Catalytic CO Oxidation over Pt nanoparticles ...

ECS Transactions, 35 (28) 43-57 (2011) 10.1149/1.3641818 © The Electrochemical Society

Catalytic CO Oxidation over Pt nanoparticles prepared from the Polyol Reduction Method supported on Yttria-Stabilized Zirconia. Rima J. Isaifana, Holly A.E. Dolea, Emil Obeidb, Leonardo Lizarragab, Elena A. Baranovaa, Philippe Vernouxb a

Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis-Pasteur St.Ottawa,ON,K1N 6N5 Canada b Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France

Pt nanoparticles were synthesized using a polyol process with ethylene glycol as a reducing agent. Nanoparticles of three average sizes were synthesized (3.8, 2.8 and 1.7 nm) and were deposited on Yttria-Stabilized Zirconia (YSZ), carbon black and γ-Al2O3 resulting in 1 wt. % of Pt on each support. In addition, the conventional wet impregnation method was used to disperse Pt on YSZ. Pt nanoparticles were characterized using transmission electron microscopy and X-ray diffraction. The catalytic activity of all these catalysts was investigated for carbon monoxide oxidationin the temperature range of 25-250 °C. It has been found that Pt/YSZ catalysts prepared by the polyol process presented the highest catalytic performances is spite of the low specific surface area compared to carbon and γ-Al2O3 supports. These performances can be explained based on metal/support interactions effect generated between Pt and YSZ or self-induced electrochemical promotion.

Downloaded on 2015-10-20 to IP 167.114.53.152 address. Redistribution subject to ECS43 terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

ECS Transactions, 35 (28) 43-57 (2011)

Introduction Yttria Stabilized Zirconia (YSZ) is known to be an O2- conductor due to the presence of oxygen vacancies inside its crystallographic structure. This oxide has been extensively studied as a material for oxygen sensors and fuel cells (1). Recently, YSZ has received attention as a catalytic support especially in studies of Electrochemical Promotion of Catalysis (EPOC); also called the NEMCA (Non Faradaic Electrochemical Modification of the Catalytic Activity) effect (2).Vayenas et al.(2)were the first to show that the migration of ionic species from a solid electrolyte to the catalyst surface induced by electrical polarizations can improve catalytic performances. Vayenas et al. (3) have described EPOC as an electrically controlled strong metal support interaction (SMSI). Recently,it wasfound that EPOC can be thermally induced without any electrical polarization by using nanoparticles of metallic catalysts supported on ionically conducting ceramics, such as YSZ (4). This phenomenon adds to the appeal of using YSZ as an alternative support in various heterogeneous catalysis such as indoor air quality (volatile organic compounds abatement) and automotive post-treatment. In this work, platinum nanoparticles with three average sizes were synthesized usingPolyol Reduction Method (PRM). This insured the production of well dispersed nanoparticles on YSZ. The variation in nanoparticle size was achieved by adjusting the pH of the synthesis solution (5-7).The selected preparation method was compared with the conventional wet impregnation method.The structure and surface properties of these catalysts were studied using Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). The deposited Pt/YSZ nanoparticles were tested usingthe model reaction of CO oxidation and were compared with the performance of Pt deposited on larger specific surface area supports; namelyγ-Al2O3 (a conventional support used in catalysis) and carbon (used in PEM fuel cells).The effect of the support nature as well as the Pt particle size on the catalytic activity was investigated. In addition, the catalytic activity of a conventional Pt/YSZ catalyst prepared by the wet impregnation method was compared with that of PRM Pt/YSZ samples.

Experimental Synthesis of PlatinumNanoparticleColloids Using Polyol Reduction Method (PRM) Synthesis of Pt nanoparticles was performed using a polyol reduction method (5-7). In short, a metal salt of PtCl4 (Alfa Aesar, 99.9% metals basis) was dissolved in 25ml of ethylene glycol (anhydrous 99.8% Sigma Aldrich) containing various amounts ofNaOH (0.06, 0.08 and0.15M) (EM Science, ACS grade). The solution was stirred for 30 min at room temperature, and subsequently heated and refluxed for 3 h at 160oC. A dark brown solution containing Pt nanoparticlecolloids was formed and was deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ) (Tosoh, specific surface area ~13m2/g). The total Pt metal loading on YSZ was 1 wt%. The resulting supported nanoparticles were centrifuged and extensively washed with nanopure water (18MΩ.cm). The concentrate was then dried in air for 24-48 h at 40-60oC.



