Enhanced Hydrogen Spillover on Carbon Surfaces Modified by

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J. Phys. Chem. C 2010, 114, 1601–1609

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Enhanced Hydrogen Spillover on Carbon Surfaces Modified by Oxygen Plasma Zhao Wang, Frances H. Yang, and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: October 2, 2009; ReVised Manuscript ReceiVed: NoVember 12, 2009

Hydrogen storage capacity by hydrogen spillover on metal-doped carbons can be significantly enhanced by introducing surface oxygen functional groups to the carbon. It is shown that the enhancement is greater by oxygen plasma treatment compared to air oxidation because different surface groups were formed. Oxygen plasma treatment generated mainly semiquinone (CdO) groups while air oxidation formed mainly hydroxyl (C-OH) groups. Experimental heats of adsorption, X-ray photoelectron spectroscopy (XPS) analyses, and ab initio molecular orbital calculations showed that the binding energies between the spiltover hydrogen and the different groups followed the following order: lactone > semiquinone > carboxyl > basal plane. Thus, the H2 storage capacity at 298 K and 10 MPa was increased from 1.17 wt % (without O2 treatment) to 1.74 wt % on Pt-doped on a templated carbon that was pretreated with O2 plasma. Similar enhancements were seen on Pt doped on a superactivated carbon, AX-21. However, there was a decrease in storage capacity during the first three adsorption-desorption cycles (at 298 K and 10 MPa). The capacity decreased from 1.74 wt % to 1.30 wt % and remained unchanged after three cycles. XPS results showed that this decrease was caused by the very strong (and irreversible) binding of the spiltover hydrogen with the lactone groups (HO-CdO). Nonetheless, the main groups of semiquinone remain functional as receptor sites upon cycling, and the 1.3 wt % storage capacity is among the highest reversible capacities reported in the literature. 1. Introduction Hydrogen has been considered as one of the potential clean fuels particularly for on-board applications. One of the remaining barriers to the utilization of hydrogen energy is an efficient and inexpensive means of hydrogen storage.1 The U.S. Department of Energy (DOE) has established a comprehensive set of targets for hydrogen storage that are able to store 4.5 wt % (28 g L-1) H2 by 2010 and 5.5 wt % (40 g L-1) H2 by 2015 at moderate temperatures and pressures.2 Many investigations have been conducted to search for good hydrogen storage materials.3-7 Recently, some groups have focused on using the phenomenon of hydrogen spillover to develop sorbents for ambient temperature storage.8-16 The phenomenon of spillover has been studied for nearly a half century. Hydrogen spillover is defined as the transport of an active hydrogen species from metal nanoparticles onto adjacent surfaces of a receptor via spillover and surface diffusion.17-19 To increase hydrogen spillover, it is important to maximize the contacts between metal nanoparticles and receptor. A high surface area is also required. Modification of the surface of the receptor to increase its binding energy with the spiltover hydrogen is a promising approach that we have taken recently. Nanostructured and porous carbon materials including carbon nanotubes (CNTs), graphite nanofibers, activated carbon (AC), and templated carbon (TC) have been of considerable interest because of their light weight, high surface areas, and chemical stabilities.20,21 Spillover of hydrogen over transition metals doped on carbon was studied.22-25 The capacity of these carbon materials is limited by both surface area and the surface properties of the carbon support. Numerous attempts have been made to change chemical and physical properties of carbon for improving metal-support interactions.26,27 Plasma techniques * To whom correspondence should be addressed. Fax: (734) 764-7453. E-mail: [email protected].

