摘要
负载型贵金属催化剂在能源转化和环境污染控制等领域有广泛应用.相对于采用传统方法制备负载型贵金属催化剂,还原析出策略在控制贵金属的粒径,增强贵金属与载体相互作用方面具有独特的优势.但是在高温还原气氛下,贵金属析出的同时往往伴随着母体结构中其它元素的析出,这会对催化剂的性能产生较大影响.因此,理解催化剂在还原气氛下以及后续反应条件下的结构演变,对于催化剂的设计及制备具有重要意义.本文通过800℃还原Ru掺杂的LaFe_(0.9)Ru_(0.1)O_(3)(LFRO)钙钛矿前驱体制备了贵金属析出的LFRO催化剂(LFRO800R),并用于丙烷催化氧化反应.活性测试结果表明,析出Ru催化剂的丙烷氧化性能远远低于原始的LFRO.当第一次反应结束,催化剂床层温度降至室温后再次评价其性能,LFRO 800R催化剂会发生“自活化”现象,在210℃下催化丙烷反应速率达到了22.3 molCO_(2)·h^(-1)·kgcat^(-1),是该温度下贵金属未析出LFRO催化剂的5倍,反应10 h后仍表现出较高的稳定性.为进一步研究LFRO800R在丙烷氧化反应中发生“自活化”的原因,在不同温度对LFRO 800R进行氧化处理,发现经过400℃氧化后得到的LFRO800R400O表现出最佳的丙烷催化活性.采用高分辨透射电镜、能量散射谱、漫反射红外傅立叶变换光谱、扫描电镜、X射线衍射、Raman光谱和X射线光电子能谱等方法对催化剂进行了表征,结果表明,经过800℃还原预处理,伴随着Ru从LFRO钙钛矿结构的体相中析出到表面,钙钛矿A位元素La也会析出并在RuFe(Ru占据主导)颗粒表面形成一层LaO_(x)覆盖层,阻碍了反应物分子在活性位点Ru上的吸附与活化,导致LFRO 800R在丙烷完全氧化反应中的低温区间表现出失活的现象.而在400℃的氧化气氛处理下,该LaO_(x)包裹层会从析出颗粒的表面完全去除,伴随着少部分界面处的Ru重新进入钙钛矿结构,大量有活性的Ru物种
The exsolution process enables to produce and control the formation of stable and catalyticallyactive nano particles via reductive extraction of uniformly incorporated precious metal ions from asolid oxide solution. Here we consider the simple and stable perovskite LaFeO_(3) (LFO) where 10% ofFe on B sites are substituted by ruthenium (LFRO). Hydrogen reduction of LFRO at 800 °C leads tothe formation of socketed ruthenium particles whose low-temperature activity in the total propaneoxidation reaction at 210 °C is substantially lower than that of the original LFRO. Upon increasingthe reaction temperature once to 400 °C, the exsolved catalyst undergoes self-activation so that theactivity at 210 °C turns out to be five times higher than that of the original LFRO. High-resolutiontransmission electron microscopy and nanometer-resolved element mapping, together with averagingcharacterization methods, including X-ray diffraction and X-ray photoelectron spectroscopy,Raman spectroscopy, and diffuse infrared spectroscopy, unveil that after reduction at 800 °C theexsolved Ru particles are slightly alloyed with Fe and encapsulated by an inert and protecting LaOxlayer. Mild oxidative treatment at 400 °C leads to the removal of the conforming LaOx layer, whilethe uncovered RuFe alloy particle transforms to catalytically active oxidic Ru species, with no indicationof a separate FeOx phase. We exemplify with our case study of LaFe_(0.9)Ru_(0.1)O_(3) that carefulredox treatment enables to control the exsolution process and to avoid deactivation. This may be ofimportance for the whole class of exsolvable materials.
作者
王宇
Jaime Gallego
汪炜
Phillip Timmer
丁敏
Alexander Spriewald Luciano
Tim Weber
Lorena Glatthaar
郭杨龙
Bernd M.Smarsly
Herbert Over
Yu Wang;Jaime Gallego;Wei Wang;Phillip Timmer;Min Ding;Alexander Spriewald Luciano;Tim Weber;Lorena Glatthaar;Yanglong Guo;Bernd M.Smarsly;Herbert Over(State Key Laboratory of Green Chemical Engineering and Industrial Catalysis,Research Institute of Industrial Catalysis,School of Chemistry and Molecular Engineering,East China University of Science and Technology,Shanghai 200237,China;Institute of Physical Chemistry,Justus Liebig University,Heinrich‐Buff‐Ring 17,D‐35392 Giessen,Germany;Center for Materials Research,Justus Liebig University,Heinrich‐Buff‐Ring 16,D‐35392 Giessen,Germany)
基金
国家重点研发计划(2022YFB3504200)
国家自然科学基金(22076047,21976057,U21A20326)
111工程(B08021)
中央高校基本科研业务费专项资金.
