Structural Characterization of Ru-Modified Carbon-Supported Pt

Article pubs.acs.org/JPCC

Structural Characterization of Ru-Modified Carbon-Supported Pt Nanoparticles Using Spontaneous Deposition with CO Oxidation Activity Amado Velázquez-Palenzuela, Enric Brillas, Conchita Arias, Francesc Centellas, José Antonio Garrido, Rosa María Rodríguez, and Pere-Lluís Cabot* Laboratory of Environmental Electrochemistry and Materials, Department of Physical Chemistry, Faculty of Chemistry, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain ABSTRACT: The modification of carbon-supported Pt nanoparticles, high performance (HP) 20% Pt on Vulcan XC-72 carbon black (Pt/C electrocatalyst), by spontaneous deposition of Ru species is examined employing electrochemical and structural techniques. Thin-layer electrodes were prepared by applying aqueous catalyst inks of Pt/C on glassy carbon (GC) disks. Ru deposition was carried out by immersion of the prepared electrode in deaerated RuCl3/ HClO4 solutions. The subsequent cyclic voltammetry experiments of the modified electrocatalysts (Ru(Pt)/C) were performed in 0.5 M H2SO4 to determine the Ru coverage and the electroactive surface. CO stripping voltammetry showed the promotional effect of Ru(Pt)/C for the CO oxidation compared to Pt/C. The structural characterization of the modified electrocatalysts was performed by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analyses, fast Fourier transform (FFT), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TEM observations revealed no appreciable signals of Ru agglomerates, and EDX confirmed the regular incorporation of Ru species to the nanoparticles. XRD analyses showed the characteristic profile of the Pt face-centered cubic (FCC) structure and the absence of crystalline Ru or Ru oxides. The application of the Williamson−Hall models indicated that Ru incorporation did not significantly affect the internal strain of the Pt nanoparticles, the increase of the crystallite size being attributed to an epitaxial growth of the Ru deposit. XPS measurements reported the presence of nonreducible RuO2 and hydrous RuO2 (RuOxHy) as the main Ru species in Ru(Pt)/C, the hydrous species justifying the promotional effect for the CO oxidation. or by underpotential,24 forced,25 or spontaneous depositions.26−39 The latter is certainly the most preferred because of its simplicity, since the process occurs through a strong chemisorption of the Ru species on the Pt substrate at open circuit potential. Unlike the underpotential deposition, the spontaneous deposition leads to the formation of irreversible deposits that are not dissolved when a potential is applied, so electrocatalysts prepared from this methodology could be used as alternative electrode materials for PEFC anodes. The spontaneous deposition has been mainly employed to modify the Pt low-index monocrystal surfaces to improve the MOR performance.27−29,31,34−36 The deposition step has been typically performed using RuCl3 as metal precursor and HClO4 as acidic electrolyte, since sulfate/bisulfate anions are strongly adsorbed on Pt sites causing the inhibition of the process.32 MOR measurements revealed a maximum catalytic activity for Ru coverages about 0.10−0.40 on Ru-modified Pt (111) surfaces.28 The modified surfaces were mainly analyzed by

1. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are excellent energysupply systems because of their low working temperature, high energy conversion, practically null production of pollutant exhausts, and adaptability to portable electronic devices.1 Ptbased alloys are the most active electrocatalysts for the oxidation reactions of hydrogen (HOR), 2−4 methanol (MOR), 5−11 and ethanol (EOR).12−16 The use of Pt nanoparticles supported on high surface carbon materials allows increasing the electroactive surface area and reducing the loading of the expensive Pt.17 The CO poisoning of Pt caused by the use of low-cost H2 from reforming or formed in the course of MOR or EOR can be significantly solved by means of alloyed Pt−Ru electrocatalysts, which present a superior CO tolerance.18−23 It is generally accepted that the hydroxylated Ru species (Ru−OH) generated through water discharge allows increasing the CO tolerance, according to reaction 1:8,9,21 Ru−OH + Pt−CO → Pt + Ru + CO2 + e− + H+

(1)

Reasonable strategies to improve the electroactivity toward CO, methanol, and ethanol oxidation are based on the Ru coverage of Pt surfaces (Ru decoration) by electrodeposition20 © 2012 American Chemical Society

Received: May 2, 2012 Revised: July 31, 2012 Published: August 2, 2012 18469

dx.doi.org/10.1021/jp304232u | J. Phys. Chem. C 2012, 116, 18469−18478

The Journal of Physical Chemistry C

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Auger electron spectroscopy (AES),22,34,39 scanning tunneling microscopy (STM),25,26,34,35 and X-ray photoelectron spectroscopy (XPS).29,33,34 Ru nanodeposits of 2−5 nm were detected as a result of the deposition step.28,31,34,35 XPS measurements showed the existence of Ru oxides as the main compounds in the modified electrocatalyst,29,31,33,34 which could be subject to further partial or complete reduction through a potential treatment28,29 or remain on the electrocatalyst surface as nonreducible oxides.22,32 Waszczuk et al.38 synthesized Ru-decorated unsupported Pt nanoparticles by spontaneous deposition, obtaining a superior performance toward CO oxidation and MOR. Maillard et al.32 prepared Ru-modified carbon-suported Pt nanoparticles using the same technique and reported an optimum Ru coverage of 0.10 for the MOR. However, when Du et al.24 analyzed the MOR performance of Pt nanoparticles electrodeposited on indium tin oxide (ITO) with Ru modification through potential cycling deposition, they found a Ru coverage around 0.40 as the most favorable surface composition. To the best of the authors’ knowledge, the structural analysis of carbon-supported Pt nanoparticles after the spontaneous deposition of Ru has not been previously reported. This is a key issue since a different morphology to that found in Rudecorated Pt monocrystal surfaces is expected. The larger surface area/volume ratio of the nanoparticles could magnify the strain generated by the lattice mismatch between the original Pt structure and the Ru species created on the surface.39 In fact, internal strain could affect not only the outer surface layer of the nanoparticle, as described for core−shell dealloyed nanoparticles,40 but also the structure of the bulk material could be affected by the existence of a surrounding phase.41,42 Thus, analogue effects could be found in the Rumodified Pt nanoparticles. In this paper, a structural study of carbon-supported Pt nanoparticles (Pt/C) modified by spontaneous deposition of Ru has been performed. For this purpose, thin-layer glassy carbon (GC) disk electrodes containing Pt/C catalyst inks were prepared, and RuCl3/HClO4 solutions were used for carrying out the deposition step. Cyclic voltammetry and CO stripping in 0.5 M H2SO4 were the electrochemical techniques utilized for the evaluation of the activity of the Ru(Pt)/C electrocatalysts. The structure of the Ru-modified specimens was characterized using transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analyses, fast Fourier transform (FFT), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and XPS.

