M ethane D ry Reform ing over A lum ina Supported Co Catalysts

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1 CH EM. R ES. CH IN ESE U. 2004, 20 (4), M ethane D ry Reform ing over A lum ina Supported Co Catalysts CH EN X iao2w ei, X IAO T ian2cun, Sergio L. Gonz lez Co rt s and M alco lm L. H. Green 3 Inorg an ic Chem istry L abora tory, U n iversity of O xf ord, S ou th P a rks R oad, O X 1 3Q R, U K R eceived June 3, 2004 A series of CogΧ2A l2o 3 catalysts w ere p repared w ith the imp regnation m ethod and characterized by m eans of the BET specific surface area, X2ray diffraction (XRD ), therm ogravim etric analysis (T GA ) and L aser R am an spectro scopy. T he CogΧ2A l2o 3 catalysts w ere activated by using H 2, 20%CH 4gH 2 o r CH 4, re2 spectively. T here w as no obvious difference betw een the activities of the CogΧ2A l2o 3 catalyst activated by us2 ing the different activation m ethods fo r m ethane dry refo rm ing. T he catalytic p roperties of the CogΧ2A l2o 3 catalysts w ith different Co loadings w ere also investigated. T he op tim ized Co loading fo r the CogΧ2A l2o 3 cata2 lyst p retreated w ith 20% CH 4gH 2 is around 12% (m ass fraction). Ke yw o rds M ethane dry refo rm ing, A lum ina suppo rted cobalt catalysts, Catalytic deactivation A rtic le ID (2004) In troduction T he ca rbon d iox ide refo rm ing of m ethane to syn thesis gas has received a great deal of atten tion du ring the past decade becau se it u tilizes bo th the green hou se gases and gives a low er mo lar ratio of H 2 to CO ( 1), w h ich is p referab le fo r the syn the2 sis of h igher hyd roca rbon s and the syn thesis of oxygenates [1_ 15 ]. N ob le2m etal based catalysts (such as R h, R u, Pd and P t) exh ib it a h igh activity and selectivity w ith little carbon depo sition [1_ 5 ]. How 2 ever, becau se of their h igh co st and lim ited ava il2 ab ility, it w ou ld be desirab le to develop a h igh ly act ive, low ca rbon depo sit ion and rela t ively cheap catalyst fo r m ethane dry refo rm ing. W h ile N i, Fe and Co based catalysts are easily p repared and mo re o r less catalytically active fo r th is reaction [6_ 15 ], the catalytic activities fo r m ethane dry refo rm ing decrea se becau se of ca rbon depo sit ion andgo r the sin tering of the m etal particles [3, 10 ]. A lim ited num ber of Co ca ta lyst s have been studied w ith carbon, SiO 2, M go, CaO, SrO, BaO and alum ina as the suppo rts [9_ 15 ]. Among these suppo rts, M go p rovides a CO yield of 93% and an H 2 yield of 90% at the h igh space velocity of ml g(g h) and rem ain s unchanged du ring the pe2 riod study of 50 h [9 ]. It has been found that the sta2 b ility of CogΧ2A l2o 3 catalyst is strongly dependen t on the Co loading and calcination temperatu re. T he activities are stab le w hen a balance betw een carbon fo rm ation and its ox idation is ach ieved [10 ]. M o st p reviou s literatu res repo rted the catalytic behaviou r of suppo rted cobalt catalysts fo r m ethane dry re2 fo rm ing activated by H 2. It has been p ropo sed that tran sition m etal carb ides have sim ilar p roperties to nob le m etals, and they can be p repared th rough carbu rising the co rresponding m etal o r ox ide p re2 cu rso rṡ In particu lar, these repo rts focu sed on the mo lybdenum and tungsten carb ide system s [16, 17 ]. T here are few repo rts abou t the cobalt carb ide cat2 alyst, although it has been p ropo sed that the cobalt carb ide system m ay p lay as the active cen tres fo r the refo rm ing and F ischer2t rop sch react ion ṡ In th is w o rk, differen