Journal of Catalysis

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1 Journl of Ctlysis 33 (13) Contents lists ville t SciVerse ScienceDirect Journl of Ctlysis journl homepge: High performnce of Ir-promoted Ni/TiO 2 ctlyst towrd the selective hydrogention of cinnmldehyde Weiwei Lin,,c, Hiyng Cheng,,, Limin He,,c, Yncun Yu,, Fengyu Zho,, Stte Key Lortory of Electronlyticl Chemistry, Chngchun Institute of Applied Chemistry, Chinese Acdemy of Sciences, Chngchun 1322, People s Repulic of Chin Lortory of Green Chemistry nd Process, Chngchun Institute of Applied Chemistry, Chinese Acdemy of Sciences, Chngchun 1322, People s Repulic of Chin c University of Chinese Acdemy of Sciences, Beijing 49, People s Repulic of Chin rticle info strct Article history: Received 12 Jnury 13 Revised 4 Mrch 13 Accepted 5 Mrch 13 Aville online 23 April 13 Keywords: Ni/TiO 2 Cinnmldehyde Selective hydrogention Interction Iridium The ctlytic performnce of Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts in the selective hydrogention of cinnmldehyde (CAL) hs een investigted. The Ni Ir/TiO 2 presented higher ctivity thn the Ni/TiO 2. Here, we studied nd gve some insights into the interction etween Ni nd Ir species nd the role of Ir using X- ry diffrction (XRD), trnsmission electron microscope (TEM), hydrogen temperture-progrmmed reduction (H 2 TPR), hydrogen temperture-progrmmed desorption (H 2 TPD), nd X-ry photoelectron spectroscopy (XPS). The size of Ni prticles on the Ni Ir/TiO 2 ws 1.1 nm, smller thn tht (12.7 nm) on the Ni/TiO 2. The reduciility of Ni ws improved y ddition of smll mount of Ir, s confirmed with the H 2 TPR nlysis. The Ni Ir/TiO 2 could e reduced t temperture of 352 C, which is lower thn the temperture for Ni/TiO 2 (385 C); moreover, new reduction pek ppered t 24 C due to the stronger interction of Ni Ir species, which ws certified y XPS nlysis. The H 2 TPD results indicte tht the hydrogen spillover effect my occur in Ni Ir/TiO 2. The electronic structure of the surfce Ni toms ws modified upon ddition of Ir, resulting in n enhnced ctivity of the Ni Ir/TiO 2 ctlyst, out four times s high s tht of the Ni/TiO 2 ctlyst. Ó 13 Elsevier Inc. All rights reserved. 1. Introduction The chemo- nd regioselective ctlytic hydrogention of,unsturted ldehydes into the corresponding unsturted lcohols y hydrogention t the C@O ond is highly desirle from n industril point of view [1]. On the other hnd, the synthesis of sturted ldehydes hs importnt industril nd iologicl pplictions such s in perfumes nd phrmceuticls. Recently, hydrocinnmldehyde (HCAL) ws even considered s n importnt intermedite for the synthesis of phrmceuticls for the tretment for HIV [2]. The hydrogention of cinnmldehyde cn occur t either olefinic (C@C) or cronyl (C@O) groups. The former leds to the formtion of HCAL, nd the ltter results in cinnml lcohol (COL) (Scheme 1). Forming HCAL is lso thermodynmiclly preferred nd cn e chieved esily compred to forming COL. Gret efforts hve een mde to develop selective ctlysts for the hydrogention of cinnmldehyde. Pt [3 6], Ru[7,8], Co[9], Cu Corresponding uthors t: Lortory of Green Chemistry nd Process, Chngchun Institute of Applied Chemistry, Chinese Acdemy of Sciences, Chngchun 1322, People s Repulic of Chin (H. Cheng), Stte Key Lortory of Electronlyticl Chemistry, Chngchun Institute of Applied Chemistry, Chinese Acdemy of Sciences, Chngchun 1322, People s Repulic of Chin. Fx: (F. Zho). E-mil ddresses: hycyl@cic.jl.cn (H. Cheng), zhofy@cic.jl.cn (F. Zho). [1], Au[11,12], Ir[13], nd imetllic ctlysts [14,15] hve een reported to e selective for the hydrogention of the cronyl group to form COL. On the other hnd, Pd [16,17], Ru[18], Ni [19,], Au[21], nd NiP [22] ctlysts re selective for the hydrogention of the C@C ond, resulting in the production of HCAL. Pd ctlysts were reported to e the most selective to the formtion of HCAL. For exmple, HCAL ws produced with 5% selectivity to HCAL t % conversion level over the Pd/TiO 2 ctlyst [23] nd 81% selectivity t % conversion over Pd/HNT (titnte nnotues) ctlyst, s reported y Jou et l. [24]. Moreover, selectivity of HCAL ws reported to e close to 9% t 9% conversion over Pd/cron [17], nd 97% selectivity ws t complete conversion over Pd/c-Al 2 O 3 ctlysts [25]. Although Pd presents good selectivity, the high costs of nole metls nd their limited resources in nture, sensitivity to sulfur poisoning, nd decrese in selectivity t high tempertures inhiit their ppliction. Therefore, nonnole metl ctlysts hve een given more ttention; the nickel-sed ctlysts hve received intense interest in oth cdemic reserch nd industril pplictions ecuse of their potentil utiliztion in hydrogention rections. Stisfctory ctlytic performnce over Ni/TiO 2 ctlysts ws reported for the hydrogention of nitro compounds to corresponding mines [26,27], nd the ctlytic performnce of ulk nickel ctlyst could e modified y dding smll mounts of other elements such s Pd or Pt, which dispersed on the surfce of Ni metl to form monolyer /$ - see front mtter Ó 13 Elsevier Inc. All rights reserved.

2 W. Lin et l. / Journl of Ctlysis 33 (13) Scheme 1. Rection pthwys for the hydrogention of cinnmldehyde. dispersed imetllic ctlyst [28]. For exmple, Singh nd Xu [29] reported tht lloying Ni with doped Ir showed % selectivity to H 2 production in the complete decomposition of hydrous hydrzine t room temperture. Shi et l. reported tht smll mount of Pt (.3) could improve the ctivity nd selectivity of Ni/Al 2 O 3 for mleic nhydride hydrogention [3]. In ddition, rutheniummodified nnonickel ctlyst exhiited much higher selectivity to HCAL [31], nd the ddition of gold to the Pd ctlyst ws fvorle to promote the hydrogention of CAL [32]. Here, the ctlytic ctivity of Ni/TiO 2 doping with tiny mount of Ir (.2 wt.%) ws investigted for the liquid-phse selective hydrogention of CAL to HCAL under mild rection conditions. The min im of the present work ws to discuss the interction etween the ctive species nd the role of Ir in the Ni Ir/TiO 2 ctlyst using X-ry diffrction (XRD), temperture-progrmmed reduction (TPR), trnsmission electron microscopy (TEM), nd X- ry photoelectron spectroscopy (XPS) techniques. Some new insights into ctlytic properties of Ni Ir/TiO 2 were reveled. 2. Experimentl 2.1. Ctlyst preprtion All chemicls used in this experiment re nlyticl grde nd used without further purifiction. Nno-TiO 2 (1 nm, 15 m 2 /g, Nnjing High Technology Nno Mteril Co. Ltd., Chin) ws used s support nd Ni(NO 3 ) 2 6H 2 O nd IrCl 3 3H 2 O s metl precursors. Ni/TiO 2 (11 wt.%) ws prepred y deposition precipittion (DP) using N 2 CO 3 s the precipitting gent. Briefly, the support ws dded to n queous solution contining Ni(NO 3 ) 2, nd the suspension ws put into wter th mintined t 7 C. Then, the N 2 CO 3 solution ws dded dropwise into the suspension under vigorous stirring. After the DP procedure, the smple ws wshed with wter nd then centrifuged, vcuum-dried, nd finlly clcined t 4 C for 4 h. The iridium-promoted Ni/TiO 2 ctlyst (.2 wt.