H2O2 synthesis over PdAu membranes

2–3 μm thick PdAu membranes have been prepared by sequential electroless plating on porous ceramic tubes followed by annealing at 823 K under H2. Gas phase H2 permeation rates had stabilized after 250 h annealing even though the alloy layers still remained inhomogeneous as evidenced by X-ray diffraction analyses after conclusion of all other experiments. The H2 permeability of a Pd90.5Au9.5 membrane reached 1.2 × 10−8 mol m−1 s−1 Pa−0.5 at 673 K. A marked discontinuity in the temperature dependence of the H2 permeation rate at 573 K signals a change from diffusion to adsorption or desorption-limited H2 transport through the membrane below that threshold. H2O2 was directly synthesized from the elements over these PdAu membranes in O2-saturated, acidic water/ethanol mixtures while feeding H2 through the membrane. The H2O2 formation rate increased from 19.5 to 97.5 mmol m−2 h−1 between 277 and 363 K in water at 300 kPa H2 feed pressure even though the H2O2 selectivity decreased from 56% to 30% in parallel. H2O2 yield and selectivity improved both with H2 feed pressure, however. H2O2 formation rate and selectivity increased also moderately with ethanol content, but the PdAu layer tended to delaminate from the ceramic support if the ethanol content was raised above 50%.