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Dec 16, 2009 - Dispersed in Ce0.83Ti0.15O22d: Significant Improvement in PROX Activity by Ti Substitution in CeO2 in Hydrogen Rich. Stream. Sudhanshu ...
Catal Lett (2010) 134:330–336 DOI 10.1007/s10562-009-0249-8

Pt2+ Dispersed in Ce0.83Ti0.15O22d: Significant Improvement in PROX Activity by Ti Substitution in CeO2 in Hydrogen Rich Stream Sudhanshu Sharma • Asha Gupta • M. S. Hegde

Received: 26 October 2009 / Accepted: 29 November 2009 / Published online: 16 December 2009 Ó Springer Science+Business Media, LLC 2009

Abstract Pt2? ion dispersed in CeO2, Ce1-xTixO2-d and TiO2 have been tested for preferential oxidation of carbon monoxide (PROX) in hydrogen rich stream. It is found that Pt2? substituted CeO2 and Ce1-xTixO2-d in the form of solid solution Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d are highly CO selective low temperature PROX catalysts in hydrogen rich stream. Just 15% of Ti substitution in CeO2 improves the overall PROX activity. Keywords PROX  CO oxidation  Selectivity  Ionic substitution  Ti substitution

1 Introduction CO content in the outlet to the WGSR (water gas shift reaction), amounting to 0.5–1%, is too high to be directly fed to a PEMFC (polymer electrolyte membrane fuel cell) stack, because platinum-based electrodes are irreversibly deactivated by CO content above 10 ppm [1]. Therefore, an additional stage of purification is needed in order to

S. Sharma  M. S. Hegde (&) Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka 560012, India e-mail: [email protected] S. Sharma e-mail: [email protected] A. Gupta Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka 560012, India

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reduce CO content down to acceptable values. Different methods can be chosen for CO removal but preferential CO oxidation (PROX) is widely used [2–5]. PROX is an efficient method for removing low levels of CO from H2-rich gas streams which comes from fuel reformation. The method consists of addition of oxygen to the fuel processor effluent in order to preferentially oxidize CO than H2. Reactions occurring in the PROX are: CO þ 1=2 O2 ! CO2 ; H2 þ 1=2 O2 ! H2 O An effective PROX catalyst should show: high activity and selectivity for CO oxidation, and an operation temperature between two ranges, 170–230 °C, which is the outlet temperature for low temperature WGSR, and 80–100 °C, which is the operation temperature for PEMFC. Wide temperature operation window is required to avoid precise temperature controls. Traditional catalysts for PROX reaction include a noble metal (Pt, Ru, Rh, Pd) supported on alumina [4–9]. Ceria-based supports are also being studied for PROX reaction because ceria presents high oxygen storage capacity (OSC) [10, 11], which is very helpful for oxidation in high reducing environment, with the cyclic incorporation/removal of lattice or structural oxygen. There are reports for Pt–CeO2 catalyst for PROX reaction [12–19] where Pt is present in the metal state. Pt ion in CeO2 in the form of Ce1-xPtxO2-d solid solution has been found to be more efficient for CO oxidation than Pt metal nano particles on CeO2, TiO2 and Ce1-xTixO2-d. Moreover, OSC of Ce1-xTixO2-d is much higher at a lower temperature compared to CeO2. It has been found that a small amount (15–20%) of Ti substitution increases the OSC of CeO2 at much lower temperature [20]. In this paper we report Pt2? substituted CeO2, Ce1-xTixO2-d and TiO2 for PROX (preferential oxidation of CO) reaction in the H2

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rich stream for the first time. We have also compared our results with the latest relevant literature.

