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Applied Catalysis A: General 388 (2010) 262–271

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

ynthesis dependent core level binding energy shift in the oxidation state oflatinum coated on ceria–titania and its effect on catalytic decomposition ofethanol

jay S. Karakoti a,∗∗, Jessica E.S. Kingc, Abhilash Vincenta, Sudipta Seala,b,d,∗

Advanced Materials Processing and Analysis Centre, University of Central Florida, Orlando, FL 32816, USAMechanical Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USADepartment of Chemistry, University of Central Florida, Orlando, FL 32816, USANanoscience and Technology Centre, University of Central Florida, Orlando, FL 32816, USA

r t i c l e i n f o

rticle history:eceived 7 April 2010eceived in revised form 27 August 2010ccepted 31 August 2010vailable online 21 September 2010

eywords:eria–titaniaupported catalystsethanol decomposition

a b s t r a c t

Synergistic interaction of catalyst and support has attracted the interest of the catalytic community forseveral decades. The decomposition/oxidation of alcohols for the production of hydrogen as a source offuel requires such support catalyst interaction. Recent studies have suggested the active role of oxidebased supports on the catalytic ability of noble metals such as gold, platinum and palladium. Herein, wereport the effect of synthesis technique on the catalytic activity of platinum coated on mixed ceria–titaniasupport system. Wet impregnation technique followed by calcination was compared with the chemicalreduction of platinum during the coating over oxide support. Methanol decomposition studied using anin-house built catalytic reactor coupled to a mass spectrometer showed that catalyst prepared by ther-mal reduction of platinum demonstrated better catalytic ability than the catalyst prepared by chemical

latinum oxidesinding energy shift

reduction of platinum. Transmission electron microscopy revealed that the size of both platinum andceria–titania particles remained unchanged, while the X-ray photoelectron spectroscopy (XPS) revealedthat the oxidation state of platinum was modified by different coating procedures. A shift in the corelevel binding energy of the Pt 4f towards lower binding energy was observed with chemical reduction.Based on the XPS data it was found that platinum (on ceria–titania supports) in mixed oxidation stateoutperformed the Pt in reduced metallic state. Results from catalysis and in situ Fourier transform infra

ente

red spectroscopy are pres

. Introduction

The development of alternative sources of energy has drivenhe research in energy sector for more than a decade. Promisinglternatives have been proposed in the form of polymer electrolyteembrane fuel cells that require hydrogen rich gas streams. Alco-

ols such as methanol and ethanol are currently being targeteds the hydrocarbon sources for production of hydrogen requiredn these fuel cells. Among several methods looked into the genera-

ion of hydrogen from methanol such as decomposition (MD) [1–4],team reforming (SRM) [4–12], combined steam reforming (CSR)4] and partial oxidation (POM) [13–18] of alcohols, the decom-osition of methanol at elevated temperatures by noble metals

∗ Corresponding author at: Advanced Materials Processing and Analysis Centre,niversity of Central Florida, Orlando, FL 32816, USA. Tel.: +1 4078235277.

∗∗ Corresponding author.E-mail addresses: karakoti@mail.ucf.edu (A.S. Karakoti),

seal@mail.ucf.edu (S. Seal).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.08.060

d and discussed.© 2010 Elsevier B.V. All rights reserved.

supported on reducible and non reducible oxide can be visualizedas one of the simplest processes. The decomposition of methanol atelevated temperature over noble metals proceeds through the scis-sion of C–H bonds and C–O bonds and is supposed to be less energyintensive and selective as compared to ethanol where a C–C scissionis also required. However, this presumably simple decompositioncan undergo various reactions (Eqs. (1)–(4)) depending upon thecatalyst and the temperature of decomposition:

CH3OH → CO + 2H2 (1)

CO + 3H2 → CH4 + H2O (2)

2CH3OH → CH3OCH3 + H2O (3)

CO + H2O ↔ CO2 + H2 (4)

Use of noble metals decreases the temperature of decompo-sition and increases the percentage decomposition of methanolobtained at specific temperature but suffers from poisoning effectof carbon monoxide produced as a byproduct of the reaction. Metaloxide as supports for noble metal catalysts has been researched

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horoughly where the synergistic effect between the metal andhe metal oxide has been shown to help lower the decompositionemperature of the oxide, improve the selectivity and minimize orompletely remove carbon monoxide or water based poisoning ofhe supported catalysts [1,2,19–22]. Both non/partially reduciblealumina and zirconia) and reducible oxides (ceria and titania)ave been studied as supports for decomposition of methanol usingither Pd, Pt or Rh as the catalyst [22]. For the current system weave chosen ceria modified titania as the support oxide because ofhe improved catalytic performance of this mixed oxide as com-ared to ceria and titania only systems. In addition, the oxidationtate of cerium in ceria modified titania is found to be trivalent thusorming a mixture of reducible (titania) and non reducible (ceria)urface oxide. It was reported that for platinum, the decomposi-ion of methanol proceeds via the cleavage of C–H bond to produceH2OH and that this scission is much more favorable than the scis-ion of O–H bond due to the existence of a very high reversiblearrier. The reaction proceeds through a second C–H bond scissionollowed by a quasi simultaneous scission of the C–H/O–H bond toorm CO [2].

