Electrocatalytic ammonia synthesis catalyzed by mesoporous nickel oxide nanosheets loaded with Pt nanoparticles

介孔氧化镍纳米片负载Pt纳米粒子电催化合成氨

Owing to its cost‐effectiveness and adjustable eight‐electron distribution in the 3d orbital,nickel oxide(NiO)is considered an effective electrocatalyst for an ambient electrochemical nitrogen reduction reaction(NRR).However,because of the low conductivity of the transition metal oxide electrocatalyst,its application in this field is limited.In this study,we found that the doping of NiO nanosheets with a small amount(3–10 nm)of Pt nanoparticles(Pt/NiO‐NSs)leads to considerable improvements in the Faradaic efficiency(FE)and NH_(3) yield compared with those obtained using pure NiO,breaking the common perception that commercial Pt‐based electrocatalysts demonstrate little potential for NRR due to their high hydrogen evolution tendency.In a 0.1 mol/L Na_(2)SO_(4) solution at−0.2 V vs.RHE,a typical Pt/NiO‐2 sample exhibits an optimum electrochemical NH_(3) yield of 20.59μg h^(–1)mg^(–1)cat.and an FE of 15.56%,which are approximately 5 and 3 times greater,respectively,than those of pure NiO nanosheets at the same applied potential.X‐ray photoelectron spectroscopy analysis revealed that Pt in Pt/NiO‐NSs exist as Pt0,Pt^(2+),and Pt^(4+)and that high‐valence Pt ions are more electropositive,thereby favoring chemisorption and the activation of N2 molecules.Density function theory calculations showed that the d‐band of Pt nanoparticles supported on NiO is significantly tuned compared to that of pure Pt,affording a more favorable electronic structure for NRR.The results of this study show that Pt can be an effective NRR electrochemical catalyst when loaded on an appropriate substrate.Most importantly,it provides a new synthetic avenue for the fabrication of highly active Pt‐based NRR electrocatalysts.

由于NiO成本低且3d轨道具有可调谐的八电子结构,被公认为是温和条件下电化学氮还原反应(NRR)的有效电催化剂之一.然而,由于过渡金属氧化物电催化剂的电导率相对较低,阻碍了其在该领域的应用.贵金属具有良好的电导率,通常可用于构建异质材料,利用协同作用,来提高载体材料的电导率并改善电催化性能.因此,在氧化镍上嵌入或掺杂具有催化性能的高导电金属是获得高性能NRR电催化剂的一种有效方法.在早期的研究中,由于贵金属基催化剂(Au,Pd和Ru)导电性好,一般都具有较好的电化学NRR活性.值得注意的是,尽管Pt在其它电催化过程中表现出优异的电催化活性,但很少有关于NRR的Pt基电催化剂的报道.这主要是因为Pt基电催化剂具有较强的催化析氢反应活性.因此,研究Pt基NRR电催化剂的结构,以构建高效的Pt基NRR催化剂,是一个有意义且具有挑战性的研究课题.迄今为止,关于Pt/NiO复合材料对NRR催化活性的研究报道较少.基于此,本文使用纯的氧化镍纳米片作为载体来负载不同含量的Pt纳米粒子,利用Pt的高导电性与NiO的可调谐电子特性的协同作用来提高NRR活性.本文通过简单的还原法成功合成了Pt/NiO-NSs,采用透射电镜、X射线光电子能谱(XPS)、密度泛函理论(DFT)计算、紫外可见光谱等表征手段研究了不同Pt负载量对NRR活性的影响.与纯NiO相比,用少量铂纳米粒子(3~10 nm)掺杂氧化镍纳米片(Pt/NiO-NSs)可显著提高纯NiO的法拉第效率(FE)和氨产率,打破了商用Pt基电催化剂几乎没有NRR潜力的普遍观点.电催化实验结果表明,在‒0.2 V vs RHE的0.1 mol/L Na_(2)SO_(4)溶液中,样品Pt/NiO-2表现出20.59μg h^(‒1)mg^(‒1)cat.的氨产率和15.56%的FE,分别是相同外加电位下纯NiO纳米片的5倍(3.79μg h^(‒1)mg^(‒1)cat)和3倍(5.97%)左右.XPS分析结果表明,Pt/NiO-NSs中Pt的价态为Pt0,Pt^(2+)和Pt^(4+),其中Pt/NiO-2具有较高含量的高氧化态Pt物种,从而有利于N2化学吸附和活化.DFT结果表明,与纯Pt相比,负载在NiO基底上的Pt纳米粒子的d带电子更接近费米能级,为NRR提供了更有利的电子结构.该研究表明,当Pt负载在合适的基底上时,可成为有效的NRR电化学催化剂.综上,本文为制备高NRR活性的Pt基电催化剂提供了一种新的策略.