Understanding effects of activation-treatments in K-free and K-MoVSbO bronze catalysts for propane...

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ARTICLE IN PRESSG ModelATTOD-8883; No. of Pages 8

Catalysis Today xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Catalysis Today

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

nderstanding effects of activation-treatments in K-free and-MoVSbO bronze catalysts for propane partial oxidation

. Ivars-Barcelóa,∗, J.M.M. Milletb, T. Blascoa, P. Concepcióna,.S. Valentec, J.M. López Nietoa,∗

Instituto de Tecnología Química, UPV-CSIC, Campus de la Universidad Politécnica de Valencia, Avda. de los Naranjos s/n, 46022 Valencia, SpainInstitut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS—Université Claude Bernard Lyon 1, 2, Av. Albert Einstein,-69626 Villeurbanne Cedex, FranceInstituto Mexicano del Petróleo, Eje Central 152, México, D.F. 07730, México

r t i c l e i n f o

rticle history:eceived 16 December 2013ccepted 22 January 2014vailable online xxx

a b s t r a c t

The effect of activation treatments of K-free and K-MoVSbO bronzes on either their physico-chemicalcharacteristics or catalytic properties for propane partial oxidation have been studied. The as-synthesizedmaterials, hydrothermally prepared and presenting (SbO)2M20O56 type structure (the so called M1-phase), were activated by different heat-treatments, characterized (XRD, SEM-EDS, HRTEM, V K-edge,Sb L1- and K-edges XANES, EPR, XPS, NH3-TPD) and tested in propane partial oxidation. In general, the

eywords:ixed metal oxides

–Mo–V–Sb–O catalystropane oxidationcrylic acid1-phase characterization

selectivity to acrylic acid (the most valuable product) was higher in K-containing MoVSbO catalysts. Inaddition, different trend in the catalytic behavior was found between K-free and K-containing MoVSbOseries, mostly related to different changes in crystalline phases distribution and catalysts surface char-acteristics (composition and acid properties) induced by the several activation treatments which alsomodified the average Sb oxidation state.

ANES

. Introduction

MoVTe(Sb)NbO mixed oxides have been proposed as the mostffective catalysts in selective (amm)oxidation of propane [1–6]nd oxidative dehydrogenation of ethane [7,8], and present asell relatively high selectivity to partial oxidation products in

he oxidation of n-butane [9]. Two crystalline phases are mainlybserved in these catalysts [1–20]: the orthorhombic (XO)2M20O56X = Te or Sb; M = Mo, V, Nb) known as M1 phase [5,20–25], and anrthorhombically distorted HTB-type phase (X2O)M6O19 (X = Te orb; M = Mo, V, Nb), named as M2 phase [25,26]. Although the M1rystalline phase is the responsible for the selective oxidation ofropane into acrylic acid, it has been suggested that the presence ofhe M2 phase has a positive synergetic effect in Te-containing mate-ials [25,27–29]. Unlike, the presence of M2 phase in Sb-containingoVO catalysts has apparently a negative effect on the yield to

Please cite this article in press as: F. Ivars-Barceló, et al., Understandingcatalysts for propane partial oxidation, Catal. Today (2014), http://dx.

crylic acid in partial oxidation of propane [28–30]. Besides therystalline phases present in MoVTe(Sb) mixed oxides, the catalyticerformance for propane partial oxidation also depends on the

∗ Corresponding authors at: Instituto de Tecnología Química, Avda. de los Naran-os, s/n, 46022 Valencia, Valencia, Spain. Tel.: +34 963877808; fax: +34 963877809.

E-mail addresses: [email protected], [email protected] (F. Ivars-Barceló),[email protected] (J.M. López Nieto).

920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2014.01.036

© 2014 Elsevier B.V. All rights reserved.

chemical composition [31,32], crystallinity [32], crystal orientation[32–36] and surface acid/base properties [14,16], among others.Therefore, the only presence of the M1 phase is not enough to getan adequate catalyst [24].

Regarding the chemical composition, the addition of promotersto the mixed metal oxide has a strong influence on their catalyticbehavior in the propane oxidation reaction. Thus, the presence ofniobium in MoVTeNbO mixed oxides is a key factor to achieve highselectivity to acrylic acid [4,5,10–12], which has been related to themodification of the acid sites [14,15] and/or to the stabilization ofthe acrylic acid formed, avoiding further oxidation [13,19]. How-ever, the presence of Nb in the corresponding Sb-containing oxideshas a less-pronounced effect on this reaction, and so other promo-ters have been investigated, finding that potassium or other alkalimetals strongly increase the selectivity to acrylic acid [16–18].Anyway, the best catalytic results, so far, are those reported forMoVTeNbO catalysts [1–5].

