Enhnced ethnol production inside cron-nnotue rectors contining ctlytic prticles XIULIAN PAN, ZHONGLI FAN, WEI CHEN, YUNJIE DING, HONGYUAN LUO AND XINHE BAO* Stte Key Lortory of Ctlysis, Dlin Institute of Chemicl Physics, The Chinese Acdemy of Sciences, Dlin 11623, PR Chin *e-mil: xho@dicp.c.cn LETTERS Pulished online: 21 My 7; doi:1.138/nmt1916 Cron nnotues (CNTs) hve well-defined hollow interiors nd exhiit unusul mechnicl nd therml stility s well s electron conductivity 1. This opens intriguing possiilities to introduce other mtter into the cvities 2 5, which my led to nnocomposite mterils with interesting properties or ehviour different from the ulk 6 8. Here, we report striking enhncement of the ctlytic ctivity of Rh prticles confined inside nnotues for the conversion of CO nd H 2 to ethnol. The overll formtion rte of ethnol (3. mol mol 1 Rh h 1 ) inside the nnotues exceeds tht on the outside of the nnotues y more thn n order of mgnitude, lthough the ltter is much more ccessile. Such n effect with synergetic confinement hs not een oserved efore in ctlysis involving CNTs. We elieve tht our discovery my e of quite generl nture nd could pply to mny other processes. It is nticipted tht this will motivte theoreticl nd experimentl studies to further the fundmentl understnding of the host guest interction within cron nd other nnotue systems. With soring oil prices nd dwindling resources, ethnol nd other lterntive energy sources hve moved into the spotlight in recent yers s clen, sustinle nd trnsportle fuel lterntives 9. This hs sprked rpid glol growth of industries producing ethnol on lrge scles y fermenttion of griculturl crohydrtes such s cne sugr nd cornstrch. However, it seems tht nture cnnot provide oth food nd fuel for still growing nd incresingly energy-hungry world popultion in the ner future 9,1. Syngs ( mixture of CO nd H 2 ) conversion over Rh-sed ctlysts to ethnol 11 offers good lterntive ecuse other C2 oxygente y-products (for exmple, cetldehyde nd cetic cid) cn e redily hydrogented to ethnol, nd syngs cn e conveniently mnufctured from nturl gs nd col t present nd from iomss in the future. On the other hnd, cron nnotues (CNTs) hve een widely studied s supports for nnoscopic metl ctlysts 12 14, lthough those were generlly deposited on the outer CNT surfce. The few experimentl exmples of ctlytic rections inside CNTs were liquid-phse hydrogentions, which showed moderte improvement of selectivity or ctivity with respect to conventionl ctlysts 15,16. Recently, it hs een shown tht the utoreduction of iron oxide nnoprticles within CNT chnnels in n inert gs tmosphere is significntly fcilitted nd sensitive to the CNT inner dimeter, wheres the oxidtion of metl prticles is retrded compred with tht of prticles locted on the outer Yield of C2 oxygentes (mol mol 1 Rh h 1 ) 8 6 4 1 nm 29 3 31 3 33 Temperture (ºC) Figure 1 TEM imge nd ctlytic performnce of RMLF-in-CNTs in C2 oxygente formtion., TEM imge of the RMLF-in-CNTs ctlyst., Yield of C2 oxygente nd selectivity to CH 4, in comprison with the RMLF-on-SiO 2 ctlyst. Circles: RMLF-in-CNTs; squres: RMLF-on-SiO 2. The filled symols denote the C2 oxygente yield nd the open symols denote the selectivity of CH 4. CNT surfce 17 19. We elieve tht the C2 oxygente formtion from syngs inside Rh-loded CNTs reported here is the first exmple 4 3 1 CH 4 selectivity (%) nture mterils VOL 6 JULY 7 www.nture.com/nturemterils 57
