Hydrogen Storage Materials Comprising Conjugated Hydrocarbon

Aug 5, 2014 - Hydrogen Storage Materials Comprising Conjugated Hydrocarbon Polymers with LiH: Comparison of Cyclic Durability between LiH–Polyacetyl...
1 downloads 6 Views 321KB Size
Download PDF
Article pubs.acs.org/JPCC

Hydrogen Storage Materials Comprising Conjugated Hydrocarbon Polymers with LiH: Comparison of Cyclic Durability between LiH− Polyacetylene, −Poly(p‑phenylene), and −Poly(diphenylacetylene) and Mechanistic Investigation upon LiH−Poly(p‑phenylene) Akihiro Yoshida,* Yoshinori Mori, Masato Watanabe, and Shuichi Naito*,† Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan S Supporting Information *

ABSTRACT: Hydrogen storage and release properties of composites were compared: lithium hydride with conjugated macromolecules of polyacetylene (PA), poly(pphenylene) (PPP), and poly(diphenylacetylene) (PDPA). Cyclic hydrogen absorption and desorption experiments showed that LiH−PPP has better durability than either LiH−PA or LiH−PDPA, probably because the higher structural stability of PPP prevented unfavorable side reactions that might inactivate LiH. Mechanistic investigations of hydrogen release reactions on LiH−PPP were conducted using isotopic tracer experiments, electrical conductivity measurements, and IR and NMR spectroscopy. Those results indicate that the hydrogen release reaction proceeded via electron transfer from hydride ion to conjugation systems of polymer molecules. This proposed mechanism is identical to those for the LiH−PA composite.

1. INTRODUCTION The development of high-performance hydrogen storage systems is a social necessity for realizing a future hydrogen economy.1,2 Hydrogen storage capabilities of metal hydrides, especially those of lighter metallic elements, have been widely investigated.3 Lithium hydride (LiH) is a potentially useful material for hydrogen storage because it contains exceedingly high gravimetric density of hydrogen (12.7 wt %). However, severe conditions (910 °C at equilibrium pressure of 1 bar) are necessary to release the hydrogen because of thermodynamical restrictions. This property prevents the practical application of LiH. Several reports have described efforts to lower the hydrogen release temperature of LiH by the addition of third elements that modify the thermodynamics by formation of alloys with lithium. Vajo et al. reported the addition of Si to LiH.4 Recently, Ichikawa et al. reported the combination of Ge with LiH.5 Thermodynamic characteristics are improved in these composites by formation of LinM alloys (M = Si and Ge) instead of Li metal at the dehydrogenated state. Addition of graphite is also reported as effective.6 In this case, LiC6 is formed instead of Li metal. More recently, we have improved the thermodynamics of hydrogen release from LiH using a different approach employing a conjugated macromolecule of polyacetylene (PA).7 The composite of LiH and PA prepared using ball-milling released 2.7 wt % of hydrogen at 573 K and reversibly rehydrogenated at 523 K with 3 MPa of hydrogen. Raman spectroscopy, electric conductivity measurements, and isotropic tracer experiments revealed hydrogen storage and release reactions proposed as proceeding via electron transfer between hydride/hydrogen molecule and PA. Thermodynamically unfavorable metallic Li formation might be avoided by the © 2014 American Chemical Society

participation of electron transfer reaction, which enables the release of hydrogen at a much lower temperature than LiH alone. Although the applicability of PA to the improvement of hydrogen release from LiH has already been demonstrated, the applicability of other kinds of conjugated hydrocarbon macromolecules has not yet been investigated. In this work, composites of poly(p-phenylene) (PPP) and poly(diphenylacetylene) (PDPA) with LiH were prepared. Then its hydrogen storage and release properties were compared to those of the composite of PA with LiH.