Besides the composition, also the activation procedure usedto transform the as-synthesized mixed metal oxides into effec-tive catalysts (usually treatment at ca. 600 ◦C upon relativelyinert conditions, preceded or not by a treatment in air at lower

effects of activation-treatments in K-free and K-MoVSbO bronzedoi.org/10.1016/j.cattod.2014.01.036

temperatures [16,37–39]) will determine the physico-chemicalproperties and catalytic performance of these materials [13,19,40].Thus, although tellurium has been mainly found as Te4+, differ-ent ratios of redox pairs Mo6+/Mo5+, V5+/V4+ and Sb3+/Sb5+ have

dx.doi.org/10.1016/j.cattod.2014.01.036


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ARTICLE IN PRESSG ModelCATTOD-8883; No. of Pages 8

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Table 1Characteristics of K-free and K-MoVSb mixed oxides.

Samplea Activation procedure SBET m2 g−1 Acidity �molNH3 m−2 (◦C)b Bulk Crystalline phasesf

Mo5+/Mototal (%)c V4+/Vtotal (%)d Sb3+/Sbtotal (%)e

f-AS Untreated n.d. n.d. n.d. n.d. n.d. M1f-T0 280 ◦C/air n.d. n.d. <1.0 91 4 M1f-T1 450 ◦C/air 30.2 n.d. 10.0 <1 <1 MoO3 » M1f-T2 280 ◦C/air; 600 ◦C/N2 14.6 45.5 (230) <1.0 98 19 M1, MoO3 > M2f-T3 600 ◦C/N2 13.2 24.3 (214) 1.1 95 >99 M1, M2f-T4 600 ◦C/N2; 280 ◦C/air 11.0 10.2 (180) <1.0 94 37 M1, M2 » MoO3

K-AS Untreated n.d. n.d. n.d. n.d. n.d. M1K-T0 280 ◦C/air n.d. n.d. 1.3 91 2 M1K-T1 450 ◦C/air 14.7 n.d. 8.9 <1 <1 MoO3

K-T2 280 ◦C/air; 600 ◦C/N2 13.4 12.3 (180) <1.0 93 19 M1, MoO3 > M2K-T3 600 ◦C/N2 8.6 9.4 (190) 4.4 96 >99 M1, M2K-T4 600 ◦C/N2; 280 ◦C/air 4.7 12.2 (180) 2.3 98 95 M1, M2, Sb2Mo10O31

a The samples after the thermal activation step presented a Mo/V/Sb/K ratio of 1.00/0.27/0.15/0.00 (f-series) or 1.00/0.29/0.11/0.02 (K-series) determined by ICP.b In parenthesis, temperature of maximum desorption during NH3-TPD experiment.c Determined by EPR, XANES and ICP.d Determined by V K-edge XANES.

n

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aaKzooa

2

2

hsaKt212aous

2

tsi

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e Determined from Sb L-edge and/or Sb K-edge XANES (Sbtotal = Sb3+ + Sb5+).f Determined by XRD.

.d.: not determined.

een revealed by XANES and/or XPS, depending on the compo-ition and the activation treatment of the MoVTe(Sb)NbO mixedxides [16,21,23,39,41]. Further, in-situ and operando studies on1-type MoVTe(Sb)NbO mixed oxide catalysts showed small and

eproducible changes in the average oxidation state of V (Te-ontaining ones) and of V and Sb simultaneously (Sb-containingnes) under reaction conditions, suggesting that those species aretrongly involved in redox and catalytic reactions, while no impor-ant variation was detected in Nb, Te or Mo cations [39,42].

In the present work we have studied the influence of differentctivating heat-treatments on the evolution of physico-chemicalnd catalytic properties of hydrothermally prepared K-free and-containing MoVSbO mixed oxide materials. Several characteri-ation techniques, as well as catalytic tests in propane selectivexidation have been employed, and correlations among the metalxidation states, nature of crystalline phases, characteristics of cat-lyst surface and catalytic behavior of these materials are discussed.

. Experimental methods

.1. Catalysts preparation

K-free and K-containing MoVSbO mixed oxides precursors wereydrothermally prepared from aqueous gels containing vanadylulfate, antimony sulfate and ammonium heptamolybdate, with

Mo/V/Sb/K atomic ratio of 1/0.25/0.15/x (x = 0 or 0.04) [16]. The-free and K-containing precursors (named f-AS and K-AS, respec-

ively) were then subjected to different heat-treatments: (T0) at80 ◦C for 1 h in air; (T1) at 450 ◦C for 1 h in air; (T2) at 280 ◦C for

h in air and then at 600 ◦C for 2 h in N2 stream; (T3) at 600 ◦C for h in N2 stream; and (T4) at 600 ◦C for 2 h in N2 stream and thent 280 ◦C for 1 h in air. The samples were named as f-Tn (K-free)r K-Tn (K-containing) in which Tn is the abbreviation previouslysed to describe the corresponding heat-treatment (T1–T4). Table 1hows the main characteristics of the prepared catalysts.

.2. Catalyst characterization

The catalysts chemical composition was determined by induc-ively coupled plasma atomic emission spectroscopy. BET specific


urface areas were measured on a Micromeritics TriStar 3000nstrument, with adsorption of N2 at −76 ◦C.

