DOI: 10.1002/cctc.201200620

Control of the Interphases Formation Degree in Co3O4/ZnOCatalystsFernando Rubio-Marcos,*[a] Vanesa Calvino-Casilda,*[b] Miguel A. BaÇares,[b] andJos� F. Fernandez[a]

Introduction

Glycerol is an important byproduct of biodiesel manufacturingthat is produced by transesterification during biodiesel produc-tion. A number of selective processes for converting glycerolinto commercially valued products are researched to find newapplications of this low-cost feedstock.[1–5] In particular, glycerolcarbonate is one of the valuable glycerol derivatives that at-tract more scientific and industrial interest because of its possi-ble applications. The search for successful routes to efficientlyproduce glycerol carbonate from renewable raw materials haslately been a key subject in different manufacturing areas. CO2

has been used as a carbonylating agent under supercriticalconditions,[6] whereas dimethyl carbonate,[7, 8] dialkyl carbo-nates,[9] or urea[10] have been used for milder reaction condi-tions. In particular, urea is an attractive carbonylating agent;Mouloungui et al. patented the synthesis of glycerol carbonate

by carbonylation of glycerol with urea over heterogeneouszinc catalysts such as zinc sulfate, zinc organosulfate, and zincion exchange resins.[10] However, this is a homogeneous cata-lytic reaction because ZnSO4 salt is soluble in glycerol so thecatalyst is partially recovered after reaction. There is a strongstrategic and environmental benefit in developing a heteroge-neous catalyst for this process. Heterogeneous catalysts areparticularly efficient for carbonylation of glycerol withurea,[11–13] affording good yields under more moderate reactionconditions.

The preparation stage is a key component in the environ-mental value of a catalyst. In industrial practice, supportedmetal and metal oxide catalysts involve the most importantgroup of heterogeneous catalysts. Thus, the synthesis of sup-ported catalysts is of greatest scientific and manufacturing im-portance. Certainly, the most common synthesis routes com-prise impregnation of a preexisting support with a solution ofa metal salt. The different impregnation techniques divergefrom one another in terms of solvent (polar or nonpolar),metal salt choice and postimpregnation treatments.[14–16] Theactive phase dispersed on the support is obtained after im-pregnation if the catalyst is thermally treated (i.e. , the metal inthe reduced state). The fundamental phenomena underlyingimpregnation and drying are extremely complex though thepractical execution is apparently simple. Typically, metal pre-cursor and support interactions are limited, thereby permittingredistribution of the active phase over the support bodyduring drying. Recently, a poor dispersion of the active phase

ZnO supported Co3O4 nanoparticles are highly active in thetransformation of renewable materials through carbonylationof glycerol with urea. The activity of the nanoparticles is modu-lated by their interaction with the ZnO support, which remark-ably depends on the impregnation method. One catalyst serieswas impregnated by conventional impregnation of a ZnO sup-port with an aqueous solution of Co(NO3)2·6 (H2O), while thesecond set was obtained using a novel room-temperature low-energy dry nanodispersion method. This work focuses on thecharacterization and catalytic activity of both series of catalystsand describes the nature of the active sites required for thecarbonylation of glycerol with urea. Raman spectroscopy andHRTEM were used to proof the nature of the interphase inter-action between ZnO and Co3O4 particles in both catalyst series.

Thus, it was verified such interphases are present in the cata-lysts prepared through the dry mixing method. Co3O4/ZnOsystem prepared with the dry mixing method at room temper-ature exhibited a catalytic behavior in the production of glyc-erol carbonate reaching conversion values up to 69 % in 4 h af-fording nearly total selectivity. There was a clear correlation be-tween the amount of the phase and the catalytic activity.Therefore, the catalytic activity can be tuned by controlling in-terphase formation in Co3O4/ZnO catalysts. Upon thermal treat-ment, both series rearranged into inactive phases such asZnxCo1�xCo2O4, decreasing the number of free Co3O4 relatedsites, which decreased the catalytic activity. Therefore, thepreparation stage of the Co3O4/ZnO catalysts played an impor-tant role in the formation of this phase.

[a] Dr. F. Rubio-Marcos, Prof. Dr. J. F. FernandezElectroceramic DepartmentInstituto de Cer�mica y Vidrio, CSICKelsen 5, 28049 Madrid (Spain)Fax: (+ 34) 91-735-58-43E-mail : frmarcos@icv.csic.es

[b] Dr. V. Calvino-Casilda, Prof. Dr. M. A. BaÇaresCatalytic Spectroscopic LaboratoryInstituto de Cat�lisis y Petroleoqu�mica, CSICMarie Curie 2, 28049 Madrid (Spain)Fax: (+ 34) 91-585-4760E-mail : vcalvino@icp.csic.es

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cctc.201200620.

