Enhancement of catalytic activity of Pd-PVP colloid for direct H2O2 synthesis from H2 and O2 in water with addition of 0.5 atom% Pt or Ir

Abstract

Catalytic activity of Pd-polyvinylpyrrolidone (PVP) colloid for the reaction of H₂ and O₂ to H₂O₂ in the presence of H₂SO₄ and NaBr was greatly enhanced with the addition of a very small quantity of Pt or Ir. In contrast to ordinary alloy catalysts, the activity of Pd was doubly increased with the addition of only 0.5 atom% Pt or Ir, although the selectivity for the H₂O₂ formation somewhat decreased. Addition of Ru, Rh, or Au affected little the activity. Kinetic analyses of the reactions over modified Pd-PVP indicated that the high H₂-activating abilities of Pt and Ir increased the H₂–O₂ reaction rate, which was supported by DFT study. In contrast, H₂O₂ hydrogenation rates were almost constant irrespective of Pt or Ir addition, indicating little participation of the hydrogen activated on Pt or Ir in H₂O₂ destruction. Pt-modified Pd-PVP showed a discontinuous change in catalytic activity in the time dependency measurement, resulting in higher H₂–O₂ reaction rates and lower H₂O₂ selectivity. Lower partial pressure of hydrogen under the reaction conditions or higher temperatures in the absence of H₂ accelerated the change. Structures of the Pt- and Ir-loaded Pd-PVP catalysts were suggested to change during the reaction; the Pt or Ir atoms would initially be located at coordinatively more unsaturated sites of the Pd particles and move into the surface Pd lattice to be stabilized. It is shown that the amount of H₂O₂ produced reaches several wt% when estimated using the Pt- and Ir-loaded Pd-PVP catalysts under an O₂ partial pressure of 360 kPa.