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C catalysts with NO2
Applied Surface Science 428 (2018) 972–976
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Comparative XPS study of interaction of model and real Pt/C catalysts with NO2 M.Yu. Smirnov ∗ , A.V. Kalinkin, E.I. Vovk, P.A. Simonov, E.Yu. Gerasimov, A.M. Sorokin, V.I. Bukhtiyarov Boreskov Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia
a r t i c l e
i n f o
Article history: Received 6 July 2017 Received in revised form 18 September 2017 Accepted 24 September 2017 Available online 28 September 2017 Keywords: Platinum Carbon supports XPS Nitrogen dioxide Core-shell Pt oxide/Pt metal particles
a b s t r a c t XP Spectroscopy is used to compare the states of the supported platinum particles in the samples of the Pt/Sibunit catalyst and the Pt/HOPG model system after treating in NO2 at room temperature and pressure of 3 × 10-6 mbar. It is shown that in both cases the platinum particles are not oxidized completely. The comparison of the spectra obtained for the Pt/Sibunit sample in the Pt 4f and Pt 3d5/2 regions with the use of Al K˛ and Ag L␣ radiations serves as a basis for an assumption that the particles of platinum after being treated in NO2 have the core (metallic Pt) − shell (platinum oxides) structure. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The physicochemical studies of the model systems which chemical composition is identical to that of real catalysts are widely used for revealing the nature of the compounds formed on the surface of catalysts in the conditions of catalytic reaction. Quite often the model systems of the supported heterogeneous catalysts represent the particles of active component on the surface of the support, bulk or prepared as a thin film (tens of nm thick) on the substrate of metal foil [1,2]. The main advantage of the model systems when studied with XPS is good spectral characteristics (high signal intensity and high resolution in binding energy) of the active componentrelated signal, which is due to the morphology of the system itself. At the same time the treatment of such samples in the reaction medium can be carried out directly in the vacuum chambers of XPS spectrometer at the conditions close to those of real catalysis. Nevertheless, even in the case when compositions of the real catalyst and the corresponding model system are similar, the question arises on how adequately the model system describes the behavior of the catalyst in reaction medium. In this work XPS was used to study the interaction of the real porous catalyst Pt/Sibunit and the planar model system Pt/HOPG (HOPG = Highly Oriented
∗ Corresponding author. E-mail address: [email protected] (M.Yu. Smirnov). https://doi.org/10.1016/j.apsusc.2017.09.205 0169-4332/© 2017 Elsevier B.V. All rights reserved.
Pyrolytic Graphite) with NO2 at identical conditions; the obtained results were compared. 2. Materials and methods A sample of the real catalyst was prepared by redox-hydrolytic precipitation of particles of platinum oxide (II) from an alkalized H2 PtCl6 solution on the surface of porous carbon material Sibunit (Institute of Hydrocarbons Processing SB RAS, Omsk) with NaOOCH. A detailed description of the method is given in [3]. The content of metal platinum in the catalyst was 40 wt.%. Dispersion of the platinum particles determined by pulse chemisorption of CO at 20 ◦ C was 0.32, which corresponds to the average particle size of 3.4 nm. A sample of the model system was prepared by vacuum evaporation of metal platinum on the surface of HOPG fixed onto the stainless steel sample holder. The graphite surface was preactivated with short-term etching by argon ions of low energy (1 s, 500 eV) in order to create the defects serving as setting sites for platinum particles [4]. After deposition of platinum the sample was heated in vacuum at 300 ◦ C for 1 h for stabilization of the metal particles. The detailed description of the method for preparation of the Pt/HOPG model system is given in [5]. X-ray photoelectron spectra of the Pt/Sibunit catalyst were obtained in SPECS spectrometer using non-monochromatic Al K␣ radiation (h = 1486.6 eV) as well as monochromatic Ag L␣ radiation (h = 2983.4 eV). The sample was fixed on the holder using
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Fig. 1. Images of the Pt/Sibunit catalyst obtained by TEM (a) and of the Pt/HOPG model system obtained by STM (b), and corresponding histograms of size distribution of the platinum particles. The image and histogram for Pt/HOPG is taken from [5].
double-sided conductive adhesive tape. The size of the platinum particles was determined by TEM on JEM-2010 microscope (JEOL, Japan) at accelerating voltage of 200 kV and line-to-line resolution of 0.14 nm. X-ray photoelectron spectra of the Pt/HOPG model system were registered in SPECS spectrometer with non-monochromatic Al K␣ radiation. STM images of the system were obtained in vacuum scanning tunneling microscope GPI-300.02 (Russia) with the use of platinum needle as a probe. The real and model Pt/C samples were treated in NO2 in the preparation chamber of SPECS spectrometer at a temperature close to RT. NO2 was synthesized directly in the chamber by thermal decomposition of lead nitrate utilizing the source described in details elsewhere [5].