% Ir) ws prepred y DP with ure (DPU) s reported in the literture [33]. Briefly, the clcined Ni/TiO 2 ctlyst ws dded to n queous solution contining IrCl 3 nd ure. The suspension ws mintined t 8 C nd vigorously stirred for 16 h. After the DP procedure, the smple ws wshed with wter, centrifuged, nd dried under vcuum. The smple ws identified s Ni Ir/TiO 2, nd it ws prereduced under flow of H 2 t 4 C for 2 h efore use. Ir/TiO 2 (2 wt.% nd.2 wt.%) ws prepred y DPU following the procedure descried ove. The smple ws dried nd clcined in ir t 4 C for 2 h nd reduced under flow of H 2 t 4 C for 2 h efore use Ctlyst chrcteriztion Powder XRD ws performed using Bruker D8 Advnce X-ry diffrct meter with CuK source t 4 kv nd 4 ma. 2h scns were performed from 1 to 9 t 4 /min. A TEM study ws crried out in JEOL JEM-1 instrument operting t n ccelerting voltge of kv. The TEM smples were prepred y dispersing the ctlyst powder in ethnol under ultrsound for 1 min, nd then, the resulting solution ws dropped onto cron film of copper grid. Hydrogen TPR (H 2 TPR) experiments were performed in tuulr qurtz rector (TP-58, Tinjin Xinqun, Chin), into which 75 mg ctlyst ws loded. The reduction ws conducted in 1% H 2 /N 2 flow t heting rte of 1 C/min. Hydrogen consumption ws determined using therml conduction detector (TCD). Hydrogen temperture-progrmmed desorption (H 2 TPD) experiments were performed in the sme pprtus s the H 2 TPR experiments. The ctlyst smple (.1 g) ws reduced in situ t 4 C for 1 h using pure hydrogen nd then cooled to room temperture in the sme tmosphere. The smple ws swept with nitrogen t flow rte of 3 ml/min for 1 h to remove physisored nd/or wekly ound species. TPD ws performed y heting the smple from room temperture to 7 C t rmp rte of 1 C/ min in nitrogen, nd the TPD spectr were recorded. XPS (VG Microtech 3 Multil) ws used to exmine the electronic properties of Ni nd Ir on the surfce of the Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts. The C1s pek t ev rising from dventitious cron ws used s reference. This reference gives inding energy vlues with precision of ±.2 ev. The surfce composition of the smples ws determined from the pek res of the corresponding lines using Shirley type ckground nd empiricl cross-section fctors for XPS. Ni nd Ir loding on the ctlysts ws mesured y inductively coupled plsm-opticl emission spectrometry (ICP-MS, Thermo Scientific ICAP6, USA) fter the extrction of metl species from the support y qu regi Liquid-phse hydrogention of cinnmldehyde The hydrogention of cinnmldehyde (J&K, >98%) ws crried out in 5-ml stinless steel utoclve t 8 C. Prior to rection, the ctlyst ws prereduced in qurtz tue t 4 C with flow of H 2 for 2 h. After the reduction finished, the qurtz tue ws cooled down nd H 2 ws chnged to N 2, nd when it cooled to room temperture, one end of the qurtz tue ws connected with glss tue to introduce the ctlyst into the rectnt solution. A certin mount of solvent (2-propnol) nd CAL ws dded into the rector; then, the prereduced ctlyst ws trnsferred to the

