active sites for ethanol oxidation over sno2-supported molybdenum oxides

Applied Catalysis A: General 193 (2000) 195–202

Active sites for ethanol oxidation overSnO2-supported molybdenum oxides

Fernanda Gonçalvesa, Paulo R.S. Medeirosa, Jean G. Eonb, Lucia G. Appela,∗a Instituto Nacional de Tecnologia/MCT, Av. Venezuela 82/sala 509B, CEP 20081-310, centro, Rio de Janeiro, Brazil

b Instituto de Quımica/UFRJ-Centro de Tecnologia, bloco A, Ilha do Fundão, Rio de Janeiro, Brazil

Received 23 April 1999; received in revised form 13 September 1999; accepted 13 September 1999

Abstract

SnO2-supported molybdenum oxides with varying coverage were synthesized and used for the catalytic oxidation of ethanol.The catalysts were obtained from precipitation of SnCl4 by ammonia in the presence of (NH4)2Mo7O24 (A). Some catalystswere also prepared by impregnation of (NH4)2Mo7O24 on SnO2 (B) for comparison. It was verified that molybdenum oxidesinhibited the sintering of SnO2 crystals during calcination for preparation A, resulting in homogeneous systems with highspecific areas. The solids were characterized by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-raydiffraction (XRD). The molybdenum coverage was determined by oxygen pulses after reduction at 400◦C under hydrogen.The results indicated two structurally different superficial sites. Four-coordinated molybdates were preferentially formed onthe surface of co-precipitated catalysts at low molybdenum loading, while six-coordinated polymolybdates were obtained inother cases. Bulk MoO3 oxide was also observed at very high loading. The turnover numbers (TONs) measured for ethanoloxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These species alsogave higher selectivity to acetic acid. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:Molybdenum; Ethanol; Tin oxide; Oxidation

1. Introduction

Acetic acid is currently produced from ethanol bydehydrogenation to acetaldehyde in the gas phase andsubsequent oxidation in the liquid phase. Using tworeactors consequently increases costs. Moreover, thedehydrogenation catalysts deactivate while environ-mental problems are associated with the homogeneoussystems. Allakhverdova et al. [1,2] demonstrated thetechnical feasibility of the production of acetic acid

∗ Corresponding author. Fax:+55-21-2636-552.E-mail addresses:[email protected] (J.G. Eon), [email protected](L.G. Appel).

from ethanol in a single step using catalysts based onSn–Mo oxides in the presence of water steam. In fact,catalysts based on Sn–Mo have been pointed out asefficient for alcohol oxidation into aldehydes [3,4],unsaturated hydrocarbons [5], esters [6] and acids[1,2,7].

Sn–Mo oxides can be obtained from the impregna-tion of a molybdenum salt on pre-formed tin oxideSnO2 [3,4] or on hydrated SnO2 (stannic acids) [5].Some authors mention also the possibility of syn-thesizing the catalysts by co-precipitation [1,2] ofmolybdenum and tin salts.

The catalytic performance of these materials has notonly been correlated to the presence of molybdenum

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196 F. Gonçalves et al. / Applied Catalysis A: General 193 (2000) 195–202

cations dissolved in the crystalline structure of SnO2[1,2,5], but also more recently, to the formation of su-perficial molybdate species, which are easily reducibleand more reactive [3,4]. Ai [6] showed that the behav-ior of this system is strongly related to its acid–basecharacteristics. New studies are thus necessary tobetter characterize the active species.

In this paper, we discuss the nature of superficialsites on SnO2-supported molybdenum oxides preparedat different molybdenum loadings and compare theimpregnation and co-precipitation techniques.

