Comparing Pt/SrTiO 3 to Rh/SrTiO 3 for hydrogen photocatalytic production from ethanol

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1 Appl Petrohem Res (2013) 3:83 89 DOI /s y ORIGINAL ARTICLE Compring Pt/SrTiO 3 to Rh/SrTiO 3 for hydrogen phototlyti prodution from ethnol A. K. Wh T. Odediro J. Lis M. Hedhili A. Delvr H. Idriss Reeived: 22 June 2013 / Aepted: 23 July 2013 / Pulished online: 13 August 2013 The Author(s) This rtile is pulished with open ess t Springerlink.om Astrt Phototlyti hydrogen prodution from ethnol s n exmple of iofuel is studied over 0.5 wt% Rh/ SrTiO 3 nd 0.5 wt% Pt/SrTiO 3 perovskite mterils. The rte of hydrogen prodution, r H2, over Pt/SrTiO 3 is found to e fr higher thn tht oserved over Rh/SrTiO 3 ( mol of H 2 g tl. min -1 ( mol -2 of H 2 m tl. min -1 ) ompred to mol of -1 H 2 g tl. min -1 ( mol of H 2 m tl. min -1 ), respetively, under UV exittion with flux equivlent to tht from the sun light (. 1 mw m -2 ). Anlyses of the XPS Rh3d nd XPS Pt4f indite tht Rh is minly present in its ioni form (Rh 3? ) while Pt is minly present in its metlli form (Pt 0 ). A frtion of the non-metlli stte of Rh in the tlyst persisted even fter rgon ion sputtering. The tendeny of Rh to e oxidized ompred to Pt might e the reson ehind the lower tivity of the former ompred to the lter. On the ontrry, lrger mount of methne re formed on the Rh ontining tlyst ompred to tht oserved on the Pt ontining tlyst due to the pity of Rh to rek the ron ron ond of the orgni ompound. Eletroni supplementry mteril The online version of this rtile (doi: /s y) ontins supplementry mteril, whih is ville to uthorized users. A. K. Wh T. Odediro H. Idriss (&) SABIC: T&I Riydh nd CRI-KAUST, Riydh, Sudi Ari e-mil: idrissh@si.om J. Lis KAIN, Riydh, Sudi Ari M. Hedhili A. Delvr KAUST, Thuwl, Sudi Ari Keywords Ethnol-photoretion XPS Rh3d XPS Pt4f Hydrogen prodution Perovskite mterils Bnd gp SrTiO 3 Cron ron ond dissoition Introdution Photo-tlyti prodution of hydrogen from renewles is poised to e one of its min soures in the future one suessful tlyti mterils re mde possile. The retion requires the presene of semiondutor with ng gp energy within the energy of the solr rdition, ondutor suh s nole metl to ept eletrons from the ondution nd in ddition to hydrogen-ontining ompounds [1]. Ultimtely, the desired ompound for hydrogen prodution is wter. Next to wter re lohols nd, in prtiulr, ethnol euse it is provided from iomss nd is therefore renewle [2]. SrTiO 3 is stle in wter s well s in presene of orgni ompounds; more importntly it endures orrosion under UV exittion nd unlike other non oxygen ontining ompounds is lredy oxygen terminted nd therefore nnot e over oxidized. It hs two nd gps: one indiret t 3.25 ev, similr to ntse the perovskite struture with TiO 2, nd one diret t 3.75 ev [3]. The indiret nd gp is etween the upper vlene nd, minly omposed of O2p, nd the empty sttes Ti3d (t 2g ). The diret nd gp is etween O2p nd Ti3d (e g ) levels [4] (Fig. 1). Numerous works hve ddressed the photo-tlyti tivity of SrTiO 3 lone, with other trnsition metls s well s doped with other ions [5 11]. Results differ strongly from one study to the other due to differene in rystllinity, metl dispersion, effet of dopnt nd retion onditions. In reent work [6], the retivity of SrTiO 3 with different prtile sizes ws tested for hydrogen

