interplay between particle size, composition, and structure of mgal2o4-supported co–cu catalysts...

Journal of Catalysis 307 (2013) 222–237

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Journal of Catalysis

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Interplay between particle size, composition, and structure ofMgAl2O4-supported Co–Cu catalysts and their influence on carbonaccumulation during steam reforming of ethanol

0021-9517/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcat.2013.07.025

⇑ Corresponding author. Fax: +55 16 33518266.E-mail address: [email protected] (J.M.C. Bueno).

1 Current address: Faculdade de Engenharia Química, Universidade Federal deUberlândia, Av. João Naves de Ávila 2121, Campus Santa Mônica – Bloco 1K, 38400-902 Uberlândia, MG, Brazil.

C.N. Ávila-Neto a,1, D. Zanchet b, C.E. Hori c, R.U. Ribeiro a, J.M.C. Bueno a,⇑a Departamento de Engenharia Química, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos, SP, Brazilb Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, SP, Brazilc Faculdade de Engenharia Química, Universidade Federal de Uberlândia, Av. João Naves de Ávila 2121, Campus Santa Mônica – Bloco 1K, 38400-902 Uberlândia, MG, Brazil

a r t i c l e i n f o

Article history:Received 3 June 2013Revised 24 July 2013Accepted 27 July 2013

Keywords:Steam reforming of ethanolHydrogen productionCobalt-supported catalystsIn situ EXAFSCrystallite sizeCobalt–copper alloy

a b s t r a c t

The effect of Cu/Co ratio on the structure of MgAl2O4-supported Cu–Co nanoparticles, and on their cata-lytic properties during steam reforming of ethanol (SRE), was studied using a series of ex situ and in situtechniques. X-ray photoelectron spectroscopy (XPS) analyses showed that Cu atoms migrated to the sur-faces of reduced nanoparticles. Extended X-ray absorption fine structure (EXAFS) data revealed the for-mation of Cu–Co alloy. The size and structure of the nanoparticles were estimated by development ofa model based on the EXAFS and XPS data, considering the nanoparticles to be spherical with reducedcores and oxidized shells. It was found that both the reduced core and nanoparticles size were dependenton the Cu/Co ratio. The observed apparent dependence of carbon accumulation rate during SRE (rcar-

bon,SRE) on Co–Cu crystallite size was mainly associated with the presence of the element in an oxidizedstate and with the nature of the metal on the nanoparticle surface.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Hydrogen is an important feedstock used as a reagent or inter-mediate in a variety of chemical, petrochemical, and metallurgicalprocesses. It is produced commercially from steam reforming ofmethane, or in refineries from partial oxidation of heavier hydro-carbons [1,2]. Alternatively, hydrogen may be produced fromrenewable sources such as ethanol. Steam reforming of ethanol(SRE) (reaction 1) catalyzed by transition metals is of particularinterest due to its environmental appeal. Noble metals such asRh show good activity for cleavage of the C–C bond [3,4] and havebeen considered to be the most effective metal catalysts for SRE[5,6]. However, the high cost of these metals has shifted attentionto transition elements such as Co [7–19] and Ni [20–28], whichalso present high activity in SRE.

C2H5OHðgÞ þH2OðgÞ ! 2COþ 4H2 DH0298 ¼ 256 kJ mol�1 ðR1Þ

The literature [3,4] suggests that ethanol is adsorbed overPt(111) surfaces, with subsequent abstraction of H atoms fromthe hydroxyl group leading to formation of intermediate�OC—CH�x species. At low temperatures and in the presence of tran-sition metals with high occupancy of the 3d orbital, such as Cu, theintermediate O�CH–CH3 species are easily desorbed in the form ofacetaldehyde [29,30]. On the other hand, metals with low occu-pancy of the 3d orbital, such as Ni, can maintain these species ad-sorbed at higher temperatures. The higher temperature thenfacilitates cleavage of the C–C bond, with formation of CO andCH4 [26]. Desorption of methane is undesirable since the moleculeis an H carrier and consequently decreases the selectivity towardH2. However, transition metals with even lower occupancy of the3d orbital, such as Co, interact strongly with the intermediateCH4 and CH�x species formed after cleavage of the C–C bond, result-ing in their decomposition to carbon (C�) and H2 in a pyrolytic pro-cess [7]. The challenge in obtaining stable Co-supported catalystsfor use in SRE is to equilibrate the steps of ethanol activation andoxidation of adsorbed C� with O� species supplied by water [10].If the rate of C� formation is faster than the rate of C� oxidation,the access of reagents to active Co0 sites may be blocked by carbonaccumulation and the growth of carbon filaments.

The rate of carbon oxidation can be enhanced by increasing thetemperature [9,31]. As the temperature increases, the entropiccontribution to the energy barrier for carbon oxidation decreases

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[32], but high temperatures lead to thermal decomposition of eth-anol, with significant formation of methane [26]. Increasing theH2O/ethanol ratio in the feed [8,31] and co-feeding oxygen to thereactor, a process called oxy-reforming of ethanol (ORE)[10,14,16], are other options for controlling the accumulation ofcarbon with time on stream. However, increasing the H2O/ethanolratio higher than the stoichiometric value decreases the yield of H2,and the presence of oxygen results in oxidation of the surfaces ofthe metal particles [10]. Both options lead to increased oxidationof cobalt, and thermodynamic calculations have shown that forma-tion of an oxide shell surrounding a metallic core would causeinstability in terms of phase separation [33]. On the other hand,earlier work [10] has demonstrated that the rate of carbon accu-mulation can be controlled by increasing the degree of oxidationof Co. It is known that the oxidation state of these catalysts de-pends on the size of the cobalt crystallites, temperature, and reac-tant composition [13,33–36]. The oxidation state of the crystallitesis actually a thermodynamic effect controlled by the curvature ofthe surfaces, with smaller crystallite size resulting in a higher oxi-dation state [33,34]. It has been suggested that the reduction oftransition metals first occurs mainly in the nanoparticle core andthen proceeds toward the surface [37–40]. Consequently, the par-tially reduced nanoparticles form a core–shell type structure,where the core is predominantly reduced and the shell is oxidized[41]. In the case of bimetallic catalysts, the component that forms astronger bond with the adsorbent generally segregates on the sur-face of the nanoparticles [42]. As a result, the electronic propertiesof the surfaces can be markedly different to those of the bulk mate-rial, due to the high ratio between the area and volume of nanopar-ticles [43]. Papaefthimiou et al. [44] demonstrated thisphenomenon with Co–Pt alloys in 3 nm nanoparticles. In an H2

atmosphere, Pt atoms segregated onto the surface of the nanopar-ticles. In contrast, in an O2 atmosphere, the opposite tendency wasobserved, indicating enrichment of the surface with cobalt oxide(CoO).

Core–shell type structures have also been obtained in Ni–Cusystems. Held [45] conducted CO-TPD and XPS analyses ofNi-covered Cu(111) surfaces and reported that since Cu has a low-er surface energy than Ni (2.4 J m�2 versus 1.9 J m�2) and a largermetallic radius (1.28 Å versus 1.24 Å), Cu atoms usually segregateonto the surface. Hence, Cu layers formed on Ni surfaces are morestable than the conjugate pair. This phenomenon has been re-ported by other researchers, using various characterization tech-niques [46–52]. The Co–Cu system has also been extensivelystudied [53–60]. However, unlike the Ni–Cu system, only a fewstudies have reported the formation of a core–shell structure.Ahmed [60] synthesized Co–Cu nanoparticles and showed thatthe core consisted of an almost immiscible Co–Cu mixture,whereas the shell was composed of a Co–Cu alloy. On the otherhand, some investigators [55,61] have found evidence for the for-mation of a Co core encapsulated by a Cu shell. A core composedof a Co–Cu mixture, surrounded by a Cu oxide shell, has also beenobserved [59]. Under SRE conditions, the Cu in both Co- andNi-based catalysts increases the activity for dehydrogenation inthe presence of ethanol at low temperatures [29,62–65]. At highertemperatures, it substantially increases the rate of carbon accumu-lation, but has only minor effects on the distribution of products[65]. These results suggest that the presence of Cu stronglymodifies the catalytic properties of Co- and Ni-based catalysts usedfor SRE.

