charge transfer in titania supported noble metal catalysts - Springer Link

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Topics in Catalysis Vol. 46, Nos. 1–2, September 2007 (Ó 2007) DOI: 10.1007/s11244-007-0314-8

Impedance measurements in catalysis: charge transfer in titania supported noble metal catalysts Wilfrid Jochuma, Dominik Ederb, Gernot Kaltenhausera, and Reinhard Kramera,* a Institut fu¨r Physikalische Chemie, Universita¨t Innsbruck, 6020 Innsbruck, Austria Department of Materials Science and Metallurgy, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3QZ United Kingdom

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Hydrogen treatment of Pt/TiO2 leads to an exceptional increase in electric conductivity even at moderate temperature presumably due to the formation of donor electrons during a superficial reduction process. Interestingly, after cooling to lower temperatures that are typically applied for catalytic reactions the observed high conductivity was largely preserved. The enhanced charge carrier density in titania is assumed to cause a considerable charge transfer between support and metal. In this work we show that this charge transfer is correlated with typical catalytic parameters (e.g. hydrogen adsorption capacity and catalytic activity for a hydrocarbon skeleton reaction). Based on these observations we suggest and discuss a modified charge transfer mechanism in addition to the decoration mechanism as a possible reason for the SMSI effect occurring during hydrogen treatment already at moderate temperatures. KEY WORDS: platinum; titania; hydrogen pretreatment effects; SMSI; electric impedance; charge transfer; electronic intergap sites; hydrogenolysis of methylcyclopentane.

1. Introduction The fact that the catalytic properties of noble metals are influenced by the support was first emphasized by Schwab [1] and results have been summarized by Solymosi [2]. This effect was reinvented by Tauster et al. [3] in 1978 for group VIII noble metals supported on titanium oxide (TiO2). The term ‘‘strong metal-support interaction’’ (SMSI) stimulated an overwhelming interest in both fundamental and industrial catalysis triggering an explosion of studies during the 1980’s. This interest mainly arose due to a reported suppression of H2 and CO chemisorption on these metals when dispersed on the surface of titania and activated in hydrogen at elevated temperatures, as well as with the associated unique changes in their catalytic behaviour (decreased catalytic activity for hydrocarbon skeleton reactions and increased catalytic activity for CO hydrogenation). Much effort has since been directed at understanding these phenomena, which are by no means limited to titanium oxide [3–11]. The suppression of H2 and CO chemisorption was initially attributed to an electronic polarisation within a heteroatomic metalmetal bonding between Ti3+ and the noble metal [12,13]. Other ‘‘electronic theories’’ included the formation of an alloy between the noble metal and titanium, a delocalised charge transfer of electrons between reduced titania and the metal [14], and a bulk effect charge transfer based on the n-type semiconducting Dedicated to Konrad Hayek. * To whom correspondence should be addressed. E-mail: [email protected]

properties of TiO2 [15,16]. Although Fung observed a stronger binding energy shift of core level electrons in smaller particles than in larger particles [17], other XPS studies reported no change when approaching the SMSI state [18,19]. Furthermore, it has been estimated that any charge transfer occurring at the metal support interface would be rather small and hence not enough to significantly affect the catalytic properties [20]. In view of these arguments Simoens et al. introduced the ‘‘decoration model’’ and claimed the encapsulation of the noble metal by substoichiometric titania species to be responsible for the SMSI effect [4]. This model was greatly supported by electron microscopy studies [21–23] as well as by a variety of surface science studies on TiO2 single crystals [24]. Soon, this ‘‘decoration model’’ was accepted almost universally as a pure geometric origin for SMSI and the other explanations were widely discarded. Recently, the interest in the SMSI effect arose again stimulated by studies on Au and Pt on TiO2 that questioned the universality of the ‘‘decoration model’’ [25]. First of all, the decoration of the noble metal was observed only at reduction temperatures above 700 K [26,27]. XPS measurements of Rh/TiO2 model catalysts showed that the encapsulation of the metal by TiOx needed even higher temperatures (900 K) [24]. In contrary, SMSI-like behaviour of titania supported noble catalysts was observed already at far lower reduction temperatures [14]. For instance, the catalytic activity of Rh/TiO2 for the hydrogenolysis of methylcyclobutane started to decrease as the temperature for hydrogen pretreatment exceeded 523 K [28]. Hitherto, no decoration or encapsulation of the noble metal by titanium 1022-5528/07/0900-0049/0 Ó 2007 Springer Science+Business Media, LLC

