Article pubs.acs.org/JPCC
Highly Oxidized Palladium Nanoparticles Comprising Pd4+ Species: Spectroscopic and Structural Aspects, Thermal Stability, and Reactivity Lidiya S. Kibis,† Andrey I. Stadnichenko,†,‡ Sergei V. Koscheev,† Vladimir I. Zaikovskii,† and Andrei I. Boronin*,†,‡ †
Boreskov Institute of Catalysis SB RAS, Prospect Akademika Lavrentieva 5, Novosibirsk 630090, Russia Novosibirsk State University, 630090, Pirogova Street 2, Russia
‡
ABSTRACT: Oxidized palladium nanoparticles, PdOx (x ≈ 1.3), measuring approximately 3 nm in size were prepared by RF-discharge under an oxygen atmosphere. The Pd3d X-ray photoelectron spectra (XPS) of oxidized palladium nanoparticles show two main peaks with binding energy Eb(Pd3d5/2) at ∼336.5 and 338.6 eV, which were assigned to Pd2+ and Pd4+ species, respectively. Attempts to synthesize pure Pd4+ nanoparticles by plasma treatment without the presence of Pd2+ were unsuccessful. High-resolution transmission electron microscopy (HRTEM) data show the defect structure of the palladium nanoparticles. The particles’ thermal stability is relatively high, being stable up to ∼425−450 K. The oxidized palladium species (Pd4+) were found to be highly reactive toward CO at room temperature. These results demonstrate the necessity of further investigation of highly oxidized palladium species as possible active centers in CO oxidation reactions.
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INTRODUCTION Palladium is widely used as an active component of catalysts in numerous industrial reactions; therefore, a large number of research groups all over the world have investigated the catalytic activity of palladium species in both real1−4 and model5−11 catalytic systems. One of the most important reactions from both practical and theoretical points of view is the low temperature oxidation of CO (LTO CO).12−21 Metallic palladium species or bulk-like oxide PdO are known to be not responsible for the LTO CO;22−26 small palladium clusters or ionic palladium states are usually considered as the active sites in this reaction. However, the nature of these active species is still the subject of vigorous debate. Highly oxidized palladium species (+4) were observed by many researchers during investigations of highly active catalysts consisting of palladium particles deposited on reducible oxide supports.26−34 The analysis of the electronic palladium state in these investigations was generally based on the X-ray photoelectron spectroscopy data. The assignment of the palladium peaks with high binding energies in the Pd3d spectra to the Pd4+ state was performed when the chemical shift of this peak relative to the metallic palladium state was greater than 3 eV. Nevertheless, most of the investigators use supported palladium systems,32,33,35−38 where such factors as dispersion, contaminations, and support effect have to be accounted for and can therefore complicate the analysis. For correct identification of palladium states based on XPS data, PdO2 © 2012 American Chemical Society
nanoparticles need to be prepared in clean, high-vacuum experimental conditions. There are few papers in which the properties of PdO2 species were investigated directly.39−48 Yet, the electronic, geometric, and catalytic properties of this species have not been clearly established. Moreover, highly oxidized species of many metals (Ru, Pd, Pt, Fe, etc.) have been intensively studied recently with respect to its possible activity in oxidation reactions.13,49−53 Therefore, the preparation of PdO2 nanoparticles and the investigation of their electronic properties, thermal stability, and reactivity toward CO are of great importance from a methodological perspective and for establishing the mechanism of LTO CO oxidation on palladium-based catalysts. The highly oxidized palladium species prepared by chemical methods is difficult to study because of its instability in anhydrous ambient.39 The oxidation of bulk palladium samples (e.g., foils, single crystals) by molecular oxygen is complicated by thermodynamic and kinetic limitations. In this study, the oxidized palladium nanoparticles were synthesized by plasma sputtering of a palladium electrode in an oxygen atmosphere. The electronic properties, thermal stability, and reaction probability of oxidized palladium nanoparticles toward CO were studied by XPS in situ directly in an analyzer chamber Received: May 28, 2012 Revised: August 1, 2012 Published: August 15, 2012 19342
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dependence of number of oxidized palladium species on the number of CO impingements. The activation energy was calculated on the basis of the reaction probability data for three different temperatures based on Arrhenius equation χ = χ0 exp(−Ea/RT). The value of the activation energy was estimated as the slope of the dependence of ln(χ) on the 1/RT. A JEM-2010 (JEOL, Japan) microscope with a lattice resolution of 0.14 nm and an accelerating voltage of 200 kV was used to obtain HRTEM data of the palladium nanoparticles. For the HRTEM analysis, the oxidized palladium nanoparticles were sputtered by the RF-discharge in the photoelectron spectrometer chamber over a carbon film measuring approximately 10 nm in thickness fixed on a standard copper grid. The preparation conditions were the same as in the case of the oxidized tantalum substrate. The average size of the particle was determined from TEM data via the ImageJ program.62 The area of each particle as measured from the TEM data was used to estimate the average particle size. For calculation purposes, all particles were assumed to have a round shape.
