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Hydrogen Adsorption on Pd- and Ru-Doped C60 Fullerene at an Ambient Temperature Dipendu Saha* and Shuguang Deng* Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico 88003, United States ABSTRACT: Palladium- and ruthenium-doped C60 fullerene compounds were synthesized by incipient wetness impregnation of C60 fullerene with the corresponding metal acetylacetonate precursors. Transmission electron microscopy (TEM) imaging of the metal-doped C60 fullerene samples showed different dispersion morphologies of palladium and ruthenium particles on the C60 matrix. Raman spectra revealed a drastic decrease in peak intensity followed by disappearance of several bands indicating the distortion of the C60 cage structure. The amorphous nature of the C60 fullerene compounds was confirmed by the X-ray diffraction study. Hydrogen adsorption amount of 0.85 wt % and 0. 69 wt % on PdC60 and RuC60, respectively, as compared to 0.3 wt % on the pure C60 fullerene were measured at 300 bar and 298 K. The enhancement in the hydrogen uptakes can be attributed to several factors, including adsorption of molecular H2 on the defect sites, metallic hydride formation, spillover of hydrogen, and bond formation with atomic hydrogen with different active sites of carbon of host fullerene. The hydrogen adsorption isotherms are of type III and can be correlated by the Freundlich (for RuC60) and modified Oswin equations (for PdC60 and pristine C60).
1. INTRODUCTION Doping the carbon-based adsorbents with different types of transition metals is believed to be an effective way to enhance the hydrogen storage capacity at ambient temperature. Many metals including palladium (Pd), platinum (Pt), nickel (Ni), and ruthenium (Ru) have been examined successfully with numerous carbonaceous adsorbents.15 It was speculated that these metals increase the hydrogen uptake capacity by the so-called spillover technique in which molecular hydrogen dissociates into atomic form and subsequently diffuses to several places of the substrate that are not accessible to molecular hydrogen. It was also confirmed experimentally that part of the atomic hydrogen chemically attached to the metals results in different nonstoichiometric proportions of hydrides.6,7 Very recently, researchers also demonstrated that atomic hydrogen can form partial bonds with different active sites of the carbon substrates, if present.8 All of these factors are responsible for the overall enhancement of hydrogen uptake in metal-doped adsorbents. C60 fullerene is one of the unique allotropes of carbon that was least examined for hydrogen adsorption because C60 does not have “real pores” in its structure that are accessible to hydrogen molecules. The very low hydrogen uptake on fullerene contributed to the octahedral lattice of fcc crystal (d = 4.12 Å), as the tetrahedral lattice was too small (d = 2.24 Å) to accommodate hydrogen molecule (d = 2.4 Å).9 Saha and Deng increased the hydrogen uptake on C60 fullerenes by partially truncating and opening the cage of C60.10 There have been numerous research papers that associate the doping of fullerene with different metals, more commonly known as metal fullerides or r 2011 American Chemical Society
metal-fullerene complexes; however, these complexes have not been studied for their hydrogen sorption properties, especially at ambient temperatures. Complexing fullerene with palladium is probably the earliest effort to synthesize metal fullerides that also has been most frequently examined by different researchers for its diverse properties.1114 It was suggested by EXAFS study that Pd forms covalent η2C bonds at the double bond region between two hexagonal carbon rings of C60.15 An XPS study also revealed that a 1.4 eV downshift of the 3d peak of palladium combined with a zero shift of the 1s peak of carbon resulted in a very weak bond formation between Pd and carbon.16 Herbst et al.17 synthesized a PtC60 complex to examine its solid-state structure. Palladium, platinum, and ruthenium fullerene complexes demonstrated the potential role as catalyst in hydrogenation of carbonyl groups and reduction of CdC bonds.1820 Other different metals which are also known to form complexes with fullerenes are iron,21 titanium,22 niobium,23 rhodium,24 cobalt,25 sodium, potassium, lithium,26 barium, calcium, and strontium.2729 Wang and Tu30 synthesized a PtC60 compound that has 1.6 wt % hydrogen uptake at 473 K, which was probably contributed by the hydrogenation of the basic fullerene skeleton and its metal counterparts. Lan et al.31 performed a simulation calculation on a fullerene-like compound Li12Si60H60 that could adsorb 12.83 wt % at 10 MPa and 77 K. Li et al.32 employed density functional theory to calculate hydrogen uptake of calcium doped boron Received: January 8, 2011 Revised: April 7, 2011 Published: April 28, 2011 6780
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fullerene (B80). Hydrogen sorption measurement on metal fullerides at ambient temperatures has never been reported so far. In this work, we synthesized palladium and ruthenium complex of C60 fullerene by an incipient wetness impregnation method. The resulting complex was characterized with transmission electron microscopy, X-ray diffraction, and Raman spectroscopic analyses. The hydrogen adsorption study was performed at ambient temperature (298 K) and hydrogen pressure up to 300 bar. The hydrogen adsorption experiments allow us to examine the hydrogen storage performance enhancement by the influence of metallic counterparts over fullerene at different pressures and to elucidate the possible adsorption mechanisms of hydrogen on the metalfullerene complexes.
