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Ceramics International 40 (2014) 6157–6163www.elsevier.com/locate/ceramint

Study of correlation between activity and structural propertiesof Cu-(Cr, Mn and Co)2 nano mixed oxides in VOC combustion

S.A. Hosseinia,n, A. Niaeib, D. Salarib, M.C. Alvarez-Galvanc, J.L.G. Fierroc

aDepartment of Applied Chemistry, Faculty of Chemistry, University of Urmia, 57159 Urmia, IranbDepartment of Applied Chemistry, University of Tabriz, Tabriz, Iran

cInstituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, E-28049 Madrid, Spain

Received 20 August 2013; received in revised form 29 October 2013; accepted 13 November 2013Available online 4 December 2013

Abstract

The correlation between structure and catalytic activities of Cu-(Cr, Mn and Co)2 mixed oxides synthesized by a sol–gel combustion methodwas investigated for the oxidation of 2-propanol. The catalysts were characterized by FTIR, XRD, XPS, TPR and UV–vis DRS. Although thesame preparation conditions were used for all the systems, obtained samples presented different pureness degrees. The structural characterizationby XRD revealed that copper-chromite was pure CuCr2O4, while copper-manganese and copper-cobalt mixed oxides resulted in a mixture of thespinel (CuMn2O4 and Cu0.15Co2.85O4, respectively) and copper oxide phases. UV–vis DRS results also indicated a larger presence of smallcrystallites of CuO in the copper-cobalt oxide. Copper-cobalt oxide exhibited the highest activity in 2-propanol combustion which is explained, asexpected for a Mars van Krevelen mechanism, by its higher reducibility at the reaction conditions and by a possible synergistic effect betweenthis mixed oxide and CuO particles.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Ceramics; Nanostructures; Sol–gel chemistry; XPS; TPR

1. Introduction

Among technologies used for remediation of volatileorganic compounds (VOCs) the best technology is catalyticcombustion because of its low energy consumption and thesmall amount of noxious by-products [1]. Driven by the needto decrease the manufacturing cost and increasing the resis-tance to poisoning of commercial catalysts for VOC elimina-tion, efforts have been made to develop transition metal oxidecatalysts, which can exhibit activity similar or higher thannoble metal based catalysts [2]. Improved properties of mixedoxides compared to the individual oxides are well known,especially in environmental catalysis. Thus, mixed oxides areusually used to prevent coke formation, thus obtaining betteron- stream stability during many industrial processes. Spinelswith the general formula AB2O4 are of importance in variousfields, such as sensor and heterogeneous catalysis, due to their

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.068

g author. Tel.: þ98 441 2972166.sses: a.hosseini@urmia.ac.ir (S.A. Hosseini),ahoo.com (D. Salari).

high thermal resistance and specific catalytic and/or electronicproperties. Some applications of spinel based catalysts, forwhich they have been found to be very active, are CO andhydrocarbons oxidation [3], and catalytic removal of NOx anddiesel soot [4]. These formulations are promising for hightemperature combustion reaction because of their high thermalresistance and specific catalytic properties.The preparation method directly affects the physical chemi-

cal properties and activity of spinels [5]. Sol–gel combustion, anovel method for the preparation of nano materials, exhibitsthe advantages of obtaining pure phase mixed oxides with agood control of stoichiometry and nano particle size [6] being,in general, preferred to the co-precipitation method, since thesol–gel preparations gives more active catalysts [7].Zavyalova et al. synthesized CuCo2O4, CoCr2O4 and Co3O4

spinels by the sol–gel combustion method and found very highactivity in the combustion of hexane [8]. They concluded thatCuCo2O4 was more active than CoCr2O4 and Co3O4.Rivas et al. studied the combustion of 1, 2-dichloroethane

over nanocrystalline Co3O4 and concluded that the activity ofsome nanocatalysts was superior to that of supported noble

ghts reserved.

S.A. Hosseini et al. / Ceramics International 40 (2014) 6157–61636158

metal catalysts and other bulk oxide catalysts [9]. They foundthat a small crystallite size is related to improved redoxproperties at low temperatures, this being the key factor indetermining the activity.

