Study of size effect of palladium atoms as single-atom and clusters nanoparticle catalysts for Heck reactions

Abstract

We reported a series of nitrogen-doped carbon-supported palladium (Pd) species with various sizes for the Heck reaction of iodobenzene and methyl acrylate. By precisely controlling the synthesis procedure, we have successfully synthesized a range of Pd-based catalysts, including single Pd atom, sub-nanometer Pd clusters (∼ 0.4 nm, and ∼1.6 nm), and Pd nanoparticles (∼5.8 nm and 10 nm). Remarkably, these catalysts exhibit a distinct size-dependent catalytic activity in the Heck reaction. In the case of the Heck reaction of 4‑methoxy-iodobenzene and methyl acrylate, the Pd-10 site demonstrated the highest site-specific turnover frequency (TOF), reaching 792.5 h−1. This performance is impressively 5.2 and 15.6 times superior to that of the Pd1 site (152 h-1) and Pd-10 site (50.6 h-1), respectively. Significantly better in the Heck reactions with other aryl halides were achieved with Pd-1.6/N-C (comprising of approximately 1.6 nm-sized Pd clusters), compared to those containing 10 nm-sized nanoparticles (Pd-10/N-C) or Pd1/N-C. These findings suggested that Pd clusters of around 1.6 nm may serve as the potential catalytic sites for the Heck reaction.