2. EXPERIMENTAL SECTION 2.1. Synthesis of PdC60 and RuC60 Complexes. The synthesis of metalfullerene complexes was performed by the incipient wetness impregnation technique. 100 mg of C60 fullerene (99.9%, Sigma-Aldrich) was mixed with a given amount of palladium or ruthenium acetylacetonate dissolved in an equal volume of chloroform solution. The amount of metal acetylacetonates was maintained in such a way that final metal loading was 1 wt % of the pristine C60. These mixtures are agitated well in a shaker for an extended period of time (>240 min) and dried at 60 °C in air. One of the metal-C60 samples was then put into a closed cell of the Rubotherm magnetic suspension balance for evacuation for an hour, followed with heating to 150 °C under hydrogen flow at 1 bar. The process was ceased when the remaining mass showed a constant weight after an initial loss due to the decomposition of acetylacetonates. The resultant metalfullerene complexes were no longer soluble in toluene, clearly indicating the loss of one of the identifying properties of the pristine C60. 2.2. Material Characterization. Material characterization including transmission electron imaging (TEM), Raman spectral study, and X-ray diffraction analysis was performed on the pristine fullerene and PdC60 and RuC60 complexes. TEM images were taken in a Hitachi H7650 transmission electron microscope. Raman spectra were recorded in a Rennishaw Raman microscope with a 633 nm HeNe laser beam. The X-ray patterns were generated with a Rigaku Miniflex-II desktop X-ray diffractometer with a zero background sample holder. The X-ray was generated using the Cu KR emission (λ = 1.540 56 Å) with 30 kV potential difference and 15 mA filament current. θ calibration and generation of a default fwhm curve of the X-ray diffractometer were performed using a NIST standard silicon powder sample. For each sample, the XRD scan was performed from 3° to 75° with 0.03° width and 1 s count time. The XRD data were processed using commercial software Jade 8þ developed by Materials Data Inc., Livermore, CA, USA. 2.3. Hydrogen Sorption Measurements. The high pressure hydrogen sorption was performed gravimetrically in a Rubotherm magnetic suspension balance at a room temperature (298 K) and hydrogen pressures up to 300 bar. Like all other gravimetric devices, this balance was also pre-examined with a blank run of the empty sample holder and a helium run after a sample was loaded to the balance in order to measure the weight and volume of the empty sample holder and the adsorbent sample before introducing hydrogen for the actual adsorption measurements. The detailed operation procedures were described in our previous publications.33,34
3. RESULTS AND DISCUSSION 3.1. Transmission Electron Imaging (TEM). The TEM imaging of PdC60 and RuC60 is shown in Figure 1A,B These two pictures definitely establish the fact that the nature of doping
Figure 1. TEM images of PdC60 (a) and RuC60 (b).
