An in Situ Study of PtCo Bimetallic Nanoparticles Supported on TiO

Jun 11, 2012 - LMSPC, UMR 7515 CNRS-UdS, 25 Rue Becquerel, 67087 Strasbourg, ... IPCMS, UMR 7504 CNRS-Uds, BP43 F-67034 Strasbourg 2, France...
0 downloads 0 Views 413KB Size
Download PDF
Article pubs.acs.org/JPCC

Probing Metal−Support Interaction in Reactive Environments: An in Situ Study of PtCo Bimetallic Nanoparticles Supported on TiO2 V. Papaefthimiou,*,† T. Dintzer,† M. Lebedeva,† D. Teschner,‡ M. Hav̈ ecker,‡,§ A. Knop-Gericke,‡ R. Schlögl,‡ V. Pierron-Bohnes,∥ E. Savinova,† and S. Zafeiratos† †

LMSPC, UMR 7515 CNRS-UdS, 25 Rue Becquerel, 67087 Strasbourg, France Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany § Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Strasse 15, 12489 Berlin, Germany ∥ IPCMS, UMR 7504 CNRS-Uds, BP43 F-67034 Strasbourg 2, France ‡

S Supporting Information *

ABSTRACT: Our recent surface characterization studies of extended and nanosized PtCo alloys under hydrogen and oxygen atmospheres, indicated significant and reversible surface segregation in response to the gas phase environment [J. Phys. Chem. Lett. 2011, 2, 900]. In the present communication, an insight into the effect of the support on the PtCo alloy stability is attempted. A model PtCo/TiO2 interface is investigated under reducing, oxidizing, and catalytic reaction conditions using ambient pressure Xray photoelectron and absorption spectroscopies (APPES and NEXAFS respectively). Encapsulation of PtCo by the TiO2 support was observed upon vacuum annealing. Upon oxidation/reduction conditions, a mixture of CoOy (1 ≤ y < 1.33), TiO2, and mixed CoxTiyOz phases with Pt located in the subsurface was formed. TiO2 was found to be remarkably stable under the temperature and pressure conditions used here (up to 620 K, 0.2 mbar), with titanium remaining always in the Ti4+ state. The interplay between the gas atmosphere and the surface is limited to modifications of the cobalt oxidation state. However, in contrast to the observations on the unsupported PtCo alloy, neither oxidation of CoO to Co3O4 in O2 nor full reduction to metallic Co under various reducing agents (H2, CH3OH), occurred. Synchronized changes of the binding energy position of core level photoelectron peaks in response to the gas phase are related to the band-bending development at the gas/solid interface. This documents the direct coupling of the electronic properties and the gas phase chemical potential of a chemically functional material useful as catalyst or gas sensing device.

1. INTRODUCTION Bimetallic catalysts have been proposed to be superior to their monometallic counterparts in several applications such as fuel cells and hydrocarbon reforming reactions. 1−4 Various structural configurations have been observed in such systems, e.g., ordered alloys or core−shell structures.4 Undoubtedly, the surface composition and oxidation state may differ from that of the bulk, due to surface segregation phenomena.5 Segregation is driven by thermodynamic characteristics of the system, like the surface energies of the constituents,6 but certainly kinetics also plays a major role in determining the working configuration. In practical applications like in heterogeneous catalysis, adsorbateinduced segregation significantly influences the surface morphology, structure, and chemical composition.7−9 In some cases chemically reversible dynamic behavior is observed, where segregation of a catalyst constituent is favored in response to the redox potential of the gas phase.10−13 In the case of oxidesupported bimetallic nanoparticles, surface segregation may be complicated by preferential bonding of one of the constituents with the support at the metal−support interface. Therefore, the interaction between the metals and the support is of importance for the stability and the chemical activity of the interfacial sites.14−17 © 2012 American Chemical Society

Strong metal−support interaction (SMSI) phenomena on oxide supported catalysts are related both to the electron transfer process from the substrate to the metallic overlayer and to the encapsulation of the overlayer by the support material.16,18 In heterogeneous catalysis TiO2 is considered as the typical SMSI material. In TiO2 supported metal catalysts, SMSI effects are typically observed following annealing in reducing atmospheres, where titanium interstitial atoms in the reduced state diffuse from the bulk to the interface and spill over to the metal leading to encapsulation of the metal.19,20 Metals with large work functions (i.e., Pt, Pd, Rh, Ir, and Ni) are encapsulated, whereas sintering or oxidation occurs for metals with lower work functions.17,19 In some cases, the encapsulation process has been found to be reversible; metal particles encapsulated by TiO2 under reductive conditions can be regenerated through oxidation treatments.19,21,22 The encapsulation of Pt under TiO 2 is a well-established process,18,19 whereas the behavior of Pt-based bimetallic particles under SMSI conditions has not been investigated. Received: March 10, 2012 Revised: June 5, 2012 Published: June 11, 2012 14342