X-ray diffraction patterns (XRD) were collected using a PhilipsıPert diffractometer equipped with a graphite monochromator,

operating at 40 kV and 45 mA and employing nickel-filtered CuK�radiation (�=0.1542 nm).

Sb L1-edge and V K-edge XANES spectra of MoVSb oxides wereacquired at room temperature using a double crystal Si (3 1 1)monochromator detuned until 50% [16]. The Sb L1-edge data wereacquired using energy steps of 0.5 eV s−1 and 0.2 eV s−1 in the4650–4680 eV and the 4680–4735 eV ranges, respectively; andof 0.4 eV s−1 in the region 4735–4780 eV. The V K-edge spectrawere measured in the 5400–5600 eV range, with an energy step of3 eV s−1 in the 5400–5460 eV region, and of 0.3 eV s−1 and 1.5 eV s−1

in the 5460–5500 eV and 5500–5600 eV ranges, respectively.Sb K-edge and V K-edge XANES spectra of K-containing MoVSbO

catalysts were acquired at the BM31 beamline at the EuropeanSynchrotron Radiation Facility (ESRF) in Grenoble using a Si (1 1 1)monochromator, and were recorded in the 30,300–30,652 eV and5420–5560 eV ranges, respectively, with an energy step of 1 eV s−1.Antimony or vanadium metal foils were used as references to cal-ibrate the pre-edge absorption energy of the Sb and V spectra to30,491 eV and 5468 eV, respectively. Sb2O3, Sb2O5, FeSbO4, andVOMoO4, V2O5, MgV2O6 were used as standard patterns to calcu-late Sb and V oxidation states, respectively, as previously reportedfor V K-edge [41,42], Sb L1-edge [16,21,23], and Sb K-edge [43]XANES spectra, processing the data using the free software Dacty-loscope [44].

Electron paramagnetic resonance (EPR) spectra were collectedat −173.15 ◦C with a Bruker Elexsys spectrometer working in theX-band (9 GHz) at 100 K, using a receiver gain of 60 dB, attenua-tion of 15 dB (20 mW) and modulation amplitude of 1 G. Vanadylacetylacetonate was used as g factor standard to calibrate magneticfields. Quantitative analyses of spins were carried out by doubleintegration of the EPR spectra and using the linear range of a cali-brating curve made using copper sulphate (CuSO4·5H2O) standards[16]. Since V4+ and Mo5+ were the only paramagnetic species, thecontent of Mo5+ was determined by subtracting the amount of V4+

measured by XANES spectroscopy to the total paramagnetic speciesconcentration (Table 1).

X-ray photoelectron spectra (XPS) have been acquired at roomtemperature and base pressure of 2 × 10−10 mbar in the analysischamber, using a pass energy of 50 eV with a SPECS spectrometerequipped with a PHOIBOS 150 9 MCD detector, and monochro-


mated Al K� (h� = 1486.6 eV) X-ray radiation operating at 300 W.Binding energy of C1s (284.5 eV) has been used as internal refer-ence, and the peak intensity was estimated integrating individualcomponents obtained by subtracting a S-shape background and



F. Ivars-Barceló et al. / Catalysis Today xxx (2014) xxx–xxx 3

f-T3

f-T4

f-T2

f-T1

0 10 20 30 40 50 60

2θ 2θ

f-AS

f-T0

0 10 20 30 40 50 60

K-T3

K-T4

K-T2

K-T1

K-AS

K-T0

F SbO ca ols: (S

paam

iMauH1ta

2

lwritcwl(s

3

3

KIo

ig. 1. XRD patterns of K-free (f-series, left) and K-containing (K-series, rigth) MoVs well as the samples calcined at 280 ◦C (f-T0 and K-T0) have been included. Symb

eak fitting with a combination of Lorentzian/Gaussian lines of vari-ble proportion. Spectrometer transmission function, cross sectionnd inelastic mean free path values have been used for quantitativeeasurements with the CasaXPS software.Temperature programmed desorption of ammonia (TPD) exper-

ments were carried out with a TPD/2900 apparatus fromicromeritics. Sample (ca. 0.30 g) was pre-treated in an Ar stream

t 450 ◦C for 1 h. Ammonia was chemisorbed by pulses at 100 ◦Cntil equilibrium was reached. Then, the sample was fluxed withe stream for 15 min prior to heat up to 500 ◦C in a helium stream of00 ml min−1, using a heating rate of 10 ◦C min−1. The NH3 desorp-ion was monitored with a thermal conductivity detector (TCD) and

mass-spectrometer.

.3. Catalytic tests

The catalytic tests were implemented in a fixed bed quartz tubu-ar reactor (i.d. 20 mm; length, 400 mm) in steady-state conditions,

orking at atmospheric pressure in the 340–420 ◦C temperatureange. Both, the amount of catalyst (diluted with silicon carbiden order to keep a constant volume in the catalytic bed) and theotal flow of reactants were varied in order to achieve differentontact times. The feed consisted in a mixture of C3H8/O2/He/H2Oith a molar ratio of 4/8/58/30. Reactants and products were ana-

yzed online by gas chromatography using two packed columns:i) molecular sieve 5 A (3 m) and (ii) Porapack Q (3 m). Blank testshowed no conversion in the temperature range studied [18].