of these two metals is different on the substrate fullerene. The particle nature of the metal is more prominent in the case of palladium compared to that of ruthenium. In Figure 1A, there are two different dimensions of particles observed: the smaller particles are in the size range 56 nm whereas the larger ones are in the range 1520 nm, though the larger ones are probably clusters of the smaller particles. The smaller particle dimensions of PdC60 are similar to those reported by Wohlers et al.20 (2 to 5 nm) and Gurav et al.24 (3 to 5 nm) for ruthenium and rhodium fullerene complexes, respectively. The type of ruthenium doping on C60 appears to be different than that of palladium and no district nanoparticle was observed in its TEM images; instead, the coverage was close to “layer” or “film” type. The coverage type was different from those reported by Wohlers et al.20 who observed distinct ruthenium nanoparticles in their samples. Most probably, the difference in the interactions of the metal precursors with the fullerene surface caused this dissimilarity of surface coverage by palladium and ruthenium. 3.2. Raman Spectral Analysis. The Raman spectral data of pure fullerene, PdC60, and RuC60 are shown in Figure 2A,B, C, respectively. The three main Raman bands of pure fullerene (Figure 2A) were obtained in the shift of 269, 494, and 1466 cm1, which are the Hg(1), Ag(1), and Ag(2) characteristic modes of fullerene. A few small, low-intensity peaks were also noticed in the spectra, which can be denoted as Hg(2), Hg(4), Hg(5), Hg(6), and Hg(8) modes as marked in the figure. In the 6781
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Figure 3. XRD patterns of pristine C60 fullerene, PdC60, and RuC60.
Figure 2. Raman spectra of pristine C60 fullerene (A), PdC60 (B), RuC60 (C).
Raman spectra of the PdC60 complex, all the major bands of fullerene disappeared, except a very small band at 1466 cm1, which is the remnant of the Ag(2) mode. The disappearance of almost all Raman bands and residue of an extremely small Ag(2) mode was also observed in a PtC60 complex by Wang and Tu.30 The Raman peaks were more prominent in the RuC60 complex, where, besides the main three bands (Hg(1), Ag(1), and Ag(2)), a small residue of Hg(4) band was also observed at 770 cm1. In the Raman spectra of both complexes, two common observations are broadening of peak shape and lowering of peak intensity as compared to those of the pristine species. The line broadening could be contributed to the electronphonon
coupling as observed in the case of KC60 and RbC60 complexes33 or due to the distortion of the original cage of C60.30 The distortion of the cage could also be the possible reason for lowering of the peak intensity.30 It has been stated that the exact role of metals in the Raman spectra of metal fullerides is not quite explicit.30,33 However, by comparison of the disorder of Raman spectra of these two types of metal fullerene complex, it can be proposed that complex formation was probably more predominant in the palladium doped fullerene complex than in the ruthenium doped fullerene complex. 3.3. X-ray Diffraction Study. The XRD patterns of pure fullerene, PdC60, and RuC60 complex is shown in Figure 3. Pure fullerene has well-defined crystalline structure, with three sharp peaks at the angles of 10.8° (111), 17.6° (200), and 20.7° (220) among which the peak at 10.8° possesses the highest intensity. Three other smaller peaks are also located at 21.7°, 28.05°, and 32.9°. The fcc cubic crystal lattice was indexed as 14.22 Å, which is similar to the reported value of 14.13 Å34 or 14.17 Å.35 In the XRD of PdC60 and RuC60, the patterns are quite altered as compared to those of the pristine C60. Except for the large peak at 17.55°, all the remaining peaks of pristine fullerene became much shorter in the PdC60 complex. The strongest peak at 10.8° and moderate peak at 20.7° became extremely small in the same species. In the XRD pattern of RuC60, no prominent peak of the pristine C60 was observed. In a close view of the XRD patterns of PdC60 and RuC60 compounds, the diffraction peaks from pure palladium and ruthenium were observed in the XRD patterns. This is quite similar to XRD patterns reported by Wang and Tu,30 where the platinum peaks were observed in a PtC60 compound. The palladium peaks at 40°, 46°, and 67.3° belong to the (111), (200), and (220) reflections of Pd, whereas the ruthenium peak at 45° is attributed to the (111) reflection. On the basis of the overall X-ray diffraction patterns, it can be concluded that the fcc structure of the pristine fullerene was destroyed in the metal-fullerene compounds and each complex is basically amorphous in nature. The existence of pure metallic X-ray diffraction peaks in the complex suggests that a monolayer dispersion of metals on the carbon surface was not achieved according the monolayer dispersion theory that the metals cannot be detected by XRD if they are dispersed on a support as a monolayer.36 By comparing the TEM, Raman, and XRD results with those of other metal-fullerene complexes published 6782