In addition, Pt colloids were deposited on carbon black (Vulcan XC-72R, Cabot Corp. Specific area of 254 m2/g and particle size of 30 nm) which has been widely used as the support for platinum catalysts (8) and on γ-alumina (Alfa-Aesar, specific surface area120 m2/g) support. The total Pt metal loading on all catalysts was 1 wt%. The resulting supported nanoparticles were filtered and washed with nanopure water (18MΩ.cm).The concentrate was then dried in air for 24-48 h at 40-60oC. The list of prepared catalysts is summarized in Table I. TABLEI. List of Catalysts Prepared by the PRM Method Catalyst Name YSZ1 YSZ2 YSZ3 C1 C2 C3

C NaOH (M) 0.06 0.08 0.15 0.06 0.08 0.15

Average particle size from TEM (nm) 2.8 2.6 2.1 2.0 1.9 1.8

Pt Dispersion* (%) 27 19 25 22 44 77

Pt Dispersion** (%) 41 44 54 57 60 63

*determined from CO titration ** estimated from TEM Eqn.[2] Synthesis of Reference Catalyst by Wet Impregnation The catalytic activity of Pt/YSZ and Pt/C nanoparticles prepared in the present work was compared to the reference catalyst synthesized using the conventional impregnation method (4,9,10). The metal precursor used was hydrogen hexachloroplatinate (IV) solution (30% w/w Pt in solution, Alfa Aesar) which was supported by YSZ (Tosoh). The desired Pt metal loading was 1 wt%. Using a measured mass of approximately 2 g of YSZ powder, the appropriate mass of the precursor solution was determined. The Pt precursor solution was added to approximately 40 ml of nanopure water (18MΩ.cm). The YSZ powder was placed in a round bottom flask and the precursor/water mixture was added to the flask. More water was added to the flask until it was at least half full. The round bottom flask was attached to a roto-vap and the temperature was increased to 70°C using a water bath. The temperature was held constant for 1.5 hrs in order to reduce the particles. Subsequently, the temperature was decreased to 40°C and the vacuum pump was turned on to eliminate the water. The evaporation continued until the sample was completely dry in the flask. Instrumentation/ Techniques TEM of Colloidal Nanoparticles The surface properties of the resulting Pt colloids were characterized by TEM (JEOL JEM 2100F FETEM) operating at 200kV. For TEM measurements, a very small volume of Pt colloids was dispersed in ethanol at the ratio of 1:5. Then, the mixture was sonicated for 10 minutes. A drop of the solution was taken and deposited on a copper grid (Electron



Microscopy sciences, CF-300) and was left to dry for 30 min in air. The grids were then transferred to a grid holder and dried in an oven (Lindhburg) for 3 h. The sample was placed under pressure overnight using a turbo pump (Varian V-81m) turbo station to remove dust and other impurities. The size distribution and mean particle size were performed using the imaging software MeasureIt (Olympus Soft Imaging Solutions) by measuring at least 200 nanoparticles from the TEM images. TEM of Pt/YSZ Catalysts High resolution transmission electron microscopy (HRTEM) were taken for the three samples of the deposited Pt/YSZ using High-Resolution Transmission Electronic Microscopy, JEOL 2010 LaB6) for investigating the morphology and size of Pt particles. An extraction replica technique was used for sample preparation (9,11).The catalyst was dispersed in ethanol and deposited on a mica film and covered with a carbon layer. Then, the YSZ support and film of mica were dissolved in hydrogen fluoride solution for 24 hours whereas Pt particles remainder was fixed on the carbon film. Then, these particles were directly observed by HRTEM. The use of Image J software allowed for the determination of Pt particles size distribution. TEM of Pt/C Catalysts HRTEM measurements were taken for the three samples of the deposited Pt/C using High-Resolution Transmission Electronic Microscopy, JEOL 2010 LaB6) for investigating the morphology and size of Pt particles. The catalysts were dispersed in ethanol and deposited on a mica film and covered with a carbon layer. Then, carbon supports as well as the film of mica were dissolved in HF solution for 24 hours resulting in the remaining Pt particles being fixed on the carbon film. Then, these particles were directly observed by HRTEM. The use of Image J software allowed for the determination of Pt particles size distribution. X-ray Diffraction of Colloidal Pt Nanoparticles X-ray diffraction patterns of all colloidal Pt nanoparticles were collected using Rigaku Ultima IV diffractometer using Cu Kαsource. The experiments were run in the focused beam geometry with a divergence slit of 2/3 degree, a scan speed of 0.17 degree/min and a scan step of 0.06 degree for all experiments. The diffractograms were collected between 30 and 50o2θ. Dispersion Measurements Dispersion measurements ofall Pt/YSZ and Pt/C catalysts were performed using the CO titration method as described in reference (12). Catalytic ActivityMeasurements The catalytic activity measurements of the Pt nanocatalysts deposited on YSZ and carbon were carried out at atmospheric pressure in a continuous flow quartz reactor (4,13). The reaction gases were mixtures of CO (Linde, 1.02% CO in He, 2% uncertainty) for CO oxidation, in addition to pure O2 (Linde, pure O2), and pure He (Linde, pure He) as a carrier gas. The gas compositions were controlled by mass flow controllers (Brooks, 5850 TR Series) and were the following: 760-780 ppm CO for CO oxidation with 4.3-



