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Catalysis Today 281 (2017) 575–582

Contents lists available at ScienceDirect

Catalysis Today

j our na l ho me page: www.elsev ier .com/ locate /ca t tod

olid state preparation of NiO-CeO2 catalyst for NO reduction

hangjin Tanga,b,∗, Bowen Suna, Jingfang Suna,b, Xi Hongb,c, Yu Dengb, Fei Gaoa,b,in Donga,b,∗

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR ChinaJiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing University, Nanjing 210093, PR ChinaNanjing Vsky GreenChem Co. Ltd., Nanjing 210047, PR China

r t i c l e i n f o

rticle history:eceived 7 January 2016eceived in revised form 27 March 2016ccepted 5 May 2016vailable online 26 May 2016

eywords:iO-CeO2

olid state preparationolten salt decomposition

nterfacial interactionO + CO

a b s t r a c t

The development of efficient method for preparation of well-performed catalyst is an attractive topic inheterogeneous catalysis. Conventional route to obtaining mixed oxide catalysts is based on wet chemicalmethods. In the present study, a novel solid state method was proposed to fabricate NiO-CeO2 cata-lyst by directly mixing metal precursors (nickel nitrate hydrate and cerium nitrate hydrate) and thesubsequent calcination, omitting the common drying procedure in wet preparations. The preparationprocess was tracked by thermogravimetry-differential thermal analysis (TG-DTA) and obtained cata-lyst was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM),N2 physisorption, powder X-ray diffraction (XRD), Raman spectroscopy, H2 temperature programmedreduction (H2-TPR) and tested in the model reaction of NO reduction by CO. Results showed that simul-taneous decomposition of the mixed nitrates was occurred, suggesting cooperative interaction betweenthe precursors. Moreover, in comparison with wet impregnation method, the catalyst from solid statepreparation displayed not only improved textual properties (larger surface area and enriched porous

structure), but also enhanced interfacial interactions between nickel and ceria, which promoted the bulkdoping, surface dispersion of nickel species and the reduction of surface oxygen species. As a result, thecatalytic performance in NO + CO reaction was significantly upgraded. Based on the preliminary resultsof this study, it is supposed the solid state preparation may open a convenient and versatile pathway tofabricate mixed oxide catalysts.

© 2016 Elsevier B.V. All rights reserved.

. Introduction

The urgent appeal for air pollution reduction and stringent stan-ard for exhaust emission control has made purification of toxicases (NOx, CO, etc.) a timely topic and stimulated tremendoustudies on the development of well-performed catalysts [1–4]. Con-entionally, catalytic reduction of NO by CO or hydrocarbons is andeal way to simultaneous elimination of exhaust pollutants andupported noble metal catalysts are proved to be most powerful5]. However, owing to the high cost and limited thermal stability ofoble metals, considerable attention has been paid to development

f base metal catalysts [6–9]. Among them, ceria-based mixed oxideatalysts are receiving more and more interest due to their specialedox property and high oxygen storage capacity (OSC), which are

∗ Corresponding authors at: Nanjing University, Key Laboratory of Mesoscopichemistry of MOE, School of Chemistry and Chemical Engineering, 22# Hankouoad, Nanjing University, Nanjing 210093, PR China.

E-mail addresses: tangcj@nju.edu.cn (C. Tang), donglin@nju.edu.cn (L. Dong).

ttp://dx.doi.org/10.1016/j.cattod.2016.05.026920-5861/© 2016 Elsevier B.V. All rights reserved.

important for controlling the ratio of oxidants and reductants inautomobile exhaust [10,11]. Particularly, NiO-CeO2 catalysts, withmerits of low cost and easy availability, are reported to be highlyactive for NO removal by CO [12,13].

