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Hydrogen generation by ethanol steam reformingover Rh/Al2O3 and Rh/CeZrO2 catalysts:A comparative study

Pankaj Kumar Sharma*, Navin Saxena, Prasun Kumar Roy, Arti Bhatt

Centre for Fire, Explosive and Environment Safety, Brig. S. K. Majumdar Marg, Delhi 110 054, India

a r t i c l e i n f o

Article history:

Received 30 August 2015

Accepted 28 September 2015

Available online xxx

Keywords:

Rhodium catalysts

Ethanol steam reforming

DRIFT

Mechanism

Coke

* Corresponding author. Tel.: þ91 11 2390714E-mail addresses: [email protected],

http://dx.doi.org/10.1016/j.ijhydene.2015.09.10360-3199/Copyright © 2015, Hydrogen Ener

Please cite this article in press as: Sharmacatalysts: A comparative study, Internation

a b s t r a c t

Rhodium (Rh) catalysts supported over Al2O3 and CeZrO2 were developed and their activity

towards hydrogen generation through ethanol steam reforming (ESR) was compared.

Reforming reactions were performed over a range of temperatures (450 �Ce600 �C) and feed

flow rates (0.1, 0.2 and 0.3 mL min�1) at a constant ethanol-to-water molar ration of 1:6.

Although complete ethanol conversion could be effected, the H2 selectivity was found to be

higher for Rh/CeZrO2 catalyst (62.9%) as compared to Rh/Al2O3 (59.3%) under optimized

reaction conditions. The average exit flow rate was relatively higher for Rh/CeZrO2 catalyst

(263 mL min�1) as compared to Rh/Al2O3 catalyst (236 mL min�1). In-situ Diffuse Reflec-

tance Infrared Fourier Transform Spectroscopy (DRIFTS) revealed the underlying mecha-

nism responsible for better performance of Rh/CeZrO2 catalyst over Rh/Al2O3 catalyst. Rh/

CeZrO2 catalyst was found to facilitate the decomposition of acetate intermediates,

through carbonates at lower temperatures. On the other hand, over Rh/Al2O3, reaction

proceeds through formation of both acetate as well as formate species both of which

decompose at much higher temperatures. The amount of coke deposited was also lower in

case of Rh/CeZrO2 catalyst (6.75 mmol gcatalyst�1) than over Rh/CeZrO2 catalyst

(10.57 mmol gcatalyst�1).

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Researchers are engaged worldwide to develop such alterna-

tive energy sources which are renewable and environment

friendly. Site specific, intermittent and unstable nature of

most of the renewable energy sources (viz. solar, wind, tidal,

biomass), presently being utilized, have catapulted hydrogen

as one of the most promising source of energy [1]. Due to its

high energy content per unit weight (~120 KJ/g) as well as

6, þ91 11 23907189; fax:[email protected] Publications, LLC. Publ

PK, et al., Hydrogen general Journal of Hydrogen En

carbon free nature in that it ultimately oxidizes to water as the

sole combustion product, hydrogen has been identified as

ideal future energy carrier with high efficiency. In addition to

these properties, the technological advances in its utilization

(particularly in fuel cells) have also made hydrogen more

important as a new fuel [2].

Hydrogen can be generated from a variety of feed stocks by

different methods [3]. Bio-ethanol appears to be the most

promising one due to its low toxicity, easy handling, high

volumetric energy density and readily production from

þ91 11 23819547.o.in (P.K. Sharma).

ished by Elsevier Ltd. All rights reserved.

ation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2

ergy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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http://dx.doi.org/10.1016/j.ijhydene.2015.09.137



i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 5 ) 1e1 12

renewable biomass [4,5]. Out of the several catalytic processes

(viz. steam reforming, partial oxidation and oxidative steam

reforming or auto-thermal reforming) investigated [6] for

hydrogen generation from ethanol, ethanol steam reforming

(ESR) leads to highest hydrogen yield [7,8]. It has therefore

become a research area of interest of researchers worldwide

[9] to develop suitable catalysts and in fact, there is ample

literature on catalytic ESR over different supported oxides

[6,7,9e12].

Catalyst preparation starts with the selection of a proper

support material followed by loading with a suitable active

metal which can be achieved by several methods. Although

various oxides of acidic, basic or redox nature have been uti-

lized for catalysts synthesis, rhodium (Rh) has been reported

to be more active towards hydrogen production by catalytic

ESR reaction as compared to its Ru, Pd, Ni and Pt counterparts

[13,14]. Rh has been shown to be a suitable choice in breaking

CeC bond and thereby rendering increased selectivity to-

wards C1 products (viz. CO2, CO and CH4) during catalytic ESR

[7,15]. Moreover, a recent Density Functional Theoretical (DFT)

study has also quantified the ability of Rh to reduce the acti-

vation energy for dissociation of CeH, CeC and CeO bonds,

present in ethanol [16]. This ability in turn has been attributed

to its high lying d band structure with empty d states, which

lowers the CeC bond dissociation barrier by stabilizing the

intermediates [17,18]. Similar to the active metals, support

materials have also been compared [14,19e25] in order to

achieve a suitable catalyst.

