Promoting effect of Sn on supported Ni catalyst during steam reforming of glycerol

P omo ing e ec o Sn on suppo ed Ni ca alys du ing s eam e o ming o glyce ol L. F. Bobadilla,†* F. Rome o-Sa ia, M. A. Cen eno and J. A. Od iozola Depa amen o de Química Ino gánica e Ins i u o de Ciencia de Ma e iales de Se illa (ICMSE). Cen o mix o CSIC - Uni e sidad de Se illa, 49 A . Amé ico Vespucio, 41092 Se illa (Spain) † Cu en add ess: Ins i u o de Tecnología Química (ITQ). Cen o mix o CSIC- Uni e sidad Poli écnica de Valencia, s/n A . de los na anjos, 46022 Valencia (Spain) * Co esponding au ho : [email p o ec ed] Abs ac : The p omo ing e ec o Sn on he ca aly ic pe o mance o suppo ed Ni ca alys in he eac ion o glyce ol s eam e o ming was s udied. The physico-chemical p ope ies o he p epa ed samples we e in es iga ed by X- ay luo escence (XRF), BET su ace a ea, in si u X- ay di ac ion (XRD), lase Raman spec oscopy, X- ay pho oelec on spec oscopy (XPS), scanning elec on mic oscopy (SEM) and empe a u e p og ammed oxida ion (TPO) echniques. The cha ac e iza ion esul s o he samples a e educ ion ea men (in he same condi ions han he ac i a ion be o e ca aly ic ac i i y measu emen s) e ealed he o ma ion o Ni-Sn alloy. The Sn-doped ca alys exhibi ed a high ac i i y and i was demons a ed ha he Sn addi ion inc ease he ca alys s abili y and du abili y by dec easing he coke deposi ion. Keywo ds: Hyd ogen p oduc ion; Sn addi ion; Coke deposi ion 1. In oduc ion Du ing he las decades impo an e o s a e ongoing o educe ossil uel dependency and g eenhouse gas emissions. Among he di e en possibili ies o ind a iable op ion, he use o hyd ogen as an ene gy ca ie is conside ed o be an in e es ing al e na i e o he u u e and could ha e a c ucial ole in educing a mosphe ic pollu ion. Biomass is conside ed as one o he mos a ac i e enewable sou ce o hyd ogen p oduc ion and nume ous s udies a e being di ec ed owa d he de elopmen o no el echnologies o p oduce hyd ogen om biomass [1-4]. S eam e o ming o glyce ol is an impo an app oach o hyd ogen p oduc ion om biomass. Glyce ol is he main by-p oduc gene a ed in he biodiesel p oduc ion. Biodiesel is one enewable bio uel ob ained by ca aly ic ans-es e i ica ion o iglyce ides wi h me hanol [5]. Abou 10 w .% o glyce ol can be p oduced du ing he con e sion o ege able oils in o biodiesel. The use o glyce ol o hyd ogen gene a ion is a e y ad an ageous op ion since i s use would dec ease he p ice o biodiesel making i mo e compe i i e [6]. The o e all eac ion o glyce ol s eam e o ming is gi en by he ollowing equa ion: C3H8O3 + 3H2O → 3CO2 + 7H2 (1) which can be exp essed as a combina ion o glyce ol decomposi ion (2) and he wa e - gas shi eac ion (3): C3H8O3 → 3CO + 4H2 (2) CO + H2O ↔ CO2 + H2 (3) Theo e ically, a maximum o 7 moles o H2 pe mol o glyce ol can be p oduced, al hough his a io depends on he eac ion condi ions such as empe a u e, p essu e and s eam- o-glyce ol s eam a io. Al hough he glyce ol s eam e o ming p ocess is e y a ac i e and i could be de eloped on an indus ial-scale, i has some challenges ha mus o be o e come in o de o accomplish i s e ec i e comme cializa ion. Fo