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suboxide was observed at these lower temperatures. Thus, we suggest that encapsulation and electronic interactions might occur simultaneously with the onset of the electronic perturbation starting at a lower reduction temperature than that of the geometric effect [29]. Recently, we found that the reduction of titania starts already at 473 K and results in the formation of surface oxygen vacancies Vs, Ti3+ ions, and electrons according to the equation [30] Ti4þ þ O2 þ H2 $ Ti3þ þ Vs þ e þ H2 O Below 723 K Ti3+ ions and oxygen vacancies remain at the surface of the titania while the electrons probably migrate in the bulk of titania and occupy donor sites just below the conduction band. Hence, the electrical conductivity of titania and consequentially that of titania supported noble metals increases by at least four orders of magnitude [31,32]. The catalytic activity of Rh/TiO2 for the hydrogenolysis of ethane decreases in the very same range of reduction temperatures [14]. However, both the decreased catalytic activity and the increased electrical conductivity can be reversed by a treatment in flowing oxygen. In order to investigate whether this concomitance of catalytic and conduction effects is coincidental or after all linked by the same mechanism, we studied the effect of hydrogen pretreatment temperature on the electrical conductivity and on the catalytic performance of titania supported platinum catalysts. To indicate any existing SMSI we measured the adsorption capacity of hydrogen and the catalytic activity for the hydrogenolysis of methylcyclopentane.

2. Experimental 2.1. Materials The titanium (iv) oxide (TiO2) used in this work as catalyst support was supplied by Degussa and consists of 85% anatase and 15% rutile. This material was impregnated with hexachloroplatinic acid designed to obtain a final content of either 2% Pt or 4% Pt. The so prepared catalysts were then calcined in flowing oxygen at 723 K (ramp 10 K/min) and, after evacuation at room temperature, activated in flowing hydrogen at 723 K. In order to obtain well reproducible results, the catalysts were oxidised at 673 K followed by a treatment in dry hydrogen at temperatures between 473 and 723 K prior to each experiment. All pre-treatments were performed for 2 h. Hydrogen, helium, and oxygen were highest-grade gases supplied by Messer-Griesheim. Hydrogen was further purified by passing through an oxygen removing purifier (Matheson), and helium was freed from oxygen traces by an Anoxy-Cil-unit. Condensable contaminants

were removed from hydrogen and helium by liquid nitrogen traps, while oxygen was passed through a trap cooled with liquid/solid ethanol. 2.2. Apparatus An all glass apparatus was used for the volumetric measurements and was equipped with metal-bellow valves (Witeg) and a Baratron pressure transducer (MKS); an oil diffusion pump provided a base pressure of 3*10)7 mbar. Mass flow controllers (MKS) were used to control the flow rates of the gases needed for catalyst activation and pre-treatments. The catalytic measurements were performed in an allglass recirculation batch reactor. In a typical reaction run a mixture of 10 mbar methylcyclopentane and 990 mbar of hydrogen was admitted to the reactor, a reaction temperature of 523 K was chosen to obtain measurable conversion even when the catalyst was in the SMSI state. In time intervals of 5 min small amounts of the reaction mixture were removed via an evacuated sample valve (Valco) and analysed by GC-analysis with FID detection. For impedance studies, 30–150 mg of the catalysts were compressed with 10 kbar for 40 min to produce pellets of 5 mm diameter. The impedance data were acquired using the 2)pole mode with gold electrodes, as described elsewhere [31]. The pellets were compressed (3 bar) by the upper electrode to minimize the contact resistance between the pellet and the electrodes. The conductivity of the sample was measured by a IM6e impedance spectrometer (Zahner-Elektrik), supplying the magnitude of the impedance and the phase angle between the current and the voltage (50 mV a.c.) in a frequency range from 100 mHz to 1 MHz. The apparatus was suited for temperature treatment upto 1273 K and for gas treatments similar to those in the volumetric and in catalytic measurements.

