Sonochemical synthesis of Pt, Ru doped TiO2 for methane reforming

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

Applied Catalysis A: General xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Sonochemical synthesis of Pt, Ru doped TiO2 for methane reforming Satyapaul A. Singh, Giridhar Madras ∗ Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

a r t i c l e

i n f o

Article history: Received 18 July 2015 Received in revised form 29 October 2015 Accepted 30 October 2015 Available online xxx Keywords: Eley-Rideal mechanism DRIFTS Sonochemical synthesis Bimetallic catalysts

a b s t r a c t Platinum group materials (Pt, Ru, Rh) are highly active materials for reforming. In the present study, bimetallic Pt and Ru substituted TiO2 catalysts were synthesized by sonochemical method. The bimetallic 2% Pt 2% Ru/TiO2 showed high surface area of 66 m2 /g compared with the other catalysts. The prepared catalysts were characterized by XRD, XPS, TEM, TPR and BET. Dry reforming, partial oxidation and combined reforming (CR) with CO2 were studied on these materials. Synthesized catalysts were found to be highly active for dry reforming reaction. 26% and 95% of methane conversions were obtained with Ru substituted TiO2 (4% Ru/TiO2 ) at 400 ◦ C and at 700 ◦ C. The H2 /CO ratio was close to unity with both Pt and Ru substituted catalysts after 650 ◦ C. The apparent activation energies found to be 51 and 56 kJ mol−1 with respect to methane for 4% Ru/TiO2 and 4% Pt/TiO2 , respectively. High oxygen storage capacity of 4% Pt/TiO2 further showed the good activity for dry reforming. The bimetallic catalyst 2% Pt 2% Ru/TiO2 sintered under time on stream condition and also resulted in the formation of amorphous carbon on to the surface of catalysts, which made them an inactive catalyst for reforming process. The surface intermediates during these reactions were found by in situ FTIR at different temperatures for reforming processes. The peaks corresponding to CO2 adsorption were not observed in case of 4% Ru/TiO2 and 2% Pt 2% Ru/TiO2 for dry reforming and this indicates the Eley-Rideal mechanism on both catalysts. A kinetic model was developed by considering CH4 dissociation as rate determining step (RDS) and the model was validated to the obtained experimental data. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Abundant resources of methane from natural wet lands, fermentation, biomass, fossil fuels and agriculture would be useful for exploiting the syngas production [1]. Fischer–Tropsch synthesis (FTS) for the production of liquid fuels production, ammonia and methanol industries utilizes syngas as a feed stock [2,3]. Syngas can be produced by processes like steam reforming (SR), dry reforming (DR), autothermal reforming (ATR), combined reforming (CR) and partial oxidation (PO). The product H2 /CO ratio depends on the reaction process and conditions used. For the production of oxygenated compounds and for liquid hydrocarbons by FT process, the H2 /CO ratio of syngas must be close to unity [4], which can be achieved by dry reforming. Both steam and dry reforming reactions are highly endothermic, whereas partial oxidation is exothermic. High temperatures are required for both steam and dry reforming to achieve maximum CH4 and CO2 conversions. The economics of the overall process can be minimized by combining one of the reforming reactions with partial oxidation of methane [5]. This process is called autother-

∗ Corresponding author. Fax: +91 80 23600683. E-mail address: [email protected] (G. Madras).

mal reforming where SR + PO are combined or combined reforming where DR + PO are combined [6–9]. Other than these, energy efficient trithermal reforming processes for syngas production without CO2 separation have been studied by various groups [10,11a,b]. Autothermal reactions have garnered scientific interest in recent years on Ni and platinum group metals (PGM) [11a,b–14]. Though various catalysts have been studied extensively, many of these catalysts deactivate over a period of time under reaction conditions. The major reasons for catalyst deactivation are (a) due to sintering of the catalyst at higher temperatures (b) oxidation of the metallic sites and (c) coke deposition on the catalyst surfaces. In reforming processes, carbon deposition on the catalyst surface occurs majorly due to Boudouard reaction/methane cracking [3]. Ni based catalysts have been reported as promising catalysts for their stability and less carbon deposition [15–18]. A detailed study of dry reforming and partial oxidation was studied on various other noble metals, and the activity order is observed as, Rh ∼ Ru > Ir > Pt > Pd [19,20]. Bimetallic catalysts have been employed for reforming of methane, which include the combination of Ni-Co, Ni-Pt, Co-Ce, PtRh and even trimetallic catalysts on single or composite of supports like CeO2, ZrO2 , Al2 O3 [3,4,14,21–26]. The mechanisms of reforming reactions are well studied on various supports and the mechanistic pathway includes either CH4 or CO2 activation. The mechanisms have been proposed based on the

http://dx.doi.org/10.1016/j.apcata.2015.10.047 0926-860X/© 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

2

4% Ru/TiO2 After reaction

*

*

4% Ru/TiO2 Before reaction

*

*

*

*

Int ensit y, a.u.

2% Ru 2% Pt/TiO2 After reaction

*

*

*

*

*

*

*

2% Ru 2% Pt/TiO2 Before reaction

*

*

*

* 4% Pt/TiO2 After reaction

*

*

4% Pt/TiO2 Before reaction

*

*

20

30

40

50

60

Diffraction angle, 2θ

*

70

80

Fig. 1. X-ray diffraction pattern of Pt, Ru catalysts on TiO2 supports (*—anatase phase TiO2 ).