are useful and effective in preparing highly dispersed catalysts as well as improving the surface properties of carbon.28-34 There are two principal advantages in using plasma treatment to modify materials. One is that the reaction takes place only on the material surface without changing its bulk properties. The other is that it is easy to achieve the desired modification by changing the gas atmosphere. Kim et al.28 used microwave nitrogen plasma treatment on carbon black to modify its surface properties. After the plasma treatment, new basic functional groups, including C-N, CdN, -NH3+, -NH, and )NH, were formed. Kodama et al.29 reported results on a carbon treated by oxygen dielectric barrier discharge to enhance adsorbability for metal ions. Wang et al.30 prepared well-dispersed metal nanoparticles with a narrow size distribution by glow discharge plasma. The heteroatoms in doped carbon could increase the interactions between the receptor and hydrogen and thus lead to enhanced hydrogen adsorption.35-37 Adding surface oxygen functional groups on carbon is an easier way to modify the surface. In a hydrogen spillover system, the presence of oxygen groups leads to stronger interactions for the spillover hydrogen, and thus an enhanced storage capacity can be obatined on an oxygen-modified carbon receptor.38 There have been two studies on plasma-treated carbon for hydrogen spillover.38,39 In this work, we report significant enhancements in hydrogen storage on oxygen plasma treated carbon via spillover. The cause for the significant enhancement was identified both by X-ray photoelectron spectroscopy (XPS) analyses and density functional theory (DFT) calculations. 2. Experimental Methods 2.1. Sample Preparation. Templated Carbon (TC). TC derived from Na-Y zeolite (powder, Aldrich 334448) was prepared according to a procedure of Ma et al.40 Na-Y zeolite was placed in a flask and was kept at 473 K for 12 h under

10.1021/jp909480d  2010 American Chemical Society Published on Web 01/04/2010

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Figure 1. Schematic of plasma treatment setup.

vacuum. Furfuryl alcohol (FA) was introduced into the flask at a reduced pressure. The mixture of zeolite and FA was stirred for 8 h under a N2 flow and then was filtered followed by washing with mesitylene to remove FA on the external surface of zeolite powder. The polymerization of FA in zeolite was carried out by heating the composite under a flow of He at 353 K for 24 h and then at 423 K for another 8 h. The zeolite/ polyFA composite was placed in a quartz tube and was heated to 973 K at a rate of 5 °C/min under a N2 flow. When the temperature reached 973 K, propylene gas (2% in N2 by volume; flow rate 150 cm3/min) was passed through the tube for 6 h. After the chemical vapor deposition (CVD) treatment, the composite was further heated at 1173 K for 2 h under a flow of N2. The obtained zeolite/carbon composite was treated in HF solution (46%) for 4 h and subsequently was refluxed by concentrated HCl solution for 4 h at 333 K to dissolve the zeolite template. The solution was filtered, and the insoluble carbon was washed with copious quantities of water. Plasma Treatment. The TC was treated with glow discharge plasma. The plasma setup is shown in Figure 1. The sample (about 0.2 g) was loaded on a quartz boat that was placed in the glow discharge cell. A few drops of water were added to the sample prior to ach treatment with the intention of preventing entrainment of the carbon particles to the vacuum pump. When the pressure was adjusted to the range 100-200 Pa, the glow discharge plasma was generated by applying 5000 V to the electrodes using a DC high voltage generator (DL-200, Tianda Cutting and Welding Setup Inc. Ltd., China) with oxygen as the plasma-forming gas. The details of plasma equipment and the treatment procedure have been described previously.41,42 The time of each plasma treatment was 10 min, and each sample was treated six times. The sample prepared this way was designated as TC-O. The AX-21 (AX-21 superactivated, obtained from Anderson Development Company) was treated by using a similar procedure to that for TC. The sample prepared this way was designated as AX-21-O. Platinum (6 wt %) Doped on TC and AX-21. Pt/TC: Typically, 200 mg of well-dried TC was dispersed in 20 mL of deionized water and was stirred for 0.5 h in a 125 mL Erlenmeyer flask at room temperature. A 2 mL water solution containing 26 mg H2PtCl6 (Aldrich, 99.9%) was slowly added dropwise to the above solution under vigorous agitation for about 10 min. The mixed slurry was subjected to ultrasonication (100 W, 42 kHz) at room temperature for 1 h, and then the sample was reduced by slowly adding NaBH4 (as a reducing agent) under vigorous agitation. After that, it was magnetically agitated at room temperature for 24 h. Finally, the suspension was filtered, was copiously washed with water, and was dried at 333 K overnight. Prior to isotherm measurements, the sample was degassed at 393 K. This method of doping was different from the method that was used previously10,16,21,25 where the

