Synthesis of bimetallic Pd⿿Ag colloids in CO2-expanded hexane and their application in partial hydrogenation of phenylacetylene

Bimetallic Pd⿿Ag colloids were successfully synthesized in CO2-expanded hexane by using their isostearate salts, Pd(C18H35O2)2 and Ag(C18H35O2). The addition of CO2 could help reduce the particle size by enhancing the nucleation rate of the metal ions. Analyses from X-ray diffraction and UV⿿vis spectroscopy provided the evidences that the bimetallic colloids possessed certain degree of alloying which were different from physical mixtures. The bimetallic Pd⿿Ag colloids were further applied as suitable partial hydrogenation (also known as semihydrogenation) catalysts for phenylacetylene. During total conversion, the selectivity to styrene was nearly twice as much when using the Pd25Ag75 colloids (87.5%) than the pure Pd colloids (46.1%). Moreover, a 20% of CO2 partial pressure ratio could further increase the selectivity to 91.8%. These suggest that CO2 could serve as a versatile promoter in both nanoparticle synthesis and partial hydrogenation.