2 84 Appl Petrohem Res (2013) 3:83 89 Fig. 1 Top view (), side view () nd perspetive view () of SrTiO 3. Smll lk spheres (Sr 2? ), lrge gry spheres (O 2- ), smller yellow spheres in re those of Ti 4? tions. Also indited in d re the eletroni trnsitions etween O2p nd Ti3d levels (redrwn from Ref. [4]) z y x x z y x z y prodution from wter. It ws found tht ulk mteril (prtiles dimension [100 nm) ws more tive thn 30 nm size prtiles whih in turn were more tive thn 6.5 nm size prtiles (produing 28, 19.4 nd 3 lmol of -1 H 2 g tl. h -1, respetively [t 26.3 mw m -2 with k in the ( nm)]. Resons for this derese re ttriuted to n inrese in the wter oxidtion overpotentil for the smller prtiles nd redued light sorption due to quntum size effet. In nother work [7], proing the nisotropy of the retivity of SrTiO 3 mirorystls indited tht oth redued nd oxidized produts re formed preferentilly on {100} surfes. This nisotropy ws explined s eing due to differenes in the eletroni nd struture. Beuse diret optil trnsitions for hrge rriers hving momentum vetors in the \100[ diretion overlp well with the spetrl distriution of the sored photons, more photogenerted rriers re moving towrd {100} surfes thn other surfes nd, s result, the {100} surfes re more tive. Other work [8] hs ddressed the phototlyti wter splitting tivity for hydrogen prodution over the mesoporous-ssemled SrTiO 3 nnorystl-sed phototlysts with vrious hole svengers: methnol, ethnol, 2-propnol, D-gluose, nd N 2 SO 3. Pristine mesoporous-ssemled SrTiO 3 phototlysts exhiited higher phototlyti tivity in hydrogen prodution thn the non-mesoporous-ssemled ommeril phototlysts or ommeril SrTiO 3. These results indite tht the mesoporous ssemly of nnorystls with high pore uniformity plys signifint role, ffeting the phototlyti hydrogen prodution tivity. Moreover, it ws seen tht the Pt o-tlyst enhnes the visile light hrvesting ility of the mesoporous ssemly with n optimum Pt loding of 0.5 wt% on the mesoporous-ssemled SrTiO 3 phototlyst providing the highest phototlyti tivity, with hydrogen prodution rte of 276 lmol h -1 g tl. nd quntum effiieny of 1.9 % under UV light irrdition. Other works indited tht Rh (1 %)-doped SrTiO 3 phototlyst loded with Pt o-tlyst (0.1 wt%) gve 5.2 % of the quntum yield t 420 nm for the H 2 evolution retion from methnol solution (10 vol%) [9]. Another work [10] foused on doping SrTiO 3 /TiO 2 with N ions (using hexmethylenetetrmine) nd optimized phototlyti tivity of hydrogen prodution (verge hydrogen prodution -1 rte = 5.1 mmol g tl. h -1 with 2 wt% loded Pt) under visile light ws seen lthough little is known out the tlyst stility nd omprison with the non-doped semiondutor mteril. A reent omputtionl study (using the hyrid DFT method) of doping SrTiO 3 with metl tions oupled with experimentl study showed tht o-doping Cr nd L ions hd onsiderle enhnement effet on hydrogen prodution of wter/methnol solution [11]. The reson invoked is tht doping with L ions rised the Fermi level of Cr ions nd stilizes its oxidtion stte d

3 Appl Petrohem Res (2013) 3: of Cr 3? whih is needed for the red shift of the nd gp to extend light sorption into the visile region. In this work, we hve onduted study of Rh/SrTiO 3 nd Pt/SrTiO 3 mterils in ddition to monitoring their photo-tlyti retion to further understnd the extent of their tivity. In prtiulr, we ttempt to nswer whih of the two metls is more tive when dded to the semiondutor support. We find tht Rh hs weker effet thn Pt on hydrogen prodution (whih is similr to the se where these metls re deposited on TiO 2 ntse [12]). Although we hve not onduted detiled study of the prtile size effet of the semiondutor, we do not find onsiderle differene in the retion rtes upon hnging its morphology. Experimentl SrTiO 3 ws prepred y the sol gel method where TiCl 4 ws dded to strontium-nitrte solution in stoihiometri mounts. After the ddition of TiCl 4 to the strontiumnitrte solution, the ph ws rised with sodium hydroxide to vlue etween 8 nd 9 where strontium hydroxide nd titnium hydroxide preipitted. The preipitte ws left to stnd for out 12 h t room temperture to ensure ompletion of the retion fter whih it ws filtered nd wshed with de-ionized wter until neutrl ph (*7). The resulting mteril ws then dried in n oven t 100 C for period of