2 Experimental Ce0.98Pt0.02O2-d, Ce0.83Ti0.15Pt0.02O2-d and Ti0.99Pt0.01 O2-d were prepared by single step solution combustion method [21–23]. In brief, for the preparation of Ti0.99 Pt0.01O2-d, titanyl nitrate (TiO(NO3)2), platinum nitrate (NH3)4Pt(NO3)2 and glycine in the molar ratio of 0.99:0.01:1.5 were dissolved in water to make a clear solution. Solution was put in the muffle furnace at 400 °C giving nano crystalline powder of Ti0.99Pt0.01O2-d after combustion. In a similar way, Ce0.98Pt0.02O2-d [24, 25] and Ce0.83Ti0.15Pt0.02O2-d [20] were also prepared. Structure of Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d were fully characterized by XRD, XPS, EXAFS, TEM where Pt is present mostly in the ?2 state and Pt ions are substituted for Ce4? ions in CeO2 and Ce1-xTixO2-d thus forming Ce0.98Pt0.02O2-d (d * 0.02), Ce0.83Ti0.15Pt0.02O2-d solid solutions [20, 24, 25]. Catalytic studies were carried out using a gas chromatograph (ProGC, Mayura Analytical Pvt. Ltd., India) equipped with flame ionization and thermal conductivity detectors. A known amount of catalyst was taken in a 4 mm diameter quartz tube and bed length was kept 2 cm. CO, CO2 and H2 were separated by Carbosphere column and detected by FID and TCD detectors. Gases were procured from M/S Chemix Speciality Gases Pvt. Ltd. (India) having specifications: 10.14 vol% CO in N2, ultra high pure H2, high pure N2 to makeup the flow. N2 was added to makeup the gas flow 100 cc/min giving the space velocity of 21,500 h-1. Heating rate was 2 °C/min. PROX reaction was done using a reaction mixture consisting of 1 vol% CO, 2 vol% O2, 5 vol% CO2 and 35% H2 and balanced with nitrogen to a total flow 100 cc/min. % selectivity of CO and H2 was calculated by the following relationship: SCO ð%Þ ¼

0:5ð½COin  ½COÞ  100 ½O2   ½O

ð1Þ

SH2 ð%Þ ¼

0:5ð½H2 in  ½H2 Þ  100 ½O2   ½O

ð2Þ

The oxygen excess with respect to amount of oxygen required for the oxidation of CO to CO2 is commonly termed as oxygen excess factor (k) and defined as: k¼

2½O2  ½CO

ð3Þ

A comparison of Eqs. 1 and 3 demonstrate that SCO and k are correlated. More clearly, lower the selectivity of the process, higher will be the k value requires to completely oxidize CO to CO2.

3 Results and Discussion Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d have been well characterized and substitution of Pt ion for Ce in mixed valence states has been confirmed [20, 25, 26]. Accordingly, XRD of freshly prepared compounds could be reproduced as shown in Fig. 1a–b. Diffraction lines for CeO2 in fluorite structure are observed for Ce0.98Pt0.02 O2-d, Ce0.83Ti0.15Pt0.02O2-d and Pt(111) peaks are not discernable. Ti0.99Pt0.01O2-d crystallizes in the anatase phase as shown in Fig. 1c and here also Pt(111) line is not be observed. Rietveld refined powder X-ray diffraction patterns (XRD) of TiO2 and Ti0.99Pt0.01O2-d (not shown) gives the lattice parameters: a = b = 3.7843 (9) and c = 9.5088 (22) for TiO2 and a = b = 3.7928 (21) and c = 9.5063 (61) for Ti0.99Pt0.01O2-d. Overall, there is an increase in the cell volume by *0.5% indicating the ˚ substitution of Pt ion (R2? Pt = 0.80 A) for Ti ion 4? ˚ ) in TiO2. (RTi = 0.67 A The surface areas of catalysts Ce0.98Pt0.02O2-d, Ce0.83Ti0.15Pt0.02O2-d and Ti0.99Pt0.01O2-d are determined by N2 desorption technique (NOVA-1000 Ver. 3.70) and the areas are 14, 33 and 29 m2 g-1, respectively. X-ray photoelectron spectra (XPS) of freshly prepared Ce0.98Pt0.02O2-d, Ce0.83Ti0.15Pt0.02O2-d and Ti0.99Pt0.01 O2-d are given in Fig. 2a–g. Pt (4f7/2,5/2) core level spectra for Ce0.98Pt0.02O2-d (Fig. 2a) shows two peaks with peak values at 72.8 and 75.8 eV. In Ce0.99Pt0.01O2-d, Pt(4f) peaks are broad and shifted to higher binding energy suggesting Pt in multiple oxidation states. Taking into consideration the peak positions of Pt0, Pt2? and Pt4?, 4f(7/ 2,5/2) spin orbit splitting value of 3.2 eV and full width at half maxima (FWHM), Pt(4f) peaks are resolved into sets of 4f7/2, 4f5/2 spin orbit doublets for Pt0, Pt2? and Pt4? states. Accordingly, Pt (4f7/2, 4f5/2) peaks at 72.5 and 75.8 eV represent Pt in ?2 state; 74.6 and 77.8 eV are due to Pt4? state and 71.1 and 74.3 eV are due to Pt0 state. From the area under each Pt states, Pt0, Pt2? and Pt4? are in the ratio of 0.10:0.75:0.15. Thus, in the case of Ce0.98Pt0.02O2-d Pt is present mainly in Pt2? state. Ce is mostly in ?4 state given in Fig. 2b. Deconvoluted spectrum for Ce(3d) in Ce0.98Pt0.02O2-d (not shown) gives Ce4?:Ce3? ratio equal to 0.90:0.10. Thus, approximate composition from Pt and Ce ratios comes out to be Ce0.98Pt0.02O1.95. Deconvoluted spectrum of Pt(4f7/2,5/2) core level in Ce0.83Ti0.15Pt0.02O2-d is shown in Fig. 2c. From the area under each Pt states, Pt0, Pt2? and Pt4? are in the ratio of 0.12:0.80:0.08. Thus, in the case of Ce0.83Ti0.15Pt0.02O2-d also Pt is present mainly in Pt2? state. Ce(3d) in Ce0.83Ti0.15Pt0.02O2-d is mostly in ?4 state (Fig. 2d). Ce4? to Ce3? ratio is 0.93:0.07. Ti(2p) in Ce0.83Ti0.15Pt0.02O2-d also gives Ti in ?4 state only (Fig. 2e). Deconvoluted Pt(4f7/2,5/2) core level spectrum