From the above discussion, it is clear that bimetallic transitionmetal systems should provide a suitable means of demonstratinghow the interplay between particle size, composition, and struc-ture of Co-based catalysts influences their catalytic properties inSRE. This work contributes to elucidating the possible formationof Co–Cu alloys, the ways in which the electronic properties of

metal surfaces change the structure of nanoparticles, and the ex-tent to which the electronic properties correlate with the catalyticactivity for SRE. These issues were investigated by synthesis of Co-based MgAl2O4-supported catalysts promoted with 0.5–3.0 wt.% ofCu. A model based on extended X-ray absorption fine structure(EXAFS) and X-ray photoelectron spectroscopy (XPS) data wasdeveloped to estimate the size and structure of Co–Cu nanoparti-cles. These properties were correlated with product selectivity asa function of temperature and with the rate of carbon accumula-tion under SRE conditions.

2. Materials and methods

2.1. Catalyst preparation

MgAl2O4-supported Co–Cu catalysts were prepared by incipientwetness co-impregnation of MgAl2O4 with aqueous solutions ofCo(NO3)2�6H2O (Aldrich, 99%) and Cu(NO3)2�2.5H2O (Aldrich, 99%)at 343 K. Following this, the precursors were dried at 383 K for12 h and calcined at 823 K (using a heating rate of 10 K min�1)for 6 h under a flow of 80 mL min�1 of synthetic air. The MgAl2O4

support was prepared by a sol–gel method described elsewhere[10]. The Co12Cux/MgAl2O4 samples were denoted as Co12Cux,where 12 is the weight percentage of Co and x is the weight per-centage of copper (x = 0.5, 1, and 3).

2.2. Characterization

2.2.1. Nitrogen physisorptionThe textural properties of the MgAl2O4 support and the Co12Cux/

MgAl2O4 calcined samples were determined by adsorption of nitro-gen at 77 K using a Quantachrome Corporation NOVA 1200 instru-ment. The specific area (SBET) was calculated by the BET method[66]. The mean pore volume (Vpore) and mean pore diameter (dpore)were calculated from the adsorption isotherms using the BJH [67]and BET methods, respectively.

2.2.2. Thermogravimetric analysisFor the thermogravimetric analyses (TGA), the samples (6 mg)

were placed inside an alumina crucible held in a thermobalancecoupled to a Shimadzu DTG-60H differential thermal analyzer.The crucible/sample system was heated from room temperatureto 1273 K, at 10 K min�1, under a 30 mL min�1 flow of synthetic air.

2.2.3. X-ray diffractionX-ray powder diffraction (XRD) analyses were carried out (i) in

conventional mode at room temperature, and (ii) during in situheating of the samples under a flow of H2 in He (5 vol.%).

In conventional mode, XRD patterns were recorded with aRigaku DMAX 2500 PC diffractometer, employing Cu Ka radiation(k = 1.54056 Å) with a Ni filter. The 2h angle was swept from 20�to 80�, with a step size of 0.02� and a counting time of 1 s. To esti-mate the average sizes of the Co3O4 crystallites, the XRD pattern ofeach catalyst was subtracted from the XRD pattern of the corre-sponding support, and Scherrer’s equation [68] was applied tothe (311) reflection.

In situ XRD was carried out at the XPD-10B beamline of theBrazilian Synchrotron Light Laboratory (LNLS). A detailed descrip-tion of this beamline may be found elsewhere [69]. The sampleswere first crushed, sieved to particle sizes smaller than 20 lm,and homogeneously distributed over the support. XRD patternswere then acquired during heating of the samples from roomtemperature to 973 K, at 10 K min�1, under a 200 mL min�1 flowof H2/He (5 vol.%). Scans were carried out from 32� to 47�, with astep size of 0.003� and a counting time of 1 s, using a wavelength

224 C.N. Ávila-Neto et al. / Journal of Catalysis 307 (2013) 222–237

of 1.54056 Å and a resolution of 4.3 eV. Under these conditions, itwas possible to observe diffraction peaks at 36.8� and 44.8�, corre-sponding to lattice planes (311) and (400) of spinel Co3O4 (ICSDcollection code 24210), at 36.6� and 42.4�, corresponding to latticeplanes (111) and (200) of face-centered cubic (fcc) CoO, and at43.9�, corresponding to lattice plane (111) of face-centered cubicmetallic Co (ICSD collection code 52934).

2.2.4. X-ray absorption near edge spectroscopyX-ray absorption near edge spectroscopy (XANES) analyses

were performed at the Co K edge (7709 eV), using the D06A-DXASbeamline of LNLS. The D06A-DXAS is a dispersive beamlineequipped with a curved Si(111) monochromator, operated inBragg mode, which selects radiation from a bending magnet sourcein the X-ray range 4–14 keV and focuses it at the sample position[70,71]. An exposure time of around 0.3 s was employed for eachmeasured spectrum, but the final spectrum was obtained after100 accumulations (frames), giving 30 s of total acquisition time.Conversion of data from pixel to energy was accomplished by com-paring measurements of a metallic Co (hcp) reference material,made in conventional mode using non-dispersive beamlines, withthose made in dispersive mode.

The catalysts were first crushed and sieved to particle sizessmaller than 20 lm and then pressed into self-supporting pelletsthat were placed inside a tubular quartz reactor (20 mm insidediameter, 440 mm X-ray path length) equipped with refrigeratedkapton windows transparent to the X-ray beam [72]. Tempera-ture-resolved XANES spectra were acquired during reduction ofthe samples, which was achieved by heating from room tempera-ture to 1023 K, at 10 K min�1, with a holding time of 30 min, undera 200 mL min�1 flow of H2/He (5 vol.%). Energy calibration of theXANES spectra was performed with open source ATHENA/IFEFFITsoftware. Additionally, a linear combination analysis was per-formed using Co0, CoO, and Co3O4 references. The linear combina-tion procedure is described elsewhere [10].

2.2.5. Extended X-ray absorption fine structureEXAFS analyses were performed at the D04B-XAFS1 and D08B-

XAFS2 beamlines of LNLS. General descriptions of these beamlinescan be found elsewhere [73]. Three types of experiments were per-formed to obtain the EXAFS spectra. Firstly, the samples were re-duced under a 200 mL min�1 flow of H2/He (5 vol.%), withheating to either 873 K (Experiment 1) or 1073 K (Experiment 2).The heating rate was kept at 10 K min�1, and the samples remainedat the reduction temperatures for 2 h. The reactor was then cooledto room temperature under a 150 mL min�1 flow of He, and threeEXAFS spectra were acquired for each experiment. In Experiment3, the samples were first reduced at 1073 K for 2 h under a flowof H2/He (5 vol.%) and then cooled to room temperature under aflow of He. The flow of H2/He (5 vol.%) was then interrupted, anda flow of 3.9 mL min�1 of the SRE mixture (H2O/ethanol molar ra-tio = 3:1) diluted in 133 mL min�1 of He was started. The sampleswere heated from room temperature to 823 K, at 10 K min�1, andthree EXAFS spectra were acquired 10 min after reaching steadystate.

