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Microporous and Mesoporous Materials 99 (2007) 126–131

Supported oxides as combustion catalysts and as humiditysensors. Tuning the surface behavior by inter-phase charge transfer

Monica Caldararu *, Cornel Munteanu, Paul Chesler, Mariana Carata, Cristian Hornoiu,Niculae I. Ionescu, Georgeta Postole, Veronica Bratan

Institute of Physical Chemistry ‘‘Ilie Murgulescu’’ of the Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania

Received 28 April 2006; received in revised form 3 July 2006; accepted 4 July 2006Available online 31 October 2006

Abstract

The changes of the electrical properties of semiconductor oxide catalysts, when measured in situ provide useful information aboutsurface processes as adsorption/desorption/reaction occur with charge transfer. The electrical properties of polycrystalline powders,mainly controlled by inter-grain Schottky-type barriers, are sensible to the ‘‘quality’’ of the surface/inter-grain domains, to sample mor-phology and to changes in charge and coverage of the surface due to operating conditions. This can be exploited to study oxide catalystsdispersed on supports at nanoscale levels for answering questions like: (i) adsorption ability of the surface layer and (ii) the influence ofthe active phase and of the support. It is demonstrated that for low loading supported SnO2 or V2O5 catalysts (also important as sensormaterials), the conductivity plots show the ‘‘fingertip’’ of the support, which controls the behavior of the supported phase. As an exam-ple, the lower propylene combustion activity of SnO2 when dispersed on c-Al2O3 is explained by hydrophile/proton conductor propertiesof the support; c-Al2O3 diminishes the oxygen adsorption ability of SnO2 by providing a higher concentration of molecularly adsorbedwater and surface OH groups towards its surface. It is proposed that by selecting appropriate n-type semiconductor oxide supports (asTiO2), and by increasing the surface conductivity one can increase the oxygen adsorption ability of the supported phase, with increasedactivity for combustion/chemical sensing. This suggests the possibility of tuning the surface properties of the supported oxide by inter-phase charge (electron or ion) transfer.� 2006 Elsevier Inc. All rights reserved.

Keywords: Supported oxide catalysts; c-Alumina; Titania; Tin oxide; Propylene combustion; In situ electrical conductivity

1. Introduction

The use of supports is a current procedure to obtainmaterials with high surface areas or to stabilize a certaincatalytically active phase. The way the oxide support isinvolved in the formatting/structuring of the active surface(which in turn determines the specific properties of the cat-alyst) is still a debated subject. In spite of being well knownthat the contact area between the supported oxide and thesupport acts as an interface for the transfer of species and/or of electric charges between the two solids, this process is

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doi:10.1016/j.micromeso.2006.07.039

* Corresponding author. Tel./fax: +40 21 312 1147.E-mail address: mcaldararu@chimfiz.icf.ro (M. Caldararu).

still not systematically studied and the choice of the sup-port is in many cases arbitrary, or intuition based. Sinceit is well known that most of the adsorption/desorption/reaction processes involved in the catalytic transformationof reactants into products occur with charge transfer, it isobvious that the nature of the electrical properties of thesupport must be important for the surface behavior ofthe supported phase.

The changes of the electrical/dielectric properties ofsemiconductor oxides, when measured in situ provide use-ful information about surface processes occurring withcharge transfer. The electrical response of polycrystallinepowders is determined by the transport of carriers overthe Schottky-type barriers at the contacts between the adja-cent grains; the height of these barriers depends on several

Table 1The surface area and the oxide content (expressed as the percent ofsupported oxide cation) of the catalysts studied

Materiala Abbreviation Men+ (wt%)b SBET

(m2/g)

c-Al2O3-Degussa, oxide C Al2O3 – 110TiO2–DT51(Rhone Poulenc) TiO2 – 108SnO2–BDH SnO2 78.8 9SnO2/c-Al2O3 A1 3.1 108V2O5/c-Al2O3 V5Al 2.46 112SnO2–TiO2 B1 3.4 90

a Preparation described elsewhere [6].b Sn4+ or V5+ from supported oxide phase as Men+.

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parameters, among which the ‘‘environmental work func-tion’’ (expressing the surface sensibility to atmosphere)could be more or less important depending on the electricalproperties of the solid [1–3]. It is stated [3] that the higherthe ionicity of the solid, the higher the sensitivity of thebarrier heights to the influence of the environment.Changes produced by hydration/dehydration and/or byadsorption/desorption/reaction are mainly reflected in thetopography of inter-grain areas and thus in the height ofinter-grain Schottky-type barriers.