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(a)

(b) 1000

9000

Ce0.98Pt0.02O2-δ

Ce0.78Ti0.20Pt0.02O2-δ

8000

800

Intensity

Intensity

7000 6000 5000

600

400

4000 200

3000 2000

0 20

30

40

50

60

70

80

20

30

40

2θ / degree

50

60

70

80

2θ / degree

(c) 500 Ti0.99Pt0.01O2-δ

Intensity

400

300

200

100

0 20

30

40

50

60

70

80

2θ / degree Fig. 1 XRD patterns of a Ce0.98Pt0.02O2–d; b Ce0.83Ti0.15Pt0.02O2–d and c Ti0.99Pt0.01O2–d

Ti0.99Pt0.01O2-d is given in Fig. 2f. It is seen that Pt is mostly in ?2 oxidation state with no Pt0 component. A slight Pt4? component exist with Pt2?. Similarly, Ti(2p)spectrum in Ti0.99Pt0.01O2-d given in Fig. 2g appear mostly in ?4 state. CO oxidation in H2 free stream at different O2 concentration is carried out for all the three catalysts. Effect of CO2 addition on CO conversion is also examined for all the three catalysts. CO conversion with Ce0.98Pt0.02O2-d catalyst at different O2 concentration and after CO2 addition is shown in Fig. 3a. With 0.8 vol% of O2 concentration, complete CO conversion occurs at 225 °C. Decrease in CO conversion temperature is observed with increase in the O2 concentration. With 1.2 vol% of O2 concentration complete CO conversion temperature decreases to 175 °C. Further increase in the O2 concentration does not indicate any significant change in the conversion temperature. There is no adverse effect of 5 vol% CO2 addition on the CO conversion profile with 2 vol% of O2 concentration. CO conversion with Ce0.83Ti0.15Pt0.02O2-d catalyst at different O2 concentration and after CO2 addition is shown in Fig. 3b. With 0.8 vol% of O2 concentration, complete CO conversion occurs at 130 °C which is 95 °C lower than Ce0.98Pt0.02O2-d catalyst. Gradual increase of O2 concentration decreases the T50 and T100 successively. For example, with 1.2 vol% of O2, T50 and T100 of CO