. Results

.1. Characterization of precursors and activated catalysts


Table 1 summarizes the main characteristics of the K-free and-containing materials activated with different heat-treatments.

n general, the specific surface area of the activated mixedxides decreased as the activation temperature increased, and was

atalysts. For comparison, the as-synthesized (untreated) samples (f-AS and K-AS),b2O)M6O18 (�), (SbO)2M20O56 (©), Sb2M10O31 (�), MO3 (�). M = Mo, V.

systematically lower for the K-containing samples. The NH3-TPDresults are summarized in Table 1. The number of acid sites andthe temperature of maximum desorption during NH3-TPD exper-iments decreased drastically from T2 to T4 treatment in MoVSbOcatalysts, meanwhile the activation procedure had a low influenceon acidity of K-promoted MoVSbO catalysts, with lower number ofacid sites.

Fig. 1 displays the XRD powder patterns of the samples and themain crystalline phases observed are listed in Table 1. XRD patternsof the samples as-synthesized (f-AS and K-AS) and treated in air at280 ◦C (f-T0 and K-T0) showed only characteristic peaks of the M1-phase with low crystalline ordering, although the presence of minoramounts of M2 phase and/or some amorphous compounds couldbe not completely discarded. Activation treatments above 400 ◦Cgenerally lead to more crystalline M1 phase and/or to partial trans-formation into other crystalline phases which depended on boththe activation treatment and the presence or not of K. Thus, calci-nation of K-free sample in air at 450 ◦C produced M1 [ICSD: 55097]and MoO3 [JCPDS: 05-0508] orthorhombic-type phases (sample f-T1, Fig. 1), while the same treatment on the K-containing oxidegives practically just MoO3 (K-T1, Fig. 1). All samples heated at600 ◦C in N2 (T2 to T4 series) present M1 and M2 phases (Fig. 1),while previous (f-T2 and K-T2, Fig. 1) or subsequent (f-T4 and K-T4, Fig. 1) calcination in air at 280 ◦C also give rise to MoO3 and/orSb2Mo10O31 phases.

Similar SEM images were observed for as-synthesized K-free(supplementary data, Fig. S1a) and K-containing (not shown)solids, showing low-dimensional M1 crystals surrounded by nano-particles. On the other hand, some differences were observed on theactivated samples. As an example, the oxides treated at 600 ◦C/N2(f-T3 and K-T3 samples) contain M1 and M2 phases mixture, but thef-T3 sample shows longer and wider M1 crystals with higher ten-


dency to form superstructures lengthwise c axis (supplementarydata, Fig. S1b), while smaller M1 crystals appear in K-T3 catalyst(supplementary data, Fig. S1c). The most relevant results obtainedby EDX analysis of selected area (supplementary data, Table S1) is




Nor

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f-T0

f-T1

FeSb O4

Sb2O3

47084703A)

Nor

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abs

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(a.u

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K-T3

K-T2

K-T4

K-T0

K-T1

3045 3050 305 5 3060

3049130496

Sb2O3

Sb2O5

B)

Fig. 2. Sb L-edge XANES spectra of MoVSbO catalysts and reference patterns Sb2O3 and FeSbO4 (A). Sb K-edge XANES spectra of K-MoVSbO catalysts and reference patternsS

tMs

otVpssutsctaTets

aMstao(aSp6noa

b2O3 and Sb2O5 (B).

hat potassium is only detected in the M1 phase (in the case of K-oVSbO samples), and that the composition of any given phase is

imilar in all catalysts (T1 to T4).Vanadium and antimony XANES spectra of K-free and K-MoVSb

xides (T2–T4 series) were recorded to investigate the effect ofhe activation treatments on the oxidation state of those elements.anadium K-edge XANES spectra show a characteristic pre-edgeeak due to the forbidden 1s–3d electronic transition, the inten-ity of which increases as the symmetry of the octahedral VO6ites decreases, and the absorption energy shifts to higher val-es as the oxidation state of vanadium increases, i.e. from V4+

o V5+ [39,41,42]. In this way, V K-edge XANES spectra of T2–T4eries catalysts (supplementary material, Fig. S2) show no signifi-ant changes in the local symmetry of vanadium with the activationreatment or the presence of potassium in the oxides, and the sameccounts for the vanadium formal oxidation states, included inable 1. Thus, more than 90% of total V was present as V4+ (pre-dge peaks observed at ca. 5469 eV) in all catalysts, excepting thosereated at 450 ◦C in air (samples f-T1 and K-T1) where vanadiumpecies were totally oxidized to V5+ (pre-edge peak at ca. 5470 eV).

