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Nov 30, 2018 - all of the Ce-Zr-Ti oxide catalysts showed much better catalytic ... Pure Ce oxide is not suitable for use as an NH3-SCR catalyst [27,28]. When Zr ...

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A CeO2/ZrO2-TiO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3 Wenpo Shan 1,2 , Yang Geng 3 , Yan Zhang 1,2 , Zhihua Lian 1 and Hong He 1,2,4, * 1

2 3 4

*

Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China; [email protected] (W.S.); [email protected] (Y.Z.); [email protected] (Z.L.) Ningbo Urban Environment Observation and Research Station-NUEORS, Institute of Urban Environment, Chinese Academy of Sciences, Ningbo 315800, China School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; [email protected] State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Correspondence: [email protected]; Tel./Fax: +86-10-62849123

Received: 30 September 2018; Accepted: 27 November 2018; Published: 30 November 2018

 

Abstract: In this study, CeZr0.5 Tia Ox (with a = 0, 1, 2, 5, 10) catalysts were prepared by a stepwise precipitation approach for the selective catalytic reduction of NOx with NH3 . When Ti was added, all of the Ce-Zr-Ti oxide catalysts showed much better catalytic performances than the CeZr0.5 Ox . Particularly, the CeZr0.5 Ti2 Ox catalyst showed excellent activity for broad temperature range under high space velocity condition. Through the control of pH value and precipitation time during preparation, the function of the CeZr0.5 Ti2 Ox catalyst could be controlled and the structure with highly dispersed CeO2 (with redox functions) on the surface of ZrO2 -TiO2 (with acidic functions) could be obtained. Characterizations revealed that the superior catalytic performance of the catalyst is associated with its outstanding redox properties and adsorption/activation functions for the reactants. Keywords: Ce-based catalyst; stepwise precipitation; selective catalytic reduction; diesel exhaust; nitrogen oxides abatement

1. Introduction NOx (mainly NO and NO2 ) in the atmosphere plays critical roles in the formation of severe air pollution problems, such as haze, acid rain, and photochemical smog. In the last few decades, great efforts have been devoted to the development of NOx emission control technologies [1–3]. Selective catalytic reduction of NOx with NH3 (NH3 -SCR) has been widely applied for the removal of NOx generated from stationary sources for many years, and it has also been used for the control of NOx emission from diesel vehicles [2,4]. Catalysts play an important role in the development of NH3 -SCR technology [5,6]. Vanadium-based catalyst (especially V2 O5 -WO3 /TiO2 ), with excellent SO2 resistance, is the most widely used NH3 -SCR catalyst for NOx emission control from power plants, and it was also applied on diesel vehicles as the first generation of SCR catalyst [4]. However, this catalyst system still has some problems, including the toxicity of active V2 O5 , narrow temperature window, and low thermal stability [2]. There has been strong interest in developing a vanadium-free catalyst that can be used on diesel vehicles [5–11]. Ce is a key component in three-way catalysts for emission control in automobiles for gasoline. CeO2 provides an oxygen storage function through redox cycling between Ce3+ and

Catalysts 2018, 8, 592; doi:10.3390/catal8120592

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4+ . In recent years, Ce has also attracted great attention for applications as a support [12,13], Cerecent years, Ce has also attracted great attention for applications as a support [12,13], promoter [14– promoter [14–18], or component main active[19–26] component [19–26] for NH3 -SCR catalysts. 18], or main active for NH 3-SCR catalysts. Pure WhenZr Zroxide oxidewas was PureCeCeoxide oxideis isnot notsuitable suitablefor foruse useasasan anNH NH33-SCR -SCR catalyst catalyst [27,28]. [27,28]. When introduced into Ce oxide, the thermal stability and the oxygen storage capacity of the oxide could introduced into Ce oxide, the thermal stability and the oxygen storage capacity of the oxide could be besignificantly significantlyimproved. improved. Therefore, Ce-Zr was investigated NH[12,13,29–34]. Therefore, Ce-Zr oxideoxide was investigated for NHfor 3-SCR In the 3 -SCR [12,13,29–34]. In NH the 3NH reaction, redox functions acidic functions of the catalyst needed [4,35]. -SCR reaction, bothboth redox functions andand acidic functions of the catalyst areare needed [4,35]. 3 -SCR Therefore, a high dispersion of redox redoxwith withacid acidsites sitesisisthe theway waytoto Therefore, a high dispersionofofactive activesites sitesand and close close coupling coupling of design a highly efficientNH NH 3-SCRcatalyst. catalyst. design a highly efficient 3 -SCR study, starting a preparation of oxide Ce-Zr by oxide by the co-preparation method, we In In thisthis study, starting fromfrom a preparation of Ce-Zr the co-preparation method, we developed developed a Ce-Zr-Ti usingprecipitation a stepwise precipitation approach, under the guidance theoreticalof a Ce-Zr-Ti oxide catalystoxide usingcatalyst a stepwise approach, under the theoretical of the closeofcombination of thewith Ce-Zr oxideredox with functions strong redox oxide with theguidance close combination the Ce-Zr oxide strong andfunctions Ti oxide and withTiexcellent acid excellent acid properties [4,5]. This obtained catalyst showed superior catalytic performance properties [4,5]. This obtained catalyst showed superior catalytic performance for NH3 -SCR. for NH3SCR. 2. Results and Discussion 2. Results and Discussion 2.1. NH3 -SCR Activity 2.1. NH3-SCR Activity Figure 1A presents the NOx conversion over the catalysts with different Ti contents under a Figure presents NOvelocity x conversion overofthe catalysts contents underover a relatively high1A gas hourly the space (GHSV) 200,000 h−1 .with Thedifferent CeZr0.5 OTi exhibited x just −1 ◦ C. relatively high gas hourly space velocity (GHSV) of 200,000 h . The CeZr 0.5OxTi just exhibited over 50% 50% NOx conversion in a narrow temperature range of 350–425 When was introduced, all of x conversion a narrowexhibited temperature range of 350–425 °C.With Whenthe Ti increase was introduced, all of the theNO Ce-Zr-Ti oxideincatalysts much better activities. in Ti content, theCelow Zr-Ti oxide catalysts exhibited much better activities. With the increase in Ti content, the low temperature firstly increased and then decreased. As a result, the CeZr0.5 Ti2 Ox catalyst presented the temperature andrange, then decreased. As aaresult, the CeZr 0.5Ti2Ox catalyst presented the best activity in firstly a low increased temperature together with high NO conversion in a wide temperature x best activity a lowhand, temperature range, in together with a high NO x conversion in a wide temperature range. On the in other the variation high temperature activity with Ti content was contrary other hand,activity, the variation temperature activity Ti content was contrary to range. that ofOn lowthe temperature with in thehigh activity of CeZr0.5 Ti2 Ox with slightly lower than those of to the that of low temperature activity, with the activity of CeZr0.5Ti2Ox slightly lower than those of the other other Ce-Zr-Ti oxide catalysts in a high temperature range. In addition, adding Ti to the catalyst also Ce-Zr-Ti oxide catalysts in a high temperature range. In addition, adding Ti to the catalyst also enhanced the N2 selectivity, and the Ce-Zr-Ti oxide catalysts all presented higher N2 selectivity than enhanced the N2 selectivity, and the Ce-Zr-Ti oxide catalysts all presented higher N2 selectivity than CeZr0.5 Ox (Figure 1B). CeZr0.5Ox (Figure 1B).