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oxidation is 100 and 120 °C which are lower when one compares with the O2 concentration of 0.8 vol%. This change indicates that increasing the partial pressure of O2 affects the lattice oxygen replacement in Ce0.83Ti0.15 Pt0.02O2-d catalyst. This was not seen with Ce0.98Pt0.02 O2-d catalyst as observed from Fig. 3a. Further, no significant effect of CO2 addition is observed on CO conversion. With 2 vol% of O2 concentration T50 and T100 of CO conversion are 90 and 100 °C without CO2. After the addition of CO2, T50 and T100 are almost the same. CO conversion with Ti0.99Pt0.01O2-d catalyst at different O2 concentration and after CO2 addition is shown in Fig. 3c. With 0.8 vol% of O2 concentration, complete CO conversion occurs at 90 °C which is 40 °C lower than Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d catalyst. There is not much difference after increasing the O2 concentration from 0.8 vol%. With 2 vol% of O2 concentration T100 is only 80 °C. Addition of CO2 gives a slight adverse effect on CO conversion. In the presence of 2 vol% of O2 and 5 vol% of CO2, T100 is 100 °C which is almost 20 °C higher than without CO2. Further, we demonstrate and compare the catalytic activity of Ce0.98Pt0.02O2-d, Ce0.83Ti0.15Pt0.02O2-d and Ti0.99Pt0.01O2-d towards PROX in hydrogen rich stream. PROX reaction over Ce0.98Pt0.02O2-d is given in Fig. 4a. In the lower temperature range (30–65 °C) CO conversion is

Pt2? Dispersed in Ce0.83Ti0.15O2-d

(a)

(b) 17 16

Intensity

4

Intensity(x10 )

15 14 13 12 11 10 70

72

74

76

78

80

880

890

B. E. / eV

900

910

920

B. E. / eV

(d) 16

(c)

Intensity

4

Intensity (x 10 )

15

14

13

12

11 70

72

74

76

78

80

B. E. / eV

880

890

900

910

920

B. E. / eV

(e) 6.2

4

Intensity (x 10 )

6.0 5.8 5.6 5.4 5.2 5.0 4.8 450

455

460

465

470

B. E. / eV

(g)18

(f) Ti0.99Pt 0.01O2-δ

Intensity (x 10 )

16

14

4

Intensity

Fig. 2 XPS spectra of a Pt(4f) core level region in Ce0.98Pt0.02O2–d; b Ce(3d) core level region in Ce0.98Pt0.02O2–d; c Pt(4f) core level region in Ce0.83Ti0.15Pt0.02O2–d; d Ce(3d) core level region in Ce0.83Ti0.15Pt0.02O2–d; e Ti(2p) core region in Ce0.83Ti0.15Pt0.02O2–d; f Pt(4f) core level region in Ti0.99Pt0.01O2–d and g Ti(2p) core region in Ti0.99Pt0.01O2–d

333

12

10

8

6 70

72

74

76

B. E. / eV

78

80 4 454 456 458 460 462 464 466 468

B. E. / eV

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% CO conversion

(a) 100 80

0.8 vol% 1.2 vol% 1.5 vol% 2 vol% 2 vol% + 5 vol % CO 2

60

40

20

0 50

100

150

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250

Temperature / °C

80

80

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% CO conversion

(c) 100

% CO conversion

(b)100

2 vol% 1.2 vol% 1.5 vol% 2 vol% + 5 vol% CO 2 0.8 vol%

40

20

0

0.8 vol% 1.2 vol% 1.5 vol% 2 vol% 2 vol% + 5vol% CO 2

60

40

20

0 40

60

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100

120

140

Temperature / °C

30

50

70

90

110

Temperature /

130

150

°C

Fig. 3 CO conversion at different conditions for a Ce0.98Pt0.02O2–d; b Ce0.83Ti0.15Pt0.02O2–d and c Ti0.99Pt0.01O2–d