On the other hand, the content of Sb3+ and Sb5+ in the cat-lysts (Table 1) was calculated from normalized Sb L1-edge (foroVSb oxides) and Sb K-edge (for the K-MoVSb oxides) XANES

pectra (Fig. 2). The Sb L1-edge (electronic transition from 2s1/2 tohe empty 5p orbitals) is very sensitive to the oxidation state ofntimony (ca. 4708 eV for Sb5+ and 4703 eV for Sb3+), that is to theccupancy of 5s orbital [16,21,23]. The Sb L1-edge XANES spectraFig. 2A) show that Sb3+ predominates in the MoVSb oxide treatedt 600 ◦C in N2 stream (f-T3 catalyst), more than half was oxidized tob5+ upon treatment at 280 ◦C in air before or after the higher tem-erature treatment in N2 (i.e. f-T2 and f-T4 catalysts, with 80% and


0% of Sb5+, respectively), and all was oxidized to Sb5+ upon calci-ation at 450 ◦C in air (f-T1 catalyst). The Sb K-edge XANES spectraf the potassium-containing MoVSb oxides (Fig. 2B) show a broadbsorption edge, appearing at ca. 30,491 eV for pyramidal Sb3+,

which progressively shifts to higher binding energy as the relativeconcentration on Sb5+ (in octahedral coordination) increases, up to5 eV when all antimony is as Sb5+ (edge position at ca. 30,496 eV)[43]. The calculated Sb3+/Sbtotal ratio for the K-containing catalystsare summarized in Table 1. The results obtained for the K-series aresimilar to those of the K-free samples, excepting the K-T4 catalyst(treated at 600 ◦C in N2 stream, and then at 280 ◦C in air) whichcontains 95% of Sb3+, while in the K-free analogue (f-T4) Sb3+ isaround 37% (Table 1), suggesting that K, in MoVSb oxides activatedat 600 ◦C in N2, makes Sb more resistant against oxidation.

The EPR spectra of K-free and K-containing catalysts wererecorded at −173 ◦C under ambient atmosphere and those forf-T3 and K-T3 catalysts are illustrated in Fig. 3. Spectra sim-ulation (Fig. 3a) shows the presence of two broad signals: S1(giso ∼ 1.87 and �G ∼ 1100 G), and a narrower S2 (giso ∼ 1.96 and�G ∼ 225 G), consistent with the presence of paramagnetic V4+ andMo5+ species, respectively [16,45–48]. Expansion of the spectra ofcatalysts f-T3 and K-T3 (Fig. 3b) showed a weak hyperfine axiallysymmetric signal, S3 (g⊥ = 1.94, A⊥ = 0.7 × 10−2 cm−1; and g|| = 1.99,A|| = 1.8 × 10−2 cm−1), associated to isolated V4+ sites [46,49]. In thecase of f-T2, f-T4 and K-T2 catalysts (composed at least by threecrystalline phases: M1, M2 and MoO3-ortho), two distinct hyper-fine signals were observed, indicating two different isolated V4+

sites with slightly different environments, whereas no vanadiumhyperfine signal was observed for K-T4 sample. The use of thirdderivative of the EPR spectra (Fig. 3c) enhances resolution evidenc-ing the presence of signals at g⊥ = 1.92, 1.96 and 1.94, correspondingto the perpendicular components of three axially symmetric Mo5+

species with coordination number four (Mo4c5+), five (Mo5c

5+) orsix (Mo6c

5+), respectively [45,47]. These signals were observed inall the samples but were difficult to distinguish in the catalyst K-T4.


In general, the density of paramagnetic species (V4+ and Mo5+),calculated from the EPR spectra, was slightly higher in K-containingthan in the corresponding K-free analogues (i.e. 1.55 mmol g−1 forK-T3, and 1.38 mmol g−1 for f-T3). The Mo5+ content (Table 1)




B (Gauss)2500 300 0 350 0 400 0 450 0

b

b

c

c

0 200 0 400 0 600 0 800 0B (Gauss)

f-T3 catalyst

K-T3 catalyst

S1

S1

S2

S2

a

a

F a) extz

wMrKat

(caadiwt(w6

F5paapmdpflho

ig. 3. EPR spectra of non dehydrated f-T3 >(down) >and K-T3 >(up) >catalysts: (ooming, and (c) third-derivative zooming.

as estimated from the amount of paramagnetic species (V4+ ando5+), the chemical composition (ICP analysis) and the V4+/Vtotal

atio obtained from the XANES data. These results indicate that the-T3 catalyst, the most selective in propane oxidation to acryliccid (vide infra), contains the highest Mo5+/Mototal ratio (ca. 4% ofotal molybdenum).