3. Results Fig. 1 presents the TEM image of the Pt/Sibunit catalyst (Fig. 1a) and the STM image of the Pt/HOPG model system (Fig. 1b) before interaction with NO2 . In both cases the platinum particles are uniformly distributed over the support surface. Fig. 1 also gives the histograms of size distribution of the particles, which are used to calculate the particle average size: 2.3 nm and 2.5 nm for Pt/Sibunit and Pt/HOPG, respectively. Therefore, it can be presumed that the platinum particles in the real catalyst and in the model system are close in size, and further comparison of their behavior in the reaction can be considered. Fig. 2 shows the X-ray photoelectron spectra registered in the Pt 4f region with the use of Al K␣ radiation for the Pt/Sibunit catalyst (Fig. 2a) and the Pt/HOPG model system (Fig. 2b) before and after interaction with NO2 . The treatment conditions in both cases are identical: NO2 pressure, 3 × 10−6 mbar; exposure time, 30 h. For the initial samples the spectra are presented with a single doublet of
asymmetric spin-orbit components Pt 4f7/2 –Pt 4f5/2 with binding energy BE(Pt 4f7/2 )–71.8–72.0 eV. The obtained values of binding energy are higher than those for the bulk metal platinum (71.2 eV). This is due to the final state effect, which is typical of highly dispersed particles [6]. After treatment in NO2 the shape of the spectra changes, however, the spectra characterizing the samples of the real catalyst (Fig. 2a, spectrum 2) and the model system (Fig. 2b, spectrum 2) are similar to each other, which implies the qualitatively close changes occurring with the platinum particles in both cases. Spectra 2 in Fig. 2 are deconvoluted into three doublet lines. One of the doublets characterized by spin-orbit components of asymmetric shape and binding energy of BE(Pt 4f7/2 ) = 71.8–71.9 eV is related to metal platinum. Two other doublets, each being represented by a pair of symmetric spin-orbit components, are related to platinum oxides, PtO (∼73.0 eV) and PtO2 (∼74.5–75.0 eV). According to the literature data the binding energies of the Pt 4f7/2 line in the bulk platinum oxides are 72.4–72.8 eV (PtO) and 74.2–74.8 eV (PtO2 ) [7–10]. For the Pt/Sibunit catalyst sample we also registered spectra in Pt 4f (Fig. 3a) and Pt 3d5/2 (Fig. 3b) regions with the use of Ag L˛ radiation. The Pt 4f spectra in Figs. 2a and 3a have similar shape; after treatment in NO2 both spectra are fitted with three doublets, one related to metal platinum, two others to platinum oxides. The only difference is that the contribution of platinum oxides in the Pt 4f spectrum (Fig. 3a, spectrum 2) registered with the use of Ag L˛ radiation is smaller than that in the spectrum of the same region (Fig. 2a, spectrum 2) registered with the Al K˛ radiation. The Pt 3d5/2 spectrum of the initial catalyst is represented with a slightly asymmetric line with the binding energy of 2122.4 eV, which is 0.6 eV higher than that for Pt foil [11]. Like in the case of Pt 4f, the observed difference in binding energy is associated with the final state effect [6,12]. After treatment of the catalyst in NO2 two additional lines with the binding energies of 2123.5 and 2125.0 eV appear in the
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Fig. 2. XP-spectra registered in the Pt 4f region with the use of Al K˛ radiation for the samples of Pt/Sibunit catalyst (a) and Pt/HOPG model system (b) before oxidation (1) and after interaction with NO2 at room temperature (2). The spectra for Pt/HOPG are taken from [5].
Fig. 3. XP-spectra of the Pt/Sibunit catalyst registered in the Pt 4f (a) and Pt 3d5/2 (b) regions with the use of Ag L˛ radiation before oxidation (1) and after interaction with NO2 at room temperature (2).
Pt 3d5/2 spectrum, which are related to platinum oxides, PtO and PtO2 . For comparison, BE(Pt 3d5/2 ) in the compounds of Pt(II) and Pt(IV) are ∼2123.5 and 2125.5 eV, respectively [11]. The position of the doublet characterizing the metal platinum does not change after the catalyst being treated in NO2 . For the Pt/Sibunit catalyst sample we also registered C 1s, N 1s, and O 1 s spectra with the use of Al K␣ radiation. After reaction with NO2 the intensive C 1 s line of carbon from the support does not undergo any significant changes, and in the N 1 s region we observe no appearance of the peaks that could be attributed to the nitrogencontaining compounds. After reaction the significant changes are found only in the O 1 s region (Fig. 4). In this case we observe a more than two-fold increase in the surface oxygen concentration. Since the quantity of the additional oxygen formed after treatment in NO2 exceeds the content of oxidized platinum more than ten times, one can conclude that the prevailing amount of oxygen is spent on the formation of the surface compounds with carbon. A similar conclusion was made when the interaction of the Pt/HOPG model system with NO2 was examined [5].
