letters to nature
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Direct oxidation of hydrocarbons in a solid-oxide fuel cell
Seungdoo Park,John M.Vohs &Raymond J.Gorte
Department of Chemical Engineering,University of Pennsylvania,Philadelphia,Pennsylvania 19104,USA
..............................................................................................................................................书籍是人类进步的阶梯
The direct electrochemical oxidation of dry hydrocarbon fuels to generate electrical power has the potential to accelerate substan-tially the u of fuel cells in transportation and distributed-power applications 1.Most fuel-cell rearch has involved the u of hydrogen as the fuel,although the practical generation and storage of hydrogen remains an important technological hurdle 2.Methane has been successfully oxidized electrochemi-cally 3–6,but the susceptibility to carbon formation from other hydrocarbons that may be prent or poor power densities have prevented the application of this simple fuel in practical applications 1.Here we report the direct,electrochemical oxida-tion of various hy
drocarbons (methane,ethane,1-butene,n -butane and toluene)using a solid-oxide fuel cell at 973and 1,073K with a composite anode of copper and ceria (or samaria-doped ceria).We demonstrate that the final products of the oxidation are CO 2and water,and that reasonable power densities can be achieved.The obrvation that a solid-oxide fuel cell can be operated on dry hydrocarbons,including liquid fuels,without reforming,suggests that this type of fuel cell could provide an alternative to hydrogen-bad fuel-cell technologies.The principle of operation of a solid-oxide fuel cell (SOFC)involves reduction of molecular O 2at the cathode,diffusion of the O 2−through a zirconia-bad electrolyte,and oxidation of the fuel by O 2−at the anode 7.SOFCs must operate at high temperatures (to enable diffusion of O 2−through the electrolyte),so that C–H bond activation in hydrocarbons is feasible.The primary difficulty encountered during direct oxidation of hydrocarbons is rapid deactivation due to carbon deposition on the anode.The anode in conventional designs is a composite of Ni and yttria-stabilized zirconia (YSZ);this composite is an electronic conductor (due to Ni)and also an ionic conductor (due to YSZ).However,Ni catalys formation of graphite from hydrocarbons:except for a narrow range of operating temperatures and only for methane 3,carbon formation with Ni-bad anodes is unavoidable.
We have recently shown 5,8that excellent performance can still be achieved if the Ni-YSZ anode is replaced by a Cu-ceria anode.Cu is
an excellent electronic conductor but a poor catalyst for C–H bond activation;therefore,carbon formation,which limits the u of Ni cermets,is avoided with the Cu cermets.(The ceramic and metal composite is known as a cermet.)Ceria is ud in the anode becau of its high activity for hydrocarbon oxidation and its high ionic conductivity 9.The SOFCs ud here were prepared according to refs 5and 8,with a 60-m-thick,YSZ electrolyte,12.5mm in diameter,and a cathode formed from a 50:50mixture of YSZ and La 0.8Sr 0.2MnO 3powders.The anodes were 40wt%Cu and 20wt%CeO 2,held in place by a YSZ matrix formed from zircon fibres.In a cond cell,a 20%Sm 2O 3-80%CeO 2mixed oxide replaced CeO 2.Electronic contacts were formed using a Pt mesh and Pt paste.(On the basis of our previous work,in which cells without either Cu or ceria showed very poor performance 5,we do not believe the Pt played a catalytic role;however,this clearly needs to be checked.)Finally,the electrolyte disk was aled onto alumina tubes.The total surface area of the cathode and anode was 0.25cm 2.The cathode was left open to air,and fuel at a total pressure of 1.01bar was allowed to flow over the anode.An on-line gas chromato-graph (GC)was ud to measure the composition of the effluent product leaving the anode side of the fuel cell.
Figure 1shows the performance of the cell with the Cu-ceria anode when supplied with H 2or n -butane at 973or 1,073K.While the open circuit voltages (OCVs)for H 2were clo to the value predict
ed by the Nernst equation,1.05V,the OCV for n -butane was ϳ0.9V,which is lower than the potential of 1.1V calculated for complete oxidation.As GC analysis of the effluent from the anode showed negligible amounts of CO and CO 2at open circuit,leaks are an unlikely cau of the low OCV.As pointed out by Marina and Mogenn 10,the direct,electrochemical oxidation of hydrocarbons is unlikely to take place in one step.Therefore,we suggest that the low OCV for butane results from equilibrium involving partial oxidation reactions.For example,the oxidation of ethane to ethene and water,one of many possible reactions,has a theoretical OCV of 0.93V.
The maximum power densities were higher for H 2than for n -butane,with values of 0.31W cm −2and 0.18W cm −2,respectively,for H 2and n -butane at 1,073K,and 0.22W cm −2and 0.12W cm −2at 973K.Significantly higher power densities can certainly be achieved with thin-film electrolytes 3.