In the case of n-type polycrystalline semiconductoroxides, the Schottky barrier formation is attributed to anegative surface charge accumulation (due to the discreteenergy levels introduced by oxygen within the band gapat the surface) [3]. The ability of n-type oxides of adsorbingoxygen as O2� and O� species (facilitated by the surfaceelectronic conductivity) is directly connected with theiractivity in oxidation reactions. Obviously, for oxide sup-ported oxides the optimal choice of the support and theextent of cooperation between the supported phase andthe support might enhance the surface conductivity andcorrespondingly the oxidation activity.

SnO2 is among the oxides frequently used as a materialfor sensors and as a component of oxidation catalysts;however, its activity and electrical response to the ambientare limited by its regularly low surface area. The SnO2

deposition on higher surface area supports like c-Al2O3

or TiO2 was expected to increase the activity/electricalresponse.

Recently we noticed that the activity of SnO2 for propyl-ene oxidation was strongly diminished when supported onc-Al2O3 [4], in spite of the higher exposed SnO2 surface.Since propylene has a basicity comparable with ammonia[5] the surface acidity of the SnO2/c-Al2O3 catalysts wasstudied by ammonia adsorption microcalorimetry at150 �C [6]; it was shown that the deposition of SnO2 in var-ious amounts had a small influence and that the changes ofthe surface acidity were mainly determined by the modifi-cation of the support (by higher temperature treatment).Indeed, surface hydration or dehydroxylation during thethermal treatment can change the nature and distributionof surface OH groups, modifying the surface acidity in aconsiderable manner. At the same time, the sensitivity forwater vapor adsorption of c-Al2O3 is directly connectedwith the surface concentration of OH-groups [7]. SnO2/TiO2 series was markedly more acidic than the aluminasupported series, mostly due to Lewis acid sites [8] and thismust be related with the lattice ionicity.

In some of our previous papers we have shown that theresponse of ac electrical conductivity to the change ofoperating conditions allows to study in situ the surfacedynamics of catalysts [9–11]; in this paper we used the samemethod of investigation to study the effect of the nature ofthe support (i.e. c-Al2O3 a protonic conductor in presenceof humidity and TiO2, an n-type semiconductor) on thecatalytic activity/surface behavior of the supported cata-lysts with SnO2 as the active phase. At the same time, we

were interested to compare the surface behavior and theactivity in propylene oxidation of two catalysts obtainedby deposition of two different semiconductor oxide phases(i.e. SnO2 and respectively V2O5) on c-Al2O3 support.

2. Experimental

2.1. Method

The electrical conductance G and capacitance C weremeasured in situ on c-Al2O3 and on SnO2/c-Al2O3,V2O5/c-Al2O3 catalysts with low (3% Sn4+, and respec-tively 2.4% V5+) oxide content (abbreviated as Al2O3, A1and respectively V5Al). SnO2 supported on titania (B1)and the corresponding support were also studied (see Table1). The catalysts were prepared by impregnation, as alreadyreported [6].

The method was to follow in situ the changes of conduc-tance G in gas flow as a function of temperature, by repeat-ing the measurements on the same sample, in differentatmospheres according to a specific protocol (differentialstep technique, DST [9,12]). By comparing the plotsobtained in successive cycles it is possible to have a deeperinsight on the surface dynamics in conditions similar withthose encountered in the practical use in catalysis or forgas sensing.

A special reaction cell [9] consisting of two coaxial tan-talum cylinders as electrodes, embedded in a Pyrex glasstube and connected by tungsten wires to a precision RLCbridge (TESLA BM 484, 1592 Hz) allows to measurein situ (in conditions simulating the catalytic experimenti.e. in gas flow and at atmospheric pressure) the changesof the parallel electrical conductance Gp and capacitanceCp of the catalyst powder under the influence of the gas/temperature. The catalyst powder (fraction between 0.25and 0.5 mm) was placed between the electrodes and sup-ported on a frit. The inlet/outlet gases were permanentlymonitored by using on-line gas chromatography (TC detec-tor, Porapak Q and molecular sieves 5 A). Temperature inthe powder bed was monitored with a thermocouplelocated in the center of the cell [9].