very low. At temperature higher than 70 °C there is a gradual increase in the CO conversion. CO conversion goes up to 90% at 100 °C and becomes constant after this temperature. H2 conversion is almost constant in this temperature range. From the conversion it is clear that 70– 100 °C is the most efficient temperature range for PROX reaction over Ce0.98Pt0.02O2-d catalyst. In this temperature range conversion of H2 is minimum and conversion of CO is maximum. Percent selectivity of CO at 100 °C is 90% which is almost constant above 100 °C up to 125 °C. PROX reaction over Ce0.83Ti0.15Pt0.02O2-d is given in Fig. 4b. Ce0.83Ti0.15Pt0.02O2-d catalyst, in the temperature range of -7 to 75 °C shows very low H2 conversion and it remains constant up to a very long range of temperature. At 55 °C, CO conversion is 99.6% which is far higher than Ce0.98Pt0.02O2-d catalyst. At 55 °C percent selectivity of CO conversion is 96%. Hence, Ce0.83Ti0.15Pt0.02O2-d is more selective than Ce0.98Pt0.02O2-d. Further, Ce0.83 Ti0.15Pt0.02O2-d shows higher CO conversion at a lower temperature compared to Ce0.98Pt0.02O2-d. Moreover, temperature window Ce0.83Ti0.15Pt0.02O2-d is large compared to Ce0.98Pt0.02O2-d. Higher activity of Ce0.83 Ti0.15Pt0.02O2-d is because of the higher reducibility of this catalyst compared to Ce0.98Pt0.02O2-d. Because of Ti substitution oxygen becomes activated and can be easily

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removed in the reaction condition [20]. The reaction conditions are highly reducing (35% H2) and hence reducibility becomes an important aspect and which makes Ce0.83Ti0.15Pt0.02O2-d more active than Ce0.98Pt0.02O2-d. Activity of Ti0.99Pt0.01O2-d towards PROX reaction is given in Fig. 4c. A maximum of 30% CO conversion is achieved at 100 °C in presence of excess hydrogen. Hydrogen to the extent of excess oxygen is converted to H2O. This was confirmed from an independent experiment where hydrogen and oxygen were passed over the catalyst at 20 °C and an instant increase of temperature to 100 °C occurred indicating that it is a good H2 ? O2 recombination catalyst. Thus, Ti0.99Pt0.01O2-d is not a good PROX catalyst for CO. A comparison of the PROX activity over Pt–CeO2 catalyst in almost the same experimental conditions with the literature has been made and summarized in the Table 1. Wootsch et al. [16] studied a series of Pt/CexZr1-xO2 catalysts and found they are very active in the 80–100 °C range but CO selectivity is only 50%. Acres et al. showed good CO selectivity in PROX reaction but temperature window is not large. Recently, Park et al. studied a variety of metal catalysts supported mostly over alumina [27]. They went up to 5% of metal loading also but selectivity was still only 50% [28]. We see that CO2 selectivity on our

Pt2? Dispersed in Ce0.83Ti0.15O2-d

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(a)100

(b)100

90 CO H2

CO H2

80

% Conversion

% Conversion

80 70 60 50 40 30

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40

20

20 10

0 -10

0

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0

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Temperature / °C

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Temperature / °C

(c)100 % CO conversion

CO

80

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20

0

50

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350

Temperature / °C Fig. 4 PROX reaction showing CO and H2 conversion for a Ce0.98Pt0.02O2–d; b Ce0.83Ti0.15Pt0.02O2–d and c Ti0.99Pt0.01O2–d Table 1 Comparison with the literature Catalyst

k (O2 excess factor)

% Selectivity (maximum)

Temperature window (oC) of maximum selectivity

Flow rate (ml min-1) and/or GHSV(h-1)

2%Pt–CeO2a

1

78

Very narrow

100

1%Pt–CeO2b

2

50

90–110

100

0.6%Pt–CeO2c

3

90–95

50–85

200 and 12,000

1%Pt–CeO2d

2

55

50–80

16,000

(1, 5)%Pt/Al2O3e

2

50

80

100

Ce0.98Pt0.02O2–d*

4

93

90–130

100 and 21,500

Ce0.78Ti0.20Pt0.02O2–d* Ti0.99Pt0.01O2–d*

4 4

95 90

50–100 Very narrow

100 and 21,500 100 and 21,500

a

From ref. 20

b

From ref. 14

c

From ref. 1

d

From ref. 21 and * this work

catalyst is either comparable or higher with the catalysts reported in the literature. It is well established that more the k (oxygen excess factor), lesser the selectivity (refer to Sect. 2). From Table 1 it is clear that we have taken the highest k (=4) in our study and temperature window of Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d catalysts towards PROX is the highest. Thus, Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d are highly active more selective

PROX catalysts compared to the other reports on the same catalyst in the literature.

