Structure–acidity correlation of supported tungsten(VI)-oxo-species: FT-IR and TPD studies of adsorbed pyridine and catalytic decomposition of 2-propanol

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Applied Surface Science

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tructure–acidity correlation of supported tungsten(VI)-oxo-species:T-IR and TPD studies of adsorbed pyridine and catalyticecomposition of 2-propanol

.I. Zaki ∗, G.A.H. Mekhemer, N.E. Fouad, A.I.M. Rabeehemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt

r t i c l e i n f o

rticle history:eceived 9 March 2014eceived in revised form 23 April 2014ccepted 27 April 2014vailable online xxx

eywords:upported tungstateurface acidityatalytic activityurface structure–acidity correlationR spectroscopy of adsorbed pyridine

a b s t r a c t

The amount of 10 wt%-WO3 was supported on alumina, titania or silica by impregnation with aqueoussolution of ammonium paratungstate and subsequent calcination at 500 ◦C for 10 h. Tungstate-relatedchemical and physical changes in the calcination products were resolved by ex-situ infrared (IR) spec-troscopy. Nature of exposed surface acid sites were probed by in-situ IR spectroscopy of adsorbed pyridine(Py) molecules at room temperature (RT). The relative strength of the acid sites thus probed was gauged bycombining results of temperature-programmed desorption (TPD) measurements of the RT-adsorbed Pywith those communicated by in-situ IR spectra of residual Py on the surface after a brief thermoevacuationat high temperatures (100–300 ◦C). Reactivity of the surface acid sites was tested toward 2-propanal cat-alytic decomposition, and observed by in-situ IR gas phase spectra. Results obtained were correlated withpredominant structures assumed by the supported tungstate species. Accordingly, polymerization of thesupported tungstate into 2-/3-dimensional structures, was found to be relatively most advanced on favor-

PD of adsorbed pyridine able locations of titania surfaces as compared to the case on alumina or silica surfaces. Consequently, theLewis acidity was strengthened, and strong Bronsted acidity was evolved, leading to a 2-propanol dehy-dration catalyst (tungstate/titania) of optimal activity and selectivity. Strong tungstate/support interfacialinteractions were found to hamper the formation of the strongly acidic and catalytically active polymericstructures of the supported tungstate (i.e., the case on alumina or silica).

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Supported tungsten(VI)-oxo-species (tungstate, [WxOy]n−)re best known in the field of heterogeneous catalysisy their activity in various acid-catalyzed, hydrotreatingnd alkene metathesis reactions [1,2]. Numerous researchndeavors have revealed that (i) the catalytic activity iseveloped and/or optimized after controlled reduction in H2tmosphere and the consequent generation of WO3 − x(OH)y

etal–acid bifunctional surface sites [3–6], and (ii) the ease ofeduction on a given support (Al2O3, TiO2 or SiO2) is in accord withhe following descending order: 3D-structured tungstate > 2D-

Please cite this article in press as: M.I. Zaki, et al., Strspecies: FT-IR and TPD studies of adsorbed pyridine and catahttp://dx.doi.org/10.1016/j.apsusc.2014.04.180

tructured overlayers of polytungstate > 2D-structured monolayersf polytungstate > 2D-structured monolayers of monotungstate.owever, for a given structure assumed by the supported

∗ Corresponding author. Tel.: +20 1202149149; fax: +20 862360833.E-mail addresses: [email protected], [email protected] (M.I. Zaki).URL: http://www.sccmu.org (M.I. Zaki).

ttp://dx.doi.org/10.1016/j.apsusc.2014.04.180169-4332/© 2014 Elsevier B.V. All rights reserved.

tungstate, the ease of reduction descends in the following order:on silica > titania > alumina [7–13]. Relevant surface characteriza-tion studies [7–13] have, furthermore, revealed that the structureand extent of dispersion of the supported tungstate species aregoverned foremost by the surface chemistry and monolayercapacity of the support material.

It has been found, however, that most of the research workperformed on the catalytic activity of supported tungstates hasbeen focused on the structure–reduction correlation of the cat-alysts [1–13], thus overlooking the structure–acidity correlation.Therefore, the present investigation was designed to bridge thisgap in knowledge. To accomplish this objective, (i) three samplesof supported tungsten(VI)-oxo-species were prepared by calcina-tion at 500 ◦C of three different oxide supports (namely, Al2O3,TiO2 and SiO2) each loaded with 10 wt%-WO3, (ii) nature of theacid sites exposed was probed by in-situ FT-IR spectra of adsorbed

ucture–acidity correlation of supported tungsten(VI)-oxo-lytic decomposition of 2-propanol, Appl. Surf. Sci. (2014),

pyridine (Py) molecules at room temperature, (iii) strength of theacid sites identified was gauged via combining results communi-cated by FT-IR spectra of residual Py molecules on the surface afterthermoevacuation at higher temperatures, with results brought

dx.doi.org/10.1016/j.apsusc.2014.04.180


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bout by temperature-programmed desorption (TPD) profiles, andiv) reactivity of the sites was assessed toward the decompositionf 2-propanol by FT-IR gas phase spectroscopy. Results obtainedere, then, correlated with the structure assumed by the supported

ungstate species as a function of the oxide support used, as char-cterized earlier [13].