Linde 5.0 (purity ≥99.999%) and 3.0 (purity ≥99.9%), respectively. Electrode Preparation. Aqueous suspensions with 3.0 mg mL−1 of Pt/C electrocatalyst (6.0 mg mL−1 for Pt−Ru/C) were prepared by sonicating for 45 min appropriate amounts of catalyst powder and ultrapure water. About 3.0−3.5 μL of the homogeneous ink was then deposited by means of a digital micropipet on the surface of the GC disk electrode. The prepared electrodes were dried under the heat of a lamp for at least 15 min, and further, they were coupled to an Ecochemie Autolab RDE to be used as working electrodes for the spontaneous deposition and the electrochemical trials. The final Pt loads on the GC surface were about 30 μg cm−2. Previously to the ink deposition, the GC tip was consecutively polished with aluminum oxide pastes of 0.3 and 0.05 μm (Buehler Micropolish II deagglomerated α-alumina and γ-alumina, respectively) on a Buehler PSA-backed White Felt polishing cloth until achieving a mirror finish. It was rinsed with Millipore Milli-Q water in ultrasonic bath between polishing steps. The GC coverage by the Pt/C ink was approached 100% in all cases. Spontaneous Deposition. Ru-modified Pt/C electrocatalysts, Ru(Pt)/C, were prepared by immersing the thinlayer Pt/C electrodes into a glass cell containing Ar-deaerated X M RuCl3 + 0.1 M HClO4 (RuCl3/HClO4) solutions at 25.0 °C, being X = 0.1 × 10−3−8 × 10−3 M, and for a period (t) between 60 and 1.8 × 104 s. The RuCl3/HClO4 solutions were primarily left aging for at least a week to facilitate the formation of aqueous Ru complexes. UV−vis measurements, performed with a UV-1800 Shimadzu spectrophotometer, confirmed that the aged solutions contained [RuO(H2O)4]2+ as the main Ru complex, showing two characteristic absorption bands at 300 and 469 nm, in agreement with the Ru speciation in perchloric acid media.43−45 Previous to the deposition step, the electrode was subjected to potential cycling in Ar-purged 0.5 M H2SO4, as described in the following section, and then extracted from the glass cell, washed with ultrapure water, carefully dried, and introduced in the RuCl3/HClO4 solution. After the surface modification, the resulting electrodes were cleaned with ultrapure water, dried, and placed in the Ar-sparged 0.5 M H2SO4 for the acquisition of the cyclic voltammogram series and CO stripping experiments, as indicated in the Electrochemical Measurements section. The coverage of the Ru(Pt)/C electrocatalysts (Θ) was calculated from eq 2:24,31

2. EXPERIMENTAL SECTION Materials and Reagents. High performance (HP) 20 wt % Pt nanoparticles supported on carbon Vulcan XC-72 (Pt/C electrocatalyst, actual analysis giving 19.6 wt % Pt on carbon) and HP 20% 1:1 Pt−Ru alloy on Vulcan XC-72 carbon Vulcan XC-72 (Pt−Ru/C electrocatalyst, actual analysis giving 19.9 wt % Pt−Ru), required for comparative purposes, were purchased from E-Tek. GC disk electrodes 3 mm diameter were provided by Metrohm. Hydrous ruthenium chloride (RuCl3·xH2O, Ru content 35−40 wt %) and analytical grade 70% HClO4 from Merck were employed for the preparation of the spontaneous deposition solutions. Analytical grade 96% H2SO4 from Merck was used to prepare 0.5 M H2SO4 as background electrolyte for the cyclic voltammetry and the CO stripping trials. All solutions were prepared with high-purity water obtained with a Millipore Milli-Q system (resistivity >18 MΩ cm). Ar and CO were

where QH,0 and QH,1 are the charges involved in the monatomic hydrogen adsorption and desorption processes with the corresponding subtraction of the double-layer contribution before and after the spontaneous deposition process (note that the Θ value calculated in this way must be taken as an approach, giving that the hydrogen desorption reaction partially overlaps with the Ru−OH formation).46 The control of the precursor concentration and the immersion duration allowed controlling the coverage of the electrocatalyst, which was found stable during the potential cycling, reproducible, and favored by the increase of both parameters. The coverage range analyzed varied between 0.0 and 0.4 for the Ru(Pt)/C specimens employed for the CO oxidation activity evaluation. Electrochemical Measurements. All the electrochemical experiments were performed using a conventional thermostated double-wall three-electrode glass cell from Metrohm of 200 mL

