Ferric oxide mediated formation of PCDD/Fs from 2-monochlorophenol.

The copper oxide surface-mediated formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs)from precursors such as chlorinated phenols is considered to be a major source of PCDD/F emissions from combustion sources. Even though iron oxide is present at 2-50 times higher concentrations than copper oxide, virtually no studies of the iron oxide mediated formation of PCDD/Fs have been reported in the literature. We have performed packed-bed, flow-reactor studies of the reaction of 50 ppm gas-phase 2-monochlorophenol (2-MCP) over a surface of 5% iron oxide on silica over a temperature range of 200-500 degrees C. Dibenzo-p-dioxin (DD), 1-monochlorodibenzo-p-dioxin (1-MCDD), 4,6-dichlorodibenzofuran (4,6-DCDF), and dibenzofuran (DF) were formed in maximum yields of 0.2%, 0.1%, 0.3%, and 0.4%, respectively. The yield of PCDD/Fs over iron oxide peaked at temperatures 50-100 degrees C higher in temperature than they peak over copper oxide. The maximum yields of DD, 1-MCDD were 2 times and for 4,6-DCDF was 5 times higher over iron oxide than over copper oxide, whereas DF was not observed at all for copper oxide. The resulting PCDD/PCDF ratio was 0.39 for iron oxide versus 1.2 observed copper oxide, which is in agreement with PCDD/PCDF ratios in full-scale combustors that are typically <<1. The combination of 2-50 times higher concentrations of iron oxide than copper oxide in most full-scale combustors and 2.5 times higher yields of PCDD/Fs observed in the laboratory suggests that iron oxide might contribute as much as 5-125 times more than copper oxide to the emissions of PCDD/Fs from full-scale combustors.