2. Methodology

Two sets of catalysts were prepared. The first one(preparation A) was obtained from the precipitation ofSnO2·xH2O, by addition of ammonium hydroxide toan aqueous solution of SnCl4 and ammonium hepta-molybate (NH4)2Mo7O24 (HMA), followed by evap-oration of the solution. Four catalysts, A1, A2, A3 andA4, were synthesized with molybdenum loading of6.4, 9.2, 14.6 and 24.6 wt.%, respectively. The secondpreparation (B) was performed by wet impregnationof a solution of HMA on pre-formed SnO2. The pHof the solution was adjusted to 10 using NH4OH. Themolybdenum loading of the samples, B1, B2 and B3were, respectively, 2.9, 6.5 and 9.2 wt.%. The solidswere dried in the oven at 110◦C and calcined at 500◦Cin a muffle for 16 h. The SnO2 used in preparation Bwas obtained according to the same procedure as thatadopted in preparation A, excepting the HMA in theaqueous solution. MoO3 was obtained after calcinationat 500◦C for 5 h from a sample of commercial MoO3.

Infrared spectra of the materials were collectedwith a Magna 750-Nicolet spectrometer using waferscontaining 8 wt.% of the catalyst in KBr. X-raydiffraction (XRD) was performed with a Philipsdiffractometer, model PW 1410, operating with radia-tion Cu Ka (40 kV, 30 mA) and Ni filter. The angularrange varied from 5 to 70◦, with increments of 0.02◦and a counting time of 2.5 s per step.

UV–VIS diffuse reflectance spectra in the range of200–500 nm were recorded in a Varian, Cary 5 spec-trometer using a Harrick diffuse reflectance accessorywith Praying Mantis geometry. Barium sulfate wasused as a reference. Spectra of the catalysts wereobtained after subtraction of pure SnO2 spectrum.

A pulse technique was used to determine molybde-num coverage [8]. The catalyst (200 mg) was oxidizedfor 1 h at 500◦C under air flow and cooled to 400◦Cunder nitrogen for 30 min. The sample was thenreduced at the same temperature under a 5% H2/N2(vol%) mixture flowing at 30 ml min−1. Oxygenpulses (0.20 ml) were then performed at the sametemperature. Molybdenum coverage was obtained byassuming the simple stoichiometry Mo : O= 1. It wasverified that pure SnO2 is not reduced under the sameconditions. Due to the wide range of surface areavalues, the results are compared in terms of surfacearea.

Measurements of specific area and chemical anal-ysis were carried out through the BET technique, byusing a CG 2000 equipment and by X-ray fluorescencespectroscopy using a Philips PW 2400 equipment,respectively. For temperature programmed reduction(TPR), a 5% H2/N2 (vol%) reducing mixture wasfed at a flow rate of 20 ml s−1 and a heating rateof 10 K min−1. The catalyst mass was 25 mg. X-rayphotoelectron spectra were obtained with a Perkin–Elmer 1257 equipment, in the following conditions:anode of Mg, 200 W, 93–90 eV per step, with 100 msper step. Charge effects’ correction was carried out byconsidering the carbon peak C1s (284.6 eV).

The catalytic tests were performed in conventionalsystem with fixed bed reactor, monitored by on-linegas chromatography, with flame ionization and con-ductivity detectors. A 3% ethanol/air (vol%) mixturewas fed at a space velocity of 11520 h−1 on a catalystmass of 200 mg. No deactivation of the catalysts wasobserved during the study.

3. Results

The XRD patterns of SnO2-supported molybdenumoxides are shown in Fig. 1. The samples prepared byco-precipitation (A) are poorly crystallized and presentonly the peaks referring to SnO2 (cassiterite phase).The samples prepared by impregnation (B) presentthe SnO2 pattern with a higher degree of crystallinity.Some authors studied SnO2 for its capacity to dissolvecations such as Sb and Mo. Montgolfier et al. showed[9] by EPR the presence of Mo+5 dissolved into crys-talline SnO2. Okamoto et al. suggested [5] throughquantitative XRD and catalytic study that SnO2 could

F. Gonçalves et al. / Applied Catalysis A: General 193 (2000) 195–202 197

Fig. 1. XRD patterns of Sn–Mo samples.

dissolve Mo+6 in spite of charge unbalance. However,no modification of the XRD pattern was observed inthis work, even at high Mo concentrations. So the XRDresults do not suggest significant dissolution of Mo inthe SnO2 structure.