It is well established that the addition of transition metal oxideinto ceria could create new redox sites, and for NiO-CeO2 cata-lysts, the catalytic performance is largely dependent on interfacialinteractions between nickel and ceria [14,15]. Up to now, variousstrategies have been attempted to modulate the interfacial interac-tions, and controlling of ceria morphologies, doping of heteroatomsin ceria lattice and choosing of different preparations constitutethe most used ones [13,16–18]. In general, to strengthen the inter-facial interactions, it is essential that the precursors be in closecontact with each other and severe phase segregation be avoided.For the conventional methods to NiO-CeO2 catalysts, including wetimpregnation, co-precipitation and sol-gel process, which are oper-

ated in aqueous phase, the intimate contact of precursors can hardlybe controlled due to the inherent different solubility products (Ksp)and complex influencing factors (pH, concentration, temperature,aging time, drying mode). Besides, leaching of soluble species is

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76 C. Tang et al. / Catalysi

lso inevitable during filtration and may incur some environmenturdens. In contrast, methods based on solid state preparation arexpected owing to the unique advantages displayed by them, likeimple operation, easy control of parameters and friendly to envi-onment [19,20]. Moreover, there were studies showed that theirect use of solid salts as precursors could provide intimate con-act and thus enhance the interactions between active componentnd the support [21,22].

It is noted that apart from interfacial interaction, textual prop-rty of catalysts is another important character of catalyst. Tochieve superior catalytic performance, the fabrication of materialsith large surface area and abundant porous structure is preferred.

n our previous studies, a solvent-free route was developed to pre-are supported metal oxide catalysts on mesoporous silica SBA-15nd ceria [22–25]. It is revealed that the molten salt phase involvedn the absence of solvent ensures successful dispersion of solid

etal precursors, which is recently stressed by Munnik et al. inheir review work as an emerging method for constructing sup-orted catalysts [26]. This intrigues us to speculate whether theolid state method could be applied to prepare mixed metal oxideatalysts, and if it works, how the properties of catalysts woulde affected? With these questions in mind, we choose NiO-CeO2atalyst as an example to investigation. Characterization resultseveal that nickel species can be well dispersed and the solid stateoute is advantageous to obtain catalyst with higher surface areand enriched porous structure. As well, the redox properties andnteractions between nickel and ceria are enhanced, resulting inmproved activity in the reaction of NO reduction by CO, as com-aring with NiO-CeO2 catalyst prepared by wet impregnation.

. Experimental

.1. Catalyst preparation

All chemicals were purchased from Sinopharm Chemicaleagent Co., Ltd., China and used as received. The solid statereparation of NiO-CeO2 catalyst is a fast and simple process as

t only comprises manual grinding of the mixture of nickel anderium precursors and the subsequent thermal treatment. Thentire process is free of solvent. Typically, 4.342 g Ce(NO3)3·9H2Ond 0.6770 g Ni(NO3)2·6H2O were added to an agate mortar andanually ground. Then the homogeneously mixed powder was

ransferred into a crucible lid, which was placed in a muffle furnace.he thermal treatment in air started at room temperature to 550 ◦Cith a rate of 1 ◦C/min and maintained at that temperature for 5 h,

hen cooled naturally. The obtained sample is denoted as NiO-CeO2MSD), where MSD means molten salt decomposition. For compar-son, pure CeO2 and NiO-CeO2 (IMP) were prepared. Pure CeO2 wasrepared according to NiO-CeO2 (MSD) but without nickel precur-or. NiO-CeO2 (IMP) was prepared by impregnating CeO2 with anqueous solution containing requisite amount of Ni(NO3)2·3H2O.fter stirred for 2 h, the sample was dried at 100 ◦C in oil bath, sub-equently dried at 110 ◦C in an oven overnight, and then calcinedn air at 550 ◦C for 5 h.

.2. Catalyst characterization

Scanning electron microscopy (SEM) images were observed by Hitachi S-4800 instrument at an acceleration voltage of 10 kV.

Transmission electron microscopy (TEM) images were taken on

JEM-2100 instrument at an acceleration voltage of 200 kV. Theample was dispersed in A.R. grade ethanol with ultrasonic treat-ent and the resulting suspension was allowed to dry on carbon

lm supported on copper grids.

y 281 (2017) 575–582

Nitrogen adsorption and desorption isotherm was measuredat −196 ◦C using a Micromeritics ASAP 2020 system. The sampleswere degassed for 160 min at 300 ◦C in the degas port of the adsorp-tion analyzer.