Fig. 4. O 1 s spectra of the Pt/Sibunit catalyst sample registered with Al K˛ radiation in the initial state (1) and after interaction with NO2 at room temperature (2).
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Fig 5. Schematic image of the particle (core) of metal platinum covered with the shell of platinum oxides. The shadowed area marks the zone of analysis for electrons photo-emitted from the Pt 3d5/2 (left) and Pt 4f (right) levels.
4. Discussion Thus, the comparison of the Pt 4f spectra obtained for the Pt/Sibunit real catalyst sample and the Pt/HOPG model system sample showed that in spite of a quite different morphology both samples demonstrate a similar behavior when interacting with NO2 under the identical conditions (Fig. 2). Therefore, we can claim that, at least for the given Pt/Sibunit + NO2 reaction, the model system adequately describes the behavior of the real catalyst. In both cases the interaction leads to partial oxidation of the Pt particles. While examining the spectra in Figs. 2 and 3 one can note that for both samples the binding energy and the shape of Pt 4f7/2 line do not change significantly keeping the metallic state after interaction with NO2 . We find the situation to be quite different considering the interaction of NO2 (or mixture of NO + O2 ) with the platinum particles supported on the oxides Al2 O3 , SiO2 , TiO2 , and ZrO2 . Along with the lines of platinum oxides appearing in the spectrum, we have observed a shift of the doublet assigned to metal platinum into the direction of higher binding energies [13–17]. We supposed that in the systems Pt – oxide support the reaction leads to dissolution of oxygen atoms in the particles of metallic platinum. We also presumed that the dissolved oxygen atoms (Odis ) and the surrounding platinum atoms do not form a regular lattice, which distinguishes these systems from oxide phases. Nevertheless, when such systems form, electron density is transferred from Pt atoms to Odis atoms, which results in an increase of the corresponding Pt 4f line binding energy. The absence of the dissolved oxygen in the Pt/Sibunit samples can be a result of its high reactivity with regard to the reducing agents [14,16,17]. In this case, if Odis does form at the initial stage, it might interact quite efficiently with carbon support significantly decreasing concentration of Odis in the platinum particles in the Pt/Sibunit samples. On the other hand, we cannot rule out the possibility that the oxygen atoms forming on the surface of the platinum particles begin to oxidize carbon before they start to dissolve. The oxidation of carbon support (Sibunit) actually takes place, which is confirmed by the increase of the O 1 s signal shown in Fig. 4. Finally, it is important to discuss the results related to platinum oxidation in the Pt/Sibunit sample (Fig. 3). From the XP-spectra
obtained with the use of Ag L˛ radiation we have determined the oxidation state of platinum on the basis of changes in the Pt 4f and Pt 3d5/2 regions. The electrons emitted in the result of photo-excitation from these two levels differ greatly in their kinetic energies: ∼2910 eV (Pt 4f) and ∼860 eV (Pt 3d5/2 ). As it was found out, the contribution of the oxidized states in the Pt 4f spectrum (∼ 25%, Fig. 3a) is lower than that in the Pt 3d5/2 spectrum (∼ 40%, Fig. 3b). This observation can be explained by the peculiarities of the spatial distribution of the oxide and metal phases in the platinum particle with regard to the different depth of the emission of the electrons with different kinetic energy. It is logical to assume that the particle has the core – shell structure, where the core is formed by metal platinum, and the shell covering the core is the mixture of platinum oxides. Fig. 5 shows schematically the section of the spherical particle with the shadowed area marking the zone of analysis that is limited by the outer surface of the particle and the surface all points of which are vertically remote from the outer surface for the distance corresponding to the length of the mean free path of electrons . The larger is the shadowed area, the bigger number of atoms gets into the analysis zone, and the more intensive signal is registered in the XP-spectrum. The image in the left part of Fig. 5 corresponds to photoemission of the electrons from the Pt 3d5/2 level (3d ), the image in the right part – from the Pt 4f level (4f > 3d ). The value of depends on the kinetic energy