Supporting Information

Pt-CeO2 nanoporous spheres - an excellent catalyst for partial oxidation of

methane: Effect of bimodal pore structure

Rajib Kumar Singha,a Astha shukla,a Aditya Yadav,a Takehiko Sasaki,b Aditya Sandupatla,c

Goutam Deo,c Rajaram Bal*a

a Nanocatalysis Area, Conversions & Catalysis Division, CSIR-Indian Institute of Petroleum,

Dehradun 248005, India. Fax: +91 135 2660202; Tel: +91 135 2525917.b Department of Complexity Science and Engineering, Graduate School of Frontier Sciences,

The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan.c Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur,

208016, India.

1. Impregnation method for Pt-CeO2ImpNP catalyst preparation

In a typical preparation method, 0.04 g [Pt(NH3)4](NO3)2 was dissolved in 30 ml water then

1 g of prepared CeO2NP was added to the solution with continuous stirring for 1 h. The whole

mixture was then evaporated to dryness at 80 °C followed by calcination of the dried material

at 600 °C for 6 h in air. The catalyst was denoted as Pt-CeO2ImpNP.

2. Catalytic procedure

Catalytic POM reaction was carried out in a fixed-bed down flow reactor at atmospheric

pressure. Typically, 60 mg of catalyst was used for the catalytic experiment. The catalyst

mixed with diluent (acid washed ground quartz) was placed in between two quartz wool

plugged in the center of 32 cm long quartz tube reactor (internal diameter = 4 mm). The

reaction was carried out at different temperatures ranging between 250 C and 800 C, with

the catalyst reduced at 200 °C with 20% H2/He gas for 2h. Weight hourly space velocity

(WHSV) was varied between 50000 and 500000 ml.g-1.h-1 with a molar ratio of O2: CH4: He

= 1:2:17. The reaction products were analyzed using an online gas chromatography (Agilent

7890A) fitted with a TCD detector using different columns; Molecular sieves (for analyzing

H2) and PoraPack-Q (for analyzing CH4, CO2 and CO).

Electronic Supplementary Material (ESI) for Catalysis Science & Technology.This journal is © The Royal Society of Chemistry 2017

2

Figure S1: Experimental set up for POM

POM is an exothermic reaction and for the same reason to avoid the temperature gradient in

the reaction zone, we diluted the catalyst with inert material (ground quartz). To check the

temperature difference in the reaction zone and just outside the reaction zone, we placed two

thermocouples inside the furnace as shown in Figure S1. We found that maximum difference

between the temperatures detected by two different thermocouples was ± 2°C, which is very

much negligible.

3. Catalyst characterization

The surface area of the catalyst was analyzed at −196 °C with a Belsorbmax (BEL, Japan)

using the BET equation. The sample was degassed at 350 °C for 6 h before analysis. The

powder X-ray diffraction patterns of different Pt-CeO2 catalysts were recorded on a Rigaku-

Geigerflex X-ray diffractometer fitted with a Cu Kα radiation source. Diffraction patterns

were recorded in 10°–80° 2 region with a 0.04 step size (step time = 4 s). The stability of the

catalyst was checked by comparing XRD patterns of the fresh catalyst and spent catalyst. To

check the reducibility of the catalysts, TPR experiments were carried out in a Micromeritics,

Auto Chem II 2920 instrument connected with a thermal conductivity detector (TCD). All

samples were subjected for TPR in the temperature range of 40–1000 °C with an increment

of 10 °C min−1, using helium as carrier and hydrogen as the reducing gas (10%H2/He). Metal

3

dispersion analysis was carried out by Micromeritics, Auto Chem II 2920 instrument

connected with a thermal conductivity detector (TCD) and Quadrupole mass analyzer

connected online with the TPR instrument. SEM images were taken on a FEI Quanta 200 F,

using a tungsten filament doped with lanthanum hexaboride (LaB6) as the X-ray source, fitted

with an ETD detector with high vacuum mode, using secondary electrons and an acceleration

voltage of 10 or 30 kV. Samples were analyzed by spreading them on a carbon tape. Energy

dispersive X-ray spectroscopy (EDX) was used for the elemental analysis. The elemental

mapping was also carried out using the same spectrophotometer. TEM images were collected

using a Jeol JEM 2100 microscope. The samples were prepared by mounting an ethanol-

dispersed sample on a lacey carbon Formvar coated Cu grid. X-ray photoelectron spectra

(XPS) were recorded on a Thermo Scientific K-Alpha X-ray photoelectron spectrometer.