3 112 W. Lin et l. / Journl of Ctlysis 33 (13) rector y inserting the end of the glss tue into the liquid surfce under the protection of N 2, to keep the solid ctlyst from contcting the ir. After eing flushed with hydrogen for three times, the rector ws closed. After the rector ws preheted in wter th for 1 min, H 2 ws introduced into the rector t up to 2. MP, nd the rection time ws recorded with strting stirring. When the rection ws finished, the rector ws cooled to room temperture in n ice-wter th, nd then, hydrogen ws vented to mient pressure. The products were nlyzed with gs chromtogrph (Shimdzu GC-1, Rtx-5 cpillry column) using flme ioniztion detector. HCAL, HCOL, nd COL re detected s rection products under the experimentl conditions used, which re quntified y using o-xylene s the internl stndrd sustnce, nd the cron lnce vlues sed on these products re ner %. The conversion nd selectivity were clculted with the formuls: m CAL CAL conversion ð%þ ¼ 1 m CAL þ m HCAL þ m HCOL þ m COL % m HCAL HCAL selectivity ð%þ ¼ % m HCAL þ m HCOL þ m COL m HCOL HCOL selectivity ð%þ ¼ % m HCAL þ m HCOL þ m COL m COL COL selectivity ð%þ ¼ % m HCAL þ m HCOL þ m COL Rection rte ðh 1 Þ¼ m CAL m NiþIr conversion=time ðhþ where m CAL, m HCAL, m HCOL, nd m COL represent the moles of cinnmldehyde, hydrocinnmldehyde, hydrocinnmyl lcohol, nd cinnmyl lcohol in the mixture. m Niþlr is the moles of nickel nd Ir used in the rection. 3. Results 3.1. Ctlyst chrcteriztion Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts were prepred y the DP method. The XRD ptterns of Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts fter reduction t 4 C re shown in Fig. 1. For the Ni/TiO 2 ctlyst (Fig. 1), esides the diffrction of ntse TiO 2, the metllic nickel ws found t 2h of 44.6 nd 52.1, indicting only the metllic nickel present on the surfce of the reduced Ni/TiO 2 [26,27]. The Intensity (.u.) Ni(111) Ni() θ (degree) Fig. 1. XRD ptterns of () Ni/TiO 2 nd () Ni Ir/TiO 2. ð1þ ð2þ ð3þ ð4þ ð5þ verge prticle size of Ni ws out 11.7 nm, s clculted from the hlf-width of the Ni(111) diffrction pek t 2h = 44.6 with the Deye Scherrer eqution. In the cse of Ni Ir/TiO 2 ctlyst (Fig. 1), the diffrction intensity of the ntse TiO 2 decreses nd the diffrction pek of nickel () t 2h = 52.1 ws rther wek nd lmost cnnot e detected. The crystl size of Ni ws out 9.1 nm from the Ni(111) t 2h = 44.6, which ws smller thn tht of the Ni/TiO 2 ctlyst. However, the chrcteristic diffrction of Ir ws not detected in the Ni Ir/TiO 2 smple, it my e scried to the lower Ir loding nd the formtion of the Ni Ir lloy in the smple lthough we did not find the shift of the Ni(111) nd Ni() [34]. TEM in Fig. 2 showed tht the Ni nnoprticles were well dispersed on the support of TiO 2, nd the verge size of Ni prticles ws out 12.7 nm. For the s-prepred Ni Ir/TiO 2 ctlyst (Fig. 2c), the verge size of Ni prticles ws out 1.1 nm, smller thn tht of the Ni/TiO 2 ctlyst; these re in good greement with the XRD results. The results suggested tht the Ni ws more highly dispersed on the smple of Ni Ir/TiO 2 thn on tht of Ni/TiO 2. The lttice of the Ni(111) crystl plne cn e seen in the enlrged HRTEM imge in Fig. 2c nd d. However, the Ir prticle ws not found in the TEM imge due to the low loding, nd the Ir my exist with Ni Ir lloy species in some extent. ICP nlysis shows tht the Ir content in the Ni Ir/TiO 2 ws out.2 wt.% Ctlytic performnce of Ni Ir/TiO 2 ctlyst Selective hydrogention of cinnmldehyde ws chosen to evlute the ctlytic performnce of Ni Ir/TiO 2 ctlyst; it is prllel nd consecutive reduction in the different functionl groups C@C nd C@O in the sme strting sustrte [35]. The ctivity of Ni Ir/TiO 2 ws compred with tht of Ni/TiO 2 nd Ir/TiO 2 ctlysts, nd the results re summrized in Tle 1. The conversion of cinnmldehyde reched 97.8% over Ni Ir/TiO 2 ctlyst fter rection for min; in comprison, only 66.2% conversion ws otined over Ni/TiO 2 ctlyst fter rection for 1 h when the qulity of the rectnt ws 3 mmol. The rection rte of Ni Ir/TiO 2 ctlyst (93.4 h 1 ) ws four times higher thn tht of Ni/TiO 2 ctlyst (21.2 h 1 ). Therefore, smll mount of Ir (.2 wt.