H2O2 synthesis over PdAu membranes

2-3 mu m thick PdAu membranes have been prepared by sequential electroless plating on porous ceramic tubes followed by annealing at 823 K under H-2. Gas phase H-2 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 H-2 permeability of a Pd90.5Au9.5 membrane reached 1.2 x 10(-8) mol m(-1) s(-1) Pa-0.5 at 673K. A marked discontinuity in the temperature dependence of the H-2 permeation rate at 573 K signals a change from diffusion to adsorption or desorption-limited H-2 transport through the membrane below that threshold. H2O2 was directly synthesized from the elements over these PdAu membranes in O-2-saturated, acidic water/ethanol mixtures while feeding H-2 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 H-2 feed pressure even though the H2O2 selectivity decreased from 56% to 30% in parallel. H2O2 yield and selectivity improved both with H-2 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%. (C) 2010 Elsevier B.V. All rights reserved.