Near surface chemical compositions, calculated from XPS dataTable 2) show no significant differences in the V/Mo ratio for all theatalysts. The V 2p3/2 XPS spectra (supplementary data, Fig. S3) ofll samples consist of two components at binding energies 517.3 eVnd 516.2 eV attributed to V5+ and V4+, respectively [50,51]. V4+ pre-ominates (more than 60%) in all samples except in those calcined

n air at 450 ◦C, i.e. catalysts f-T1 and K-T1 (Table 2). Comparisonith XANES results (Table 1) indicates that for T1-series samples

he amount of V4+ near surface (Table 2) is higher than in bulkTable 1) (V4+/Vtotal of ca. 40% near surface vs. <1% of V4+ in bulk),hile the opposite is observed in T2–T4-series catalysts (V4+/Vtotal

0–70% near surface vs. more than 90% in the bulk).On the other hand, the Sb 3d3/2 XPS spectra (supplementary,

ig. S4) show two components at binding energies 539.4 eV and40.3 eV associated to Sb3+ and Sb5+, respectively [21,23,52]. Thus,ractically only Sb3+ is present near surface in the samples heatedt 600 ◦C in N2 (f-T3 and K-T3), and only Sb5+ in those calcined inir at 450 ◦C (f-T1 and K-T1). Meanwhile, both oxidation states areresent in samples calcined at 280 ◦C/air before or after the treat-ent at 600 ◦C/N2 (T2- and T4-series), with a relative amount that

epends on the activation procedure and the presence/absence ofotassium. For instance, more than 70% Sb5+ is found in sample


-T4 (600 ◦C/N2 and then 280 ◦C/air), while the K-containing ana-ogue (K-T4) contains about 20% Sb5+, suggesting that potassiumas a stabilizing effect on surface Sb3+, in agreement with resultsbtained by XANES for bulk composition (see Table 1). Finally, it was

ended first-derivative spectrum and simulation, (b) first-derivative centered field

observed that the Sb/Mo ratio on the catalyst surface progressivelydecreased with the activation treatment from T1- to T3-procedure,suggesting migration of Sb cations from the catalyst surface to thebulk. Moreover, it must be noted that the higher relative amountof Sb3+ occurs on surfaces with less Sb content.

3.2. Catalytic results in the partial oxidation of propane

Fig. 4 displays the yields to acrylic acid obtained with K-freeand K-MoVSb oxide catalysts during propane oxidation at 380 ◦C.The as-synthesized and T0-treated oxides showed no productivityto acrylic acid, so that they were not included. Inspection of Fig. 4shows that the acrylic acid production is not affected by thepresence of potassium when the catalysts are submitted to treat-ments T1 (450 ◦C/air) or T2 (280 ◦C/air; 600 ◦C/N2), whereas thisproduction is enhanced when activation protocols T3 (600 ◦C/N2)or T4 (600 ◦C/N2; 280 ◦C/air) are used.

Detailed catalytic results including propane conversions, spe-cific catalytic activity and the distribution of all reaction productsare submitted as supplementary data (Table S2). Along with acrylicacid; propylene, acetic acid and carbon oxides were obtained asmain reaction products. Propane conversion per catalyst mass unitwas lower in K-free catalysts, which could be partially explainedby their lower specific surface areas (Table 1). In fact, small differ-ences between K- and f-series are observed when considering thespecific catalytic activity (molC3H8 h−1 m−2) (supplementary data,Table S2).

Fig. 5 displays the selectivity to acrylic acid with the propane


conversion during selective propane oxidation at 380 ◦C, whichis significantly better for K-containing samples in all the rangeof propane conversions studied. The K-T3 catalyst (activated at600 ◦C/N2) gives the highest selectivity to acrylic acid (55% at 30% of




Table 2Near surface compositions as determined by XPS spectroscopy.

Samplea Surface composition Surface atomic ratio Surface oxidation state

Mo V Sb O K V/Mo Sb/Mo V4+/Vtotal (%) Sb3+/Sbtotal (%)

f-T1 23.2 4.5 6.8 65.5 0.0 0.19 0.29 41.1 0.0f-T2 25.4 4.0 4.7 65.9 0.0 0.16 0.18 68.1 31.6f-T3 27.0 4.2 2.5 66.3 0.0 0.16 0.09 63.5 100f-T4 22.8 4.2 6.6 66.4 0.0 0.18 0.26 68.2 17.8K-T1 21.4 4.6 6.6 67.0 0.4 0.22 0.31 27.3 0.0K-T2 25.4 3.8 5.6 64.9 0.3 0.15 0.22 75.2 20.0K-T3 26.3 3.9 3.0 66.5 0.3 0.15 0.11 66.0 100K-T4 25.2 5.0 3.1 66.2 0.5 0.20 0.12 75.5 78.1

a The K-free or K-containing samples after the thermal activation step presented a simi

Fig. 4. Variation of the formation of acrylic acid (in 10−5 molAA h−1 m−2) duringpMt

ptwc

4

peaatfsMmcsdwsrs(oMmpaa

ropane selective oxidation at 380 ◦C over K-free (empty) and K-containing (full)oVSbO catalysts activated by different treatments (T1–T4). Reaction conditions in

ext.

ropane conversion), and the yields to acrylic acid achieved withhis catalyst are comparable or even higher than those reportedith the most effective Nb-free and Nb-containing MoVSb oxide

atalysts [4,28,32,53,54].