example, he p ocess is an endo he mic eac ion and equi es high empe a u es inc easing he ope a ion cos s. Besides, he ca alys deac i a ion by coke deposi ion is also an issue since i a ec s hyd ogen yield and long e m ope a ion. Wi h he aim o o e coming hese challenges, he de elopmen o ac i e, s able and inexpensi e ca aly ic ma e ials is manda o y. Ca alys s con aining g oup 8-10 me als such as Ni [7-12], Co [13-16], P [17-21], Ru [22-24], Rh [25], Pd [26] o I [27] on di e en oxides ha e been la gely in es iga ed as ac i e ca alys s o glyce ol s eam e o ming. Ni-suppo ed is one o he mos p omising ac i e me als o such an applica ion because o i s high ac i i y, low cos and wide a ailabili y. Howe e , Ni-based ca alys s su e deac i a ion by coke deposi ion on he ca alys su ace ha block ac i e si es and a ou side eac ions. Deac i a ion o nickel- based ca alys s by sin e ing o nickel c ys alli es is ano he impo an d awback. P omo ing nickel ca alys s wi h a second me al has been p o en o be one o he mos p omising app oach o ob ain mo e s able and op imal ca alys s [28]. Bime allic ca alys s comp ising Sn as a Ni p omo e ha e been p o ed o ou pe o m Ni monome allic ca alys in s eam e o ming p ocesses [29]. The coke deposi ion can be ma kedly educed by using Sn-doped ca alys s in he s eam e o ming eac ion o gene a e hyd ogen om hyd oca bons. Sn alloyed wi h nickel p e en s he o ma ion o nickel a om ensembles, which a e he esponsible o he coke o ma ion, and a oids he di usion o ca bon o o m la ge coke agglome a es [30]. The e o e, NiSn-based ca alys s ha e he po en ial o dec ease he ca alys deac i a ion caused by coking main aining i s high speci ic ac i i y. Sn-doped Ni ca alys s ha e been epo ed o aqueous phase e o ming [31, 32] and me hane s eam e o ming [33] whe e he o ma ion NixSny alloys play a key ole o inhibi coke deposi ion. Pengpanich e al. [34] epo ed a clea example o his posi i e e ec in he pa ial oxida ion o iso- oc ane. These au ho s ound ha he addi ion o small amoun s o in dec eased by mo e han 50% he o ma ion o ca bon deposi s wi hou changes in he con e sion. This enhancemen was asc ibed o he abili y o Sn o educe he g ow h o ca bon ilamen by e a ding ca bon solubili y in he Ni pa icles [28]. Mo eo e , Saadi e al. [35] ha e demons a ed, using densi y unc ional heo y (DFT) calcula ions, he abili y o Ni-Sn o inhibi g aphi e o ma ion du ing s eam e o ming eac ions. They demons a ed ha he p esence o Sn inc eases he C-C bond o ma ion ba ie . We ha e in es iga ed p e iously he e ec o he na u e o he suppo (acidi y, basici y and edox p ope ies) in he ca aly ic pe o mance o Sn-doped Ni ca alys s du ing he s eam e o ming o alcohols [36-38]. The objec i e o he p esen wo k is o in es iga e he e ec o in on he ca aly ic pe o mance o Ni-suppo ed ca alys s in e ms o ac i i y, selec i i y and du abili y in he s eam e o ming o glyce ol. 2. Expe imen al 2.1. Ca alys s p epa a ion The alumina suppo was ob ained by ball milling sphe ical alumina pelle s (SASOL, 1.78 mm diame e ), by using a PM4 Re sch ins umen pa icle sizes in 7 – 8 μm ange we e achie ed. A monome allic Ni-based ca alys was p epa ed by imp egna ing he alumina wi h an aqueous solu ion o all he ino ganic p ecu so s wi h he desi ed concen a ions o ce ium (III) ni a e hexahyd a e (Sigma-Ald ich), magnesium ni a e hexahyd a e (Sigma-Ald ich), and nickel (II) ni a e hexahyd a e (Sigma-Ald ich). Then, he sample was d ied o e nigh a 100ºC and calcined a 700ºC o 12 h in lowing ai using a hea ing a e o 10 C min-1 o ob ain 20 w .