. Experimental

.1. Materials

.1.1. Test samples10 wt%-WO3 was supported on alumina (denoted WAl), titania

WTi) or silica (WSi) by calcination at 500 ◦C (in a still atmospheref air) for 10 h of ammonium paratungstate-impregnated supportaterials. The paratungstate ((NH4)10H2W12O42·7H2O) was a 99%

ure product of Fluka Chemika, whereas the oxide support mate-ials were high-area products of Degussa (Table S1, given in theinked Supporting information file). The impregnation was carriedut from aqueous solutions of the appropriate concentration ofhe paratungstate as detailed previously [13]. A control sample ofach unloaded support material (denoted Al, Ti or Si) was obtainedfter stirring in the same aqueous medium (i.e., same impregnationedium, however without tungstate) and a similar calcination at

00 ◦C for 10 h.

.1.2. Surface probe and gas moleculesSurface probe and reactive gas molecules of pyridine

Py = C5H5N) and 2-propanol (C3H7OH) were obtained byxpansion of the duly purified (by freeze–pump–thaw cycles)apors of the corresponding liquids (98%-pure products ofDH). Pretreatment and adsorptive gas molecules (O2, N2) were9.9%-pure products of the Egyptian Industrial Gases CompanyEl-Hawamddyia/Egypt).

.2. Methods and techniques

Ex- and in-situ infrared (IR) spectra were recorded at000–400 cm−1 by averaging 200 scans at the resolution of 4 cm−1,sing a Genesis II Thermo Mattson FT-IR spectrophotomer (USA)owered with a WinFirst Lite software (Mattson Corp.) for datacquisition and handling. The ex-situ spectra were taken of lightlyoaded (<1 wt%) KBr-supported discs of the test samples. The in-situpectra were recorded using a cell system similar to that describedy Peri and Hannan [14]. A thin wafer of the test sample waseated in a stream of O2 atmosphere at 400 ◦C for 30 min and,hen, subjected to degassing at 400 ◦C for 30 min. Subsequently,pectra of the test wafer and the adsorbed species were derived byackground absorption subtraction procedure. In Py experiments,pectra were taken from the Py/wafer after a 5 min degassing at RTnd higher temperatures (100–300 ◦C). In 2-propanol experiments,he spectra were taken only from the gas phase after a 5 min con-act with the wafer at RT, or at higher temperatures (200–300 ◦C)nd cooling back to RT.

N2 sorptiometry (at −195 ◦C) was performed with a model Novaeries 2000 Quantachrome automatic sorptiometer (USA) and thetandard BET (Brunauer–Emmett–Teller) method [15] was appliedo the adsorption data thus obtained in order to derive the specificurface area (SBET, m2/g). Temperature-programmed desorptionTPD) profiles were obtained by heating up to 700 ◦C at 10 ◦C/min


f RT-adsorbed Py on the test sample using a ChemBET 3000Quantachrome/USA). For this experiment, a 1-g sample was firstegassed in a stream of N2 atmosphere at 100 ◦C for 2 h and thenxposed at RT to a saturation pressure of Py vapor for 1 h.

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3. Results and discussion

3.1. Surface structure

In a recent communication [13] we have reported results ofX-ray diffractometry, UV–vis diffuse reflectance spectroscopy andtemperature-programmed reduction studies of the structure(s)assumed by supported tungstate species as a function of its load-ing level (2–20 wt%-WO3) and the oxide support used (viz., Al2O3,TiO2 or SiO2). The investigation findings for the tungstate loadedat 10 wt%-WO3, as a function of the support used, are summa-rized in Table S1. It is obvious from the table that most of thealumina-supported tungstate (WAl) is unrestrictedly, monolayerdispersed in monomeric tungstate (WO4) species that were foundto be hardly reducible in hydrogen atmosphere [13]. On titania, thetungstate (WTi) is shown (Table S1) to be immobilized in the formof polymeric and, to lesser extent, monomeric tungstates on favor-able locations restricted to the rutile-structured support surface,thus leading to hindered monolayer coverage by multilayer buildup(paracrystalline WO3) of tungstates and uncovered areas of the sup-port surface. Whether organized in 2D-overlayers, or 3D-towers,the titania-supported tungstate was found to be reducible to WO2[13]. It is worth noting that minority, non-crystalline WTiOx mixedoxide species are suggested to form, only on the basis of UV–visdiffuse reflectance results [13], thus suggesting the involvementof non-adsorptive interactions at the tungstate/titania solid/solidinterfaces. On silica surfaces, the immobilization of tungstate (WSi)was found [13] to be undertaken dominantly by non-adsorptivechemical interactions leading to formation of W O Si bonds andtungstosilicate surface compound (Table S1). This surface com-pound was found [13] to be thermally unstable to heating in air(at 500 ◦C) and in hydrogen atmosphere. Resulting WO3/WO3·H2Oparticles were reducible to WO2 [13].

