Electro-catalytic oxidation of CO on Pt catalyst supported on carbon nanotubes pretreated with oxidative acids

Transcription

1 Cron 44 (26) Electro-ctlytic oxidtion of CO on Pt ctlyst supported on cron nnotues pretreted with oxidtive cids Li Li, Gng Wu, Bo-Qing Xu * Innovtive Ctlysis Progrm, Key L of Orgnic Optoelectronics nd Moleculr Engineering, Deprtment of Chemistry, Tsinghu University, Beijing 184, Chin Received 9 Decemer 25; ccepted 19 My 26 Aville online 7 July 26 Astrct Chrcteristics of nnosized Pt electro-ctlyst deposited on cron nnotues (CNTs) were studied with CO-stripping voltmmogrm nd chronomperometry mesurements. The CNTs were pretreted y oxidtion in HNO 3, mixed HNO 3 +H 2 SO 4 nd H 2 SO 4 +K 2 Cr 2 O 7 solution, respectively, to enle surfce modifiction. Well-homogenized Pt prticles (verge size: 3 nm) were loded onto the pretreted CNT smples y modified colloidl method. TEM, BET, FTIR nd XRD techniques were used to chrcterize the physicochemicl properties of the pretreted CNT smples. In the electro-oxidtion of CO, ll the Pt/CNT smples showed lower on-set s well s pek potentils thn the conventionl Pt/XC-72 electro-ctlyst, indicting tht the Pt/CNT smples were more resistnt to CO poisoning nd could e superior node electro-ctlyst for the proton exchnge memrne fuel cells (PEMFCs). Moreover, we found tht the pretretment of CNTs in mixed HNO 3 +H 2 SO 4 solution ws very eneficil for the performnce enhncement of Pt/CNT electro-ctlyst; the ctlyst otined s such gve the lowest pek potentil nd the highest ctlytic ctivity for the electrooxidtion of CO. Lrger mount of oxygen-contining functionl groups, higher percentge of mesopores, nd higher grphitic crystllinity of the pretreted CNTs were considered crucil for the performnce enhncement, e.g., y strengthening the interction etween Pt nnoprticles nd the CNT support nd enhncing the mss diffusion in the electro-chemicl rection. Ó 26 Elsevier Ltd. All rights reserved. Keywords: Cron nnotues; Chemiclly modified crons; Ctlyst support; Electro-chemicl properties; Chemicl tretment 1. Introduction There hs een incresing interest in proton exchnge memrne fuel cells (PEMFCs) s zero emission power sources [1 3]. Pure hydrogen is n idel fuel for PEMFCs. However, the production, storge nd refueling infrstructure of hydrogen re still of significnt prolems [4]. To void such difficulties, the use of hydrogen-rich synthetic gs otined from stem-reforming or prtil oxidtion of lcohols or hydrocrons cn e fesile choice [5 7]. For exmple, the methnol reforming results in gs mixture of pproximtely 74% H 2, 25% CO 2 nd 1 2% * Corresponding uthor. Tel.: ; fx: E-mil ddress: qxu@mil.tsinghu.edu.cn (B.-Q. Xu). CO [8]. However, detrctor to this technology is tht the presence of even smll mount of CO would result in poisoning of the hydrogen oxidtion rection occurring t the node Pt ctlyst, nd consequently lower energy conversion efficiency; which is referred to s CO-poisoning effect [9,1]. Using specil selective oxidtion ctlyst, the CO concentrtion in hydrogen cn e reduced further to pproximtely 2 1 ppm. But even t this level, the COpoisoning effect could lso significntly plgue the longterm performnce of the PEMFC stck [11]. The literture is rich in the electro-oxidtion of CO on Pt single crystls with regulr (1), (11) nd (111) surfces in cidic electrolytes [12 14]. CO dsored on these surfces ppers s linerly onded species nd its dsorption isotherm cn e fitted with Tempkin eqution [15]. The CO-poisoning effect is risen from locking y CO /$ - see front mtter Ó 26 Elsevier Ltd. All rights reserved. doi:1.116/j.cron

