Following the temperature-induced activation of carbon-supported trigonal Pd3 nanoclusters for catalysis

Abstract

The thermal activation of nanoclusters on a support material can enhance their activity and selectivity in heterogeneous catalysis. However, their evolution upon thermal activation remains challenging to study due to their small size. Herein, we probe the speciation and structural evolution of trigonal [Pd₃(μ-Cl)(μ-PPh₂)₂(PPh₃)₃]Cl nanoclusters on carbon supports upon thermal activation at different temperatures via the combination of in situ differential Pair Distribution Function (dPDF) analyses, Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), and X-ray Absorption Spectroscopy (XAS). EXAFS and dPDF measurements show that upon activation at 150 °C, the phosphine ligands were removed from the nanocluster surface. Upon further heating the nanoclusters, a transformation from Pd nanoclusters towards Pd nanoparticles occurred, as evidenced by an increase in the Pd–Pd coordination number. XPS, XANES, and EXAFS measurements also show the formation of PdO starting at 250 °C. The speciation and structural evolution of Pd nanoclusters during the thermal treatment has direct effects on the catalytic potential of the nanoclusters in terms of activity and selectivity. Nanoclusters activated at 150 °C (with the smallest Pd–Pd contribution and no phosphines present) were found to be extremely selective for the partial hydrogenation of 3-hexyn-1-ol to trans-3-hexen-1-ol. In Suzuki–Miyaura cross-coupling reactions, the Pd nanoclusters activated at 150 °C were the most active catalytic system. Three-phase studies suggests that the presence of surface ligands on the surface of nanoclusters reduces the strong-metal surface interaction between metal and support, which causes excessive leaching of Pd in reaction solvent during the reaction.