Importance of Oxygen-Free Edge and Defect Sites for the ...

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Importance of Oxygen-Free Edge and Defect Sites for the Immobilization of Colloidal Pt Oxide Particles with Implications for the Preparation of CNF-Supported Catalysts Ingvar Kvande,†,| Jun Zhu,‡ Tie-Jun Zhao,† Nina Hammer,† Magnus Rønning,† Steinar Raaen,§ John C. Walmsley,§,| and De Chen*,† Department of Chemical Engineering and Department of Physics, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway; Unilab, State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, PR China; and SINTEF Materials and Chemistry, N-7491 Trondheim, Norway ReceiVed: July 12, 2009; ReVised Manuscript ReceiVed: NoVember 16, 2009

It was found that the Pt loading obtained by immobilization of colloidal Pt oxide particles on carbon nanofibers (CNFs) at a pH of 9-10 varied from 1.4 to 19.6 wt % (nominal loading 19.4 wt %). The amount of Pt deposited depended significantly on the CNF properties. The study targeted the identification of the determining carbon support properties for the successful preparation of Pt catalysts. CNFs, multiwall carbon nanotubes (MWNTs), and Vulcan XC-72R were utilized as supports. Platelet and fishbone CNFs with different surface areas, graphene layer stacking angles, and amount of surface oxygen groups were obtained by catalytic chemical vapor deposition. The carbon supports were thoroughly characterized by transmission electron microscopy (TEM), N2-adsorption measurements, X-ray diffraction (XRD), temperature-programmed oxidation (TPO), ζ-potential measurements, and X-ray photoelectron spectroscopy (XPS). Characterization of the deposited Pt particles by TEM revealed similar sizes but differences with respect to particle location. Based on TEM, BET, XRD, and XPS, a clear indication of the importance of surface defects and edge sites for successful immobilization of Pt oxide colloid particles was found for all carbon supports. From XPS a linear relationship was found between the fraction of species originating at a binding energy of 285.1 eV and the final Pt loading. These species can be sp3-hybridized carbon, defects, and/or dangling bonds (edge structure). The surface oxygen groups were found to have a decisive effect on the immobilization of Pt. Negative linear trends were found between the Pt loading obtained on CNFs and the O 1s/C 1s ratio and number of carboxylic groups determined from XPS. It is based on the results believed that the oxygen-free defect and edge structure can play a vital and important role in the preparation of more effective CNF-supported catalysts. 1. Introduction Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) are, because of their interesting properties, considered as suitable supports for catalytic systems.1 One particular application is as replacement for conventional carbon black supports to improve the efficiency of both polymer electrolyte fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Especially, the mesoporous nature, purity, conductivity, and possibly higher corrosion resistance compared to conventional supports are thought to be beneficial.2,3 In addition, the presence of a unique catalytic behavior has been reported for CNF/CNT-supported systems.4-10 The Pt loading of conventional fuel cell catalysts is usually relatively high to avoid drops in performance because of ohmic resistance and mass transfer limitations.11 The prospect of using CNFs has led to a number of studies on how to prepare highly loaded and well-dispersed Pt/CNF catalysts. The as-grown CNFs are considered to be hydrophobic, and it is the common viewpoint that the preparation of Pt catalysts with aqueous methods requires the introduction of surface oxygen groups.12 * To whom correspondence should be addressed. E-mail: chen@ chemeng.ntnu.no. † Department of Chemical Engineering, Norwegian University of Science and Technology. ‡ East China University of Science and Technology. § Department of Physics, Norwegian University of Science and Technology. | SINTEF Materials and Chemistry.

In a recent study on the deposition of Pt-supported particles on fishbone CNFs, the limitations of conventional methods like ion-exchange and impregnation as well as depositionprecipitation for achieving catalysts with high loading and high dispersion were demonstrated.13 Meanwhile, the potential of colloidal methods was illustrated, as by the modified polyol method, a Pt content of 24 wt % with a particle size of 2-4 nm was achieved.13 Elsewhere, the polyol or modified polyol process has been successfully applied, where Pt and Pt-Ru loadings as high as 40 wt % have been obtained on CNFs/ CNTs.14-19 In the modified polyol method, ethylene glycol (EG) acts as both a reducing agent and stabilizer. The quick nucleation and complete separation of the nucleation and growth steps give effective control of particle size and size distribution. A method employing metal oxide colloids (PtO2 colloids) obtained from base hydrolysis of H2PtCl6 has been proposed by Reetz and co-workers20 and later patented.21 Particles 1-2 nm in diameter can be deposited on carbon supports by using water as a solvent and a base as the stabilizer. In this way, the need of a surfactant removal step can be eliminated.21 However, the exact nature of the deposition of the colloids was not revealed. As the authors state: “it was not to be assumed that it was possible to immobilize colloids stabilized by negatively charged hydroxide ions in a basic solution on a support”.21 The merits of this method have already been investigated by comparing a number of carbon supports, including CNFs.22 It

10.1021/jp906572z  2010 American Chemical Society Published on Web 01/07/2010

Immobilization of Colloidal Pt Oxide Particles

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TABLE 1: Supports, Synthesis Conditions and Post-Treatment Conditions sample

structure

catalyst

reactants/feed ratio

temp (°C)

post-treatment

a

platelet platelet platelet fishbone fishbone fishbone fishbone MWNTs -

Fe3O4 Fe3O4 Fe3O4 Fe/SiO2 Ni/SiO2 Ni/Al2O3 Ni/Al2O3 -

CO/H2 4:1 CO/H2 4:1 CO/H2 4:1 CO/H2/N2 80:27:160 CO/H2/N2 80:27:160 CH4/H2 4:1 CH4/H2 4:1 -

600 600 600 600 550 600 600 -

conc. HNO3, reflux 1 h 4 M HCl 4 times 2 h at 50 °C 4 M NaOH overnight 4 M NaOH overnight conc. HNO3/H2SO4 1:3, reflux 30 min 4 M HCl 4 times 2 h at 50 °C -

P1ag P1oxb P2 FB Fe-CO FB Ni-CO FB Ni-CH4ag FB Ni-CH4ox MWNTc Vulcan-XC72Rd a As-grown. Cabot Corp.

b

Oxidized.

c

Commercial sample, Nanfeng Chemical Corp.