MoO3 peaks are also observed at high molybde-num loading in sample B3, and perhaps in sampleB2 within the limit of detection of the technique.Table 1 reports the crystal diameter calculated ac-cording to the Sherrer formula and considering thepeak at 33.8◦ (2θ ). Particle size values of about 3 and9 nm are obtained for samples A and B, respectively.However, increasing the molybdenum loading brings

Table 1Molybdenum content by chemical (Mob) and XPS (Mos) analyses,particle diameter (D) from XRD and specific area from BET

Catalyst Mob (wt.%) Mos (wt.%) D (nm) Area (m2 g−1)

MoO3 − − − 2SnO2 − − 9.0 34A1 6.4 7.4 3.8 70A2 9.2 9.1 3.2 120A3 14.6 15.1 2.3 197A4 24.6 22.0 2.3 92B1 2.9 5.6 9.4 40B2 6.5 8.5 9.1 39B3 9.2 12.6 8.6 38

Fig. 2. Infrared spectra of Sn–Mo samples.

about an important decrease in the particle size incase of preparation A. This fact is not observed in thecase of preparation B, where the particle size is equalto the value of the pre-formed support.

The values of BET specific area reported in Table1 are in agreement with the previous data. The areasrelated to the catalysts prepared by impregnation (B)are approximately equal to that of the support (SnO2).A significant increase in the specific area is observedfor catalysts prepared by co-precipitation (A). More-over, the area of these samples strongly increases withmolybdenum loading up to 15% Mo (wt.%) (sam-ple A3). Thus, the XRD and BET data suggest thatthe presence of molybdate species strongly affects thegrowth of SnO2 crystals.

The infrared spectra are displayed in Fig. 2. Sam-ples B2 and B3 present absorption bands at 993,871 and 819 cm−1, which can be attributed to thevibrational spectrum of MoO3 [4,10]. This observa-tion is in agreement with the XRD results. Anotherabsorption band at 956 cm−1 was observed in A1,A2, A3, A4 and B2. This band as well as that at993 cm−1 mentioned above are attributed to Mo=Ostretching, which has been studied extensively in theliterature, in systems involving molybdenum oxidessupported in ZrO2 [10], Al2O3 [8,11], TiO2 [8] andSiO2 [12]. All samples also present a band at about


860 cm−1; a shoulder at 905 cm−1 is seen in A3and A4.

Niwa and col. [4], applying diffuse reflectanceinfrared spectroscopy to Sn–Mo systems, and at-tributed a band at 960 cm−1 to superficial Mo speciessupported on SnO2. Ono et al. [10] studying thesystem Mo–Zr also observed the above-mentionedband, and suggested that it results from the shiftingof the Mo=O stretching band normally observed at995 cm−1. Such a shifting indicates that bond weaken-ing is associated with the decrease in the Mo contentand should contribute to a higher activity in oxidativedehydrogenation. Kasztelan et al. [11] using Ramanspectroscopy to characterize Mo/Al2O3 catalysts withintermediate molybdenum loading attributed a bandin the range of 941–947 cm−1 to heptamolybdatespecies (Mo7O24

−2). At low molybdenum loading,they reported bands at 925 and 860 cm−1, which theyassigned to Mo=O and Mo–O–Al vibration modes ofdistorted tetrahedral species MoO4

2−. Desikan et al.[8] also employing Raman spectroscopy observed thevibration band at around 960−950 cm−1 in Mo/Al2O3and Mo/TiO2 and attributed it to dispersed isopoly-molybdates and hydrated tetrahedral dioxo species.According to the authors, the same type of bond andsimilar coordination characterize these Mo speciesso that their vibrational spectra should be similar.In accordance with these observations, we attributethe band at 956 cm−1 in Fig. 2 to molybdate speciesdispersed on the surface of SnO2.