2. EXPERIMENTAL METHOD 2.1. Material. Benzene, toluene (dehydrated grade), methanol, CuCl2 (anhydrous), AlCl3 (anhydrous), TaCl5, and hydrochloric acid were purchased from Wako Pure Chemical Industries, Ltd. Diphenylacetylene and triethylsilane were purchased from TCI. Benzene-d6 and LiH were purchased from Acros Organics and Aldrich, respectively. Benzene and benzene-d6 were stored with activated MS4A before use. Unless otherwise noted, those reagents were used as purchased. The synthesis of PA was described elsewhere.7 2.2. Synthesis of PPP. PPP was synthesized by oxidative polymerization of benzene applying Cu2+ as a chemical oxidant.8 All procedures were conducted under an argon atmosphere. A 200 mL round-bottomed flask equipped with a balloon was charged with anhydrous CuCl2 (10.1 g, 75.2 mmol) and anhydrous AlCl3 (20.0 g, 150 mmol). Dried benzene (53.2 mL, 0.6 mol) was added to the flask with Received: August 2, 2014 Published: August 5, 2014 19683

dx.doi.org/10.1021/jp507802b | J. Phys. Chem. C 2014, 118, 19683−19687

The Journal of Physical Chemistry C

Article

for 12 h. After cooling to ambient temperature, the remaining hydrogen was released slowly with subsequent evacuation of the reactor. 2.6. NMR, TGA, FT-IR, and Electrical Conductivity Measurements. Solid state 13C NMR measurements were conducted using a spectrometer (JNM-270; JEOL) equipped with a 5 mm MAS probe. A sample was placed in an O-ringsealed sample tube. Spectra were recorded at 67.8 MHz with 5 kHz of MAS speed and cross-polarization pulse sequences. The values of the chemical shifts were referenced to solid hexamethylbenzene (17.3 ppm). Thermogravimetric analyses were conducted on a Thermoplus TG 8120 (Rigaku Corp.). FT-IR spectra were recorded on a spectrometer (FT/IR-660 Plus; Jasco Corp.) by the transmission method using KBr as a diluting agent. Electrical conductivity was measured using the following procedures: 0.10 g of a sample was placed on a 9 mmdiameter rod-shaped aluminum electrode that was fixed at the bottom of a 9 mm-diameter hole of a cylindrical PTFE mold. Another electrode was subsequently placed above the sample. The sample was then compressed with two electrodes in a mold by tightening a screw. The electrodes were connected to a frequency response analyzer (1255B; Solartron Analytical) and a potentiostat (1287A; Solartron Analytical). The two-terminal AC impedance method with voltage amplitude of 10 mV at frequencies of 0.1−106 Hz was applied for measurements. The samples were placed in an Ar atmosphere during the measurements.

vigorous stirring. The solution color darkened along with hydrogen chloride gas emissions. After stirring for 10 min at ambient temperature, the flask was warmed to 313 K in a water bath with stirring. After 2.5 h, the crude product was collected by filtration and was washed successively with benzene and icecold 18% hydrochloric acid. The obtained solid was dispersed in 300 mL of boiled 18% hydrochloric acid, followed by filtration. This procedure was repeated three times. The collected solid was dispersed again in boiled water and filtered off. The resultant reddish-brown powder of PPP was dried at 473 K under vacuum (yield: 2.19 g). 13C CP-MAS NMR: 127.0 and 137.3 ppm. Deuterated PPP (DPPP) was synthesized using the procedures described above, but using benzene-d6 instead of benzene as the starting material. 2.3. Synthesis of PDPA. According to the reported procedures, PDPA was synthesized by polymerizing diphenylacetylene in the presence of the Ta complex.9 All procedures were conducted using the Schlenk technique under an argon atmosphere. First, TaCl5 (72.4 mg, 0.202 mmol) and toluene (10 mL) were charged in a Schlenk tube and were stirred to dissolve TaCl5. Then triethylsilane (25.5 mg, 0.219 mmol) was added successively to the solution and heated to 353 K with stirring. The dark brown precipitate was formed soon thereafter. After 15 min, diphenylacetylene (0.898 g, 5.04 mmol) was added to the suspension. After stirring for 20 h at 353 K, the product was collected by filtration and washed with toluene, methanol, and a mixture of methanol and hydrochloric acid. The resultant bright yellow powder of PDPA was dried at 573 K for 2 h (0.762 g, yield: 86%). 13C CP-MAS NMR: 124.3, 128.8 (sh), 142.1 and 145.2 (sh) ppm. 2.4. Preparation of the Composites of Conjugated Polymers with LiH. Lithium hydride−polymer composites were prepared using ball-milling. In preparation of the composites having an atomic ratio of Li:C = 1:1, 0.124 g of PPP (9.8 mmol of carbon atoms), 0.125 g of PDPA (9.8 mmol of carbon atoms) or 0.127 g of PA (9.8 mmol of carbon atoms) and 0.078 g of LiH (9.8 mmol) were placed in a 40 mL stainless steel mill-pod with 18 zirconia balls (10 mm-diameter). For the preparation of composites consisting with the atomic ratio of Li:C = 1:2, 0.154 g of PPP (12.1 mmol of carbon atoms), 0.152 g of PDPA (12.1 mmol of carbon atoms), or 0.157 g of PA (12.1 mmol of carbon atoms) and 0.048 g of LiH (6.0 mmol) were placed in a mill-pod with zirconia balls. The mill-pod was charged with 1.0 MPa of argon to prevent contamination by air with subsequent rotation using a planetary ball-mill apparatus (Pulverisette P-7; Fritsch GmbH) at 250 rpm for 5 h. 2.5. Hydrogen Absorption and Desorption Experiments. A temperature-programmed desorption (TPD) experiment for hydrogen release was conducted in a U-shaped stainless steel reactor. Then 0.020 g of the sample was placed in the reactor followed by evacuation and introduction of helium at atmospheric pressure. Hydrogen desorption was started by increasing the reactor temperature by 5 K/min to 573 K and was held for 5 h with a 50 mL/min of helium flow. The amount of released hydrogen was determined by integration of the mass signal of m/z = 2 recorded on a quadrupole mass analyzer (QME200; Pfeiffer Vacuum, GmbH). Hydrogen storage was conducted using the following procedures. The U-shaped stainless steel reactor containing the sample after hydrogen release was evacuated with subsequent introduction of 3.0 MPa of hydrogen. The reactor temperature was increased by 5 K/min to 523 K and was held