With respect to the activity of the metals, specifically platinum,t has been debated whether the pure metallic state or the oxidizedorm of platinum is more active in improving the performance andate of catalysis [23–25]. It was shown that the reconstruction oft (1 1 0) structure into the oxide form results in improved oxida-ion catalysis of carbon monoxide [26]. DFT analysis by Pedersent al. [27] confirmed this observation with respect to Pt (1 1 0) sur-ace and it was also found that �-PtO2 (0 0 0 1) hexagonal basallanes remain inert for CO oxidation. In another theoretical study

t was found that �-PtO2 forms on Pt (1 1 1) surface while PtO typetoichiometry is favored on Pt (1 0 0) surfaces. Although both PtOnd PtO2 were catalytically inefficient, the authors found Pt3O4hich formed at higher oxygen coverage, as more catalytically

ctive [28,29]. Li and Hammer [30] found a triple phase boundary ofas/metal/metal oxide to be much more active for the oxidation ofarbon monoxide at Pt (1 1 1)/CO and �-PtO2 phase boundary. Croyt al. [20] reported that the oxidation state (Pt�+) and particle sizef platinum play more important role for methanol decompositionhan the reducibility of the support. Xu et al. [23] emphasized theact that the oxidation ability of the Pt clusters is dependent uponhe size of the clusters and can exhibit disparate activity as well asnteraction with the support oxide using DFT calculations.

Herein we present further evidence that the activity of the plat-num towards methanol decomposition is primarily affected by itsxidation state. We show that the synthesis method chosen foroating platinum particles over mixed oxide support can play aajor role in determining the final oxidation state of the platinum.e compared the two most common methods of coating platinum,

mpregnation of platinum salt followed by thermal reduction andhemical reduction using sodium borohydride, over the oxide sup-ort. We show that the catalytic properties are a function of theynthesis and the oxidation state of platinum at fixed (2–5 nm) par-icle size over ceria–titania mixed oxides and the poisoning of theatalyst is observed in higher metallic state of platinum.

. Experimental details

.1. Synthesis of mixed oxides

Ceria–titania (1–5 wt%) mixed oxide was prepared by co-

recipitation technique using cerium sulfate and titanium (III)ulfate as precursors. Stoichiometric amounts of titanium sulfateas mixed with cerium sulfate and diluted to a final volume of

0 mL. 1 M ammonium hydroxide solution was added drop wiseith continuous stirring to this solution and the pH of the solu-

: General 388 (2010) 262–271 263

tion was gradually increased to 10.0. The remaining suspension wasstirred overnight to ensure complete hydrolysis of cerium and tita-nium ions. The precipitate was then filtered and the filtrate wastreated with 30% ammonium hydroxide solution to check if anytitanium and cerium ions were left non-hydrolyzed. The collectedprecipitate was washed repeatedly until the pH of the DI water afterwashing was neutral. The final powder was dried and calcined at600 ◦C for 3 h in air ambient.

2.2. Platinum coating by wet impregnation

120 mg of ceria–titania mixed oxide was dispersed in 50 mL ofdeionized water (18.2 M�). Stoichiometric amount of hexachloroplatinic acid was dissolved to yield 0.5 wt% Pt on the mixed oxide.The solution was stirred continuously and the water was boiledoff. This process ensures the coating of platinum ions on the sur-face of the oxide support. The powder was collected and heatedat 500 ◦C for 3 h inside the furnace in air ambient to obtain metal-lic platinum and decompose excess chlorine. The final powder wasfinely grounded and labeled as PTCT to represent platinum coatedon ceria–titania by thermal decomposition.

2.3. Platinum coating by chemical reduction

The chemical reduction and simultaneous coating of platinumwas obtained using a previously established procedure [31]. Briefly,120 mg of ceria–titania was suspended in 50 mL of deionized water(18.2 M�). Stoichiometric amount of hexachloro platinic acid wasdissolved to yield 0.5 wt% Pt on the mixed oxide. The solution wasstirred under argon atmosphere and 10 mL of freshly prepared(0.1 mg/mL) solution of sodium borohydride was added to the sus-pension. The reduction of platinum ions to metallic platinum couldbe observed immediately by a change (to black) in the color of thesuspension. The suspension was stirred for 2 h, filtered using 20 nmalumina filter (ANODISC – 47, 0.1 �m) and washed several timeswith excess deionized water to get rid of excess chlorine. The pow-der sample was dried at 100 ◦C and labeled as PTCR. To comparethe effect of thermal treatment on the oxidation state of platinumanother sample was prepared using same conditions and was cal-cined at 500 ◦C for 3 h in air ambient. This sample was labeled asPTCR-T.

2.4. Catalytic activity—decomposition of methanol

The decomposition of methanol was studied using an in-housebuilt catalytic reactor. The catalytic reactor was coupled to a massspectrometer (QIC-20, Hiden Analytical). Methanol was bubbled(using Ar as the carrier gas) through the catalytic reactor contain-ing Pt coated mixed oxide support on glass wool. The Ar flow ratewas maintained at 20 SCCM and the methanol flow rate throughthe reactor was calibrated before use. 50 mg of sample was loadedinterspersed in 4 layers of glass wool. The temperature was var-ied from 125 ◦C to 400 ◦C in 25 ◦C increments and the pressure wasnoted until it stabilized. The relative pressure of the gases was notedfrom the mass spectrometer.

