Vu Hue-Tong, Arčon Iztok, de Souza Danilo Oliveira, Pollastri Simone, Dražić Goran, Volavšek Janez, Mali Gregor, Zabukovec Logar Nataša, Novak Tušar Nataša
National Institute of Chemistry Hajdrihova 19 1000 Ljubljana Slovenia
University of Nova Gorica Vipavska 13 5000 Nova Gorica Slovenia.
Nanoscale Adv. 2022 Apr 28;4(10):2321-2331. doi: 10.1039/d2na00102k. eCollection 2022 May 17.
Catalyst design is crucial for improving catalytic activity and product selectivity. In a bifunctional Ni/ZSM-5 zeolite type catalyst, catalytic properties are usually tuned varying Al and Ni contents. While changes in acid properties associated with Al sites are usually closely investigated, Ni phases, however, receive inadequate attention. Herein, we present a systematic structural study of Ni in the Ni/ZSM-5 materials by using Ni K-edge XANES and EXAFS analyses, complemented by XRD and TEM, to resolve the changes in the local environment of Ni species induced by the different Al contents of the parent ZSM-5 prepared by a "green", template free technique. Ni species in Ni/ZSM-5 exist as NiO crystals (3-50 nm) and as charge compensating Ni cations. The Ni K-edge XANES and EXAFS results enabled the quantification of Ni-containing species. At a low Al to Si ratio ( / ≤ 0.04), the NiO nanoparticles predominate in the samples and account for over 65% of Ni phases. However, NiO is outnumbered by Ni cations attached to the zeolite framework in ZSM-5 with a high Al to Si ratio ( / = 0.05) due to a higher number of framework negative charges imparted by Al. The obtained results show that the number of highly reducible and active NiO crystals is strongly correlated with the framework Al sites present in ZSM-5 zeolites, which depend greatly on the synthesis conditions. Therefore, this kind of study is beneficial for any further investigation of the catalytic activities of Ni/ZSM-5 and other metal-modified bifunctional catalysts.
催化剂设计对于提高催化活性和产物选择性至关重要。在双功能镍/ ZSM - 5沸石型催化剂中，催化性能通常通过改变铝和镍的含量来调节。虽然与铝位点相关的酸性性质变化通常会得到密切研究，但镍相却受到的关注不足。在此，我们通过使用镍K边X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）分析，并辅以X射线衍射（XRD）和透射电子显微镜（TEM），对镍/ ZSM - 5材料中的镍进行了系统的结构研究，以解析由采用“绿色”、无模板技术制备的母体ZSM - 5不同铝含量所引起的镍物种局部环境变化。镍/ ZSM - 5中的镍物种以NiO晶体（3 - 50纳米）和作为电荷补偿的镍阳离子形式存在。镍K边XANES和EXAFS结果能够对含镍物种进行定量。在低铝硅比（/≤0.04）时，NiO纳米颗粒在样品中占主导，占镍相的65%以上。然而，在高铝硅比（/ = 0.05）的ZSM - 5中，由于铝赋予的骨架负电荷数量较多，附着在沸石骨架上的镍阳离子数量超过了NiO。所得结果表明，高可还原且活性的NiO晶体数量与ZSM - 5沸石中存在的骨架铝位点密切相关，而这在很大程度上取决于合成条件。因此，这类研究对于进一步研究镍/ ZSM - 5和其他金属改性双功能催化剂的催化活性是有益的。