3. Results 3.1. Catalyst characterisation For the dispersion measurements the calcined and afterwards reduced catalysts were again oxidized under standard conditions (2 h O2 at 673 K) and subsequently reduced in flowing hydrogen at 323 K. After evacuation at 323 K, the retained hydrogen was desorbed into vacuum at 673 K. From the hydrogen uptake at room temperature the dispersion of the catalysts was calculated by extrapolation of the adsorption isotherm to zero pressure. Assuming that exactly one hydrogen atom adsorbs on each surface platinum atom (H/Pt = 1), the dispersion of the 2% Pt/TiO2 and 4% Pt/TiO2 catalysts was calculated to be 59 and 49%, respectively. Analysis of the corresponding electron micrographs (figure 1) revealed a dispersion of 47% for the 2% Pt/TiO2 along


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Figure 1. Electron micrographs (a) of 2% Pt/TiO2 and (b) of 4% Pt/TiO2.

with a mean diameter for the platinum particles of 2.3 nm. The 4% Pt/TiO2 had a dispersion of 41% with a mean diameter of 2,6 nm. 3.2. Adsorption measurements After standard oxidation (2 h O2 at 673 K) the catalysts were treated in flowing dry hydrogen at temperatures between 473 and 723 K for 2 h, followed by hydrogen desorption at 673 K. The adsorption isotherms reveal a typically strong adsorption at pressures below 5 mbar and saturation starting above 10 mbar. Figure 2 shows the effect of hydrogen pretreatment temperature on the hydrogen adsorption capacity. For both catalysts, the hydrogen adsorption capacity at 323 K decreases significantly after hydrogen pretreatments above 473 K and nearly vanishes when the pretreatment temperature approaches 723 K. The samples were again evacuated at 673 K and the amount of oxygen taken up at 323 K was measured (figure 3). The oxygen uptake was found to be constant for hydrogen pretreatments below 523 K, its values correspond to those expected for oxygen adsorption on the platinum surface. If the catalysts were hydrogen pretreated beyond those temperatures the amount of

Figure 2. Effect of pretreatment temperature on the hydrogen adsorption capacity at 323 K for both catalysts.

oxygen uptake increased significantly with increasing reduction temperature. We assume that this ‘‘excess oxygen’’ is needed to reoxidise the surface vacancies in the prereduced titania to its stoichiometric state [30]. Therefore, this excess oxygen uptake corresponds to the number of oxygen vacancies and hence the density of donor electrons, the calculated values are shown in table 1. Interestingly, the charge carrier density in the 4% Pt/TiO2 catalyst is higher than in the 2% Pt/TiO2, at least for temperatures above 573 K. Presumably, the reduction is catalysed by hydrogen spillover. Consequently, the extent of reduction is higher with higher platinum loading and the reduction time (2 h) is not enough for both samples to reach equilibrium. In summary, these results confirm that the charge carrier density in titania supports increases considerably upon hydrogen treatment above 523 K, in agreement with data recently obtained with pure titania [32]. 3.3. Hydrogenolysis of methylcyclopentane To investigate any metal support interaction effect on the catalytic performance of Pt/TiO2 catalysts, the hydrogenolysis of methylcyclopentane was chosen because of its well-known sensitivity to hydrogen pretreatment effects [33]. The conversion initially increases linearly with time to reach values of 20% and this linear conversion-time slope was used to calculate the turnover frequency. Figure 4 shows that the catalytic activity for both catalysts decreases with increasing hydrogen pretreatment temperature (the applied reaction temperature naturally limits the lowest possible pretreatment temperature). As expected, after reduction at temperatures typically applied to induce SMSI according to the ‘‘decoration model’’ (above 700 K), the catalysts experienced an almost entire loss of catalytic activity. A standard oxidation treatment (2 h in oxygen at 673 K) followed by a reduction at 523 K restored the catalytic activity completely. However, the interesting finding in this work is the significant decrease of the catalytic activity already at temperatures as low as 523 K. At the very same temperatures also a decrease in the hydrogen adsorption

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3.4. Impedance measurements Recently, we applied the impedance spectroscopy to study the reduction effects on TiO2 and suggested that electrical conduction in TiO2 occurs via two pathways: (i) ionic conduction at the surface of the titania particles and (ii) electronic transport through the bulk of titania [32]. When titania was treated in hydrogen, compared to oxygen or helium, the electrical conductivity increased exceedingly by orders of magnitude, presumably due to the enhanced electronic conduction (figure 5). The reason for the high conduction induced by hydrogen is thus believed to be the formation of electrons occupying intergap sites in titania [30]: Figure 3. Oxygen uptake as a function of the hydrogen pretreatment temperature, measured by volumetric oxygen titration.