4.5% O2 and He balance. The overall flow rate was held constant at 4.3 L/h, resulting in a space velocity of approximately 19 000 h-1. The product gases were analyzed with an online micro-chromatograph (µGC-R3000 SRA instruments) which contained two separate columns; one to separate O2, N2 and CO at a temperature of 90°C and the other to separate CO2 at a temperature of 100°C. CO2 concentration in the product gas of the reactor was also monitored by an infrared (IR) analyzer (Horiba, VA-3000). A K-type thermocouple was attached to the reactor at the catalyst bed in order to monitor the temperature during the heat-up period. The entire reactor was placed in a furnace, which was attached to a temperature controller (Eurotherm). The sample was heated from room temperature to 250°C (for Pt on YSZ) and 200°C (for Pt on C) at a rate of 3°C/min. The temperature was held constant at 250°C and 200°C for 5 min and then decreased to room temperature. For each catalyst, three heating cycles were performed to observe the stability of the catalyst. Results and Discussion Characterizations of Pt Colloidal Solutions. Figure 1 shows the TEM micrographs and the corresponding histograms of the three Pt colloids synthesized in this paper in different NaOH concentrations. The TEM shows that well defined and almost spherical Pt nanoparticles were prepared using ethylene glycol reduction method. X-ray diffraction patterns of the platinum nanoparticle colloids are shown in Figure 2. The Pt nanoparticle size was calculated using the Debye formula for scattering by randomly oriented molecules because the Scherrer’s formula proved to have limitations for the interpretation of size and composition of bimetallic particles with the cluster size smaller than 5 nm (14). In this paper Baranova et al. established a methodology for estimating the average size and composition of fcc Pt clusters. The average crystallite size was estimated using the full width at half maximum (FWHM) and full width at ¾ maximum (FW3/4M) of the fcc (111) peak as described in (14) (Table I). The peak around 40o 2θ corresponds to Pt reflection (111), whereas sharp peaks at around 32 and 46 o2θ are attributed to NaCl phase, which is present in the sample as a result of the synthesis solution (14).



(a) Percentage of Particles (%)

40

30

20

10

0 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Particle Size (nm)

20 nm

(b) Percentage of Particles (%)

40 35 30 25 20 15 10 5 0 1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Particle Size (nm)

10 nm

(c) Percentage of Particles (%)

30 25 20 15 10 5 0 0.6 50 nm

0.9

1.2

1.5

1.8

2.1

2.4

2.7

Particle Size (nm)

Figure 1.TEM images (left) and corresponding histograms (right) of Pt colloids, synthesized in EG solutions using following concentrations: (a) 0.06 M; (b) 0.08 M; (c) 0.15 M.



Figure 2.X-ray diffraction patterns of Pt nanoparticles inside colloidal solutions showing Pt (111) peak. Comparison of the TEM and XRD size distributions indicates a broad agreement of the two methods about the typical size, but also exposes deviations at both ends of the size spectrum (Table II). One of the limitations of TEM technique is that only relatively small amount of particles can be analyzed, while XRD is a bulk technique, which allows evaluation of the entire sample. TABLE II.Summary of TEM and Slow Scan XRD Results on Pt Colloidal Nanoparticles Samples # 1

C NaOH (M) 0.06

2 3

0.08 0.15

Average particle size from TEM (nm) 2.5 + larger particles of 5.5 nm 1.8±1.0 1.5±1.0

Average particle size from XRD (nm) 3.8 ±0.2

2θmax (o ) 39.90

2.8 ±0.2 1.7 ±0.1

39.17 39.33

Characterizations of Pt/YSZ Catalysts Figure 3 shows the TEM micrographs of Pt/YSZ catalysts prepared at different concentrations of NaOH as indicated below.