t activation m ethods v ia H 2, a m ix tu re of m ethane and H 2 o r pure m ethane w ere u sed to activate the CogΧ2A l2o 3 catalysts in o rder to study the effect of the p re2treatm en t atmo sphere on the catalytic perfo rm ance. T he Co loading influ2 ence on the catalytic activity has also been investi2 gated. T he deactivation m echan ism of m ethane dry refo rm ing over the CogΧ2A l2o 3 catalyst w as dis2 cu ssed. Exper im en ta l T he cobalt catalyst p recu rso rs w ere p repared by imp regnating suppo rt alum ina (BET specific su r2 face area= 158 m 2 g) in to a cobalt n itrate aqueou s so lu tion of a desired concen tration. T he cobalt loadings w ere set to 116%, 316%, 812%, 1210% and 1610% in the catalystṡ A fter evapo ration and drying at 393 K fo r 12 h, the samp les w ere calcined at 773 K fo r 4 h. T he catalytic perfo rm ances of cat2 3 To w hom co rrespondence should be addressed. E2m ail: m alco lm. green@ chem. ox. ac. uk

2 4 58 CH EM. R ES. CH IN ESE U. V o l. 20 alysts (011 g) fo r m ethane dry refo rm ing w ere eval2 ua ted in a fixed bed qua rtz reacto ṙ B efo re reac2 tion, the 12% CogA l2o 3 catalyst w as activated by flow ing H 2, 20% CH 4gH 2 o r CH 4, respectively. T he temperatu re of it w as increased to 973 K at a rate of 5 Kgm in, and then kep t at 973 K fo r 1 h. A fter the activation, the feedstock [ n (CH 4 ) gn (CO 2 ) = ] w as in troduced in to the catalyst bed at a flow rate of 23 ml gm in. T he reaction temperatu re w as set at 1073 K. T he feedstock and the p roduct analyses w ere carried ou t by u sing an on2line gas ch rom atograph ( Perk in E lm er A u tosystem XL ) w ith a therm al conductivity detecto r. T he BET specific su rface areas of the catalyst w ere m easu red w ith a glass h igh vacuum line and calcu lated from the N 2 BET iso therm s. T he struc2 tu ral studies of the catalyst w ere m ade by m ean s of X2ray diffraction on a Ph ilip s PW 1710 diffractom e2 ter w ith Cu K Αradiation. T he R am an spectra w ere reco rded on a Yvon Job in L ab ram sp ect rom eter w ith a reso lu tion of 2 cm - 1, u sing a nm A r + la ser, run in a back sca t tered confoca l a rrange2 m en ṫ T hermogravim etric analyses (T GA ) in ox2 idative atmo sphere of the u sed CogΧ2A l2o 3 cata2 lysts w ere carried ou t on a R heom etric Scien tific STA 1500 in strum en ṫ T GA p rofiles w ere reco rded from room temperatu re to 1173 K, at an air flow of 30 ml gm in and a heating rate of 5 Kgm in. T he samp le of 25_ 35 m g w as loaded in to a sm all alum i2 na crucib le, w ith alum ina as reference. Results and D iscuss ion F ig. 1 show s the catalytic perfo rm ance of 12% CogΧ2A l2o 3 catalyst activated by a flow ing of H 2, 20% CH 4gH 2 o r CH 4. It can be seen that there is no sign ifican t d ifference betw een the ca ta lyt ic act ivi2 ties of 12% CogΧ2A l2o 3 fo r m ethane dry refo rm ing under differen t activation condition ṡ T he CH 4 and CO 2 conversion s are stab le at mo re than 90% con2 version du ring the period of study of 100 h. In the p roducts, the mo lar ratio s of H 2 to CO are around Since the mo lar ratio of CH 4 to CO 2 in the feedstock is 1 113, the larger conversion of CO 2 than CH 4 as w ell as the mo lar ratio of H 2 to CO less than 1 m igh t be due to the reverse w ater gas sh ift reaction (CO 2 + H 2 = CO + H 2O ). T h is resu lt sug2 gests that bo th 20% CH 4gH 2 and CH 4 are effective fo r the activation of the CogΧ2A l2o 3 catalyst fo r m ethane dry refo rm ing. F ig. 2 p resen ts the XRD pattern s of the Cog Χ2A l2o 3 catalysts activated v ia differen t