characterization and selective oxidation catalysis of modified pt particles on sbox

Applied Catalysis A: General 191 (2000) 131–140

Characterization and selective oxidation catalysis of modifiedPt particles on SbOx

Tomoya Inouea, S. Ted Oyamab, Hideo Imotoa, Kiyotaka Asakuraa,1,Yasuhiro Iwasawaa,∗a Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

b Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering and Chemistry, Virginia Polytechnic Institute andState University, Blacksburg, VA 24061-0211, USA

Received 10 May 1999; received in revised form 14 June 1999; accepted 20 June 1999

Abstract

Characterization and catalytic performance of modified Pt on SbOx for the selective oxidation ofi-C4H10 andi-C4H8 tomethacrolein are summarized and reconsidered in this paper. XRD revealed that the main oxide phase in Pt/SbOx catalystsunder the reaction conditions was Sb6O13, while Raman spectroscopy also indicated the presence of smalla-Sb2O3 crystallites(senarmontite), forming on or around Pt particles, with small highly dispersed O=SbOy decorating the Pt surface. These specieswere responsible for breaking up large Pt ensembles and suppressing the total oxidation reaction. Synergistic combination ofthe modified Pt and the defect pyrochlore-type Sb6O13 was essential for the selective oxidation ofi-C4H10. ©2000 ElsevierScience B.V. All rights reserved.

Keywords:Pt/SbOx ; Sb6O13; Sb2O3; Sb suboxides; Modification of Pt surface; Catalytic selective oxidation of isobutane and isobutylene;Methacrolein; Active structure; XAFS; Raman; XRD; TEM

1. Introduction

Although catalysts for selective oxidation of alkeneshave long been studied to establish and improve in-dustrial processes, the development of selective oxi-dation catalysts for small alkanes is still a challengingsubject to be solved. Only the V–P–O catalyst forn-butane oxidation to maleic acid anhydride has beencommercially successful [1]. Catalysts for alkene oxi-dation cannot naturally be applied to alkane oxidationbecause of the difficulty of activation of alkanes com-

∗ Corresponding author. Fax: +81-3-5800-6892.E-mail address:[email protected] (Y. Iwasawa)

1 Present address: Catalysis Research Center, Hokkaido Univer-sity, Kita-ku, Sapporo 060-0811, Japan.

pared with the corresponding alkenes. Unsaturatednitriles and aldehydes which are important basicchemicals have been produced by oxidation or am-moxidation of propene or isobutylene [2,3]. Increas-ing demand of use of alkanes rather than alkenes asstarting materials for their syntheses attempted usto explore a new catalytic system for the selectiveoxidation of small alkanes.

Some catalysts have been found to be promising inthe selective oxidation processes starting from alkanes.V–Sb–O based catalysts have been developed to thelevel of pilot plant [4,5]. Scheelite type Bi–V–O hasbeen reported to work as a good catalyst for propaneoxidation or ammoxidation to acrolein or acryloni-trile [6]. Recently, V–Mo–Nb–Te oxide catalysts havebeen reported to be active and selective for propane

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132 T. Inoue et al. / Applied Catalysis A: General 191 (2000) 131–140

ammoxidation to acrylonitrile at lower temperaturescompared to the case of the V–Sb–O and Bi–V–Ocatalysts [7–9]. In spite of these exploits, further im-provement on catalytic performance and catalyst lifeis needed.

Activation of C–H bond in alkanes (C–H bond dis-sociation) has been demonstrated as a key issue forthe alkane selective oxidation reactions. Noble metalssuch as Pt, Pd and Rh may be the candidate as activeelements for the dehydrogenation of alkane moleculesat relatively low temperatures. However, they are alsogood catalysts for the total oxidation of alkanes toCO2, which is a reason that supported noble metalshave not been employed as selective oxidation cata-lysts. Nevertheless, it is possible to control the prop-erty of the noble metals by addition of the secondmetal and to create new catalysis by the obtained noblemetal–additive metal ensembles.