Background subtraction, normalization, and alignment of theEXAFS data were performed with the ATHENA/IFEFFIT software[74]. The Co K edge was established at the maximum of the firstpeak in the first derivative of the absorption spectra. The k2-weighted data were then Fourier transformed from 3 to 11.5 Å�1

using a Hanning apodization function, and back-Fourier trans-formed over a non-phase-corrected radial distance (R) range from1 to 3 Å to isolate the contribution of scattering on the first shell ofneighbors. The structural parameters were obtained from a nonlin-ear least-squares fitting of the EXAFS data performed with ARTE-MIS/IFEFFIT software [75]. The amplitude reduction factor (S2

0)

and the edge-energy correction (DE0) were determined from thecobalt (Co0-hcp and CoO-fcc) and copper (Cu0-fcc and CuO-monoclinic) references and were fixed for fitting the spectra ofthe Co12Cux samples (obtained at room temperature). The coordi-nation numbers were fixed for all references and floated for theCo12Cux samples. The average distance (R) and structural Debye–Waller factor (Dr2

s ) were allowed to float during the fittings.To fit the EXAFS data obtained during SRE at 823 K, the edge-en-

ergy correction was allowed to float and the structural Debye–Waller factor was set to the values obtained at room temperatureafter reduction of the samples at 1023 K. A temperature-dependentDebye–Waller factor (Dr2

T) was also considered in the fittings. Thiswas estimated using the Einstein model [76] with an Einstein tem-perature of 294 K, determined for pure hexagonal cobalt foil [77].Thus, Dr2

T becomes a function of both the measured temperatureand the reduced mass of the atomic pair under investigation. Inthis case, ðDrCo�O

T Þ2 = 2.34ðDrCo�CoT Þ2 and ðDrCu�O

T Þ2 =2.48ðDrCu�Cu

T Þ2.

2.2.6. X-ray photoelectron spectroscopyXP spectra were acquired at the LNLS with a SPECSLAB II PHOI-

BOS HSA 3500 150 spectrometer containing nine detection chan-nels and a hemispherical analyzer, employing AlKa radiation(1486.6 eV). The analyzer was operated in constant pass energymode (Epass = 40 eV). Binding energies were referenced to the Al2s

core level (118.96 eV) of c-alumina. The vacuum level during theexperiments was below 10�7 Pa.

Two types of sample were characterized by XPS: (i) calcinedsamples and (ii) samples pre-reduced at 973 K in H2/He (5 vol.%).The samples were first crushed and sieved to particle sizes smallerthan 20 lm and then pressed into self-supporting pellets. The pel-lets of the pre-reduced samples received a second treatment withheating to 973 K (at 10 K min�1), under a 200 mL min�1 flow of H2/He (5 vol.%), in the reaction cell of the preparation chamber. Thepellets were subsequently transferred to the analysis chamber ofthe spectrometer. The fittings were performed with CASA (version2.3.13) software, using a 50/50 Gaussian/Lorentzian function. Theratio between the Co 2p3/2 and Co 2p1/2 photoelectron peaks wasfixed at ½.

2.3. Catalytic tests

The influence of Cu/Co ratio on the distribution of SRE productswas investigated in catalytic and stability tests performed using afixed-bed quartz reactor (8 mm inside diameter, 230 mm length),operated isothermally at atmospheric pressure. Prior to the reac-tions, the catalysts (120 mg; 80–100 mesh particles) diluted withquartz beads (300 mg) with similar granulometry were reducedin situ at 823 K (10 K min�1) for 2 h under a 30 mL min�1 flow ofH2. The reactor was subsequently cooled to room temperature un-der a flow of He, and the flow of the SRE mixture was initiated.

The SRE activity and product distribution were measured from473 to 823 K, in steps of 50 K, using low (H2O/ethanol = 3) and high(H2O/ethanol = 12) molar ratios. For the low molar ratio, the feedconsisted of 11 mL min�1 of the reaction mixture eluted in292 mL min�1 of He. For the high molar ratio, the feed was com-posed of 12.8 mL min�1 of the reaction mixture eluted in383 mL min�1 of He.

Stability tests were carried out using the H2O/ethanol = 12 Mratio. After reduction at 823 K for 2 h and cooling to room temper-ature, the samples were heated to 823 K at 10 K min�1 and kept atthis temperature for 6 h. The feed consisted of 12.8 mL min�1 ofreaction mixture eluted in 383 mL min�1 of He. The rate of carbonaccumulation in SRE (rcarbon,SRE) was estimated by dividing theweight gain, determined after the TGA procedure, by the reactiontime (6 h).


During both the activity and stability tests, the effluent wasanalyzed by online gas chromatography employing a Varian 3600CX gas chromatograph equipped with a thermal conductivitydetector (TCD) and a 20% Carbowax� 20 M on 80/100 Chromosorbcolumn. Eqs. (5) and (6) (Supplementary Material) were used tocalculate the molar fractions of the different species and the etha-nol conversion efficiencies, respectively.

Fig. 1. Ex situ XRD patterns of the synthesized MgAl2O4 support and calcined Co12

and Co12Cu3 samples. (a) XRD pattern of MgAl2O4 taken from the Inorganic CrystalStructure Database [collection code 22354].

3. Results

3.1. Textural properties and ex situ XRD of the MgAl2O4 support andcatalysts

The textural properties of the MgAl2O4 support and the calcinedCo12Cux samples are given in Table 1. All the samples presentedtype IV isotherms with H3-type hysteresis loop (IUPAC system)around P/P0 = 0.3 (not shown), which is characteristic of mesopor-ous materials (>2 nm) [78]. Impregnation of cobalt and copper overthe surface of the MgAl2O4 decreased the average pore diameterfrom 6.2 to around 4.6 nm. The MgAl2O4 support presented SBET

and Vpore values of 130 m2 g�1 and 0.2 cm3 g�1, respectively.Impregnating the support with cobalt and copper followed by cal-cination (sample Co12Cu1, for example) decreased both the SBET andVpore values, to around 113 m2 g�1 and 0.13 cm3 g�1, respectively.Nevertheless, the SBET per gram of MgAl2O4 remained constant ataround 129 m2 g�1

MgAl2O4 after impregnation, indicating that thepores of the support had not been obstructed.

The ex situ XRD patterns obtained for the MgAl2O4 support andthe calcined Co12 and Co12Cu3 samples are shown in Fig. 1. Thesupport presented diffraction peaks at 31.3, 36.9, 44.9, 55.8, 59.5,and 65.4�, corresponding to lattice planes (220), (311), (400),(422), (511), and (440) of the spinel MgAl2O4 structure (ICSD col-lection code 22354). In the spinel structure, the oxygen anionsadopt a cubic close-packed (ccp) arrangement, and the magnesiumand aluminum ions occupy the tetrahedral and octahedral holes inthe ccp structure, respectively [79,80]. Since in the calcined cata-lysts cobalt was in the form of spinel Co3O4 (one tetrahedral Co2+

ion and two octahedral Co3+ ions [81]), the diffraction peaks ofthe calcined Co12 sample overlapped those of MgAl2O4. Peaks re-lated to CuO tenorite structure were not identified in the XRD pat-terns. Moreover, when copper was added to the calcined samples,the peaks corresponding to the spinel structure showed a decreasein full width at half maximum. This suggests an increase in the (Co,Cu)Co2O4 crystallite size. The average apparent diameters calcu-lated from Scherrer’s equation (dScherrer) are given in Table 1.

3.2. Temperature-resolved XRD

Fig. 2 presents the temperature-resolved XRD patterns obtainedfor the calcined Co12 and Co12Cu3 samples heated in a H2/He mix-ture (5 vol.%). The XRD patterns clearly demonstrated that cobaltpassed through a two-step reduction process (Co3O4 ?

Table 1Textural properties of calcined MgAl2O4-supported cobalt catalysts promoted with copper

Sample Co loading (wt.%) Cu loading (wt.%) SBET (m

MgAl2O4 – – 130Co6 6.0 0.0 –Co12 12.0 0.0 112Co12Cu0.5 12.0 0.5 113Co12Cu1 12.0 1.0 114Co12Cu3 12.0 3.0 133

a Calculated from the adsorption isotherm using the BJH method.b Calculated from the adsorption isotherm using the BET method.c Determined by applying the Scherrer equation to the peak corresponding to lattice

CoO ? Co0), as previously described [10,82]. The results indicatedthat copper promoted the reduction of cobalt oxides in both reduc-tion steps. Spinel Co3O4 started to be reduced to CoO at 580 and510 K for the unpromoted and promoted samples, respectively.The XRD patterns of the reduced Co12 and Co12Cu3 catalysts indi-cated the existence of a face-centered cubic (fcc) crystal structure.The occurrence of this type of crystal structure is revealed by thepresence of a peak corresponding to lattice plane (111) of Co0-fcc (ICSD collection code 52934), at around 43.9�. In the XRD pat-tern of Co0-hcp (ICSD collection code 52935), a much lower inten-sity peak at 44.3� is expected, together with a second peak ofsimilar intensity at 41.6�. However, neither peak was observed inthe XRD patterns of the catalysts. These results were in agreementwith the literature [83] and demonstrated that Co0-hcp was stableup to approximately 693 K, after which the fcc phase became morestable. The hcp ? fcc phase change is reversible and influenced bythe thermal history of the sample, considering the heating/coolingrates and the number of transformation cycles [84,85].