The aim of the present work is to compare the catalytic

activity of two Rh catalysts namely Rh/Al2O3 and Rh/CeZrO2,

prepared in the laboratory, for ESR reaction under varying

operating conditions. This comparison has been made based

on ethanol conversion, product distribution, selectivity and

average mass flow rate of exit product gases. Further, in view

of the fact that detailed studies onmechanistic aspects can go

a long way in improving catalyst design by establishing the

role of active metal as well as support in ESR, we have also

identified the nature of intermediates and products formed

over the two different catalyst surface, by performing in-situ

diffuse reflectance infrared Fourier transform spectroscopy

(DRIFTS) under steadyestate reaction conditions at different

temperatures.

Experimental

Materials

Nitrates of cerium (Ce(NO3)3.6H2O), zirconium (ZrO(-

NO3)2.xH2O) aluminium (Al(NO3)3.9H2O) and rhodium

(Rh(NO3)2.10% w/w in HNO3 solution) were used as the metal

precursors. These metal nitrates (Aldrich), urea (Aldrich) and

ethanol (E. Merck) were used without further purification.

Double distilled water was used throughout the course of the

experimental work.

Catalyst preparation

Mixed CeZrO2 oxide (with a CeO2:ZrO2 molar ratio of 70:30)

was synthesized according to the homogeneous urea co-

Please cite this article in press as: Sharma PK, et al., Hydrogen genecatalysts: A comparative study, International Journal of Hydrogen En

precipitation technique described previously for zirconia

based compositions [26]. Al2O3 was prepared by a combina-

tion of homogeneous urea precipitation technique and self-

propagating combustion synthesis (SPCS) technique [27].

Subsequently, Rh (5% w/w) was loaded on the support

following incipient wetness impregnation technique using

an acidic solution of Rh nitrate. Post-impregnation, the

sample was dried in an air oven at 90 �C for 12 h and calcined

in a muffle furnace at 600 �C for 6 h to yield Rh loaded

catalyst.

Catalyst characterization

Textural properties and porous nature of the as prepared

materials were studied by N2-adsorption-desorption experi-

ments performed at 77 K on a Physisorption Analyzer

(Micromeritics ASAP 2010). Particle size distribution was

determined using a particle size analyser (DIPA 2000,

Donner).

Active metal surface area was determined using pulse CO

chemisorption studies, on a Chemisorption Analyzer (Micro-

meritics Chemisorb 2920), as per reported procedure [28,29]

and the average crystallite size of Rh-metal was calculated

using this active metal surface area, assuming cubic crystal

structure [29]. Prior to analysis, 50 mg of catalyst was equili-

brated at 120 �C for 2 h to remove volatiles, after which the

temperature was raised to 400 �C in the presence of H2eAr

(10% v/v) for 3 h to ensure complete reduction. Subsequently,

the sample was cooled to 50 �C under He purging, following

which a COeHe mixture (10%v/v) pulse was introduced every

2 min until complete saturation.

Temperature programmed reduction (TPR) studies were

performed to establish the reduction behavior of calcined

samples. For this purpose, accurately weighed amount

(~15 mg) of sample was subjected to a temperature program

under reducing atmosphere of H2 at 10 �C min�1 from 50 �C to

1000 �C.In order to quantify the acidity of the prepared catalysts,

temperature programmed desorption (TPD) experiments were

performed on Chemisorption Analyzer using ammonia (NH3)

as the probe molecule. Accurately weighed amount (~50 mg)

of sample was pretreated for 30 min at 300 �C to desorb im-

purities. Subsequently, NH3 was adsorbed into the sample bed

maintained isothermally at room temperature for a period of

120 min. Excess NH3 was eliminated by flowing He over the

sample. NH3 desorptionwas performed by heating the sample

at 10 �C min�1 up to 900 �C.Temperature programmed oxidation (TPO) studies were

also performed on the catalyst samples, spent in ESR re-

actions, by oxidizing the sample (~15mg) from 50 �C to 1000 �Cat 10 �C min�1 under continuous flow of O2eHe (10%v/v)

mixture (25 ml min�1).

Powder X-ray diffraction studies were carried out to iden-

tify the crystalline phases of prepared materials, on a Philips

PANanalytical Pro-HRXRD diffractometer using CuKa radia-

tion (l¼ 1.54�A). The sampleswere first pelletized and the data

were collected over the range 2q ¼ 20e80�. The average crys-

tallite size were estimated by Scherrer equation using full

width at half maximum (FWHM) of corresponding highest

intensity diffraction peaks.

ration by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2




Fig. 1 e Nitrogen adsorption-desorption isotherms of as

prepared support materials before and after metal loading

(Inset shows the pore size distribution from the BJH

desorption curve).

i n t e r n a t i o n a l j o u r n a l o f h yd r o g e n e n e r g y x x x ( 2 0 1 5 ) 1e1 1 3

Performance evaluation of catalysts in ESR reaction

Performance of the prepared Rh/CeZrO2 and Rh/Al2O3 cata-

lysts was evaluated towards ESR reaction at ambient pressure

at different temperatures (450, 500, 550 and 600 �C) and feed

flow rates (0.1, 0.2, & 0.3 ml min�1) with a feed of 1:6 ethanol-

to-water molar ratio. The reforming experiments were carried

out in an 8 mm (I.D.) packed bed vertical down-flow stainless

steel continuous flow tubular micro (CFTM) reactor (Micro-

activity Reference PID, Spain). Approximately 0.2 g of catalyst

(D50 ¼ 50 mm) thoroughly mixed with ~2.8 g of inert material

(SiC) of same particle size, was loaded in the reactor and the

feed (ethanol þ water) was introduced into the reactor by

means of an HPLC pump, which regulated the flow rates. Prior

to entering the reactor, the feed solution was passed through

an evaporator (maintained at 200 �C) to ensure complete

gasification of the feed stream. Reaction temperature was

measured using a sliding thermocouple placed within the

catalyst bed. Prior to the ESR reactions, catalysts were reduced

under H2 atmosphere (5 mL/min) for 2 h at 600 �C. Subse-quently, N2 was purged for 30min at 10mL/min to remove the

excess hydrogen at the same temperature.