We have reported that a Pt–Sn bimetallic ensemblecatalyst, which was prepared by reaction of Sn(CH3)4with Pt particles supported on SiO2, followed by H2reduction, was effective for the conversion of hydro-carbons like isobutane to the corresponding unsatu-rated nitriles with high selectivity of 70% [10]. In thebimetallic Pt–Sn ensemble catalyst, Pt acts as dehy-drogenation site for hydrocarbons such as propene,isobutene and isobutane to form the allyl intermedi-ates and Sn acts as an oxygen acceptor from NO toform SnOx and atomic N. The SnOx stabilizes the al-lyl intermediates which react with atomic N to pro-duce the unsaturated nitriles. Monometallic Pt cata-lysts show no significant activity for nitrile synthesis[10,11].

The results on the Pt–Sn/SiO2 catalyst promoted usto prepare new Pt-base bimetallic catalysts for selec-tive oxidation of alkanes. Second metal additives suchas V, Mo, Fe, Zn, Ga, Sn, Sb, Pb, Bi, etc which havedifferent oxygen affinities (metal–oxygen bond ener-gies) were combined with Pt particles supported onSiO2 by using the metal alkyls, followed by reductionwith H2 at 673 K. Performance of the resulting Pt-basebimetallic catalysts was examined for propane am-moxidation in a closed circulating system [12]. Amongthe examined second additive metals, Sb promoted theunsaturated nitrile (acrylonitrile) synthesis and sup-pressed the undesired C–C bond breaking.

Thus we chose a combination of Pt and Sb, andprepared supported Pt/SbOx catalysts instead of the

Pt–Sb/SiO2 to simplify the catalytic system. Recently,we have found that the Pt/SbOx catalysts were activefor the isobutane (i-C4H10) oxidation to methacrolein(MAL) with a high selectivity of 57%, and the selec-tivity including the intermediate isobutylene (i-C4H8)was as high as 90% [13]. SbOx itself never producedMAL from i-C4H10, while i-C4H8 was converted toMAL with a high selectivity on the SbOx . The yieldof MAL in the i-C4H8 oxidation also increased seventimes by the coexistence of Pt. Characterization of thePt/SbOx catalysts by X-ray diffraction (XRD), trans-mission electron microscopy (TEM), X-ray absorptionfine structure (XAFS), Raman, and gas adsorptionrevealed that the surface of Pt was decorated with ox-ide species under selective oxidation reaction condi-tions and that a Sb6O13 phase was found in the bulk[14]. This modification of the Pt particles by Sb sub-oxides on Sb6O13 induced high selectivity to MALin both the i-C4H10 and i-C4H8 oxidation reactions[14,15].

In the Pt/SbOx system, activation ofi-C4H10 to formthe dehydrogenated species may occur on the Pt sur-face, and oxygen addition (C–O bond formation) maytake place on the Sb6O13 surface [13–15]. In additionto the cooperative reaction processes, the metal–metaloxide junction may promote the reactivity of latticeoxygen of the metal oxide [16,17].

The aim of this paper is to discuss the states andinteraction of Pt and SbOx in the Pt/SbOx cata-lyst, and to shed light on the key issues relevant tothe good performance of the new Pt/SbOx systemfor the selective oxidation ofi-C4H10 and i-C4H8to MAL.

2. Experimental

2.1. Catalyst preparation

SbOx was prepared by hydrolysis of SbCl5(Soekawa Chemical) with an aqueous ammonia solu-tion, followed by drying at 373 K and calcination at773 K. Pt was supported on the SbOx by an impreg-nation method using an acetone solution of Pt(acac)2(Soekawa Chemical), followed by drying at 323 Kand calcination at 773 K [13]. Pt loadings were reg-ulated in the range 0.3–2.0 wt.%. BET surface areaswere 50–60 m2 g−1 for all the samples examined.

T. Inoue et al. / Applied Catalysis A: General 191 (2000) 131–140 133

2.2. Catalytic reactions

Performance of the catalysts was examined in afixed-bed flow reactor system composed of gas flowlines of stainless steel tubes and teflon tubes, except fora quartz-made reactor tube. Two sets of experimentswere performed; steady-state and pulse reactions. Thesteady-state kinetic data were measured over severalhours after the first 1 h reaction for each constant re-action temperature. The reaction products were ana-lyzed by two gas chromatographs (Shimadzu GC-9Aand GC-8Ait), using columns of Unibeads C at 423 Kfor O2 and CO2, Gaskuropack 54 at 423 K for C3products,i-C4H10 and MAL, and VZ-10 at 343 K fori-C4H10 and i-C4H8 [13].