The crucial test is whether carbon formation occurs on the Cu-cermet anode when n -butane is ud.This cell was operated at the maximum power density of 0.12W cm −2in dry butane at 973K for a period of 48hours with no obrvable change in performance.The power output was also stable at 1,073K,but gas-pha reactions formed tar on the walls of the alumina tube at the
Current density (A cm –2)
P o w e r d e n s i t y (W c m –2)
V o l t a g e (V )
0.4
0.3
0.2
0.1
0.0Figure 1Power densities and current density–voltage relationships for an SOFC using the Cu-ceria composite anode.The cell had a 60-m electrolyte,and data are shown for the following fuels:filled circles,n -butane at 973K;open circles,n -butane at 1,073K;filled triangles,H 2at 973K;and open triangles,H 2at 1,073K.
Current density (A cm –2)
A m o u n t o f C O 2 p r o d u c e d (×1017 c m –2 s –1)
Figure 2CO 2production for methane and n -butane as a function of current density using the Cu-ceria composite anode.The data were obtained by gas analysis for:n -butane at 973K (open triangles),methane at 973K (open circles),and methane at 1,073K (filled circles).The lines were calculated from the current density,assuming complete oxidation.
higher temperature.Visual inspection of a cell after two days in n -butane at 1,073K showed that the anode itlf remained free of the tar deposits that covered the alumina walls.
Although it is possible that the power generated from n -butane fuels resulted from oxidation of H 2—formed by gas-pha reactions of n -butane that produce hydrocarbons with a lower C:H ratio—other evidence shows that this is not the ca.First,experiments were conducted in which the cell was charged with n -butane and then operated in a batch mode without flow.After 30minutes of batch operation with the cell short-circuited,GC analysis showed that all of the n -butane in the cell had been converted completely to CO 2and water.(Negligible amounts of CO 2were formed in a similar experiment with an open circuit.)Second,analysis of the CO 2formed under steady-state flow conditions,shown in Fig.2,demonstrates that the rate of CO 2formation incread linearly with the current density.(It was not possible for us to quantify the amount of water formed in our system.)Figure 2includes data for both n -butane at 973K,and methane at 973K and 1,073K.The lines in the figure were calculated assuming complete oxidation of methane (the dashed line)and n -butane (the solid line)to CO 2and water according to reactions (1)and (2):
CH 4þ4O 2Ϫ→CO 2þ2H 2O þ8e Ϫð1ÞC 4H 10þ13O 2Ϫ→4CO 2þ5H 2O þ26e Ϫ
ð2Þ
With methane,only trace levels of CO were obrved along with CO 2,so that the agreement between the data points and the calculation demonstrates consistency in the measurements and no leaks in the cell.With n -butane,simultaneous,gas-pha,free-radical reactions to give hydrocarbons with various C:H ratios make quantification more difficult;however,the data still suggest that complete oxidation is the primary reaction.Furthermore,the batch experiments show that the condary products formed by gas-pha reactions are ultimately oxidized as well.Taken together,the results demonstrate the direct,electrocatalytic oxidation of a higher hydrocarbon in a SOFC.
pastparticiple
Along with our obrvation of stable power generation with n -butane for 48hours,Fig.3further demonstrates the stability of the composite anodes against coke formation.Aromatic molecules,such as toluene,are expected to be precursors to the formation of graphitic coke deposits.In Fig.3,the power density was measured at 973K and 0.4V while the fuel was switched from dry n -butane,to 0.033bar of toluene in He for 30minutes,and back to dry n -butane.The data show that the performance decread rapidly in the prence of toluene.Upon switching back to dry n -butane,however,
boroughthe current density returned to 0.12W cm −2after one hour.Becau the return was not instantaneous,it appears that carbon formation occurred during exposure to toluene,but that the ano
de is lf-cleaning.We note that the electrochemical oxidation of soot has been reported by others 11.
The data in Fig.4show that further improvements in cell performance can be achieved.For the experiments,samaria-doped ceria was substituted for ceria in the anode,and the current densities were measured at a potential of 0.4V at 973K.The power densities for H 2and n -butane in this particular cell were approxi-mately 20%lower than for the first cell,which is within the range of our ability to reproduce cells.However,the power densities achieved for some other fuels were significantly higher.In particu-lar,stable power generation was now obrved for toluene.Simi-larly,Fig.4shows that methane,ethane and 1-butene could be ud as fuels to produce electrical energy.The data show transients for some of the fuels,which are at least partially due to switching.The role of samaria in enhancing the results for toluene and some of the other hydrocarbons is uncertain.While samaria is ud to enhance mixed (ionic and electronic)conductivity in ceria and could increa the active,three-pha boundary in the anode,samaria is also an active catalyst 12.Other improvements in the performance of SOFCs are possible.For example,the composite anodes could be easily attached to the cathode-supported,thin-film electrolytes that have been ud by others to achieve very high power densities 3.In addition to raising the power density,thinner electro-lytes may also allow lower operating temperatures.