%) could enhnce the ctivity of Ni/TiO 2 ctlyst mrkedly. The min product on oth Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts ws HCAL vi C@C ond hydrogention with >95% selectivity. However, for Ir/TiO 2 ctlyst, the conversion of CAL ws only 16.9% fter rection for 6 h, nd the min product ws HCAL with the side products of COL vi C@O ond hydrogention nd HCOL vi hydrogention of C@C nd C@O onds. These re different from the reports in the literture [36,37] tht the COL ws the min product over Ir-sed ctlysts. On ccount of the rection ws very fst over Ni Ir/TiO 2 ctlyst, the qulity of the rectnt ws incresed to 6 mmol to exmine the performnce of the ctlysts long with the rection time. Figs. 3 nd 4 illustrte the results of hydrogention of CAL over Ni/ TiO 2 nd Ni Ir/TiO 2. For Ni Ir/TiO 2 ctlyst, the conversion of CAL could rech 6.8% in 15 min nd 95.8% in 9 min with the selectivity to HCAL ove 97%. At the sme time, the conversion of CAL could only rech 71% for 5 h over Ni/TiO 2 ctlyst. At rection time 1 h, the verge rection rte of Ni Ir/TiO 2 ctlyst ws 57 h 1, which is three times higher thn tht of Ni/TiO 2 ctlyst (17.9 h 1 ). It is evident tht the ddition of Ir could promote the ctlytic ctivity. The stility of Ni Ir/TiO 2 ws lso exmined; the ctlysts were reused directly without ny tretment fter precipitted nd seprted from the rection solution. It ws noted tht the ctlysts should e immersed in the solution during the recycling process to void the oxidtion of ctive Ni species. For the Ni Ir/TiO 2 (Fig. 5), it presented s reltively stle under the rection conditions; slight decrese in the conversion nd selectivity ppered fter the ctlyst ws reused four times, ut it still could rech 9%

4 W. Lin et l. / Journl of Ctlysis 33 (13) Fig. 2. TEM imges of () Ni/TiO 2, () HRTEM of Ni/TiO 2, (c) Ni Ir/TiO 2, nd (d) HRTEM of Ni Ir/TiO 2. Tle 1 Ctlytic results of the hydrogention of cinnmldehyde. Ctlyst Time Conv. (%) conversion nd 97.8% selectivity to HCAL. ICP nlysis shows tht Ir content ws only.3 wt.% in the used Ni Ir/TiO 2 ctlysts fter four runs nd Ni content remined unchnged, indicting tht Ir leched into the rection solution nd so the ctivity decresed slightly Discussion Sel. (%) HCAL COL HCOL Ni/TiO 2 1 h n.d. c Ni Ir/TiO 2 min n.d. c Ir/TiO 2 6 h Rection rte (h 1 ) Rection conditions: ctlyst 5 mg, CAL 3 mmol, 2-propnol 1 ml, H 2 2 MP, T 8 C. Rection rte ws clculted y the moles of cinnmldehyde converted per mol Ni nd Ir per hour. c Not detected. Most recently, the imetllic ctlysts hve een pid more ttention, nd the ddition of nole metls such s Pt [38,39], Pd [4], Au[41], nd Ir [29] to nickel-sed ctlysts hs een studied, in which the ctivity nd product selectivity in the hydrogention nd some other rections were improved gretly. It ws reported tht the ctivity for the conversion of cellulose ws gretly enhnced y ddition of smll mount of Pt to the Ni ctlyst (Ni:Pt = 22:1 tom rtio), ecuse the presence of smll mount of Pt promotes the protontion of wter nd moleculr hydrogen, Conversion (%) Time (h) Fig. 3. Vrition of conversion nd selectivity with rection time in the