Thermogravimetry and differential thermal analysis (TG-DTA)was carried out on a Beijing Hengjiu thermoanalyzer HCT-1 withheating rate of 5 ◦C min−1.

X-ray diffraction (XRD) pattern was recorded on a Philips X’PertPro diffractometer, equipped with a Ni-filtered Cu K� radiation(� = 0.15418 nm). The X-ray tube was operated at 40 kV and 40 mA.The average grain sizes were determined from XRD line broadeningmeasurements using the Scherrer equation, d = K�/�cos�, where �is the X-ray wavelength, � is the diffraction angle, K is the parti-cle shape factor, usually taken as 0.89, and � is full width at halfmaximum in radians.

Raman spectra were collected on a Jobin-Yvon (France-Japan)T64000 type Laser Raman spectroscopy using Ar+ laser beam. TheRaman spectra were recorded with an excitation wavelength at514 nm and the laser power at 300 mW.

Temperature-programmed reduction (TPR) measurement wascarried out in a quartz U-tube reactor, and 50 mg sample wasused for each measurement. Before reduction, the sample was pre-treated in N2 stream at 150 ◦C for 1 h and then cooled to roomtemperature. After that, a H2-Ar mixture (7% H2 by volume) with aflow rate of 70 mL/min was switched on and the temperature wasincreased linearly at a rate of 10 ◦C/min. A thermal conductivity celldetector was used to detect the consumption of H2 on stream.

2.3. Catalytic activity test

The activities of the catalysts were determined under a light-off procedure, involving a feed steam with a fixed composition, NO2.5%, CO 5.0%, and He 92.5% by volume as diluents. The catalyst(50 mg) was fixed in a quartz tube and pretreated in a N2 streamat 150 ◦C for 1 h. The reaction was carried out at different tempera-tures with a space velocity of 9000 mL g−1 h−1. Gas chromatographequipped with thermal conduction detection was used to analyzethe products (Column A with Poropak Q for separating N2O and CO2,and Column B packed with 5A and 13X molecule sieves (40–60 M)for separating N2, NO, and CO).

3. Results and discussion

3.1. Textural feature of catalyst (SEM, TEM and N2 physisorption)

To get elementary information about the product from solidstate preparation, the macroscopic morphology of as-preparedNiO-CeO2 is shown by a digital photograph (Fig. 1a). Normally,the colors of NiO and CeO2 are blackish green and pale yellow,respectively. However, the composite oxide presents a homoge-neous color of brown, which is different from their mixed colors. Itindicates some interactions have taken place between nickel oxideand ceria. Meanwhile, it is observed that after calcination, the loosepowdery precursor (Fig. 1a) is transformed into a compact andbumped crab-shell like product (Fig. 1c). The fringe of “crab-shell”contains many air vents and the interior is actually hollow. Theunusual macroscopic morphology prompts us to explore furtherthe detailed microstructure.

Fig. 2 shows the SEM and TEM images of the prepared NiO-CeO2catalyst. From the SEM result, it can be seen that the catalyst ismade up of particles with irregular morphology and the surface is

not smooth (Fig. 2a). Further observing the structure from enlargedimage (Fig. 2b), it is obvious that the coarse surface is composed oftightly stacked particles with size of tens of nanometers. From theTEM result (Fig. 2c), we can find that the sample is mainly com-

C. Tang et al. / Catalysis Today 281 (2017) 575–582 577

Fr

ptIipar

s

Fig. 2. Typical (a, b) SEM images of NiO-CeO2 (MSD) and TEM imagesof (c) NiO-CeO2

(MSD) and (d) NiO-CeO2 (IMP).

ig. 1. The digital photos of physically mixed nickel and cerium precursors at (a)oom temperature, (b) 110 ◦C and (c) the final air calcined product.

osed of irregular rod-like nanoparticles with rough surface andhe grain size is of 10–30 nm, in good agreement with SEM result.nterestingly, just in these small rod-like nanoparticles, numerousnternal pores are present (indicated by arrows in the image). Theores are ca. 2 nm in diameter. It is supposed that the tiny poresre propitious to formation of defects and beneficial to construct

eactive centers or yield active oxygen species [27,28].