Θ=

18470

Q H,0 − Q H,1 Q H,0

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was alternatively carried out by depositing 2 mL of Pt/C-water ink on a low-density polyethylene (LDPE) square support (4 cm2 geometrical area), subsequently desiccated under the heat of a lamp for 30 min, and introduced in the RuCl3/H2SO4 solution. The LDPE support was then washed and dried, and the resulting Ru(Pt)/C electrocatalyst was released by scratching. The EDX analysis of the Ru(Pt)/C specimens prepared on GC electrode and on LDPE support employing the same Ru precursor concentration and exposure time showed identical Ru incorporation to the electrocatalyst, in agreement with the same Ru coverage of the Pt nanoparticles in both cases. X-ray photoelectron spectroscopy (XPS) experiments for the Pt/C and the Ru(Pt)/C electrocatalysts were performed using a Physical Electronics PHI 5500 Multitechnique System spectrometer with a monochromatic X-ray source (Al Kα line of 1486.6 eV energy and 350 W). This X-ray source was placed perpendicularly to the analyzer axis and calibrated using the 1s line of C region located at 284.6 eV. The analyzed area was a circle of 0.8 mm diameter. The Ru(Pt)/C sample was prepared following the same methodology employed for the XRD analysis to obtain a representative amount of electrocatalyst. A survey spectrum (187.5 eV of pass energy and 0.8 eV/step) was obtained before recording the high-resolution spectra (23.5 eV of pass energy and 0.1 eV/step). All measurements were made in an ultrahigh-vacuum chamber pressure between 5.0 × 10−9 and 2.0 × 10−8 Torr. XPS spectra were analyzed using the Ulvac-phi MultiPak V8.2B software.

capacity and an Ecochemie Autolab PGSTAT100 potentiostat−galvanostat with computerized control by an Autolab Nova 1.5 software. The auxiliary electrode was a Pt rod of 3.78 cm2 apparent area, and the reference electrode was a double junction Ag|AgCl|KCl (saturated) electrode (Eref = 0.199 V vs SHE). All potentials given in this work are referred to the SHE scale. The 0.5 M H2SO4 electrolyte was first deareated by bubbling Ar for 30 min and further; 10 cyclic voltammograms at 100 mV s−1, 5 more at 50 mV s−1, and 3 more at 20 mV s−1 between 0.02 and 0.98 V were consecutively performed under an Ar atmosphere at 25.0 °C. The recorded cyclic voltammograms were practically quasi-stationary after the second cycle, evidencing the stability and cleanness of the electrodes. The same protocol was carried out after the spontaneous deposition process, as commented on above. Cyclic voltammograms and CO stripping trials were conducted at 20 mV s−1 and 25.0 °C under an Ar atmosphere. In the case of CO stripping, this gas was bubbled through the solution for at least 15 min, keeping the electrode potential at 0.10 V to ensure its complete adsorption. The CO remaining in the electrolyte was further removed by Ar bubbling for 30 min. After CO stripping, three repetitive supplementary cyclic voltammograms were recorded to check the complete removal of the adsorbate. Physical Characterization. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy dispersive X-ray (EDX) analysis, fast Fourier transform (FFT), and selected-area electron diffraction (SAED) were employed for the structural analysis. These techniques were performed using a JEOL JEM 2100 TEM 200 kV microscope, which allowed obtaining the corresponding images and the EDX spectra as well as the electron diffraction pattern. FFT algorithm was applied to the HRTEM images. For these analyses, the GC electrode containing the freshly prepared Ru(Pt)/C electrocatalyst was washed with Millipore Milli-Q water, dried, and placed in a vial with 0.5−1.0 mL of n-hexane covered with parafilm, which was introduced in an ultrasonic bath for 10 min to release the electrocatalyst from the GC surface. Then, the remaining suspension was stirred for another 10 min, and a drop of it was placed over a holey carbon copper grid (400 mesh), being its solvent evaporated using a 40 W lamp for 15 min. TEM and HRTEM images were recorded with a Gatan MultiScan 794 CCD (charge-coupled device) camera. Gatan Digital Micrograph 3.7.0 software was used for the digital treatment of images and SAED analysis. Crystallographic data obtained from electron diffraction pattern, as well as from FFT, were contrasted with the CaRIne Crystallography 3.1 software and the PCPDFWIN 2.3 database. The Oxford INCAEnergy software was applied to the EDX spectra for the quantitative analysis of the material. The X-ray diffraction (XRD) patterns of the electrocatalysts were obtained using a PANAlytical X’Pert PRO Alpha-1 diffractometer equipped with a Cu Kα radiation (λ = 0.154 06 nm). The samples were placed on a Kapton support, and the recorded 2θ angle was varied between 4° and 90°. Experimental diffraction patterns were modeled using FullProf Suite WinPLOTR 2008 software to fit the corresponding diffraction signals to individual peaks, employing pseudo-Voight equations to calculate the exact value of 2θ peak position and the full width at half-maximum (fwhm) as well. Given the low amount of sample employed in the electrochemical analyses (about 10 μg of Pt/C) and the minimum mass needed for the XRD analysis (minimum 5 mg), the spontaneous deposition process