The protocol of experiments consisted of successive lin-ear heating (2 �C/min between 20 and 400 �C)–cooling

Fig. 1. Arrhenius dependence of conductance G during heating inpropylene:air mixture 1:10 for Al2O3, A1, TiO2, B1 and V5Al samples.

128 M. Caldararu et al. / Microporous and Mesoporous Materials 99 (2007) 126–131

(approx. 10 �C/min) cycles in various gases; each cycle waspreceded by a sequence of flushing for 30 min at room tem-perature with the corresponding gas. Before stopping thegas flow at the end of the cooling sequence, the cell wasclosed by a six-ways valve and the sample was kept stati-cally in the same atmosphere up to the next cycle. The over-all flow-rate in all cases was 69.3 cm3/min. For details, seeRef. [12–14].

Cycles succession was as following: DAr (1-3), DO,DAr-4, CT- where DAr, DO abbreviate dry argon, dryoxygen, respectively, and CT represents the mixture usedfor the catalytic test, i.e. C3H6–air 1:10 (these cycles werefollowed by some other runs, not reported here). Dry gases(except propylene) were obtained by passing the researchgrade compounds through molecular sieves units.

The apparent dielectric constant e was evaluated fromcapacitance data as shown previously [13], by consideringthe cell as a cylindrical condenser, with the powder to bemeasured as the dielectric.

The BET specific surface areas of the samples were eval-uated by N2 adsorption. The pore size distribution wasdetermined from the desorption branch of the N2 isotherm,using an ASAP 2010M instrument, from Micromeritics.

TG experiments were performed in nitrogen flow byheating the samples up to 400 �C with a ramp of 5 �C/min on a TG-DSC 111 instrument from Setaram.

2.2. Materials

In Table 1 are presented the materials used and theircharacteristics. c-Al2O3 is a nonporous material, whileTiO2 was mesoporous, with pore volume of 0.30 cm3/g.

3. Results

As shown by electrical conductivity measurements inDAr and in DO, respectively, both SnO2 and TiO2 behaveas n-type semiconductors (i.e. lower G values in DO andhigher in DAr cycles, respectively), as already reported inthe literature [15,16]. The G values measured for bulkSnO2 (not presented here) were in all cycles more thantwo orders of magnitude higher than those of Al2O3 andof the A1 sample, indicating a considerable influence ofthe support.

In the case of Al2O3 [12] and of alumina supported A1sample [4], the G-temperature dependences are similar(very different from those of SnO2), with a characteristicU-shape; the decreasing arm of the G-temperature plotsis clearly related with the evolution of water in effluent[4,12]. In Fig. 1 are presented for comparison the Arrheniusplots obtained for these samples in CT runs. Alumina andthe supported tin oxide sample A1 showed the same Ushape pattern; the behavior was similar for V5Al sample,but with a much lower decrease of conductivity at lowtemperature. On the other hand, B1 sample followed thepattern of the titania support (Fig. 1). Except for a smalllow temperature domain, G values were higher in this

temperature range for both titania and B1 samples. Thedecrease of conductance G on heating at low temperatureand the associated loss of water (detected by GC in efflu-ent) were smaller.

The temperature dependences of the measured capaci-tance C (and of the calculated apparent dielectric constante) are very different for alumina and respectively for titaniacontaining series in CT (Fig. 2): high values at low temper-ature and low high temperature values for alumina and A1sample, and a quite opposed trend for titania and B1 sam-ple. The last pair shows also much higher values for thecapacitance (and correspondingly for the calculated dielec-tric constant). In Fig. 3a and b we presented for compari-son the temperature dependence of the apparent dielectricconstants in DAr and DO runs calculated from capacitancedata. As proved by the evolution of water in effluent, themaxima of e at low temperature for Al2O3 and A1 samplesare related with the presence on the surface of weaklyadsorbed water species which are progressively removedon heating at higher temperature. V5Al catalyst behavesdifferently, the evolution of e is different of that of aluminasupport, when submitted to the same protocol (Fig. 3a); the

Fig. 2. Temperature dependence of capacitance on heating in CT run(propylene:air mixture 1:10) for Al2O3, A1, TiO2 and B1 samples.