%Ni, 12 w .%CeO2, 8 w .%MgO, and 60 w .%Al2O3. The bime allic NiSn ca alys was p epa ed ollowing he same p ocedu e bu subs i u ing a ac ion o he nickel sal by he adequa e amoun o anhyd ous in chlo ide (Fluka) in o de o ob ain 17 w .%Ni, 3 w .%Sn, 12 w .%CeO2, 8 w .%MgO, and 60 w .%Al2O3. The esul ing ca alys s we e labelled as Ni/CeMgAl and Ni- Sn/CeMgAl, espec i ely. 2.2. Ma e ials cha ac e iza ion The chemical composi ion o he samples was de e mined by X- ay luo escence (XRF) spec oscopy in a Panaly ical AXIOS PW4400 sequen ial spec ome e wi h Rh ube as sou ce o adia ion. The analysis we e ca ied ou on o p essed wa e s con aining 6 w .% o wax. The ex u al p ope ies (BET su ace, po e size and po e olume) we e s udied by ni ogen physical adso p ion-deso p ion iso he ms a 77 K wi h Mic ome i ics ASAP 2010 equipmen . Be o e each measu emen , he samples we e ou gassed o 2 h a 150ºC in acuum o emo e he adso bed impu i ies. X- ay di ac ion (XRD) analysis was pe o med on a Siemens D-500 di ac ome e using Cu Kα adia ion (40 mA, 40 kV) and a posi ion-sensi i e de ec o using a s ep size o 0.05º and a s ep ime o 1 s. The educibili y o he ca alys s was in es iga ed by in si u XRD analysis using a high empe a u e came a An on Paa HTK 1200 coupled wi h an X´Pe P o Philips di ac ome e . The sys em was equipped wi h X´Cele a o de ec o wi h a s ep o 0.05º and an equi alen ime acquisi ion o 30 s. The di ac og ams we e eco ded in he 25 – 900ºC empe a u e ange unde a low o 100 mL min-1 (5% H2 in A ). Su ace cha ac e iza ion was ca ied ou by X- ay pho oelec on mic oscopy (XPS) on a Leybold-He aeus spec ome e (LHS-10/20), wo king wi h a cons an pass ene gy o 50 eV and acuum o 5 x 10-9 To . The sys em was equipped wi h an EA-200 MCD hemisphe ical elec on analyse wi h a dual X- ay sou ce wo king wi h Al Kα adia ion (1486.6 eV) a 120 W and 30 mA. Binding ene gies we e e e enced o he spu ious C (1s) signal a 284.6 eV and he a omic composi ion was es ima ed by he elemen al sensi i i y ac o o each a om. Be o e each analysis, he sample was educed in an ex e nal ube u nace o 1 h a 750ºC unde a o al low o 100 mL min-1 (50% H2 in N2). Then, he sample was cooled down o oom empe a u e and placed in he p echambe o he XPS spec ome e . The mo phology o he ca alys s was e alua ed by SEM using a JEOL 5400 ins umen equipped wi h an Ene gy Dispe si e X- ay Spec ome e (OXFORD LINK TETRA 1128-231). Raman spec a we e eco ded in a dispe si e Ho i a Jobin Y on LabRam HR800 Con ocal Raman mic oscope wi h a g een lase (532.05 nm) wo king a 5 mV powe and using a 600 g oo es/mm g a ing. The mic oscope used a 50x objec i e wi h con ocal pinhole o 1000 μm. The Raman spec ome e was calib a ed using a silicon wa e . The empe a u e-p og ammed oxida ion (TPO) was ca ied ou in a U-shaped eac o unde a o al low o 50 mL min-1 (5% O2 in He) om oom empe a u e o 900ºC wi h hea ing a e o 10 ºC min-1. The o med p oduc s we e analysed by mass spec ome y (MS) in a P ei e acuum mass spec ome e . 