3.3. Temperature-resolved XANES-H2

The XANES spectra fitting procedure is illustrated in Fig. S1-A(Supplementary material), which shows the in situ temperature-resolved XANES-H2 spectrum of the Co12 sample, together withthe corresponding fittings. The dynamics of the XANES spectra dur-ing the reduction process are also represented by a three-dimen-sional plot in Fig. S1-B and by a contour plot in Fig. S1-C. In bothplots, X-ray absorption increases from the areas in black and pur-ple to the area in dark red. Linear combination fittings were per-formed with spinel Co3O4, CoO-fcc, and Co0-hcp references. The

.

2 g�1) Vporea (cm3 g�1) dpore

b (nm) dScherrerc (nm)

0.20 6.2 –– –0.13 4.7 120.13 4.6 –0.13 4.6 –0.16 4.6 18

plane (311) of Co3O4.

(A) Co12(B) Co12Cu3

Fig. 2. Temperature-resolved XRD patterns of (A) Co12 and (B) Co12Cu3 catalysts during reduction. Heating rate: 10 K min�1; carrier flow: H2/He (5 vol.%). XRD patterns ofCo3O4, CoO, Co0-hcp, and Co0-fcc taken from the Inorganic Crystal Structure Database [collection codes 24210, 28505, 52935, and 52934, respectively].


CoAl2O4 reference was also used, but the fittings did not show theexistence of a significant amount of this phase in the catalysts (notshown). It was therefore not considered in the final results illus-trated in Fig. S1.

The influence of copper on the dynamics of phase transforma-tion during reduction of the Co12Cux catalysts is shown in Fig. 3.The left-hand column of Fig. 3 shows the contour plots correspond-ing to the XANES-H2 spectra, and the right-hand column of thesame figure shows their fitting from the Co3O4, CoO-fcc, and Co0-hcp references. Fig. 4 shows the temperature-resolved XANES-H2

spectra and fitting results obtained at the Cu K edge of the Co12Cu3

sample during the reduction procedure described above.The Co12 sample produced a spectrum with an intense white

line that was characteristic of spinel Co3O4 species [86]. The XANESprofile indicated the formation of CoO and subsequently Co0, as al-ready demonstrated in the temperature-resolved XRD experiments(Fig. 2A). Reduction of Co3O4 to CoO started at 580 K (Fig. 3A), withmaximum percentage of CoO (87%) being achieved at 680 K. Themaximum rate of transformation of Co3O4 to CoO occurred at theinflection point of the CoO percentage curve, at 637 K (Fig. 3A).Impregnation with 0.5, 1, and 3 wt.% of copper decreased theinflection point temperature to 616, 600, and 538 K, respectively.Similarly, the maximum percentages of CoO occurred at 659,640, and 607 K, respectively. After 20 min at 1023 K under a flowof H2/He (5 vol.%), the catalysts with 3 and 1 wt.% of copper werecompletely reduced, and the catalysts with 0.5 and 0 wt.% ofcopper were 93% and 87% reduced, respectively. In the case ofthe Co12Cu3 sample, reduction of the Cu2+ species to Cu0 startedto occur at around 490 K, so that at 510 K the sample presented28% of Cu0 species, while at 673 K these species were completelyreduced (Fig. 4). It is known that H2 adsorbs dissociatively on aCu0 surface [87–89], with very low activation barriers [90]. It istherefore plausible that H2 was activated on the Cu0 surface andthat the hydrogen atoms were transferred to the cobalt oxidethrough the spillover effect, thus facilitating the reduction of cobalt

[57,57,91]. The presence of 28% of Cu0 in the Co12Cu3 sample at510 K can then explain the decrease in the temperature requiredfor reduction of the Co3O4 species to CoO, compared to the Co12

sample.

3.4. Extended X-ray absorption fine structure

The normalized absorption, changes in k2-weighted EXAFS data,and magnitude of the Fourier transform of each sample, togetherwith the corresponding fits, are shown in Fig. 5 (Co K edge) andFig. 6 (Cu K edge). The results of the fitting analysis in terms ofthe coordination number (N), average distance (R), structuralDebye–Waller factor (Dr2

S ), and edge-energy correction (DE0) forthe references and samples are summarized in Table 4 (Co K edge)and Table 5 (Cu K edge).

As previously demonstrated from the XANES analysis (Fig. 3),the samples were not fully reduced when the reduction was per-formed at 823 K (Fig. 5). There were two main differences betweenthe absorption spectra of the samples obtained at the Co K edge: (i)the white line intensity decreased with increasing copper loading,and (ii) the intensity of the pre-edge peak at 7709 eV increasedsubstantially with increasing copper loading. This demonstratedthat the degree of reduction of the samples was proportional tothe copper loading. In addition, the first reduction step (Co3O4 ?CoO) was not completed in the case of sample Co12, with approxi-mately 8 wt.% of the cobalt remaining in the form of Co3O4 (notshown). In the case of the Co12Cu3 sample, there was 95% reductionof copper, and the copper atoms were present in the form of the fccstructure (Fig. 6). This structure is characterized by two features(points A and B in Fig. 6) formed after the white line in the absorp-tion spectra of both the sample and the Cu0-fcc reference [54,92].Moreover, it is known that fcc structures present a multiple scat-tering effect that enhances the signal of the fourth shell of neigh-bors (see the magnitude of the Fourier transform in Fig. 6). Thiseffect is due to the existence of an intermediate atom (of the

(A)

Co 1

2(B

) C

o 12C

u 0.5

(C)

Co 1

2Cu 1

(D)

Co 1

2Cu 3

Fig. 3. In situ temperature-resolved XANES at the Co K edge of samples (A) Co12, (B) Co12Cu0.5, (C) Co12Cu1, and (D) Co12Cu3 during reduction in a H2/He mixture (5 vol.%). Left-hand column: contour plots of XANES spectra. Right-hand column: linear combination fitting of sample spectra with those of spinel Co3O4, CoO-fcc, and Co0-hcp references.The intensity of X-ray absorption increases toward the area in dark red and decreases toward the areas in purple and black. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)


Fig. 4. In situ temperature-resolved XANES at the Cu K edge of sample Co12Cu3 during reduction in a H2/He mixture (5 vol.%). (A) XANES spectra. (B) Linear combinationfitting of sample spectra with those of monoclinic CuO and Cu-fcc references.


second shell), situated between the absorber atom and the atom ofthe fourth shell [93]. These two characteristics can be used to dis-tinguish between fcc and hcp structures.

The values of the structural Debye–Waller factor (Dr2S ), ob-

tained from the fits of the samples reduced at 823 K, were in therange 9.6 � 10�3 to 15.4 � 10�3 Å2 (Table 2), in agreement withthe range of values reported in the literature for Co–Cu mixturesobtained at room temperature [77,94,95]. However, the mostinteresting results were those related to the coordination numberor number of first neighbors (N) and to the average distance be-tween first neighbors (R). The number of first neighbors of cobaltrelative to Co–Co scattering in the metal (NCo–Co) was directly pro-portional to the copper loading (Table 2). This proportionality wasdue to an increase in the degree of reduction of cobalt. On the otherhand, the numbers of first neighbors of cobalt relative to Co–O(NCo–O) and Co–Co scattering in CoO showed the opposite behavior:the value of NCo–O decreased as the copper loading increased, indi-cating that the reduced fraction of the cobalt nanoparticles in-creased proportionally to the copper content. Concerning theaverage distance between first neighbors relative to Co–Co scatter-ing in the metal, the samples showed Co–Co bond lengths (RCo–Co)close to the value of the reference, with RCo–Co = 2.4925 Å. Thisbehavior was also observed in the fitting results for the oxidizedfraction of the samples.