2.3. Characterization of catalysts

The amounts of adsorbed H2 (Takachiho Chemical,purity 99.9999%) and CO (Takachiho Chemical,purity 99.95%) on Pt/SbOx catalysts were measuredvolumetrically in a closed circulating system (deadvolume: 70 cm3). The amount of irreversible adsorp-tion was estimated by subtracting the amount of re-versible adsorption obtained at the second adsorptionexperiment from the adsorbed amount determinedfrom the first adsorption measurement. The Pt/SbOx

catalysts were not reduced with H2 before the ad-sorption measurements because the H2 reductiontreatment changed the surface state of the catalysts.Instead, the catalysts were calcined at 773 K andevacuated at 473 K. The samples thus treated weresubjected to the adsorption experiment.

Powder XRD patterns were obtained on a Geiger-flex diffractometer (Rigaku) using Cu Ka1 radiation(λ = 0.15418 nm). Crystalline phases were identifiedusing ASTM files. Pt particle size on average was es-timated by the linewidth of Pt(111) diffraction peak[14].

Transmission electron microscope (TEM) photo-graphs were taken with 350 000 magnification on aJEM 2010/LaB6 microscope (JEOL), and more than500 metallic particles were scaled for every sample toget the averaged particle diameter [14]. The sampleswere stored in ampoules until TEM measurementsand exposed to atmosphere at room temperature justbefore TEM measurements.

Pt LIII -edge XAFS spectra for Pt/SbOx catalystswere measured in a transmission mode at the BL-7Cof the Photon Factory in Institute for Material Struc-ture Science (PF-IMSS) with a storage-ring energyof 2.5 GeV and a maximum current of 350 mA (Pro-posal No. 95G002). Higher harmonics were removedby detuning the parallelism for the intensity to be 60%of the maximum. In EXAFS analysis, Sb has a largeX-ray absorption in the Pt LIII -edge EXAFS regionwhich makes it difficult to measure EXAFS spectrawith a good S/B ratio for the Pt/SbOx samples withlow loading of Pt in a transmission mode. Hence theEXAFS spectra for 0.5 wt.% Pt/SbOx were taken bya fluorescence method. For the fluorescence mode aLytle detector filled with Ar was used. The sampleswere transferred to glass-made XAFS cells with kap-ton windows without contacting air. The EXAFS datawere analyzed by the EXAFS analysis program ‘REX’(Rigaku). The detailed analysis procedure is describedin the previous paper [14].

Raman spectra for the samples in capillary glasstubes (0.8–1.1 OD× 90 mm, Kimble) were obtainedon a Spex 500 M spectrometer with a resolutionof 4 cm−1 coupled with a liquid nitrogen cooledcharged coupled device (CCD) detector (SpectrumOne, Spex). An Ar+ laser (Model 95, Lexel; 50 mW;514.5 nm) was used for excitation, and a holographicfilter (SuperNotch Plus, Kaiser) was employed toreject Rayleigh scattering from the sample [15].

3. Results and discussion

3.1. Promotion effect of Pt on the Sb oxide catalysisfor the i-C4H10 selective oxidation

SbOx was inactive for the oxidation ofi-C4H10 withO2 at 773 K, whereas small amounts of Pt supported onthe SbOx promoted the selective oxidation ofi-C4H10to produce MAL andi-C4H8 as shown in Table 1 [13].When the Pt loadings were 0.2 and 0.35 wt.%, theMAL + i-C4H8 selectivities were as high as 87.1% and89.6%, respectively. The addition of Pt to SbOx alsopromoted the dehydrogenation ofi-C4H10 to i-C4H8(Table 1). Simultaneously, Pt increased the total oxi-dation of i-C4H10 to CO2. Thus, there existed an op-timum Pt loading in Pt/SbOx for the MAL yield from


Table 1Product yields and selectivities in the catalytici-C4H10 oxidationon the Pt/SbOx catalysts with different Pt loadings at 773 K

Pt loading Product yield (%) Selectivity (%)(wt.%)

MAL i-C4H8 MAL i-C4H8 MAL + i-C4H8

0.0 0.0a 0.0a – – –0.2 0.15 0.15 43.8 43.3 87.10.35 0.38 0.27 52.1 37.5 89.60.5 0.98 0.43 56.6 24.9 81.50.72 0.59 0.51 35.1 30.4 65.5

a No reaction on SbOx . Catal.: 0.3 g; total flow rate:2400 ml h−1; i-C4H10: 20%, O2: 4%, and balanced with He.

i-C4H10. The maximum MAL yield was achieved onthe catalyst with a Pt loading of 0.5 wt.% under thepresent reaction conditions, where the selectivity toMAL was 56.6% (Table 1). As far as we know, thePt/SbOx catalyst is only a noble metal-base catalystwhich is promising for the selectivei-C4H10 oxida-tion to MAL. The product yields and selectivities ini-C4H10 oxidation on Pt/SbOx at 773 K are summa-rized in Table 1.