2.3. Ca aly ic pe o mance The glyce ol s eam e o ming eac ion was e alua ed as a unc ion o ime o pe iods o 4, 12 and 24 h in iso he mal condi ions a 750ºC, wi h a s eam- o-ca bon mola a io o 4 and 100000 mL g-1 h-1 space eloci y. P io o eac ion, he samples we e p essed, c ushed, and sie ed o achie e pa icles in he 100-200 μm ange. In each un, 100 mg o sample was dilu ed wi h he same amoun o c ushed qua z sie ed o he same pa icle size and placed in o he eac o be ween wo qua z wool plugs. The ca alys s we e ac i a ed unde a o al low o 100 mL min-1 (50% H2 in N2) a 750ºC o 1 h. The expe imen s we e ca ied ou a a mosphe ic p essu e in a comme cial Mic oac i i y Re e ence Reac o made by PiD Eng&Tech using a Has elloy C-276 ubula eac o wi h 9 mm in e nal diame e . Gas p oduc s we e analysed on-line using a mic oGC (Va ian 4900) equipped wi h Po apak Q and Molecula Sie e – 5A columns. A he eac o ou le a Pel ie liquid coole was i ed allowing he analysis o condensable p oduc s by HPLC (Va ian 356-LC) equipped wi h a e ac i e index de ec o and a Hi-Plex H column wi h milli-Q wa e as eluen . 3. Resul s and discussion 3.1. Ca alys s cha ac e iza ion Table 1 shows he chemical composi ions and ex u al p ope ies (BET su ace, po e size and po e olume) o he p epa ed ca alys s. Fo compa ison, he suppo was also measu ed. The expe imen al composi ions o all samples we e close o he nominal ones con i ming he e ec i eness o he imp egna ion me hod employed o p epa e he ca alys s. The pa en suppo p esen s supe io BET su ace a ea, which dec eases a e Ni o Ni-Sn addi ion caused by he inco po a ion o me al pa icles in o he po es o he suppo . Figu e 1 shows he XRD pa e ns eco ded o he p epa ed ca alys s. The di ac ion peaks a 2θ = 28.8º, 33.3º, 47.7º and 56.6º can be assigned o he (111), (200), (220) and (311) planes o he cubic luo i e phase cha ac e is ic o ce ium oxide (JCPDS 34- 0394). I can also be obse ed he di ac ion lines co esponding o he MgAl2O4 spinel phase (JCPDS 21-1152). As desc ibed in a p e ious wo k [38] he p esence o nickel induces he ans o ma ion o he MgAl2O4 spinel s uc u e in o a con inuous MgNi spinel laye al hough only a pa o nickel pa ially subs i u e Mg in he MgAl2O4 phase o ming a new NixMgyAl2O4 phase. Di ac ion peaks cha ac e is ic o nickel oxide (JCPDS 71-1179) we e obse ed a 37º and 43º. The e o e, i can be es ablished ha a ac ion o Ni is well dispe sed as NiO pa icles wi h mode a e in e ac ion wi h he suppo and ano he po ion o Ni is inco po a ed wi hin he non-s oichiome ic spinel- like phase NixMgyAl2O4 [8] wi h s ong me al-suppo in e ac ion. XRD peaks associa ed o Sn phases we e no obse ed in he Ni-Sn/CeMgAl ca alys likely due o i s high dispe sion. In o de o analyse he e olu ion and he changes in he c ys alline phases p oduced unde educ i e condi ions, in si u X- ay di ac ion analyses we e pe o med. Reduc ion was ca ied ou in an