2.5. In situ diffuse reflectance infra red spectroscopy (DRIFTS)

DRIFTS was used to study the nature of reactants and productson the surface of the catalyst using HARRICK reaction cell chamber.Methanol was flown through a bubbler using argon as the car-

rier gas at 40 SCCM. The dry Pt coated mixed oxide powder wasmixed with KBr and packed in the sample cell holder. The samplein the reaction cell was heated at 150 and 300 ◦C and the adsorptionand formation of various products was monitored using FTIR Spec-trum One (Perkin Elmer). Time base software was used to record

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easurements at every 10 s for a total of 2 min at a resolution ofcm−1. The spectra were recorded during and after the methanolow through the cell to qualitatively record the rate at which thedsorbed species are desorbed from the surface of the catalyst. Ashe methanol flow was turned off, the connection to the vacuumas also turned off during the collection of spectra to reduce thehysical surface desorption at elevated temperatures. A new sam-le was used at each temperature under study and allowed to run

n vacuum at the fixed temperature overnight to allow degassingnd stabilization of catalyst in the reaction cell. Different catalystst defined temperatures (room temperature, 150 ◦C and 300 ◦C)ere used as a baseline so that the peaks corresponding to only

dsorption of methanol are observed.

.6. High resolution transmission electron microscopy (HRTEM)

The size and the distribution of the Pt coated on mixed oxideupports were characterized by HRTEM (Philips, Tecnai) operatedt 300 kV with a point to point resolution of 0.2 nm. Samples wererepared by placing a drop of suspension of Pt coated nanopar-icles in acetone over the holey carbon coated copper grids. TheEM grids were dried overnight in a vacuum dessicator beforemaging.

.7. X-ray photoelectron spectroscopy (XPS)

XPS measurements were done using 5400 PHI ESCA spectrome-er using Mg K� as source of X-rays operating at a power of 300 W.he base pressure during the measurements was below 10−9 Torr.

ig. 1. High resolution transmission electron micrographs (HRTEM) of mixed metal oxidgglomeration tendency; (b) SAED pattern of titania confirming the polycrystalline nat–10 nm; (d) the crystal structure of the mixed oxide was confirmed by X-ray diffraction

A: General 388 (2010) 262–271

Pt coated powder samples were mounted on a carbon tape for XPSanalysis. The charging shift was calibrated with the binding energyof carbon (1s) as a baseline. The instrument was calibrated prior torunning samples using gold as the standard. The peaks were decon-voluted using Peakfit software by assigning peaks for Pt(0), Pt(2)and Pt(4). The fitting of the peaks by assigning just Pt(2) or Pt(0)resulted in extremely large FWHM and thus at least three peakscorresponding to the three oxidation states of Pt were assigned. Forcomparison between different samples the FWHM was kept con-stant from sample to sample for Pt(0) and Pt(2) as 1.65 and 1.9 whilethe FWHM for Pt(4) was left floating and the distance between thepeaks from Pt 4f5/2 and 4f7/2 was fixed at 3.3 eV.

2.8. X-ray diffraction (XRD)

Powder XRD (Rigaku D-Max B) was recorded using Cu K� radi-ation in the 2� range of 20–80◦. The powder samples were calcinedat 600 ◦C for 3 h to determine the phase and crystallinity of themixed oxide. The equipment was calibrated using standard quartzsample.

2.9. Surface area analysis

Surface area was measured using BET sorptometer (Nova 4200,Quantachrome) using nitrogen physisorption. The surface area wasmeasured for the mixed oxides sample both before and after coatingwith platinum and did not show significant difference in the surfacearea values.

e (ceria–titania) after calcination at 600 ◦C: (a) low magnification image depictingure of sample; (c) high magnification image confirming the individual particle ofwhich showed anatase as the only crystalline phase.

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. Results and discussion

.1. Physical characteristics of the support and metal decoratedatalysts

Metal oxides as catalyst support have been recognized as impor-ant components in the design and synthesis of modern catalystystems. The mixed oxide system of ceria and titania was used ashe supporting oxide. The co-precipitation based synthesis of the

ixed oxide results in synthesis of 5–7 nm crystallite of titania afteralcination at 600 ◦C as shown in Fig. 1a and c. The SAED patternn Fig. 1b matches with the anatase phase of the titania confirm-ng the polycrystalline nature of the oxide. The XRD did not showny indication of the cerium oxide or mixed phase of ceria titanatespon calcination at 600 ◦C which is consistent with the literature32–39]. It has been shown that the presence of ceria helps to delayhe onset of the rutile as well as anatase phase in titania and alsoelps to increase the temperature range over which anatase existss stable phase [35,36]. It has also been hypothesized that ceriumxide is present as an extremely diffuse phase over the grain bound-ries which also helps in preventing the coarsening of the particlest elevated temperatures and the onset of rutile phase [35,36].ecently, in a theoretical investigation, superior catalytic activityf M/CeO2/TiO2 system in water gas shift catalysis was reported40]. No indications of ceria being doped in the titania lattice wasbserved as confirmed by the absence of any shift in the XRD pat-ern of the ceria–titania with respect to only titania (not shown).

he high magnification image in Fig. 1c shows lattice fringes ofhe titanium oxide nanoparticles along with few amorphous par-icles. This could indicate that there are particles out of the planef focus or that a partially amorphous phase of ceria or titania isresent. The BET surface area of the as synthesized powders was

ig. 2. Transmission electron micrographs of sample PTCR shows 2–5 nm platinum nanopab) high magnification clusters of three nanoparticles; (c) faceted geometry of the platinuf Pt.

: General 388 (2010) 262–271 265

410 ± 3 m2/g and reduced to 83 ± 4 m2/g upon calcination at 600 ◦Cfor 3 h.

