Operando Surface Studies on Metal-Oxide Interfaces of Bimetal and Mixed Catalysts

The formation of metal-oxide interfaces in catalytic systems exhibits a synergistic phenomenon between metal and reducible oxides, often referred to as the strong metal–support interaction (SMSI). This unusual characteristic has been connected to the origin of highly enhanced catalytic performance for decades, but the mechanistic explanation of SMSI remains a long-standing issue in heterogeneous catalysis. To understand this matter at the molecular level, geometric and electronic functions of metal-oxide interfaces during catalytic reactions should be clearly interpreted using advanced microscopic and spectroscopic analysis techniques. In this Review, we highlight recently performed investigations at metal-oxide interfaces by operando characterization tools to identify active sites in working conditions. We introduce two kinds of catalysts, platinum-based bimetallic alloys and mixed metal-oxide catalysts. Selected operando techniques reveal their atomic-scale morphology, surface electronic structure, and charge transfer/transport at surfaces under oxidation, reduction, and gas mixture environments. With bimetallic model catalysts, topographic morphology observations present critical evidence for the structural modulation between the topmost layer and the subsurface lattice in oxygen conditions. For the mixed metal-oxide catalysts, we note that metal nanoparticles on reducible oxides demonstrate the catalytic activity enhancement, which is obviously influenced by the change of oxidation states at the metal-oxide interface. Environmental transmission electron microscopy images unveil the atomic-scale redox behaviors at the nanoparticle interfaces with evolutions of the reducible oxide under the catalytic reaction. The important role of reactive interfaces between the transition metal atom and oxide-support explains the surface chemistry and heterogeneous catalysis over active sites on well-defined single crystal model surfaces, as well as nanoparticle catalysts. Overall, operando studies for metal-oxide interfaces can shed light on mechanistic insights into the tuning of catalytic activity at the molecular level and on improving catalytic performance by the SMSI effect.