应中的催化性能和高温水热稳定性的影响。结果表明,Cu2+是Cu-SAPO-34催化剂在NH3-SCR反应中的主要活性中心,随着Cu交换量的增加催化剂的低温SCR活性先升高后降低。铜交换量为2.37%时,Cu-SAPO-34催化剂的低温活性最好,NOx转化率在185℃时即可达到80.0%,且最高达98.7%。ICP、H2-TPR、FT-IR和NH3-TPD等表征结果显示,Cu引入不但产生了新的氨吸附位点,增加了Lewis酸量,而且略微降低了SAPO-34上氨的吸附强度。继续增加Cu交换量反而导致催化剂的SCR活性下降,这是由于大量的Cu2+取代了桥式羟基Si-OH-Al中的H,抑制了SCR反应中NH3在催化剂表面的吸附、储存与迁移。此外,较高Cu交换量也不利于改善Cu-SAPO-34的高温水热稳定性。

A series of Cu-SAPO-34 catalysts with various Cu contents were prepared by using different kinds of ammonium salts and adjusting Cu2＋ ion exchange time; their catalytic performance and hydrothermal stability in selectively catalytic reduction of NOx with NH3（ NH3-SCR） were investigated. The results showed that Cu2＋ions are the predominant active sites in Cu-SAPO-34 for the NH3-SCR of NOx . With the increase of Cu content, the activity of Cu-SAPO-34 at low temperature is increased at first and then decreased. With a Cu content of 2. 37%, the Cu-SAPO-34 catalyst exhibits the best low temperature activity; over it, the conversion of NOx is higher than 80. 0% at 185℃ and may even reach 98. 7%. ICP, H2-TPR, FT-IR and NH3-TPD results illustrated that the introduction of Cu into SAPO-34 leads to an addition of new adsorption sites for ammonia, an increase of Lewis acid sites and a decrease of the adsorption strength of NH3 on SAPO-34. However, a further increase of Cu loading （e. g. 2. 90%） results in a decrease of Cu-SAPO-34 activity in SCR, which may be ascribed to the replacement of H in Si-OH-Al by Cu2＋that hinders the adsorption, storage and migration of NH3 in NH3-SCR. In addition, excessive loading of Cu is probably detrimental to the hydrothermal stability of the Cu-SAPO-34 catalyst.