was proposed that the “roughness” of the CNF surface is the determining factor both for the immobilization and stabilization of colloid particles.22 By altering the conditions of CNF synthesis by catalytic chemical vapor deposition (CCVD), structures with different properties can be obtained. CNF that have different fiber diameter, surface area, stacking orientation with respect to the fiber axis, number of edges, amount of surface oxygen groups, and disordered carbon can be obtained. The prismatic edges of CNFs are typically considered to be the sites that readily accommodate adsorption of particles.23 The electrostatic interaction between the charged surfaces of the support and the metal precursor or colloid is crucial.24 Carbon surfaces are amphoteric, and the net surface charge is therefore dependent on pH. At pHs below the isoelectric point (IEP) the surface is positively charged, while at pHs above the IEP, the surface is negatively charged. It is important to control the carbon surface chemistry to facilitate electrostatic interaction at the conditions used for catalyst preparation. Characterization of the carbon surface chemistry and surface oxygen groups is highly complex due to the large variety of groups that can coexist in relatively small amounts.25,26 The surface oxygen groups can be characterized by several methods, e.g., acid-base titration, IR, and temperature-programmed desorption. However, due to the complexity of carbon surface chemistry, quantification is difficult. X-ray photoelectron spectroscopy (XPS) is a surfacesensitive technique, where the surface concentrations of various species can be estimated from relative peak intensities. It has been applied to study the oxygen groups of acid and plasma oxidized CNFs and CNTs.27-31 The C 1s peak has been used to asses the ratio of sp2- and sp3-hybridized carbon atoms, typically found at binding energies of 284.6 and 285.1 eV,32-34 respectively. In this work, CNF and CNT structures as well as the carbon black, Vulcan XC-72R, are compared for the preparation of Pt catalysts by the metal oxide colloid method. Results from the characterization of the supports by transmission electron microscopy (TEM), N2 adsorption, X-ray diffraction (XRD), temperature-programmed oxidation (TPO), XPS, and ζ-potential measurements are used in order to gain insight into the properties that govern the immobilization of Pt oxide colloids. 2. Experimental Section Platelet and Fishbone carbon nanofibers were grown by CCVD.13,35,36 Commercially available multiwall carbon nanotubes (MWNT) were obtained from Nanfeng Chemical Corp., while Vulcan XC-72R carbon black was obtained from Cabot Corp. Different methods of post-treatment for purification and/ or introduction of oxygen surface groups by acid oxidation at reflux conditions were employed and are listed together with the synthesis conditions in Table 1.

d

Commercial Vulcan XC-72R (conductive furnace black) from

The supports were characterized by TEM, TPO, XRD, N2 adsorption, ζ-potential measurements, and XPS. TEM was performed using a JEOL 2010F electron microscope equipped with a field emission gun. The instrument has a point resolution of approximately 2 Å. The samples were prepared by dispersing the powder samples in ethanol. A drop of the sonicated suspension was applied to a holey carbon film on a copper grid. TPO was performed with 10-20 mg samples in a thermogravimetric analyzer (TGA Q500, TA Instruments). A flow of 90/ 10 mL/min air/argon with heating up to 900 °C (rate 10 °C/ min) was used. XRD analysis was carried out using a BrukerAXS D8-focus instrument with a D8 goniometer with Cu KR radiation and a Lynxeye detector. Diffractograms were acquired in the 2θ range of 20-80° with a step size of 0.03° and a step time of 0.5 s. N2 adsorption was conducted using a Micromeretics Tristar 3000 after degassing the samples at 250 °C overnight. The total pore volume and pore size distributions were obtained from the nitrogen desorption branch using the Barrett-Joyner-Halenda (BJH) method. The t-plot method was used to determine the micropore volume. ζ-potential curves were measured as a function of pH in the range 2-10 on a Malvern Nano-ZS Zetasizer connected to a multipurpose titrator (Malvern Instruments, UK). The electrophoretic mobility was measured for each pH value by laser Doppler velocimetry. The ζ-potentials were determined by the Smoluchowski approximation.37 Additional measurements were done using manual titration and measurements of the ζ-potential in the pH range 2-6. An XPS study was performed in order to further investigate the surface chemistry of the carbon supports. The CNFs were dried at 120 °C overnight prior to analysis. The measurements were conducted in a hemispherical SCIENTA SES 2002 electron energy analyzer. A monochromatic Al KR X-ray source (Gammadata Scienta) was used to obtain the exciting radiation. The total energy resolution was found to be 0.4 eV. The spectra were measured in an angle integrated mode around normal emission. The surface compositions were found by determination of the peak areas and adjusted by the corresponding atomic sensitivity factors. A Shirley background subtraction was applied. The program XPS Peak 4.138 was used for fitting the spectra. An iterative least-squares optimization algorithm was used. Peak positions and boundaries used for the deconvolutions were obtained from the literature.27,29,31 Pt-supported catalysts were prepared under basic conditions, pH 9-10, by a metal oxide (MO) colloid method using H2PtCl6 as precursor and Li2CO3 as base.13 The Pt loading was determined by TPO. The particle size and distribution were examined by TEM.

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Figure 1. Typical TEM images of (a) P1 ag, (b) P2, (c) FB Fe-CO, (d) FB Ni-CO, (e) FB-Ni-CH4, and (f) MWNT.

3. Results and Discussion 3.1. CNF Characterization. Typical TEM images of the carbon structures used in this study are shown in Figure 1. The two platelet samples were synthesized in two different setups but otherwise using the same conditions. Inspection of a number of TEM images indicated that the purity with respect to the well-defined platelet structure was high for P1ag (Figure 1a). While the major part of the P2 (Figure 1b) sample consists of CNFs with a well-defined platelet structure, the TEM study revealed a significant fraction of short fibers as well as fibers with their graphene sheets not stacked perpendicularly but at a high angle with respect to the fiber axis. Platelet CNFs grown at similar conditions have earlier been found to have a quite broad diameter size distribution ranging from 35 to 336 nm with an average diameter of about 116 nm.35 The majority of these structures are quite short and have been described more as rods

than fibers.39 The fishbone CNFs have their graphene sheets stacked at a relatively small angle with respect to the fiber axis. Closer inspection of a number of TEM images revealed that FB Fe-CO (Figure 1c) has an average diameter of 54, an average inner diameter of 11 nm and an average stacking angle of 21°. FB Ni-CO (Figure 1d) and FB Ni-CH4ag (Figure 1e) have similar diameters, 32 and 35 nm, respectively. They possess a mixture of hollow and bamboo-like structures. The average angle with respect to the fiber axis is different, 19° and 28° for FB Ni-CH4ag and FB Ni-CO, respectively. For all carbon nanofibers, structures most likely corresponding to closed layers or disordered carbon can sometimes be seen at the surface, e.g., in Figure 1a for P1ag and Figure 1f for MWNTs. It should be mentioned that the CNF structure was destroyed when the beam was left on the sample for prolonged time. To avoid this effect,