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

Figure 1. Ti 2p photoelectron spectra of (a) the as-prepared PtCo/TiO2 samples and (b) after vacuum annealing at 520 K. The corresponding atomic abundance of Pt, Co, and Ti as a function of the electron kinetic energy (or the estimated depth) are shown in Figure 1c,d. All spectra were measured under UHV conditions.

conditions (base pressure 10−8 mbar) with nominal thicknesses of 2.5 nm. The samples were transferred in air and introduced to the ambient pressure photoelectron spectrometer (APPES) at the ISISS beamline at BESSY synchrotron facility in Helmholtz Zentrum Berlin, in a setup described elsewhere.30 The gas flow into the reaction cell was controlled by mass flow controllers. The catalytic activity of the PtCo/TiO2 samples was tested under methanol reforming (CH3OH + H2O) and methanol oxidation (CH3OH + O2) reaction conditions at temperatures between 520 and 620 K, using various mixing ratios. In all cases prior to gas mixture exposure the samples were preconditioned in 0.2 mbar O2 at 570 K for 30 min. The presented photoelectron measurements were recorded after 15 min equilibration in the gas mixture, and the stability of the surface during measurements was confirmed by recording a set of photoelectron spectra two times in succession (total measurement duration around 1.5 h). The sample did not exhibit any electrostatic charging under our conditions, as judged by the C 1s peak of adventitious carbon at 284.4 eV. Therefore, unless otherwise specified, the binding energies are presented as measured without any correction. A differentially pumped quadrupole mass spectrometer was connected through a leak valve to the experimental cell and the gas phase composition was monitored by online mass spectrometry simultaneously to the spectroscopic characterization of the surface. Quantitative calculations were performed using normalized Co 2p, Pt 4f, and Ti 2p intensities, and taking into account the photon-energy dependence of the atomic subshell photoionization cross sections.31 The surface morphology before and after the treatment in the spectrometer was inspected by scanning electron microscopy using a JEOL JSM-6700F microscope (Supporting Information, S1). The surface of the as prepared samples (before any treatment) appears granular with the presence of discrete and nonuniform islands with different grain sizes. After treatment in the spectrometer chamber (annealing in UHV,

Apart from the fundamental interest in heterogeneous catalysis, TiO2 based materials have potential applications in several other technologies like photovoltaics,23 electrochromics,24 sensors,25 and photocatalysis.26 In the latter, the presence of well-dispersed cobalt ions (Co2+) was found to enhance the photoreduction efficiency in TiO2 based photocatalysts.27 We have recently shown that the surface of PtCo bimetallic11 and ternary12 model catalysts is drastically and rapidly modified when cycling from reductive to oxidative gas phase atmospheres in the mbar pressure. Co and PtCo nanoparticles supported on amorphous carbon showed remarkable resistance to complete reduction, connected to the existence of strained interfaces and the formation of wurtzite-CoO (w-CoO).11,28 The present work is a step forward in the comprehension of the physicochemical behavior of supported nanosized PtCo in reactive environments. The system under study is PtCo on TiO2, a model SMSI system where the relationships between morphology, composition, and chemical activity can be explored on the atomic level. Ambient pressure X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies (APPES and NEXAFS) were applied in situ to monitor the surface composition and chemical state of PtCo/ TiO2 thermally treated (500−620 K) in 0.2 mbar H2, O2, CH3OH, and subsequently under CH3OH reforming and oxidation conditions. We focus here on the chemical aspects controlling the dynamic response of bimetallic particles.

2. EXPERIMENTAL SECTION The PtCo/TiO2 samples were prepared by atomic sputtering in a high vacuum chamber (base pressure 10−8 mbar; Ar work pressure 0.008 mbar for metals) described elsewhere.29 Thick (30 nm) titanium layer was deposited on a silicon wafer and covered by a 1 nm conductive titanium oxide deposited in a 0.004 mbar of Ar and 0.0005 mbar of O2 atmosphere at 300K. Consequently, Co and Pt were codeposited (in a 1:1 atomic ratio) at room temperature under ultrahigh vacuum (UHV) 14343

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

Figure 2. (a) Ti 2p photoemission, (b) Co L3-edge absorption spectra of PtCo/TiO2, and (c) Co L3-edge absorption spectra of reference PtCo foil recorded under 0.3 mbar of oxygen, methanol reforming (CH3OH + H2O), methanol oxidation (CH3OH + O2), pure methanol, and pure hydrogen at 520 K.

