CeZrO2 catalysts: A comparative study

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Hydrogen generation by ethanol steam reforming over 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

article info

abstract

Article history:

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

Received 30 August 2015

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

Accepted 28 September 2015

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

Available online xxx

flow rates (0.1, 0.2 and 0.3 mL min1) 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

Keywords:

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

Rhodium catalysts

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

Ethanol steam reforming

(263 mL min1) as compared to Rh/Al2O3 catalyst (236 mL min1). In-situ Diffuse Reflec-

DRIFT

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

Mechanism

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

Coke

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 alternative 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

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

* Corresponding author. Tel.: þ91 11 23907146, þ91 11 23907189; fax: þ91 11 23819547. E-mail addresses: [email protected], [email protected] (P.K. Sharma). http://dx.doi.org/10.1016/j.ijhydene.2015.09.137 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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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 utilized 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 towards 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 activation 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 on mechanistic 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-

precipitation technique described previously for zirconia based compositions [26]. Al2O3 was prepared by a combination of homogeneous urea precipitation technique and selfpropagating 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 experiments 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 (Micromeritics 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 equilibrated 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 min1 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 impurities. 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 desorption was performed by heating the sample at 10
C min1 up to 900
C. Temperature programmed oxidation (TPO) studies were also performed on the catalyst samples, spent in ESR reactions, by oxidizing the sample (~15 mg) from 50
C to 1000
C at 10
C min1 under continuous flow of O2eHe (10%v/v) mixture (25 ml min1). Powder X-ray diffraction studies were carried out to identify the crystalline phases of prepared materials, on a Philips PANanalytical Pro-HRXRD diffractometer using CuKa radiation (l ¼ 1.54
A). The samples were first pelletized and the data were collected over the range 2q ¼ 20e80
. The average crystallite size were estimated by Scherrer equation using full width at half maximum (FWHM) of corresponding highest intensity diffraction peaks.

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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Performance evaluation of catalysts in ESR reaction Performance of the prepared Rh/CeZrO2 and Rh/Al2O3 catalysts 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 min1) with a feed of 1:6 ethanolto-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 (Microactivity 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. Subsequently, N2 was purged for 30 min at 10 mL/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 evaluate the catalyst performance, ethanol conversion (XEtOH) and selectivity to a product gas (Sx) were determined as follows: XEtOH ¼

Sx ¼

MolesEtOH ðinÞ  MolesEtOH ðoutÞ  100 MolesEtOH ðinÞ

3

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 (~20 mL/min) through a saturator filled with the feed (ethanolto-water molar ratio 1:6) for 1 h at room temperature. DRIFT spectra were then recorded at increasing temperatures 50
C e 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 temperature 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 performing 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 presented 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 distribution 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.

Moles of gas x in gaseous product stream Total moles of all gases in the gaseous product stream  100

In situ DRIFT analysis DRIFT spectra were recorded using a Nicolet 8700 spectrometer 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 cm1 and a data spacing of 1.928 cm1 in order to achieve sufficient signal to noise ratios. Prior to analysis, the sample was reduced under H2 atmosphere by ramping at 10
C min1 and held at 600
C

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).

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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

BET surface area (m2g1)

Pore volume (cm3g1)

Average pore size (nm)

CO uptake (mmolg1)

Active metal dispersion (%)

Active metal surface area (m2gmetal 1 )

Al2O3 Rh/Al2O3 CeZrO2 Rh/CeZrO2

261.64 9.91 42.05 36.81

0.23 0.10 0.160 0.122

3.35 2.62 11.89 10.64

e 102.27 e 351.77

e 25.96 e 93.05

e 114.26 e 409.58

a b

a Rh average particle size (nm)

b CeZrO2 Crystallite size (nm)

3.53 e 0.98

e e 9.86 10.46

Calculated using CO pulse chemisorption. 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 conformation (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 loading were 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 associated with (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 attributed 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

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 materials 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 interacting 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 centred at around 410
C has been reported [34] for a Rhhydrotalcite 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. 2 e Powder XRD patterns of support materials before and after metal loading.

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

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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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 reaction (DH
¼ 173.2 kJ mol1 at 298.15 K), ESR (R1) is favored at high temperatures and low pressures. The reaction temperature can however be lowered by the choice of a suitable catalyst. Two possible pathways for this reaction are dehydration of ethanol leading to ethylene (R5) and dehydrogenation 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 dehydrogenation route (R4).

