Hydrogenation of hydrogen peroxide over palladium/carbon in aqueous acidic medium containing different halide anions under static/flowing hydrogen

Abstract

Kinetics of the hydrogenation of hydrogen peroxide (at 278 and/or 300 K) over Pd(5 wt%)/carbon in aqueous medium with or without containing acid or different halide anions (F^-, Cl_-, Br^-, or I^-) under static H₂ (i.e. batchwise) or flowing H₂ (i.e. semi-batchwise) at atmospheric pressure have been thoroughly studied. In the absence of protons or halide anions and also in the presence of F^- anions, a very rapid decomposition of H₂O₂ (H₂O₂ → H₂O + 0.5O₂) predominates over the H₂O₂ hydrogenation (H₂O₂ + H₂ → 2H₂O). However, in the presence of both the protons and Cl^- or Br^- anions, the H₂O₂ conversion occurs mainly by the hydrogenation reaction. The hydrogenation is found to be a zero-order reaction with respect to the H₂O₂ concentration, and its activation energy and frequency factor are strongly influenced by the halide anions. The hydrogenation is found to be more and more inhibited with increasing halide (Cl^- or Br^-) anion concentration. The cations associated with the halide anions have, however, only a very little influence on the hydrogenation. Iodide anions act as a very strong catalyst poison for the hydrogenation. In the presence of acid, the hydrogenation is very strongly influenced by the different halides, but, in the presence of halide anions (5.4 mmol/dm³), it is very mildly influenced by the different mineral acids (except HI). In the absence of protons, the Cl^- and Br^- anions are ineffective for inhibiting the rapid H₂O₂ decomposition. A role played by protons and halide anions for drastically inhibiting the rapid H₂O₂ decomposition is discussed.