Nano-sized magnetic iron oxides as catalysts for heterogeneous Fenton-like reactions-Influence of Fe(II)/Fe(III) ratio on catalytic performance

摘要

Nano-sized Fe(II, III) oxides with various Fe(II)/Fe(III) ratios were characterized and tested as catalysts for the oxidative degradation of phenol via Fenton-like reactions at neutral pH. Under conditions typically applied for wet peroxide oxidation, Fe(II) in magnetite is oxidized to Fe(III), successively converting the mineral into maghemite. The residual Fe(II) content in the catalyst core is of only minor benefit for the catalytic activity in phenol oxidation, i.e. magnetite is not superior to maghemite. Achievable reaction rates for phenol degradation appeared to be rather low, e.g. phenol half-life of about 12 h when 3 g L~(-1) magnetite and 5 g L~(-1) H_2O_2 were applied. Preceding surface-reduction of maghemite by NaBH_4, leading to an over-stoichiometric Fe(II) content compared to magnetite, only enhanced the non-productive decomposition of H_2O_2 rather than the rate of phenol degradation. Reaction rates were shown to be relatively insensitive to catalyst concentration in the range of 1-10 g L~(-1), probably resulting from a scavenging of reactive species by the catalyst surface, whereby particle agglomeration seems to play a key role. Degradation experiments with various structurally distinct compounds were carried out, indicating a similar selectivity of the heterogeneous Fenton-like system to that known for oxidation with ·OH.