The coating of metal oxides with 0.5% platinum did not modifythe particle size of the mixed oxides. Coarsening of particle size ofplatinum was reported previously on the cerium oxide supports[22]. Such coarsening was not observed in present experimentshowever it is possible that the relatively lower atomic concentra-tion of the cerium is not sufficient enough to cause the coarsening ofthe Pt nanoparticles. Fig. 2a–d shows the TEM images of the 0.5% Ptcoated ceria–titania synthesized by the chemical reduction of plat-inum (PTCR) after adsorption. The Pt nanoparticles were distributedas 2–5 nm particles which are larger than the sizes reported byCroy et al. [20,22]. The Pt nanoparticles formed after chemicalreduction appears elliptical and/or faceted. The particle size of themixed oxide did not change after the process of reduction makingit difficult to identify the Pt nanoparticle dispersed in ceria–titaniamatrix. Surprisingly the particle size of the Pt nanoparticles syn-thesized from the chemical reduction followed by calcination wasin the similar range of 2–5 nm. Fig. 3a–d shows the size of the plat-inum nanoparticles synthesized by the chemical reduction methodand calcined at 500 ◦C (PTCR-T) for 2 h. However the geometry ofthe nanoparticles is more spherical as compared to the non cal-cined nanoparticles which suggest volume redistribution in the Ptnanoparticles during the process of calcination. The particle sizeand shape of the Pt nanoparticles synthesized by thermal reduction(Fig. 4a–c) were identical to the calcined sample. The highly dis-persed nature of Pt nanoparticles on the surface of oxide prevented

further agglomeration during the calcination however, the possi-bility of support effect in pinning the coarsening of Pt nanoparticlescannot be ruled out. Surprisingly, no indication of a top oxide coat-ing was evidenced by the TEM even though the samples showeda high degree of oxidized platinum (as discussed in following sec-

rticles formed upon chemical reduction of platinum chloride: (a) low magnification;m nanoparticles; (d) high magnification image confirming the interplanar spacing

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ig. 3. Transmission electron micrographs of sample PTCR-T shows that the particleupport: (a) low magnification image; (b) high magnification image; (c) spherical ghe polycrystalline nature of the mixed oxide.

ions). The surface area of Pt modified nanoparticles did not showny significant change as compared to the base value of 84 m2/g for00 ◦C calcined sample which can be attributed to the fact that thearticle size of as-synthesized powders is 5–7 nm and the modifi-ation by 0.5% coating of 2–5 nm platinum nanoparticles would notroduce a significant change.

.2. Activity and selectivity of the catalysts

The activity of different catalysts at various temperatures islotted in Fig. 5a. It was observed that the catalyst prepared byhermal decomposition of platinum over the mixed oxide cata-yst (PTCT) outperformed the catalysts prepared by the chemicaleduction of the metal. The onset temperatures of the PTCT andTCR were around 175–200 ◦C while the onset temperature ofhe PTCR-T sample was around 200–225 ◦C. The decomposition of

ethanol in all the samples proceeded with production of only COnd hydrogen. No additional gases such as methane or carbon diox-de were detected in the mass spectrometer for the temperaturesange covered in this study. It has been shown that the catalyticecomposition of methanol over Pt supported on ceria or plaineria (no noble metal catalyst) produces carbon dioxide as one ofhe byproducts [1,22]. As the crystalline features corresponding toerium oxide were not identified in the as-synthesized catalyst, it

an be assumed that the support oxide effect was mainly influ-nced by the presence of titanium oxide and not cerium oxide thusliminating the production of carbon dioxide. The activity of theatalysts increased with increase in temperature. Even though theample PTCR showed some activity in the lower temperature, the

platinum does not change upon calcination at 500 ◦C over ceria–titania mixed oxidetry of the platinum nanoparticles; (d) selected area electron diffraction confirming

catalytic activity did not improve significantly with temperatureas this catalyst reached only 50% efficiency at 325 ◦C. The cata-lyst (PTCR) failed to reach 100% conversion even upon increasingthe temperature up to 400 ◦C and demonstrated heavy poison-ing as shown in Fig. 5b. It was observed that at this temperaturetrace amounts of water and carbon dioxide were also producedand that the production of water significantly and rapidly poisonsthe catalyst in the presence of carbon monoxide. The calcinationof this sample at 500 ◦C improved the performance of the cata-lyst (PTCR-T) showing classical S-shaped kinetics with increase indecomposition of methanol at as the temperature was increasedwith conversion efficiency in excess of 90% at 350 ◦C and 375 ◦C.The catalyst PTCR-T showed 100% conversion at 400 ◦C, however,at this temperature the catalyst poisoning was very rapid similarto the poisoning observed for uncalcined sample (PTCR). Interest-ingly the catalyst PTCR-T showed very slow poisoning at 375 ◦Cfor at least 3 h but the conversion efficiency at this temperaturewas around 95%. As soon at the temperature was increased to400 ◦C the poisoning of the catalyst poisoning occurred within afew minutes (Fig. 4b) of increasing the temperature to 400 ◦C. Itwas also noted that even though the calcination of PTCR sampleimproved the overall conversion efficiency, it reduced the activityof the catalyst in the lower temperature range. This also suggestsan increase in the energy barrier for decomposition of methanol by

the PTCR-T sample which resulted in increasing the onset temper-ature for methanol decomposition. In contrast, the PTCT catalystdemonstrated excellent catalytic behavior as the decompositionof methanol increased rapidly with increase in temperature. Thecomplete conversion of 3.5 mmol of methanol/min/g of the catalyst

A.S. Karakoti et al. / Applied Catalysis A: General 388 (2010) 262–271 267

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ig. 4. Transmission electron micrographs of sample PTCT over ceria–titania mionfirming the polycrystalline nature of the mixed oxide; (c) high magnification immage depicting 2–5 nm platinum (1 1 1) nanoparticles.