of the electrons and the nature of material the electrons move through. The numerical values of can be found with the help of Tougaard Quases-IMFP-TPP2 M software [18]. Fig. 5 shows that the number of the platinum ions participating in the formation of photoemission signal in the shell of platinum oxides depends but slightly on , and, consequently, on the depth from which the electrons emit. On the contrary, a much higher number of the atoms of metallic platinum in the core contribute to the Pt 4f line as compared with the Pt 3d5/2 line, due to the higher kinetic energy of the electrons emitting from the Pt 4f level. That is the reason why the contribution of the line of oxide state in the Pt 3d5/2 spectrum (Fig. 3b, spectrum 2) is higher than in the Pt 4f spectrum (Fig. 3a, spectrum 2). It is noteworthy that the core – shell structure pictured in Fig. 5 agrees well with the numerous results published for the supported
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platinum catalysts. In an earlier work [19] such structure was suggested for the Pt/SiO2 catalysts based on the measuring of the radial distribution of the electron density at X-ray scattering. Later the use of in situ methods EXAFS and XANES showed that in the contact with oxidizing medium a layer of two-dimensional oxide can form over the surface of the supported platinum nanoparticles (1–3 nm in size), protecting the metal core of the particle from total transformation into oxide [20–25]. In the identical reaction conditions with the oxidizing medium, the contribution of the oxide phase [24] as well as the stoichiometric ratio O/Pt in the surface oxide [21] grow when the particle size diminishes. It was established that the platinum particles with the shell of non-stoichiometric surface oxide provide a high activity of the supported platinum catalysts in the reactions of CO [26] and 2-propanol [23] oxidation. On the other hand, in the reaction of methane oxidation at elevated concentration of oxygen in the reaction mixture the platinum particles are also covered by the shell of two-dimensional platinum oxide, as a result of which the catalyst switches to inactive state [25]. It was shown that in the reaction of methane oxidation the active is the catalyst in which the supported platinum particles are in the metallic state. Taking the structure of the oxidized platinum particles shown in Fig. 5 and using the approach developed in [27,28], it is possible to estimate the thickness of the oxide shell from the intensities of the Pt 4f and Pt 3d5/2 lines measured for the metallic and oxide states of platinum. Such estimates are made for the Pt/Sibunite sample studied in this paper and also for two other samples of different platinum loading and average size of Pt particles, after the treatment of the samples in NO2 . The results are described in a separate paper to be published in the near future. For the sample considered here, the oxide shell thickness is about 0.1-0.2 nm, which corresponds to a two-dimensional (surface) oxide with PtO or PtO2 stoichiometry [29]. 5. Conclusions The real (Pt/Sibunit) and model (Pt/HOPG) catalyst samples with similar size distributions of platinum particles interact with NO2 under the same conditions (pressure 3×10−6 mbar, room temperature) to result in a partial oxidation of platinum particles in both samples. It is possible to distinguish three components in XP spectra of the Pt 4f and Pt 3d5/2 regions, one of them corresponding to metallic platinum and two others, – to platinum oxides, PtO and PtO2 . The comparison of the intensity ratios of oxide to metal components in the Pt 4f and Pt 3d5/2 spectra obtained with Al K␣ and Ag L␣ radiations indicates that the platinum particles acquire the core (metal) – shell (oxide) structure as a result of the interaction with NO2 . Acknowledgement The authors thank the Russian Science Foundation (project 1423-00146) for the financial support. References [1] D.W. Goodman, Model studies in catalysis using surface science probes, Chem. Rev. 95 (1995) 523–536. ¨ [2] M. Baumer, H.-J. Freund, Metal deposits on well-ordered oxide films, Prog. Surf. Sci. 61 (1999) 127–198. [3] I.N. Voropaev, P.A. Simonov, A.V. Romanenko, Formation of Pt/C catalysts on various carbon supports, Russ. J. Inorg. Chem. 54 (2009) 1531–1536. [4] D.V. Demidov, I.P. Prosvirin, A.M. Sorokin, V.I. Bukhtiyarov, Ag/HOPG catalysts: preparation and STM/XPS study, Catal. Sci. Technol. 1 (2011) 1432–1439.
