Bio-ethanol catalytic steam reforming over supportedmetal catalysts

Fabien Aupreetre, Claude Descorme *, Daniel Duprez

Laboratoire de Catalyse en Chimie Organique (LACCO) – UMR 6503 CNRS, Universit�ee de Poitiers, Facult�ee des Sciences,40 avenue du Recteur Pineau, F-86022 Poitiers Cedex, France

Received 4 April 2002; accepted 29 April 2002

Abstract

Considering both the influence of the nature of the metal (Rh, Pt, Ni, Cu, Zn, Fe) and the role of the support

(c-Al2O3, 12%CeO2–Al2O3;CeO2;Ce0:63Zr0:37O2), CO2 is presented as a primary product in the bio-ethanol steam-re-

forming catalytic reaction (SRR) over some supported metal catalysts. Based on this unexpected observation, a new

strategy for maximizing the hydrogen production and minimizing the CO formation is proposed. Any highly selective

catalytic formulation should be free of any promoter in the water gas shift reaction (WGSR) which tends to equilibrate

the SRR gas towards higher CO concentrations. � 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

Since a few decades, increasing attention is be-ing paid to pollution-related environmental andpublic health problems. Particularly, as one of themajor contributor to the atmospheric pollution,the automotive sector had to work hard on pol-lution control. Both catalytic converters on gaso-line-fueled engines and filters for particulates ondiesel-fueled engines were implemented.

The next step, to overpass the forthcomingregulations on CO2 emissions according to theKyoto’s Protocol (in Europe, 8% reduction com-

pared to the 1990s emissions by 2008–2012), re-quires the development of both new engines andalternative fuels. . .

Fuel cells recently appeared as one solution inthe development of zero-emission electrical vehi-cle. Technical options range from batteries toliquid H2 tanks, high pressure H2 storage, me-dium pressure H2 storage on hydrides and nano-structured carbons and on-board H2 production[1–4].

This study is in keeping with this last approach.Bio-ethanol, presented as an environmentallyfriendly fuel (renewable, CO2 neutral, nontoxic. . .), was used as a fuel and the catalytic steamreforming reaction (SRR) was studied for H2 on-board production (1).

C2H5OHþ 3H2O ! 6H2 þ 2CO2

ðDH0r ¼ 173:1 kJ mol�1Þ ð1Þ

Catalysis Communications 3 (2002) 263–267

www.elsevier.com/locate/catcom

*Corresponding author. Tel.: +33-5-49-45-39-97; fax: +33-5-

49-45-34-99.

E-mail address: claude.descorme@univ-poitiers.fr (C. Desc-

orme).

1566-7367/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S1566-7367 (02 )00118-8

The goal was a high yield in H2 and a high selec-tivity towards CO2. In fact, CO, possibly producedin the SRR, is commonly described as a poison ofthe fuel cell electrodes made of noble metals.

A wide range of catalysts were tested usingdifferent metals (Rh, Pt, Pd, Ru, Ni, Cu, Zn, Fe)and various supports (Al2O3, 12%CeO2–Al2O3;CeO2;CeO2–ZrO2;ZrO2). The reason for such ascreening was to obtain a catalyst both (i) highlyactive in the SRR to maximize the ethanol con-version to hydrogen, that is a high yield in H2 and(ii) highly selective towards the formation of CO2

(low CO outlet concentration) both in the SRR(1)and in the water gas shift reaction (WGSR):

COþH2O ! CO2 þH2 ð2Þ

2. Experimental

A 12%CeO2–Al2O3 was prepared by impreg-nation of a c-Al2O3 support commercialized byProcatalyse (130 m2 g�1) with an aqueous solutionof CeðNO3Þ6ðNH4Þ2, dried overnight at 120 �C andcalcined at 500 �C under flowing dry air(30 cm3 min�1) for 5 h.

The catalysts were prepared by impregnationof the different supports using aqueous solutionsof the corresponding metal precursor salts(RhðNO3Þ3;PtðNH3Þ2ðNO2Þ2;PdðNO3Þ2;NiðNO3Þ2;CuðNO3Þ2;ZnðNO3Þ2;FeCl3). The as-preparedcatalysts were pre-calcined at 700 �C under flowingdry air (30 cm3 min�1) for 5 h.