3. RESULTS AND DISCUSSION Cyclic Voltammetry in 0.5 M H2SO4. Figure 1a presents the cyclic voltammograms obtained in Ar-purged 0.5 M H2SO4 solution for the Ru(Pt)/C electrocatalysts with coverages Θ between 0.0 and 0.4 after the spontaneous deposition process. A reduction of the hydrogen adsorption/desorption charge can be observed when the coverage increases. This behavior is evidenced by the suppression of the hydrogen adsorption/ desorption peaks for the Pt (110) and Pt (100) surfaces located at 0.13 and 0.21 V, respectively,1,47 as well as by the decrease of the charge involved in these processes, estimated between 0.00 and 0.40 V and directly related to the Θ value through eq 2. This diminution of the hydrogen adsorption/desorption charge can be associated with the incorporation of Ru species on the Pt nanoparticles, resulting in a poorer performance toward hydrogen oxidation/reduction.21 From the above cyclic voltammograms, the change of double layer charge between 0.40 and 0.80 V (QDL) with coverage is also highlighted in Figure 1b. A continuous increase of the capacitive charge can be observed from Θ = 0.0 to about 0.2, followed by a plateau with ΔQDL ≈ 60%. This increase could be explained by the discharge of water on Ru atoms, if they are deposited in this form on the Pt nanoparticles, from reaction 3, which behaves as a pseudocapacity8,48,49 Ru + H 2O → Ru−OH + H+ + e−

(3)

and also by the hydroxylation of hydrous Ru oxides (RuOxHy) according to reaction 4:50−52 RuOx Hy + H 2O → RuOx Hy(OH) + H+ + e−

(4)

The formation of RuOxHy(OH) is apparent in Figure 1a by the broad cathodic peak at 0.45 V, which corresponds to its reduction following the inverse of reaction 4,52 not observed for 18471



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Figure 2. Cyclic voltammograms for CO stripping on (a) the untreated Pt/C, (b) the alloyed Pt−Ru/C, and (c) the Ru(Pt)/C electrocatalyst with Θ = 0.22 in Ar-purged 0.5 M H2SO4 at 20 mV s−1 and 25.0 °C. CO was previously adsorbed during 15 min at 0.1 V. Pt loading of 30 μg cm−2 in all cases.

hydroxylated Ru species through reaction 3 or 4 facilitating the oxidation of adsorbed CO. One could explain the smaller activity for CO oxidation of Ru(Pt)/C with respect to Pt−Ru/C considering the partial coverage of the Pt surface but also to the additional deposition of Ru species that do not behave as promoters of CO oxidation, as it is the case of the anhydrous Ru oxide, which is considerably less active for the nucleation of oxygenated species than RuOxHy.53 However, a more refined explanation is based on the electronic modification of Pt by Ru atoms, as predicted by Kitchin et al. using density functional (DFT) calculations, resulting in a negative shift of the Pt d-band energy level and causing a weakening of the Pt−CO bond.54 This effect would be more notorious in the case of the Pt−Ru/C electrocatalyst, where a more intimate interaction between the two kinds of atoms is achieved. TEM Observation. Figure 3a shows the corresponding TEM image of a Ru(Pt)/C electrocatalyst with Θ = 0.25. One can observe the characteristic framework of the Vulcan XC-72 carbon spheres, with particle size 30−40 nm, as well as the regular distribution of the metallic nanoparticles on their surface. The histogram of size distribution was obtained by counting more than 100 nanoparticles, and the result is presented in Figure 3b. An average particle size (DTEM) equal to 3.2 ± 0.6 nm was determined within an overall range of 2.0− 4.5 nm. A similar DTEM of 2.6 ± 0.3 nm, as well as analogous distribution on the carbon support, was previously reported by us for the unmodified Pt/C electrocatalyst.47 The wide distribution of size (relative standard deviation 19%) indicates that the modification of the Pt/C sample does not alter substantially the average particle size of the electrocatalyst. However, the slight increase in DTEM could be attributed to the formation of Ru species islands on the Pt nanoparticle surface. This phenomenon was also reported by Jones et al.55 for Ptcore−Ru-shell nanoparticles prepared from a chemical reduction method. However, no metallic agglomerates of nanoparticles as a result of sonication during the preparation method have been detected in this work. On the other hand, EDX was employed to study the composition of the metallic nanoparticles by performing general and individual spectra as well as elemental mapping analyses. The obtained results (data not shown) corresponded to a rather homogeneous distribution of Ru on the electrocatalyst surface. For the Ru(Pt)/C sample with Θ = 0.25, a

Figure 1. (a) Cyclic voltammograms of the Ru(Pt)/C electrocatalysts prepared through spontaneous deposition with the indicated coverage in Ar-purged 0.5 M H2SO4 at 20 mV s−1 and 25.0 °C. Platinum loading of 30 μg cm−2. (b) Variation of double layer charge vs coverage of the Ru(Pt)/C electrocatalyst. Data obtained from the anodic sweep between 0.40 and 0.80 V from cyclic voltammograms recorded under the same conditions as in (a).

Θ = 0.0. Therefore, it is apparent that RuOxHy is one of the Ru species formed during the spontaneous deposition. It seems that the formation of metallic Ru during the spontaneous deposition is not significant because no cathodic reduction of oxidized Ru species leading to metallic Ru can be discerned in the cyclic voltammograms of Figure 1a (at least a cathodic peak at about 0.3 V in the cyclic voltammogram should be expected).27,34 CO Stripping Measurements. This technique was used to evaluate the performance of the Ru(Pt)/C electrocatalyst in front of the CO poisoning. Figure 2 presents the comparative CO stripping voltammograms recorded for the untreated Pt/C (curve a) and Pt−Ru/C (curve b) electrocatalysts as well as for a modified specimen with Θ = 0.22 (curve c). These I−E profiles show a greater CO oxidation activity for the Rucontaining specimens in comparison to pure Pt. Thus, the onset potentials of CO oxidation for the alloyed Pt−Ru and the Ru(Pt)/C electrocatalysts are comparable (0.45 and 0.50 V, respectively), whereas it is of 0.65 V for the Pt/C sample. A similar trend can be observed for the CO stripping peak potential, which appears at 0.82 V for the unmodified Pt/C electrocatalyst and at lower values of 0.54 and 0.65 V for the Pt−Ru/C and Ru(Pt)/C specimens, respectively. From these data, one can conclude that the Pt−Ru/C electrocatalyst still possesses a superior CO oxidation capability than the prepared Ru(Pt)/C sample. The promotional effect caused by the spontaneous deposition process evidence the incorporation of Ru-containing species on the Pt nanoparticles, the formation of 18472



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Figure 4. (a) HRTEM image for two nanoparticles of the Ru(Pt)/C electrocatalyst prepared with Θ = 0.25 (scale bar 2 nm). (b) FFT result showing signals 1−6 asigned to the Pt FCC structure (scale bar 5 nm−1).