In the present investigation, further insights into the surfacestructure of the supported tungstate were sought by resolvingIR absorption characteristics that are due specifically to W O Hbond vibrations. This was accomplished by IR difference spectraobtained by absorption subtraction of ex-situ spectrum of the puresupport from that of the tungstate-loaded support. These spec-tra were taken from KBr-discs of equal masses (100 ± 1 mg/cm2),compressed under identical hydraulic pressures, and compara-ble ultimate IR absorbance values (i.e., comparable thicknessesand concentrations in the optical path). It is worth noting thatthis was successfully achieved for the test discs except for thetungstate/silica system where an intact disc of the WSi always hada detectably higher ultimate absorbance than that of the pure silica(Si) support. In a properly obtained difference spectrum, however,IR characteristics due to the support M O H vibrations wouldbe cancelled out leading to negative absorptions (i.e. pointingupwards), whereas those due to the supported tungstate W O Hvibrations would be resolved as positive absorptions (i.e., pointingdownwards). As an exemplary presentation, the ex-situ IR spectraobtained at 1850–400 cm−1 (i.e., encompassing the metal–oxygenbond vibration frequency range) for pure and tungstate-loadedtitanias (Ti and WTi, respectively) are compared to the resultingdifference spectrum (WTi–Ti) in Fig. S1.

IR difference spectra obtained over the �M O frequency regionfor the supported tungstate on the three test supports are com-pared in Fig. 1. Corresponding difference spectra over the �O Hfrequency region were obtained from in-situ spectra taken of self-supporting wafers (i.e., without KBr) and are exhibited in Fig. S2.Fig. 1 shows that the strongest positive IR absorptions resolved are


those monitored (at 1640, 1205, 1117, 810 and 476 cm−1) in thedifference spectrum (WSi–Si) of the silica-supported tungstate. Allof these five absorptions are closely similar to those monitored at1636, 1202, 1111, 808 and 473 cm−1in the spectrum obtained for


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Fig. 1. Ex-situ IR difference �MO-spectra obtained for tungstate supported on titania(WTi–Ti), alumina (WAl–Al), or silica (WSi–Si). Downward-pointing arrows indi-cate positive absorptions that may be related to the supported tungstate species.It

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solely bound to W sites or bridging W and support metal sites.

nsets are close-ups at the corresponding frequency regions meant to make clearhe included absorptions.

he pure silica support in Fig. S3. The resolution of positive absorp-ions due to the support in the difference spectrum is justifiableonsidering the inevitably thicker disc of WSi than Si. The insetlose up reveals, however, a set of well-defined, weak absorptionst 975, 955, 939, 925, 903 and 880 cm−1. Three of these absorp-ions coincide satisfactorily with diagnostic absorptions (at 980,26 and 883 cm−1) reported for dodecatungstosilicic acid [12,16],hereas the other three resemble characteristic absorptions ofetatungestic acid (at 960, 935 and 900 cm−1) [12,17]. These

esults are consistent with results of Martin et al. [12] in showinghe tungestosilicate (SiW12O40

4−) species to earn more thermal sta-ility on silica than when self-supported. Martin et al. [12] reveal,oreover, that the decomposition of these heteropolyacid species

ields tungsten oxide (WO3) and/or tungestic acid (WO3·H2O)pecies. Unfortunately, related OH-groups are not reflected in pos-


tive absorptions in the corresponding difference �OH-spectrumFig. S2[C]). Instead, the spectrum is dominated by strong negativebsorptions assignable to �OH vibrations of Si OH (at 3744 cm−1)

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and Si(OH)2 (at 3672) [18], whose hydroxyl groups are most likelyused up in the tungstate/silica chemical interactions.

Though the difference �MO-spectrum of WSi (Fig. 1) does notdocument formation of tungstate species, neither in monomericnor polymeric states, the difference spectrum (WTi–Ti) obtained forthe titania-supported tungstate (Fig. 1 and Fig. S1) resolves weak(at 1180, 1131, 723, 696 and 665 cm−1) and moderate-to-strong (at629 and 585 cm−1) positive IR absorptions that are assignable totungstate species [11,19]. Moreover, a well-defined, sharp absorp-tion is resolved at 1401 cm−1. According to Eibl et al. [11] andNakamoto [19], the weak IR absorptions at 1180–665 cm−1 mayaccount for �W O vibrations associated with variously config-ured (from octahedra to distorted tetrahedra) and polymerizedtungstate, whereas the lower-frequency absorptions at 629 and585 cm−1 are due to the companion �W O W vibrations [19]. Aspectrum taken of WO3 particles (Fig. S4) exhibits a similar set ofIR absorptions due to W O and W O W vibrations [20] but at1036, 1010, 914, 857, 768 and 633 cm−1. As per Martin et al. [12],on the other hand, the absorption at 1401 cm−1 should be due to �4of residual ammonium cations [19]. However its evident sharpnessand symmetry may also tentatively relate it to more structurally-defined vibrators, which might be associated with the WTiOx mixedoxide species formed in the support surface layer (Table S1). Over-all, the difference spectrum of WTi is in line with the previouslyobserved predominance on titania of polymeric tungstates in 2D-and 3D-structures, along with minority mixed oxide species (TableS1). The corresponding difference �OH-spectrum (Fig. S2[B]) dis-plays negative absorptions at 3727, 3675, 3613 and 3465 cm−1,which are assignable to variously configured, isolated and asso-ciated Ti OH groups [21] that were consumed in the ion-exchangeimmobilizing interactions of tungstate species with titania surface[13]. The spectrum resolves, moreover, three positive absorptionsat 3700, 3647 and 3536 cm−1, accounting most likely for OH-groupsbound to/or bridging W sites or W and Ti surface sites [11].