Reactivity testing was carried out at atmo-spheric pressure in a quartz fixed-bed reactor(down flow). 40 mg catalyst (grain size: 250–315lm) diluted in 360 mg cordierite (grain size: 100–200 lm) were used for each test. Before testing, thecatalyst was reduced under flowing hydrogen(50 cm3 min�1) at 300 �C for 1 h. A stoichiometricreaction mixture was used (12.8 vol%C2H5OH +38.4 vol% H2O + 48.8 vol%N2 ) water:etha-nol¼ 3:1 mol/mol). Total flow rate was100 cm3 min�1. Ethanol and water were intro-duced as liquids using automated syringes, va-porized at 120 �C and further eluted by N2

(48 cm3 min�1). Each test was performed at con-stant temperature, between 500 and 800 �C. Before

the gas phase chromatographs, condensable va-pors (ethanol, water, acetaldehyde, acetone. . .)were trapped using a condenser which temperaturewas set at 0.5 �C. The liquid volume was used toapproximate the ethanol conversion. Gas analysiswas performed on-line using a combination ofthree gas chromatographs: two of them equippedwith a thermal conductivity detector (TCD) andthe other one equipped with a flame ionizationdetector (FID). H2 was analyzed using a TCD,equipped with a 5A molecular sieve column, withN2 as both the carrier gas and the reference gas.N2, CO, CO2 and CH4 were analyzed using aTCD, equipped with a CTR column (Alltech),with H2 as both the carrier gas and the referencegas. N2 was not only a dilutant but also a tracerand the corresponding GC signal was used to es-timate the outlet flow rate to determine the hy-drogen yield. Finally, hydrocarbons (CH4;C2H6

and C2H4 essentially) were separated on a PorapakQ column and analyzed with the FID detector.

3. Results and discussion

A wide range of catalysts was tested, changingboth the active phase (Rh, Pt, Pd, Ru, Ni, Cu, Zn,Fe) and the oxide support (Al2O3, 12%CeO2–Al2O3;CeO2;CeO2–ZrO2;ZrO2). As the SRR wasthought to produce CO as a primary product, theoverall idea was to couple both a highly active SRcatalyst and a highly active catalyst in the WGSR(2) for the subsequent transformation of CO –produced upon bio-ethanol SR – to CO2. Onpurpose, metals were selected either for theiractivity in the SRR (Rh, Ni) [5] or/and for theiractivity in the WGSR (Pt, Cu, Zn, Fe) [6]. Addi-tionally, supports were tested either as promotersof the WGSR (ceria-based supports) [7] or/and theSRR (2) [8]. In fact, in the alkylated aromaticsselective reforming reaction, Duprez et al. [9]proposed a bi-functional mechanism where thehydrocarbon to be reformed would be activated onthe metal particle and the water would be activatedon the support as hydroxyl groups. In such ascheme, oxide supports with high OH groupssurface mobility should be considered as promot-ers in the SRR. In that sense, a Ce0:63Zr0:37O2

264 F. Aupreetre et al. / Catalysis Communications 3 (2002) 263–267

mixed oxide – with enhanced oxygen surface mo-bility [10] – was tested as support.

In a first part, the influence of the metal particlechemical nature was checked. Catalytic perfor-mances in the bio-ethanol SRR of c-Al2O3 (Pro-catalyse, SBET ¼ 130 m2 g�1) supported metalcatalysts (Rh, Pt, Pd, Ru, Ni, Cu, Zn, Fe) werestudied. The results are presented in Table 1.

At 700 �C under atmospheric pressure, c-Al2O3

supported Rh and Ni catalysts clearly appeared asthe most active and selective catalysts in the ethanolSRR. Compared to Rh/c-Al2O3, Ni/c-Al2O3 gave ahigher yield in hydrogen but a lower selectivity to-wards CO2. In fact, earlier studies had demon-strated that Rh and Ni are the best catalysts in thehydrocarbons SRR.Additionally, Ni and especiallyRh were poor candidates in the WGSR [11,12].

