An Alloying-Degree-Controlling Step in the Impregnation Synthesis of

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J. Phys. Chem. C 2007, 111, 16416-16422

An Alloying-Degree-Controlling Step in the Impregnation Synthesis of PtRu/C Catalysts Deli Wang, Lin Zhuang,* and Juntao Lu Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China ReceiVed: April 20, 2007; In Final Form: August 16, 2007

Alloying degree is a key structural parameter of PtRu catalysts for fuel cells. In this work, we demonstrate that the alloying degree of PtRu/C catalysts prepared by impregnation method can be controlled via a drying step conducted before the H2 reduction procedure. Whereas the particle size increased only slightly with increasing drying temperature, the alloying degree changed significantly, i.e., the higher the drying temperature, the lower the alloying degree. On the basis of the results from X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements, the partial conversion of RuCl3 to RuOx during drying is attributable to the decrease in the alloying degree. The present findings can be employed to control the alloying degree in PtRu/C preparations and thus benefit the study of structure-activity relationships aimed at catalyst optimization.

Ru + H2O f Ru-OHads + H+ + e-

1. Introduction PtRu catalysts are currently the most active anode catalysts for direct methanol fuel cells (DMFCs) and proton-exchange membrane fuel cells (PEMFCs) fed with reformed gas.1-5 Numerous studies have been directed toward the synthesis6-25 and structure-activity relationship26-37 of this kind of catalyst. To achieve high dispersion, PtRu catalysts are usually supported on a high-surface-area carbon support such as Vulcan XC-72. Among the synthetic methods reported in the literature for carbon-supported PtRu catalysts (denoted as PtRu/C), the most popular are colloidal chemistry method10-16 and impregnation method.17-25 In comparison to the colloidal chemistry method, the impregnation method is relatively simple; in particular, no filtering and washing procedures are required. However, there was a misunderstanding that the impregnation method was not able to obtain high dispersions for high metal loadings,2 especially when chlorine-containing precursors such as H2PtCl6 and RuCl3 were used directly. In our previous work,25 we demonstrated that PtRu/C catalysts with excellent dispersion can be readily obtained by an improved impregnation method using H2PtCl6 and RuCl3 as precursors even for high metal loadings up to 60 wt %. Actually, in addition to the particle size, some important structural parameters, such as alloying degree, can also be controlled through the impregnation procedure. In the present article, we show that the alloying degree of PtRu/C catalysts prepared by the impregnation method can be controlled via a drying step conducted before the H2 reduction procedure. The structure-activity relationship (SAR) of PtRu/C catalysts is rather complicated and still remains a topic for further studies, despite numerous relevant research works. Alloying degree is believed to be an important structural parameter of PtRu catalysts.38-42 It has been well documented that the promotion effect of Ru in the PtRu catalyst follows a bifunctional mechanism.24,43-45 Briefly, Ru is more active than Pt in the water-discharge reaction (R1) that produces OHads, a required reactant in the electrochemical reaction with Pt-COads to yield CO2 (R2). * Corresponding author. E-mail: [email protected]. Fax: +86-2768754067.

(R1)

Pt-COads + Ru-OHads f Pt + Ru + CO2 + H+ + e(R2) It is clear that the efficiency of reaction R2 depends on the proximity between the Pt and Ru sites. Alloying Pt with Ru is an intuitive approach to mixing the two components at the atomic level. In addition, alloying Ru into a Pt lattice is also believed to be able to weaken the adsorption of CO through electronic effects.46-49 However, some recent studies have found that the most active form of Ru could be hydrous ruthenium oxides (RuOxHy) rather than metallic Ru;26,32,33 thus, there is still a debate about the effective form of Ru in PtRu catalysts. Nevertheless, the alloying degree of PtRu is a highly relevant factor in SAR studies of PtRu catalysts and deserves careful control in catalyst synthesis. Unlike unsupported PtRu alloys, the alloying degree of PtRu/C might largely differ from the nominal Pt/Ru ratio in the precursor and, therefore, requires dedicated studies. In this article, we report the crucial role of the drying step in the impregnation method for PtRu/C preparation. 2. Experimental Section 2.1. Catalyst Preparation. H2PtCl6 and RuCl3 dissolved in ultrapure water were used as the precursors. High-surface-area carbon, i.e., Vulcan XC-72 (Carbot), was preheated at 110 °C in air and then poured into the warm precursor solution. The Pt/Ru/C weight ratio was controlled according to the targeted metal loading. After ultrasonic blending for 30 min, the suspension was heated under magnetic stirring to allow the solvent to evaporate and to form a smooth thick slurry that had an apparent volume over 10 times larger than that of the pristine carbon powder. The slurry was dried in air at a selected temperature for 1 h. The resulting agglomerates were ground in an agate mortar and then placed in a glazed ceramic boat and heated in a tube furnace at 120 °C under flowing H2 for 2 h. Finally, the powder material was cooled to room temperature in argon atmosphere.

