合成氨钌催化剂的晶体和形貌敏感性

Crystallographic and Morphological Sensitivity of N_(2) Activation over Ruthenium

哈伯-博施法合成氨反应是高温高压的耗能过程,因此降低该过程的能量消耗及开发温和条件下合成氨反应催化剂具有重要意义.金属钌是合成氨反应中最有前途的催化剂之一,一直备受广泛关注.确定金属钌催化剂的结构敏感性并提高其比质量活性是多相催化中亟待解决的重要问题.氮气(N_(2))活化是合成氨反应中的关键步骤.本文通过第一性原理理论计算和微观动力学模拟方法系统研究了具有六方密排和面心立方晶体结构的钌催化剂上N_(2)活化过程和N_(2)解离反应速率.理论计算研究表明,在六方密排Ru形貌中,{2130}晶面具有最高的N_(2)解离活性,其次是{0001}台阶面,它们比六方密排Ru其他表面上N_(2)解离反应速率高3个数量级以上;在面心立方Ru形貌中,{211}和{311}表面上N_(2)解离活性最高.这些结果都表明台阶面/台阶位对氮气活化至关重要.虽然六方密排Ru{2130}晶面具有最低的N_(2)解离能垒,然而由于面心立方Ru上可以暴露更高密度的活性位点,使得面心立方Ru比六方密排Ru具有更高的N_(2)转化速率.本研究深入理解了N_(2)解离过程中,金属Ru催化剂形貌和晶相结构敏感性,这为设计和优化高活性的合成氨Ru催化剂提供了理论基础.

Ruthenium(Ru)serves as a promising catalyst for ammonia synthesis via the Haber-Bosch process,identification of the structure sensitivity to improve the activity of Ru is important but not fully explored yet.We present here density functional theory calculations combined with microkinetic simulations on nitrogen molecule activation,a crucial step in ammonia synthesis,over a variety of hexagonal close-packed(hcp)and face-center cubic(fcc)Ru facets.Hcp{2130}facet exhibits the highest activity toward N_(2) dissociation in hcp Ru,followed by the(0001)monatomic step sites.The other hcp Ru facets have N_(2) dissociation rates at least three orders lower.Fcc{211}facet shows the best performance for N_(2) activation in fcc Ru,followed by{311},which indicates stepped surfaces make great contributions to the overall reactivity.Although hcp Ru{2130}facet and(0001)monatomic step sites have lower or comparable activation barriers compared with fcc Ru{211}facet,fcc Ru is proposed to be more active than hcp Ru for N_(2) conversion due to the exposure of the more favorable active sites over step surfaces in fcc Ru.This work provides new insights into the crystal structure sensitivity of N_(2) activation for mechanistic understanding and rational design of ammonia synthesis over Ru catalysts.