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has been found to occur frequently as a result of solvent evap-oration on the outer surface of the support particles.[17]

We have previously reported a solvent-free and residue-freepreparation method to prepare hierarchical nanoscaled cata-lysts dispersed on microscaled particles (support).[18, 19] Thus,mixing oxides of dissimilar materials by this nanodispersionmethod makes it possible to obtain hierarchical nanoparticles–microparticles systems with unusual properties. After the par-tial reaction of the two oxides, the interfaces created revealnew and interesting properties attributed to proximity and dif-fusion phenomena,[20, 2] which is relevant at the nanoscalerange. Moreover, the support and supported nanoparticles in-teractions can be used to tune or create magnetic, piezoelec-tric, and catalytic properties, depending on the temperaturetreatment for the nanodispersed material.[19, 22, 23] Because ofthe small particle sizes, it is possible to characterize changes instructure spectroscopically.[24, 25]

In a pioneering study concerning the Co3O4/ZnO catalysts,we have demonstrated that Co3O4/ZnO systems prepared bythe dry mixing method at room temperature exhibit excellentcatalytic behavior in the production of glycerol carbonate.[23].Arelative increase in conversion and an increase in the reactionselectivity toward glycerol carbonate were observed in Co3O4/ZnO catalysts, the performance of which depended on thedegree of interaction at the interface that is directly related totemperature treatment.[23] If the catalysts were prepared by thenanodispersion method, Co3O4 active sites were dispersed inthe Co3O4/ZnO system. Upon thermal treatment, the numberof free Co3O4 related sites and the corresponding catalytic ac-tivity decreased because of a rearrangement of this catalystinto inactive phases, such as ZnxCo1�xCo2O4.[23] However, thereare still some structural and nano–microstructural aspects thatremain controversial regarding the role of interfaces formationfor the synthesis of glycerol carbonate in the Co3O4/ZnOsystem. Herein, the effects of the preparation procedure onCo3O4/ZnO catalysts obtained by impregnation and dry nano-dispersion methods are reported. In addition, we assess theeffect of the thermal treatment on the phase structure andmorphology of Co3O4/ZnO mixtures. It seems that Co3O4/ZnOinterface properties directly affect the catalytic activity of thesystem and depend on the preparation method and thermaltreatment.

Results and Discussion

Thermal behavior and morphology of Co3O4/ZnO systems

Thermal analysis methods (thermogravimetry–differential ther-mal analysis, TG–DTA) were used to establish the conditions inwhich the coordination compound, Co(NO3)2·6 (H2O), decom-poses to Co3O4. The obtained TG and DTA curves, shown inFigure 1 a, suggest four steps that occur during the progressiveheating of the coordination compound. In the temperaturerange 25–160 8C, Co(NO3)2·6 (H2O) exhibits two endothermicdehydration DTA peaks (black trace, broad feature) for the twolosses of water (processes I and II). The TG weight losses (graytrace) correspond to the loss of 4 (process I) and 2 (process II)

water molecules, respectively. The strong exothermic effects atsteps III and IV (maxima at 220 and 315 8C) were attributed toligands combustion and oxidation of CoII to CoIII by oxygenleading to the spinel formation, respectively.[26, 27] This indicatesthat thermal treatment should be performed at temperaturesover 400 8C. Based on these results the heat treatment was se-lected at 500 8C for 36 h. In this way, we guarantee the com-plete decomposition of the Co(NO3)2·6 (H2O) precursor used inimpregnation method.

The TG–DTA profiles of Co3O4/ZnO catalysts obtained by im-pregnation, ZCo10-Imp, showed similar processes to that ofthe Co(NO3)2·6 (H2O) precursor in the 25 and 400 8C tempera-

Figure 1. TG–DTA curves of a) Co(NO3)2·6 (H2O) compound and as-synthe-sized b) ZCo10-Imp and c) ZCo10-Nps.

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ture range (Figure 1 b). Furthermore, the ZCo10-Imp sample ex-hibited a weight-loss peaks associated with endothermic peaks(black trace, process V) above 910 8C. This mass loss near910 8C was attributed to the formation of CoO through loss ofoxygen according to the reaction shown in Equation (1):[28]

2 Co3O4 ! 6 CoOþ O2 " ð1Þ

The thermal analysis of Co3O4/ZnO catalysts obtained by thedry nanodispersion method, ZnCo10-Nps, is shown in Fig-ure 1 c. The TG curve (gray trace) exhibits four mass-loss phe-nomena at 80, 260, 400, and 910 8C, associated with endother-mic peaks. Minor mass-losses steps observed between 80 and150 8C were attributed to water desorption. The mass-loss stepassociated with endothermic peaks at 260 8C was ascribed toimpurities or adsorbates of the ZnO, because pure ZnO exhib-ited the same weight loss. The weight loss at 400 8C (broadpeak) is related to the generation of defects[29] through theheat treatment in the ZnO powders. These defects can be as-cribed to the loss of oxygen ions, which forms defects (Vþo )s,and to the desorption of Znþi ions.[25, 30] A fourth mass-loss stepat 910 8C is again attributed to the formation of CoO (seeabove).

