Poisoning Effects of Alkali and Alkaline Earth Metal Doping on Selective Catalytic Reduction of NO with NH3 Over the Nb-Ce/Zr-PILC Catalysts

catalysts

Article Poisoning Effects of Alkali and Alkaline Earth Metal Doping on Selective Catalytic Reduction of NO with NH3 over the Nb-Ce/Zr-PILC Catalysts

Chenxi Li 1, Jin Cheng 1, Qing Ye 1,* , Fanwei Meng 1, Xinpeng Wang 1 and Hongxing Dai 2,*

1 Key Laboratory of Beijing on Regional Air Pollution Control, Department of Environmental Science, School of Environmental and Chemical Engineering, Faculty of Environment and Life, Beijing University of Technology, Beijing 100124, China; [email protected] (C.L.); [email protected] (J.C.); [email protected] (F.M.); [email protected] (X.W.) 2 Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, and Laboratory of Catalysis Chemistry and Nanoscience, Department of Environmental Chemical Engineering, School of Environmental and Chemical Engineering, Faculty of Environment and Life, Beijing University of Technology, Beijing 100124, China * Correspondence: [email protected] (Q.Y.); [email protected] (H.D.)

Abstract: The poisoning effects of alkali metals (K and Na) and alkaline earth metals (Ca and Mg) on catalytic performance of the 2Nb4Ce/Zr-PILC catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated, and physicochemical properties of the catalysts were char- acterized by means of the X-ray diffraction XRD (XRD), Brunner−Emmet−Teller (BET), hydrogen temperature-programmed reduction (H2-TPR), X-ray Photoelectron Spectroscopy (XPS), ammonia temperature-programmed desorption (NH3-TPD), and in situ diffuse reﬂectance infrared Fourier Citation: Li, C.; Cheng, J.; Ye, Q.; transform spectroscopy (in situ DRIFTS) techniques. Doping of M (M = K, Na, Ca, and Mg) deacti- Meng, F.; Wang, X.; Dai, H. Poisoning vated the 2Nb4Ce/Zr-PILC catalyst according to the sequence of 0.8 K > 0.8 Na > 0.8 Ca > 0.8 Mg Effects of Alkali and Alkaline Earth Metal Doping on Selective Catalytic (M/Ce molar ratio = 0.8). The characterization results showed that the decreases in redox ability, 3+ 4+ Reduction of NO with NH3 over the NH3 adsorption, Ce /Ce atomic ratio, and amount of the chemisorbed oxygen (Oβ) were the Nb-Ce/Zr-PILC Catalysts. Catalysts important factors inﬂuencing catalytic activities of the alkali metal-and alkaline earth metal-doped 2021, 11, 329. https://doi.org/ samples. Consequently, compared with the Mg- and Ca-doped samples, doping of K caused the

10.3390/catal11030329 2Nb4Ce/Zr-PILC sample to possess the lowest redox ability, NH3 adsorption, and amount of the Oβ species, which resulted in an obvious deactivation effect. Academic Editor: Luciana Lisi Keywords: alkali metal; alkaline earth metal; surface acidity; poisoning; selective catalytic reduction; Received: 9 February 2021 NH3-SCR Accepted: 1 March 2021 Published: 5 March 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- It is well known that the selective catalytic reduction of NOx with NH3 (NH3-SCR) iations. is one of the most effective technologies to remove NOx from stationary source flue gas, but poor resistance to poisoning induced by the alkali metals and alkaline earth metals is still an urgent problem to be solved [1]. Fly ash is a major problem for the SCR catalysts because it may clog the pores of the catalysts and react with the active components [1–3]. However, the alkali or alkaline earth metals are the main components of fly ash, and Copyright: © 2021 by the authors. have a strong toxic effect on the SCR catalysts. Effects of alkali or alkaline earth metals Licensee MDPI, Basel, Switzerland. on catalytic activity of the well-known vanadium-titanium-based catalysts utilized in This article is an open access article distributed under the terms and industry have been widely reported [4,5]. Especially for the tungsten-free catalysts, alkali conditions of the Creative Commons and alkaline earth metal (mainly sodium, potassium, calcium, and magnesium) salts are Attribution (CC BY) license (https:// generally considered to reduce acidity, and may partially react with V2O5, giving rise to creativecommons.org/licenses/by/ the deactivation of the SCR catalysts [1,6]. 4.0/).

Catalysts 2021, 11, 329. https://doi.org/10.3390/catal11030329 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 329 2 of 17

In recent years, ceria as the NH3-SCR catalyst has attracted much more attention, owing to its high oxygen storage capacity, excellent redox property, and nontoxicity [7,8]. Therefore, there have been many studies on the ceria-based NH3-SCR catalysts in recent years, such as Ce–Ti [9], Ce–W [10], Ce–Nb [11], and Ce–Mo [12]. By introducing W, Nb, P or other solid acid component, SCR performance of the Ce-based catalysts could be remarkably improved. Among them, the Ce-Nb-based catalysts have broad applications in the flue gas post-treatment system due to their excellent performance [11,13]. In addition, a support is also considered as an important component of a NH3-SCR catalyst because it can provide acidic sites and good dispersion of the active sites. Pillared interlayer clay (PILC) is a two-dimensional zeolite-like material. It is synthesized by replacing charge-compensating cations in the clay layers with macropolymers formed by hydrolysis of oligohydroxy metal cations or salts, thus separating these layers and finally forming porous networks with two-dimensional channels [14–16]. Compared with the zeolite-like support, PILC is a low-cost material with a large pore volume, a high surface area, good thermal stability, and high acidity. Hence, PILC could have specific applications if the column size and composition were modified. Although SCR performance of the Nb-Ce-based catalysts has been reported, few studies have been carried out on the Zr-pillared clay-supported Nb-Ce catalysts, especially effects of the alkali or alkaline earth metals on NH3-SCR performance, furthermore the interaction between the poison and cerium is still unclear. Therefore, the main purpose of this study is to investigate effects of doping alkali and alkaline earth metals on catalytic performance of the 2Nb4Ce/Zr-PILC sample and elucidate their deactivation mechanisms.

2. Results 2.1. SCR Performance According to one of our previous studies, 2Nb4Ce/Zr-PILC showed excellent catalytic activity [17]. Therefore, the poisoning effects of alkali and alkaline earth metals on catalytic performance of 2Nb4Ce/Zr-PILC were investigated. DeNOx performance of the different alkali metal- and alkaline earth metal-poisoned 2Nb4Ce/Zr-PILC samples were tested in a simulating gas system, as shown in Figure1. When the molar ratio of Na/Ce, K/Ce, Ca/Ce or Mg/Ce was equal to 0.8, different alkali metals and alkaline earth metal oxides exhibited different degrees of inhibition on NH3- SCR activity, among which the poisoning effect of Mg was the weakest: NO conversion over 0.8 Mg-2Nb4Ce/Zr-PILC decreased only slightly (about 20%) from 150 to 350 ◦C, but the activity was still high in the range of 400−450 ◦C. The degree of Ca-poisoning was almost the same as that of Mg-poisoning. The activity of Ca-doped sample decreased slightly in the entire temperature range, and the maximum NO conversion was 80% at 450 ◦C. However, when K and Na were doped to the 2Nb4Ce/Zr-PILC sample, their NO conversions decreased signiﬁcantly. Especially, K-poisoning was the most serious (NO conversion was only 65% at 450 ◦C). According to the above results, doping of alkali and alkali earth metals on poisoning degree of the 2Nb4Ce/Zr-PILC sample decreased in the order of 0.8 K > 0.8 Na > 0.8 Ca > 0.8 Mg, indicating that inhibition of the alkali and alkaline earth metals increased proportionally to the alkalinity. The 0.8 K-2Nb4Ce/Zr-PILC sample was deactivated most seriously among all of samples doped with equal molar amount of alkali and alkali earth metals. Catalysts 2021,, 11,, x 329x FORFOR PEERPEER REVIEWREVIEW 33 of of 17 17

