Hydrogen Generation from s‑Trioxane and Water CatalyticReforming: A Solid Organic Hydrogen CarrierYangbin Shen,†,‡ Fandi Ning,†,§ Chuang Bai,†,§ Yulu Zhan,† Shuping Li,†,‡ Yunjie Huang,∥

and Xiaochun Zhou*,†,⊥

†Division of Advanced Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou215123, China‡University of Chinese Academy of Sciences, Beijing 100049, China§University of Science and Technology of China, Hefei 230026, China∥Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, China⊥Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy ofSciences, Suzhou 215123, China

*S Supporting Information

ABSTRACT: As a clean and sustainable energy carrier, hydrogen is one ofthe most promising methods to address the energy and environmental crisis.However, hydrogen storage and transportation still face many challenges.Herein, we design a new hydrogen storage and production method based ons-trioxane. s-Trioxane has many advantages, such as convenient and safestorage method, high hydrogen content (6.7 wt % and 78.1 g L−1), and lowcost. The organic solid is originally employed to produce high-qualityhydrogen when catalyzed by organo ruthenium complex under mildcondition. Notably, 1 cm3 of s-trioxane can generate 1872 cm3 of hydrogenat 25 °C, which performs better than methanol. The byproduct COconcentration is less than 10 ppm, while the turnover frequency (TOF) is upto 280 h−1 when catalyzed by Ru(p-cymene)(NH3)Cl2 at 90 °C. We alsopropose and prove the reaction mechanism during the s-trioxane decomposition in water, and the results show thatformaldehyde and formic acid could be generated during the reaction.

KEYWORDS: hydrogen carrier, hydrogen generation, aldehyde−water shift reaction, catalytic reforming, s-trioxane

■ INTRODUCTIONThe energy and environmental issues have made hydrogenresearch become an important topic in the 21st century.Accordingly, hydrogen production and storage have attractedgreat attention from scientists, companies and governments. Inpractical application, the hydrogen storage materials shouldhave high content of hydrogen. The U.S. Department ofEnergy (DOE) 2020 targets for system-based gravimetric andvolumetric hydrogen densities are 5.5 wt % and 40 g L−1,respectively.1 These have stimulated more scientists to focustheir research efforts on the topic.The organic hydrogen carrier is one of the most promising

ways to meet the system-based hydrogen content of DOE2020 targets, hence having attracted increasing attention.Methanol,2−7 ethanol,8−11 formic acid,12−17 and even form-aldehyde18−22 have attracted widespread investigation forhydrogen generation. Especially, methanol and formic acidhave been deeply studied with respect to hydrogen generation.Formic acid is a common hydrogen storage medium with

many advantages, such as considerable hydrogen content (4.4wt %), low-temperature hydrogen generation (

temperature (Table 1). Notably, 1 cm3 s-trioxane can generate1872 cm3 hydrogen at 25 °C, which performs better than

methanol (Table 1) . Additionally, unlike hydrogen generationfrom borohydride hydrolysis which is a common solidhydrogen carrier, the trioxane could be decomposedcompletely, and no solid byproduct would be generated afterreaction.

sC H O ( trioxane) 3H O 6H 3CO3 6 3 2 2 2‐ + → +

Scheme 1 proposes the hydrogen generation process using s-trioxane and water as hydride source. The committed steps are

s-trioxane hydrolysis into formaldehyde or methanediol underacidic condition, followed by hydrogen and formic acidgeneration from formaldehyde or methanediol, which is analdehyde−water shift (AWS) reaction.32−34 Then the formicacid further decomposes into hydrogen and carbon dioxide.Hydrogen production from formaldehyde or methanediol

decomposition has been fruitful under basic or neutralcondition.18,21 However, strong acidity will deactivate thecatalyst for formaldehyde decomposition. Accordingly, theacidic condition of s-trioxane hydrolysis will make manycatalysts lose activity for formaldehyde decomposition. More-over, many reported catalysts do not have high activity forhigh-concentration formaldehyde decomposition, attributingto the formaldehyde disproportionation. In this work, we willstudy some metal complexes for hydrogen generation from s-trioxane and water reforming. The turnover frequency for s-trioxane dehydrogenation is up to 280 h−1 when catalyzed byorgano ruthenium catalysts, and the byproduct CO concen-tration is less than 10 ppm. In addition, the research onformaldehyde−water reforming will also enlighten themethanol reforming study. Since methanol dehydrogenationalways contains the formaldehyde−water reforming reac-tion.5,35,36

