a sol-gel ti-al-ce-nanoparticle catalyst for simultaneous

A sol-gel Ti-Al-Ce-nanoparticle catalyst for simultaneous removal of NOand Hg0 from simulated flue gas

Junyi Zhang, Caiting Li ⇑, Lingkui Zhao, Teng Wang, Shanhong Li, Guangming ZengCollege of Environmental Science and Engineering, Hunan University, Changsha 410082, PR ChinaKey Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China

h i g h l i g h t s

� TiAl10Ce20 showed superior activityfor simultaneous removal of NO andHg0.

� The deactivation effects of H2O andSO2 were reduced by Al addition.

� The mechanisms for simultaneousremoval of Hg0 and NO weresystematically studied.

� The connection between NH3-SCRand Hg0 removal was also obtained.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 July 2016Received in revised form 12 October 2016Accepted 6 November 2016Available online 8 November 2016

Keywords:Simultaneous removalAl2O3

NH3-SCRElemental mercuryCeO2

Nanoparticle

a b s t r a c t

To optimize simultaneous control of NO and elemental mercury (Hg0) and gain more insight into themechanisms, nano-sized TiO2-Al2O3-CeO2 materials synthesized via sol-gel method were used for simul-taneous removal of NO and Hg0 from simulated flue gas. The physicochemical characteristics of catalystswere characterized by ICP-OES, BET, XRD, SEM, TEM, XPS, H2-TPR and FT-IR. TiAl10Ce20 nanoparticle withthe addition of 10 wt%Al2O3 showed superior NO removal efficiency (93.41%) and Hg0 removal efficiency(80.54%) in the presence of SCR atmosphere at 300 �C. The deactivation effects of 8% H2O and 400 ppmSO2 were also reduced by Al addition. In the presence of SCR atmosphere, the capture of Hg0 was inhib-ited by the existence of NH3, while the presence of Hg0 had little impact on NO removal. The character-ization results showed that the excellent performance of TiAl10Ce20 nanoparticle might result from thestronger redox ability, lower crystallinity and better texture properties with highly dispersed Ce species,which were all attributed to Al addition. The mechanisms for simultaneous removal of NO and Hg0 werealso proposed on the basis of above results. TiAl10Ce20 nanoparticle developed in this work was consid-ered to be a promising catalyst for simultaneous removal of NO and Hg0.

� 2016 Elsevier B.V. All rights reserved.

1. Introduction

Coal is one of the dominating fuel sources and will remain theprimary one until 2030 according to the international energy

agency (IEA) [1,2]. Various air pollutants are released during thecombustion processes in coal-fired power plants, in which nitrogenoxides (NOx) and elemental mercury (Hg0) have received extensiveattention as two atmospheric contaminants emitted from coal-fired boilers. NOx emissions can cause high ground-level ozoneconcentration, photochemical smog and acid rain which can dam-age the environment [3]. Hg0 is a global pollutant with toxicity,bio-accumulation and volatility that can result in lots of

http://dx.doi.org/10.1016/j.cej.2016.11.0391385-8947/� 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: College of Environmental Science and Engineering,Hunan University, Changsha 410082, PR China. Tel.: +86 731 88649216; fax: +86731 88649216.

E-mail addresses: [email protected], [email protected] (C. Li).

Chemical Engineering Journal 313 (2017) 1535–1547

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environmental and health risks [4,5]. Accordingly, countriesaround the world set strict standards on NOx and mercury control,such as standards of limiting mercury, acid gases and other toxicpollution from power plants enacted by the US Environmental Pro-tection Agency (USEPA) on December 21, 2011 [6].

Several methods have been investigated to control NO and Hg0

emissions respectively in coal-fired flue gas [5,7–9]. Among them,selection catalytic reduction of NO by NH3 (NH3-SCR) has beenwidely applied because of its high efficiency [10], and activatedcarbon injection (ACI) has become a common method for Hg0

removal [9]. However, the large consumption of ACI due to adsorp-tion saturation of carbon materials and the high cost of controllingthe two emissions alone urged researchers to develop more cost-effective technologies to remove NO and Hg0 [11–13]. Accordingto researches in recent years [14,15], Hg0 is difficult to removeby the existing air pollution control devices due to its high volatil-ity and nearly insolubility in water. As oxidized mercury (Hg2+) issoluble in water, Hg0 could be oxidized to Hg2+ by SCR systemand subsequently removed by wet flue gas desulfurization devices(WFGD), Hg2+ dissolved in desulfurization waste water wasreported that could be removed by the conventional WFGD wastewater treatment technology such as the addition of precipitatingand agents or adsorption technology [16–18]. Hence, simultaneousremoval of NO and Hg0 with an already existing SCR system wasconsidered to have higher efficiency of apparatus, lower operatingcosts and less investment than removing NO and Hg0 separately[13,19,20]. Although some studies have been conducted to investi-gate simultaneous removal of NO and Hg0 as well as the mecha-nisms of the process [21–23], the co-benefit of combiningexisting SCR system and WFGD inspired us to develop more novelmaterials which could control NO and Hg0 emissionssimultaneously.

Cerium oxide (CeO2) has attracted much attention as an effec-tive support, promoter, as well as active species for NH3-SCR andHg0 oxidation respectively due to its high redox ability of Ce4+/Ce3+ couple, large oxygen storage capacity, nontoxic and high effi-ciency [7,24,25]. Among various Ce-based catalysts, TiO2-CeO2 cat-alyst has been researched broadly for its excellent catalyticproperty. For example, Li et al. [26] found that TiO2-CeO2 catalystpossessed high Hg0 oxidation activity; Gao et al. [27] indicated thatTiO2-CeO2 catalyst showed superior activity for NH3-SCR. However,a high concentration of SO2 was proved to deactivate the activity ofCeO2/TiO2 catalyst and restrict the application of it [28]. Aluminawas widely applied as a support of catalyst due to its low costand effective promotion effect on thermal stability as well ashomogeneous mixing of components [29,30]. Al2O3 modified TiO2

can significantly improve the structural stability and surface acid-ity of TiO2 because of its interception effect on phase transition.Moreover, it is worth noting that Al modified catalyst was provedto possess stronger sulfur resistance [31,32]. The tolerance of cat-alyst to H2O and sintering was also improved by Al modification[33]. For instance, Camposeco et al. [33] proved that the perfor-mance of V2O5/NPTiO2 for NH3-SCR was greatly enhanced by Aladdition. On the basis of above states, it was deduced that the addi-tion of Al to TiO2-CeO2 catalyst might result in higher catalyticactivity in NH3-SCR and Hg0 removal. However, Al2O3 modifiedTiO2-CeO2 catalyst via sol-gel method has seldom been reportedfor simultaneous removal of NO and Hg0.