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Langmuir in the literature,11,30 we can suggest that metal-C60 compounds were formed in our samples. Detailed structural information could be confirmed by an XPS study of these compounds. The exact composition of metal in the metal fullerene complex is quite difficult to determine. Palladium fulleride (PdxC60) was synthesized by Talyzin et al.11 and the exact composition of Pd in this complex was not measured experimentally. The stoichiometry coefficient of Pd (x = 15) was calculated from the initial molar concentrations of the Pd precursor solution. We have not incorporated the molecular formula based on the initial concentration of the ingredients because we could not confirm the stoichiometric addition of the reactants. Apart from the molecular identity, the structural morphology of the metal fullerides is also quite debatable. It was suggested by a few researchers11,17,21,30,3739 that one Pd atom is required to form intermolecular connections in a polymer PdC structure as one-dimensional, two-dimensional, or even multidimensional structures like the rhombohedral polymer. However, in our work, we did not observe the possible polymer configuration in the TEM images shown in Figure 1. Moreover, we can confirm our conclusion by analyzing the Ag(2) mode on the Raman spectra because it is the main signature of the fullerene molecule. A downshift of the Ag(2) peak could also possibly give the polymer formation. For the two-dimensional polymer, the downshift could go to ∼1449 cm1; for the rhombohedral structure, it even goes to ∼1407 cm1. A possible split of the Ag(2) mode also indicates the polymer formation. In our study, as the location of the Ag(2) mode is on the higher side (PdxC60 ∼ 1466 cm1, RuxC60 ∼ 1456 cm1) and no peak split was observed, we can rule out the possibility of polymer formation. The destruction of the fullerene cage and lowering of cage symmetry are the key causes for the intensity decrease and peak broadening as explained by Wang and Tu.30 We also proposed similar amorphous structure in this paper, which is also supported by the drastic change in the XRD pattern of the metalfullerene complex compared to that of pure fullerene. The amorphous nature of the PtC60 complex was also reported by Herbst et al.17 It was stated by Wang and Tu30 that the exact role of Pt doping on the mode intensity was unclear. From the above discussion, we can find the disagreement in structural information of the PtC60 complex reported in the literature.17,21,30 3.4. Hydrogen Adsorption Study. Hydrogen sorption results at 298 K and 300 bar are shown in Figure 4. Pure fullerene adsorbs ca. 0.3 wt %. The sorption amount increased for both metal complexes; RuC60 complex adsorbs ca. 0.69 wt %, whereas PdC60 adsorbs ca. 0.85 wt % at the terminal pressure (300 bar). These adsorption amounts by metalfullerene complexes are higher than the H2 sorption shown by ball-milled pure C60 fullerene (0.7 wt %)40 at 300 bar and ambient temperature or the reversible storage capacity PtC60 compound at 20 bar pressure and within 200 °C (0.50.6 wt %) temeprature.30 All the sorption isotherms in our work are of typical type III according to IUPAC classifications. It is also noticeable that the adsorption amount was negligible below 100 bar, after which it began to increase almost exponentially. It is worthwhile mentioning that type III isotherms are the essential characteristics of weak interactions between adsorbate and adsorbent. The initially very small amount of adsorbate that accumulates on the surface of the adsorbent was due to the weak interactions between them, after which the cohesive force between the adsorbate molecules assists in building up the remaining adsorption amounts.
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Figure 4. Hydrogen sorption isotherms on pristine C60 fullerene, PdC60, and RuC60.