3 FigureFigure 1. 1. CatalyticCatalytic activity activity for for the the NH NH3-SCR-SCR3-SCR reactionreaction reaction overover over thethe the freshfresh fresh 2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC 2Nb4Ce/Zr-PILC andand andalkalialkali alkali metal-metal- and and alkaline alkaline earth earth metal-doped metal-doped samples samples under under the the reaction reaction conditions conditions of of(1000 (1000 ppm ppm NO NO + + 1100 ppm NH3 + 4 vol% O2 + 5 vol% H2O + N2 (balance)) and SV = 100,000 mL/(g h). 1100 ppm NH 3 ++ 4 4 vol% vol% O O22 ++ 5 5vol% vol% H H2O2O + +N N2 (balance))2 (balance)) and and SV SV = 100,000 = 100,000 mL/(g mL/(g h). h).

FigureFigure 22 showsshows deNOdeNOxx activitiesactivities of of thethe 2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC samplessamples samples withwith differentdifferent amountsamounts of of K K at at different different temperatures. temperatures. The The2Nb4Ce/Zr-PILC 2Nb4Ce/Zr-PILC sample sample showed showed high activ- high ◦ ityityactivity inin thethe in wholewhole the whole testtest testtemperaturetemperature temperature rangerange range (esp(esp (especiallyecially at at300–450 300–450 °C,C, where where NO NO conversion conversion ◦ reachedreached 96% 96% at at 400 400 °C).C). However, However, K K doping doping exhibited exhibited a strong strong inhibitory effect on deNO xx performanceperformance of of the the sample. When When the the K/Ce K/Ce molar molar ratio ratio was was 0.3, 0.3, activity activity of of the the sample decreaseddecreased by by 10–30% 10–30% in the wholewhole temperaturetemperature range.range. With increasingincreasing thethe K/CeK/Ce molar ratioratio to to 0.8, 0.8, the the activity activity inhibition inhibition became became gr graduallyadually serious, serious, and and the the activity activity was was less less than than ◦ 65%65% at at 150–450 150–450 °C.C. These These results results demonstrate demonstrate that that deNO xx performanceperformanceperformance ofof thethe samplesample waswas greatlygreatly inhibited inhibited and and the the inhibition inhibition effect effect increased with a rise in K/Ce molar molar ratio. ratio.

Figure 2. Catalytic activity for the NH33-SCR reaction over the fresh 2Nb4Ce/Zr-PILC and K-poisoned Figure 2. Catalytic activity for the NH3-SCR-SCR reactionreaction overover thethe freshfresh 2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC andand K-poi-K-poi- samples under the reaction conditions of (1000 ppm NO + 1100 ppm NH + 4 vol%3 O + 5 vol%2 H O sonedsoned samplessamples underunder thethe reactionreaction conditionsconditions ofof (1000(1000 ppmppm NONO ++ 11001100 ppmppm3 NHNH3 ++ 44 vol%vol%2 OO2 ++ 55 2 2 2 vol%+ N2 (balance))H2O + N 2 (balance))(balance)) and SV = 100,000andand SVSV mL/(g== 100,000100,000 h). mL/(gmL/(g h).h).

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Catalysts 2021, 11, 329 4 of 17 2.2. NH3 Oxidation

To clarify effects of the alkali and alkaline earth metals on NH3 oxidation, NH3 oxi-

dation2.2. NH 3experimentsOxidation were performed. Figure 3A shows catalytic activities of the fresh and M-poisoned samples for NH3 oxidation. Over all of the samples, NH3 began to be oxidized To clarify effects of the alkali and alkaline earth metals on NH3 oxidation, NH3 oxidation atexperiments 300 °C, and were NH performed.3 conversion Figure 3increasedA shows catalytic with aactivities rise in temperature. of the fresh and However, M-poisoned NH3 oxi- ◦ dationsamples activity for NH3 overoxidation. each of Over the all M-poison of the samples,ed samples NH3 beganshowed to bea downward oxidized at 300 trendC, as com- paredand NH with3 conversion the fresh increased sample, with indicating a rise in temperature.that doping However,of the alkali NH and3 oxidation alkaline activity earth metals inhibitedover each of ammonia the M-poisoned oxidation. samples It showedwas reported a downward that trenda strong as compared redox ability with the could fresh lead to sample, indicating that doping of the alkali and alkaline earth metals inhibited ammonia oxi- over-oxidation of NH3 [18,19]. Therefore, the decrease in ammonia oxidation activity dation. It was reported that a strong redox ability could lead to over-oxidation of NH [18,19]. might be caused by the decrease in redox ability (discussed below). As shown3 in Figure Therefore, the decrease in ammonia oxidation activity might be caused by the decrease in 3B, during the NH3 oxidation process, N2O formation over all of the samples increased redox ability (discussed below). As shown in Figure3B, during the NH 3 oxidation process, with a rise in temperature. The poisoning of the alkali or alkaline earth metal progres- N2O formation over all of the samples increased with a rise in temperature. The poisoning of sivelythe alkali led or alkalineto formation earth metal of progressivelythe undesired led to N formation2O product, of the undesiredespecially N2 Oover product, the 0.8 K- 2Nb4Ce/Zr-PILCespecially over the 0.8 sample. K-2Nb4Ce/Zr-PILC sample.

Figure 3.3. ((AA)) NHNH33 conversionconversion and and (B (B)N) N2O2O concentration concentration as as a functiona function of temperatureof temperature over over the the fresh andand alkali alkali metal- metal- and and alkaline alkaline earth earth metal-poisoned metal-poisoned samples samples during during NH3 oxidation NH3 oxidation under the under the reaction conditions conditions of of (1100 (1100 ppm ppm NH NH3 + 43 vol%+ 4 vol% O2 + O N2 2+(balance)) N2 (balance)) and SV and = 100,000SV = 100,000 mL/(g mL/(g h). h). 2.3. XRD 2.3. XRD Crystal phases of the samples before and after M-poisoning were determined by the XRD technique,Crystal phases and their of the patterns samples are presentedbefore and in Figureafter M-poisoning4. The XRD peaks were belonging determined to by the XRDthe two-dimensional technique, and hk their reﬂections patterns (at 2areθ = presente 19.8◦ andd 34.9 in Figure◦) and the 4. The quartz XRD and peaks cristobalite belonging to theimpurities two-dimensional (at 2θ = 26.6 hk◦ and reflections 28◦) were (at clearly 2θ = observed19.8° and in 34.9°) all of and the samplesthe quartz [15 ].and These cristobalite peaksimpurities were (at characteristic 2θ = 26.6°of and montmorillonite. 28°) were clearly No observed peaks assignable in all of to the the samples alkali metal [15]. These peaksoxides were were characteristic detected, which of wasmontmorillonite. amorphous and No dispersed peaks assignable evenly on to the the surface alkali ofmetal ox- ides2Nb4Ce/Zr-PILC. were detected, which was amorphous and dispersed evenly on the surface of 2Nb4Ce/Zr-PILC.