t lest 12 h. Next the mteril ws lined t 800 C. X-ry diffrtion tehniques were used to indite formtion of SrTiO 3 (Fig. S1). Severl SrTiO 3 mterils were tested in ddition to ommeril SrTiO 3 (Sigm- Aldrih). Rh metls were impregnted from solution ontining RhCl 3 in 1 N HCl. The resulting tlyst differed from the initil SrTiO 3 s its prtile size ws fr smller nd its BET surfe re higher. Pt metls were impregnted the sme mnner (from PtCl 4 preursor). Other tehniques used to study the mteril inluded XPS, XRD, TEM, nd UV vis. Photoretion ws onduted in th retor ( ml) ontining mg of mterils under stirring onditions with UV lmp flux of out 1 mw m -2. Anlysis ws onduted using GC- TCD equipped with Porpk Q with N 2 rrier gs isothermlly t 50 C (N 2 flow rte = 20 ml min -1 ). At these onditions, the produts were eluted from the olumn with the following order: hydrogen, oxygen, methne, ron dioxide, ethylene, ethne, propylene, propne, etldehyde, followed y ethnol. The powder XRD ptterns of the smples were reorded on Philips X pert- MPD X-ry powder diffrtometer. A 2h intervl etween 10 nd 90 ws used with step size of nd step time of 0.5 s. The diffrtometer ws equipped with Nifiltered Cu K rdition soure (k = Å). The X-ry soure ws operted t 45 ma nd 40 kv. Smple preprtion for the X-ry nlysis involved gentle grinding of the solid into fine powder nd pking of pproximtely g of the smple into n luminum smple holder with light ompression to mke it flt nd tight. XRD ptterns of the smples were reorded with the X pert HighSore Plus softwre nd sved in XRDML text formt for further mnipultion nd proessing. X-Ry Photoeletron spetrosopy ws onduted using Thermo Sientifi ESCALAB 250 Xi, equipped with monohromted AlK X-ry soure, Ultr Violet He lmp for UPS, ion sttering spetrosopy (ISS), nd refleted eletron energy loss spetrosopy (REELS). The se pressure of the hmer ws typilly in the low to high mr rnge. Chrge neutrliztion ws used for ll smples (ompensting shifts of *1 ev). Spetr were lirted with respet to C1s t ev. The Rh3d, Pt4f, O1s, Sr3d, Ti2p, C1s nd vlene nd inding energy regions were snned for ll mterils. Typil quisition onditions were s follows: pss energy = 20 ev nd sn rte = 0.1 ev per 200 ms. Ar ion omrdment ws performed with n EX06 ion gun t 2 kv em energy nd 10 ma emission urrent; smple urrent ws typilly la. The sputtered re of lm 2 ws lrger thn the nlyzed re: lm 2. Self-supported oxide disks of pproximtely 0.5 m dimeter were loded into the hmer for nlysis. Dt quisition nd tretment were done using the Avntge softwre. Results Figure 2 presents TEM imge of the Rh/SrTiO 3 tlyst. Rh prtiles with men size of 3 4 nm re seen. SrTiO 3 is omposed of smll rystllites with sizes etween 10 nd 20 nm. The BET surfe re of the mteril is found to e. 13 m 2-1 g tl. Figure S2 presents high resolution imge of n Rh prtile; the spots in the FT imge unmiguously identify it y its rystlline struture. Figure 3 presents TEM of two types of Pt/SrTiO 3 : one where Pt ws deposited on ommeril SrTiO 3 nd the other on SrTiO 3 tht ws prepred y the sol gel method followed y nneling t 800 C. The ommeril SrTiO 3 is omposed of prtiles with dimension lrger thn 1 lm while the one prepred y the sol gel method is mde of typil perovskite (ui) prtiles with men size of out 50 nm; Pt prtiles nnot e seen t this resolution. XP spetr Ti2p, Sr3d, O1s, nd Rh3d (or Pt4f) were olleted mong other lines; here we fous on the min lines relevnt to this study. Figure 4 presents XPS Rh3d nd Ti2p of the s-prepred tlyst (, ) nd Ar ions sputtered (, d). The presene of Rh 3? (minly s Rh 2 O 3 ) nd Rh metl n e seen with inding energy = nd