without contact with air. The structural properties of the oxidized palladium nanoparticles were studied using transmission electron microscopy (TEM).
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EXPERIMENTAL SECTION Oxidized palladium nanoparticles were synthesized directly in the preparation chamber of a VG ESCALAB HP spectrometer54 by application of capacitively coupled radio frequency discharge.55,56 Oxidized palladium nanoparticles were prepared by plasma sputtering of the RF-electrode (a thin palladium wire) under an oxygen atmosphere. The RF-discharge was generated at 300 W at a pO2 of 0.2−0.5 mbar for approximately 4 min. An oxidized tantalum foil was used as a substrate for the oxidized palladium nanoparticles.54 The oxygen signal from the oxidized tantalum foil was carefully subtracted from all O1s spectra.54 The XPS measurements were performed using an Al Kα (hν = 1486.6 eV) X-ray source. The core-level gold (Au4f7/2) and copper (Cu2p3/2) lines with binding energies of 84.0 and 932.7 eV, respectively, were used for calibration of the electron spectrometer. The experimental curves were fitted with a combination of Gaussian and Lorentzian peaks after the Shirley background subtraction procedure. The data were processed and analyzed using the XPS-Calc program.54,57,58 The reaction probability of the oxidized palladium nanoparticles was studied by specrokinetic X-ray photoelectron spectroscopy experiments. The particles’ reaction probability (χ) was determined from the ratio of the number of reactions between CO molecules and oxidized palladium nanoparticles to the total number of CO impingements on the surface of the sample. The number of the oxidized palladium species on the surface was estimated from the XPS data. The thickness of the oxidized palladium layer on the surface of tantalum oxide was estimated on the basis of the dependence of support’s signal on covering layer thickness:59
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RESULTS AND DISCUSSION The Pd3d and O1s XPS spectra of the oxidized palladium nanoparticles deposited on the oxidized tantalum foil are shown in Figure 1.
I = Io exp( −d /λ sin α)
where Io is the XPS signal of the pure support, d is the thickness of the covering layer, λ is the mean free pass of the photoelectrons, and I is the XPS signal of the support covered with the film of d thickness recorded at α electron takeoff angle. The calculated thickness of the oxidized film was about 0.6λ. Taking λ as 20 Å,59 we obtained d ≈ 12 Å. The 1 cm2 of the oxidized palladium layer with a thickness 12 Å gives us area with volume 1.2 × 1017 Å3. According to the structural data, the unit cell volume of PdO2 oxide is 62.32 Å3, and there are 2 Pd atoms and 4 oxygen atoms in the unit cell.60 Thus 1 cm2 of the PdO2 layer with thickness 12 Å contains 1.2 × 1017 Å3/62.32 Å3 ≈ 2 × 1015 unit cells and 4 × 1015 palladium atoms. The unit cell volume of PdO oxide is 48 Å3 with 2 Pd atoms and 2 oxygen atoms in the unit cell.61 There are 1.2 × 1017 Å3/48 Å3 ≈ 2.5 × 1015 unit cells and 5 × 1015 palladium atoms in 1 cm2 of the PdO layer with thickness 12 Å. As we have a mixture of PdO and PdO2 species, let us suppose that 1 cm2 of our oxidized palladium layer contains about 4.5 × 1015 palladium atoms (average number between PdO2 and PdO layers). According to Pd3d spectra, the intensity of Pd4+ species is about 55% of total Pd intensity, and then the number of Pd4+ species is about 2.5 × 1015 in 1 cm2 of oxidized layer. The number of CO impingements on 1 cm2 of the surface was calculated from the kinetic theory of gases. The calculation procedure is described in detail elsewhere.58 The value of the reaction probability was estimated as the slope of the
Figure 1. The (a) Pd3d and (b) O1s spectra of the oxidized palladium nanoparticles prepared by the RF-discharge for 4 min. The oxygen spectrum is presented after the subtraction of the oxygen signal from tantalum oxide.