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FigureFigure 4.4.XRD XRD patterns patterns of of (A ()A alkali) alkali metal- metal- and and alkaline alkaline earth earth metal-poisoned metal-poisoned and (B )and K-poisoned (B) K-poisoned samples. samples. 2.4. Surface Area and N2 Adsorption-Desorption Isotherm

2.4. SurfaceTextural Area parameters and N2 Adsorption-Desorption of the samples before Isotherm and after M-poisoning are summarized in 2 3 Table1Textural. Surface parameters area and pore of the volume samples of 2Nb4Ce/Zr-PILC before and after wereM-poisoning 271 m /g are and summarized 0.164 cm /g, in respectively. When the M was doped, surface area and pore volume dropped obviously. For Table 1. Surface area and pore volume of 2Nb4Ce/Zr-PILC were 271 m2/g and 0.164 cm3/g, example, surface area and pore volume of 0.8 K-2Nb4Ce/Zr-PILC decreased to 215 m2/g respectively. When the M was doped, surface area and pore volume dropped obviously. and 0.134 cm3/g, respectively. These results indicate that the doped M caused the partial For example, surface area and pore volume of 0.8 K-2Nb4Ce/Zr-PILC decreased to 215 blocking of pores, which explains part of the reasons why activity of the M-poisoned m2/g and 0.134 cm3/g, respectively. These results indicate that the doped M caused the sample declined. partial blocking of pores, which explains part of the reasons why activity of the M-poisoned sample declined.

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Catalysts 2021, 11, 329 6 of 17

Table 1. BET surface areas and pore volumes of the samples.

Sample BET Surface Area (m2/g) Pore Volume (cm3/g) Table 1. BET surface areas and pore volumes of the samples. 2Nb4Ce/Zr-PILC 271 0.164 0.3K-2Nb4Ce/Zr-PILCSample BET Surface231 Area (m2/g) Pore Volume0.150 (cm 3/g) 0.8K-2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC 271215 0.1640.134 0.8Na-2Nb4Ce/Zr-PILC0.3K-2Nb4Ce/Zr-PILC 231215 0.1500.140 0.8Ca-2Nb4Ce/Zr-PILC0.8K-2Nb4Ce/Zr-PILC 215217 0.1340.148 0.8Mg-2Nb4Ce/Zr-PILC0.8Na-2Nb4Ce/Zr-PILC 215224 0.1400.149 0.8Ca-2Nb4Ce/Zr-PILC 217 0.148 0.8Mg-2Nb4Ce/Zr-PILC 224 0.149 Figure 5 shows N2 adsorption–desorption isotherms of the samples before and after M-poisoning. Each isotherm was nearly type I in the lower relative pressure range, which wereFigure characteristic5 shows Nof 2microporousadsorption–desorption materials. isothermsNevertheless, of the the samples hysteresis before loop and corre- after M-poisoning. Each isotherm was nearly type I in the lower relative pressure range, which sponding to type H3 appeared in the region of the higher relative pressure range, indicat- were characteristic of microporous materials. Nevertheless, the hysteresis loop corresponding that the layered structure was preserved and the typical pores were slit-like [20]. It can ing to type H3 appeared in the region of the higher relative pressure range, indicating be seen from Figure 5 that the adsorption volume of each M-poisoned sample decreased that the layered structure was preserved and the typical pores were slit-like [20]. It can at a lower relative pressure, which proves that the amount of micropores in the sample be seen from Figure5 that the adsorption volume of each M-poisoned sample decreased decreased. The reason for such a phenomenon was due to the partial coverage of pores by at a lower relative pressure, which proves that the amount of micropores in the sample the doped alkali and alkaline earth metal oxides on the samples. It should be noted that decreased. The reason for such a phenomenon was due to the partial coverage of pores by since the pillared montmorillonite formed mostly by a large amount of micropores and a the doped alkali and alkaline earth metal oxides on the samples. It should be noted that few amount of mesopores, surface areas of the Zr-pillared samples were mainly contrib- since the pillared montmorillonite formed mostly by a large amount of micropores and a uted by the micropores and the contribution of mesopores to surface area could be ig- few amount of mesopores, surface areas of the Zr-pillared samples were mainly contributed nored. by the micropores and the contribution of mesopores to surface area could be ignored.

Figure 5. N2 adsorption–desorptionadsorption–desorption isotherms isotherms of of the the samples samples before before and after poisoning. 2.5. Reducibility 2.5. Reducibility It is known that the redox property usually plays an important role in the NH3-SCR It is known that the redox property usually plays an important role in the NH3-SCR reaction [21–23]. Therefore, redox properties of the samples before and after M-poisoning were reaction [21–23]. Therefore, redox properties of the samples before and after M-poisoning evaluated using the H2-TPR technique, and their profiles are shown in Figure6. For each were evaluated using the H2-TPR technique, and their profiles are shown in Figure 6. For sample, four reduction peaks were observed at 473–489, 570–580, 671–706, and 779–815 ◦C, each sample, four reduction peaks were observed at 473–489, 570–580, 671–706, and 779– respectively. The first peak at 473–489 ◦C corresponded to reduction of the surface Ce4+ to 815 °C, respectively. The first peak at 473–489 °C corresponded to reduction of the surface Ce3+ [22], the second one at 570–580 ◦C was ascribed to reduction of the iron oxide species in Ce4+ to Ce3+ [22], the second one at 570–580◦ °C was ascribed to reduction of the iron oxide montmorillonite, the third one at 671–706 C was attributed to reduction of the bulk CeO2, and species in montmorillonite, the third one at 671–706 °C was attributed to reduction of the the last one at 779–783 ◦C was ascribed to reduction of niobium oxide [21,22]. It is generally bulkbelieved CeO that2, and the the reduction last one peak at 779–783 temperature °C was represented ascribed to the reduction reduction of ability. niobium A loweroxide reduction temperature indicates a stronger reduction ability [23]. For the K-poisoned sample, the reduction peak at 483 ◦C was slightly shifted to a higher temperature than that at 476 ◦C

