Mutual catalyst inhibition in the chromium(VI)–copper(II)–hydrogen peroxide reacting system

Cr(VI) and Cu(II) are both efficient catalysts for the decomposition of H2O2. However, in their joint presence, the two catalysts exert a mutual inhibition effect on the catalytic activity of the other, and the study of these inhibition effects has provided important information on the mechanisms of the Cr(VI)/H2O2 and Cu(II)/H2O2 reactions. The inhibition effect produced by Cr(VI) on the catalytic activity of Cu(II) is the more pronounced (a [Cr(VI)]/[Cu(II)] ratio as low as 2×10-4 is enough to provoke a 10% decrease in the decomposition rate), the limit being around 90% inhibition. On the contrary, higher concentrations of Cu(II) are necessary to produce a noticeable inhibition on the catalytic effect of Cr(VI), the limit being 50% inhibition. Besides, Cu(II) prevents the reduction of Cr(VI) to Cr(III) by H2O2, so that when [Cu(II)] is high enough the catalyst Cr(VI) is recovered unaltered at the end of the reaction. The inhibition effect produced by Cu(II) on the Cr(VI)/H2O2 reaction can be ascribed to its ability to catalyze the dismutation of superoxide free radicals, thus preventing the reduction of Cr(V) to Cr(IV) provoked by them, whereas the inhibition effect produced by Cr(VI) on the Cu(II)/H2O2 reaction can be ascribed to its ability to act as an oxidative scavenger for Cu(I), thus preventing its participation as the initiator of a free-radical chain mechanism leading to the decomposition of H2O2.