FULL PAPER www.afm-journal.de © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1903242 (1 of 10) Real-Time Imaging of Nanoscale Redox Reactions over Bimetallic Nanoparticles Shu Fen Tan, See Wee Chee, Zhaslan Baraissov, Hongmei Jin, Teck Leong Tan, and Utkur Mirsaidov* The catalytic performance of bimetallic nanoparticles (NPs) strongly depends on their structural and compositional changes under reaction conditions. At the fundamental level, these changes are driven by redox reactions that occur on the surface of the NPs. The degree of complexity in the redox reactions is further amplified in bimetallic NPs because both metals can have their own reactions with the reactant molecules, in addition to any synergistic effects between the metal nanocatalysts and their reducible oxides. Here, the gas phase oxidation and reduction reactions, and the oxidation of carbon mono- xide (CO) over Pt–Ni rhombic dodecahedron NPs with segregated Pt frames and Pt–Ni alloy NPs are investigated using in situ gas cell transmission electron microscopy. The real-time observations show that NiO shell forma- tion and Pt segregation are two important features during the oxidation and reduction of Pt–Ni NPs, respectively. Moreover, the two types of NPs evolved in different ways. By combining high-resolution imaging, mass spectroscopy, and modeling, it is shown that the evolution of NP morphology and composi- tion during redox reactions plays an important role in controlling the catalytic activity of the NPs. DOI: 10.1002/adfm.201903242 Dr. S. F. Tan, Dr. S. W. Chee, Z. Baraissov, Prof. U. Mirsaidov Department of Physics National University of Singapore Singapore 117551, Singapore E-mail: mirsaidov@nus.edu.sg Dr. S. F. Tan, Dr. S. W. Chee, Z. Baraissov, Prof. U. Mirsaidov Centre for BioImaging Sciences Department of Biological Sciences National University of Singapore Singapore 117557, Singapore Dr. H. Jin, Dr. T. L. Tan Institute of High Performance Computing Agency for Science Technology and Research Singapore 138632, Singapore Prof. U. Mirsaidov Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore Singapore 117546, Singapore Prof. U. Mirsaidov Department of Materials Science and Engineering National University of Singapore Singapore 117575, Singapore The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201903242. 1. Introduction The key challenge limiting our ability to design robust bimetallic catalysts is the lack of understanding of how these complex nanostructures evolve during catalyst activation and during the cata- lytic process itself. Oxidation and reduc- tion reactions are the simplest and most frequently applied pretreatment processes for heterogeneous nanocatalysts. [1,2] These pretreatments can significantly alter the catalytic properties, such as chemical sta- bility and reactivity of the monometallic heterogeneous nanocatalysts through the interaction of oxidizing (O 2 ) and/or reducing species (H 2 or carbon monoxide (CO)) with the metal surfaces. [3,4] The case of bimetallic nanocatalysts is even more complicated than monometallic ones because both metals can have their own redox reactions, in addition to any syner- gistic effect between the metals and their reducible oxides. [5–7] To date, many syn- thetic methods have been employed to synthesize these bime- tallic nanoparticles (NPs), with a focus on improving their activity and stability under reaction conditions in different liquid or gas environments. [8–10] However, it is not clear if the structural changes during subsequent redox treatments are dictated by the starting NP architecture and how these changes impact the catalytic performance of these NPs. At the fundamental level, redox reactions can change both the morphology and composition of NPs. An array of interme- diate oxidation and reduction reactions can take place on the metal NPs and their supports as part of the catalytic process. [3,11] The oxidation state of the metal NPs may change during redox reactions, thereby leading to the dynamic transformation of a NP’s shape and composition under reaction conditions. [11,12] The complexity in structural changes will also be reflected in their associated catalytic behavior during each catalytic cycle. [12] Currently, the primary methods for probing these catalytic processes involve the combination of indirect spectroscopic techniques such as vibrational and electron emission spectros- copies, together with customized reactions cells that allow the specimens to be studied under reactive environments. [11,13] This approach is broadly known as operando spectroscopy. However, these investigations are mostly derived from bulk catalytic surfaces [1,5,7] or ensembles of catalytic NPs, [14,15] and so, their Catalysis Adv. Funct. Mater. 2019, 1903242