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

Resumo

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg₂(OAc)₄(HOAc)₄ on α-Al₂O₃, γ-Al₂O₃, and MgAl₂O₄ has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg₂(OAc)₄(HOAc)₄ supported on γ-Al₂O₃ and MgAl₂O₄ 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-Al₂O₃ 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 PdAg₂/α-Al₂O₃ 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-Al₂O₃ and MgAl₂O₄ and in the monometallic reference sample Pd/γ-Al₂O₃.