Oxidative-Atmosphere-Induced Strong Metal-Support Interaction and Its Catalytic Application

ConspectusIn 1978, the classical strong metal-support interaction (C-SMSI) was first explored by observing significantly suppressed H₂ and CO adsorption on Group-VIII noble-metal-reducible oxide systems after high-temperature treatment. Subsequent studies showed that local electron redistribution and encapsulation overlayers on metal nanoparticles (NPs) are typical features of SMSI, which endows supported metal heterogeneous catalysts with various advantageous properties for catalytic applications. In recent decades, significant advancements have been made in the utilization of SMSI effects via oxidation, adsorbate mediation, wet-chemistry processes, and so on. Oxidative SMSI (O-SMSI) was first observed by Mou et al. for Au/ZnO, wherein encapsulation overlayers were formed on Au NPs after being treated under oxidative conditions. In this system, positively charged Au NPs are formed through electron transfer from the metal to the support, and Au-O-Zn linkages drive the formation of the encapsulation overlayer. O-SMSI and the behavior it imparts in catalyst systems contradict our previous understanding on C-SMSI with respect to the need for a reducing atmosphere and the known encapsulation driving force. Moreover, O-SMSI encapsulation overlayers show considerable stability in oxidizing atmospheres and provide a potential solution to the problem of high-temperature sintering of supported catalysts. To date, O-SMSI has been observed for catalyst systems with various supports, including metal oxides, phosphides, and nitrides, and provides application opportunities for supported metal catalysts in oxidative catalytic process.In this Account, we first briefly introduce the research background of O-SMSI and the motivation for developing new systems exhibiting this effect. In particular, the Au/hydroxyapatite (HAP, nonoxide) system with O-SMSI induced by applying high-temperature oxidation prevents the sintering of Au NPs. Furthermore, Pt and Pd catalysts exhibit O-SMSI with HAP and ZnO supports under oxidizing heat treatment. Based on the composition and structure of HAP, the tetrahedral units ((PO₄)^3-) and OH^- are shown to be responsible for O-SMSI. Importantly, the local electronic redistribution in the metal NPs (i.e., electron transfer from the metal to support), which is a characteristic feature of O-SMSI, can be controlled to tailor the strength of the metal-support interaction. We used exogenous adsorbents to tune the electronic state (Fermi level) of metal NPs to artificially introduce O-SMSI to Au, Pd, Pt, and Rh catalysts supported on TiO₂. Moreover, the findings of our study indicate that O-SMSI can be broadly applied to the development of heterogeneous catalysts. Finally, we summarize some common O-SMSI catalysts with different proposed mechanisms and provide insights into the existing challenges and possible research directions in the field.