4 86 Appl Petrohem Res (2013) 3: ev, respetively, in line with referene elements [13]. To further onfirm the presene of Rh metl s well to proe the extent of its redution, the sme re ws Ar ions sputtered for 2 min (2 kv, 10 ma; smple urrent 10 ma). Clerly, the signl from Rh metl inresed ompred to tht of Rh 3? due to redution of Rh ions under omrdment. However, only out 50 % of Rh ions re redued to metlli Rh. Our ojetive ws not to redue ll Rh ions ut to oserve for the effets of mild ion sputtering on the redution of the nole metl. Assoited with Rh ions redution is redution of Ti 4? ions to Ti 3? nd Ti 2? ions (, d) s hs een seen in numerous work for Ar ions redution of TiO 2 [14, 15]. Figure 5 presents the Sr3d, Ti2p nd Pt4f of the sprepred 0.5 wt% Pt/SrTiO 3 tlyst. Sr3d is typil of the doulet (3d 5/2,3d 3/2 )ofsr 2? ions (spin orit splitting lose to 2 ev), the nrrow lines of the Ti2p 3/2,1/2 nd their inding energy positions indite the sene of Ti 3? while the Pt4f 7/2,5/2 is tht of metlli Pt (inding energy of Pt 7/2 Fig. 2 TEM of 0.5 wt% Rh/SrTiO 3. Rh n e seen s drk prtiles in the min figure s well s the inset t 72.5 ev with spin orit splitting of 3.4 ev). The presene of Pt metl in the s-prepred tlyst is in shrp ontrst to tht of the Rh/SrTiO 3 where lrge frtion of Rh is found to e in the form or Rh 3? ions. This is typil of Pt deposited on reduile metl oxides where it is often lrgely present in its metlli form [16, 17]. Figure 6 presents results of the hydrogen prodution using photons of 3.3 ev in th retor ontining 0.5 wt% Pt/SrTiO 3 nd 0.5 wt% Rh/SrTiO 3. Hydrogen prodution is seen together with methne. The prodution rte is omprle to tht oserved on Au/TiO 2 ntse previously nd fr higher thn tht of Au/TiO 2 rutile [18]. The rtio Rh/Ti is found equl to 0.07 while tht of Pt/Ti is equl to Two min oservtions re ler from Fig. 6. First Pt/SrTiO 3 is out six times higher thn Rh/SrTiO 3 sed on weight nd out 20 times higher sed on re. Even if we ount for differene in the M to Ti 4? rtios, the rte of hydrogen prodution is still fr higher for the Ptontining tlysts ompred to tht of the Rh one. Seond, Rh/SrTiO 3 produes lrge mounts of methne when ompred to Pt/SrTiO 3. The lrger mounts of methne n e rtionlized y the ft tht Rh is more tive in reking the C C ond ompred to Pt [19 21]. In this se, the rtio H 2 to CH 4 is equl to 2 for Rh/SrTiO 3 while it is equl to 8 in the se of Pt/SrTiO 3. The retion ours due to eletron exittion from the vlene nd (VB) O2p to the ondution nd (CB) Ti3d upon UV illumintion, s presented in Fig. 1. Alohols re known s hole svenges [22 24]. Ethnol is oxidized to etldehyde vi two eletron injetion into the VB [25] with time sle in the nnoseond rnge [26]. Mehnistilly ethnol is first dissoitively dsored on the surfe forming ethoxides (ording to Eq. 2) nd then upon two eletron injetions (through -oxy rdil [18, 25]) etldehyde is formed. The hydrogen ions relesed through this proess re redued to one hydrogen moleule [27, 28]. Retions 1 4 represent these steps. Fig. 3 TEM of 0.5 wt% Pt/SrTiO 3. Commeril SrTiO 3 nd SrTiO 3 prepred y the sol gel method; the inset in presents the [100] ui struture of SrTiO 3 evidene of high degree of rystllinity

5 Appl Petrohem Res (2013) 3: As prepred Ar + sputtered (redued) d Fig. 4 XPS Rh 3d nd Ti2p of 0.5wt% Rh/SrTiO 3., As-prepred nd, d fter Ar ions sputtering Sr 3d 0.5 wt.% Pt/SrTiO 3 Pt 4f 0.5 wt.% Pt/SrTiO 3 Ti 2p 0.5 wt. %Pt/SrTiO 3 Intensity (ps) Intensity (ps) Intensity (CPS) Binding energy (ev) Binding energy (ev) Binding energy (ev) Fig. 5 XPS Sr3d, Ti2p nd Pt 4f of the s-prepred 0.5 wt% Pt/SrTiO 3. The Sr to Ti tomi rtio is out 1 SrTiO 3 þ UV! e þ h þ CH 3 CH 2 OH! CH 3 CH 2 OðÞþH ðþ CH 3 CH 2 OðÞþ2h þ! CH 3 CHO þ H ðþ 2HðÞþ2e! H 2 ð1þ ð2þ ð3þ ð4þ () dsored The retion over Pt/SrTiO 3 is similr to tht over Au/ TiO 2 where the min produts re H 2 nd etldehyde [1, 18], with tres of methne. However, over Rh/SrTiO 3 the retion proeeds to CH 4. Therefore, lrge frtion of CH 3 CHO frther rets with CH 4 nd CO.