Figure 3. (a) TEM image showing the dispersion of the metallic nanoparticles on the Vulcan XC-72 carbon (particle size 30−40 nm) for the Ru(Pt)/C electrocatalyst with Θ = 0.25 (scale bar 50 nm). (b) Histogram of the nanoparticle size distribution for the same electrocatalyst.

as Debye−Scherrer rings (circles a, b) or Laue spots (signals 1−7). The interplanar distances derived from a (d = 0.22 nm) and b (d = 0.12 nm) can be assigned to the Pt (111) and Pt (311) planes, respectively, confirming again the domain of the Pt FCC structure in the electrocatalyst, as pointed out in the FFT analysis. The identification of the Laue signals 1−7 was carried out considering the additional presence of Pt oxides in the sample, not previously described for the untreated Pt/C electrocatalyst.47 Thus, while signals 2 (d = 0.20 nm), 6 (d = 0.12 nm), and 7 (d = 0.10 nm) could be caused by Pt (200), Pt (311), and Pt (222) planes, respectively, the Laue spot 1 (d = 0.25 nm) could be produced by PtO2 (101)/(011) or Pt3O4 (210) planes. The diffraction signal 3 (d = 0.18 nm) can be attributed to Pt3O4 (310) planes and the signal 4 (d = 0.17 nm) to PtO2 (121) interplanar distances. Finally, the spot 5 (d = 0.15 nm) is probably derived from the electron diffraction through Pt3O4 (320) planes. The PtOx species seem to be residual in comparison to the main Pt FCC structure, and they are probably formed due to a partial oxidation of the Pt surface by perchloric acid (E°(ClO4−/ClO3−) = 1.23 V) during the spontaneous deposition. X-ray Diffraction. XRD measurements were performed to achieve a general characterization of the Ru(Pt)/C electrocatalysts. While FFT and SAED tools involve a more reduced number of nanoparticles, the XRD data is a bulk analytical technique representative of all the crystalline phases in the sample. As an example, Figure 6a,b exhibits the XRD patterns for the untreated Pt/C and the Ru(Pt)/C electrocatalyst with Θ = 0.25. The same diffraction peaks (a−f) appear in both diffractograms at the same 2θ value, suggesting that no new

Ru:Pt atomic ratio of 20:80 (XRu = 20 at. %) was found from both spectra, the Pt−Ru loading in carbon being of 21.6 wt %. This is comparable to a coverage of Θ ≈ 0.3 for XRu = 20 at. % reported by Jusys et al.22 for high surface area unsupported Pt− Ru nanoparticles (3−5 nm), obtained in alloyed form, and for different Pt:Ru ratio. In addition, the regular Pt:Ru composition is in agreement with the CO oxidation experiments, which showed a single CO stripping peak as expected for a unique kind of electroactive site. Figures 4a and 4b display a HRTEM image of two metallic nanoparticles, showing their lattice structure, and the consequent FFT applied on this selected region, respectively, for a Ru(Pt)/C specimen with Θ = 0.25. The corresponding interplanar distances, obtained from reciprocal analysis of the signal highlighted in Figure 4b, are d1−2 = 0.19 nm, d3−4 = 0.22 nm, and d5−6 = 0.22 nm, with angular separation δ1,3 = 54°, δ1,5 = 54°, and δ3,5 = 108°. These data match very well with the expected interplanar distances for the Pt bulk face-centered cubic (FCC) unit cell, corresponding d1−2 and d3−4/d5−6 to the family planes Pt (200) and Pt (111), respectively. These results confirm Pt (FCC) as the main crystallographic phase in the electrocatalyst and indicate that the expected Ru species are not segregated as nanocrystals56 but cover in part the surface of the Pt nanoparticles. SAED coupled to TEM was performed on regions of Ru(Pt)/C with a representative number of nanoparticles, thus providing an accurate diffraction pattern. An example of such region is presented in Figure 5a, and its SAED pattern is shown in Figure 5b, where the corresponding diffraction signals appear 18473



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Figure 6. X-ray diffractograms of (a) the untreated Pt/C and (b) the Ru(Pt)/C electrocatalyst with Θ = 0.25. Peaks of the (a) (200) carbon planes and (b) (111), (c) (200), (d) (220), (e) (311), and ( f) (222) Pt FCC planes. (c) Fitting of the XRD pattern for the Ru(Pt)/C electrocatalyst with Θ = 0.25 displaying (1) the experimental data, (2) the corresponding profile fitting, (3) the deconvoluted band assigned to the diffraction signal c, (4) the modeled band corresponding to the diffraction peak d, and (5) the difference between the experimental and the fitted data.

