Journal of Molecular Catalysis A:Chemical193(2003)
27–32
Photocatalytic carbon dioxide photoreduction by Co(bpy)32+
nsitized by Ru(bpy)32+fixed to cation exchange polymer
Takuji Hiro a,∗,Yoshihito Maeno a,Yuichiro Himeda b
a Department of Applied Chemistry,Faculty of Engineering,Saitama University,255Shimo-ohkubo,Saitama,Saitama338-8570,Japan
b Photoreaction Control Rearch Center,National Institute of Advanced Industrial Science and Technology,
AIST Central5,1-1-1Higashi,Tsukuba,Ibaraki305-8565,Japan
Received17April2002;received in revid form24June2002;accepted26July2002
Abstract
Photocatalytic CO2reduction was performed in a partially heterogeneous system using Ru and Co complexes:[Ru(bpy)3]2+ (bpy=2,2 -bipyridine)as a photonsitizer wasfixed to cation exchange poly
mers while[Co(bpy)3]2+was ud as a homo-geneous catalyst.It was demonstrated that the metal complex system could be repeatedly ud after replacement of the gaous components by fresh CO2.Reu of the complexes in the homogeneous systems was also experimented.As a result,incread CO production(TN=8.3)was achieved in the heterogeneous system due to the prolonged catalytic activity more than400h under atmospheric pressure.Apparent deactivation of the system was discusd in terms of the intermediates of the Co complex.©2002Elvier Science B.V.All rights rerved.
Keywords:Photocatalytic reduction;Carbon dioxide;Cobalt complex;Ruthenium complex;Heterogeneous system
1.Introduction
About20years ago,Lehn ported the pioneering work on a photocatalytic CO2reduction system using Ru and Co polypyridine complexes in a homogeneous solution[1],which has collected increasing interest from not only academic but also energetic and environmental point of view[2–10]. The system is important becau visible light(λ> 400nm),a major part of sunlight,can be ud as an energy source.It is also interesting that two kinds of metal complexes play different roles[1]:the Ru and Co complexes work as a photonsitizer and a catalyst,respectively.
∗Corresponding author.Tel.:+81-48-858-3522;
fax:+81-48-858-9534.
E-mail address:hiro@apc.saitama-u.ac.jp(T.Hiro).
Although the mechanism is not completely eluci-dated[1b],the following is accepted;photo-excited Ru complex,∗Ru(II),transfers an electron to Co(II) complex,which works as a reaction center of CO2, and then the oxidized Ru(III)complex is reduced by a tertiary amine known as the“sacrifice reagent”.Many rearch groups have worked on related systems for mechanistic study and/or for more efficient produc-tion of CO.Other combinations of metal complexes [3,5,6],other kinds of nsitizers and reducing agents [3,6,7]and dinuclear systems[8–10]have been inves-tigated but little work has been done on heterogeneous systems.
Development of heterogeneous catalysts is usually advantageous for processing and also for recovery of the catalysts.When metal complexes arefixed to a solid matrix,dissociation of ligands from a metal ion could be suppresd due to restricted mobility,which
1381-1169/02/$–e front matter©2002Elvier Science B.V.All rights rerved. PII:S1381-1169(02)00478-8
28T.Hiro et al./Journal of Molecular Catalysis A:Chemical193(2003)27–32
should lead to a longer lifetime.Here we report a
partially heterogeneous CO2photoreduction system,
where[Ru(bpy)3]2+wasfixed to an ion exchange
polymer[11]and[Co(bpy)3]2+was dissolved in a
mixture of dimethylformamide(DMF)as solvent and
triethanol amine(TEOA)as a reducing agent and the
amount of produced CO and its ratio to H2was dis-
cusd.
2.Experimental
2.1.Materials
All reagents and solvents were purchad from
Wako Pure Chemical Industries Ltd.,Sigma-Aldrich
Corporation Inc.and Kanto Chemical Co.Ltd.of at初会成绩查询
延时技巧
认识钟表最简单的方法least reagent grade quality,and were ud as received
except for DMF and TEOA,which were distilled
from KOH.