The formation of both MAL andi-C4H8 in thei-C4H10 oxidation on Pt/SbOx possessed the samekinetics that were of nearly first order with respectto i-C4H10 pressure and zeroth order with respect toO2 pressure. Selective oxidation reactions of alkaneson mixed oxide catalysts often show similar reactionkinetics, where dehydrogenation from alkanes is sug-gested to be the rate-determining step for the over-all reaction [18]. Thei-C4H10 selective oxidation onPt/SbOx may also proceed by the successive steps,i-C4H10→ i-C4H8 → i-C4H7 (methallyl)→ MAL, asgenerally accepted [3,19]. The result that Pt promotedthe selectivei-C4H10 oxidation may be relevant to thedehydrogenation ofi-C4H10 to i-C4H8 which occurson the Pt surface. Thei-C4H8 intermediate was oxi-dized to MAL on SbOx without Pt as reported in theprevious paper [13]. Propene oxidation has also beenreported to take place on Sb oxide (Sb6O13) [20].

It is to be noted that Pt also promoted the selec-tive oxidation ofi-C4H8 to MAL, and the selectivityto MAL at 773 K was as high as 90%. The additionof a small amount (0.5 wt.%) of Pt to SbOx increasedthe MAL yield from i-C4H8 to five times of what itwas on SbOx alone, while retaining the high selec-tivity of the SbOx itself. The yield to MAL was in-

Fig. 1. XRD patterns for SbOx and Pt/SbOx ; (a)SbOx after cal-cination at 773 K(*: assigned to Sb6O13); (b) Pt/SbOx (2.0 wt.%)after i-C4H8 selective oxidation at 773 K (1.7%i-C4H8, 4% O2,balanced with He); (c) Pt/SbOx (0.5 wt.%) afteri-C4H10 selectiveoxidation at 773 K (20%i-C4H10, 4% O2, balanced with He); (d)Pt/SbOx (0.5 wt.%) afteri-C4H8 oxidation under thei-C4H8 rich(20%) condition at 773 K (20%i-C4H8, 4% O2, balanced withHe) followed by i-C4H10 oxidation at 773 K (20%i-C4H10, 4%O2, balanced with He) (d: assigned toa-Sb2O4). The i-C4H10

and i-C4H8 oxidation were conducted in a fixed-bed flow reac-tor using 0.3 g catalyst at a total flow rate of 2400 ml h−1 and anatmospheric pressure.

creased seven times for 1.0 wt.% Pt. Further, the acti-vation energy drastically decreased from 150 for SbOx

to 60 kJ mol−1 for Pt (0.5 wt.%)/SbOx [13]. As men-tioned above, the formation of MAL fromi-C4H8 oc-curs through methallyl intermediate (i-C4H7). Theseresults indicate that the dehydrogenation ofi-C4H8 toi-C4H7 preferentially proceeds on the Pt surface andthe methallyl species may migrate to the SbOx surfaceon which MAL is produced.

3.2. Sb oxide phase in Pt/SbOx active for thei-C4H10 selective oxidation

Powder XRD patterns in the wide range of 2θ forPt/SbOx and SbOx samples are shown in Fig. 1 [14].The SbOx phase in all samples except for sample(Fig. 1(d)) was identified as Sb6O13. The SbOx sam-ple without Pt always comprised of Sb6O13 alone irre-


Fig. 2. Raman spectra of SbOx and Pt/SbOx ; spectra (a) and(b): SbOx and Pt/SbOx after calcination at 773 K; spectrum (c):Pt/SbOx after heat treatment in He at 773 K; spectrum (d): Pt/SbOx

after reaction withi-C4H8 at 698 K; spectrum (e): SbOx afterreaction withi-C4H8 at 698 K.

