Reforming of ethanol in a microwave surface-wave plasma discharge

Re o ming o e hanol in a mic owa e su ace-wa e plasma discha ge A. Yanguas-Gil, J. L. Hueso, J. Co ino, A. Caballe o, and A. R. González-Elipea) Ins i u o de Ciencia de Ma e iales de Se illa and Dp os. de Física A ómica, Molecula y Nuclea and Química Ino gánica (CSIC-Uni e sidad de Se illa) (Recei ed 22 Ma ch 2004; accep ed 1 Sep embe 2004) Hyd ogen p oduc ion h ough plasma e o ming o e hanol a oom empe a u e and mode a e p essu e has been ca ied ou in a mic owa e su ace-wa e eac o . Bo h pu e e hanol and mix u es e hanol-wa e ha e been s udied. The e o ming yield was almos 100% in all condi ions wi h H2, solid ca bon, CO and CO2as he main eac ion p oduc s. In he mix u e e hanol-wa e he o ma ion o solid C was a oided. The op ical emission spec oscopy analysis has shown ha he o ma ion o he exci ed species CO*, CH* and C2 *depends on he plasma mix u e. The empe a u e o he OH* species was de e mined by analyzing he shape p o ile o i s emission band. © 2004 Ame ican Ins i u e o Physics.[DOI: 10.1063/1.1808875] Hyd ogen is being conside ed as a s a egic uel o he u u e and, consequen ly, he de elopmen o p ocedu es o i s p oduc ion is nowadays dese ing much e o .1,2 A p esen , wo majo echnologies a e being used o ob aining his uel a la ge scale: he ca aly ic ou e h ough he wa e gas shi eac ion and he elec oly ic decomposi ion o wa- e . In bo h cases he p oduced H2canno be conside ed as a enewable uel since i is ob ained om ano he p ima y en- e gy sou ce. The p esen wo k epo s on he easibili y o p oducing H2 om e hanol by plasma e o ming. E hanol is a liquid uel ha can be ob ained om he biomass and, in his ega d, can be conside ed as enewable. Plasma p ocedu es ha e been used in he pas o he e o ming o me hane and o he hyd oca bons. Depending on he ype o exci a ion (mic owa e, dielec ic ba ie dis- cha ge, e c.)i has been epo ed ha plasma e o ming o me hane yields C2 o highe hyd oca bons3and, in some cases, H2as p oduc s o he eac ion.4A p oblem wi h he di ec e o ming o CH4in o C and H2is ha his eac ion is endo he mic a oom empe a u e. The e o e, o i s p ac ical implemen a ion i is necessa y ei he o hea he hyd oca bon a highe empe a u es o o ea mix u es o me hane (o o he hyd oca bons)and H2O simula ing he wa e gas shi eac ion. Success ul a emp s combining bo h ypes o ap- p oaches ha e been epo ed. Thus, E ie an and Roshl2ha e shown ha hyd oca bon (pa icula ly CH4)and/o alcohols a e e icien ly e o med by dc o mic owa e (MW)plasmas when mix u es o hese gases a e mixed wi h s eam and hea ed a empe a u es highe han 473 K. In he p esen wo k, he easibili y o he hyd ogen ob- en ion h ough e o ming o e hanol and mix u es o e hanol plus wa e is s udied in a MW su ace-wa e (SW)plasma de ice a oom empe a u e. This plasma igni ion p ocedu e has been used o o he applica ions including hin ilm deposi ion5o emo al o ola ile o ganic compounds.6 The mic owa e discha ge is gene a ed in a qua z ube o 1.5 mm o inne adius. A SW launche (Su a on7) ans- o ms he elec omagne ic powe om he mic owa e gen- e a o in o a a eling wa e ha p opaga es along he dielec- ic ube, c ea ing he plasma inside i . Th ee di e en mix u es ha e been used: pu e e hanol (0.02 mol/h, 3 To ), mix u e e hanol-wa e (0.02 mol/h, 0.06 mol/h, 10 To ) and pu e wa e (0.06 mol/h, 10 To ). Vapo s we e dosed om liquid ampoules using calib a ed mass low con olle s. The elec omagne ic powe used anged be ween 20 and 80 W, al hough mos expe imen s we e done wi h a powe o 60 W. Composi ion o he eac ion mix u e was de e mined by mass spec ome y (Balze s P ei e QMS422 mass spec- ome e )a e calib a ion o he in ensi y o peaks wi h mix- u es o known composi ion. The emission spec um o he discha ge was analyzed by op ical emission spec oscopy (OES)using a Jobin-Y on HR250 monoch oma o and a Hamama su pho omul iplie ube. A scheme o he expe i- men al se up is p esen ed in Fig. 1. Mass spec ome y showed a s ong dec ease o he CH3CH2OH peaks e ealing almos 100% o e hanol decom- posi ion. Simul aneously peaks o H2, CO and CO2de el- oped, indica ing a ne p oduc ion o hese gases. A quan i a- i e e alua ion o he eac ion