The plasma discharge in oxygen at room temperature resulted in the formation of two palladium species. The Pd3d core-level line could be fitted with two main doublets with the Eb of the Pd3d5/2 peaks at 336.5 and 338.6 eV (Figure 1a). The palladium state with the lower Eb can be assigned to Pd2+ in palladium oxide (PdO) but with a shift to a lower energy by 0.3−0.5 eV in relation to the Eb of bulk PdO.63,64 On the basis of the data obtained for the plasma oxidized palladium foil,43 the Eb for the oxidized palladium nanoparticles was expected to be smaller than in case of the bulk oxide. The Pd3d5/2 peak with the higher Eb can be assigned to the strongly oxidized palladium (4+) state.37,42,44 Attempts to obtain only PdO2 nanoparticles proved to be unsuccessful, as characteristics of Pd2+ species were always observed in the Pd3d spectra. Palladium dioxide is known to be unstable in its anhydrous form;32,37,39,42,43 however, no changes were observed when recording the Pd3d spectra. PdO2 particles have been shown to be stabilized by the matrix of other oxides, such as Al2O3, SnO2, and PdO.32,37,43,44,65 Therefore, it is reasonable to propose that the Pd4+ state in our work is stabilized in its anhydrous form by the 19343
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Figure 2. TEM image of the palladium nanoparticles obtained by the RF-discharge and the particle size distribution (d is an average size of the particles; SD is standard deviation in size distribution).
Figure 3. The high-resolution TEM images of oxidized palladium particles. (a) PdO oxide particles are marked. Insets show the Fourier patterns corresponding to the particles marked on the HRTEM image by numbers. (b,c) HRTEM images with analysis of interplanar spacings of defect particles. Insets show the histogram of interplanar spacing based on the contrast analysis of the area marked by arrows on HRTEM images.
PdO oxide. Thus, the mixture of PdO and PdO2 oxides in the oxidized palladium nanoparticles was obtained. The small feature at about 341 eV is likely to arise from a satellite structure of the oxidized palladium species. It is known that the
Pd3d spectrum of PdO oxide is characterized by the satellite structure at an energy distance of about 2.6 eV corresponding to the PdO main line.43,64 The nature of this satellite peak remains unclear.64 It can be proposed that the Pd4+ species 19344
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could also have a satellite structure. Nevertheless, further investigations are required to clarify the nature of this peak. Note that only one state of oxygen with an Eb at 529.2 eV is present in the O1s region (Figure 1b). The Eb of oxygen in the regular PdO oxide is known to be approximately 529.8−530.1 eV.20,43 The Eb of the O1s peak for palladium dioxide is unknown; however, the Eb of oxygen in PdO and PdO2 oxides may be close, and interference cannot be ruled out. Formation of mixed palladium oxides, xPdO·yPdO2, with two nonequivalent palladium species and a single oxygen state may also explain our experimental data. The stoichiometric Pd/O ratio of the obtained compounds as estimated by standard XPS techniques59 is equal to approximately 1.3−1.4. Panin et al.66 were able to synthesize and investigate the salt of the mixed palladium oxide, KPdIIPdIVO3, by the X-ray diffraction method. This compound is composed of an alternating sequence of Pd2O3− and K+ layers with ordered Pd2+ and Pd4+ ions. The Pd2O3− layers are constructed from PdO4 and PdO6 polyhedra connected by common edges. The TEM data show that the palladium particles formed extensive islands of irregular shape measuring more than 5 nm in size. The smaller particles of approximately 1−2 nm have a round shape. Assuming that each particle has a round shape, the average particle size was calculated to be approximately 3 nm (see Figure 2). HRTEM images and corresponding Fourier patterns show point reflexes corresponding mainly to the interplanar spacing of metallic palladium: d111 = 0.225 nm (Figure 3a, insets (3) and (4)). These metallic particles are defective, composing smaller crystallites approximately 2 nm in size separated by intergrain boundaries along the lattice planes (111). The interplanar spacing of the tetragonal lattice of PdO (d002 = 