as achieved at 300 ◦C. It was observed that the temperature for00% conversion of methanol was higher than the values reported

n literature [1,4,20,22,41–49] but our rate of flow of methanol isignificantly higher with respect to the catalyst loading and themount of noble metal on the supported oxides which could result

n reduced activity of the catalyst at lower temperatures. The cat-lyst loading of 50 mg in present study is lower than the mostlyeported data in the literature (100–300 mg) but has significantlyigher conversion rate of methanol [20,22,41–43,46–49].

ig. 5. (a) The activity of Pt modified ceria–titania catalysts for decomposition of methanoy change in the partial pressure of hydrogen as a function of time. The catalyst PTCT dispeduction of platinum (PTCR and PTCR-T) displayed very fast poisoning behavior.

ide support: (a) low magnification image; (b) selected area electron diffractionepicting that the particle size of the base oxide is unaltered; (d) high magnification

The rate of conversion of methanol for various catalysts isdepicted in Fig. 6a and it can be observed that both the cata-lysts PTCR and PTCT decomposed methanol at a very high rate of3.5 mmol of methanol/min/g of the catalyst. Arrhenius plots wereconstructed by plotting the natural log of rate of decomposition of

methanol against (1/T) and are depicted in Fig. 6b. It must be men-tioned that the values of ln (R) for PTCR and PTCR-T was plottedonly in the range of 250–350 ◦C and the values were neglected inthe lower temperature range to obtain the best fit. The values acti-

l at various temperatures and (b) the poisoning behavior of the catalysts is depictedlayed no signs of poisoning even after 11 h while the samples prepared by chemical

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be attributed to a limited interaction between the metal and thesupport during the reduction process. This shift is only 0.2 eV andthe marginal shift is under the limit of experimental and instru-mental accuracy. The fact that the metallic platinum peak was

ig. 6. (a) The rate of decomposition of methanol of various catalyst at various temf catalyst) flowing through the reactor and (b) the Arrhenius plots for the decomange for PTCT was chosen as 175–300 ◦C while for PTCR and PTCR-T the temperatu

ation energies were calculated from the plot and are 13.06 kJ/mol,3.67 kJ/mol and 19.21 kJ/mol for PTCT, PTCR and PTCR-T, respec-ively. As expected from the decomposition plots the activationnergy is lowest for PTCT followed by PTCR and highest for PTCR-Tuggesting that the decomposition of methanol is favored on ther-ally prepared noble metal catalyst on mixed oxide support.

.3. Chemical oxidation state of platinum and oxide support

The X-ray photoelectron spectroscopy was used to ascertainhe relative changes in the oxidation state of the Pt, Ce and Ti.he modification of support oxides with platinum did not changehe characteristics of the support oxide material. The XPS spec-ra of cerium 3d, titanium 3p, carbon 1s and chlorine 2p levelere obtained and is presented as supporting information. It was

bserved that the XPS spectra do not show any significant dif-erences between the samples coated with platinum. No tracef residual chlorine was observed in the XPS peak indicatinghe complete removal of chlorine during the decomposition ofrganometallic Pt to the metal. A high resolution scan was requiredo observe the Ce 3d spectra which demonstrated that cerium isredominantly present in trivalent oxidation state by the absencef characteristic 917 eV satellite peak for Ce4+. It has been shownreviously that at low atomic concentration of cerium in a mix-ure of ceria–titania, cerium is predominantly present in trivalentxidation state. The presence of reduced cerium on the surfacehanges the behavior of support oxide from reducible to partiallyon reducible. The XPS spectra of platinum 4f level (Fig. 7) depictsifferent percentage of Pt(0) for selective treatments of platinum.he thermally reduced platinum (PTCT) showed the highest per-entage of mixed oxidation states of platinum with Pt(2) being theredominant oxidation state. The position of the Pt(0) peak washifted to higher binding energy for PTCT relative to the pure metal-ic platinum while the Pt(0) peak for PTCR and PTCR-T was shiftedo lower binding energy relative to pure metallic titanium. Thishift towards the lower binding energy is consistent with the pre-ious reports [20]. Such large shift towards lower binding energyf Pt 4f core level peak was also reported by Dauscher et al. [50]nder reducing conditions over ceria–titania mixed oxide surface.

t was observed that under various annealing treatment of plat-num supported over bulk ceria–titania mixed oxide, the Pt 4f peak

howed various shift and was shifted to values as low as 70.3 eVpon annealing in hydrogen gas at 450 ◦C. The shift towards lowerinding energy suggests a charge transfer from the oxide supporto the metal under the reducing atmosphere thereby facilitatinghe removal of the binding electron from platinum. In contrast the

res shows both PCTT and PCTR-T decomposed 100% of methanol (3.5 mmol/min/gn of methanol as a function of temperature. For Arrhenius plots the temperaturege under study was from 275 ◦C to 350 ◦C.

shift in the Pt(0) peak towards higher binding energy in PTCT can

Fig. 7. The XPS spectra for platinum after reduction over the ceria–titania supportsfollowing the peak fit assignments: (a) PTCT shows mixed oxidation states with Pt(2)as the major component; (b) PTCR sample shows Pt(0) as the major component; (c)the calcination of the sample PCTR marginally increases the concentration of mixedoxidation states.