Table 1 Charge carrier density in titania after a pretreatment in hydrogen for two hours as a function of the treatment temperature 2% Pt/TiO2 H2 pretreatment temperature [K] 523 573 623 673 723

Excess oxygen uptake lmol/g 2.8 6.0 12.2 17.0 19.7

Electron density 1024/m3 14.3 30.7 62.4 86.9 100.5

4% Pt/TiO2 Excess oxygen uptake lmol/g 1.4 6.9 14.1 21.7 24.9

Electron density 1024/m3 6.9 35.5 72.3 110.8 127.2

capacity was found, as described above. Therefore, we conclude that the loss in catalytic activity is a consequence of the reduced hydrogen adsorption. Since skeletal reactions need accessible hydrogen to proceed, the kinetics of the hydrogenolysis of methylcyclopentane is certainly affected by that demand of hydrogen.

Figure 4. Effect of the hydrogen pretreatment temperature on the catalytic activity for MCP hydrogenolysis at 523 K.

Ti4þ þ O2 þ H2 $ Ti3þ þ Vsurface þ e þ H2 O In this case, the ionic conduction becomes negligible compared to the electronic conduction and the impedance at low frequencies (typically at 1 Hz) may be correlated to the number of the donor electrons formed. Figure 5 shows the electric resistivity at 1 Hz for the 2% Pt/TiO2 catalyst treated in hydrogen during a temperature ramp that is typically applied for the catalyst’s pretreatment (upto final temperature with 5 K/min, 2 h at pretreatment temperature and back to 323 K with )2 K/min). With increasing temperature the impedance decreases because of an increased number of donor electrons and a higher probability that electrons occupy the conduction band. When approaching the final temperature of the heating ramp the impedance continues to decrease slightly and finally seems to reach saturation indicating that the number of electrons formed attains an equilibrium value. Upon cooling the impedance again increases but never reaches the initial values again. The final impedance at room temperature was found to be significantly smaller after hydrogen pretreatment at

Figure 5. Electric impedance (at a frequency of 1 Hz) of 2% Pt/TiO2 treated in hydrogen during a temperature program from room temperature to final temperature (5 K/min), 120 min at final temperature and back to 323 K ()2 K/min).


higher temperature. Assuming that the number of intergap electrons does not change during cooling in hydrogen, the change in electronic conduction is caused only by the decreased probability that intergap electrons occupy the conduction band. Hence, the sample keeps memory of its pretreatment in the way that the electronic conduction is higher after pretreatment at higher temperatures.

4. Discussions 4.1. Energy level of the intergap sites Upon cooling in hydrogen the number of oxygen vacancies, of Ti3+ ions, and of donor electrons remain preserved. The influence of temperature on the electron density in the conduction band is related to the energy required to excite the electrons from the donor sites to the conduction band by the Boltzmann equation. Thus, the electronic conductivity increases with temperature according to exp()D/kT), with D being the energy gap between the conduction band and the donor sites. Arrhenius type plots of the logarithm of the impedance versus the reciprocal temperature show an excellent linear behaviour. The obtained activation energy decreases from 0.09 to 0.03 eV after pre-treatments at 473 and 723 K, respectively. Presumably, an interaction among donor sites results in the formation of a diffuse band of donor levels. Thus, the electrons formed at low temperatures (low electron density) probably occupy only donor sites that are 0.09 eV below the conduction band, whereas higher-energy donor sites can be filled up with increasing reduction temperature and hence higher electron density. On the other hand, when pure titania was reduced above 723 K the already low impedance slightly decreased even further if the temperature was decreased. This corresponds to a positive temperature coefficient of impedance and is typical for a metallic conductor. This result suggests that the electrons formed during reduction above 723 K start to occupy sites in the conduction band. 4.2. Catalytic effects The effect of hydrogen pretreatment temperature on the catalytic properties shows that the SMSI effect is not an ‘‘either’’-‘‘or’’ state, which is switched ‘‘on’’ by hydrogen pretreatment at high temperature and switched ‘‘off’’ by an oxygen treatment at elevated temperatures. There is a gradual change between non-SMSI state and SMSI state depending on the pretreatment temperature applied. For hydrogen pretreatment above 723 K the decoration of the noble metal by substoichiometric titania is widely agreed to be the pre-eminent mechanism for SMSI. We support this model and further want to emphasize that this mechanism might be correlated to the observed formation of bulk

