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/Al<sub>2</sub>O<sub>3</sub> 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” Pd<sub>1</sub> 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/CaCO<sub>3</sub>). The most probable reason for the high selectivity in this case is the final formation of the structure of “monatomic” Pd<sub>1</sub> sites on the surface of the Pd–Ag alloy and an increase in its stability.