Co- or Ni-modified Sn-MnOx low-dimensional multi-oxides for high-efficient NH3-SCR De-NOx: Performance optimization and reaction mechanism

https://doi.org/10.1016/j.jes.2021.05.032Get rights and content

Highlights

  • Low-dimensional multi-oxides nanosheet were synthesized successfully by one-step synthesis.

  • Co-Mn-SnO2 solid solution promoted the efficient electron interaction for catalysis.

  • Increased adsorbed oxygen, Mn3++Mn4+, Sn4+ and more acidity on Lewis acid sites were obtained.

  • E-R and l-H mechanisms co-existed on Co-Mn-SnO2 surface in SCR reaction process.

  • L-NH3 species and bidntate nitrate were the main active species at high temperatures.

Abstract

NH3-SCR performances were explored to the relationship between structure morphology and physio-chemical properties over low-dimensional ternary Mn-based catalysts prepared by one-step synthesis method. Due to its strong oxidation performance, Sn-MnOx was prone to side reactions between NO, NH3 and O2, resulting in the generation of more NO2 and N2O, here most of N2O was driven from the non-selective oxidation of NH3, while a small part generated from the side reaction between NH3 and NO2. Co or Ni doping into Sn-MnOx as solid solution components obviously stronged the electronic interaction for actively mobilization and weakened the oxidation performance for signally reducing the selective tendency of side reactions to N2O. The optimal modification resulted in improving the surface area and enhancing the strong interaction between polyvalent cations in Co/Ni-Mn-SnO2 to provide more surface adsorbed oxygen, active sites of Mn3+ and Mn4+, high-content Sn4+ and plentiful Lewis-acidity for more active intermediates, which significantly broadened the activity window of Sn-MnOx, improved the N2 selectivity by inhibiting N2O formation, and also contributed to an acceptable resistances to water and sulfur. At low reaction temperatures, the SCR reactions over three catalysts mainly obeyed the typical Elye-rideal (E-R) routs via the reactions of adsorbed l-NHx (x = 3, 2, 1) and B-NH4+ with the gaseous NO to generate N2 but also N2O by-products. Except for the above basic E-R reactions, as increasing the reaction temperature, the main adsorbed NOx-species were bidentate nitrates that were also active in the Langmuir-Hinshelwood reactions with adsorbed l-NHx species over Co/Ni modified Mn-SnO2 catalyst.

Graphical abstract

Probable surface SCR reaction pathways over Ni/Co-modified Mn-SnO2 were put forward to follow the basic Elye-Rideal reactions between gaseous NO with adsorbed l-NHx on Lewis acid sites at low temperature while also obey the Langmuir-Hinshelwood routes of main active bidentate nitrates with l-NHx as increasing the reaction temperature.

Image, graphical abstract
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Introduction

Selective catalytic reduction with ammonia (NH3-SCR) has been a full-fledged technology that is most application for the abatement of NOx (mainly NO and NO2) participating in the formation of ozone depletion, particulate matter, acid rain, photochemical smog and haze pollution (Zhang et al., 2020a), (Yang et al., 2019), etc. Nowdays, it is a major challenge to develop the low–temperature SCR catalysts working below 250 °C with excellent efficiency and strong stability using in non-electrical industries (steel, coking, cement, building materials, gas boiler, etc.) (Gao et al., 2017b). Among of typical metal oxides (Mn (Zhang et al., 2020b), V (Wu et al., 2020a), Co (Hu et al., 2015), Cu (Gao et al., 2021), Ce (Yao et al., 2017), Fe (Zhang et al., 2020d), etc.), Mn-based catalysts have become the major research objects due to the superior property at low temperatures profiting from variable valence states and excellent redox ability (Andreoli et al., 2015), which are also significantly affected by the crystal structure, crystallinity, specific surface area, oxidation state, active oxygen, active sites and acidity on the surface (Gao et al., 2017b), (Andreoli et al., 2015), etc. Nevertheless, the narrow reaction temperature window, poisoning by H2O and SO2 and poor selectivity to N2 have are the concerned weaknesses of Mn-based catalysts (Zhang et al., 2020a). In our previous paper (Gao et al., 2017b), we reviewed the latest research progress on Mn-based catalysts that are expected to break through the resistance, such as multi-metal modified MnOx with special crystal or/and shape structures. In the previous paper (Gao et al., 2017a), we found Co– or Ni–MnOx–CeO2 catalysts had higher SCR performance because of higher special area, greater contents of chemisorbed–oxygen and more active sites. Besides, the Co/Ni element played an effective role in delaying the poison effects by SO2, while the mixed oxides would still loss its SCR activity during a long-time reaction (Gao et al., 2018). Most importantly, the design of Mn–based catalysts with micro–crystal morphology (core–shell (Liu et al., 2015), micro–sheet (Gao et al., 2020a), spinel crystal (Gao et al., 2019), etc.) are the potential ideas to overcome the above poisoning process.