Fig. 3 shows the UV–VIS spectra of the twoseries of catalysts compared with that of MoO3. Twobroad bands varying in relative intensity at about250 and 325 nm characterize the catalysts preparedby co-precipitation (A). The bulk oxide displays acomplex band shifted to lower energy. This suggeststhe absence of bulk MoO3 in the catalysts and thepresence of at least two different molybdate species.More precise characterization of the species was at-tempted after assuming the independence of the twobands and determining the respective absorption edgeenergy following the method described by Weber[13]. The results are reported in Table 2, in additionto the edge values calculated for the heptamolybdateand orthomolybdate ions. The proximity of these ref-erence values to the edge energies obtained in thiswork suggest that four-coordinated molybdates areformed at low molybdenum loading but condense to

Fig. 3. UV spectra of Sn–Mo samples.

form six-coordinated polymolybdates with increasedmolybdenum loading. Polymolybdate species are seento predominate in the case of impregnated catalyst(B). However, the spectra are not so easy to interpreton account of the presence of bulk MoO3, as wasalready observed in XRD patterns.

Table 3 reports the molybdenum coverage anddispersion calculated from the oxygen pulse tech-nique. The high dispersion values obtained at lowermolybdenum loadings confirm the formation of super-ficial layers, discarding the hypothesis of significantmolybdenum dissolution in tin oxide. It is seen thatdispersion tends to decrease with increasing molyb-denum loading along each catalyst series. However,we note that catalysts A1, A2 and A3 present thesame dispersions and coverage values character-izing a four-coordinated monolayer. Samples A4,B2 and B3 show high coverage values, somewhat

Table 2Absorption edge energy determined from UV–VIS spectra

Catalyst Edge energy (eV)

Heptamolybdate 3.3Orthomolybdate 4.3A1 4.7; 3.3A2 4.8; 3.3A3 4.6; 3.3A4 4.7; 3.3


Table 3Superficial molybdenum coverage, dispersion measured by oxygen pulses and TON for ethanol oxidation at 150◦C

Catalyst Mo coverage (mmol m−2) Mo dispersion (%) TON (mmolmmol−1 s−1)

Four-coordinated layer [4] 6.6 − −Six-coordinated layer [4] 11.3 − −A1 7.5 79 0.0075A2 7.4 88 0.0082A3 7.3 85 0.0075A4 16.4 58 0.0008B1 10.0 110 0.0043B2 13.3 59 0.0042B3 15.6 47 0.0028

higher than that one should expect for the completesix-coordinated layer. This might be due to the pres-ence of three-dimensional species on the surface ofthe catalyst.

Fig. 4 displays the reduction profiles obtained byTPR. In contrast to pure tin and molybdenum ox-ides, the reduction of Sn–Mo catalysts begins at about200◦C; this was attributed to the reduction of superfi-cial Mo species. The reduction peaks observed at 667,648, 604 and 574◦C in samples A1, A2, A3 and A4,

Fig. 4. TPR profiles of Sn–Mo samples.

respectively, were assigned to SnO2. It is interestingto note that increasing the molybdenum loading ofthe catalyst makes it easier to reduce SnO2. This factwas attributed to the decrease in SnO2 particle size(Table 1) since dissolution of molybdenum in SnO2was ruled out on the basis of XRD patterns and datafrom the oxygen pulse technique. Peaks above 700◦Cin samples A3 and A4 were attributed to condensedspecies, that is, to MoO3 or to polymolybdates. TheTPR profiles referring to preparation B show basi-cally a small reduction at low temperatures. Reductionpeaks at above 700◦C are similar to that of the SnO2support and indicate a higher difficulty of reduction ofthese samples. This fact was attributed to the biggerparticle size of SnO2 crystals.

XPS analysis provided bond energy values equalto 232.4, 486.8 and 530.6 eV referring to Mo3d5/2Sn3d5/2 and O1s, respectively, which are character-istic of Mo+6, Sn+4 and O−2 [14]. Superficial Moconcentrations are presented in Table 1. The valuesobtained for samples prepared by precipitation (A) areapproximately equal to bulk concentrations. Superfi-cial enrichment is observed in the case of catalystsprepared by impregnation (B).