spective of the calcination and thei-C4H10 andi-C4H8oxidation at 773 K. The Pt/SbOx catalyst afteri-C4H10oxidation at 773 K also constituted the Sb6O13 phase(Fig. 1(c)). However, the Sb6O13 structure was partlyconverted toa-Sb2O4 wheni-C4H8 oxidation was per-formed on Pt (2.0 wt.%) /SbOx (Fig. 1(b)). In this casethe combustion to CO2 was enhanced. Thea-Sb2O4phase became major with the Pt/SbOx after i-C4H8oxidation under thei-C4H8-rich (20%) reaction con-dition as shown in Fig. 1(d). The selectivity to MALin the i-C4H10 andi-C4H8 oxidation reactions on thisphase became nearly zero and the total oxidation toCO2 was predominant.

Fig. 2(a) and (b) show the Raman spectra of SbOx

and Pt/SbOx after calcination at 773 K, which alsoidentify the Sb6O13 phase [15]. SbOx exhibited a pat-tern corresponding to Sb6O13 with strong but broadpeaks at 416, 476, and 556 cm−1 [21].

Sb6O13 has been regarded as a defect pyrochlore-typecubic structure, consisting of a network of SbO6octahedra, sharing all corners (Sb5+–O–Sb5+ zigzagchains) along the [1 1 0] axis, with the Sb3+ andremaining oxygen ions positioned inside vacanciesformed by the network of octahedra [22–24]. Fig. 3shows three types planes (1 0 0), (1 1 1) and (1 1 0) ofSb6O13 crystal. In the defect pyrochlore structure halfof the Sb3+–O2− network should be omitted, but theexact arrangement is not known at moment. In Fig. 3,Sb5+ ions are located in the oxygen octahedra (smallgrey scales) at the Sb–O distance of 0.19574 nm andeach of Sb3+ ions illustrated as black filled circlesis connected to six oxygen atoms (small grey scales)at 0.26023 nm and two oxygen atoms (white scales)at 0.22307 nm. The Sb5+–O–Sb5+ chains in Sb6O13may be responsible for selective oxidation of unsatu-rated hydrocarbons likei-C4H8 [4,19]. Alternatively,oxygen atoms loosely connected to the Sb3+ ions inthe vacancies may be responsible for the selectiveoxidation. However, Sb2O3 phase produced in thePt/SbOx catalyst was inactive for the selective oxi-dation as discussed in the next section. On the otherhand,a-Sb2O4 phase has been demonstrated to workas ‘donor’ which supplies reactive oxygen atoms onactive sites [25]. Extreme reaction condition leadsto the formation ofa-Sb2O4 and total oxidation asshown in Fig. 1(d).

3.3. Surface state of Pt particles in Pt/SbOx activefor the i-C4H10 selective oxidation

XRD patterns for the Pt/SbOx catalysts with 0.5 and2.0 wt.% Pt loadings revealed that Pt metallic particleswere formed after the calcination at 773 K as provedby Pt(1 1 1) diffraction peak [14]. The Pt(1 1 1) peakshowed little change after thei-C4H10 oxidation reac-tion, indicating almost no aggregation of Pt particlesto large particles. EXAFS also demonstrated that Ptin the Pt/SbOx catalysts forms metallic particles afterthe calcination at 773 K as shown in Table 2 [14]. Themetallic Pt–Pt bonds were observed at 0.275–0.277 nmand the co-ordination numbers of Pt–Pt were as largeas 10.3–11.8, which are close to 12.0 for bulk Pt (Table2). Thus, there is no evidence for the coexistence ofisolated Pt ions and Pt oxides in the calcined samples.Small increase in the white line peaks in the XANES


Fig. 3. Three representative planes (1 0 0), (1 1 1) and (1 1 0) of Sb6O13 crystal; Sb5+ ions are not shown, which are located inside oxygenoctahedra. Sb3+ ions are shown with black scale. White and grey scales are O2− ions. In fact, half of Sb3+ and O2− are removed in thedefect pyrochlore Sb6O13 stucture, but the exact arrangement of the defect sites is not known at present.

spectra for 0.5 and 2.0 wt.% Pt/SbOx calcined at 773 Kmay be due to adsorption of oxygen on the Pt surface.