As a complementary to SEM and TEM characterizations, Fig. 3hows the N2 adsorption/desorption isotherm and related pore size

578 C. Tang et al. / Catalysis Today 281 (2017) 575–582

tions of (a, b) NiO-CeO2 (MSD) and (c, d) NiO-CeO2 (IMP).

dciaeAt

3s

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Table 1Thermal properties of the two metal precursors.

Melting temperature (◦C) Decomposition point (◦C)

Ni(NO3)2·6H2O 57 ∼220

Fig. 3. N2 sorption isotherms and pore size distribu

istribution (PSD) curve. The isotherm is type IV according to IUPAClassification and exhibits an obvious H3 hysteresis loop, suggest-ng presence of mesoporous structures. The mesopores should berrived from void of the stacking particles. Additionally, the pres-nce of internal pores is reflected by pore size distribution curve.pparently, the sample exhibits bimodal pore size distributions and

he two kind pores are centered at 2 nm and 9 nm, respectively.

.2. Possible formation process of NiO-CeO2 catalyst from solidtate preparation

From the aforementioned characterizations, it is known thatome unique features are presented by the facile solid state methodn preparation of mixed oxide catalyst. To elucidate the possibleormation mechanism, it is essential to understand the thermalroperties of metal precursors. Table 1 lists the melting points and

ecomposition temperatures of the two precursors. Both nickelitrate hydrate and cerium nitrate hydrate are present as solidhase at room temperature. They will turn into molten salts whenhe temperatures rise to their melting points. Further increas-

Ce(NO3)3·9H2O 96 200

ing temperature leads to transformation of metal salts into metaloxides and the crystal growth proceeds at higher temperatures.

The above deduction is tracked by TG-DTA results (Fig. 4). Ascan be seen from TG curve, no distinct weight loss is detected forpure Ni at 50–60 ◦C (inset in Fig. 4a), whereas a sharp endothermicpeak is existed in the DSC curve (Fig. 4b). This phenomenon can bewell explained by the melting of nickel nitrate. For ceria nitrate,an endothermic peak with weaker intensity appears at almostthe same temperature, which can be accounted by partial loss ofcrystalline water. The obvious endothermic peak observed at tem-

perature near 100 ◦C may be due to overlapped melting and H2Odesorption processes. For the mixed nitrates, no marked change inmelting behavior can be observed.

C. Tang et al. / Catalysis Today 281 (2017) 575–582 579

F

wTwfaOpbtMs

srossctstmamr

ig. 4. TG-DSC results of pure nickel nitrate, cerium nitrate and the mixed nitrates.

For thermal decomposition behavior, it is obvious significanteight loss and endothermic peaks are observed in 250–350 ◦C.

he endothermic peak for cerium nitrate is centered at ca. 254 ◦C,hile two endothermic peaks at ca. 250 ◦C and 320 ◦C are observed

or nickel nitrate. Interestingly, when nickel and cerium nitratesre mixed together, their decomposition behaviors change greatly.nly one endothermic peak appears, suggesting the two decom-ositions influence each other and merge together. This result cane regarded as an important evidence of the corporative interac-ion between nickel and cerium salts in thermal decomposition.

oreover, due to the single decomposition process, severe phaseegregation can be greatly inhibited.

Based on TG-DTA results and the literature reports [24,29], it ispeculated that the intermediate molten salt phase plays the keyole preparing NiO-CeO2 catalyst from solid metal precursors. Onne hand, as assisted by thermal treatment, the mobility of moltenalts greatly reduces the diffusion resistance and ensures free diffu-ion of the solid precursors, which in turn facilitates homogenousontact of the mixed precursors (Fig. 1b). This can explain wellhe transformation of macroscopic morphology from dusty precur-ors to compact crab-shell like product. On the other hand, whenhe temperature reaches decomposition point, decomposition of

etal precursor is initiated and gases like NOx and O2 are gener-ted. Since the molten salts are viscous, most of the evolved gasesay initially be trapped in molten salts as bubbles and can not be

eleased easily. Simultaneously, the crystal nucleates and grows

Fig. 5. XRD patterns of pure ceria and NiO-CeO2 samples.