Figure (A) NO x conversionsand and(B) (B)NN22 selectivity selectivity over over the Ox and Ce-Zr-Ti oxide catalysts. Figure 1. 1. (A) NO the CeZr CeZr0.5 x conversions 0.5 Ox and Ce-Zr-Ti oxide catalysts. 3 ] = 500 ppm, [O 2 ] = 5 vol.%, N 2 balance, and GHSV = 200,000 h−1.h−1 . Reaction conditions: [NO] = [NH Reaction conditions: [NO] = [NH3 ] = 500 ppm, [O2 ] = 5 vol.%, N2 balance, and GHSV = 200,000

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velocity on the NOx conversion over CeZr0.5 Ti2 Ox influences of H2Oinand space2.velocity on the NOofx conversion over 2Ox were tested and theThe results are shown Figure The existence 5% H2 O in theCeZr flow0.5Ti gas decreased the The influences of H 2 O and space velocity on the NO x conversion over CeZr 0.5 Ti 2 Ox werethe tested and the results are shown in Figure 2. The existence of 5% H 2O in the flow gas decreased low temperature activity, but enhanced the high temperature activity. As a result, over 80%low NOx and the results are shown Figure 2.the Thehigh existence of 5% Hactivity. 2O in theAs flow gas decreased the NO lowx temperature activity, but in enhanced temperature a result, over 80% conversion could still be achieved from 250 to 450 ◦ C. When the GHSV was decreased from 200,000 h−1 temperature activity, butachieved enhanced the250 high temperature activity. a decreased result, over 80% NOx conversion could still be from to 450 °C. When the GHSVAs was from 200,000 to 100,000 h−1 , the activity of the catalyst at low temperatures was obviously improved. conversion could stillactivity be achieved 250 at to low 450 temperatures °C. When the was GHSV was decreased from 200,000 h−1 to 100,000 h−1, the of thefrom catalyst obviously improved. h−1 to 100,000 h−1, the activity of the catalyst at low temperatures was obviously improved.

Figure 2. NOconversion x conversion over CeZr0.5Ti2Ox catalyst under different reaction conditions. Reaction Figure 2. NO over CeZr0.5 Ti2 Ox catalyst under different reaction conditions. Reaction x Figure 2. NO x conversion CeZr0.5[O under reactionused), conditions. Reaction conditions: [NO] [NH3]] = =over 500 22]O =x 5catalyst vol.%, [H 2O] =different 5 vol.% (when N2 balance, and conditions: [NO] ==[NH 500 ppm, ppm, Ti [O 3 2 ] = 5 vol.%, [H2 O] = 5 vol.% (when used), N2 balance, −1. ppm, [O2] = 5 vol.%, [H2O] = 5 vol.% (when used), N2 balance, and conditions: [NO]or = 200,000 [NH3] =h500 GHSV = 100,000 and GHSV = 100,000 or 200,000 h−1 . GHSV = 100,000 or 200,000 h−1.

