Journal of Membrane Science 360 (2010) 397–403
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Journal of Membrane
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Ammonia synthesis at atmospheric pressure using a reactor with thin solid electrolyte BaCe 0.85Y 0.15O 3−␣membrane
W.B.Wang a ,b ,1,X.B.Cao a ,W.J.Gao b ,F.Zhang b ,H.T.Wang a ,G.L.Ma a ,∗
a
Key Laboratory of Organic Synthesis of Jiangsu Province,College of Chemistry,Chemical Engineering and Materials Science,Suzhou University,Suzhou 215123,People’s Republic of China b
Center of Analysis and Testing,Suzhou University,Suzhou 215123,People’s Republic of China
a r t i c l e i n f o Article history:
Received 27January 2010
Received in revid form 8May 2010Accepted 14May 2010
Available online 9 June 2010Keywords:Reactor BCY15
Thin proton conduction electrolyte membrane
Ammonia synthesis at atmospheric pressure
a b s t r a c t
A thin proton conduction electrolyte BaCe 0.85Y 0.15O 3−␣(BCY15)membrane was deposited on a porous green NiO–BCY15substrate by a modified spin-coating method.The den,crack-free electrolyte BCY15layer with thickness of ca.30m was obtained after the bi-layer was co-sintered at 1400◦C for 5h.With Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−␣as the cathode,a membrane reactor for ammonia synthesis at atmospheric pressure was asmbled.The membrane reactor was characterized by field-emission scanning electron microscope (FESEM),X-ray diffraction (XRD)and electrochemical AC impedance.The peak ammonia formation rate of about 4.1×10−9mol s −1cm −2was achieved with an impod DC current 1mA at 530◦C.The process bad on a further optimized BCY15membrane reactor may be a potential route for ammonia synthesis at atmospheric pressure.
© 2010 Elvier B.V. All rights rerved.
1.Introduction
It is well known that proton conduction ceramics are a kind of important functional materials and have attracted much atten-tion becau of their potential applications in solid oxide fuel cells (SOFCs),hydrogen nsors,steam electrolyzers,paration and purification of hydrogen,hydrogenation and dehydrogena-tion of some organic compounds and ammonia synthesis,etc.[1–8].
Ammonia is a major and important raw material for industry and agriculture.Today,synthetic ammonia is still the ba from which virtually all nitrogen-containing products are derived.For example,the fertilizer generated from the ammonia is responsible for sustaining one-third of the Earth’s population.The traditional ammonia synthesis method is the Haber–Bosch process,in which nitrogen gas and hydrogen gas react on an enriched iron or cobalt catalyst at about 450◦C,and especially under high pressures of 15–30MPa.And due to the thermodynamic restrictions the hydro-
∗Corresponding author at:College of Chemistry,Chemical Engineering and Mate-rials Science,Dushu Lake Campus of Suzhou University,Suzhou 215123,China.Tel.:+8651265880326;fax:+86512658800
89.
E-mail address:32uumagl@ (G.L.Ma).1
Address:Center of Analysis and Testing,Dushu Lake Campus of Suzhou Univer-sity,Suzhou 215123,China.