Accordingly, a bench-scale test was conducted over TiO2-Al2O3-CeO2 nanoparticle synthesized by sol-gel method to study simulta-neous removal of NO and Hg0 from simulated flue gas. The focus ofthis work is to clarify the mechanism for simultaneous removal ofNO and Hg0 over TiO2-Al2O3-CeO2 nanoparticle and discuss theimpacts brought by Al addition. The effects of various flue gas com-ponents including SO2 and H2O and the interaction between SCRprocess and Hg0 removal were also investigated. Simultaneously,

crucial characterizations of catalysts were conducted to examinethe physicochemical properties of catalysts and further reveal themechanisms.

2. Experimental section

2.1. Preparation of the catalysts

A single step sol-gel method was applied to synthesize TiO2,TiO2-CeO2 and TiO2-Al2O3-CeO2 catalysts. The preparation of cata-lysts consists of following steps. Butyl titanate (0.1 mol), an appro-priate amount of Aluminium tri-sec-butoxide, anhydrous ethanol(2.6 mol) and acetic acid (0.02 mol) were mixed under continuousstirring. Subsequently, the mixture of an amount of cerium nitrate,nitric acid (0.05 mol) and deionized water (0.65 mol) was addeddropwise into above solution. Then the mixed solution was main-tained under constant and vigorous stirring at room temperatureuntil the gel was formed. The obtained gels were aged at the syn-thesis temperature for 24 h and then dried at 80 �C for 24 h respec-tively. Finally, the dried gels were calcined at 400 �C for 6 h in air.All of the obtained catalysts were ground and sieved to 80–100mesh for use and were labelled as TiAlxCey, where x representedthe mass percentage of Al2O3/(TiO2 + Al2O3) (x = 5, 10, 15); y repre-sented the mass percentage of CeO2/(TiO2 + Al2O3 + CeO2) (y = 5,10, 20, 40).

2.2. Catalytic activity tests

As shown in Fig. 1, the activity tests of catalysts for NH3-SCRand Hg0 removal were conducted in a fixed-bed system. A 0.5 gsample was loaded in a quartz reactor (i.d. 10 mm), the simulatedflue gas (SFG) consisted of 500 ppm of NO, 500 ppm of NH3, 5 vol%O2, Hg0 (80 lg/m3), 400 ppm SO2(when used), 8 vol% H2O(whenused) and pure N2 as balance. The gas-phase Hg0 was generatedby a mercury permeation tube (VICI Metronics, USA) and thentransported into the reaction system with N2. All the gas flowsexcept H2O were controlled by mass flow controllers (MFC). Thewater vapour was generated by injecting water into teflon tubewrapped with a temperature-controlling heating strip at 120 �Cthrough peristaltic pump. For all experiments, a total gas flow of500 mL/min (GHSV = about 50,000 h�1) was introduced into thefixed-bed reactor at temperatures from 100 to 350 �C. The concen-trations of NO and Hg0 at inlet and outlet of the quartz reactorwere analysed respectively using a flue gas analyser (MGA5, Ger-many) and an online RA-915 Mmercury analyser (LUMEX Ltd, Rus-sia). Sample tests and analyses were kept for 2 h after the catalyticprocess had reached equilibrium in each measurement. To reducethe experimental error, three or more replicates of sample testswere conducted. Besides, the error bars in each figure representthe standard deviation from the mean of the series of experimentsat each condition.

The performance of simultaneous removal of NO and Hg0 wasexpressed by NO removal efficiency (ENO) and Hg0 removal effi-ciency (EHg) according to Eq. (1) and Eq. (2) respectively:

ENO ¼ 1� ½NO�out½NO�in

� �� 100% ð1Þ

EHg ¼ 1� ½Hg�0out½Hg�0in

!� 100% ð2Þ

In which [NO]in and [NO]out denote the inlet NO concentrationand outlet NO concentration; the inlet Hg0 concentration and out-

let Hg0 concentration are defined as ½Hg�0in and ½Hg�0out. An Hg speci-ation conversion system was applied in this study to recognize the

1536 J. Zhang et al. / Chemical Engineering Journal 313 (2017) 1535–1547

species of Hg in the outlet gas when needed. The details of conver-sion system were described in our previous study and the resultswere discussed in Supplementary material (S1) [16].

2.3. Catalyst characterization

The Inductively Coupled Plasma–Optical Emission Spectrome-try (ICP–OES) experiments were performed on a Varian 720-ES(Varian Incorporation, USA) to determine the content of differentelements. Operating conditions are as follows: RF power 1200W,plasma gas flow rate 18 L/min, nebulizer gas pressure 200 kPa,auxiliary gas flow rate 2.25 L/min, viewing mode: radical, elementwavelength: Ti: 336.122 nm, Al: 308.22 nm, Ce: 413.76 nm. A 15 swashing was settled between two successive samples to eliminatememory effects.

The Brunauer–Emmett–Teller (BET) specific surface area, aver-age pore diameter and pore volume of the catalysts was deter-mined from N2 adsorption isotherm on a Micromeritics 180Tristar II 3020 analyzer (Micromeritics Instrument Crop, USA).Prior to BET measurement, all of the catalysts were degassed invacuum at 180 �C for 5 h.

X-ray diffraction (XRD) analysis was performed on a Rigaku D/Max 2500 (Rigaku Corporation, JPN). The diffraction patterns werecollected in a 2h range of 10–80� with Cu Ka radiation.

X-ray photoelectron spectroscopy (XPS) was performed on a K-Alpha 1063 system (Thermo Fisher Scientific, UK) with an Al Karadiation. The binding energies were corrected by the C 1s with abinding energy value of 284.6 eV. The reaction condition for spentcatalysts was 500 ppm NH3, 500 ppm NO, 5% O2 with N2 as bal-anced and 80 lg/m3 Hg0, the total gas flow rate was 500 mL/min.