■ RESULTS AND DISCUSSION1.1. Selectivity and Activity of Catalysts for Hydrogen

Generation from Formaldehyde and s-Trioxane. Manyiridium, rhodium, and ruthenium complexes have high catalyticactivity for the aldehyde−water shift reaction, and the productsare hydrogen and corresponding carboxylic acid.32−34 Form-aldehyde is the simplest aldehyde, and the generated formicacid would be further decomposed into hydrogen and carbondioxide through aldehyde−water shift reaction. However,many catalysts could catalyze the formaldehyde disproportio-nation, which produces methanol and carbon dioxide. Weprepared several complexes for formaldehyde decomposition,and found that iridium and rhodium catalysts showed lowcatalytic selectivity for hydrogen generation from form-aldehyde. As shown in Table 2, little hydrogen could be

produced during formaldehyde decomposition when iridiumcomplexes are employed, and the main gaseous products arecarbon dioxide. The selectivity of rhodium complexes are alittle better than iridium complexes, but the hydrogen contentin the gaseous product is still low. Only the rutheniumcomplexes could catalyze formaldehyde decomposition intohydrogen and carbon dioxide, and the mole rate betweenhydrogen and carbon dioxide is 2, indicating the rutheniumcomplexes could avoid formaldehyde disproportionation.If the catalysts do not have high catalytic activity and

selectivity for formaldehyde decomposition, it could be difficultfor s-trioxane to produce hydrogen. Since the reportedruthenium catalysts for formaldehyde decomposition all havepoor solubility in aqueous solution, we synthesized severalsoluble ruthenium complexes for formaldehyde and s-trioxanedecomposition.As exhibited in Table 3, several ruthenium complexes with

different ligands were prepared for formaldehyde and s-trioxane dehydrogenation at 80 °C. We find the ligands are

Table 1. Parameters of Representative Organic Moleculesfor Hydrogen Storage

hydrogencarriers

state atr.t.

density(g/cm3)

volumetricdensity ofH2 (g/L)

gravimetricdensity ofH2 (wt %)

volume ofevolved H2from 1 cm3

substrate(cm3)

formaldehyde gas / 54.4 6.7 1304

methanol liquid 0.79 98.8 12.6 1782

formic acid liquid 1.22 53.0 4.4 636

s-trioxane solid 1.17 78.1 6.7 1872aRefer to the reaction formulas in reactions (S1) for the hydrogenproduction from the representative organic molecules.

Scheme 1. Reaction Pathway: Proposed Reaction SequencesUsing s-Trioxane and Water as Hydride Source for theSubsequent Hydrogen Generation

Table 2. Catalytic Selectivity of Iridium, Rhodium, andRuthenium Complexes for Formaldehyde Decomposition

entry catalyst H2 content of evolved gas

1 [Cp*IrCl2]bpym trace2 [Cp*IrCl2]2bpym trace3 Cp*IrCl2(ppy) 0.2%4 [IrCp*Cl2]2 2.65%5 [Cp*RhCl2]bpym 7.42%6 [Cp*RhCl2]2bpym 9.28%7 [RhCp*Cl2]2 16.7%8 [Ru(p-Cymene)Cl2]2 66.7%

Table 3. Catalytic Activity of Complexes for Formaldehydeand s-Trioxane Decomposition at 80°C

entry catalyst TOFs‑trioxane (h−1) TOFHCHO (h

−1)

1 [Ru(p-cymene)2Cl2]2 190 58902 Ru(p-cymene)(H2O)3SO4 100 19603 Ru(p-cymene)(NH3) Cl2 92 30604 Ru(p-cymene)Cl2bpym 2 1765 Ru(p-cymene)Cl2(H2Bim) 0 126 Ru(p-cymene)Cl2phen 0 37 Ru(p-cymene)Cl2bpy 0 0.1

aThe H2SO4 concentration for s-trioxane decomposition remains at0.04 M, the s-trioxane concentration is 20 wt %.