c 1 nm 1 nm nm 6 5 4 3 1 2 4 6 8 1 12 4 3 1 2 4 6 8 1 12 3 1 2 4 6 8 1 12 d e 1 nm 1 nm 3 12 1 8 4 2 4 6 8 1 12 2 4 6 8 1 12 14 f 4 C2 oxygentes yield (mol mol 1 Rh h 1 ) 3 1 d c e 4 6 8 1 1 Time on strem (h) Figure 2 TEM imges nd prticle size distriution of ctlysts nd their C2 oxygente formtion ctivities. c, Fresh RM-in-CNTs ctlyst (), tht fter 28 h () nd fter 112 h (c) of rection. d,e, Fresh RM-out-CNTs ctlyst (d) nd tht fter 1 h of rection (e). f, C2 oxygente formtion ctivities s function of time on strem. Rection temperture: 3 C. 58 nture mterils VOL 6 JULY 7 www.nture.com/nturemterils
where the ctivity nd selectivity of metl-ctlysed gs-phse rection enefits significntly from proceeding inside nnosized CNT rection vessel. Active components of Rh nd Mn with Li nd Fe s the dditives in weight rtio of 1:1:.75:.5 were introduced into the CNT lumen (4 8 nm inner dimeter nd 25 5 nm length) y wet-chemistry method using the cpillry forces of the tues 4, ided y ultrsonic tretment nd stirring. Thus, ctlyst lelled s RMLF-in-CNTs with nominl 1.2 wt% Rh loding ws otined. Over 8% of the prticles were homogeneously distriuted inside the chnnels with the rest on the outer surfce, s indicted y trnsmission electron microscopy (TEM) (Fig. 1). For comprison, the sme composition of metls ws dispersed on conventionl silic support to otin n RMLF-on-SiO 2 ctlyst. Figure 1 shows tht the ctivity of RMLF-in-CNTs is significntly improved, prticulrly t higher tempertures. The C2 oxygentes yield (contining up to 76% ethnol, see Supplementry Informtion, Tle S1 nd Fig. S2 for detiled selectivities) t 33 C is 84.4 mol mol 1 Rh h 1, more thn twice tht on RMLF-on-SiO 2. More remrkly, the selectivity to the undesired y-product CH 4 is reduced to out 15% from 41% over RMLF-on-SiO 2 (ref. 21). Furthermore, no ovious dectivtion of RMLF-in-CNTs is oserved over period of 18 h. To scertin the effect of the CNT chnnels, we deposited simplified metl mixture of 5% Rh nd 5% Mn oth inside nd outside the CNTs, denoted s RM-in-CNTs nd RM-out-CNTs, respectively. Supplementry Informtion, Fig. S1 g shows tht the RhMn prticles re uniformly distriuted inside the tues in the fresh RM-in-CNTs. Figure 2 shows tht over 75% of the prticles re in the size rnge of 1 2 nm. This ctlyst yields 7.5 mol C2 oxygentes mol 1 Rh h 1 (open circles in Fig. 2f). The ctivity increses steeply nd reches mximum vlue of 38.9 mol mol 1 Rh h 1 fter out 28 h. TEM shows slight ggregtion of the prticles, ut they re still firly uniform nd the most undnt ones re 3 4 nm in size (Fig. 2). Then the ctivity grdully levels off fter 6 h t stedy production rte of 3. mol mol 1 Rh h 1 (Fig. 2f). After 112 h of rection, the prticle size within the chnnels is found to e out 4 5 nm, in ccordnce with the verge inner dimeter of the CNTs, wheres those on the outer surfce hve grown to out 8 nm (Fig. 2c). Oviously, the CNT chnnels hve restricted the growth of the prticles inside, nd the 3 4 nm size corresponds to the mximum ctivity, in greement with erlier studies on SiO 2 -supported Rh ctlysts 22. However, the RM-out-CNTs ctlyst (Fig. 2d) gives only n initil C2 oxygente yield of 5.3 mol mol 1 Rh h 1 (filled squres in Fig. 2f), lthough the prticle size of RM-out-CNTs (2 4 nm) is lso in the optiml rnge. The ctivity of RM-out-CNTs decreses continuously from the initil vlue with time on strem. The C2 oxygente yield is 16 times lower thn tht on RM-in-CNTs fter 1 h of rection. The contriution of the outer prticles (%) of RM-in-CNTs cn e estimted to e only 1% of the totl yield of C2 oxygentes using the yield of