The scanning electron microscopy (SEM) images of preparedsamples were observed using JSM-6700F (JEOL, Japan). The sepa-rated areas for the catalysts were magnified to 100,000�. Thetransmission electron microscopy (TEM) images were obtainedusing a Tecnai G2 F20 S-TWIX transmission electron microscope(FEI, USA).

H2-temperature programmed reduction (H2-TPR) was con-ducted on an AutoChem 2920 automated chemisorption analyser(Micromeritics Instrument Crop, USA). The Fourier TransformInfrared Spectroscopy (FT-IR) was collected on a Nicolet 6700(Nicolet, USA) FT-IR spectrometer. The details of pre-treated

method of H2-TPR and FT-IR were shown in Supplementary mate-rial (S2, S3).

3. Results and discussion

3.1. The performance of different catalysts

3.1.1. Effect of Ce loading on the activity of TiO2-Al2O3-CeO2

Prior to combined removal experiments, the deNOx experimentand Hg0 removal experiment were conducted respectively over dif-ferent catalysts to obtain the optimal Ce loading. The effects of var-ious Ce loadings from 5% to 40% on the performance of TiAl10Ceyare shown in Fig. 2. Al2O3 addition was kept at 10% of TiO2. Fig. 2shows that the ENO and EHg increased sharply after Ce was addedto pure TiO2 and TiAl10 catalyst. As for TiAl10Cey catalysts, theTiAl10Ce20 catalyst showed the highest NO removal efficiency(93.44%) and Hg0 removal efficiency (91.21%) at 300 �C, furtherincreasing the Ce loading to 40% led to a decrease of ENO and EHg,which could result from the excess Ce loading [34]. As statedabove, when the Al addition kept unchanged, a 20% Ce loadingwas proper for TiAlxCey catalyst due to the preferable catalyticactivity both in single removal of NO and Hg0.

3.1.2. Simultaneous removal of NO and Hg0 over TiO2-Al2O3-CeO2

To deep discuss the effect of Al addition, the performance of cat-alysts with different Al additions in simultaneous removal of NOand Hg0 were compared at 100–350 �C and shown in Fig. 3. Itwas obvious that the addition of Al to both TiCe20 and TiO2 cata-lysts resulted in a distinct enhancement of ENO and EHg, wideningthe temperature range of operating. For the TiAlxCe20 catalysts,the ENO and EHg increased with the increase of Al addition. TheTiAl10Ce20 catalyst showed the highest SCR activity (93.41%) andcatalytic activity for Hg0 removal (80.54%) at 300 �C, furtherincrease Al amount to 15% led to a decrease of ENO and EHg. Addi-tionally, the EHg of TiAlxCe20 catalysts increased with the increaseof reaction temperature from 100 to 300 �C, while showed consid-erable decrease of Hg0 catalytic activity at 350 �C, this could bebecause the inhibition effect of high temperature on the physicaladsorption of Hg0 [26]. The results above suggested that the addi-tion of Al was of essential importance for excellent performance ofTiAlxCe20 catalyst. Accordingly, the TiAl10Ce20 catalyst was chosento study the catalytic reaction in the following work, and the

Fig. 1. Schematic diagram of the experimental setup.

J. Zhang et al. / Chemical Engineering Journal 313 (2017) 1535–1547 1537

mechanisms for simultaneous removal of NO and Hg0 over TiAl10-Ce20 catalyst will be discussed later in this study.

3.2. Interaction between NH3-SCR and Hg0 removal

3.2.1. Effect of Hg0 on NO removalTo identify the application possibility of TiAl10Ce20 catalyst for

simultaneous removal of NO and Hg0, the interaction betweenNO removal and Hg0 removal were investigated. The effect ofHg0 on NO removal is shown in Fig. 4, the results indicated thatwhether in the case of single NO removal system (500 ppm NH3,500 ppm NO, 5% O2, N2 as balance) or simultaneous removal sys-tem (500 ppm NH3, 500 ppm NO, 5% O2, 80 lg/m3 Hg0, N2 as bal-ance), the ENO of TiAl10Ce20 catalyst increased with the increaseof reaction temperature from 100 �C to 300 �C, then decreasedslightly at 350 �C. It was clear that the presence of Hg0 did notchange the tendency of ENO. However, the ENO decreased slightlyin the presence of Hg0 especially at 100 �C. In general, Hg0 was oxi-dized to HgO and accumulated on the surface of catalysts in thepresence of O2 [35]. Meanwhile, the results of XPS Hg 4f (Fig. 10(c)) has proved that HgO was formed on the surface of TiAl10Ce20

Fig. 2. Single component removal efficiency on various catalysts with different Celoading: (a) NO removal, (b) Hg0 removal (Reaction condition: balance N2, 100–350 �C, 500 mg of catalyst, GHSV = 50,000 h�1, (a) NO = NH3 = 500 ppm, O2 = 5%; (b)Hg0 = 80 lg/m3, O2 = 5%).

Fig. 3. Simultaneous removal efficiency on various catalysts with different Aladdition (Reaction condition: NO = NH3 = 500 ppm, Hg0 = 80 lg/m3, O2 = 5%, bal-ance N2, 100–350 �C, 500 mg of catalyst, GHSV = 50,000 h�1).

Fig. 4. Effect of Hg0 on NO removal efficiency over TiAl10Ce20 catalyst (Reactioncondition: NO = NH3 = 500 ppm, Hg0 = 80 lg/m3 (when used), O2 = 5%, balance N2,100–350 �C, 500 mg of catalyst, GHSV 50,000 h�1).


catalyst. Therefore, it was deduced that the HgO accumulated onthe surface of TiAl10Ce20 catalyst slightly affected the ENO. Never-theless, it was worth noting that the ENO of TiAl10Ce20 catalystshowed no obvious change at the optimal temperature (300 �C)and the Hg0 concentration of real coal flue gas was much lowerthan 80 lg/m3. Hence, it was suggested that the Hg0 removal pro-cess would not affect the NO conversion on TiAl10Ce20 catalyst sig-nificantly in the real application.

