The authors focused their efforts on a Pd2dba3-derived catalyst deposited on graphite and carbon-coated copper grids, as models for Pd/C in the Mizoroki–Heck and Suzuki–Miyaura reactions and in order to perform electron microscopy on the samples. They developed an area search protocol that combined scanning electron microscopy or transmission electron microscopy images from the millimetre to the nanometre scale with computational matching at different reaction times, to monitor the evolution of a group of palladium nanoparticles before and after reaction (pictured). While the nanoparticles seemed to be stable, they also tracked single palladium atoms, and found they had relocated closer to the nanoparticles. This caused the atoms to sinter and merge with the nanoparticles, increasing their size; the authors found this correlated with a decreased activity of the supported catalyst. Whilst such mobility can be thermodynamically explained, taking into consideration the bond strengths of Pd atoms with either the carbon support or larger palladium species, it also suggests that single palladium atoms could migrate into the solution and work as homogeneous catalytic species. This fact was corroborated by studying the contribution of leached atoms in the model reactions, where excess palladium ions containing single atoms were identified in solution via mass spectrometry techniques.
The experiments conducted with space and time monitoring showed that only the leached single atoms are involved in the catalysis, while most of the palladium in the nanoparticles is inactive, and that the surface reactions may not have such an important effect as previously thought. These findings on cross-coupling reactions widen our understanding of the potential involvement of single-atom catalysts in organic synthesis.
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