The fitting results of spectra obtained at the Cu K edge of theCo12Cu3 sample (Table 4) showed that metallic copper presentedapproximately 8.5 atoms in its neighborhood (for the sample re-duced at 1073 K). More important, the average distance betweenfirst neighbors (RCu–Cu = 2.499 Å) was small compared to the Cu foil(RCu–Cu = 2.543 Å), but equivalent to the distance obtained for thesame sample at the Co K edge, with RCo–Co = 2.496 Å (Table 2). SinceCu and Co scattering phases and amplitudes are similar due to theclose atomic numbers, it is not possible to distinguish the nature ofthe backscattered atom. This similarity can be explained by consid-ering that copper atoms replaced cobalt atoms in a cobalt matrix.In this situation, the expected distance between first neighborswould be the distance between two atoms of cobalt in an fcc ma-trix (as determined from the in situ XRD results) and not the dis-tance between two atoms of copper. This indicates that the

copper and cobalt atoms were sufficiently close to each other thatthe scattering of an electron in the absorber copper atom couldreach a cobalt atom located in its surroundings. Similar evidenceof the substitution of Cu atoms by Co atoms in a matrix of Cucan be found in the literature [53].

In order to determine whether the crystalline structure of theCo matrix was actually fcc, experiments were performed withreduction of the samples at 1023 K for 2 h followed by acquisi-tion of EXAFS spectra at room temperature (see the EXAFS datain Fig. 5 and the fitting results in Table 3). The normalizedXANES spectra showed that the Co12 and Co12Cu3 samples wereapproximately 94% reduced at 1073 K. The same two featuresthat characterized the fcc structure in the Cu0 reference (featuresA and B in Fig. 6) also appeared after the white line of theXANES spectra at the Co K edge of samples Co12 and Co12Cu3

(Fig. 5). Fourier transform of the absorption spectra of thesetwo samples also revealed an enhancement of the signal of thefourth shell of neighbors, which is characteristic of scatteringin fcc structures. Hence, together with the temperature-resolvedXRD results (Fig. 2), the EXAFS spectra obtained after reducingthe samples at 1023 K showed that Co and Cu atoms formedan alloy with fcc structure.

EXAFS spectra were also obtained during SRE (H2O/ethanolmolar ratio of 3) at 823 K after reducing the samples at 1023 Kfor 2 h. Table 3 provides the fitting results for EXAFS spectra ac-quired at 823 K in SRE with the Co12 and Co12Cu3 samples thathad been previously reduced at 1023 K. Table 4 provides the fit-ting results of EXAFS spectra acquired at the Cu K edge of theCo12Cu3 sample under the same experimental conditions. Com-paring the metal coordination numbers, NCo–Co (Table 3) andNCu–Cu (Table 4), and the distances between first neighbors,RCo–Co (Table 3) and RCu–Cu (Table 4), for reduced samples andthose submitted to SRE, it can be observed that both NCo–Co andNCu–Cu increased, while both RCo–Co and RCu–Cu decreased, whenthe reduced samples were submitted to SRE. Interestingly,although the coordination numbers of the metals (NCo–Co andNCu–Cu) showed only a slight change after SRE, the coordinationnumbers of the oxides (NCo–O and NCu–O) increased after SRE.For example, when the reduced Co12 sample was submitted to

Fig. 5. EXAFS results at the Co K edge of samples Co12, Co12Cu0.5, Co12Cu1, and Co12Cu3. The spectra were obtained at room temperature after reduction at 823 K for 2 h (bluelines), at room temperature after reduction at 1023 K for 2 h (black lines), and during SRE (H2O/ethanol = 3) at 823 K after reduction at 1023 K for 2 h (red lines). (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


SRE, NCo–Co increased from 9.4 (±0.1) to 10.1 (±0.2), and NCo–O in-creased from 0.7 (±0.1) to 1.3 (±0.3). These results indicate thatthe degree of oxidation of Co and Cu increased when the feedcomposition was changed from H2 to SRE.

3.5. X-ray photoelectron spectroscopy

XPS analyses were performed on the calcined Co12 and Co12Cu3

samples and after their reduction under a flow of H2 at 1023 K for

Table 2Results of EXAFS fitting for spectra acquired at the Co K edge of samples reduced at 823 K for 2 h (spectra acquired at room temperature).

Sample N R (Å) Dr2 (�10�3 Å2) DE0 (eV) R factor

Co–Co in first coordination shell of Co0-hcpCo0-hcp 12 2.4925 ± 0.0003 6.2 ± 0.1 7.5 ± 0.1 0.000045Co12 7.7 ± 0.2 2.498 ± 0.002 10.3 ± 0.2 7.5a 0.001412Co12Cu0.5 9.3 ± 0.2 2.496 ± 0.001 9.6 ± 0.2 7.5a 0.000744Co12Cu1 10.0 ± 0.2 2.498 ± 0.002 9.6 ± 0.2 7.5a 0.000810Co12Cu3 10.8 ± 0.3 2.496 ± 0.002 9.6 ± 0.2 7.5a 0.000791

Co–O in first coordination shell of CoO-fccCoO-fcc 6 2.112 ± 0.002 13.5 ± 0.9 11.5 ± 0.2 0.000759Co12 4.1 ± 0.1 2.144 ± 0.004 10.3 ± 0.2 11.5a 0.001412Co12Cu0.5 3.0 ± 0.1 2.143 ± 0.005 9.6 ± 0.2 11.5a 0.000744Co12Cu1 2.6 ± 0.2 2.144 ± 0.006 9.6 ± 0.2 11.5a 0.000810Co12Cu3 2.1 ± 0.2 2.13 ± 0.01 9.6 ± 0.2 11.5a 0.000791

Co–Co in first coordination shell of CoO-fccCoO-fcc 12 3.0216 ± 0.0007 9.1 ± 0.2 11.5 ± 0.2 0.000759Co12 3.5 ± 0.2 3.079 ± 0.005 10.3 ± 0.2 11.5a 0.001412Co12Cu0.5 2.2 ± 0.2 3.089 ± 0.007 9.6 ± 0.2 11.5a 0.000744Co12Cu1 1.5 ± 0.3 3.08 ± 0.01 9.6 ± 0.2 11.5a 0.000810Co12Cu3 1.1 ± 0.3 3.05 ± 0.02 9.6 ± 0.2 11.5a 0.000791

a The DE0 values of the samples were set equal to the values fitted for the references.

Table 3Results of EXAFS fitting for spectra acquired at the Co K edge of samples either reduced at 1023 K for 2 h, with spectra acquired at room temperature (Red), or submitted to SREconditions (H2O/ethanol molar ratio = 3, 823 K), with spectra acquired in situ during SRE (SRE).