The morphology of pure ZnO, Co(NO3)2·6(H2O), and Co3O4

raw materials were characterized by field-emission scanningelectron microscopy (FESEM). The typical ZnO morphologyconsisted mainly of elongated prismatic particles and nearlycubical particles with sizes of 0.5–1.0 mm (see the SupportingInformation, Figure S1 a). The Co(NO3)2·6 (H2O) particles(Figure 2) were composed of small flake-like particles with

sizes between 0.5 and 5 mm that form weakly bound agglom-erates (inset in Figure 2). Finally, the morphology of Co3O4 par-ticles was that of small spherical particles with sizes of 30–40 nm, which form globular agglomerates of �500 nm (seethe Supporting Information, Figure S1 b).

An FESEM image of the Co3O4/ZnO catalysts prepared bythe impregnation method and thermally treated at 500 8C,ZCo10-Imp-500, is shown in Figure 3. The thermal treatmentstrongly influenced the morphology and size of the particles.The particles of the cobalt precursor compound had complete-ly irregular shapes with sizes between 500 nm and few micro-

meters, which were highly agglomerated (see Figure 2), butupon thermal treatment they decomposed into smaller, nearlyspherical, nanoparticles of less than 35 nm, which connectedrandomly (Figure 3). Highly agglomerated and heterogeneous-ly distributed �35 nm cobalt oxide nanoparticles dominatedthe impregnated series compared to typical 30–40 nm cobaltoxide nanoparticles used in “dry nanodispersion” methods asraw materials. The decomposition of the agglomerates was ac-companied by a phase transformation from Co(NO3)2·6 (H2O) toCo3O4, which could be concluded from the TG–DTA resultsshown in Figure 1 a. In particular, decomposition of the parti-cles into smaller nanoparticles resulted from the decomposi-tion of NO3

�C radical in the Co(NO3)2·6 (H2O) precursor duringheat treatment. However, most of these nanoparticles seemedto be connected with each other forming agglomerates andnot fully dispersed on the ZnO surface.

Contrarily, from the micrographs of ZCo10-Nps catalyst itcan be seen that the Co3O4 nanoparticles were well-dispersedon the ZnO surface (see the Supporting Information, Fig-ure S2). This dispersion and greater adherence of these nano-particles could indicate the appearance of Co3O4/ZnO interfa-ces at room temperature, which can be attributed to the highinitial reactivity of the Co3O4 and ZnO components. In thiscase, thermal treatment at 500 8C for the ZCo10-Nps systemdid not produce apparent changes in the size of the Co3O4

nanoparticles. Both preparation methods resulted in differentdispersion and adherence of Co3O4 nanoparticles onto the ZnOsupport, which affected the ZnO–Co3O4 interface.

Structural Characterization of Co3O4/ZnO catalysts

The catalysts prepared by impregnation and dry nanodisper-sion methods and thermally treated have been investigated byRaman spectroscopy and the resulting spectra are depicted inFigure 4. ZnO has a wurtzite structure, with two formulae perunit cell with C3v symmetry. For this structure, the grouptheory predicts the following Raman active modes:A1+E1+2 E2.[31, 32] These different Raman active modes of ZnOare listed in Table 1 along with the proposed assignment.Moreover, the main Raman modes of Co(NO3)2·6(H2O) com-pound are also listed in Table 1, and its Raman spectra areshown in Figure 4. According to Refs. [33, 34], Co(NO3)2 crystal-

Figure 2. FESEM micrograph of Co(NO3)2·6 (H2O) compounds.

Figure 3. FESEM micrograph of the thermally treated ZCo10-Imp catalysts.

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lizes in the trigonal space group R3 (no. 148) and Z = 12 (ZB =

4). As it is the case in Cd(NO3)2 and Pb(NO3)2, the nitrate unitsin Co(NO3)2·6 (H2O) are mostly ionic bonded. Thus, the vibra-tional spectra of Co(NO3)2·6 (H2O) compound might proceedfrom NO3

� anion with D3h symmetry with the following distri-bution of the fundamentals from the vibrational analysis[Eq. (2)]:[35]

GvibðNO3�=D3hÞ : A01 ðRÞþA002 ðIRÞþ2 E0 ðR, IRÞ ð2Þ

The pulsation u1 (A01) controlled the Raman spectrum (R) ofionic nitrates, whereas the most intense peak in the IR spectraresulted from the asymmetrical stretching vibration u3 (E’), (IR).The out-of-plane mode u2 (A002) was only IR active. However, thecombination 2 u2 is often found in Raman spectra of nitrates,[36]

and u4 (E’) is a bending mode.