All polymer supports,Amberlite IR-120(Rohm&
Hass),Nafion112(Du Pont),and Nafion NR-50N
(Du Pont),were ud without further purification.
Both complexfixed Nafions were cut into small
pieces,approximately veral square millimeter
(Nafion112)and cubic(Nafion NR-50)for photoreac-
tion while Amberlite IR-120was ud in the original
shape.
The Ru mononuclear complex,Ru(bpy)3Cl2[12],
and the Ru-Co dinuclear complex(bpy)2Ru(phen-C2
H4-phen)Co(bpy)2(phen=phenathroline-4-yl)[10],
were prepared according to the literature procedures.
The Co mononuclear complex,Co(bpy)3Cl2,was pre-
pared by the reaction of1eq.CoCl2·6H2O and3 eq.of bpy in ethanol(80◦C,2h).After concentra-
tion of the reaction mixture,the precipitate was col-
lected and ud for the photochemical reaction without
further purification:ES-MS;264[M-2Cl−]2+/2,406
[M-bpy-Cl−]+,186[M-bpy-2Cl−]2+/2.Anal.Calcd
for C30H24N6Cl2Co·1.5H2O:C,57.56;H,4.32;N,
13.43%.Found:C,57.41;H,4.23;N,13.32%.
2.2.Spectroscopic study
The IR,UV,and emission spectrum were studied
for Ru complexesfixed to Nafion112and compared
with tho of Ru(bpy)3Cl2.As shown in Fig.1,al-
most the identical emission spectra with20%鼻腔干燥怎么办
lower Fig.1.Emission specra of Ru(bpy)32+adsorbed to Nafion112. intensity was obtained for the solid membrane as that of the original complex(602nm,intensity107.2a.u.). Obviously,however,it is impossible to quantitatively compare the data due to the differences in concentra-tion and conditions between the Ru complexes.Some identical peaks were also obrved in IR and UV mea-surement but the spectrum were not clear due to the membrane thickness(50m).
2.3.Photochemical reaction
All photoreactions were carried out by using a certain amount of the Ru and Co complexes or the dinuclear Ru–Co complex in the DMF/TEOA(4/1, v/v)or DMF/H2O/TEOA(3/1/1,v/v/v)(30ml)mixed solvent.The reaction mixture was saturated with CO2by exposure to atmospheric pressure of CO2 for at least20min and the solution was transferred to a gastight photolysis photoreaction vesl(Pyrex) [10].Photoirradiation was carried out with a Xe lamp (Ushio UXL500D-O)with an IR-cutfilter(HOY A, HA-50)and the irradiation was continued until the evolution of CO became negligible,less than approx-imately2%,in more than4h for each experiment and the maximum experimental error was about 25%.
2.4.Measurements
Gas sampling(0.3ml)was performed at various in-tervals with a gastight syringe through a ptum.The amounts of CO and H2produced were detected by TCD-GC(Hitachi263-50,3mm×5m activated char-coal column,at70◦C using Ar as a career gas).Each gas was identified and quantified using the working curve,which had been previously obtained using stan-dard CO and H2gas(GL Science).