Turnover numbers for ethanol oxidation are alsopresented in Table 3. Acetaldehyde was the main prod-uct of the reaction, with a selectivity around 70%, butethyl ether, ethyl acetate and ethylene were also ob-served. Fig. 5 shows the variation of the turnover num-ber (TON) against molybdenum coverage. It is seenthat activity increases with dispersion.

Figs. 6 and 7 show the variation of selectivity versusthe reaction temperature for A2 and B1. These cat-alysts were selected because they exhibit the high-est TON for each preparation; they do not display


Fig. 5. TON of Sn–Mo samples; the symbolsh, N refer to samplesA and B, respectively.

Fig. 6. Catalytic results for sample A2. The symbols X,N, H, r,+, * refer to ethanol conversion and selectivity toward acetic acid,acetaldehyde, carbon dioxide, ethyl acetate and carbon monox-ide, respectively. Experimental conditions: 11520 h−1, 3% ethanol(mol%).

Fig. 7. Catalytic results for sample B1. The symbols X,N, H, r,+, * refer to ethanol conversion and selectivity toward acetic acid,acetaldehyde, carbon dioxide, ethyl acetate and carbon monox-ide, respectively. Experimental conditions: 11520 h−1, 3% ethanol(mol%).

bulk MoO3 and present similar coverage values. Thecurves clearly indicate that acetaldehyde is the pri-mary product; this is then oxidized to acetic acid andcarbon oxides. It is interesting to note that the se-

lectivity to acetic acid depends on the nature of thepreparation.

4. Discussion

XRD patterns and BET analyses have shown thatthe preparation method has a strong influence uponthe texture of the final SnO2-supported molybdenumoxides. As expected, the XRD pattern and specificarea are almost undisturbed after impregnation ofmolybdenum on pre-formed SnO2. Molybdenum tri-oxide, which is observed at high molybdenum loadingin these catalysts, is then concluded to grow from thesurface of the support as the molybdenum loading in-creases. Superficial Mo enrichment observed by XPSis in agreement with this model. On the other hand, thecatalysts obtained by co-precipitation only showed thepresence of the cassiterite phase, the crystal size andarea of which strongly depend on the molybdenumloading: increasing molybdenum concentration leadsto increasing tin oxide dispersion. This observationsuggests that superficial molybdate ions could inhibitsintering of the support during the calcination. Accord-ingly, catalyst A4, which bears the highest molybde-num loading, displays the same crystal size as catalystA3; we tentatively attribute the discrepancy betweenthe areas of these two solids to sintering throughcondensation of the external, octahedral molybdenumlayers between different particles during the calcina-tion of sample A4. The agreement between bulk andsuperficial molybdenum concentrations measured byXPS is a consequence of the small crystal size, whichis comparable to the electron mean free path withinthe kinetic energy range of the measure, and is thusconsistent with the model.

The results from TPR, infrared and principallyUV–VIS spectroscopy confirmed the formation ofmolybdate species with at least two different struc-tures. The oxygen pulse technique showed that agreat part of the molybdenum was located on theexternal surface of the solid, the dispersion tendingto decrease with increasing molybdenum loading.Various studies of supported molybdenum oxideshave already shown that tetrahedral Mo species areformed at low superficial coverage [8], that is, whenmolybdenum loading is lower than the theoreticalmonolayer value. The coverage values calculated for


solids A1, A2 and A3 are consistent with the hypoth-esis of the four-coordinated monolayer described inthese works. This is in apparent contradiction withUV–VIS spectra, which show the existence of poly-molybdate species, and with molybdenum dispersion,which was not complete in these three catalysts.We propose that partial condensation of molybdatespecies associated with sintering could occur duringcalcination, as commented above for sample A4. Inthis model, six-coordinated molybdates are occludedspecies. Interestingly, we observe that the coveragevalue for the latter sample (A4) slightly exceedsthe value referred to the six-coordinated monolayer.This suggests that the high heptamolybdate con-centration imposed during the precipitation of thecatalyst should induce the formation of the con-densed, six-coordinated molybdenum layer, which inturn could explain its higher tendency to sintering.Finally, the coverage values obtained for catalystsprepared by impregnation suggest the presence ofsix-coordinated layers, in agreement with the UV–VISspectra.