3. RESULTS AND DISCUSSION 3.1. Cyclic Hydrogen Release Properties of LiH−PPP, LiH−PDPA, and LiH−PA. We have reported already that LiH−PA reversibly stores and releases hydrogen in the conditions of hydrogen release at 573 K with helium flow and hydrogen storage at 523 K with 3.0 MPa of hydrogen.7 Initially, the cyclic hydrogen storage and release capabilities of LiH−PPP and LiH−PDPA consisting of the atomic ratio of Li:C = 1:1 were examined under the same conditions. The TPD profiles and the amount of released hydrogen are respectively presented in Figure 1 and Table S1. Results show that LiH−PPP and LiH−PDPA respectively released 2.36

Figure 1. Hydrogen release profiles for 300−573 K for LiH−PA, LiH−PPP, and LiH−PDPA consisting of Li:C = 1:1. 19684

dx.doi.org/10.1021/jp507802b | J. Phys. Chem. C 2014, 118, 19683−19687

The Journal of Physical Chemistry C

Article

and 2.81 wt % of hydrogen at the first cycle, and released 2.13 and 1.98 wt % at the tenth cycle. The near maintenance of hydrogen release amounts over 10 cycles indicates clearly that both LiH−PPP and LiH−PDPA exhibited cyclic hydrogen storage and release capabilities under the same conditions as those used for LiH−PA. Although all conjugated hydrocarbon macromolecules of PA, PPP, and PDPA enhanced hydrogen release from LiH, the tendencies of degradation in hydrogen storage and release cycles differed. In comparison of the hydrogen release amounts at the first and tenth cycles, the hydrogen release amount of LiH−PPP was reduced by 10% after 10 cycles, although those of LiH−PDPA and LiH−PA were reduced by 30% (Figure 2). This comparison revealed that