TABLE 2: Textural and Crystallite Properties of the Carbon Supports crystallite properties

textural properties sample P1ag P1ox P2 FB Fe-CO FB Ni-CO FB Ni-CH4ag FB Ni-CH4ox MWNT Vulcan XC-72R

BET surface area (m2/g) 144 142 186 135 93 74 81 211 23540

pore vol (cm3/g) 0.22 0.20 0.31 0.44 0.33 0.22 0.26 0.77 0.6740

micropore vol (cm3/g) 0.016 0.018 0.015 0.002 0.000 0.004 0.2540

d002 (nm)

Lc (nm)

0.336 0.338 0.339 0.341 0.344 0.341 0.343 0.361

9.2 10.7 7.6 5.5 5.0 5.9 4.2 2.1

TEM images were taken within a few seconds of moving to the area of interest. A TEM image of the commercial MWNT sample can be seen in Figure 1f). The MWNTs have an average diameter of 20 nm and the tubes are made up of 5-20 graphene layers. No TEM images were obtained for Vulcan XC-72R. Carbon blacks generally consist of shells made up of small entities of disordered carbon.26 The surface area, pore volume, and microporosity are shown in Table 2. The platelet supports have surface areas of 142-186 m2/g. The difference in surface area between P1ag and P2 most likely originates from the larger fraction of small, more disordered structures in the P2 support as observed by TEM. The fishbone CNFs have lower surface areas (74-135 m2/g). The larger surface area of FB Fe-CO (135 m2/g) despite the larger average diameter is most probably a result of the accessible open inner core. In comparison, FB Ni-CO (93 m2/ g) and FB Ni-CH4ag (74 m2/g) have partially closed hollow structures. The surface area of the MWNT support was found to be 211 m2/g, while the Vulcan XC-72R support has the highest surface area among the supports, elsewhere found to be 235 m2/g.40 The CNF structures have pore volumes of 0.2-0.44 cm3/g and are predominantly mesoporous. However, the micropore volume as determined from the t-plot method is significant in the platelet samples, constituting 7-9% of the total pore volume. Slitlike pores formed by open-edge graphene planes has elsewhere been found to make up 2-7% of the active surface area.41,42 The pore width distributions of the supports are shown in Figure 2. The platelet samples have very broad distributions with an average pore size of about 8 nm. The maxima of FB Ni-CO and FB Ni-CH4 (10-30 nm) can be ascribed to pores made up of the interstices between entangled fibers. The first maxima of FB Fe-CO and MWNT at 10 and 2-4 nm correlates with their inner open tube diameter. As also found by others, Vulcan XC-72R has a large fraction of micropores (37%) with a total pore volume of about 0.67 cm3/g.40 The d002 graphene interlayer distance and the average stacking height parallel to the fiber axis, Lc, as found from XRD are shown in Table 2. The highest intensity of the graphite (002) peak (2θ ) 26°) and thereby d002 values close to the value for graphite (0.335) was seen for the platelet samples. The higher amount of macro-sized defects and shorter fibers for the platelet structure, P2 as indicated by TEM and BET was reflected in the higher d002 value and a lower Lc compared to P1ag. After oxidation, a higher d002 value and also a higher stacking height, Lc, was found. This can probably be attributed to the removal of disordered and defect-rich carbon. No peaks corresponding

Figure 2. Pore size distributions for the carbon supports.

Figure 3. Derivative weight loss [%/°C] for the carbon supports obtained by temperature programmed oxidation.

to the Fe growth catalyst used for synthesis of platelet CNFs were seen. The fishbone supports showed larger d002 values and lower Lc values, explained by a lower graphitization degree and the frequent change in fiber orientation. The level of impurities is significant in FB Ni-CH4ag and FB Ni-CO, both showing the characteristic Ni peak at 2θ ) 44.5°. The intensity of the graphite (002) peak of the MWNT sample is surprisingly low, indicating a poor graphitic nature. As expected, strongly disordered carbon, carbon black, Vulcan XC-72R, has a high d002 value of 0.361 and a low average stacking height, Lc of 2.1. The derivative TPO curves of the supports are shown in Figure 3. While XRD only gives an average value of crystallinity/graphitic nature, TPO can give information about the distribution of carbon structures by their different oxidation temperatures. The results are hampered by the CNF synthesis catalyst which catalyzes the oxidation, but a qualitative discussion can be made. P1ag oxidized in two steps. This probably originates from the smaller and more disordered fibers being oxidized at lower temperatures, while the more graphitic and defined fibers with larger diameters being oxidized at higher

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Figure 4. Zeta-potential curves for P1ag (2), P1ox (b), P2 (]), and FB Ni-CO(0). Figure 6. XPS O 1s spectra of (from top to bottom): Vulcan XC-72R (1), FB Fe-CO (2), P1ag (3), FB Ni-CO (4), FB Ni-CH4ag (5), MWNT (6), and P2 (7).

Figure 5. Zeta-potential curves for FB Fe-CO ([), FB Ni-CH4ag (+), FB Ni-CH4ox (9), and MWNT(4).

temperatures. The broad derivative TPO curves of P2 and FBNiCH4ox compared to P1ag and FB Fe-CO indicate that they are less defined. A more facile oxidation of P1ox compared to P1ag is probably caused by the introduction of highly reactive oxygen groups. The MWNT sample has a very broad peak with an onset at relatively low temperature. As hard-to-oxidize basal planes usually make up the surface of carbon nanotubes, this indicates that a substantial part of the sample consists of disordered or defect-rich carbon. The lower oxidation temperature of CNFs compared to Vulcan XC-72R is explained by the higher abundance of reactive graphene edges in CNFs. Once the oxidation has initiated along the more ordered graphene sheets, the continued oxidation is facile. The ζ-potential curves as a function of pH for the majority of the supports are shown in Figure 4 and 5. Carbon materials are typically amphoteric with a mixture of acidic and basic sites on the surface. At a certain pH (the IEP), the net surface charge will be zero (PZC). The as-grown platelet (P1ag) and the purified P2 sample have positively charged surfaces at low pH with an IEP at about pH 5. Oxidation in nitric acid (Pox) results in an increase in the fraction of oxygen groups with acidic character which makes the net surface charge negative over the entire pH range. FB Ni-CH4ag and FB Ni-CO also show an amphoteric nature, while FB Fe-CO is negatively charged in the pH range measured. Oxidation in acid leads to a net negatively charged surface for FB Ni-CH4ox at pH 2-6. Meanwhile, the MWNT support has an IEP at a pH of about 3. XPS was conducted on the majority of the supports. The O 1s and C 1s spectra acquired are shown in Figures 6 and 7. As

Figure 7. XPS C 1s spectra obtained for the carbon supports.