2 2974 L. Li et l. / Cron 44 (26) of the surfce sites ctive to hydrogen dsorption since CO molecules re onded much more strongly nd the oxidtion potentil of dsored CO is much higher thn tht of dsored hydrogen [16 18]. Three different pproches hve een eing ttempted to mitigte the effect of CO poisoning in PEMFC technology [18]: (1) use of suitle pltinum lloy electro-ctlyst to increse the competitiveness of hydrogen dsorption, (2) ssuming higher cell operting temperture to reduce the dynmic coverge of CO t the Pt surfce, nd (3) dding smll mount of oxygen into the fuel gs flow to enle selective oxidtion of CO in hydrogen. With respect to the electro-ctlyst, n lterntive pproch to enhnce the ctlyst performnce could e the serch for suitle ctive supporting mterils for Pt [19]. Nture of the supporting mterils hs proven to ply n importnt role in enhncing the electro-ctlytic ctivity for the oxygen reduction rection (ORR) t the cthode nd methnol oxidtion rection (MOR) t the node ctlyst [2 23]. Being new form of cron mterils [24], cron nnotues (CNTs) hve een min focus of mny current reserch efforts since recent development in lrge-scle syntheses of the mteril hs significntly incresed their vilility [25,26]. Due to their unique structure, high surfce re, low resistnce nd high stility, the CNTs re considered s promising supporting mterils for the electroctlyst in the PEMFC technology [27 35]. A numer of erlier investigtions hve shown tht Pt deposited on CNTs cn exhiit high ctivity for methnol electro-oxidtion s well s oxygen electro-reduction [28 35]. But, pretretment of the CNT support in oxidtive minerl cid ws lwys found eneficil for homogeneous metl deposition nd thus for etter ctlytic performnce [21 23,3 33]. The oxidtive pretretment cn ffect the density of surfce functionl groups which could e necessry for etter metl deposition nd metl-support interction [32,33]. A suitle electro-ctlyst in the PEMFC technology should e t lest less sensitive to CO poisoning thn the conventionl Pt/XC-72 ctlyst. However, little is known out effects of the supporting cron mterils on the electro-chemicl oxidtion of CO. In the present study, we investigte the effects of oxidtive pretretments of CNTs in different minerl cids. The pretreted CNT smples re chrcterized with BET, TEM, XRD, FTIR, nd electro-chemicl mesurements to understnd the effect of supporting cron mteril on the electro-ctlytic oxidtion of CO. 2. Experimentl 2.1. Pretretment of CNTs in oxidtive cids The multi-wll cron nnotue (MWNT) (i.d. 3 1 nm, o.d. 6 2 nm, rtio of length to dimeter 1 1) mteril ws received from the lortory of Professors Wei nd Luo of the Chemicl Engineering Deprtment (Tsinghu University), where the mteril ws prepred y ctlytic vpor deposition (CVD) of propylene using Fe/Al 2 O 3 s the ctlyst [36]. Pretretments of the s-received CNT mteril were conducted y immersing the mteril in oxidtive minerl cids, followed y refluxing t 8 9 C for 3 h under vigorously gittion. After seprtion y filtrtion, the pretreted mterils were wshed extensively with deionized wter nd dried in ir t 12 C for 5 h. The minerl cids used for the pretretments were 3% HNO 3, mixed 98% H 2 SO 4 + 7% HNO 3 nd 98% H 2 SO M K 2 Cr 2 O 7 solutions, resulting in smples denoted CNTs-I, CNTs-II nd CNTs-III, respectively. Pretretment of the well-known Vulcn XC-72 cron with mixed 98% H 2 SO 4 + 7% HNO 3 cids produced cron smple coded s XC-72-II Ctlyst preprtion All the ctlyst smples were prepred y Pt deposition onto the supporting cron mterils using modified colloid method descried in our erlier work [37]. Briefly, Pt sulfite complex ws first synthesized from K 2 PtCl 6 nd NHSO 3. This Pt sulfite complex ws dissolved in deionized wter t ph = 3. nd then proper mount of CNTs ws dded to form CNT suspension in the solution. Under vigorous stirring nd creful ph (3.) control with queous NOH, hydrogen peroxide ws then dded dropwise to induce oxidtive decomposition of the sulfite complex to deposit Pt nnoprticles onto the suspended CNTs. After extensive wshing, the solid from the solution ws finlly dried t 393 K to give the Pt/CNT ctlyst. Pt loding in the ctlysts ws 2 wt%. The Pt/CNTs-s-received nd Pt/XC-72, which represent Pt ctlysts supported y CNTs-s-received nd Vulcn XC-72 cron, respectively, were prepred s reference ctlysts for comprison purpose. And lso, use of the pretreted Vulcn XC-72 cron s the ctlyst support produced the Pt/XC-72-II ctlyst Chrcteriztion nd CO electro-oxidtion evlution of the ctlysts Surfce res nd pore size distriution of the cron mterils nd their supported Pt ctlysts were mesured y nitrogen dsorption t 77 K on Micromeritics ASAP 21C instrument. Trnsmission electron microscopy (TEM) ws performed on Hitchi H8 microscope operting t 15 kv to determine smple morphology nd metl dispersion. Fourier trnsform infrred trnsmission spectroscopy (FTIR) ws crried out on PE 2 FTIR spectrometer. The frequency ws scnned from 4 to 14 cm 1 with totl of 2 scns for ech of the smples. The smple ws mixed nd grounded with KBr t rtio of 1:2 (wt.) efore it ws pressed into wfer for the mesurement. The crystllinity of the smples ws determined y X-ry diffrction (XRD) performed on Bruker d8 diffrctometer equipped with Cu K rdition nd grph-

3 L. Li et l. / Cron 44 (26) ite monochromtic opertion t 45 kv nd 4 ma. The diffrction ptterns were scnned t rte of 1.2 deg/min with scn step of.2 deg. The ctlyst electrodes were prepred y conventionl ink method. The ctlyst inks were prepred y mixing 5 mg ctlyst smple with.25 ml isopropnol, followed y dding 2 ll Nfion (5% in ethnol solution) nd dispersing ultrsoniclly for 15 min. To prepre the working electrode, 5 ll of the ink ws psted onto the cron pper to otin Pt loding of pproximtely.2 mg/ cm 2. The electro-chemicl mesurements were crried out with.5 M H 2 SO 4 solution s the electrolyte in threeelectrode cell t 298 K. A sturted clomel electrode (SCE) nd Pt foil were used s the reference nd the counter electrodes, respectively. The cyclic voltmmogrm nd chronomperometry mesurements were performed on PARC EG&G M273 electro-chemicl system. The electro-chemicl potentil dt reported in this pper were clirted to the RHE vlues. Unless otherwise specified, the solutions were firstly deerted with high purity nitrogen prior to ny mesurement. For the CO-stripping mesurement, the working electrode ws repetedly scnned in etween nd 1.2 V t sweep rte of 1 mv/s to ssure stiliztion of the electrode in.5 M H 2 SO 4 solution. Adsorption of CO on the electrode ctlyst ws conducted y uling cron monoxide (UHP grde) through the electrolyte (.5 M H 2 SO 4 ) for 15 min, followed y purging with nitrogen for 2 min to remove ny residul CO from the solution. The CO-stripping CV curve nd lnk CV curve were otined from two consecutive scn cycles in etween nd 1.2 V (sweep rte: 1 mv/s). The chronomperometric curves of the CO electro-oxidtion were mesured t.7 V in flowing CO. 3. Results nd discussion 3.1. Effects of pretretment on the physicochemicl property of CNTs Fig. 1 shows the TEM imges of the CNT smples efore nd fter the oxidtive pretretments. All of these CNT smples re fetured y hollow tuulr nnostructures with dimeters in etween 6 nd 2 nm (pore dimeter: 3 1 nm) nd length up to severl micrometers, demonstrting no chnge of the structurl feture of the CNTs fter the different pretretments. The s-received CNT smple (Fig. 1A) nd the one pretreted with nitric cid (CNTs-I: Fig. 1B) were contminted with some Fig. 1. TEM microgrphs of the s-received (A) nd ctivted CNTs (B D). B, CNTs-I; C, CNTs-II; D, CNTs-III.