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essential for the activity of ruthenium complexes. The catalystsowing different ligands show diverse catalytic activities forhydrogen generation, even if they have the same organo-metallic core (Ruthenium). Moreover, the catalytic activities ofcomplexes for formaldehyde dehydrogenation have a strongcorrelation with s-trioxane. The catalysts that have higheractivity for formaldehyde always show better catalytic perform-ance for s-trioxane. The strong correlation further proves thatthe s-trioxane dehydrogenation undergoes the process offormaldehyde dehydrogenation in Scheme 1.We also studied other regularities for the s-trioxane

dehydrogenation, which is beneficial to extrapolate thereforming reaction mechanism between s-trioxane and water.By comparing the catalysts in Table 3, we find that the chelateeffect of the ligand has negative effects on the dehydrogenationreaction. The catalysts with monodentate ligands, such as H2O,NH3, and Cl have high catalytic activity, while the catalystswith bidentate ligands, such as 1,10-phenanthroline mono-hydrate (phen), 2,2′-dipyridyl (bpy), 2,2′-bipyrimidine(bpym), and 2,2′-biimidazole (H2Bim), show low catalyticactivity. The bidentate ligands could decrease the activity ofthe complexes in two ways. First, the chelate effect wouldincreases the steric hindrance between the catalyst andsubstrate, which makes the substrate more difficult to bindon the catalytic core (Ruthenium). Second, the chelate effectmakes the structure of catalysts become too stable to catalyzethe dehydrogenation efficiently.1.2. High-Quality Hydrogen from s-Trioxane Dehy-

drogenation. Figure 1a shows the volume of evolved gasincrease continuously from s-trioxane−water reforming, which

is catalyzed by organoruthenium complexes, that is, Ru(p-cymene)(H2O)3SO4 (Catalyst-1) or Ru(p-cymene)(NH3)Cl2(Catalyst-2). Both of the catalysts have high catalytic stabilityin 40 h. Figure 1a also shows that Catalyst-1 has higher activitythan Catalyst-2 under the same reaction conditions. Thereactions are carried out under mild condition, and no extraalkali is added in the reaction solution.The gas chromatogram (GC) spectra of H2 and CO2 show

that the evolved gas from the s-trioxane−water reformingcatalyzed by Catalyst-1 contains 67.2% H2and 32.8% CO2,while the one catalyzed by Catalyst-2 contains 67.1% H2 and32.9% CO2 (Figure 1b). Therefore, the s-trioxane dehydrogen-ation indeed undergoes the process in Scheme 1 and generatesboth H2 and CO2. Figure 1c confirms the CO contents arelower than 10 ppm in the evolved gas. The high-qualityhydrogen from s-trioxane decomposition will benefit the highperformance of PEMFC particularly.

1.3. Optimization of the Reaction Conditions. The s-trioxane-water reforming could be affected by many factors,such as temperature, pH of solution, and s-trioxaneconcentration. Figure 2a shows that higher temperature canaccelerate the s-trioxane dehydrogenation greatly. The turn-over frequency of catalyst-1 and 2 are up to 270 and 280 h−1 at90 °C, respectively. Simultaneously, we can gain the apparentactivation energy (Ea) by the plot of ln(TOF) and (1/T).Owing to the high correlation coefficients >0.99 in Figure 2a,the Ea could be determined accurately. The apparent activationenergies of Catalyst-1 and Catalyst-2 for s-trioxane dehydro-genation have close values of 91.6 and 100.5 kJ mol−1,indicating a similar catalytic mechanism.

Figure 1. High-quality H2 from s-trioxane−water reforming catalyzed by organoruthenium complexes. (a) Volume of the evolved gas by the twocatalysts during 40 h at 80 °C. (b,c) Gas chromatogram spectra of evolved gas. Yellow, red, and black curves represent the standard gas samples, theevolved gas by Ru(p-cymene)(H2O)3SO4 (Catalyst-1), and Ru(p-cymene)(NH3)Cl2 (Catalyst-2), respectively. The reaction solution contains 1 gof s-trioxane, 3 mL of water, and 0.04 M H2SO4. The evolved gas for GC measurement was collected at the 20th hour.

Figure 2. Influence factors on the catalytic activity of s-trioxane−water reforming. (a) initial TOF values and Arrhenius plot at differenttemperature. (b) Effect of H2SO4 concentration on the activity of the catalysts. The s-trioxane concentration keeps at 22%. (c) Effect of the s-trioxane concentration on the activity. The H2SO4 concentration for the Catalyst-1 and 2 remains at 0.03 and 0.05 M, respectively.