Pt dispersion is usually controlled by the loading,that is, the more Pt loaded on SbOx , the larger Pt par-ticles would grow. The H/Pt values indicate an oppo-

site tendency [14]. The Pt particle sizes for 0.5 wt.%Pt/SbOx and 2.0 wt.% Pt/SbOx were calculated to be13.0, 5.2 nm, respectively, assuming a spherical shapeof the particles. The mean Pt particle sizes estimatedby the particle size distribution in TEM were 5.5 and


Table 2Curve-fitting results for the Pt L3-edge EXAFS data of the 0.5 and 2.0 wt.% Pt/SbOx catalysts after calcination at 773 K and selectiveoxidation reactions ofi-C4H10 and i-C4H8 at 773 Ka

Sample Bond C.N. (± 0.7) r/nm (± 0.001 nm) 1E0 (eV) σ 2 (nm2)

0.5 wt.% Pt/SbOxCalcinedb Pt–Pt 10.3 0.277 −0.7 4.4× 10−5

i-C4H10 ox.c Pt–Pt 11.7 0.277 −1.6 3.3× 10−5

i-C4H8 ox.d Pt–Pt 10.9 0.276 −4.1 3.8× 10−5

2 wt.% Pt/SbOxCalcinedb Pt–Pt 11.8 0.275 −4.4 5.2× 10−5

i-C4H10 ox.e Pt–Pt 10.8 0.275 −5.2 4.1× 10−5

H2 reducedd Pt–Pt 6.1 0.274 −5.2 6.5× 10−5

Pt–Sb 0.6 0.260 8.9 5.1× 10−5

a C.N., r and σ are coordination number, interatomic distance and Debye–Waller factor, respectively.1E0 is the difference betweenthe origin of the photoelectron wavenumber and that conventionally determined. Selective oxidation reaction conditions: 2400 ml h−1; 20%i-C4H10 or 1.7% i-C4H8, 4% O2, balanced with He.

b FT range: 25–135 nm−1, FF range: 0.18–0.33 nm.c FT range: 25–170 nm−1, FF range: 0.20–0.30 nm.d FT range: 25–160 nm−1, FF range: 0.17–0.35 nm.e FT range: 25–180 nm−1, FF range: 0.17–0.33 nm. The residual factors (Rf ) as evaluation of the EXAFS analysis were 0.4–4.8% for

all the samples.Rf = ∫ kmaxkmin

∣∣k3χobs(k) − k3χcalc(k)∣∣2 dk

/ ∫ kmaxkmin

∣∣k3χobs(k)∣∣2 dk.

Table 3Comparison of Pt particles sizes in Pt/SbOx evaluated by H/Pt,XRD, TEM and EXAFS

Pt loading Pt particle size (nm)

0.5 wt.% 2.0 wt.%

H/Pt 13 5.2XRD 6.4 3.5TEMa 5.5 3.7EXAFS ≥4.0 ≥4.0

a d = ∑inid

3i /

∑inid

2i .

3.7 nm for the 0.5 and 2.0 wt.% Pt/SbOx catalysts, re-spectively, which also show that the Pt particles onSbOx are larger with the less Pt loading sample (Table3). The Pt particle sizes were also calculated to be6.4 and 3.5 nm for the 0.5 and 2.0 wt.% Pt/SbOx cat-alysts, respectively, from the broadening of the XRDPt(1 1 1) peaks. The coordination numbers of Pt–Pt(10.3–11.8) in EXAFS also provide the particle sizes≥4.0 nm though the values in this range could not beprecisely evaluated by EXAFS. There is differencein Pt particle sizes determined by H/Pt, XRD, TEM,and EXAFS in Table 3 [14]. The Pt particle sizesin Pt/SbOx catalysts estimated from H/Pt were 1.5–2times larger than the values derived from XRD, TEMand EXAFS. The result that the H/Pt values are muchsmaller than those expected from the XRD, TEM and

EXAFS data implies that the surface property of Ptparticles in the calcined Pt/SbOx catalysts is differentfrom that of Pt metal. This is also suggested by COadsorption (CO/Pt) which was remarkably suppressed.The suppression of H2 and CO adsorption may be dueto the decoration effect of Sb suboxide like the SMSI(strong metal–support interaction) [26].