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[21,22]. It is generally believed that the reduction peak temperature represented the reduction ability. A lower reduction temperature indicates a stronger reduction ability [23]. Catalysts 2021, 11, 329 For the K-poisoned sample, the reduction peak at 483 °C was slightly shifted to a higher7 of 17 temperature than that at 476 °C of the fresh 2Nb4Ce/Zr-PILC sample. In order to accurately compare redox ability of the fresh and M-poisoned samples, H2 consumption was ofcalculated the fresh and 2Nb4Ce/Zr-PILC presented in Table sample. 2. H2In consumption order to accurately of the K-poisoned compare redox sample ability decreased of the significantly (from 0.390 mmol/g for the fresh sample to 0.236 mmol/g for the 0.8 K- fresh and M-poisoned samples, H2 consumption was calculated and presented in Table2. H 2 consumption2Nb4Ce/Zr-PILC of the sample). K-poisoned This sampleresult indicate decreaseds that significantly there was a (from synergistic 0.390 mmol/g effect between for the freshCe and sample Nb, towhich 0.236 mmol/gdecreased for amount the 0.8 K-2Nb4Ce/Zr-PILC of the reducible species sample). on This the result surface indicates of 0.8 that K- there2Nb4Ce/Zr-PILC. was a synergistic Similar effect results between were Ce also and ob Nb,tained which for decreasedthe other samples amount ofafter the poisoning reducible speciesof Na, onCa, the and surface Mg, ofin 0.8which K-2Nb4Ce/Zr-PILC. all of the reduction Similar peaks results were were shifted also obtained slightly for to the higher other samplestemperatures, after poisoning and H2 consumption of Na, Ca, and decreased Mg, in which significantly. all of the For reduction example, peaks H2 consumption were shifted slightlydecreased to higherto 0.255, temperatures, 0.262, and 0.265 and H mmol/g2 consumption for the decreased2Nb4Ce/Zr-PILC significantly. samples For example,doped with H2 consumption0.8 Na, 0.8 Ca, decreased and 0.8 Mg, to 0.255, respectively. 0.262, and The 0.265 results mmol/g show for that the doping 2Nb4Ce/Zr-PILC of the alkali samples metals dopedto 2Nb4Ce/Zr-PILC with 0.8 Na, 0.8 could Ca, and lead 0.8 to Mg, stabilizatio respectively.n of the The active results compon show thatent doping and make of the it alkalimore metalsdifficult to to 2Nb4Ce/Zr-PILC be reduced. In addi couldtion, lead it is to also stabilization revealed ofthat the effects active of component K- and Na-poisoning and make it moreon redox difficult ability to beof reduced.the fresh Insamples addition, were it is mo alsore revealed serious than thateffects those of K-Mg- and and Na-poisoning Ca-poison- oning. redox Redox ability ability of theof these fresh samples samples werewas also more in serious good agreement than those with of Mg- their and SCR Ca-poisoning. activities, Redoxwhich abilityindicates of thesethat redox samples ability was of also the in samp goodles agreement played an with important their SCR role activities, in the SCR which re- indicatesaction. that redox ability of the samples played an important role in the SCR reaction.

Figure 6. H2-TPR profiles of the 2Nb4Ce/Zr-PILC samples before and after poisoning. Figure 6. H2-TPR proﬁles of the 2Nb4Ce/Zr-PILC samples before and after poisoning.

Table 2. The reduction peak temperatures and H2 consumption of the samples. Table 2. The reduction peak temperatures and H2 consumption of the samples.

Reduction Peak Temperature◦ (°C) H2 Consumption Sample Reduction Peak Temperature ( C) H2 Consumption Sample Peak 1 Peak 2 Peak 3 Peak 4 (mmol/g) Peak 1 Peak 2 Peak 3 Peak 4 (mmol/g) 2Nb4Ce/Zr-PILC 473 570 671 779 0.390 0.3K-2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC 473476 570576 671677 779815 0.291 0.390 0.3K-2Nb4Ce/Zr-PILC 476 576 677 815 0.291 0.8K-2Nb4Ce/Zr-PILC 483483 575575 701701 806 0.236 0.236 0.8Na-2Nb4Ce/Zr-PILC 479479 572572 694694 808 0.255 0.255 0.8Ca-2Nb4Ce/Zr-PILC 480480 570570 692692 800 0.262 0.262 0.8Mg-2Nb4Ce/Zr-PILC 489489 580580 706706 812 0.265 0.265

2.6. Surface Elemental Composition In order to further explore the chemical valence distributions and surface composition changes on the surface of the samples after M-poisoning, the fresh and M-poisoned samples were analyzed using the XPS technique, and their Ce 3d and O 1s XPS spectra are shown in Figures7 and8, respectively.

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2.6. Surface Elemental Composition In order to further explore the chemical valence distributions and surface composition changes on the surface of the samples after M-poisoning, the fresh and M-poisoned Catalysts 2021, 11, 329 8 of 17 samples were analyzed using the XPS technique, and their Ce 3d and O 1s XPS spectra are shown in Figures 7 and 8, respectively.

A Ce 3d 0.8Mg-2Nb4Ce/Zr-PILC

0.8Ca-2Nb4Ce/Zr-PILC

0.8Na-2Nb4Ce/Zr-PILC Intensity(a.u.)

0.8K-2Nb4Ce/Zr-PILC

920 910 900 890 880

Binding Energy(eV)

B Ce 3d 0.8K-2Nb4Ce/Zr-PILC

0.3K-2Nb4Ce/Zr-PILC Intensity(a.u.) Ce3+ 2Nb4Ce/Zr-PILC u' u' v' v'

u''' u'' u v''' v'' v Ce4+

920 910 900 890 880 Binding Energy(eV) FigureFigure 7. CeCe 3d 3d XPS XPS spectra spectra of ofthe the (A ()A alkali) alkali metal- metal- and and alkaline alkaline earth earth metal-poisoned metal-poisoned and ( andB) K- (B) K-poisonedpoisoned samples. samples.

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FigureFigure 8. 8.O O 1s 1s XPS XPS spectra spectra of theof the (A) ( alkaliA) alkali metal- metal- and alkaline and alkaline earth metal-poisoned earth metal-poisoned and (B) and (B) K- K-poisonedpoisoned samples.