10.1021/jp073062l CCC: $37.00 © 2007 American Chemical Society Published on Web 10/11/2007

Alloying-Degree Control in PtRu/C Catalyst Synthesis

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16417

Figure 1. (a) Bright-field HRTEM images of PtRu/C samples S60, S100, S150, and S200. (b) Particle-size statistics over more than 200 particles for samples S60, S100, S150, and S200.

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Wang et al. TABLE 1: Data Obtained from HRTEM (Figure 1) and XRD (Figure 2) Analyses sample

Pt(220) peak (deg)

lattice constant a (Å)

xRu (%)

TEM particle size (nm)

68.59 68.54 68.38 67.95

3.866 3.869 3.877 3.898

40 ( 2 38 ( 2 31 ( 2 14 ( 1

2.3 2.5 2.5 2.8

S60 S100 S150 S200

TABLE 2: XPS Results Deduced from Curve Fitting (Figure 3) signal/sample Pt 4f7/2/P60 Pt 4f7/2/P200 Ru 3p3/2/R60 Ru 3p3/2/R200

suggested atomic binding reference energy (eV) BE (eV)55-58 formula55-58 ratio (%) 70.9 72.7 74.8 70.9 72.5 74.6 462.1 463.4 465.4 461.9 463.5 465.3

70.8 72.4 74.2 70.8 72.4 74.2 462.4 463.3 46559,60 462.4 463.3 46559,60

Pt PtCl2, PtO PtCl4, PtO2 Pt PtCl2, PtO PtCl4, PtO2 RuO2 RuCl3 Ru(V)59,60 RuO2 RuCl3 Ru(V)59,60

25.0 46.4 28.6 30.5 32.4 37.1 30.8 43.5 25.7 25.2 35.7 39.1

3. Results and Discussion

Figure 2. (a) XRD patterns of PtRu/C samples S60, S100, S150, and S200. (b) Curve fitting for Pt(220) diffraction peaks in part a.

The temperature of the drying step was found to markedly influence the alloying degree of the final PtRu/C catalysts. Four drying temperatures, i.e., 60, 100, 150, and 200 °C, were systematically tested in this work, and the corresponding catalyst samples are denoted as S60, S100, S150, and S200, respectively. 2.2. Catalyst Characterization. High-resolution transmission electron microscopy (HRTEM) examinations were performed on a JEOL JEM-2010FEF ultrahigh-resolution transmission electron microscope working at 200-kV accelerating voltage. The ultimate spatial resolution of the instrument was 0.19 nm. Specimens were prepared by ultrasonically suspending the catalyst powders in ethanol, applying the suspension to a carbon grid, and drying the suspension-coated grid in air. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained in Bragg-Brentano mode on a Shimadzu XRD-6000 X-ray diffractometer using a Cu KR radiation source operating at 40 kV and 30 mA. The whole XRD profile (from 15° to 85°) was recorded at a scanning rate of 4°/min, whereas the peak profile of the (220) reflection of Pt face-centered-cubic (fcc) structure (from 62° to 75°) was also recorded at the low scan rate of 0.5°/min and fitted to a Lorentzian line shape on a linear background using the Levenberg-Marquardt algorithm so that the position of the peak maximum (θmax) could be obtained precisely for alloying degree calculations (vide infra). X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Kratos XSAM-800 spectrometer with a Mg KR radiation source. The Pt(4f) and Ru(3p) signals were collected and analyzed by deconvolution of the spectra using the free software XPSPeak. For the Pt(4f) signal, which shows doublet peaks, constraints in the energy and intensity ratio were applied in the curve fitting.