m ethods. F ig. 1 Catalytic activ ities of 12%CogA l2o 3 catalyst for m ethane dry reform ing under d ifferen t activation condition ṡ Reaction temperature: 1073 K, n (CH 4 ) gn (CO 2 ) = 1g113. (A ) T he p lo t of mo lar ratio s of H 2 to CO vṡ re2 action tim e; (B) the p lo t of CO 2 conversion vṡ reaction tim e; (C) the p lo t of CH 4 conversion vṡ reaction tim e. F ig. 2 XRD pattern s of the 12%CogA l2o 3 catalyst activated using differen t methodṡ T he m ain phase of 12% CogΧ2A l2o 3 catalyst befo re the activation is Co 3O 4. T h is agrees w ith the resu lt repo rted by R ucken stein et a l., that Co 3O 4 w as generated as a m ajo r phase at the calcination tem 2

3 N o. 4 H EN X iao2w ei et al. 459 peratu re of 773 K, Co 2A lo 4 and CoA l2o 4 w ere fo rm ed at the calcination temperatu re of 1273 K [10, 12 ]. A s seen in F ig. 2, Co 3O 4 can be reduced to m etallic Co o r Co carb ide w hen the catalyst w as treated w ith H 2, 20% CH 4gH 2 o r CH 4. How ever, graph itic carbon appears after the p re2treatm en t of CogΧ2A l2o 3 catalyst w ith pure CH 4. A s m en tioned above, there is no sign ifican t deact iva t ion of the 12%CogΧ2A l2o 3 catalyst ( activated by 20% CH 4g H 2) du ring a stream of 100 h. How ever, XRD pat2 tern s of graph itic carbon can be ob served on the 12% CogΧ2A l2o 3 catalyst after reacting fo r 100 h. R am an spectro scopy is a u sefu l techn ique fo r the determ ina t ion of the fo rm s of ca rbonaceou s m atter. F ig. 3 show s the resonan t R am an spectra of the 12% CogΧ2A l2o 3 catalyst activated by differ2 en t m ethods. F ig. 3 Resonan t Raman spectra of the 12%CogA l2o 3 ca ta lyst activa ted using d ifferen t m ethodṡ T he w eak ab so rp tion bands at 469 and 511 cm - 1 appear fo r the 12% CogΧ2A l2o 3 catalyst w ith2 ou t any treatm en t, suggesting that Co 3O 4 particles cover the alum ina su rface [18 ]. Co 3O 4 is no t detected in the 12% CogΧ2A l2o 3 catalyst reduced w ith H 2. T hese resu lts are in agreem en t w ith the XRD m ea2 su rem en tṡ How ever, strong signals of carbona2 ceou s m atter can be ob served fo r the 12% CogΧ2 A l2o 3 catalyst p retreated w ith 20% CH 4gH 2. T he band at 1582 cm - 1 can be attribu ted to the tangen2 tial C_ C stretch ing modes, indicating that graph itic carbon ex ists in CogΧ2A l2o 3 catalysṫ T he band at 1355 cm - 1 w ith its overtone around 2695 cm - 1 is associated w ith diso rdered carbon s [19 ]. T h is reveals that the tw o types of carbon depo sit on the su rface of the CogΧ2A l2o 3 catalyst activated by the flow of 20% CH 4gH 2 even though they are no t de2 tected by XRD. D iso rdered ca rbon and g rap h it ic carbon signals are also seen fo r the CogΧ2A l2o 3 cat2 alyst p retreated by CH 4. Characteristic R am an ab2 so rp tion bands of diso rdered carbon and crystalline graph ite (at abou t 1355 and 1582 cm - 1 ) are p resen t fo r all the CogΧ2A l2o 3 catalysts after the reaction fo r 100 h. M eanw h ile, the R am an ab so rp tion bands of Co 3O 4 after reaction are m uch stronger than tho se befo re the reaction. T he XRD and R a2 m an spectra resu lts suggest that Co o r Co carb ide is the active phase on the CogΧ2A l2o 3 catalyst fo r m ethane dry refo rm ing no m atter w hat k ind of acti2 vation m ethods is u sed, and the ox idation of Co o r Co carb ide and carbon depo sition take p lace du ring m ethane dry refo rm ing at the sam e tim e. It can be seen that carbon depo sition ex ists on the su rface of the CogΧ2A l2o 3 catalyst activated by 20% CH 4gH 2 and CH 4. How ever, their ca ta lyt ic act ivit ies fo r