Low-Temperature Selective Catalytic Reduction Of NOx With NH₂-Containing Compounds Supported On Carbon Based Catalysts

Selective catalytic reduction (SCR) with NH3 is the most widely used method for the removal of NOx. However, NH3 does not appear to be an ideal reducing agent when considering its corrosiveness and toxicity. Furthermore, it is very difficult to exactly control an appropriate NH3 input because of the fluctuating NOx concentration in exhaust gas, which is very likely to cause additional environmental problems due to NH3 slip. Therefore, it is of great significance to develop new SCR technologies with other proper reducing agents as a substitution for NH3. In this work, the low-temperature selective catalytic reduction of NOx with NH2-containing compounds supported on catalysts was systematically studied. The details are shown as follows:(1) Low-temperature SCR of NO with urea supported on pitch-based spherical activated carbon (PSAC):NO oxidation to NO2 catalyzed by the 0.5-0.8 nm micropores in PSACs was found to be the rate-limiting step in urea-SCR reaction, which was confirmed by both the apparent activation energy calculations and the kinetics results of urea-SCR reaction and NO oxidation on PSAC. These two reactions gave very similar negative apparent activation energies (-16.5 kJ/mol for urea-SCR reaction and-15.2 kJ/mol for NO oxidation), indicating that the adsorption of reactants on PSAC is of key importance in these two reactions. Moreover, these two reactions were both approximately first order with respect to NO and one-half order with respect to O2. It was found that NO3 from the disproportionation of the produced NO2 was quickly reduced by supported urea into N2. After the complete consumption of supported urea, NO2 started to release, and the carbon surface was gradually oxidized by adsorbed NOx species. NO3 was found to be stably adsorbed on the oxidized carbon surface.(2) Low-temperature SCR of NO2 with urea supported on PSAC:The urea-SCR of NO2 was not catalyzed by the micropores in PSACs. Increasing urea loading raised the reaction probabilities of SCR of NO2 by urea, which resulted in significant increase of the SCR activity; moreover, the NOx removal period was extended. It was found that the SCR activity was improved by increasing NO2 or O2 concentration in the feed gas. However, further increase in O2 concentration above 9 vol% made a weak contribution to the improvement of the SCR activity.(3) Low-temperature SCR of NO with melamine supported on carbon aerogels-supported MnOx-CeO2 based catalyst:In the presence of gaseous O2, carbon aerogels-supported MnOx-CeO2 based catalyst with 15 wt.% melamine loading exhibited high activity and selectivity in the SCR of NO to N2. It was found that, after the SCR reaction, melamine supported on the catalyst was totally consumed, and no N-containing by-products released as gas or deposited on catalyst surface during the reaction. Increasing melamine loading extended the NOx removal period. However, melamine loading above 15 wt.% markedly decreased the SCR activity due to serious coverage of active sites. The calcination temperature above 400?caused the decrease of the interactions between the Mn and Ce oxides and increased the crystallinity of the metal oxides. Such changes on the catalyst were found to be harmful to the melamine-SCR activity. Since the increase of O2 feed concentration and reaction temperature both strengthened the chemical adsorption of NO on catalyst surface to form adsorbed NO3- which was reduced to N2 by supported melamine, the melamine-SCR activity increased.(4) Low-temperature SCR of NO2 with melamine supported on carbon aerogels-supported MnOx-CeeO2 based catalyst:The melamine-SCR of NO2 was catalyzed by carbon aerogels-supported MnOx-CeO2 based catalyst. Melamine loading above 15 wt.% and calcination temperature above 400?were both found to be harmful to the SCR activity. Since the increase of O2 feed concentration and reaction temperature both strengthened the chemical adsorption of NO2 on catalyst surface to form adsorbed NO3- which was reduced to N2 by supported melamine, the melamine-SCR activity increased.(5) Mechanism study on low-temperature melamine-SCR:SBA-15-supported MnOx-CeO2 based catalyst was also found to efficiently catalyze the SCR of NOx with supported melamine at proper reaction temperatures. In this work, the systematic in situ FTIR studies on NOx adsorption and melamine-SCR reaction over SBA-15-supported MnOx-CeO2 based catalyst showed that:(1) In the absence of gaseous O2, both NO and NO2 could be chemically adsorbed on catalyst surface to produce adsorbed NO3-. The introduction of O2 in the feed gas benefited the chemical adsorption of NO while had no obvious effect on the chemical adsorption of NO2. (2) The low-temperature melamine-SCR reaction which contains a series of elementary reactions is very complex. At the early stage of the reaction, the functional groups involving in the reduction of adsorbed NO3- were mainly the NH2 groups in melamine molecules, the products contained N2 and H2O. When the NH2 groups were consumed to some extent, the adsorbed HNCO from the hydrolysis and decomposition of reaction intermediates started to participate in the reduction of adsorbed NO3- to produce CO2, N2 and H2O.