Abstract
The selective catalytic reduction of NOx (deNOx) at temperatures less than or at 200 °C was investigated while using C2H4 as the reductant and mixed oxides of Fe and Mn supported on activated carbon; their activity was compared to that of MnOx and FeOx separately supported on activated carbon. The bimetallic oxide compositions maintained high NO conversion of greater than 80–98% for periods that were three times greater than those of the supported monometallic oxides. To examine potential reasons for the significant increases in activity maintenance, and subsequent deactivation, the catalysts were examined by using bulk and surface sensitive analytical techniques before and after catalyst testing. No significant changes in Brunauer-Emmett-Teller (BET) surface areas or porosities were observed between freshly-prepared and tested catalysts whereas segregation of FeOx and MnOx species was readily observed in the mono-oxide catalysts after reaction testing that was not detected in the mixed oxide catalysts. Furthermore, x-ray diffraction and Raman spectroscopy data detected cubic Fe3Mn3O8 in both the freshly-prepared and reaction-tested mixed oxide catalysts that were more crystalline after testing. The presence of this compound, which is known to stabilize multivalent Fe species and to enhance oxygen transfer reactions, may be the reason for the high and relatively stable NO conversion activity, and its increased crystallinity during longer-term testing may also decrease surface availability of the active sites responsible for NO conversion. These results point to a potential of further enhancing catalyst stability and activity for low temperature deNOx that is applicable to advanced SCR processing with lower costs and less deleterious side effects to processing equipment.
Highlights
Nitrogen oxides (NOX ) are created during fossil fuel combustion and participate in the formation of acid rain and photochemical smog [1,2,3,4]
The use of mixed oxides of Fe and Mn impregnated onto activated carbon was shown to The use of mixed oxides of Fe and Mn impregnated onto activated carbon was shown to significantly improve catalytic activity and activity maintenance for decomposing NO with C2 H4 as significantly improve catalytic activity and activity maintenance for decomposing NO with C2H4 as the reductant in comparison to the mono-oxides of FeOx and Manganese oxides (MnOx) supported on activated carbon the reductant in comparison to the mono-oxides of FeOx and MnOx supported on activated carbon when temperatures were less than or at 200 ◦ C. Reasons for these differences were sought by acquiring when temperatures were less than or at 200 °C. Reasons for these differences were sought by acquiring data from the as-prepared and reaction tested catalysts by surface and bulk analytical techniques, including BET surface area, porosity and pore volume analyses, SEM-energy dispersive spectrometer (EDS), X-ray diffraction (XRD), Raman data from the as-prepared and reaction tested catalysts by surface and bulk analytical techniques, including BET surface area, porosity and pore volume analyses, SEM-EDS, XRD, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS)
Porosities and pore volumes of the supported mono-oxide or mixed metal oxide catalysts did not change as the concentrations of FeOx and MnOx were increased, and no difference existed between the as-prepared and tested catalysts
Summary
Nitrogen oxides (NOX ) are created during fossil fuel combustion and participate in the formation of acid rain and photochemical smog [1,2,3,4]. Ren et al [6] showed that pure Fe2 O3 exhibited 95% NO conversion the operation temperature window was toward a higher range of 250 ◦ C to 400 ◦ C Supports for these transition metal catalysts have been inert metal oxides, such as Al2 O3 , TiO2 , and SiO2 [34,35,36] that promote high temperature resistance and mechanical strength [12]; as a downside, some of these have exhibited low surface areas which decreases activity [32]. The use of C2 H4 as the reductant for MnOx-based deNOx catalysts supported on AC showed NO conversions up to 100% at a temperature as low as 130 ◦ C [39] These mono-transition metal catalysts usually suffered from rapid deactivation; for example, the NO conversion over a 3.0 wt.%. In this study, mono-transition metal oxides, i.e., MnOx or FeOx, and their mixtures were supported on a commercial AC and tested for NO conversion activity during C2 H4 -SCR reactions at temperatures below 200 ◦ C; the freshly-prepared and reaction-tested catalysts were analyzed using bulk and surface sensitive techniques to determine structural and chemical aspects relating to their activity and deactivation
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