m ethane dry refo rm ing are clo se to that of the Cog Χ2A l2o 3 catalyst reduced by H 2. It seem s that the m ain reason fo r the deactivation of the CogΧ2A l2o 3 catalyst is the ox idation of Co o r Co carb ide be2 can se the excess of CO 2 is p resen t du ring the cata2 lyst test ing. T GA studies of the 12% CogΧ2A l2o 3 catalyst activated v ia differen t m ethods after the reaction fo r 100 h are show n in F ig. 4. T he tw o w eigh t lo ss stages can be attribu ted to the com bu stion of the tw o types of carbon depo sition fo r the 12% Cog Χ2A l2o 3 catalyst activated by H 2 and 20% CH 4gH 2. O n ly one w eigh t lo ss stage w as ob served fo r the 12% CogΧ2A l2o 3 catalyst p retreated by u sing CH 4. D u ring the T G experim en ts in the ox idation atmo2 sphere, tw o reaction s m ay occu r as indicated in the fo llow ing equation ṡ Co (Co 3C) + O 2= Co 3O 4+ (CO 2) (1) C+ O 2= CO 2 (2) Eq. (1) is a w eigh t gain reaction, w h ile eq. (2) is a w eigh t lo ss p rocesṡ A s seen in F ig. 4, there is an overall w eigh t lo ss fo r all the u sed CogΧ2A l2o 3 cat2 alysts p retreated by u sing H 2, 20% CH 4gH 2 o r CH 4. T h is suggests that eq. ( 2) is the m ain reaction. T here is a large w eigh t lo ss of the u sed CogΧ2A l2o 3 catalyst activated by CH 4 w h ich occu rs around 620 K. T h is temperatu re is h igher than tho se fo r the catalysts activated by H 2 o r 20% CH 4gH 2. T h is m ay be due to the p resence of ca rbon on the su rface fo rm ed by the CH 4 p retreatm en t of the CogΧ2A l2o 3 catalysṫ It is mo re difficu lt fo r th is carbon layer to react w ith oxygen than tho se fo r the CogΧ2A l2o 3 catalysts activated by H 2 and 20% CH 4gH 2. How ev2

4 4 60 CH EM. R ES. CH IN ESE U. V o l. 20 F ig. 5 XRD pattern s of the CogA l2o 3 catalysts with differen t Co loadingṡ over the 812% CogΧ2A l2o 3 catalyst and also the in2 ten sity of Co 3O 4 increases w ith the increase of Co load ing. T he catalytic p roperties of a series of Cog Χ2A l2o 3 catalysts w ith differen t Co loadings acti2 vated by a flow of 20% CH 4gH 2 w ere studied. A s F ig. 4 Thermograv imetr ic analysis (TGA ) in a ir atmosphere of the used CogΧ-A l2o3 catalysts activated by differen t methodṡ A ir flow rate: 30 ml gm in. (A ) A ctivated by CH 4; (B) activated by 20% CH 4gH 2; (C) activated by H 2. er, a s soon a s it is induced, the react ion occu rs qu ick ly and therefo re on ly one w eigh t lo ss peak ap2 pearṡ T he to tal w eigh t lo sses are 9%, 7% and 20% fo r the 12% CogΧ2A l2o 3 catalyst activated by H 2, 20% CH 4gH 2 o r CH 4. T h is resu lt indicates that the CogΧ2A l2o 3 catalyst activated by 20% CH 4gH 2 is less liab le to cau se carbon depo sition du ring the m ethane dry refo rm ing. T he BET specific su rface areas of the Cog Χ2A l2o 3 catalysts w ith Co loadings of 116%, 316%, 8%, 12%, 16% a re 156, 140, 119, 108 and 106 m 2 g, respectively. T he BET specific su r2 face area decreases w ith the Co loading increasing. F ig. 5 show s the XRD pattern s of the CogΧ2A l2o 3 catalysts w ith differen t Co loadings befo re activa2 tion. O n ly b road peak s of alum ina can be ob served fo r the CogΧ2A l2o 3 catalysts w ith the Co loadings of 116% and 316%, Co 3O 4 phase begin s to appear F ig. 6 Catalytic activ ities of CogA l2o 3 catalysts with differen t Co loadings for methane dry reform ing. Reaction temperature: 1073 K, n (CH 4 ) gn (CO 2 ) = 1g113. (A ) T he p lo t of mo lar ratio s of H 2 to CO vṡ re2 action tim e; (B) the p lo t of CO 2 conversion vṡ reaction tim e; (C) the p lo t of CH 4 conversion vṡ reaction tim e.

