低温氨-选择催化还原用氧化锰-沸石杂化催化剂上Brønsted/Lewis酸位的作用

doi:10.1016/S1872-2067(24)60112-9

摘要/Abstract

摘要：

尽管金属氧化物-沸石杂化材料通过中间物种的颗粒间扩散在氮氧化物去除反应中增强了催化活性和选择性, 但它们在酸性位点上的后续反应机理尚不清楚, 需要进一步深入研究. 本文通过引入钾离子精确调节了杂化材料中Brønsted/Lewis酸位点的分布, 钾离子不仅选择性地与Brønsted酸位点结合, 而且可能影响活化NO物种的形成和扩散. 原位漫反射红外傅里叶变换光谱和NH₃选择性催化还原NO_x (NH₃-SCR)反应结果表明, MnO_x上的Lewis酸位点对NO还原的活性更高, 但对N₂的选择性低于Brønsted酸位点. MnO_x上Brønsted酸位点主要产生N₂, 而Lewis酸位点主要形成了N₂O, 使得N₂选择性较低. Y沸石上Brønsted酸比MnO_x上的更强, 从而加速了NH₃-SCR反应, 其中从MnO_x颗粒扩散来的亚硝酸盐/硝酸盐物种迅速转化为N₂. 因此, 设计催化剂非常重要的是使MnO_x上形成的活性NO物种扩散到H-Y沸石的Brønsted酸位, 而不是扩散到MnO_x的Brønsted酸位并选择性分解. 对于物理混合的H-MnO_x+H-Y样品, H-MnO_x中丰富的Brønsted/Lewis酸位点在颗粒间扩散前会导致活性NO物种的大量消耗, 从而阻碍了协同效应的增强. 此外, 研究发现K-MnO_x中嵌入的K⁺对NO还原速率具有意想不到的促进作用, 这可能是由于活化的NO物种在K-MnO_x上的扩散速度比H-MnO_x快. 综上, 本文通过确定酸位点在两种不同成分中的作用, 对设计高效的金属氧化物-沸石杂化催化剂提供参考.

关键词: 金属氧化物-分子筛杂化材料, 酸性位的作用, 氧化锰, 物理混合, 氨选择催化还原NO_x

Abstract:

Although metal oxide-zeolite hybrid materials have long been known to achieve enhanced catalytic activity and selectivity in NO_x removal reactions through the inter-particle diffusion of intermediate species, their subsequent reaction mechanism on acid sites is still unclear and requires investigation. In this study, the distribution of Brønsted/Lewis acid sites in the hybrid materials was precisely adjusted by introducing potassium ions, which not only selectively bind to Brønsted acid sites but also potentially affect the formation and diffusion of activated NO species. Systematic in situ diffuse reflectance infrared Fourier transform spectroscopy analyses coupled with selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) reaction demonstrate that the Lewis acid sites over MnO_x are more active for NO reduction but have lower selectivity to N₂ than Brønsted acids sites. Brønsted acid sites primarily produce N₂, whereas Lewis acid sites primarily produce N₂O, contributing to unfavorable N₂ selectivity. The Brønsted acid sites present in Y zeolite, which are stronger than those on MnO_x, accelerate the NH₃-SCR reaction in which the nitrite/nitrate species diffused from the MnO_x particles rapidly convert into the N₂. Therefore, it is important to design the catalyst so that the activated NO species formed in MnO_x diffuse to and are selectively decomposed on the Brønsted acid sites of H-Y zeolite rather than that of MnO_x particle. For the physically mixed H-MnO_x+H-Y sample, the abundant Brønsted/Lewis acid sites in H-MnO_x give rise to significant consumption of activated NO species before their inter-particle diffusion, thereby hindering the enhancement of the synergistic effects. Furthermore, we found that the intercalated K⁺ in K-MnO_x has an unexpected favorable role in the NO reduction rate, probably owing to faster diffusion of the activated NO species on K-MnO_x than H-MnO_x. This study will help to design promising metal oxide-zeolite hybrid catalysts by identifying the role of the acid sites in two different constituents.

Key words: Hybrid metal oxide-zeolite, The role of acid sites, Manganese oxides, Physical mixing, Selective catalytic reduction of NO_x with NH₃

Hyun Sub Kim, Hwangho Lee, Hongbeom Park, Inhak Song, Do Heui Kim. 低温氨-选择催化还原用氧化锰-沸石杂化催化剂上Brønsted/Lewis酸位的作用[J]. 催化学报, 2024, 65: 79-88.

Hyun Sub Kim, Hwangho Lee, Hongbeom Park, Inhak Song, Do Heui Kim. Understanding the roles of Brønsted/Lewis acid sites on manganese oxide-zeolite hybrid catalysts for low-temperature NH₃-SCR[J]. Chinese Journal of Catalysis, 2024, 65: 79-88.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(24)60112-9

https://www.cjcatal.com/CN/Y2024/V65/I10/79

图/表 7

Fig. 1. (a) XRD patterns of as-synthesized K-MnOx and H-MnOx. (b) XRD peak shift after removal of K+ in K-MnOx sample via HNO3 treatment. N2 adsorption-desorption isotherms (c), and DRIFTS spectra (d) of K-MnOx and H-MnOx samples after 500 ppm NH3 + 5% O2 adsorption at 100 °C for 30 min.

Fig. 2. NOx conversion (a) and N2 selectivity (b) over MnOx and physically mixed MnOx+Y samples in NH3-SCR reaction at low temperatures with the following reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, and GHSV = 120000 mL h?1 g?1 based on MnOx amount.

Fig. 3. N2O concentration of K-MnOx, H-MnOx, and K-MnOx+K-Y in NH3-SCR reaction at 200 and 250 °C with the following reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol%, N2 balance, and GHSV = 120000 mL h?1 g?1 based on MnOx amount.

Fig. 4. In situ DRIFT spectra of K-MnOx (a,b) and H-MnOx (c,d) after exposure to 500 ppm NH3 + 5% O2 followed by 500 ppm NO + 5% O2 at 100 °C. (e) The reactivity of adsorbed NH3 species towards activated NO species over H-MnOx plotted as a normalized intensity.

Fig. 6. The reactivity of adsorbed NH3 species towards activated NO species over H-MnOx+H-Y and K-MnOx+H-Y, plotted as a normalized intensity.

Fig. 5. In situ DRIFT spectra of H-Y zeolite (a,b), H-MnOx+H-Y (c,d), and H-MnOx+H-Y (e,f) after exposure to 500 ppm NH3 + 5% O2 followed by 500 ppm NO + 5% O2 at 100 °C.

Scheme 1. The plausible reaction scheme over the physically mixed MnOx and zeolite hybrid catalyst in low-temperature NH3-SCR reaction.

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