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ot shifted towards lower binding energy in the case of thermallyeduced platinum also suggests that the catalysts will behave dif-erently towards methanol oxidation. It is unclear whether theaceting of nanoparticles as observed in chemical reduction pro-ess of platinum as opposed to the thermal reduction is responsibleor opposite shift in the core level binding energy of metallic Pt.he cluster size dependent broadening of FWHM and shift in theE of metal nanoparticles have also been reported [51–53]. Weave previously reported the size dependent shift (negative) in theinding energy coupled with increase in FWHM of gold nanopar-icles relative to the bulk gold using sol–gel synthesized gold NPs

52]. However the cluster size effect is generally observed for par-icles with large difference in particle or cluster sizes. As observedrom TEM images the range of particle size in the current study isetween 2 and 4 nm and thus the shift in BE may not necessarilyrise from the difference in the particle size of Pt.

ig. 8. In situ diffuse reflectance spectroscopy spectra of catalyst (PTCT) in methanol flemperature (RT); (c and d) at 150 ◦C; (e and f) at 300 ◦C shows the behavior of methanol dorrespond to the C–H sym and asym stretch mode from the OCH3 group of methanol wormate.

: General 388 (2010) 262–271 269

The calcination of PTCR sample resulted in increasing the rela-tive amount of oxidized platinum and also resulted in improvingthe catalytic efficiency of the catalyst. It can be noted from Fig. 7band c that both the Pt(2) and Pt(4) peaks were also shifted to lowerbinding energy as compared to pure PtO and PtO2. While the cal-cination at 500 ◦C increases the relative percentage of oxidizedplatinum it did not induce any shift in the position of the Pt 4f peaksas compared to the non calcined sample. In contrast Pt(2) was thepredominant form of platinum in the sample prepared by thermalreduction of platinum.

A comparison of the activity of catalysts towards methanol

decomposition with the results from the XPS spectra suggests thatthe oxidized form of platinum is more active for the decomposi-tion of methanol over mixed ceria–titania as the supports. Thisobservation adds to the controversy in the open literature aboutthe role of the oxidation state of platinum towards its catalytic

ow and after the flow of methanol at different temperatures: (a and b) at roomecomposition over the surface of catalyst. The peaks in the region 2950–2810 cm−1

hile the peaks in region 1570–1300 cm−1 shows presence of O–C–O stretch from

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ctivity. The fact that we could produce similar sized Pt particlesver the same support with different oxidation state is very impor-ant contribution and may be used as an experimental evidence forigher activity of oxidized platinum at least in the decompositionf methanol. The oxidized form of platinum was also important tovoid the poisoning of the catalyst as the thermally calcined mate-ial did not show poisoning for more than 11 h at a rate of 3.5 mmolf methanol/min/g of the catalyst, a high rate of decomposition.espite the size being the same, it should also be noted that the

hermally calcined catalysts did not show any faceting probablyue to 500 ◦C calcination. The contribution of facets in affectinghe catalytic performance cannot be ignored and will be a topicf future investigations. The spherical geometry is the most stableonfiguration and may expose the most stable planes as opposedo the faceted geometry which may expose more reactive planes.his could explain the higher activity of chemically reduced cata-yst (PTCR) in the low temperature regime however; the catalyticerformance was hampered at elevated temperature possibly dueo partial poisoning of the catalyst. The role of support was not the

ain focus of the study as the support was kept same for differentreparation of platinum coated nanoparticles. The effect of differ-nt coating procedures on the chemical and physical properties ofupport oxide was inconclusive as the support did not show anyignificant changes in the XPS spectra. Thus all the changes in theatalytic performance can be related to the changes in the chemicaltate of platinum.

.4. In situ diffuse reflectance spectroscopy

FTIR analysis was performed on all the samples to identify anyifference in the mechanism of decomposition of methanol whichay have resulted in lowering the activity of PTCR and poisoning

f PTCR-T samples. General observations were consistent with thebservations of Wachs et al. [16,54–56]. It was surprising that nopparent difference was found between the samples except thathere were no signs of immediate breakdown of methanol at 150 ◦Cor PTCR and PTCR-T samples after stopping the flow of methanol.he spectra are not shown for the brevity of manuscript; however,he peaks from the sample PTCT is being discussed and shown withespect to changes in temperature in Fig. 8. At room temperaturehe peaks corresponding to only physical adsorption of methanolan be observed at 2952 and 2841 cm−1 for C–H stretching mode ofhe methoxy group. The methanol is not desorbed from the surfacepon stopping the methanol flow and the absorption peaks corre-pond only to physisorbed methanol during and after the methanolow at room temperature. Upon increasing the temperature to50 ◦C the peaks from the physisorbed methanol disappeared andhe peaks corresponding to the chemisorbed methanol arise fromhe C–H stretch of the methoxy group at 2935 and 2830 cm−1.he peaks from the bending mode were not very apparent how-ver the formate peaks at 1572 and 1385 cm−1 were visible. Theseormate peaks are at higher wavelength than the normal formateeaks possibly due to the support interactions. The C–H stretchrom the formate also appeared at 2880 and 2961 cm−1 suggestinghat the stepwise dehydrogenation of methanol is the preferred

echanism of decomposition of methanol. For efficient catalysis its imperative that the reactants must be adsorbed on the catalysturface leading to the formation of products which should be read-ly desorbed from the surface. The relative stability and energy fordsorption should be an intermediate value such that the reactionroceeds in the forward direction and the products are desorbed

rom the surface with relative ease. From Fig. 8c and d it is clearhat both the species are coexistent on the surface however; thesere strongly adsorbed on the surface as even after stopping theow of the methanol the C–H stretching peaks from methoxy andhe O–C–O stretching peaks from the formate were present (for

A: General 388 (2010) 262–271

the duration of measurement). Thus even though the decomposi-tion of methanol may take place at 150 ◦C the yield will be low asthe catalyst is supersaturated. Upon increasing the temperature to300 ◦C the peaks due to formate adsorption shifted to lower val-ues than at 150 ◦C and appeared at 1564 and 1383 cm−1. At thistemperature both the reactants and the products were readily des-orbed from the surface and the formate as well as methoxy peaksdisappeared within few seconds of stopping the flow of methanol.This shows that at 300 ◦C the relative stability of the reactants andproducts on the surface of nanoparticles is low enough to allowimmediate desorption of products. Such an observation also hintsat lower poisoning of the PTCT catalyst at 300 ◦C and the completedecomposition of methanol.