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Kalinkin, A.V. Nazimov, Model sulfur-resistant NSR catalysts: an XPS study of the interaction of BaO/TiO2 -ZrO2 and Pt-BaO/TiO2 -ZrO2 with NO2 , Kinet. Catal. 56 (2015) 540–548. [16] M.Yu. Smirnov, A.V. Kalinkin, E.I. Vovk, V.I. Bukhtiyarov, Size effect in the oxidation-reduction process in platinum particles supported onto silicon dioxide, Kinet. Catal. 56 (2015) 801–809. [17] E.I. Vovk, A.V. Kalinkin, M.Yu. Smirnov, I.O. Klembovskii, V.I. Bukhtiyarov, XPS study of stability and reactivity of oxidized Pt nanoparticles supported on TiO2 , J. Phys. Chem. C 121 (2017) 17297–17304. [18] H. Shinotsuka, S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. X. data for 41 elemental solids over the 50 eV to 200 keV range with the relativistic full Penn algorithm, Surf. Interface Anal. 47 (2015) 871–888. [19] P. Ratnasamy, A.J. Leonard, L. Rodriquez, J.J. Fripiat, Structure and surface properties of supported platinum catalysts, J. Catal. 29 (1973) 374–384. 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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Comparative XPS study of interaction of model and real Pt/C catalysts with NO2 M.Yu. Smirnov ∗ , A.V. Kalinkin, E.I. Vovk, P.A. Simonov, E.Yu. Gerasimov, A.M. Sorokin, V.I. Bukhtiyarov Boreskov Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia
a r t i c l e
i n f o
Article history: Received 6 July 2017 Received in revised form 18 September 2017 Accepted 24 September 2017 Available online 28 September 2017 Keywords: Platinum Carbon supports XPS Nitrogen dioxide Core-shell Pt oxide/Pt metal particles
a b s t r a c t XP Spectroscopy is used to compare the states of the supported platinum particles in the samples of the Pt/Sibunit catalyst and the Pt/HOPG model system after treating in NO2 at room temperature and pressure of 3 × 10-6 mbar. It is shown that in both cases the platinum particles are not oxidized completely. The comparison of the spectra obtained for the Pt/Sibunit sample in the Pt 4f and Pt 3d5/2 regions with the use of Al K˛ and Ag L␣ radiations serves as a basis for an assumption that the particles of platinum after being treated in NO2 have the core (metallic Pt) − shell (platinum oxides) structure. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The physicochemical studies of the model systems which chemical composition is identical to that of real catalysts are widely used for revealing the nature of the compounds formed on the surface of catalysts in the conditions of catalytic reaction. Quite often the model systems of the supported heterogeneous catalysts represent the particles of active component on the surface of the support, bulk or prepared as a thin film (tens of nm thick) on the substrate of metal foil [1,2]. The main advantage of the model systems when studied with XPS is good spectral characteristics (high signal intensity and high resolution in binding energy) of the active componentrelated signal, which is due to the morphology of the system itself. At the same time the treatment of such samples in the reaction medium can be carried out directly in the vacuum chambers of XPS spectrometer at the conditions close to those of real catalysis. Nevertheless, even in the case when compositions of the real catalyst and the corresponding model system are similar, the question arises on how adequately the model system describes the behavior of the catalyst in reaction medium. In this work XPS was used to study the interaction of the real porous catalyst Pt/Sibunit and the planar model system Pt/HOPG (HOPG = Highly Oriented
∗ Corresponding author. E-mail address: [email protected] (M.Yu. Smirnov). https://doi.org/10.1016/j.apsusc.2017.09.205 0169-4332/© 2017 Elsevier B.V. All rights reserved.
Pyrolytic Graphite) with NO2 at identical conditions; the obtained results were compared. 2. Materials and methods A sample of the real catalyst was prepared by redox-hydrolytic precipitation of particles of platinum oxide (II) from an alkalized H2 PtCl6 solution on the surface of porous carbon material Sibunit (Institute of Hydrocarbons Processing SB RAS, Omsk) with NaOOCH. A detailed description of the method is given in [3]. The content of metal platinum in the catalyst was 40 wt.%. Dispersion of the platinum particles determined by pulse chemisorption of CO at 20 ◦ C was 0.32, which corresponds to the average particle size of 3.4 nm. A sample of the model system was prepared by vacuum evaporation of metal platinum on the surface of HOPG fixed onto the stainless steel sample holder. The graphite surface was preactivated with short-term etching by argon ions of low energy (1 s, 500 eV) in order to create the defects serving as setting sites for platinum particles [4]. After deposition of platinum the sample was heated in vacuum at 300 ◦ C for 1 h for stabilization of the metal particles. The detailed description of the method for preparation of the Pt/HOPG model system is given in [5]. X-ray photoelectron spectra of the Pt/Sibunit catalyst were obtained in SPECS spectrometer using non-monochromatic Al K␣ radiation (h = 1486.6 eV) as well as monochromatic Ag L␣ radiation (h = 2983.4 eV). The sample was fixed on the holder using
M.Yu. Smirnov et al. / Applied Surface Science 428 (2018) 972–976
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Fig. 1. Images of the Pt/Sibunit catalyst obtained by TEM (a) and of the Pt/HOPG model system obtained by STM (b), and corresponding histograms of size distribution of the platinum particles. The image and histogram for Pt/HOPG is taken from [5].