3.2.2. Effects of SCR components on Hg0 removalAs shown in Fig. 5(a), the effect of SCR on Hg0 removal was

studied. The trend of EHg for the system with SCR was similar toHg0 removal system without SCR. The EHg decreased sharply atall reaction temperatures in the presence of SCR atmosphere, sug-gesting that the co-presence of NO and NH3 could significantlyinhibit the catalytic activity of TiAl10Ce20 for Hg0 removal. How-ever, the TiAl10Ce20 nanoparticle still exhibited a relatively highEHg of 80.54%.

To gain more insight into the effects of SCR components on Hg0

removal efficiency, a series of tests with different amounts of NO

and NH3 were conducted. As shown in Fig. 5(b), the introductionof 500 ppm NO resulted in a lower EHg of 89.35% in comparisonwith that of Hg0 removal system without NO, further increasingNO concentration to 1000 ppm led to a slightly decrease of EHg.These results indicated that NO had a negligible inhibition effecton Hg0 removal over TiAl10Ce20 catalyst. It was hypothesized thatnon-active species produced from the reactions between NO andsurface oxygen slightly limited the removal of Hg0 [36].

The effect of NH3 was also investigated since NH3 is usuallyemployed as the reducing agent in SCR process. It is clearly shownin Fig. 5(b) that the Hg0 oxidation activity over TiAl10Ce20 was sig-nificantly limited by NH3. The EHg decreased sharply to 49.21%when 500 ppm NH3 was introduced, then dropped to 30.36% at ahigher concentration of 1000 ppm NH3. It was demonstrated thatNH3 can rapidly react with the catalyst and generate adsorbed spe-cies which can occupy the surface active sites of catalyst and con-sume surface active oxygen, resulting in the inhibition of Hg0

oxidation [37]. Additionally, the effect of NH3/NO ratio on Hg0

removal is shown in Fig. 5(b). With the increase of NH3/NO ratiofrom 0 to 1, the mercury removal efficiency decreased slightly. Inthe coexistence of SCR and Hg0, SCR reaction with high concentra-tion of NO and NH3 plays the dominant role in the inlet region [38].It was proposed that NH3 could be adsorbed on the catalyst surfaceto form coordinated NH3 and NH2 which is beneficial for NOremoval and consume surface oxygen, resulting in a small decreaseof Hg0 removal efficiency [39].

3.3. Effect of O2 on simultaneous removal of NO and Hg0

Oxygen plays an important role on simultaneous removal of NOand Hg0 as a crucial flue gas component. In this work, the effect ofO2 on simultaneous removal of NO and Hg0 were investigated andshown in Fig. 6. The TiAl10Ce20 catalyst exhibited poor activity forNO conversion and Hg0 removal in the absence of O2, higher ENOand EHg were obtained in the whole temperature range with theaddition of 5% O2. A further increase of concentration of O2 to10% showed little impact on NO conversion and Hg0 removal. Itwas detected that O2 could greatly promote the SCR reaction andthe removal of Hg0; this was in line with other researches [40,41].

3.4. Effects of H2O and SO2 on simultaneous removal of NO and Hg0

A series of researches attached great importance to the effectsof H2O and SO2 on NH3-SCR and Hg0 removal respectively becauseof their deactivation effects over catalysts which could affect thepractical application [31,42]. In this work, the impacts of 8%H2Oand 400 ppm SO2 on simultaneous removal of NO and Hg0 overTiAl10Ce20 and TiCe20 catalyst were compared at 300 �C. As shownin Fig. 7, the inhibitive effects of 8% H2O on NO removal and Hg0

removal were observed both in TiAl10Ce20 and TiCe20 catalyst. Itcould be seen that TiAl10Ce20 exhibited a better resistance to H2Othan TiCe20 catalyst, the ENO and EHg of TiAl10Ce20 catalystremained 87.25% and 77.36% respectively after 8% H2O was keptfor 300 min. The peak at 1628 cm�1 of FT-IR spectrum of H2O+ O2 adsorption was corresponded to dHOH of H2O, indicated thatthe H2O was formed on the catalyst surface [22]. The slightly inhi-bition effect of H2O on SCR reaction could be ascribed to competi-tive adsorption between H2O and NH3 for the reaction sites whilethe competition between H2O and Hg0 might led to the decreaseof EHg [26,33,43].

Meanwhile, the ENO and EHg of TiAl10Ce20 decreased to 83.6%and 66.59% respectively after 400 ppm of SO2 was maintained inthe feed stream for 300 min. However, the TiAl10Ce20 still exhibiteda rather better resistance to SO2 than TiCe20 catalyst. According tothe research of Xu and his co-workers [28], the formation ofNH4HSO4, Ce(SO4)2 and Ce2(SO4)3 was responsible for the

Fig. 5. Effects of SCR components on Hg0 removal efficiency over TiAl10Ce20 catalyst(Reaction condition: Hg0 = 80 lg/m3, O2 = 5%, balance N2, 500 mg of catalyst, GHSV50,000 h�1, (a) 100–350 �C, NO = NH3 = (0, 500 ppm), (b) 300 �C, individual SCRcomponent (NO = 0 ppm, NH3 = (500, 1000 ppm) or NH3 = 0 ppm, NO = (500,1000 ppm)) or co-presence of NH3 and NO with different NH3/NO ratio (NH3/NO = 0–1.2)).


deactivation of SO2 on NO removal. Meanwhile, in the FT-IR spec-trum of SO2 + O2 adsorption, the stretching vibration of adsorbedsulfate and/or bisulfate was detected at 1045 cm�1 which was pro-posed that could partly reducing the deactivation effect of SO2 byenhancing the acidity of catalyst and promoting the adsorptionof NH3 [31,44]. It could be the reason that TiAl10Ce20 catalystshowed better activity than TiCe20 catalyst in the presence ofSO2. It was worth noting that the promotion effect on NH3 adsorp-tion could not inhibit the deposition of NH4HSO4 or Ce(SO4)2 spe-cies which resulted in the deactivation effect on catalytic activityof TiAl10Ce20 catalyst [31]. In addition, the band of liquidlike phy-sisorbed SO2 was also obtained at 2336 cm�1 and 2363 cm�1 inthe FT-IR spectrum of SO2 + O2 adsorption. The results indicatedthat the inhibitive effect of SO2 for Hg0 removal might result fromthe competitive adsorption between SO2 and Hg0 on the surface ofTiAl10Ce20 catalyst [42].

