Oriented Pt Nanoparticles Supported on Few-Layers Graphene as

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Oriented Pt nanoparticles supported on few-layers graphene as highly active catalyst for aqueous phase reforming of ethylene glycol Ivan Esteve-Adell, Nadia H. L. Bakker, Ana Primo, Emiel J. M. Hensen, and Hermenegildo Garcia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11904 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

Oriented Pt nanoparticles supported on few-layers graphene as highly active catalyst for aqueous phase reforming of ethylene glycol Iván Esteve-Adella, Nadia Bakkerb, Ana Primoa, Emiel Hensenb, Hermenegildo Garcíaa* a

Instituto Universitario de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Av. De los Naranjos s/n, 46022, Valencia, Spain.

b

Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands.

Keywords: Catalysis, Aqueous Phase Reforming, Hydrogen Generation from Biomass, Graphene-supported nanoparticles, Oriented Platinum Nanoparticles

ACS Paragon Plus Environment

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ABSTRACT

Pt nanoparticles (NPs) strongly grafted on few layers graphene (G) have been prepared by pyrolysis under inert atmosphere at 900 ºC of chitosan films (70-120 nm thickness) containing adsorbed H2PtCl6. Preferential orientation of exposed Pt facets was assessed by XRD of films having high Pt loading in where the 111 and 222 diffraction lines were observed and also by SEM imaging comparing elemental Pt mapping with the image of the 111 oriented particles. Characterization techniques allow to determine the Pt content (from 45 ng to 1 µg cm -2, depending on the preparation conditions), particle size distribution (9±2 nm) and thickness of the films (12-20 nm). Oriented Pt NPs on G exhibit at least two orders of magnitude higher catalytic activity for aqueous phase reforming of ethylene glycol to H2 and CO2 than analogous samples of randomly oriented Pt NPs supported on preformed graphene. Oriented

/fl-G undergoes

deactivation upon reuse, the most probable cause being Pt particle growth, probably due to the presence of high concentrations of carboxylic acids acting as mobilising agents during the course of the reaction.


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INTRODUCTION Metal nanoparticles (MNPs) supported on large surface area solids are general heterogeneous catalysts for a wide variety of reactions including oxidations, reductions, hydrogenations, homoand cross-couplings, cycloadditions, rearrangements and many others.1-5 A general feature of these catalysts is that the support plays several roles in the process, including stabilization of MNPs against agglomeration under reaction conditions, but also providing additional active sites than can cooperate in the reaction mechanism.6-9 In this regard, the use of graphene (G) as support of MNPs is currently under intense research due to the unique properties of G that can make the resulting MNP/G material among the most active and efficient recoverable catalysts for many reactions.10,

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Gs are suitable supports due to their specific large surface area having a

theoretical value of 2600 m2/g, easy dispersability in liquid media due to the one-atom thickness morphology and strong adsorption capacity that brings substrates near the MNP active site

12, 13

.

Another unique feature offered by Gs that contribute to the activity of MNP/G as catalysts is the interaction of metal atoms with the extended π orbital of G that can serve to modulate the electronic density of metal atoms at the interface. In this regard, we have recently reported a onestep method for the simultaneous preparation of MNPs (Au and Cu) and Gs based on the pyrolysis at temperatures between 900 and 1200 ºC of natural polysaccharides containing adsorbed the corresponding transition metal salt. The resulting facet-oriented MNP/G sample prepared by this pyrolytic process exhibits unique catalytic activity with turnover numbers (TON) and turnover frequencies (TOF) 3 or 4 orders of magnitude higher than analogous MNP/G materials obtained by adsorption of preformed MNPs (Au or Cu) onto preformed Gs. This dramatic enhancement of the catalytic activity has been proposed to derive from the


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preferential MNP morphology as thin nanoplatelets exposing 111 facets and from the strong MNP-G grafting as consequence of the high preparation temperatures. 14-16 Continuing with this line of research it is important to present additional examples showing other metals susceptible to undergo similar phase separation during the process of pyrolytic G formation and to evaluate the catalytic activity of the resulting strongly-grafted MNPs as consequence of the interaction of Gs. Herein we report the preparation of Pt particles with preferential exposed 111 facets strongly bound to N-doped G obtained by pyrolysis at 900ºC of chitosan embedding H2PtCl6 ( meaning 111 oriented Pt NPs, fl meaning few-layers). The resulting