0.266 nm, d101 = 0.263 nm) is also present (Figure 3a, insets (1) and (2)). Crystallites with widened interplanar spacings (d = 0.250 nm, 0.280 nm) can also be observed, as shown in Figure 3, panels b and c. Such lattice parameters are close to the interplanar spacings of palladium dioxide PdO2 (d101 = 0.253 nm)60 and PdO of cubic lattice (d200 = 0.282 nm).67 Note that the formation of particles with such lattice parameters can also occur as a result of size effects or the formation of nonstoichiometric oxidized palladium species. Thus, palladium is mainly in a metallic state according to the TEM data. Conversely, XPS data show only peaks with Eb typical of oxidized palladium species. This discrepancy can be explained by the low stability of the oxidized palladium states. As shown below, the oxidized palladium species are highly reactive toward CO. Therefore, the oxidized palladium species are expected to be reduced when removing the samples from the high-vacuum spectrometer chamber and exposing them to air to perform the TEM analysis. Nevertheless, a general conclusion about the size and structure of the particles can be made from the TEM data. The large number of extended defects and inconsistent lattice parameter support the assumption that the structure of the oxidized palladium species is unlike that of bulk PdO oxide. The thermal stability of the oxidized palladium species was studied by heating the samples in a vacuum chamber to 600 K at 25 K intervals. Pd3d, O1s, and Ta4f spectra and survey spectra were recorded for each temperature point. Figure 4a shows the Pd3d spectra. Each spectrum was fitted by individual Pd components: Pd4+ species (Eb(Pd3d5/2) = 338.6 eV), Pd2+ species (Eb(Pd3d5/2) = 336.5 eV), and metallic palladium (Eb(Pd3d5/2) = 335.1 eV). Figure 4b shows the intensity of
Figure 4. (a) Pd3d spectra after sample heating to (1) 300 K, (2) 325 K, (3) 350 K, (4) 375 K, (5) 400 K, (6) 425 K, (7) 450 K, (8) 475 K, (9) 500 K, (10) 525 K, (11) 550 K, (12) 575 K, and (13) 600 K. (b) The overall intensity of the Pd3d line (◆) and the intensity of peaks with Eb(Pd3d5/2) 338.6 eV (●), 336.5 eV (■), and 335.1 eV (▲).
these Pd components in the Pd3d spectrum as a function of sample temperature. The change in the overall intensity of the Pd3d spectrum is also presented. Although the overall intensity of the Pd3d spectra remains practically the same up to 425 and 450 K, a decrease in the intensity of the Eb(Pd3d5/2) = 338.6 eV component and an increase of the Eb(Pd3d5/2) = 336.5 eV component can be observed. Thus, the Pd4+ species decomposed with the formation of the palladium (2+) state. Shaplugin et al.60 found that palladium dioxide synthesized at a high oxygen pressure decomposed at temperatures above 340 K. Consequently, the palladium (4+) state obtained in our work is relatively stable. Curve fitting of the Pd3d spectra shows the appearance of metallic palladium species at 500 K. At this temperature, the Eb of the metallic state is 0.2−0.3 eV higher than the value expected for bulk metallic palladium (Eb(Pd3d5/2) = 335.1 eV), indicating the formation of small metallic particles.68−70 As the temperature was increased further, larger metallic clusters were formed, and the Eb shift disappeared. The overall intensity of the Pd3d spectrum increased slightly at temperatures greater than 450 K, which might be related to the depletion of oxygen amount in the system and changes of the sample’s morphology during the heating. Also, the redeposition of Pd containing materials from other parts of the substrate holder to the analysis area with increasing temperature cannot be excluded. Yet, as the temperature is not very high, its impact to the overall intensity could not be significant. The overall intensity of the Pd3d spectrum decreased at temperatures greater than 550 K, probably due to sintering and formation of larger particles. A shift to a higher Eb at these temperatures was observed for the palladium (2+) oxide. The Eb of Pd3d5/2 at 337.2 eV is typical for bulk PdO oxide. Oxidized nanoparticles are likely to merge with the formation of three-dimensional oxide particles. Unfortunately, due to the overlapping of spectral regions of O1s and Pd3p lines, the reliable quantitative data on the O1s signal depletion could not be obtained. Nevertheless, the substantial depletion of the O1s signal is observed at temperature above 525 K (O1s spectra not shown) pointing to the formation of metallic palladium with temperature increase. 19345