5 N o. 4 H EN X iao2w ei et al. 461 seen in F ig. 6, the 116% Co gχ2a l2o 3 ca ta lyst ex2 h ib its a qu ite low activity fo r m ethane dry refo rm 2 ing. T he 316% CogΧ2A l2o 3 catalyst w as qu ick ly de2 activated w ith in 20 h. W hen the Co loading is up to 812%, CH 4 conversion decreases slow ly du ring the in itial 90 h reaction tim e, and then qu ick ly declines to 5% in the fo llow ing 10 h. W ith the con tinuou s p ro longation of tim e, no sign ifican t deactivation w as ob served du ring 100 h over tho se w ith Co load2 ings around 12% and 16%. T he mo lar ratio of H 2 to CO of m ethane dry refo rm ing has the sam e ten2 dency as the CH 4 conversion over the CogΧ2A l2o 3 catalysts w ith differen t Co loadings. T hese resu lts suggest that the stab ility, activity and selectivity of CogΧ2A l2o 3 catalysts vary greatly from the Co load2 ings. T he best Co loading is around 12%. Conclus ion A lum ina suppo rted Co ca ta lyst s act iva ted by H 2, 20% CH 4gH 2 and CH 4 are stab le and active fo r m ethane dry refo rm ing. T he active phase of the CogΧ2A l2o 3 catalyst is Co o r Co carb ide under dif2 feren t activation condition s. T he op tim ized Co loading of the CogΧ2A l2o 3 catalyst p retreated w ith 20% CH 4gH 2 is around 12%. Acknowledgm en t X iaow ei Chen w ou ld like to thank the R oy a l S o2 ciety f or a R oy a l S ociety K C W ong F ellow sh ip. Re fe re nce s [ 1 ] Edw ards J. H., M aitra A. M., S tudy S u rf. S ci. Catal., 1994, 81, 291 [ 2 ] Inui T., Cataly sis, 2002, 16, 133 [ 3 ] Zhang Z. L., T sipouriariv. A., Efstath iou A. M., et al., J. Catal., 1996, 158, 51 [ 4 ] Gronch i P., Cento la P., D el Ro sso R., A pp l. Catal. A : Gen., 1997, 152, 83 [ 5 ] Ro strup2n ielsen J. R., H ansen J. H. B., J. Catal., 1993, 144, 38 [ 6 ] H alliche D., Bouarab R., CherifiO., et al., Catal. T od ay, 1996, 29, 373 [ 7 ] D ing R. G., Yan Z. F., Catal. T od ay, 2001, 68, 135 [ 8 ] Gokon N., O ku Y., Kaneko H., et al., S olar E nergy, 2002, 72, 243 [ 9 ] Ruckenstein E., W ang H. Y., A pp l. Catal. A : Gen., 2000, 204, 257 [ 10 ] Ruckenstein E., W ang H. Y., J. Catal., 2002, 205, 289 [ 11 ] W ang H. Y., Ruckenstein E., A pp l. Catal. A : Gen., 2001, 209, 207 [ 12 ] W ang H. Y., Ruckenstein E., Catal. L etṫ, 2001, 75, 13 [ 13 ] Guerrero2Ruiz A., Sep lveda2e scribano A., Rodr guez2 Ramo s İ, Catal. T oday, 1994, 21, 545 [ 14 ] O sak i T., M asuda H., Ho riuch i T., et al., Catal. L etṫ, 1995, 34, 59 [ 15 ] L u Y., Yu C., Xue Y., et al., Chem. L etṫ, 1997, 515 [ 16 ] B rungs A. J., Yo rk A. P. E., Green M. L. H., Catal. L etṫ, 1999, 57, 65 [ 17 ] B rungs A. J., Yo rk A. P. E., C laridge J. B., et al., Catal. L etṫ, 2000, 70, 117 [ 18 ] X iao T. C., J i S. F., W ang H. T., et al., J. M ol. Caṫ A, 2001, 175, 111 [ 19 ] Jeh lic ε ka J., U rban O., Poko rny J., S p ectrach im. A cta A, 2003, 59, 2341

A L A BA M A L A W R E V IE W

A L A BA M A L A W R E V IE W Volume 52 Fall 2000 Number 1 B E F O R E D I S A B I L I T Y C I V I L R I G HT S : C I V I L W A R P E N S I O N S A N D TH E P O L I T I C S O F D I S A B I L I T Y I N