Support effect on the reactivity and stability of Au₂₅(SR)₁₈ and Au₁₄₄(SR)₆₀ nanoclusters in liquid phase cyclohexane oxidation

Abstract

Using oxide supported atomically precise gold nanoclusters is an emerging field in heterogeneous catalysis. Such well-defined nanocatalysts represent a rather new model system enabling fundamental insights in catalytic reactions. In the present work, the stability of Au₂₅ and Au144 clusters, supported either on TiO₂ or SiO₂, was examined upon thermal air pretreatment and, for the first time, upon liquid phase oxidation reaction. A pronounced influence of the support (TiO₂ vs. SiO₂) and cluster size (Au₂₅ vs. Au₁₄₄) was revealed by XAS, DRS and STEM. Upon pretreatment, Au₁₄₄ was more stable which may be related to its specific cluster core structure and staple configuration. The catalytic properties in liquid phase cyclohexane oxidation were clearly size dependent, with Au144 yielding higher TOF values, particularly in the case of SiO₂ supported catalysts. However, with respect to selectivity, TiO₂ supported catalysts led to higher KA production than SiO₂ supported ones. This can be explained by the different reaction pathways, as observed by in situ ATR. HERFD-XAS measurements of Au₁₄₄/TiO₂ catalysts revealed a pronounced cluster structure modification towards bulk gold during the reaction, in contrast to a high stability of Au₁₄₄/SiO₂. This study demonstrates the important role the support material has on the reactivity and stability of gold nanoclusters, which is key for their catalytic function.