The higher stability of PPP enabled the storage and release of hydrogen reversibly on the LiH−PPP composite consisting of the atomic ratio of Li:C = 1:2. For the LiH−PA composite, the hydrogen release amounts of LiH−PA (Li:C = 1:2) were reduced drastically over several cycles, as shown in Figure 2 and Table S2, although LiH−PA (Li:C = 1:1) maintained 70% amount of the hydrogen release amount after 10 cycles. These results demonstrate that the excess amount of LiH is necessary to maintain good cyclability on LiH−PA. In contrast, LiH−PPP (Li:C = 1:2) released almost identical amounts of hydrogen in 10 cycles. This is a beneficial point of LiH−PPP in comparison to LiH−PA in terms of practical applications. 3.2. Mechanistic Investigation of Hydrogen Storage and Release on LiH−PPP. The mechanisms of hydrogen release on LiH−PPP (Li:C = 1:2) were investigated. During the TPD measurement conducted at 300−573 K, the composite of LiH and deuterated PPP (designated as LiH−DPPP), respectively released 63, 29 and 8% of H2, HD, and D2, whereas 18, 49, and 33% were expected to be released in the binomial distribution, resulting from complete isotopic exchange (Figure S2). The predominant formation of H2 suggests that most of the released hydrogen originated from LiH, although the isotopic exchange reaction proceeded to some extent. After the TPD measurement, the LiH−DPPP sample was rehydrogenated with H2. The rehydrogenation of dehydrogenated LiH−DPPP with H2 might not form PPP but might maintain DPPP if hydrogen is released reversibly from and stored as LiH. In fact, DPPP and PPP are readily distinguishable using IR spectroscopy because of the observation of isotopic shifts in the vibrations related to C−H/C−D bonds. The IR spectrum of the rehydrogenated LiH−DPPP sample showed bands at almost identical positions to those of DPPP (Figure S3). This result also supports hydrogen release from and storage as LiH. Therefore, it is reasonably presumed that the hydrogen release reaction proceeded in accordance with eq 1, in which PPP accepts an electron from H− to form lithium-doped PPP and molecular hydrogen.

Figure 2. Relative hydrogen release amounts of (a) LiH−PA, (b) LiH−PDPA, and (c) LiH−PPP in the cyclic experiments. Filled markers represent values for the composite consisting with Li:C = 1:1. Hollow markers denote data for the composite consisting of Li:C = 1:2.

x LiH + (C6H4)n → x Li+ + [(C6H4)n ]x − + x /2H 2

(1)

The electronic state of PPP might be changed in the hydrogenation−rehydrogenation process if the hydrogen absorption and desorption proceed in accordance with eq 1. To investigate the change of the electronic state of PPP, electric conductivity was measured using the AC impedance method. The electric conductivities of bare PPP, as-milled, hydrogen released, and rehydrogenated LiH−PPPs were, respectively, 1.6 × 10−8, 1.5 × 10−8, 3.2 × 10−3, and 5.9 × 10−6 S/cm. The hydrogen release caused more than a 2 × 105-fold increase of electrical conductivity and rehydrogenation caused an almost 600-fold decrease, indicating that the electronic state of PPP was changed reversibly during hydrogen absorption and desorption. The increase of electrical conductivity of PPP by doping electrons on its conjugated system has been reported.12 Collectively, these observations suggest that electron transfer from H− to PPP occurred during the hydrogen release step. The reversible change of PPP during hydrogen absorption and desorption and the electron transfer at the hydrogen desorption were demonstrated using NMR spectroscopy. The 13 C MAS NMR spectrum of as-milled LiH−PPP was almost identical to that of pure PPP: two main signals attributed to the D2h symmetry of aromatic rings at 127.0 and 137.3 ppm and corresponding spinning side bands were observed, indicating that no structural or electronic change was made on PPP by

LiH−PPP exhibited superior cyclability to LiH−PDPA and LiH−PA. This superior cyclability is presumably attributable to the higher structural stability of PPP than the others. Reportedly, PA lost more than 20% of its weight at 573−673 K, and lost almost 80% of its weight at temperatures up to 923 K along with decomposition into small hydrocarbon molecules such as benzene, methane, and ethylene in the TGA analysis with helium flow.10 Formation of these products is a result of C−C and C−H bond cleavage. In the presence of LiH, bond cleavage on PA engenders formation of lithium carbide, which is inactive to reversible hydrogen storage and release in the present conditions. On the other hand, thermal decomposition of PPP reportedly started at around 973 K.11 In the TGA analyses under nitrogen atmosphere, the PPP sample we applied lost only 3% of its weight up to 800 K, although the PA and PDPA samples lost 66 and 67%, respectively (Figure S1). This thermal stability of PPP attributable to rigid and stable aromatic rings presumably prevented unfavorable side reactions, which converted LiH to inert forms in the LiH−PPP composite. 19685