p oduc s is included in Table I. Besides CO, CO2and H2, which a e he main eac ion p od- uc s o he plasma p ocess, some C2 o highe hyd oca bons a e de ec ed as aces in he case o pu e e hanol. Unde hese condi ions a deposi o solid ca bon was also o med in he inne walls o he qua z ube. This solid ca bon became ho ed and was dele e ious o he plasma, which ex inguished when a c i ical mass o ca bon was o med. Wi h he wa e - e hanol mix u e he o ma ion o he solid ca bon was a oided while keeping a high yield o H2. In bo h cases he composi ion o he inal mix u e was no a ec ed by he inciden powe in he s udied ange, om 20 o 80 W. Al hough he gas mix u es we e no ex e nally hea ed, an inc ease o hei empe a u e was de ec ed as a esul o he plasma discha ge. Acco ding o he model p oposed by Moisan,8in a low p essu e su ace-wa e discha ge he mi- c owa e powe is abso bed mainly by he elec ons, which a)Au ho o whom co espondence should be add essed; elec onic mail: [email p o ec ed] FIG. 1. Schema ic plo o he expe imen al se up used o he e o ming o e hanol and mix u es o e hanol plus wa e . APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 18 1 NOVEMBER 2004 0003-6951/2004/85(18)/4004/3/$22.00 © 2004 Ame ican Ins i u e o Physics4004 Downloaded 10 Feb 2010 o 161.111.180.191. Redis ibu ion subjec o AIP license o copy igh ; see h p://apl.aip.o g/apl/copy igh .jsp pa ially lose hei excess ene gy h ough collisions wi h he gas molecules and a oms, hus inc easing hei empe a u e. The empe a u e o he ex e nal wall o he qua z ube p o- ides a lowe limi o his magni ude and i was measu ed using a he mocouple a di e en axial posi ions. Fo a pu e e hanol discha ge his alue anged be ween 500 and 700 K, inc easing wi h he inciden powe . Acco ding o he da a in Table I, and dis ega ding he mino i y p oduc s, he global plasma eac ions o pu e e h- anol and he wa e -e hanol mix u e can be desc ibed acco d- ing o CH3CH2OH →C+CO+3H 2,共1兲 CH3CH2OH+H 2O→2CO+4H 2.共2兲 A simila expe imen ca ied ou wi h pu e wa e also led o he almos o al spli ing o wa e acco ding o he eac ion H2O→H2+1/2O2. The ligh p oduced in he glow zone was cha ac e ized by OES. Figu e 2 shows he eco ded spec a co esponding o he h ee apo composi ions: e hanol, e hanol-wa e and pu e wa e . By compa ison wi h epo ed spec a9 he di e - en bands obse ed in his igu e ha e been iden i ied in Table II. Acco ding o he assignmen in his able he spec- um o he e hanol plasma is cha ac e ized by bands and lines due o CO*, OH* and H* species, oge he wi h some smalle bands due o CH* and C2 *species. Fo he mix u e e hanol plus wa e he spec um is domina ed by he lines o he Balme se ies o hyd ogen (H ␣ ,H ␤ ,H ␥ )and by he 3064 sys em o he OH* species. Some mino i y con ibu ion o CO* and CH* species can be also de ec ed in his spec um. Excep o he absence o hese la e bands, he spec um o wa e plasma esembles ha o he mix u e o e hanol plus wa e wi h he addi ion o some peaks due o a omic oxygen. The appea ance in he plasma o pu e e hanol o in e media e species o he ype C2 *and CH* can be aken as an indica ion o he o ma ion o , espec i ely, solid ca bon and highe hyd oca bons, as hese species a e ypically ound as in e - media e adicals in he o ma ion mechanism o hese p oduc s.3 The o a ional empe a u e o he OH molecule was es- ima ed by i ing he emission band o he OH* sys em lo- ca ed be ween 306 and 314 nm. The i ing p ocedu e has been discussed in di e en publica ions.10,11 These calcula- ions yield he o a ional empe a u es summa ized in Table III o he h ee eac ions conside ed and 60 W o exci ing powe . The da a e eal ha he o a ional empe a u e o OH inc eases g ea ly om wa e o he e hanol plasma. Calcula- ions o OH o a ional empe a u e as a unc ion o powe showed ha his pa ame e inc eased almos linea ly wi h he powe , anging om 1150 K a 30 W o 1300 K a 60 W o pu e wa e . Simila linea a ia ions we e ound o pu e e hanol and he mix u e o e hanol-wa e . This empe a u e canno be iden i ied wi h he gas empe a u e since he o a- ional modes a low p essu es may ha e a highe empe a u e han he ib a ional and ansla ional modes.12 TABLE II. Main ea u es o OES spec a. Species Sys em T ansi ion Posi ion (nm) C2Swan 3P→3P, g ound s a e 450–500 CH 4300 A 2D→2P, g ound s a e 430 3900 A 2S→2P, g ound s a e 390 3143 A 2S→2P, g ound s a e 314 CO 4 h Posi i e A1P→X1S, g ound s a e 200–280 3 d Posi i e and 5B b3S→a3P150–300 OH 3064 A 2S→2P, g ound s a e 295, 306 H Balme se ies n→2s,2p656, 486, 434, 410 TABLE III. Calcula ed OH* o a ional empe a u es. Reac ion mix u e, 60 W TR(K) E hanol ⬎2500 E hanol+wa e 1600 Wa e 1300 TABLE I. Pe cen ages o he di e en ca bon compounds de ec ed as p od- uc s du ing he plasma mic owa e e o ming o e hanol (Solid C was de e - mined as he di e ence o he decomposed e hanol and he o med CO and CO2). % De ec ed E hanol 60 W E OH+Wa e 60 W E OH+Wa e 80 W CO 55 95 95 CO2⬍15 5 C deposi 45 ¯¯ CxHy,x⬎2 T aces ¯¯ CxHy,x=1, 2 T aces T aces T aces FIG. 2. OES spec a o plasma discha ges: (a)pu e e hanol, (b)e hanol/ wa e and (c)pu e wa e . The inse igu es show he spec al ange be ween 250 and 350 nm in mo e de ail. Appl. Phys. Le ., Vol. 85, No. 18, 1 No embe 2004 Yanguas-Gil e al. 4005 Downloaded 10 Feb 2010 o 161.111.180.191. Redis ibu ion subjec o AIP license o copy igh ; see h p://apl.aip.o g/apl/copy igh .jsp The inc ease in he a e age o a ional empe a u e o OH* species as he o e all a io C/O in he discha ge in- c eases (i.e., o mix u es CH3CH2OH+H2O and, pa icu- la ly, pu e e hanol)can be ela ed o he dissocia ion ene gies o he di e en in e media e species de ec ed in he dis- cha ge. Thus, while he alue o he dissocia ion ene gies o C2, CH and CO species a e 6.2, 3.46 and 11.09 eV, espec- i ely, ha o OH is 4.39 eV.13 The e o e, he o ma ion and pos e io decomposi ion o dia omic species such as CO* and C2 *implies he elease o mo e ene gy, which is dis ib- u ed by exci a ion exchange om he ib a ional and o a- ional modes o CO and C2 o he es o he species o he plasma and, in pa icula , o he OH*. Mo eo e , o he same gas empe a u e, he ee ene gy and he e o e he en- halpy o eac ion (1)is g ea e han ha o eac ion (2)(i.e., −100 kJ/mol and −70 kJ/mol a 700 K),14 so ha o plas- mas o pu e e hanol mo e ene gy is eleased. This excess o ene gy is abso bed by o he molecules, yielding highe em- pe a u es and highe exci a ion deg ees in he pu e e hanol discha ge. The eac ion schemes p esen ed below, compa ible wi h epo ed eac ion kine ics,15 accoun o bo h he eac ion p oduc s and he in e media e species de ec ed by OES. CH3CH2OH —— → elec on impac H2 ↑ CH2OH,CH3—— → e, adical impac H2 ↑ CO*,C 2*—— → ca bon condensa ion C共solid兲,CO, 共3兲 冦 CH3CH2OH —— → e impac H2 ↑CH2OH,CH3 H2O—— → e impac H*,O* 冧 →H2,CO. 共4兲 The eac ion scheme (3)co esponds o he e o ming o e hanol. I indica es ha a omic hyd ogen is emo ed om e hanol, and e en ually he C–C bond dissocia ed, by di ec elec on impac . This p ocess con inues o he in e media e species and leads o he o ma ion o H2. Some o hese in e media e species such as CH* ha e been de ec ed by OES [c . Fig. 2(a)]. Fu he hyd ogen emo al by elec on and adicals impac leads o he o ma ion o CO* and C2 * species [c . Fig. 2(a)], his la e being conside ed a p ecu so o he o ma ion o solid ca bon. The eac ion scheme (4)accoun s o he p ocesses ha ake place in mix u es o e hanol plus wa e . The p ocess can be conside ed as equi alen excep o he excess o oxygen adicals in he plasma, coming om he spli ing o he wa e molecules, ha p e en he o ma ion o C2 * adicals and he e o e he p oduc ion o solid ca bon. The p e ious esul s show ha almos 100% e o ming o e hanol can be achie ed by plasma decomposi ion in a SW de ice by wo king a mode a e p essu e and oom em- pe a u e. In mos wo ks dealing wi h he plasma e o ming o hyd oca bons a oom empe a u e, highe hyd oca bons a he han H2a e ob ained as main eac ion p oduc s. By con as , plasma decomposi ion o e hanol, acco ding o e- ac ion (1), is he modynamically a o ed a oom empe a- u e and has a e y nega i e alue o ee ene gy a a em- pe a u e close o ha measu ed a he walls o he plasma ube (i.e., ␦ G=−100 kJ/mol a 700 K). The p esence o an oxygen a om in he e hanol molecule and he nega i e ene gy o o ma ion o CO a e he d i ing o ces ha a o he plasma decomposi ion eac ion o e hanol a oom empe a- u e. 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