Elsevier

Catalysis Today

Volume 332, 15 July 2019, Pages 49-58
Catalysis Today

The promotional role of Nd on Mn/TiO2 catalyst for the low-temperature NH3‑SCR of NOx

https://doi.org/10.1016/j.cattod.2018.07.031Get rights and content

Highlights

  • The significant promotional effect of Nd to 30%Mn/TiO2 catalyst was observed for the low-temperature NO-SCR activity.

  • Coprecipitation and Sol-gel methods are superior to Impregnation for synthesizing more active 30%Mn-3%Nd/TiO2 catalysts.

  • The MnOx species with lower Mn oxidation states increased with Nd addition, which was observed by XANES spectra fitting.

Abstract

A series of rare-earth metal doped 30% Mn-3% RE/TiO2 (RE = Ce, Sm, Nd, Er, Y) catalysts were prepared by adopting two step incipient wetness impregnation method and investigated for the low-temperature SCR of NOx with NH3. It was found that 3% of rare earth elements (Nd, Er, Y) was added to 30% Mn/TiO2 catalyst even could increase the catalytic performance at the low temperature, in addition to usual used Ce and Sm. And the promoting effect of Nd was among the most significant. Then the 30%Mn-3%Nd/TiO2 catalyst was synthesized by another two methods, Coprecipitation and Sol-gel, for a comparative study. Various techniques were applied to characterize the catalysts. The catalysts prepared by Coprecipitation and Sol-gel method exhibited higher surface area and smaller average particle size in comparison with catalyst prepared by impregnation with TiO2 support. The addition of Nd made surface area and dispersion of MnOx species increase. It also led both the content of amorphous MnO2 and the content of crystalline Mn2O3 to decrease, but the proportion of MnOx species with low oxidation state increased. H2-TPR results showed that the low temperature reduction peak of MnOx was shifted to much lower temperatures with the doping of Nd, which improved the redox property of the catalysts. The all characteristics shown above could well explain the promotional role of Nd doping in the 30%Mn-3%Nd/TiO2 catalyst, that improved the low temperature SCR catalytic activity.

Introduction

The current effective technology for eliminating NOx from flue gas of stationary sources is the selective catalytic reduction (SCR) of NOx with NH3, whereas V2O5/TiO2 promoted by WO3 or MoO3 are the major commercial catalysts used for the SCR process in the industry [1,2]. Of course, the vanadia-based SCR catalysts have some inevitable disadvantages in practical application, such as the narrow operation temperature window of 300 and 400 °C [3], high conversion of SO2 to SO3 at high temperature [4], and the toxicity of vanadium pentoxide to the environment and human health [5]. Other types of widely used catalysts are transition-metal-exchanged (Cu or Fe) zeolites [[6], [7], [8]], which exhibit a wide operation temperature window and satisfactory de-NOx efficiency. However, when the stack gases from stationary sources are purified, the gas temperature is usually low after passing through the desulfurizer and particle removal device to eliminate sulfur dioxide and solid particles in the stack gases. The catalytic activities of the previous two kinds of catalysts for the NH3-SCR reaction are not satisfactory at low temperature. Therefore, there is a great interest in developing novel SCR catalysts that are active at relatively low temperatures (< 200 °C) to control the emission of NOx from stationary sources at present, which is the typical temperature of flue gases in the downstream of the desulfurizer and electrostatic precipitators [5,9,10].

Among many reported various transitional metal oxides (Cr, Mn, Fe, Cu, Co, etc) catalysts, the Mn-based catalysts were widely investigated and exhibited excellent low-temperature activity for SCR with NH3. However, the commercial applications of pure MnOx are restricted severely because of its low surface area and poor thermal stability [11,12]. Therefore, supported MnOx catalysts and Mn-based mixed oxides have attracted more attention, such as MnOx supported on TiO2 [[13], [14], [15]], Al2O3 [16], SiO2 [17], NaY [18], and AC (activated carbon) [19] and Ce-Mn [20] and Cu-Mn [9] mixed oxides. Smirniotis at el [17] reported that the SCR performance of the supported MnOx catalysts decreased in the following order: TiO2 (anatase, high surface area) > TiO2 (rutile) > TiO2 (anatase, rutile) > γ-Al2O3 > SiO2 > TiO2(anatase, low surface area). Furthermore, TiO2 powder, as a support, has many advantages, such as good thermal stability, cost-effective and easily promoting synergy catalysis with MnOx at el. Hence, our group has deeply great interest in studying MnOx/TiO2 catalyst for low temperature SCR. Even though the MnOx/TiO2 catalysts have shown excellent low-temperature selective catalytic reduction (SCR) activity for NOx removal, they all suffer from the serious SO2 poisoning effect on activity.