As shown in Figure7, the peaks denoted as u, u II, uIII, v, vII, and vIII represented the II III II III 3d104f0Asstate shown of the surfacein Figure Ce4+ 7,species, the peaks while denoted those denoted as u, as u uI,and u , v v,I represented v , and v the represented the 3d3d10104f4f1 0initial state electronic of the surface state of Ce the4+ surface species, Ce3+ whilespecies those [24,25 denoted]. It can be as seen uI fromand v theI represented the XPS3d10 spectra4f1 initial that electronic the Ce3+ and state Ce4+ ofspecies the surface co-existed Ce3+ on species the sample [24,25]. surface, It can indicating be seen from the XPS thatspectra Ce was that not the completely Ce3+ and oxidized. Ce4+ species According co-existed to the calculated on the results sample using surface, the relative indicating that Ce areas of the corresponding peaks (Table3), the Ce 3+/Ce4+ atomic ratio dropped in the orderwas not of 2Nb4Ce/Zr-PILC completely oxidized. (0.82) > 0.8 According Mg-2Nb4Ce/Zr-PILC to the calculated (0.59) > 0.8 results Ca-2Nb4Ce/Zr- using the relative areas PILCof the (0.56) corresponding > 0.8 Na-2Nb4Ce/Zr-PILC peaks (Table (0.51) 3), the > 0.8 Ce K-2Nb4Ce/Zr-PILC3+/Ce4+ atomic ratio (0.41). dropped For the in the order of K-poisoned2Nb4Ce/Zr-PILC samples, (0.82) with the > 0.8 rise Mg-2Nb4Ce/Zr-PILC in K doping, the Ce3+/Ce (0.59)4+ atomic > 0.8 ratio Ca-2Nb4Ce/Zr- decreased PILC (0.56) sharply> 0.8 Na-2Nb4Ce/Zr-PILC in the order of 2Nb4Ce/Zr-PILC (0.51) > 0.8 (0.82) K-2Nb4Ce/Zr-PILC > 0.3 K-2Nb4Ce/Zr-PILC (0.41). (0.50) For > the 0.8 K-K-poisoned sam- 2Nb4Ce/Zr-PILC (0.41). After doping of the M to 2Nb4Ce/Zr-PILC, the Ce3+/Ce4+ atomic ples, with the rise in K doping, the Ce3+/Ce4+ atomic ratio decreased sharply in the order ratio also decreased, especially on the Na- and K-doped samples. This result indicates of 2Nb4Ce/Zr-PILC (0.82) > 0.3 K-2Nb4Ce/Zr-PILC (0.50) > 0.8 K-2Nb4Ce/Zr-PILC (0.41). After doping of the M to 2Nb4Ce/Zr-PILC, the Ce3+/Ce4+ atomic ratio also decreased, especially on the Na- and K-doped samples. This result indicates that poisoning of the M led to a decrease in amount of the Ce3+ species and an increase in amount of the Ce4+ species on the sample. It has been reported that the Ce3+ species originated from structural defects of CeO2 and were accompanied by formation of oxygen vacancies. A higher Ce3+/Ce4+ atomic ratio could bring about charge imbalance and unsaturated chemical bond, which increased amount of the chemisorbed oxygen species on the sample surface, thereby promoting the repeatable Ce3+/Ce4+ redox cycles [25]. In contrast, the redox cycle of Ce3+/Ce4+ was greatly inhibited once the Ce3+ species disappeared. Therefore, the change

Catalysts 2021, 11, 329 10 of 17

that poisoning of the M led to a decrease in amount of the Ce3+ species and an increase in amount of the Ce4+ species on the sample. It has been reported that the Ce3+ species originated from structural defects of CeO2 and were accompanied by formation of oxygen vacancies. A higher Ce3+/Ce4+ atomic ratio could bring about charge imbalance and unsaturated chemical bond, which increased amount of the chemisorbed oxygen species on the sample surface, thereby promoting the repeatable Ce3+/Ce4+ redox cycles [25]. In contrast, the redox cycle of Ce3+/Ce4+ was greatly inhibited once the Ce3+ species disappeared. Therefore, the change in Ce3+ species concentration on the surface might be one of the reasons affecting the SCR activity.

Table 3. Surface element compositions of the samples.

Composition of Cerium Species (at%) Composition of Oxygen Species (at%) Sample Ce3+/Ce4+ Ce3+ Ce4+ O O O Atomic Ratio α β γ 2Nb4Ce/Zr-PILC 45.1 54.9 0.82 12.5 42.3 45.2 0.3K-2Nb4Ce/Zr-PILC 33.4 66.6 0.50 12.4 28.3 59.3 0.8K-2Nb4Ce/Zr-PILC 29.0 71.0 0.41 13.1 22.6 64.3 0.8Na-2Nb4Ce/Zr-PILC 33.7 66.3 0.51 12.8 25.1 62.1 0.8Ca-2Nb4Ce/Zr-PILC 35.8 64.2 0.56 14.7 31.2 54.1 0.8Mg-2Nb4Ce/Zr-PILC 37.0 63.0 0.59 12.7 36.2 51.1

Figure8 shows O 1s XPS spectra of the fresh and M-poisoned samples. Each spectrum could be decomposed into three components. The component at binding energy (BE) = 530.1–530.3 eV was assigned to the surface lattice oxygen (labeled as Oα) species, the one at BE = 531.3–531.8 eV was attributed to the surface chemisorbed oxygen (labeled as Oβ) species, and the strong and broad one at BE = 532.3–532.6 eV was ascribed to the surface oxygen species in the Si−O bonds of SiO2 (labeled as Oγ)[26]. It has been widely reported that the surface chemisorbed oxygen (Oβ) exhibited a high activity in the SCR reaction since it was more mobile than lattice oxygen [2,24] and the lower concentration of the former would lead to a less reactivity. The relative concentrations of three oxygen species on the sample surface were estimated from area ratios of the corresponding characteristic peaks, and the results are listed in Table3. The order in amount of the O β species decreased in the sequence of 2Nb4Ce/Zr-PILC (42.3%) > 0.8 Mg-2Nb4Ce/Zr-PILC (36.2%) > 0.8 Ca- 2Nb4Ce/Zr-PILC (31.2%) > 0.8 Na-2Nb4Ce/Zr-PILC (25.1%) > 0.8 K-2Nb4Ce/Zr-PILC (22.6%), which was in accordance with their SCR performance and amounts of oxygen vacancies. For the K-poisoned samples, with the rise in K concentration, the atomic ratio of the Oβ species decreased sharply in the order of 2Nb4Ce/Zr-PILC (42.3%) > 0.3 K- 2Nb4Ce/Zr-PILC (28.3%) > 0.8 K-2Nb4Ce/Zr-PILC (22.6%). According to the literature, the change in O 1s signal after M-doping could be explained as formation of the strong bond between the doped M and surface oxygen center, which made reducibility of the surface species decrease (H2-TPR results) [2]. Therefore, doping of the M induced an inhibition effect on formation of the chemisorbed oxygen species.

2.7. Surface Acidity 2.7.1. NH3-TPD

The adsorption capacity of NH3 exerted an important effect on NH3-SCR performance, which was strongly related to surface acidity of a catalyst [27,28]. In order to investigate effect of the alkali metal- and alkaline earth metal-doping on surface acidity of 2Nb4Ce/Zr- PILC, NH3-TPD experiments were performed and the results are illustrated in Figure9. It ◦ can be seen that NH3 could be desorbed at 180, 230, and 300 C, respectively. According to the literature [27], the three desorption peaks of each sample were attributed to desorption + of the physisorbed ammonia and some NH4 bounded to the weak Brønsted acid sites + (weak acid sites), the NH4 from the strong Brønsted acid sites (medium acid sites), and the coordinated NH3 from the Lewis acid sites (strong acid sites), respectively. The Catalysts 2021, 11, 329 11 of 17

corresponding desorption peak temperatures and acidity are listed in Table4. For the M-poisoned samples, the desorption amount of NH3 decreased, as compared with that of the fresh sample. It is well known that the Brønsted acid sites were favorable for the adsorption of NH3, thus improving the low-temperature SCR activity [28]. The order in amount of the Brønsted acid sites was as follows: 2Nb4Ce/Zr-PILC (0.092 mmol/g) > 0.8 Mg-2Nb4Ce/Zr-PILC (0.071 mmol/g) > 0.8 Ca-2Nb4Ce/Zr-PILC (0.065 mmol/g) > 0.8 Na-2Nb4Ce/Zr-PILC (0.057 mmol/g) > 0.8 K-2Nb4Ce/Zr-PILC (0.046 mmol/g); and the total acid amount decreased in the sequence of 2Nb4Ce/Zr-PILC (0.259 mmol/g) > 0.8 Mg- 2Nb4Ce/Zr-PILC (0.182 mmol/g) > 0.8 Ca-2Nb4Ce/Zr-PILC (0.177 mmol/g) > 0.8 Na- 2Nb4Ce/Zr-PILC (0.159 mmol/g) > 0.8 K-2Nb4Ce/Zr-PILC (0.130 mmol/g). The NH3-SCR activity of the M-poisoned samples decreased in the following order: 0.8 K-2Nb4Ce/Zr- PILC > 0.8 Na-2Nb4Ce/Zr-PILC > 0.8 Ca-2Nb4Ce/Zr-PILC > 0.8 Mg-2Nb4Ce/Zr-PILC, Catalysts 2021, 11, x FOR PEER REVIEWwhich was consistent with those in amount of the Brønsted acid sites and total acid amount 11 of 17