gen conversion rate is low (10–15%).In 1998,ammonia synthesis at atmospheric pressure has been realized successfully in an elec-trolytic cell reactor using a high temperature proton conductor SrCe 0.95Yb 0.05O 3−␣.The process provides an alternative route that permits operation at atmospheric pressure and avoids the prob-lem of thermodynamic restrictions impod on the conventional Haber catalytic reactors [8].Since then,bad on the pioneer-ing work of Marnellos and Stoukides,considerable efforts have focud on various aspects of the cell reactor in order to improve the ammonia formation rate [8–21].Stoukides and coworkers rearch group has studied in detail the ammonia synthesis reaction in solid state proton conducting cells on Pd electrodes and on an industrial Fe catalyst by using strontia–ceria–ytterbia electrolytes [8–10].More recently,they first demonstrated the feasibility of an electrocatalytic process in which ammonia was synthesized from steam and nitrogen by using either oxygen ion or proton conducting solid electrolyte cells [11].Yiokari
et al.have reported that the catalytic activity of industrial ammonia synthesis catalysts could be enhanced by up to 1300%by inter-facing the catalyst with a CaIn 0.1Zr 0.9O 3−␣proton conductor and electrochemically supplying protons to the catalyst surface [12].Wherein,the effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA)was first demonstrated under high (50bar)pressure.In addition,many other ion conductors such as La 1.9Ca 0.1Zr 2O 6.95[13],Ce 0.2M 0.2O 2−␦(M =La,Y,Gd,Sm)[14],
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0376-7388/$–e front matter © 2010 Elvier B.V. All rights rerved.doi:10.sci.2010.05.038
398W.B.Wang et al./Journal of Membrane Science360 (2010) 397–403
Ca2+-doped La2M2O7(M=Ce,Zr)[15],Ba3(Ca1.18Nb1.82)O9−␦[16], La0.9M0.1Ga0.8Mg0.2O3−␣(M=Ca,Sr,Ba)[17,18],BaCe1−x M x O3−␣[19]have been extensively studied as possible electrolytes for ammonia synthesis.Not long ago,Liu and coworkers reported that high ammonia formation rate was obtained at atmospheric pres-sure and low temperature(25–100◦C)by using Ni–Ce0.8Sm0.2O2−␦(Ni–SDC)as an anode,various cathode catalysts(Sm1.5Sr0.5MO4 (M=Ni,Co,Fe),SmFe0.7Cu0.3−x Ni x O3(x=0,0.1,0.2,0.3)),and Nafion prot
那么骄傲歌词on exchange membrane as a proton permeating membrane [20,21].The highest rate of evolution of ammonia was up to 1.13×10−8mol cm−2s−1using SmFe0.7Cu0.1Ni0.2O3as the cathode at80◦C[21].
In our prior work we studied proton conduction in BaCe1−x Y x O3−␣ceramics in the intermediate temperature range of300–600◦C[22].We confirmed that Y doped BaCeO3materials exhibit excellent proton conduction and are almost pure proton conductors under a wet reducing atmosphere.We also found that BaCe0.85Y0.15O3−␣has the highest proton conductivity among the ceramic samples with nominal Y concentration0.05,0.10, 0.15,and0.20,respectively.The electrolytic cell reactor bad on BaCe0.85Y0.15O3−␣(thickness:ca.0.8mm)showed a good performance in ammonia synthesis at atmospheric pressure.Nev-ertheless,the relatively high resistance loss due to relatively low proton conductivity at intermediate temperature still becomes one of the obstructions to improve further the ammonia formation rate. More recently,den proton conduction electrolyte membranes have received considerable interests for SOFCs and hydrogen permeation[23–27].The performances of SOFCs bad on proton conduction electrolyte membrane have been greatly enhanced [23–25].However,for ammonia synthesis using proton conductors as electrolytes,most of previous studies were devoted to the bulk materials[8–19].The bulk materials as electrolytes in the above studies,how
ever,were rather thick(ca.0.5mm or more)and were sintered by high temperature.In the practical u of perovskite-type proton conduction electrolyte reactor for ammonia synthesis, it is indispensable to develop large area and thin electrolyte mem-branes as well as highly active electrodes to improve performance.
A thin electrolyte membrane,instead of the bulk materials,may have lower resistance loss.In addition,thin membranes also have obvious advantages over bulk materials such as lowering operating temperature and improving current efficiency,and so on.Thus,it is necessary to develop a cost-effective process to asmble ammonia synthesis reactors with thinner electrolyte membranes to reach higher performances.Many techniques have been developed to prepared thin electrolyte membranes,such as sol–gel,spin-coating,suspension spray,dry-pressing,chemical vapor deposition,plasma spraying,and RF sputtering[28].Among the preparation methods,the spin-coating method is a simple, cost-effective and efficient method,by which the thickness of the thin electrolyte membrane can be easily controlled by the time and spin-coating velocity.