CH4 dissociation or CO2 dissociation on support as a rate determining step (RDS) [27,28]. The reaction pathway of reforming on the catalyst surface depends on the surface acidity and/or basicity. If the support possesses Lewis acidity, CO2 can adsorb on the surface of the support and dissociate. In case of basic support, carbonates and bicarbonates play a role in the CO and H2 formation [29]. PGM catalysts enhance the transport of hydrogen and oxygen between the precious metal and support by spillover mechanism. This metal support interaction is important to achieve higher rates and low coke formation on the catalyst. Despite this, strong metal support interactions at higher temperatures can reduce the catalytic activity due to loss of active sites [30]. In the present study, monometallic and bimetallic Pt, Ru catalysts with TiO2 supports were synthesized by sonochemical synthesis. These catalysts were employed for dry reforming, partial oxidation and dry autothermal reforming reactions for the first time with these catalysts based on this particular synthesis route. The mechanisms of all these reactions were proposed according to the information obtained from in situ FTIR or DRIFTS. The kinetics of dry reforming was studied further due to the catalytic activity towards this reaction. Eley-Rideal mechanism was proposed for dry reforming reaction on these materials as per the information collected from DRIFTS study. All catalysts were characterized with XRD, XPS, TEM before and after the reaction and the chemical stability was studied. The amount of coke deposition was determined by using high pressure thermogravimetric analysis after stability studies. 2. Experimental 2.1. Catalyst synthesis Sonochemical synthesis is one of the well employed techniques to synthesize metal oxides of homogeneous size distribution

Fig. 2. TEM images of fresh catalysts (a), (b) 4%Pt/TiO2 , (c), (d) 2%Pt 2%Ru/TiO2 and (e), (f) 4%Ru/TiO2 .

[31,32]. In a typical synthesis of bimetallic Ru, Pt doped TiO2 , titanyl nitrate (TiO(NO3 )2 ), chloroplatinic acid (H2 PtCl6 ·xH2 O) and ruthenium chloride (RuCl3 ·xH2 O) are used as starting materials. Titanyl hydroxide (TiO(OH)2 ) was precipitated by controlled hydrolysis of 5 ml titanium iso-propoxide under ice bath temperature at 4 ◦ C. 20 ml of 1:1HNO3 was added drop wise to TiO(OH)2 , which results in a clear solution of TiO(NO3 )2 . To synthesize 2%Pt 2%Ru/TiO2 , 20 ml of 0.017 M H2 PtCl6 ·xH2 O and RuCl3 ·xH2 O solutions were added drop wise to TiO(NO3 )2 solution. To ensure effective doping, 5 ml of diethyltetramine (DETA) was added that results in the gelation of the solution. The solution was irradiated with high intensity ultrasonic horn (25 kHz, 125 W) with 20 min ON and 15 min OFF cycle for 5 h (i.e., 3 h effective sonication). The precipitate was collected by centrifuging the colloidal mixture and washed several times with ethanol. The obtained material was dried at 120 ◦ C for 12 h on a hot plate. The resultant material was calcined at 500 ◦ C for 2 h. The bimetallic composites were also prepared by the same protocol. The amounts taken for synthesis are listed in Table 1.

Table 1 Amounts of precursors required for the bimetallic Pt, Ru – TiO2 catalysts synthesis for 5 ml of titanium iso-propoxide. Catalyst

Amount of RuCl3 xH2 O, g

Amount of H2 PtCl6 xH2 O, g

Specific surface area, m2 /g

4% Pt/TiO2 3% Pt 1% Ru/TiO2 2% Pt 2% Ru/TiO2 1% Pt 3% Ru/TiO2 4% Ru/TiO2

0 0.0356 0.0712 0.1069 0.1425

0.2815 0.2111 0.1407 0.0704 0

10 43 66 13 22

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model APCATA-15623; No. of Pages 13

ARTICLE IN PRESS S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

3

Fig. 3. TEM images of spent catalysts (a), (b) 4%Pt/TiO2 , (c), (d) 2%Pt 2%Ru/TiO2 and (e), (f) 4%Ru/TiO2 after reaction 20 h time on stream condition.

2.2. Catalyst characterization X-ray diffraction patterns were collected by using Rigaku X-ray diffraction equipment with 0.35 ◦ /min in the scan range of 20–80◦ with a Cu K␣ source. XPS data was collected using AXIS ULTRA instrument with Al-K␣ as radiation source. The specific surface area was measured by using Belsorb surface area analyzer (Smart instruments) by adsorption at 77 K with liquid N2 . TEM micrographs for the prepared catalysts (before and after reaction) were recorded by using FEI F 30 instrument operating at 200 kV.

2.3. Catalytic activity studies 100 mg of catalyst of 60/80 mesh size was packed with ceramic wool at the center of the quartz tube (ID of 4 mm) quartz reactor. A bed length of 1 cm was maintained in a quartz reactor with gas hourly space velocity of 38,200 h−1 (48,000 ml/(g catal h−1 )). Concentrations of CH4 , CO2 and N2 maintained as 2:2:96 (in vol%), The concentrations of CH4 or CO2 were varied to different concentrations at 2 vol% of the other reactant concentration and accordingly N2 flow rate was adjusted to maintain the specified space velocity. Reactor temperatures were maintained by using a PID controller (Care instruments, India) and the product gas mixture was analyzed under steady state conditions. After conversion was obtained at a particular temperature, the controller was set to a different temperature, and the conversions were measured at each temperature under steady state conditions. Gas chromatograph equipped with

both flame ionized detector (FID) and thermal conductivity detector (TCD) was used to analyze CO, CO2 , CH4 and H2 concentrations at the reactor outlet. 2.4. H2 —TPR study Temperature programmed reduction (TPR) with H2 was performed with 10 mg of catalyst, loaded in a quartz reactor. The experiments were done under the flow rate of 30 ml/min of 5% H2 gas mixture diluted with Ar (Chemix gases, India) in temperature range of 50–600 ◦ C with a rate of 10 ◦ C/min. Hydrogen consumption during the reduction was analyzed with thermal conductivity detector connected to outlet stream. The temperatures were controlled by using a PID controller (Culture instruments, India) during the experiments. 2.5. In situ FTIR (DRIFTS) analysis Fourier transform infrared spectroscopy (FTIR, Perkin Elmer) machine was connected to Praying mantis system which contains a high temperature reaction chamber. The reaction chamber was filled with a catalyst and FTIR spectra were collected at different temperatures (30 ◦ C, 300 ◦ C, 450 ◦ C and 600 ◦ C, respectively). At each temperature the background was collected with 32 scans and 4 cm−1 resolution with nitrogen purging. For each reaction, IR spectra were collected after 30 min of the respective composition of gas flow depending upon the process (CR/PO/DRM). After finishing the study at a particular temperature, the reaction chamber

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

4

(b)

