Chemical Activation of Atom-Precise Pd3 Nanoclusters on γ-Al2O3 Supports for Transfer Hydrogenation Reactions

Abstract

Deposition of atom-precise nanoclusters onto solid supports is a promising route to synthesize model heterogeneous catalysts. However, to enhance nanocluster-support interactions, activation of the nanoclusters by removal of surface ligands is necessary. Thermal treatment to remove surface ligands from supported metal nanoclusters can yield highly active heterogeneous catalysts, however, the high temperatures employed can lead to poor control over the final size and speciation of the nanoclusters. As an alternative to high-temperature thermal treatments, chemical activation of [Pd₃(μ-Cl)(μ-PPh₂)₂(PPh₃)₃]⁺ (Pd₃) nanoclusters on γ-Al₂O₃ supports under mild reaction conditions has been demonstrated in this work. Hydride-based reducing agents such as NaBH₄, LiBH₄, and LiAlH₄ have been examined for the activation of the Pd₃ nanoclusters. The structural evolution and speciation of the nanoclusters after activation have been monitored using a combination of XAS, XPS, STEM-EDX mapping, and solid-state NMR techniques. The results indicate that treatment with borohydride reducing agents successfully removed surface phosphine and chloride ligands, and that the extent of size growth of the nanoclusters during activation is directly correlated with the amount of borohydride used. Borate side products remain on the γ-Al₂O₃ surface after activation; moreover, exposure to high amounts of NaBH₄ resulted in the incorporation of B atoms inside the lattice of the activated Pd nanoclusters. LiAlH₄ treatment, on the other hand, led to no significant size growth of the nanoclusters and resulted in a mixture of Pd single-atom sites and activated nanoclusters on the γ-Al₂O₃ surface. Finally, the catalytic potential of the activated nanoclusters has been tested in the transfer hydrogenation of trans-cinnamaldehyde, using sodium formate/formic acid as the hydrogen donor. The catalytic results showed that smaller Pd nanoclusters are much more selective for hydrogenating trans-cinnamaldehyde to hydrocinnamaldehyde, but overall have lower activity compared to larger Pd nanoparticles. Overall, this study showcases chemical activation routes as a viable alternative to traditional thermal activation routes for activating supported nanoclusters by offering improved speciation and size control.