spectrometer the samples were exposed to the atmosphere for several days. Therefore, the observed surface segregation of cobalt is not surprising, because cobalt oxides are known to migrate toward the surface covering Pt.11,12 A monotonic increase of the Ti abundance in the deeper layers is also the expected behavior for a support material. However, the only moderate increase observed in the present case indicates that part of the Ti 2p signal might also come from cracks/holes through the PtCo film. Vacuum annealing induces significant overlayer reorganization, with a considerable decrease of the Pt and Co signals, followed by a dramatic increase (from 20% to 90%) in the surface amount of Ti (Figure 1d). Despite the dramatic intensity decrease, the Pt atomic abundance has the same trend as before (increases with the probing depth), whereas the Co atomic abundance is practically unaffected by the probing depth, showing that cobalt is homogeneously distributed over the outermost 4 nm of the sample. On the other hand, the Ti atomic abundance decreases with depth, which is the reverse trend compared to that for the as-prepared sample. Surface diffusion and agglomeration of PtCo can partly explain the drastic decrease of Pt and Co and increase of Ti ARs observed upon annealing for all probing depths. The monotonic decrease of the Ti AR as a function of the electron kinetic energy is a clear evidence of Ti diffusion to the surface (segregation), because otherwise Ti AR should increase or remain constant. Surface segregation of Ti over PtCo together with partial reduction of TiO2 are typical indications of SMSI effects induced between the TiO2 support and the PtCo overlayer. It should be noted that most probably after annealing PtCo and TiO2 form a mixed interface, as indicated by the independence of the Co abundance on the kinetic energy, in contrast to the observations of Figure 1c. A more realistic representation of the Pt−Co−Ti arrangement is therefore a mixture of three surface constituents, with a preferential localization of Ti on the surface and Pt in the subsurface.

oxidation and reduction in 0.2 mbar), the surface microstructure is significantly modified. Bigger and randomly shaped features were observed, whereas it seems that the initially formed grains have merged to form large agglomerates on the substrate.

3. RESULTS AND DISCUSSION 3.1. Interaction of PtCo Overlayer with TiO2 Substrate upon Vacuum Annealing. Deposition of PtCo overlayers on TiO2 films was performed at room temperature; therefore, kinetic limitations could prevent the formation of the thermodynamically most stable state. To stimulate SMSI effects, the as-prepared PtCo/TiO2 samples were vacuum annealed at temperatures ranging from 470 to 620 K prior to any gas exposure. In Figure 1a,b the characteristic photoelectron Ti 2p core levels of the as-prepared and vacuum annealed PtCo/TiO2 are shown. It is evident that on the asprepared sample, Ti is exclusively in the Ti4+ state (TiO2), whereas upon annealing, a significant amount of reduced Ti3+ appears in the spectrum as a second Ti 2p component shifted by ∼2.1 eV from the main peak. The evolution of the layer structure of the different surface constituents upon annealing is probed by nondestructive depthdependent measurements, whose results are shown in Figure 1c,d. It should be noted that the higher is the kinetic energy, the deeper originate the photoelectrons (see estimated depth on the upper axis of Figure 1c).32 In the outermost surface layers (∼1.8 nm) of as-prepared samples the Co atomic abundance is significantly enhanced compared to that of Pt, whereas when deeper layers are probed, relatively more Pt is detected. On the other hand, the Ti abundance is not significantly influenced by the probing depth and only a slight increase is observed for higher photon energies. These results indicate that in the as-prepared films Co and Pt are not homogeneously mixed, but Co is preferentially segregated over Pt. After preparation and before introduction to the 14344