(a) 2

2% Pt 2% Ru/TiO

4% Pt/TiO

Intensi ty, cps

Intensi ty, cps

4% Ru/TiO

Ru-3d after reaction

2

Ru-3d before reaction

2

279

0

200

400

600

800

282

1000

Pt

Ti

Ti -2p Before reaction

4+

455

Pt-4f after reaction for 4% Pt/TiO 2

0

Pt

70

450

291

4+

Intensi ty, cps

Intensi ty, cps

Ti

288

(d)

Ti -2p After reaction

(c)

285

Binding energy, eV

Binding energy, eV

460

72

0

Pt-4f after reaction for 2% Pt 2% Ru/TiO

74

465

76

78

80

82

2

84

Binding energy, eV

Binding energy, eV

Pt - 4f Before reaction for 4% Pt/TiO

(e)

Intensi ty, cps

Pt

Pt - 4f Before reaction for 2% Pt 2% Ru/TiO Pt

66

68

70

72

74

2

4+

2

4+

76

78

80

82

84

Binding energy, eV

Fig. 4. (a) Wide spectra of 4%Pt/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Ru/TiO2 , (b) Core level Ru—3d spectra for 4%Ru/TiO2 , (c) Core level Ti—2p spectra for 4%Ru/TiO2 , (d) Core level Pt—4f spectra (after reaction) in 4%Pt/TiO2 , 2%Pt 2%Ru/TiO2 , (e) Core level Pt—4f spectra (before reaction) in 4%Pt/TiO2 , 2%Pt 2%Ru/TiO2 .

temperature was set to 500 ◦ C and purged with the mixture of N2 and O2 for 30 min. The temperature of reaction chamber was set to another value and the same procedure was repeated for collecting the spectra.

2.6. Thermogravimetric analysis In typical analysis, 100 mg of the spent catalyst was taken in a crucible, placed in a heating chamber of Mettler Toledo thermal analyzer (Germany). The amount of coke deposition was found at

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

3.0x10

4

2.5x10

4

2.0x10

4

1.5x10

4

(a)

C o ncentrati o n, ppm

C o ncentratio n, ppm

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

CH 4 1.0x10

CO 2

4

H2 5.0x10

CO

3

3.0x10

4

2.5x10

4

2.0x10

4

5

(b)

CH 4 CO 2

1.5x10

4

1.0x10

4

5.0x10

3

0.0

H2 CO

0.0 300

400

500

600

o

700

800

900

300

C o ncentrati o n, ppm

Temperature, C

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

400

500

600

o

700

800

900

Temperature, C

(c)

CH 4 CO 2 H2

5.0x10

CO

3

0.0 300

400

500

600

700

800

900

o

Temperature, C Fig. 5. CR reaction with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 and (c) 4%Ru/TiO2 .

operating conditions of 50 ml/min of moisture free air flow rate, with a ramping rate of 10 ◦ C/min from 25 ◦ C to 800 ◦ C. 3. Results and discussion 3.1. Material characterization All catalysts were characterized with XRD and that confirms the presence of TiO2 in all composites of bimetallic catalysts (as shown in Fig. 1). No metallic form of Pt and Ru was found in the synthesized catalysts. Fig. 2 shows the TEM micrographs of 4%Pt/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Ru/TiO2 . The particle sizes of synthesized catalysts are found to be in the order of 20 nm. The diffraction patterns confirms the (1 0 0) planes of TiO2 . The catalysts are formed in both anatase and rutile phases. Under the same preparation conditions, 2%Pt 2%Ru/TiO2 showed anatase phase as the dominant phase. With the same synthesis method, TiO2 alone was also prepared that showed specific surface area of 122 m2 /g with combined anatase and rutile phases (46% anatase, not shown here). The speTable 2 Activation energies of bimetallic Pt, Ru—TiO2 catalysts. Catalyst

Eapp,CH4 , kJ mol−1

Eapp,CO2 , kJ mol−1

4% Pt/TiO2 2% Pt 2% Ru /TiO2 4% Ru/TiO2

56.5 ± 4.2 17.4 ± 0.6 50.8 ± 3.8

49.8 ± 2.0 36.8 ± 2.0 48.3 ± 4.4

cific surface areas of the bimetallic catalysts are in the range of 10–66 m2 /g. 4% Pt/TiO2 was having low surface area of 10 m2 /g, 3%Pt 1%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 1%Pt 3%Ru/TiO2 have shown 43, 66 and 13 m2 /g. 4%Ru/TiO2 has shown surface area of 22 m2 /g. In general rutile phase is more thermally stable than the anatase phase. Anatase TiO2 is having relatively lower surface energy compared with rutile, which causes more stable crystallite sizes of smaller sizes that further results higher surface area [33]. The % of anatase phase in 2%Ru 2%Pt/TiO2 was found to be 79%, which is higher than all other catalysts. The general temperatures of reforming reactions are above 500 ◦ C, which leads to formation of most stable rutile phase. Without the presence of any metal, pure TiO2 was found to be inactive at the temperature above 700 ◦ C, irrespective of the initial phase composition. Thus the phase of titania has no significant effect on conversion. The surface areas of the catalysts are decreased after reaction at 650 ◦ C under time on stream condition for 20 h. The final surface areas were found to be 4, 24 and 7 m2 /g for 4%Pt /TiO2 , 2%Pt 2%Ru/TiO2 , 4%Ru/TiO2 . After reaction, the catalysts were characterized with XRD, which indicates that the materials phase ratios were changed and the rutile phase TiO2 was found at the end of the reaction. The% anatase phase of 2%Pt 2%Ru/TiO2 after reaction was found to be 25%. Fig. 3 shows the TEM micrographs taken after reaction for 20 h. From Fig. 2 and Fig. 3, the catalyst particle sizes are found to be increased by 4–5 times. This is due to the sintering of the catalyst particles under the reaction conditions. Fig. 3(b), (d) and (f) shows