of the acid sites. It is worth noting that NH3 desorption amount of the 0.8 K-2Nb4Ce/Zr- PILC sample was much lower than those of the other samples in the whole temperature range. This result indicates that K-doping induced a more negative effect on surface acidity of2Nb4Ce/Zr-PILC 2Nb4Ce/Zr-PILC, in(0.057 good consistencymmol/g) > with 0.8 itsK-2Nb4Ce/Zr-PILC poor SCR performance. (0.046 mmol/g); and the total acid amount decreased in the sequence of 2Nb4Ce/Zr-PILC (0.259 mmol/g) > 0.8 Mg- Table 4. NH2Nb4Ce/Zr-PILC3 desorption temperatures (0.182 and NHmmol/g)3 desorption > amounts0.8 Ca-2Nb4Ce/Zr-PILC from the samples. (0.177 mmol/g) > 0.8 Na- ◦ 2Nb4Ce/Zr-PILCTemperature ( C) (0.159 mmol/g) Acidity > 0.8 (mmol K-2Nb4Ce/Zr-PILCNH3/g) (0.130 mmol/g). The NH3-SCR Total Desorption Amount Sample WeakactivityMedium of the M-poisonedStrong Weak samplesMedium decreasedStrong in the following(mmol/g) order: 0.8 K-2Nb4Ce/Zr- PeakPILC > 0.8Peak Na-2Nb4Ce/Zr-PILCPeak Peak > Peak0.8 Ca-2Nb4Ce/Zr-PILCPeak > 0.8 Mg-2Nb4Ce/Zr-PILC, 2Nb4Ce/Zr-PILCwhich 175 was 218 consistent 287 with 0.048 those in 0.092 amount 0.119 of the Brønsted 0.259 acid sites and total acid 0.3K-2Nb4Ce/Zr-PILC 184 229 306 0.023 0.062 0.064 0.149 0.8K-2Nb4Ce/Zr-PILCamount 181 of 225 the acid 293 sites. It 0.030 is worth 0.046 noting 0.054 that NH3 desorption 0.130 amount of the 0.8 K- 0.8Na-2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC 183 229 sample 295 was 0.033 much 0.057 lower than 0.069 those of the 0.159 other samples in the whole 0.8Ca-2Nb4Ce/Zr-PILC 182 228 303 0.032 0.065 0.080 0.177 0.8Mg-2Nb4Ce/Zr-PILCtemperature 180 227 range. 304 This result 0.037 indicates 0.071 that 0.074K-doping induced 0.182 a more negative effect on surface acidity of 2Nb4Ce/Zr-PILC, in good consistency with its poor SCR performance.

Figure 9. Conts.

Figure 9. NH3-TPD profiles of (A) alkali metal- and alkaline earth metal-poisoned and (B) K-poisoned samples.

Catalysts 2021, 11, x FOR PEER REVIEW 11 of 17

2Nb4Ce/Zr-PILC (0.057 mmol/g) > 0.8 K-2Nb4Ce/Zr-PILC (0.046 mmol/g); and the total acid amount decreased in the sequence of 2Nb4Ce/Zr-PILC (0.259 mmol/g) > 0.8 Mg- 2Nb4Ce/Zr-PILC (0.182 mmol/g) > 0.8 Ca-2Nb4Ce/Zr-PILC (0.177 mmol/g) > 0.8 Na- 2Nb4Ce/Zr-PILC (0.159 mmol/g) > 0.8 K-2Nb4Ce/Zr-PILC (0.130 mmol/g). The NH3-SCR activity of the M-poisoned samples decreased in the following order: 0.8 K-2Nb4Ce/Zr- PILC > 0.8 Na-2Nb4Ce/Zr-PILC > 0.8 Ca-2Nb4Ce/Zr-PILC > 0.8 Mg-2Nb4Ce/Zr-PILC, which was consistent with those in amount of the Brønsted acid sites and total acid amount of the acid sites. It is worth noting that NH3 desorption amount of the 0.8 K- 2Nb4Ce/Zr-PILC sample was much lower than those of the other samples in the whole temperature range. This result indicates that K-doping induced a more negative effect on surface acidity of 2Nb4Ce/Zr-PILC, in good consistency with its poor SCR performance.

Catalysts 2021, 11, 329 12 of 17

Figure 9. NH -TPD proﬁles of (A) alkali metal- and alkaline earth metal-poisoned and (B) K- Figure 9. NH3 3-TPD profiles of (A) alkali metal- and alkaline earth metal-poisoned and (B) K-poi- poisonedsoned samples. samples. In addition, in the case of different K doping to the fresh sample, ammonia desorption amount also decreased to some extent. The calculated ammonia desorption amount of

the 2Nb4Ce/Zr-PILC, 0.3 K-2Nb4Ce/Zr-PILC, and 0.8 K-2Nb4Ce/Zr-PILC samples were 0.259, 0.149, and 0.130 mmol/g, respectively, indicating that the increase in K concentration made ammonia adsorption ability of the sample decrease sharply. Therefore, M-doping could reduce the adsorption of ammonia, which was conﬁrmed by the results obtained in the following in situ DRIFT experiments.

2.7.2. DRIFTS Study of NH3 Adsorption

The in situ DRIFTS spectra were used to investigate the NH3 species adsorbed on the sample surface to further distinguish the different acid types [29–32]. Figure 10 shows the in situ DRIFTS spectra of the adsorbed NH3 species on the fresh and M-poisoned samples. −1 + The absorption bands at 1685 and 1440 cm were attributed to the NH4 coordinated to the Brønsted acid sites [22,29], the ones at 1180, 1250, and 1601 cm−1 were assigned to the −1 coordinated NH3 linked to the Lewis acid sites, the ones at 3360 and 3265 cm were attributed −1 to the N–H stretching vibration modes of the coordinated NH3, and the one at 965 cm was ascribed to the gas-phase or weakly adsorbed NH3 [30]. The above results indicate that both the Lewis acid sites and the Brønsted acid sites existed on the sample surface, which was consistent with the NH3-TPD results. As shown in Figure 10A, after doping the M to 2Nb4Ce/Zr-PILC, both the NH3 coordinated to the Lewis acid sites (1601, 1250, −1 + −1 and 1180 cm ) and the NH4 coordinated to the Brønsted acid sites (1685 and 1440 cm ) + were also detected on all of the M-poisoned samples. Nevertheless, the adsorption of NH4 and NH3 on the M-poisoned samples also showed a different degree of decline in band intensity. This result shows that all of the doped M exhibited a strong inhibitory effect on NH3 adsorption at the acid sites, especially at the Brønsted acid sites. The order of adsorption strength at the Brønsted acid sites was as follows: 2Nb4Ce/Zr-PILC > 0.8 Mg-2Nb4Ce/Zr- PILC > 0.8 Ca-2Nb4Ce/Zr-PILC > 0.8 Na-2Nb4Ce/Zr-PILC > 0.8 K-2Nb4Ce/Zr-PILC, which agreed well with the NH3-TPD results. Moreover, as shown in Figure 10B, the absorption band intensity decreased in the order of 2Nb4Ce/Zr-PILC > 0.3 K-2Nb4Ce/Zr-PILC > 0.8 K- 2Nb4Ce/Zr-PILC, suggesting that the acid sites of the sample also decreased with a rise in K content. Compared with the other poisoned samples, the absorption band at the acid sites of the 0.8 K-2Nb4Ce/Zr-PILC sample was the weakest. This result indicates that the acid sites Catalysts 2021, 11, 329 13 of 17