Figure 5. (a) HRTEM image of a representative region of the Ru(Pt)/ C electrocatalyst with Θ = 0.25 (scale bar 5 nm) and (b) the subsequently selected-area electron diffraction (SAED) of this region: (a, b) Debye−Scherrer diffraction rings caused by the Pt FCC phase and (1−7) Laue diffraction spots attributed to Pt or PtOx species (scale bar 5 nm−1).

crystalline phases are formed during the spontaneous deposition process. The peaks b (2θ = 39.7°), c (2θ = 46.3°), d (2θ = 67.5°), e (2θ = 81.4°), and f (2θ = 86.1°) correspond to the (111), (200), (220), (311), and (222) Pt FCC planes, respectively, whereas the wider signal a (2θ ≈ 25°) is attributed to the (200) planes of the carbon support. Note that characteristic peaks either at 2θ = 44.0° or 2θ = 58.3° corresponding to the Ru hexagonal close-packed (hcp) structure or at 2θ = 35.1° or 2θ = 54.3° for RuO2 (rutile) were not detected, and therefore, the Ru species detected by EDX should be amorphous. Taking into account that the Pt structure is the main crystallographic phase in the Ru(Pt)/C electrocatalyst, deconvolution of their XRD signals considering only this structure was made. Figure 6c exemplifies the deconvolution carried out for the Pt (111) and the Pt (200) diffraction signals. Note that the difference between the fitted and the experimental result is close to 0, indicating an accurate modeling. The lattice parameter of the Pt FCC unit cell on the Ru(Pt)/ C electrocatalyst (aRu(Pt)/C) was calculated from the relationship between the interplanar distances of each family plane (dhkl) and the corresponding Miller index (hkl) according to eq 5 (Bragg’s law) and eq 6: nλ = 2dhkl sin θ dhkl =

where n is the diffraction order (generally 1), λ is the wavelength of the Cu Kα radiation, and θ is the diffraction angle. From these parameters, an average aRu(Pt)/C = 0.3921 ± 0.0003 nm was obtained. This lattice parameter agrees with aPt/C = 0.3921 ± 0.0004 nm previously reported for the unmodified Pt/C electrocatalyst.47 Therefore, the deposition process only incorporates Ru-containing species on the Pt nanoparticles surface and not into the bulk Pt FCC structure, which, according to Vegard’s law,57 would cause a decrease in the lattice parameter. The average nanoparticle size (D0) of the Ru(Pt)/C specimen was estimated considering the Debye−Scherrer equation58 D0 =

(7)

where k = 0.9 is the shape factor, λ is the wavelength of the Cu Kα radiation, θ is the angle at the maximum of the peak, and B2θ is the width of the corresponding fwhm. From eq 7, a value of D0 = 3.5 ± 0.4 nm for the modified Ru(Pt) nanoparticles was obtained, in excellent agreement with the value of 3.2 ± 0.6 nm found by TEM. A more accurate particle size can be determined if the B2θ values are considered to be affected by lattice strain owing to defects as dislocations or atom vacancies that cause an additional broadening contribution to the XRD diffraction signals. For nanoparticulate specimens, the broadening caused

(5)

aRu(Pt)/C h2 + k 2 + l 2

kλ B2θ cos θ

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by instruments can be neglected.59 To correct the contribution of nanocrystal stress to the size measured, the three models of the Williamson−Hall analysis60−63 were then applied. The Williamson−Hall model 1 assumes the existence of a 3D uniform deformation and the nanoparticle size D1 is calculated from eq 8: B2θ cos θ =

kλ + 4ε sin θ D1

(8)

where ε represents the isotropic microstrain of the interplanar distances. The other two of Williamson−Hall consider the effect of the anisotropy of the Young’s modulus in the normal direction to the corresponding plane of Miller index (Ehkl). Thus, the uniform deformation stress (model 2) and uniform deformation energy density (model 3) assumptions are defined by eqs 9 and 10, respectively: B2θ cos θ =

kλ 4σ sin θ + D2 Ehkl

B2θ cos θ =

⎛ 2u ⎞1/2 kλ + 4⎜ ⎟ sin θ D3 ⎝ Ehkl ⎠

(9)

(10)

where D2 and D3 are the nanoparticle size obtained by model 2 and 3, respectively, σ is the uniform deformation stress, and u is the uniform deformation energy density or resilience. The Ehkl values were calculated considering that the Young’s modulus is a function of the elastic compliances s11, s12, and s44 for a cubic system according to eq 11:60 Ehkl

k 2l 2 + l 2h2 + h2k 2 = s11 − (2s11 − 2s12 − s44) (h2 + k 2 + l 2)2

Figure 7. Williamson−Hall plots for the Ru(Pt)/C electrocatalyst with Θ = 0.25 considering (a, ○) a uniform deformation model, (b, △) a uniform deformation stress model, and (b, □) a uniform deformation energy density model.

(11)

To apply eq 11, the elastic compliances were related to the Pt FCC unit cell, i.e., s11 = 7.35 TPa−1, s44 = 13.1 TPa−1, and s12 = −3.08 TPa−1,64 as an acceptable approach. The plots of B2θ cos θ vs 4 sin θ (Figure 7a), B2θ cos θ vs 4 sin θEhkl−1 (Figure 7b), and B2θ cos θ vs 25/2sin θEhkl−1/2 (Figure 7b) were built to calculate D1, D2, and D3, respectively. The nanoparticle sizes thus obtained, as well as from the Debye−Scherrer equation, are collected in Table 1. A closer value to that obtained from TEM analysis was found for the models 2 and 3 (D2/D3 = 3.2 ± 0.5/3.3 ± 0.5 nm), in contrast with the slightly higher value determined assuming the model 1 (D1 = 3.5 ± 0.5 nm). This is an expected result since the anisotropic models, which take into account the nature of the unit cell, are more realistic approaches.60 Note, besides, that the small slopes obtained for the three methods (see Figure 7a,b) suggest that the internal strain is not significant.62,63 As the Williamson−Hall analysis of the untreated Pt/C specimen also showed a slight tensile strain without substantially influence on the corresponding lattice parameter value,47 we may conclude that the incorporation of Ru species on Pt in the conditions indicated does not significantly modify the internal strain of the Pt nanoparticles. However, as alternative explanation of the Williamson−Hall results, the similarity between DTEM and D2/ D3 could be tentatively attributed to the epitaxial growth of the Ru deposits on the Pt nanoparticle surface, causing the apparent expansion on the crystallite size.39 Consequently, a reduction of the induced internal strain on the underlying Pt structure because of a relaxation process would take place, as it is derived from Figure 7.