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exposure at different temperatures. On the basis of the XPS data, the reaction probability (χ) of the oxidized palladium nanoparticles comprising Pd2+ and Pd4+ species was estimated as was described in the Experimental Section. Figure 6a and b shows the dependence of the number of oxidized palladium species on the number of CO impingements on the surface. To estimate the reaction probability of Pd4+ species, the initial parts of the curves were used (CO exposures up to 5000 L). The reaction probability of Pd2+ species was estimated on the linear part of the curve. The parts of the curves corresponding to CO exposures of 35 000−80 000 L at T = 300 K; 24 500−80 000 L at T = 325 K; and 7000−17 000 L at T = 350 K were chosen. The maximum reaction probability (χ ≈ 10−3) was obtained for the oxidized nanoparticles comprising Pd4+ species at 350 K. This is a rather high value of the reaction probability. For instance, the reaction probability of nanostructured oxidized copper estimated by the same calculation procedure was 5 × 10−5.58 The reaction probability of PdO nanoparticles was estimated to be 10 times less than that of the highly oxidized palladium species at room temperature. The bulk PdO oxide is known to be unreactive toward CO at low temperatures.23,26 Therefore, the reaction probability of palladium (2+) oxide nanoparticles prepared in this study was relatively high. These data are in a good agreement with works of Rogal et al.,73,74 Hinojosa et al.,7 and Hendriksen et al.18 who showed by theoretical and experimental methods that thin films of PdO oxide can be the catalytically active phase in CO oxidation. The increase in reaction probability may be observed because the Pd−O bond in the thin PdO films or small particles is weaker than that in bulk palladium oxide. Also, defect sites at the surface of the samples18 or the predominant presence of certain surface planes17 could be responsible for the increased catalytic activity of PdO surface in LTO CO. The reaction probability values for the three different temperatures were used to estimate the activation energy (Ea) for the reaction between CO and the highly oxidized palladium species Pd4+ (Figure 6c, curve (●)). Ea was estimated to be approximately 25 kJ/mol. This relatively small Ea indicates that the reaction proceeds without a noticeable activation barrier that is in good agreement with theoretic data obtained by Gong et al.,13 who showed almost negligible activation energy for CO oxidation on PdO2. The estimated Ea for the reaction of Pd2+ reduction in CO was about 35 kJ/mol (Figure 6c, curve (■)).
According to literature data, the decomposition temperature of bulk PdO oxide varies from 87571 to 1075 K,72 depending on the experimental conditions. Thus, less stable PdO nanoparticles were obtained, and higher reaction probability of oxygen species in these nanoparticles can be expected. The interaction of the oxidized palladium species with CO was studied by exposing oxidized palladium particles step-bystep to CO. Figure 5a shows the Pd3d spectra as a function of CO exposure at room temperature.
Figure 5. (a) The Pd3d spectra of the oxidized palladium species after CO exposure at room temperature: (1) 0 L, (2) 3 × 103 L, (3) 6 × 103 L, (4) 9 × 103 L, (5) 1 × 104 L, (6) 2 × 104 L, (7) 4 × 104 L, (8) 2 × 107 L, (9) 5 × 107 L, (10) 2 × 108 L, (11) 9 × 108 L, (12) 3 × 109 L, and (13) 1 × 1010 L. (b) The intensity of peaks with Eb(Pd3d5/2) = 336.5 eV (1) and 338.6 eV (2) at 300 K (□), 325 K (○), and 350 K (△), depending on the CO exposure.
The reduction of the Pd4+ species to Pd2+ and the metallic state can be clearly observed, even at room temperature. The overall intensity of the Pd3d spectrum remains practically unchanged. Note that the Eb(Pd3d5/2) of metallic palladium was 335.6 eV. The formation of small metallic clusters Pd0n may be responsible for this chemical shift of the Pd3d spectra to a higher Eb. The reaction probability of the oxidized palladium species was studied by specrokinetic XPS experiments in a CO flow (pCO = 5 × 10−6 mbar) at three temperatures (300, 325, and 350 K). The experiments were conducted at temperatures below 400 K to prevent thermal decomposition of the Pd4+ species. Figure 5b shows the change in intensity of the Pd2+ (336.5 eV, Figure 5b, curve (1)) and Pd4+ (338.6 eV, Figure 5b, curve (2)) components in the Pd3d spectra as a function of CO