Binding energies (±0.1 eV) were determined with respect to the position of the C 1s peak at

284.6 eV. The spectra of the O 1s, C 1s, Pt 4f, Ce 3d regions were measured. Raman

spectroscopy of both fresh and spent catalyst was carried out using a Sentera, Bruker Raman

spectrometer. An Argon ion laser (wavelength of 532 nm) was used for excitation. Typically,

2 mW laser powers were used for the Raman analysis. EXAFS measurements of the Pt LIII-

edge were carried out at the High Energy Accelerator Research Organization (KEK-IMMS-

PF). The measurement was made in transition mode and spectra were taken at BL-9C. The

electron storage ring was operated at 2.5 GeV and 450 mA and synchrotron radiation from

the storage ring was monochromatized by a Si (111) channel cut crystal. The ionized

chambers, which were used as detectors for the incident X-ray (Io) and transmitted X-ray (I),

were filled with N2 (85 %) – Ar (15 %) mixture gas and Ar (100 %) gas, respectively. The

angle of the monochromators was calibrated with Cu foil. The EXAFS raw data were

analyzed with UWXAFS analysis package,1 including background subtraction program

AUTOBK 2 and curve fitting program FEFFIT.3 The amplitude reducing factor, S02 was fixed

at 0.95. The backscattering amplitude and phase shift were calculated theoretically by FEFF

8.4 code.4

4

Figure S2: SEM images of CeO2 synthesized by (a) process (i), (b) process (ii), (c) process

(iii) and (d) process (iv).

5

Table S1. Comparison of the physical properties of 2Pt-CeO2NP catalyst with catalyst

reported for POM, by different research groups.

Catalyst Physical properties Ref.

AM (wt%) SA (m2/g) Pore size (nm) MD (%) SAvg. (nm)

Pd-CeO2 5 NR NR NR NR [5]

Pt-CeO2 5 NR NR NR NR [5]

Rh-CeO2 5 NR NR NR NR [5]

Ni-Al2O3 5 192 10.4 NR NR [6]

Ni-SiO2 4.85 326 7.5 5.3 6-8 [7]

MgO-NiAl 31 62 15 8.7 9-11 [8]

Yb/Ni-Al2O3 7.56 103 11 NR 14 [9]

Rh-ZrO2 0.5 22 NR 72 1.51 [10]

Ni-SiO2 3.5 46 NR 29 3-5 [11]

NiAl2O4 NR 58 9.9 NR 23 [12]

Rh/Ti-SiO2 1 82 5.4 2.9 22 [13]

Ni/Ce1-xZrxO2 5 160 NR NR >5 [14]

Pt-CeO2 1.7 49 NR NR 4.6 [15]

Ni-CeO2 6 28 NR NR 28 [16]

Ru-Al2O3 12 72 NR 18 5.8 [17]

Ni–Ce(La)Ox 5 99 NR NR 7.4 [18]

Ni-CeO2 7.5 145 3.6 NR 5.1 [19]

Pt-CeO2 1 37 18 NR NR [20]

Rh-CeO2/ZrO2 0.5 49 14.2 15.2 7.2 [21]

Ni-CeO2 5 75.8 NR 24.6 4.0 [22]

Pt-CeO2PS 2 72.8 NR 64.7 1.54 [23]

Pt-CeO2NP 2 146 5.5, 11.5 78.4 1.27 This

report

Note: AM → Active metal, SA → Surface area, MD → Metal dispersion, SAvg. → Average

active particles size, NR → Not reported.

From Table S1, it can be seen that most of the catalyst reported for POM are either non-

porous or normal single pore catalyst. Most of the reports also described quite larger active

particles size (about 5 nm or above) and low metal dispersion. In our recent report,23 we have

6

reported that controlled deposition led to very high metal dispersion of Pt-nanoparticles over

the CeO2 support surface. Pt-CeO2PS catalyst reported in our previous article 23 was non-

porous with surface area 72.8 m2/g, metal dispersion 64.7% and average Pt-species size 1.54

nm. In this report, we are reporting Pt-CeO2NP catalyst, which showed much higher surface

area (146 m2/g) and with bimodal pore structure. This bimodal feature of the catalyst highly

influenced Pt-dispersion (78.4%) and formed even smaller Pt-species particles (1.27 nm).