On the other hand, the difference �MO-spectrum obtained foralumina-supported tungstate (WAl–Al, Fig. 1) resolves a set ofhigh-frequency positive IR absorptions at 1740, 1654, 1584 and1465 cm−1, as well as low-frequency positive absorptions at 988,698 and 444 cm−1. The low-frequency absorptions are related toW O, W O and W O W vibrations of supported monomeric andpolymeric tungstates [19]. Whereas, the high-frequency absorp-tions are due to partially hydrated and/or distorted tungstatesanchored in the surface layer of the alumina support [22]. The high-frequency character of these absorptions is ascribed most likelyto the low electron-withdrawing power of the Al3+ sites to whichthese tungstates are bound. Hence, a proportion of the monolayerdispersed monomeric tungstate (Table S1) is involved in forma-tion of a surface (Al O (WO2) O Al like) compound anchored inthe support surface layer. Corresponding difference �OH-spectrum(Fig. S2[A]) monitors negative absorptions at 3786, 3727, 3680,3577 and 3294 cm−1. These negative absorptions account for used-up isolated and associated surface Al OH and Al2(OH) groups (Fig.S5), most likely, in the ion-exchange immobilizing interactions ofthe tungstate with the support surface. Two positive IR absorp-tions are resolved at 3655 and 3497 cm−1, which may account forOH-groups related to supported tungstate species.

In addition to the structural characteristics identified previously[13], and summarized in Table S1, the present IR spectroscopicinvestigation may help specifying the following. First, OH-groupsexposed on the support surfaces are functional in the ion-exchangeimmobilizing interactions with supported tungstate species. Sec-ond, supported tungstate species give rise to OH-groups either


Third, the tungstosilicates formed on the silica support are not com-pletely decomposed at 500 ◦C, and the decomposition product israther WO3·H2O than WO3 [17]. Fourth, a proportion of monolayer



4 M.I. Zaki et al. / Applied Surface S

Fig. 2. In-situ IR �CCN spectra obtained for irreversibly adsorbed Py molecules at RTon the indicated support and supported tungstate materials. The indicated assign-mab

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ents of the absorptions monitored to various Py adsorbed species (PPy = physicallydsorbed; HPy = hydrogen-bonded; LPy = Lewis acid-bonded; BPy = Bronsted acid-onded) are according to the literature data set out in Table S2.

ungstates exchange solid/solid interactions with the support lead-ng to formation of tungstate surface compounds with alumina oritania.

.2. Surface acid sites

Fig. 2 compares in-situ IR �CCN spectra obtained for irreversiblydsorbed Py at RT on the support and the supported tungstate mate-ials. The indicated assignments of the monitored IR absorptionso nature of the Py adsorbed species (LPy, Lewis acid-bound Py;Py, Bronsted acid-bound Py; HPy, hydrogen bonded Py; and PPy,hysically adsorbed Py) are based on the literature data [12,24]ummarized in Table S2. Corresponding difference �CCN-spectrabtained by absorption subtraction of the spectrum of Py/supportrom that of the Py/supported tungstate are compared in Fig. S6.n the other hand, the IR �OH spectral consequences of Py adsorp-

ion are manifested in the in-situ difference �OH-spectra exhibitedn Fig. S7, which were obtained by absorption subtraction of the


pectrum recorded before Py adsorption from that recorded afterdsorption.

The �CCN spectrum obtained for Py/Al (Fig. 2) monitors forma-ion of LPy (1613, 1490 and 1445 cm−1) and HPy species (1595, 1577