4 2976 L. Li et l. / Cron 44 (26) Volume dsored, cm 3 /g STP A CNTs s received CNTs-I CNTs-II Reltive pressure, P/P dv/dd, cm 3 /g-nm A CNTs s received CNTs-I CNTs-II 1 1 Pore Dimeter, nm Volume dsored, cm 3 /g STP B Vulcn XC-72 CNTs-III Reltive pressure, P/P Fig. 2. Representtive nitrogen dsorption desorption isotherms of CNTs nd Vulcn XC- 72. impurities including the Fe/Al 2 O 3 ctlyst, which ws employed in the CVD preprtion of the s-received CNTs. The impurities disppered fter the smple ws pretreted in either of the mixed cids, H 2 SO 4 + HNO 3 (CNTs-II: Fig. 1C) or H 2 SO 4 +K 2 Cr 2 O 7 (CNTs-III: Fig. 1D), indicting tht the mixed cids were effective in eliminting the impurities from the s-received smple. The specific surfce re nd porosity of the CNT smples s well s Vulcn XC-72 cron were mesured y N 2 physisorption t 77 K nd the results re shown in Fig. 2 nd Tle 1. All the dsorption desorption isotherms re elong to Type IV in the clssifiction of IUPAC [38], with oviously dsorption desorption hysteresis loop t the high pressure side. The Type-IV dsorption isotherm chrcterizes the existence of mesoporosity of the CNT smples, which my ply n importnt role in the lter Pt deposition processes [39]. As in mny other documenttions [4 44], the well-known BJH method ws used to nlyze the desorption rnch of the isotherms t reltive high pressure to otin the pore size distriution of the CNT smples (Fig. 3). According to the definition for dv/dd, cm 3 /g-nm B Tle 1 Specific surfce re nd porosity of CNTs nd Vulcn XC-72 cron smples Cron mterils S BET, m 2 /g 1 1 Pore Dimeter, nm S meso-, m 2 /g Vulcn XC-72 CNTs-III Fig. 3. Mesopore size distriution of CNTs nd Vulcn XC-72. V totl, cm 3 /g V meso-, cm 3 /g Vulcn XC CNTs s-received CNTs-I CNTs-II CNTs-III micropores (elow 2 nm), mesopores (etween 2 nd 5 nm), nd mcropores (ove 5 nm) [44], microporosity is significnt in the Vulcn XC-72 cron while mesoporosity domintes in ll of the CNT smples. Within the CNT smples, it is esy to see tht the pore size distriution ws significntly ffected y the pretretment condition (Fig. 3 nd Tle 1). In comprison with the s-received CNTs without ny pretretment, the percentge of pores within 1 5 nm ws significntly enhnced in CNTs-II nd CNTs-III, while in CNTs-I the pores with sizes in etween 5 nd 1 nm were incresed pronouncedly. The fct tht the incresed porosity in CNTs-I (Fig. 3A) ws contriuted

5 L. Li et l. / Cron 44 (26) y those pores with sizes (5 1 nm) slightly smller thn the dimeters of cron tues in the CNT mteril (Fig. 1) suggests tht the pretretment with 3% HNO 3 effected opening of the closed ends of the cron tues. On the other hnd, the incresed porosity t 1 5 nm in CNTs-II nd CNTs-III would indicte disentnglement of the entngled cron nnotues y the pretretments with the mixed HNO 3 +H 2 SO 4 nd H 2 SO 4 +K 2 Cr 2 O 7. Recently, Rolison et l. [44,45] reported tht molecules within microporous chnnels could suffer significntly hindered trnsport, while molecules in mesoporous chnnels cn pproch diffusion rtes comprle to those in n open medium. Therefore, in terms of the pore structure, the mesoporous CNTs would e more optimistic s the supporting mteril for electro-ctlysts. The specific BET surfce res of these cron smples re clculted y the BET eqution nd the totl pore volumes re otined from the mount of nitrogen dsored t reltive pressure of.99 t 77 K. As listed in Tle 1, the totl pore volumes of the CNT mterils incresed fter the pretretments due to the removl of impurities either inside the cnls or t the ends of the cron nnotues. FTIR ws used to detect the overll chnge of functionl groups on the surfce of CNTs efore nd fter the oxidtive pretretments, the results re shown in Fig. 4. In comprison with the other smples, much stronger sorption nds were found t 1 12 cm 1 for CNTs-II. According to Ref. [46], these results indicte tht high density of surfce functionl groups, such s CAOH nd AC@O, re creted on the surfce of CNTs fter the pretretment in the mixed HNO 3 +H 2 SO 4 cids. However, no significnt formtion of surfce functionl groups