The as-synthesized samples presented Raman spectra corre-sponding to ZnO, Co(NO3)2·6 (H2O), and Co3O4 materials in linewith the XRD patterns (Figure 4). After thermal treatment,some relevant changes were observed in the spectra. TheRaman spectra of the thermally treated ZCo10-Imp catalystshowed a complete disappearance of the u1 (A01) mode at1057 cm�1, which could be attributed to ligands combustion,whereas new peaks appeared at �184, 487, 525, 623, and680 cm�1 (Figure 4). The new peaks correspond to the Co3O4

spinel phase, indicating that the formation of Co3O4 spinelphase had been completed. These results are in line with theXRD results that also indicated the presence of the Co3O4

spinel phase in the ZCo10-Imp catalyst after the thermal treat-ment. This can be also observed from Table 1, which lists theRaman vibrational active modes of Co3O4 spinel. Consideringthat spinels belong to the Fd3m (O7

h) space group, one shouldtheoretically expect five Raman active modes (A1 g+Eg+3 F2 g).Pure cobalt oxide exhibits the five expected Raman activemodes in this spectral range: A1 g (689 cm�1), F2 g (619, 521, and191 cm�1), and Eg (481 cm�1). The high-frequency peak, A1 g,has been assigned to vibrations involving the motion ofoxygen atoms inside the octahedral unit (e.g. , CoO6 in Co3O4).Their breadth is related to the cation–anion bond lengths andpolyhedral distortion occurring in the spinel lattice,[37] whereasF2 g and Eg modes combine the vibration of tetrahedral (e.g. ,CoO4 in Co3O4) and octahedral sites.[38]

A careful examination of the Raman spectrum in the ther-mally treated ZCo10-Imp catalyst reveals a slight broadeningof the A1 g Raman peak and the appearance of a double peak.The second peak could be attributed to the appearance of thespinel phase ZnxCo1�xCo2O4 at �711 cm�1, which emerges con-comitantly. A detail of this region is presented in the respective

Figure 4. Raman spectra of raw materials, Co3O4/ZnO mixtures, and thermal-ly treated. The insets show magnified Raman spectra in the Raman shiftranges from 640 to 740 cm�1 of the thermally treated catalysts and Lorent-zian fits of the individual peak of the Raman mode associated to the appear-ance of the spinel phase (ZnxCo1�xCo2O4).

Table 1. Main Raman modes observed on ZnO, Co(NO3)2·6 (H2O), andCo3O4 phases. The intensity of the peaks is marked from very low to veryintense.

Raw materials Raman shift[cm�1] (process)

Intensity[a]

[a.u.]References

ZnO 203 (2 TA; 2 Elow2 ) l [29–30]

333 (Ehigh2 –Ehigh

2 ) m378 (A1TO) m410 (E1TO) l438 (Ehigh

2 ) vs483 (2 LA) vl536 (2 Blow

1 ; 2 LA) l574 (A1LO) vl590 (E1LO) l

Co(NO3)2·6 (H2O) 1057 (A01) vs [33, 34]1040 (A002) w720 (E’) vw603 (lattices vibrations) w491 (lattices vibrations) m139 (lattices vibrations) w108 (lattices vibrations) vw

Co3O4 481 (Eg) m [37, 38]521 (F2 g) m619 (F2 g) l689 (A1 g) vs

[a] l = low, m = medium, w = weak, s = strong, v = very.

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inset of the Figure 4, in which the spectrum is fitted to thesum of two Lorentzian functions centered at 680 cm�1 and711 cm�1, and ascribed to Co3O4 and ZnxCo1�xCo2O4, respective-ly. Actually, at annealing temperature as low as 500 8C, the for-mation of the spinel is clearly observed and its correspondingpeak becomes more intense than the peak at 680 cm�1 afterheat treatment. Formation of the ZnxCo1�xCo2O4 spinel requiresa large diffusion of Zn cations (not only those at the particlesurface) and is more favored by the smaller particles size andthe high surface reactivity of the cobalt oxide nanoparticlesobtained to the decomposition reaction from Co(NO3)2·6 (H2O)to Co3O4. These results have also been confirmed by XRD pat-tern, as shown in Figure S3.

In the case of the ZCo10-Nps sample, the peak associatedwith the A1 g mode of the Co3O4 phase was more intense thanthe peak associated with the ZnxCo1�xCo2O4 spinel phase, asshown on top of Figure 4. This fact indicates that the interac-tions between Zn and Co species under the same thermal con-ditions are totally different in both series.

To sum up, the most distinctive feature of the materials pre-pared by the impregnation method was the high surface reac-tivity of the cobalt oxide nanoparticles obtained by the de-composition reaction of Co(NO3)2·6 (H2O), which reacted withZnO to form the ZnxCo1�xCo2O4 spinel phase, whereas an incip-ient inhibition was found for materials obtained by the drynanodispersion method.