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

3.2. Gas Phase Induced Surface Modifications. After vacuum annealing, the PtCo/TiO2 samples were exposed to various gas atmospheres (oxygen, hydrogen, methanol, methanol and water, methanol and oxygen) in total pressures between 0.2 and 0.3 mbar and at temperatures varying from 500 to 620 K. Online mass spectrometry was used to monitor the gas atmosphere above the sample; however, the catalytic activity was negligible. It should be noted though that under identical reaction conditions PtCo foils were notably active (around 15% methanol conversion), implying that TiO2 has a detrimental effect on the catalytic activity of supported PtCo, at least under the reaction conditions used here. The effect of the reaction mixture on the surface state of the elements provides valuable information on the redox behavior of the system, as will be described below. Parts a and b of Figure 2 show the Ti 2p photoemission and Co L3-edge absorption spectra respectively, recorded at 520 K under various reactive environments, starting from oxidative and gradually going to more reducing atmospheres (H2, CH3OH). Reference Co L3-edge spectra of a PtCo alloyed foil are also included for comparison in Figure 2c. The Ti 2p peak is characteristic of TiO2, indicating that Ti3+ formed as a result of the vacuum-annealing (Figure 1b) was reoxidized to Ti4+ and afterward was not influenced by the gas treatments. A systematic BE shift toward lower BEs is, however, observed when going to more oxidative conditions, which is related to the band bending at the gas/solid interface, as will be discussed in the next section. We found no indication of chemical bonding between Pt and Ti,33 because neither the position (at 71.4 eV similar to that measured on the reference PtCo polycrystalline foil)11 nor the shape of the Pt4f peak (Supporting Information, S2) changes in the experiments. In contrast to titanium, the oxidation state of cobalt is sensitive to the gas phase composition. Although Co 2p core level photoelectron peaks were also recorded, modifications of the Co oxidation state and electronic structure are more pronounced in NEXAFS Co L-edge spectra and only these results will be presented here. Previous studies11,34−36 provide the necessary basis for the identification of the cobalt oxidation states. In pure O2, the Co L3-edge (upper graphs, Figure 2b) is similar to that of Co3O4 oxide.36 However, comparison with the spectrum obtained at the reference PtCo foil under identical conditions reveals that the intensity of the shoulder at ∼779 eV is enhanced. Subtraction of the Co3O4 contribution from the L3-edge spectrum of PtCo/TiO2 (Supporting Information, S4) indicates that the spectral difference is due to the presence of octahedrally coordinated Co2+ species37 (ca. 20%) in the TiO2 supported PtCo, resisting further oxidation to form the Co3O4 phase. Exposure in oxygen rich CH3OH mixtures (CH3OH:O2, 1:5) reduces partially the Co3O4-like oxide to octahedrally coordinated Co2+ (40%, Co2+, see Supporting Information, S5), whereas under identical conditions the unsupported PtCo alloy remains solely in the Co3O4 state. Complete transformation of Co3O4 to Co2+ occurs, in both cases, when methanol−water (middle spectra) or methanol-rich methanol−oxygen mixtures (CH3OH:O2, 2:1) are used. This Co2+ state is stable in the TiO 2 supported PtCo even in pure CH 3 OH and H 2 atmospheres (lower graphs), as confirmed by the characteristic fingerprint of the octahedral Co2+. Meanwhile, full reduction to metallic Co occurs under these conditions in the case of the PtCo foil. It is noteworthy that the NEXAFS and APPES spectral features of octahedral Co2+ in pure CoO, cobalt doped TiO2, and mixed CoxTiyOz are very much alike;38−40 therefore,

a precise determination of the cobalt electronic structure only from Co L-edge is not straightforward. Overall, comparison of unsupported and TiO2 supported PtCo under identical pretreatment and reaction conditions indicate significant differences. In particular, the reduction of Co3O4 to CoO-like species is facilitated in the TiO2 supported sample, whereas further reduction of CoO to metallic Co is inhibited. These results are in good agreement with previous findings of Morales et al. on manganese promoted Co/TiO2 catalysts,37 showing that reduction of Co3O4 to CoO is achieved at lower temperatures compared to that of bulk Co3O4, whereas further reduction of CoO to metallic Co was very difficult. The decreased cobalt reducibility of CoO was attributed to the formation of Co3−xMnxO4 mixed-oxide compounds.37 In Figure 3 NEXAFS Ti L3,2 spectra recorded at 570 K in O2 and H2 are presented, to describe the TiO2 electronic structure.

Figure 3. Ti L3,2 edge during oxidation (black spectrum) and reduction (red spectrum) at 570 K. The green line that results from the subtraction of the two spectra corresponds to rutile-TiO2 formation.

The characteristic multiple features of the spectra are related to the crystal field induced by oxygen ligands around the Ti4+ cations and hence are extremely sensitive to the different ligand configurations and local distortions. The spectra are characterized by t2g orbitals that point between the oxygen neighbors and form π-type bonds and the eg orbitals that point directly toward them, forming σ-type bonds. Previous studies indicated that the eg peak appears different in rutile and anatase phases. In particular, for rutile the peak has more intensity on the high than on the low photon energy side, whereas the reverse trend is observed for anatase TiO2.41 The Ti L-edge spectra shown in Figure 3 exhibit anatase-like features. However, because the two components of the eg peak are not well separated as previously found for pure single crystalline phases, a mixture of various crystalline phases and maybe a high degree of disorder (like amorphous TiO2) cannot be excluded. A noticeable increase in the t2g peak is observed upon H2 reaction at 570 K. This spectral modification remains and is not influenced by the temperature or any gas atmosphere, applied afterward, but only 14345