Other reasons for enhancement in the performance of spinelswere attributed to the stabilization of active phases and tosynergistic interactions between two different oxides in thespinel structure [10].

Copper based catalysts are the most active among the metalbased catalysts for the combustion of aromatic hydrocarbons[11,12]. Copper-spinels have been reported in combustion oforganic compounds [13,14]. On other hand, it has beendemonstrated that Co, Cu, Mn, Cr and Ni exhibit excellentbehavior in oxidation reactions [15]. So in this work we studythe application of some copper spinels with active metals inoxidation in O-VOC oxidation.

The main objective of this work was to study the propertiesand catalytic behavior of a series of Cu-(Cr, Mn, Co)2 mixedoxides under the same synthesis conditions in order to gaininsight about the influence of the cation nature —with oxidationstate (III), cation B in AB2O4 spinels—in their reactivity for thecombustion of 2-propanol, as a model molecule of oxygenatedvolatile organic compounds [16]. The samples were prepared bya sol–gel combustion method under the same preparationconditions and characterized by XRD, UV–vis DRS, XPS,TPR and FTIR. An attempt to correlate the structural andsurface composition with the catalytic activity will be done.

2. Material and methods

2.1. Preparation of catalysts

The synthesis of the catalysts was carried out by the sol–gelcombustion method which has been described in detail inour previous studies [7,17]. Briefly, nitrates of the differentcations were used as precursors Cu(NO3)2 � 3H2O (99.5%),Cr(NO3)3 � 9H2O (98%), Co(NO3)2 � 6H2O (99%) and Mn(NO3)2 � 4H2O (98.5%), with the stoichiometric ratio Cu: B(Cr, Mn, Co)=1:2 and citric acid monohydrate (C6H8O7 �H2O(99.5%)) as fuel ([citric acid]/[total nitrates]=0.4. The solutionswere left to evaporate at 80 1C with continuous mechanicalstirring until sticky gels were obtained. The spontaneouscombustion gave rise to the powdered product. The combustedpowders were then calcined in static air at 700 1C for 6 h.

2.2. Characterization of catalysts

The crystalline structure of the different phases was analyzedby X-ray diffraction (XRD) on a Siemens D500 diffractometer,working with the Kα line of copper (k=0.154 nm). Fouriertransform infrared spectra were recorded using a Bruker spectro-meter (model TENSOR 27).

Surface composition was determined by X-ray photoelectronspectroscopy (XPS) on a Microlab 310-F scanning Augermicroprobe using Mg Kα X-ray source (hc=1253.6 eV) oper-ated at 120 W.

The nature and coordination of copper cations in the sampleswere determined by recording UV–vis diffuse reflectance spectraof samples in the wavelength range of 300–1000 nm, using aScinco 210 spectrophotometer (model AA-1301) equipped withan integral sphere.Temperature Programmed Reduction (TPR) profiles of the

catalysts were obtained by a Micrometritics Autochem 2910equipment. The samples were pre-treated with 30 cm3 min�1

of helium at 700 1C for 30 min. Hydrogen consumption wasmeasured with a flow of a mixture of 10 vol% H2 in argon witha flow rate of 50 cm3 min�1 and a linear heating rate of 10 1C/min, from 50 to 700 1C.

2.3. Catalytic activity tests

Catalytic tests were carried out in a pyrex reactor (L=40 cm, i.d.=0.8 cm) placed inside an electrical furnace under atmosphericpressure. Catalyst (0.2 g) was placed over a plug of glass wool.2-propanol was supplied by purging an air flow through a saturatorat 0 1C. The gas hourly space velocity was 2400 h�1. The inletconcentration of 2-propanol to gaseous phase was about 0.085 molmol�1. The analyses of inlet feed and products were carried out ina Shimadzu 2010 gas chromatograph apparatus with a flameionization detector (FID).