Wang et al. doped sample was reduced by H2 at 300 °C after doping and where NaBH4 was not used. Pt/TC-O: Pt/TC-O was prepared by using a similar procedure to that for Pt/TC, but the support was oxygen plasma treated TC. Pt/AX-21: Pt/AX-21 was prepared by using a similar procedure to that for Pt/TC. Pt/AX-21-O: Pt/AX-21-O was prepared by using a similar procedure to that for Pt/TC-O. Here, AX-21-O denotes O2 plasma treated AX-21 carbon. 2.2. Characterization. High-resolution transmission electron microscopy (HRTEM) images of the samples were obtained on a JEOL 3011 analytical electron microscope equipped with EDX (energy-dispersive X-ray spectroscopy) analysis operated at 300 kV. X-ray photoelectron spectroscopy (XPS) was recorded on a Kratos Axis ultra XPS spectrometer. Nitrogen adsorption and low-pressure H2 adsorption isotherms (0-1 atm) were measured with a standard static volumetric technique (Micromeritics ASAP 2020). Hydrogen adsorption isotherms at 298 K and pressures greater than 0.1 MPa (up to 10 MPa) were measured by using a static volumetric technique with a specially designed Sievert’s apparatus. The apparatus was previously proven to be leak-free and accurate through calibration by using LaNi5, AX-21, zeolites, and MOFs at 298 K.47 Approximately 200 mg of sample was used for each high pressure isotherm measurement in this study. Before measurements, the samples were degassed in vacuum at 473 K for at least 12 h. 3. Computational Details The Gaussian 03 package43 and Cerius2 molecular modeling software44 were used for all molecular orbital (MO) calculations. Density functional theory (DFT) calculations were performed with p function added in the basis set, thus, B3LYP/6-31g(d,p) was used for geometry optimization, self-consistent field energy (SCF), and single point energy (SPE) calculations. Bond Energy Calculations. To understand hydrogen spillover on oxygen-modified carbon surfaces, ab initio molecular orbital calculations were performed for the binding energies between the spiltover hydrogen atom and various sites on the oxygen-modified carbon. Frequency analysis was used to verify that all geometryoptimized structures were true minima on the potential energy surface. The optimized structures were then used for bond energy calculations according to the following expression:

Eads ) Eadsorbate + Eadsorbent - Eadsorbent-adsorbate where Eadsorbate is energy of free adsorbate, Eadsorbent is energy of free adsorbent, and Eadsorbent-adsorbate is energy of the adsorbate/ adsorbent system. A higher value of Eads corresponds to a stronger adsorption. 4. Results and Discussion Porosity Analysis. Figure 2a shows the N2 adsorption isotherms at 77 K on both unoxidized and O2 plasma oxidized carbons. All samples exhibited type I isotherms according to the BDDT (Brunauer-Deming-Deming-Teller) classification45 indicating that they were microporous materials. However, the adsorption of the N2 was influenced by plasma oxidation. The oxidation treatment led to a decrease in porosity, as is confirmed by the decrease in N2 adsorption at low relative pressures, and a decrease in the SBET value (see Table 1). Pore size distribution (PSD) was calculated from the adsorption branch of the isotherm

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Figure 2. (a) N2 isotherms at 77 K and (b) pore size distribution (PSD) based on DFT method for TC (9), TC-O (via O2 plasma) (0), AX-21 (b), and AX-21-O (via O2 plasma) (O).