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

recording periods; therefore, a relatively lower temperature was used to slow down kinetic effects and ensure for the stability of the surface during the data collection. The Ti abundance is practically unaffected by the photon energy, whereas Co and Pt abundance monotonically decrease and increase respectively, as deeper layers are probed. No particular dependence on the reactive environment was observed. It is interesting to note that at the highest probing depth (∼4 nm) the Pt:Co abundance is about 1:1, as expected from the nominal value. These results are consistent with a surface arrangement model where cobalt is preferentially segregated to the outermost surface layers. One can therefore assume that the initially formed Pt-core/Co-shell structure, which was deformed upon vacuum annealing (Figure 1d), is restored after the gas exposure. On the other hand, the results show that TiO2 is no longer segregated on the surface but is interdiffused and is mixed with Pt and Co (possibly acting as an oxide matrix). The fact that complete oxidation of Co to Co3O4 is observed only on the PtCo foil, whereas for TiO2-supported PtCo Co2+ species resist further oxidation to Co3O4, is probably related to partial formation of mixed CoxTiyOz phases. Reduction of Co3O4 is a two-step process (Co3O4 → CoO → Co).28 Concerning the first step (Co3O4 → CoO), the ease of Co3O4 reduction for TiO2 supported PtCo compared to the foil (Figure 2), might be due to the stability of the CoxTiyOz phases and due to the lower cobalt content in this sample. Of course differences in the electronic or geometric structure of Co3O4 in the two samples that do not induce measurable differences at the Co L-edge spectrum cannot be excluded. Further reduction (CoO → Co) readily occurs on unsupported PtCo but also, as shown previously,28 on an oxidized Co(0001) crystal. In contrast, in the presence of TiO2, the Co2+ species resist reduction under identical temperature and pressure conditions. Hydrogen activation on Pt is known to facilitate reduction of Pt-based bimetallics. However, for PtCo foil, Pt was found to have no effect on the reduction process,11 because it was “buried” under a thick Co3O4 layer. Similarly, here Pt does not seem to facilitate CoO reduction, because it is “buried” in a mixed TiO2−CoO matrix. Therefore, the persistence of Co2+ species on TiO2 containing samples can be partially ascribed to CoxTi1−xO2-type mixed oxides species known to be more resistive to CoO under reducing conditions. However, a significant part of cobalt does not participate in CoxTiyOz phases and in oxidative atmospheres is oxidized further to Co3O4, which can form a solid solution with CoxTiyOz.46 Therefore, a plausible explanation of the persisting oxidized cobalt could be the direct interference of TiO2 either by blocking or, alternatively, by affecting the electronic structure of hydrogen activation sites on cobalt. 3.3. Gas Phase Induced Band Bending. The binding energy (BE) position of core level photoelectron peaks can provide information about the interaction of the components with one another and with the gas phase. In Figure 5 the BE shifts of the Ti 2p3/2 and O 1s photoelectron peaks for the various environments, relative to the vacuum annealed sample are depicted. From this figure it is evident that in an O2 atmosphere, and irrespective of the temperature, the Ti2p3/2 and O1s peaks shift by 0.5 ± 0.1 eV toward lower BEs, as is also shown in the corresponding bar diagram. The Ti 2p3/2 and O 1s peaks regain their initial position in a reducing atmosphere (H2 or CH3OH), whereas in CH3OH + O2 or CH3OH + H2O intermediate BE shifts are measured. The Co 2p3/2 peak follows the same trend, but because the oxidation state of Co is also

slightly increases with the annealing time. It should be noted that similar modification was also observed at lower temperatures (500 K); however, in that case the relative increase of the t2g peak was less pronounced. The difference of the two Ti L-edge spectra of Figure 3 (see bottom curve) shows modified t2g to eg intensity ratio as well as different structure of the eg peak. These observations are indicative of partial crystallization and formation of the rutileTiO2 phase.42 The presence of rutile-TiO2 was also implied by ex-situ XRD measurements (Supporting Information, S3). It is, however, noteworthy to mention that the anatase-to-rutile phase transition is a process that normally occurs at temperatures well in excess of 870 K.43 Room-temperature phase conversion of anatase to rutile was reported when TiO2 is doped with transition-metal ions like Co or Ni.44 Incomplete anatase-to-rutile transformation was observed by the same group, with lower Co2+ concentrations. The driving mechanism behind this effect is the increased interaction between cobalt atoms that form a linear chain in the rutile phase, as shown by density functional theory calculations.45 Co and Ti are known to form a variety of highly stable mixed oxide systems such as Co2TiO4, CoTiO3, and CoTi2O5.46 Partial formation of the mixed CoxTiyOz phase, where Co2+ ions occupy interstitial positions in the TiO2 lattice can rationalize the low temperature phase transition of anatase to rutile TiO2 indicated in Figure 3. In addition, this might be also the reason of the persistence of Co2+ in O2 ambience (Supporting Information, S4) and the resistance of some Co2+ species to reduction (Figure 2), because mixed CoxTiyOz structures are reported to be resistant to full reduction, even after prolonged annealing in 1 bar H2 at 570 K.47 Nondestructive depth profiling was used to shed light on the layer distribution between Pt, Co, and Ti in the first few atomic layers in oxidative and reductive environments. In Figure 4 the Co, Pt, and Ti atomic abundance in 0.2 mbar of H2 (open symbols) and O2 (filled symbols) at 500 K are plotted as a function of the electron kinetic energy. It should be noted that collecting a full set of depth-dependent spectra requires long

Figure 4. Cobalt, platinum, and titanium atomic abundance as a function of the photon kinetic energy recorded under O2 (filled symbols) or H2 atmospheres (open symbols) at 500 K. Solid lines were added to guide the eye. 14346