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vacancies or Ti3+ interstitials after reduction at those temperatures [30,34]. More precisely, we assume that the presence of lattice defects in the bulk phase is necessary to provide sufficient mobility for substoichiometric titania species to decorate the noble metal particles. Upon hydrogen treatment at temperatures below 723 K only surface oxygen vacancies along with Ti3+ ions and electrons are formed. However, SMSI type behaviour was also observed after reduction at those temperatures, thus suggesting an additional mechanism. For instance, hydrogen pretreatment affected the hydrogen adsorption capacity already at temperatures as low as 473 K, and the catalytic activity was decreased significantly after reduction at 523 K or higher. The catalyst remembers the pretreatments at least in terms of an increased electronic conductivity. This enhanced conduction is in turn caused by electrons that are formed along with surface oxygen vacancies during the reduction and occupy donor sites just below the conduction band. Assuming that two materials with different Fermi potentials, e.g. noble metal and transition metal oxide, are brought to electrical contact, the initial potential difference initiates a charge transfer between the materials until both Fermi energies are adjusted. For instance, in case of an n-type semi-conducting oxide (e.g. TiO2) its lower work function (higher Fermi potential) causes the electrons to flow from the oxide’s donor level into the conduction band of the adjacent metal, leaving behind a positively charged depletion zone. Consequently, an electric field is formed at the metal-semiconductor interface as well as a potential barrier caused by a bending of the bands. The height of this Schottky barrier reflects the mismatch in the energy position of the majority carrier band edge of the semiconductor and the metal Fermi level across the metal-semiconductor interface and is approximately the difference between the conduction band minimum and the Fermi level in case of n-type semiconductors. The width of the positively charged depletion zone (space charge region) mainly depends on the concentration of charges as well as on temperature. The charge transfer can be calculated by assuming that in the n-type semiconductor donor electrons situated adjacent to the metal are transferred to the metal. The resulting charge in this electron depleted layer screens the electric potential and outside of this layer the electric potential gradient disappears. The thickness d of this depletion layer depends on the difference in the Fermi energies and on the density of free charge carriers [35]. d¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ee0 ðEF;metal EF;oxide Þ=Ne

The charge transferred per unit area of the phase boundary is the charge that has been removed from the depletion zone.

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qtransferred ¼ Ne d ¼


qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ee0 Ne ðEF;metal EF;oxide Þ

with dielectric constant 0 vacuum permittivity Ne density of charge carriers and EF the Fermi energies in the two phases With a similar equation Ponec [20] calculated the charge transfer to amount the negligible value of 3.5*1012 electrons/cm2, using estimated parameters (Ne = 1018/cm3, = 10 and DEF = 2 eV). However, based on our experimental data the input parameters are quite different. For instance, the charge carrier density obtained after a hydrogen treatment at 673 K for 2 h results in values between 87*1018/cm3 and 111*1018/cm3, depending on the amount of platinum in the catalyst (table 1). Since the screening of the charges occurs in the depletion layer, which is free of charge carriers, the dielectric constant of stoichiometric titania P25 has to be taken for this calculation. This dielectric constant was obtained by electric impedance spectroscopy to be 55 [31] in agreement with literature data [36–38]. Applying these values along with a Fermi potential difference of 2 V the number of electrons transferred to the metal is then calculated to be about 1.2*1014 electrons/cm2. This value corresponds to a charge transfer of approximately 0.1 electrons per platinum atom at the interface and is higher by a factor of 30 than those estimated by Ponec. The fact that the electron’s binding energy of the noble metal in the SMSI state is nearly the same as in the non-SMSI state [17–19] has been used as an argument against the charge transfer mechanism. The calculation given here shows clearly that the amount of charge transferred is increased significantly due to an increase in the charge carrier density in the support, even if the Fermi potential difference remains constant. Keeping this in mind, the considerably higher charge transfer leads to a higher electron density in the metal. As the bulk of the metal remains free of electric charge, these excess electrons are localised at the metal surface changing the electronic properties on the metal surface and thereby affecting the adsorption capacity of hydrogen and probably the catalytic activity. According to the given equation the charge transfer increases with the square root of the charge carrier density retained in the titania after hydrogen pretreatment. Thus, in order to explore the correlation between charge transfer and the SMSI effect we plotted the hydrogen adsorption capacity versus the square root of the conductivity retained at 323 K after hydrogen treatment at the respective temperature. Figure 6 shows that the hydrogen adsorption capacity decreases linearly with increasing charge transfer. The 2% Pt/TiO2 catalyst, although exhibiting the higher metal dispersion,

