(651d) Highly Dispersed Bimetallic Catalysts By Selective Adsorption Of Metal Complex On Mixed Metal Oxides

The Strong Electrostatic Adsorption (SEA) technique has changed the black art of catalyst synthesis into engineering protocols. Numerous publications have been produced in the past two decades. In past years, SEA has been successfully applied to various metals such as Pt, Pd, Ru, Rh, Co, and Cu on a variety of supports such as SiO2, Al2O3, TiO2, Nb2O5 and carbons. This paper presents a two step process to form bimetallic catalysts containing Co as the base metal and either Pt or Pd as the noble metal. [Co(NH3)6]Cl3, H2PtCl6, and H2PdCl4 were employed as the metal precursors. The adsorption behavior of [Co(NH3)6]Cl3 on SiO2 and carbon supports have been extensively studied. The pH of the solution, the kind of alkali and the pretreatment conditions are found to be the key points of obtaining higher dispersions. [Co(NH3)6]Cl3 was found to undergo reductive deammination in the presence of NaOH forming Co3O4 instantaneously on carbon, leading to a novel, one pot synthesis of Co3O4 nanoclusters. In the presence of NH4OH it isomerizes to [Co(NH3)5Cl]Cl2. The Co/C system has been compared with that of Co/SiO2 and found that they behave very differently even though their point of zero charge is almost the same (~4.0). The semiquinone, the pyridine-N-oxide, and the π-electrons on the graphite surface might result in the reductive deammination of [Co(NH3)6]Cl3 leading to the Co3O4 nanoclusters, but the hydroxyl groups on the SiO2 surface enhance the thermal stability of the absorbed [Co(NH3)6]Cl3 species. Bimetallic catalysts were synthesized by directing noble metal complex ions on preformed Co3O4 particles by selective adsorption followed by pretreatment. This synthetic procedure has a direct impact over the tedious procedure to synthesize bimetallic catalysts. All the systems have been characterized by using EXAFS, STEM and powder XRD techniques.