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

6

the HRTEM images of 4%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Pt/TiO2 . The graphitic carbon traces were observed on the catalysts surfaces as shown in HRTEM images, where 0.34 nm d—spacing indicates the presence of graphitic carbon. X-ray photoelectron spectroscopy (Fig. 4) reveals the existence of the oxidation states of synthesized catalysts. Fig. 4(a), shows the wide, Fig. 4(b) and (c) shows the core level spectra of Ru3d and Ti2p for 4%Ru/TiO2 . From Fig. 4(c), the core level Ti2p shows the oxidation state of Ti in 4+. The 2p3/2 and 2p1/2 core level binding energy difference was 5.7 eV. A small shift in peaks was observed due to reduction in Ti4+ to Ti3+ . The peak at 460.6 eV is assigned as satellite peak [34]. Fig. 4(b) shows the core level 3d spectra of Ru and the as synthesized catalyst existed in 3+ oxidation state. The binding energies of 3d3/2 and 3d5/2 are 286.2 and 280.7 eV, respectively [35]. After treating in chemical reaction for 20 h at 650 ◦ C, the material was found to be reduced. XPS spectra revealed that Ru existed in metallic form after the chemical reaction. Before reaction, Pt in the catalysts have shown +4 oxidation state (at 74.1 eV) and metallic Pt was not observed (shown in Fig. 4(e)) in 4%Pt/TiO2 and 2%Pt 2%Ru/TiO2 . In case of spent catalysts of 2%Pt 2%Ru/TiO2 and 4%Pt/TiO2 , platinum was found to be in +4 oxidation state (Fig. 4(d)). But, after reaction, Pt was found to be in Pt0 , Pt2+ and Pt4+ oxidation states [36].

2.5x10

3.2.1. Dry autothermal reforming and partial oxidation with 4%Pt/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Ru/TiO2 The synthesized catalysts were employed for dry autothermal reforming reaction. From Figs. 5 and 6, it is very clear that the catalysts 4%Pt/TiO2 and 4%Ru/TiO2 have shown very high activity for the production of hydrogen for CR and PO reactions. From Fig. 5, the concentration of CO at outlet was found to be small due to significant effect of RWGS and methane combustion reactions. In case of 2%Pt 2%Ru/TiO2 , CO2 concentration was increased further, which indicates the possibility of RWGS and methane combustion reactions. Similar results were observed with partial oxidation reaction also. With PO reaction, 4%Pt/TiO2 and 4%Ru/TiO2 were able to produce more amount of hydrogen. CH4 concentrations were decreased up to a temperature (500 ◦ C in case of 4%Pt/TiO2 and 600 ◦ C in case of 4%Ru/TiO2 ) and then further increased with temperature. This is because of the equilibrium shift that occurs due to the endothermic nature of partial oxidation. 3.3. Dry reforming of methane (DRM) studies over bimetallic TiO2 Synthesized catalysts for DRM reaction have shown trends as depicted in Fig. 7(a) and Fig. 7(b). Here CH4 to CO2 ratio is 2:2 (vol%) balanced with nitrogen. Fig. 7 shows that the species con-

4

(a) 2.0x10

3.2. Catalytic activity studies

(b)

CH 4

1.5x10

4

CH 4

H2 4

1.0x10

4

5.0x10

3

C o ncentrati o n, ppm

C o ncentrati o n, ppm

4

CO 2

4

CO 1.5x10

2.0x10

CO 2 H2 CO

1.0x10

4

5.0x10

3

0.0

0.0 400

500

600

700 o

800

900

300

Temperature, C

400

500

600

o

700

800

900

C o ncentratio n, ppm

Temperature, C

3.0x10

4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

5.0x10

3

(c)

CH 4 CO 2 H2 CO

0.0

300

400

500

600

o

700

800

900

Temperature, C Fig. 6. POM reaction with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 and (c) 4%Ru/TiO2 .

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

4.0x10

4

3.5x10

4

3.0x10

4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

(a)

CH 4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

H2 CO

(b)

CH 4 CO 2 H2 CO 5.0x10

5.0x10

7

CO 2

C o ncentrati o n, ppm

C o ncentrati o n, ppm

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

3

3

0.0

0.0 300

400

500

600

700

800

300

o

400

500

C o ncentrati o n, ppm

Temperature, C

600

o

700

800

900

Temperature, C

3.5x10

4

3.0x10

4

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

CO 2

5.0x10

3

CO

(c)

CH 4 H2

0.0 300

400

500

60o0

700

800

Temperature, C Fig. 7. DRM reaction with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 and (c) 4%Ru/TiO2 .

centration at outlet of the reactor. Fig. 8(a) and (b) shows the methane and CO2 conversion variation with temperature. Clearly from these figures, 4% Ru/TiO2 and 4% Pt/TiO2 have shown better performance compared with the bimetallic catalysts. 4%Ru/TiO2 has shown methane conversion as high as 98% and CO2 conversion of 95% at 800 ◦ C, whereas 4%Pt/TiO2 has shown 92% and 93% CH4 and CO2 conversions. All these conversions are found to be lower than equilibrium values at various temperatures denoted by a dotted line in Fig. 8. Among all the catalysts, 2%Pt 2%Ru/TiO2 has shown very poor activity for the reforming reaction. All the materials started showing activity from 400 ◦ C (other than 3%Pt 1%Ru/TiO2 ). With 2%Pt 2%Ru/TiO2 , the CO2 and CH4 conversions reduced to 4% and again increased to higher conversion. This is due to the coke formation during reaction and sintering of active metal on the support as shown in Fig. 3. 2%Pt 2%Ru/TiO2 showed 37%CH4 conversion even at 900 ◦ C, so this bimetallic catalyst can be considered to be ineffective for dry reforming reaction. With 3%Pt 1%Ru/TiO2 the DRM reaction started at 600 ◦ C, which is higher compared with the other materials. At 900 ◦ C, CH4 and CO2 conversions reached 72% and 83%. Among all bimetallic catalysts, 1%Pt 3%Ru/TiO2 showed higher CH4 and CO2 conversions due to high Ru content. With this material CH4 and CO2 conversions reached 83% and 91% at 800 ◦ C. In all cases, at temperatures >700 ◦ C, CH4 and CO2 conversions are comparable, which indicates that the contribution of reverse water gas shift (RWGS) reaction is negligible.

