Topochemical Oxidation of Transition Metals in Layered Double Hydroxides by Anthraquinone-2-sulfonate

Abstract

colored solid precipitates for the Co-Fe-LDH and dark yellow for the Ni-Fe-LDH, respectively, as shown in the insets of Figure 1. The color of the starting CoCl2-FeCl2 solution was pink due to colorless ferrous ions dissolved. After refluxing with the AQS2, the pink color of the solution changed to a brownish one, suggesting the existence of ferric ions, and finally the brown solid precipitates were yielded. Without the AQS2, pink colored precipitates of brucite-like Co 2+ -Fe 2+ hydroxides were obtained as mentioned in a previous report. 5 The results strongly suggest that the redox reactions between Fe 2+ and AQS2 have been involved in the coprecipitation by the HMT hydrolysis. The oxidation potential of AQS2 2− /AQS2 was measured as −0.60 V (vs Ag/AgCl), 6 which is probably capable of oxidizing the Fe 2+ ions in Fe(OH)2 phase because of +0.58 V of standard oxidation potential in Fe(OH)3/Fe(OH)2. 7 According to our previous report for hydrotalcite-like α-cobalt hydroxide containing AQS2, 8 the AQS2 did not oxidize the Co 2+ ions under similar reaction conditions. In the present study, Fe 2+ was oxidized into Fe 3+ , resulting in the Co 2+ -Fe 3+ hydroxide layers forming the LDH structures. Figure 1 shows typical SEM images of the Co-Fe-LDH and the Ni-Fe-LDH samples, consisting of uniform platelets with an average lateral size of 0.5 μm and a thickness of ~70 nm for both samples. Wellcrystallized LDH platelets in SEM inspections further revealed little impurity such as gel-like Fe(OH)3 phase, often observed in alkaline solution syntheses in the presence of oxygen. 4