3.1. HRTEM Examination of Catalyst Dispersion. The bright-field HRTEM images of samples S60, S100, S150, and S200 are shown in Figure 1a. It can be seen that the catalyst particles are quite uniform in size not only in each individual sample but also among different samples. Statistics over more than 200 particles for each sample (Figure 1b) show narrow distributions centered round 2-3 nm in diameter. These observations indicate that the drying temperature had a negligible effect on the alloy particle size. 3.2. XRD Analysis of Alloying Degree. A convenient approach to obtain the alloying degree of PtRu/C catalysts is to measure the lattice constant change caused by alloying.32,35,50 To determine the change in the lattice parameter, XRD measurements were carefully conducted as described in the Experimental Section. As shown in Figure 2a, all samples exhibit a characteristic Pt fcc pattern with different peak shifts depending on the drying temperature. In addition, there are no distinct peaks related to tetragonal RuO2 or hexagonal close-packed (hcp) Ru phases, indicating the absence of metallic Ru and the presence of unalloyed Ru most probably in amorphous oxides states.51,52 The Pt fcc lattice parameter can be calculated from the diffraction peak position. In the XRD pattern of a PtRu/C catalyst, the Pt (220) peak is far from the background signal of the carbon support and was thus chosen for the calculation of the lattice parameter. The peak position (θmax) was obtained from curve fitting (Figure 2b) and used for the calculation of lattice parameter (a)53

a)

x2λKR sin θmax

(E1)

The alloying degree of a PtRu catalyst is defined as the Ru atomic fraction (xRu) in the PtRu alloy, which is related to the lattice parameter through the following equation proposed by Antolini and co-workers20,54

a ) a0 - 0.124xRu

(E2)



Figure 3. Pt(4f) and Ru(3p) signals in the XPS spectra of precursor-containing samples P60, P200, R60, and R200. Points, experimental data; lines, fitting curves.

Figure 4. Pt(4f) and Ru(3p) signals in the XPS spectra of samples S60 and S200 before H2 reduction. Points, experimental data; lines, fitting curves.

where a0 is the lattice constant of pure Pt. In the case of unsupported pure Pt, a0 ) 0.39231 nm, whereas for carbonsupported pure Pt, a0 ) 0.39155 nm, which was obtained from XRD measurements of Pt/C made by E-TEK.54

Equation E2 is similar to the relationship reported by Radmilovic´ et al.53 for single-phase PtRu bulk alloys: a ) 0.39262 - 0.1249xRu (or a ) 0.38013 + 0.1249xPt). Equation E2 was verified by the reasonable agreement between the xRu

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Wang et al.

Figure 5. Pt(4f) and Ru(3p) signals in the XPS spectra of samples S60 and S200 after H2 reduction. Points, experimental data; lines, fitting curves.

values calculated using this equation and the values from XRF for arc-melting PtRu alloys reported by Gasteiger et al.40 Table 1 summarizes the lattice parameter values obtained from the XRD data, along with the average particle diameters obtained from HRTEM. It can be clearly seen that, whereas the particle size changed only slightly, the alloying degree changed significantly, decreasing from 0.4 to 0.14 for an increase in drying temperature from 60 to 200 °C. 3.3. XPS Analysis for Pt Precursor and Ru Precursor. To ascertain the changes caused by drying at different temperatures, we prepared slurries of carbon-supported Pt precursor and carbon-supported Ru precursor. The two samples were divided into two portions, and one portion of each was dried at 60 and 200 °C. The samples thus obtained are denoted as P60 and P200, respectively, for those containing Pt and as R60 and R200 for those containing Ru. XPS was employed to analyze the Pt(4f) and Ru(3p) signals of the aforementioned four samples. The Ru(3p) signal was used instead of the Ru(3d) signal to avoid the interference of the C(1s) signal. Figure 3 shows the Pt(4f) and Ru(3p) spectra of the corresponding samples. All of these spectra exhibit a broad line shape, and each of them can be deconvoluted into three components (three pairs of peaks). The atomic ratio and binding energy (BE) of the components in each spectrum, obtained from the curve fitting, are listed in Table 2, along with the reference BE values obtained from the U.S. National Institute of Standards and Technology (NIST) XPS database55 or other relevant sources.56-60 According to the XPS results, there were three kinds of Pt species in the Pt-containing samples (P60 and P200), i.e., Pt0, PtII, and PtIV. The presence of Pt0 was unexpected as no deliberate reducing agent was applied; moreover, the Pt0 content increased with drying temperature. A possible reason for this could be the carbon support acting as a reductant during the sample preparation steps from mixing to drying. This phenom-