3. Results and discussion

In the synthesis of samples by the sol–gel combustionmethod, a ratio of fuel/oxidant was kept in 0.4. It is reported inliterature that low fuel/oxidant ratios, similar to that used inthis synthesis, avoid the segregation of dopant and providesamples with small particle sizes [7].The XRD patterns of the samples are shown in Fig. 1. Both

copper-chromite and copper-manganese oxides showed thetetragonal phase. Cr-containing mixed oxide presents a highlypure CuCr2O4 spinel with a space group of I42d (cellparameters: a¼b¼6.034, c¼7.782 Å, JCPDS 34-0424),while the Cu-Co and Cu-Mn mixed oxide presented a mixtureof the corresponding spinel and some impurity of CuO. Thus,copper-cobaltite showed a mixture of cubic Cu0.15Co2.84O4

with a¼8.090 Å (a CuxCo3�xO4 spinel type) and someimpurity of CuO (JCPDS 45-937) and copper manganite wasa mixture of CuMn2O4, with some impurities of CuO andCu2O (JCPDS 5-667). The mean crystallite sizes of oxideswere estimated by the Scherrer equation, using the mostintense and non-overlapped diffraction peak (�2θ¼36.71).The mean crystalline domain size obtained for chromium,manganese and cobalt samples (Cu-Cr, Cu-Mn, and Cu-Comixed oxides) was 40, 50, and 55 nm, respectively.Another proof to confirm the existence of the spinel

structure in the samples is by FTIR. The spinel structure ischaracterized by IR spectra in the region 400–700 cm�1 [7].IR spectra of samples (Fig. 2) present two absorption bands inabove region corresponding to the stretching vibration of metaloxygen bond [18]. For all spectra, the band around 500 cm�1

corresponds to the stretching band of Cu–O in the tetrahedralsystem (Td) and the band between 600–700 cm�1 corresponds

Fig. 1. XRD patterns of CuMn2O4 (a), CuCo2O4 (b), CuCr2O4 and (c); Spinelphase (◊), CuO phase (★), and Cu2O phase (♦).

Fig. 2. FTIR spectra of CuM2O4samples; CuMn2O4 (a), CuCo2O4 (b), andCuCr2O4 (c).

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to the stretching bond of metal (Co, Cr and Mn)-O in theoctahedral system.

X-ray photoelectron spectra were recorded to know thesurface chemical composition of prepared samples. Thecharacteristic core-level spectra of Cu 2p, Mn 2p, Co 2p andCr 2p in the samples are displayed in Fig. 3(A)–(D). Thebinding energies of the corresponding Cu 2p, Mn 2p3/2, Co2p3/2 and Cr 2p3/2 core levels and surface atomic ratios of Cu/M for the spinels are reported in Table 1.

Fig. 3(A) shows the Cu 2p spectra of samples. It is found thatCu 2p3/2 peak contains two different signals. The signal at lowerbinding energy is due to monovalent copper and the signal withbinding energies higher than 934 eV together with its satellitebetween 940–945 eV is indicative of divalent copper [19]. Itshould be pointed out that the presence of Cu (I) was clearlydistinguishable in copper–manganese and in copper–chromiumsamples with a characteristic peak around 932 eV [20]. Thesmall size of these surface particles is below the detection limitof XRD technique (around 3 nm) and therefore, this phase is notobserved in diffractograms shown above.

For these spinels, the presence of Cu(I) is due to thesimultaneous presence of Mn (IV), which would be in accordance

with Mn 2p signal [20]. Thus, in Fig. 3(B), binding energieshigher than 641.7 eV for the Mn 2p3/2 peak can be assigned to thecoexistence of Mn (III) and Mn (IV) cations in the surface ofcopper–manganese sample [14]. It is also observed the existenceof a small satellite around 648 eV which was also found forcopper-manganese oxide [21].It has been already reported that copper-manganese oxide

has the formula Cu(I)xCu(II)1�xMn(III)2�xMn(IV)xO4 wherethe following equilibrium is established [22]:

