2.2. Catalyst Preparation

Both Pd₁Ag₃/Al₂O₃ and Pd/Al₂O₃ catalysts were synthesized using the incipient-wetness co-impregnation.

Scheme 1 summarizes the catalyst preparation steps. For Pd₁Ag₃/Al₂O₃ preparation alumina powder pre-calcined in dry-air at 500 °C for 4 h was impregnated with the aqueous solution containing both Pd(NO₃)₂ and Ag(NO₃)₂ which was acidified by diluted HNO₃ to pH of 2.9 to avoid precipitation of Pd or Ag hydroxides. The impregnated material was dried overnight at room temperature, then calcined in dry air at 500 °C for 4 h followed by reduction reduced in 5%H₂/Ar at 550 °C for 3 h. The material was cooled down in 5%H₂/Ar to 200 °C, and after that the gas was switched to N₂ (99.999%) and the sample was cooled down to room temperature. The selection of reduction temperature was based on our previously reported data [10]. It was shown that this temperature is sufficient for the reduction of metallic components providing a required degree of Pd and Ag atoms mobility and formation of uniformly distributed Pd–Ag nanoparticles.

Preparation of Pd₁Ag₃/Al₂O₃ catalyst via impregnation.

The weight loading of both metallic components on the supported catalysts were determined via inductively coupled plasma atomic emission spectrometry (ICP-AES). The data obtained showed the metal content of 2.1 wt.% Pd and 5.99 wt.% Ag.

As a reference 0.5 wt. % Pd/Al₂O₃ catalyst prepared in a similar way was used. To ensure a similar size of metal particles in both Pd₁Ag₃/Al₂O₃ and Pd/Al₂O₃ catalysts, the monometallic Pd sample was reduced in 5%H₂/Ar at 700 °C for 3 h. The same preparation technique was used for synthesis of Ag/Al₂O₃ catalyst used for XPS study. Pd₁Ag₃/Al₂O₃, Pd/Al₂O₃ and Ag/Al₂O₃ catalysts were designated as Pd₁Ag₃, Pd, and Ag respectively.