/fl-G;

/fl-G exhibits three orders

of magnitude higher catalytic activity for aqueous phase reforming (APR) of ethylene glycol (equation 1) than analogous Pt/G. C2H6O2 + 2H2O → 5H2 + 2CO2 (eq. 1) APR of biomass residues present in aqueous solution is considered as a useful reaction to valorise aqueous wastes generated during the processes of biomass conversion, particularly valorisation of aqueous effluents containing high carbohydrate loading. 17, 18

RESULTS AND DISCUSSION Sample preparation and characterization The process followed to obtain

/fl-G films deposited on quartz is illustrated in Scheme 1. It

consists in spin coating an acid solution of chitosan on a clean quartz substrate that is subsequently dipped into an aqueous solution of H2PtCl6. The resulting chitosan film containing adsorbed H2PtCl6 has typical thickness between 70 and 120 nm depending on chitosan concentration of the aqueous solution and spin coating rate. Pyrolysis of these thin films under


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inert atmosphere at 900 ºC results in the spontaneous segregation of Pt NPs on top of a few layers G film. This procedure for the preparation of Pt-containing N-doped G was the most convenient one with respect to an alternative method consisting in adding H2PtCl6 to chitosan before spin casting. In the last case, the solution becomes so viscous that no homogeneous nanometric film of the H2PtCl6-chitosan precursor could be cast on quartz. In contrast, the film of chitosan was stable if it is dipped into an H2PtCl6 aqueous solution.

Chitosan solution

i) SiO2

/fl-G

Pt solution Pt

ii)

iii)

Pt Pt

Chitosan film

Pt Pt

Scheme 1. Illustration of the preparation procedure of /fl-G films. The films were prepared by spin coating on clean substrate a chitosan solution (i) that is subsequently immersed in H2PtCl6 solution (ii) and pyrolyzed at 900 ºC under inert atmosphere (iii).

The typical thickness of these films after pyrolysis is between 12 and 20 nm depending on the thickness of the precursor. The thickness of the chitosan precursor can be controlled by varying its concentration in the aqueous phase and the rate in the spin coating process. Thus, if 200 mg of chitosan diluted in 10 mL of distilled water is spin cast at 2000 or 6000 rpm the film thickness decreases from 45 to 25 nm and if only 100 mg of chitosan in 10 mL distilled H2O is spin cast at 6000 rpm the thickness before pyrolysis is about 6 nm (see supporting information Figure S1). Figure 1 presents a frontal AFM view of one of these films showing the uniform distribution of Pt NPs on top of the few layers G film. This image also allows estimating of the average height


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of the Pt NPs of about 3 nm. Notice that according to the low magnification image shown in the left frame, Pt nanoplatelets are consistently located on valleys of the G film of height lower than 10 nm and that the substrate is not completely flat at the nanometric scale (white areas at the bottom left and top right of left AFM image in Figure 1).

Figure 1. Top view at two different magnifications of the AFM image taken for

/fl-G (45 ng x

cm-2). The size (height and lateral dimensions) of three Pt nanoplatelets marked in the right panel are presented in the bottom part of the image.

The success of the preparation method derives from the low solubility of Pt on carbon that together with the lack of metal carbide formation gives rise to the spontaneous segregation of Pt NPs during the process of G formation by chitosan graphitization. In the past, we have already


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shown that pyrolysis of chitosan films renders N-doped G where the percentage of N in the film varies from 0 to 7 %, depending on the pyrolysis conditions.19 In a closely related precedent, it has been observed that an analogous process leading to Au/G results in the formation of G essentially free of N. The preferential orientation of Pt NPs in the 111 facet was deduced from the comparison of the XRD of thick

/fl-G films with those characteristic Pt NPs lacking any preferential orientation

(Figure 2). As it can be seen in this Figure 2,

/fl-G films exhibit in XRD one intense peak at 2θ

40º corresponding to the 111 facet that is accompanied by a much less intense peak at 86º corresponding to the harmonic 222 diffraction. The only other observable Pt diffraction peak for /fl-G is the 200 diffraction line at 2θ 47º that in any case is much weaker than it should be for a sample of unoriented Pt NPs (see Figure 2b). The broad, low intensity band recorded at 23º is due to the stacking of the G layers of the few layers G film. Note that the much sharper peaks in /fl-G indicate a larger lateral size of

nanoplatelets. It should be commented that in order to

record XRD patterns, thicker films with relatively high Pt content are required, since other films also prepared in this study having lesser Pt content and lower thickness fail to exhibit any XRD pattern. The Pt content in the the