After reforming, the exit stream was passed through a

condenser and gaseliquid separator to separate the gaseous

and liquid products before being subjected to GC analysis. The

composition of the gaseous stream was determined using an

online gas chromatograph (NuCon, India), equipped with a

thermal conductivity detector (TCD) and carbosieve column.

The concentration of ethanol in the liquid condensate was

determined using a Flame Ionization Detector (FID), after

separation through a Porapaq-Q column. The response factors

for all species were calculated, and the system was calibrated

with appropriate standards before each sample run. To eval-

uate the catalyst performance, ethanol conversion (XEtOH) and

selectivity to a product gas (Sx) were determined as follows:

XEtOH ¼ MolesEtOH ðinÞ �MolesEtOH ðoutÞMolesEtOH ðinÞ

� 100

Sx ¼ Moles of gas x in gaseous product streamTotal moles of all gases in the gaseous product stream

� 100

In situ DRIFT analysis

DRIFT spectra were recorded using a Nicolet 8700 spectrom-

eter equipped with a DTGS-TEC detector and the data were

analyzed on OMNIC software. A Harrick reaction chamber

(HVC-DRP) fitted with ZnSe windows served as the reaction

cell for in-situ catalytic ethanol steam reforming experiments.

This chamber, which is used in conjunction with the Praying

Mantis diffuse reflection accessory, allows diffuse reflection

measurements under controlled pressures and a wide range

of temperatures. A thermocouple mounted in this cell allows

direct measurement of sample temperature. Catalyst sample

(~50 mg) was placed inside the reaction cell and typically 128

scans were collected at a resolution of 4 cm�1 and a data

spacing of 1.928 cm�1 in order to achieve sufficient signal to

noise ratios. Prior to analysis, the sample was reduced under

H2 atmosphere by ramping at 10 �C min�1 and held at 600 �C

Please cite this article in press as: Sharma PK, et al., Hydrogen genercatalysts: A comparative study, International Journal of Hydrogen En

for 2 h and subsequently cooled to room temperature under

inert atmosphere of Ar. A background spectrum of the

reduced catalyst was taken at room temperature before

introducing the reactant mixture.

For investigating in situ ethanol steam reforming reactions

over the surface of prepared catalysts, the catalyst surface

was saturated with reactant molecules by bubbling Ar gas

(~20mL/min) through a saturator filled with the feed (ethanol-

to-water molar ratio 1:6) for 1 h at room temperature. DRIFT

spectrawere then recorded at increasing temperatures 50 �Ce

600 �C, while maintaining a continuous flow of the Ar (~20 ml/

min), through a saturator filled with the feed, over the entire

temperature range. For this purpose, the sample was heated

at 10 �C/min and maintained for 5 min at the desired tem-

perature prior to recording of spectra.

Results and discussion

Catalyst characterization

The surface area of the fresh samples of support materials,

before and after metal loading, were determined by per-

forming nitrogen adsorptionedesorption experiments and the

obtained isotherms are presented in Fig. 1. The BET surface

areas of the supports before and after Rh loading are pre-

sented in Table 1. It can be observed that there is a substantial

difference in the BET surface areas, Al2O3 possessing

approximately six times larger surface area as compared to

CeZrO2. As expected, loading with Rh leads to a decrease in

the surface area of both the supports, the extent of decrease

being much larger for Al2O3. The pore size distribution was

determined using BarretteJoynereHalenda (BJH)method from

the desorption branch (inset, Fig. 1) and are presented in Table

1, which revealed that the materials exhibit a pore size dis-

tribution in the mesoporous range (2e50 nm). It can also be

noted that CeZrO2 and its Rh analog possess pores which are

bigger in size as compared to those of Al2O3 and its Rh analog.





Table 1 e Physico-chemical properties of support materials before and after metal loading.

Support/Catalyst

BETsurface area

(m2g�1)

Porevolume(cm3g�1)

Averagepore size

(nm)

COuptake

(mmolg�1)

Active metaldispersion

(%)

Active metalsurface area(m2gmetal

�1)

aRhaverage

particle size(nm)

bCeZrO2

Crystallitesize(nm)

Al2O3 261.64 0.23 3.35 e e e e

Rh/Al2O3 9.91 0.10 2.62 102.27 25.96 114.26 3.53 e

CeZrO2 42.05 0.160 11.89 e e e e 9.86

Rh/CeZrO2 36.81 0.122 10.64 351.77 93.05 409.58 0.98 10.46

a Calculated using CO pulse chemisorption.b Calculated using powder XRD.


The cumulative CO uptake, active metal dispersion and

metallic surface area are presented in Table 1. For metallic

surface area calculations, a stoichiometric factor of 1 was

employed, characteristic of linear chemisorption conforma-

tion (single CO per metal particle) [29]. It can be seen that the

cumulative CO uptake, active metal dispersion and metallic

surface area are higher for Rh/CeZrO2 catalyst as compared to

that for Rh/Al2O3. The average crystallite size of Rh-metal

(Table 1) was found to be higher for Rh/Al2O3 catalyst as

compared to that for Rh/CeZrO2 catalyst.

