Silver palladium bimetallic core-shell structure catalyst supported on TiO2 for toluene oxidation

We show that silver palladium bimetallic core-shell structure catalysts supported on TiO2 substrate (Ag@Pd/TiO2) are highly active with a low palladium loading for toluene purification in oxidation reaction. The Ag@Pd catalysts were synthesized by galvanic replacement of Pd coating on Ag nanoparticles. Transmission electron microscopy (TEM), high-resolution TEM, and scanning TEM with energy dispersive spectrometer confirmed that the catalysts have a core-shell configuration, and are spherical in shape with an average diameter size of about 9.0 nm. The UV-vis absorption spectroscopy and selected area electron diffraction further proved that the intimate contact and different electronegativity of Ag and Pd in core-shell structure result in electrons transferring from Ag core to Pd shell, which increases local electron density around Pd atom. In addition, X-ray photoelectron spectroscopy reveals that such an increase of electron density on Pd active site in Ag@Pd/TiO2 catalyst can substantially strengthen the chemisorption with reactant of O-2, further promote the phase transformation from Pd-0 metal to PdO oxidation state, and finally maintain the normal operation of Pd-0-PdO synergic mechanism during toluene oxidation reaction. Hence, compared to monometallic Pd/TiO2 catalyst, the as-prepared bimetallic core-shell Ag@Pd/TiO2 catalyst exhibits greatly higher toluene oxidation activity as a whole although its palladium loading value is just one third of Pd/TiO2. The high catalytic oxidation activity with a low Pd loading makes the Ag@Pd/TiO2 a promising catalyst for use in VOCs purification.