Lower activities and lower selectivities towardsCO2 are observed using c-Al2O3 supported Pt, Cu,Zn or Fe catalysts. Such metals are known as effi-cient catalysts in the WGSR while they have onlylimited activity in the SRR. As a result, such cata-lytic formulations lead to an equilibrated CO=CO2

mixture, according to the thermodynamic.Additionally, Ru catalyst was almost inactive in

the SRR but essentially active in the ethanol de-hydration reaction leading to the formation ofethylene (38%). Such a catalyst deactivate rapidlydue to coke formation by ethylene polymerization.Dehydration is also observed on Cu, Pt, Zn andFe. Furthermore, one can note that Pd, Cu and Zncatalysts were very poorly selective towards CO2

and active in the hydrogenolysis reaction towardsmethane formation.

Consequently, (i) metals highly active in theSRR and poorly efficient in the WGSR would giveactive and selective catalysts in the ethanol SRRand (ii) active metals in the WGSR would result inpoorly selective catalysts towards the formation ofCO2 upon bio-ethanol steam reforming.

In the second part, concentrating on Rh and Nicatalysts, the role of the oxide support (c-Al2O3,12%CeO2–Al2O3;CeO2;Ce0:63Zr0:37O2 and ZrO2)was investigated. To improve the performances ofthe catalyst in the SRR, ceria-containing supports(12%CeO2–Al2O3;CeO2;CeO2–ZrO2), with en-hanced OH surface mobility [10,13,14], were usedas supports. Furthermore, ceria is known as apromoter in the WGSR [7]. The results obtainedupon reaction at 600 �C under atmospheric pres-sure are presented in Table 2 for Rh-based cata-lysts and Table 3 for Ni-based catalysts.

Looking at Rh catalysts (Table 2), 1%Rh/c-Al2O3 showed the highest selectivity while1%Rh=CeO2–ZrO2 exhibited the highest yield inhydrogen. 1%Rh/ZrO2 activity could not be prop-erly measured due to rapid deactivation. In fact, H2

production is initially low and large amounts ofethylene are detected. In descending order, thefollowing sequences could be establish for both theactivity: 1%Rh=Ce0:63Zr0:37O2 > 1%Rh=12%CeO2

–c-Al2O3 > 1%Rh=CeO2 > 1%Rh=c-Al2O3 andthe selectivity towards CO2:1%Rh=c-Al2O3 > 1%Rh=12%CeO2–c-Al2O3 > 1%Rh=CeO2 > 1%Rh=

Table 1

Ethanol steam reforming at 700 �C under stoichiometric reaction conditions (nðH2OÞ ¼ 3, nðC2H5OHÞ ¼ 1) at atmospheric pressure

on c-Al2O3 supported Rh, Pt, Pd, Ru, Ni, Cu, Zn and Fe catalysts

Gas phase composition (%) H2 yield

(g h�1 g�1

catalyst)

Selectivity

towards

CO2 (%)H2 CO2 CO CH4 C2H4 C2H6

Thermodynamic equilibrium 69.2 10 20 0.8 0 0 – 33

1%Rh/c-Al2O3 72 21 7 0 0 0 2.3 75

1%Pt/c-Al2O3 46 7 13 12 21 1 0.6 35

0.75%Pd/c-Al2O3 55 2 18 15 9 1 1.1 10

0.67%Ru/c-Al2O3 38 2 9 12 38 1 0.3 18

9.7%Ni/c-Al2O3 70.5 18 11 0.5 0 0 3.1 62

9.1%Cu/c-Al2O3 40 1 12 21 23 1 0.4 8

9.8%Zn/c-Al2O3 42 0 16 21 20 1 0.4 0

8.7%Fe/c-Al2O3 44 5 10 20 20 1 0.3 33

Variation of the gas phase composition, the H2 yield and the selectivity towards CO2.

F. Aupreetre et al. / Catalysis Communications 3 (2002) 263–267 265

Ce0:63Zr0:37O2. So that: (i) in agreement with Du-prez’s bifunctional mechanism, the activity of thecatalyst in the SRR increases as the OH groupsmobility at the catalyst surface increases and (ii) theselectivity of the catalyst towards CO2 decreases asthe efficiency of the catalyst in the WGSR in-creases. The last observation tends to indicate thatCO2 is produced as a primary product in the eth-anol SRR and transformed into CO via the WGSRto reach the thermodynamic equilibrium. In fact,looking at the results obtained on ceria-containingcatalysts, one can observe that the thermodynamicequilibrium is almost reached. The different prod-uct fractions are comparable to the thermodynamicequilibrium concentrations obtained from the cal-culations performed using an homemade program.