Figure 6. The depletion of oxidized Pd species (Pd4+ and Pd2+, (a) and (b), respectively) with increasing number of CO impingements at three temperatures (300 K (□), 325 K (○), and 350 K (△)). Figures are also provided with calculated data of the curve’s slope. (c) The Arrhenius plot for the reaction of Pd4+ (●) and Pd2+ (■) species reduction. 19346
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(17) Hirvi, J. T.; Kinnunen, T.-J. J.; Suvanto, M.; Pakkanen, T. A.; Nørskov, J. K. J. Chem. Phys. 2010, 133, 084704. (18) Hendriksen, B. L. M.; Ackermann, M. D.; van Rijn, R.; Stoltz, D.; Popa, I.; Balmes, O.; Resta, A.; Wermeille, D.; Felici, R.; Ferrer, S.; et al. Nat. Chem. 2010, 2, 730−734. (19) Parker, S. F. Chem. Commun. 2011, 47, 1988−1990. (20) Westerstrom, R.; Messing, M. E.; Blomberg, S.; Hellman, A.; Gronbeck, H.; Gustafson, J.; Martin, N. M.; Balmes, O.; van Rijn, R.; Andersen, J. N.; et al. Phys. Rev. B 2011, 83, 115440. (21) Kunz, S.; Schweinberger, F. F.; Habibpour, V.; Rottgen, M.; Harding, C.; Arenz, M.; Heiz, U. J. Phys. Chem. C 2010, 114, 1651− 1654. (22) Bera, P.; Patil, K. C.; Jayaram, V.; Subbanna, G. N.; Hegde, M. S. J. Catal. 2000, 196, 293−301. (23) Zheng, G.; Altman, E. I. J. Phys. Chem. B 2002, 106, 1048−1057. (24) Oh, S.-H.; Hoflund, G. B. J. Phys. Chem. A 2006, 110, 7609− 7613. (25) Boronin, A. I.; Slavinskaya, E. M.; Danilova, I. G.; Gulyaev, R. V.; Amosov, Y. I.; Kuznetsov, P. A.; Polukhina, I. A.; Koscheev, S. V.; Zaikovskii, V. I.; Noskov, A. S. Catal. Today 2009, 144, 201−211. (26) Oh, S.-H.; Hoflund, G. B. J. Catal. 2007, 245, 35−44. (27) Guimaraes, A. L.; Dieguez, L. C.; Schmal, M. J. Phys. Chem. B 2003, 107, 4311−4319. (28) Zhu, H.; Qin, Z.; Shan, W.; Shen, W.; Wang, J. J. Catal. 2005, 233, 41−50. (29) Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Krohnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F. C.; Knop-Gericke, A.; Paal, Z.; et al. J. Catal. 2006, 237, 17−28. (30) Wang, F.; Lu, G. J. Phys. Chem. C 2009, 113, 4161−4167. (31) Persson, K.; Jansson, K.; Järås, S. G. J. Catal. 2007, 245, 401− 414. (32) Otto, K.; Haack, L. P.; Vries, J. E. d. Appl. Catal., B 1992, 1, 1− 12. (33) Luo, J.-Y.; Meng, M.; Xian, H.; Tu, Y.-B.; Li, X.-G.; Ding, T. Catal. Lett. 2009, 133, 328−333. (34) Meng, L.; Lin, J.-J.; Pu, Z.-Y.; Luo, L.-F.; Jia, A.-P.; Huang, W.X.; Luo, M.-F.; Lu, J.-Q. Appl. Catal., B 2012, 119−120, 117−122. (35) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Venezia, A. M.; Deganello, G.; Borla, E. M.; Pidria, M. F. Top. Catal. 2007, 42−43, 425−428. (36) Venezia, A. M.; Liotta, L. F.; Pantaleo, G.; La Parola, V.; Deganello, G.; Beck, A.; Koppany, Z.; Frey, K.; Horvath, D.; Guczi, L. Appl. Catal., A 2003, 251, 359−368. (37) Domashevskaya, E. P.; Ryabtsev, S. V.; Turishchev, S. Y.; Kashkarov, V. M.; Yurakov, Y. A.; Chuvenkova, O. A.; Shchukarev, A. V. J. Struct. Chem. 2008, 49, 80−91. (38) Cao, X.; Cao, L.; Yao, W.; Ye, X. Surf. Interface Anal. 1996, 24, 662−666. (39) Kim, K. S.; Gossmann, A. F.; Winograd, N. Anal. Chem. 1974, 46, 197−200. (40) Barr, T. L. J. Phys. Chem. 1978, 82, 1801−1810. (41) Mucalo, M. R.; Cooney, R. P.; Metson, J. B. Colloids Surf. 1991, 60, 175−197. (42) Mucalo, M. R.; Bullen, C. R. J. Mater. Sci. Lett. 2001, 20, 1853− 1856. (43) Kibis, L. S.; Titkov, A. I.; Stadnichenko, A. I.; Koscheev, S. V.; Boronin, A. I. Appl. Surf. Sci. 2009, 255, 9248−9254. (44) Sohn, Y.; Pradhan, D.; Leung, K. T. ACS Nano 2010, 4, 5111− 5120. (45) Semagina, N.; Renken, A.; Laub, D.; Kiwi-Minsker, L. J. Catal. 2007, 246, 308−314. (46) Ten Eyck, G. A.; Pimanpang, S.; Juneja, J. S.; Bakhru, H.; Lu, T.M.; Wang, G.-C. Chem. Vap. Deposition 2007, 13, 307−311. (47) Grden, M.; Lukaszewski, M.; Jerkiewicz, G.; Czerwinski, A. Electrochim. Acta 2008, 53, 7583−7598. (48) Casella, I. G. J. Electrochem. Soc. 2008, 155, D723−D729. (49) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474− 1476.