seen from Figure 6, there are clear differences among the samples with respect to the oxygen content. The O 1s/C 1s ratio in Table 4 was found by determination of the peak areas and adjustment by the corresponding atomic sensitivity factors. While the P1ag sample contains a significant amount (O 1s/C 1s ) 0.049) the P2 sample contains minor amounts of surface oxygen (O1s/C1s ratio of 0.008). The contribution from iron oxide species originating from the growth catalyst are considered negligible since the O 1s signals in Figure 6 are very weak at the low binding energies (529.6-530.2 eV43) where the contribution from iron oxides are reported. The large difference in oxygen content between P1ag and P2 grown at similar conditions cannot be readily explained based on the present results. The use of different growth catalysts and carbon sources has profound effects on the resulting oxygen content. FB-NiCO and FB Fe-CO have O 1s/C 1s ratios of 0.038 and 0.083, respectively. By changing the growth catalyst from Ni to Fe and increasing the temperature by 50 °C, it seems that the incorporation of oxygen is more readily accommodated. The contribution to the O 1s signal from SiO2 is considered insignificant, as if the case, similar oxygen peaks should be seen for FB Fe-CO and FB Ni-CO (similar SiO2 content). Also, full range scans showed no Ni 2p peak, indicating that Ni particles are not present on the surface of FB Ni-CH4ag (results not shown here). In general, there is no direct correlation between oxygen amount and the proton affinity. However, for CNFs of the same



TABLE 3: O 1s and C 1s Peak Deconvolution Assignments27,29,31 O 1s

peak I

peak II

peak III

peak IV

BE assignment

531-531.9 carbonyl oxygen of quinones

532.3-532.8 carbonyl oxygen atoms in esters, anhydrides and oxygen atoms in phenolic groups

533.1-533.8 non-carbonyl oxygen atoms in esters and anhydrides

534.3-535.4 COOH

C 1s

peak 1

peak 2

peak 3

peak 4

peak 5

peak 6

BE assignment

284.6 graphite

285.1 sp3-hybridized and/or disordered carbon and/or dangling bonds

286-286.3 OH, C-O

287.3-287.6 CdO

288-289.1 COOH, ester

290-291 π-π* shake up

structure a high oxygen content results in a lower ζ-potential, in good agreement with what has been observed elsewhere.44 The O 1s peaks were deconvoluted in order to determine the relative amount of groups present in the different structures. Caution is important when deconvoluting XPS spectra. Deconvolution is done in a similar manner for all spectra, using similar constrains. The deconvolution of the O 1s spectra into four peaks was realized based on the assignments in Table 3. Typical deconvolution results obtained for O 1s and C 1s are shown in Figure 8a,b and in Table 4. As there was for the O 1s/C 1s ratio, there are significant differences in the relative abundance of groups on different samples. The P2 support has an even distribution of groups, although no carboxylic groups seem to be present. P1ag has a significant contribution from carboxylic groups, although dominated by carbonyl oxygen atoms in esters, anhydrides and oxygen atoms in phenolic groups. FB Fe-CO has a distinct contribution at a binding energy of 533-533.5 eV (47%) corresponding to non-carbonyl (ether-type) oxygen atoms in esters and anhydrides. The high fraction of carboxylic groups found for FB Fe-CO is consistent with the negatively charged surface indicated by ζ-potential measurements. The results for FB Ni-CO indicate a larger fraction of carbonyl oxygen atoms in esters, anhydrides and oxygen atoms in phenolic groups, with the MWNT support showing similar characteristics. The Vulcan XC-72R support is seemingly dominated by the same species, probably corresponding to the significant amount of surface OH groups reported for carbon black structures.26 The XPS spectra of C1s are shown in Figure 7. The main peak is located at about 284.6 eV which is the peak position reported in the literature for sp2-carbon.25 The MWNTs peak exhibited the highest intensity, while the C 1s peak of Vulcan XC-72R was very broad, consistent with its strongly disordered nature. The seemingly higher ordering of the MWNT sample from XPS suggests that the surface is predominantly sp2hybridized. However, the XRD results imply that a large number of defects are present in this sample.

Figure 8. Deconvolution results for FB Fe-CO of (a) O 1s and (b) C 1s.

TABLE 4: Relative Content of Species Determined from Deconvolution of the O 1s and C 1s XPS Spectra relative contents (%) of species in C 1s and O 1s O 1s

I

II

III

IV

O 1s/C 1s

P1ag P2 FB Fe-CO FB Ni-CO MWNT Vulcan XC-72R

11 31 14 17 10 2

51 34 26 42 44 61

28 35 47 36 35 33

10 0 14 5 11 5

0.049 0.008 0.083 0.038 0.014 0.124

C 1s

fwhma (eV)

1

2

3

4

5

P1ag P2 FB Fe-CO FB Ni-CO MWNT Vulcan XC-72R

0.99 0.94 0.95 0.96 0.96 1.21

57 67 68 71 67 49

31 24 21 19 22 30

9 3 9 8 8 19

4 5 3 3 3 2

7 3 5 4 5 10

a

Full width at half-maximum, C 1s. I, carbonyl oxygen of quinones; II, carbonyl oxygen atoms in esters, anhydrides, and oxygen atoms in hydroxyl groups; III non-carbonyl (ether-type) oxygen atoms in esters and anhydrides; IV, oxygen atoms in carboxyl groups; 1, graphitized (sp2) carbon; 2, contribution from sp3-hybridized carbon/disordered carbon and/or dangling bonds; 3, carbon belonging to hydroxyl or single bonded groups; 4, carbon in carbonyl or double bonded groups; 5, carbon belonging to carboxyl or ester groups.

The high binding energy side of the C 1s peak contains information about the oxygen groups. The deconvolution of the C 1s spectra yielded six peaks (assignments in Table 3). The results from the C 1s deconvolution of Vulcan XC-72R indicate the dominance of OH groups, but also a significant contribution from carboxyl and ester groups was found. The results are consistent with respect to the high group II and group 3 values which confirm the dominating amount of OH groups. The OH groups are generally considered to be abundant due to their structural simplicity.26

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TABLE 5: Growth Catalyst Residue, Catalyst Preparation Conditions and Pt Loading support

growth catalyst residue (wt %)

sample

concn (mM)

time (h)

Pt loading (wt %)

P1ag P1ag P1ox P2 FB Fe-CO FB Ni-CO FB Ni-CH4ag FB Ni-CH4ox MWNT XC72

3.4 3.4 0.6 0.4 1.1 0.9 4.9 1.8 3.2 0.9

MO/P1ag MO/P1ag 2 MO/P1ox MO/P2 MO/FB Fe-CO MO/FB Ni-CO MO/FB Ni-CH4ag MO/FB Ni-CH4ox MO/MWNT MO/XC72