RM-out- CNTs. This demonstrtes remrkle enhncement of the ctivity for the prticles inside compred with those outside the CNTs. This ehviour is even more surprising considering tht diffusion of rectnts nd products in nd out of the nnotues should hinder the rection to some extent with respect to tht on the freely ccessile outer CNT surfce. Oviously the significntly different ctlytic ctivities of RM-in-CNTs nd RM-out-CNTs cnnot e relted to the numer of prticles ecuse the lodings of ctive components nd the size of the ctlytic prticles in oth smples re similr. Furthermore, the exclusive contriution of the geometricl effect of the chnnel structure on the unique ctlytic property inside the CNTs is ruled out y comprison experiment with mesoporous silic SBA-15 s support, which yields Intensity (.u.) 4 6 8 1, Rmn shift (cm 1 ) Figure 3 Rmn spectr. 1: Reduced RM-in-CNTs; 2: reduced RM-out-CNTs; 3: CO-dsored RM-out-CNTs; 4: CO-dsored RM-in-CNTs. much poorer ctlytic performnce thn RMLF-in-CNTs (see Supplementry Informtion, Fig. S3), lthough SBA-15 possesses similr morphology to CNTs with respect to the chnnel dimeter (6 7 nm) nd length (3 6 nm), nd thus cn provide similr dimensionl confinement for the metl prticles. Our preliminry Rmn spectroscopy dt indicte tht n interction etween the CNT surfce nd metl prticles could ply crucil role in the improvement of ctlytic ctivity inside CNTs. Figure 3 shows tht no ovious Rmn nds re oserved for oth RM-in-CNTs nd RM-out-CNTs fter in situ reduction in hydrogen, lthough it is widely ccepted tht the MnO is unlikely to e fully reduced to metllic Mn in RhMn ctlysts 22 25.On susequent exposure to CO t room temperture, the Rmn spectrum of RM-in-CNTs shows two new nds t 55 nd 64 cm 1, wheres tht of RM-out-CNTs shows only one nd t 54 cm 1. The 54 55 cm 1 fetures cn e ssigned to ν Rh CO ecuse Rh CO modes hve een reported in the rnge of 54 6 cm 1 for Rh cronyl complexes 26,27. The 64 cm 1 nd on the CO-treted RM-in-CNTs indictes the presence of ν Mn O modes inside the CNTs, s demonstrted y similr feture oserved in ulk MnO 28 nd M-in-CNTs (see Supplementry Informtion, Fig. S4). It is known tht the exterior surfces of CNTs re electron-rich, wheres the interior ones re electron-deficient 17,29,3, which could influence metl nd metl oxide prticles in contct with either surfce. Thus, the oxophilic Mn inside CNTs could ttrct the O of CO dsored on djcent Rh sites leding to tilted dsorption of CO with the C tom onding to Rh tom nd the O onding to Mn tom. The 64 cm 1 nd oserved here proly indictes n lmost completed dissocition. Such tilted dsorption of CO on RhMn ctlysts ridging Rh nd Mn sites fcilittes the dissocition of CO nd hence increses the C2 oxygente formtion ctivity, s suggested previously 24,25. In contrst, the electron-rich exterior surfce reduces the tendency of Mn to ccept oxidic CO donor electrons. Consequently, the dissocition of CO is retrded on the CNT outer surfce, leding to lower formtion rte of C2 oxygentes. In ddition, the higher ν Rh CO frequency (55 cm 1 ) inside the CNTs suggests 4 3 2 1 nture mterils VOL 6 JULY 7 www.nture.com/nturemterils 59