CONCLUSIONS In summary, the oxidized palladium nanoparticles were prepared using the RF-discharge under an oxygen atmosphere and were studied by XPS and TEM. Our attempts to obtain pure PdO2 particles were unsuccessful; instead, palladium (+2) states were formed together with highly oxidized palladium species, where the palladium (+2) states acted as a stabilization matrix for the Pd4+ species. The highly oxidized palladium species were observed to have a relatively high thermal stability and a high reaction probability toward CO. Future studies should include not only the reaction probability of these species but also their catalytic activity, that is, the possibility to regenerate these palladium species by oxygen or reaction mixture treatments. These results can also be used as a basis for the investigation of highly oxidized palladium nanoparticles deposited on active supports, such as CeO2.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +7 383 3269631. Fax: +7 383 3308056. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Education and Science of the Russian Federation (project no. 14.740.11.0419).
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REFERENCES
(1) Park, J.-N.; McFarland, E. W. J. Catal. 2009, 266, 92−97. (2) Obuya, E. A.; Harrigan, W.; Andala, D. M.; Lippens, J.; Keane, T. C.; Jones, W. E., Jr. J. Mol. Catal. A: Chem. 2011, 340, 89−98. (3) Castellazzi, P.; Groppi, G.; Forzatti, P.; Finocchio, E.; Busca, G. J. Catal. 2010, 275, 218−227. (4) Ntainjua, E. N.; Piccinini, M.; Pritchard, J. C.; Edwards, J. K.; Carley, A. F.; Kiely, C. J.; Hutchings, G. J. Catal. Today 2011, 178, 47− 50. (5) Zheng, G.; Altman, E. I. Surf. Sci. 2002, 504, 253−270. (6) Schalow, T.; Brandt, B.; Starr, D. E.; Laurin, M.; Shaikhutdinov, S. K.; Schauermann, S.; Libuda, J.; Freund, H. J. Phys. Chem. Chem. Phys. 2007, 9, 1347−1361. (7) Hinojosa, J. J. A.; Kan, H. H.; Weaver, J. F. J. Phys. Chem. C 2008, 112, 8324−8331. (8) van Rijn, R.; Balmes, O.; Resta, A.; Wermeille, D.; Westerstrom, R.; Gustafson, J.; Felici, R.; Lundgren, E.; Frenken, J. W. M. Phys. Chem. Chem. Phys. 2011, 13, 13167−13171. (9) Ludwig, W.; Savara, A.; Dostert, K.-H.; Schauermann, S. J. Catal. 2011, 284, 148−156. (10) Paredis, K.; Ono, L. K.; Behafarid, F.; Zhang, Z.; Yang, J. C.; Frenkel, A. I.; Cuenya, B. R. J. Am. Chem. Soc. 2011, 133, 13455− 13464. (11) Weaver, F. J.; Hinojosa, J. A., Jr.; Hakanoglu, C.; Antony, A.; Hawkins, J. M.; Asthagiri, A. Catal. Today 2011, 160, 213−227. (12) Fernandez-Garcia, M.; Martinez-Arias, A.; Salamanca, L. N.; Coronado, J. M.; Anderson, J. A.; Conesa, J. C.; Soria, J. J. Catal. 1999, 187, 474−485. (13) Gong, X.-Q.; Liu, Z.-P.; Raval, R.; Hu, P. J. Am. Chem. Soc. 2003, 126, 8−9. (14) Dyakonov, A. J.; Little, C. A. Appl. Catal., B 2006, 67, 52−59. (15) Zhu, H.; Qin, Z.; Shan, W.; Shen, W.; Wang, J. Catal. Today 2007, 126, 382−386. (16) Liang, F.; Zhu, H.; Qin, Z.; Wang, G.; Wang, J. Catal. Commun. 2009, 10, 737−740. 19347