dx.doi.org/10.1021/jp507802b | J. Phys. Chem. C 2014, 118, 19683−19687

The Journal of Physical Chemistry C

Article

of irreversible hydrogen release reactions related to C−H bond dissociation of polyene chains. For composites having an atomic ratio of Li:C = 1:2, C/H molar ratios of 3.68, 3.23, and 2.04 were observed at the first cycle on LiH−PA, LiH−PPP, and LiH−PDPA, respectively (Table S2). At the following cycles, C/H ratios on LiH−PA increased immediately because the hydrogen release amount had decreased steeply attributable to the consumption of LiH by irreversible side reactions, as discussed in the previous section. For LiH−PPP, C/H molar ratios were maintained at 3.02−3.33 during the cycles. These values indicate that approximately three carbon atoms on PPP accept one electron, and indicate that the variable stoichiometry of x in eq 1 is presumed to be 2n. The comparison of the C/H stoichiometry between the LiH−PPP composites having atomic ratios of Li:C = 1:1 and 1:2 reveals that the C/H stoichiometry at the hydrogen release process depends on the Li:C composition of LiH−PPP. Although the LiH−PPP composite exhibited nice reversible hydrogen storage/release capabilities even with the atomic ratio of Li:C = 1:2, the presence of a greater amount of LiH is beneficial for achieving a greater amount of hydrogen release by transferring greater numbers of electrons into polyene chains. In the case of LiH−PDPA, C/H molar ratios were changed gradually from 2.0 to 3.1 during the cycles. It is considered that a sufficient amount of LiH was present for transferring one electron into two carbon atoms on PDPA at earlier cycles, although the amount of LiH became insufficient for transferring one electron to two carbon atoms at the latter cycles because of the consumption of LiH by side reactions.

milling with LiH (Figure 3). After hydrogen release, extremely weak signals were observed under the same measurement

Figure 3. 13C MAS NMR spectra of LiH−PPP (atomic ratio of Li:C = 1:2): (a) as-milled, (b) after dehydrogenation, and (c) after hydrogenation. Signals denoted by asterisks are the spinning side bands.

4. CONCLUSION Reversible hydrogen storage and release was demonstrated on composites of LiH with conjugated macromolecules of LiH− PA, LiH−PPP, and LiH−PDPA. All the composites exhibited cyclability under the same conditions as those of hydrogen release at 573 K and hydrogen storage at 523 K with 3.0 MPa of hydrogen. Comparison of hydrogen release amounts of these composites having the atomic ratio of Li:C = 1:1 showed that the hydrogen release amount of LiH−PPP was reduced by 10% after 10 cycles of hydrogen storage and release, although those of LiH−PDPA and LiH−PA were reduced by 30% after the same number of cycles. The superior durability of LiH−PPP is ascribable to its prominent structural stability. This feature allowed LiH−PPP to maintain excellent cyclability even with the atomic ratio of Li:C = 1:2. The isotopic tracer experiments, electrical conductivity measurements, and IR and NMR spectroscopy suggest that the hydrogen release mechanisms on LiH−PPP are identical to those of LiH−PA. The H2 molecule was formed by electron transfer from H− to the conjugated polymer molecules. The stoichiometry between released hydrogen and carbon atoms on polyene chains suggests that two or three carbon atoms accept one electron during the hydrogen release process, depending on the Li:C atomic ratios of the composites.

conditions. Paris et al. reported that the 13C NMR signals were weakened and broadened considerably by doping potassium to p-sexiphenyl. Then they described that this signal broadening can arise from unpaired electrons.13 Therefore, the near disappearance of the signals is regarded as ascribable to doped electrons on the aromatic rings, which suggests that lithium-doped PPP is formed by hydrogen desorption. After hydrogenation, the peak intensity recovered. Then the two main signals and corresponding spinning side bands were observed at the same positions as the spectrum of original PPP, indicating that the electronic state of PPP was recovered by hydrogenation. 3.3. Stoichiometry between Released Hydrogen and Carbon Atoms on Polyene Chains. The stoichiometry between released hydrogen and carbon atoms on the composites were compared on LiH−PA, LiH−PPP, and LiH−PDPA. For the composites having an atomic ratio of Li:C = 1:1, almost identical C/H molar ratios were observed at the tenth cycle (2.27−2.44 molC/molH), although LiH−PA and LiH−PDPA exhibited slightly smaller values (1.74 and 1.72 molC/molH, respectively) than LiH−PPP (2.05 molC/molH) at the first cycle (Table S1). As discussed already in the previous section and the previous report, it is assumed that the electron transfer reaction from hydride to polyene chains proceeds during the hydrogen release process.7 According to this assumption, these observed C/H ratios indicate that approximately two carbon atoms accept one electron from hydride irrespectively of the polyene structure. These ratios also revealed that the variable stoichiometry of x in eq 1 is presumed to be 3n on the LiH−PPP composite having an atomic ratio of Li:C = 1:1. Smaller C/H ratios than two at the early cycles of LiH−PA and LiH−PDPA are probably the result