enon was not quite apparent in previous works.17-25 The reductive activity of carbon supports could be due to surface functional groups, which have been reported to be able to reduce precursors of noble metals such as Ag, Au, and Pt.61-65 In addition to the reduction of chlorides and/or oxides by surface reductive groups to produce Pt0, the surface of the thus-formed Pt particles could be reoxidized by air, and PtII might be oxidized to PtIV, especially at higher temperature. The effect of the drying step on the Ru precursor was mainly a conversion from RuCl3 to ruthenium oxides. The higher the drying temperature, the less RuCl3 remaining and the more RuV in comparison to RuIV. 3.4. XPS Analysis of PtRu/C before and after H2 Reduction. Once the separate effects of the drying step on the Pt and the Ru precursors were clear, the subsequent analysis was to determine whether those effects still hold when Pt and Ru precursors are mixed together and how such effects influence the alloying degree of PtRu/C after H2 reduction. XPS was employed to analyze two typical PtRu/C samples, S60 and S200. Figures 4 and 5 show the Pt(4f) and Ru(3p) spectra of the corresponding mixture samples before and after H2 reduction, respectively, and the data are summarized in Tables 3 and 4. By comparing the XPS data of single precursor samples to those of the mixed precursors before hydrogen reduction, some changes due to the mixing can be found. For example, the fraction of PtII is higher in S200 than in P200. The mechanisms for these mixing effects remain for further study and are beyond the scope of this work. The distribution of different valences of Ru in the mixture, however, did not seem to be much altered by the coexisting Pt precursors. The important drying-temperature-dependent trend remained, i.e., a higher drying temperature caused a larger fraction of RuCl3 to be converted to ruthenium oxides. Upon comparison of the XPS data obtained before and after hydrogen reduction for samples S60 and S200, it is clear



TABLE 3: XPS Results before H2 Reduction (Deduced from Curve Fitting in Figure 4) signal/sample Pt 4f7/2/S60 Pt 4f7/2/S200 Ru 3p3/2/S60 Ru 3p3/2/S200


70.8 72.4 74.2 70.8 72.4 74.2 462.4 463.3 46559,60 462.4 463.3 46559,60

Pt PtCl2, PtO PtCl4, PtO2 Pt PtCl2, PtO PtCl4, PtO2 RuO2 RuCl3 Ru(V)59,60 RuO2 RuCl3 Ru(V)59,60

30.7 42.8 25.5 25.4 49.7 26.9 35.3 41.1 23.6 23.2 31.4 45.4

TABLE 4: XPS Results after H2 Reduction (Deduced from Curve Fitting in Figure 5) signal/sample Pt 4f7/2/S60 Pt 4f7/2/S200 Ru 3p3/2/S60 Ru 3p3/2/S200