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 evaluate the prepared Rh/Al2O3 and Rh/CeZrO2 catalysts for ESR at various operating conditions, catalytic performance was investigated under selected conditions of different temperatures (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 conversion 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 achieved 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 understood in terms of progressive decrease in contact time between 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.min1) 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. 4 e Ammonia temperature programmed desorption (NH3-TPD) profiles of support materials before and after metal loading.

Fig. 5 e Schematic showing different reactions of ESR process; DHr 0 values (kJ.mol¡1), given in parenthesis, have been calculated for gaseous phase of reactants and products.

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
C can be attributed to the reduction of bulk CeZrO2 [37]. It appears 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 heterogeneity 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 g1) was found to be 1.5 times larger than that of CeZrO2 (mmol.g1), which indicate 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 g1) was found to be much higher as compared to that of Rh/CeZrO2 catalyst (mmol.g1).

Performance evaluation of Rh/Al2O3 and Rh/CeZrO2 catalysts towards ESR

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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49 93 145 65 129 189 66 140 202 85 170 263 3.34 18.99 69.65

8.01

7.92 21.14 65.79

5.14

22.94 59.53

2.75

14.78

43 73 125 58 121 177 62 126 180 75 158 236 23.95 1.23 24.08 50.74

30.45 26.57 22.78 22.35 18.93 17.6 14.01 13.34 12.26 7.53 7.37 7.4 1.55 2.62 3.44 2.17 2.55 3.35 3.3 4.18 4.69 5.03 5.84 6.55 26.72 27.0 26.58 27.59 26.68 26.8 25.6 25.0 25.0 24.5 24.4 24.0 41.28 43.81 47.2 47.88 51.83 52.25 57.09 57.48 58.05 62.7 62.79 62.9 99.05 96.04 90.04 99.2 96.42 95.1 99.87 99.62 97.21 99.91 99.97 99.99 30.56 25.4 22.74 23.72 21.86 19.67 11.36 13.13 15.25 10.05 8.11 8.41 b

a

600

550

500

450

Feed composition (EtOH-to-water molar ratio of 1:6). Average mass flow rate of exit gases.

1.86 4.44 6.06 1.45 6.44 7.7 6.24 7.84 8.49 8.7 7.99 8.5 28.68 27.15 26.29 28.01 26.95 25.81 27.7 26.53 25.2 23.2 24.74 24.15 38.9 43.01 44.91 46.74 44.75 46.82 54.7 52.5 51.06 58.05 59.1 59.26 98.77 98.21 85.25 99.46 99.23 96.16 99.68 99.52 99.48 99.96 99.88 99.96 0.1 0.2 0.3 0.1 0.2 0.3 0.1 0.2 0.3 0.1 0.2 0.3

Rh/ CeZrO2 Rh/ Al2O3 SCH4 (%) SCO (%) SCO2 (%) SH2 (%) SCO2 (%) XEtOH (%)

SH2 (%)

SCO2 (%)

SCO (%)

SCH4 (%)

XEtOH (%)

SH2 (%)

Rh/CeZrO2 Rh/Al2O3

Feed flow rate (mL/min)

a

Temp. (
C)

Table 2 e Activity and selectivity of Rh/Al2O3 and Rh/CeZrO2 towards ethanol steam reforming.

SCO (%)

SCH4 (%)

Thermodynamic equilibrium

b

AMFR (mL/min)

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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 the main products remain to be CO2 and CO together with H2, implying that most of the H-atoms of ethanol participate in hydrogen production. Our study indicates that both the catalysts exhibit their optimal performance at 600
C~. Further, thermodynamic analysis predicts that the selectivity to CO2 should decrease while to CO should increase with increasing temperature. The same is evident from the experimental 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) converting 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 this may 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 formation 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 decomposition (ED) (R3) and water gas shift (WGS) (R10) reaction, prevails over the surface of these catalysts. The increase in selectivity towards H2 (Table 2) with increasing temperature, at all flow rates, can be attributed to methane steam reforming (MSR) (R8) reaction together with ED (R3) reaction. A simultaneous 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 catalysts under optimal temperature of 600
C and flow rate of 0.3 ml.min1. 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 conversion 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 catalysts, the sequence of reactions was followed by identifying