(a)

Percentage of Particles (%)

30 25 20 15 10 5 0 1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4

Particle Size (nm)


(b)

30 25 20 15 10 5 0 1.6

2.0

2.4

2.8

3.2

3.6

4.0

2.8

3.2

Particle Size (nm)

(c)


30 25 20 15 10 5 0 0.8

1.2

1.6

2.0

2.4

Particle Size (nm)

Figure 3.TEM images (left) and corresponding histograms (right) of Pt/YSZ catalysts prepared at different NaOH concentrations: (a) Catalyst YSZ1 - 0.06 M, (b) Catalyst YSZ2 - 0.08M, (c) Catalyst YSZ3 - 0.15M. The TEM images show homogeneously dispersed Pt particles on YSZ with the highest dispersion in YSZ1 which is 27% as shown in Table II. The supported nanoparticles are spherical in shape and have average particle sizes of 2.8, 2.6 and 2.1 nm, respectively. It is also noticed that Pt nanoparticles agglomeration decreases with increasing pH.



Characterizations of Pt/C Catalysts Figure 4 shows the TEM images of Pt/C catalysts at different concentrations of NaOH with the corresponding histograms.


(a) 25 20 15 10 5 0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6

Particle Size (nm)

(b)


30 25 20 15 10 5 0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

Particle Size (nm)


(c) 35 30 25 20 15 10 5 0 0.8

1.2

1.6

2.0

2.4

2.8

3.2

Particle Size (nm)

Figure 4.TEM images (left) and corresponding histograms (right) of Pt/C catalysts prepared at different NaOH concentrations: (a) Catalyst C1 - 0.06 M, (b) Catalyst C2 0.08 M, (c) Catalyst C3 - 0.15M.



The images show homogeneously dispersed Pt particles on carbon support particle size and it is noticed that Pt particles agglomeration also decreases with pH value. The mean particle sizes of the Pt/C nanoparticles are 2.0, 1.9 and 1.8nm, respectively. As expected, these values are lower than those observed with Pt/YSZ catalysts regarding the larger specific surface area of C support. Nevertheless, the difference of size between Pt nanoparticles supported on YSZ and on C is not very significant suggesting the efficiency of the PRM method to finely disperse metallic particle on low specific surface area supports. Dispersion Measurements Dispersion measurements were performed for all Pt/YSZ and Pt/C samples using CO titration method. The results were compared to the dispersion estimated from TEM images using the following formula:

dispersion (%) =

M Pt × 600 ρ × d nm × a Pt × N a

[1]

where : MPt is the molecular weight of Pt= 195.08 g/mole, aPt= 8.06x10-20 m²/atom, ρ = 21.09 g/cm3, Na =Avogadro’s number and davgis the mean Pt particle diameter estimated from TEM. Then, one can conclude this expression of dnm:

d avg (nm) =

114 dispersion (%)

[2]

Table I shows a summary result for the dispersion experiments performed on each catalyst. From a general point of view, Pt dispersions estimated from TEM images are overestimated certainly because larger and isolated Pt particles were not observed, essentially on YSZ support as already observed (9).Nevertheless, the two measurement methods indicate that Pt dispersion is larger on C than on YSZ. The chemical CO titration seems to indicate that the Pt dispersion of YSZ-supported do not vary significantly with NaOH concentration. On the other hand, the metallic dispersion on C strongly increases from 22 to 77% with the NaOH loading. Catalytic Activity Measurements for CO Oxidation The catalytic oxidation of CO to CO2 is a well studied reaction for its environmentally and industrially importance (15-18). It is chosen as the model reaction in this work to study the catalytic activity of the synthesized catalysts. Performances of YSZ-Supported Catalysts For each catalyst, the catalytic oxidation of CO was repeated for three successive heating ramps between 25 and 250°C. Figure 5 shows the catalytic activity of YSZ2. The



results showed that the catalyst exhibits highly stable performance and is active from around 40oC. 1st Cycle 2nd Cycle 3rd Cycle

