No Reduction With Light Alkanes Over Iron And Fe-based Catalysts

Nitrogen oxides?NOx? are key air pollutants and mainly come from combustion process of fossil fuels in different sources such as coal-fired boilers, cement kilns and motor vehicles. The researches on nitrogen oxides controlling technologies are important for both academics and engineering application. Traditional method of selective catalytic reduction of NOx by ammonia?NH3-SCR? has some disadvantages associated with the use of NH3, for example, corrosion, expensive reductant, possible NH3 slip, expensive cost of the catalysts based on V2O5/Ti O₂, narrow operation temperature window, etc. Therefore, it is interesting to carry out researches on new De NOx methods that has been widely concerned by academic and industrial communities. Selective catalytic reduction of NOx by hydrocarbons?HC-SCR? as one of the potential technologies for flue gas De NOx has been highly concerned by researchers.In this paper, NO reduction by the combination of the light alkanes?C₁-C₃? and metallic iron was proposed and the parameters that influenced the NO reduction efficiency were experimentally investigated under different atmosphere and operating conditions, and further the reaction mechanism was analyzed. On this basis, iron-based supported catalysts, i.e., monolithic cordierite-based Fe/Al₂O₃ catalysts, were prepared for NO reduction by light alkanes. The catalyst samples were characterized and measured by N₂ adsorption, X-ray diffraction?XRD?, scanning electron microscope?SEM?, X-ray photoelectron spectroscopy?XPS? and temperature programmed reduction?H2-TPR?. The iron loading, calcination temperature, reducing agents and H₂O/SO₂ impact on the NO reduction performance were tested. The reaction intermediate species in HC-SCR were investigated via in situ diffuse reflectance infrared Fouier transform spectroscopy?DRIFTS? method, and the possible reaction pathways and mechanisms were proposed. The following conclusions could be drawn from the present research work.1. NO reduction by light alkanes over metallic iron was investigated. Results showed the combination of light alkanes and iron/iron oxides was proved very effective to reduce NO at high temperature. In simulated flue gas, NO reduction by light alkanes over iron could exceed 90% when the temperature was above 900 °C. NO was reduced by light alkanes over iron via two major routes: one is light alkanes reduced iron oxides to metallic iron through redox reactions and then NO was reduced by metallic iron, the other one is NO was reduced by reburning reaction of alkanes while the reburning intermediates?HCN? were oxidized by iron oxides. The effect of H₂O/SO₂ on NO reduction by methane over iron was small. In N₂ atmosphere, water vapor involved in the oxidation of iron and caused NO reduction efficiencies decreased slightly. In simulated f lue gas atmosphere, water vapor increased the NO reduction by methane over iron. When the excess air ratios in reaction zone and burnout zone were SR1=0.7 and SR2=1.2 respectively, the NO reduction efficiencies were found to be 96.7% and 90.6% respectively at 1050 °C when there were 7% and 0% water vapor. SO₂ caused a slight decrease of NO reduction. Experimental data showed that more than 90% of NO was reduced by 1.14% methane over iron at 1050 °C in a durable test over 50 hours in simulated flue gas atmosphere containing 7% H2 O and 0.02% SO₂.2. A series of monolithic cordierite-based Fe/Al₂O₃ catalysts, which were prepared by a sol-gel and impregnation method, were used for selective catalytic reduction of NO with light alkanes. Results showed that the reactivity order of the De NOx activity with light alkanes was as follows: propane ? ethane > methane. A good catalytic performance for NO reduction by C₂H₆/C3 H8 was demonstrated to be more than 90% in the presence of oxygen at 600 °C. Compared to the temperature for effective NO reduction by light alkanes?C₁-C₃? and metallic iron, there was a decrease of 300 °C for similar NO reduction by light alkanes?C₁-C₃? over Fe/Al₂O₃/CM catalysts. Iron loading showed an obvious effect on the textural properties, surface morphology, nanorod size, crystallinity and reducibility of the catalysts. 97% NO conversion was observed when 5.5 wt.% iron was loaded on the monolithic cordierite-based Fe/Al₂O₃ catalyst at 600 °C in the presence of oxygen. The iron oxide nanorods distributed on the surface of the Al₂O₃ layer may act as SCR active sites. With higher Fe loading, the agglomeration of iron oxide nanorods led to the decrease in the surface area and a lesser surface availability of the active phases. The NO₂ adspecies and nitrate species formed during the SCR reaction can strongly adsorb on the catalyst surface, while C2 H6 is partially oxidized to acetate and formate species. The isocyanate?-NCO? species are an important intermediates which result in the formation of N₂ and CO₂. Based on the results, the reaction pathway was proposed to describe the C₂H₆-SCR reaction over Fe/Al₂O₃/cordierite catalyst.3. The effect of calcination temperatures on the catalystic performance of Fe/Al₂O₃/CM was carefully studied in C2 H6-SCR. The calcination temperature showed signif icant influence on the crystallinity, redox ability, and reactant adsorption ability of iron oxide nanorods, which resulted in different catalytic activity. The crystallinity and agglomeration of iron oxide nanorods increased at high calcination temperature. The catalyst sample calcined at 500 °C showed the highest activity, e.g., 94% NO reduction in the presence of oxygen at 600 °C. Based on in situ DRIFTS analys is, the larger Fe₂O₃ nanorods due to higher calcination temperature promoted the formation of NO₂/NO3- species, but inhibited the formation of acetate/formate species during NO reduction by C₂H₆.4. The selective reduction of NO by C₃H₈ and the sensitivity to H2 O and SO₂ were studied over Fe/Al₂O₃/CM catalysts. The samples showed the stability even after hydrothermal treatment yielding a slight loss of 15% of initial activity, however, maintaining 65% of the initial activity after sulphidation treatment. The results indicated that the mild activity depression after hydrothermal treatment was due to the thermal deactivation caused by agglomeration of iron oxide nanorods. The deactivation after sulphidation treatment was mainly attributed to the deposited SO42- species. The effect of coexisting H2 O and SO₂ on the catalytic activity catalysts was investigated by step-change experiment and further discussed based on in situ DRIFTS study. The decline in the SCR activity in the presence of H2 O was signif icant but recoverable due to the strong and reversible inhibition effect of coexisting H2 O on the adsorption of NO₂ adspecies and unidentate nitrate. On the contrary, the formation of NO₂/NO3- species was slightly inhibited due to sulfate species with only one S=O band in the presence of SO₂, although the coexisting SO₂ promoted the formation of the acetate/formate species. Based on the results, the mechanism was proposed to describe the C₃H₈-SCR reaction over Fe/Al₂O₃/cordierite catalyst and the effect of H2 O and SO₂ on the reaction pathway.