Clearly Pt-CeO2NP catalyst reported here have much better physical properties than our

previously reported Pt-CeO2PS catalyst. Higher Pt-dispersion of Pt-CeO2

NP catalyst created

much more active sites for catalysis. Reports suggested that smaller active particles are much

more active for POM reaction than its larger counterparts,16, 23 which was also evident form

the catalytic activity of the catalyst reported here.

Scheme S1: Possible Pt-deposition pattern on prepared catalysts surfaces.

7

Morphological differences between Pt-CeO2PS 23 and Pt-CeO2

NP.

Figure S3: TEM images of (A, B) 2Pt-CeO2PS catalyst and (C, D) 2Pt-CeO2

NP catalyst.

Morphological differences between Pt-CeO2NP and Pt-CeO2

PS are also visible from Figure S3.

Figure S3A, S3B shows 2-5 nm Pt-species deposited on 20-60 nm CeO2 nanoparticles,

whereas Figure S3B, S3C shows 5-15 nm CeO2 particles formed porous spheres together. So,

Pt-CeO2NP catalyst is morphologically completely different from Pt-CeO2

PS catalyst.

Mass and Heat Transfer Calculations for Partial oxidation of methane upon 2Pt-

CeO2NP-F catalyst at 800 °C

For PBR reaction mode (5 O2 / 10 CH4 / 85 He) with respect to Methane

Mears Criterion for External Diffusion

If , then external mass transfer effects can be neglected.15.0'

Abc

bA

CkRnr

= reaction rate, kmol/kg-cat · s 'Ar

n = reaction order

8

R = catalyst particle radius, m

ρb = bulk density of catalyst bed, kg/m3

=(1-ф) (ф= porosity or void fraction of packed bed)

ρc = solid catalyst density, kg/m3

CAb = bulk gas concentration of A, kmol/m3

kc = mass transfer coefficient, m/s

= [6.21 10-06 kmol-C3/kg-cat.s 1000] [183.73 kg/m3][ 5.3 10-5 m][0.92]/([ Abc

bA

CkRnr '

1.02 m/s] [ 0.0254 kmol/m3])= 2.15 x 10-03 < 0.15 {Mears for External Diffusion}

Weisz-Prater Criterion for Internal Diffusion

If , then internal mass transfer effects can be neglected.1' 2

)(

Ase

cobsAWP CD

RrC

-r’A(obs) = observed reaction rate, kmol/kg-cat · s

R = catalyst particle radius, m

ρc = solid catalyst density, kg/m3;

De = effective gas-phase diffusivity, m2/s

= where cpABD

DAB = gas-phase diffusivity m2/s; = pellet porosity; =constriction factor; =tortuosity. p c

CAs = gas concentration of A at the catalyst surface, kmol-A/m3

= [6.21 10-06 kmol-C3/kg-cat.s 1000] [4.342 x 103 kg-cat/m3] Ase

cobsAWP CD

RrC

2)('

[5.3 10-5 m]2 / ([3.91 10-4 m2/s] [0.0254 kmol-C3/m3]) = 7.63 x 10-03 < 1

{Weisz-Prater Criterion for Internal Diffusion}

9

Mears Criterion for External (Interphase) Heat Transfer

15.0)'(

2

gbt

bAr

RThRErH

[-53 kJ/mol (-6.21 10-06 kmol/kg-cat.s) 1000 183.73 kg-cat/m3 5.3 x 10-5 m 65.17

kJ/mol] / [5.325 kJ/m2.K.s 10732 K2 8.314 10-3 kJ/mol.K] = 4.10 10-06 < 0.15

{Mears Criterion for External (Interphase) Heat Transfer}

Mears Criterion for Combined Interphase and Intraparticle Heat and Mass Transport

(Mears, 1971)

nnDCRr

bbeAb

A

33.0133.01' 2

; ; ; ; sgTR

E

bgb TR

E

b

Aberb T

CDH

bt

Ar

ThRrH '

Abc

A

CkRr '

γ = Arrhenius number; βb = heat generation function;

λ = catalyst thermal conductivity, W/m.K;

χ = Damköhler number for interphase heat transport

ω = Damköhler number for interphase mass transport

= [6.21 10-06 kmol/kg-cat.s 1000 (5.3 10-5)2 m2]/[ 0.0254 kmol/m3 3.91 eAb

A

DCRr 2'

10-4 m2/s] = 1.76 10-6 < 3

10

Figure S4: Effect of different factors on methane conversion over 2Pt-CeO2NP catalyst.