a mosphe e o hyd ogen dilu ed in A . The ob ained X- ay di ac og ams as a unc ion o empe a u e a e shown in Fig. 2. In bo h cases, he s uc u al modi ica ions a e only app eciable in he high empe a u e ange (700-900 ºC). Fo he Ni/CeMgAl ca alys , he NixMgyAl2O4 and NiO phases a e educed o me allic nickel (JDPDS 45-1027), whe eas he educ ion o he NiSn/CeMgAl ca alys leads o he o ma ion o NixSny alloys coexis ing wi h me allic Ni pa icles. I is known ha nickel can be comple ely alloyed wi h in o ming Ni3Sn, Ni3Sn2 and/o Ni3Sn4 in e me allic compounds depending o he Ni/Sn mola a io [39]. In ou case, he cha ac e is ic peaks o he Ni3Sn alloy (JCPDS 35-062) we e e idenced al hough he p esence o o he NixSny in e me allic compounds canno be uled ou . In bo h cases, me allic nickel was also p esen a e educ ion a 700 ºC. A u he inc ease o he empe a u e o 900 ºC led o an inc ease o he Ni di ac ion lines, which indica e a sin e ing o he nickel pa icles. Su ace analysis measu emen s by XPS we e pe o med o e Ni/CeMgAl and Ni- Sn/CeMgAl ca alys s o in es iga e he chemical su ace p ope ies and oxida ion s a es o bo h me als. P io o XPS analysis, esh ca alys s we e educed o 1 h a 750ºC unde lowing hyd ogen dilu ed in ine o ge in o ma ion abou he chemical s a e o he ac i a ed ca alys s. Ni 2p and Sn 3d XPS spec a a e shown in Fig. 3. Bo h Ni/CeMgAl and Ni-Sn/CeMgAl ca alys s exhibi ed wo decon olu ed Ni2p3/2 co e-le el peaks wi h he co esponding sa elli e peaks posi ioned o ~6 eV, espec i ely; he peak a 856 eV can be assigned o nickel alumina e phase, while ha he peak a 853 eV was a ibu ed o me allic nickel [40, 41]. I was obse ed ha he ela i e amoun o nickel species inco po a ed in he non-s oichiome ic spinel-like phase NixMgyAl2O4 is simila in bo h ca alys s while ha he ela i e quan i y o me allic nickel is sligh ly supe io in he Ni/CeMgAl ca alys s. This obse a ion is cohe en wi h he chemical composi ion o he ca alys s sugges ing ha he addi ional nickel in he Ni/CeMgAl ca alys is highly dispe sed o e he suppo as nickel oxide c ys alli es. F om he XPS esul s, i is e iden ha only he c ys alline nickel oxide was educed and he nickel wi h s ong in e ac ion wi h he suppo equi es highe empe a u es. Bo h Ni/CeMgAl and Ni- Sn/CeMgAl ca alys s exhibi ed iden ical amoun o nickel alumina e species s ongly bound o he suppo which we e no educed unde ou condi ions a 750ºC. The educ ion o Ni2+ ions inco po a ed non-s oichiome ically in e ahed al o oc ahed al si es o spinel phases equi es empe a u es as high as 800-850 ºC [42]. The Sn 3d egion spec a o he Ni-Sn/CeMgAl ca alys is shown in Fig. 3. The peak a 485 eV can be assigned o Sn0 species while ha he highe binding ene gy Sn 3d5/2 peak a 487 eV is a ibu able o in oxides (Sn2+ o Sn4+) [32]. The educed species o in is p obably a Ni-Sn alloy. Disc imina ing be ween Sn2+ and Sn4+ wi h XPS is complica ed, bu he p esence o his species indica es ha he educ ion o in was no comple e, and oxidized species a e exis en e en a e educ ion a 750 ºC, in ag eemen wi h p e ious esul s on alumina-suppo ed ca alys s [43]. Alumina in e ac s s ongly wi h he in and hinde s he edu