on the surface of the bubbles. Probably, with the accumulation ofevolved gases, the internal pressure of bubbles is sharply increasedand hence allows the excavation of covered shells. As a result, theinternal pores are created. Noticeably, the balance between inter-nal pressure and viscosity of molten salts results in the formation ofcluster-sized pores. Similar bubble assisted process has been pre-viously reported through solvothermal method for the preparationof CuS hollow spheres and the in-situ generated bubbles are sug-gested to function as template for aggregation of primary particles[30].

Following the above discussion, it is worthy noting that for theconventional solution phase based preparation method, like co-precipitation and sol-gel, the anion of metal salts (e.g., NO3

−, SO42−,

CH3COO−) is always leached away with solvent, hence having littleinfluence on the textual property of the final catalysts. However, forthe solid state preparation, the function of anions is well exploitedand they play a vital role in the generation of porous structures.This can be viewed as one of the distinct differences between solidstate preparation and the aqueous phase based preparations.

The textual property of NiO-CeO2 catalyst from solid statepreparation is well studied. In the following section, we will focuson the interfacial interactions. To reveal the effect of solid statepreparation on the interfacial interactions between nickel andceria, NiO-CeO2 catalyst from conventional wet impregnation isused as a comparison, and such characterization techniques likeXRD, Raman and H2-TPR are employed.

3.3. XRD result

Fig. 5 shows the XRD patterns of NiO-CeO2 catalysts. As a com-parison, the curve of pure CeO2 is listed. Pure CeO2 presents acubic fluorite structure (JCPDS #43-1002) and the lattice parame-ter calculated from (111) diffraction peak is 5.424 Å. For NiO-CeO2catalysts, no obvious shift in Bragg positions could be observed,indicating preservation of cubic structures of ceria. Additionally,weak peaks at 2� = 37.2◦ and 43.3◦ attributed to crystalline NiO areobserved, suggesting formation of bulk NiO in both samples. Thesimilarity of diffraction peaks between NiO-CeO2 (MSD) and NiO-CeO2 (IMP) implies the solid state preparation can disperse nickeloxide well on ceria.

The average grain size, lattice parameter and surface area of thethree samples are summarized in Table 2. It is obvious that theaverage grain sizes are comparable and less than 10 nm. In contrastto the marginal differences in grain size, the differences in surfaceareas are obvious. NiO-CeO2 (MSD) shows apparently larger sur-

face area than NiO-CeO2 (IMP) and pure CeO2, while the latter twoshow almost the same value. This dissimilarity can be reasonablyinterpreted from the proposed molten salt decomposition process.With the participation of nickel nitrate, more gases will be liber-

580 C. Tang et al. / Catalysis Today 281 (2017) 575–582

Table 2Textural properties of pure CeO2 and NiO-CeO2 samples.

average grainsizea (nm)

latticeparameter(nm)

surface area(m2/g)

AOV/AF2gb

Pure CeO2 8.2 0.5424 66 0.02NiO-CeO2 (IMP) 8.1 0.5415 69 0.18NiO-CeO2 (MSD) 7.6 0.5400 82 0.33

a Calculated by Scherrer equation.b AOV and AF2g represent the peak area of bands at 580–610 nm−1 and 460 cm−1.

aRfdiadtd

titi[CN

3

dit6([bttlob

Fig. 6. Raman spectra of pure ceria and NiO-CeO2 samples.

ted when the temperature arrives to the decomposition point.esultantly, enriched pores are generated and hence increased sur-

ace area is obtained. Previously, El-Shobaky discovered that byoping Li2O into the CuO-ZnO/Al2O3 system, the surface area was

ncreased. They attributed the phenomenon as a result of the cre-tion of pores produced from liberation of gaseous nitrogen oxidesuring thermal decomposition of LiNO3 [31]. Further evidences ofhe more abundant tiny pores can be revealed by TEM (Fig. 2c and) and N2 sorption (Fig. 3) results.