ws detected in CNTs-I nd CNTs-III. It is interesting to see tht the functionl groups of CNTs-II were similr y nture to those of the strting s-received CNTs, lthough their densities were much lower in the lter smple. A referee questioned out tht the functionl groups of the s-received CNTs might ffect the formtion of surfce functionl groups during the oxidtive pretretments. This Trnsmission intensity (r. units) e c 1 d Wvenumer, cm -1 Fig. 4. FTIR spectr of CNTs nd Vulcn XC-72: () CNTs-s-received; () CNTs-I; (c) CNTs-II; (d) CNTs-III; (e) Vulcn XC-72. Intensity (.u.) (2) θ (deg.) is n interesting point tht needs to e clrified in future work. Fig. 5 compres the XRD ptterns of the cron mterils used in the present study. The smples pretreted with the mixed cids (i.e., CNTs-II nd CNTs-III) exhiited excellent grphitic crystllinity y showing cler diffrction peks ssocited with the (2), (1) nd (4) plnes. XC-72 cron, CNTs-I nd the s-received CNTs seem to contin significnt morphous fetures ecuse they showed exceptionlly wide diffrction peks from the (2) nd (1) plnes. It hs een reported [47] tht the grphitic crystllinity of the cron support is importnt for electron trnsport in the electro-chemicl rections due to its good electronic conductivity nd interction with the supported metl ctlyst. Hence, in terms of electronic conductivity, CNTs-II nd CNTs-III could e more effective supporting mterils for Pt electro-ctlyst. To summrize, the morphology, crystllinity nd surfce properties of CNTs re modified y the pretretments with oxidtive minerl cids. In prticulr, the pretretment of the s-received CNTs with mixed HNO 3 +H 2 SO 4 cids produced smple CNTs-II hving firly lrge mesoporosity t pore sizes lrger thn 1 nm, higher concentrtion of surfce functionl groups nd good grphitic crystllinity Morphology of the Pt/CNT electro-ctlysts Fig. 6 shows the typicl TEM imges of Pt prticles deposited on Vulcn XC-72 nd the CNT mterils. The lrge light gry prticles nd tues re the imges of the cron mterils (A for Vulcn XC-72; B E for CNTs in Fig. 6) while the scttered lck spots re the imges of Pt prticles. With the id of filter technology nd tilting opertion of TEM, we found tht Pt prticles were deposited on oth the inner nd outer surfces of the cron nnotues in the Pt/CNT ctlysts. The r grphs locted on the right of the TEM imges in Fig. 6 show the size distriution of Pt prticles, otined y rndomly mesuring the sizes of t lest 15 prticles in the mgnified TEM (1) (4) CNTs-III CNTs-II CNTs-I As received CNTs XC-72 Fig. 5. X-ry diffrction (XRD) ptterns of vrious cron supports.

6 2978 L. Li et l. / Cron 44 (26) imges. The verged size of Pt prticles in ech of the Pt loded smples is listed in the lst column of Tle 2. Irrespective of the pretretments of CNTs, the verged Pt prticle sizes (3 nm) in ll Pt/CNT ctlysts were found quite close to tht in Pt/XC-72 ctlyst. However, the size distriution ws nrrower (1 5 nm) in Pt/XC-72 nd Pt/CNTs- III thn those in other Pt/CNT ctlysts (1 8 nm) Electro-chemicl oxidtion of CO on Pt/CNT electro-ctlysts Fig. 7 shows the CO-stripping nd ckground voltmmogrms of the Pt/CNT ctlysts in.5 M H 2 SO 4 solution t sweep rte of 1 mv/s. The results of Pt/XC-72 re lso included in this figure for comprison. It is seen tht the ckground current, ssocited with the doule-lyer cpcitnce, of the electrode with Pt/CNTs-s-received is much lower thn those with Pt/CNTs-I, -II nd -III, possily due to poor mesoporosity nd smller percentge of ccessile surfce in Pt/CNTs-s-received [28]. The cler peks t the low potentil side (.1.3 V) proved the occurrence of hydrogen dsorption on the Pt ctlysts. Blnk CO-stripping mesurements with either pretreted or non-pretreted CNTs, insted of Pt/CNT ctlysts, for the electrode demonstrted no dsorption of CO on the surfce of the CNT mterils. Therefore, the peks in Fig. 6. TEM microgrphs of Pt/XC-72 (A), Pt/CNTs-s-received (B), Pt/CNTs-I (C), Pt/CNTs-II (D) nd Pt/CNTs-III (E); the prticle size distriution of metllic Pt in the smples re shown y the corresponding grphs (A E ) locted eside the TEM microgrphs.