The crystalline phases of Al2O3 and CeZrO2 support both

before and after Rh loadingwere identified by x-ray diffraction

studies of fresh samples, and the patterns are presented in

Fig. 2. PXRD pattern confirms the existence of Al2O3 in the g

phase, (JCPDS file 050-0741), which is reported to exhibit high

surface area [30]. In the PXRD patterns of CeZrO2 and Rh/

CeZrO2, the diffraction peaks at 2q values 28.62, 33.27, 47.87,

56.67, and 77.32 are associatedwith (111), (200), (220), (311) and

(331) crystal planes of the cubic fluorite structure of CeZrO2.

Absence of the diffraction peaks associated with pure zirconia

together with a downward shift in the 2q values suggests the

formation of a solid solution (JCPDS# 28-0271) where the Ce

lattice positions are replaced with Zr atoms. The appearance

of peaks centered at 2q values 44.37 and 64.57 can be attrib-

uted to the formation of some amount of non-stoichiometric

oxides of Ce and Zr (JCPDS#88-2392). The average crystallite

sizes of CeZrO2 as determined using the Scherrer equation

Fig. 2 e Powder XRD patterns of support materials before

and after metal loading.


indicates a slight increase in the crystallite size after Rh

loading (Table 1). The PXRD patterns of both Rh/Al2O3 and Rh/

CeZrO2 catalysts are dominated by the patterns of the support

materials only and the peaks pertaining to Rh species could

not be identified, probably due to amorphous nature of the

noble metal oxide.

The TPR profiles of fresh samples of both the support ma-

terials before and after Rh impregnation are presented in

Fig. 3. Alumina exhibits negligible H2 consumption in the

temperature range studied [31]. Therefore, the peaks present

in the TPR profile of Rh/Al2O3 catalyst may be assigned solely

to the reduction of different RhOx species present. The first

broad peak with a maximum at 157 �C can be assigned to the

reduction of RhOx species of either different sizes or inter-

acting differently with the support [32,33]: the bigger the

particle (or the stronger the interaction with the support), the

higher the reduction temperature. The next broad peak cen-

tred at around 410 �C has been reported [34] for a Rh-

hydrotalcite type of material. Finally, a weak hump at very

high temperature, centred at 765 �C can be attributed to the

reduction of [Rh(AlO2)y] type of rhodium aluminate species.

The reduction profile of CeZrO2 (Fig. 3) revealed two peaks.

Since ZrO2 is reportedly non-reducible under the conditions

employed in the present investigation [23,35], the first peak

with a maximum at 546 �C may be assigned due to reduction

of surface layer of CeO2 and another centred at 761 �C due to

reduction of CeO2 in bulk phase [36]. Higher intensity of the

Fig. 3 e TPR profiles of support materials before and after

metal loading.






low temperature peak indicates that replacement of Ce with

Zr renders a larger fraction of the lattice reducible at lower

temperatures [36]. In the TPR profile of Rh/CeZrO2 (Fig. 3), it

can be observed that the reduction of Rh takes place at much

lower temperature (~80 �C) and the broad hump near 742 �Ccan be attributed to the reduction of bulk CeZrO2 [37]. It ap-

pears that the dispersed RhOx species interact very weakly

with the CeZrO2 support, which permits easy reduction of the

supported species resulting in the formation of metallic Rh.

The NH3-TPD profiles of Al2O3 and CeZrO2 are presented in

Fig. 4. The profiles are indicative of the considerable hetero-

geneity of the surface in terms of different acidic site types

and densities. The appearance of peak maxima for Al2O3 at

comparatively higher temperature indicates the presence of

acidic sites of higher strength in comparison to CeZrO2. In

addition, the acidity of Al2O3 (1.99 mmol g�1) was found to be

1.5 times larger than that of CeZrO2 (mmol.g�1), which indi-

cate that Al2O3 contains acidic sites of not only higher

strength but also of higher density as compared to that of

CeZrO2. The acidity of Rh/Al2O3 (2.51 mmol g�1) was found to

be much higher as compared to that of Rh/CeZrO2 catalyst

(mmol.g�1).

Performance evaluation of Rh/Al2O3 and Rh/CeZrO2

catalysts towards ESR

It has been reported [38] that the reaction pathway for ethanol

steam reforming, ESR (R1) [C2H5OH þ 3H2O / 6H2 þ 2CO2] is

rather complex and comprises of several secondary reactions

(R2-R11), as summarized in Fig. 5. Being an endothermic re-

action (DH� ¼ 173.2 kJ mol�1 at 298.15 K), ESR (R1) is favored at

high temperatures and low pressures. The reaction tempera-

ture can however be lowered by the choice of a suitable

catalyst. Two possible pathways for this reaction are dehy-

dration of ethanol leading to ethylene (R5) and dehydrogena-

tion of ethanol yielding acetaldehyde (R4). Since ethylene is

known to be a very strong coke precursor (R12), it is generally

preferred that the reaction should proceed via dehydrogena-

tion route (R4).

Fig. 4 e Ammonia temperature programmed desorption

(NH3-TPD) profiles of support materials before and after

metal loading.