8.2 2.5 2.5 8.2 8.2 8.2 2.5 2.5 8.2 8.2

8 2 2 8 8 8 2 2 8 8

11.3 6.5 5.4 17.1 3.1 8.9 3.0 1.4 8.1 19.6

The full width at half-maximum (fwhm) of the C 1s peak has been used to assess the graphitic nature of carbons, with a larger fwhm corresponding to a more unordered structure.45 Also, for carbon blacks, a narrower fwhm was related to a higher amount of basic sites.45 In this study, the fwhms found for the CNFs and MWNTs were relatively similar (0.94-0.99 eV). As a result of its strongly disordered nature, a significantly higher fwhm (1.21 eV) was determined for Vulcan XC-72R. The C 1s peak has also been used to asses the ratio of sp2 and sp3 carbon, typically at binding energies of 284.6 and 285.1 eV. A larger fraction of sp3-hybrids has been correlated with a lower degree of graphitization in amorphous carbon films.32 Elsewhere, as a result of plasma oxidation treatment, a larger fraction of sp3-hybrids has been correlated with the destruction of π-conjugation followed by transformation to the amorphous carbon phase.33 However, there is debate regarding which carbon features are really contributing and it is likely several species are contributing at this binding energy. An increase in the intensity at 285.1 eV was seen with Ar+ irradiation of highly oriented pyrolytic graphite (HOPG) and ascribed to the presence of unsaturated carbon (dangling bonds) rather than sp3-hybridized carbon.34 From the deconvolution, the percentage of the contribution at 285.1 eV was found to vary in descending order P1ag > Vulcan XC-72R > P2 > MWNT > FB Fe-CO > FB Ni-CO. Thus, there are significant differences in the edge structure and amounts of defects among the supports. sp3Hybridized carbon is thought to be incorporated to obtain closure of adjacent graphene layers.46 The high value obtained for MWNTs compared to the fishbone samples can be explained by a larger amount of surface disordered carbon as indicated by TEM and TPO, although sp3-bonding also has been reported47 and is probably present in the bent parts of the tubes. 3.2. Catalyst Preparation. As mentioned above, the mechanism for colloid adsorption on the support for the metal oxide colloid (MO) method is not well understood. It is observed that immobilization can take place at the conditions for deposition where both the colloids and the support are normally negatively charged (pH 9-10). In a recent study in our group,13 the procedure as developed by Reetz et al. was adopted,21 but the reproduction of their results was not achieved. The instant method involving formation and deposition of the colloid in the same step, successfully applied for both Vulcan and CNF supports22 showed no promise for the fishbone support (FB NiCH4).13 For both platelet and fishbone CNFs, a higher loading was achieved with a two-step procedure where the precursor solution was initially heated without the presence of carbon.13 The formation of colloid particles was followed by UV to study the extent of colloid formation. The decrease in the Pt4+ plasmon band was clearly seen, although only to a certain level. Neither did the solution change color to red-brown characteristic for the formation of colloid particles as indicated for the adopted procedure.20 However, this color was observed in the filtrate

after the immobilization step. It is possible that the hydrolysis reaction takes place more readily in the second step because of the increase in temperature (70 °C). The method parameters of the patent21 were adopted, except for the immobilization time, which initially was chosen to be 2 h. Increasing the time of this step from 2 to 8 h increased the Pt loading from 6.5 to 11.3 wt % for P1ag (Table 5). As the parameters already had been extensively investigated,48 the effect of support properties rather than immobilization conditions was pursued. It is likely that more Pt could be deposited by extending the time for immobilization further, but the results reported here are considered to be representative of the supports that are tested and their properties. 3.3. Identification of the Most Important CNF Properties for Immobilization of Pt Oxide Colloids. The Pt loadings obtained by TPO vary significantly among the different carbon supports, as shown in Table 5, and in line with the typical TEM images in Figure 9. The preparation of catalysts using the platelet supports results in a dense immobilization of Pt particles, as can be seen in Figure 9a,b. However, for MO-P2 (Figure 9b) there are also clearly agglomerates of particles. In Figure 9c-f, a lower density of particles can be seen on the fishbone structures. There is a clear discrepancy between the loading found from TPO and the Pt coverage in the FB Ni-CO sample in Figure 9d. A possible explanation indicated by EXAFS-results is that a large fraction of the particles is smaller than the detection limit of the TEM instrument.49 Figure 9e shows that small particles were deposited on FB Ni-CH4ag, but the density was found to vary significantly from fiber to fiber. By close inspection of Figure 9f, a sparse decoration, predominantly in clusters of particles on FB Ni-CH4ox, indicating lack of suitable anchoring sites is seen. A Pt loading of 8.1 wt % was obtained on the MWNT-supported sample (Figure 9g), with the dispersion of the particles found to be inhomogeneous. The TEM images indicate that defects, such as in the bent tube in Figure 9g or disordered carbon as in Figure 1f, are required for deposition of the metal oxide colloids. Figure 9h shows that immobilization was complete and homogeneous on the Vulcan XC-72R supported sample. Although a high Pt loading can be readily achieved on Vulcan XC-72R, this is in large part because of its microporosity. With regard to maximal utilization of Pt, this becomes important as the micropores make a signification fraction of the Pt particles inaccessible to reactants. A mean particle size of 1.8 nm was reported in the patent.21 In the present study, the size obtained from TEM is similar for particles supported on the different supports, ranging predominantly from 1.5 to 2.5 nm. The possible presence of less than 1 nm sized particles is hard to assess due to the detection limitations of the TEM instrument as mentioned above. A lower mean particle size obtained from EXAFS (∼1 nm) indicates that smaller particles are present49


Figure 9. Typical TEM images of (a) MO/P1 ag, (b) MO/P2, (c) MO/ FB Fe-CO, (d) MO/FB Ni-CO, (e) MO/FB Ni-CH4 ag, (f) MO/FB NiCH4 ox, (g) MO/MWNT, and (h) MO/XC72.

