Niedermaier Matthias, Schwab Thomas, Kube Pierre, Zickler Gregor A, Trunschke Annette, Diwald Oliver
Department of Chemistry and Physics of Materials, Paris-Lodron University Salzburg, Jakob-Haringer-Strasse 2a, A-5020 Salzburg, Austria.
Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany.
J Chem Phys. 2020 Feb 21;152(7):074713. doi: 10.1063/1.5138894.
Microstructure, structure, and compositional homogeneity of metal oxide nanoparticles can change dramatically during catalysis. Considering the different stabilities of cobalt and iron ions in the MgO host lattice [M. Niedermaier et al., J. Phys. Chem. C 123, 25991 (2019)], we employed MgO nanocube powders with or without transition metal admixtures for the oxidative coupling of methane (OCM) reaction to analyze characteristic differences in catalytic activity and sintering behavior. Undoped MgO nanocrystals exhibit the highest C selectivity and retain the nanocrystallinity of the starting material after 24 h time on stream. For the Co-Mg-O nanoparticle powder, which exhibits the highest activity and CO selectivity and where OCM-induced coarsening is strongest, we found that the Co ions remain homogeneously distributed over the MgO lattice. Trivalent Fe ions migrate to the surface of Fe-Mg-O nanoparticles where they form a magnesioferrite phase (MgFeO) with a characteristic impact on catalytic performance: Fe-Mg-O is initially less selective than MgO despite its lower activity. An increase in C selectivity and a decrease in the CO/CO ratio with time on stream are attributed to the increasing fraction of coarsened particles that become depleted in redox active Fe. Surface water is a by-product of the OCM reaction, favors mass transport across the particle surfaces, and serves as a sintering aid during catalysis. The characteristic changes in size and morphology of MgO, Co-doped, and Fe-doped MgO particles can be consistently explained by activity and C selectivity trends. The original morphology of the nanocubes as a starting material for the OCM reaction does not impact the catalytic activity.
金属氧化物纳米颗粒的微观结构、结构和成分均匀性在催化过程中可能会发生显著变化。考虑到钴和铁离子在MgO主体晶格中的不同稳定性[M. Niedermaier等人，《物理化学杂志C》123, 25991 (2019)]，我们使用了添加或未添加过渡金属的MgO纳米立方粉末进行甲烷氧化偶联(OCM)反应，以分析催化活性和烧结行为的特征差异。未掺杂的MgO纳米晶体表现出最高的C选择性，并且在24小时的反应时间后仍保持起始材料的纳米晶性。对于表现出最高活性和CO选择性且OCM诱导的粗化最强的Co-Mg-O纳米颗粒粉末，我们发现Co离子在MgO晶格上保持均匀分布。三价Fe离子迁移到Fe-Mg-O纳米颗粒的表面，在那里它们形成了镁铁氧体相(MgFeO)，这对催化性能有显著影响：尽管Fe-Mg-O的活性较低，但最初它的选择性低于MgO。随着反应时间的增加，C选择性增加，CO/CO比降低，这归因于粗化颗粒比例的增加，这些颗粒在氧化还原活性Fe中逐渐耗尽。表面水是OCM反应的副产物，有利于颗粒表面的质量传输，并在催化过程中作为烧结助剂。MgO、Co掺杂和Fe掺杂的MgO颗粒在尺寸和形态上的特征变化可以通过活性和C选择性趋势得到一致的解释。作为OCM反应起始材料的纳米立方体的原始形态不会影响催化活性。