CO + H 2 dissocition/ctivtion of CO nd incresed hydrogention rte led to significnt increse in the overll yield of C2 oxygentes. This synergetic confinement effect of metl nnoprticles inside CNTs could led to new dvnces nd pplictions pertining to functionl mterils in electronic, mgnetic nd ctlytic fields, nd relted processes. METHODS Cron nnotue Rh-sed ctlyst Ethnol Figure 4 Schemtic digrm showing ethnol production from syngs inside Rh-loded cron nnotues. The lck spheres denote cron toms, which form the grphene lyers of the cron nnotues. The strems in light ornge nd green entering the nnotues indicte the gs mixture of CO nd H 2, respectively. The three stcks of smll spheres in rose, lue, green nd red inside the tues represent ctlyst prticles tht my comprise more thn one component. The strems in light cyn tiling ehind the ctlyst prticles long the xis of the nnotues represent ethnol. stronger Rh CO onding strength, which is nother sign of esier dissocition of the C O ond compred with RM-out-CNTs. Figure 4 schemticlly summrizes the ethnol production inside cron-nnotue rector contining ctlytic prticles. The enhncement of the ctivity y more thn n order of mgnitude inside the CNTs compred with the conversion on the more ccessile outer surfce cn e ttriuted to the confinement of the nnoprticles within the CNTs, leding to the peculir interction of the interior nnotue surfce with the metl prticles, which enefits the dissocition of CO. Theoreticl studies on non-ctlytic gs-phse rections hve prefigured tht confinement within smll chnnels could increse the density of rectnts 31, nd hence crete loclly higher pressure, which will fvour syngs conversion to C2 oxygentes in the present cse. In fct, much higher intensity of the H 2 desorption pek is oserved on RM-in-CNTs thn tht on RM-out-CNTs during temperture-progrmmed desorption experiments (see Supplementry Informtion, Fig. S5), indicting higher concentrtion of ctive hydrogen species inside the CNTs, which will further supplement the hydrogention rte 32. It is generlly greed tht the formtion of C2 oxygentes involves the dissocitive dsorption of CO, hydrogention of cron species nd insertion of the moleculr CO dsored on the ctlyst surfce into CH x species, followed y hydrogention. Thus, oth the fcilitted CATALYST PREPARATION CNTs (inner dimeter 4 8 nm nd outer dimeter 1 nm) were purchsed from Chengdu Orgnic Chemicls. Rw CNTs were refluxed in HNO 3 (68 wt%) for 14 h t 14 C in n oil th. Then the mixture ws filtered nd wshed with deionized wter, followed y drying t 6 C for 12 h. The purity of the CNTs otined ws 99.9%, contining out 1 p.p.m. iron detected y X-ry fluorescence. TEM indicted tht ll of the CNTs hd open ends with length of 25 5 nm. The CNTs were then immersed into n queous solution of precursor slts of Rh, Mn, Li nd Fe under stirring to prepre RMLF-in-CNTs. The queous solution ws drwn into the CNT chnnels y cpillry forces ided y ultrsonic tretment nd stirring. The ultrsonic tretment fcilitted the filling of the CNT chnnels with precursor solution during the impregntion. A crefully controlled slow drying process ws crried out. First, the mixture ws stirred t room temperture overnight, followed y heting in ir to 1 Ct1 C min 1 nd held for 1 h. In this wy, the ctlyst components were homogeneously distriuted inside the CNT lumen long the tue xis. For the preprtion of RM-out-CNTs, nnotues with closed ends were used, which were otined y refluxing rw CNTs in 6 M nitric cid (37 wt%) t 11 C for 5 h. This milder tretment removed morphous cron while keeping the cps intct. Scnning cross severl specimens y trnsmission electron microscopy (TEM), we did not oserve nnotues with open ends. Then n impregntion procedure ws crried out to disperse the ctive components on the outer surfce of these nnotues. SBA-15 ws purchsed from Jilin University HighTech. RMLF-in-SBA ws