/fl-G films can be determined by ICP-OES analysis after treating

/fl-G films with aqua regia. Typically Pt content ranges from 45 ng to 1 µg·cm-2

depending on the concentration of H2PtCl6 in the aqueous solutions in which the chitosan films is dipped. For diluted (nM) H2PtCl6 solutions, essentially all the H2PtCl6 becomes adsorbed in chitosan, while for concentrated solutions some remnant H2PtCl6 remains in the aqueous solution. Most of the catalytic studies were carried out with samples having Pt content 45 ng·cm 2

.


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a

b

Pt(111)

Pt(111)

Intensity (a.u.)

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Pt(311)

Pt(200) Pt(220) Pt(222)

Pt(200) 20

40

2 (o)

Figure 2. XRD patterns of

60

80

30

40

50

60

70

80

90

2 (o)

/fl-G (45 ng·cm-2; a) and Pt/fl-G (3 wt%; b) showing the different

indexation of the peaks.

SEM images also provide evidence for the homogeneous distribution of small Pt NPs on the G film. The lateral size ranges from 4 to 15 nm with an average 9±2 nm. Additional evidence of the preferential 111 facet orientation was obtained also by scanning electron microscopy (SEM) by comparing the elemental Pt mapping on the film with the image showing the different orientation of the top facets of Pt crystallite. This comparison is provided in Figure 3 and supporting information (Figure S6) presents the SEM image corresponding to the image Figure 3 e. Comparison of two set of images with Pt mapping and 111 orientation shows that about 80 % of the area of Pt element correspond to particles having 111 facet exposed.


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a

b Fecuency (counts)

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Fecuency (counts)

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20 15 10 5 0

6

8

10

12

Particle size (nm)

14

16

40 35 30 25 20 15 10 5 0

5

10 15 20 25 30 35 40 45

Particle size (nm)

200 nm

200 nm

c

d 001 101

111

e

f 001 101

Figure 3. SEM images of mapping of two

111

/fl-G (45 ng·cm-2) fresh (a) and used (b), (c) and (e) elemental Pt

/fl-G samples having different Pt content (c: 0.43; e: 1 µg·cm-2). (d) and (f)

particles of c and e having 111 facet orientation. Insets of panels (a) and (b): histograms of particle size distribution. Insets of panels (d) and (f): color code for the different facet orientation of Pt particles. /fl-G films are not suitable for direct TEM imaging due to the presence of quartz support. Nevertheless we detached some film from the quartz substrate by scratching it with a cutter and imaged the resulting sample. The TEM images obtained confirm the nanoplatelet morphology with average lateral dimension of 9 nm (see supporting information Figure S2).


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To compare the catalytic performance of

/fl-G films, a series of analogous unoriented Pt/fl-G

materials with different Pt loadings were also prepared. These materials were obtained by wet incipient impregnation of H2PtCl6 aqueous solutions on preformed G powder followed by H2 reduction at 350 ºC and subsequent sonication. According to AFM measurements this procedure renders a distribution of single, double and few layers G sheets dispersed in aqueous phase. TEM images of unoriented Pt/fl-G are presented in Figure 4. As it can be seen there, the images show a uniform distribution of very well dispersed Pt NPs with and average particle size 1±0.5 nm. Worth noting is that the particle size of Pt NPs in unoriented Pt/fl-G sample is significantly smaller than that of

/fl-G, reflecting the fact that formation temperature of

/fl-G is very high

(900 ºC). It should be, however, commented that is a very general observation that the catalytic activity of MNPs decreases with the increase in the particle size beyond 5 nm

20, 21

and, in this

regard, the unoriented Pt/fl-G sample that was not treated at high temperature has, in principle, more favourable particle size for exhibiting higher catalytic activity.

a

30 20 10 0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Particle size (nm)

50 Fecuency (counts)

40 Fecuency (counts)

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b

40 30 20 10 0 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Particle size (nm) Particle size (nm)

100 nm


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Figure 4. Representative TEM images in bright (a) and dark (b) field of Pt-fl/G. The inset of panel (a) and (b) corresponds to the histogram of Pt NP size distribution based on TEM.