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The H2SO4 concentration ([H2SO4]) has a strong effect onthe activity of the two catalysts. As exhibited in Figure 2b, bothcatalysts have an optimized performance for the s-trioxanedehydrogenation under acid condition. The Catalyst-1 exhibitsan optimal TOF (i.e., 90 h−1 at 0.05 M H2SO4), and thecatalyst-2 exhibits an optimal TOF (i.e., 82 h−1 at 0.03 MH2SO4). The optimal TOF under acid condition can beattributed to the competition of two reactions: s-trioxanehydrolysis and formaldehyde−water shift. The s-trioxanehydrolysis should be performed under acidic condition asshowed in Scheme 1. However, the formaldehyde−water shiftusually needs basic or neutral condition (Figure S8).18,21 Thetwo contradictory reaction conditions determine the optimalpH for the s-trioxane dehydrogenation (Figure 2b).The s-trioxane concentration also has strong effects on the

activity of the catalysts. As shown in Figure 2c, both catalystshave an optimized performance for the s-trioxane dehydrogen-ation under different s-trioxane concentrations. The Catalyst-1and Catalyst-2 exhibit optimal catalytic activities in 20 and 30wt % s-trioxane solution, respectively. The optimal catalyticactivity under different s-trioxane concentrations can beattributed to the competition of two reaction substrates: thes-trioxane and the water. When we increase the s-trioxaneconcentration, the water concentration will decrease. InScheme 1, we know that both s-trioxane and water arereactant in the s-trioxane-water reforming. Water not onlyserves as solvent but also participates in the s-trioxanehydrolysis and aldehyde−water shift. Additionally, the watercontent also has great influence on formic acid decomposition,which was an essential step during s-trioxane dehydrogenation(Figure S9). Therefore, an optimal catalytic activity wouldappear at certain s-trioxane concentration.1.4. Reaction Mechanism. In order to optimize the

activity of catalysts and study the reaction mechanism, it isnecessary to determine the reaction pathway for the s-trioxane-water reforming. However, s-trioxane is not a simple moleculelike methanol or formic acid, but it forms by three methanegroups through oxygen bridges. It can be predicted that the s-trioxane dehydrogenation will not be a one-step reaction. Inthis work, the reaction pathway is mainly revealed by the 1HNMR spectra during the reaction. For tracking theintermediates during the reaction, 1H NMR spectra of thereaction solutions are recorded at different reaction time.Figure 3 shows that only the 1H NMR signals of s-trioxane andwater are detected at the beginning of the s-trioxanedehydrogenation. After 10 h of reaction, the 1H NMRspectrum shows that a small quantity of intermediate products,that is, HCOOH and HCHO (or methanediol),37 aregenerated, but their concentrations are relatively low. About40 h later, the 1H NMR peaks of HCOOH and HCHObecome much higher, indicating more HCOOH and HCHO(or methanediol) are generated in the reaction solution.Therefore, HCOOH and HCHO (or methanediol) are themain intermediates during the s-trioxane-water reforming.According to the 1H NMR characterization above, we can

infer the s-trioxane-water reforming contains three steps. First,s-trioxane is hydrolyzed to formaldehyde or methanediol underacidic condition (Scheme 1). Then the formaldehyde ormethanediol produce H2 and HCOOH through the aldehyde−water shift (AWS) reaction. Lastly, HCOOH furtherdecomposes to H2 and CO2.In order to further determine the reaction pathway in

Scheme 1, we measure the mole ratio of H2 to CO2 (rH2/CO2)

and the gas generating rate during the reaction. We employ gaschromatography to measure the contents of H2 and CO2during the reaction. Figure 4 shows that, at the beginning of

reaction, the mole ratio rH2/CO2 approaches 20, which is muchhigher than 2. Especially, no CO2 is detected in the evolved gasin the initial 3 min (Figure S11). A higher mole ratio rH2/CO2indicates that the reaction rate of AWS reaction is much fasterthan HCOOH decomposition. The low HCOOH decom-position rate is mainly attributed to the low HCOOHconcentration at the beginning of reaction because HCOOHis produced through AWS reaction during s-trioxanedehydrogenation. Thus, the AWS reaction of formaldehydedominates the hydrogen production at the beginning of s-trioxane-water reforming.With the reaction proceeding, HCOOH accumulates, and

the HCOOH decomposition rate wil increase consequently.More CO2 will be generated at a considerable rate. When thereaction rate of AWS reaction equals that of HCOOHdecomposition, the mole ratio rH2/CO2 will be 2. As shown in

Figure 4, the mole ratio rH2/CO2 decreases from about 20 to 2

after 10 h. From 10 to 40 h, the mole ratio rH2/CO2 remains at 2,which is equal to the stoichiometric ratio of H2 to CO2 inScheme 1. This evidence proves that the s-trioxane

Figure 3. 1H NMR spectra during the s-trioxane-water reforming. Thereaction solution contains 2 μmol Ru(p-cymene)(H2O)3SO4, 1 g of s-trioxane, 3 mL of water, and 0.04 M H2SO4.

