关键词: 金属铬, NaOH水溶液, 电化学氧化, 循环伏安, 阳极极化

Abstract:

Ferrochrome electrolysis technology is a novel method for preparing sodium chromate (Na₂CrO₄). Although the method performs well at soft reaction conditions, controllable process, environmentally friendly production process, etc., the electrochemical oxidation process of metal chromium in NaOH aqueous electrolyte is still unclear. At present, there are few research articles about specific electrochemical oxidation of metal chromium in NaOH aqueous electrolyte. It is, therefore, meaningful to carry out the research in electrochemical oxidation mechanism of chromium. The electrochemical oxidation of metal chromium in 0.01 mol·L^-1 ~ 10 mol·L^-1 NaOH aqueous electrolytes at 20 °C was studied by cyclic voltammetry (CV, Scan rate: 100 mV·s^-1) and linear sweep voltammetry (LSV, Scan rate: 1 mV·s^-1) through controlling the potential range. The working electrode (WE) was a chromium rod, the counter electrode (CE) was a platinum sheet, and the reference electrode (RE) was a saturated calomel electrode (SCE). EDS, SEM, XRD and XPS were used to characterize the metal chromium before and after the electrolysis to determine intermediate during electrochemical oxidation. Ultraviolet-visible (UV) spectrophotometer was used to analyze the electrolyte solution after the electrolysis to confirm the formation of Na₂CrO₄. The results indicated that Cr(0) and Cr(OH)₃ might undergo electrochemical oxidations in sequence to directly form Na₂CrO₄. When the anode potential was negative, chromium generated Cr(OH)₃ film through electrochemical oxidation, while hydroxide ions (OH^-) underwent electrochemical oxidation to form oxygen. On the othere hand, when the anode potential was positive, two electrochemical reactions: (1) Cr(0) → Cr(VI); (2) Cr(OH)₃ → Cr(VI) took place. Thus, the anodic polarization included activation of Cr(0). The dissolution reaction of chromium was stimulated by OH^- at a higher concentration of NaOH aqueous solution. Futhremore, the amounts of Cr(OH)₃ and Na₂CrO₄ formed were increased with the increased concentration of NaOH aqueous solutions. At the same time, a large amount of oxygen was deposited on the anode electrode surface with the alkaline concentration ≥ 2 mol·L^-1 and anodic potential ≥ 1.6 V (vs. SCE).

Key words: metal chromium, NaOH aqueous solution, electrochemical oxidation, cyclic voltammetry, anodic polarization