CATALYTIC ACTIVITY. As commented in the introduction APR of ethylene glycol (EG) was selected as a model reaction for the APR valorisation of biomass residues in order to evaluate the catalytic activity of /fl-G films obtained by one step pyrolysis of chitosan films adsorbing H2PtCl6. The reactions were carried out in a stainless steel autoclave at 250 ºC using 10 v/v % of EG in water and varying the Pt content, either by using lesser amount of Pt/fl-G (3wt% Pt) sample or by using Pt/fl-G samples with much lower Pt loading. A prior study of the influence of the reaction temperatures in the range 200 - 250 ºC showed an increase in H2 and CO2 evolution with the temperature (see supporting information Figure S3). For this reason, we select 250 ºC that is about the maximum temperature of the equipment to perform further studies. Also the evolution of H2 and CO2 is influenced by the concentration of EG in water (see supporting information Figure S4) and 10% was considered the most convenient concentration to evaluate catalyst activity, since higher EG concentrations tend to quickly deactivate the catalyst, while H2 production is lower for concentrations below 10 %. A blank control under the reaction conditions in the absence of any catalyst or in the present of G in the amounts used in these experiments showed negligible activity (see Figure 5), indicating that, as expected in view in the reports in the literature, the catalytic activity for APR derives from the presence of Pt. The course of the reaction was followed by analysing periodically the gas phase and quantifying the amount of evolved H2, CO2, CH4, ethane and other gases. As an example Figure 5 shows the temporal evolution for a set of Pt containing samples. As it can be seen there, a large amount of Pt gives


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rise to the generation of H2 in the proportion expected for the complete APR of EG in less than 2 h of reaction time. For longer reaction times a significant decrease in the amount of H 2 and CO2 accompanied by the corresponding formation of CH4 and ethane was observed indicating that, once H2 and CO2 have evolved in significant concentration, the Sabatier methanation of CO2 is taking place in a certain extent. Importantly,

/fl-G also exhibits activity for APR of EG,

although the reaction is significantly slower (see Figure 5).

25

Total amount (mmol)

H2 production

100 mmol H2/mmol EG (%)

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(a)

80 60

(b)

40 20

(c) (d) (e)

0 50

100 time (min)

150

200

20 15 10 5 0

Figure 5. Left: Temporal H2 production ((mmols H2/ mmols of EG) x 100) in the presence of various catalysts. a) Pt/fl-G (1 mol %), b) Pt/fl-G (0.1 mol %), c)

/fl-G (10-5 mol %), d) Pt/fl-G

(10-3 mol %), e) blank. Right: Amount (mmols) of H2 (black) and CO2 (red) formed at 3 h reaction time in the presence of a series of Pt containing catalysts (see reaction conditions in the experimental section).

However when the TON and TOF values corresponding to the activity per Pt atom are determined, then, the catalytic activity of

/fl-G becomes at least 2 orders of magnitude higher


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than that determined for any of the unoriented Pt/fl-G samples. Thus, TON values of 7.58×106 and 512 for

/fl-G film and Pt/fl-G, respectively, were measured. The corresponding TOF

values of 10 266 and 1 min-1 were determined for

/fl-G film and Pt/fl-G, respectively. These

TOF values compare favourably with those reported in the literature for Pt/Al2O3 that are 7 min1 22

. Note that a control in which exactly the same amount of Pt present in

/fl-G was used with

an unoriented Pt/fl-G catalyst did not allow observing the evolution of any H2 gas. Also higher amount of Pt on the

/fl-G film, namely from 45 ng·cm-2 to 0.43 µg·cm-2 and 1 µg·cm-2 results

in lower specific H2 production for Pt amount (see supporting information Figure S5). This is probably due to the increase of Pt particle size when increasing the loading of Pt on oriented films (see supporting information Figure S6 for images of

/fl-G films with higher Pt loading,

1 µg·cm-2). From the influence of the initial reaction rate with the temperature an estimation of the apparent activation energy for hydrogen generation by Pt/fl-G and estimated Ea for Pt/fl-G and