TABLE 1: Surface Areas and Pore Volumes of Carbons sample

SBET (m2 g-1)

Vtotal (cm3 g-1)

Vmicro (cm3 g-1)

AX-21 AX-21-O TC TC-O

2880 2541 3554 3110

1.27 1.15 1.53 1.38

0.7 0.6 1.2 1.0

using the density functional theory (DFT) method, and the results are shown in Figure 2b. The results are consistent with PSD for activated carbons with a significant quantity of micropores and some mesoporosity.46 Despite this decrease in micropore volume, the PSD maintained a similar shape after oxidation. The properties of both untreated and O2 plasma oxidized carbons are summarized in Table 1. Hydrogen Storage at 298 K and 10 MPa. The high-pressure H2 isotherms for both adsorption and desorption branches were measured with a Sievert’s apparatus. Details of the system are given elsewhere.47 High-pressure hydrogen isotherms at 298 K for Pt/TC and Pt/AX-21 samples are presented in Figure 3. As shown in Figure 3, Pt/TC had a hydrogen storage capacity of 1.17 wt % at 298 K and 10 MPa, while that for the Pt/AX-21 sample was 0.98 wt % at 298 K and 10 MPa. For comparison, the Pt/AX-21 (6 wt % Pt) doped by using H2 reduction at 300 °C after doping (rather than using NaBH4 as a reducing agent during doping) had a consistently higher capacity of 1.2 wt % storage at 298K and 10 MPa.16,21,25

Figure 3. High-pressure hydrogen isotherms at 298 K for 6 wt % Pt doped on various carbons. (a) Adsorption (9) and desorption (0) on Pt/TC-O (via O2 plasma); adsorption (2) and desorption (∆) on Pt/TC (not oxidized). (b) Adsorption (9) and desorption (0) on Pt/AX-21-O (via O2 plasma); adsorption (2) and desorption (∆) on Pt/AX-21 (not oxidized).

On the Pt doped on O2 plasma treated carbons, Pt/TC-O and Pt/AX-21-O, the hydrogen uptakes at 10 MPa were enhanced to 1.74 and 1.48 wt %, respectively. It can be seen that all of the plasma-treated samples had much higher hydrogen adsorption capacities. The enhanced hydrogen storage capacity could not be attributed to the differences in surface areas or pore sizes because the plasma-treated samples had lower surface areas and similar pore size distributions. Clearly, the surface oxygen groups on Pt/AX-21-O and Pt/TC-O were responsible for this enhanced capacity. For the Pt-doped carbons without preoxidation, the isotherms were fully reversible, that is, the adsorption and desorption branches were the same. For the Pt-doped carbons that were preoxidized by O2 plasma treatment, however, significant losses in the capacity during the first two adsorption-desorption cycles were seen. The largest drop in the storage capacity was seen on Pt/TC-O. At 298 K and 10 MPa, the capacity dropped from 1.74 wt % in the first cycle to 1.39 wt % in the second cycle and to 1.30 wt % in the third cycle while maintaining at that level beyond the third cycle. This result was caused by the strong bindings between the spiltover hydrogen and the surface lactone groups, and it will be further discussed with the XPS and DFT

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Figure 4. XPS survey spectra of carbon, untreated and that treated with oxygen plasma. (a) AX-21; (b) AX-21-O; (c) TC; (d) TC-O; (e) Pt/TC-O-fresh; (f) Pt/TC-O-4th (after four adsorption-desorption cycles).

calculation results. Nonetheless, the reversible capacity (of 1.3 wt %) is still among the highest reproducible capacities among all known sorbents. XPS. The XPS spectra of activated carbons usually show two distinct peaks from carbon (C1s) and oxygen (O1s) as shown in Figure 4. During the plasma treatment process, active plasma oxygen species effectively react with the surface of carbon generating oxygen-containing functional groups. Therefore, the ratio O1s/C1s represents a measure of describing the degree of oxidation. The results in Figure 4 show that the amount of oxygen increased substantially upon O2 plasma treatment. Thus, the surface oxygen content increased from 2.74% to 30.66% on AX-21 and from 5.5% to 29.15% on TC-O. To analyze the chemical bonds of the functional groups, the C1s spectra were deconvoluted into four individual component peaks29,48 as shown in Figure 5. It shows XPS results of untreated and O2 plasma treated carbons. Although the total strength of C1s was reduced, the content of the carbon in the semiquinone groups (CdO) was sharply increased by O2 plasma treatment from 4.88% to 23.19% for AX-21 carbon and from 3.77% to 25.76% for TC carbon. Meanwhile, the intensities of the peaks because of surface hydroxyl and phenolate groups (COH) and carboxylic groups (COOH) showed only slight increases by O2 plasma treatment. The data are given in Table 2.