Separated NO/NH 3 Oxidation 2.2.2.2. Separated NO/NH 3 Oxidation 2.2. Separated NO/NH3 Oxidation analyze effects thecatalyst, catalyst,separated separated NO oxidation tests were To To analyze thethe effects ofofTiTiononthe oxidation and andNH NH3 3oxidation oxidation tests were To analyze the effects of Ti on the catalyst, separated NO oxidation and NH 3 oxidation tests were carried out for the CeZr 0.5 O x and CeZr 0.5 Ti 2 O x (Figure 3). The NO 2 production during NO oxidation carried out for the CeZr0.5 Ox and CeZr0.5 Ti2 x (Figure 3). The NO2 production during NO oxidation carried out forTi the CeZr 0.5Oclearly x and CeZr 0.5Tithan 2Ox (Figure 3). The 2 xproduction during NO oxidation over CeZr 0.5 Ti2O x was higher that over CeZrNO 0.5O at a low temperature. Since the over thethe CeZr 0.5 2 Ox was clearly higher than that over CeZr0.5 Ox at a low temperature. Since the over the CeZr 0.5 Ti 2 O x was clearly higher than that over CeZr 0.5 O x at a low temperature. Since presentation NOin 2 in the reaction gas could promote the SCR reaction at a low temperaturethe byby presentation of of NO the reaction gas could promote the SCR reaction at a low temperature 2 presentation of NO 2 in the reaction gas3 +could promote the2 +SCR reaction at a lowlow-temperature temperature by accelerating the fast SCR process (2 NH NO + NO 2 → 2N 3H 2O), the enhanced accelerating the fast SCR process (2 NH3 + NO + NO2 → 2N2 + 3H2 O), the enhanced low-temperature accelerating theintroduction fast SCR process (2should NH3 +be NO + NO2 → 2N 2 +the 3H2O), the enhanced low-temperature activity associated with 2 activity byby thethe introduction ofofTiTishould be associated with the promoted promotedoxidation oxidationofofNO NOtotoNO NO 2 activity by0.5the introduction Ti should associated of with of NOover to NO over CeZr Ti2O x [10,35]. In of addition, thebe introduction Ti the alsopromoted promotedoxidation NH3 oxidation the2 over CeZr0.5 Ti2 Ox [10,35]. In addition, the introduction of Ti also promoted NH3 oxidation over the over CeZr 2Ox temperature. [10,35]. In addition, introduction of Tiatalso promoted NH3 oxidation over the 3-SCR reaction route a high temperature mainly follows catalyst at 0.5 a Ti high The NHthe catalyst at a high temperature. The NH3 -SCR reaction route at a high temperature mainly follows the 3 -SCR reaction route at a high temperature mainly follows the catalyst at a high temperature. The NH Eley-Rideal mechanism, and the activation of NH3 to form NH2 species by oxidation plays the key Eley-Rideal mechanism, and the activation of NH3 to form NH2 species by oxidation plays the key Eley-Rideal mechanism, and the activation of NH 3 to form NH 2 species by oxidation plays the key role for the reaction with NO to form N2 and H2O, owing to NH2 + NO(g) → N2 + H2O. Therefore, role forfor thethe reaction with NO toto form NN owing to NO(g)→ →NN2 2+ +HH O. Therefore, 2 2and 2activity. role reaction with NO form andH H2 O, 2O, to NH NH22 of ++ NO(g) 2O. Therefore, promoted NH 3 oxidation would be beneficial for theowing improvement high temperature promoted NH oxidation would be beneficial for the improvement of high temperature activity. 3 promoted NH3 oxidation would be beneficial for the improvement of high temperature activity.

Figure 3. (A) NO2 productions during separate NO oxidation reaction and (B) NH3 conversions Figure 3.separate (A) 2 productions during separate NO oxidation reaction and NH 3 conversions 3 oxidation reaction over CeZr 0.5O x and CeZr Ti2NH O(B) x 3catalysts. Reaction during NH Figure 3. (A) NONO during separate NOthe oxidation reaction and 0.5 (B) conversions during 2 productions 3=reaction oxidation reaction over the CeZr 0.5 O x vol.%, and CeZr 0.5Ti2O xand catalysts. Reaction during separate NH 3] = 500 ppm, [O 2 ] = 5 N 2 balance GHSV = 200,000 conditions: (A) [NO] 500 ppm, (B) [NH separate NH oxidation over the CeZr O and CeZr Ti O catalysts. Reaction conditions: (A) 3 0.5 x 0.5 2 x 3] =ppm, 500 ppm, = 5 vol.%, N2 balance GHSV = 200,000 conditions: (A) [NO] = 500 ppm, (B)3[NH h−1._ENREF_30= [NO] 500 ppm, (B) [NH ] = 500 [O2 ] [O = 52]vol.%, N2 balance andand GHSV = 200,000 h− 1 . h−1.

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Catalysts 2018, 8, x FOR PEER REVIEW 4 of 11 The X-ray diffraction (XRD) results of the CeZr0.5Ox and Ce-Z-Ti oxide catalysts are presented in

Figure 4. Both CeO2 and ZrO2 were detected in CeZr0.5Ox. With the increase of Ti, the peaks for CeO2 2.3. XRD 2.3. andXRD ZrO2 became more and more weak, and only anatase TiO2 was observed for CeZr0.5Ti10Ox. Only X-ray results the O and Ce-Z-Ti oxide catalysts inx, weakThe peaks fordiffraction CeO2 with(XRD) cubic fluorite structures (PDF# 43-1002) were observed in are the presented CeZr0.5Ti2O The X-ray diffraction (XRD) resultsof of theCeZr CeZr0.5 0.5Oxx and Ce-Z-Ti oxide catalysts are presented in Figure 4. Both CeO and ZrO were detected in CeZr O . With the increase of Ti, the peaks for CeO indicating that the introduction of Ti had induced the structural change of the CeZr 0.5 O x , and the Figure 4. Both CeO22 and ZrO22 were detected in CeZr0.5 0.5Ox x. With the increase of Ti, the peaks for CeO22 and ZrO became more and more weak, and only anatase was observed for CeZrAs TiTi O crystallizations of Ce, Zr and Ti oxides in CeZr 0.5 Ti 2Ox were inhibited. a1010 result, the 2 2significantly 0.50.5 and ZrO2 became more and more weak, and only anataseTiO TiO 2 was observed for CeZr Oxx.. Only Only 2with weak peaks CeO cubic fluorite structures (PDF# 43-1002) were observed in the CeZr Ti O CeZr 0.5 Ti 2Oxfor (165.1 m /g) showed a higher Brunauer–Emmett–Teller (BET) surface area than CeZr 0.5 2 0.5 2 weak peaks for CeO2 with cubic fluorite structures (PDF# 43-1002) were observed in the CeZr0.5Ti2OOxx,,x indicating that O , and the (113.5 m2/g). 0.5 indicating that the the introduction introduction of of Ti Tihad hadinduced inducedthe thestructural structuralchange changeofofthe theCeZr CeZr 0.5Oxx, and the crystallizations Ce, Zr Zr and and Ti Ti oxides oxidesin inCeZr CeZr0.50.5 significantly inhibited. a result, x were 2 xOwere crystallizations of of Ce, TiTi 2O significantly inhibited. As aAsresult, the 2 /g) showed a higher Brunauer–Emmett–Teller (BET) surface area than the CeZr Ti O (165.1 m 2 x 2 CeZr0.5Ti0.5 2Ox (165.1 m /g) showed a higher Brunauer–Emmett–Teller (BET) surface area than CeZr0.5Ox CeZr O2x/g). (113.5 m2 /g). 0.5m (113.5

