摘要
少量H_2可以增强丙烯在Ag/Al_2O_3催化剂上选择性还原NO的活性,降低NO的起燃温度(<100℃),在2个温度区间内NO转化率比较高:第1个是低温区间80~180℃,第2个是250~500℃,在这两个温度区间内NO的转化率没有太大变化,在中间温度区间内比较低,低温区间NO转化率较高是由于H_2的还原作用,当温度高于180℃时C_3H_6起主要作用。N_2和NO_2是主要的竞争产物低温时有NO_2形成,当温度在140℃左右和高于380℃的时候对N_2的选择性非常高,在200℃和260℃时NO_2的浓度77×10^(-6)~83×10^(-6)占主要产物,165~500℃时仅仅检测到N_2O_2×10^(-6),对H_2-C_3H_6-SCR的反应体系,500℃时比较高的空速条件下会生成48×10^(-6)NH_3并增加了CO/CO_2比值。氧气和催化剂表面NO_2等中间体与还原剂之间存在着竞争反应,但是仍能被还原成N_2,这些中间产物在低温时容易被H_2还原而在高于180℃时易被C_3H_6还原。通过XPS分析可以发现,部分活性组分Ag在载体Al_2O_3上即使在低温条件下也非常不稳定,由于不同价态银物种相互作用使催化剂在不同温度区间内活性不同选择性也有很大差别,在C_3H_6-SCR反应之后Ag^+和Ag_n^(δ+)物种随之出现,当在反应气中通入H_2以后金属纳米Ag单质也会生成。XRD结果也同样表明,在H_2-C_3H_6-SCR反应后有大于5 nm的Ag纳米粒子形成。
A little H2 had been processed on the Ag/Al2O3 catalyst which significantly enhanced the C3H6- SCR-NO activity and lower NO light-off to below 100 ℃. There are two well-dedined zones of NO conversion rate is high：the first is low-temperature interval 80 - 180 ℃ ,the second is 250 -500 ℃ ,in the two zones for NO conversation there is no significantly change but in the intermediate temperature range it drops. H2 is re- sponsible for the reaction of NO at low temperature,while C3H6 is the reductant above about 180 ℃. N2and NO2 are the main competing products N20 is formed at low temperature. The selectivity of N2 is very high when the temperature at about 140 ℃ and above 380 ℃. At 200 -260 ℃ ,the concentration of 77 × 10-6 -83 × 10-6 NO2 is the main products,there is only small amounts N20（2 × 10-6）between 165 ℃and 500 ℃. The reaction system of H2-C3H6-SCR,500 ℃ high airspeed will generate 48 x 10-6 Nil3 and increased the ratio of CO/CO2. There is a competitive reaction between the oxygen and the catalyst surface NO2 and other intermediates and reducing agents,but still can be reduced to N2 ,these intermediate products at low temperature is easily to be reduced by H2 at higher than 180 ℃ by C3H6. The micro-structure of the catalysts was fully characterized by XRD, BET, TEM, and XPS technologys. We can find Ag species intervene and responsible for the variations activity and selectivity in SCR. XPS show that Ag＋ and Agn^δ species emerged after C3H6-SCR reaction,Ag will generate after the reaction of H2-C3H6-SCR,XRD results also show that greater than 5 nm Ag nanoparticles are formed.
出处
《应用化工》
CAS
CSCD
北大核心
2016年第2期207-211,215,共6页
Applied Chemical Industry
基金
国家科技支撑计划项目(2012BAF03B00)