hydrogention of cinnmldehyde over Ni/TiO 2 ctlyst. Rection conditions: ctlyst 5 mg, CAL 6 mmol, 2-propnol 1 ml, H 2 2 MP, T 8 C. which could spillover to Ni sites, creting lrge numer of cid sites in situ [38]. Zhu et l. [42] reported tht ddition of Ir to Co/SiO 2 could enhnce the performnce of Co Ir/SiO 2 ctlyst in the selective hydrogention of CAL to the corresponding COL, they suggested strong interction existed etween the imetllic species nd the presence of Ir promoted the reduction of Co 3 O 4 to Co, nd the mount of Co ws correlted with the ctivity nd selectivity of ctlyst [42]. Herein, the promoting role of Ir ws discussed from the interction etween the ctive species of Ir nd Ni y using the comintions of XRD, H 2 TPR, H 2 TPD, TEM, nd XPS techniques with the Selectivity (%)

5 114 W. Lin et l. / Journl of Ctlysis 33 (13) Conversion (%) Time (min) Fig. 4. Vrition of conversion nd selectivity with rection time in the hydrogention of cinnmldehyde over Ni Ir/TiO 2 ctlyst. Rection conditions: ctlyst 5 mg, CAL 6 mmol, 2-propnol 1 ml, H 2 2 MP, T 8 C. Conversion (%) Run Fig. 5. The recycling results for Ni Ir/TiO 2 ctlyst in the hydrogention of cinnmldehyde. Rection conditions: ctlyst 5 mg, CAL 6 mmol, 2-propnol 1 ml, H 2 2 MP, T 8 C, time 1.5 h. H 2 Consumption (.u.) Temperture (ºC) experimentl dt. First, the redox properties of Ni/TiO 2, Ni Ir/ TiO 2, nd Ir/TiO 2 ctlysts were compred; the H 2 TPR profiles of clcined Ni/TiO 2 (Fig. 6) showed min reduction pek centered t out 385 C, which ws ssigned to NiO significntly intercting with the titni surfce, while the pek centered t lower temperture ( C) ws the ulk NiO without ny interction Fig. 6. H 2 TPR profiles of () Ni/TiO 2, () Ni Ir/TiO 2, nd (c) Ir/TiO 2 (2 wt.%). c Selectivity (%) Selectivity (%) with titni surfce [43]. The Ni/TiO 2 prepred y DP ws more esily reduced thn the one prepred y incipient wetness [27]. For the Ir/TiO 2 ctlyst (Fig. 6) the pek centered t 122 C ws ssigned to the reduction of IrO 2, wheres rod pek round 35 C ws ttriuted to IrO 2 species hving strong interction with TiO 2 support [33]. The high-temperture pek round 59 C might e the reduction of prtilly reminded IrO 2 nd the support TiO 2 (Ti 4+? Ti 3+ ) [44]. It ws suggested tht the mount of hydrogen consumption ssocited with the reduction temperture <4 C ws not enough to reduce Ir 4+ to Ir completely in the clcined Ir/TiO 2 smples [45], nd it is well known tht TiO 2 is prtilly reduced to TiO x y hydrogen t high tempertures (ove 5 C), nd this process is promoted y the presence of dispersed metl crystllites [46]. As for the Ni Ir/TiO 2 ctlyst (Fig. 6c) the reduction pek t 144 C ws ttriuted to the reduction of IrO 2, out 22 C higher thn tht of Ir/TiO 2. This my e due to the interction etween Ir nd Ni. The second pek t 24 C cn e relted to the reduction of Ni Ir species with strong interction, suggesting tht Ni Ir lloy my exist in the smple. The third pek t 352 C could e ttriuted to the reduction of NiO, which ws significntly intercted with the titni surfce nd Ir species. This pek ws 33 C lower compred with tht of Ni/TiO 2 ctlyst. Shi et l. reported tht Pt cn e reduced t lower tempertures thn Ni; therefore, hydrogen tht dissocites on the surfce of Pt could migrte to the surfce of NiO nd support, resulting in Ni eing reduced t much lower tempertures in the imetllic Pt Ni ctlysts [29]. When dding.5 wt.