Nanoparticle catalysts containing two types of metals play an important role in petrochemical reforming, hydrogenation, and other industrial processes. Researchers have long known that the combination of metals leads to enhanced catalytic activity compared with the corresponding monometallic catalysts. Knowing the molecular level details that underpin the bimetallic enhancement could lead to even better catalysts—ones that use less energy and generate fewer by-products. But because of a lack of nanoparticle uniformity and difficulty tracking reactions on the nanometer scale, those details have remained hidden. Now they’re being exposed. Peng Chen of Cornell University and coworkers prepared palladium nanorods tipped with a gold sphere and used the catalysts to mediate a model photocatalytic reaction—reduction of nonfluorescent resazurin to fluorescent resorufin. By scrutinizing the system with electron microscopy and a fluorescence microscopy method that has single-molecule resolution, the team tracked thousands of reactions individually. In that way, they pinpointed with nanometer resolution, where on more than 50 particles, a reaction occurred (ACS Cent. Sci. 2017, DOI: 10.1021/acscentsci.7b00377). The data show that the nanoscale interface of the two metals is the catalytic hot spot: It’s about 50% more active than single-metal regions. Also, the most active facets of the monometallic catalysts are the best spots to form bimetallic interfaces.