. Discussion

The results obtained demonstrate that the thermal activationrocedure affects in a different way depending on the pres-nce/absence of K, giving rise, in general, to different crystallinitynd distribution of phases, metal oxidation states and surfacecid/base features, as well as catalytic behavior in propane selec-ive oxidation to acrylic acid. However, some similarities wereound between K-free and K-MoVSbO oxides. For instance, as-ynthesized materials in both systems consist of low crystalline1 phase (Fig. 1). The treatment at 280 ◦C in air leads to the for-ation of Sb5+ (Table 1) with no apparent modification of the

atalyst structure (Fig. 1), whereas stronger oxidant treatment (T1-eries, at 450 ◦C/air) oxidizes vanadium to V5+ (starting also theecomposition of M1 phase). Even in the presence of M1 phase,hich is associated to the activity and the selectivity in propane

elective oxidation to acrylic acid [16], none of the above mate-ials showed significant formation of acrylic acid. Only, catalystsubmitted to an activation protocol including a step at 600 ◦C/N2T2–T4-series) give significant selectivity to acrylic acid, even ifther crystalline phases, i.e. the so-called M2 phase (Sb2M6O19),O3 and/or Sb2M10O31 (M = Mo, V) and the oxidation state of the


etal cations change from one sample to another. This agrees withrevious results showing that M1 phase by itself is not enough tochieve high catalytic performance in propane oxidation to acryliccid, but that an optimal M1 phase crystallization at ca. 600 ◦C is

lar bulk Mo/V/Sb/K ratio of 1.00/0.27/0.15/0.00 or 1.00/0.29/0.11/0.02, respectively.

essential to achieve a good selectivity to acrylic acid [13,19,40].The capacity of transforming selectively propane into acrylic acidis associated to a rearrangement of the M1 phase surface duringthe activation treatment at high temperature actually forming theactive sites as a monolayer coverage or metal oxide mono-disperseclusters anchored on the crystalline bulk [24,55]. In a similar way,the composition and activation temperature strongly influencethe surface density of active sites in similar mixed metal oxidesemployed for other reactions of alkane selective oxidation [56].

We must indicate that in the T2–T4-series catalysts, the vana-dium was mainly found as V4+, while the oxidation state of Sb varieddepending on the activation treatment and the presence or not ofpotassium (Table 1). Thus, all antimony is as Sb3+ in T3-series, butis oxidized to Sb5+ upon calcination in air. We observed that theamount of Sb5+ inversely correlates with the formation of M2 phase,in agreement with previous publications reporting that M2 phasecontains almost exclusively Sb3+, whereas wide range of Sb3+/Sb5+

ratios have been found for M1 phase [10,11,32,57].M2 phase and the other crystalline phases formed, different

from M1 phase, are practically inactive for propane oxidation.However, they could be active in propylene oxidation (interme-diate reaction product from propane oxidation) but are unselectiveto acrylic acid formation [16,21,23]. The different distribution ofcrystalline phases in these catalysts would partially explain the dif-ferences in selectivity to acrylic acid obtained within each series ofcatalysts.

On the other hand, stoichiometry in the bulk remains practi-cally constant for all catalysts, while important changes are foundin the near surface average chemical composition (Table 2), in asimilar way to other studies using Te-containing Mo–V–oxides [55].In our case, a relation between the Sb oxidation state and the nearsurface Sb/Mo atomic ratio was observed. Thus, the higher the rela-tive amount of Sb3+ near surface, the lower the Sb/Mo atomic ratio(Fig. 6). At this point, we must indicate that considerably amountof Sb is detected in the phases presenting hexagonal channels intheir structure (M1, M2 and Sb2Mo10O31), while extremely lowcontent is observed in the MoO3 type structure (without hexago-nal channels) (supplementary data, Table S1). That is because Sbcations in bulk are preferably located at hexagonal channels ofthe oxide crystalline network [16,21,23]. Taking this into account,a decrease in Sb/Mo atomic ratio favors isolated Sb species, andonly Sb3+ species can have 3-coordination to oxygen, i.e. SbO3E(2 oxygen from a hexagonal window and one in the channel inopposite direction to the lone pair, E) [19,58,59] forming iso-lated dimers [EO2Sb–O–SbO2E], whereas Sb5+ must have at leasta 4-coordination which favors the formation of chains inside thechannel increasing the relative amount of antimony [21]. In this


way, it is understandable that the characteristics of the activationheat-treatment strongly influences the near surface chemical com-position (Table 2). It is also important to indicate that, contrary towhat may be expected, lower V5+ content near surface than in the




0

10

20

30

40

50

60

0 10 20 30 40 50 60Prop ane con version (%)

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Treatment70

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0 10 20 30 40 50 60Prop ane con version (%)

