Electrochemical and XAFS investigation of nitrite reduction by heat-treated μ-oxo derivative of iron phthalocyanine supported on high area carbon

The electrochemical nitrite reduction by heat-treated (FePc)2O/KB (KB=Ketjen Black) in the range of 500–1000 °C has been done in 0.5 M sulfuric acid and the effect of heat-treatment on the structural aspects has been investigated by X-ray absorption fine structure. (FePc)2O before heat-treatment displayed exactly the same voltammetric behavior and catalytic activity toward nitrite reduction as FePc. Nitric oxide that was produced by disproportionation of nitrite formed a stable adduct with FePc before being irreversibly reduced. The catalytic activity, which was highest for a nonheat-treated sample, decreased monotonically as the heat-treatment temperature increased, which followed approximately the nitrogen-to-carbon ratio in the sample. XANES and EXAFS analyses showed that (i) the μ-oxo structure was still maintained upon adsorption and (ii) (FePc)2O was converted to FePc in the range of 500–600 °C and to an Fe2O3-like structure above 900 °C. The loss of catalytic activity has been attributed to an alteration from the phthalocyanine structure. It is concluded that the FeN4 structure is essential for maintaining the catalytic activity.