On the other hand, compared to pure ceria, the lattice parame-ers of NiO-CeO2 catalysts are slightly decreased. This observations in line with literature reports and suggests the possible forma-ion of solid solution by considering the fact that the radius of Ni2+

on (rNi2+ = 0.72 Å) is smaller than that of Ce4+ ion (rCe4+ = 0.97 Å)15,32,33]. Notably, the contraction of lattice parameter of NiO-eO2 (MSD) is larger than that of NiO-CeO2 (IMP), implying morei2+ ions are probably incorporated into the lattice of ceria.

.4. Raman result

The Raman spectra of pure ceria and NiO-CeO2 catalysts areisplayed in Fig. 6. The dominant band of pure ceria at 463 cm−1

s ascribed to the characteristic F2g vibration mode of the fluoriteype lattice and the weak band extending between about 500 and50 cm−1 is attributed to the nondegenerate Longitudinal OpticalLO) mode of ceria (linked to oxygen vacancies in the ceria lattice)34]. Compared with pure ceria, NiO-CeO2 catalysts exhibit a muchroad F2 g band, which is generally related to either smaller crys-alline size or more oxygen vacancies [35]. Since the grain sizes of

he three samples are almost the same, the widening of ceria mainine evidences formation of more oxygen vacancies with additionf nickel. It is proposed that due to the different charges and radiusetween Ni2+ and Ce4+ cations, the incorporation of Ni2+ ion in ceria

Fig. 7. H2-TPR profiles of pure ceria and NiO-CeO2 samples.

lattice will cause charge unbalance and lattice distortion withinthe structure of ceria. Thus, oxygen vacancy will be generated [14].From this point of view, it can be reasonably deduced that the con-centration of oxygen vacancy is largely related to nickel ions dopedin ceria lattice. Further evidence of Ni incorporation in ceria latticeis given by the red shift of the main F2g mode band [18,36], andthe more obvious red shift together with wider band for NiO-CeO2(MSD) sample indicate more Ni2+ ions are doped into ceria, whichis in good agreement with the XRD result. Simultaneously, the con-centration of oxygen vacancies reflected by the ratio of AOV/AF2gis shown in Table 2. The AOV/AF2g values of NiO-CeO2 are muchhigher than pure CeO2, indicating the formation of more oxygenvacancies with the incorporation of nickel. Moreover, the AOV/AF2gof NiO-CeO2 (MSD) is higher than that of NiO-CeO2 (IMP), which isrelated to a higher concentration of oxygen vacancies in NiO-CeO2(MSD)·3·5H2-TPR result.

It is documented that the interactions between nickel and ceriain NiO-CeO2 can be appropriately reflected by its reduction behav-iors, and thus H2-TPR characterization is carried out. The resultsare shown in Fig. 7. For pure CeO2, a broad H2 consumption peakin the range between 300 and 550 ◦C assigned to surface layerreduction of ceria [37] is observed. With introduction of NiO, thereductions become complex and H2 consumptions start at temper-atures around 150 ◦C, which is much lower than those of pure ceriaand bulk NiO [38], suggesting synergistic interactions between NiOand ceria. For nickel cerium mixed oxides, generally five hydrogenconsumption peaks will be present, corresponding to reductions ofsurface adsorbed oxygen species (�1, �2) on doped ceria, bulk NiO(�), highly dispersed NiO with strong interactions with ceria (�) andbulk ceria (�), respectively [39,40]. In the present result, the firstfour peaks can be clearly observed and the reduction of bulk ceria(�) is not displayed, because of its relatively high reduction temper-ature (>650 ◦C) [37]. Compared to NiO-CeO2 (IMP), the reductions ofNiO-CeO2 (MSD) shift to lower temperatures, revealing preparationmethods have great influence the reducibility of samples and morefacilely reduced species can be obtained by NiO-CeO2 (MSD). More-over, to get quantitative information of the reduced species, thepeaks are deconvoluted with Guassian-Lorentzian curves. Table 3gives the corresponding H2 uptake result. It is evident that thesample prepared from solid state method displays more surfaceadsorbed oxygen (�1 + �2) and highly dispersed NiO (�), while

the conventional impregnation method produces more bulk NiO.The result clearly demonstrates the solid state preparation route isadvantageous to enhance the mutual interactions between nickeland ceria.