Almost the same conclusions can be drawn fromthe results obtained with Ni catalysts (Table 3).The activity in the SRR directly varies as thedegree of mobility of surface OH groups ð9:7%Ni=Ce0:63Zr0:37O2 > 9:7%Ni=CeO2 > 9:7%Ni=12%CeO2–c-Al2O3 > 9:7%Ni=c-Al2O3Þ and the selec-

tivity towards CO2 is controlled by the activity inthe WGSR ð9:7%Ni=c-Al2O3 > 9:7%Ni=12%CeO2

–c-Al2O3 > 9:7%Ni=CeO2 > 9:7%Ni=Ce0:63Zr0:37O2Þ. Use of ceria-based supports, highly active inthe WGSR, immediately lead to the thermody-namic CO/CO2 equilibrium. As a result, activity inthe SRR and selectivity towards CO2 vary in theopposite direction.

In conclusion, these unexpected and unprece-dented results clearly evidence the formation ofCO2 as a primary product in the ethanol SRR onsome metal supported catalysts. This is a totallydifferent picture compared to the results obtainedfrom earlier studies on hydrocarbons steam re-forming. Such results extensively showed that COwas produced as a primary product. For thatreason, high temperature WGS and low tempera-ture WGS units are required on methane steamreforming industrial plants for the synthesis ofammonia. Nevertheless, recent results on dimethylether catalytic SR also suggested CO2 and H2 asprimary products [15]. As a result, the most se-

Table 3

Ethanol steam reforming at 600 �C under stoichiometric reaction conditions (nðH2OÞ ¼ 3, nðC2H5OHÞ ¼ 1, P¼ 1 atm.) on Rh-based

supported c-Al2O3, 12%CeO2–Al2O3;CeO2;CeO2–ZrO2 and ZrO2 catalysts

Gas phase composition (%) H2 yield

(g h�1 g�1

catalyst)

Selectivity

towards

CO2 (%)H2 CO2 CO CH4 C2H4 C2H6

Thermodynamic equilibrium 64.7 14.9 13.4 7 0 0 – 53

9.7%Ni/c-Al2O3 76 24 0 0 0 0 0.6 100

10%Ni/12%CeO2–c-Al2O3 65 15.5 3 3 13 0.5 2.5 84

10%Ni/CeO2 63 21 8 8 0 0 4.1 72

10%Ni/Ce0:63Zr0:37O2 62 21 9 8 0 0 4.4 70

10%Ni/ZrO2 68.5 20 7 3 1.5 0 2.5 74

Table 2

Ethanol steam reforming at 600 �C under stoichiometric reaction conditions (nðH2OÞ ¼ 3, nðC2H5OHÞ ¼ 1, P ¼ 1 atm.) on Rh-based

supported c-Al2O3, 12%CeO2–Al2O3;CeO2;CeO2–ZrO2 and ZrO2 catalysts

Gas phase composition (%) H2 yield

(g h�1 g�1

catalyst)

Selectivity

towards

CO2 (%)H2 CO2 CO CH4 C2H4 C2H6

Thermodynamic equilibrium 64.7 14.9 13.4 7 0 0 – 53

1%Rh/c-Al2O3 73.5 22 3 1 0.5 0 2.2 88

1%Rh/12%CeO2–c-Al2O3 63.5 17 7.5 12 0 0 4.3 69

1%Rh/CeO2 63 16 13.5 5.5 2 0 4.0 54

1%Rh/Ce0:63Zr0:37O2 62 17 13 7.5 0.5 0 5.1 57

1%Rh/ZrO2 57 17 5.5 3.5 16 0 0.5 74

266 F. Aupreetre et al. / Catalysis Communications 3 (2002) 263–267

lective catalyst towards CO2 in the bio-ethanolSRR should contain no WGSR promoters.

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