■

ASSOCIATED CONTENT

S Supporting Information *

Hydrogen release amounts and C/H, H/Li stoichiometries in each cycle, thermogravimetric profiles, results of isotopic tracer measurement, and FT-IR spectra are provided. This material is available free of charge via the Internet at http://pubs.acs.org. 19686

dx.doi.org/10.1021/jp507802b | J. Phys. Chem. C 2014, 118, 19683−19687

The Journal of Physical Chemistry C

■

Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.Y.); TEL.: +81-45-4815661; FAX; +81-45-413-9770. *E-mail: [email protected] (S.N.) Present Address †

Research Institute for Engineering, Kanagawa University, 3-271 Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan. Notes

The authors declare no competing financial interest.

■ ■ ■

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 26870663 (Grant-in-Aid for Young Scientists (B)). ABBREVIATIONS PA, polyacetylene; PPP, poly(p-phenylene); PDPA, poly(diphenylacetylene) REFERENCES

(1) Dalebrook, A. F.; Gan, W.; Grasemann, M.; Moret, S.; Laurenczy, G. Hydrogen Storage: Beyond Conventional Methods. Chem. Commun. 2013, 49, 8735−8751. (2) Durbin, D. J.; Malardier-Jugroot, C. Review of Hydrogen Storage Techniques for On Board Vehicle Applications. Int. J. Hydrogen Energy 2013, 38, 14595−14617. (3) Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Metal Hydride Materials for Solid Hydrogen Storage: A Review. Int. J. Hydrogen Energy 2007, 32, 1121−1140. (4) Vajo, J. J.; Mertens, F.; Ahn, C. C.; Bowman, R. C.; Fultz, B. Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2 Destabilized with Si. J. Phys. Chem. B 2004, 108, 13977−13983. (5) Jain, A.; Kawasako, E.; Miyaoka, H.; Ma, T.; Isobe, S.; Ichikawa, T.; Kojima, Y. Destabilization of LiH by Li Insertion into Ge. J. Phys. Chem. C 2013, 117, 5650−5657. (6) Miyaoka, H.; Ishida, W.; Ichikawa, T.; Kojima, Y. Synthesis and Characterization of Lithium-Carbon Compounds for Hydrogen Storage. J. Alloys Compd. 2011, 509, 719−723. (7) Yoshida, A.; Okuyama, T.; Mori, Y.; Saito, N.; Naito, S. Hydrogen Storage Material Composed of Polyacetylene and LiH and Investigation of Its Mechanisms. Chem. Mater. 2014, 26, 4076−4081. (8) Kovacic, P.; Kyriakis, A. Polymerization of Benzene to pPolyphenyl. Tetrahedron Lett. 1962, 467−469. (9) Niki, A.; Masuda, T.; Higashimura, T. Effects of Organometallic Cocatalysts on the Polymerization of Disubstituted Acetylenes by TaCl5 and NbCl5. J. Polym. Sci., Polym. Chem. 1987, 25, 1553−1562. (10) Chien, J. C. W.; Uden, P. C.; Fan, J. Pyrolysis of Polyacetylene. J. Polym. Sci., Polym. Chem. 1982, 20, 2159−2167. (11) Kovacic, P.; Jones, M. B. Dehydro Coupling of Aromatic Nuclei by Catalyst-Oxidant Systems: Poly(p-phenylene). Chem. Rev. 1987, 87, 357−379. (12) Shacklette, L. W.; Eckhardt, H.; Chance, R. R.; Miller, G. G.; Ivory, D. M.; Baughman, R. H. Solid-State Synthesis of Highly Conducting Polyphenylene from Crystalline Oligomers. J. Chem. Phys. 1980, 73, 4098−4102. (13) Paris, M.; Péres, L. O.; Chauvet, O.; Froyer, G. Solid-State NMR Study of Na versus K Doping of para-Phenylene Oligomers. J. Phys. Chem. B 2006, 110, 743−747.

19687

dx.doi.org/10.1021/jp507802b | J. Phys. Chem. C 2014, 118, 19683−19687