In DRM process, H2 /CO ratio is one of the important parameters for downstream processes. With all materials, H2 /CO ratio gradually increased with temperature in a range of 400–650 ◦ C and after that the ratio was almost same. From Fig. 8(c), the value reached 0.97 for 4%Ru/TiO2 at 900 ◦ C and this material showed higher than H2 /CO ratio of 0.9 above 700 ◦ C. 4%Pt/TiO2 showed more than 0.80 of H2 /CO ratio from 600 ◦ C and reached a maximum of 0.92 by 900 ◦ C. Bimetallic catalysts also showed reasonably good H2 /CO ratios, but not as high as 4%Pt/TiO2 and 4%Ru/TiO2 . The H2 /CO ratio in case of 3%Pt 1%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 1%Pt 3%Ru/TiO2 reached 0.89, 0.85 and 0.86 at 900 ◦ C. Considering the CH4 and CO2 conversions, it was found that 4%Pt/TiO2 and 4%Ru/TiO2 were the best catalysts. Fig. 9 shows the methane conversion with varying W/FCH4 and W/FCO2 . The reaction rates were calculated by assuming the packed bed as differential flow reactor. The Arrhenius plot was developed with the obtained rates. From this plot, the activation energy of 4%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Pt/TiO2 are listed in Table 2. Among these materials, 4%Pt/TiO2 has shown higher oxygen storage capacity compared to 4%Ru/TiO2 and 2%Pt 2%Ru/TiO2 . The reduction peaks are well spread all over the regions of 50–600 ◦ C for 4%Ru/TiO2 . From Fig. 10, the first reduction peak can be observed at 113 ◦ C, which indicates reduction of ruthenium oxide [37]. In case of 4%Pt/TiO2 , the reduction peaks are observed at temperatures below 500 ◦ C. This indicates the reduction of PtOx to metallic Pt [38]. The

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13 8

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

Fig. 8. (a) Variation of methane conversion (XCH4 ) with temperature, (b) Variation of carbon dioxide conversion (XCO2 ) with temperature, (c) H2 /CO molar ratio variation for dry reforming reaction with temperature with bimetallic catalysts.

reduction peak shifted to 120 ◦ C corresponding to oxides of Ru and at 262 ◦ C, 328 ◦ C, 483 ◦ C corresponds to oxides of Pt are observed with 2%Pt 2%Ru/TiO2 . 3.4. Reaction mechanism and kinetic model The intermediate species of the catalysts 4%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Pt/TiO2 were analyzed with FTIR under in situ conditions of partial oxidation (CH4 :O2 = 2:1 vol%), dry autothermal reforming (CH4 :CO2 :O2 = 2:2:1 vol%) and dry reforming (CH4 :CO2 = 2:2 vol%) conditions. In combined reforming and dry reforming, methane was observed in both adsorbed (at 3013, 3090 cm−1 ) and gaseous form (1250–1350 cm−1 ), gaseous form of CO2 (2358, 2311 cm−1 ) as represented in Figs. 11–13 [39,40]. In the case of all catalysts, the adsorbed methane species intensities decreased with increasing temperature. Particularly at 600 ◦ C, the adsorbed methane species intensities decreased for all the catalysts, whereas the intensities completely vanished in case of 4%Ru/TiO2 . These observations indicate that CH4 activation plays a major role in all reforming processes. In Figs. 11(a) and 12(a), at higher temperatures i.e., at 450 ◦ C and 600 ◦ C physically adsorbed CO to Pt metal ions was observed assigned at 2186 cm−1 [41]. In addition, small peaks were observed at 2065 cm−1 and 2058 cm−1 in the case of 4% Pt/TiO2 at 300 ◦ C (Figs. 11(a) and 13(a)). These peaks can be assigned to linear carbonyl bond attached to Pt [41]. As temperature increases, these peaks assigned to linearly bonded carbonyl species with Pt disappeared.

This interesting step occurred in CR and DRM process, where there is CO2 in the feed mixture. This step was not observed in the case of POM process, which indicates this peak is due to the conversion of CO2 to linearly bonded CO to Pt ion. In Fig. 11(b), other than linear carbonyl group to Pt ion, all other groups mentioned earlier were observed for 2%Pt 2%Ru/TiO2 . The peaks at 2175 cm−1 can be assigned to either carbonyl group attached to Pt ion or CO coordinated to reduced TiO2 , but they are not distinguishable clearly. In Figs. 11(c) and 13(c), a shoulder was observed at 2175 cm−1 in case of 4%Ru/TiO2 at 600 ◦ C and it can be assigned to CO coordinated to reduced TiO2 [41,42]. This is possible due to reduction of TiO2 at temperatures higher than 500 ◦ C. In partial oxidation of methane, at 30 ◦ C no peaks were observed at 2358, 2311 cm−1 which correspond to gaseous CO2 . But after 300 ◦ C onwards, gaseous and adsorbed CO2 peaks were observed. This clearly denotes the formation of CO2 species due to chemical reaction between CH4 and oxygen. From Fig. 12(b), carbonyl group attached to Pt ion and/or CO coordinated to reduced TiO2 are negligible. This indicates the possibility of Eley-Rideal mechanism on the catalyst surface. In Figs. 11–13, no peaks were observed at 1580, 1400, 1640, 1485 and 3700–3745 cm−1 which indicates the adsorbed formate, bicarbonate and hydroxyl species [43] were absent. This indicates these catalysts do not follow formate mechanism. The mechanism for dry reforming is discussed based on the information obtained from DRIFTS analysis. On Pt/TiO2 catalyst, CH4 adsorbed species were observed on Pt ions but no CO2 was adsorbed. Only gaseous CO2 information is

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

(b) ln(rate), (rate in mol/(g-s))

-7

-8.0

0.35

-8.5

2

-8

C O 2 co nv ersi o n, X CO

0.3

0.40

(a) ln(rate), (rate in mol/(g-s))

Methane conversion, XCH

4

0.4

9

-9

-10 1.2

0.2

1.3 1.4 -1 1000/T (K )

1.5

o

400 C o 450 C o 500 C o 550 C

0.1

0.30

-9.0

0.25

-9.5 1.2

0.20

1.3 1.4 -1 1000/T (K )