4 Conclusion In conclusion, Ce0.98Pt0.02O2-d and Ce0.83Ti0.15Pt0.02O2-d are highly active PROX catalysts while Ti0.99Pt0.01O2-d is

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not an efficient catalyst for PROX reaction. Fifteen percent of the Ti substitution helps to increase the reducibility of CeO2 which results in the higher activity of Ce0.83 Ti0.15Pt0.02O2-d compared to Ce0.98Pt0.02O2-d. Thus, reducibility plays an important role in selecting the PROX catalyst in the highly reducing conditions. Acknowledgments Department of Science and Technology, India is great fully acknowledge for financial support.

References 1. Acres G, Frost J, Hards G, Potter R, Ralph T, Thompsett D (1997) Catal Today 38:393 2. Echigo M, Tabata T (2003) Appl Catal A Gen 251:157 3. Epling WS, Cheekatamarla PK, Lane AM (2003) Chem Eng J 93:61 4. Manasilp A, Gulari E (2002) Appl Catal B Environ 37:17 5. Oh SH, Sinkevitch RM (1993) J Catal 142:254 6. Brown ML, Green AW (1960) Ind Eng Chem Res 52:841 7. Igarashi H, Uchida H, Suzuki M, Sasaki Y, Watanabe M (1997) Appl Catal A Gen 159:159 8. Kahlich MJ, Gasteiger HA, Behm RJ (1997) J Catal 171:93 9. Korotkikh O, Farrauto R (2000) Catal Today 62:249 10. Trovarelli A (1996) Catal Rev 38:439 11. Fornasiero P, Balducci G, Monte RD, Kasˇpar J, Sergo J, Gubitosa G (1996) J Catal 164:173 12. Bunluesin T, Gorte RJ, Graham GW (1998) Appl Catal B 15:107 13. Fu Q, Weber A, Flytzani-Stephanopoulos M (2001) Catal Lett 77:87

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S. Sharma et al. 14. Jacobs G, Williams L, Graham U, Sparks DE, Davis BH (2003) J Phys Chem B 107 15. Pozdnyakova O, Teschner D, Wootsch A, Kro¨hnert J, Steinhauer B, Sauer H, Toth L, Jentoft FC, Knop-Gericke A, Paa´l Z, Schlo¨gl R (2006) J Catal 237:1 16. Wootsch A, Descorme C, Duprez D (2004) J Catal 225:259 17. Marin˜o F, Descorme C, Duprez D (2005) Appl Catal B Environ 58:175 18. Teschner D, Wootsch A, Pozdnyakova-Tellinger O, Kro¨hnert J, Vass EM, Ha¨vecker M, Zafeiratos S, Schno¨rch P, Jentoft PC, Knop-Gericke A, Schlo¨gl R (2007) J Catal 249:318 19. Ayastuy JL, Gil-Rodrı´guez A, Gonza´lez-Marcos MP, Gutie´rrezOrtiz MA (2006) Int J Hydrogen Energy 31:2231 20. Baidya T, Gayen A, Hegde MS, Ravishankar N, Dupont L (2006) J Phys Chem B 110:5262 21. Hegde MS, Madras G, Patil KC (2009) Acc Chem Res 42:704 22. Patil KC, Aruna ST, Ekambaram S (1997) Curr Opin Solid State Mater Sci 2:158 23. Patil KC, Hegde MS, Rattan T, Aruna ST (2008) Chemsitry of Nano Crystalline Oxide Materials: Combustion Synthesis. Prperties and Applications, World Scientific Singapore 24. Bera P, Gayen A, Hegde MS, Lalla NP, Spadaro L, Frusteri F, Arena F (2003) J Phys Chem B 107:6122 25. Bera P, Priolkar KR, Gayen A, Sarode PR, Hegde MS, Emura S, Kumashiro R, Jayaram V, Subbanna GN (2003) Chem Mater 15:2049 26. Baidya T, Priolkar KR, Sarode PR, Hegde MS, Asakura K, Tateno G, Koike Y (2008) J Chem Phys 128:124711 27. Kim YH, Park ED, Lee HC, Lee D, Lee KH (2009) Catal Today 146:253 28. Park ED, Lee D, Lee HC (2009) Catal Today 139:280