7 L. Li et l. / Cron 44 (26) Fig 6. (continued) Tle 2 CO electro-oxidtion y CO-stripping studies over Pt/CNTs nd Pt/XC-72 ctlysts Ctlysts Onset potentil, V vs RHE Pek potentil, V vs RHE ECA, m 2 /g Arel specific ctivity t.65 V (ma/cm 2 Pt) TOF t Prticle.65 V, s 1 size, nm Pt/XC ±.5 Pt/XC-72-II Pt/CNTs-s-received.56.71, ±.5 Pt/CNTs-I.57.72, ±.5 Pt/CNTs-II.51.68, ±.5 Pt/CNTs-III ±.3 CO-stripping mesurements of Fig. 7 re undoutedly ssocited with the electro-oxidtion of CO molecules dsored on the Pt ctlysts. The disppernce of the hydrogen dsorption peks t low potentils on the COstripping curves indictes locking of the hydrogen dsorption sites y CO dsorption, i.e., CO-poisoning, in the Pt/ CNT nd Pt/XC-72 electrode ctlysts. The onset potentils of CO electro-oxidtion on the different Pt/CNT ctlysts ppered in etween.5 nd.6 V, which re significntly lower thn the onset potentil (.7 V) on Pt/ XC-72 ctlyst. Also, the CO oxidtion peks on every Pt/CNT ctlyst were negtively shifted y 4 16 mv in comprison with the one on the Pt/XC-72 ctlyst (Fig. 7 nd Tle 2), indicting tht ll of the Pt/CNT ctlysts re more tolernt to CO poisoning. Moreover, it is noticele tht the ctlyst Pt/CNTs-II gve the lowest onset (.51 V) nd pek (.68 V) potentils nd showed lso the highest rel specific ctivity for the electro-oxidtion of CO. A referee reminded us tht the Vulcn XC-72 cron in the reference Pt/XC-72 ctlyst ws not pretreted in the oxidtive cids; the sme pretretment s for CNTs-II must e conducted in order to compre the resistiility to COpoisoning of Pt/CNT-II nd Pt/XC-72. Pretretment of Vulcn XC-72 cron with mixed 98% H 2 SO 4 + 7% HNO 3 cids ws then conducted nd the pretreted cron smple ws nmed s XC-72-II. We then otined Pt/ XC-72-II ctlyst y deposition of Pt onto this pretreted cron support XC-72-II. The CO-stripping CV curve of this Pt/XC-72-II ctlyst ws lso included in Fig. 7 nd

8 298 L. Li et l. / Cron 44 (26) Pt/CNTs-s-received 1.82 V 3 15 Pt/CNTs-II.82 V 3 15 Pt/XC-72-II.82 V Current density, A/g-Pt 1 Pt/CNTs-I Pt/CNTs-III Pt/XC Potentil, V vs RHE Fig. 7. CO-stripping curves of Pt/CNTs nd Pt/XC-72 smples in.5 M H 2 SO 4 solution t 298 K with the sweep rte of 1 mv/s; the dshed lines show the ckground voltmmogrm curves efore the CO dsorption. the prmeters derived from the CO-stripping mesurement on this ctlyst dded to Tle 2 to compre with the other ctlysts. Clerly, the onset (.65 V) nd pek (.79 V) potentils of the CO electro-oxidtion over Pt/ XC-72-II re very close to those over its counterprt Pt/XC-72 ctlyst, thus suggesting tht the pretretment of XC-72 cron in the mixed 98% H 2 SO 4 + 7% HNO 3 cids hs little effect on the electro-oxidtion of CO, lthough Pt/XC-72-II showed much lower ECA thn its counterprt Pt/XC-72 ctlyst. Still, the onset (.51 V) nd pek (.68 V) potentils over the ctlyst Pt/CNTs-II pper to e the lowest. Bsed on the electro-chemicl re (ECA) otined from the hydrogen dsorption peks on the ckground voltmmogrm, nd the rel specific ctivity (ma/cm 2 ) of the Pt ctlyst for the CO electro-oxidtion, we clculted the ctlytic CO turnover frequency (TOF) on surfce Pt toms t given working potentil (.65 V), the results re given in Tle 2. It ppers tht the intrinsic ctlytic ctivity y the TOF rte is in the order of Pt/CNTs-II > Pt/CNTs- I P Pt/CNTs-s-received > Pt/CNTs-II ffi Pt/CNTs-III > Pt/XC-72. Quntittively, the TOF rte on the Pt/CNTs-II ctlyst ws pproximtely five times tht of Pt/XC-72-II ut pproximtely 2 times tht of Pt/XC-72 ctlyst. Therefore, the ctlyst Pt/CNTs-II is unique in showing the lowest onset nd pek potentils (Fig. 7) s well s the highest TOF rte for the electro-oxidtion of CO (Tle 2). These results suggested tht the ctlyst Pt/CNTs-II would e the most CO tolernt mong the ctlysts used in the present work. Xin et l. [22,23] nd He et l. [28,29] found tht Pt/CNTs exhiited enhnced ctivities nd cell performnce for the reduction of oxygen when it ws used s the cthode ctlyst. The high grphitic crystllinity nd high density of surfce functionl groups of the support CNTs-II