4. Summary

It was shown that synthesis of the Pt modified catalysts playa very important role in determining the final oxidation state ofplatinum on the mixed oxide support. It was found that oxidizedform of platinum showed superior catalytic performance withoutany hints of poisoning for decomposition of methanol at rela-tively higher rate of 3.5 mmol/min/g of the catalyst. The completeconversion for Pt modified ceria–titania (PTCT) in which Pt waspresent in both oxidized and reduced form was obtained at 300 ◦Cwhile the catalyst containing predominantly reduced form of plat-inum (PTCR) could not decompose methanol completely even uponincreasing the temperature to 400 ◦C. The catalytic performanceimproved upon calcination of the chemically reduced and coatedplatinum over ceria–titania supports (PTCR-T) however, the bar-rier for catalytic transformation increased as indicated by relativelylower conversions in the 200–300 ◦C temperature regime and thehigh activation energy value. The modification of Pt nanoparticlesfrom a faceted structure to spherical morphology could be oneof the reasons for higher activation energy. In contrast the pres-ence of oxidized form of platinum (may be at the surface) resultedin improvement in catalyst performance and showed completeconversion at 400 ◦C but demonstrated a heavy poisoning of thecatalyst. The observations are consistent with portions of literaturethat have shown the relatively superior performance of oxidizedform of platinum. The main mechanism for decomposition wasfound to be dehydrogenation of methanol and was confirmed usingDRIFTS. Such a behavior could be a characteristic of the ceria–titaniasupport and future investigations will be aimed at developing ageneral theory based on the catalysis behavior of the platinumcoated supports over a range of reducible and non reducible sup-ports.

Acknowledgements

Authors would like to acknowledge all members of SNF lab-oratory who helped in initial phases of project. Authors alsoacknowledge portions of material work funded by NSF NIRT0708172 (partial support) and the student funding support fromNSF REU site 0453436 for Jessica King. Authors would like to thankDr. Ponnusamy Nachimuthu and Shail Sanghavi for their help inXPS measurements to determine the presence of surface cerium. (Aportion of) The research was performed using EMSL, a national sci-entific user facility sponsored by the Department of Energy’s Officeof Biological and Environmental Research and located at PacificNorthwest National Laboratory.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.apcata.2010.08.060.

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eferences

[1] S. Imamura, T. Hagashihara, Y. Saito, H. Aritani, H. Kanai, Y. Matsumura, N.Tsuda, Catalysis Today 50 (1999) 369–380.

[2] J. Greeley, M. Mavrikakis, Journal of the American Chemical Society 126 (2004)3910–3919.

[3] S.A. Vilekar, I. Fishtik, R. Datta, Journal of Catalysis 252 (2007) 258–270.[4] G. Avgouropoulos, J. Papavasiliou, T. Ioannides, Chemical Engineering Journal

154 (2009) 274–280.[5] J.H. Jang, Y. Xu, D.H. Chun, M. Demura, D.M. Wee, T. Hirano, Catalysis Letters

134 (2010) 258–263.[6] S.H. Ahn, O.J. Kwon, I. Choi, J.J. Kim, Catalysis Communications 10 (2009)

2018–2022.[7] S. Esposito, M. Turco, G. Bagnasco, C. Cammarano, P. Pernice, A. Aronne, Applied

Catalysis A: General 372 (2010) 48–57.[8] S. Kudo, T. Maki, K. Miura, K. Mae, Carbon 48 (2010) 1186–1195.[9] H. Lorenz, S. Turner, O.I. Lebedev, G. Van Tendeloo, B. Klotzer, C. Rameshan, K.

Pfaller, S. Penner, Applied Catalysis A: General 374 (2010) 180–188.10] Y. Matsumura, H. Ishibe, Journal of Catalysis 268 (2009) 282–289.11] R. Tesser, M. Di Serio, E. Santacesaria, Chemical Engineering Journal 154 (2009)

69–75.12] P.P.C. Udani, P. Gunawardana, H.C. Lee, D.H. Kim, International Journal of Hydro-

gen Energy 34 (2009) 7648–7655.13] A. Kuzume, Y. Mochiduki, T. Tsuchida, M. Ito, Physical Chemistry Chemical

Physics 10 (2008) 2175–2179.14] M.V. Ganduglia-Pirovano, C. Popa, J. Sauer, H. Abbott, A. Uhl, M. Baron, D. Stac-

chiola, O. Bondarchuk, S. Shaikhutdinov, H.J. Freund, Journal of the AmericanChemical Society 132 (2010) 2345–2349.