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

Figure 5. Binding energy shifts of the PES Ti 2p and O 1s peaks for the various gas treatments applied, obtained from the PES spectra by Gaussian fits to the peaks (VA, vacuum; O2, oxygen; H2, hydrogen; M, methanol). In the bar diagram, the total BE shift is depicted. A schematic diagram of the energy bands of the TiO2/gas (O2, H2) interface is depicted in the bottom right.

positive space charges in the mixed oxide surface region (see schematic diagram, bottom right of Figure 5). Some of the O2− ions penetrate in the mixed oxide matrix to convert CoO to Co3O4. The charge redistribution at the interface induces changes in the surface potential. The electrostatic field formed at the interface is reported to hinder oxygen anion diffusion from the bulk of the oxide toward the interface, in the case of metal/oxide interfaces.19 In reducing environments (pure methanol or H2), the inverse shifts are observed and the PES peaks regain their initial positions. In this case, it seems that the surface adsorbed oxygen ions that are formed during oxidation react with incoming H2 molecules. This results in leveling off the band bending. The electrostatic field formed in this case could induce oxygen anion diffusion from the bulk of the oxide toward the interface (see energy diagram in Figure 5).

influenced by the gas phase with a direct effect on its BE, the interpretation of the BE shifts in that case is complicated and will not be discussed. As stated above, the Pt 4f peak appears always at a fixed binding energy independent of the gas treatments. The fixed position of the Pt 4f peak suggests that the electronic conductivity of the TiO2 matrix is sufficient to compensate for the loss of electrons due to photoemission and electrostatic charging cannot account for the observed BE shifts of Ti 2p and O 1s PES peaks. Changes in the surface potential of TiO2 due to the adsorption or desorption of gases is a property that finds applications in gas sensors.48,49 In the present experiments, reversible core level BE shifts are observed (within ±0.1 eV) when cycling from oxidative to reducing atmospheres. These shifts occur shortly after the application of the gas phase, indicating that the changes are not due to incorporation of oxygen in the bulk of the film that could effectively change the bulk Fermi level position.48 Similar BE shifts have been recently observed in O2 atmospheres of Au/TiO250 and were related to the bending of TiO2 bands due to the interaction with the gas phase. In general, oxidation of TiO2 at room temperature leads to the rapid surface coverage of (mainly) ionized molecular oxygen species (O2−), which can subsequently be dissociated leading to the formation of atomic species (O2−)51 according to the reaction O2− + 3e− → 2O2−. Assuming similar oxygen adsorption mechanism for PtCo/TiO2, electrons from TiO2 are transferred to the surface adsorbed oxygen atoms in oxidative atmospheres. Such an electron transfer process causes upward bending of the TiO2 surface energy bands and formation of

4. SUMMARY AND CONCLUSIONS Summarizing, prior to any treatment and after exposure to the laboratory atmosphere, codeposition of Pt and Co onto thermally oxidized TiO2 films produces an inhomogeneous overlayer with oxidized cobalt segregating over Pt. Upon vacuum annealing, TiO2 is partially reduced and significant overlayer reorganization occurs, characterized by overgrowth of Ti. In oxidative conditions, a considerable amount of cobalt segregates back to the surface, forming a quite stable overlayer consisting of a Co3O4−TiO2 oxide mixture and CoxTiyOz mixed oxide, which “blocks” (encapsulates) Pt in its interior. This surface arrangement is rather stable, and apart from a 14347

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

spectra of a CoO reference sample and of a PtCo/TiO2 sample in 0.2 mbar CH3OH + O2. This information is available free of charge via the Internet at http://pubs.acs.org

partial reduction of cobalt species, is not significantly influenced when switching to reductive conditions. The effect of TiO2 substrate on the surface stability is dominant, taking into account results presented here and in our recent study on unsupported PtCo that showed a remarkably fast and quite extensive surface rearrangement responding to changes in the gas phase environment.11 A schematic representation of the above-mentioned process is given in Figure 6.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Contribution from M. Acosta in sample preparation is highly appreciated. Financial support from FP7-FCH-JU-2008-1-CP: (ROBANODE, IRAFC projects) and ETNAA ANR-07NANO-018-01 and BESSY II EUSA programmes is gratefully acknowledged. Finally, we acknowledge the HelmholtzZentrum Berlin (Electron storage ring BESSY II) for provision of synchrotron beamtime at ISISS beamline.

■

Figure 6. Schematic of the changes that occur at PtCo/TiO2 upon vacuum annealing (encapsulation of PtCo in the TiO2 film), oxidation (formation of a mixed TiO2/Co3O4/CoxTiyOz overlayer), and reduction (formation of a mixed TiO2/CoO/CoxTiyOz overlayer).