Hydrogen adsorption in the pure fullerene can be attributed solely to the octahedral fcc lattice or the possible crystal defect sites (like grain boundary) of C609 as the hydrogenation of the pure fullerene cage is not quite feasible at the ambient temperature. The (enhanced) hydrogen uptake in the metalfullerene complex can no longer be contributed the fcc lattice, as the crystal itself was destroyed during the metal doping as observed in the XRD patterns in Figure 3. We suggest four possible mechanisms to account for the hydrogen adsorpotion enhancement in the metalfullerene complex. First, it is quite possible that several defect sites have been created in metalfullerene matrix during the metal doping process as the key crystalline shape of C60 was destroyed. Many of these defect sites that have micropores larger than the hydrogen molecule can easily accommodate H2 thereby contributing to the higher hydrogen adsorption amount. Second, the hydride formation of metals at the experimental conditions is feasible as is evident from the previously published results.6,7 It has been proven experimentally that hydrogen dissolves in the crystalline lattice of Pd thereby forming two varieties of nonstoichiometric hydrides PdH0.02 (R-phase) and PdH0.67 (β-phase).7 It was also demonstrated that a transition from R to β hydride takes place within 300 to 500 mbar pressure at ambient temperatures. As our experimental work was performed at a pressure much higher than 500 mbar, contribution from β-phase is possible. However, we could not observe hydrogen uptake below 100 bar, which could be attributed to the fact of a very low hydrogen uptake on both carbon surface and a very small amount of free metallic nanoparticles. The best method to examine the formation of metallic hydride is to perform an in situ XRD study under a hydrogen flow. We could not perform this study due to lack of this special XRD instrument. So far, there is no experimental evidence on hydrogenruthenium hydride formation; however, by comparing the similarity between Ru and Pd, we can speculate that ruthenium could form stoichiometric or nonstoichiometric hydrides at our experimental conditions and count toward the enhancement of H2 uptake on the RuC60 complex. 6783
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Table 1. Isotherm Model Parameters model isotherm models
parameters
modified Oswin
A
0.023
B
1.306
A
2.3 104
B
0.082
Xm
0.012
K
1.124
C Xm
56.601 31.187
modified Peleg modified GAB
modified BET
C n Freundlich
pure C60
% ARE pure C60 0.78
n
0.211
% ARE RuC60
0.94
6.87 103
3.64
2.555
1 104
2.59
6.96
0.235 6.85
0.034
18.1
2.34
5.88 104 2.26
0.283
k and n constants and can be found by the linear plots ln p versus ln q.
16.59
56.609 31.187
3.94
403.3
1.59 109
ð1Þ
0.034
5.88 104
403.3 1.57
5.12
1.104
56.607 31.187
0.89
1.54 103 0.199
1.123
Spillover of hydrogen from the metal sites to the carbon substrate is the third factor leading to the increased hydrogen adsorption on the metalfullerene complex. It is well-known that Pd and Ru surfaces can dissociate molecular hydrogen into its atomic form. The spilt-over atomic hydrogen then diffuses to the different sites of the host matrix and gets adsorbed. As explained earlier, the generation of crystal defect sites in the metalfullerene complex during metal doping is quite possible. Any of these sites, if smaller than molecular hydrogen but larger than atomic hydrogen, can accommodate the split-over hydrogen and increase the overall hydrogen uptake capacity. An explicit bond formation between hydrogen atom and active sites of the carbon matrix is the fourth mechanisms for the hydrogen uptake enhancement and an indirect contribution from the hydrogen spillover. Contescu et al.8 confirmed with the help of inelastic neutron scattering that CH bonds had been formed between atomic hydrogen and the active sites of activated carbon fiber doped with palladium. On the basis of this observation, we can suggest that bond formation between the split-over hydrogen and the carbon sites of remaining C60 cage was another possible factor for increasing the hydrogen adsorption amounts. There are four possible factors that can be counted toward enhancing hydrogen adsorption in the metalfullerene complex. We believe that hydrogen adsorption on the defect carbon sites formed during the synthesis procedure plays a key role as observed in our previous study.10 The spillover of atomic hydrogen from the metal sites to the defect sites and a possible bond formation between hydrogen and metal also contribute to the increase of hydrogen adsorption in these metalfullerene adsorbents. It would be interesting and necessary to find suitable adsorption isotherm models to describe the typical type III isotherms of hydrogen adsorption on C60 fullerene and metal-C60 compounds. In order to fit these isotherms, we used five different equations and examined their degrees of fit. The first equation is a very well-known Freundlich equation, given by q ¼ kp1=n