To establish the relationship between process parameters

(temperature, pressure and feed composition) and the product

distribution, equilibrium thermodynamic analysis of ESR was

performed. Based on the results of this analysis and to eval-

uate the prepared Rh/Al2O3 and Rh/CeZrO2 catalysts for ESR at

various operating conditions, catalytic performance was

investigated under selected conditions of different tempera-

tures (450, 500, 550 & 600 �C) and feed flow rates (0.1, 0.2 &

0.3 ml/min) with a feed compositions of 1:6 ethanol-to-water

molar ratio. The results obtained in terms of ethanol conver-

sion and product selectivities as a function of temperature at

different feed flow rates are presented in Table 2.

It can be observed that for ESR reaction catalyzed by both

the catalysts EtOH conversion increases with increasing

temperature and almost complete conversion could be ach-

ieved at T > 550 �C. From Table 2 it is also clear that increasing

the feed flow rate led to a decrease in ethanol conversion at all

temperatures and this effect is more pronounced at lower

temperatures. Such a decrease in conversion may be under-

stood in terms of progressive decrease in contact time be-

tween the reactants and the catalyst with increasing feed flow

rate. At higher temperatures this effect of decreasing contact

time is shrouded to a certain extent by the increased kinetic

energy of reactants.

Under the reaction conditions employed during ethanol

steam reforming, the exit product stream comprised only of

H2, CO2, CO and CH4. It may therefore be concluded that, as far

as EtOH conversion and product distribution is concerned,

both the Rh catalysts act independently of the nature of the

support material as reported earlier [19]. The selectivity of

these products as a function of temperature at different flow

rates (0.1, 0.2 and 0.3 ml.min�1) are also presented in Table 2

together with the theoretically calculated values determined

using thermodynamic equilibrium conditions.

Another important observation worth noting from Table 2

is that differences between the predicted and experimental

selectivities are more pronounced at lower temperatures and

at T ~ 600 �C, the selectivity values tend to come closer to each

other as well as to the equilibrium values.

Fig. 5 e Schematic showing different reactions of ESR

process; DHr0 values (kJ.mol¡1), given in parenthesis, have

been calculated for gaseous phase of reactants and

products.





Table

2e

Activityandse

lectivityofRh/A

l 2O

3andRh/C

eZrO

2toward

seth

anolsteam

reform

ing.

Tem

p.

(�C)

aFee

dflow

rate

(mL/m

in)

Rh/A

l 2O

3Rh/C

eZrO

2Therm

odynamic

equilibrium

bAMFR

(mL/m

in)

XEtO

H(%

)SH2(%

)SCO2(%

)SCO(%

)SCH4(%

)XEtO

H(%

)SH2(%

)SCO2(%

)SCO(%

)SCH4(%

)SH2(%

)SCO2(%

)SCO(%

)SCH4(%

)Rh/

Al 2O

3

Rh/

CeZ

rO2

450

0.1

98.77

38.9

28.68

1.86

30.56

99.05

41.28

26.72

1.55

30.45

50.74

24.08

1.23

23.95

43

49

0.2

98.21

43.01

27.15

4.44

25.4

96.04

43.81

27.0

2.62

26.57

73

93

0.3

85.25

44.91

26.29

6.06

22.74

90.04

47.2

26.58

3.44

22.78

125

145

500

0.1

99.46

46.74

28.01

1.45

23.72

99.2

47.88

27.59

2.17

22.35

59.53

22.94

2.75

14.78

58

65

0.2

99.23

44.75

26.95

6.44

21.86

96.42

51.83

26.68

2.55

18.93

121

129

0.3

96.16

46.82

25.81

7.7

19.67

95.1

52.25

26.8

3.35

17.6

177

189

550

0.1

99.68

54.7

27.7

6.24

11.36

99.87

57.09

25.6

3.3

14.01

65.79

21.14

5.14

7.92

62

66

0.2

99.52

52.5

26.53

7.84

13.13

99.62

57.48

25.0

4.18

13.34

126

140

0.3

99.48

51.06

25.2

8.49

15.25

97.21

58.05

25.0

4.69

12.26

180

202

600

0.1

99.96

58.05

23.2

8.7

10.05

99.91

62.7

24.5

5.03

7.53

69.65

18.99

8.01

3.34

75

85

0.2

99.88

59.1

24.74

7.99

8.11

99.97

62.79

24.4

5.84

7.37

158

170

0.3

99.96

59.26

24.15

8.5

8.41

99.99

62.9

24.0

6.55

7.4

236

263

aFeedco

mposition(EtO

H-to-w

atermolarratioof1:6).

bAveragemass

flow

rate

ofexit

gase

s.



In line with thermodynamic predictions, with increasing

temperature, selectivity towards H2 and CH4 should increase

and decrease respectively, irrespective of the type of catalyst

used for reforming. At 600 �C, selectivity to H2 reaches its

maximum value while selectivity to CH4 reaches its minimum

value for all flow rates and themain products remain to be CO2

and CO togetherwith H2, implying thatmost of the H-atoms of

ethanol participate in hydrogen production. Our study in-

dicates that both the catalysts exhibit their optimal perfor-

mance at 600 �C~.Further, thermodynamic analysis predicts that the selec-

tivity to CO2 should decrease while to CO should increase with

increasing temperature. The same is evident from the exper-

imental results of both the catalysts at all flow rates. This

concomitance indicates that as the temperature increases, in

view of the exothermic nature of WGS (R10) reaction, both the

catalysts prepared favors reverse of WGS reaction (R10) con-

verting CO2 to CO, the extent of which being slightly higher for

Rh/Al2O3 catalysts.

