Bimetallic Pd–Ag/Al<sub>2</sub>O<sub>3</sub> Catalysts with “Monoatomic” Pd<sub>1</sub> Sites in Liquid-Phase Hydrogenation of Substituted Alkynes
The catalytic characteristics of Pd–Ag/Al2O3 bimetallic catalysts with different atomic Ag/Pd ratios (0.03–3.0) were studied in the selective liquid-phase hydrogenation of diphenylacetylene and 1-phenyl-1-propyne. It was found that an increase in the Ag/Pd ratio leads to a significant increase in selectivity, which, however, is accompanied by a decrease in the activity of the catalysts. Comparison with the data of a previous structural study allows us to conclude that the observed increase in selectivity with an increase in the Ag/Pd ratio to 0.5–1.0 is associated with (1) a suppression of the formation of Pd hydride, and (2) an increase of “monatomic” Pd1 sites isolated from each other by Ag atoms on the surface of Pd–Ag nanoparticles. An increase in Ag/Pd ratio to 2.0–3.0 leads to a further increase in selectivity, which exceeds the selectivity of the commercial Lindlar catalyst (Pd–Pb/CaCO3). The most probable reason for the high selectivity in this case is the final formation of the structure of “monatomic” Pd1 sites on the surface of the Pd–Ag alloy and an increase in its stability.