ion o SnO [44]. The e e o e, XPS analysis o he Ni- Sn/CeMgAl a e educ ion a 750ºC indica es he p esence o Ni and Sn sepa a e phases. The ongoing educ ion o Ni and Sn can be concu en wi h he o ma ion o a Ni-Sn alloy, howe e his alloys a e indis inguishable by XPS om educed Ni su aces [32]. Consequen ly, i can be p oposed ha he Ni-Sn ca alys a e ac i a ion may be ep esen ed by he o ma ion o bime allic pa icles wi h a cen al co e o nickel su ounded by an ou e laye ich in in (Fig. 3). Du ing he educ ion phase, a pa o Sn a oms mig a e o he me allic nickel pa icles su ace o o m a Sn laye ha is pa ially educed o ming a Ni-Sn alloy co e ing he su ace o Ni pa icles. As can be obse ed in Table 2, he su ace Ni/Sn de e mined by XPS was lowe han he bulk one sugges ing ha Sn is loca ed mainly in he su ace laye s o he ca alys . Simila obse a ions we e ound by o he au ho s in he li e a u e [34, 45]. 3.2. Ac i i y measu emen s The ca alys pe o mances we e es ed o he hyd ogen p oduc ion in he glyce ol s eam e o ming. Figu e 4 shows he speci ic ac i i y o bo h Ni/CeMgAl and Ni- Sn/CeMgAl ca alys s. I is no ewo hy ha he ca aly ic beha iou was di e en in bo h cases. The ac i i y o Ni/CeMgAl ca alys was ound o be highe han ha o he Sn- p omo ed one and, appa en ly, e y s able o he i s 12 hou s. Once his pe iod is eached he ca aly ic ac i i y d as ically dec eases sugges ing a as deac i a ion p ocess. The pa ial subs i u ion o Ni by Sn in he NiSn/CeMgAl ca alys educes he ca aly ic ac i i y bu no ably enhances he s abili y o he ca alys . As can be obse ed, a e an ini ial s abiliza ion pe iod o he i s 4 hou s, he ca aly ic ac i i y emains s able. The obse ed ac i i y loss upon in addi ion migh be due o he ac ha he nickel ac i e si es o he glyce ol s eam e o ming a e co e ed by Sn species as con i med by XPS analysis. The Sn-en iched su ace p e en s he adso p ion o he glyce ol molecules on he me allic nickel su ace and he subsequen e o ming eac ions. Simila obse a ions we e ea ly epo ed in he li e a u e o he NiSn/Ce0.75Z 0.25O2 [34] and P Sn/C [46] sys ems. The same gaseous p oduc s we e de ec ed in bo h ca alys s (H2, CO, CO2, CH4 and C2H4, Figu e 5), al hough wi h a di e en p oduc dis ibu ion. Condensable p oduc s such as oxygena ed compounds we e no de ec ed a his eac ion empe a u e in good ag eemen wi h p e ious esul s [36]. Hyd ogen was he main componen in bo h ca alys s and emained in a ian du ing all ime-on-s eam. I can be no iced ha du ing he i s 12 hou s, he Ni/CeMgAl ca alys p oduced he maximum hyd ogen mola ac ion possible (70%) acco ding o he s oichiome y o he eac ion (1). A e 12 h o eac ion changes in he mola composi ion we e obse ed. F om he e, he p oduc ion o CO2 dec eases along wi h he inc ease o CO, CH4 and C2H4. Mo eo e , he change o selec i i y appea s when ac i i y loss is obse ed. Fo he Ni-Sn/CeMgAl ca alys , an inc ease o CO and dec ease o CO2 s a ed om he beginning o he eac ion while ha CH4 and C2H4 we e p oduced con inuously. The di e ences obse ed in bo h cases would be associa ed o he ca aly ic p ocesses in ol ed in he glyce ol decomposi ion p ocess. The o e all eac ion o glyce ol