CuðIIÞþMnðIIIÞ’-CuðIÞþMnðIVÞTherefore, the presence of Mn (III) and (IV) in the surfacewould be in accordance with the above mentioned coexistenceof Cu (I) and Cu (II) in this spinel.Concerning the Co 2p spectrum (Fig. 3(C)), the shape and

binding energies are close to that reported for Co3O4 spinel.Distinguishable satellite peaks at about 6 or 10 eV above theCo 2p3/2 main line, placed around 780 eV, are observed.According to assignments reported in literature, they can beassociated, respectively, to Co2þ and Co3þ species, in CoOand Co3O4 segregated oxides or existing in surface Cu–Cospinel phase [19,23].Fig. 3(D) shows the core-level of Cr 2p spectrum of copper–

chromium sample. The binding energy of the 2p3/2 levelindicates that Cr is mainly in the form of Cr3þ (around576.7 eV), forming part of the spinel. However, the width ofthe peaks suggest the existence of some amount of Cr6þ

(579.1 eV) as CrO3 in the surface [23].

Fig. 3. XPS results: (A) Cu 2p of CuM2O4(M: Mn, Co, Cr), (B) Mn 2p, (C) Co 2p, and (D) Cr 2p core levels XPS.

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Surface Cu/M atomic ratios were measured to be 0.46 and0.43 for copper–chromium and copper–cobalt oxides, respec-tively; in good agreement with the nominal ratio value 0.50,although showing some enrichment of M element in thesurface of both samples. The ratio found for the Cu-Mn mixedoxide (0.28), reveals that this sample shows a much highersurface enrichment in this element.

UV–vis spectra were recorded for the samples in order tounderstand the nature and coordination of copper cations in thesamples. In the range 210–360 nm, absorption bands are

observed for all samples. The UV–vis spectra of samples areshown in Fig. 4. The intensity of the band at around 320 nm inthe samples is indicative of the presence of well dispersedcrystalline CuO species on the surface [6]. As observed inFig. 4, the higher intensity of this band for the sample with Comay indicate the greater development of small crystallites ofCuO, in accordance with XRD results. For the sample copper-chromium oxide, Cr3þ in octahedral coordination existing inthe spinel structure also produces a band around 310–320 nm[24]. Therefore, the intensity of this band should be mostly

Table 1Binding energies (eV) of core electrons and surface atomic ratios of CuM2O4 samples.

Samples Binding energy (ev) Atomic ratio

Cu 2p Cr 2p Mn 2p Co 2p Cu/M

Copper-chromium oxide 932.5 (38) 576.7 (72) 0.461935.3 (62) 579.1 (28)

Copper-manganese oxide 931.5 (44) 641.7 (54) 0.276934.3 (56) 643.2 (46)

Copper-cobalt oxide 934.5 780.2 0.43

Numbers in parentheses are peak percentages.

Fig. 4. UV–vis DRS of spinels.

Fig. 5. TPR profiles of Cu-X2 oxides.

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derived by the presence of this cation in copper chromitespinel. For the sample copper-manganese oxide, the absorptionband in the visible region around 450–500 nm is attributed tosingle spin d–d transition of Mn3þ in octahedral environment.The lower wave length in comparison to those reported tohausmannite Mn3O4 structure is explained by the incorporationof Cu2þ /Cuþ in the place of Mn2þ [25].

Temperature-programmed reduction was used to study theredox properties of the different catalysts. The H2-consumptionprofiles of Cu-(Cr, Mn, Co)2 mixed oxides are shown in Fig. 5.From these profiles it is evident that the nature of element Bhas a great influence on the reduction capability. Differentpeaks are observed in the reduction profile of CuCr2O4

catalyst. The first contribution centered around 160 1C, couldbe attributed to the reduction of small and highly dispersedCuO particles [26]. The small size of these particles do notallow their determination by XRD. The second peak around270 1C is originated from the reduction of surface CrO3

particles resulted from the oxidation of a fraction of exposedCr3þ ions during the calcination step. The presence of theseparticles is confirmed by XPS results [27]. The main reductionpeaks, with maximum around 400 1C and 550 1C, are respec-tively attributed to the reduction of bulk Cu2þ ions ofCuCr2O4 to CuCrO2, and to the mixed oxide toward CuOand Cr2O3 [26,28].