Figure 5. XPS spectra of C1S of the carbon (a) AX-21; (b) AX-21-O; (c) TC; (d) TC-O; (e) oxygen-plasma-treated AX-21-10O; (f) airoxidized AX-21-10O; (g) Pt/TC-O-fresh; (h) Pt/TC-O-4th (after four adsorption-desorption cycles).

To analyze the influence to hydrogen storage by different surface oxygen groups, two samples were prepared: one AX21 carbon was oxidized by air at 473 K for 15 h and the other AX-21 sample was subjected to O2 plasma treatment. The reaction/treatment conditions were adjusted such that approximately equal oxygen content was obtained on the two samples. Thus, XPS showed that the oxygen content increased to 10.2% oxygen groups on the carbon sample by air oxidation (designated AX-21-AO10 by air oxidation to 10% O). For the other sample, AX-21 was treated by oxygen plasma for 10 min, and XPS showed that it increased to 10.8% oxygen groups on the sample (designated AX-21-PO10 by plasma oxidation).

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TABLE 2: XPS Relative Subpeak Area of C1s of the Untreated and Oxygen-Plasma-Treated Carbon carbon sample

C-C (284.6 eV)

C-OH (286.1 eV) hydroxyl

CdO (287.6 eV) semiquinone

HO-CdO (289.6 eV) lactone

untreated AX-21 O-plasma AX-21 untreated TCa O-plasma TC AX-21-PO10b AX-21-AO10c Pt/TC-O-freshd Pt/TC-O-4the

88.90% 67.30% 89.43% 65.08% 79.87% 77.73% 64.22% 67.53%

3.70% 6.90% 4.61% 5.72% 4.24% 13.84% 6.56% 6.37%

4.88% 23.19% 3.77% 25.76% 13.37% 5.32% 25.22% 25.13%

2.52% 2.61% 2.19% 3.44% 2.52% 3.11% 4.0% 0.97%

a TC, templated carbon. b Plasma-oxygen-treated AX-21 carbon, for 10 min, with 10.8% oxygen content. c Air-oxidized AX-21 carbon, 200 °C for 15 h, with 0.2% oxygen content. d Templated carbon first treated by oxygen plasma and then doped with Pt (with NaBH4). e Same as d except the sample was subjected to four H2 adsorption-desorption cycles (to 10 MPa at 298 K).

Figure 6. High-pressure hydrogen isotherms at 298 K for Pt/AX-21: plasma-treated Pt/AX-21-PO10 (b); Pt/AX-21-CO10 (9); and Pt/AX21 (2).

High-pressure hydrogen isotherms at 298 K for the two samples are shown in Figure 6 along with a sample without any oxygen treatment (Pt/AX-21). The hydrogen uptake was increased by oxygen treatment. Compared to the air-oxidized sample, the hydrogen uptake at 10 MPa by the plasma-oxidized sample was the highest, at 1.32 wt %, compared to 1.11 wt % on the air-oxidized sample. The plasma-treated sample showed significantly higher hydrogen adsorption capacities than the airoxidized sample, although both had similar oxygen contents. The different surface oxygen groups on Pt/AX-21 were responsible for this enhanced capacity. As shown in Figure 5e and 5f, in the case of the plasma-treated AX-21, the amount of semiquinone groups (CdO) was 13.37%, showing higher intensities than the other surface oxygen groups. Compared to the plasma-treated AX-21, air oxidation increased mainly the concentration of the C-OH functional groups on the AX-21 surfaces. This could be because plasma could easily remove weak or unstable bonds on carbon surfaces. The semiquinone groups played a very significant role in improving hydrogen spillover and storage. There are no hydrogen bonds on the oxygen of semiquinone, so it is available to capture the spiltover hydrogen atoms. Further discussion will be given in our molecular orbital calculation results. As mentioned, the Pt-doped templated carbon that was first treated by O2 plasma (before doping) suffered a significant loss of hydrogen storage capacity during the first two adsorptiondesorption cycles (at 298 K and up to 10 MPa). The capacity