The effect of different CNF properties on the final Pt loading is presented in Figure 10 in order to gain insight regarding the sites of deposition when preparing catalysts by the MO method. Generally, the dispersion of metal particles on carbon is found to increase with higher surface area and amount of micropores.23,50 Figure 10a shows the effect of surface area on the Pt loading, which indicate a linear increase in the final loading with an increase in surface area. The two points below the trend line correspond to MO/FB Fe-CO and MO/MWNT. The lower loading of MO/FB Fe-CO can be partly due to the large surface area of the inner hollow tube. It is likely that the immobilization in the inner tubes is hindered due to diffusion limitations. The lower loading obtained on MWNTs is probably due to the high fraction of nonreactive basal planes. Modification by oxidizing acids and/or polyelectrolytes is normally considered necessary in order introduce sites that can successfully support metal particles for MWNTs. However, the relatively high loading for MO/MWNT compared to, e.g., MO/FB Fe-CO pinpoints the

J. Phys. Chem. C, Vol. 114, No. 4, 2010 1759 presence of defects or disordered carbon as sites of deposition. This is supported by the TEM image in Figure 9g where metal particles were predominantly found on seemingly defect-rich and few or no particles were seen on the defect-free MWNT surfaces. The interlayer spacing, d002, and graphene layer stacking height, Lc, are affected by the disorder of the carbon structure which again influences the deposition of Pt particles. However, in the present study, no clear relationship was found between d002 (Figure 10b), and Lc (Figure 10c) with the observed Pt loading. As mentioned above, a wider C 1s fwhm has been used as an indication of a more defect-rich structure. Figure 10d shows that except for the one platelet support there is a clear positive effect of increasing C 1s fwhm. The effect of the O 1s/C 1s ratio on the loading is plotted in Figure 10e. The oxygen content of the supports was found important for the final loading, with a lower amount of oxygen being beneficial for the anchoring of Pt oxide clusters. This can be explained by a repellent action between the negatively charged CNF surface and the negatively charged Pt oxide colloids. However, a high loading was achieved on Vulcan XC72R despite its high oxygen content indicating that the deposition sites in carbon black are different and less affected by the oxygen nature and content. Deconvolution of the O 1s and C 1s peaks was performed in a further effort to relate the effect of different oxygen groups to the final loading. Diagrams similar to Figure 10e showing the effect of various oxygen groups obtained from deconvolution of the C 1s and O 1s spectra on the Pt loading are not presented. A negative linear effect on the Pt loading was found with increasing amounts of carboxyl and ester groups. A negative trend was also found for carbon in carbonyl or double-bonded groups and carbon belonging to phenolic or single-bonded groups. In a recent study, the loading obtained for MO/FB Ni-CH4ox was found to be about half of that found for MO/FB Ni-CH4ag.13 Temperature-programmed desorption-mass spectroscopy results showed that the treatment in the mixture of HNO3/H2SO4 introduced three times more oxygen compared to treatment in HNO3 only,13 and the lower loading on FB Ni-CH4ox was explained by the negative effect of acidic groups. This is also supported by the lower loading obtained on P1ox compared to P1ag in this study (Table 5). The loading obtained on MWNTs is generally lower than for the indicated trend obtained on the CNF supports. Since the XPS deconvolution results are normalized by surface area, this is probably related to the few suitable anchoring sites despite a high BET surface area. The role of defects and/or unsaturated C atoms (dangling bonds) can also be assessed by plotting the value of carbon species located at 285.1 eV found from deconvolution of the C1s spectra. Figure 10f suggests that a higher percentage of these carbon species found at this binding energy are beneficial for the immobilization of metal oxide colloids. This corresponds to what was found in a similar study, where the deposition of metal oxide colloids on different carbon supports, mainly carbon blacks, were investigated.22 It was reported that metal deposition was considerably faster on carbon blacks with “rough” compared to “smooth“surfaces.22 This ”roughness” probably relates to the abundance of edge and defect sites as mentioned above. The loading obtained on P1ag is low compared to the trend in Figure 10f. From the N2-adsorption results, this structure has a larger fraction of micropores compared to P2, but the explanation for the lower value is most likely a negative effect of surface oxygen groups. As mentioned above, Vulcan XC-72R behaves quite differently from the CNF supports, as a negative effect of

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Kvande et al.

Figure 10. Effect of (a) surface area, (b) interlayer spacing d002, (c) stacking height, Lc, (d) C 1s fwhm, (e) O 1s/C 1s ratio, and (f) sp3-hybridized/ disordered carbon and/or dangling bonds. Catalyst symbols: platelet CNFs (O), fishbone CNFs (0), MWNTs (2), Vulcan XC-72R (/).

oxygen groups was not indicated. It is probable that Vulcan XC-72R made up of small entities of disordered carbon and strong micropore adsorption sites have abundant preferable sites for adsorption corresponding to A-sites, as discussed in the next section. Based on the large fraction of micropore sites in Vulcan XC-72R compared to what is the case for the predominantly mesoporous CNFs, the rest of the discussion will be based on the assumption that the sites and mechanism for deposition is different for the two subgroups of carbon structures. Defect and edge sites are clearly needed for the immobilization on MWNTs and CNF supports. The higher loading obtained on the platelet compared to fishbone and MWNT supports can be explained by a higher fraction of defects caused by the closure of adjacent graphene layers. The importance of defects resulting from closed graphene layers was suggested by Gan et al.;51 heat treatment of fishbone CNFs resulted in the closure of adjacent graphene layers, producing sites to facilitate the homogeneous deposition of Pt particles by the ethylene glycol

method. We propose that such defects are more readily formed in a platelet structure compared to a fishbone structure due to a shorter graphene layer distance in addition to a smaller fiber curvature. In summary, no direct relationship exists between the Pt loading and CNF properties such as the interlayer spacing, the stacking height, or fwhm. A negative effect of surface oxygen groups and a positive effect of defects and dangling bonds (represented by 285.1 eV in C 1s spectra) are indicated. The results provide new insight in the role of oxygen-free sites for anchoring Pt oxide clusters. However, based on the results it is not straightforward to identify the exact nature of these active sites. 3.4. Nature of the Active Sites for Anchoring Metal Oxide Clusters. In a recent review on the physicochemical properties of carbon for the preparation of noble metal catalysts, the nature and quantity of traps or defects were recognized as important for the nucleation and stabilization of particles.23 In sp2-carbons