Additional rearch is clearly necessary for commercial develop-ment of fuel cells which generate electrical power directly from hydrocarbons;however,the work described here suggests that SOFCs have an intriguing future as portable,electric generators and possibly even as energy sources for transportation.The sim-plicity afforded by not having to reform the hydrocarbon fuels is a significant advantage of the cells.Ⅺ
Received 13September 1999;accepted 26January 2000.
拿走英语1.Steele,B.C.H.Running on natural gas.Nature 400,620–621(1999).
2.Service,R.F.Bringing fuel cells down to earth.Science 285,682–685(1999).
3.Perry Murray,E.,Tsai,T.&Barnett,S.A.A direct-methane fuel cell with a ceria-bad anode.Nature 400,649–651(1999).
4.Putna,E.S.,Stubenrauch,J.,Vohs,J.M.&Gorte,R.J.Ceria-bad anodes for the direct oxidation of methane in solid oxide fuel cells.Langmuir 11,4832–4837(1995).
5.Park,S.,Craciun,R.,Vohs,J.M.&Gorte,R.J.Direct oxidation of hydrocarbons in a solid oxide fuel hane oxidation.J.Electrochem.Soc.146,3603–3605(1999).
6.Steele,B.C.H.,Kelly,I.,Middleton,P.H.&Rudkin,R.Oxidation of methane in solid-state electrochemical reactors.Solid State Ionics ,28,1547–1552(1988).
7.Lloyd,A.C.The power plant in your bament.Sci.Am.281(1),80–86(1999).
letters to nature
隐晦是什么意思
P o w e r d e n s i t y (W c m –2)
Time (×103 s)
Figure 3Effect of switching fuel type on the cell with the Cu-ceria composite anode at 973K.The power density of the cell is shown as a function of time.The fuel was switched from n -butane (C 4H
10)to toluene (C 7H 8),and back to n -butane.
P o w e r d e n s i t y (W c m –2)
Time (×103 s)
Figure 4Effect of switching fuel type on the cell with the Cu-(doped ceria)composite anode at 973K.The power density is shown as a function of time.The fuels were:n -butane (C 4H 10),toluene (C 7H 8),n -butane,methane (CH 4),ethane (C 2H 6),and 1-butene (C 4H 8).
8. al.A novel method for preparing anode cermets for solid oxide fuel cells.J.Electrochem.
Soc.146,4019–4022(1999).
9.Trovarelli,A.Catalytic properties of ceria and CeO 2-containing materials.Catal.Rev.Sci.Eng.38,
439–520(1996).
10.Marina,O.A.&Mogenn,M.High-temperature conversion of methane on a composite gadolinia-doped ceria-gold electrode.Appl.Catal.A 189,117–126(1999).
11.Christenn,H.,Dinen,J.,Engel,H.H.&Hann,K.K.Electrochemical Reactor For Exhaust Gas
Purification 1–5(Society of Automotive Engineers paper no.1999-01-0472,Warrendale,Philadelphia,1999).
12.Y amanaka,I.&Otsuka,K.Catalysis of Sm +3for the oxidation of alkanes with O 2in the liquid pha.
J.Mol.Catal.A 95,115–120(1995).
Acknowledgements
We thank C.Wang and W.L.Worrell for technical advice.This work was supported by the Gas Rearch Institute.
Correspondence and requests for materials should be addresd to R.J.G.(e-mail:gorte@as.upenn.edu).