T.Hiro et al./Journal of Molecular Catalysis A:Chemical193(2003)27–3229
3.Results and discussion
3.1.Blend system
In order to prepare a heterogeneous photocatalyst system,wefirst blended metal complex(es)with a polymer.It was expected that CO2dissolved in the sol-vent would diffu into the polymer matrix and react with the metal complexes.A polymer-complex blend was prepared as follows:the polymer was dissolved in an appropriate solvent and the complex was added into the mixture.However,only a limited number of poly-mers were applicable,as the blends must be homoge-neous in the solvent for photoreaction.For example, homogeneous polystyrene(PS)-bad blend could not be prepared becau the complexes did not dissolve in tetrahydrofuran,which is a good solvent for PS. The Ru a
nd/or Co complexes were blended into four polymers,polyvinyl chloride(PVC),poly(vinyl acetate)(PV Ac),poly(vinyl alcohol)(PV A),and poly(vinyl cinnamate)(PVCm)and all of the mix-tures formed membranes.Among them,PVC-and PV Ac-bad membranes were fed to the DMF-H2O-TEOA mixed solvent and were irradiated.On the other hand,PV A-and PVCm-bad polymers were thermally and photochemically cross-linked,respec-tively[13].The mixtures were applied to the photore-action step.As a result,most of the blends maintained their membrane forms at least for one day,but the complexes soon came out of the polymer matrix due to polymer dilation in DMF-H2O-TEOA.喜欢的句子
3.2.Adsorption system
3.2.1.Fixation of metal complexes
Three kinds of cation exchange polymers,PS-bad Amberlite IR-120,perfluorinated Nafion112(mem-brane type),and Nafion NR-50(grain type)were ud forfixing the Ru and Co complexes.Thefirst was expected to be compatible with organic solvents and the others should be stable for photoreduction.All polymers ud emed effective infixing the Ru and Co complexes.Only small amounts of the complexes, probably physically adsorbed ones,came off during thefirst photo-ir
radiation but not any more as far as the solvent is the DMF-TEOA mixture.Most com-plexes,however,were completely taken off from the polymers under strongly acidic conditions at pH<
1.Fig.
2.Schematic of partially heterogeneous CO2photoreduction. This feature may be important for handling Ru-bad compounds considering Ru is generally expensive.
3.2.2.Photoreaction
却的成语
The photonsitizer,Ru(bpy)3Cl2,fixed to Nafion 112,photoactive in solid as mentioned in Section2, was irradiated using a Xe lamp under atmospheric pressure of CO2in the prence of Co(bpy)3Cl2dis-solved in DMF-TEOA.Only small amount of dilation of the polymer substrate was obrved.The reaction system is schematically shown in Fig.2.Production of CO was obrved as en Fig.3;that is,photore-duction of CO2in the partially heterogeneous Ru–Co system was achieved for thefirst time.Both CO (20.0mol)and H2(50mol)were produced during thefirst irradiation for2days.For comparison,the homogeneous system was also examined under the identical conditions and about two times CO pro-duction(38mol)was obrved in2days as en in Fig.4.Low CO productivity in the heterogeneous system may be the result of decrea in the rate of electron transfer from the Ru complex on the polymer surface to the Co complex in solution.
The reddish-orange color of the polymer membrane and the homogeneous system was maintained after CO production apparently stopped.This obrvation sug-gested that the photonsitizer was still active.With this in mind,in order to examine the durability of the complexes,we tried to reu the metal complexes for both heterogeneous and homogeneous systems.After CO production was stopped not only the gas pha but also the mixed solvent were resaturated by CO2and the photoreaction was resumed.As a result,five cy-cles,totally more than400h of photoirradiations were r
epeated and it was demonstrated that the catalytic ac-tivity was maintained at more than200h in the prent
30T.Hiro et al./Journal of Molecular Catalysis A:Chemical193(2003)27–32
Fig.3.Time dependence of CO2photoreduction of the partially heterogeneous system(Run1in Table1).
Fig.4.Time dependence of the CO2photoreduction by the homogeneous reaction system(Run6in Table1).
T.Hiro et al./Journal of Molecular Catalysis A:Chemical193(2003)27–3231 Table1
Generation of CO and H2by photoreduction of CO2a
Run Cation exchange
polymer Amount
(mg)
Fixed complex(mol)Reaction
time b(h)
CO
(mol(TN))
H2
(mol(TN))
CO/H2
1c Nafion11250Ru(bpy)3/15404125(8.3)276(18.4)0.45 2d Nafion112195Co(bpy)3/11422–––
3c Nafion NR-50669Ru(bpy)3/1516636(2.4)23(1.5) 1.6 4c Amberlite IR-12019Ru(bpy)3/1515044(2.9)8(0.5) 5.5 5e Nafion112100Ru(bpy)3/15and Co(bpy)3/154625(1.7)21(1.4) 1.2 6f Nafion11296Ru-Co complex/1521–––
7g None
(homogeneous system)
–Ru(bpy)3/15and Co(bpy)3/1521393(6.2)236(15.7)0.39
a All reactions were performed in the mixed solvent of DMF/TEOA(4/1,v/v)(30ml).
