Enhancement of catalytic activity of Pd-PVP colloid for direct H 2 O 2 synthesis from H 2 and O 2 in water with addition of 0.5 atom% Pt or Ir

Catalytic activity of Pd-polyvinylpyrrolidone (PVP) colloid for the reaction of H 2 and O 2 to H 2 O 2 in the presence of H 2 SO 4 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 H 2 O 2 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 H 2 -activating abilities of Pt and Ir increased the H 2 –O 2 reaction rate, which was supported by DFT study. In contrast, H 2 O 2 hydrogenation rates were almost constant irrespective of Pt or Ir addition, indicating little participation of the hydrogen activated on Pt or Ir in H 2 O 2 destruction. Pt-modified Pd-PVP showed a discontinuous change in catalytic activity in the time dependency measurement, resulting in higher H 2 –O 2 reaction rates and lower H 2 O 2 selectivity. Lower partial pressure of hydrogen under the reaction conditions or higher temperatures in the absence of H 2 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 H 2 O 2 produced reaches several wt% when estimated using the Pt- and Ir-loaded Pd-PVP catalysts under an O 2 partial pressure of 360 kPa.