double-sided conductive adhesive tape. The size of the platinum particles was determined by TEM on JEM-2010 microscope (JEOL, Japan) at accelerating voltage of 200 kV and line-to-line resolution of 0.14 nm. X-ray photoelectron spectra of the Pt/HOPG model system were registered in SPECS spectrometer with non-monochromatic Al K␣ radiation. STM images of the system were obtained in vacuum scanning tunneling microscope GPI-300.02 (Russia) with the use of platinum needle as a probe. The real and model Pt/C samples were treated in NO2 in the preparation chamber of SPECS spectrometer at a temperature close to RT. NO2 was synthesized directly in the chamber by thermal decomposition of lead nitrate utilizing the source described in details elsewhere [5].
3. Results Fig. 1 presents the TEM image of the Pt/Sibunit catalyst (Fig. 1a) and the STM image of the Pt/HOPG model system (Fig. 1b) before interaction with NO2 . In both cases the platinum particles are uniformly distributed over the support surface. Fig. 1 also gives the histograms of size distribution of the particles, which are used to calculate the particle average size: 2.3 nm and 2.5 nm for Pt/Sibunit and Pt/HOPG, respectively. Therefore, it can be presumed that the platinum particles in the real catalyst and in the model system are close in size, and further comparison of their behavior in the reaction can be considered. Fig. 2 shows the X-ray photoelectron spectra registered in the Pt 4f region with the use of Al K␣ radiation for the Pt/Sibunit catalyst (Fig. 2a) and the Pt/HOPG model system (Fig. 2b) before and after interaction with NO2 . The treatment conditions in both cases are identical: NO2 pressure, 3 × 10−6 mbar; exposure time, 30 h. For the initial samples the spectra are presented with a single doublet of
asymmetric spin-orbit components Pt 4f7/2 –Pt 4f5/2 with binding energy BE(Pt 4f7/2 )–71.8–72.0 eV. The obtained values of binding energy are higher than those for the bulk metal platinum (71.2 eV). This is due to the final state effect, which is typical of highly dispersed particles [6]. After treatment in NO2 the shape of the spectra changes, however, the spectra characterizing the samples of the real catalyst (Fig. 2a, spectrum 2) and the model system (Fig. 2b, spectrum 2) are similar to each other, which implies the qualitatively close changes occurring with the platinum particles in both cases. Spectra 2 in Fig. 2 are deconvoluted into three doublet lines. One of the doublets characterized by spin-orbit components of asymmetric shape and binding energy of BE(Pt 4f7/2 ) = 71.8–71.9 eV is related to metal platinum. Two other doublets, each being represented by a pair of symmetric spin-orbit components, are related to platinum oxides, PtO (∼73.0 eV) and PtO2 (∼74.5–75.0 eV). According to the literature data the binding energies of the Pt 4f7/2 line in the bulk platinum oxides are 72.4–72.8 eV (PtO) and 74.2–74.8 eV (PtO2 ) [7–10]. For the Pt/Sibunit catalyst sample we also registered spectra in Pt 4f (Fig. 3a) and Pt 3d5/2 (Fig. 3b) regions with the use of Ag L˛ radiation. The Pt 4f spectra in Figs. 2a and 3a have similar shape; after treatment in NO2 both spectra are fitted with three doublets, one related to metal platinum, two others to platinum oxides. The only difference is that the contribution of platinum oxides in the Pt 4f spectrum (Fig. 3a, spectrum 2) registered with the use of Ag L˛ radiation is smaller than that in the spectrum of the same region (Fig. 2a, spectrum 2) registered with the Al K˛ radiation. The Pt 3d5/2 spectrum of the initial catalyst is represented with a slightly asymmetric line with the binding energy of 2122.4 eV, which is 0.6 eV higher than that for Pt foil [11]. Like in the case of Pt 4f, the observed difference in binding energy is associated with the final state effect [6,12]. After treatment of the catalyst in NO2 two additional lines with the binding energies of 2123.5 and 2125.0 eV appear in the
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Fig. 2. XP-spectra registered in the Pt 4f region with the use of Al K˛ radiation for the samples of Pt/Sibunit catalyst (a) and Pt/HOPG model system (b) before oxidation (1) and after interaction with NO2 at room temperature (2). The spectra for Pt/HOPG are taken from [5].
Fig. 3. XP-spectra of the Pt/Sibunit catalyst registered in the Pt 4f (a) and Pt 3d5/2 (b) regions with the use of Ag L˛ radiation before oxidation (1) and after interaction with NO2 at room temperature (2).
