Probing Bimetallicity in Catalytic Systems: A Practical Tutorial on Advanced Analytical Tools

Bimetallic catalysts provide a versatile platform for tuning catalytic activity, selectivity, and stability through metal–metal interactions, electronic modification, geometric/ensemble effects, and dynamic restructuring under reaction conditions. However, establishing their working structure is challenging because the two metals may form alloys, solid solutions, intermetallic phases, Janus particles, core–shell structures, segregated domains, or partially oxidized surface species. This Tutorial Review introduces advanced analytical tools and complementary computational approaches for assessing bimetallicity, emphasizing the capabilities and limitations of each method. The discussion first outlines the structural complexity of bimetallic catalysts and the thermodynamic and kinetic factors that influence alloying, segregation, particle size, and surface composition, including the dynamic evolution of subnanometer-sized clusters. It then compares bulk, surface-sensitive, local-coordination, imaging, and adsorbate-probe methods, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), selected-area electron diffraction (SAED), infrared spectroscopy, chemisorption, temperature-programmed desorption/reduction (TPD/TPR), and in situ/operando approaches. Representative studies demonstrate why individual signatures, such as diffraction peak shifts, binding energy changes, lattice fringes, or CO adsorption bands, should not be interpreted in isolation. A dedicated Perspective distinguishes structural evidence of bimetallicity from demonstrations of catalytic synergism and discusses how controlled comparisons link working-state descriptors with kinetic and performance metrics, including turnover frequency, apparent activation energy, reaction order, selectivity, stability, and regenerability. Overall, this Tutorial Review provides a practical framework for selecting complementary characterization methods and building reliable structure-kinetics-performance relationships to guide bimetallic catalyst design.