/fl-G could be made. The

/fl-G were 10 and 26.9 KJ·mol-1, respectively. This reflects that

/fl-G has a stronger dependence with the temperature than unoriented Pt/fl-G. We propose that the higher activity of oriented

/fl-G arises from the preparation procedure

that affords Pt NPs with preferentially 111 oriented facets strongly grafted on G. This strong grafting can be deduced from the relatively small particle size of Pt NPs in spite of the high pyrolysis temperature, and from the epitaxial growth of Pt exposing facets with the replica of the hexagonal arrangement present in G. The catalyst stability of

/fl-G films was studied by performing a second use of the sample,

observing upon reuse a decrease in the performance of the material, determined from a slower initial reaction rate and lower H2 and CO2 production at final time. In order to gain


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understanding on the origin of

/fl-G deactivation, the two-times used

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/fl-G sample was again

studied by SEM. The images show the appearance of certain bumps on the G films, as consequence of the detachment of G from the quartz support as well as an increase in the average Pt particle size. Figure 3 also includes a representative SEM image of the twice used

/fl-G

catalyst, as well as the corresponding histogram of Pt particle size distribution. The observation of Pt growth indicates that the Pt-graphene interaction is not strong to completely stabilize Pt NPs under the reaction conditions. This agglomeration of Pt can probably derive from the high reaction temperatures required for APR, as well as the generation of high concentrations of carboxylic acids in the aqueous phase during the reaction. There are precedents in the literature showing that carboxylic acids act as mobilising agents23 increasing particle size by favouring the Ostwald ripening mechanism. To support this proposal for

/fl-G deactivation, a fresh sample

/fl-G was submitted hydrothermal treatment in an aqueous 0.1 M solution of acetic acid for 3 h, whereby a similar growth in Pt particle size as that determined in the APR reaction of EG was observed. It seems, therefore, that the presence of carboxylic acids in high concentration in the reaction media is detrimental for the stability of Pt NPs supported on G, as it was already observe in other cases 24.

CONCLUSIONS In the present manuscript it has been shown that pyrolytic synthesis of G and Pt NPs affords a material in which a strong interaction between Pt NPs and G is established and, as consequence, preferential 111 facet orientation of Pt NPs occurs.

/fl-G exhibits at least 2 orders of

magnitude higher TOF for the APR reaction of EG, although the sample decreases catalytic activity upon reuse due probably to the increase in the Pt particle size distribution caused by the


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presence of carboxylic acids in the reaction media and the high reaction temperature required for the APR process. Overall the present study illustrates the general scope of preparation of oriented MNPs on G that has been applied to Au and Cu and non to Pt, and the remarkably high catalytic activity of the resulting G supported MNPs.

EXPERIMENTAL SECTION Catalysts preparation. Synthesis of few-layers graphene (fl-G) Alginic acid sodium salt from brown algae (Sigma) was pyrolyzed under argon atmosphere using the following oven program: annealing at 200 oC for 2 h and, then, heating at 10 oC/min up to 900 oC for a holding time of 6 h. Cooling at room temperature was also performed under argon. The resulting graphitic powder was sonicated at 700 W for 1 h in water and the solid residue removed by centrifugation to obtain a suspension of fl-G dispersed in water. Synthesis of oriented Pt NPs over few-layers graphene films (

/fl-G).

0.5 g of chitosan from Aldrich (low molecular weight) was dissolved in 25 mL of water with a small quantity of acetic acid (0.23 g), necessary for complete dissolution of chitosan. The solution was filtered through syringe of 0.45 µm diameter pore to remove impurities present in commercial chitosan. The films were supported on a quartz plate (2×2 cm2) by casting 500 µL of filtered solution at 4000 rpm during 1 min. In the second step of the synthesis the obtained chitosan films, once dried, were immersed in a hexachloroplatinic acid hexahydrate solution (0.01-1 mM) during 1min. Using this concentration and spin coating rate, the thickness of chitosan films is about 40 nm. Thinner films can be obtained by using lesser weights of chitosan (from 0.5 to 0.25 g) and higher