Catalysts 2021, 11, x FOR PEER REVIEW 13 of 17 on the surface of the K-doped sample were the most seriously destroyed (in consistency with the above TPD results), resulting in the lowest deNOx activity.

FigureFigure 10. 10. InIn situ situ DRIFTS DRIFTS spectra spectra of of (A ()A alkali) alkali metal- metal- and and alkaline alkaline earth earth metal-poisoned metal-poisoned and and (B) K- (B) poisoned samples first exposed to (1100 ppm NH3 + N2 (balance)) for 1 h and subsequently purged K-poisoned samples ﬁrst exposed to (1100 ppm NH3 + N2 (balance)) for 1 h and subsequently purged by N2 at 100 °C◦ for 30 min. by N2 at 100 C for 30 min. 3.3. Discussion Discussion TheThe poisoning effects effects of alkali metals (K, Na) and alkaline earth metals (Ca, Mg) on catalyticcatalytic performance of the 2Nb4Ce/Zr-PILC2Nb4Ce/Zr-PILC sample sample were investigated. The poisoning extentextent of the M-doping decreased in the orderorder of K >> Na > Ca > Mg, Mg, and and the the results show thatthat the the poisoning poisoning effect effect increased increased proportionally with the alkalinity of the M. It is is widely widely 3+ 4+ acceptedaccepted that a higherhigher CeCe3+/Ce4+ atomicatomic ratio ratio can can cause cause charge charge imbalance imbalance and generate unsaturatedunsaturated chemicalchemical bonds,bonds, which which would would lead lead to anto increasean increase in amount in amount of the of chemisorbed the chemi- 3+ 4+ sorbedoxygen oxygen species species on the sampleon the sample surface, surface, thus promoting thus promoting the redox the cycleredox of cycle Ce of/Ce Ce3+/Ce. The4+. 3+ TheXPS XPS results results (Figure (Figure7) verify 7) verify the the existence existence of Ceof Ce3+in in the the 2Nb4Ce/Zr-PILC 2Nb4Ce/Zr-PILC samplesample and 3+ thethe Ce Ce3+ contentcontent decreased decreased in inthe the M-poisoned M-poisoned samples. samples. Under Under the actual the actual preparation preparation conditions, however, the nitrate of potassium, sodium, calcium or magnesium could form a molten salt flux on the surface of 2Nb4Ce/Zr-PILC, giving rise to the covering of the active sites [3]. Alkali and alkaline earth metal oxides with strong alkalinity could occupy the

Catalysts 2021, 11, 329 14 of 17

Catalysts 2021, 11, x FOR PEER REVIEW 14 of 17

conditions, however, the nitrate of potassium, sodium, calcium or magnesium could form a molten salt flux on the surface of 2Nb4Ce/Zr-PILC, giving rise to the covering of the surface of 2Nb4Ce/Zr-PILC, thus reducing the amount of the chemisorbed oxygen species active sites [3]. Alkali and alkaline earth metal oxides with strong alkalinity could occupy and the reducibility of cerium species. It can be found that the reduction peak was shifted the surface of 2Nb4Ce/Zr-PILC, thus reducing the amount of the chemisorbed oxygen to a higher temperature and the hydrogen consumption decreased (Figure 6) with the species and the reducibility of cerium species. It can be found that the reduction peak was doping of the M. This result was a piece of important evidence that the reducibility of shifted to a higher temperature and the hydrogen consumption decreased (Figure6) with cerium species decreased. Surface acidity was another main factor suppressing SCR per- the doping of the M. This result was a piece of important evidence that the reducibility formance of the M-poisoned sample [4,33,34]. It is generally believed that NH3 was firstly of cerium species decreased. Surface acidity was another main factor suppressing SCR adsorbed at the acid sites via hydrogen abstraction or protonation, thereby forming the performance of the M-poisoned sample [4,33,34]. It is generally believed that NH3 was active NH3 species[34], and then the active NH3 species reacted with the adsorbed ni- firstly adsorbed at the acid sites via hydrogen abstraction or protonation, thereby forming trate/nitrite intermediates (Langmuir−Hinshelwood mechanism) or gas-phase NO/NO2 the active NH3 species [34], and then the active NH3 species reacted with the adsorbed (Eley−Rideal mechanism) to produce H2O and N2. If the surface acidity was inhibited, the nitrate/nitrite intermediates (Langmuir−Hinshelwood mechanism) or gas-phase NO/NO2 first step would become the rate-determining step and hence significantly decrease the (Eley−Rideal mechanism) to produce H2O and N2. If the surface acidity was inhibited, reaction ratethe [33,34]. first stepTherefore, would the become decrease the rate-determining in surface acidity step of the and M-poisoned hence significantly sample decrease the (Figures 9 andreaction 10) exerted rate [ 33a negative,34]. Therefore, impact theon the decrease SCR activity in surface of 2Nb4Ce/Zr-PILC. acidity of the M-poisoned sample 3+ 4+ Therefore,(Figures the decreases9 and 10) exertedin NH3 aadsorption, negative impact Ce /Ce on the redox SCR cycle activity (surface of 2Nb4Ce/Zr-PILC. chemisorbed oxygen), and cerium species reducibility might be the3+ main4+ reasons why the Therefore, the decreases in NH3 adsorption, Ce /Ce redox cycle (surface chemisorbed 2Nb4Ce/Zr-PILCoxygen), sample and was cerium poisoned species by reducibility the M. Based might on the be theabove main discussion, reasons why a deac- the 2Nb4Ce/Zr- tivation mechanismPILC sample of the was M-doped poisoned 2Nb4Ce by the/Zr-PILC M. Based samples on the above was proposed, discussion, as a deactivationshown mecha- in Figure 11. nism of the M-doped 2Nb4Ce/Zr-PILC samples was proposed, as shown in Figure 11.