dx.doi.org/10.1021/jp305166k | J. Phys. Chem. C 2012, 116, 19342−19348
The Journal of Physical Chemistry C
Article
(50) Hellman, A.; Klacar, S.; Gronbeck, H. J. Am. Chem. Soc. 2009, 131, 16636−16637. (51) Sun, Y.-N.; Giordano, L.; Goniakowski, J.; Lewandowski, M.; Qin, Z.-H.; Noguera, C.; Shaikhutdinov, S.; Pacchioni, G.; Freund, H.J. Angew. Chem., Int. Ed. 2010, 49, 4418−4421. (52) Butcher, D. R.; Grass, M. E.; Zeng, Z.; Aksoy, F.; Bluhm, H.; Li, W.-X.; Mun, B. S.; Somorjai, G. A.; Liu, Z. J. Am. Chem. Soc. 2011, 133, 20319−20325. (53) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; et al. Phys. Rev. Lett. 2005, 95, 256102. (54) Kibis, L. S.; Stadnichenko, A. I.; Pajetnov, E. M.; Koscheev, S. V.; Zaykovskii, V. I.; Boronin, A. I. Appl. Surf. Sci. 2010, 257, 404−413. (55) Gupta, B.; Hilborn, J.; Hollenstein, C.; Plummer, C. J. G.; Houriet, R.; Xanthopoulos, N. J. Appl. Polym. Sci. 2000, 78, 1083− 1091. (56) Bogaerts, A.; Neyts, E.; Gijbels, R.; van der Mullen, J. Spectrochim. Acta, Part B 2002, 57, 609−658. (57) Kibis, L. S.; Avdeev, V. I.; Koscheev, S. V.; Boronin, A. I. Surf. Sci. 2010, 604, 1185−1192. (58) Svintsitskiy, D. A.; Stadnichenko, A. I.; Demidov, D. V.; Koscheev, S. V.; Boronin, A. I. Appl. Surf. Sci. 2011, 257, 8542−8549. (59) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley & Sons: New York, 1983. (60) Shaplugin, I. S.; Aparnikov, G. L.; Lazarev, V. B. Zh. Neorg. Khim. 1978, 23, 884−887. (61) Waser, J.; Levy, H. A.; Peterson, S. W. Acta Crystallogr. 1953, 6, 661−663. (62) Abramoff, M. D.; Magalhaes, P. J.; Ram, S. J. Biophotonics Int. 2004, 11, 36−42. (63) Peuckert, M. J. Phys. Chem. 1985, 89, 2481−2486. (64) Pillo, T.; Zimmermann, R.; Steiner, P.; Hufner, S. J. Phys.: Condens. Matter 1997, 9, 3987−3999. (65) Kappler, J.; Bârsan, N.; Weimar, U.; Dièguez, A.; Alay, J. L.; Romano-Rodriguez, A.; Morante, J. R.; Göpel, W. Fresenius’ J. Anal. Chem. 1998, 361, 110−114. (66) Panin, R. V.; Khasanova, N. R.; Bougerol, C.; Schnelle, W.; Van Tendeloo, G.; Antipov, E. V. Inorg. Chem. 2010, 49, 1295−1297. (67) Kumar, J.; Saxena, R. J. Less-Common Met. 1989, 147, 59−71. (68) Mason, M. G. Phys. Rev. B 1983, 27, 748. (69) Wertheim, G. K.; DiCenzo, S. B. Phys. Rev. B 1988, 37, 844. (70) Wu, T. P.; Kaden, W. E.; Kunkel, W. A.; Anderson, S. L. Surf. Sci. 2009, 603, 2764−2770. (71) Lundgren, E.; Gustafson, J.; Mikkelsen, A.; Andersen, J. N.; Stierle, A.; Dosch, H.; Todorova, M.; Rogal, J.; Reuter, K.; Scheffler, M. Phys. Rev. Lett. 2004, 92, 046101. (72) Zhang, H.; Gromek, J.; Fernando, G.; Marcus, H.; Boorse, S. J. Phase Equilib. 2002, 23, 246−248. (73) Rogal, J.; Reuter, K.; Scheffler, M. Phys. Rev. Lett. 2007, 98, 046101. (74) Rogal, J.; Reuter, K.; Scheffler, M. Phys. Rev. B 2007, 75, 205433.
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