Subhash D. Sansare

Nonhazardous Direct Oxidation of Hydrogen to Hydrogen Peroxide Using a Novel Membrane Catalyst

Nonhazardous and highly selective H2O2production! An H 2 -permselective membrane (2) has been developed which consists of thin films of Pd–Ag alloy (2b) and oxidized Pd (2c), and a hydrophobic polymer membrane (2d) supported on a tubular membrane (2a) within a reactor wall (1). Hydrogen atoms permeate through the membrane and react with molecular OH 2 in a liquid medium (LM; 2 M HH 2 SOH 4 ).

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H2 to H2O2 (at 295 K) in acidic medium (0.02 M H2SO4) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H2O2, N2O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H2-to-H2O2 conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO2 < Pd/ThO2 < Pd/Ga2O3. The H2 oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H2O2 decomposition activity of the catalysts. There exists a close relationship between the H2-to-H2O2 conversion activity and/or H2O2 selectivity in the oxidation process and the H2O2 decomposition activity of the catalysts; the higher the H2O2 decomposition activity, the lower the H2-to-H2O2 conversion activity and/or H2O2 selectivity.

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H2O2 selectivity (or yield) in direct oxidation of H2-to-H2O2 (using 1.7 mol% H2 in O2 as a feed) and also the H2O2 decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H-β) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H2SO4, HCl, HNO3, H3PO4 and HClO4), and also by the form of palladium (Pd0 or PdO). The oxidized (PdO-containing) catalysts are active for the H2-to-H2O2 conversion and show very poor activity for the H2O2 decomposition. However, the reduced (Pd0-containing) catalysts show higher H2 conversion activity but with no selectivity for H2O2, and also show much higher H2O2 decomposition activity. No direct correlation is observed between the H2-to-H2O2 conversion activity (or H2O2 selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H2O2 yield and lower H2O2 decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.