100

CO Conversion (%)

80 60 40 20 0 20

40

60

80

100

120

140

Temperature (°C)

Figure 5. Catalytic activity measurements of YSZ2 (2.8 nm). Figure 6 shows the impact of Pt particle size deduced from TEM for the three Pt/YSZ samples of different sizes. The results show that the best performance is achieved by the smallest nanoparticles. The smaller the size of the nanoparticle, the earlier the 100% conversion of CO is achieved. 20 18 CO Conversion (%)

16

1.7 nm 2.8 nm 3.8 nm

14 12 10 8 6 4 2 0 20

40

60

80

100

Temperature (°C)

Figure 6.Catalytic performances for CO oxidation of PRM Pt/YSZ catalysts. Table III shows the average turnover frequency (TOF) values for the three cycles per each catalyst of Pt/YSZ at 60 oC and 100 oC. The TOF are measured in units of product molecules of CO2 molecules produced per catalyst surface site per second of reaction time. It was estimated from the intrinsic catalytic rate, r, according to the following equation:



r( mol / s ) =

F 1 273 [CO]in COconversion × × × × 3600 22.4 298 106 100

[3]

Where r is the intrinsic catalytic rate of CO oxidation in mol/s, F is the total gas flowrate = 4.3 L/h, 22.4 L corresponds to the standard molar volume at 273 K, the ratio 273/298 is the corrected factor considering gas analysis at 298K, and [CO]in is the inlet concentration of CO expressed in ppm. TOFs −1 =

r

[4]

ntot × dispersion(%)

with ntot is the total number of Pt moles in the catalytic reactor. The Pt dispersion was estimated from the CO titration experiments. The highest TOF values were obtained for the smallest nanoparticles which indicate their highest activity towards CO oxidation more than twotimes the value for larger nanoparticles.

TABLE III. Summary of TOF Values for the PRM Pt/YSZ and Pt/C Catalysts. TOF(s-1) (×1000) at 60oC at 100°C

YSZ1

YSZ2

YSZ3

C1

C2

C3

4.1 29

7 39

9.2 44

2.9

1.5

1

Performances of C-supported catalysts. Figure 7 shows the catalytic activity measurements of C2 (2.8 nm). The results show a rather good stability of the performances within the 3 cycles. However, performances are significantly lowest than those measured on Pt/YSZ catalysts. The CO conversion only starts from 100°C compared to 40°C for YSZ-supported catalysts.



100

CO Conversion (%)

80

1st Cycle 2nd Cycle 3rd Cycle Blank Experiment

60 40 20 0 40

60

80

100

120

140

160

Temperature (°C)

Figure 7.Catalytic performances for CO oxidation of PRM Pt/C catalysts. Table III shows the TOF values for PRM Pt/C catalysts at 100 oC. The TOF values recorded on Pt/C samples at 100°C are much lower than those measured on YSZsupported catalysts despite that the specific surface area of Pt/C is 20 times higher than Pt/YSZ catalysts,. This result confirms that a metal/support interaction occurs between Pt and YSZ and strongly affects the Pt catalytic activity. This suggests the promoting role of O2- ionic species containing in YSZ on the catalytic activity of Pt for CO oxidation as already shown using electrochemical promotion of catalysis on Pt/YSZ electrochemical catalyst in oxygen excess conditions (19). Performances of γ-alumina-Supported Catalysts and Wet Impregnation Pt/YSZ Catalyst. Figure 8 compares the catalytic activity of Pt nanoparticles prepared at CNaOH = 0.08M with average crystalline size of 2.8 nm deposited on YSZ (YSZ2), carbon (C2) and γ-alumina along with the activity of the reference catalyst synthesized by the wet impregnation technique on YSZ. The performance of the deposited PRM Pt/YSZ sample showed better activity than C2 and Pt/γ-Al2O3 sample. Again, in spite of the specific surface area of Pt/γ-Al2O3 being 10 times larger than Pt/YSZ, the γ-Al2O3-supported sample exhibits lower performances than YSZ2. In addition, the wet impregnation catalyst shows poor catalytic performances with a CO conversion starting from 160oC instead of 40°C for PRM catalysts. This demonstrates that the PRM produces smaller Pt nanoparticles than the conventional impregnation route, therefore improving the Pt/YSZ interactions.