Figure 4. XRD patterns of the CeZr0.5Ox and Ce-Z-Ti oxide catalysts.

2.4. H2-TPR Figure4.4.XRD XRDpatterns patternsreduction ofthe theCeZr CeZr 0.52 The H2 temperature-programmed (H profilesoxide of CeZr 0.5Ox and CeZr0.5Ti2Ox are Figure of O-TPR) catalysts. xx and Ce-Z-Ti 0.5 presented in Figure 5. The CeZr0.5Ox exhibited two peaks at 496 and 755 °C due to the surface and 2.4. H22-TPR -TPR 2.4. bulkHreductions of CeO2 (as detected by XRD), respectively [31,36–38]. During the test, coordinatively unsaturated surface oxygen anions arereduction easily reduced by H2profiles in the of low temperature region, while the The H temperature-programmed (H OO x xand 2 -TPR) 0.50.5 0.5 The H22 temperature-programmed reduction (H 2-TPR)profiles ofCeZr CeZr andCeZr CeZr 0.5Ti Ti22O Oxx are are ◦ bulk oxygen species5.5.are reduced only after the two transportation to and the surface [39].to With the addition presented in The CeZr Oxx exhibited exhibited two peaks at at 496 496 755 C due the and presented in Figure Figure The CeZr0.5 0.5O peaks and 755 °C due to the surface surface and of Ti, a sharp H 2 CeO consumption peak appeared at 567 °C, which indicates that another type of Ce bulk reductions of (as detected by XRD), respectively [31,36–38]. During the test, coordinatively 2 bulk reductions of CeO2 (as detected by XRD), respectively [31,36–38]. During the test, coordinatively species might be formed. Considering the XRD results, this sharp peak might be associated with the unsaturated surface oxygen anions are easily reduced by H in the low temperature region, while the unsaturated surface oxygen anions are easily reduced by H22 in the low temperature region, while the reduction of species the highly Ce species from Ce4+ to Ce3+ to [22,34]. In addition, the Hthe 2 consumption bulk oxygen are reduced after transportation thethe surface [39]. With addition of bulk oxygen species aredispersed reducedonly only afterthe the transportation to surface [39]. With the addition ◦ of CeZr 0.5Ti 22 O x was much higher than that of CeZr 0.5 O x at indicates a low temperature. The H 2-TPR results Ti, a sharp H consumption peak appeared at 567 C, which that another type of Ce species of Ti, a sharp H2 consumption peak appeared at 567 °C, which indicates that another type of Ce clearlybeindicated the enhancement of redox functions for CeZr 0.5Ti2Obe x. associated with the reduction might formed. Considering the XRD results, this sharp peak might species might be formed. Considering the XRD results, this sharp peak might be associated with the 4+ 3+ Previous studies have indicated that the redox properties of NH 3-SCR catalyst play a dominant of the highly dispersed Ce species from Ce to Ce [22,34]. In addition, the H of 4+ 3+ 2 consumption reduction of the highly dispersed Ce species from Ce to Ce [22,34]. In addition, the H2 consumption role in the low temperature activity [35,40,41]. Therefore, the enhanced redox function of CeZr 0.5 Ti 2 Ox CeZr Ti O was much higher than that of CeZr O at a low temperature. The H -TPR results clearly x 0.5Ox at a low temperature. 2The H2-TPR results 0.5 0.5 2Tix2Ox was much higher than that of 0.5 of CeZr CeZr would beneficial for low temperature activity. indicated the enhancement of redoxoffunctions for CeZrfor . Ti2Ox. 0.5 Ti 2 Ox 0.5 clearly indicated the enhancement redox functions CeZr

Previous studies have indicated that the redox properties of NH3-SCR catalyst play a dominant role in the low temperature activity [35,40,41]. Therefore, the enhanced redox function of CeZr0.5Ti2Ox would beneficial for low temperature activity.

Figure 2-TPR profilesofofthe theCeZr CeZr Oxxand andCeZr CeZr0.5 0.5Ti 2Ox catalysts. Figure 5.5.HH profiles 2 -TPR 0.50.5O 2 x catalysts.