3.5. Characterization of materials

3.5.1. ICP-OES, BET and XRD structureInductively Coupled Plasma–Optical Emission Spectrometry

(ICP–OES) experiments were conducted to test the real contents

of Ti, Al and Ce of different catalysts. As the results of ICP-OESshown in Table 1, the actual contents of Al2O3 and CeO2 were sim-ilar to the nominal contents for all the catalysts. The slight differ-ence between the nominal and actual contents of Al2O3 and CeO2

might be attributed to the loss of the alumina precursor and ceriaprecursor during the preparation process.

The BET specific surface area, pore volume and average porediameter of different catalysts are listed in Table 2. The introduc-tion of Al2O3 to both TiO2 and TiCe20 lead to a higher specific areaand lower pore volume. The results revealed that the addition of Alcould expand the BET specific surface area. In particular, theTiAl10Ce20 nanoparticle owned the highest specific area. It wasinferred that more reactive sites for the reactant were provided overTiAl10Ce20 nanoparticle and thus partially promote the catalyticactivity [22,31]. The higher BET surface area of TiAl10Ce20 could bebecause that the amorphous or crystallite structure was formeddue to Al addition [27,45], which could also be proved by the XRD.

To investigate the crystal structural of the catalysts, the XRDpatterns are shown in Fig. 8. The pure TiO2 was well crystallizedand contained the typical diffraction patterns of anatase titania(PDF ICDD 02-0406), rutile titania (PDF ICDD 21-1276) [27] as wellas weak brookite titania (PDF ICDD 29-1360) [27], in which anatasetitania was the predominant phase under 400 �C calcination

Fig. 6. Effect of O2 on simultaneous removal efficiency over TiAl10Ce20 catalyst: (a)NO removal, (b) Hg0 removal (Reaction condition: NO = NH3 = 500 ppm, Hg0 =80 lg/m3, O2 = (0, 5, 10%), balance N2, 100–350 �C, 500 mg of catalyst, GHSV50,000 h�1).

Fig. 7. Effects of H2O and SO2 on simultaneous removal efficiency over TiAl10Ce20catalyst: (a) NO removal, (b) Hg0 removal (Reaction condition: NO = NH3 =500 ppm, Hg0 = 80 lg/m3, O2 = 5%, SO2 = 400 ppm (when used), H2O = 8vol.% (whenused), balance N2, 300 �C, 500 mg of catalyst, GHSV 50,000 h�1).


temperature. For TiAl10, the addition of Al2O3 into TiO2 decreasedthe fraction of rutile titania and the peak intensity. Such resultsindicated that the addition of Al2O3 effectively inhibited the phasetransformation from anatase phase to rutile phase which wasreported to be detrimental to performance of SCR catalysts[46,47]. For the XRD patterns of TiAlxCe20 catalysts, only diffractionpeaks of anatase phase of TiO2 were observed. It was inferred thatceria and Al species existed as amorphous or other highly dis-persed species. Meanwhile, the particle size of TiCe20 and TiAl10-Ce20 estimated by the Scherrer equation and MDI Jade were6.9 nm and 5.1 nm respectively from the scattering angles (2h) at25.24�. The results indicated that the addition of Al decreased theparticle size which was almost in accord with the results of TEM.With the increase content of Al2O3 in TiAlxCe20 catalyst, the bandintensity ascribed to TiO2 decreased and the diffraction peaksbroadened, suggesting that the addition of Al weakened the crys-tallinity and strong interaction existed among Al, Ti and Ce species.The higher catalytic activity could be obtained over amorphous orcrystallite materials due to structural distortion in comparison

with crystalline materials [48], which was probably partly respon-sible for the highest activity of TiAl10Ce20.

3.5.2. SEM and TEMIn order to further study the texture properties of catalysts, SEM

and TEM were conducted. The SEM images of TiAl10Ce20 and TiCe20catalyst were presented in Fig. 9(a) and (b). As shown in Fig. 9, theactive components were well dispersed on the surface of TiAl10Ce20catalyst. Furthermore, the microstructure of catalyst has changedafter 10%Al was added. The TiAl10Ce20 catalyst showed smallerand more uniform particles with well-proportioned distributionin compared with TiCe20 catalyst, this was in consistence withthe results of XRD which makes TiAl10Ce20 catalyst more efficientfor NO and Hg0 removal.

The HR-TEM micrographs of TiAl10Ce20 and TiCe20 catalyst areshown in Fig. 9(c) and (d). Crystalline nanoparticles with visiblelattice fringe were observed in the TEM images of TiAl10Ce20 andTiCe20 catalyst. As shown in Fig. 9, the primary particle size ofTiCe20 catalyst was around 7 nm and two kinds of lattice fringeswere observed, 0.327 nm and 0.344 nm corresponding to rutile(1 1 0) phase and anatase (1 0 1) phase respectively [7]. For TiAl10-Ce20 catalyst, the primary particle size was about 5 nm and slightlysmaller than that of TiCe20 catalyst, indicating that Al additioncould somewhat decrease the particle size of catalyst and lowerthe crystallinity. Only a kind of lattice fringe of 0.345 nm corre-sponding to anatase (1 0 1) phase was observed on TiAl10Ce20 cat-alyst. In addition, there was no lattice fringes matched Al2O3, cubicCeO2 and rutile TiO2. Such results indicated that CeO2 was well dis-persed in the TiAl10Ce20 catalyst, and the unobservable Al2O3 par-ticles might exist as an amorphous or highly dispersed species.

3.5.3. XPS analysisThe XPS measurements were conducted on fresh and spent cat-

alysts to better understand the removal mechanisms and verify thechemical states of different elements on the surface materials. TheXPS spectra of Ti 2p are presented in Supplementary material(Fig. S1). As shown in Fig. 10(a), the O 1s spectra for freshTiAl10Ce20 and TiCe20 catalysts were divided into three peaks. Thesub-bands at approximately 528.9–530 eV could be assigned tothe lattice oxygen (Ob) [26,49]. Two shoulder sub-bands at thehigher banding energy corresponded to chemisorbed oxygen and/or weakly bonded oxygen species (Oa) and surface oxygen inhydroxyl and/or surface adsorbed water (Ok) [7,27,49]. Accordingto the calculation results of surface oxygen concentration, theOb + Oa/OT ratio sharply increased with the addition of Al, whereOT indicated the total oxygen. The content of Oa (51.0%) on the sur-face of TiAl10Ce20 catalyst was higher than that of TiCe20 catalyst(43.9%). Chemisorbed oxygen (Oa) on the surface of catalyst playsa dominant role in oxidation reaction due to its high mobility [50].Therefore, the higher Oa/OT ratio on the TiAl10Ce20 catalyst surfacecould result in the better activity for NO and Hg0 removal, which

Table 1Metal oxides content of different catalysts.