The absence of any C2 product, particularly ethylene over

both the catalysts is indicative of the fact that during these

reaction conditions dehydrogenation route (R4) of ESR is being

favored over dehydration (R5) route and thismay be attributed

to the inherent lower acidity of the both the supports prepared

as discussed above in characterization section under TPD. The

absence of acetaldehyde in the product stream may be

attributed to comparatively faster decomposition (R6) or

transformation (R7) of acetaldehyde as compared to its for-

mation through dehydrogenation step (R4). The absence of

both ethylene as well as acetaldehyde in the reformer exit

stream lead to conclude that primarily ethanol steam

reforming (R2), which is a combination of ethanol decompo-

sition (ED) (R3) and water gas shift (WGS) (R10) reaction, pre-

vails over the surface of these catalysts. The increase in

selectivity towards H2 (Table 2) with increasing temperature,

at all flow rates, can be attributed tomethane steam reforming

(MSR) (R8) reaction together with ED (R3) reaction. A simulta-

neous decrease in selectivity to CH4 (Table 2) supports this

fact.

Highest ethanol conversion (XEtOH) and selectivity towards

H2 (SH2) were obtained for both Rh/Al2O3 and Rh/CeZrO2 cat-

alysts under optimal temperature of 600 �C and flow rate of

0.3 ml.min�1. It can be observed that although complete

conversion of ethanol is effected over both the catalysts, the

hydrogen selectivity of Rh/CeZrO2 is slightly higher (62.9%) as

compared to Rh/Al2O3 (59.3%), which suggests the superiority

of the former. This superiority is further supported by the

values of exit flow rates of exit gases summarized in Table 2,

which reveals that the amount of product gases is ~10% higher

in the presence of Rh/CeZrO2. In view of the complete con-

version of ethanol over the surface of both the catalysts, it can

be concluded that the reactants undergo transformation to

other undesired carbonaceous species, e.g. coke, in the case of

Rh/Al2O3.

Reaction mechanism using in-situ DRIFT spectroscopy

In order to find out the possible reason behind the apparent

differences in the activity of Rh/Al2O3 and Rh/CeZrO2 cata-

lysts, the sequence of reactions was followed by identifying






the intermediates and products formed over the surface of

both catalysts using in-situ DRIFT spectroscopic technique

during catalytic ESR reaction conditions. Different vibrational

modes of these species were assigned based on reported

studies. The changes in the DRIFT spectra, as a function of

temperature are presented in Fig. 6 (a and b).

A broad band (3650e3050 cm�1) in the form of a hump is

observed in the DRIFT spectra (50 �C), the intensity of which is

higher for Rh/CeZrO2. This band indicates the presence of

adsorbed water [39] on the surface of the catalysts. Disap-

pearance of this band above 100 �C is suggestive of either

thermal desorption of this water from the catalyst's surface or

its participation in reforming reactions. In the case of Rh/Al2O3

catalyst, this ease of water desorption may be an indicative

[40] of the lower acidity of the prepared Al2O3 support.

Weak absorptions centered at 1253 cm�1 (Rh/Al2O3 cata-

lyst) [41,42] and 1226 cm�1 (Rh/CeZrO2 catalyst) [43] can be

attributed to the angular vibration [d(OH)] of physically

adsorbed ethanol molecules held to the Lewis acid sites of the

surfaces of both catalysts. This band disappears at tempera-

tures close to 300 �C. In view of the dipoleedipole interactions,

ethanol molecules are expected to exhibit acidebase interac-

tion between H atoms of ethanol and OH sites (and/or coor-

dinatively unsaturated O-sites) available on the support

surface. As a result of this interaction EtOH molecules further

undergo dissociation to result in surface ethoxide and water

(and/or hydroxyl species) [42,44e46]. The presence of these

ethoxide species was confirmed by the appearance of ab-

sorption peak at ~1086 cm�1 and ~1052 cm�1 in Rh/Al2O3

catalyst [22] and at ~1093 cm�1 and ~1053 cm�1 in Rh/CeZrO2

Fig. 6 e In-situ DRIFT analysis during ethanol steam refo


catalyst [41,47e50], which is indicative of ethoxide linkage

with the surface in both mono-dentate as well as bi-dentate

fashions, respectively. On increasing the temperature, these

bands gradually decrease in intensity and completely disap-

pear above 400 �C (Rh/Al2O3 catalyst) or 300 �C (Rh/CeZrO2

catalyst). This may be attributed either to further trans-

formation of ethoxides into other surface species or partly to

their thermal desorption in the form of ethanol from the

surface.

Other medium intensity bands observed in the case of Rh/

Al2O3 catalyst at lower temperatures may be assigned to CeH

stretching and bending vibrationalmodes of surface ethoxide/

ethanol species: 2972 cm�1 [nas(CH3)], 2926 cm�1 [nas(CH2)],

2896 cm�1 [ns(CH3)], 1417 cm�1 [ds(CH3)] and 1385 cm�1 [u(CH2)]

[51]. Similarly in the case of Rh/CeZrO2 catalyst, weak in-

tensity bands observed at 50 �C may be assigned to CeH

stretching and bending vibrationalmodes of ethoxide/ethanol

species: 2980 cm�1 [nas(CH3)], 2932 cm�1 [nas(CH2)], 2902 cm�1

[ns(CH3)], 2879 cm�1 [ns(CH2)], 1457 cm�1 [das(CH2)], 1420 cm�1

[ds(CH3)] and 1388 cm�1 [u(CH2)]. Intensity of these CeH bands

decreases in both the catalysts with increasing temperature

and finally disappears above 300 �C.For Rh/Al2O3 catalyst, bands pertaining to surface acetate