Figure 5. H2-TPR profiles of the CeZr0.5Ox and CeZr0.5Ti2Ox catalysts.

desorption (NH3-TPD) were performed for the catalysts (Figure 6). The NOx-TPD profiles are presented in Figure 6A. The first NOx peak of CeZr0.5Ti2Ox was at ca. 110 °C, mainly due to the desorption of physisorbed NOx, while the other NOx peak was at ca. 300 °C and was associated with the decomposition of chemsorbed NOx species [42,43]. On the other hand, Catalysts 2018,peaks 8, 592 were observed for CeZr0.5Ox at ca. 270 °C and ca. 410 °C, respectively, which5were of 12 two weak due to the decomposition of different types of chemsorbed NOx species. With the addition of Ti, the adsorbed NOx on CeZr0.5Ti2Ox was obviously more than that of CeZr0.5Ox. Particularly, the desorbed Previous studies have indicated that the redox properties of NH3 -SCR catalyst play a dominant NO2 of CeZr0.5Ti2Ox was much higher, owing to the enhanced low-temperature activity for NO role in the low temperature activity [35,40,41]. Therefore, the enhanced redox function of CeZr0.5 Ti2 Ox oxidation (as shown by the separated NO oxidation results), which could facilitate the conversion of would beneficial for low temperature activity. NOx in NH3-SCR. Surface acidity plays a dominate role in the high-temperature SCR activity due to its effects on 2.5. NO x /NH3 -TPD the adsorption and activation of NH3 [35,41]. Previous studies have revealed that Ti species of NH3investigate the NO NH of CeZr0.5 Ox and CeZr x and 3 adsorption/desorption 0.5 Ti2 Ox , SCRTo catalysts mainly act as acid sites in the reaction for NHproperties 3 adsorption [4]. Therefore, the adsorbed NO -TPD) and NH3 temperature-programmed NHx3temperature-programmed of CeZr0.5Ti2Ox was muchdesorption more than(NO thatxof CeZr 0.5Ox, which might be an importantdesorption reason for (NH -TPD) were performed for the catalysts (Figure 6). 3 the better NH3-SCR activity of CeZr0.5Ti2Ox at high temperatures.

Figure (A) NO x-TPD and(B) (B)NH NH 3-TPD profilesofofthe theCeZr CeZr andCeZr CeZr0.5 0.5Ti Ti22O Figure 6. 6. (A) NO and profiles OOxxand Oxx catalysts. catalysts. x -TPD 3 -TPD 0.50.5

The NOx -TPD profiles are presented in Figure 6A. The first NOx peak of CeZr0.5 Ti2 Ox was at ca. 2.6. XPS 110 ◦ C, mainly due to the desorption of physisorbed NOx , while the other NOx peak was at ca. 300 ◦ C The X-ray photoelectron spectroscopy (XPS) results for Ce 3d of the CeZr0.5Ox and CeZr0.5Ti2Ox and was associated with the decomposition of chemsorbed NOx species0 [42,43]. On the other hand, two are shown in Figure 7. The sub-bands labeled ◦with u’/v’ and◦ u /v0 represent the 3d104f1 initial weak peaks were observed for CeZr0.53+Ox at ca. 270 9 C2 and ca. 410 3+C, respectively, which were due to electronic state corresponding to Ce and the 3d 4f state of Ce , respectively [44]. The sub-bands the decomposition of different types of chemsorbed NOx species. With the addition of Ti, the adsorbed NOx on CeZr0.5 Ti2 Ox was obviously more than that of CeZr0.5 Ox . Particularly, the desorbed NO2 of CeZr0.5 Ti2 Ox was much higher, owing to the enhanced low-temperature activity for NO oxidation (as shown by the separated NO oxidation results), which could facilitate the conversion of NOx in NH3 -SCR. Surface acidity plays a dominate role in the high-temperature SCR activity due to its effects on the adsorption and activation of NH3 [35,41]. Previous studies have revealed that Ti species of NH3 -SCR catalysts mainly act as acid sites in the reaction for NH3 adsorption [4]. Therefore, the adsorbed NH3

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of CeZr0.5 Ti2 Ox was much more than that of CeZr0.5 Ox , which might be an important reason for the better NH3 -SCR activity of CeZr0.5 Ti2 Ox at high temperatures. 2.6. XPS The X-ray photoelectron spectroscopy (XPS) results for Ce 3d of the CeZr0.5 Ox and CeZr0.5 Ti2 Ox are shown in Figure 7. The sub-bands labeled with u’/v’ and u0 /v0 represent the 3d10 4f1 initial Catalysts 2018, 8, x FOR PEER REVIEW 6 of 11 electronic state corresponding to Ce3+ and the 3d9 4f2 state of Ce3+ , respectively [44]. The sub-bands 0 4+ , and the sub-bands labeled with u, u”, v and labeled represent thethe 3d10 of of CeCe 0 state 4+, and the sub-bands labeled with u, u’’, v labeled with withu”’ u’’’and andv”’ v’’’ represent 3d4f104fstate 9 1 4+ v” the 3dthe 4f 3d state to Ce to[44]. of Ce3+ofwould induce induce a chargea 94f1 corresponding andrepresent v’’ represent state corresponding Ce4+The [44].presence The presence Ce3+ would imbalance, which could lead to unsaturated chemical bonds and oxygen vacancies. The calculated charge imbalance, which could lead to unsaturated chemical bonds and oxygen vacancies. The 3+ ratio of CeZr Ti O (36.0%) was higher than that of CeZr O (33.8%), indicating that more Ce x 0.5Ti2Ox (36.0%) was higher than that of x 0.5of 2CeZr 0.5 CeZr calculated Ce3+ ratio 0.5Ox (33.8%), indicating that 3+ ratio of the catalyst could surface oxygenoxygen vacancies presented in CeZrin0.5CeZr Ti2 O0.5 In2O addition, the Cethe x . Ti more surface vacancies presented x. In addition, Ce3+ ratio of the catalyst influence the redox ability and reactant adsorption and activation functions, and thereby contribute to could influence the redox ability and reactant adsorption and activation functions, and thereby NH -SCR performance. 3 contribute to NH3-SCR performance.