Sample N R (Å) Dr2S (�10�3 Å2) Dr2

T (�10�3 Å2) DE0 (eV) R factor

Co–Co in first coordination shell of Co0-hcpCo0-hcp 12 2.4925 ± 0.0003 6.2 ± 0.1 – 7.5 ± 0.1 0.000045Co12 Red 9.4 ± 0.1 2.492 ± 0.001 7.0 ± 0.1 – 7.5 a 0.000114

SRE 10.1 ± 0.2 2.479 ± 0.002 7.0b 10.0 ± 0.3 5.1 ± 0.4 0.001502Co12Cu3 Red 11.1 ± 0.1 2.4932 ± 0.0008 7.0 ± 0.1 – 7.5 a 0.000052

SRE 10.5 ± 0.5 2.475 ± 0.004 7.0b 10.5 ± 0.6 4.9 ± 0.5 0.005778

Co–O in first coordination shell of CoO-fccCoO-fcc 6 2.112 ± 0.002 13.5 ± 0.9 10.1 ± 0.4 11.5 ± 0.2 0.000045Co12 Red 0.7 ± 0.1 2.02 ± 0.02 7.0 ± 0.1 – 11.5a 0.000114

SRE 1.3 ± 0.3 2.09 ± 0.03 7.0b 23.5c 11.5a 0.001502Co12Cu3 Red 0.3 ± 0.1 1.97 ± 0.04 7.0 ± 0.1 – 11.5a 0.000052

SRE 0.9 ± 0.6 2.12 ± 0.08 7.0b 24.7c 11.5a 0.005778

Co–Co in first coordination shell of CoO-fccCoO-fcc 12 3.0216 ± 0.0007 9.1 ± 0.2 10.1 ± 0.4 11.5 ± 0.2 0.000045Co12 Red 0.6 ± 0.2 3.11 ± 0.03 7.0 ± 0.1 – 11.5a 0.000114

SRE 0.1 ± 0.5 3.1 ± 0.5 7.0b 10.1 ± 0.4 11.5a 0.001502Co12Cu3

A Red 0.3 ± 0.2 3.10 ± 0.06 7.0 ± 0.1 – 11.5a 0.000052

a DE0 values of samples reduced at 1023 K and during SRE were set equal to the values fitted for the references. The only exceptions are the DE0 values corresponding toCo–Co scattering in Co0-hcp during SRE, which were allowed to float.

b The Dr2S values during SRE at 823 K were set equal to the fitting results of the spectra obtained after reducing the same samples at 1023 K.

c The thermal Debye–Waller factor for the Co–O scattering during SRE was considered to be 2.34 times the thermal Debye–Waller factor for the Co–Co scattering.A The coordination number corresponding to Co–Co scattering in CoO-fcc of sample Co12Cu3 during SRE was not significant. Thus, this path was not considered in the fitting

procedure.


2 h. The Co 2p photoelectron peaks are shown in Fig. 7, and the Cu2p and Cu LMM photoelectron peaks are shown in Fig. 8. The fittingresults are summarized in Table 5. The Cu 2p3/2 peak at 934 eV, andthe presence of a satellite peak between the Cu 2p1/2 and Cu 2p3/2

peaks, indicated the presence of Cu2+ species in the calcined sam-ples [55]. The satellite peak disappeared after reduction, which ischaracteristic of the presence of Cu+ or Cu0 species. However, theAuger peak at 567.1 eV confirmed the presence of only Cu0 speciesafter reduction.

Since the molar fraction of cobalt and the Co/(Mg + Al) molar ra-tio decreased after reducing the samples, it appears that the cobaltnanoparticles may have agglomerated or have been covered by thesupport. On the other hand, both the molar fraction of copper andthe Cu/Co molar ratio increased significantly after reduction. This isindicative of the migration of copper atoms to the surface of the

crystallites. It is therefore reasonable to suppose that the reducedparticles formed a core–shell structure, with the copper concentra-tion increasing toward the surface. Analogously to copper, the lit-erature [44] reports similar behavior for platinum, whichsegregates in the top surface layers of cobalt crystallites under ahydrogen atmosphere. Another important aspect of the XPS fittingresults concerns the concentration of surface oxygen. After reduc-tion, the sample promoted with copper had approximately 4% lessoxygen than the unpromoted sample.

3.6. Determination of nanoparticle diameter and structure

Based on previous reports and the results obtained from the EX-AFS and XPS analyses, a simple but well-grounded representationof the Co12Cux nanoparticles was elaborated. Considering that (i)

Table 4Results of EXAFS fitting for spectra acquired at the Cu K edge of sample Co12Cu3, (i) reduced at 1023 K for 2 h, with spectra acquired at room temperature (Red), and (ii) during SRE(H2O/ethanol = 3) at 823 K after reduction at 1023 K for 2 h.

Sample N R (Å) Dr2s (�10�3 Å2) Dr2

T (�10�3 Å2) DE0 (eV) R factor

Cu–Cu in first coordination shell of Cu0-fccCu0-fcc 12 2.543 ± 0.001 8.9 ± 0.1 – 4.5 ± 0.2 0.000148Red 8.0 ± 0.2 2.499 ± 0.001 11.3 ± 0.2 – 2.6 ± 0.2 0.000774SRE 8.9 ± 0.2 2.457 ± 0.002 11.3a 9.8 ± 0.3 �1.6 ± 0.2 0.002356

Cu–O in first coordination shell of CuO-monoclinicCuO-monoclinic 4 1.947 ± 0.002 3.9 ± 1.2 – �1.4 ± 0.3 0.001905Red 0.7 ± 0.1 1.84 ± 0.02 11.3 ± 0.2 – �1.4c 0.000774SRE 1.5 ± 0.3 2.01 ± 0.02 11.3a 24.3b �1.4c 0.002356

a The Dr2S value during SRE at 823 K was set equal to the fitting result of the spectrum obtained after reducing the same sample at 1023 K.

b The thermal Debye–Waller factor for the Co–O scattering was considered to be 2.48 times the thermal Debye–Waller factor for the Co–Co scattering.c The DE0 values of the samples were set equal to the values fitted for the references.

Table 5Results of XPS fitting for the Co 2p3/2 and Cu 2p3/2 photoelectron peaks of the Co12 and Co12Cu3 samples, after calcination (Cal.) and after reduction at 1023 K for 2 h (Red.).

Sample Binding energy (eV) Molar fraction (%) Molar ratio

Co 2p3/2 Cu 2p3/2 O 1s Co O Al Mg Cu Co/(Mg + Al) Cu/(Mg + Al) Cu/Co

Co12 (Cal.) 780.4 – 531.1 3.8 59.0 23.9 13.3 – 0.102 – –(Red.) 777.5 – 530.8 2.0 52.2 28.0 17.7 – 0.044 – –

Co12Cu3 (Cal.) 780.2 933.4 531.1 4.5 60.8 21.7 12.0 1.0 0.133 0.030 0.222(Red.) 777.4 931.6 530.7 2.6 48.8 29.1 15.8 3.7 0.058 0.082 1.243

Fig. 6. EXAFS results at the Cu K edge of sample Co12Cu3. The spectra were obtained at room temperature after reduction at 823 K for 2 h (blue lines), and during SRE (H2O/ethanol = 3) at 823 K after reduction at 1023 K for 2 h (red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis article.)


the reduction of transition metal nanoparticles occurs primarily inthe core and continues toward the surface [37–40], and (ii) the par-tially reduced nanoparticles form a core–shell type structure,where the core is predominantly reduced and the shell is oxidized[39], the nanoparticles could be considered to be spheres with re-duced cores and oxidized shells.

The core diameter was estimated from the average number offirst neighbors of Co in the nanoparticle core, comprising reducedatoms in a face-centered cubic structure. The average number offirst neighbors of Co in the core (Ncore) is given by

Ncore ¼NCo—Co

NbCoð1Þ

where NCo–Co is the sum of the first nearest neighbors calculatedfor each atom in the core, according to its radial position, and NbCo

is the total number of Co atoms in the core. Fig. 9 illustrates theaverage number of first neighbors of Co in an fcc spherical core,according to core diameter, for the EXAFS fitting results given inTable 2. When compared to the method proposed by Calvinet al. [96,97], the approach used in this work was able to provideaccurate results, although the accuracy decreased for smallerparticles due to the proximity to the coordination number of thesurface.

