Elsevier

Applied Catalysis A: General

Volume 605, 5 September 2020, 117801
Applied Catalysis A: General

Effect of HCl and o-DCBz on NH3-SCR of NO over MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts

https://doi.org/10.1016/j.apcata.2020.117801Get rights and content

Highlights

  • HCl severely poisons the MnOx/TiO2 catalyst towards NH3-SCR of NO.

  • Surface acid sites and reactive oxygen species are ruined in catalyst chlorination.

  • Ce addition improves Cl-resistance of catalyst by preferentially reacting with HCl.

  • Cl atom prefers to anchor on the uncoordinated sites of Ce atom.

  • Competitive effect between o-DCB, NO and NH3 aggravates the catalyst deactivation.

Abstract

A novel selective catalytic reduction (SCR) catalyst with strong chlorine resistance ability is urgently needed to extend the catalyst life, when SCR technique is applied in plants with high chlorine content (e.g. municipal solid waste and medical waste incinerators). This study investigates the influences of typical chlorine-containing inorganic/organic compounds (i.e. hydrogen chloride (HCl) and o-dichlorobenzene (o-DCBz)) on NH3-SCR of NO over MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts at low temperatures (100−250 °C). The results indicate the presence of HCl severely inhibits the NH3-SCR reaction. On one hand, the reaction between NH3 and HCl causes the deposition of NH4Cl on catalyst surface and reduces the amount of NH3; on the other hand, HCl consumes the surface acid sites which is essential to the adsorption and activation of NH3; moreover, chlorine (Cl) atom prefers to anchor on the uncoordinated metal sites that restrains the regeneration of surface reactive oxygen species and interrupts the redox cycle of catalysts. Ce addition enhances the chlorine resistance property of MnOx-CeO2/TiO2 catalyst by interacting with HCl preferentially. The negative effect of o-DCB on NH3-SCR reaction is more serious than that of HCl below 200 °C. That’s because the competitive effect between o-DCB catalytic oxidation and deNOx via NH3-SCR reaction as well as the accumulation of o-DCB incomplete oxidation byproducts on catalyst surface further aggravates the catalyst deactivation induced by Cl atom.

Introduction

Waste incineration has received rapid expansion in China over the last few decades since it can reduce solid waste volume and allow energy recovery. NO emissions from waste incinerators are comparable to those from coal-fired plants [1]. Selective catalytic reduction (SCR) with NH3 has been proven to be an efficient post-combustion technology to control NO emissions from stationary sources [2] and, as a consequence, SCR technique is also applied for NO abatement in some waste incinerators. However, two main problems emerge during the utilization of this technique in industrial field: (1) the high active temperature (300–400 °C) of the conventional SCR catalysts (e.g. V2O5-WO3/TiO2) always requires auxiliary device and additional energy for heating the exhaust behind baghouse filter (160 °C) to achieve high NO conversions [3]; (2) the poor resistance of SCR catalysts to the inevitable deactivation induced by the complicated flue gas compositions, highly increases the catalyst replacement cost and even leads to the abnormal emission of target pollutants [4]. Therefore, a novel catalyst with excellent low-temperature activity and strong resistance to deactivation is significantly demanded for industrial application to avoid the additional energy consumption and frequent catalyst replacement.

Manganese-based catalyst has been regarded as a competitive candidate for future application due to its excellent low-temperature SCR performance [5,6]. It allows the SCR device using manganese-based catalyst to be installed downstream of the baghouse filter to protect the catalyst from abrasion and corrosion. However, there is still a small number of particulates and contaminants left in the flue gas after the baghouse filter. Previous studies have reported that manganese-based catalysts are susceptible to the fly ash components including heavy metals (Pb and Cd) [7], alkali metals (Na and K) [8] and alkali earth metals (Ca and Mg) [9] as well as SO2 [10] in the flue gas. Besides that, various chlorinated contaminants generated from chlorine-containing wastes incineration [11] also reveal a deactivation effect on manganese-based catalysts. As reported, HCl concentration ranges within 200−1500 ppm in the raw flue gas of medical waste incinerators due to the high chlorine content in medical wastes [12]. Ji et al. [13] surveyed 9 municipal solid waste (MSW) incinerators and pointed out that the HCl concentration in flue gas can be controlled efficiently by means of alkaline reagents (i.e. Ca(OH)2 or NaHCO3) to fulfill the emission limit (60 mg/Nm3) of China (GB 18485−2014). However, HCl residual in flue gas still deactivates the SCR catalyst mainly due to the transformation of active components to inactive chlorinated species; the deposition of NH4Cl formed from the reaction between NH3 and HCl on catalyst surface; and the change of catalyst surface acid property [4,14]. Alkali chlorines (e.g. KCl) in flue gas shows more severe inhibition effect on SCR activity than other alkali salts (e.g. K2SO4, KNO3 and K2CO3) [15,16]. In addition, chlorinated aromatics in flue gas act as another chlorine source; nevertheless, few literatures report the impact of those compounds on NH3-SCR reaction [17].