In the prent work,a den,crack-free thin proton conduc-tion BCY15electrolyte membrane(∼30m)has been prepared successfully over NiO–BCY15anode substrate by a modified spin-coating method.A thin membrane reactor for ammonia synthesis at atmospheric pressure,which
was compod of two porous function layers(Ba0.5Sr0.5Co0.8Fe0.2O3−␣cathode and Ni–BCY15 anode substrate)and thin electrolyte(BCY15)membrane,was asmbled.We prent the results of its ammonia synthesis rearch.Electrical properties of the thin BCY15electrolyte mem-brane reactor were also studied by AC impedance spectra.The work is also important to identify the optimum materials and conditions for various potential applications of the proton conduc-tion perovskite-type materials such as paration and purification of hydrogen,hydrogenation and dehydrogenation of organic compounds.2.Experimental
BaCe0.85Y0.15O3−␣precursor powder was prepared by a citric–nitrate process.All the reagents ud are analytical-grade. The required amounts of Ba(NO3)2,Ce(NH4)2(NO3)6and Y(NO3)3 (Y2O3were dissolved into dilute HNO3)were added and dissolved in deionized water under stirring and citric acid was added as the complexant with citric acid/metal mol ratio t at1.5.The solution was heated under stirring,evaporated to form a vis-cous gel and the pale yellow ash was obtained after ignited to flame.The BaCe0.85Y0.15O3−␣powder was obtained by calcining the ash at1150◦C for8h.NiO powder was prepared by calcining Ni(NO3)2·6H2O powder at400◦C.Ba0.5Sr0.5Co0.8Fe0.2O3−␦powder was also synthesized by a citric-nitrate process.Analytical-grade Ba(NO3)2,Sr(NO3)2,Co(NO3)2·6H2O and Fe(NO3)3·9H2O were ud as t
he starting materials.Stoichiometric amounts of nitrates were dissolved in deionized water and citric acid was added as a com-plexant.The molar ratio of the total cation concentration to the citric acid was1:1.5.The solution was evaporated at75–100◦C to form a dry gel,which wasfinallyfired at1000◦C for5h to get Ba0.5Sr0.5Co0.8Fe0.2O3−␦powder.The as-prepared BCY15and NiO powders were blended with a weight ratio of35:65and ball-milled for10h using ethanol as media.To form sufficient porosity in the substrate,10wt%starch was added as the pore former.The well-mixed powder was presd into green pellets of13mm in diameter and0.8mm in thickness under50MPa,and subquentlyfired at 1000◦C for4h as the anode substrate.
Before preparing slurry the as-prepared BCY15powders must pass through a sieve(500meshes)to guide uniform BCY15pow-ders.Without a sieve,it is difficult to uniformly and fully distribute BCY15on the organic solution and obtain uniform,crack-free mem-brane.For the preparation of electrolyte slurry BCY15powder was disperd into the organic solution of6%ethyl cellulo-terpineol.The weight ratio of powder to the organic solution was 1.The BaCe0.85Y0.15O3−␣slurry was spin-coated onto the pre-sintered NiO–BCY15anode substrate.Three spin-coating cycles were needed to get an adequate thickness.After dried for30min, the bi-layer of BCY15electrolyte membrane and anode substrate was sintered at1400◦C for5h at a heating rate of2◦C min−1.The Ba0.5Sr0.5Co0.8Fe0.2O3
−␣paste was coated onto the pre-sintered membranes and the surface area was ca.0.5cm2.After co-firing at1000◦C for2h,thin BCY15membrane reactors were obtained. Ag–Pd paste was painted on two surfaces(ca.0.5cm2),dried by infrared lamp and thenfired at900◦C for20min as current collec-tors.
