Fe-Mo alloy coatings as cathodes in chlorate production process

Abstract

The aim of this study was to gain a better understanding of the feasibility of partial replacement of dichromate, Cr(VI), with phosphate buffer, focusing on the cathode reaction selectivity for hydrogen evolution on mild steel and Fe-Mo cathodes in undivided cell for chlorate production. To evaluate the ability of phosphate and Cr(VI) additions to hinder hypochlorite and chlorate reduction, overall current efficiency (CE) measurements in laboratory cell for chlorate production on stationary electrodes were performed. The concentration of hypochlorite was determined by a conventional potentiometric titration method using 0.01 mol dm -3 As 2 O 3 solution as a titrant. The chlorate concentration was determined by excess of 1.0 mol dm -3 As 2 O 3 solution and excess of arsenic oxide was titrated with 0.1 mol dm -3 KBrO 3 solution in a strong acidic solution. Cathodic hypochlorite and chlorate reduction were suppressed efficiently by addition of 3 g dm -3 dichromate at both cathodes, except that Fe-Mo cathode exhibited higher catalytic activity for hydrogen evolution reaction (HER). The overvoltage for the HER was around 0.17 V lower on Fe-Mo cathode than on mild steel at the current density of 3 kA m -2 . It was found that a dichromate content as low as 0.1 g dm -3 is sufficient for complete suppression of cathodic hypochlorite and chlorate reduction onto Fe-Mo catalyst in phosphate buffering system (3 g dm -3 Na 2 HPO 4 + NaH 2 PO 4 ). The overall current efficiency was practically the same as in the case of the presence of 3 g dm -3 dichromate buffer (98%). However, for the mild steel cathode, the overall current efficiency for the chlorate production was somewhat lower in the above mentioned mixed phosphate + dichromate buffering system (95%) than in the pure dichromate buffering solution (97.5%).

A review of chromium(VI) use in chlorate electrolysis: Functions, challenges and suggested alternatives

“…containing chromium (Cr-MoO 2 ) electrodes were tested as cathodes for the electrochemical reduction of hypochlorite in neutral and slightly alkaline solutions, respectively [93,94]. In both cases, an increased activity towards the HER was found with the electrodes of optimized composition as compared to mild steel cathodes.…”

“…The applicability of phosphate buffer has been studied [42,66,94]. These studies concluded, that in a membrane separated electrochemical cell the cathode current efficiency is slightly increased.…”