Catalytic Performance, Characterization, and Mechanism Study of Fe2(SO4)3/TiO2 Catalyst for Selective Catalytic Reduction of NOx by Ammonia

Abstract

Iron oxides supported on TiO2 catalysts show good activity in selective catalytic reduction (SCR) of NOx by ammonia. In this work, Fe2(SO4)3/TiO2 and other catalysts containing iron and sulfates were prepared by the impregnation method, and their activities for SCR of NOx by ammonia were investigated. On Fe2(SO4)3/TiO2 catalyst, NOx conversion reached 98.0% in the temperature range of 350−450 °C, while yielding little N2O. Characterization results showed that α-Fe2O3 and sulfates were mainly formed on the Fe2(SO4)3/TiO2 catalyst. Iron oxide catalysts with sulfates [e.g., Fe2(SO4)3/TiO2] caused more dispersed iron phase, and the sulfation effect might have mostly occurred on iron oxides but not TiO2. In contrast, Fe(NO3)3 precursor supported on TiO2 without sulfates led to the Fe2O3 particle products, which physically connected to the surface of TiO2 and resulted in some agglomerations or heterogeneous distribution of Fe2O3 particles. In addition, sulfates played an important role in the SCR reactions, which were responsible for strong Brönsted acid sites. The SCR reaction mechanism on Fe2(SO4)3/TiO2 might have taken place as follows: NH3 was adsorbed onto the surface of Fe2(SO4)3/TiO2 and mainly formed NH4+ on Brönsted acid sites. NO was oxidized to NO2 by the oxygen in the reaction gas or on the iron oxide sites of the catalyst. Then, nitrates originated from NO2 were generated on Fe2(SO4)3/TiO2 catalyst for NH3−SCR reaction. Finally, an active intermediate species like ammonium nitrite was formed, which eventually decomposed to gaseous nitrogen.