Here,Ni/electrolyte cermet was lected as anode on the basis of following facts:(1)NiO is ud as a pore former becau NiO can be reduced to Ni by H2.A highly porous Ni/BCY15cermet with a large surface area aids its effectiveness as a catalyst.(2)Ni has excel-lent electronic conduction and exhibits very good chemisorption properties and rves as an excellent electrocatalyst for electro-chemical reaction of hydrogen.(3)Porous cermet supports the BCY15electrolyte membrane and helps it maintain its shape over time,and additionally,offers a significant part of proton con-tribution to the overall conductivity,thus effectively broadening the three-pha boundaries[25].Ba0.5Sr0.5Co0.8Fe0.2O3−␣is one of promising cathode materials becau of their good electronic-ion mixed conduction[29].Gas pha diffusion into the porous BSCF cathode must also be considered and is known to become impor-tant at intermediate temperature.BSCF exhibits good compatible with doped cerates electrolyte and have excellent chemical stabil-ity.BSCF has some electrocatalytic activity for ammonia synthesis [10,21].
Morphologies of the membrane reactors before and after testing were obrved by afield-emission scanning electron microscopy (FESEM,Hitachi S-4700).The phas of all powder samples were
W.B.Wang et al./Journal of Membrane Science360 (2010) 397–403
399都梁软胶囊
Fig.1.The schematic diagram of the experimental apparatus for ammonia synthesis reaction with thin proton conduction electrolyte BCY15membrane reactor at atmospheric pressure.
identified by a powder X-ray diffraction(XRD)on a Panalytical
X’pert Pro MPD diffractometer with Nifilter using Cu K␣radiation.
Electrical properties of the membrane reactors were characterized
by an AC impedance method using electrochemical workstations
(Zahner IM6EX)in the frequency range from1Hz to3MHz.The
interfacial resistance error derived from electrode area was about
±1.0%.
Fig.1shows the schematic diagram of the experimental appa-
ratus for ammonia synthesis reaction with thin proton conduction
BCY15membrane reactor at atmospheric pressure.Two ceramic
glass rings were ud as the ceramic binding agents to al the reac-
tor to two den alumina tubes at the solidification temperature of
900◦C for20min.The gas tightness of the membrane reactor was
evaluated by measuring theflow rates of Ar in the inlet and outlet.
More accurate measurement was conducted to evaluate the hydro-
gen leakage by using hydrogen nsor(Shanghai Gainforce SG33A)
with hydrogen gas instead of Ar on the anode side and Ar gas as
dilute gas on the other side.In our experiments,the leakage was
lower than1.0%.
Ammonia synthesis experiments were carried out by using the
pre-prepared BCY15membrane reactor at atmospheric pressure.
Wet H2,Ag–Pd
Membrane reactor
Ag–Pd,dry N2
In the ammonia synthesis experiments,the cathode and anode werefilled with a dry high pure nitrogen stream(99.999%,dried by a cold trap bad on liquid nitrogen)and a high pure hydrogen stream(99.999%,pH2O=0.023atm),respectively.During the mea-surements,the volumetricflow rate of dry pure nitrogen was the same as the humidified 30ml/min.A direct current was nt to the electrolytic cell.The generated NH3was absorbed by10ml dilute sulfuric acid solution with an initial pH of3.25for 20min,and then the concentration of NH4+in the absorption solu-tion was analyzed by spectrophotometry(VIS-723N)after Nessler’s reagent was added to the above solution.The absorbance accuracy of the instrument is less than0.5%and the maximum range of mea-sured value varies from0.0to4.0.Therefore the testing error can be controlled less than2.0%.The blank test was also preformed under open circuit condition[17].Before testing,the anode substrate was reduced in pure hydrogen at700◦C to get a porous anode substrate.3.Results and discussion
Fig.2(a)and(b)displays the XRD patterns of bi-layer membrane reactor consisted of the BCY15–NiO(35:65)anode substrate and BCY15electrolyte membrane sintered at1400◦C for5h.Fig.2(a) shows that the BCY15membrane after sintering showed a sin-gle pha without impurity phas.It could be clearly en from Fig.2(b)that there were only peaks corresponding well to a mixture of BCY15and NiO(Fig.2(c))for the substrate.The result indicates that BCY15and NiO did not react with each other during the sin-tering process.From Fig.2(d)we know a single pha BSCF powder was obtained after sintering at1000◦C for5h.