([na(OCO)] and [ns(OCO)] vibrations at 1552 and 1454 cm�1,

respectively) [51] and formate ([na(OCO)] and [ns(OCO)] vibra-

tions at 1586 and 1341 cm�1, respectively) [52] are visible at

50 �C. On the contrary, there appears a band pertaining to

surface acetyl (at 1635 cm�1) species together with acetate

species ([na(OCO)] and [ns(OCO)] vibrations at 1582 and

1436 cm�1, respectively) in the case of Rh/CeZrO2 catalyst. It

rming over (a) Rh/Al2O3; and (b) Rh/CeZrO2 catalysts.






appears that the presence of Rh over the surface of CeZrO2

induces successive removal of hydrogen atoms from ethoxide,

which in turn produces intermediates, particularly acetalde-

hyde and acetyl species, which on oxidation gives rise to

surface acetate species. For the Al2O3 supported catalysts ESR

reaction has been reported [53e55] to follow either acetate

driven or formate driven route which is governed by the

acidity of catalyst. Presence of both acetates and formate in-

termediates in the present case indicates that the nature of

catalyst favors both the routes.

Another visible difference in the DRIFT spectra of two

catalysts at 50 �C is the presence of a weak intensity band at

2018 cm�1 attributed to the linearly adsorbed CO species

[n(CO)linear] on the surface of Rh/CeZrO2 catalyst. Presence of

these species at such low temperatures clearly reveals higher

activity of Rh/CeZrO2 catalyst in the decomposition of ethanol

to form CO, CH4 and H2 after CeH and CeC bond fissions as

compared to its Rh/Al2O3 counterpart.

On increasing the temperature to 100 �C, no appreciable

change in the DRIFT spectra of Rh/Al2O3 catalyst are apparent,

except slight increase in the intensity of bands corresponding

to surface acetates and formates. Similarly in case of Rh/

CeZrO2 catalyst, the intensity of acetate and linearly adsorbed

CO species increases but that of acetyl absorption decreases.

The [n(CO)linear] band blue shifts from 2018 cm�1 to 2028 cm�1

and a new band emerges at 1800 cm�1, which may be attrib-

uted to the stretching vibrations of bridged CO species,

[n(CO)bridge]. This is again suggestive of ethanol decomposition

to H2, CO and CH4 through acetyl and/or acetaldehyde

intermediates.

In the presence of Rh/Al2O3 catalyst, at 200 �C, intensity of

CeH vibrations related with ethoxide/ethanol decreases while

those of acetates and formates increase slightly with minor

shift in their positions. A new band, at 1407 cm�1, develops

whichmay be assigned to CeH vibrations related with acetate

species. Important development appears in terms of new

bands at 2028, 1817 and 1675 cm�1. Of these bands the former

two can be assigned to stretching vibrations of linearly

adsorbed and bridge-bonded surface CO species [51,55]

[n(CO)linear], [n(CO)bridge], respectively while the latter at

1675 cm�1 has been attributed to [n(CO)acetyl] of acetyl species

[51]. Another noticeable observation at this temperature is the

appearance of vibrational bands pertaining to gaseous

methane (3019 cm�1) and CO2 (2361 and 2322 cm�1). Under

similar conditions, in the presence of Rh/CeZrO2 catalyst, the

intensity of bands pertaining to both linearly and bridged CO

species sharply increases. The acetyl band disappears

completely and the acetate bands become clearly visible

although with reduced intensity.

In the presence of Rh/Al2O3 catalyst, at 300 �C, intensity of

various bands increases including CeH vibration of acetate

species. This band loses its intensity above 500 �C with other

symmetric and asymmetric stretching vibrations of surface

acetate species. Further, increasing the temperature to 400 �Cincreases the intensity of gaseous methane and CO2 bands.

Intensity of linearly adsorbed CO vibration decreases and in

addition to bridge-bonded CO species, there appears multi-

coordinated CO species in the form of medium intensity

band at 1930 cm�1. Two new bands also emerge at 3733 and

1768 cm�1, which can be assigned to n(OeH) vibration of type


IIa hydroxyls [56] and n(CO) vibration of surface acetaldehydes

[51] respectively.

In the presence of Rh/CeZrO2 catalyst, at 300 �C, bands

associated with gaseous methane (at 3016 cm�1) [57] and CO2

(at 2362 and 2323 cm�1) starts appearing together with a

decrease in acetate bands at 1582, 1436 and 1388 cm�1. The

appearance of gaseous CH4 and CO2 may be a result of acetate

decomposition. The [n(CO)bridge] band red shifts from

1800 cm�1 to 1779 cm�1 and a new band in the form of

shoulder at 1868 cm�1 appears, indicating presence of multi-

coordinated CO species [n(CO)multi]. At 400 �C, intensity of

gaseous methane and CO2 bands increases, while the acetate

bands completely disappear. Additional bands at 1481 and

1365 cm�1 were observed, which may be attributed to the

presence of surface carbonate ðCO3�2Þ species. It can be

concluded that surface acetates, post de-methanation,

convert into surface carbonates which on decomposition

gives rise to gaseous CO2 species. It is particularly interesting

to note the presence of three different kinds of surface CO

species on the catalyst surface.

In the DRIFT spectrum recorded at 500 �C (not shown in Fig.

6), over Rh/Al2O3 catalyst, the acetyl band disappears instan-

taneously. The intensity of bands pertaining to gaseous

methane, surface acetates and formates decreases slightly.

