Effect of precursor anion in alcohol/water solutions during hydro/solvothermal synthesis on cobalt oxide morphology and catalytic ability

Abstract

The textile industry requires water for practically every step during its manufacturing process, and would benefit from water reuse systems. This is challenging, as the reactive dyes used in this industry often escape the conventional wastewater treatment methods used. The use of advanced oxidation processes combats these challenges, as it allows for the conversion of organic toxic waste to harmless H2O and CO2. Potassium peroxymonosulphate (Oxone®) has become popular in these treatment methods, as highly active sulphate radicals are activated once in contact with a transition metal. Such reactions are dependent on the catalyst used, and therefore require control of its morphology for enhanced capabilities. Cobalt oxide is deemed the best activator of peroxymonosulphate. The use of alcohols in its synthesis has been studied, but a systematic study increasing the alcohol chain lengths, in addition to the cobalt anion during hydro/solvothermal synthesis, has not been studied. Although the cobalt complex formed from cobalt chloride in water and alcohol has been studied, the use of these complexes as a precursor to cobalt oxide nanoparticles has not been studied. Cobalt hydroxide precursors were therefore synthesised in pure alcohol, pure water and alcohol/water solutions in the ratios 1:1 and 1:0. Five alcohols were selected, namely methanol, ethanol, propanol, butanol and octanol. and cobalt chloride hexahydrate and cobalt nitrate hexahydrate were used to study the effect of the anion. The effect of calcination temperature was also studied by varying it between 300°C and 500°C. The resulting particles were characterised using TEM, SEM, XRD, BET, EDS, FT-IR and ELNEFS, and its catalytic ability was tested treating a methylene blue solution in an in-house developed continuous reactor. Both α- and β-cobalt hydroxide polymorphs were encountered as precursors from cobalt chloride. The β-phase was evident when the water was exhausted from the system, whereas the α-phase was evident when water was present in the system. Only α-cobalt hydroxide was formed from cobalt nitrate. This provided a relationship between the cobalt complex formed to the phase of cobalt hydroxide polymorph. A blue, tetrahedral orientated precursor complex, produced pink β-cobalt hydroxide particles, while a red, octahedral orientated precursor complex, produced α-cobalt hydroxide. Cobalt oxide nanoparticles were formed from both α- and β-cobalt hydroxides. Anion morphology-dependent changes were observed in the presence of alcohol only. Rods were formed in the presence of the nitrate anion, while rhombic shapes dominated in the presence of chloride anions. Only rods were produced in water. An increase in the calcination temperature increased the crystallite size, which negatively affected the catalytic activity. It was also noted that a crystallite size between 8 and 11 nm resulted in highly active cobalt oxide particles for both anions explored. The catalytic ability of the cobalt oxide resulting from the β-phase was better than that of the α-phase. The best catalytic activity was produced by the cobalt oxide synthesised from cobalt chloride hexahydrate precursor salt in 100% methanol for which the ELNEFS analysis revealed a Co3+/Co2+ ratio of ten times that of its 50% counterpart.