Figure 4. Mole ratio of H2 to CO2 (rH2/CO2) and reaction rate for thetwo catalysts at 80 °C. Red and black curves represent Ru(p-cymene)(H2O)3SO4 (Catalyst-1) and Ru(p-cymene)(NH3)Cl2 (Cat-alyst-2), respectively. The reaction solution contains 2 μmol catalyst,1 g of s-trioxane, 3 mL of water, and 0.04 M H2SO4.

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dehydrogenation contains the process of aldehyde−water shift(AWS) reaction and HCOOH decomposition, and theHCOOH is produced by the AWS reaction.Figure 4 also shows that the reaction rate is slow at the

beginning of reaction, accelerates in the first 10 h, and then hasa slight decrease afterward. The slow initial reaction rateimplies that s-trioxane does not directly participate in the s-trioxane dehydrogenation. Otherwise, the reaction rate will bethe highest at the beginning because the s-trioxaneconcentration is the highest at this moment. The s-trioxanehydrolysis is a key step to generate HCHO (or methanediol).The accelerated reaction rate in the first 10 h implies that theintermediate concentrations including HCHO and HCOOHbecome higher and higher. Both catalysts have a relative highcatalytic stability, and they maintain about 80% of themaximum catalytic rate for s-trioxane dehydrogenation after40 h. The reasons for the slight decrease of activity can bepartially attributed to the s-trioxane concentration decrease.According to the research above, we propose the catalytic

cycles for the s-trioxane dehydrogenation. Scheme 2 shows thatthe s-trioxane dehydrogenation contains three catalytic cycles.First, the s-trioxane hydrolyzes into formaldehyde ormethanediol under acidic condition. Notably, the H atoms ofboth hydroxyl groups on methanediol originate from water.The gravimetric hydrogen density of methanediol is as high as8.3%, which is higher than that of formaldehyde (6.7%).Moreover, the generated methanediol is favored in the nextcatalytic cycle of aldehyde−water shift reaction. Second, themethanediol will catalytic convert to formic acid, andconsequently, the catalyst could generate Ru(p-cymene)L3hydride complex (Scheme 2b). The hydride complex willreact with H+ to produce hydrogen in solution. One of the Hatoms of hydrogen molecule is from formaldehyde, and theother is from water. The H atom of carboxyl group of formicacid also originates from water. This is a typical aldehyde−water shift reaction, which is similar to that in theliteratures.32−34 Lastly, the generated formic acid will undergoa further dehydrogenation reaction. The catalyst will react withformate to produce the Ru(p-cymene)L3 hydride complex afterβ-hydrogen elimination and release CO2. The hydride complexreacts with H+ in solution to produce hydrogen molecule. Asthe catalysts with the Ru(p-cymene) group have high

selectivity for formic acid dehydrogenation, little CO (

■ EXPERIMENTSChemicals and Materials. All chemicals are commercial and use

without further purification unless specified. Chemicals includedethanol (C2H5OH, Tianjing Baishi Chemical Industry, Co.,Ltd. >99.7%), pyrrole (C4H5N, Shanghai Macklin Biochemical Co., Ltd., >99%), chloroiridic acid (H2IrCl6·6H2O, Shanghai Tuosi ChemicalCo., Ltd., Ir wt %>35%), 1,2,3,4,5-pentamethylcyclopentadiene(C10H16, Sun Chemical Technology(Shanghai) Co., Ltd., 97%), s-trioxane (C3H6O3, Sinopharm Chemical Reagent. Co.,Ltd. > 99%),formaldehyde (HCHO, Sinopharm Chemical Reagent. Co.,Ltd. 37%),formic acid (HCOOH, Sinopharm Chemical Reagent. Co.,Ltd. >98%), methanol (CH3OH, Sinopharm Chemical Reagent, Co.,Ltd. >99.7%), 1,10-phenanthroline (C12H8N2, Sinopharm Chemical Re-agent, Co.,Ltd. > 99%), 2,2′-bipyridine (C10H8N2, China LangchemCo.,Ltd. > 98%), 2,2′-bipyrimidyl (C8H6N4, China Langchem,Co.,Ltd. > 98%), α-phellandrene (C10H16, Jiangxi Gannan naturaland perfume oil Co.,Ltd. > 98%), ammonium hydroxide (NH3,Chinasun Specialty products, Co.,Ltd. 25−28%), and rutheniumtrichloride (RuCl3, Energy-chemical, Co.,Ltd. Ru wt %>48.2%).Ultrapure water was prepared by Thermo PureLab Ultra Genetic.2,2′-Biimdimazole and [RuCl2(p-cymene)]2 were synthesized accord-ing to literature procedures.41−44

