Tailoring the synthesis of supported Pdcatalysts towards desired structure and size of metal particles

Abstract

In a systematic study, the influence of different preparation parameters on phase composition and size of metal crystallites and particles in Pd–Cu/TiO₂ and Pd–Sb/TiO₂ catalyst materials has been explored. Temperature and atmosphere of thermal pretreatment (pure He or 10% H₂/He), nature of metal precursors (chlorides, nitrates or acetates) as well as of ammonium additives (ammonium sulfate, nitrate, carbonate) and urea were varied with the aim of tailoring the synthesis procedure for the preferential formation of metal particles with similar size and structure as observed recently in active catalysts after long-term equilibration under catalytic reaction conditions in acetoxylation of toluene to benzylacetate. Among the metal precursors and additives, the chloride metal precursors and (NH₄)₂SO₄ were most suitable. Upon thermal pretreatment of Pd–Sb or Pd–Cu precursors, chloroamine complexes of Pd and Cu are formed, which decompose above 220 °C to metallic phases independent of the atmosphere. In He, metallic Pd particles were formed with both the co-components. In H₂/He flow, Pd–Cu precursors were converted to core–shell particles with a Cu shell and a Pd core, while Sb₁Pd₁ and Sb₇Pd₂₀ alloy phases were formed in the presence of Sb. Metal crystallites of about 40 nm agglomerate to particles of up to 150 nm in He and to even larger size in H₂/He.