Nanostructure characterization of Co3O4/ZnO systems

To identify the structure of the nanoparticles, we did HRTEMcharacterizations in addition to XRD and Raman spectroscopy.Figure 5 shows HRTEM images of the Co3O4/ZnO catalysts pre-pared by the impregnation method. From Figure 5 a, the highagglomeration and heterogeneous distribution of the nanopar-ticles on the ZnO support can be clearly observed. These nano-particles have a broad average particles size in the range be-tween 30 and 45 nm, Figure 5 a and b. As alluded to earlierand indicated in Figure 4, the Raman spectra could be satisfac-torily assigned to the ZnxCo1�xCo2O4 spinel phase. The assign-ment is further supported by HRTEM, in which the lattice spac-ing of the nanoparticles was �4.68 �, which is indexed as theZnxCo1�xCo2O4 (111) plane; see Figure 5 c. However, theZnxCo1�xCo2O4 solid solution was reported to have variouscompositions, so the solid solution limit is in the range 0�x�0.96. To prove the solid solution limit of the nanoparticlesmentioned above prepared by the impregnation method, wealso confirmed their composition by using energy dispersive X-ray (EDX) spectroscopy. By performing EDX spectroscopy tar-geted to the domain marked by a “1” in Figure 5 c, Zn elementpeaks were detected on all Co3O4 nanoparticles. Cu signals inthe spectra originated from the TEM grid. This reveals againthat the Zn cations reacted with the Co3O4 nanoparticles pro-ducing the formation of the ZnxCo1�xCo2O4 solid solution. Fromthe EDX spectra, Figure 5 d, we could calculate theZnxCo1�xCo2O4 spinel compositions for the Co3O4 nanoparticlesdispersed on the ZnO surface under study reaching the limit ofsolid solutions, Zn0.85Co0.15Co2O4, in which most of the tetrahe-

dral sites of the spinel structure were occupied by Zn+ 2

cations.TEM was used to study further details of the Co3O4/ZnO het-

eronanostructure of the particles prepared by the dry nanodis-persion method, Figure 6. Figure 6 a shows a low-magnificationimage of the Co3O4 nanoparticles anchored on ZnO particles.From Figure 6 a, the ZCo10-Nps structure can be clearly ob-served. Isolated Co3O4 nanoparticles are highly and uniformlymonodispersed on the ZnO support with an average size ofabout 30–35 nm, Figure 6 b. To investigate also the nanocrys-talline structure of the nanoparticles, HRTEM was employed.

Figure 5. a, b) TEM images of samples synthesized by the impregnationmethod and thermally treated. c) HRTEM images of the nanoparticle-coatedZnO support shown in part b. d) EDX spectra of the nanoparticles, corre-sponding to the domains marked with a “1” in part c.

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We note from the HRTEM image in Figure 6 c that the spinelZnxCo1�xCo2O4 and Co3O4 are present as a core–shell structure.The outer shell consists of polycrystalline domains with latticespacing consistent with that of ZnxCo1�xCo2O4, in which the lat-tice distances observed are �2.45 � and �2.02 � matches wellwith that of the (3 11) and (4 0 0) planes, respectively. However,in the inner core of the nanoparticles the lattice spacing isaround 4.67 �, which is indexed as the Co3O4 (111) plane. EDXanalysis further confirms the shell nanoparticles composition,Figure 6 d. The EDS spectra, marked as point 1 in Figure 6 c, in-

dicated that the shell contained nanopolycrystalline domainswith a composition Zn0.20Co0.80Co2O4, which is consistent withthe results from the Raman spectroscopy shown in Figure 4.

The shell was formed by Zn2 + cation diffusion, attributed tohigh vapor partial pressure of ZnO at the treatment tempera-ture. The appearance of polycrystalline domains is an experi-mental proof of the Kirkendall effect.[39] This phenomenon ex-plains that atomic diffusion occurs not by the direct inter-change of atoms but by the vacancy exchange.[40] In our case,the Kirkendall effect has provoked a crystalline reordering inthe nanoparticle surface that gives place to a �5 nm shell con-taining polycrystalline domains �3 nm, as evidenced inFigure 6 b.

