Nanometallurgy in solution: organometallic synthesis of intermetallic Pd–Ga colloids and their activity in semi-hydrogenation catalysis

Nanoparticles (NPs) of Pd_1--xGa_x (x = 0.67, 0.5, 0.33), stabilized in non-aqueous colloidal solution, were obtained via an organometallic approach under mild conditions using [Pd₂(dvds)₃] and GaCp* as all-hydrocarbon ligated metal-precursor compounds (dvds = 1,1,3,3-tetramethyl-1,3-divinyl-disiloxane; Cp* = η⁵-C₅Me₅; Me = CH₃). The reaction of the two precursors involves the formation of a library of molecular clusters [Pd_nGa_mCp*_y(dvds)_z], as shown by liquid injection field desorption ionization mass spectrometry (LIFDI-MS). Full characterization of the catalytic system (HR-TEM, EDX, DLS, PXRD, XPS, NMR, IR, Raman) confirmed the formation of ultra-small, spherical NPs with narrow size distributions ranging from 1.2 ± 0.2 nm to 2.1 ± 0.4 nm (depending on the Pd : Ga ratio). The catalytic performance of the Pd_1--xGa_x NPs in the semi-hydrogenation of terminal and internal alkynes and the influence of the gallium content on product selectivity were investigated. The highest activities (65%) and selectivities (81%) are achieved using colloids with a "stoichiometric" Pd/Ga ratio of 1 : 1 at 0 °C and 2.0 bar H₂ pressure. While lower Ga ratios lead to an increase in activity, higher Ga contents increase the olefin selectivity but are detrimental to the activity.