Catalysts Element content (wt.%) Al2O3/(TiO2 + Al2O3) (wt.%) CeO2/(TiO2 + Al2O3 + CeO2) (wt.%)

Ti Al Ce

TiO2 59.81 0 0 0 0TiAl10 54.23 2.52 0 9.52 0TiCe20 48.64 0 15.34 0 18.84TiAl5Ce20 46.47 1.02 15.15 4.74 18.61TiAl10Ce20 43.95 2.10 15.24 9.77 18.72TiAl15Ce20 41.78 3.13 15.05 14.51 18.49TiAl10Ce5 51.79 2.38 3.73 9.43 4.58TiAl10Ce10 49.22 2.34 7.36 9.72 9.04TiAl10Ce40 33.74 1.58 30.71 9.59 37.72

Table 2The surface area, pore volume and pore diameter of the catalysts.

Catalysts BET surface area Pore volume Average pore diameter(m2/g) (cm3/g) (nm)

TiO2 87.61 0.1366 4.91TiAl10 152.73 0.1357 2.94TiCe20 150.83 0.3090 6.41TiAl10Ce20 238.49 0.2026 2.94

Fig. 8. XRD patterns of TiO2, TiAl10, TiCe20, TiAl5Ce20, TiAl10Ce20 and TiAl15Ce20.


Fig. 9. SEM photographs of (a) TiCe20, (b) TiAl10Ce20, 100,000 multiplier; TEM images of (c) TiCe20, (d) TiAl10Ce20.

Fig. 10. XPS spectrums of fresh and spent TiCe20 and TiAl10Ce20 catalysts: (a) O 1s, (b) Ce 3d, (c) Hg 4f. (Reaction condition for spent catalysts: NO = NH3 = 500 ppm,Hg0 = 80 lg/m3, O2 = 5%, balance N2, 300 �C, 500 mg of catalyst, GHSV 50,000 h�1).


was consistent with the results of activity tests in this work. Theabsence of Ok peak on the spent TiAl10Ce20 catalyst which corre-sponded to adsorbed water might result from the volatilizationof H2O at high temperature of 300 �C [12]. Meanwhile, the peakof Oa decreased significantly over the spent TiAl10Ce20 catalystwhile the Ob/OT ratio increased, demonstrating that the adsorbedsurface oxygen mainly participated in the oxidation reaction overTiAl10Ce20 catalyst at 300 �C. The decrease of H2O oxygen and the

compensation of O2 for consumed lattice oxygen might result inthe increased content of lattice oxygen (Ob) [12].

The Ce 3d XPS peaks of fresh and spent TiAl10Ce20 and TiCe20 cat-alyst are shown in Fig. 10(b), the spectra of Ce 3d were assignedaccording to the research of Burrough et al. [51] and Mullins et al.[52]. The peaks of 3d3/2 and 3d5/2 spin-orbit states were denotedas u and v respectively. Among which, the sub-bands labelled u/v,u00/v00 and u000/v000 were assigned to 3d104f0 electronic state

Fig. 10 (continued)


corresponding to Ce4+; the peaks denoted u0/v0 represented 3d104f1

of Ce3+. All of the catalysts exhibited a total of eight peaks attributedto Ce4+ and Ce3+ oxidation states, while the ratio of Ce3+/(Ce3++Ce4+)increased from 29.45% to 36.54% after introducing 10%Al to TiCe20catalyst. The Ce3+ could induce unsaturated chemical bonds, favor-able charge imbalance as well as oxygen vacancies on the surfaceof catalyst, and thus resulted in an increase of chemisorbed oxygenwhich is beneficial for the catalytic process [7], this was agreementwith the XPS results of O 1s. In comparison with the fresh catalysts,the Ce3+/(Ce3++Ce4+) ratio over the surface of spent TiAl10Ce20 andTiCe20 catalyst both decreased, indicating that a redox reactionexisted between Ce4+ and Ce3+ (2CeO2 ? Ce2O3 + Ob,Ce2O3 +1/2O2 ? 2CeO2). The Ce species shift constantly between Ce4+ andCe3+ with the redox proceeding and simultaneously storing andreleasing oxygen. The decreased ratio could be because that the O2

oxidized Ce3+ to Ce4+ at 300 �C and compensated the lattice oxygenquickly.

The Hg 4f spectrums for spent TiAl10Ce20 and TiCe20 catalyst areshown in Fig. 10(c). The spectrum of spent TiAl10Ce20 catalystshowed two peaks at 100.3 and 102.9 eV respectively, while therewas no obvious characteristic peak at 99.9 eV corresponding to ele-mentalmercury [25], the possible reasonmight be that Hg0 could beeasily desorbed from the surface of TiAl10Ce20 catalyst at 300 �C orthe concentration of adsorbed Hg0 was below the detectability ofXPS equipment [45]. The peak at lower binding energy of 100.3 eVwas due to the formation of HgO and the peak at 102.9 eV wasascribed to the characteristic peak of Si 2p [21,25], suggesting thatthe oxidationofHg0 could occur on the surface of TiAl10Ce20 catalyst.The spectrum of spent TiCe20 catalyst exhibited two peaks at 100.5and 103.1 eV, in which the former one was attributed to HgO andthe latter onewasdue to Si 2p [53]. The spent TiCe20 catalyst showeda smallerHgOpeak thanTiAl10Ce20 catalyst, this indicated thatmoreHgO was formed on the surface of TiAl10Ce20 catalyst which corre-sponding to a better oxidation capacity.