Figure Ti22O Oxxcatalysts. catalysts. Figure7.7.XPS XPSresults resultsofofCe Ce3d 3dofofthe theCeZr CeZr 0.5O Oxx and and CeZr CeZr0.5 0.5Ti 0.5

The The surface surface oxygen oxygen vacancies vacancies of of the the catalysts catalysts might might generate generate weakly-adsorbed weakly-adsorbed oxygen oxygen species species or additional chemisorbed oxygen on the surface of the catalyst [27,45]. The XPS results of1s Oof 1sthe of or additional chemisorbed oxygen on the surface of the catalyst [27,45]. The XPS results of O the CeZr Ox CeZr and CeZr Ti2 Oshown in Figure 8.1sThe O was 1s peak wastwo fit into two sub-bands. x are shown 0.5 0.5Ox0.5 and 0.5Ti2O x are in Figure 8. The O peak fit into sub-bands. The subCeZr The sub-bands at 531.2–531.5 eV and 529.1–529.6 eV were assigned to the surface adsorbed bands at 531.2–531.5 eV and 529.1–529.6 eV were assigned to the surface adsorbed oxygen (Ooxygen α), such 2− and O− belonging to defect-oxide or a hydroxyl-like group, and the lattice (O ), such as the 2 as αthe O22− and O− O belonging to defect-oxide or a hydroxyl-like group, and the lattice oxygen O2− (Oβ), oxygen O2− (O respectively The Oα ratios of calculated the catalysts calculated by Oα /(O +Ti O2β α 0.5 β ),The respectively [46]. Oα ratios[46]. of the catalysts were bywere Oα/(O α + Oβ), and the CeZr O),x and the CeZr Ti O showed higher O ratio than CeZr O . The results confirmed that the addition x 0.5 O2α ratio 0.5 x that the addition of Ti indeed induced showed higher than CeZr0.5Oxα . The results confirmed of Ti indeed induced more surface-adsorbed oxygen, which facilitate NO oxidation to NO2 more surface-adsorbed oxygen, which would facilitate NOwould oxidation to NO 2 (as shown by the (as shown by the separated NO oxidation and NO -TPD results), and thus facilitates the conversion of x thus facilitates the conversion of separated NO oxidation and NOx-TPD results), and NO by fast SCR NO by fast SCR effects. effects. 2.7. Formation Process Analysis of the CeZr0.5 Ti2 Ox Catalyst Figure 9 shows the pH variations of the mixed solutions for the preparation of the CeZr0.5 Ox and CeZr0.5 Ti2 Ox catalysts. During the preparation of CeZr0.5 Ox , the initial pH value of the solution was 1.6. With the hydrolysis of urea, the pH increased gradually to be 7.6 after heating for 12 h. Due to the increase in pH, suspended particles began to appear in the solution in the second hour. The particles with the precipitation time of 2 h, 4 h, 6 h, and 12 h were collected and then calcined to be catalyst samples. The activity tests of these samples showed similar NOx conversions with each other.

as the O22− and O− belonging to defect-oxide or a hydroxyl-like group, and the lattice oxygen O2− (Oβ), respectively [46]. The Oα ratios of the catalysts were calculated by Oα/(Oα + Oβ), and the CeZr0.5Ti2Ox showed higher Oα ratio than CeZr0.5Ox. The results confirmed that the addition of Ti indeed induced more surface-adsorbed oxygen, which would facilitate NO oxidation to NO2 (as shown by the Catalysts 2018,NO 8, 592 12 separated oxidation and NOx-TPD results), and thus facilitates the conversion of NO by fast7 of SCR effects. Catalysts 2018, 8, x FOR PEER REVIEW

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2.7. Formation Process Analysis of the CeZr0.5Ti2Ox Catalyst Figure 9 shows the pH variations of the mixed solutions for the preparation of the CeZr0.5Ox and CeZr0.5Ti2Ox catalysts. During the preparation of CeZr0.5Ox, the initial pH value of the solution was 1.6. With the hydrolysis of urea, the pH increased gradually to be 7.6 after heating for 12 h. Due to the increase in pH, suspended particles began to appear in the solution in the second hour. The particles with the precipitation time of 2 h, 4 h, 6 h, and 12 h were collected and then calcined to be catalyst samples. The activity tests of these samples showed similar NOx conversions with each other. Due to the acidity induced by the added Ti(SO4)2, the initial pH value of the mixed solution during the preparation of CeZr0.5Ti2Ox dropped to be 1.1. With the hydrolysis of urea, the pH increased gradually after heating, and some white particles generated in the first hour and suspended in the solution. With the increase of time, the particles gradually turned yellow. The pH reached ca. 7.0 after 12 h of reaction. The particles with the precipitation times of 1 h, 4 h, 6 h, and 12 h were collected and then calcined to be catalyst samples. Interestingly, the activity test showed a remarkable Figure XPSresults results 1sofofthe theCeZr CeZr and CeZr 0.5Ti Ti22Oin enhancement of NO x conversions forofof the four samples with theCeZr increase precipitation time. Figure 8.8.XPS OO1s OOx xand catalysts. xx catalysts. 0.50.5 0.5

Figure 9. 9. The of of thethe mixed solution during the preparation of theof(A) CeZr Ox and (B) Figure ThepH pHvariation variation mixed solution during the preparation the (A)0.5CeZr x 0.5 O 0.5TiCeZr 2Ox catalysts, the NOand x conversions the obtainedof samples at different precipitation time. CeZr(B) and catalysts, the NOxofconversions the obtained samples at different 0.5 Ti2 Ox and precipitation time.