(A) Effect of Catalyst weight, (B) Effect of pellet size, (C) Methane feed rate. Reaction

condition: Temperature (800 °C), WHSV (100000 ml.g-1.h-1), O2 : CH4: He = 1:2:17 (feed

ratio), Pressure (1 atm.) and (D) Effect of WHSV and methane flow rate/bed length ratio,

Reaction condition: Temperature (800 °C), O2 : CH4: He = 1:2:17 (feed ratio), Pressure (1

atm.).

It is reported that methane reforming reactions are affected by HT-MT effect and it is

undesired. So, first off all we have diluted the catalyst with inert material to avoid the thermal

gradients. To check HT-MT limitations during catalysis, we have performed number of

experiments, which suggested negligible effect of it on methane conversions. Figure S4

shows effect of different factors like catalyst weight, catalyst mesh size, methane flow rate or

total flow rate, methane flow rate/bed length ratio etc. All the experiments showed minimal

differences in methane conversions indicating negligible HT-MT effect. In presence of HT-

MT limitations all these factors are supposed to influence methane conversions during POM

catalysis.

11

Figure S5: Change in methane conversion with Catalyst weight and WHSV at 400 °C.

Figure S5 showed that there was a decreasing trend in methane conversion with increasing

WHSV but it can be seen that the values are almost merging for different catalyst weight. The

observation revealed that HT-MT limitation was negligible during catalysis.

12

Table S2. Comparison of the catalytic activities of 2Pt-CeO2NP catalyst with the latest and

best catalyst reported for POM, by different research groups.

Catalyst POM light

off

CH4 Conv. (%) at H2/CO ratio at Stability Ref.

600 °C 700 °C 600 °C 700 °C

Pd-CeO2 400 °C 61.5 84.0 2.6 2.2 24 h at

600 °C

[5]

Pt-CeO2 400 °C 53.2 74.6 2.6 2.3 24 h at

600 °C

[5]

Rh-CeO2 400 °C 83.6 95.2 2.3 2.2 24 h at

600 °C

[5]

Ni-Al2O3 500 °C 63.1 85.4 2.5 2.2 48 h at

700 °C

[6]

Ni-SiO2 NR ~58 ~90 NR NR 70 h at

700 °C

[7]

MgO-NiAl NR NR 63 NR 1.7 75 h at

700 °C

[8]

Yb/Ni-Al2O3 NR NR 95 NR ~1.5 165 h at

700 °C

[9]

Rh-ZrO2 450 °C ~75 ~90 ~2 ~2 6 h at

700 °C

[10]

Ni-SiO2 NR NR 82 NR ~1.8 10 h at

700 °C

[11]

NiAl2O4 500 °C ~63 88 ~2.5 ~2.2 40 h at

850 °C

[12]

Rh/Ti-SiO2 NR ~72 ~90 2.2 NR 22 h at

700 °C

[13]

13

Catalyst POM light

off

CH4 Conv. (%) at H2/CO ratio at Stability Ref.

600 °C 700 °C 600 °C 700 °C

Ni-

Ce(1-x)ZrxO2

450 °C 59 ~85 ~2.1 ~2.1 7 h at

700 °C

[14]

Pt-CeO2 NR NR NR NR NR 100 h at

800 °C

[15]

Ni-CeO2 650 °C 22 98 NR NR 84 h at

800 °C

[16]

Ru-Al2O3 500 °C ~60 ~78 1.8 NR NR [17]

Ni–

Ce(La)Ox

550 °C 84 96 2 2 100 h at

650 °C

[18]

Ni-CeO2 450 °C 67 84 1.9 2 50 h at

800 °C

[19]

Pt-CeO2 500 °C 85 96 ~3.8 ~2.9 NR [20]

Rh-

CeO2/ZrO2

410 °C ~82 ~95 ~2.1 ~1.9 6 h at

775 °C

[21]

Ni-CeO2 400 °C 59 82 1.87 1.95 90 h at

800 °C

[22]

Pt-CeO2PS 350 °C 55 73 1.98 1.99 105 h at

800 °C

[23]

Pt-CeO2NP 350 °C 70 88 1.99 1.99 60 h at

800 °C

This

report

Note: NR → Not reported.