% Ir to the Co 1 /SiO 2, the reduction pek of IrO 2 shifted to higher temperture nd tht of CoO x shifted to much lower temperture, indicting significnt synergistic effect etween Ir nd Co species [42]. Beltrmini nd co-workers [4] lso reported tht the ddition of oth Pt nd Pd to the Ni ctlyst, even in very smll frction, could increse its reduciility mrkedly. Therefore, Ir cn ctlyze the NiO reduction y surfce migrtion of chemisored hydrogen tom. Consequently, the presence of Ir in the Ni Ir/TiO 2 ctlyst ppers to promote the reduction in nickel oxides through hydrogen spillover, resulting in much lower reduction temperture of NiO in Ni Ir/TiO 2 ctlyst. H 2 TPD is used to chrcterize the ctive sites for the heterogeneous ctlyst. Generlly, the res of H 2 TPD peks cn e correlted with the mounts of ctive species. As shown in Fig. 7, the res of the H 2 TPD peks for the Ni Ir/TiO 2 ctlyst were much lrger thn for Ni/TiO 2 ctlyst, which my e ttriuted to the hydrogen spillover effect in Ni Ir/TiO 2. Besides, Ni/TiO 2 nd Ni Ir/TiO 2 showed two deposition peks t C nd C. However, the desorption temperture of Ir/TiO 2 ws t out 222 C, which ws much higher thn those of Ni/TiO 2 nd Ni Ir/TiO 2. This H 2 Desorption (.u.) c Temperture (ºC) Fig. 7. H 2 TPD profiles of () Ni/TiO 2, () Ni Ir/TiO 2, nd (c) Ir/TiO 2 (.2 wt.%).

6 W. Lin et l. / Journl of Ctlysis 33 (13) Intensity (.u.) Binding Energy (ev) Fig. 8. Ni2p XPS spectr of Ni/TiO 2 () nd Ni Ir/TiO 2 () ctlysts reduced t 4 C. Intensity (.u.) Ir 4f 7/2 Ti 3s Ir δ+ 4f7/ Binding Energy (ev) Fig. 9. Ir4f XPS spectr of Ni Ir/TiO 2 reduced t 4 C. suggested tht the interction etween Ir nd TiO 2 ws wekened in Ni Ir/TiO 2. Similrly, the mount of hydrogen desorption incresed when nole metls such s Pt, Pd, or Ru were dded to Co/Al 2 O 3 ctlyst [47]. There is no dout tht the increse in chemisored hydrogen on Ni Ir/TiO 2 is one of the resons for the improved reduciility. It is widely ccepted tht the more hydrogen dsored, the higher the ctlytic ctivity otined [48]. Therefore, the enhncement in the mount of chemisored hydrogen is eneficil for improving the ctlytic ctivity. To further understnd the interctions mong Ni, Ir, nd support, the surfce properties nd the chemicl sttes of Ni nd Ir were investigted y XPS. Fig. 8 displys the Ni2p spectr of the Ni/TiO 2 nd Ni Ir/TiO 2 ctlysts. Ni/TiO 2 ctlyst presented two sttes of Ni, the metllic Ni presented t the inding energy (BE) of 853 ev (stellite pek t ev [49]) similr to the reported BE vlue of ev [5], nd the oxidized Ni (NiO) presented t the BE vlue of 856 ev. It should e noted tht NiO ws formed from the oxidtion of metllic nickel y contcting ir during the smple trnsfer nd settlement during the XPS nlysis [51]. After smll mount of Ir ws dded to Ni/TiO 2 ctlyst, the BE of metllic Ni shifted to ev. This pointed out the existence of strong interction etween Ni nd Ir, which cn e lso certified y the shift of Ir. The Ir4f spectr of the Ni Ir ctlyst re shown in Fig. 9. In the sme region ppered pek due to Ti3s, nd therefore the spectr ecme more complex [45,52]. It is found tht the peks my e deconvoluted into two components; one corresponds to Ir t 61.2 ev nd the other corresponds to prtilly oxidized Ir d+ t 62.9 ev. In comprison with the stndrd BE of 6.9 ev [44,45,53], the BE of Ir4f 7/2 incresed.3 ev for Ni Ir/TiO 2 ctlyst. This suggested electron trnsfer from Ir to Ni toms, nd thus resulted in Ir with deficient electrons nd Ni with enriched electrons, in good greement with the Ni2p spectr given ove nd confirming tht the