Pt 3d5/2 spectrum, which are related to platinum oxides, PtO and PtO2 . For comparison, BE(Pt 3d5/2 ) in the compounds of Pt(II) and Pt(IV) are ∼2123.5 and 2125.5 eV, respectively [11]. The position of the doublet characterizing the metal platinum does not change after the catalyst being treated in NO2 . For the Pt/Sibunit catalyst sample we also registered C 1s, N 1s, and O 1 s spectra with the use of Al K␣ radiation. After reaction with NO2 the intensive C 1 s line of carbon from the support does not undergo any significant changes, and in the N 1 s region we observe no appearance of the peaks that could be attributed to the nitrogencontaining compounds. After reaction the significant changes are found only in the O 1 s region (Fig. 4). In this case we observe a more than two-fold increase in the surface oxygen concentration. Since the quantity of the additional oxygen formed after treatment in NO2 exceeds the content of oxidized platinum more than ten times, one can conclude that the prevailing amount of oxygen is spent on the formation of the surface compounds with carbon. A similar conclusion was made when the interaction of the Pt/HOPG model system with NO2 was examined [5].
Fig. 4. O 1 s spectra of the Pt/Sibunit catalyst sample registered with Al K˛ radiation in the initial state (1) and after interaction with NO2 at room temperature (2).
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Fig 5. Schematic image of the particle (core) of metal platinum covered with the shell of platinum oxides. The shadowed area marks the zone of analysis for electrons photo-emitted from the Pt 3d5/2 (left) and Pt 4f (right) levels.
4. Discussion Thus, the comparison of the Pt 4f spectra obtained for the Pt/Sibunit real catalyst sample and the Pt/HOPG model system sample showed that in spite of a quite different morphology both samples demonstrate a similar behavior when interacting with NO2 under the identical conditions (Fig. 2). Therefore, we can claim that, at least for the given Pt/Sibunit + NO2 reaction, the model system adequately describes the behavior of the real catalyst. In both cases the interaction leads to partial oxidation of the Pt particles. While examining the spectra in Figs. 2 and 3 one can note that for both samples the binding energy and the shape of Pt 4f7/2 line do not change significantly keeping the metallic state after interaction with NO2 . We find the situation to be quite different considering the interaction of NO2 (or mixture of NO + O2 ) with the platinum particles supported on the oxides Al2 O3 , SiO2 , TiO2 , and ZrO2 . Along with the lines of platinum oxides appearing in the spectrum, we have observed a shift of the doublet assigned to metal platinum into the direction of higher binding energies [13–17]. We supposed that in the systems Pt – oxide support the reaction leads to dissolution of oxygen atoms in the particles of metallic platinum. We also presumed that the dissolved oxygen atoms (Odis ) and the surrounding platinum atoms do not form a regular lattice, which distinguishes these systems from oxide phases. Nevertheless, when such systems form, electron density is transferred from Pt atoms to Odis atoms, which results in an increase of the corresponding Pt 4f line binding energy. The absence of the dissolved oxygen in the Pt/Sibunit samples can be a result of its high reactivity with regard to the reducing agents [14,16,17]. In this case, if Odis does form at the initial stage, it might interact quite efficiently with carbon support significantly decreasing concentration of Odis in the platinum particles in the Pt/Sibunit samples. On the other hand, we cannot rule out the possibility that the oxygen atoms forming on the surface of the platinum particles begin to oxidize carbon before they start to dissolve. The oxidation of carbon support (Sibunit) actually takes place, which is confirmed by the increase of the O 1 s signal shown in Fig. 4. Finally, it is important to discuss the results related to platinum oxidation in the Pt/Sibunit sample (Fig. 3). From the XP-spectra
obtained with the use of Ag L˛ radiation we have determined the oxidation state of platinum on the basis of changes in the Pt 4f and Pt 3d5/2 regions. The electrons emitted in the result of photo-excitation from these two levels differ greatly in their kinetic energies: ∼2910 eV (Pt 4f) and ∼860 eV (Pt 3d5/2 ). As it was found out, the contribution of the oxidized states in the Pt 4f spectrum (∼ 25%, Fig. 3a) is lower than that in the Pt 3d5/2 spectrum (∼ 40%, Fig. 3b). This observation can be explained by the peculiarities of the spatial distribution of the oxide and metal phases in the platinum particle with regard to the different depth of the emission of the electrons with different kinetic energy. It is logical to assume that the particle has the core – shell structure, where the core is formed by metal platinum, and the shell covering the core is the mixture of platinum oxides. Fig. 5 shows schematically the section of the spherical particle with the shadowed area marking the zone of analysis that is limited