70.8 72.4 74.2 70.8 72.4 74.2 461.1 462.4 46559,60 461.1 462.4 46559,60

Pt PtO PtO2 Pt PtO PtO2 Ru RuO2 Ru(V)59,60 Ru RuO2 Ru(V)59,60

38.4 37.2 24.4 32.2 43.7 24.1 39.8 35.1 25.1 21.9 37.1 41.0

that the Ru0 in the catalyst came mainly from RuCl3. Therefore, it can be concluded that RuCl3 is the main reducible form of Ru precursor, whereas RuO2 and RuV are much more difficult to reduce with hydrogen. Because no Ru phase was detected by XRD, the Ru0 signal detected by XPS should be mainly from the PtRu alloy particles, although the existence of extremely small particles of Ru metal cannot be ruled out absolutely. 3.5. Control of Alloying Degree through Drying Temperature. In the impregnation synthesis of PtRu/C catalysts, the Pt and Ru precursors impregnated in the carbon support are expected to be reduced simultaneously by heat treatment in H2 to achieve complete alloying, i.e., all of the Pt and Ru components are combined into the alloy, and the resulting alloying degree exactly corresponds to the nominal Pt/Ru ratio in the raw material. In reality, however, the alloying degree is always lower than expected to some extent. The above XPS analyses reveal that two important changes occur during the drying step and might influence the alloying degree of the final products. One change is the partial reduction of H2PtCl6 to Pt0. This change might influence the alloying degree in two opposite ways. Intuitively, compared to co-reduction, the combination of preformed Pt0 and the late reduction of the Ru precursor is less favorable for alloy formation, and thus, the partial prereduction of Pt precursor would lead to a lower alloying degree. On the other hand, however, the partial prereduction of the Pt precursor would decrease the amount of remaining Pt precursors and would increase the ratio of Ru to Pt in the remaining precursors, in favor of a high alloying degree. If the two effects both take place, two different alloying degrees would result. However, based on the facts that the XRD pattern shows a single peak and that the particle size calculated from the XRD peak width is consistent with that from HRTEM observations (Figure 1), the dispersion in the alloying degree should be negligible.

This might imply that the sequence of reduction of the two precursors is not critical for alloying degree. Therefore, the observed correlation between alloying degree and drying temperature should be attributed to the conversion of RuCl3 to RuOx. This hypothesis seems reasonable because RuOx was reported to be very difficult to reduce to Ru0.66,67 In conclusion, the alloying degree depends largely on the reducible Ru precursor remaining just before the H2 reduction step, and the amount of reducible Ru precursor decreases with increasing drying temperature. According to the above discussion, the alloying degree of PtRu/C can be controlled to a large extent by regulating the drying temperature. For catalysts targeted at a higher alloying degree, a lower drying temperature should be employed, and vice versa. For example, as shown in Table 1, when the drying temperature was changed from 60 to 200 °C the alloying degree decreased by about a factor of 3, changing from 0.40 to 0.14. We once tried vacuum-drying at 40 °C instead of drying in air and obtained an alloying degree of 0.42, which is slightly higher than that obtained by drying at 60 °C in air (alloying degree 0.40) but still notably lower than the theoretical value of 0.50. This result might indicate that RuCl3 conversion starts in fact before the drying step. It is known that RuCl3 can easily undergo hydrolysis in aqueous solutions,68 and partial hydrolysis is expected to occur during blending and stirring steps. If the products of hydrolysis are less reducible, an alloying degree below 0.50 would be unavoidable. A remedy for the adverse changes might be an increase of the H2 reduction temperature. For example, we successfully obtained PtRu/C with an alloying degree of 0.47 by H2 reduction at 300 °C following drying at 60 °C, at the expense of a slight increase of particle size (3.2 nm). In contrast, a high drying temperature can be employed for catalysts targeted at a low alloying degree. An alloying degree of 0.08 was obtained with a drying temperature of 250 °C followed by hydrogen reduction at 120 °C. However, a further increase in the drying temperature is limited by serious oxidation of carbon support in air. Although drying in an inert gas atmosphere (or in vacuum) can prevent carbon burning, the precursors would be thermally decomposed and form alloys, which is not favorable for the purpose of lowering the alloying degree. In summary, compared to the parameters in other steps, the temperature of drying step is most influential on the alloying degree in the impregnation method for PtRu/C synthesis. An increase in the drying temperature results in a lowering of the alloying degree of PtRu/C, most probably owing to partial conversion of RuCl3 to less reducible RuOx. For samples with the same Pt/Ru ratio in the precursor, the alloying degree of PtRu/C can be controlled over a fairly large range mainly by selecting the drying temperature, from an upper limit close to the theoretical value (0.50) to a lower limit slightly below 0.1. The present findings can be employed to control the alloying degree in PtRu/C preparations and thus benefit the study of structure-activity relationships aimed at catalyst optimization. Acknowledgment. This work was financially supported by the Natural Science Foundation of China (20433060, 20473058) and the Program for New Century Excellent Talents in Universities of China (NCET-04-0688) and made use of the facility at the Center for Electron Microscopy of Wuhan University. References and Notes (1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5. (2) Arico, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 1.

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