Kinetics and mechanism of the chromium(vi) catalyzed decomposition of hypochlorous acid at elevated temperature and high ionic strength

Abstract

An important reaction step in the industrial production of NaClO (electrochemical chlorate process) is the thermal decomposition of HOCl/OCl to yield ClO and Cl. It is widely accepted that this reaction is accelerated by aqueous chromium(vi) species. A detailed kinetic study was conducted under industrially relevant conditions, i.e. at high ionic strength (6.0 M) and elevated temperature (80 °C), to investigate this phenomenon. The decomposition of hypochlorous acid was followed in the presence of Cr(vi) or phosphate (PO) or without any additive. In addition to the beneficial pH buffering effect of Cr(vi), the CrO form of chromium(vi) was found to slightly catalyze the decomposition of hypochlorous acid. The overall rate of HOCl decomposition can be expressed as -dc/dt = k[HOCl][OCl] + k[HOCl][CrO]. The corresponding rate constants were determined, k = 9.4 ± 0.1 M s and k = 4.6 ± 0.8 M s, and mechanistic interpretation of the catalytic rate law is given. The contribution of the catalytic path to the overall rate of decomposition changes from ca. 30% at pH = 8 to ca. 70% at pH = 6.

Suppressed oxygen evolution during chlorate formation from hypochlorite in the presence of chromium(VI)

“…Importantly, the decomposition rate no longer follows the third‐ order kinetic model, involving only hypochlorite species as reactants. Instead, in agreement with previous literature data, the hypochlorite decomposition rate can be described by the sum of an uncatalyzed third‐order reaction , and a parallel catalyzed reaction (Eqn (6), further detailed in the Supporting information in File S1). Using this model, the experimental data was fitted to give a partial order of α = 2, with respect to hypochlorite species, in the second term.…”

“…In the reaction scheme proposed earlier for the uncatalyzed case, the rate‐determining reaction (Eqn (7)) proceeds via a transition state complex, formed by a hypochlorite ion and a Cl 2 O · H 2 O molecule. In the reaction scheme suggested by Kalmár for the catalyzed case (Eqn (8)), chromate takes the place of the hypochlorite ion in the transition state. In a chromium(VI)‐containing electrolyte the uncatalyzed and catalyzed reactions proceed in parallel.…”

“…Recently, three papers reported on the rate enhancing effect of Na 2 Cr 2 O 7 addition on the homogeneous decomposition of hypochlorite . Regarding the kinetics of the decomposition, Spasojević found that the reaction order with respect to hypochlorite was 3, both in the absence and presence of chromium(VI).…”

“…Regarding the kinetics of the decomposition, Spasojević found that the reaction order with respect to hypochlorite was 3, both in the absence and presence of chromium(VI). Wanngård and Kalmár on the other hand found that in the presence of chromium(VI) the hypochlorite decomposition could be described by the sum of an uncatalyzed third‐order reaction, and a parallel catalyzed reaction of order ∼2 with respect to hypochlorite. In all studies, chromium(VI) species were included in the rate expressions.…”

“…Recently, three papers reported on the rate enhancing effect of Na 2 Cr 2 O 7 addition on the homogeneous decomposition of hypochlorite. [22][23][24] Regarding the kinetics of the decomposition, Spasojević 23 found that the reaction order with respect to hypochlorite was 3, both in the absence and presence of chromium(VI). Wanngård 22 and Kalmár 24 on the other hand found that in the presence of chromium(VI) the hypochlorite decomposition could be described by the sum of an uncatalyzed third-order reaction, and a parallel catalyzed reaction of order ∼2 with respect to hypochlorite.…”