by the outer surface of the particle and the surface all points of which are vertically remote from the outer surface for the distance corresponding to the length of the mean free path of electrons . The larger is the shadowed area, the bigger number of atoms gets into the analysis zone, and the more intensive signal is registered in the XP-spectrum. The image in the left part of Fig. 5 corresponds to photoemission of the electrons from the Pt 3d5/2 level (3d ), the image in the right part – from the Pt 4f level (4f > 3d ). The value of depends on the kinetic energy of the electrons and the nature of material the electrons move through. The numerical values of can be found with the help of Tougaard Quases-IMFP-TPP2 M software [18]. Fig. 5 shows that the number of the platinum ions participating in the formation of photoemission signal in the shell of platinum oxides depends but slightly on , and, consequently, on the depth from which the electrons emit. On the contrary, a much higher number of the atoms of metallic platinum in the core contribute to the Pt 4f line as compared with the Pt 3d5/2 line, due to the higher kinetic energy of the electrons emitting from the Pt 4f level. That is the reason why the contribution of the line of oxide state in the Pt 3d5/2 spectrum (Fig. 3b, spectrum 2) is higher than in the Pt 4f spectrum (Fig. 3a, spectrum 2). It is noteworthy that the core – shell structure pictured in Fig. 5 agrees well with the numerous results published for the supported
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platinum catalysts. In an earlier work [19] such structure was suggested for the Pt/SiO2 catalysts based on the measuring of the radial distribution of the electron density at X-ray scattering. Later the use of in situ methods EXAFS and XANES showed that in the contact with oxidizing medium a layer of two-dimensional oxide can form over the surface of the supported platinum nanoparticles (1–3 nm in size), protecting the metal core of the particle from total transformation into oxide [20–25]. In the identical reaction conditions with the oxidizing medium, the contribution of the oxide phase [24] as well as the stoichiometric ratio O/Pt in the surface oxide [21] grow when the particle size diminishes. It was established that the platinum particles with the shell of non-stoichiometric surface oxide provide a high activity of the supported platinum catalysts in the reactions of CO [26] and 2-propanol [23] oxidation. On the other hand, in the reaction of methane oxidation at elevated concentration of oxygen in the reaction mixture the platinum particles are also covered by the shell of two-dimensional platinum oxide, as a result of which the catalyst switches to inactive state [25]. It was shown that in the reaction of methane oxidation the active is the catalyst in which the supported platinum particles are in the metallic state. Taking the structure of the oxidized platinum particles shown in Fig. 5 and using the approach developed in [27,28], it is possible to estimate the thickness of the oxide shell from the intensities of the Pt 4f and Pt 3d5/2 lines measured for the metallic and oxide states of platinum. Such estimates are made for the Pt/Sibunite sample studied in this paper and also for two other samples of different platinum loading and average size of Pt particles, after the treatment of the samples in NO2 . The results are described in a separate paper to be published in the near future. For the sample considered here, the oxide shell thickness is about 0.1-0.2 nm, which corresponds to a two-dimensional (surface) oxide with PtO or PtO2 stoichiometry [29]. 5. Conclusions The real (Pt/Sibunit) and model (Pt/HOPG) catalyst samples with similar size distributions of platinum particles interact with NO2 under the same conditions (pressure 3×10−6 mbar, room temperature) to result in a partial oxidation of platinum particles in both samples. It is possible to distinguish three components in XP spectra of the Pt 4f and Pt 3d5/2 regions, one of them corresponding to metallic platinum and two others, – to platinum oxides, PtO and PtO2 . The comparison of the intensity ratios of oxide to metal components in the Pt 4f and Pt 3d5/2 spectra obtained with Al K␣ and Ag L␣ radiations indicates that the platinum particles acquire the core (metal) – shell (oxide) structure as a result of the interaction with NO2 . Acknowledgement The authors thank the Russian Science Foundation (project 1423-00146) for the financial support. References [1] D.W. Goodman, Model studies in catalysis using surface science probes, Chem. Rev. 95 (1995) 523–536. ¨ [2] M. Baumer, H.-J. Freund, Metal deposits on well-ordered oxide films, Prog. Surf. Sci. 61 (1999) 127–198. [3] I.N. Voropaev, P.A. Simonov, A.V. Romanenko, Formation of Pt/C catalysts on various carbon supports, Russ. J. Inorg. Chem. 54 (2009) 1531–1536. [4] D.V. Demidov, I.P. Prosvirin, A.M. Sorokin, V.I. Bukhtiyarov, Ag/HOPG catalysts: preparation and STM/XPS study, Catal. Sci. Technol. 1 (2011) 1432–1439.
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