Cerium oxide (CeO2) is frequently selected as a promoter to optimize the SCR performance of MnOx catalysts [18]. Our previous study [19] has reported that the TiO2 supported MnOx-CeO2 composite catalysts exhibits optimal low-temperature SCR performance and above 98 % of NO is abated below 200 °C. Different researches indicated that the combination of MnOx with CeO2 can form uniform MnCeOx solid solution and couple the multiple oxidation states of MnOx (Mn4+/Mn3+/Mn2+) with the fast redox cycle of CeO2 (Ce4+/Ce3+), thereby improves the SCR performance of the catalysts [5,20]. Besides that, several groups also have proposed that CeO2 modification shows promotion effect on the chlorine resistance ability of MnOx [17] and CuOx [21] catalysts.

In this study, the catalytic activities of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts with or without HCl were experimentally evaluated at low temperatures. The deactivation mechanism of HCl on NH3-SCR of NO as well as the promotion effect of Ce addition on the chlorine resistance property of catalyst was explored based on the characterizations of the fresh and deactivated catalysts. Besides, the Gibbs free energy calculation was carried out to further verify the proposed deactivation mechanism. Moreover, the differences between the deactivation mechanisms of HCl and o-DCBz on NH3-SCR reaction were also contrasted in detail.

Section snippets

Catalyst preparation and characterizations

The MnOx/TiO2 (with the Mn/Ti mole ratio = 0.15, notated as Mn/Ti) and MnOx-CeO2/TiO2 (with the Mn/Ti mole ratio = 0.15 and the Ce/Ti mole ratio = 0.10, notated as MnCe/Ti) catalysts were prepared using sol-gel method, which has been depicted in detail in our previous study [19]. Both the fresh Mn/Ti and MnCe/Ti catalysts and the two catalysts used after NH3-SCR reaction with 1000 ppm HCl were characterized to investigate the deactivation mechanism of HCl on catalytic activity as well as the

Catalytic activity evaluation with or without HCl

The influence of HCl on the catalytic performance towards NO reduction at different reaction temperatures was studied on Mn/Ti and MnCe/Ti catalysts. As seen from Fig. 1, the results indicate that the presence of HCl dramatically reduces the SCR activities of the catalysts, especially for Mn/Ti catalyst. The NO conversions from Mn/Ti catalyst are declined from 31.6 %, 55.1 %, 70.0 % and 46.0 % to 6.9 %, 23.4 %, 40.5 % and 30.8 % at the temperatures of 100°C, 150°C, 200°C and 250°C respectively

Conclusion

The influences of HCl on NH3-SCR activities of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts were investigated based on the catalytic characterization of the fresh and deactivated catalysts. The results indicate that HCl inhibits the NH3-SCR reaction because it consumes the reactant NH3, disrupts the surface acid sites and blocks the redox cycle of catalyst. Higher HCl concentration and lower reaction temperature aggravate the deactivation effect of HCl on SCR activity. Ce addition can enhance the

Credit author statement

I have made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; I have drafted the work or revised it critically for important intellectual content; I have approved the final version to be published; I agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriated investigated and resolved.

All persons who have made

Declaration of Competing Interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgements

This research is supported by Natural Science Foundation of Shanghai (17ZR1419400) and National Natural Science Foundation of China (51976129).

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