decreased from 1.74 wt % to 1.3 wt % after three cycles and remained at 1.3 wt % after three cycles. To understand the origin for this loss, a fresh sample and a sample that was subjected to four H2 adsorption-desorption cycles (to 10 MPa at 298 K) were studied with XPS. The results are compared in Figure 5g and 5h and are also summarized in Table 2. It is clear that the lactone groups (HO-CdO) were the only groups that were reacted during adsorption. This result will be further discussed along with molecular orbital calculation results. Low-Pressure H2 Isotherms. Figure 7 shows the lowpressure H2 isotherms on the Pt-doped carbons with and without oxygen plasma treatment. Oxygen plasma treatment resulted in significantly increased H2 adsorption. From the adsorbed amount of hydrogen extrapolated to zero pressure, the dispersion of Pt metal on AX-21 could be calculated according to the method of Benson-Boudart.49 Using the assumption of 1 H per surface Pt atom, the metal dispersion was 64, 153, 75.4, and 202% for Pt/AX-21, Pt/AX-21-O, Pt/TC, and Pt/TC-O, respectively. The dispersion exceeded 100% on the oxygen plasma treated carbon likely because of enhanced spillover over oxygen bridges.50 For carbon support, even with pure H2, spillover leads to overestimates of dispersion by the Benson-Boudart method, and any small amounts of gaseous impurities could lead to much greater overestimates.50 The results provide evidence that surface oxygen groups play the role of a stepping stone or bridge for spillover between Pt and carbon as well as between different domains/grains of carbon within each carbon particle. Transmission Electron Microscopy (TEM). TEM images of the Pt/TC and Pt/TC-O samples are shown in Figure 8. The nanosized particles of Pt on Pt/TC-O were 2-4 nm in size. They were smaller and more dispersed than that on Pt/TC in agreement with the Benson-Boudart results. One possible reason for this result is that more surface oxygen groups can enhance the wettability of carbon during doping thereby increasing the adsorption of metal ions.51 Apparent or Overall Heats of Adsorption. Use of the Clausius-Clapeyron equation would yield the overall heats of adsorption. The overall heats of adsorption of H2 on the Pt/TC, Pt/TC-O, Pt/AX-21, and Pt/AX-21-O samples were calculated from the H2 isotherms at 298 and 323 K by using the Clausius-Clapeyron equation as shown in Figure 7.

( )

∆Hads ) R

d ln P d(1/T)

(1)

n

The isosteric heats of adsorption were determined by evaluating the slope of the plot of ln(P) versus (1/T) at the same adsorption amount. It can be seen that the H2 adsorption amounts

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Figure 7. Low-pressure H2 adsorption isotherms for Pt/carbon at 298 and 323 K. (a) Pt/TC-O; (b): Pt/TC; (c) Pt/AX-21-O; (d) Pt/AX-21. Inset: calculated isosteric heats of adsorption (O).