On account of the present situation, a portion of metallic elements (Fe, Cu, Ni, Sn, at el) and rare earth elements were added into the MnOx/TiO2 catalyst to improve its catalytic performance at low temperature. For example, Qi at el [21] found that Fe-Mn/TiO2 catalyst with the addition of iron oxide not only increased the NO conversion and N2 selectivity but also increased the resistance to H2O and SO2. Chang at el [22] reported that SnO2-MnOx-CeO2 with a molar ratio of Sn/(Sn + Mn + Ce) = 0.1 showed a remarkably high activity, N2 selectivity and SO2 resistance. Chen at el [23] reported that the Ni0.4Mn0.6Ti10 catalyst exhibited excellent NH3-SCR activity and good H2O and SO2 durability even in the presence of 100 ppm SO2 and 15% H2O under a GHSV of 40,000 h−1, due to the interactions among Mn, Ni and Ti oxides. Moreover, CeO2 has been studied extensively due to its excellent oxygen storage capacity and redox properties via the shift between Ce4+ and Ce3+ in oxidizing and reducing conditions, respectively. Jin at el [24] reported that ceria modification could be able to promote SO2 tolerance of SCR catalysts via reducing thermal stabilities of the sulfate species covered on catalyst surface, thereby promoting its decomposition. After that, Meng at el [25] also reported that an introduction of appropriate amount of Sm into the MnOx catalyst can markedly improve its catalytic activity for NH3-SCR and increase its abilities for sulfur resistance and H2O resistance. However, addition of other rare earth elements (such as Nd, Y, Er, etc.) to MnOx/TiO2 catalyst has less study up to now. Taking into consideration that rare earth elements own some common characteristics in aspect with physicochemical property, we have a great interesting in study the effect of other rare earth elements added on the catalyst of Mn/TiO2 for low-temperature NH3-SCR.

Herein, 30% Mn/TiO2 and a series of 30% Mn-3% RE/TiO2 (RE = Ce, Sm, Nd, Er, Y) catalysts were prepared by the two-step incipient wetness impregnation method and their catalytic performances for low-temperature NH3-SCR of NOx were investigated, in which the addition of Nd showed obvious enhancement for catalytic activity. The eff ;ect of Nd addition on the structure and physicochemical properties of 30% Mn/TiO2 was systematically investigated by various characterizations, such as BET, XRD, SEM, XAFS, H2-TPR and NH3-TPD. On the basis of the above results, we studied the reason why the Nd addition promoted the catalytic performance of 30% Mn-3% Nd/TiO2 catalyst for NH3-SCR of NOx at low temperature, and we also well explained the reason why 30% Mn-3% Nd/TiO2 catalyst showed good H2O resistance and sulfur resistance at 120 °C.

Section snippets

Impregnation method (i)

The catalysts were prepared by a two-step incipient wetness impregnation method. Firstly, the commercial anatase titanium dioxide (TiO2) was pretreated by washing.

Secondly, the catalysts were prepared by the first-step incipient wetness impregnation of TiO2 (pretreated power) with different rare earth nitrates (RE(NO3)3·5(6)H2O, RE = Ce, Sm, Nd, Er, Y). A desired amount of respective rare earth nitrate aqueous solution (0.16 M) was dropwise added into 5 g of TiO2 fine powder with stirring for

SCR activity

The six catalysts, including 30%Mn/TiO2(i) and 30% Mn-3% RE/TiO2(i) (RE = Ce, Sm, Nd, Er, Y), were investigated in the SCR of NOx with NH3, and the oxynitride (NOx) conversion as a function of the reaction temperature is shown in Fig. 1(A). The catalysts show similar activity and temperature window (100–350 °C with NOx conversion >90%). Over the catalysts, NO started to convert from a very low temperature with around 20% NO conversion at 40 °C. They exhibited 100% NOx conversion at 100–130 °C.

Conclusions

The promotional effect of Nd on 30%Mn/TiO2 catalyst was observed for the selective catalytic reduction of NOx by NH3. The 30%Mn-3%Nd/TiO2 catalysts prepared by Coprecipitation and Sol-gel method exhibited higher activity at low-temperature than the catalyst made by Impregnation. The increasing large BET surface area, smaller average pore size and the highly dispersed MnOx species induced by Nd addition accounted for the activity enhancement. The improvements of redox property and Mn species

Acknowledgement

This work was supported by the National Natural Science Foundation of China (grant number 21676288).

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