Figure 6. Hydrogen adsorption capacity versus the square root of the conductivity retained after hydrogen treatment at the respective temperature.

adsorbs less hydrogen than the 4% Pt/TiO2 catalyst because of the lower metal loading. However, the hydrogen adsorption capacity of the 2% Pt/TiO2 catalyst, normalized to the non-SMSI hydrogen adsorption, is stronger affected by the hydrogen pretreatment that of the 4% Pt/TiO2 catalyst. This result indicates that smaller metal particles are more SMSI-sensitive than the larger ones and agrees well with the fact that in smaller particles the proportion of the phase boundary to the volume of the particles and therefore also the charge transfer per number of platinum atoms is higher than in larger particles. Similarly, the catalytic activity vanishes almost completely after reduction at temperatures above 723 K. Furthermore, the temperature effect on the electronic conductivity shows that the average energy of the donor sites increases with increasing occupancy by electrons. The donor sites probably form a ‘‘donor band’’ that is filled up continuously. After hydrogen reduction at 723 K the activation energy for the electrons to access the conduction band was found to be 0.03 eV, in comparison to Pt/TiO2 that was pretreated at 473 K (E 0.09 eV). Thus, with higher charge carrier densities higher energetic sites are occupied and consequently the Fermi energy in titania increases (in this case by 0.06 eV). This energy shift enhances the potential difference between metal and semiconductor (the driving force for the charge transfer) and leads to an additionally increased charge transfer.

5. Conclusions When titania supported platinum was treated in hydrogen at moderate temperatures between 473 K and 723 K the electric conduction increased by orders of magnitude because of the formation of donor electrons.


These electrons occupy a band of intergap sites in titania, whose energies were found to range from 0.09 to 0.03 eV beneath the lower edge of the conduction band. The electrons remain in those donor sites even after cooling to lower temperatures hence preserving the electrical conductivity of the catalysts. Simultaneously, in the very same temperature range, both the hydrogen adsorption capacity as well as the catalytic activity of Pt/TiO2, e.g. towards the hydrogenation of methylcyclopentane, decreased significantly with increasing reduction temperature and nearly vanished completely after pretreatment at 723 K. This concomitance of both phenomena (change of conduction and catalytic properties) is by no means coincidental. On the contrary, our observations suggest that those effects are well linked together. Our new calculations based on recent data provided in this work result in considerably higher amounts of charge transfer between titania and platinum than those estimated in an earlier work [20]. Hence, we emphasize to consider this electron transfer as an additional mechanism to explain the SMSI-effect induced at moderate temperatures. However, we do not disclaim the decoration of the metal by titania species as a possible explanation for the SMSI-effect after hydrogen treatment at temperatures above 723 K. Still, a precondition of the decoration is that solid patches of titania become mobile enough to migrate on the platinum particles. We assume that the formation of a sufficient number of lattice defects (oxygen vacancies or Ti3+ interstitials) in the bulk phase of titania is needed to successfully decorate the metal. Previously, we reported that the migration of those defects in the bulk phase starts at temperatures above 723 K only [30]. Thus, we assume that both mechanisms are responsible for the SMSI effect and propose (i) the charge transfer mechanism for hydrogen pretreatment temperatures below 723 K, based on the high charge carrier density due to the formation of oxygen vacancies at the surface of titania, and (ii) the decoration effect for hydrogen pretreatments above 723 K, assisted by the formation of defects in the bulk of titania.

References [1] G.M. Schwab, Disc. Far. Soc. 8 (1950) 166. [2] F. Solymosi, Catal. Rev.-Sci.Eng. 1 (1967) 233. [3] S.J. Tauster, S.C. Fung and R.L. Garten, J. Amer. Chem. Soc. 100 (1978) 170.