Table 1. Average Nanoparticle Size of the Ru(Pt)/C Electrocatalysts with Θ = 0.25 Calculated through Different Methods method

particle size (nm)

method

particle size (nm)

D0a D1b D2c

3.5 ± 0.4 3.5 ± 0.5 3.2 ± 0.5

D3d DTEMe

3.3 ± 0.5 3.2 ± 0.6

a

Average nanoparticle size from the Debye−Scherrer equation. Average nanoparticle size considering the isotropic deformation Williamson−Hall (WH) model. cAverage nanoparticle size calculated through the uniform deformation stress WH model. dAverage nanoparticle size assuming uniform the deformation energy density WH model. eAverage nanoparticle size from TEM counting. b

XPS Study. The XPS results for the Pt/C and Ru(Pt)/C electrocatalysts are summarized in Table 2. Despite XPS is Table 2. Elemental Surface Composition Obtained from XPS Analysis of the Untreated Pt/C and the Modified Ru(Pt)/C Electrocatalysts with Θ = 0.25 electrocatalyst Pt/C Ru(Pt)/C

C (at. %) O (at. %) Pt (at. %) 90.2 84.4

6.6 12.6

3.2 2.4

Ru (at. %)

XRua (at. %)

0.6

20

a

Ru atomic fraction in the overall Pt + Ru content of the electrocatalyst.

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defined as a surface characterization technique, giving only compositional information on the outer layer of the sample (about 5−10 nm in depth),65 a full analysis of the whole nanoparticle composition is expected considering the depth of analysis. As shown in Table 2, the oxygen content clearly increases with the spontaneous deposition, changing from 6.6 to 12.6 at. %. One could suppose that this higher oxygen content is due to the deposition of Ru oxides, but the large O:Ru atomic ratio of about 20 and the increase in the oxygen content with respect to carbon allow also deducing the carbon oxidation by perchloric acid to form of oxygenated species (such as carboxyl, carbonyl, and hydroxyl groups).66 The XPS signal fitting was focused on the Pt 4f and Ru 3p binding energy regions, and the spectral data were contrasted with those given in the literature.67−72 Figure 8a,b illustrates

Table 3. Binding Energies and Relative Atomic Concentrations for Pt and Ru Species Obtained from Pt 4f and Ru 3p XPS Spectra, Respectively, for the Untreated Pt/ C and the Modified Ru(Pt)/C Electrocatalysts with Θ = 0.25 electrocatalyst Pt/C

Ru(Pt)/C

species

BE (eV)a

rel concn (at. %)b

Pt Pt(II) Pt(IV) Pt Pt(II) Pt(IV) RuO2 RuOxHy

71.5−74.9 72.2−75.4 74.2−77.3 71.6−74.9 72.6−75.9 74.2−77.4 463.8 467.0

93 5 2 78 18 4 66 34

a

Peak position of the deconvoluted band. bAtomic concentration referred to the different oxidation states of the same metal.

species (93% and 78% for Pt/C and Ru(Pt)/C, respectively). The higher fraction of Pt oxides found in the sample after the spontaneous deposition step (22% in front of the original 7%) suggests that some Pt oxidation by HClO4 takes place, in agreement with the above discussion. The XPS spectrum in the Ru 3p region for the Ru(Pt)/C electrocatalyst with Θ = 0.25 is shown in Figure 9. This region

Figure 9. X-ray photoelectron spectrum in the Ru 3p region for the Ru(Pt)/C electrocatalyst with Θ = 0.25, being the overall signal deconvoluted into the RuO2 and RuOxHy contributions. The binding energy for the Ru 3p3/2 band corresponding to missing metallic Ru is also indicated.

Figure 8. X-ray photoelectron spectra in the Pt 4f region for (a) the untreated Pt/C and (b) the Ru(Pt)/C electrocatalyst with Θ = 0.25, displaying the deconvolution of the doublet signals of metallic Pt, Pt(II), and Pt(IV).

was selected because the C 1s peak (BE = 285 ± 0.1 eV) partially overlaps the more intense Ru 3d doublet (3d3/2/3d5/2, BE = 284/280 eV), and hence, it cannot be utilized for the semiquantitative estimation of oxidation states. The wide Ru 3p3/2 band at 464.0 ± 0.1 eV was deconvoluted into two main contributions of different intensities. The corresponding binding energies and normalized areas are also given in Table 3. As can be seen, both bands with BE of 463.8 ± 0.1 and 467.0 ± 0.1 eV are assigned to Ru species with oxidation state (IV), that is, anhydrous RuO 2 (66%) and RuO xH y (34%), respectively. This labeling is supported by the expected increase of the binding energy of the latter compared to that of the anhydrous structure.68−71 Besides, the presence of RuOxHy instead of RuO3 with higher BE is based on the instability of the latter, which is only thermodynamically stable in the gas phase at temperatures >1000 °C.67 The same argument is valid for the volatile RuO4 species.73 The metallic Ru should be detected through a band located at 461.7 ± 0.2 eV,51,67 as indicated in Figure 9, which is missing here. As can be observed, the XPS