at all pressures up to 1 atm decreased with an increase in temperature. Figure 7d shows that the absolute values of heat of adsorption decrease sharply with adsorption amount for each sample. The heats of adsorption on Pt/TC-O and Pt/AX-21-O were greater than on Pt/TC and Pt/AX-21. The higher values of heats of adsorption on oxygen plasma treated carbon can be attributed to the stronger binding of H atoms on the oxygen group as will be discussed further with the molecular orbital calculation results. Hydrogen can also be adsorbed strongly on defect and edge sites on carbon.52-54 The plasma process could create defective sites and edge sites. This could also lead to stronger binding with the spillover hydrogen. As shown in Figure 7, the heats of adsorption at high H2 loadings on the Pt/TC-O, Pt/AX-21-O, Pt/TC, and Pt/AX-21 samples were, respectively, 13, 11.5, 9.6, and 8.1 kJ/mol. The relative higher heat of adsorption on Pt/C-O than on Pt/C suggests more H atoms favorably bonded to oxygen group, which is in agreement with high-pressure hydrogen adsorption results. Further discussion will be given along with molecular orbital results. Adsorption Rates and Apparent Activation Energy for Surface Diffusion. The surface diffusion of atomic hydrogen is likely the rate-determining step for hydrogen adsorption by spillover. To understand the mechanism, activation energies (∆E) for surface diffusion (via spillover rates) were obtained for Pt/TC and Pt/TC-O. The ∆E values for spillover were calculated from the temperature dependence of the uptake rates. The uptake rates of Pt/TC and Pt/TC-O at various temperatures

in this pressure range were measured; the results for one step are shown in Figure 9. The rates for both Pt/TC and Pt/TC at 298 K are high at low pressures. From these rate data, estimates of the surface diffusion time constants D/R2, where D is the surface diffusivity and R is an average radius of diffusion for spillover, were first made. To obtain the activation energy for surface diffusion, the following temperature dependence can be correlated by the Eyring equation

( )

D ) D0 exp -

Ea

RT

(2)

where R′ is the gas constant, T is the absolute temperature, and Ea is the difference in energy between the states corresponding to adsorption at the ground vibrational level of the bond and to free mobility on the surface. Thus, plots of log(D/R2) versus 1/T yielded the activation energies. The results for ∆E values are given in Figure 10. The values of ∆E were increased by plasma treatment. This result indicates the high coverage of hydrogen resulting in an increase in the active energy of surface diffusion. As shown in Figure 11, the uptake rates for high-pressure ranges (>1 atm) on Pt/TC-O were lower than that on Pt/TC. These results are in agreement with the ∆E data. Molecular Orbital Calculation Results. As mentioned, the spillover storage process involves chemisorption of hydrogen

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Figure 8. TEM images of Pt/TC (a) Pt/TC; (b) Pt/TC-O.

on Pt followed by migration of H atoms to the carbon surface. Hydrogen atoms must be mobile on both metal and carbon surfaces at ambient conditions. Although atomic hydrogen is chemisorbed to platinum, Pliskin and Eischens observed weak forms of atomic hydrogen bound to platinum using infrared spectroscopy.55 Volumetric studies of platinum catalysts56 and, more recently, molecular simulation results have supported this observation demonstrating that atomic hydrogen is mobile on the platinum surface and migration from the source to the receptor is entirely possible at ambient conditions with a small energy barrier.14,24 Likewise, the H atoms are also mobile on the carbon surfaces as shown by our recent experimental results as well as by others.17,22,23 The mobility of H atoms on carbon at ambient conditions has also been shown by molecular simulation results by using the basal plane of graphite as the carbon surface.14,24,57 It is known that hydrogen spillover involves several complex steps: dissociation on the metal, migration of H from metal to carbon, migration on carbon, recombination on carbon, and binding on carbon sites. However, complete simulation of the spillover process has not been done; simulations of some of the steps above have provided insights into the spillover process.14,24,57 The calculation results for H atoms binding on different graphene sites only represent the last step of the spillover process. Without a complete simulation of the spillover process, it is not possible to directly relate the binding energy from the calculation and the overall heats of adsorption that are obtained from the temperature dependence of the isotherms. However, it is clear that they are related, that is, a stronger binding energy leads to a higher overall heat of adsorption. Thus, we performed calculations on the binding energies of H on the basal plane of graphite as well as on oxygen-modified carbon with various oxygen functional groups. The calculation results

Figure 9. Hydrogen adsorption kinetics at different temperatures on the samples (P ) ∼80 Torr): (a) Pt/TC; (b) Pt/TC-O.