Whereas, the intensity of bands representing gaseous CO2 and

type IIa hydroxyls increases. But over Rh/CeZrO2 catalyst, the

intensity of gaseous methane and CO2 bands decreases

slightly. At the same time the intensity of other bands asso-

ciated with surface carbonate and CO species, remain largely

unaffected.

Increasing the temperature to 600 �C brings a further in-

crease in gaseous CO2 and type IIa hydroxyl bands in case of

Rh/Al2O3 catalyst. It also decreases the intensity of bands of

surface acetates, formates and gaseous methane. The only

observable species are CH4, CO2, and linearly adsorbed CO,

other than some carbonaceous species in the form of weak

bands in the region 1500e1400 cm�1. Whereas over Rh/

CeZrO2, at 600 �C, the intensity of bands associated with sur-

face carbonate and CO species slightly decrease and the band

at 3016 cm�1 [n(CH4)] disappears completely, which is indica-

tive of higher activity of Rh/CeZrO2 catalyst in MSR reaction

(R8 or R9). A simultaneous decrease in intensity of gaseous

CO2 bands may be indicative of similar activity of this catalyst

in reverse WGS reaction (R10) predominating at higher tem-

peratures. These facts support the decreasing selectivity to-

wards CO2 and CH4 with increasing temperature during

ethanol steam reforming reaction over Rh/CeZrO2 catalyst.

Stability test and coke analysis

The catalysts prepared were evaluated for their application in

ESR for a period of 20 h time-on-stream and both were found

to exhibit negligible variation in activity in terms of ethanol

conversion and product selectivities. Quantitative and quali-

tative determination of the coke deposited over the surface of

both the spent catalysts (after 20 h of ESR at 600 �C,0.3 mL min�1 and EtOH:H2O::1:6) was performed using TPO

analyses, the results of which are presented in Fig. 7. The TPO

profiles of spent Rh/Al2O3 and Rh/CeZrO2 catalysts exhibit a

broad peak, attributed to the oxidation of coke deposited, with





Fig. 7 e TPO profiles of coke deposited after 20 h run of ESR

reaction over the surface of rhodium catalysts supported

over CeZrO2 and Al2O3 materials.


a maximum roughly at 586 �C and 615 �C, respectively. Thetemperature range associated with the oxidation is indicative

of the amorphous nature [26] of coke deposited mainly on the

support [58]. Total consumption of oxygen associated

with these peaks was 10.57 mmol gcatalyst�1 and

6.75 mmol gcatalyst�1, for spent Rh/Al2O3 and Rh/CeZrO2 cata-

lysts, respectively, which clearly indicates that the amount of

coke deposited over Rh/Al2O3 catalyst is larger (1.5 �) than for

spent Rh/CeZrO2 catalyst, which in turn may be attributed to

the comparatively acidic nature of Al2O3 [59]. Comparatively

lower amount of coke deposited on the surface of spent Rh/

CeZrO2 may also be ascribed to the oxygen storage capacity

(OSC) of the ceria, which has been reported to aid the gasifi-

cation of coke deposited over active sites by activating the

oxidationereduction cycle. Further, the presence of zirconia

reportedly increases the OSC of ceria [36]. Coke formation

during catalytic ESR reportedly occurs by either decomposi-

tion of ethylene (R12) or dissociation of CO (R11), i.e Boudouard

reaction (Fig. 5) [60]. The absence of ethylene in the product

stream as well as during DRIFT studies, indicates the larger

contribution of Boudouard reaction (R11) towards coking

irrespective of the nature of support. The larger amount of

coke formed over the surface of Rh/Al2O3 also supports the

higher amount of gaseous products formed over Rh/CeZrO2 as

discussed earlier (Table 2).

Conclusion

The catalytic activity of Rh loaded over two different supports,

Al2O3 and CeZrO2, towards ethanol steam reforming was

studied. Both the Rh catalysts are efficient systems for

hydrogen generation by ESR reaction at 600 �C. Complete

conversion of ethanol (99.9%) could be effected over the sur-

face of both the catalysts, however the amount of gaseous

products, comprising primarily of H2, CO, CO2 and CH4 was

found to be higher in the case of Rh/CeZrO2. Further, the

amount of coke deposited over Rh/Al2O3 was Rh/CeZrO2 as

established by TPO studies. Under the experimental


conditions employed, ethylene was not detected in the prod-

uct stream, irrespective of the nature of support system used.

Further, formation of different products has been explained

on the basis of in situ DRIFT study. Our studies revealed that

the ethanol molecules, in the presence of Rh, follow dehy-

drogenation route, after dissociative adsorption on the cata-

lyst's surface in the form of ethoxides. These successively

dehydrogenate into acetaldehyde and acetyl species over the

surface of both the catalysts. In the presence of Al2O3, these

acetaldehyde and acetyl species through acetate or formate

driven route directly convert into H2, CO, CO2 and CH4. How-

ever, in the presence of Rh/CeZrO2, acetaldehyde and acetyl

species first oxidise into acetates, which on decomposition

through surface carbonates convert into H2, CO, CO2 and CH4.

The prepared catalyst was found to be stable up to 20 h in

catalytic ethanol steam reforming for hydrogen production.

Acknowledgement

The authors are grateful to Director, CFEES for providing the

laboratory facilities. The authors are also thankful to Akhilesh

Pandey, SSPL, Delhi for carrying out XRD analyses.

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