3.5.4. H2-TPR analysisTo discuss the redox behaviour of catalysts in this work, the H2-

TPR profiles of different catalysts are shown in Fig. 11. The pureTiO2 catalyst only showed a small peak at 573 �C, indicating thatit has low redox ability at temperatures less than 800 �C [21]. After10% Al was added into Ti, the reduction peak shifted to a lowertemperature of 562 �C and another shoulder peak was obtainedat 666 �C. The TiCe20 catalyst exhibited two reduction peaks cen-tered at 460 �C and 604 �C respectively. It has been reported that

the reduction of bulk CeO2 only occurred at about 750 �C [7,54].Therefore, the reduction peak at 460 �C could be ascribed to thereduction of surface oxygen of type Ce4+-O-Ce4+ [55], while anotherpeak at 604 �C could belong to the reduction of ceria of type Ce3+

-O-Ce4+ due to the existence of a fraction Ce3+ on the surface of cat-alysts [54]. For TiAl10Ce20 catalyst, the two reduction peaks showedlower reduction temperatures of 459 �C and 558 �C, illuminatingthat the redox ability of TiAl10Ce20 catalyst was greatly improvedwith the addition of 10%Al. The higher redox ability of TiAl10Ce20catalyst could enhance the mobility of surface oxygen, which wasbeneficial to the activity of catalysts. The diffusion of oxygen mightbe promoted by the synergetic effect among Ti, Al and Ce species,which results in structural distortion of catalysts and generatedabundant oxygen defects [56].

3.5.5. FT-IRTo further investigate the reaction mechanism of NH3-SCR over

TiAl10Ce20 catalyst in the presence of Hg0 and the influence of SO2

and H2O, the FT-IR spectra of TiAl10Ce20 catalyst is shown in Fig. 12.After adsorption of NO + O2 for 1 h, several weak bands at 1385,1551, 1622, 2365 and 3742 cm�1 were detected. The band at1385 cm�1 was attributed to nitrate (NO3

�) species [57], whichindicated that the nitrate species loaded on the active adsorptionsites might compete with Hg0 and result in a slight inhibitive effecton Hg0 removal [37]; this is in concordance with the experimentresults shown in Fig 4(b). The band centered at 1551 cm�1 wasascribed to NO2-containing species, such as nitrito (O-boundNO2) and nitrato (NO3) species [36], which could also occupy theactive sites. In addition, the peak appeared at 1622 cm�1 in thespectrum was assigned to the bridging bidentate nitrates [31],while the signal observed at 2365 cm�1 was attributed to overtoneand combination vibrations of nitrato species. The band at3742 cm�1 was corresponded to the typical hydroxide specieswhich was associated with the interaction of surface hydroxylsand NO2 and nitrate species [58].

Several bands were obtained in the spectrum of NH3 + O2

adsorption. The band centered at 1406 cm�1 was attributed tocoordinated NH3 species on Lewis acid sites [31,59]. The weak peakdetected at 1474 cm�1 could be assigned to asymmetric deforma-tion of NH4

+ (das(NH4+)) generated from interaction of NH3 and

catalyst surface and then adsorbed on Brønsted acid sites on the

Fig. 11. H2-TPR profiles over TiAl10Ce20, TiCe20, TiAl10 and TiO2. Fig. 12. FTIR spectra for TiAl10Ce20 catalyst: (a) NO + O2, (b) NH3 + O2, (c) NO+ NH3 + O2, (d) H2O + O2, (e) SO2 + O2. (Pretreated condition: Hg0 = 80 lg/m3,O2 = 5%, balance N2, 500 mg of catalyst, GHSV 50,000 h�1, (a) 500 ppm NO, (b)500 ppm NH3, (c) NH3 = NO = 500 ppm, (d) 8% H2O, (e) 400 ppm SO2).


surface of catalyst [36]. The NH4+ species were regarded as active

intermediate of SCR reaction and could load on the active siteand limit adsorption of Hg0 [37]. Meanwhile, the band at1516 cm�1 was proposed to be amide species (–NH2) observedby hydrogen abstraction of coordinated NH3 [60], which was con-sidered as the intermediate of SCR reaction. The band appeared at1549 cm�1 was attributed to asymmetric bending vibration of N–Hbond in –NH3 group decomposed from –NH4

+ bound to Brønstedacid sites [61]. The signal obtained at 3742 cm�1 was ascribed toO–H stretching vibration modes formed by interaction betweensurface hydroxyls and NH3 [48].

In (NO + NH3 + O2) FT-IR curve, the sharp band appeared at1269 cm�1 and 1543 cm�1 were attributed to nitrate and nitritespecies. The first signal at 1269 cm�1 was ascribed to bridgednitrate and the second one at 1543 cm�1 was corresponded tomonodentate nitrate which was regarded as real reactive speciesunder SCR condition [48,62]. In addition, the band assigned tocoordinated NH3 species bound to Lewis acid sites appeared at1400 cm�1 [31] and the shoulder peak arose at 1456 cm�1 wasascribed to NH4

+ species on Brønsted acid sites [36]. The intensepeak at 1620 cm�1 was supposed to be the overlapping of nitratesand coordinated NH3 species on Lewis acid sites [31,62], and theband at 3744 cm�1 was due to the water formed on the surfaceof catalyst [60].

Meanwhile, the adsorption spectrums of SO2 + O2 and H2O + O2

were also exhibited in Fig 12. After the adsorption of SO2 + O2, theband observed at 1045 cm�1 was assigned to stretching vibrationof adsorbed sulfate and/or bisulfate on the catalyst surface [63].The sulfate could enhance the acidity of catalyst, especially theLewis acid sites, and thus promote the adsorption of NH3 and infor-mation of intermediate –NH3

+ [31], partly reducing the inhibitioneffect of SO2 to catalyst. Besides, the peak arose at 1136 cm�1

was demonstrated as gas-phase SO2 [63], while two bandsappeared at 2336 and 2363 cm�1 were proposed to be liquidlikephysisorbed SO2 [64]. Such SO2 adsorbed on the surface could limitthe removal of Hg0 due to the competition effect [42]. The charac-teristic peaks of H2O appeared at 3744 cm�1 and 1557 cm�1 indi-cated that surface water was formed due to the reaction betweenSO2 and hydroxyl groups [28]. When the catalyst was treated byH2O + O2, the band observed at 1628 cm�1 was assigned to dHOH

of H2O [22]. Meanwhile, only a peak at 1628 cm�1 was obtainedwhich was similar to the spectrum of the fresh catalyst (Fig. S2),indicating that the gas-phase water had little impact on the activ-ity of TiAl10Ce20 catalyst which was agreement with the results ofFig. 7.