Figure 10. Determination of activation energy: plot of ln(D/R2) vs T-1 at pressure of ∼80 Torr.

should provide an indication whether oxygen modification will lead to higher heats of adsorption, or more favorable adsorption, and, more importantly, which oxygen functional groups (and by what method of oxidation) would yield higher binding energies.

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Wang et al. TABLE 3: Average Bond Lengths [Å] and Angles [°] of Model A Compared to Experimental Data of Graphite model A exptl data56

C-C

C-H

C-C-C

C-C-H

1.42 1.42

1.09 1.07

120 120

120 120

TABLE 4: Calculated Binding Energy Eads for H on Basal Plane, Semiquinone, and Carboxyl Functional Groups graphite and oxygen groups

Eads (kcal/mol)

B (H on central basal site of model A) E (H on semiquinone oxygen, model D) G (H on carboxyl oxygen of model F) J (H on lactone group of model K)

-15.8 -46.8 -39.3 -62.8

showed that the binding energy was increased significantly on oxidized surfaces and, more importantly, it followed the following order: Figure 11. Rates of adsorption on Pt/TC and Pt/TC-O at 298 K and various pressures. Pressure steps: (a) 0-5.1 atm for Pt/TC; (b) 0-5.1 atm for Pt/TC-O; (c) for Pt/TC at 95.7-100 atm; (d) 95.7-102.8 atm for Pt/TC-O.

lactone > semiquinone > carboxyl > basal plane The oxygen plasma treatment generated more semiquinone groups than air oxidation (Table 2) and led to a higher heat of adsorption (and higher storage capacity). Semiquinone groups were also the main surface groups generated by O2 plasma. The molecular orbital results are in agreement with the experimental results. The binding of H atoms on the lactone group (O-CdO, model I of Figure 12) led to the breakage of a surface C-O bond and the formation of a new surface C-H bond. The binding required a two-step process and was the strongest among all surface oxygen groups. Consequently, the binding energy was the highest. As mentioned, the H2 storage capacity on the Pt-doped templated carbon that was first treated by O2 plasma decreased from 1.74 wt % to 1.3 wt % after three cycles and remained at 1.3 wt % after three cycles (at 298 K and up to 10 MPa). The XPS analyses for a fresh sample and a sample that was subjected to four H2 adsorption-desorption cycles (to 10 MPa at 298 K) showed that only the lactone groups (nearly) disappeared by H2 spillover. This result is clear evidence that the loss in H2 capacity was due to the strong binding and reaction with the lactone groups, and also, the lactone groups represented about 4 wt % storage capacity on a fresh sample. Nonetheless, the most important semiquinone groups remained functional as receptor sites for the spiltover hydrogen during the cyclic adsorption-desorption process.

Figure 12. H binding on different surface oxygen groups on graphite models D (semiquinone), F (carboxyl), and I (lactone) compared to the pristine model A (basal plane).

The molecular structure selected for the graphite models used in this study consisted of five aromatic rings in one single layer, as shown in Figure 12, which is adequate. Selections of molecular system and model chemistry for graphite have been discussed elsewhere.58 The unreactive boundaries for all models were terminated with hydrogen. The optimized structure of the unsubstituted graphite, model A, has parameters of bond distances and angles in good agreement with the experimental values of graphite59 as shown in Table 3. The results of Eads calculations are presented in Table 4 and Figure 12. The results

5. Conclusions In this work, we have investigated oxygen groups generated by oxygen plasma on two high surface area carbons (templated carbon and AX-21 superactivated carbon) and have compared with that by air oxidation. Plasma formed mainly semiquinone while air oxidation formed mainly hydroxyl groups. It was found that the oxygen groups, especially semiquinone group, increased the reversible hydrogen storage capacity significantly. However, there was a decrease in the reversible capacity during the first three adsorption-desorption cycles. On the Pt doped on O2 plasma treated sample, the storage capacity decreased from 1.74 wt % in the first cycle to 1.39 wt % in the second cycle and to 1.30 wt % in the third cycle while maintaining at that level beyond the third cycle. XPS and ab initio molecular orbital

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