prepred in the sme mnner s RMLF-in-CNTs. These two ctlysts s well s RMLF-on-SiO 2 hd the sme metl composition. The ltter ctlyst ws prepred y impregntion of SiO 2, which ws otined from Qingdo Hiyng Chemicl Group, with n verge pore size of nm. REACTION AND ANALYSIS OF REACTION PRODUCTS Rectionswerecrriedoutinflow-typefixed-edrector.Adigrm of the rection rig is shown in Supplementry Informtion, Fig. S6. The stndrd conditions were 3 MP, H 2 /CO = 2 (molr rtio), gs hourly spce velocity = 12, h 1, ctlyst chrge =.3 g. Before the rection, the ctlyst ws reduced in situ t 35 CinH 2 strem (5 ml min 1 ) for 2 h. The effluent pssed through condenser filled with deionized wter. The oxygentes collected in the condenser were nlysed offline y Vrin CP-38 gs chromtogrphy using flme ioniztion detector nd the til gs ws nlysed online using therml conductivity detector. TEM CHARACTERIZATION Mesurements were crried out on n FEI Tecni F3 microscope operted t n ccelerting voltge of 3 kv. RAMAN SPECTROSCOPY Spectr were otined with LRm I confocl microproe Rmn instrument (Dilor). The excittion wvelength ws 632.8 nm. Before CO dsorption, the smple ws reduced in situ in flowing H 2 strem t 35 C for 2 h, followed y purging in Ar for 3 min t this temperture, nd then cooled to room temperture in Ar. The dsorption of CO ws llowed for 3 min t room temperture. Then the spectrum ws recorded in Ar. Received 3 Jnury 7; ccepted 13 April 7; pulished 21 My 7. References 1. Di, H. Cron nnotues: Synthesis, integrtion, nd properties. Acc. Chem. Res. 35, 135 144 (2). 2. Ajyn, P. M. et l. Opening cron nnotues with oxygen nd implictions for filling. Nture 362, 522 525 (1993). 3. Serphin, S., Zhou, D., Jio, J., Withers, J. C. & Loutfy, R. Yttrium cride in nnotues. Nture 362, 53 (1993). 4. Tsng, S. C., Chen, Y. K., Hrris, P. J. F. & Green, M. L. H. A simple chemicl method of opening nd filling cron nnotues. Nture 372, 159 162 (1994). 51 nture mterils VOL 6 JULY 7 www.nture.com/nturemterils
5. Guerret-Plécourt, C., Bour, Y. L., Loiseu, A. & Pscrd, H. Reltion etween metl electronic structure nd morphology of metl compounds inside cron nnotues. Nture 372, 761 765 (1994). 6. Kog, K., Go, G. T., Tnk, H. & Zeng, X. C. Formtion of ordered ice nnotues inside cron nnotues. Nture 412, 82 85 (1). 7. Slon, J. et l. Metstle one-dimensionl AgCl 1 xix solid-solution wurzite tunnel crystls formed within single-wlled cron nnotues. J. Am. Chem. Soc. 124, 2116 2117 (2). 8. Mühl, T. et l. Mgnetic properties of ligned Fe-filled cron nnotues. J. Appl. Phys. 93, 7894 7896 (3). 9. Frrell, A. E. et l. Ethnol cn contriute to energy nd environmentl gols. Science 311, 56 58 (6). 1. Deluc, T. H. Looking t iofuels nd ioenergy. Science 312, 1743 1744 (6). 11. Bhsin, M. M. & O Connor, G. L. Procede de preprtion selective de derives hydrocrones oxygenes deux tomes de crone. Belgin Ptent 824822 (1975). 12. Plneix, J. M. et l. Appliction of cron nnotues s supports in heterogeneous ctlysis. J. Am. Chem. Soc. 116, 7935 7936 (1994). 13. Yoon, B. & Wi, C. M. Microemulsion-templted synthesis of cron nnotue-supported Pd nd Rh nnoprticles for ctlytic pplictions. J. Am. Chem. Soc. 127, 17174 17175 (5). 14. Girishkumr, G., Hll, T. D., Vinodgopl, K. & Kmt, P. V. Single wll cron nnotue supports for portle direct methnol fuel cells. J. Phys. Chem. B 11, 17 114 (6). 15. Zhng, A. M., Dong, J. L., Xu, Q. H., Rhee, H. K. & Li, X. L. Plldium cluster filled in inner of cron nnotues nd their ctlytic properties in liquid phse enzene hydrogention. Ctl. Tody 93 95, 347 352 (4). 