Ethanol oxidation was studied on these catalysts,both at low and high conversion values. The mainproduct observed at low conversion was acetaldehyde.In this case, as shown in Fig. 5, the lower the molybde-num coverage, the higher is the TON. Alcohol oxida-tive dehydrogenation catalyzed by Mo is known to be astructure-sensitive reaction [15–17], the rate of whichis determined by the decomposition step of adsorbedethoxide through ana-hydrogen abstraction, probablythrough hydride transfer. Accordingly, the activity ofthe catalyst was suggested to depend upon the avail-ability of empty electronic states in the intermediatesurface complex, which has been correlated with thereducibility of the support [15]. It was found thatthe reactivity follows the order TiO2 > Al2O3 > SiO2.The TONs displayed in Table 3 are in agreementwith these observations. Indeed, the highly reducibleSnO2 support provides catalysts up to four timesmore active than those obtained with these supports.However, it is worth noting that the differences inactivity with the molybdenum loading in series Ado not correlate with the reducibility of SnO2 fromTPR profiles (see sample A4 for example). Clearly,TPR profiles that correspond to bulk reduction in-volve nucleation and growth of a reduced phase, andthus, cannot be used to compare electron accepting

characters. On the other hand, the low precision ofthe measure does not permit a clear comparison ofthe profiles of the different catalysts in the low tem-perature range, where the reduction was attributed tosurface species.

It then appears that molybdenum oxide is moreactive in the dispersed, four-coordinated layer thanit is in the six-coordinated layer. In accordance withthe above-mentioned argument, this suggests that thehigher dispersion on a highly reducible support in thecase of samples A1, A2 and A3 leads to more activecatalysts by optimizing the interaction with the sup-port. The decrease in the coordination number from6 to 4, which is another consequence of the higherdispersion, may not be responsible for the increase inactivity.

Figs. 6 and 7 show that it is necessary to attaincomplete conversion of ethanol to produce acetic acidand carbon dioxide from acetaldehyde. Selectivity toacetic acid then presents a maximum at 270◦C for thetwo catalysts A2 and B1. However, the selectivity ofA2 is about twice that of B1, suggesting that dispersed,four-coordinated species are also more selective thansix-coordinated ones.

5. Conclusion

SnO2-supported molybdenum oxides were obtainedby co-precipitation and impregnation methods. Twoseries of catalysts were prepared with various molyb-denum loadings. It was shown that the presence ofmolybdenum oxides inhibits crystal growth of theSnO2 support, leading to material with a high spe-cific area. In this case, four-coordinated species aredispersed on the external surface of the solid af-ter calcination. A model was proposed, accordingto which six-coordinated species are occluded afterpartial sintering through coalescence of molybdatelayers. Molybdenum impregnation on the pre-formedSnO2 support yields six-coordinated species andmolybdenum trioxide at higher loading. The catalystswere studied in the reaction of oxidative dehydro-genation of ethanol. It was found that dispersed,four-coordinated species are up to 10 times more ac-tive than six-coordinated ones, and moreover, twiceas more selective to acetic acid at high ethanolconversion. The high dispersion of molybdenum


species on the highly reducible SnO2 support wassuggested to be responsible for the exceptionalactivity of these catalysts.

Acknowledgements

We thank Ricardo Aderne, Carlos Andre A.C. Perezand Prof. Martin Schmal (NUCAT/COPPE/UFRJ, Riode Janeiro) for DRS-UV and XPS analyses. We ac-knowledge INT/MCT (Instituto Nacional de Tecnolo-gia), CNPq (Conselho Nacional de DesenvolvimentoCientıfico e Tecnológico) and FUJB (Fundação Uni-versitária José Bonifácio) for financial support.

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active sites for ethanol oxidation over sno2-supported molybdenum oxides

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