letters to nature
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Single crystals of an ionic anthracene aggregate with a triplet ground state
H.Bock *,K.Gharagozloo-Hubmann *,M.Sievert *,T.Prisner *&Z.Havlas †
*Department of Chemistry,Johann Wolfgang Goethe University,Marie Curie Stras 11,D-60439Frankfurt am Main,Germany
†Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences,Flemingovo Nam 2,CZ-16610Prague,Czech Republic
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Crystalline supramolecular aggregates consisting of charged organic molecules,held together throug
h metal-cluster-mediated Coulomb interactions,have attracted interest owing to their unusual structural,chemical and electronic properties 1–3.Aggre-gates containing metal cation clusters ‘wrapped’by lipophilic molecular anions have,for example,been shown 4,5to be kineti-cally stable and soluble in nonpolar liquids such as saturated hydrocarbons.The formation of supramolecular aggregates can even be exploited to generate aromatic hydrocarbons that carry four negative charges and crystallize in the form of organic poly(metal cation)clusters 6,7or helical polymers 8.Here we report the anaerobic crystallization of an ionic organic aggre-gate—a contact ion ptuple consisting of a fourfold negatively charged ‘tripledecker’of three anthracene molecules bridged by four solvated potassium cations.Its electronic ground state is shown experimentally,using temperature-dependent electron paramagnetic resonance spectroscopy,to be a triplet.Although the spins in this biradical ionic solid are parated by a consider-able distance,density functional theory calculations 9indicate that the triplet ground state is 84kJ mol −1more stable than the first excited singlet state.We expect that the successful crystallization of the ionic solid we report here,and that of a covalent organic compound with a triplet ground state 10at room temperature,will
如何成为一名模特>郑州美甲培训
Table 1Density functional theory calculations
Singlet
Triplet D E total (kJ mol −1bl是什么意思的缩写
)−84Q (M 2−)
−1.58−1.52Q (M 1
−)
−0.86−0.83r (M 1−)
0.97
.............................................................................................................................................................................Selected results from density functional theory (DFT)calculations for the triplet ground state and the lowest singlet state of the biradical salt [(anthracene 4−(K +2THF 3)2.Shown are difference in total energies D E total and natural bond orbital charges Q as well as spin distribution r in the inner (M 2−)or the outer (M 1−)anthracene ligands.
stimulate further attempts to develop new triplet-ground-state materials for practical u.Molecular anions M n −
and their countercations Met +solv (where Met is the alkali metal)crystallize from aprotic solvents either as
solvent-parated [M n −][Met +solv ]n or as solvent-shared [M n −Met +
n ]solv ion pairs 11,12.They are formed in a multidimensional network of solution equilibria 13involving electron transfer and contact ion pair formation as well as aggregation,and are often dominated by cation solvation 14;such solvation is known to control numerous chemical reactions,including geochemical and biochemical reactions.For instance,if the p -hydrocarbon anthracene is reduced in ether or alkylamine solutions at alkali-metal surfaces [Met]x ,slight modifi-cations of the conditions lead to various reduction products which can be structurally characterized in single crystals 15,16(Fig.1).
The manifold of possible anthracene anion salt structures (Fig.1)ranges from the solvent-parated sodium salt of the bare anthra-cene radical anion 15(Fig.1a),via the double h 6-half-sandwichs of two [Li +TMEDA]ligands on both faces of the anthracene dianion 16(Fig.1b),to the ion quadruple of two anthracene radical anions
connected by a [K +2THF 3]
+2
bridge (Fig.1c)(e Methods),and stimulated attempts to crystallize even larger aggregates.(THF,tetrahydrofuran;TMEDA,tetramethylethylenediamine.)
Low-gradient crystallization by diffusion of n-hexane from an added layer into the blue THF solution of anthracene that is reduced at a potassium metal mirror yields—after three days standing at room temperature—black needles with a violet lustre (e Methods).On further standing of the decanted solution,another batch of black needles is collected.X-ray structure analysis proves the first crystal fraction to be the title compound (that is,the contact
ion ptuple [(anthracene)4−3(K +2THF 3)+2
2]1(Fig.2),whereas the cond
batch contains the smaller aggregate [(anthracene)2−2(K +2THF 3)+2
]1(Fig.1c).This suggests that the smaller compound might act as a ‘ed’,from which the crystal of the three-layered anthracene tetraanion (Fig.2)might grow.
The crystal structure analysis of the anthracene potassium p-tuple (Fig.2)reveals a hitherto unknown type of hydrocarbon aggregate:in the centrosymmetric double sandwich,the planar anthracene anions exhibit centroid/centroid distances of 565pm and are tilted by interplanar angles of +54Њand −54Њ.The four K +centres (with a total coordination number of 14)are each h 6-connected to the peripheral six-membered rings with contacts K +…C d −of 312–322pm length to the outer anthracene ligands,and of 290–335pm to the central anthracene;the perpendicular distances K +…ring amount to 289and 275pm.(C d −indicates partial changes at the individual centres.)The K +centres are 431pm apart,and each one exhibits two contacts K +…O to the O centres of two THF solvate ligands of lengths 265and 285pm.The planar anthracene anion rings are distorted differently:relative to the neutral molecule,the bonds C1–C2are shortened by 4pm and C2–C3are elongated by 2pm for the outer ligands,or shortened by 5pm and elongated by 4pm for the central dianionic bridge.This structural diversity is already known from published anthracene anion structures 15,16(Fig.1),and suggests that the outer ligands are
beamreaderO O O
O O Figure 1Selected examples of anthracene anion salts.a ,Anthracene radical anion-sodium bis(triglyme)(ref.15);b ,anthracene dianion-dilithium-bis(TMEDA)(ref.16);
欢迎光临日语c ,[(anthracene)2−2(K +2THF 3)+2
]1(e Methods).