reduction of Ni ecme much esier in the presence of Ir. The electronic structure of the surfce Ni toms ws modified upon the ddition of Ir. Comining the XPS nd TPR, tht the significntly improved ctlytic ctivity of Ni Ir/TiO 2 ctlyst ws due to the stronger interction etween Ir nd Ni toms vi the electron trnsfer from Ir to Ni, leding to high ctivity of Ni ctive species. As we know, smll nnoprticles nd high metl dispersion re eneficil to high ctivity nd selectivity in some cses [54]. The size of nickel crystllites s estimted y TEM decresed from 12.7 nm to 1.1 nm when smll mount of Ir ws dded. The results suggested tht the dispersion of nickel in Ni Ir/SiO 2 ws higher thn tht for the Ni/TiO 2 smple, which is proly reson for the promoted ctivity of Ni Ir/TiO 2. The TPR results demonstrte tht the reduction temperture of IrO 2 shifted to higher temperture while the reduction of NiO shifted to lower temperture when smll mount of Ir ws dded. At the sme time, there ws reduction pek relted to the reduction of Ni Ir species. The H 2 TPD results indicte tht the hydrogen spillover effect my occur from Ir to Ni in Ni Ir/TiO 2, nd the XPS results proved there is n electron trnsfer etween Ir nd Ni toms. Therefore, ddition of smll mount of Ir into Ni/TiO 2 roused strong interction etween Ni nd Ir species, which resulted in the excellent ctlytic performnce of Ni Ir/TiO 2 nd good recyclility. Bsed on the ove results, proposed ctlytic model for the hydrogention of cinnmldehyde over Ni Ir/TiO 2 ctlyst is outlined in Fig. 1. The most Ir nnoprticles dispersed on the Ni prticles of the Ni Ir/TiO 2, which is the min ctive site for dsoring nd cleving H 2 molecules ecuse it is more ctive thn Ni in cleving H 2 into ctive H toms. The produced ctive H tom ws then dded into the cinnmldehyde molecule directly nd/or spilled to the Ni prticles or TiO 2 surfce. The hydrogention of cinnmldehyde hppened with ddition of these ctive H toms, nd so the ctivity of Ni Ir/TiO 2 ws significntly enhnced y dding smll quntity of Ir. 4. Conclusions Fig. 1. Proposed model for the hydrogention of cinnmldehyde over Ni Ir ctlyst. A Ni Ir/TiO 2 ctlyst with high performnce in cinnmldehyde hydrogention ws prepred y DP with sequentil deposition. The ctivity of the Ni Ir/TiO 2 ctlyst (93.4 h 1 ) ws significntly enhnced: it ws out four times s high s tht (21.2 h 1 )of Ni/TiO 2 ctlyst. The strong interction etween Ni nd Ir ws

7 116 W. Lin et l. / Journl of Ctlysis 33 (13) demonstrted to e one of the min resons for the enhnced ctlytic ctivity of Ni Ir/TiO 2 ctlyst. The electronic structure of the surfce Ni toms ws modified upon the ddition of Ir, nd so the reduciility of Ni incresed. Consequently the rection rte of the Ni Ir/TiO 2 ctlyst ws improved. The significntly improved ctivity cn e lso scried to the high dispersion of Ir on the Ni nnoprticles of the Ni Ir/TiO 2 ctlyst. The excellent ctlytic performnce nd good recyclility of the Ni Ir/TiO 2 ctlyst will mke it ttrctive for fundmentl reserch nd prcticl pplictions. Acknowledgments The uthors grtefully cknowledge finncil support from NSFC nd 8636, 562 from Jilin Provincil Science nd Technology Deprtment, Chin. References [1] P. Gllezot, D. Richrd, Ctl. Rev. 4 (1998) 81. [2] J.B.A. Muller, WO Ptent Appl. No. WO 99/8989, 1999, First Chemicl Corportion. [3] S. Bhogeswrro, D. Srinivs, J. Ctl. 285 (12) 31. [4] Z.T. Liu, C.X. Wng, Z.W. Liu, J. Lu, Appl. Ctl. 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