6 88 Appl Petrohem Res (2013) 3:83 89 Fig. 6 Hydrogen nd methne photo-prodution from ethnol over 0.5 wt% Rh/SrTiO 3 nd 0.5 wt% Pt/SrTiO 3. BET surfe re of Rh/SrTiO 3 = 13 m 2 g -1 while tht of Pt/SrTiO 3 is 3.5 m 2 g -1. The numers inside the figure indite the slope (the rte in moles of hydrogen per g tl. per minute) Rte = 4.2 x 10-5 mol/g Ctl..h Rte = 24 x 10-5 mol/g Ctl..h 0.5 wt.% Rh-SrTiO 0.5 wt.% Pt-SrTiO 3 3 H 2 H 2 4 x 10-6 CH 4 CH4 0.5 x 10-6 CH 3 CHO! CH 4 þ CO ð5þ As presented in Fig. 6, CH 4 is formed. Therefore, one should exept if the retion of Eq. 5 is omplete, equl mounts of H 2 nd CH 4 would e formed. We hve onduted numerous runs nd hve found tht in ll ses CH 4 onentrtion ws lower thn tht of H 2. Reent DFT studies on SrTiO 3 indited the possiility of pthwy for ethnol deomposition involving CH 3 CO rdil (with the relese of three hydrogen ions) [29]. In this se, CH 3 CO my ret with surfe oxygen mking ette speies or my split to CH 3 rdil nd CO; CH 3 rdils my ret with OH rdils giving methnol, the ltter would esily deompose to CO nd hydrogen [30]. This route my explin the higher rtio H 2 to CH 4 oserved. To extrt ll hydrogen toms from ethnol, the formtion of CH 4 is not desired nd t present work is in progress to find method to more effiiently rek the ron ron ond efore otining etldehyde t 300 K under photon irrdition in the presene of wter providing dditionl hydrogen nd oxygen ions to omplete the reforming retion, s in high temperture ethnol stem reforming [31]. Conlusions The rte of photo-tlyti retion of ethnol to hydrogen is found to e fr higher on Pt/SrTiO 3 ompred to Rh/ SrTiO 3. The most likely reson for the higher tivity of the Pt ontining tlysts is the ese y whih metlli Pt is formed ompred to Rh (where lrge frtion of the sprepred mteril is in Rh 3? ). Ar ions sputtering Rh/ SrTiO 3 onsiderly redued Ti in SrTiO 3, ut only redued out hlf of Rh ions to metlli Rh. Another importnt differene is notied etween the two tlyti systems nd it is relted to retion seletivity. Rh/SrTiO 3 produes lrge mounts of CH 4 ompred to Pt/SrTiO 3 ; this is most likely relted to the pity of Rh to rek the ron ron ond ompred to tht of Pt. Open Aess This rtile is distriuted under the terms of the Cretive Commons Attriution Liense whih permits ny use, distriution, nd reprodution in ny medium, provided the originl uthor(s) nd the soure re redited. Referenes 1. Connelly KA, Idriss H (2012) The photoretion of TiO 2 nd Au/ TiO 2 single rystl nd powder surfes with orgni dsortes. Emphsis on hydrogen prodution from renewles. Green Chem 14: Sott M, Idriss H (2009) Heterogeneous tlysis for hydrogen prodution. In: Anstis P, Crtree RH (eds) Hndook of green hemistry green tlysis, vol 1, hp. 10, ISBN-10: Ymd Y, Knemitsu Y (2010) Bnd-to-nd photoluminesene in SrTiO 3. Phys Rev B 82: (R) vn Benthem K, Elsässer C, Frenh R (2001) Bulk eletroni struture of SrTiO 3 : experiment nd theory. J Appl Phys 90: Kudo A, Miseki Y (2009) Heterogeneous phototlyst mterils for wter splitting. Chem So Rev 38: Townsend TK, Browning ND, Osterloh FE (2012) Nnosle strontium titnte phototlysts for overll wter splitting. ACS Nno 6: Gioondi JL, Slvdor PA, Rohrer GS (2007) The origin of photohemil nisotropy in SrTiO 3. Topis Ctl. 44: Pungpeth T, Sreethwong T, Yoshikw S, Chvdej S (2009) Hydrogen prodution from phototlyti wter splitting over mesoporous-ssemled SrTiO 3 nnorystl-sed phototlysts. J Mol Ctl A Chem 312: Kont R, Ishii T, Kto H, Kudo A (2004) Phototlyti tivities of nole metl ion doped SrTiO 3 under visile light irrdition. J Phys Chem B 108: Jin-Hui Y, Yi-Rong Z, You-Gen T, Shu-Qin Z (2009) Nitrogendoped SrTiO 3 /TiO 2 omposite phototlysts for hydrogen prodution under visile light irrdition. J Alloy Compd 472:

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