As can be seen in Fig. 9, the core of nanoparticles of sample Co12

presented an average diameter of approximately 1.0 nm. There-fore, the oxidized shell of these nanoparticles started at a diameter

Fig. 7. Co 2p photoelectron peaks of Co12 and Co12Cu3 samples, after calcination(cal.) and after reduction at 1023 K for 2 h (red.).


of 1.0 nm, and the thickness of this shell could be estimated fromthe average number of Co–O bonds (Nshell), given by:

Nshell ¼NCo—O

NbCoð2Þ

where NCo–O is the sum of the first Co–O nearest neighbors calcu-lated for each Co atom in the shell, according to its radial posi-tion. The same analysis was performed for the other samples,which presented core diameters of about 1.6 nm (sample Co12-

Cu0.5), 2.3 nm (sample Co12Cu1), and 3.8 nm (sample Co12Cu3).Fig. 10 shows the average number of first neighbors of Co (Oatoms) in an fcc spherical shell, as a function of shell thickness,for the EXAFS fitting results given in Table 2. In order to simplifythe specific correlation calculations, the shell was hypotheticallyconsidered to be separated from the core, so that the atoms ofthe core did not backscatter the electrons scattered by the atomsof the shell. It is noteworthy that only one correlation wasneeded to represent the shell thicknesses of all samples. As ex-pected, the depth of the shell was inversely proportional to thediameter of the core.

The scheme shown in Fig. 10 also considers (iii) that the cobaltand copper atoms formed an alloy with fcc structure after reduc-tion, and (iv) that the copper atoms migrated to the surface ofthe crystallites in a reducing atmosphere such as obtained in SREat 823 K. In fact, the copper atoms formed part of the fcc latticein the surface of the crystallites. These atoms covered almost theentire surface of the Co12Cu3 sample, as a result of which therecould have been significant surface effects for the reformingspecies.

3.7. Catalytic tests

The distributions of products and ethanol conversion in SREwith low (H2O/ethanol = 3) and high (H2O/ethanol = 12) molarratios, as a function of reaction temperature, are shown in Fig. 11for samples Co12 and Co12Cu3. For the Co12 sample with

H2O/ethanol = 3 (Fig. 11A), ethanol conversion began at approxi-mately 623 K. In the lower temperature region, ethanol was firstdehydrogenated to acetaldehyde. Increasing the temperaturecaused cleavage of the C–C bond, resulting in the formation ofCH4, CO, and H2, with a CO/CH4 ratio that exceeded unity, evenbelow 610 K. Formation of these products in SRE indicated thatethanol decomposition followed the reaction pathway previouslyreported by Ribeiro et al. [7] for Co/SiO2 catalysts.

When the Co12 sample was used, there were three main effectsof changing the H2O/ethanol molar ratio from 3 to 12 (Figs. 11Aand B, respectively). Firstly, the temperature required for dehydro-genation of ethanol decreased from about 623 to 573 K, which sug-gests that increasing the H2O/ethanol molar ratio changed the Co0/Co2+ ratio. As previously reported by Martono and Vohs [98], bothCo0 and Co2+ species are active for the partial oxidation of adsorbedethoxide groups to produce acetaldehyde. Secondly, both the selec-tivity to H2 and the CO2/CO ratio increased at higher temperatures,suggesting a change in the water–gas shift reaction (reaction 2).Thirdly, formation of ethylene decreased when the H2O/ethanol ra-tio was increased. Since ethylene is mainly produced from thedehydration of ethanol over acidic sites [26,99], excess water chan-ged the acidic properties of the support and the selectivity toethylene.

COþH2OðgÞ¢ CO2 þH2 DH0298 ¼ �41 kJ mol�1 ðR2Þ

The main differences between samples Co12 (Fig. 11A and B)and Co12Cu3 (Fig. 11C and D) were the following: (i) the temper-ature at which acetaldehyde was desorbed, (ii) the amount ofacetaldehyde produced, and (iii) the temperature at which COwas formed. The presence of Cu increases the activity fordehydrogenation of ethanol at low temperatures [29,62–65],which resulted in large amounts of acetaldehyde for sampleCo12Cu3. Interestingly, CO was formed at a low temperature ofaround 523 K when the Cu-containing catalyst was used. On theother hand, CH4 and CO2 were formed at higher temperatures,compared to CO, and ethanol conversion decreased at around723 K, indicating that species formed after cleavage of the C–Cbond accumulated on the surface, which probably caused somedeactivation.

The catalysts were also tested for their resistance to carbonaccumulation during SRE at 823 K with a high H2O/ethanol molarratio. The rate of carbon accumulation (rcarbon,SRE) was determinedby TG analysis. Fig. 12 shows the linear correlation betweenrcarbon,SRE and (A) the number of first neighbors of Co0 in the core,and (B) the number of first neighbors of Co in the shell of theCo12Cux nanoparticles (estimated from the fitting results for theEXAFS data, summarized in Table 2).

The correlations illustrated in Fig. 12 show the abrupt in-crease in rcarbon,SRE with successive addition of Cu. However,since the EXAFS and XPS fitting results (Tables 2–5) demon-strated that after reduction, the Cu atoms were mainly concen-trated in the surfaces of the nanoparticles, it is clear that thesurface electronic properties of samples promoted with Cu weredifferent to those of the unpromoted samples. Hence, the linearcorrelation shown in Fig. 12A is an apparent correlation. Thismeans that the rate of carbon accumulation was in fact propor-tional to the diameter of the nanoparticles and to the degree ofreduction of cobalt. However, even more important were thechanges in the surface electronic properties caused by formationof the Co–Cu alloy. The surfaces of nanoparticles with higherloadings of copper are less oxidized and consequently presentlower oxygen concentrations, which is a key factor in terms ofoxidation of deposited carbon. Hence, the concentration ofsurface reactive oxygen needed to control the accumulation ofcarbon during SRE on Co nanoparticles depends on the sizeand type of the Co alloy.

Fig. 8. (A) Cu 2p and (B) Cu LMM photoelectron peaks of Co12 and Co12Cu3 samples, after calcination (cal.) and after reduction at 1023 K for 2 h (red.).

Fig. 9. Number of first neighbors of Co in a face-centered cubic spherical coreversus core diameter for the EXAFS fitting results given in Table 2. The approachemployed by Calvin et al. [94,95] using spherical structure yielded nearly identicalmatching.

Fig. 10. Number of first neighbors of Co (O atoms) in a face-centered cubic sphericalshell versus shell thickness for the EXAFS fitting results given in Table 2.


4. Discussion

Calcined Co oxides modified with Cu and supported on MgAl2O4

present a spinel Co3O4 structure, which is reduced in two distinctsteps (Co3O4 ? CoO ? Co0). Temperature-resolved XANES experi-ments showed that the transition between these phases was fasterfor the Cu-containing catalysts, and that the reduction of Cu2+ oc-curred at a lower temperature than that of Co3+. Cu0 species caninitiate the reduction process by activation of H2 [87–89] andtransfer of atomic H to the cobalt oxides surface by the spillovermechanism. After reduction, the XAFS and in situ XRD data indi-cated that the Co and Cu atoms formed a face-centered cubic(fcc) structure. In the XANES spectra, this structure was character-

ized by two features (points A and B in Figs. 5 and 6) formed afterthe white lines in the absorption spectra of the samples. Moreover,it is known that fcc structures exhibit a multiple scattering effectthat enhances the signal of the fourth shell of neighbors (Figs. 5and 6). The occurrence of the fcc crystal structure was also indi-cated by the presence of the peak corresponding to lattice plane(111) of Co0-fcc (ICSD collection code 52934), at around 43.9� inthe in situ XRD patterns (Fig. 2).