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speeds of the spin coater (from 4000 to 8000 rpm). By varying these parameters, films of about 10 nm can be formed. The pyrolysis of chitosan films containing adsorbed H2PtCl6 was performed under argon atmosphere using an electrical furnace and the heating program indicated for the preparation of fl-G. The platinum content on the films was determined by ICP-OES by immersing the plates into aqua regia at room temperature for 3 h and analysing the Pt content of the resulting solution after dilution with milli-Q water at the appropriate volume. Synthesis of unoriented Pt NPs over few-layers graphene (Pt/fl-G). Pt NPs were deposited by incipient wetness impregnation, adding drop wise a 10 mL hexachloroplatinic acid hexahydrate solution (4.6 mM) over preformed fl-G powders under constant stirring. The solid catalysts were, then, dried overnight in an oven at 110 ºC. Prior to carry out the reaction, the fresh catalyst was reduced in pure hydrogen flow heating progressively the sample (2 ºC/min) up to 350 ºC and maintaining this temperature for a holding time of 2 h, followed by sonication to obtain the exfoliated Pt/fl-G. Samples with lower Pt content were prepared analogously by using more diluted H2PtCl6 · 6H2O solutions (from 4.6 to 0.1 mM). General procedure for the APR reaction of EG. The reaction was performed with a 2.5 mL aqueous solution 10% v/v of EG (4.47 mmol) in a stainless steel autoclave reactor (55 mL, Parker Autoclave Engineers). The autoclave has an outlet that allows sampling of the headspace gas of the reactor with just a very minor decrease of the reaction pressure. The required amount of catalyst was added to the reaction mixture, and the system sealed and heated at 250 ºC. The evolution of the gas phase products was analysed at the required time by injecting 100 µL of the headspace gas in an Agilent 490 Micro-GC (Molsieve


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5A column with argon as a carrier gas) at the end of the reaction. The Micro-GC has a precolumn filter to avoid injecting water from the reaction into the column. Identification of the products was made by the retention time, and the concentration was determined from calibration data. At the end of the reaction the reactor was cooldown at room temperature and the liquid phase was analysed by HPLC (Waters 2410, Refractive index detector) with a column (ICECOREGEL 87H3) using as eluent 0.004 M H2SO4 aqueous solution. TOF values were determined by dividing the number of mols of H2 formed by the number of mols of Pt present in the catalyst and the time in minutes. Physicochemical characterization. X-ray diffraction (XRD) patterns were obtained employing a Philips X’Pert diffractometer using Cu Ka radiation (λ= 1.5418Å, 40 kV, 40 mA) at a scanning speed of 0.20º per min in the 10–80º 2Θ range. The Raman measurements (Renishaw in Via Raman Microscope) were carried out at room temperature with the 514.5 nm line of an Ar ion laser as excitation source. XP spectra were recorded on a SPECS spectrometer equipped with a Phoibos 150 9MCD detector using a non-monochromatic X-ray source (Al and Mg) operating at 200 W. The samples were evacuated in the prechamber of the spectrometer at 1×10-9 mbar. The measured intensity ratios of components were obtained from the area of the corresponding peaks after nonlinear Shirley-type background subtraction and corrected by the transmission function of the spectrometer. Atomic force microscopy (AFM) measurements were made in air at ambient temperature with a Multimode Nanoscope 3A equipment working in tapping mode. It should be noted that AFM


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images were not acquired in a clean room and, therefore, films on glass substrates may contain dust that will be detectable by these techniques. FESEM images were taken with an ULTRA 55 ZEISS Oxford instrument. Samples were prepared by sticking a small piece of quartz having on top

/fl-G on a sample holder.

Preferential 111 orientation of the nanoplatelets was determined in the same FESEM equipment after determining Pt mapping by using the electron backscatter diffraction (EBSD) detector that is also included in the instrument chamber which detects the Kikuchi lines for each particle and assigns these lines to the corresponding diffracting plane. TEM images were recorded in a JEOL JEM 2100F under accelerating voltage of 200kV. Samples were prepared by applying one drop of the suspended material in ethanol onto a carboncoated copper TEM grid, and allowing them to dry at room temperature.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENTS Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2012-32315) and Generalitat Valenciana (Prometeo 2013-019) is gratefully acknowledged. I.E.-A. thank to Spanish Ministry of Science for PhD scholarships. ABBREVIATIONS G, Graphene; fl, few-layers; MNPs, metal nanoparticles;


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5H2 + 2CO2

+ H2O Pt Pt

Pt

Pt Pt

/fl-G


21

Oriented Pt Nanoparticles Supported on Few-Layers Graphene as

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