(1) Chen, J. G.; Menning, C. A.; Zellner, M. B. Surf. Sci. Rep. 2008, 63, 201. (2) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (3) Su, H. Y.; Bao, X. H.; Li, W. X. Journ. Chem. Phys. 2008, 128, 194707. (4) Alayoglu, S.; Nilekar, A. U.; Mavrikakis, M.; Eichhorn, B. Nat. Mater. 2008, 7, 333. (5) Zafeiratos, S.; Piccinin, S.; Teschner, D. Catal. Sci. Technol. 2012, DOI: 10.1039/C2CY00487A. (6) Ruban, A. V.; Skriver, H. L.; Norskov, J. K. Phys. Rev. B 1999, 59, 15990. (7) Nishihata, Y.; Mizuki, J.; Akao, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamada, N. Nature 2002, 418, 164. (8) Hansen, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Topsoe, H. Science 2002, 295, 2053. (9) Nolte, P.; Stierle, A.; Jin-Phillipp, N. Y.; Kasper, N.; Schulli, T. U.; Dosch, H. Science 2008, 321, 1654. (10) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. Science 2008, 322, 932−934. (11) Papaefthimiou, V.; Dintzer, T.; Dupuis, V.; Tamion, A.; Tournus, F.; Teschner, D.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R.; Zafeiratos, S. J. Phys. Chem. Lett. 2011, 2, 900. (12) Zafeiratos, S.; Paloukis, F.; Papakonstantinou, G.; Teschner, D.; Hävecker, M.; Vass, E.; Schnorch, P.; Knop-Gericke, A.; Schlögl, R.; Moreno, B.; Chinarro, E.; Jurado, J. R.; Neophytides, S. G. Catal. Today 2010, 157, 250. (13) Surnev, S.; Schoiswohl, J.; Kresse, G.; Ramsey, M. G.; Netzer, F. P. Phys. Rev. Lett. 2002, 89, 246101. (14) Tang, L. G.; Yamaguchi, D.; Burke, N.; Trimm, D.; Chiang, K. Catal. Commun. 2010, 11, 1215. (15) Rodriguez, J. A.; Ma, S.; Liu, P.; Hrbek, J.; Evans, J.; Perez, M. Science 2007, 318, 1757. (16) Burch, R. Phys. Chem. Chem. Phys. 2006, 8, 5483. (17) Molina, L. M.; Hammer, B. Phys. Rev. Lett. 2003, 90, 206102. (18) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (19) Fu, Q.; Wagner, T. Surf. Sci. Rep. 2007, 62, 431. (20) Dulub, O.; Hebenstreit, W.; Diebold, U. Phys. Rev. Lett. 2000, 84, 3646. (21) Bernal, S.; Botana, F. J.; Calvino, J. J.; Lόpez, C.; Pérez-Omil, J. A.; Rodriguez Izquierdo, J. M. J. Chem. Soc., Faraday Trans. 1996, 92, 2799. (22) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (23) Oregan, B.; Gratzel, M. Nature 1991, 353, 737.

Changes of the gas atmosphere above the sample induce core level BE shifts without affecting the chemical composition, which were attributed to band bending effects at the gas−solid interface. This behavior may be exploited as a gas sensing effect. It originates from formation of a space charge region due to adsorption and desorption of oxidizing or reducing species. One of the most remarkable findings of this study is the stability of Co2+ species against further reduction, which may be explained by the strong interaction with TiO2 support and formation of stable mixed oxide phases. Although the present study is performed at about 6 orders of magnitude higher pressure compared to typical surface science experiments, there is still a substantial pressure gap to technical catalyst operation conditions (1 atm and above). Previous ex-situ studies of Co/ TiO2 catalysts have shown that Co3O4 is reduced to Co2+ after 2 h annealing at 570 K under 3% H2−Ar mixtures under atmospheric pressure.47 Partially reduced Co2+ to metallic Co0 was observed only after 66 h annealing. Under similar conditions SiO2 supported cobalt oxide was completely reduced.47 The qualitative agreement between the results presented here and those of ref 47 shows that the hydrogen chemical potential in the mbar range is sufficient to induce reduction effects on TiO2-supported cobalt oxides, similar to the atmospheric pressure. Thus, we can presume that the pressure gap between our study and real catalytic conditions should have no significant qualitative influence on the results. Ambient pressure is therefore expected to slow down the kinetics of the catalytic processes involved in this study, but not their nature.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

(1) SEM images of the as-prepared PtCO/TiO2 samples and after in situ treatments, (2) Pt 4f spectra of PtCo/TiO2 sample and a reference PtCo polycrystalline foil, (3) XRD patterns of the Ti film on Si and of the Si/Ti/TiO2/PtCo sample before (fresh) and after (used) the in situ redox treatments (4) Co L3edge NEXAFS spectra of a PtCo/TiO2 sample and a PtCo polycrystalline foil in 0.2 mbar O2, (5) Co L3-edge NEXAFS 14348