and 1445 cm−1), without exclusion of a contribution from resid-ual PPy species in the latter set of absorptions. Possible Lewis acidsites on defective spinel-structured transitional aluminas (such asthe present support, Table S1) are associated with the availabil-ity on the surface of coordinatively unsaturated (cu) tetrahedral((Al3+)t) and octahedral ((Al3+)o) aluminum sites, with the formerbeing relatively more acidic than the later [21,23]. On the otherhand, the formation of HPy species had been accompanied byred-shifting �OH IR-absorptions (not shown) which are observedat ≤3725 cm−1before Py adsorption (Fig. S2[A]), and assigned tobridging and multi-centered OH-groups [21]. According to Zaki andKnözinger [21], a similar red-shifting was experienced followinghydrogen-bonding of CO molecules to the same bridging ((Al)2OH)and multi-centered ((Al)3OH) hydroxyl groups on alumina surfaces(Fig. S5). Hence, these low �OH-frequency non-terminal hydroxylgroups have been hydrogen bond donors [21]. They are, therefore,relatively more acidic than terminally bound OH-groups (Al OH),but are not acidic enough to act as proton donors to Py [25]. Thislatter suggestion is based on the fact that the spectrum of Py/Al(Fig. 2) is void of the diagnostic �CCN absorptions of pyridiniumions (i.e., BPy species having �CCN at 1640-30 and 1540-00 cm−1,Table S2). In contrast, the spectrum of Py/WAl (Fig. 2) resolves veryweak BPy IR-absorptions near 1640 and 1540 cm−1, in additionto the absorptions assigned to LPy, HPy and PPy. The difference�CCN-spectrum obtained for Py/WAl (Fig. S6) resolves four positiveabsorptions at 1639, 1620, 1540 and 1490 cm−1. Hence, this differ-ence spectrum relates the formation of BPy (1639 and 1540 cm−1)and LPy species (1620 and 1490 cm−1) to tungstate-exposed Bron-sted (W OH) and Lewis (cu-W6+) acid sites. The correspondingdifference �OH-spectrum obtained for WAl (Fig. S7) displays twobroad negative absorptions centered at 3722 and 3665 cm−1due tohydroxyl groups exposed on the alumina support (Fig. S2[A]). Thismay reveal involvement of these OH-groups in the formation ofthe HPy species on WAl. The �OH frequencies resolved (at 3655and 3497 cm−1, Fig. S3[A]) for tungstate-exposed OH-groups arelower than those observed for the alumina-exposed OH-groups (at≥3680 cm−1). This may account for their non-terminal character(i.e., (W)2OH or W OH Al) and, hence, their detectable Bronstedacidity (i.e., proton donor character) and involvement in the for-mation of BPy species on WAl. The weak intensities of the BPydiagnostic IR-absorptions (1639 and 1540 cm−1, Fig. 2) may sug-gest that they are rather related to the minority polytungstate thanthe majority monotungstate species (Table S1). In the mean time,the frequency (1620 cm−1) resolved for LPy species on WAl (Fig. S6)is slightly higher than that (1613 cm−1) observed for LPy species onAl (Fig. 2). This may suggest that the thus generated stronger Lewisacid sites on WAl are either cu-W6+ sites exposed most probablyon the minority polytungstate species, or cu-(Al3+)t sites exposedon the alumina support.

Based on the above adopted interpretational approach, the in-situ �CCN spectra obtained for Py/Ti and Py/WTi (Fig. 2) can jointlyhelp suggesting that the titania supported tungstate are responsiblefor the generation of strong Bronsted acid sites leading to the for-mation of BPy species whose diagnostic IR-absorptions at 1637and1540 cm−1 are only monitored in the spectrum of Py/WTi. Other-wise, the rest of the monitored absorptions (at 1602, 1595, 1577,1490, 1445 and 1438 cm−1) are assignable to LPy, HPy and PPyspecies, which seem to be formed mostly on uncovered surfacesof the titania support. It is noteworthy, however, that (i) the split-ting of the �19b mode of �CCN ring breathing vibration (at 1445 and1438 cm−1), as observed in the spectrum of Py/WTi (Fig. 2), mayaccount for distinctly structurally different acid sites, which canbe correlated with the variety of established polytungstate struc-


tures (3D overlayers and 2D monolayers), and (ii) the proportionsof HPy and LPy species seem to be slightly more developed on WTithan on Ti. The difference �OH-spectrum obtained for WTi (Fig. S7)


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Fig. 3. In-situ IR �CCN spectra obtained for residually adsorbed Py molecules after5 min thermoevacuation at the indicated higher temperatures (100–300 ◦C) of RT-adsorbed Py/WAl. The Roman numerals indicate diagnostic absorptions of the Py



onitors a large negative absorption resolving a well defined max-mum at 3652 cm−1, as well as two ill-defined maxima centeredround 3580 and 3528 cm−1. These are most probably related to thenvolvement of the variously coordinated OH-groups (terminal andridging OHs) on titania [26] in the hydrogen bonding with Py. Thearious OH-groups associated with supported tungstate on titania,aving �OH at 3700, 3647 and 3535 cm−1 (Fig. S2[B]), seem to beesponsible for the Bronsted acid sites involved in the formation ofhe BPy species observed solely on WTi (Fig. 2).