M. Caldararu et al. / Microporous and Mesoporous Materials 99 (2007) 126–131 129

difference is even more obvious for titania and B1 series(Fig. 3b). The calculated apparent dielectric constant, e,for B1 increased continuously on heating in DAr3, i.e. inslightly reducing conditions (with a sharper variationabove 250 �C), and showed lower values in DO. These indi-cate a much higher lattice mobility in comparison with thealumina supported sample, reflected also in the catalyticactivity (see below). By comparison, TiO2 had lower evalues than B1 in both DAr3 and DO cycles. Based oncapacity/dielectric constant relative variations at low tem-perature and also on the lower amount of water evolvedin the effluent on heating, it can be stated that titania sur-face contains less weakly adsorbed water than alumina.This is also supported by TG experiments performed inN2 flow. As shown in Fig. 4, both samples gain a smallweight on starting the heating up to about 30 �C (byadsorbing residual moisture from ambient), but this effectis lower on TiO2 (0.99% for alumina versus 0.21% forTiO2). A sharp loss of weight up to about 120–130 �C isfollowed by a slower one above, in all cases. This temper-ature is coincident with the end of the low temperaturepeaks in the capacitance–temperature plots for c-Al2O3

[13] and is relevant also for the decrease of the capacitancein CT run (Fig. 2) for alumina samples. Taking the maxi-mum value of weight as a reference, the total mass lossup to 400 �C was 4.10% for Al2O3 and 3.06% for TiO2,the samples having comparable surface areas (see Table 1).

The catalytic activity results obtained during the CT runare presented in Table 2. The best performances areobtained for B1 sample. The highest catalytic activity ofTiO2 supported SnO2 sample in propylene oxidation isdirectly related with the highest value of electrical conduc-tivity/capacity/calculated e of the support and of the cata-lyst surface.

4. Discussion

As shown by the presented data, in the case of these lowloaded supported oxides, the nature of the support has astrong effect on the surface behavior of the supportedphase. G-temperature plots revealed that the support putsits ‘‘fingertip’’ on the behavior of the supported catalysts.Al2O3 is a protonic conductor in this temperature rangeand the sharp decrease of conductivity at low temperature(regained also in the case of alumina-supported samplesand corresponding to the low temperature peaks in C-tem-perature plots) is associated mainly with the loss of wateras a proton vehicle (and of other weakly adsorbed species)from the surface [12–14]. In this respect, it is clear that thedeposition of SnO2 on alumina surface diminishes the num-ber of weakly adsorbed water species. This statement isalso supported by the evolution of capacitance (and ofthe corresponding calculated dielectric constant, e) on heat-ing, and confirmed by the lower amount of water evolvedin the exit gas while flushing dry gases. However, for thesame support (as alumina, in our case) the ‘‘quality’’ ofthe supported layer is also an important parameter whichresults by comparing the plots of calculated apparentdielectric constant of alumina supported tin oxide and van-adia samples. The difference between the samples must berelated with the effect of vanadia, more uniformly dispersedon the alumina surface, imprinting the main characteristicsof the vanadia layer. As shown, V5Al sample is also slightlymore sensible to the presence of oxygen (lower e values inDO) than A1 sample. The height of the inter-grain Scho-ttky-type barriers depends strongly also on the dielectricconstant of the surface layer [2]: the higher the dielectricconstant of the surface layer, the lower the surface barrierheight and the higher the conductivity. In the case of thelow loaded catalysts the surface layer of the solid exposesboth the oxides. The different behavior of temperaturedependence of G/dielectric constant for V5Al sample, andthe better sensitivity to oxygen in comparison with aluminaindicates a better dispersion of vanadia on alumina; in thiscase, the vanadia phase has a stronger effect on the surfacebehavior in comparison with tin oxide. This is also demon-strated by the higher catalytic activity of V5Al sample inpropylene oxidation (Table 2). The lack of electronic con-ductivity of alumina in this temperature range does notfavor oxygen adsorption and thus the catalytic activity ofalumina in oxidation reaction is practically null up to400 �C. At the same time, due to lower dispersion, the oxy-gen adsorption ability of SnO2 must be diminished by thepresence of alumina domains in its proximity, as alumina

Fig. 3. The temperature dependence of the calculated apparent dielectric constant e on heating in dry argon and dry oxygen (DAr3 and DO cycles): a –Al2O3, A1, B1 and V5Al samples; b – TiO2 and B1 samples.

Fig. 4. TG analysis on flushing Al2O3 and TiO2 with N2 (the heating rate5 �C/min) up to 400 �C.

