Atom-precise silver–palladium bimetallic clusters on carbon supports as selective hydrogenation catalysts

Abstract

Bimetallic atom-precise Ag₂₄Pd₁(SR)₁₈²⁻ clusters on carbon supports were prepared, characterized, and evaluated as heterogeneous catalysts for hydrogenation reactions. Element-specific X-ray spectroscopy data suggests that the single Pd atom resides in the thiolate environment in the cluster structure and thus is accessible for hydrogenation catalysis. XAS data indicated that bimetallic Ag₂₄Pd₁(SR)₁₈²⁻ clusters exhibit better thermal stability than homonuclear Ag₂₅(SR)₁₈⁻ clusters on carbon supports, and significant Pd–S interactions still exist after calcination at 250 °C. Both Pd K-edge and that Pd L₃-edge measurements indicated that after calcination at 450 °C the resulting Ag₂₄Pd₁ clusters consist mostly of zerovalent Ag and Pd, with a trace amount of PdO_x species. Carbon-supported Ag₂₄Pd₁(SR)₁₈²⁻ bimetallic cluster catalysts showed improved catalytic activity compared to homonuclear Ag₂₅(SR)₁₈⁻ cluster catalysts in the hydrogenation reactions of 2-methyl-butyn-1-ol (MBY) and 3-hexyn-1-ol (HY) as model alkynol substrates. The Ag₂₄Pd₁(SR)₁₈²⁻ cluster catalysts also showed excellent chemoselectivity for selective hydrogenations to alkenols and stereoselectivity towards the Z-isomer of 3-hexen-1-ol (Z-HE). The presence of residual thiolate ligands on activated Ag₂₄Pd₁ clusters led to higher selectivity towards semi-hydrogenation products.