On the Pt/SbOx sample new Raman peaks were ob-served at 134, 200, 264, 384, 460, and 718 cm−1 (Fig.2(b)) [15]. These are assignable to Sb2O3 (senarmon-tite) [21]. We propose that this phase is present ashighly dispersed crystallites, and that it is one of theforms of antimony oxide on the surface of the Pt par-ticles that give rise to the SMSI-like effect. Notably,the peak shape around 500 cm−1 changed significantlywith the addition of Pt. For SbOx the left side of thepeak was concave downward but for Pt/SbOx the leftside became convex. Since there are no reported anti-mony oxide peaks in that region, the feature may beassigned to adsorbed atomic oxygen on Pt, based onHREELS results [27,28]. Another feature appearingat 806 cm−1 in Fig. 2(b) is assigned to a mononuclearspecies such as O=SbOy [15]. Although the wavenum-ber position is somewhat low, it is still in the rangepossible for oxo species. Fig. 2(d) shows that the inten-sity of Sb2O3 got stronger as lattice oxygen was con-sumed in thei-C4H8 selective oxidation at 698 K. On


the contrary, the spectrum for SbOx alone remainedlargely unchanged after the selective oxidation in Fig.2(e). The Sb2O3 formed on or around Pt particles prob-ably arises from the reduction of Sb6O13 by reverseoxygen spillover to Pt. The Sb3+ and O2− in the va-cancies of the defect pyrochlore Sb6O13 are looselyconnected in the crystal structure, and the junctionwith Pt may allow facile oxygen movement leading toSb2O3 formation. This may be the reason that the di-rect reduction to Sb2O3 occurred in the Pt/SbOx cata-lyst, while this reduction is difficult by simple thermalmeans in the pure oxides. The spectrum changed fur-ther when Pt/SbOx was treated under He at 773 K inFig. 2(c), where new peaks appeared at 96, 152 and406 cm−1 and the peak at 202 cm−1 became stronger.These new features were identified as due to finelydispersed Sb2O4 [21]. In the steady-state reaction atmoderate conditions, it was found that high selectivityto MAL was attained when the Sb6O13 structure wasmaintained, while more extreme conditions led to theformation ofa-Sb2O4, which resulted in a decrease inselectivity [13,14].

The structural change in Pt particles for the Pt/SbOx

catalysts under more reducible conditions was ob-served in the EXAFS oscillation. The careful EXAFSanalysis by a curve fitting technique revealed the pres-ence of Pt–Sb bond at 0.260 nm besides Pt–Pt bondat 0.274 nm (Table 2), demonstrating the formationof Pt–Sb alloy in the reduced Pt/SbOx catalysts [14].After the i-C4H8 oxidation reactions (1.7%i-C4H8 or4% i-C4H8, 4% O2, balanced with He) at 773 K thePt–Sb bonding was not observed by EXAFS, whereonly Pt–Pt bonds with large Pt–Pt coordination num-bers similar to those for the calcined Pt/SbOx catalystswere observed in Table 2. The degree of the alloy for-mation in the Pt particles depends on the atmosphere.It is to be noted that Pt and Sb tend to make alloyparticles on SbOx with each other under reducibleconditions. However, under the reaction condition ofthe i-C4H10 selective oxidation, Sb on the Pt surfaceshould be partially oxidized. As for the Pt/SbOx cat-alyst after thei-C4H10 selective oxidation to MAL(20%i-C4H10, 4% O2, balanced with He) at 773 K, theXRD and EXAFS data were similar to those for thecalcined catalyst before the catalytic reactions (Fig. 1and Table 2). These results from the bulk characteriza-tion indicate that the Pt particles remained essentiallyunchanged afteri-C4H10 oxidation.

Fig. 4. The reaction profile ofi-C4H8 oxidation by lattice oxygenat 698 K; (a) SbOx and (b) Pt/SbOx (2.0 wt.%); i-C4H8 injection:0.7 kPa; O2 injection: 1.3 kPa; reaction time: 60 s.

The overall redox process was assisted by Pt asshown in Fig. 4 [15]. The effect of gas phase oxygenon the redox properties was tested by first pulsing onlyi-C4H8 and then injecting alternate pulses of O2 andi-C4H8 at 698 K. It was found that the consumption oflattice oxygen was promoted by the O2 injections onthe Pt/SbOx sample, while it was hardly affected forSbOx . In this case, the amount of oxygen promotedby reoxidation was about 1.0× 10−5 mol g-cat−1 andcorresponded to a ratio of O/Pt about 0.1. Not muchSbOx alone was reoxidized under the reaction condi-tion. At the steady-state conditions, SbOx is readilydeactivated because it loses its reactive lattice oxy-gen, while in the Pt/SbOx catalyst, the Pt allows re-oxidation. The Pt, however, is surrounded by Sb2O3and O=SbOy species so that large ensembles that cancause deep oxidation are not present. This may bean example of the concept of ‘site isolation’ [29]. InSb-containing multicomponent oxide catalysts, one ofthe important functions of the secondary component isthe promotion of a redox cycle in SbOx . This may be