In recent years, benefiting from its ultrathin nanoscale structure (Tang et al., 2018), low-dimensional nanomaterials have large specific surface area and highly exposed surface atoms, which provide the possibility for the full surface-exposure of active sites (Zhang, 2015). Low-dimensional nanomaterials induce rich physical and chemical properties due to the reduction in dimensions, which is suitable for being used as a catalyst with excellent catalytic performances in energy catalytic reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), etc., and in environmental catalytic reactions such as selective catalytic reduction (SCR) and catalytic conversion of volatile organic compounds (VOCs). In recent years, researchers have been working on the development of new materials, actively researching the unique advantages of low-dimensional materials in the field of catalysis and optimizing their synthesis methods in order to achieve large-scale applications as early as possible. In our review acticle (Yang et al., 2020a), we summarized that the top-down peeling methods (micromechanical cleavage, oxidation/reduction-based intercalation-assisted exfoliation, mechanical force-assisted exfoliation, ion exchange-assisted exfoliation, etching-assisted exfoliation) and bottom-up controllable synthesis strategies (chemical vapor deposition, wet chemical methods) were applied to prepare higher quality ultra-thin nanomaterials according to whether the bulk materials have a layered structure. Furthermore, the micro-fine control of low-dimensional nanomaterial properties can be carried out through surface modification, functionalization and phase engineering strategies to further optimize its catalytic performance. For instance, the adjustable electronic properties make it easy to be regulated into multi-component composites by surface modification and doping element, which greatly affect the catalytic performance and have shown outstanding application prospects in the field of catalysis of NOx on mesoporous MnO2 nanosheets (Liu et al., 2018), MnOx/TiO2 nanosheets (Deng et al., 2016), Fe2O3−Mn2O3 nanosheet (Li et al., 2016), Ni-MnOx/NF nanosheet (Cai et al., 2014), foliated NiMn2O4-spinel (Gao et al., 2020a).

