Single atom doping induced charge-specific distribution of Cu1-TiO_(2) for selective aniline oxidation via a new mechanism

单原子掺杂诱导电荷特异分布的Cu_(1)-TiO_(2)通过新机理催化苯胺选择性氧化

Utilizing single atom sites doping into metal oxides to modulate their intrinsic active sites,achieving precise selectivity control in complex organic reactions,is a highly desirable yet challenging endeavor.Meanwhile,identifying the active site also represents a significant obstacle,primarily due to the intricate electronic environment of single atom site doped metal oxide.Herein,a single atom Cu doped TiO_(2)catalyst(Cu_(1)-TiO_(2)) is prepared via a simple“colloid-acid treatment”strategy,which switches aniline oxidation selectivity of TiO_(2) from azoxybenzene to nitrosobenzene,without using additives or changing solvent,while other metal or nonmetal doped TiO_(2) did not possess.Comprehensive mechanistic investigations and DFT calculations unveil that Ti-O active site is responsible for triggering the aniline to form a new PhNOH intermediate,two PhNOH condense to azoxybenzene over TiO_(2) catalyst.As for Cu_(1)-TiO_(2),the charge-specific distribution between the isolated Cu and TiO_(2) generates unique Cu_(1)-O-Ti hybridization structure with nine catalytic active sites,eight of them make PhNOH take place spontaneous dissociation to produce nitrosobenzene.This work not only unveils a new mechanistic pathway featuring the PhNOH intermediate in aniline oxidation for the first time but also presents a novel approach for constructing single-atom doped metal oxides and exploring their intricate active sites.

利用单原子掺杂金属氧化物来调控固有的活性位点,实现复杂有机反应选择性的精准调控,是一项极具吸引力但富有挑战性的工作.苯胺选择性氧化可生成氧化偶氮苯、亚硝基苯等产物,是一个重要的有机化学反应,但苯胺氧化过程涉及多个竞争性氧化和缩合反应,因此实现反应选择性精准调控的难度较高,尤其是将反应控制在亚稳态的亚硝基苯产物.目前仅有几个复杂的催化体系被报道,但需使用昂贵的催化剂、配体、添加剂和有毒溶剂.单原子掺杂金属氧化物是一类重要的单原子催化剂,通过单原子掺杂可以改变金属氧化物的配位环境和电子性质,从而显著影响其催化性能.目前,这类催化剂主要用于H_(2)O,CO_(2),CO,CH_(4)等小分子的催化转化,而对于复杂的催化反应,特别是有机液相反应的选择性调控,仍存在较大难度.本文通过“先凝胶-后酸处理”策略制备了单原子Cu掺杂的TiO_(2)催化剂(Cu_(1)-TiO_(2)).利用球差电镜和同步辐射等表征手段,证明了Cu成功地以单原子形式掺杂到TiO_(2)中.在苯胺氧化反应中,对制备的Cu_(1)-TiO_(2)催化剂进行了活性测试,结果显示,其能够选择性地氧化苯胺生成亚硝基苯,且这一过程无需任何添加剂或改变溶剂条件,这显著区别于未掺杂的TiO_(2)催化剂,后者主要生成氧化偶氮苯.进一步的研究揭示了这一选择性调控的机理.中间体机理实验研究、原位红外实验和密度泛函理论计算结果表明,在TiO_(2)催化体系中,苯胺首先在Ti-O活性位点上被氧化为PhNOH中间体,随后这些中间体进一步反应生成氧化偶氮苯.然而,在Cu_(1)-TiO_(2)催化体系中,尽管苯胺的初始氧化步骤相似,但Cu单原子与TiO_(2)之间的特殊电荷分布形成了独特的Cu_(1)-O-Ti杂化结构,这一结构引入了九个催化活性位点.其中,八个活性位点能够促使PhNOH中间体发生自发解离生成亚硝基苯,从而改变了反应选择性.此外,Cu_(1)-TiO_(2)催化剂在循环性能测试、底物扩展实验和放大实验均具有良好的活性.综上所述,本文提出了一种基于新型PhNOH中间体的苯胺选择性氧化机理,并且开发了一种构建单原子掺杂金属氧化物并探索其复杂活性位点的方法,为设计能够精确调控有机反应选择性的多相催化剂提供了新思路.