In the case of the copper cobalt oxide reduction profile, twopeaks around 195 and 240 1C are observed. The lowertemperature reduction peak is attributed to reduction of smallparticles of copper oxide to CuO whereas the second peak canbe attributed to the reduction of Co3þ in CuCo2O4 spinel. As

reported in literature, CuCo2O4 could be partially decomposedwhen the precursor is calcined above 400 1C, forming Co3O4

phase and CuO. Although Co3O4 phase is not observed byXRD, since the main diffraction lines are quite similar to thoseof CuCo2O4, its reduction has to be also considered in thisprofile [29,30].In the TPR profile of copper-manganese oxide, the main and

wide reduction peak is attributed to the reduction of CuOparticles, at lower temperatures, and to the reduction of themixed oxide CuMn2O4 at higher values. It has been reportedthat the presence of CuMn2O4 phase facilitates the reductionof CuO. The shoulder observed at higher temperatures isexplained by the reduction of some segregated phases ofMn2O3, forming small particles not observed by XRD [31,32].

Fig. 6. Light-off curves of 2-propanol oxidation over CuM2O4 (M: Co,Mn, Cr).

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Catalytic oxidation of 2-propanol as a model molecule ofoxygenated VOCs, which is used as solvent in industry wasused as a probe reaction to evaluate the catalytic performanceof samples [7,16].

Fig. 6 shows the light-off curves for 2-propanol oxidationover catalysts. Complete 2-propanol conversions wereachieved at 250 1C over copper-cobalt and copper-manganese oxides, while the maximum conversion of2-propanol at this temperature was lower than 60% overCuCr2O4 reaching only 85% at 400 1C. Copper-cobalt oxideshowed a higher activity than Cu-Mn mixed oxide below200 1C. Temperature of 50% conversion of 2-propanol (T50%),used as an index to evaluate the activity of catalysts, was foundto be 150, 197 and 220 1C over Cu-Co, Cu-Mn and Cu-Croxides, respectively. Therefore, the order of activities ofcatalysts was as follows: copper-cobalt oxide4copper-man-ganese oxide4copper-chromite oxide.

TPR results reveal that reducibility of Cu-X2 oxides isrelated with the catalytic performance, since the system that isable to be reduced more easily (Cu-Co mixed oxide) is thatfound to be more active for this oxidation reaction. Thisobservation is supported with what it is established in literatureabout the oxidation of organic compounds over metal oxides.This reaction proceeds according to the Mars Van Krevelen(MVK) mechanism [7], where the organic molecule isoxidized by the oxygen on the oxide, the latter being re-oxidized by gas phase oxygen. The better catalytic perfor-mance of Cu–Co and Cu–Mn catalysts is also ascribed to asynergistic effect observed between Cu–Co spinel and agreater presence of CuO phase [8,33], due to enhancedelectron transfer in these oxides [34,35].

The presence of two different oxidation states for cobalt andmanganese in the samples and correlation of activity withreducibility of cations promote the MVK mechanism foroxidation of 2-propanol over these oxides.

4. Conclusions

From this study, it is concluded that the use of the samepreparation conditions for the synthesis of Cu-M2 mixed

oxides (M: Co, Cr and Mn) does not lead to the formationof spinels with the same purity. The structural analysis byXRD shows that copper chromite was constituted by pureCuCr2O4 while, for the others, a mixture of spinel phase andCuO was obtained.Obtained results show a relationship between the higher

reducibility capability of studied catalysts, based on Cu-X2

oxides, and the higher activity for 2-propanol combustion. Thesynergistic effect between Cu–Co spinel and CuO particles isalso pointed out, promoting this reaction.

Acknowledgments

This project was financially supported by the IranianNanotechnology Initiative Council. We are grateful to thecouncil for encouragement support.

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