s eam e o ming (1) may be iewed as he combina ion o he glyce ol decomposi ion (2) and he wa e gas shi (WGS) eac ion (3). Fu he mo e, i may also be accompanied by o he he modynamically easible eac ions ha include: 2C3H8O3 (g) + H2 (g) ↔ 3CH4 (g) + 3CO (g) + 3H2O (g) (4) CO (g) + 3H2 (g) ↔ CH4 (g) + H2O (g) (5) CO2 (g) + 4H2 (g) ↔ CH4 (g) + 2H2O (g) (6) CO2 (g) + CH4 (g) ↔ 2CO (g) + 2H2 (g) (7) C(s) + H2O (g) → CO (g) + H2 (g) (8) C(s) + CO2 (g) ↔ 2CO (g) (9) C(s) + 2H2 (g) ↔ CH4 (g) (10) The ini ial gas p oduc s dis ibu ion o Ni/CeMgAl ca alys indica es ha he WGS eac ion was p edominan in he i s 12 h. This is consis en wi h a p oduc ion o hyd ogen close o he equilib ium alue. A e 12 h o eac ion me hane and e hylene a e p oduced. A aque e al. [13] sugges ed ha he o ma ion o me hane p oceeds h ough ca bon monoxide me hana ion (5) while e hylene would be o igina ed by hyd ogena ion/dehyd a ion o ca bonyl in e media es species. Cheng e al. [47] p oposed, howe e , ha me hane is p oduced om he hyd ogenolysis o glyce ol (4) uling ou he me hana ion eac ion as pa h o me hane p oduc ion. The s able p oduc ion o me hane o Ni-Sn/CeMgAl ca alys sugges s ha me hane could be o igina ed p edominan ly om he glyce ol hyd ogena ion and he ca bon hyd ogena ion (10). Indeed, he s eam gasi ica ion o ca bon (8) and he Boudoua d eac ion (9) may also accoun o he inc eased s abili y o Ni-Sn ca alys educing he deposi ion o ca bonaceous species while he p oduc ion o CO and H2 keeps cons an . CO o ma ion on Sn is he a ou ed eac ion om CO2 decomposi ion, especially o [25] Chiodo V, F eni S, Gal agno A, Mondello N, F us e i F. 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Coke deac i a ion o Ni and Co ca alys s in e hanol s eam e o ming a mild empe a u es in a luidized bed eac o . In . J. Hyd ogen Ene gy. 2014;39:12586-96. TABLES AND FIGURES Table 1. Chemical composi ion and ex u al p ope ies o he p epa ed ca alys s w .% composi iona a .% composi ion SBET (m2 g-1) Al2O3 MgO CeO2 Ni Sn Ni Sn Ni/Sn CeMgAl 77.9 (78) 9.6 (10) 12.5 (12) - - - - - 106 Ni/CeMgAl 63.6 (60) 6.6 (7) 11.1 (12) 18.7 (20) - 27.5 - - 89 Ni-Sn/CeMgAl 60.1 (60) 6.8 (7) 11.9 (12) 17.3 (17) 3.7 (3) 25.6 2.7 9.4 86 a Nominal alues in pa en hesis Table 2. Su ace composi ion ob ained by XPS analysis o he educed and spen ca alys s a .% composi ion a Al Mg Ce O Ni Sn C Ni/Sn Ni/CeMgAl educed 18.7 5.1 2.0 69.5 4.6 (27.5) - - Ni/CeMgAl spen - - 0.15 4.35 - - 95.5 Ni-Sn/CeMgAl educed 17.8 6.1 2.0 69.2 3.7 (25.6) 1.1 (2.7) - 3.4 (9.4) Ni-Sn/CeMgAl spen 3.5 2.1 0.4 21.7 0.5 0.1 71.8 5.0 a Bulk alues in pa en hesis Fig. 1 XRD pa e ns o he p epa ed ca alys s Fig. 2 In si u XRD pa e ns in educ i e a mosphe e o he p epa ed ca alys s: (a) Ni/CeMgAl and (b) Ni-Sn/CeMgAl Fig. 3 XPS spec a in he Ni 2p and Sn 3d le el and pic o ial ep esen a ion o educed ca alys Fig. 4 Ca aly ic ac i i y as a unc ion o ime o glyce ol s eam e o ming a 750ºC o he p epa ed ca alys s Fig. 5 P oduc dis ibu ion in glyce ol s eam e o ming a 750ºC o he s udied ca alys s Fig. 6 SEM mic og aph o he esh and used ca alys s, a e 4 h and 24 h on s eam in glyce ol s eam e o ming a 750 ºC