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349

The Journal of Physical Chemistry C

Article

(24) Bach, U.; Corr, D.; Lupo, D.; Pichot, F.; Ryan, M. Adv. Mater. 2002, 14, 845. (25) Birkefeld, L. D.; Azad, A. M.; Akbar, S. A. J. Am. Ceram. Soc. 1992, 75, 2964. (26) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (27) Dubey, N.; Rayalu, S. S.; Labhsetwar, N. K.; Devotta, S. Int. J. Hydrogen Energy 2008, 33, 5958. (28) Papaefthimiou, V.; Dintzer, T.; Dupuis, V.; Tamion, A.; Tournus, F.; Hillion, A.; Teschner, D.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R.; Zafeiratos, S. ACS Nano 2011, 5, 2182. (29) Yaacoub, N.; Meny, C.; Ulhaq-Bouillet, C.; Acosta, M.; Panissod, P. Phys. Rev. B 2007, 75, 174402. (30) Knop-Gericke, A.; Kleimenov, E.; Hävecker, M.; Blume, R.; Teschner, D.; Zafeiratos, S.; Schlögl, R.; Bukhtiyarov, V. I.; Kaichev, V. V.; Prosvirin, I. P.; Nizovskii, A. I.; Bluhm, H.; Barinov, A.; Dudin, P.; Kiskinova, M. Adv. Catal. 2009, 52, 213. (31) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1. (32) Seah, M. P. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Wiley & Sons: Chichester, U.K., 1992; Vol 1, p 209. (33) Gao, Y.; Liang, Y.; Chambers, S. A. Surf. Sci. 1996, 365, 638. (34) Zafeiratos, S.; Dintzer, T.; Teschner, D.; Blume, R.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. J. Catal. 2010, 269, 309. (35) Regan, T. J.; Ohldag, H.; Stamm, C.; Nolting, F.; Luning, J.; Stohr, J.; White, R. L. Phys. Rev. B 2001, 64, 214422. (36) Bazin, D.; Kovács, I.; Guczi, L.; Parent, P.; Laffon, C.; de Groot, F.; Ducreux, O.; Lynch, J. J. Catal. 2000, 189, 456. (37) Morales, F.; de Groot, F. M. F.; Glatzel, P.; Kleimenov, E.; Bluhm, H.; Hävecker, M.; Knop-Gericke, A.; Weckhuysen, B. M. J. Phys. Chem. B 2004, 108, 16201. (38) Quilty, J. W.; Shibata, A.; Son, J. Y.; Takubo, K.; Mizokawa, T.; Toyosaki, H.; Fukumura, T.; Kawasaki, M. Phys. Rev. Lett. 2006, 96, 027202. (39) Ogale, S.; Kundaliya, D.; Mehraeen, S.; Fu, L. F.; Zhang, S. X.; Lussier, A.; Dvorak, J.; Browning, N.; Idzerda, Y.; Venkatesan, T. Chem. Mater. 2008, 20, 1344. (40) Lee, Y. J.; de Jong, M. P.; Jansen, R. Appl. Phys. Lett. 2010, 96, 082506. (41) Kucheyev, S. O.; van Buuren, T.; Baumann, T. F.; Satcher, J. H.; Willey, T. M.; Meulenberg, R. W.; Felter, T. E.; Poco, J. F.; Gammon, S. A.; Terminello, L. J. Phys. Rev. B 2004, 69, 245102. (42) Crocombette, J. P.; Jollet, F. J. Phys.-Condens. Matter 1994, 6, 10811. (43) Dachille, F.; Simons, P. Y.; Roy, R. Am. Mineral. 1968, 53, 1929. (44) Gole, J. L.; Prokes, S. M.; Glembocki, O. J. J. Phys. Chem. C 2008, 112, 1782. (45) Li, S.; Jena, P. Phys. Rev. B 2009, 79, 201204. (46) Jacob, K. T.; Rajitha, G. J. Chem. Thermodyn. 2010, 42, 879. (47) Riva, R.; Miessner, H.; Vitali, R.; Del Piero, G. Appl. Catal. AGen. 2000, 196, 111. (48) Gassenbauer, Y.; Schafranek, R.; Klein, A.; Zafeiratos, S.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R. Sol. St. Ionics 2006, 177, 3123. (49) Kohl, D. J. Phys. D-Appl. Phys. 2001, 34, R125. (50) Porsgaard, S.; Jiang, P.; Borondics, F.; Wendt, S.; Liu, Z.; Bluhm, H.; Besenbacher, F.; Salmeron, M. Angew. Chem.-Int. Ed. 2011, 50, 2266. (51) Nowotny, J.; Bak, T.; Sheppard, L. R.; Nowotny, M. K. J. Am. Chem. Soc. 2008, 130, 9984.

14349

dx.doi.org/10.1021/jp302320s | J. Phys. Chem. C 2012, 116, 14342−14349