Full length articleManganese oxide catalysts supported on zinc oxide nanorod arrays: A new composite for selective catalytic reduction of NOx with NH3 at low temperature
Graphical abstract
Introduction
As major air pollutants, nitrogen oxides (NOx) can cause great harm to human beings' lives and to the natural environment. Many studies have been devoted to the control of NOx emission over the past decades. Selective catalytic reduction (SCR), mainly based on reaction (1) (the so-called “Standard SCR”), has been considered as a promising technology for flue gas decontamination to meet the NOx emission targets of several countries [1].
Currently, metal oxide SCR catalysts, typically fabricated by combining TiO2 as the support, V2O5 as the main catalyst, and WO3 or MoO3 as an additive, are currently used as commercial catalysts, since they integrate several desirable advantages including a high catalytic activity, high stability and a capacity to deal with the large volume of flue gas [2]. However, applying these SCR catalysts could encounter obstacles such as the high cost and toxicity of vanadium pentoxide, restraining their large-scale industrial applications [3]. Furthermore, to fulfil the high-temperature requirement, these SCR catalysts are typically located upstream from the desulfurizer and electrostatic precipitator, where the high concentrations of SO2 and ash accelerate the deactivation of the catalysts [4]. In this regard, low-temperature SCR (LT-SCR) techniques in which the denitrification (de-NOx) process operates at 100–300 °C, thus, giving rise to prospective downstream installation, has attracted considerable attention [5].
Various transition metal oxides, such as FeOx, MnOx, CeOx, CoOx, etc., have been evidenced to be of high efficiency in LT-SCR [[6], [7], [8]]. Due to the existence of unstable oxygen and various manganese valence states, MnOx-based catalysts have shown improved activities compared to other metal oxides [9]. In particular, when MnOx is well dispersed onto nanostructured TiO2 with high surface area, the synthesized catalysts can display good SCR activities in the low-temperature range (100–300 °C) [[10], [11], [12]]. For example, Deng et al. measured the de-NOx efficiency of MnOx/TiO2 nanosheet catalysts in a temperature range of 80–280 °C, obtaining a maximum NOx conversion rate of 80% at 200 °C [13]. However, the powder-form catalysts need complex molding operation for industrial application, during which the addition of agglomerant and further calcination treatment may impair the performance of catalysts. The in situ synthesis of active components onto monolith materials thus provides a feasible solution. Cordierite ceramic is a type of Alumina Magnesia Silicate (Al4Mg2Si5O18), which offers excellent thermal shock resistance and high-temperature resistance. It also has several advantages that are suitable to be widely utilized as industrial catalytic substrate, such as low thermal expansion coefficient, small pressure drop and high mechanical strength [14,15]. Qi et al. reported a catalyst fabricated with MnOx/TiO2 composite evenly dispersed on cordierite honeycomb ceramics, which exhibited a favorable LT-SCR catalytic performance, maintaining a NOx conversion rate of 90% at 200 °C [16]. Huang et al. successfully synthesized a Cr-V/TiO2/cordierite monolithic catalysts for LT-SCR as well, which achieved over 90% NOx conversion ratio in the range of 160–300 °C [17].
Compared to the commonly used TiO2 support, ZnO has similar properties, such as approximated band gap of 3.0–3.3 eV, high thermal stability, easy preparation of divisive morphologies of nanostructures, which are propitious for its application as the support for catalysts [[18], [19], [20], [21]]. Furthermore, Lewis and Brønsted acid sites can simultaneously exist on the surface of ZnO [22], indicating that ZnO might be able to promote the de-NOx activity of a catalyst at low temperature. Du et al. decorated La0.8Sr0.2MnO3 (LSMO) nanoparticles onto the ZnO nanorod arrays integrated cordierite honeycomb monoliths (LSMO/ZnO), demonstrating a competitive NO oxidation capacity for lean NOx traps in a temperature range of 180–250 °C. Compared to the catalysts fabricated with LSMO coated on Al2O3 powder-form support, their LSMO/ZnO catalyst contributed to about 34% higher turn-over frequency (TOF), which can be attributed to the enhanced dispersion of active components on the array structure with high surface area. Additionally, no N2O or other side products were observed during NO oxidation by the LSMO/ZnO catalysts [23], giving convictive evidence that ZnO nanostructures on monolith substrate might be an attractive candidate as the LT-SCR support.
In this study, we develop a MnOx-ZnO nanocomposite LT-SCR catalyst via a hydrothermal-impregnated method. Such MnOx-ZnO catalyst is constructed by direct impregnation of MnOx onto ZnO nanorod arrays hydrothermally pre-synthesized on cordierite honeycomb monolith. For comparison, we also synthesize a MnOx-TiO2 composite based on TiO2 nanorod arrays. The de-NOx efficiencies of these two catalysts are investigated in the low-temperature range of 100–300 °C. Compared to the MnOx-TiO2 analogue, the MnOx-ZnO catalyst achieves an enhanced de-NOx efficiency, which implies the potential use of nanostructured-ZnO support in the LT-SCR field. By optimizing the synthesis conditions, we eventually synthesize a MnOx-ZnO nanocomposite LT-SCR catalyst with an activity as high as 96.8%. Comparative analyses with X ray photoelectron spectroscopy (XPS), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) are conducted to investigate the reaction mechanisms and pathways of the LT-SCR process for this MnOx-ZnO catalyst, which is significant to the design and synthesis of high-performance LT-SCR catalysts.
Section snippets
Synthesis of nanocomposite catalysts
The cordierite honeycomb monolith (Corning Inc., USA) with low thermal expansion coefficient (CET25–800 °C = 1.5 × 10−7 °C−1) and good mechanical strength (modulus of rupture strength > 175 psi) was firstly divided into small module with the dimension of 1.2 ∗ 1.2 ∗ 0.5 cm and employed as the substrate (denoted as CC), which was then ultrasonically cleaned with deionized water and ethanol before use.
Properties of TiO2/CC and ZnO/CC Supports
The hydrothermal method was employed to synthesize TiO2 nanorod arrays and ZnO nanorod arrays on cordierite ceramic (CC), which were used as substrates to load MnOx for the composite catalysts. As illustrated by the SEM images in Fig. 1a-c, the CC substrate shows a smooth surface, while both the TiO2 and ZnO exhibit a well-arranged nanorod structure that grow uniformly on the CC substrate. The as-prepared TiO2 nanorods demonstrate a diameter of ~100 nm and a length of ~1 μm, while the ZnO
Conclusions
In this work, we developed a MnOx-ZnO/CC nanocomposite-based LT-SCR catalyst via a hydrothermal-impregnated route. Compared to its analogue MnOx-TiO2/CC, this MnOx-ZnO/CC catalyst exhibits excellent SCR activities at a low-temperature range of 100–250 °C. By optimizing the loading amount of MnOx and the microstructure properties, catalysts with a largely promoted specific surface area can be obtained, resulting in a high NOx conversion rate of 96.8% at 200 °C for sample 0.06-MnOx-ZnO/CC. In
Acknowledgment
This work is supported by the National Natural Science Foundation of China (Grant No. 51608333, Grant NO. 51472006). The authors would like to thank Corning Incorporated Company for providing the cordierite ceramic samples.
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