Immobilization of Colloidal Pt Oxide Particles like CNFs, a strong localization of double bonds and deviation from planarity is caused by defects, for instance, the exchange of hexagon rings with smaller or larger cyclic rings.26 These double bonds are considered important for the interaction of metal precursors with the carbon support and are known as A or Cπ sites. The strength of these sites increases as the size of the disordered carbons constituting the support matrix decreases.23 This is related to smaller entities of carbon being stronger electron acceptors. CNFs are usually rich in these sites, corresponding to the presence of edges.23 Important sites for adsorption discussed in the literature that could fit the definition of A-type sites are topological defects (heptagons, pentagons, etc.) existing due to the “nanocurvature” of closed adjacent graphene layers (incorporating sp3-hybridized carbon atoms) as mentioned above. This was supported by the work reported by Gan et al.,51 where pentagon and heptagon defects were suggested as important active sites. In a recent paper, Radovic and Bockrath concluded that the exact nature of the edge structure has been neglected and misunderstood. A number of experimental observations and a model for the chemical composition of the edges of graphene sheets was used to support that a large fraction of edge sites can be oxygen- and hydrogen-free carbene-like (zigzag) sites.52 Carbene structures at zigzag sites can, e.g., explain the origin of ferromagnetism and basicity of carbon materials.52 As opposed to A-sites, these sites are considered to increase in strength with an increase in the size of the crystallite or graphene sheet. Due to affinity of carbon for oxygen, water, and organic solvents, the authors state that these dangling bonds sites will not survive exposure to room temperature air and that their direct identification is not to be expected.52 As well as deposition on A-type sites and defects from the closure of adjacent graphene layers, we believe that the deposition is to a similar or higher degree dependent on the carbene sites on graphene edges indicated by Radovic.52 A recent study for deposition of Pd on various CNFs showed that the electrostatic attraction caused by the potential difference between the CNF surfaces and the palladium colloids is the determining factor for the obtained Pd loading and particle size.44 That is, the deposition of a negatively charged Pd colloid was most readily achieved on the carbon support with the highest PZC (IEP) or highest basicity. In comparison, the deposition of metal oxide colloids is performed at basic conditions and it is therefore not straightforward to argue that the process is governed by electrostatic attraction. However, when comparing the ζ-potential measurements in Figures 4 and 5 and the Pt loadings in Table 5, it can be seen that adsorption of metal oxide colloids is most readily achieved on platelet CNFs with the highest PZCs. Edge carbene sites as proposed by Radovic and Bockrath52 can account for carbon basicity, and the present results are in this respect, although indirectly, supportive with regard to their existence. The present authors propose that for deposition on CNFs and the basic conditions applied, chemisorption of the metal oxide colloids onto carbene-type sites and defect sites resulting from closed graphene layers is the predominant mechanism. In this context, recent molecular dynamics simulations using a revised Reaxx reactive force field to study the interactions between platinum clusters and carbon platelets was reported.53 A cluster of 100 Pt atoms was selected to mimic structural changes and initially placed at a typical C-Pt distance of around 2 Å away from the platelet structure. The overall energy of the system was minimized without constraints. A high adsorption energy (-251 kcal/mol) was found for a 4-fold interlayer site compared to 2-fold on-planes sites (-167 and

J. Phys. Chem. C, Vol. 114, No. 4, 2010 1761 -205 kcal/mol), indicating that these sites are highly favorable in terms of adsorption and stabilization of metal particles. Based on the presented results and discussion, the conclusion is that the immobilization of Pt colloids by the metal oxide colloid method is largely governed by defects identified as disordered carbon and/or open or closed graphene edges. In Vulcan XC-72R defects (A sites) appear to be more abundant. In CNFs/CNTs a larger number of edges (carbene sites) and defects (closed adjacent graphene layers) are preferable and chemisorption is identified as the mechanism for metal oxide colloid deposition. A low amount of surface oxygen is highly beneficial at the conditions for immobilization, as especially for CNF structures, it seems the presence of oxygen groups weakens the strength of the adsorption sites. As mentioned above, homogeneous deposition of Pt onto heat-treated and thus oxygen-free CNFs has been reported.51 As similar indirect evidence for the importance of oxygen-free edge sites for deposition of Pd colloids on CNFs have been found44 the results invite a reconsideration of the pretreatment necessary for deposition of metal particles. Thus, deposition onto oxygenfree edge sites stands out itself as an alternative to the common approach of deposition onto surface oxygen groups introduced by acid treatment. It is also interesting to note that acidic surface groups used for anchoring metal precursors can decompose at relatively low temperatures, thereby having implications for the stability of metal particles. Meanwhile, enhanced stability of Pd particles with an increasing amount of edge sites was indicated through a suppression of leaching during the Heck reaction.44 Also, enhanced performance in the oxygen reduction reaction has been found for Pt supported onto heat-treated and oxygen-free CNFs.51 It is believed that these findings will have great implications for the development and performance of CNFsupported catalysts. 4. Conclusions The study targeted the identification of the determining carbon support properties for the successful preparation of Pt catalysts by a metal oxide colloid method. CNFs, MWNTs, and Vulcan XC-72R were utilized as supports. Significant differences with respect to surface area, oxygen groups and content, graphitic nature, and defect and edge structure were found. A linear relationship was obtained between carbon species originating at 285.1 eV and the final Pt loading, which could be ascribed to a higher fraction of disordered carbon, defects, and/or dangling bonds related to the edge structure of CNFs. An abundance of graphene edges as well as defects originating from closed adjacent graphene layers was thus considered beneficial for the immobilization of metal oxide colloids on CNFs. Chemisorption is identified as the mechanism for deposition of metal oxide colloids on CNFs. For CNFs, the loading showed a negative linear dependence with the amount and different types of oxygen groups obtained by XPS. Meanwhile, a high loading was achieved on Vulcan XC-72R despite its high oxygen content, indicating that the deposition sites are less affected by the oxygen nature and content and thereby different from those on CNFs. However, the micropores that provide the high surface area and more facile deposition of higher Pt loadings also make a significant fraction of the Pt particles inaccessible to reactant molecules. In this respect, CNFs have the prospect of a better utilization of Pt since they are predominantly mesoporous. Based on the results, the deposition of particles onto CNF structures with oxygen-free edge and defect sites stands out as a promising alternative to the conventional deposition onto CNFs with