would e responsile for the high resistiility to CO-poisoning of the ctlyst Pt/CNTs-II since they could function to nchor the metllic ctlyst nd to strengthen the metlsupport interction [4] tht would e eneficil for electron trnsfer in etween the metl nd supporting cron mteril during the electro-chemicl rection. Electron trnsfer etween Pt nd cron support ws ctully detected in n electron spin resonnce (ESR) study of Bschuk nd Li [3], who showed tht the mount of unpired electrons in Pt/C ws much less thn those in unsupported Pt lck ctlysts. According to Refs. [12 14,18], electro-oxidtion of CO would e possile only when there is formtion of PtAOH ds t surfce of the Pt ctlyst: Pt + H 2 O! PtAOH ds +H þ +e ð1þ Theoreticlly, the potentil required for the formtion of PtAOH ds in the cid medi is pproximtely.62 V t 25 C [48]. With the presence of OH ds (PtAOH ds ), the electro-oxidtion of dsored CO on the Pt surfce cn e expressed s [37]: PtACO + PtAOH ds! 2Pt + CO 2 +H þ +e ð2þ Since the onset potentils of CO electro-oxidtion re lower thn.62 V on the present Pt/CNT ctlysts, some ctive AOH groups or their equivlent might hve een provided y the supporting CNTs, which would e prtly the reson why the onset potentils of CO oxidtion on Pt/CNT ctlysts were reduced in comprison with the Pt/XC-72 ctlysts. Further work will e needed to confirm this explntion. Two well-seprted CO oxidtion peks re oserved in Fig. 7 on the ctlyst smples except Pt/CNTs-III nd the two Pt/XC-72 ctlysts, indicting tht two types of surfce sites were ville for CO dsorption. The presence of two types of surfce sites for CO dsorption re consistent with Becdelievre et l. [12], who discriminted the CO dsorption species ssocited with the two types of surfce sites y reversing the potentil sweep just efore the ppernce of the second CO electro-oxidtion pek. The

9 L. Li et l. / Cron 44 (26) hydrogen dsorption desorption signls on the susequent potentil sweep curve reveled tht those dsored CO molecules ssocited with the second electro-oxidtion pek were locking the surfce sites on which hydrogen toms remined strongly dsored [12]. In other words, dsored CO molecules ssocited with the first CO electro-oxidtion pek (i.e., the one on the low potentil side) would lock the surfce sites tht dsor hydrogen with reltive wek onding. The studies performed y Beden nd Lmy [13] on the electrodes with single crystls of Pt (111) nd Pt (1) ctlysts produced similr conclusions. Beden nd Lmy lso showed tht the oxidtion of CO on Pt (111) surfce occurred t potentil 5 mv lower thn tht on the Pt (1) surfce. Thus, the pek t the lower potentil of the present study could e ssocited with the electro-oxidtion of the CO molecules dsored on the exposed (111) surfce while tht t higher potentil with those onded to the exposed (1) surfce. According to in situ IR sorption studies in Refs. [13,14], the second pek t higher potentil ws connected with strongly dsored CO molecules, which mens tht CO dsorption on Pt(111) is weker thn it does on Pt(1) surfce. On the other hnd, the interction strength of CO with the Pt ctlyst could e dependent of the prticle size of Pt, e.g., Mukerjee nd Mcreen et l. [49] showed tht the strength of PtACO ond would increse s the Pt prticle size ecme smller thn 5 nm. The single CO oxidtion pek t the higher potentil side (.82 V) on Pt/CNTs-III nd Pt/XC-72 electrodes seems in consistent with the Pt prticle size effect on the onding of CO t the Pt surfce since Pt prticles in these two ctlysts were smller thn 5nm(Fig. 6). Thus, the existence of Pt prticles lrger thn 5 nm in the other Pt/CNT ctlysts (Pt/CNTs-I, Pt/CNTs- II nd Pt/CNTs-s-received) my e responsile for the first electro-oxidtion pek of CO in Fig. 7. Hence, it is inferred tht the preferred exposed ctlyst surfce is dominnt y Pt (1) sites when the Pt prticles re smller thn 5 nm ut the surfce ecomes dominnt y Pt (111) sites when the prticles ecome lrger. Chronomperometry mesurements were crried out to evlute the ctlyst stility in the electro-oxidtion of CO nd the