Table 2Propylene oxidation to CO2 and CO in C3H6–air mixture 1:10

Catalyst Temperature(�C)

Conversion(%)

CO2 selectivity(%)

CO selectivity(%)

A1 390 11.0 72.5 22.6V5Al 403 66.4 84.4 15.6Al2O3 403 6.9 �100 –B1 405 74.6 89.9 9.9TiO2 400 56.7 74.4 25.6SnO2 395 30.0 83.6 16.4

130 M. Caldararu et al. / Microporous and Mesoporous Materials 99 (2007) 126–131

provides water molecules which compete with oxygen onthe same adsorption sites of SnO2 [9]. The better dispersionof vanadia layer makes this ‘‘moderating’’ effect lessimportant.

The increase/decrease of the capacitance/dielectric con-stant values are related with the increase/decrease of themobility (and of the number) of the surface dipoles. ForAl2O3 and A1 samples, the highest number of surfacemobile dipoles at low temperature is represented by weaklyadsorbed water molecules, accumulation of which increasesthe dielectric constant [13,17,18]. The loss of these specieson heating in dry atmosphere is accompanied by the sharpdecrease of conductance [12]/capacitance/dielectric con-stant [13]. This is also proved by the fact that on flushing

M. Caldararu et al. / Microporous and Mesoporous Materials 99 (2007) 126–131 131

with humid gases these values increase sharply at roomtemperature. This very obvious effect of moisture recom-mends the samples to be used as humidity sensors, at lowtemperature.

A different situation is encountered for TiO2–B1 series.The very different behavior of e-temperature plots forTiO2 and B1 must be connected with n-type semiconductorproperties and higher reducibility of titania-containingphases at high temperature (see below). The effect of lowtemperature water desorption on electrical properties ofthese compounds is less important; however, on increasingthe temperature, the dipoles in the lattice (as for examplecation–oxygen or cation–oxygen vacancy pairs) becomemore mobile, which explains the increasing higher valuesof C and e. Moreover, as shown in Fig. 3b, the valuesare higher in slightly reducing (inert) atmosphere, andlower in the presence of oxygen. This points to the highsensibility of e to the level of surface oxidation. This is easyto understand, as the higher the level of surface oxidation,the higher the lattice perfection and the lower the mobilityof the lattice dipoles.

Higher e values (indicative for surface reduction) implic-itly decrease the height of Schottky-type barriers, increas-ing the conductivity in the presence of reducing gases, animportant effect for chemical sensing. At the same time,the higher lattice mobility is associated with higher com-bustion activity, as shown in Table 2. To be noted that inCT cycle the C (e) values are higher for TiO2 in comparisonwith B1 sample, while the order is reverse in DAr3 and DOcycles. At a first sight, it seems that in the presence of pro-pylene and oxygen, TiO2 would be more reducible thanSnO2/TiO2 and this is not in favor of activity. However,the higher C and e values for TiO2 in CT run could be alsodue to the presence of higher amounts of some hydrocar-bon derived species/coke precursors on the titania surface,whose formation is less important in the presence of SnO2.This suggests the cooperation between these two oxidesduring catalysis.

5. Conclusions

The electrical properties of the support strongly influ-ence the surface behavior of the supported phase. This inturn is reflected in the sensitivity of inter-grain Schottkybarriers to the environmental factor, and consequently inthe observed electrical characteristics of the surface in var-ious atmospheres. The influence of the support is also dem-onstrated by the catalytic properties of the correspondingmaterials in propylene oxidation. The lower propylenecombustion activity of SnO2 when dispersed on c-Al2O3

could be explained by hydrophile/proton conductor prop-erties of the support; it provides a higher concentrationof molecularly adsorbed water and surface OH groups on

the supported oxide surface which diminishes the oxygenadsorption ability of SnO2 moderating its activity in oxida-tion. However, these samples could be successfully used ashumidity sensors, at low temperature. By selecting appro-priate n-type semiconductor oxide supports (as TiO2),one can increase the oxygen adsorption ability of the sup-ported phase (by increasing the surface conductivity), withincreased ability for combustion/chemical sensing. Thissuggests the possibility of tuning the surface properties ofthe supported oxide by inter-phase charge (electron orion) transfer.

Acknowledgments

The authors wish to thank Prof. Dr. A. Gervasini(Dipartimento di Chimica Fisica ed Electrochimica, Uni-versita di Milano) for providing the V2O5/Al2O3 sampleand also to INSIDEPORES NoE team for the financialsupport for participation to its first workshop.

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