Enhancement of Catalytic Activity Over the Iron-Modified Ce/TiO₂ Catalyst for Selective Catalytic Reduction of NO_x with Ammonia

Abstract

A series of iron-modified Ce/TiO 2 catalysts with different Fe/Ti molar ratios were prepared by an impregnation method and used for selective catalytic reaction (SCR) of NO x with NH 3 . The Fe−Ce/ TiO 2 catalyst with a Fe/Ti molar ratio of 0.2 had good low-temperature activity and sulfur-poisoning resistance compared with the Ce/TiO 2 catalyst. The introduction of Fe could increase the amount of Ce 3+ and chemisorbed oxygen species on the catalyst surface and thereafter generate more ionic NH 4 + and in situ formed NO 2 , respectively. In addition, the dispersion of cerium oxide could be improved by the addition of iron, and no visible phase of iron oxide could be observed at low Fe/Ti molar ratios (≤0.2). All of these factors played significant roles in the enhanced catalytic activity, especially the low-temperature activity. Furthermore, mechanisms of the SCR reaction and the SO 2 poisoning of the Fe(0.2)−Ce/TiO 2 catalyst were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. Coordinated NH 3 and ionic NH 4 + species as well as adsorbed NO 2 might be the key intermediates in the SCR reaction in the relatively low-temperature range. The formation of ammonium sulfate appeared to be the dominant cause for the catalyst deactivation in SO 2 -containing gases.

Promotional Effect of Ce on Iron-Based Catalysts for Selective Catalytic Reduction of NO with NH3

“…The splitting signal we detected may have been due to the various interactions of Fe with Ti or the reduction of Fe 3+ located in different sites within the catalyst structure [22]. The third reduction peaks, which showed less intensity and appeared at 778˝C, were attributed to the reduction of Fe 2+ to Fe 0 [5,19]. After Ce doping, the H 2 -TPR profile of Fe(0.2)-Ce(0.4)-Ti catalysts showed enhanced reduction peaks.…”

“…This indicated that the Fe(0.2)-Ce(0.4)-Ti catalysts had more reductive species and that more chemisorbed oxygen formed on the catalyst surface after Ce doping (Table 2), which benefited the SCR reaction [19,27,28]. In addition to the enhanced reduction peak areas of the catalyst, the reduction peak of Fe 2+ to Fe 0 shifted to the lower temperature region-this reflects a strong interaction between Fe and Ce species (as further confirmed by the XPS results shown below) [4].…”

“…Larger surface area and pore volume provide more active sites for the SCR reaction and thus adsorb more reactant molecules, which promote the reaction and result in the favorable catalytic activity we observed. diffraction peaks gradually decreased until no visible Fe, Ce, or Ti species phases could be observed, suggesting that the Fe, Ce, and Ti oxides were highly dispersed on the catalyst surface or at least that its particle size was outside the XRD detection limit [8,19]. In brief, these results showed that Ce doping inhibits the crystallinity of metal oxides and enhances the dispersion of active species on the catalyst surface, which is beneficial as far as increasing the surface area and active sites to promote catalytic activity in SCR reactions [4,14].…”

“…The broad reduction peaks at 628˝C can potentially be ascribed to the reduction of Fe 2+ to Fe 0 and the deeper interior of CeO 2 and bulk CeO 2 [4,25,26]. (Table 2), which benefited the SCR reaction [19,27,28]. In addition to the enhanced reduction peak areas of the catalyst, the reduction peak of Fe 2+ to Fe 0 shifted to the lower temperature region-this reflects a strong interaction between Fe and Ce species (as further confirmed by the XPS results shown below) [4].…”

The use of ceria for the selective catalytic reduction of NOx with NH3

“…Furthermore, Peng et al [37] improved the low-temperature activity of CeO2-WO3/TiO2 by SiO2 addition. Other transition metals such as Mo [38], Fe [39], Zr [40], and Nb [41] were also investigated as modifying agents.…”