Analogously, the IR �CCN spectra obtained for Py/Si and Py/WSiFig. 2) can be used to suggested that the supported tungstate onilica help formation of small proportions of LPy (1613, 1490 and445 cm−1) and BPy species (1639 and 1545 cm−1), in addition tohe majority species of HPy species (1595, 1577 and 1455 cm−1)ound mostly on the silica support surface. These results are ingreement with Martin et al. [12]. For further confirmation ofhe suggested contribution of the supported tungstate, the differ-nce �CCN-spectrum obtained for Py/WSi (Fig. S6) resolves morelearly the IR-absorptions assignable to the tungstate-bound LPy1610, 1490 and 1450 cm−1) and BPy species (1639 and 1545 cm−1).ccording to the corresponding difference �OH-spectrum obtained

or Py/WSi (Fig. S7), the negative absorption at 3743 cm−1 is dueo the Si OH groups that are involved in hydrogen-bonding Py

olecules essentially on the support surface. Neither of the differ-nce �OH-spectra obtained for WSi before (Fig. S2[C]) and after (Fig.7) Py adsorption could resolve positive IR-absorptions indicat-ng tungstate-exposed OH-groups. Hence, the minute proportionf Bronsted acid sites is related to a residual small proportion ofhe tungstosilisic and tungestic acid species that are still avail-ble on WSi. Martin et al. [12] have related generation of Bronstedcid sites on a similar silica-supported tungstate to the pres-nce of tungstosilicate (Keggin-type) species, which dwell bridging

O Si or terminal W O bonds that are easily protonated leadingo the formation of bridging or terminal OH-groups. On the otherand, Martin et al. [12] have, also, related developed Lewis acidites to the presence of cu-W6+ in the Keggin-type species or inheir thermal decomposition products (WO3/WO3·H2O).

Overall, the above presented and discussed results relate theeneration of Lewis and Bronsted acid sites on silica surface,hich is otherwise void of surface acidic sites, to supported

ungstate species and their consequent involvement in forma-ion of interaction acidic species with the support. On alumina,nd more prominently on titania, polymeric tungstate, particularlyhose established in overlayers of 3D-structures, generate terminalW OH) and/or bridging ((W)2OH or W (OH) Ti) groups of strongroton donor character. These polymeric tungstate also expose cu-

6+ sites having detectable Lewis acidity. It seems, however, thatcid sites exposed on supported tungstate are stronger than thosexposed by W O M surface compounds with the support.

.3. Acid site strength

The relative strength of the above identified surface acid sitesas gauged by TPD measurements upon heating up to 700 ◦C of RT-

dsorbed Py. In order to reveal type of the desorbing Py species (i.e.,Py, HPy, LPy or BPy), in situ IR spectra were taken of RT-adsorbedy after a 5 min thermoevacuation at higher temperatures, namely00–300 ◦C. IR spectra obtained for Py/WAl are stacked as a func-ion of the thermo-evacuation temperature in Fig. 3, whereas thosebtained for adsorbed Py on WTi, WSi, and each respective supportaterial (Al, Ti or Si) are displayed in Figs. S8–S12. The resulting

PD profiles for Py/WAl, Py/WTi and Py/WSi are compared in Fig. 4.


Fig. 3 exhibits in situ IR �CCN spectra obtained after 5 minhermoevacuation at the indicated temperatures of Py/WAl.oman numerals (I–IV) are employed to indicate diagnostic IRCCN absorption frequencies of the various Py adsorbed species

adsorbed species specified in the inset panel.

on WAl at RT, which are specified in the inset panel: I, PPy + HPy;II, HPy + LPy; III, LPy; and IV, BPy. It is obvious from the figure thatone can arrange the thermal stability of the various Py adsorbedspecies on WAl in the following ascending order: PPy (I) < HPy(II) < LPy (III) < BPy (IV). The observed blue-shift of the �CCN of LPyfrom 1613 to 1620 cm−1 with temperature increase may be dueto a surface Py coverage effect, or the presence of two differentlystrong Lewis acid sites. In fact a similar observation can be madefrom the spectra staked for Py/Al in Fig. S8 (1613 → 1624 cm−1).Hence, this may not help referring the Lewis sites exposed onWAl solely to cu-W6+ or cu-Al3+. In contrast, the absence of anyspectral indication for the formation of BPy species (IV) on Al (Fig.S8) can certainly attribute their formation and thermal stabilityon WAl to Bronsted acid sites (W–OH) exposed on the supported(poly) tungstate species. The proton donor OH-groups are mostprobably bridging W–Al and/or W–W sites, which are expectedto accumulate a more positive charge than the terminally boundOH-groups [21]. Accordingly, the four Py-desorption events (I–IV)monitored in the TPD profile of Py/WAl (Fig. 4) may be assigned,respectively, to the following adsorbed species: PPy at 107 ◦C,HPy at 250 ◦C, LPy at 310 ◦C, and BPy at 360 ◦C. Though these


results may suggest that supporting tungstate on alumina does notsignificantly improve the Lewis acidity, they may certainly helpconsider them responsible for the generated Bronsted acidity. The



6 M.I. Zaki et al. / Applied Surface S

Fip

mS

tPosthcfsifdoatBfdbe

ht

ig. 4. TPD profiles obtained for irreversibly adsorbed Py molecules at RT on thendicated test samples (WAl, WTi and WSi). The Roman numerals significances aser Fig. 3.

inor proportion of polytungstate established on alumina (Table1) might be the platform of the emerging Bronsted acidity.