In our previous study (Yang et al., 2020b), the low dimensional flake Mn4Co1Ox composite oxides used for low-temperature NH3-SCR were prepared by four different methods of hydrothermal, template, co-precipitation and one-step process. It was found that Co-MnOx nanosheet catalyst was synthesized successfully via the gradual induction and hydrolysis of sodium lauryl sulfate controlling the effective persistence of redox reactions (Yang et al., 2020b). As a "bottom-up" strategy of wet chemical theory, this steerable preparation method is relatively easy to control and available in guiding the assemblage of nanosheet-structured metal oxides. Importantly, the catalyst deriving from this method has the relatively good catalytic activity and acceptable N2 selectivity during the NH3-SCR reaction. In addition of Co (Shi et al., 2019), in our previous studies, other metal-element (Cr (Gao et al., 2019), Fe (Gu et al., 2019), Ni (Gao et al., 2020b), Sn (Gao et al., 2017a), etc.) doping into Mn-based catalysts also had the potential of low-temperature NH3-SCR activity due to their excellent redox properties. It was found that the novel fluffy structural Co–Mn–O, Fe–Mn–O, Ni–Mn–O catalysts had high degree crystal splitting, high surface areas, abundant surface acid sites and large active surface oxygen, which were essential for the enhancement of their catalytic activities (Meng et al., 2014). Among them, the atomic number of Ni element is 28 (adjacent to Co of 27), has the similar ion radius (0.60 Å) to Co (0.61 Å) might owning the similar chemical properties. The effective electron transfer between Ni and Mn also plays an important role in the efficient SCR activity (Liu et al., 2021). Study (Kilic and Zunger, 2002) showed that SnO2 has low structural energy and strong mutual attraction between the Sn interstitial site and oxygen vacancy, so there was a large number of inherent defects in the structure, and the bridged oxygen on SnO2 (110) surface could be removed to form oxygen vacancy at temperature below 227 °C. However, Sn has an atomic number of 50 and an ion radius of 0.69 Å, which is quite different from Co and also Ni.

Thus, in this study, the next experiments were investigated to reveal the universality and commonality of oxalate co-precipitation method for the preparation of low-dimensional multi-MnOx (Me=Co, Ni, Sn) in the representation of Ni element with similar radius to Co element and also larger radius Sn, and further used for the removal of NOx at low temperature with a view to morphology and structure. The catalytic performances and physicochemical properties of Sn-MnOx catalysts before and after doping Co and Ni elements were explored based on the experimental design and characterization analysis by SEM, XRD, TPSR, XPS, H2-TPR, NH3-TPD and in-situ DRIFTS, the changes of surface species were analyzed through the study of adsorption-reactivation behaviors to explore the active species in the SCR reaction, and further realized the reaction mechanism on the surface of catalysts. Therefore, Co- or Ni-modified Sn-MnOx multi-oxides can significantly improved the specific surface area of the catalyst, promoted the generation of acid site and adsorbed oxygen, and obtain better redox quality, thus showing excellent SCR capacity including activity and selectivity at low temperature, and presented an acceptable resistances to H2O&SO2.

Section snippets

Catalysts preparation

Me-Mn bi-metal oxides (Me= Co, Ni or Sn) were prepared by one-step synthesis method using odium lauryl sulfate, KMnO4 and metal-precursor. An acidic solution of sodium lauryl sulfate (SDS, 1 mmol/L [H+]) was heated at 95 °C for 15 min. Then, an appropriate amount of KMnO4 and SnCl4•5H2O, Co(NO3)2•6H2O or Ni(NO3)2•6H2O) were dissolved in deionized water, and the prepared solution (0.05 mol/L, typically, optimized Mn:Sn:Co = 8:2:1) was quickly added to the above solution, reacted in a water bath

Influence of dopant and doping ratio on SCR performances

In our preliminary research, it was found that the one-step synthesis of redox co-precipitation method was well applicable in universally preparing Me-MnOx nanosheet catalysts with different metal doping (Me = Sn, Co, Ni, in Fig. S1). Among them, as shown in Fig. S2, Sn-MnOx catalyst has the best catalytic activity at low temperature (below 175 °C), but its activity decreased obviuosly at high temperature with poor selectivity to N2. Co and Ni doped MnOx catalysts could maintain stable

Conclusions

In this study, the ternary composite Me-MnOx catalyst was prepared by one-step synthesis method on the basis of Sn-MnOx catalyst doped with Co and Ni elements, and the catalytic performance and physicochemical properties of Ni/Co-Sn-MnOx catalyst were investigated by oxidation experiment, transient reactions, XRD, XPS, H2-TPR, NH3-TPD and DRIFTs characterization. The adsorption form and their change of surface substances were studied by in-situ infrared experiment to find out the active species

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (Nos. U20A20130, 21806009), China Postdoctoral Science Foundation (2019T120049) and Fundamental Research Funds for the Central Universities (No. 06500152).

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