1.5

o

400 C o 450 C o 500 C o 550 C

0.15 0.10 0.05 0.00

0.0

0

200

400

600

800

-0.05 -200

1000

0

200

400

4

0.25

0.35 2

-8

-9

C O 2 co nv ersi o n, X CO

0.30

ln(rate), (rate in mol/(g-s))

Methane conversion, XCH

4

0.35

0.40

-10

0.20

1.2

1.3 1.4 -1 1000/T (K ) o

1.5

400 C o 450 C o 500 C o 550 C

0.15 0.10

0.30 0.25

0

200

400

600

800

W/FCH (g-s/mol)

1000

1200

1.2

1.3 1.4 -1 1000/T (K )

1.5

o

400 C o 450 C o 500 C o 550 C

-200

1400

0

200

400

W/F

4

0.25

0.16 2

-8.8

C O 2 co nv ersi o n, X CO

0.20

ln(rate), (rate in mol/(g-s))

Methane conversion, XCH

4

-8.6

-9.0

0.15

-9.2

0.10

1.3

1.4 -1 1000/T (K ) o

1.5

400 C o 450 C o 500 C

0.05

CO 2

800

1000

1200

1400

1600

1200

1400

1600

(g-s/mol)

-9.0

-9.5

-10.0

0.12

-10.5

1.2

1.3 1.4 -1 1000/T (K ) o

1.5

200

400

400 C o 450 C o 500 C o 550 C

0.08

0.04

0.00

0.00 -200

600

(f) ln(rate), (rate in mol/(g-s))

0.20

(e)

1600

-9.5

0.10

0.00

1400

-9.0

0.15

0.00

1200

-8.5

0.20

0.05

1000

(g-s/mol)

-8.0

-10.0

0.05

800

CO 2

(d) ln(rate), (rate in mol/(g-s))

(c)

0.40

600

W/F

W/FCH (g-s/mol)

0

200

400

600

800

1000

1200

1400

1600

-200

0

W/F

W/FCH (g-s/mol) 4

600 CO 2

800

1000

(g-s/mol)

Fig. 9. Arrhenius plot for (a), (b) 4%Ru/TiO2 , (c), (d) 4%Pt/TiO2 , (e), (f) 2%Pt 2%Ru/TiO2 .

obtained from DRIFTS study. This clearly indicates no adsorption of CO2 was observed either on the surface of metal or support. The carbonates and molecular adsorption on Pt metal and TiO2 support were observed in the DRIFTS study for Pt/TiO2 synthesized with an incipient wetness technique [43]. However, in this catalytic system where the catalyst has been synthesized by a sonochemical technique, the contributions of carbonates, bidentate carbonates and adsorbed CO2 is minimal. This is confirmed with the DRIFTS study at different feed and temperature conditions. Moreover with

these catalytic systems, CO adsorption on Pt ions was observed at higher temperatures (450 ◦ C and 600 ◦ C). With this information the reaction pathway is as follows, k1,f

M + CH4  M − CH4

(1)

k1,b

k2

M − CH4 →M − C + 2H2

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

(2)

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

10 k3,f

M − C + CO2 + M  2M − CO

(3)

k3,b

k4,f

2M − CO  2M + 2CO

4% Ru/TiO2

(4)

By considering methane dissociation (Reaction (2)) as rate determining step, the rate expression for this mechanism is, rate =



k2 K1 K3 K4 PCH4 PCO2



1 + K1 PCH4 K3 K4 PCO2 + K3



(5)

2 K4 PCO + PCO

TCD Signal, mV

k4,b

2% Pt 2% Ru/TiO2

Ru/TiO2 does not result in any CO adsorption peak at any temperature. For dry reforming, Eley-Rideal mechanism without CO adsorption on support is as follows, k5,f

M − C + CO2  M + 2CO

4% Pt/TiO2

100

(6)

200

300

The rate expression for second reaction pathway is, rate =





where, k2 = A2 exp

 −E  2

RT

and Ki =

(7) ki,f ki,b

= Ai exp



Hi RT

600

 .

The kinetic parameters were found by using nonlinear regression and listed in Table 3. The initial guesses were taken from the literature [44]. Fig. 14 shows the kinetic model that fits the variation of rate with partial pressure of CH4 .

(b)

(a)

3095

2953 3017

2186

2186

1341

3095

o

1267

2311 2358

1267

2311 2358

1341

600 C

Transmi ttance

o

600 C

1299

2109

1299

2109 2953 3017

Transmi ttance

500

Fig. 10. H2 —TPR for 4% Ru/TiO2 , 2% Pt 2% Ru/TiO2 and 4% Pt/TiO2 .

k2 K1 K5 PCH4 PCO2 2 1 + K1 PCH4 K5 PCO2 + PCO

400

o

Temperature, C

k5,b

o

450 C

o

300 C

o

450 C

o

300 C

2065 o

o

30 C

4000

3500

30 C

3000

2500

2000

Wavenumber, cm

1500

-1

1000

4000

3500

3000

2500

2000

Wavenumber, cm

1500

1000

-1

(c) 1299 2953 3017

2175 1267

2311 3095

2358

1341

Transmi ttance

o

600 C

o

450 C o

300 C

o

30 C

4000

3500

3000

2500

2000

Wavenumber, cm

-1

1500

1000

Fig. 11. DRIFTS for CR with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 , (c) 4%Ru/TiO2 .

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

(a)

1267

2311 2358

1299

2953 3017

2186

3095

3095

o

600 C o

450 C o

300 C

1267

2311 2358

o

600 C

1341

Transmi ttance

Transmi ttance

(b)

1299

2109

2953 3017

11

1341

o

450 C

o

300 C

o

30 C

4000

3500

o

30 C

3000

2500

2000

Wavenumber, cm

1500

1000

4000

-1

3500

3000

2500

2000

Wavenumber, cm

-1

1500

1000

(c) 1267 1341

o

600 C

Transmi ttance

1299

2311 2358

2953 3017 3095

o

450 C

o

300 C

o

30 C

4000

3500

3000

2500

2000

Wavenumber, cm

-1

1500

1000

Fig. 12. DRIFTS for POM with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 , (c) 4%Ru/TiO2 .