Catalytic response of the Co3O4/ZnO systems

According to previous works, when urea and glycerol wereheated in the presence of a catalyst, these compounds reactfollowing a reaction mechanism with four possible steps:1) carbamoylation of glycerol to glycerol urethane, 2) carbony-lation of glycerol urethane to glycerol carbonate (4-hydroxy-methyl-1,3-dioxolan-2-one) with abstraction of the ammonia or3) carbonylation of glycerol urethane to 5-(hydroxymethyl)oxa-zolidin-2-one without abstraction of the ammonia and 4) glyc-erol carbonate can react with another molecule of urea toobtain (2-oxo-1,3-dioxolan-4-yl)methyl carbamate, decreasingglycerol carbonate selectivity.[10, 12] In our previous work,[13] wehave demonstrated that the formation of glycerol carbonateproceeds in two consecutive steps. As it was evidenced withinthe use of real-time attenuated total reflection FTIR spectrosco-py the first step was fast, whereas the second one was relative-ly slow.[13] Reaction conditions and catalysts had to be carefullyselected to avoid a decrease in selectivity by reaction betweenglycerol carbonate and urea.

The activity results obtained during carbonylation of glycerolover the raw materials showed that ZnO is slightly more activethan the blank test, whereas Co3O4 is more active and selec-tive, see Figure S4. However, the combination of both oxidesexhibited a catalytic activity that depended on the specificpreparation procedure and treatment temperature.

The ZCo-Imp series was slightly more active and selectivethan the Co3O4 reference. However, the preparation of the cat-alysts by dry nanodispersion (ZCo-Nps series) increased the ac-tivity markedly, particularly for those catalysts prepared atroom temperature. ZCo-Nps calcined at 500 8C exhibiteda moderate increase in activity demonstrating that the catalyticactivity was hampered upon calcination.

Thus, ZCo10-Imp delivered a conversion of 29 %, slightlyabove the blank test, but with nearly 100 % selectivity to glyc-erol carbonate. On the contrary, ZCo10-Nps samples treated atroom temperature, delivered a conversion of 69 % with 97 %selectivity to glycerol carbonate (see Table 2). Upon calcinationof this sample at 500 8C, the conversion decreased to 37 %while selectivity remains unchanged. It is evident that for theCo3O4/ZnO series calcination at 500 8C has a deleterious effecton both conversion and selectivity. Co3O4/ZnO interface prop-

Figure 6. a, b) TEM images of samples synthesized by the dry nanodispersionprocess and thermally treated. c) HRTEM images of the nanoparticle-coatedZnO substrate shown in part b. d) EDX spectra of the nanoparticle-shell-con-tained polycrystalline domains, corresponding to the domains marked witha “1” in part c.

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erties are modified by both methods producing changes in thefinal catalytic activity of the system.

These results evidence a clear correlation between the pres-ence of the interphase and the catalytic activity. The formationof Co3O4/ZnO interphases during the preparation and theamount of CO3O4

[13, 23] in the samples was critical to delivergood catalytic performance. In addition, the components fur-ther reacted into inactive spinel phases after thermal treat-ment. Therefore, the preparation stage of the Co3O4/ZnO cata-lysts plays an important role in the extent of interaction be-tween these phases so that the catalytic activity can be tunedby controlling such interphase.

The results obtained in this work for the synthesis of glycerolcarbonate from glycerol and urea with those obtained recentlyby other authors are compared in Table 2. As it can be ob-served in this table, not only the reaction conditions are keyfor performing the reaction successfully but also the selectionof the catalysts and their preparation method are crucial toreach high yields and glycerol carbonate selectivity. Thus, it isevident how important is tuning the catalytic activity of theCo3O4/ZnO system to be effective in the glycerol carbonatesynthesis.

In Scheme 1 a and b, cartoons of the impregnation anddrying processes are shown, in which the preexisting ZnO sup-port is impregnated with an aqueous solution of Co-(NO3)2·6 (H2O). After the subsequent drying, the catalyst istreated thermally at 500 8C (Scheme 1 c–d). This thermal pro-cess provokes the complete decomposition of the Co-(NO3)2·6 (H2O) precursor, (Scheme 1 d), so the new phase is dis-persed on the support. A magnification of the ZnO supportsurface is shown in Scheme 1 e and f, which illustrates the for-mation of interfaces between ZnO support and newZnxCo1�xCo2O4 crystalline nanoparticles. The high surface reac-tivity of the cobalt oxide nanoparticles was obtained by thedecomposition reaction of the Co(NO3)2·6 (H2O), which reactedwith ZnO support ; producing the formation of theZn0.85Co0.15Co2O4 spinel phase (as evidenced by TG–DTA,Raman spectroscopy, and HRTEM). From the viewpoint of dis-persion, impregnation generated a high agglomeration andheterogeneous distribution of cobalt oxide nanoparticles onthe ZnO support. It seems that at such loading, there was nota molecularly dispersed phase on the oxide support. The nano-particles aggregated into 3 D aggregates and formed bulkmixed oxide phases that led to a poor dispersion of the phase

(as evidenced by FESEM andHRTEM). This fact, together withthe appearance of a spinel-typeinterphase, had a negativeeffect on the carbonylation ofglycerol with urea, as evidencedin Table 2 and Figure S4.