15] H.Q. Guo, D.B. Li, D. Jiang, W.H. Li, Y.H. Sun, Catalysis Letters 135 (2010) 48–56.16] G.S. Li, D.H. Hu, G.G. Xia, Z.C. Zhang, Topics in Catalysis 53 (2010) 40–48.17] D.R. Rolison, Science 299 (2003) 1698–1701.18] A. Wittstock, V. Zielasek, J. Biener, C.M. Friend, M. Baumer, Science 327 (2010)

319–322.19] D.L. Coffing, J.L. Wile, Journal of Chemical Education 70 (1993) 585–586.20] J.R. Croy, S. Mostafa, H. Heinrich, B.R. Cuenya, Catalysis Letters 131 (2009)

21–32.21] J.R. Croy, S. Mostafa, L. Hickman, H. Heinrich, B.R. Cuenya, Applied Catalysis A:

General 350 (2008) 207–216.22] J.R. Croy, S. Mostafa, J. Liu, Y.H. Sohn, H. Heinrich, B.R. Cuenya, Catalysis Letters

119 (2007) 209–216.23] Y. Xu, W.A. Shelton, W.F. Schneider, Journal of Physical Chemistry A 110 (2006)

5839–5846.24] W.X. Li, Journal of Physics-Condensed Matter 20 (2008) 184022.25] W.X. Li, L. Osterlund, E.K. Vestergaard, R.T. Vang, J. Matthiesen, T.M. Pedersen,

E. Laegsgaard, B. Hammer, F. Besenbacher, Physical Review Letters 93 (2004)146104.

26] E.M.C. Alayon, J. Singh, M. Nachtegaal, M. Harfouche, J.A. van Bokhoven, Journalof Catalysis 263 (2009) 228–238.

27] T.M. Pedersen, W.X. Li, B. Hammer, Physical Chemistry Chemical Physics 8(2006) 1566–1574.

[

[[[

: General 388 (2010) 262–271 271

28] Y. Xu, W.A. Shelton, W.F. Schneider, Journal of Physical Chemistry B 110 (2006)16591–16599.

29] N Seriani, W. Pompe, L.C. Ciacchi, Journal of Physical Chemistry B 110 (2006)14860–14869.

30] W. Li, B. Hammer, Chemical Physics Letters 409 (2005) 1–7.31] C.A. Linkous, G.J. Carter, D.B. Locuson, A.J. Ouellette, D.K. Slattery, L.A. Smitha,

Environmental Science & Technology 34 (2000) 4754–4758.32] F.B. Li, X.Z. Li, M.F. Hou, K.W. Cheah, W.C.H. Choy, Applied Catalysis A: General

285 (2005) 181–189.33] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, Journal Of

Physical Chemistry B 107 (2003) 5162–5167.34] J. Rynkowski, J. Farbotko, R. Touroude, L. Hilaire, Applied Catalysis A: General

203 (2000) 335–348.35] Z.M. Shi, W.G. Yu, X. Bayar, Scripta Materialia 50 (2004) 885–889.36] A.K. Sinha, K. Suzuki, Journal Of Physical Chemistry B 109 (2005) 1708–

1714.37] C.S. Wright, R.I. Walton, D. Thompsett, J. Fisher, Inorganic Chemistry 43 (2004)

2189–2196.38] A.W. Xu, Y. Gao, H.Q. Liu, Journal of Catalysis 207 (2002) 151–157.39] H.Q Zhu, Z.F. Qin, W.J. Shan, W.J. Shen, J.G. Wang, Journal of Catalysis 225 (2004)

267–277.40] J. Graciani, J.J. Plata, J.F. Sanz, P. Liu, J.A. Rodriguez, The Journal of Chemical

Physics 132 (2010) 104703–104708.41] G. Avgouropoulos, Catalysis Communications 10 (2009) 682–686.42] D.H. Chun, Y. Xu, M. Demura, K. Kishida, D.M. Wee, T. Hirano, Journal of Catalysis

243 (2006) 99–107.43] A.Y. Kapran, L.M. Alekseenko, S.N. Orlik, Theoretical and Experimental Chem-

istry 45 (2009) 338–342.44] S.D. Lin, T.C. Hsiao, L.C. Chen, Applied Catalysis A: General 360 (2009) 226–

231.45] Z.M. Ni, J.H. Mao, G.X. Pan, Q. Xu, X.N. Li, Acta Physico-Chimica Sinica 25 (2009)

876–882.46] T. Tsoncheva, L. Ivanova, C. Minchev, M. Froba, Journal of Colloid and Interface

Science 333 (2009) 277–284.47] T Tsoncheva, J. Rosenholm, C.V. Teixeira, M. Dimitrov, M. Linden, C. Minchev,

Microporous and Mesoporous Materials 89 (2006) 209–218.48] H.R. Wang, Y.Q. Chen, Q.L. Zhang, Q.C. Zhu, M.C. Gong, M. Zhao, Journal of

Natural Gas Chemistry 18 (2009) 211–216.49] J. Zhou, D.R. Mullins, Journal of Physical Chemistry B 110 (2006) 15994–16002.50] A. Dauscher, L. Hilaire, F. Lenormand, W. Muller, G. Maire, A. Vasquez, Surface

and Interface Analysis 16 (1990) 341–346.51] G. Zhang, D. Yang, E. Sacher, The Journal of Physical Chemistry C 111 (2006)

565–570.52] S. Shukla, S. Seal, Nanostructured Materials 11 (1999) 1181–1193.

53] A.I. Kovalev, D.L. Wainstein, A.Y. Rashkovskiy, A. Osherov, Y. Golan, Surface and

Interface Analysis 42 (2010) 850–854.54] M. Badlani, I.E. Wachs, Catalysis Letters 75 (2001) 137–149.55] L.J. Burcham, L.E. Briand, I.E. Wachs, Langmuir 17 (2001) 6164–6174.56] I.E. Wachs, J.M. Jehng, W. Ueda, Journal of Physical Chemistry B 109 (2005)

2275–2284.