Fig.3(a–c)shows thefield-emission scanning electron micro-scope images of half membrane reactor with the structure of NiO–BCY15/BCY15sintered at1400◦C for5h.From the SEM sur-face image of BCY15membrane displayed in Fig.3(a),it could be en that the membrane with clear grain boundaries was den, uniform,and crack-free with an average grain size of2–5
m.
Fig. 2.XRD patterns for(a)BCY15electrolyte membrane sintered at1400◦C for5h,(b)NiO–BCY15anode substrate,(c)NiO powder prepared by calcining Ni(NO3)2·6H2O powder at400◦C for5h and(d)the Ba0.5Sr0.5Co0.8Fe0.2O3−␦powders calcined at1000◦C for5h.
400W.B.Wang et al./Journal of Membrane Science
360 (2010) 397–403
Fig.3.(a)SEM image of surface morphology of BCY15electrolyte membrane deposited on the NiO–BCY15anode substrate;(b)SEM image of the cross-ctional view of the half membrane reactor;(c)the cross-ctional morphology of anode substrate after electrochemical testing and (d)SEM image of the cross-ctional view of thin BCY15electrolyte membrane reactor after testing.
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Combined with XRD result,it is interesting to note that the elec-trolyte could be densified and formed a single perovskite pha after sintering at 1400◦C,which may be ascribed to the favorable effects of the surface tension of the membrane and fluidity of slurry.Fig.3(b)shows the fracture cross-ctional SEM image of half mem-brane reactor.From the fracture cross-ctional SEM image,it can be en that the thickness of the membrane was consistent and around 30m and the membrane adhered to the anode substrate firmly.Although some pinholes were obrved on the surface of the electrolyte layer,as shown in Fig.3(b),no penetrated pores were obrved inside the electrolyte layer.Obviously the modified spin-coating method has played an important role in improving membrane quality.Fig.3(c)shows additional porosity developed in the substrate after electrochemical testing and the porosity of sub-strate was rather high enough to form connected pores in substrate.SEM image of the cross-ctional view of multi-layer thin BCY15electrolyte membrane reactor after testing is shown in Fig.3(d).It could be clearly en that after testing the electrolyte membrane also adhered tightly to the anode substrate.The BSCF cathode was also porous but showed some large pores at the interface.
污的作文Fig.4prents the AC impedance spectra of membrane reactor system under the open circuit condition in the range of temper-ature 400to 700◦C.As shown in Fig.4,at temperature below 600◦C,th
e impedance spectrum consisted of two depresd mi-circles.The high-frequency micircle may be ascribed to the resistance of grain and grain boundary,while the low-frequency micircle is connected with the resistance of the interface of electrode–electrolyte.However the impedance spectrum shows only one depresd arc which ascribes mainly to the resistance of the interface of electrode–electrolyte at a temperature of 700◦C.In the spectra the total resistance of membrane reactor (R t )could be determined from the intercept with the real axis at low frequencies.
The intercept of the left micircle in the spectroscopy prents the grain resistance (R g )of the BCY15membrane electrolyte,while the interction between the spectra at higher frequency and the real axial prents the grain boundary resistance (R gb ).The difference of R t and (R g +R gb )corresponds to the electrode–electrolyte interface polarization resistance (R p ).The increa of temperature resulted in a remarkable decline of R p and R g ,typically R p from 7.51 cm 2at 400◦C to 0.22 cm 2at 700◦C and R g from 4.84 cm 2at 400◦C to 0.65 cm 2at 700◦C,respectively.It could be obrved that at the higher temperature of 700◦C R g was higher than R p ,
whereas
Fig.4.The impedance spectra of the membrane reactor with BCY15electrolyte at different temperatur
es.Impedance was measured under open circuit conditions in wet hydrogen (anode)and dry N 2(cathode)atmospheres with a flow rate of 30ml/min.