The process of dry nanodis-persion is illustrated inScheme 2. In parts a–d, themechanism can be described asthe formation of Co3O4–ZnO in-terfaces at room temperature(Scheme 2 d), which are highlystable and modified only duringthermal treatment (Scheme 2 e).The higher stability of nanodis-persed Co3O4/ZnO allowed pro-ducing nanoparticles witha core–shell structure. The nano-structured shell was composed

Table 2. Comparison of the results obtained herein for the synthesis of glycerol carbonate from glycerol and urea with those obtained recently by otherauthors.

Corma et al.[12a]

(2010)Hutchings et al.[12b]

(2012)This work

catalyst mixed oxides (Al/Mg, Al/Li)from hydrotalcites

supported AuPd nanoparticles Co3O4/ZnO

preparation method Co precipitation sol immobilization impregnation dry–nanodispersion impregnationreaction temperature [8C] 145 150 145reaction pressure [kPa] 4.0 atmospheric atmosphericreaction time [min] 300 240 240 240 240conversion [%] 82 87 86 69 29selectivity [%] 88 77 64 97 >99

Scheme 1. The formation process of Co3O4/ZnO catalysts by the impregnation method.

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of Zn0.20Co0.80Co2O4 polycrystalline domains, whereas the corematched well with a pure Co3O4 composition. The formation ofthe spinel phase reduced the conversion toward glycerol car-bonate. However, the core–shell structure formation is a re-markable strategy that allows design other catalysts with im-proved surface activity. As discussed above, the reactivity ofthe Co3O4/ZnO catalysts was affected by the generation of de-fects, which, on the ZnO support was caused by specific sur-face reduction, and by the reactivity of the Co3O4 nanoparti-cles. The lower reactivity of the Co3O4 nanoparticles used inthe dry nanodispersion series limited the formation of newZnxCo1�xCo2O4 spinel phases during the thermal treatment.This could explain the highest catalytic activity showed by theCo3O4/ZnO catalyst without thermal treatment.

Conclusions

This work delivers comprehension of differences betweenCo3O4/ZnO catalysts obtained by conventional impregnationversus novel nanodispersion procedures. The understanding ofthe catalyst features required a complete characterization ofthe materials structure and micro–nanostructure. The impreg-nated catalysts series was characterized by the high surface re-activity of the cobalt oxide nanoparticles obtained by the de-composition reaction of the Co(NO3)2·6 (H2O), which reactedwith the ZnO support. Therefore, the diffusion of Zn2 + cationsinto Co3O4 nanoparticles was promoted, and thus a newZn0.85Co0.15Co2O4 spinel phase formed rapidly during thermaltreatment.

The dry nanodispersion cata-lysts series resulted in a free in-terface between Co3O4 nanopar-ticles and ZnO. This process in-creased the exposure of Co3O4

nanoparticles. The thermal treat-ment of the nanodispersedseries generated a nanostruc-tured surface over the Co3O4

nanoparticles, forming a core–shell structure. The shell wasformed by nanopolycrystallinedomains with Zn0.20Co0.80Co2O4

spinel-type composition. The Kir-kendall effect was responsiblefor such a surface nanostructure.From the viewpoint of catalysisresponse, it seems that fora given catalyst composition, thesynthesis method has a dramaticeffect on performance. It hasbeen demonstrated that the ap-pearance of a spinel-type inter-phase has a negative effect onthe carbonylation of glycerolwith urea, being more relevantin the catalysts obtained by the

conventional impregnation method.We believe that the general strategy and design principles

described in this study will open new avenues in developingnovel catalysts with enhanced properties for a wealth of cata-lytic reactions.

Experimental Section

Impregnation of Co3O4/ZnO system (Imp series)

The composite particles with 10 wt % of Co3O4 nanoparticles (here-after named as ZCo10-Imp) were prepared by the impregnationmethod from an aqueous solution of Co(NO3)2·6 (H2O). The cobaltprecursor was Co(NO3)2·6 (H2O) (Sigma–Aldrich) and the supportZnO (>99.99 %, Sigma–Aldrich) with a specific surface area of4 m2 g�1. The sample was prepared by impregnation with an aque-ous solution of the precursor and treated in a rotary evaporator at80 8C. The impregnated solid was dried at 120 8C for 4 h. These as-synthesized materials (ZCo10-Imp, 3 g) were submitted to varioustreatments to both ensure the complete removal of precursor rem-nants in the final materials and to investigate the effect of treat-ment conditions on the final catalyst and, therefore, on the activityof the materials. Such temperature treatments are reflected onsample nomenclature by adding a dash with the numeric value tothe temperature treatment in degrees centigrade.