Table 3 Kinetic parameters of 4% Ru/TiO2 and 4% Pt/TiO2 . Parameter k2 K1 K3 K4 K5

4% Ru/TiO2

  (1.216 ± 0.11) × 10 exp −1518±112 RT   −4 (2.279 ± 0.210) × 10 exp 754±16 RT  743±18  −5 10

(0.442 ± 0.03) × 10

exp

RT

–

3.5. Stability studies The stabilities of 4%Pt/TiO2 and 4%Ru/TiO2 were studied under time on stream conditions with 1:1CO2 :CH4 ratio. In each cycle, the materials were treated with normal feed condition for 20 h at 650 ◦ C. The conversions decreased to 38% and 42% of CH4 and CO2 at the end of 20 h continuous operation. The coke deposition was found to be 72, 247 and 113 mg of coke/g of catalyst for 4%Ru/TiO2 , 2%Pt 2%Ru/TiO2 and 4%Pt/TiO2 respectively. From Fig. 15, the conversion at 650 ◦ C was found to be approximately 76% of CH4 and 78% of CO2 in the case of 4% Ru/TiO2 . The conversions decreased to 38% and 42% of CH4 and CO2 after 20 h. 2%Pt 2%Ru/ TiO2 has shown very poor conversions at 923 K. The methane and CO2 conversions are found to be 11% and 7%, but with time the gradual increment in conversions were observed up to 2 h and decreased

4% Pt/TiO2 10 (6.724 ± 0.21) × 10 exp −4

(2.465 ± 0.33) × 10

exp

 −1691±105  RT  656±13 

– (0.338 ± 0.041) × 10

−5

exp

RT

 1932±37  RT

at further time intervals. This could be due to formation of coke on the catalyst surface at longer times of the study. From DRIFTS, 2%Pt 2%Ru/TiO2 follows the Eley-Rideal mechanism and where methane combustion occurs on the 2%Pt 2%Ru/TiO2 surface.This further generates carbon on to the catalyst surface. From Fig. 1, the peaks corresponding to 26.4◦ are not observed, which indicates that there is no traceable formation of pure graphitic carbon [39–40]. From Fig. 3(b), (d) and (f) sintering of metal particles and traces of graphitic carbon was observed. Thus the amount of the graphitic carbon is very low and this is limited by the detection levels of XRD. By comparing Figs. 2 and 3, the spent catalyst particle sizes have increased by 4–5 times compared with fresh catalysts. The active metals were not distinguished in fresh TiO2 solid solutions, whereas in case of spent catalysts, active metal particles can be distinguished clearly from the support.

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

12

(b)

(a)

2298

2298 3013

3013

2385

2385

o

600 C

Transmi ttance

o

Transmi ttance

600 C o

450 C

o

300 C

o

450 C

o

300 C

2058 o

30 C

o

30 C 3095 2950

3095 2950

1349 1303

1349 1303

4000

3500

3000

2500

2000

Wavenumber, cm

1500

-1

4000

1000

3500

3000

2500

2000

Wavenumber, cm

1500

1000

-1

(c) 2298 3013

2385

2108 2179

o

Transmi ttance

600 C 3095 2950 o

450 C

o

300 C o

1349 1303

30 C

4000

3500

3000

2500

2000

Wavenumber, cm

1500

1000

-1

Fig. 13. DRIFTS for DRM with (a) 4%Pt/TiO2 , (b) 2%Pt 2%Ru/TiO2 , (c) 4%Ru/TiO2 .

500

(a)

400

300

rCH , μmo le/ (g-s)

300

200

673 K 723 K 773 K 823 K

200

4

4

rCH , μmo le/ (g-s)

400

(b)

673 K 723 K 773 K 823 K

100

100

0

0 0

1

2

3

4

5

6

Partial pressure of methane, P CH in kPa 4

7

0

1

2

3

4

Partial pressure of methane, P CH in kPa

5

4

Fig. 14. Rate variation with partial pressure of methane (a) 4% Ru/TiO2 and (b) 4% Pt/TiO2 .

4. Conclusions In the present study, Pt and Ru catalysts with TiO2 were prepared by using sonochemical synthesis. The materials were characterized by XRD and combination of anatase and rutile phases was observed. The bimetallic Pt and Ru doped TiO2 has higher surface

area among all composites. These materials are employed for all dry reforming, partial oxidation and autothermal reforming and combined dry autothermal reforming. 4%Ru/TiO2 and 4%Pt/TiO2 has shown favorable hydrogen conversions for all reactions. But higher conversions and close to equal H2 /CO ratio were observed in case of dry reforming for 4%Ru/TiO2 and 4%Pt/TiO2 . All bimetal-

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047

G Model

ARTICLE IN PRESS

APCATA-15623; No. of Pages 13

S.A. Singh, G. Madras / Applied Catalysis A: General xxx (2015) xxx–xxx

80

13

CH4 for 4% Ru/TiO2 CO2 for 4% Ru/TiO2

70

CH4 for 4% Pt/TiO2 CO2 for 4% Pt/TiO2

% Conversion

60

CH4 for 2% Pt 2% Ru/TiO2

50

CO2 for 2% Pt 2% Ru/TiO2

40 30 20 10 0 0

2

4

6

8

10

12

14

16

18

20

Time on stream, h Fig. 15. Stability of 4% Ru/TiO2 , 2%Pt 2% Ru/TiO2 and 4% Pt/TiO2 at 923 K.

lic and monometallic catalysts have shown activity at 400 ◦ C. The greater than 90% conversions were observed with both 4%Ru/TiO2 and 4%Pt/TiO2 at 800 ◦ C. 4%Ru/TiO2 material was found to be stable among all materials studied. The Eley-Rideal mechanism was observed with DRIFTS study with CH4 dissociation as the rate determining step and this model predicts the system behavior under given operating conditions. Acknowledgements Authors thank Gas Authority India Ltd (GAIL) for financial support. Authors are thankful for Advanced Facility for Microscopy and Macroanalysis (AFMM), IISc for TEM analysis. S.A. Singh thanks Dr. V.M. Shinde for his valuable suggestions in catalyst synthesis. The corresponding author thanks the Department of Science and Technology (DST), India for the J.C. Bose fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12] [13] [14]