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introduced oxygen groups. As for metal oxide colloids, oxygenfree edge sites are believed to be equally important in the deposition of metal colloids on CNFs. Acknowledgment. This work was supported by the Research Council of Norway, Grant No. 158516/S10 (NANOMAT). Heimir Magnusson is acknowledged for his help with the transmission electron microscopy experiments. The referees are highly acknowledged for their valuable comments and suggestions. References and Notes (1) de Jong, K. P.; Geus, J. W. Catal. ReV.sSci. Eng. 2000, 42, 481. (2) Bessel, C. A.; Laubernds, K.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem. B 2001, 105, 1115. (3) Serp, P.; Corrias, M.; Kalck, P. Appl. Catal., A: Gen. 2003, 253, 337. (4) Rodriguez, N. M.; Kim, M.-S.; Baker, R. T. K. J. Phys. Chem. 1994, 98, 13108. (5) Park, C.; Baker, R. T. K. J. Phys. Chem. B 1999, 103, 2453. (6) Baker, R. T. K.; Laubernds, K.; Wootsch, A.; Paal, Z. J. Catal. 2000, 193, 165. (7) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7935. (8) Chesnokov, V. V.; Prosvirin, I. P.; Zaitseva, N. A.; Zaikovskii, V. I.; Molchanov, V. V. Kinet. Catal. 2002, 43, 838. (9) Ochoa-Fernandez, E.; Chen, D.; Yu, Z.; Tøtdal, B.; Rønning, M.; Holmen, A. Surf. Sci. 2004, 554, L107. (10) Ledoux, M.-J.; Pham-Huu, C. Catal. Today 2005, 102-103, 2. (11) Lee, J. S.; Han, K. I.; Park, S. O.; Kim, H. N.; Kim, H. Electrochim. Acta 2004, 50, 807. (12) Ros, T. G.; Van Dillen, A. J.; Geus, J. W.; Koningsberger, D. C. Chem.sEur. J. 2002, 8, 1151. (13) Kvande, I.; Briskeby, S. T.; Tsypkin, M.; Rønning, M.; Sunde, S.; Tunold, R.; Chen, D. Top. Catal. 2007, 45, 81. (14) Han, K. I.; Lee, J. S.; Park, S. O.; Lee, S. W.; Park, Y. W.; Kim, H. Electrochim. Acta 2004, 50, 791. (15) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Li, H.; Sun, G.; Xin, Q. Carbon 2004, 42, 436. (16) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou, Z.; Sun, G.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292. (17) Liang, Y.; Zhang, H.; Yi, B.; Zhang, Z.; Tan, Z. Carbon 2005, 43, 3144. (18) Bonet, F.; Delmas, V.; Grugeon, S.; Urbina, R. H.; Silvert, P. Y.; Tekaia-Elhsissen, K. Nanostruct. Mater. 2000, 11, 1277. (19) Guha, A.; Lu, W.; Zawodzinski, T. A.; Schiraldi, D. A. Carbon 2007, 45, 1506. (20) Reetz, M. T.; Koch, M. G. J. Am. Chem. Soc. 1999, 121, 7933. (21) Reetz, M. T.; Lopez, M. , Method for in-situ immobilization of water-soluble nanodispersed metal oxide colloids, U.S. Patent 7,244,688, 2003. (22) Reetz, M. T.; Schulenburg, H.; Lopez, M.; Spliethoff, B.; Tesche, B. Chimia 2004, 58, 896. (23) Simonov P. A., Likholobov V. A. Physicochemical Aspects of Preparation of Carbon-supported Noble Metal Catalysts. In Catalysis and

Kvande et al. Electrocatalysis at Nanoparticle Surfaces; Wieckowski, A., Savinova, E. R., Vayenas C. G., Eds.; Marcel Dekker, Inc.: New York, 2003; pp 425-426. (24) Rodriguez-Reinoso, F. Carbon 1998, 36, 159. (25) Boehm, H. P. Carbon 2002, 40, 145. (26) Schloegl, R. Carbons In Preparation of Solid Catalysts; Ertl, G., Knoezinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1999; pp 138. (27) Lakshminarayanan, P. V.; Toghiani, H.; Pittman, C. U. Carbon 2004, 42, 2433. (28) Toebes, M. L.; van Heeswijk, J. M. P.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Carbon 2004, 42, 307. (29) Zhou, J.-H.; Sui, Z.-J.; Zhu, J.; Li, P.; Chen, D.; Dai, Y.-C.; Yuan, W.-K. Carbon 2007, 45, 785. (30) Bruser, V.; Heintze, M.; Brandl, W.; Marginean, G.; Bubert, H. Diamond Relat. Mater. 2004, 13, 1177. (31) Felten, A.; Bittencourt, C.; Pireaux, J. J. Nanotechnology 2006, 17, 1954. (32) Diaz, J.; Paolicelli, G.; Ferrer, S.; Comin, F. Phys. ReV. B: Condens. Matter 1996, 54, 8064. (33) Ago, H.; Kugler, T.; Cacialli, F.; Salaneck, W. R.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. J. Phys. Chem. B 1999, 103, 8116. (34) Speranza, G.; Minati, L. Diamond Relat. Mater. 2007, 16, 1321. (35) Yu, Z.; Chen, D.; Tøtdal, B.; Holmen, A. J. Phys. Chem. B 2005, 109, 6096. (36) Toebes, M. L.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Catal. Today 2002, 76, 33. (37) DTS Customer Training Manual. Chapter 6, Zeta Potential Theory, p 66. (38) Kwok, R. W. M. XPS Peak 4.1, XPS peak fitting program. (39) Zhou, J.-H.; Sui, Z.-J.; Li, P.; Chen, D.; Dai, Y.-C.; Yuan, W.-K. Carbon 2006, 44, 3255. (40) Raghuveer, V.; Manthiram, A. J. Electrochem. Soc. 2005, 152, A1504. (41) Fenelonov, V. B.; Derevyankin, A. Y.; Okkel, L. G.; Avdeeva, L. B.; Zaikovskii, V. I.; Moroz, E. M.; Salanov, A. N.; Rudina, N. A.; Likholobov, V. A.; Shaikhutdinov, S. K. Carbon 1997, 35, 1129. (42) Fenelonov, V. B.; Likholobov, V. A.; Derevyankin, A. Y.; Mel’gunov, M. S. Catal. Today 1998, 42, 341. (43) Handbook of X-ray photoelectron spectroscopy; Perkin Elmer: Waltham, MA, 1992. (44) Zhu, J.; Zhao, T.; Kvande, I.; Chen, D.; Zhou, X.; Yuan, W. Chin. J. Catal. 2008, 29, 1145. (45) Darmstadt, H.; Roy, C. Carbon 2003, 41, 2662. (46) Shaikhutdinov, S. K.; Zaikovskii, V. I.; Avdeeva, L. B. Appl. Catal., A: Gen. 1996, 148, 123. (47) Mazzoni, M. S. C.; Chacham, H. Phys. ReV. B: Condens. Matter Mater. Phys. 2000, 61, 7312. (48) Lopez, M. Ph.D. Thesis, University of Bochum, Germany, 2001. (49) Kvande, I. Ph.D. Thesis, Norwegian University of Science and Technology, Trondheim, Norway, 2007. (50) Ralph, T. R.; Hogarth, M. P. Platinum Metals ReV. 2002, 46, 3. (51) Gan, L.; Du, H.; Li, B.; Kang, F. Carbon 2008, 46, 2140. (52) Radovic, L. R.; Bockrath, B. J. Am. Chem. Soc. 2005, 127, 5917. (53) Sanz-Navarro, C. F.; Åstrand, P.-O.; Chen, D.; Rønning, M.; van Duin, A. C. T.; Jacob, T.; Goddard, W. A. J. Phys. Chem. A 2008, 112, 1392.

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