Zhu et al. 5 reported Pd-CeO2, Pt-CeO2 and Rh-CeO2 catalyst for POM reaction, which

showed POM light off at 400 °C but the amount of noble metals used as active species was

quite high (5 wt%). They also reported uneven distribution of POM products with high

H2/CO ratio. Most of the reports showed either high noble metal loading or high POM light

14

off temperature (≥ 400 °C) or low methane conversion or uneven distribution of POM

products or low time on stream (TOS) stability. Our recent report on POM over Pt-CeO2PS

catalyst 23 improved almost all these deficiencies but still some areas of concern were present

like low temperature selectivities of POM products (H2 + CO) and improvement of methane

conversion at temperature range 500 °C to 700 °C. This report of POM over Pt-CeO2NP

catalyst describes further improvement of these catalytic aspects by modifying the physical

properties of Pt-CeO2 catalyst.

Figure S6: Activity difference of 2Pt-CeO2NP-F (bimodal) and 2Pt-CeO2 (uniporous)

catalysts.

Reaction Conditions: Temperature (350-800 °C), Pressure (1 atm.), WHSV (100000 ml.g-

1.h-1), Feed ratio (O2: CH4: He = 1:2:17), Reaction time (6 h).

15

Figure S7: CH4 conversions over 2Pt-CeO2NP and 2Pt-CeO2

ImpNP catalysts. Reaction

Conditions: Temperature (300 °C - 800 °C), Pressure (1 atm.), WHSV (100000 ml.g-1.h-1),

Feed ratio (O2: CH4: He = 1:2:17), Reaction time (6 h).

16

Figure S8: Comparative analysis of 2Pt-CeO2NP catalyst and our previously reported 2Pt-

CeO2PS catalyst.

Our purpose was to synthesize a catalyst with high POM activity at lower temperatures,

which is very much demanding from energy point of view and in this aspect Pt-CeO2NP

catalyst is much better as compared to Pt-CeO2PS catalyst. The advantage of Pt-CeO2

NP

catalyst was higher selectivity of POM products even at lower temperature (350-550 °C) as

compared to Pt-CeO2PS catalyst (Figure S8). At higher temperature (≥ 550 °C) although the

POM product selectivity was similar but methane conversions for Pt-CeO2NP catalyst was

much higher (about 11-15%). These advantageous properties of Pt-CeO2NP catalyst was

evolved from the bimodal pore structure of the catalyst, which was absent in Pt-CeO2PS

catalyst. Bimodal pore size distribution highly increased Pt-dispersion in case of 2Pt-CeO2NP

catalyst (78.4%) as compared to 2Pt-CeO2PS catalyst (64.7%) and highly increased the

number of active sites. With increase in temperature methane conversion was increased due

to higher molecular transportation through the larger pores of 2Pt-CeO2NP catalyst,24-27 which

17

is absent in Pt-CeO2PS catalyst. The reaction was controlled by metal (Pt)-support (CeO2)

interfacial surface rather than Pt-loading.22 Higher metal dispersion of 2Pt-CeO2NP catalyst

highly increased metal-support interfacial surface area as compared to 2Pt-CeO2PS catalyst.

So, although both the catalyst contains 2wt% Pt but increased Pt-CeO2 interfacial surface area

caused higher activity of 2Pt-CeO2NP catalyst compared to 2Pt-CeO2

PS catalyst. Smaller Pt-

particles were much more active for POM rather than complete oxidation even at lower

temperature, which effected on POM product selectivity (Figure S8). So, clearly 2Pt-CeO2NP

catalyst is much more advantageous than 2Pt-CeO2PS catalyst from both the point of view of

physical properties and POM activity.

Figure S9: (A) Time on stream (TOS) stability of 2Pt-CeO2ImpNP catalyst and (B) SEM image

of 2Pt-CeO2ImpNP-S catalyst. Reaction Conditions: Temperature (800 °C), Pressure (1 atm.),

WHSV (100000 ml.g-1.h-1), Feed ratio (O2: CH4: He = 1:2:17).

REFERENCES:

1. E. A. Stern, M. Newvill, B. Ravel, Y. Yacoby and D. Haskel, Physica B, 1995, 208,

117-120.

2. M. Newvill, P. Livins, Y. Yacoby, E. A. Stern and J. J. Rehr, Phys. Rev. B:

Condensed Matter, 1993, 47, 14126-14131.