W.B.Wang et al./Journal of Membrane Science360 (2010) 397–403
401
Fig.5.The relationship between the rate of ammonia formation and operating tem-perature under a direct current of1mA.The dotted line is the attainable maximum rate of ammonia formation at the he ideal Faradaic rate of ammonia formation.
at temperatures below600◦C R g was less than R p.That is,com-pared with the electrolyte membrane,the resistance for electrode polarization incread more quickly as the temperature declined. The result indicates that under the current conditions the electrode polarization R p was a dominant factor to reduce the performance of the membrane reactor at the temperature below600◦C.
The rates of ammonia formation obtained with the membrane reactor as a function of temperature were measured under a direct current of1mA.As shown in Fig.5,the rate of ammo-nia formation incread,arrived at a maximum at530◦C,and then decread at temperatures over530◦C.The maximum rate of ammonia formation(4.1×10−9mol s−1cm−2)was obtained under a direct current of1mA at530◦C,which was considered to be the optimum temperature for ammonia synthesis.Compared with the value(2.1×10−9mol s−1cm−2)in our prior report[22],the value is almost double that one.In our ca,the current efficiency has been improved obviously.The rate of NH3form
ation bad on our BCY15 membrane reactor is also higher than tho for pyrochlore-type complex oxides La1.95Ca0.05Zr2O7−␦[13]and complex perovskite-type oxides Ba3(Ca1.18Nb1.82)O9[16].One important reason is that the co-sintering BCY15electrolyte membrane reactor had lower resistance loss and lower electrode polarization.Combination with the preceding AC impedance spectrum analysis we know that with the increasing temperature the polarization resistance and the elec-trolyte membrane resistance may decrea,which is beneficial to improving the ammonia formation rate.But on the other hand,the electron conduction and ammonia decomposition rate increas with the increa of temperature.Under low oxygen partial pres-sure and in the reducing atmosphere,the electronic conduction of cerates may appear at elevated temperatures according to the following reactions(1)and(2).
一英文
O X O↔1
2
乐在其中O2+V··O+2e−(1)
2Ce X
Ce +2e−↔2Ce
Ce
(2)
Eq.(1)shows that each oxygen vacancy generated is combined with two free electrons.Eq.(2)suggests that a part of Ce4+may be reduced to Ce3+in the reducing atmosphere and the reduc-tion reaction results in the introduction of electronic conduction. For the oxygen partial pressure experimental dependence of the total electrical conductivity of BCY15at intermediate tempera-ture(300–600◦C),our rearch shows that the electron conduction was negligibly small and not a key factor under current
condi-Fig.6.(a)The relationship between the rate of ammonia formation and direct cur-rent applied on the BCY15membrane reactor at530◦C and(b)the membrane reactor potential vs.the applied direct current at530◦C.
tion.As reported by the Stoukides and coworkers rearch group, at temperatures over500◦C,ammonia decomposition becomes a dominant factor and there is a stronger increa of decomposition rate than of the formation rate and thus the overall rate decreas [9,10].The two different effects mentioned above may result in the maximum of the rate of ammonia formation.
The dotted line in Fig.5shows the attainable Faradaic maximum rate of ammonia formation at the he ideal Faradaic rate.According to Faraday’s Law,the current I corresponds to a molarflux of I/2F moles of hydrogen per cond and the theoret-ical rate of ammonia formation is2/3×I/2F.In the current ca, the conversion rate of electrochemically supplied H2was about 60.0%under a direct current of1.0mA at530◦C.Provided that the electrochemically supplied hydrogen reacts fully with N2,the ammonia decomposition rate was ca.40.0%under current condi-tion.The value was higher than the result reported in literature[8]. The main reason for this difference is due to different configurations between ammonia synthesis reactors.
Fig.6(a)prents the relationship between the rate of ammo-nia formation and the applied DC current at530◦C with the as-prepared membrane reactor.The trend was similar to that in Fig.5.The rate of ammonia formation incread with increas-ing applied current,and then decread after1.0mA.The change of the rate of ammonia formation with the applied DC current may be due to that nitrogen chemisorption was hindered by the high rate of H+supply,which poisoned the catalyst cathode sur-face[10],i.e.the resulting electrode polarization.Fig.6(b)shows