On similar grounds, one may find the in situ IR �CCN spec-ra obtained as a function of thermoevacuation temperature fory/WTi (Fig. S9) to suggest an analogus thermal stability ascendingrder: PPy (I) < HPy (II) < LPy (III) ≤ BPy (IV). As a matter of fact, con-iderable IR absorptions due mainly to LPy (III) and BPy (IV) are stillo be observed after thermoevacuation at 300 ◦C of Py/WTi. This,owever, is significantly different from the case of Py/Ti, where theorresponding IR �CCN spectra (Fig. S10) do not show any sign oformation of BPy species (similar to the case on Al (Fig. S8)), buthow weak absorptions due to LPy species. In line with these find-ngs, the desorption event at 107 ◦C in the TPD profile obtainedor Py/WTi (Fig. 4) can be assigned to PPy species. Whereas, theesorption of HPy species is ill-resolved from the desorption eventf LPy near 310 ◦C and that of BPy is assigned to the weak eventt 360 ◦C. Though the desorption of Py/WTi seems to be similaro that of Py/WAl, the tungstate-related development not only ofronsted acidity, but also of Lewis acidity on WTi is a distinct dif-

erence between the WTi and WAl systems. The reason behind theetectably tungstate-related improved acidity on WTi may welle ascribed to the higher polytungstate/monotungstate proportion


stablished on WTi than WAl (Table S1).Considering the case of Py/WSi, it is plausible to refer first to the

igh thermal instability of the PPy, HPy and LPy species allowedo form on the pure silica support (Fig. S12). IR �CCN absorptions


of these three adsorbed species become hardly detectable in thespectra obtained after thermoevacuation at ≥100 ◦C of Py/Si (Fig.S12). This picture is a bit modified after loading the support withtungstate. Fig. S11 reveals the generation of BPy species with alimited stability to thermoevacuation at 100 ◦C, and improvementof the thermal stability of LPy species up to 300 ◦C on WSi. Accord-ingly, the TPD-monitored desorption event at 107 ◦C of Py/WSi(Fig. 4) may involve both PPy and BPy species, whereas the highertemperature events (at 200 ◦C and 255–310 ◦C) may be attributedto the desorption of HPy and LPy, respectively. Here, the mostprominent effect of supported tungstate is in the development ofLewis acidity on WSi rather than Bronsted acidity. This may fur-ther emphasize that the formed tungstosilicate and WO3/WO3·H2Ospecies (Table S1) are more capable of contributing cu-W6+ Lewisacid sites than Bronsted acid sites.

3.4. Acid site reactivity

2-propanol was the reactive probe molecule used in the presentinvestigation to gauge the reactivity of the above identified acidsites. It decomposes on metal oxide catalysts via dehydration to givepropene and/or dehydrogenation to give acetone [27]. A high dehy-drogenation selectivity means prevalence on the catalyst surfaceof reactive basic sites (cu-O2−/ OH−) and/or redox metal couples(Mn+ O M(n + 1)+), whereas a high dehydration selectivity wouldmean prevalence of reactive acid sites (cu-Mn+/ OH+) [27–29].Established mechanistic aspects of 2-propanol dehydration sug-gest two possible pathways [30]: (i) coordination of the alcoholmolecule to a Lewis acid site, whereby it is activated for protonationby an adjacent Bronsted acid site to split water and release propene,and (ii) a direct protonation of the alcohol oxygen by a Bronsted acidsite followed by rearrangement to split water, regeneration of theBronsted acid site and the release of propene. It is obvious that theformer reaction pathway requires the availability on the catalyticsurface of reactive Lewis–Bronsted acid site-pairs, whereas the lat-ter pathway only depends on the availability of reactive Bronstedacid sites.

Fig. 5 demonstrates in-situ IR gas phase spectra taken of 2-propanol/catalyst before (RT) and after 5 min heating at the setreaction temperature (250 or 300 ◦C) and cooling back to RT. Thecatalysts tested were the pure support materials (Al, Ti or Si)and the supported tungstates (WAl, WTi or WSi). The spectrumobtained at RT displays nothing but the absorptions of 2-propanolmolecule bond vibrations [31]: �OH (at 3655), �(CH3)as (2975),�(CH3)s (2883), ı(CH3)as (1466), ı(CH3)s (1383), ı(OH) (1250), �CO(1150) and �CC (1066 cm−1). The spectra obtained after the hightemperature treatment reveal the emergence of new absorptionpeaks (at 3091, 1825, 1650 and 1441 cm−1) at the expense of thealcohol diagnostic peaks. All of the emerging peaks are due to bondvibrations of propene molecule [32]. These results indicate that thesole detectable reaction occurring at the 2-propanol/catalyst inter-face is the dehydration pathway to release propene into the gasphase. Quantification of the spectral results into % conversion of thealcohol per g-catalyst (and per m2-catalyst) at the chosen reactiontemperatures (250 and 300 ◦C), as well as the catalyst selectiv-ity (%), resulted in the data set out in Table 1. The quantificationprocessing implemented changes conceded by the integrated areaof analytical peaks of those diagnosing the alcohol (namely, �OHat 3655 cm−1) and its possible decomposition products (�C C ofpropene at 1650 cm−1, and �C O of acetone at 1740 cm−1) versuspre-determined calibration curves (pressure vs. peak area) for thealcohol and the products, as detailed elsewhere [33]. The mathe-


matical formulae used are those given as footnotes to Table 1.A thorough inspection of Table 1 may help make the following

conclusive remarks: (i) all of the catalysts tested are overwhelm-ingly alcohol dehydration selective whether at 250 or 300 ◦C,


ARTICLE IN PRESSG ModelAPSUSC-27780; No. of Pages 8

M.I. Zaki et al. / Applied Surface Science xxx (2014) xxx–xxx 7

Table 1Percentage conversion of 2-propanol and the selectivities toward the formation of propene (Spp) and acetone (Sac) at 250 ◦C and 300 ◦C on the indicated test catalysts.