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

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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 cm1) 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. Disappearance 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 cm1 (Rh/Al2O3 catalyst) [41,42] and 1226 cm1 (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 temperatures close to 300
C. In view of the dipoleedipole interactions, ethanol molecules are expected to exhibit acidebase interaction between H atoms of ethanol and OH sites (and/or coordinatively 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 absorption peak at ~1086 cm1 and ~1052 cm1 in Rh/Al2O3 catalyst [22] and at ~1093 cm1 and ~1053 cm1 in Rh/CeZrO2

7

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 disappear above 400
C (Rh/Al2O3 catalyst) or 300
C (Rh/CeZrO2 catalyst). This may be attributed either to further transformation 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 vibrational modes of surface ethoxide/ ethanol species: 2972 cm1 [nas(CH3)], 2926 cm1 [nas(CH2)], 2896 cm1 [ns(CH3)], 1417 cm1 [ds(CH3)] and 1385 cm1 [u(CH2)] [51]. Similarly in the case of Rh/CeZrO2 catalyst, weak intensity bands observed at 50
C may be assigned to CeH stretching and bending vibrational modes of ethoxide/ethanol species: 2980 cm1 [nas(CH3)], 2932 cm1 [nas(CH2)], 2902 cm1 [ns(CH3)], 2879 cm1 [ns(CH2)], 1457 cm1 [das(CH2)], 1420 cm1 [ds(CH3)] and 1388 cm1 [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 cm1, respectively) [51] and formate ([na(OCO)] and [ns(OCO)] vibrations at 1586 and 1341 cm1, respectively) [52] are visible at 50
C. On the contrary, there appears a band pertaining to surface acetyl (at 1635 cm1) species together with acetate species ([na(OCO)] and [ns(OCO)] vibrations at 1582 and 1436 cm1, respectively) in the case of Rh/CeZrO2 catalyst. It

Fig. 6 e In-situ DRIFT analysis during ethanol steam reforming over (a) Rh/Al2O3; and (b) Rh/CeZrO2 catalysts. Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.137

8

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 e n e r g y x x x ( 2 0 1 5 ) 1 e1 1

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 acetaldehyde 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 intermediates 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 cm1 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 cm1 to 2028 cm1 and a new band emerges at 1800 cm1, which may be attributed 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 cm1, develops which may be assigned to CeH vibrations related with acetate species. Important development appears in terms of new bands at 2028, 1817 and 1675 cm1. 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 cm1 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 cm1) and CO2 (2361 and 2322 cm1). 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
C increases 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 multicoordinated CO species in the form of medium intensity band at 1930 cm1. Two new bands also emerge at 3733 and 1768 cm1, 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 cm1) [57] and CO2 (at 2362 and 2323 cm1) starts appearing together with a decrease in acetate bands at 1582, 1436 and 1388 cm1. The appearance of gaseous CH4 and CO2 may be a result of acetate decomposition. The [n(CO)bridge] band red shifts from 1800 cm1 to 1779 cm1 and a new band in the form of shoulder at 1868 cm1 appears, indicating presence of multicoordinated 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 cm1 were observed, which may be attributed to the presence of surface carbonate ðCO3 2 Þ 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 instantaneously. 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 associated with surface carbonate and CO species, remain largely unaffected. Increasing the temperature to 600
C brings a further increase 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 cm1. Whereas over Rh/ CeZrO2, at 600
C, the intensity of bands associated with surface carbonate and CO species slightly decrease and the band at 3016 cm1 [n(CH4)] disappears completely, which is indicative 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 temperatures. These facts support the decreasing selectivity towards 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 qualitative determination of the coke deposited over the surface of both the spent catalysts (after 20 h of ESR at 600
C, 0.3 mL min1 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

Please cite this article in press as: Sharma PK, et al., Hydrogen generation by ethanol steam reforming over Rh/Al2O3 and Rh/CeZrO2 catalysts: A comparative study, International Journal of Hydrogen Energy (2015), 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 e n e r g y x x x ( 2 0 1 5 ) 1 e1 1

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.

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conditions employed, ethylene was not detected in the product 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 dehydrogenation route, after dissociative adsorption on the catalyst'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. However, 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 a maximum roughly at 586
C and 615
C, respectively. The temperature 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 catalysts, 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 gasification 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 decomposition 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 surface 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

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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