that for both Pt/C and Ru(Pt)/C with Θ = 0.25 the Pt 4f spectra present a doublet because of the spin−orbital splitting, which originates a lower energy (Pt 4f7/2) and a higher energy (Pt 4f5/2) band with relative intensities 3:4 appearing around 71.5 ± 0.1 and 74.9 ± 0.1 eV, respectively. The broadening of these peaks suggests the existence of various Pt species. To quantify the different oxidation states of Pt, the overall doublet was then deconvoluted, resulting in three pairs of peaks with different area for both electrocatalysts corresponding to three different oxidation states: metallic Pt, Pt(II), and Pt(IV). The existence of Pt(II) can be assigned to PtO or Pt(OH)2 species, while Pt(IV) is generally attributed to PtO2.72 Alternatively, Pt(II) and Pt(IV) can also be found as an oxide mixture with formula Pt3O4 ((Pt(II)2)(Pt(IV))O4), as confirmed above from SAED. The binding energy values of the corresponding deconvoluted bands (BE), as well as the atomic concentration referred to the different oxidation states of each metal, are listed in Table 3. In both cases, metallic Pt appears as the main 18476



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showing principally the diffraction profile of the Pt structure. The Williamson−Hall analysis assuming anisotropic considerations led to similar average particle sizes to those found by TEM, indicating a possible epitaxial growth of the Ru deposits. According to this analysis, the compressive microstrain in the Pt nanoparticles was not significantly increased by the spontaneous deposition of the Ru species, suggesting the relaxation of the induced stress on the Pt structure because of the epitaxial deposit. The XPS results showed an increase in the oxygen composition on the Ru(Pt)/C specimen with respect to Pt/C, suggesting the formation of oxygenated moieties by perchloric acid oxidation of carbon and Pt. The Ru 3p signal of Ru(Pt)/C was mainly due to RuO2 and RuOxHy, the latter being the hydrous oxide responsible for the enhancement of the CO oxidation capability.

deconvolution was made accurately without considering metallic Ru, since its contribution to the overall Ru compounds should be lower than 2%. That means that the spontaneous deposition essentially leads to the formation of RuO2 and RuOxHy (represented as RuO2·xH2O) on the Pt nanoparticles, which can take place according to the reactions 12 and 13 already proposed by Chrzanowski et al.:27 [RuO(H 2O)4 ]2 + → RuO2 + 3H 2O + 2H+

(12)

[RuO(H 2O)4 ]2 + → RuO2 ·x H 2O + (3 − x)H 2O + 2H+ (13)

Actually, the RuOxHy species would work as a promoter of the CO oxidation via a mechanism similar to that of the Pt−Ru alloy surface, as commented on in the CO Stripping Measurements section. Note that the larger formation of anhydrous Ru oxide compared with that of the more active RuOxHy during the spontaneous deposition agrees with the CO stripping data exposed above. The intermediate CO oxidation activity found for the Ru(Pt)/C specimen, between the untreated Pt/C and the alloyed Pt−Ru/C electrocatalysts, can be explained by the low proportion of hydrous Ru oxide formed during the spontaneous deposition, whereas the Pt− Ru/C electrocatalyst contains greater amounts of metallic Ru and RuOxHy on its surface, of 62 and 26 at. %, respectively, both being active for CO oxidation promotion.3 Additionally, the weakening of the Pt−CO bond caused by alloyed Ru, according to the electronic effect, could also improve the CO oxidation activity of the Pt−Ru/C specimen compared to Ru(Pt)/C.54 In spite of being less active for CO oxidation, the present results are an starting point to explore different variables to improve the performance of the catalyst for this reaction. Note also that this system could be useful for other fuels such as methanol or ethanol, which have not yet been sufficiently studied. Another interesting point is that the spontaneous deposition simplifies the generation of Ru-covered Pt nanoparticles with little consumption of the expensive Ru and perhaps satisfactory with small amounts of CO impurities when using H2 from reforming and for methanol or ethanol as fuels.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +34 934039236; Fax +34934021231. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the financial support given by the Spanish MEC (Ministerio de Educación y Ciencia) through the project NAN2004-09333-C05-03. The FPU fellowship from Spanish MEC received by A. Velázquez to do this work is also acknowledged. The authors also thank the CCiT-UB (Scientific and Technological Centers of the Universitat de Barcelona) for the structural analysis facilities. This work is dedicated to the memory of Amado Velázquez Méndez.

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REFERENCES

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4. CONCLUSIONS A spontaneous deposition technique was employed to prepare Ru(Pt)/C electrocatalysts with improved CO oxidation capability compared to the untreated Pt/C specimen. The deposition of Ru species on Pt nanoparticles surface was evidenced by the suppression of the hydrogen adsorption/ desorption charge using cyclic voltammetry in 0.5 M H2SO4. An improvement of the CO oxidation catalytic activity was also detected by this technique, showing similar onset potential values for the prepared Ru(Pt)/C specimens and the alloyed Pt−Ru/C electrocatalyst. The combination of the TEM and EDX techniques confirmed that Ru was distributed homogeneously on the electrocatalyst surface without appreciable formation of agglomerates. The FFT algorithm revealed crystallographic signals only corresponding to the Pt FCC unit cell, whereas the SAED technique allowed detecting additional diffraction signals related to Pt oxides, mainly PtO2 and Pt3O4 phases. This is indicative of a partial oxidation of the Pt surface due to the exposure to the oxidizing perchloric acid media during the deposition process. The XRD characterization of the Ru(Pt)/C specimen displayed the same diffraction pattern as that found for the untreated Pt/C electrocatalyst, 18477



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Structural Characterization of Ru-Modified Carbon-Supported Pt

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