b Total irradiation time.
c Co complex(15mol)was dissolve
d in th
e mixed solvent.
d Ru complex(15mol)was dissolved in th
e mixed solvent.
e The mixed solvent was changed for each irradiation.
f Ru–Co complex is(bpy)2Ru(phen-C2H4-phen)Co(bpy)2.
g Control experiment.Both the Ru and Co complexes were dissolved in the mixed solvent.
heterogeneous system as shown in Table1(Run1)and Fig.3.As the lifetime of the metal complexes was ex-tended greater than that in the homogeneous solution, the total amount of CO produced was comparable to the homogeneous system.In addition the CO/H2ra-tio was incread to0.45.To our surpri,the com-plexes in the homogeneous system was also shown to be reusable(Table1;Run6),although the activity was largely decread after thefirst reaction as shown in Fig.4.
We thought the prolonged lifetime of the polymer-supported system was due to retardation of the decom-position of[Ru(bpy)3]2+becau the complex could not move from the ion exchange polymer surface so dissociation of the ligands was suppresd.Consider-ing the negligible increa at each photoreaction af-ter2days(∼50h),the photoreaction emed to be inhibited in the cour of time.At prent,the de-activation of the Co complex ems highly proba-ble as the color of the Ru complex did not change. The coordination of CO to the Co complex is specu-lated as one possible cau.However,this intermedi-ate should not be so stable that the deactivation was temporary and resaturation or replacement of the re-action system by fresh CO2allowed iteration of the reaction.
When a granular type of Nafion,NR-50,was ud, smaller amounts of gas with higher CO lectivity (CO/H2=1.2)were formed by photoirradiation and the lifetime of the system became shorter(Run3). Another type of cation exchange polymer,Amber-lite IR-120,was not so effective for CO formation but the lectivity was much higher(CO/H2=5.5) (Run4).When Co[(bpy)3]2+wasfixed to Nafion, however,neither CO nor H2was produced(Run2). Comparison of Runs1,2,5,and7shows thatfixa-tion of Co[(bpy)3]2+was ineffective under the prent experimental conditions.Becau the interaction be-tween the Ru and Co complexes should be almost the same for Runs1and2,we suppofixation of the Co complex drastically reduced the rate of interaction between[Co(bpy)3]+an
d CO2.Obviously that fur-ther spectroscopic and electrochemical analys are necessary.
From the prent results and the discussion by Zies-l et al.[1b],the following reaction feature was con-cluded:∗Ru(II)species was not quenched on cation exchange polymer for a certain period when electron transfer could effectively occur from the photonsi-tizer to Co[(bpy)3]2+in solution.It was also shown for thefirst time that a large amount of CO was pro-duced by repeated u of the photonsitizer.On the other hand,fixation of the Co complex was not ef-fective.Elucidation of the mechanistic feature and the state of the Ru complex on ion exchange polymer are currently being investigated.
32T.Hiro et al./Journal of Molecular Catalysis A:Chemical193(2003)27–32
4.Conclusion
In the prent study,it was found that the polymer-bad photonsitizer was easily prepared from Ru polypyridine complex and Nafion membrane,cation exchange polymer,while[Co(bpy)3]2+was neces-sarily dissolved in the reaction media for catalytic CO2photoreduction.The prent system allowed easy handling and recovery of the Ru complex including repeated cycles using CO2refreshing.As a result, production of CO(TN=8.3)was largely improved for the partially heteroge
neous system.The prolonged lifetime would be owing to suppression of the decom-position of the Ru complexfixed on polymer surface. Acknowledgements
Authors also thank to Mr.R.Otsuka of JASCO Sup-port Co.Ltd.for his help for the spectroscopic mea-surements.This work was partly supported by Saitama Prefectural Government Business Pre-incubator Project(No.552)and by the Grant-in-Aid Scientific Rearch(No.13650912)from Ministry of Education Science,Sports,and Culture of Japan. References
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