Synergy of iron and copper oxides in the catalytic formation of PCDD/Fs from 2-monochlorophenol

Abstract

Transition metal oxides present in waste incineration systems have the ability to catalyze the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) through surface reactions involving organic dioxin precursors. However, studies have concentrated on the catalytic effects of individual transition metal oxides, while the complex elemental composition of fly ash introduces the possibility of synergistic or inhibiting effects between multiple, catalytically active components. In this study, we have tested fly ash surrogates containing different ratios (by weight) of iron (III) oxide and copper (II) oxide. Such Fe₂O₃/CuO mixed-oxide surrogates (in the Fe:Cu ratio of 3.5, 0.9 and 0.2 ) were used to study the cooperative effects between two transition metals that are present in high concentrations in most combustion systems and are known to individually catalyze the formation of PCDD/Fs. The presence of both iron and copper oxides increased the oxidative power of the fly ash surrogates in oxygen rich conditions and led to extremely high PCDD/F yields under pyrolytic conditions (up to >5% yield) from 2-monochlorophenol precursor. PCDD/F congener profiles from the mixed oxide samples are similar to results obtained from only CuO, however the total PCDD/F yield increases with increasing Fe₂O₃ content. Careful analysis of the reaction products and changes to the oxidation states of active metals indicate the CuO surface sites are centers for reaction while the Fe₂O₃ is affecting the bonds in CuO and increasing the ability of copper centers to form surface-bound radicals that are precursors to PCDD/Fs.

Keywords: 2-Monochlorophenol; Dioxin; Fly ashes; Incineration; Iron and copper oxides; PCDD/F.