Dehydrogenation of Formaldehyde. Typically, 2 mL of 37 wt% formaldehyde aqueous solution is stirred in a thin glass tube with asmall magnetic bar in water bath. Then, 4 μmol catalyst is added intothe solution at the preset temperature. The tube is connected to the Utube with graduation to measure the volume of the evolved gas, whichis recorded by a digital camera. Because trace carbon dioxide isdetected by GC during the initial 3 min of reaction (Figure S10,Figure S11), we calculate the initial TOF of the formaldehydedehydrogenation based on the following chemical reaction.

HCHO H O HCOOH H2 2+ → +

TOF Calculation. The initial TOF was calculated by eq 1. Thewater vapor was neglected.

V t V

nTOF

/( )H m,20 C

cat

2=* °

(1)

VH2 is the volume of generated hydrogen, t is the time during theformaldehyde or s-trioxane decomposition, and ncat is the molarnumber of catalyst. The calculation of Vm,20 °C was carried out usingvan der Waals of hydrogen by eq 2.

VRT

pb

aRT

24L/molm,20 C = + − =°(2)

R is 8.3145 m3 Pa mol−1 K−1, T is 293.15 K, p is 101 325 Pa, b is26.7 × 10−6 m3·mol−1, and a is 2.49 × 10−10 Pa·m3·mol−2.s-Trioxane Dehydrogenation. Typically, 200 μL of 1.0 M

H2SO4 is added into the tube with 3.8 mL of s-trioxane water solution.The solution is stirred with a small magnetic bar and heated in thewater bath at 80 °C for 3 h. After the solid dissolves completely andhydrolyzes partially, 2 μmol catalyst is added into the tube. Thevolume of the evolved gas is recorded by a digital camera. Thereaction conditions are maintained during the reaction, and the initialTOF calculation is similar to formaldehyde decomposition.Characterization and Physical Measurements. 1H NMR

spectra of catalysts were recorded on Varian Plus 400 MHz. Theevolved gas is analyzed by GC-G5 (Beijing Persee General InstrumentCo., Ltd.). The gas chromatography are assembled with TDX-1chromatographic column, FID, TCD, and methane conversionreactor; N2 as carrier gas; and the detection limit of CO was below1 ppm.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsaem.8b00917.

Experimental scheme, additional results, and analyses

(PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: xczhou2013@sinano.ac.cn.ORCIDXiaochun Zhou: 0000-0001-9793-6072Author ContributionsX.Z. designed and directed the project. Y.S. and F.N. carriedout the experiments and synthesis of the catalysts, and C.B.performed NMR analysis and analyzed the data. Y.Z. and S.L.performed the GC and analyzed the data, and Y.H. testedelement content of all the catalysts. X.Z. and Y.S. wrote andrevised the manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSProject was funded by China Postdoctoral Science Foundation(No. 2018M632406). The authors are also grateful forfinancial support granted by Ministry of Science andTechno logy o f Ch ina (Nos . 2016YFE0105700 ,2016YFA020070), the National Natural Science Foundationof China (No. 21373264, No. 21573275), the Natural ScienceFoundation of Jiangsu Province (BK20150362), SuzhouInstitute of Nanotech and Nanobionics (Y3AAA11004), andThousand Youth Talents Plan (Y3BQA11001).

■ ABBREVIATIONS:bpym = 2,2′-Bipyrimidineppy = polypyrroleCp* = 1,2,3,4,5-pentamethylcyclopentadienylH2Bim = 2,2′-Biimidazolephen = 1,10-phenanthrolinebpy = 2,2′-Bipyridine

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ACS Applied Energy Materials Article

DOI: 10.1021/acsaem.8b00917ACS Appl. Energy Mater. 2018, 1, 4860−4866

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