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Treatmenta b

F uring propane oxidation at 380 ◦C over K-free (a), and K-containing (b) MoVSbO catalystsa

bmT

s(aKaccKdsoOmtthatot([

F(

0

10

20

30

40

50

60

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0 10 20 30 40 50 60Surface density of ac id sites (mm olNH3/m2)

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aci

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ig. 5. Variation of the selectivity to acrylic acid with propane conversion achieved dctivated by different treatments (T2–T4). Reaction conditions in text.

ulk was found for oxides submitted to the most oxidizing treat-ent (T1, 450 ◦C/air), while the opposite trend was observed for

2–T4-series of catalysts, (Tables 1 and 2).Moreover, the activation heat-treatment also influences the

urface acidity of these materials, especially for K-free catalystsTable 1). Fig. 7 shows the evolution of the selectivity to acryliccid with the surface density of acid sites which was different for-free and K-containing catalysts series. Although these resultsre apparently contradictory, they can tentatively be explained byonsidering that there are two ways to get efficient catalysts. In thease of K-free catalysts, presenting higher catalytic activity than-containing ones, the formation of acrylic acid increases with theensity of surface acid sites, which may be explained by the fact thatome type of acid sites could be beneficial for the rapid desorptionf acrylic acid avoiding further oxidation and increasing selectivity.n the other hand, in the case of K-containing catalysts, which showuch lower catalytic activity, probably the desorption rate of reac-

ion products is lower and, therefore, the less amount of acid siteshe lower decomposition of reaction intermediates which favoursigher selectivity to acrylic acid. In this way, it must be noted thatcid sites, in this type of materials, has been mainly associated tohe M1 phase [14,16,60], as they are practically negligible on the


ther more dense phases present, i.e. the M2 phase [20,57]. Thus,he incorporation of potassium has two effects on the active phase:i) it neutralizes acid sites at the surface, as demonstrated (Table 1)16–18], but also (ii) would compete with antimony for hexagonal

0

20

40

60

80

100

0 0,1 0,2 0,3 0,4

Sb/Mo near surfa ce (%)

Sb3+

/Sb T

near

sur

face

(%)

K-freeK-con taini ng

ig. 6. Relation between the Sb3+/SbT and Sb/Mo near surface ratios of the K-freewhite) and K-containing (black) from T1 to T4 catalyst series (determined by XPS).

Fig. 7. Variation of the selectivity to acrylic acid (at 30% propane conversion) withthe acid sites surface density (�molNH3 m−2) of K-free (�) and K-containing ( )MoVSbO catalysts (see also Table 1).

channels and/or partially occupy heptagonal channels, which hasbeen reported to be associated to loss of catalytic activity [30]. Thiswould explain the decrease in catalytic activity for the K-containingseries, and the need to strongly increase the contact time in orderto have higher conversion comparable to K-free MoVSbO oxidecatalysts.

5. Conclusions

The results obtained indicate that crystalline phases, oxidationstate of elements (especially antimony) surface density of acid sitesand composition, as well as the catalytic behavior in propane oxi-dation to acrylic acid depend on the presence/absence of potassiumin MoVSbO mixed oxides and the activation protocol used.

Active and selective catalysts must contain crystalline M1 phase,obtained through an activation protocol including a step at 600 ◦C inN2 stream. K-containing catalysts presented systematically loweractivity and higher selectivity to acrylic acid than the correspond-ing K-free ones. The incorporation of potassium in the synthesis gelleads to catalysts with lower surface acidity and improved selec-tivity to acrylic acid. However, the number of surface acid sitesalso depends on the activation procedure, especially for K-freecatalysts.


Sb is the element most affected by the activation conditions.The presence of potassium in MoVSbO materials activated at 600 ◦Capparently exerts a stabilizing effect on Sb3+ species against subse-quent oxidizing heat-treatments. However, the differences in the


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xidation state of Sb, which also depends on the reaction conditions14,16], is not the determining issue to obtain effective catalysts inropane oxidation to acrylic acid, but rather a proof of the versatileedox properties of these catalysts.

In conclusion, the results obtained demonstrate that adequateeat-treatment can optimize the surface composition and the oxi-ation state of each element modifying so the nature of surfacexygen species. M1 phase bulk of the catalyst could be considereds a support favoring the best distribution of active sites in envi-onments with appropriate chemical characteristics, i.e. low aciditynd, as recently suggested, oxygen surface species with relativelyow mobility, in which the participation of adsorbed oxygen speciess low [42]. If it is the case, appropriate activation procedures canrovide adequate improvements in the catalytic behavior of theseatalysts.

cknowledgment

Financial support from DGICYT in Spain through ProjectsTQ2012-37925-C03-1 is gratefully acknowledged. Authors arelso grateful to Belén Albela and Laurent Bonneviot (Laboratoryf Chemistry, Ecole Normale Supérieure de Lyon UMR-CNRS 5182,niversité de Lyon) for the instrumental and technical EPR support.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.cattod.014.01.036.

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Understanding effects of activation-treatments in K-free and K-MoVSbO bronze catalysts for propane...

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