16. Tessonnier, J., Pesnt, L., Ehret, G., Ledoux, M. J. & Phm-Huu, C. Pd nnoprticles introduced inside multi-wlled cron nnotues for selective hydrogention of cinnmldehyde into hydrocinnmldehyde. Appl. Ctl. A 288, 3 21 (5). 17. Chen, W., Pn, X., Willinger, M., Su, D. & Bo, X. Fcile utoreduction of iron oxide/cron nnotue encpsultes. J. Am. Chem. Soc. 128, 3136 3137 (6). 18. Chen, W., Pn, X. & Bo, X. Tuning of redox properties of iron nd iron oxides vi encpsultion within cron nnotues. J. Am. Chem. Soc. (7, in the press, doi:1.121/j71372). 19. Co, F. et l. Reducing rection of Fe 3O 4 in nnoscopic rectors of -CNTs. J. Phys. Chem. B 111, 1724 1728 (7).. Dujrdin, E., Eesen, T. W., Hiur, H. & Tnigki, K. Cpillrity nd wetting of cron nnotues. Science 265, 185 1852 (1994). 21. Yin, H. et l. Influence of iron promoter on ctlytic properties of Rh Mn Li/SiO 2 for CO hydrogention. Appl. Ctl. A 243, 155 (3). 22. Hnok, T. et l. Ethylene hydroformyltion nd cron monoxide hydrogention over modified nd unmodified silic supported rhodium ctlysts. Ctl. Tody 58, 271 28 (). 23. de Jong, K. P., Glezer, J. H. E., Kuipers, H. P. C. E., Knoester, A. & Emeis, C. A. Highly dispersed Rh/SiO 2 nd Rh/MnO/SiO 2 ctlysts: 1. Synthesis, chrcteriztion, nd CO hydrogention ctivity. J. Ctl. 124, 5 529 (199). 24. Trevino, H., Lei, G. & Schtler, W. M. H. CO hydrogention to higher oxygentes over promoted rhodium: Nture of the metl-promoter interction in RhMn/NY. J. Ctl. 154, 245 252 (1995). 25. Ichikw, M. & Fukushim, T. Infrred studies of metl dditive effects on CO chemisorption modes on SiO 2-supported Rh-Mn, -Ti, nd Fe ctlysts. J. Phys. Chem. 89, 1564 1567 (1985). 26. Moigno, D., Cllejs-Gspr, B., Gil-Ruio, J., Werner, H. & Kiefer, W. The metl-cron ond in vinylidene, cronyl, isocynide nd ethylene complexes. J. Orgnometl. Chem. 661, 181 19 (2). 27. von Ahsen, B. et l. Ctionic cronyl complexes of rhodium (I) nd rhodium (III): Synthesis, virtionl spectr, NMR studies, nd moleculr structures of tetrkis(cronyl) rhodium(i) heptchlorodiluminte nd gllte, [Rh(CO) 4][Al 2Cl 7] nd [Rh(CO) 4][G 2Cl 7]. Inorg. Chem. 42, 381 3814 (3). 28. Kpteijn, F. et l. Alumin-supported mngnese oxide ctlysts: I. chrcteriztion: effect of precursor nd loding. J. Ctl. 15, 94 (1994). 29. Hddon, R. C. Chemistry of the fullerenes: The mnifesttion of strin in clss of continuous romtic molecules. Science 261, 1545 155 (1993). 3. Ugrte, D., Chtelin, A. & de Heer, W. A. Nnocpillrity nd chemistry in cron nnotues. Science 274, 1897 1899 (1996). 31. Sntiso, E. E. et l. Adsorption nd ctlysis: The effect of confinement on chemicl rections. Appl. Surf. Sci. 252, 766 777 (5). 32. Dong, X., Zhng, H., Lin, G., Yun, Y. & Tsi, K. R. Highly ctive CNT-promoted Cu ZnO Al 2O 3 ctlyst for methnol synthesis from H 2/CO/CO 2. Ctl. Lett. 85, 237 246 (3). Acknowledgements We thnk Z. Tin nd W. Weng from Ximen University Chin for their help with Rmn spectroscopy chrcteriztion nd A. Goldch for the discussion. We lso cknowledge the finncil support of the Nturl Science Foundtion of Chin nd the Ministry of Science nd Technology of Chin. Correspondence nd requests for mterils should e ddressed to X.B. Supplementry Informtion ccompnies this pper on www.nture.com/nturemterils. Competing finncil interests The uthors declre no competing finncil interests. Reprints nd permission informtion is ville online t http://npg.nture.com/reprintsndpermissions/ nture mterils VOL 6 JULY 7 www.nture.com/nturemterils 511