(a) (b) (c) (d)

100

CO Conversion (%)

80 60 40 20 0 40

80

120

160

200

240

280

Temperature (°C)

Figure 8.Comparison between catalytic performances for CO oxidation of Pt nanoparticles deposited by wet impregnation on YSZ (a) and deposited by RPM at CNaOH = 0.08M on γ-Al2O3(b), YSZ (c) and carbon (d).

Conclusions Three different nanoparticle sizes of 1 wt% of platinum deposited on YSZ and on carbon were examined for CO oxidation in the temperature range of 25-250°C. Pt nanoparticles synthesized in ethylene glycol were found to be active and stable towards CO oxidation with earlier activity for smaller particle size. Moreover, it was found that catalyst support plays an important role in the catalytic performance of Pt nanoparticles. Thus, Pt nanoparticles of 2.8 nm in size supported on YSZ show the highest catalytic activity (from around 40 oC) followed by Pt supported on γ-alumina and then on carbon. Highest performance of Pt/YSZ catalyst might be explained based on metal/support interactions effect generated between Pt and YSZ or self-induced electrochemical promotion. In the latter, ionic species from the YSZ support (O2-) might migrate to the gas-exposed Pt surface leading to catalytic properties alteration of Pt nanoparticles. Further catalytic studies of Pt nanoparticles interfaced with YSZ would shed light on the origin of the observed catalytic activity increase.

Acknowledgements We thank the Natural Science and Engineering Research Council (NSERC) for partial financial support via the Discovery project and Centre for Catalysis Research and Innovation (University of Ottawa) for XRD and TEM analysis of colloidal Pt. We also would like to thank the microscopy service at IRCELYON.



References 1. E.C. Subbarao, H.S. Maiti, Solid State Ionics, 11, 317-338 (1984). 2. C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-Suport Interactions, Kluwer Academic Publishers/Plenum Press, New York, (2001). 3. C.G. Vayenas, S. Brosda, C. Pliangos, J. Catal., 216, 487–504 (2003). 4. P. Vernoux, M. Guth, X. Li, Electrochem. and Solid State Letters, 12, E9E11(2009). 5. C. Bock, C. Paquet, M. Couillard, G.A. Botton, B.R. MacDougall, J. Am. Chem. Soc.,126 , 8028-8037 (2004). 6. E.A. Baranova, N. Miles, P.H.J. Mercier, Y. Le Page, B. Patarachao. Electrochim. Acta, 55, 8182-8188 (2010). 7. E.A. Baranova, T. Amir, P.H.J. Mercier, B. Patarachao, D. Wang, Y. Le Page. J. Appl. Electrochem. 40, 1767-1777 (2010). 8. E. Antolini, Applied Catalysis B: Environmental, 88, 1-24 (2009). 9. M. Alves Fortunato, D. Aubert, C. Capdeillayre, C. Daniel, A. Hadjar, A. Princivalle, C. Guizard, P. Vernoux, Appl. Catal. A, 2011, in press, doi.org/10.1016/j.apcata.2011.06.005. 10. M. Boudart, G. Djega-Mariadassou, Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, NJ (1984). 11. J. Ayache, L. Beaunier, J. Boumendil, G. Ehret, D. Laub, Sample Preparation Handbook for Transmission Electron Microscopy, Chapter 5 Replica Techniques, First Edition, Springer, (2010). 12. C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.G. Lintz, Catalysis Today, 11, 303438, (1992). 13. P. Vernoux, F. Gaillard, L. Bultel, E. Siebert, M. Primet, J. Catal., 208, 412-421 (2002). 14. E.A. Baranova, Y.Le page, D. Ilin, C.Bock, B.MacDougall, P.H.J. Mercier, J.Alloys and Comp., 471, 387-394 (2009). 15. M.Valden, X. Lai, D.W. Goodman, Science, 281 (5383), 1647-1650 (1998). 16. C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, J. Chem. Phys., 73 (11), 5862-5873 (1980). 17. X.C. Su, P.S. Cremer, Y.R. Shen, G.A. Somorjai, J. Am. Chem. Soc., 119 (17), 3994-4000 (1997). 18. K.R. McCrea, J.S. Parker, G.A. Somorjai, J. Phys. Chem., 106 (42), 1085410863(2002). 19. M.N. Tsampas, F.M. Sapountzi, C.G. Vayenas, Catal. Today, 146, 351–358 (2009).