Nanodispersion of Co3O4/ZnO system (Nps series)

To prepare the system with 10 wt % of Co3O4 nanoparticles (here-after named as ZCo10-Nps), a previously described dry solid-statemethod was used, which incorporates the appropriate amounts ofCo3O4 nanoparticles to ZnO microparticles by producing an elec-

Scheme 2. Schematic representation of the attachment of nanoparticles to the outer surface of an individual ZnOmicroparticle obtained by the dry nanodispersion method. a) Co3O4/ZnO mixtures before and b) after of the drynanodispersion process. c) Magnified ZnO surface cover of Co3O4 nanoparticles hierarchically dispersed withd) Co3O4–ZnO interfaces. e) Schematic representation of the spinel phase formed after thermal treatment.

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trochemical reaction at RT.[22] The dry dispersion process consistedof shaking in a 60 cm3 nylon container for 5 min at 500 rpm witha mixer similar to a Turbula mixer. The process was done with1 mm ZrO2 balls. To ensure that there are no structural disordercontributions induced by the mixing process, pure ZnO and Co3O4

powders were also subjected to the same mixing process. The rawmaterials used in this study were cobalt oxide nanoparticles witha specific surface area of 35 m2 g�1 (Co3O4, 99.99 %, Sigma–Aldrich)and ZnO microparticles (>99.99 %, Sigma–Aldrich). Analytical-grade powders were dried at 110 8C for 2 h before dry mixing. Theas-prepared sample, ZCo10-Nps, was further submitted to temper-ature treatments. Such temperature treatments are reflected insample nomenclature by adding a dash with the numeric value tothe temperature treatment in degrees centigrade. The characteriza-tion of this catalyst is reported and discussed elsewhere,[23] includ-ing particle size and morphology (FESEM) and structural features(XRD and Raman spectroscopy) of the powders.

Thermal characterization

Simultaneous thermogravimetric and differential thermal analyseswere performed on samples of the Co(NO3)2·6 (H2O)/ZnO andCo3O4/ZnO mixed powder before the heat treatment witha NETZSCH STA 409/C analyzer. Powder (�50 mg) was placed ina Pt/Rh crucible and heated up to 1200 8C with a heating rate of3 8C min�1. The measurements were performed in a flowing airatmosphere.

Morphology characterization

The particle size and morphology of the powders were evaluatedby using secondary electrons images of FESEM (Hitachi S-4700)and a JEOL 2100F transmission electron microscope (TEM/STEM)operating at 200 KV and equipped with a field emission electrongun providing a point resolution of 0.19 nm. The microscope wasalso coupled with an EDXS energy dispersive X-ray spectrometer(INCA x-sight, Oxford Instruments) used for chemical elementalanalysis.

Structural characterization

The Raman spectra were measured in air at RT with 514 nm radia-tion from an Ar+ laser operating at 0.8 mW on the sample. Thesignal was collected by a 20 � objective in a Raman microscopespectrometer (Renishaw Micro-Raman System 1000) in the100 cm�1–1100 cm�1 range.

Carbonylation of glycerol with urea

In a typical experiment, an equimolecular mixture of glycerol andurea were placed in a 10 mL round-bottom flask in a batch reactorfor 5 min before catalyst was added. The reaction was heated in anoil bath at 140–145 8C, and stirred at 300 rpm. Reactions were runin the absence of solvent at atmospheric pressure by removingammonia from the system by air passing through the reactor. Theamount of catalyst used was 6 wt % of the initial amount of glycer-ol. After the reaction was completed (4 h), water was added andthe catalyst removed by filtration. The catalyst was washed withacetone several times and dried at RT for 24 h to be used in a newcycle of reaction. The reactions were followed by GC usinga HP5890 gas chromatograph equipped with a 50 m long Ultra2-

5 % Phenyl methyl siloxane capillary column and a flame ionizationdetector (standard accuracy �5 % relative).

Acknowledgements

The authors thank CICYT projects MAT 2010-21088-C03-01,CTQ2008-02461/PPQ, and CTQ2011-13343E for their financialsupport. Dr. F. Rubio-Marcos is also indebted to CSIC for a ‘‘Juntade Ampliaci�n de Estudios’’ contract (ref. JAEDOC071). Dr. V. Cal-vino-Casilda is indebted to CSIC for a JAE-DOC fellowship.

Keywords: biomass · cobalt · heterogeneous catalysis ·nanoparticles · zinc

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Received: September 7, 2012

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FULL PAPERS

F. Rubio-Marcos,* V. Calvino-Casilda,*M. A. BaÇares, J. F. Fernandez

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Control of the Interphases FormationDegree in Co3O4/ZnO Catalysts

Keep dry and carry on: ZnO supportedCo3O4 nanoparticles are highly active inthe transformation of renewable materi-als through carbonylation of glycerol byurea. The activity of the catalyst ismodulated by interaction of the nano-particles with the ZnO support, whichdepends on the preparation stage. TheCo3O4/ZnO catalyst system prepared bya dry mixing method at room tempera-ture is very promising for this reaction.

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