Transition metals doped Mn-based catalysts were prepared via ultrasonic immersing method for the selective catalytic reduction (SCR) of NO x from fuel gas. The Catalysts’ DeNO x efficiency and tolerance to sulfur were investigated in the paper. XRD results demonstrate high dispersion of Mn, Ce and M (Pr, Y, Zr, W) elements on TiO 2 carrier, which is favor for reduction of active materials content. Mn-Ce-W catalyst presents uniform particle size about 500 nm to 800 nm from SEM pictures and shows the best NO x conversion of 93.2% at 200 ° ;C and 98.4% at 250 ° ;C, respectively. Sulfur tolerance analysis indicated that transition metals M can improve the catalysts’ performance when 0.01% SO 2 exists in the fuel gas, because metal doping into the Mn-Ce catalyst can inhibit the sulfate deposition, especially metal sulfate, on the catalyst, which can be seen from the Fourier infrared spectrum.
Strict control of NOx emission is urgently needed due to the increasing effect on the environment from urban smog, ozone depletion and greenhouse. The selective catalytic reduction (SCR) technology with NH3 is considered as the most effective method to remove NOx in flues gases. The SCR catalysts have been developed in the past decades and Vanadium-based catalyst is the commercial one for application at temperature 300˚C - 400˚C due to its high DeNOx efficiency and good sulfur-resistance. However, these catalysts show the toxicity of raw materials and high cost, which limit their extensive applications. So developing catalysts working less than 300˚C is of interest due to their possible application in downstream after desulfurizer and electrostatic. Various catalysts have been investigated at low temperature such as Manganese oxides (MnOx) supported on Al2O3, TiO2 and SiO2. The results show that MnOx is a kind of promising catalyst applied at low temperature. At the same time, ceria is found to increase the oxygen storage capability of MnOx and enhance the migration rate of oxygen. In present, MnOx-CeO2 catalysts have been demonstrated possessing the high NOx removal efficiencies [
In this work, transition metals will be doped into Mn-based catalysts to improve the catalyst’s tolerance to sulfur. Then ultrasonic immersing method was used to prepare catalysts, which apparently reduce the preparation period comparing to sol-gel method. The transition metals effect on DeNOx efficiency and sulfur tolerance of catalysts was discussed in details as following.
The TiO2 powder (Industrial grade, 96% purity, 15 - 25 nm, 93.9 m2・g−1) was dried at 105˚C for 1 h and milled for 150 mesh. Manganese nitrate, cerium nitrate and nitrate of transition metal (M) (Sinopharm Chemical Reagent Co.) at a certain ratio are solved into Deionized water (DW). Then, the obtained solution was placed into the ultrasonic reaction pool. The processed TiO2 powder was added into the solution when the metal nitrate dissolved fully under the stirring condition with 160 W ultrasonic at 50˚C for 24 h. Then the immersed TiO2 powder was dried at 105˚C for 24 h, calcined at 450˚C for 6 h and milled for 150 mesh to obtain catalysts powder. The obtained catalyst was expressed as Mn-Ce-M0.025, and M represent W, La, Pr and Zr. The mole ratio of Ti:Mn:Ce:M is 1:0.4:0.07:0.025.
The catalyst’s catalytic activity was measured in a quartz reactor (35 cm i.d.) using 6.75 g catalyst, see in
The concentration of the NOx in the inlet and outlet of the reactor were measured on-line by a flue gas analyzer (MRU, Germany). H2-temperature-programmed reduction (H2-TPR, ChemBET Pulsar) experiments were carried out using every catalyst (40 mg) under a mixed gas flow of H2 and Ar.Morphologies of the samples were characterized by scanning electron microscopy (SEM, quanta FEG 250). Powder XRD was characterized with Cu-Ka radiation at the range of 10˚ - 90˚. Fourier infrared spectrum (FT-IR, Prestige-21) was used to analyze functional group change of catalyst after operating for a time.
XRD patterns of the Mn-Ce-M catalysts are shown in
The catalytic performance of Mn-Ce-M0.025 catalysts, expressed as the percent conversion of NO as function of temperature, is shown in
catalyst activity only at the high temperature. However, at high temperature range (above 250˚C), the NO conversions for Mn-Ce-Pr and Mn-Ce-Zr are near to that of Mn-Ce. According to SEM picture, it is possible that uniform particle size and good porosity tends to present good catalyst, especially at low temperature. The reason of the transition metal effect on catalyst performance will be further investigated in the following.
The doped metal effect on the redox property of Mn-Ce-M0.025 catalysts was observed by H2-TPR, see in
which was attributed to the continuous reduction steps of MnO2 → Mn2O3 → Mn3O4 → MnO [
The tolerance to the SO2 for these prepared SCR catalysts is further investigated. When SO2 concentration is 0.01%, activities of Mn-Ce-M were performed at 250˚C, as shown in
ceed that of Mn-Ce catalyst after 150 min in SO2 condition. After 275 min in SO2 condition, the NOx conversion decrease from 88.8% to 73.5% over Mn-Ce catalyst, 87.1% to 77.1% over Mn-Ce-Pr catalyst and 88.7% to 75% over Mn-Ce-Y. The results demonstrated that Pr presents better tolerance to the SO2 than Y element. Doping W and Zr into the Mn-Ce catalyst not only improve the catalytic performance but also enhance the catalyst’s sulfur tolerance. In all catalysts, Mn-Ce-W catalyst presented the best performance with 88.1% NO conversion in SO2 condition after 275 min.
In order to study the catalyst sulfur tolerance, the fresh catalyst and deactivated Mn-Ce-M catalysts with SO2 was carried out by FT-IR spectroscopy to detect the adsorbed species on the Mn-Ce-M catalyst. The FTIR spectra for the fresh Mn-Ce catalyst, deactivated Mn-Ce and Mn-Ce-W catalysts with SO2 deactivated catalysts with SO2 are shown in
There into, the adsorption peak of NH4+ was weakest on Mn-Ce catalyst, but the strongest
Mn-based catalysts were prepared via ultrasonic immersing method for the selective catalytic reduction (SCR) of NOx from fuel gas. Transition metals effect on the Catalysts’ DeNOx efficiency and tolerance to sulfur were investigated. SEM and XRD results demonstrate metal elements high dispersion on TiO2 and uniform particle size about 500 nm to 800 nm. Sulfur tolerance analysis indicated that transition metals M can improve the catalysts’ performance when SO2 exists in the fuel gas. Mn-Ce-W catalyst performs the best NOx conversion of 93.2% at 200˚C and 98.4% at 250˚C, respectively. From the FTIR results, the weakest
We acknowledge the support of the National Support Plan (2011BAA04B07) and Beijing Institute of Technology Basic Research Fund (20111042014).