Catalyst SBET (m2/g) Conversiona (%) (per g-Cat.) Selectivityb (%)

250 ◦C 300 ◦C Spp Sac

250 ◦C 300 ◦C 250 ◦C 300 ◦C

Al 69 26 36 91 94 9 6WAl 42 73 83 100 99 0 1Ti 34 0 2 0 36 0 64WTi 26 92 100 100 100 0 0Si 136 4 17 85 83 15 17WSi 105 68 51 97 98 3 2

a % Conversion per g-Cat. = [(Pi − PT)/Pi] × 100, where Pi = the initial alcohol pressure (in Torr), and PT = the alcohol pressure at reaction temperature (T).b Spp = [(Ppp)/(Pac + Ppp)] × 100; Sac = [(Pac)/(Pac + Ppp)] × 100, where Pac = the formed acetone pressure (in Torr), and Ppp = the formed propene pressure (in Torr) at the set

r

ewtttdtt

F5

eaction temperature.

xcept for the pure titania support which is shown to be veryeakly active (Conv. = 2%) only at 300 ◦C with obvious selectivity

oward the dehydrogenation pathway; (ii) upon supporting theungstate species, both the % conversion and the dehydration selec-ivity upsurged, particularly on titania; (iii) 100% conversion and


ehydration selectivity are allocated to WTi; and (iv) the reactionemperature effect (250 → 300 ◦C) is insignificant for most catalystsested.

ig. 5. In-situ IR gas phase spectra taken of 10 Torr 2-propanol before (RT) and after min heating at 250 ◦C over the indicated catalysts and cooling back to RT.

A comparative assessment of the catalytic activity of supportedtungstate is facilitated by plotting the alcohol % conversion at250 ◦C, whether specific (per g-Cat.) or intrinsic (per m2-Cat.),as a function of catalyst used, in Fig. 6. Both of the specific andintrinsic relationships are shown to agree on the following ascend-ing order of 2-propanol dehydration activity: WSi < WAl < WTi.This fact means that the observed enhancement of dehydrationactivity is due to increasing population and/or reactivity of acidsites in the same order. It is worth noting that this activityascending order encompasses a parallel increase in the poly-tungstate/monotungstate proportion which maximizes on titania(Table S1). Alongside, the Lewis acidity is improved and strongBronsted acidity is evolved. It is obvious from these results thatthe higher the polymerization of supported tungstate in 2D/3Dmonolayers and overlayers, the more strong and catalytically activethe acid sites generated. Moreover, the observed optimization of2-propanol dehydration on WTi may help suggesting occurrenceof the reaction via a concerted mechanism [30] whereby alcoholmolecules bound to Lewis sites are activated for electrophilic attack


by adjacent Bronsted acid sites.

Fig. 6. Specific (per g-Cat.) and intrinsic (per m2-Cat.) conversion (%) at 250 ◦C of2-propanol as a function of catalyst used.


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. Conclusions

The above presented and discussed results may help drawinghe following conclusions:

1) Polymerization of supported tungstate species into 2D/3Dstructures in mono/overlayers exposes enhanced Lewis andBronsted acid sites of high catalytic acivity and selectivitytoward 2-propanol dehydration to propene formation.

2) On titania, polymerization of tungstate is highly facilitatedseemingly on favorable (rutile structured [13]) domains of thesupport surface, resulting in a 100% active and selective 2-propanol dehydration catalyst.

3) On alumina, polymerization of tungstate species is retardedby formation of monolayers of monomeric tungstate stronglybound to the support surface and, hence, a moderately activeand selective 2-propanol dehydration catalyst is produced.

4) On silica, polymerization of tungstate species is largely ham-pered by a high interfacial chemical affinity leading to theformation of tungstosilicate interaction species of weak activityand selectivity in the alcohol dehydration reaction.

5) Exposure of coordinatively unsaturated W6+ sites and bridging-OH groups on surface structures of supported polytungstatespecies are intimately related to the consequent enhancementof the Lewis and Bronsted acidity, respectively.

cknowledgments

The authors are indebted to the Alexander von Humboldt-oundation (Bonn) for an equipment donation (V-815/03029).IMR appreciates a financial support from Minia Universityesearch Administration.

ppendix A. Supplementary data


Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.014.04.180.

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Structure–acidity correlation of supported tungsten(VI)-oxo-species: FT-IR and TPD studies of adsorbed pyridine and catalytic decomposition of 2-propanol