C. Tang et al. / Catalysis Toda

Table 3H2 uptakes for various consumption peaks of NiO-CeO2 catalysts.

Sample H2 uptake (mmol)

�1 �2 � �

NiO-CeO2 (IMP) 0.0108 0.0138 0.0708 0.0094NiO-CeO2 (MSD) 0.0091 0.0295 0.0550 0.0302

3

rCc

2

N

tilArN

Fig. 8. The (a) NO conversion and (b) N2 selectivity of the prepared samples.

.5. Activity in NO reduction by CO reaction

To examine the catalytic performance of the prepared samples,eaction of NO reduction by CO is tested. The reduction of NO byO is a typical model reaction in exhaust gas depollution, and itomprises mainly the following two reactions:

NO + CO→N2O + CO2 (1)

2O + CO→N2 + CO2 (2)

The catalytic results are shown in Fig. 8. It can be concludedhat: (1) for all catalysts, the activities are increased with increas-ng reaction temperatures; (2) NO is mainly converted to N2O at

ow temperatures (<200 ◦C), and N2 at higher temperatures; (3)mong the employed catalysts, NiO-CeO2 from solid state prepa-ation displays the best activity, i.e., both the NO conversion and2 selectivity are higher than that of the conventional catalyst. It

[[

y 281 (2017) 575–582 581

converts more than 95% NO to N2 at temperature as low as 300 ◦C,suggesting great potential of this catalyst in NO reduction.

As proposed elsewhere [12], the NO + CO reaction over NiO-CeO2catalyst proceeds in three consecutive steps: (i) CO reduces surfaceoxygen to create vacant sites; (ii) on the vacant sites, NO dissoci-ates to produce N2O/N2; and (iii) the oxygen originated from NOdissociation is removed by CO. According to this mechanism, thedifference in catalytic behaviors between the two NiO-CeO2 cata-lysts can be understood. Solid state preparation endows NiO-CeO2catalyst with enhanced interfacial interactions and more facilelyreduced surface oxygen species. As a result, more NO dissociationsites will be created under CO reduction and improved performanceis obtained.

4. Conclusions

In the present study, a novel solid state route for preparation ofbinary mixed oxide catalyst was investigated by using NiO-CeO2as a represent. On the basis of characterization techniques, it isconcluded that distinct advantages are present by solid state prepa-ration in comparison with the most widely used wet impregnationmethod. Firstly, solid state preparation owns the merit of extremelyfacile operation. Although the wet impregnation method is famousfor its convenience, it still contains three steps (impregnation, dry-ing and calcination). In contrast, the solid state preparation onlyincludes two steps (physical mixing and calcination), which is timeconserving and energy-saving. Secondly, the solid state prepara-tion can make good use of the accompanied anion of metal salt. Asdiscussed in the text, the evolved gases from decomposed anioncan role as porogen, thus contributing enhanced textual properties(enriched porous structure and enlarged surface area). Thirdly, ourpreliminary result for NiO-CeO2 catalyst shows that by using thesolid state preparation, nickel species can be well dispersed andenhanced interfacial interactions is obtained between nickel andceria, which promote bulk doping and surface dispersion of nickelspecies and the related redox properties. As a result, the preparedcatalyst displays enhanced catalytic performance in NO reductionby CO. Lastly, it should be noted that from the proposed forma-tion mechanism, the solid state preparation may not only limitedto NiO-CeO2. It is expected that this novel and facile method mayfind great applications in fabrication of efficient heterogeneous cat-alysts.

Acknowledgements

The financial supports of National High-tech Research andDevelopment (863) Program of China (2015AA03A401), NationalNatural Science Foundation of China (21273110, 21303082 and21427803), Jiangsu Province Science and Technology Support Pro-gram (Industrial, BE2014130) and Fundamental Research Funds forthe Central Universities are gratefully acknowledged.

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