The XAFS at the Cu K edge fitting results (Table 4) also indicatedthat the Cu and Co atoms were sufficiently close to each other thatthe scattering of an electron in the absorber Cu atom could reach aCo atom located in its surroundings. Hence, the Co and Cu atomsformed an alloy with fcc structure. However, the XPS spectra fitting

Low H2O/ethanol ratio (H2O/ethanol = 3)

High H2O/ethanol ratio (H2O/ethanol = 12)

(A) Co12 (B) Co12

(C) Co12Cu3 (D) Co12Cu3

Fig. 11. Distribution of products and ethanol conversion in SRE, as a function of temperature, for (A and B) Co12 and (C and D) Co12Cu3 catalysts after activation with H2 at823 K for 2 h. The tests were carried out using low (H2O/ethanol = 3) and high (H2O/ethanol = 12) molar ratios. (�) ethanol conversion, (s) H2, (h) CH4, (N) CO, (O) CO2, (H)ethylene, and (I) acetaldehyde.


results (Table 5) showed that the Cu/Co molar ratio on the surfaceincreased significantly after reduction (from 0.222 to 1.243), indi-cating that Cu atoms migrated to the surface of the metal crystal-lites. It is known that the component which forms a stronger bondwith the adsorbent (in this case the H atoms) generally segregateson the surface of nanoparticles [42]. Previous studies [37,38] sug-gest that reduction of transition metals occurs primarily in thenanoparticle core, continuing toward the surface and generatinga core–shell type structure in which the core is predominantly re-duced and the shell is oxidized [41]. However, although the coor-dination numbers corresponding to Co–Co (NCo–Co) and Co–O(NCo–O) depend on the oxidation potentials of the reactants, theypresented opposite characteristics, with NCo–Co decreasing andNCo–O increasing when H2 was replaced by the SRE mixture, evenat higher temperatures (823 K) (Table 3). An analogous phenome-non occurred with NCu–Cu and NCu–O (Table 4). Based on theseobservations, it is reasonable to suppose that the Co–Cu nanopar-ticles formed a core–shell structure in which the Cu concentrationincreased toward the surface, with formation of a core Co–Cu alloyand a CoO–CuO shell.

The XPS fitting results (Table 5) also indicated that, after reduc-tion, the sample promoted with Cu had approximately 4% less sur-face oxygen than the unpromoted sample. This is consistent withthe model presented in Figs. 9 and 10, which shows that nanopar-ticle size increased with increased Cu loadings. In earlier work, it

was found that the concentration of oxidized Co–Cu species hada direct correlation with the curvature of the nanoparticles,decreasing with increased nanoparticle size [34,38]. However, theEXAFS results indicated that Co was alloyed with Cu, generatinga negative shift of the center of the d-band that was greater athigher loadings of Cu [100]. The shift of reactivity between metaland oxygen was expected, with the effects of both increased nano-particle size and increased Cu loading acting to reduce the concen-tration of oxides in the core of the nanoparticles.

The Co12 catalyst showed a TPSR profile of SRE products similarto that previously reported for Co/SiO2 [7]. At low temperatures(below 500 K), ethanol was first dehydrogenated, with desorptionof acetaldehyde and H2. Above 720 K, the C–C bond was cleaved,with formation of CO and CH4. In the case of Co/SiO2 [7], CO/CH4

molar ratios lower than unity were obtained, with the valuedepending on particle size. Based on theoretical studies [101],the TPSR profile obtained in this work suggested that the abstrac-tion of H atoms from acetyl species formed ketene (�CH2–CO�) orketenyl (�CH–CO�) species, which were favored at higher tempera-tures. Comparison of the activities of the Co12 and Co12Cu3 samplesindicated that ethanol was strongly adsorbed on the unpromotedCo12 sample, blocking the active sites at temperatures below573 K and rendering the catalyst inactive (Fig. 11A and B). Thereforming products CH4, CO, and CO2 were produced at tempera-tures higher than 633 K, with CO/CH4� 1. This implies that the

Fig. 12. Rate of carbon accumulation under SRE (H2O/ethanol = 12, T = 823 K)versus (A) number of first neighbors of Co in the core, and (B) number of firstneighbors of Co–O in the shell of Co12Cux nanoparticles. The core diameter and shellthickness were estimated from Figs. 9 and 10, respectively.


products of oxidation were formed mainly by activation of H2O andoxidation of adsorbed C� species derived from the pyrolytic decom-position of strongly adsorbed intermediates. Addition of Cu re-sulted in higher activity for ethanol dehydrogenation at lowtemperatures. This reflected the change in the electronic propertiesof the metal surface due to increased occupancy of the 3d band,and resulted in easy desorption of acetaldehyde. The C–C bondwas also cleaved at lower temperatures (between 523 and 623 K)using the Cu-promoted sample, but CO was the only product des-orbed (Fig. 11C and D). Hence, considering the cleavage of interme-diate OC—CH�x species [3,4], it is possible that the CH�x speciesremained adsorbed and accumulated on the active sites, leadingto deactivation at around 723 K.

Studies of the decomposition of ethanol on Ni and Co sites havedemonstrated that C� species are formed in a pyrolytic process,with adsorption of ethanol on metal surfaces and subsequent step-wise H–C and C–C bond cleavage [7,26]. Density Functional Theory(DFT) calculations have demonstrated that the binding of C� spe-cies on Ni sites is sensitive to the particle structure and the typeof site available [102]. Nucleation of graphene preferentially occurson step-edge sites, and the growth of graphene layers is initiateddue to an energy gain when graphene islands are formed on thesurface. It has also been reported that the type of �C species formed

during decomposition of hydrocarbons depends on the size of themetal particles [103,104].

Increased Cu loadings resulted in higher coordination numbersof the first neighbors of Co (NCo–Co) and Cu (NCu–Cu) (Tables 2 and4), indicating an increase in metal particle size. An attempt wastherefore made to correlate the rate of carbon accumulation inSRE (rcarbon,SRE), using H2O/ethanol = 12 and T = 823 K, to the num-ber of first neighbors of Co0 in the core (NCo–Co) and the number offirst neighbors of Co in the shell (NCo–O) of Co12Cux nanoparticles(Fig. 12A and B, respectively). Interestingly, a linear correlationwas obtained between rcarbon,SRE and the number of first neighborsof Co–O (NCo–O) in the shell of the Co12Cux nanoparticles. It has al-ready been reported that the rate of carbon accumulation duringethanol reforming processes (SRE or ORE) decreases in line withthe oxidizing degree of Co-based catalysts [10]. Although the sizeof the nanoparticles can control the reactivity of Co with surfaceoxygen, the addition of Cu can change the electronic propertiesof the particles due to the formation of the surface Co–Cu alloy,which changes the reactivity of Co with oxygen. The presence ofoxidized species (Cu–O, Co–O) covering the reduced core is essen-tial for oxidation of the adsorbed carbon formed after activation ofethanol. Hence, the results of this work suggest that control of thenature and composition of metal particles is an important factordetermining their degree of reduction. A stable Co-supported cata-lyst for SRE requires that the steps of ethanol activation and oxida-tion of adsorbed C� are equilibrated, which may be achieved by theright interplay between size, nature, and composition of metal par-ticles. We hope that this work contributes to the goal of improvingthe stability of SRE catalysts composed of metal oxide-metal sys-tems by highlighting the effect of a second metal to the metal–me-tal oxide equilibrium.

5. Conclusions

MgAl2O4-supported cobalt catalysts promoted with copperwere investigated using a series of ex situ and in situ techniquesin order to understand the properties that control the rate of car-bon accumulation during steam reforming of ethanol. Oxidized co-balt and copper formed a (Co,Cu)CoO4 spinel-like structure onMgAl2O4, and reduction produced a Cu–Co alloy with fcc structureand copper atoms preferentially concentrated over the surface ofthe metal crystallites. The size and structure of the crystalliteswere estimated using a model based on the EXAFS data, consider-ing the nanoparticles to be spherical. In this model, the reductionof metal nanoparticles occurred in the core and proceeded towardthe surface. The partially reduced nanoparticles formed a core/shell-type structure, where the core was predominantly reducedand the shell was oxidized. The diameters of both the nanoparticlesand the reduced cores were proportional to the loading of copper.The presence of copper increased nanoparticle size, modified theelectronic properties of the outer layers of the nanoparticles, anddecreased the concentration of reactive oxygen over the surface.Interplay between metal nanoparticle size, nature, and composi-tion affects the oxidation properties of the material, which arekey to controlling carbon accumulation during SRE.

Acknowledgments

The authors gratefully acknowledge by support finance for thisresearch from CNPq (proc. 472050/2011-7) and FAPESP (proc.2011/50727-9) and the Brazilian Synchrotron Light Laboratory(LNLS) for the XAS beamline experiments (D06A-DXAS 10156)and (D04B-XAFS1). The authors thank to Clelia M. P. Marques forthe use of laboratory in DQ-UFSCar for catalysts preparation.


Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2013.07.025.

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