3.6. Mechanisms discussion

According to the results of above experiments and characteriza-tions, the possible reaction mechanisms of simultaneous removalof NO and Hg0 over TiAl10Ce20 catalyst were proposed. The resultsof XPS O1s indicated that both the lattice oxygen ([O]) andadsorbed oxygen (O⁄) participated in the reaction process. Mean-while, the coexistence of Ce3+ and Ce4+ proved by XPS Ce3d indi-cated that the ceria on the surface of catalyst occupied twooxidation states during the reaction, and the redox shift betweenthem (2CeO2 ? Ce2O3 + Ob,Ce2O3 + 1/2O2 ? 2CeO2) could be bene-ficial to the simultaneous removal of NO and Hg0. Moreover, theadsorbed oxygen could be compensated by gas-phase oxygen (O2

(g)) (Eq. (3)) and the consumed lattice oxygen might come fromthe redox shift between Ce3+ and Ce4+ (Eqs. (5) and (6)). To distin-guish the speciation of mercury in the outlet flue gas, results ofmercury speciation conversion experiment were shown in Fig. S3and discussed in S1. The results indicated that part of mercurywere captured by TiAl10Ce20 catalyst while part of which werereleased into outlet flue gas in the form of HgO. Combined with

the analysis of XPS Hg4f, the speciation of captured mercury wasproved to be HgO. Therefore, it was proposed that gas-phase Hg0

(Hg0(g)) was firstly adsorbed on the surface of catalyst to formadsorbed Hg0 (Hg0(ad)) (Eq. (4)), and then be oxidized into HgO by lat-tice oxygen and adsorbed oxygen (Eqs. (7) and (8)). A fraction of HgOwas adsorbed (HgO(ad)) on the surface of catalyst while part of it con-verted to gas-phase HgO (HgO(g)) (Eq. (9)). The removal mechanismof Hg0 was described as follows on the basis of the present work andother researches [21,45]:

O2 ! 2O� ð3Þ

Hg0ðgÞ ! Hg0

ðadÞ ð4Þ

2CeO2 ! Ce2O3 þ ½O� ð5Þ

Ce2O3 þ 1=2O2 ! 2CeO2 ð6Þ

Hg0ðadÞ þ ½O� ! HgOðadÞ ð7Þ

Hg0ðadÞ þ O� ! HgOðadÞ ð8Þ

HgOðadÞ ! HgOðgÞ ð9ÞAs for NO removal, the results of FT-IR spectrum of NO + O2

adsorption indicated that gas-phase NO (NO(g)) could be oxidizedto form a small amount of adsorbed NO2 (NO2(ad)) (Eq. (10)) andother intermediate nitrates on the surface of TiAl10Ce20 catalystin the presence of O2. Besides, in the FT-IR spectrum of NH3 + O2

adsorption, coordinated NH3, NH4+ and –NH2 were detected on

the catalyst surface which means both Lewis and Brønsted acidsites existed. On the basis of the results obtained in this workand other researches [22,39], the SCR reaction could carry out asfollowing steps. Gaseous NH3 (NH3(g)) was firstly adsorbed on thecatalyst surface or react with the H+ on the catalyst surface (H+

sur-

face) to generate adsorbed NH4+(NH4

+(ad)) (Eqs. (11) and (13)), then

the adsorbed NH3(NH3(ad)) or NH4+(ad) could react with NO2(ad) to form

intermediate species and further react with NO(g) to give N2 and H2O(Eqs. (12) and (14)). Meanwhile, the existence of –NH2 species (–NH2

(ad)) proved that NH3(g) might be dehydrated by [O] to generate –NH2

(ad) (Eq. (15)) which could react with NO(g) to form N2 (Eq. (16)). Themechanism for NO removal could be explained as follows:

2NOðgÞ þ O2 ! 2NO2ðadÞ ð10Þ

NH3ðgÞ ! NH3ðadÞ ð11Þ

2NH3ðadÞ þ NO2ðadÞ þ NOðgÞ ! 2N2 þ 3H2O ð12Þ

NH3ðgÞ þHþsurface ! NHþ

4ðadÞ ð13Þ

2NHþ4ðadÞ þ NO2ðadÞ þ NOðgÞ ! 2N2 þ 3H2Oþ 2Hþ ð14Þ

NH3ðgÞ þ ½O� ! �NH2ðadÞ þ �OHðadÞ ð15Þ

�NH2ðadÞ þ NOðgÞ ! N2 þH2O ð16Þ

4. Conclusion

A series of TiAlxCey nano-sized catalysts were prepared by a sin-gle step sol-gel method, the performance of TiAl10Ce20 nanoparticlewas greatly enhanced by Al addition and exhibited the highest ENO(93.41%) and EHg (80.54%) under the gas condition of SCR atmo-sphere at 300 �C. Besides, the deactivation effects of 8% H2O and400 ppm SO2 could be reduced by the Al addition. The presence


of O2 could compensate the consumed lattice oxygen and chemi-sorbed oxygen, which was essential to both NO and Hg0 removal.The presence of SCR atmosphere showed an inhibitive effect onHg0 removal due to the existence of NH3, while Hg0 exhibited littleimpact on NO removal. Meanwhile, the characterization resultsdemonstrated that the TiAl10Ce20 nanoparticle possessed lowercrystallinity, stronger redox ability, better texture property withhighly dispersed Ce on the catalyst surface owing to the additionof Al, which were favorable to the excellent SCR activity and Hg0

removal efficiency. The mechanisms for simultaneous removal ofNO and Hg0 over TiAl10Ce20 nanoparticle were also proposed, theredox shift between Ce4+ and Ce3+ was significant both in NO andHg0 removal. Hence, TiAl10Ce20 nanoparticle with Al addition wasproposed to be a promising catalyst for simultaneous removal ofNO and Hg0.

Acknowledgments

This project was financially supported by the National NaturalScience Foundation of China (51278177, 51478173) and the Scien-tific and Technological Major Special Project of Changsha City inChina (k1502028-31).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2016.11.039.

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a sol-gel ti-al-ce-nanoparticle catalyst for simultaneous

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