Fig. 5. The structural change of Pt/SbOx at the various reactionconditions.

also an essential function of Pt in Pt/SbOx catalysts,aside from the C–H activation function for alkaneoxidation. The Raman results show the presence ofPt–O (ca.490 cm−1), indicating that Pt is able to acti-vate oxygen. The atomic oxygen would spill over toSb6O13 to reoxidize the active site.

Fig. 5 illustrates the surface states of the Pt/SbOx

catalyst at the various reaction conditions. Sb2O3 isevolved during the calcination process and the initialstages of hydrocarbon oxidation, in which total oxi-dation is promoted by adsorbed oxygen on the Pt par-ticles. As the reaction proceeds, Sb2O3 is further dis-persed, possibly as O=SbOy species on Pt, and theselectivity is high in the steady state of reaction [15].This phenomenon is irreversible, and once an activephase at the Pt/SbOx catalyst surface is formed underthe reaction conditions, typically at 773 K, the highselectivity to MAL is maintained. A driving force toform the dispersed O=SbOy may be the tendency of

the Pt–Sb alloy formation (Pt–Sb = 0.260 nm) in thedeeply reduced conditions [14]. Only indirect evidencefor the precise role of Sb2O3 is available. For com-parative purposes a sample of Pd/SbOx was examinedafter i-C4H8 oxidation by laser Raman spectroscopyand XRD. This sample had been found to be unselec-tive for the catalytic selective oxidation reaction [2,3].The sample was found to contain both major Sb6O13phase and Sb2O3. If Sb2O3 had not been observed, itcould have been concluded that the Sb2O3 phase inthe Pt sample promoted the selective oxidation. How-ever, since it was found in the unselective Pd catalyst,it plays just an inert role on the metal surface [13,15].The critical phase may be the putative O=SbOy speciesfound only on the Pt containing samples. Under thecatalytic reaction conditions, the lattice oxygen as wellas gaseous oxygen is consumed for the oxidation re-action. Thus, it seems that the high selectivity to MALin the i-C4H10 oxidation is attained when the Pt sur-face modified by Sb suboxides (Sb2O3 and possiblyO=SbOy) is formed on the Sb6O13 support as illus-trated in Fig. 5. The surface oxygen atoms of Sb6O13with the cubic structure of defect pyrochlore plays adecisive role in the selective oxidation ofi-C4H10 andi-C4H8. The reactivity of oxygen atoms may be pro-moted by the junction effect of the Pt particle at thesurface. The active oxygen atoms can be recovered byoxygen spillover from the modified Pt surface. Thesynergistic performance found with Pt on the Sb ox-ide was not observed with the other metals such asPd, Rh, Ir, Cu and Ag, except for Re [30]. Under theextremely reducible conditions the Sb6O13 support isreduced toa-Sb2O4 phase, where the selectivity toMAL reduced to less than 20% and the total oxidationto CO2 was predominant.

4. Conclusions

1. The surface states of Pt particles and SbOx in thenew Pt/SbOx catalyst which showed good perfor-mance for the selective oxidation ofi-C4H10 andi-C4H8 to MAL with 57% and 90% selectivities,respectively, were reconsidered by means of XRD,TEM, XAFS and gas adsorption.

2. The Pt particle surface was modified by Sb subox-ides (Sb2O3 and possible O=SbOy) under the cat-alytic selective oxidation conditions, which was


relevant to suppression of the total oxidation.3. Pt promoted the redox cycle of Sb6O13, which

resulted in the selective oxidation.4. The defect pyrochlore Sb6O13 structure was im-

portant for the selective oxidation ofi-C4H10 andi-C4H8 to MAL.

5. Too heavy reduction of the catalyst led to theformation of Pt–Sb alloy particles anda-Sb2O4,which enhanced the total oxidation to CO2.

Acknowledgements

This work has been supported by CREST (CoreResearch for Evolutional Science and Technology) ofthe Japan Science and Technology Corporation (JST).

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