Formation of Pd–Ag nanoparticles in supported catalysts based on the heterobimetallic complex PdAg2(OAc)4(HOAc)4

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg 2 (OAc) 4 (HOAc) 4 on α-Al 2 O 3 , γ-Al 2 O 3 , and MgAl 2 O 4 has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg 2 (OAc) 4 (HOAc) 4 supported on γ-Al 2 O 3 and MgAl 2 O 4 takes place in two steps (at 15–245 and 290–550°C) and yields Pd–Ag particles whose average size is 6–7 nm. The reduction of the Pd–Ag catalyst supported on α-Al 2 O 3 occurs in a much narrower temperature range (15–200°C) and yields larger nanoparticles (~10–20 nm). The formation of Pd–Ag alloy nanoparticles in all of the samples is demonstrated by IR spectroscopy of adsorbed CO, which indicates a marked weakening of the absorption band of the bridged form of adsorbed carbon monoxide and a >30-cm –1 bathochromic shift of the linear adsorbed CO band. IR spectroscopic data for PdAg 2 /α-Al 2 O 3 suggest that Pd in this sample occurs as isolated atoms on the surface of bimetallic nanoparticles, as is indicated by the almost complete absence of bridged adsorbed CO bands and by a significant weakening of the Pd–CO bond relative to the same bond in the bimetallic samples based on γ-Al 2 O 3 and MgAl 2 O 4 and in the monometallic reference sample Pd/γ-Al 2 O 3 .