RuC60
1.270
403.3 6.55 1013
% ARE PdC60
0.069
2.08 104
K
PdC60
5.78 109
1.5
0.306
Four other equations are Modified Oswin, Peleg, GAB, and BET equations. All these equations were originally developed for type III isotherms, mostly for the adsorption of water vapor or other biological samples on carbon and other adsorbents at ambient conditions.41 These equations are modified to fit the high pressure data in our experiment and are given below. Modified Oswin !B p=pmax q¼A ð2Þ 1 p=pmax Modified Peleg q ¼ A B lnð1 p=pmax Þ
ð3Þ
Modified GAB q¼
ðc 1Þxm Kp=pmax xm Kp=pmax þ 1 þ ðc 1ÞKp=pmax 1 Kp=pmax
ð4Þ
Modified BET q¼
xm pC½1 ðn þ 1Þðp=pmax Þn þ nðp=pmax Þðn þ 1Þ ð1 p=pmax Þ½1 þ ðC 1Þp Cðp=pmax Þðn þ 1Þ
ð5Þ
where xm is the BET and GAB monolayer hydrogen content and A, B, C, and K are all the constants. All the parameters are calculated by the nonlinear regression technique. The degree of fit of each parameter is estimated by absolute relative error (ARE) percent calculated as , q q cal ð6Þ ARE ¼ N q where q is the experimental adsorption amount, qcal is calculated adsorption amount using the models, and N is the number of adsorption points. The model parameters and ARE values are given in Table 1 and model fitting plots are given Figure 5A,B,C for pristine C60, PdC60, and RuC60, respectively. The correlation results shown in Figure 5 suggest that the Freundlich equation is the best fit for the hydrogen isotherm of RuC60, but the modified Oswin model fits best for those on the PdC60 and the pristine C60. 6784
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drastic decrease in peak intensity indicates the massive deterioration of the main cage of C60. The XRD patterns revealed that both PdC60 and RuC60 compounds are amorphous in nature as the fcc crystal shape was destroyed. The XRD study also revealed that the peaks from pure metallic Pd and Ru also prevail in the overall pattern of the resultant species. Hydrogen sorption measurements showed the enhancement of hydrogen uptake on both metalfullerene complexes: 0.85 wt % (PdC60) and 0.69 wt % (RuC60) as compared to 0.3 wt % of pristine C60. The hydrogen adsorption enhancement can be attributed to hydrogen adsorption on the defect carbon sites, the formation of metallic hydrides, the spilling-over of hydrogen from the metallic surface, and the explicit bond formation between the atomic hydrogen and the active carbon sites of the host fullerene lattice. All the hydrogen sorption isotherms that are type III in nature were modeled with five adsorption isotherm equations. The Freundlich model is the best for fitting the hydrogen isotherm on RuC60, and the modified Oswin model fits best for the hydrogen adsorption isotherms on PdC60 and the pristine C60.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected],
[email protected]; phone: þ1 865 574 0798, þ1 575-646-4346.
’ ACKNOWLEDGMENT The authors appreciate the financial supports provided by the U.S. Army Research Office through a grant W911NF-06-1-0200, the U.S. Department of Energy through a grant DE FC3608GO88008 for maintaining the Renishaw Raman spectroscope. D. Saha acknowledges Bruce Wilson Research Fellowship provided by the Chemical Engineering Department of New Mexico State University. ’ REFERENCES
Figure 5. Modeling of hydrogen adsorption isotherms on C60 fullerene (a); PdC60 (b); RuC60 (c).
4. CONCLUSION In this work, Pd and RuC60 compounds were synthesized by doping 1 wt % of each metal on the pure C60 fullerene followed by hydrogen sorption measurements at 300 bar and 298 K. TEM imaging, Raman spectra, and X-ray diffraction patterns were analyzed to characterize the metal-C60 compounds. The TEM imaging results indicate that the characteristics of palladium onto fullerene are close to those of “particles” unlike ruthenium, where metal formation is close to a “layer” type. In the Raman spectra, the remainder of only the Ag(2) mode (for PdC60) and Ag(2), Hg(1), Ag(1), and Hg (4) modes (for RuC60) along with a
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dx.doi.org/10.1021/la200091s |Langmuir 2011, 27, 6780–6786