results re shown in Fig. 8. The current densities on ll the electro-ctlysts reduced shrply in the very initil minutes, followed y gentle decrese to pproch constnt vlues fter 15 2 min. This shrp decrese in the current density is most likely due to the exceptionlly high surfce coverge of CO t the eginning of the electro-oxidtion rection. The gentle decrese period would men trnsition in pproching dynmiclly equilirted CO coverge on the Pt ctlyst. Then, the constnt current density t longer rection time would chrcterize stedy stte of the dsorption nd oxidtive stripping of CO. Therefore, the constnt current densities in Fig. 8 cn e used to evlute the stedy stte ctlytic ctivities of the electro-ctlysts, which cn e rnked in the order of Pt/CNTs-II > Pt/CNTs-s-received P Pt/CNTs-III > Pt/ Current density, A/g-Pt CNTs-I P Pt/XC-72. This order is somewht different from the one (i.e., Pt/CNTs-II > Pt/CNTs-I P Pt/CNTs-sreceived > Pt/CNTs-III > Pt/XC-72) otined in the CO-stripping CV mesurements (Tle 2), ut oth the chronomperometry nd CO-stripping CV mesurements revel tht Pt/CNTs-II is the most ctive while Pt/XC-72 the lest effective electro-ctlyst for the electro-oxidtion of CO. On the other hnd, electro-oxidtion of CO on the electrode ctlyst is ffected y numer of steps t the moleculr levels, including mss trnsfer (diffusion), dsorption, nd surfce electro-chemicl oxidtion nd desorption. The electro-oxidtion of dsored CO on Pt surfce is undoutedly much slower step in comprison with the dsorption nd desorption steps ccording to Ref. [13]. However, influence of CO diffusion on the electro-oxidtion rection is not yet cler. By vrying the CO flowing (uling) rte during the chronomperometry mesurement, we investigted the CO diffusion effect over the ctlysts Pt/CNTs- II nd Pt/XC-72, respectively. Fig. 9 shows tht the current Current density ma/g Pt c d c e Time, min Fig. 8. I t plots of CO electro-oxidtion t.7 V over Pt/CNTs-sreceived (), Pt/CNTs-I (), Pt/CNTs-II (c), Pt/CNTs-III (d), nd Pt/XC- 72 (e) ctlysts. Pt/CNTs-II Pt/XC-72 c t / min c ().5 ml/min () 2 ml/min (c) 1 ml/min Fig. 9. The effect of CO flowing (uling) rte on the current of CO electro-oxidtion t.7 V over Pt/CNTs-II nd Pt/XC-72 ctlysts: ().5 ml/min, () 2 ml/min, (c) 1 ml/min.

10 2982 L. Li et l. / Cron 44 (26) density on the lest ctive conventionl Pt/XC-72 electroctlyst ws pprently ffected y the CO flowing/uling rte, e.g., the current density ws enhnced from.9 to.25 ma (3 times) when the CO flowing/uling rte ws incresed from.5 ml/min to 1 ml/min (2 times). In contrst, the current density on the most ctive Pt/CNTs-II electro-ctlyst did not chnge during the vrition of the CO flowing/uling rte. These results indicte clerly significnt CO diffusion effect on the electro-oxidtion rection with the conventionl Pt/XC-72 ctlyst ut the effect cn resonly e neglected with the ctlyst Pt/CNTs-II. The sence of CO diffusion effect on Pt/CNTs-II cn e well ccounted for y the dominnt mesoporosity in its support mteril CNTs-II. It is therefore conclusive tht mesoporosity of the supporting cron mteril is lso n importnt fctor in effecting Pt ctlyst for the electro-chemicl oxidtion of CO. 4. Conclusions The morphology, crystllinity nd surfce properties of CNTs were modified y the pretretments with oxidtive minerl cids (HNO 3, mixed HNO 3 +H 2 SO 4 nd H 2 SO 4 +K 2 Cr 2 O 7 ). In prticulr, pretretment of the sreceived CNTs with mixed HNO 3 +H 2 SO 4 cids produced mteril CNTs-II hving firly lrge mesoporosity t pore sizes lrger thn 1 nm, higher concentrtion of surfce functionl groups nd good grphitic crystllinity. The deposition of 2% Pt on the pretreted CNT supports y modified colloidl method led to Pt/CNT ctlysts with verge Pt prticle size t pproximtely 3. nm. The CO-stripping CV mesurements showed tht the Pt/ CNT ctlysts re more tolernt to CO poisoning since the CO electro-oxidtion pek potentils over these Pt/CNT ctlysts re 4 16 mv lower thn tht over the conventionl Pt/XC-72 ctlyst. 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