3. A. L. Aukudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev. B: Condensed

Matter, 1998, 58, 7565-7576.

4. A. L. Ankudinov, A. I. Nesvizhskii and J. J. Rehr, Phys. Rev. B: Condensed Matter,

2003, 67, 1151201-1151206.

18

5. Y. Zhu, S. Zhang, J. Shan, L. Nguyen, S. Zhan, X. Gu and F. (Feng) Tao, ACS Catal.

2013, 3, 2627−2639.

6. M. Peymani, S. M. Alavi and M. Rezaei, Int. J. Hydrogen Energy 2016, 41, 19057–

19069.

7. C. Ding, J. Wanga, G. Ai, S. Liu, P. Liu, K. Zhang, Y. Han and X. Ma, Fuel 2016,

175, 1–12.

8. Z. Boukha, C. Jiménez-González, M. Gil-Calvo, B. de Rivas, J. R. González-Velasco,

J. I. Gutiérrez-Ortiz and R. López-Fonseca, Appl. Catal. B: Environmental 2016, 199,

372–383.

9. C. Ding, J. Wang, Y. Jia, G. Ai, S. Liu, P. Liu, K. Zhang, Y. Han and X. Ma, Int. J.

Hydrogen Energy 2016, 41, 10707–10718.

10. M. C. Campa, G. Ferraris, D. Gazzoli, I. Pettiti and D. Pietrogiacomi, Appl. Catal. B:

Environmental 2013, 142– 143, 423– 431.

11. W. S. Xia, Y. H. Hou, G. Chang, W. Z. Weng, G. B. Han and H. L. Wan. Int. J.

Hydrogen Energy 2012, 37, 8343-8353.

12. R. López-Fonseca, C. Jiménez-González, B. de Rivas and J. I. Gutiérrez-Ortiz, Appl.

Catal. A: General 2012, 437– 438, 53– 62.

13. A. Karelovic, X. García, R. Wojcieszak, P. Ruiz and A. L. Gordon, Appl. Catal. A:

General 2010, 384, 220–229.

14. A. S. Larimi and S. M. Alavi, Fuel 2012, 102, 366–371.

15. L. Pino, A. Vita, M. Cordaro, V. Recupero and M. S. Hegde, Appl. Catal. A: General,

2003, 243, 135-146.

16. G. Pantaleo, V. L. Parola, F. Deganello, R. K. Singha, R. Bal and A. M. Venezia,

Appl. Catal. B: Environmental, 2016, 189, 233-241.

17. I. Balint, A. Miyazaki and K. Aika, J. Catal. 2003, 220, 74–83.

18. T. Zhu and M. Flytzani-Stephanopoulos, Appl. Catal. A: General 2001, 208, 403–

417.

19. P. Pal, R. K. Singha, A. Saha, R. Bal and A. B. Panda, J. Phys. Chem. C, 2015, 119,

13610-13618.

20. W. Tang, Z. Hu, M. Wang, G. D. Stucky, H. Metiu and E. W. McFarland, J. Catal.,

2010, 273, 125-137.

21. A. Scarabello, D. D. Nogare, P. Canu and R. Lanza, Appl. Catal. B: Environmental,

2015, 174, 308-322.

19

22. R. K. Singha, A. Shukla, A. Yadav, L. N. Sivakumar Konathala and R. Bal, Appl.

Catal. B. Environmental, 2017, 202, 473-488.

23. R. K. Singha, S. Ghosh, S. S. Acharyya, A. Yadav, A. Shukla, T. Sasaki, A. M.

Venezia, C. Pendem and R. Bal, Catal. Sci. Technol., 2016, 6, 4601-4615.

24. J. Wang, H. Li, D. Li, J. P. den Breejen and B. Hou, RSC Adv., 2015, 5, 65358-65364.

25. J. Levenspiel, Chemical Reaction Engineering, (2nd ed.) Wiley, New York, 1972.

26. Y. Zhang, M. Koike, R. Yang, S. Hinchiranan, T. Vitidsant and N. Tsubaki, Appl.

Catal. A: General, 2005, 292, 252-258.

27. R. K. Singha, S. Das, M. Pandey, S. Kumar, R. Bal, A. Bordoloi, Catal. Sci. Technol.,

2016, 6, 7122-7136.