The surface metal atomic concentrations of the CeZr0.5Ti2Ox samples with different precipitation Due to analyzed the acidity induced by the Ti(SO )2 , the initial pH value of the mixed solution times were using XPS, and the added variations in 4Ce, Zr, and Ti concentrations with precipitation during the preparation of CeZr Ti O dropped to be 1.1. With the hydrolysis of urea, the pH increased x 1-h precipitation sample, only Ti and Zr, without Ce, were time are shown in Figure 10.0.5For2 the gradually after the heating, andinsome white particles generated in the first hour and suspended detected. With increase precipitation time, surface Ce concentration increased graduallyin inthe the samples. At the same time, Ti and Zr concentrations gradually decreased with the increase in precipitation time. A TEM-EDS mapping image showed that Ce was highly dispersed in the CeZr0.5Ti2Ox catalyst (Figure 11).

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solution. With the increase of time, the particles gradually turned yellow. The pH reached ca. 7.0 after 12 h of reaction. The particles with the precipitation times of 1 h, 4 h, 6 h, and 12 h were collected and Catalysts 2018, 8, x FOR PEER REVIEW 8 of 11 then calcined to be catalyst samples. Interestingly, the activity test showed a remarkable enhancement of NOx conversions for the four samples with the increase in precipitation time. The surface metal atomic concentrations of the CeZr0.5 Ti2 Ox samples with different precipitation times were analyzed using XPS, and the variations in Ce, Zr, and Ti concentrations with precipitation time are shown in Figure 10. For the 1-h precipitation sample, only Ti and Zr, without Ce, were detected. With the increase in precipitation time, surface Ce concentration increased gradually in the samples. At the same time, Ti and Zr concentrations gradually decreased with the increase in precipitation time. A TEM-EDS mapping image showed that Ce was highly dispersed in the CeZr0.5 Ti2 Ox catalyst (Figure 11). Catalysts 2018, 8, x FOR PEER REVIEW 8 of 11

Figure 10. Surface metal atomic concentrations of the CeZr0.5Ti2Ox samples with different precipitation times.

Considering the variations in the solution pH value when preparing the CeZr0.5Ti2Ox, the formation process of the catalyst can be proposed as follows: The Ti and Zr species were first coprecipitated with the increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti species with the further increase in pH. Finally, a CeZr0.5Ti2Ox catalyst with a higher surface Ce concentration than Ti and Zr was obtained. Through control of the hydrolysis of urea, the variations in the solution pH can be controlled, and then we can control the precipitation process of Figure 10. Surface metal atomic concentrations of the CeZr 0.5 Ti2CeZr Ox samples withCeO different precipitation the catalyst, which is very important forconcentrations the formation highly-dispersed 2 on ZrO 2-TiO2. Thus, Figure 10. Surface metal atomic ofofthe with different 0.5 Ti2 Ox samples times. catalyst the obtained precipitation times.can present excellent NH3-SCR performance. Considering the variations in the solution pH value when preparing the CeZr0.5Ti2Ox, the formation process of the catalyst can be proposed as follows: The Ti and Zr species were first coprecipitated with the increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti species with the further increase in pH. Finally, a CeZr0.5Ti2Ox catalyst with a higher surface Ce concentration than Ti and Zr was obtained. Through control of the hydrolysis of urea, the variations in the solution pH can be controlled, and then we can control the precipitation process of the catalyst, which is very important for the formation of highly-dispersed CeO2 on ZrO2-TiO2. Thus, the obtained catalyst can present excellent NH3-SCR performance.

Figure 11. image (A) and EDS mapping (B) for the of the 2O x Figure 11. TEM TEM image (A) the andcorresponding the corresponding EDS mapping (B)Cefor the CeZr Ce 0.5 ofTithe catalyst. CeZr0.5 Ti2 Ox catalyst.

3. Experimental ConsideringSection the variations in the solution pH value when preparing the CeZr0.5 Ti2 Ox , the formation process of the catalyst can be proposed as follows: The Ti and Zr species were first 3.1. Catalyst Preparation Activity Test co-precipitated with theand increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti themolar further increase Finally, withtoa be higher 0.5 Ti 2 Ox catalyst The CeZr 0.5Tispecies aOx (a =with Ti/Ce ratio = 0, 1,in2,pH. 5, 10), withaaCeZr Zr/Ce molar ratio fixed 0.5, surface Ce concentration than Ti and Zr was obtained. Through control of the hydrolysis of urea, was prepared using a precipitation method. Desired precursors of Ce(NO3)3·6H2O (>99%, Sinopharm the variations in theCo., solution can be controlled, and 3then control the precipitation of Chemical Reagent Ltd., pH Shanghai, China), Zr(NO )4·5Hwe 2O can (>99%, Sinopharm Chemicalprocess Reagent Figure 11. TEM image (A) and the corresponding EDS mapping (B) for the Ce of the CeZr0.5Ti2Ox

Co., Ltd., Shanghai, China) and Ti(SO4)2 (>98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, catalyst. China) were dissolved together in distilled water, and urea (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added to the mixed solution as a slowly-releasing precipitator. Then, the 3. Experimental Section solution was heated to 90 °C to facilitate the release of NH3 and thereby raise the pH value gradually. The temperature of theand mixed solution 3.1. Catalyst Preparation Activity Test was held at 90 °C for 12 h under vigorous stirring (some samples with shorter precipitation times were also prepared). After that, the precipitated powders