S. Kirschke, et al., Nat. Geosci. 6 (2013) 813–823. R.W. Howarth, R. Santoro, A. Ingraffea, Clim. Change 106 (2011) 679–690. H. Ay, D. Üner, Appl. Catal. B: Environ. 179 (2015) 128–138. ˇ ´ I.G.O. Crnivec, P. Djinovic, B. Erjavec, A. Pintar, Appl. Catal. B: Environ. 125 (2012) 259–270. S. Wang, G.Q. Lu, G.J. Millar, Energy Fuels 10 (1996) 896–904. K. Tomishige, M. Nurunnabi, K. Maruyama, K. Kunimori, Fuel Process. Technol. 85 (2004) 1103–1120. Z. Hou, J. Gao, J. Guo, D. Liang, H. Lou, X. Zheng, J. Catal. 250 (2007) 331–341. J.C. Escritori, S.C. Dantas, R.R. Soares, C.E. Hori, Catal. Commun. 10 (2009) 1090–1094. X. Cai, Y. Cai, W. Lin, J. Nat. Gas Chem. 17 (2008) 201–207. C. Song, W. Pan, Catal. Today 4 (2004) 463–484. (a) A.J. Majewski, J. Wood, Int. J. Hydrogen Energy 39 (2014) 12578–12585; (b) B. Nematollahi, M. Rezaei, M. Khajenoori, Int. J. Hydrogen Energy 36 (2011) 2969–2978. T. Liu, H. Temur, G. Veser, Chem. Eng. Technol. 32 (2009) 1358–1366. S. Ayabe, H. Omoto, T. utaka, R. Kikuchi, K. Sasaki, Y. Teraoka, K. Eguchi, Appl. Catal. A: Gen. 241 (2003) 261–269. S.M. Stagg-Williams, F.B. Noronha, G. Fendley, D.E. Resasco, J. Catal. 194 (2000) 240–249.

[15] S.E. Evans, J.Z. Staniforth, R.J. Darton, R.M. Ormerod, Green Chem. 16 (2014) 4587–4594. [16] D. Pakhare, C. Shaw, D. Haynes, D. Shekhawat, J. Spivey, J. CO2 Util. 1 (2013) 37–42. [17] P. Djinovic, J. Batista, A. Pintar, Int. J. Hydrogen Energy 37 (2012) 2699–2707. [18] V.M. Shinde, G. Madras, RSC Adv. 4 (2014) 4817–4826. [19] M. Khajenoori, M. Rezaei, B. Nematolahi, J. Ind. Eng. Chem. 19 (2013) 981–986. [20] M. Rezaei, S.M. Alavi, S. Sahebdelfar, Z.-F. Yan, J. Nat. Gas Chem. 15 (2006) 327–334. [21] A.E. Abasaeed, A.S. Al-Fatesh, M.A. Naeem, A.A. Ibrahim, A.H. Fakeeha, Int. J. Hydrogen Energy 40 (2015) 6818–6826. [22] M.M.V.M. Souza, D.A.G. Aranda, M. Schmal, J. Catal. 204 (2001) 498–511. [23] J.H. Bitter, K. Seshan, J.A. Lercher, J. Catal. 176 (1998) 93–101. [24] A.M. O’Connor, J.R.H. Ross, Catal. Today 46 (1998) 203–210. [25] A.I. Paksoy, B.S. Caglayan, A.E. Aksoylu, Appl. Catal. B: Environ. 168–169 (2015) 164–174. [26] U. Olsbye, T. Wurzel, L. Mleczko, Ind. Eng. Chem. Res. 36 (1997) 5180–5188. [27] D. Pakhare, V. Schwartz, V. Abdelsayed, D. Haynes, D. Shekhawat, J. Poston, J. Spivey, J. Catal. 316 (2014) 78–92. [28] M.F. Mark, F. Mark, W.F. Maier, Chem. Eng. Technol. 20 (1997) 361–370. [29] A.W. Budiman, S.H. Song, T.S. Chang, C.H. Shin, M.J. Choi, Catal. Surv. Asia 16 (2012) 183–197. [30] S.H. Song, J.H. Son, A.W. Budiman, M.J. Choi, T.S. Chang, C.H. Shin, Kor. J. Chem. Eng. 31 (2014) 224–229. [31] N. Ghows, M.H. Entezari, Ultrason. Sonochem. 17 (2010) 878–883. [32] R. Vijaya kumar, Y. Diamant, A. Gedanken, Chem. Mater. 12 (2000) 2301–2305. [33] D.A.H. Hanaor, C.C. Sorell, J. Mater. Sci. 46 (2011) 855–874. [34] D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, John Wiley & Sons, Chichester, 1983, xivþ 533. [35] P. Singh, M.S. Hegde, Chem. Mater. 21 (2009) 3337–3345. [36] V.M. Shinde, G. Madras, Chem. Cat. Chem. 4 (2012) 1968–1978. [37] C.-H. Lin, J.-H. Chao, C.-H. Liu, J.-C. Chang, F.-C. Wang, Langmuir 24 (2008) 9907–9915. [38] N. Perkas, Z. Zhang, L. Chen, M. Besson, A. Gedanken, Catal. Lett. 103 (2005) 9–14. [39] J. Ni, L. Chen, J. Lin, S. Kawi, Nano Energy 1 (2012) 674–686. [40] A.Y. Stakheev, E.S. Shpiro, O.P. Tkachenko, N.I. Jaeger, G.S. Ekloff, J. Catal. 169 (1997) 382–388. [41] E.V. Benvenutti, L. Franken, C.C. Moro, Langmuir 15 (1999) 8140–8146. [42] F. Boccuzzi, A. Chiorino, S. Tsubota, M. Haruta, J. Phys. Chem. 100 (1996) 3625–3631. [43] M.C.J. Bradford, M.A. Vannice, Catal. Today 50 (1999) 87–96. [44] M.M.B. Quiroja, A.E.C. Luna, Ind. Eng. Chem. Res. 46 (2007) 5265–5270.

Please cite this article in press as: S.A. Singh, G. Madras, Appl. Catal. A: Gen. (2015), http://dx.doi.org/10.1016/j.apcata.2015.10.047