Figure 11. ProposedFigure deactivation11. Proposed mechanism deactivation of themechanism Nb-Ce/Zr-PILC of the Nb sample-Ce/Zr-PILC after loading sample of after alkali loading and alkaline of alkali earth metals. and alkaline earth metals. 4. Experimental 4. Experimental4.1. Catalyst Preparation 4.1. Catalyst PreparationThe acid-leached montmorillonite was dispersed in deionized water at room tem- The acid-leachedperature undermontmorillonite stirring for was 24 disper h, thensed mixed in deionized with the water zirconium at room oxychloride temper- aqueous ature under solution.stirring for Afterwards, 24 h, then themixed mixed with aqueous the zirconium solution oxychloride was stirred aqueous for 12 h andsolu- aged at room temperature for 12 h, and the product was ﬁnally dried at 100 ◦C for 12 h after being tion. Afterwards, the mixed aqueous solution was stirred for 12 h and aged◦ at room temperature for washed12 h, and with the deionizedproduct was water finally three dried times at and 100 calcined°C for 12 at h 400 afterC being for 2 h,washed thus obtaining the with deionizedZr-incorporated water three times montmorillonite and calcined (denoted at 400 as°CZr-PILC) for 2 h, thus support. obtaining The 2Nb/Zr-PILC the Zr- sample incorporatedwith montmorillonite a Nb loading (denoted of 2 wt% as was Zr-PILC) synthesized support. byimpregnating The 2Nb/Zr-PILC Zr-PILC sample with a niobium with a Nb loadingoxalate of aqueous 2 wt% solution,was synthesized followed by by impregnating stirring for 1 h, Zr-PILC drying withwith a a rotary niobium evaporator, and calcining in air at 400 ◦C for 2 h. The 2Nb4Ce/Zr-PILC sample was prepared by impreg- oxalate aqueous solution, followed by stirring for 1 h, drying with a rotary evaporator, nating 2Nb/Zr-PILC with a cerium nitrate aqueous solution. The preparation method was and calcining in air at 400 °C for 2 h. The 2Nb4Ce/Zr-PILC sample was prepared by im- the same as that described above. pregnating 2Nb/Zr-PILC with a cerium nitrate aqueous solution. The preparation method was the same as that described above.

Catalysts 2021, 11, 329 15 of 17

The alkali metal-doped 2Nb4Ce/Zr-PILC samples with different M/Ce molar ratios (labeled as x M, M = Na, K, Mg, and Ca; x = M/Ce molar ratio) were prepared by impregnating the 2Nb4Ce/Zr-PILC with KNO3, NaNO3, Mg(NO3)2 and Ca(NO3)2 aqueous solutions, respectively. The subsequent procedures were the same those stated above. The obtained samples were denoted as 0.8 M-2Nb4Ce/Zr-PILC.

4.2. Catalyst Characterization The X-ray diffraction measurement was performed on a Bruker D8 advance diffrac- tometer (Bruker, Karlsruhe, Germany) equipped with a Cu Kα irradiation. N2 adsorption− desorption isotherms were determined at −196 ◦C using a W-BK132F apparatus (JWGB, Beijing, China). X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCALAB 250XI (Thermo Fisher, Waltham, MA, USA) to analyze the surface element compositions and metal chemical states of the samples, in which the operating pass energy (PE) was 50 eV with an X-ray source of 200 W and an Al Kα radiation (hv = 1486.6 eV). Before the analysis, all of the samples were degassed in vacuum to eliminate influence of the adsorbed gas on the surface of the samples. The binding energy of each XPS spectrum was calibrated against that (284.6 eV) of the standard C 1s signal of the contaminant carbon. The CASA XPS software was used to make the peak fitting of the XPS spectra and the background signals were deducted before analysis. Hydrogen temperature-programmed reduction of (H2-TPR) and ammonia temperature-programmed desorption (NH3-TPD) experiments were conducted on a PCA-1200 analyzer (Beijing Builder Electronic Technology, Beijing, China) equipped with a TCD detector using 100 mg of the sample. Before the H2-TPR ◦ experiment, the sample was pretreated in a 5 vol% O2/N2 flow of 30 mL/min at 400 C for 1 h and cooled to room temperature (RT). Then, the atmosphere was switched to a 5 vol% ◦ H2/N2 flow of 30 mL/min, and the temperature was raised to 950 C at a heating rate of ◦ 10 C/min. Prior to the NH3-TPD experiment, the sample was pretreated in a He flow ◦ of 30 mL/min at 400 C for 1 h, and then a NH3 flow of 30 mL/min was passed through the sample for adsorption at 100 ◦C for 1 h. Subsequently, the sample was purged in a He flow of 30 mL/min for 30 min to remove the physically adsorbed NH3. Finally, NH3 desorption was carried out from 30 to 850 ◦C at a heating rate of 10 ◦C/min. The in situ diffuse reflectance Fourier transform infrared spectroscopic (in situ DRIFTS) experiments were performed on a Bruker 0 spectrometer (Bruker, Karlsruhe, Germany). The sample was pretreated at 400 ◦C for 1 h to remove the moisture. The background spectrum was recorded by exposing the sample to a N2 flow of 100 mL/min, and switched to a (1100 ppm NH3 + N2 (balance)) flow of 100 mL/min to record the NH3 absorption spectrum.

4.3. NH3-SCR and NH3 Oxidation

Catalytic activity tests of the samples for the NH3-SCR reaction were carried out in a fixed-bed quartz tubular microreactor using 300 mg of the sample and 600 mg of quartz sand. The simulated flue gas consisted of (1100 ppm NH3 + 1000 ppm NO + 4 vol% O2 + 5 vol% H2O + N2 (balance)) and the total flow rate was 500 mL/min, giving a space velocity (SV) of 100,000 mL/(g h). The concentration of NO was measured by a MODEL1080 analyser (Beijing SDL Technology, Beijing, China). NO conversion was calculated according to the following equation:

[NO] − [NO] NO conversion(%) = in out × 100% [NO]in

NH3 oxidation activity was evaluated in a flow-through microreactor system equipped with a Fourier transform infrared spectroscopy (FT-IR) spectrometer. The total flow rate of the reactant mixture (1100 ppm NH3 + 4 vol% O2 + N2 (balance)) was 500 mL/min and the SV was 100,000 mL/(g h). NH3 conversion was calculated according to the following equation:

[NH3]in − [NH3]out NH3 conversion = × 100% [NH3]in Catalysts 2021, 11, 329 16 of 17

5. Conclusions In this study, the deactivation of the 2Nb4Ce/Zr-PILC sample by the alkali and alkaline earth metals was investigated in detail. Doping of the M (M = K, Na, Ca, and Mg) deactivated the 2Nb4Ce/Zr-PILC sample, and their deactivation effects decreased in the sequence of K > Na > Ca > Mg. Through a series of characterization, it is shown that the M 3+ 4+ doping induced the decreases in NH3 adsorption (surface acidity), Ce /Ce atomic ratio (chemisorbed oxygen (Oβ)), and redox ability (redox sites), which were accountable for such a deactivation.

Author Contributions: Conceptualization, C.L.; Methodology, J.C.; Formal analysis, C.L.; Investiga- tion, C.L.; Software, F.M.; Validation, X.W.; Writing—original draft, C.L. and J.C.; Writing—review & editing, C.L., Q.Y. and H.D.; Supervision, Q.Y. and H.D. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Grant Nos. 21277008 and 20777005), the National Key Research and Development Program of China (Grant No. 2017YFC0209905), and the Natural Science Foundation of Beijing (Grant No. 8082008). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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