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[4] A.J. Simoens, R.T.K. Baker, D.J. Dwyer, C.R.F. Lund and R.J. Madon, J. Catal. 86 (1984) 359. [5] R. Burch, in: Hydrogen Effects in Catalysis, eds. Z. Paal and P.G. Menon (1988) 347. [6] T.H. Fleisch, A.T. Bell, J.R. Regalbuto, R.T. Thomson, G.S. Lane, E.E. Wolf and R.F. Hicks, Stud. Surf. Sci. Catal. 38 (1988) 791. [7] U. Bardi, Catalysis Letters 5 (1990) 81. [8] J.P.S. Badyal, in: The Chemistry of Solid Surfaces and Heterogeneous Catalysis, Vol. 6, (Elsevier, 1993) 311. [9] T. Uchijima, Catal. Today 28 (1996) 105. [10] K. Hayek, R. Kramer and Z. Paal, Appl. Catal. A General 162 (1997) 1. [11] S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. Gatica, C. Lopez Cartes, J.A. Perez Omil and J.M. Pintado, Catal. Today 77 (2003) 385. [12] J.A. Horsley, J. Amer. Chem. Soc. 101 (1979) 2870. [13] S.J. Tauster, S.C. Fung, R.T.K. Baker and J.A. Horsley, Science 211 (1981) 1121. [14] D.E. Resasco and G.L. Haller, J. Catal. 82 (1983) 279. [15] B.H. Chen and J.M. White, J. Phys. Chem. 86 (1982) 3534. [16] P. Meriaudeau, O.H. Ellestad, M. Dufaux and C. Nacchache, J. Catal. 75 (1982) 243. [17] C.S. Fung, J. Catal. 76 (1982) 225. [18] T. Huizinga and R. Prins, Stud. Surf. Sci. Catal. 11 (1982) 11. [19] B.A. Sexton, A.E. Hughes and K. Foger, J. Catal. 77 (1982) 85. [20] V. Ponec, Stud. Surf. Sci. Catal. 64 (1991) 117. [21] E.J. Braunschweig, A.D. Logan, A.K. Datye and D.J. Smith, J. Catal. 118 (1986) 227. [22] S. Bernal, F.J. Botana, J.J. Calvino, J.A Lopez, J.A. Perez-Omil and J.M. Rodriguez-Izquierdo, J. Chem. Soc., Faraday Trans. 92 (1996) 2799. [23] E.D. Boyes and P.L. Gai, Ultramicroscopy 67 (1997) 219. [24] A. Berko, I. Ulrych and K.C. Prince, J. Phys. Chem. B 102 (1998) 3379. [25] D.W. Goodman, Catal. Lett. 99 (2005) 1. [26] A.K. Datye, D. Kalakkad, M.A. Yao and D.J. Smith, J. Catal. 155 (1995) 148. [27] A. Huidobro, A. Sepulveda-Escribano and F. Rodrıguez-Reinoso, J. Catal. 212 (2002) 94. [28] G. Rupprechter, G. Seeber, H. Goller and K. Hayek, J. Catal. 186 (1999) 201. [29] J.P. Belzunegui, J. Sanz and J.M. Rojo, J. Amer. Chem. Soc. 112 (1990) 4066. [30] H. Haerudin, St. Bertel and R. Kramer, J. Chem. Soc., Faraday Trans. 94 (1998) 1481. [31] D. Eder and R. Kramer, Phys. Chem. Chem. Phys. 5 (2003) 1314. [32] D. Eder and R. Kramer, J. Phys. Chem. B 108 (2004) 14823. [33] H. Zuegg and R. Kramer, in: Strong Metal Support Interaction, ACS Symposium Series 298, (Washington D.C., 1986) p. 145. [34] M.A. Henderson, Surf. Sci. 419 (1999) 174. [35] Ch. Kittel, Einfu¨hrung in die Festko¨rperphysik (Oldenbourg, Mu¨nchen, 1988) p. 621ff. [36] A. Goossens, B.V.D. Zanden and J. Schoonman, J. Chem. Phys. Lett. 331 (2000) 1. [37] R.V.D. Krol, A. Goossens and J. Schoonman, J. Electrochem. Soc. 144 (1997) 1723. [38] J. Errera and H. Ketelaar , J. Phys. Radium 3 (1932) 239.