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the catalyst, which is very important for the formation of highly-dispersed CeO2 on ZrO2 -TiO2 . Thus, the obtained catalyst can present excellent NH3 -SCR performance. 3. Experimental Section 3.1. Catalyst Preparation and Activity Test The CeZr0.5 Tia Ox (a = Ti/Ce molar ratio = 0, 1, 2, 5, 10), with a Zr/Ce molar ratio fixed to be 0.5, was prepared using a precipitation method. Desired precursors of Ce(NO3 )3 ·6H2 O (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Zr(NO3 )4 ·5H2 O (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Ti(SO4 )2 (>98%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were dissolved together in distilled water, and urea (>99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was added to the mixed solution as a slowly-releasing precipitator. Then, the solution was heated to 90 ◦ C to facilitate the release of NH3 and thereby raise the pH value gradually. The temperature of the mixed solution was held at 90 ◦ C for 12 h under vigorous stirring (some samples with shorter precipitation times were also prepared). After that, the precipitated powders were collected via filtration, washed using distilled water, and dried for 12 h at 100 ◦ C. Finally, the catalyst was obtained after calcination at 500 ◦ C for 5 h. The SCR activity of the catalysts (40–60 mesh) were tested in a fixed-bed quartz flow reactor. The reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH3 , 5 vol.% O2 , N2 balance, and 400 mL/min total flow rate. Different gas hourly space velocities (GHSVs) were obtained by changing the volume of catalysts, i.e., 0.24 mL catalyst for a GHSV = 100,000 h−1 and 0.12 mL catalyst for a GHSV = 200,000 h−1 . The concentrations of effluent N-containing gases (NO, NH3 , NO2 and N2 O) were continuously measured by an online FTIR gas analyzer (Nicolet Antaris IGS analyzer, Thermo-Fisher Scientific, Waltham, MA, USA). NOx conversion and N2 selectivity were calculated using the following equations, respectively: NOx conversion = (1 −

N2 selectivity = (1 −

[NO]out + [NO2 ]out ) × 100% [NO]in + [NO2 ]in

2[N2 O]out ) × 100% [NOx ]in + [NH3 ]in − [NOx ]out − [NH3 ]out

3.2. Characterizations X-ray diffraction (XRD) measurements were carried out on a computerized AXS D8 diffractometer (Bruker, GER), with Cu Kα (λ = 0.15406 nm) radiation, from 20 to 80◦ at 8◦ /min. Surface areas were tested using an ASAP 2020 (Micromeritics, Norcross, GA, USA) at −196 ◦ C by N2 adsorption/desorption and calculated using a BET equation in the 0.05–0.35 partial pressure range. The X-ray photoelectron spectroscopy (XPS) results of Ce 3d and O 1s were measured on an ESCALAB 250Xi Scanning X-ray Microprobe (Thermo-Fisher Scientific, Waltham, MA, USA) using Al Ka radiation (1486.7 eV) and a C 1 s peak, with BE = 284.8 eV as the calibration standard. The transmission electron microscopy (TEM) image and energy-dispersive X-ray spectroscopy (EDS) mapping of Ce were obtained using a JEM-2100F equipment (JEOL, Tokyo, Japan), combined with a specimen tilting beryllium holder for energy dispersive spectroscopy. The accelerating voltage was 200 kV. The H2 temperature-programmed reduction (H2 -TPR) was tested using an AutoChem_II_2920 chemisorption analyzer (Micromeritics, Norcross, GA, USA), and the temperature-programmed desorption of NH3 and NOx (NOx -TPD and NH3 -TPD) were tested using the same reaction system as the activity tests. Experiment details can be found in Reference [42].

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4. Conclusions A series of Ce-Zr-Ti oxide catalysts were prepared using a stepwise precipitation approach for NH3 -SCR. CeZr0.5 Ox without Ti just showed a relatively low NOx conversion. When Ti was introduced, Ce-Zr-Ti catalysts showed much better activities and N2 selectivity. A CeZr0.5 Ti2 Ox catalyst, which contains moderate Ti amounts, showed the best performance, which is associated with its optimal ratios for the redox (CeOx ) and acidic (TiO2 ) components. CeZr0.5 Ox and CeZr0.5 Ti2 Ox catalysts were characterized using various methods and the formation process during preparation was investigated. CeZr0.5 Ti2 Ox catalyst showed superior redox properties (by H2 -TPR), good adsorption and NOx /NH3 activation functions (by NOx -TPD and NH3 -TPD, respectively), and enhanced charge imbalance (by XPS). During preparation, the Ti and Zr species were first co-precipitated with an increase in solution pH. Then, the Ce species uniformly precipitated onto the precipitated Zr-Ti species with the further increase in pH. As a result, CeZr0.5 Ti2 Ox catalyst with a surface Ce concentration higher than those of Ti and Zr was obtained. This preparation process resulted in the formation of highly-dispersed CeO2 on ZrO2 -TiO2 , and thus the catalyst can present excellent NH3 -SCR performance. Author Contributions: W.S. and H.H. conceived the project; Y.G. and Y.Z. performed the experiments; W.S. and Z.L. carried out the data analysis; W.S. and Y.G. wrote the paper; H.H. supervised the study. Funding: This work was supported by the National Key R&D Program of China (2017YFC0212502, 2017YFC0211101), the National Natural Science Foundation of China (201637005), and the Key Research Program of the Chinese Academy of Sciences (ZDRW-ZS-2017-6-2-3). Conflicts of Interest: The authors declare no conflicts of interest.

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