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Autothermal syngas production from modelgasoline over Ni, Rh and NieRh/Al2O3 monolithiccatalysts

Laurent Villegas, Nolven Guilhaume*, Claude Mirodatos

Universite Lyon 1, Institut de recherches sur la catalyse et l’environnement de Lyon IRCELYON, UMR5256 CNRS,

2 avenue Albert Einstein, F-69626 Villeurbanne Cedex, France

a r t i c l e i n f o

Article history:

Received 7 November 2013

Received in revised form

14 January 2014

Accepted 20 January 2014

Available online 22 February 2014

Keywords:

Autothermal reforming

Syngas

Hydrogen

NieRh catalyst

Monolith

Carbon

* Corresponding author. Tel.: þ33 (0) 472 445E-mail address: Nolven.Guilhaume@ircely

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2014.01.1

a b s t r a c t

On-board reforming of liquid fuels is attractive for fuel cell-powered auxiliary power units

in vehicles. In this work, monometallic Ni/Al2O3/cordierite, Rh/Al2O3/cordierite and bime-

tallic NieRh/Al2O3/cordierite monolithic catalysts were prepared, characterized and tested

in ATR of isooctane for syngas production. Compared to monometallic formulations, the

bimetallic NieRh/Al2O3 catalyst was active for ATR at lower temperature and H2 production

already reached the equilibrium composition in 400e550 �C temperature range. The NieRh/

Al2O3 catalyst exhibited stable performances for 140 h in ATR of isooctane at 700 �C, and

was unaffected by oxidizing conditions at 700 �C. Thermoneutral reactions conditions at

H2O/C ¼ 2 were obtained with O/C ¼ 0.66. Carbon deposition was marginal during ATR of

isooctane and no carbons whiskers were detected. Post-reaction characterizations showed

that the Ni particles were small enough to prevent filamentous carbon formation, while Rh

also prevented carbon film deposition by improving the gasification of adsorbed C with

steam.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Converting liquid fuels or alcohols to syngas has many po-

tential applications for fuel cells used as portable power sys-

tems or auxiliary power units in vehicles [1e3]. Gasoline,

Diesel and kerosene fuels are of particular interest for mobile

applications [4e6], as well as ethanol and dimethylether,

which can be produced from renewable resources. The fuel

processing system includes a reformer, in which the fuel is

converted into syngas, a wateregas shift (WGS) reactor to

simultaneously reduce the concentration of CO and increase

389; fax: þ33 (0) 472 445on.univ-lyon1.fr (N. Guil2014, Hydrogen Energy P23

H2 production, and a H2 purification reactor by selective

oxidation or methanation.

In autothermal reforming, the hydrocarbon fuel is reacted

with oxygen (from air) and steam to produce syngas. By

adjusting the proportions of fuel, air, and steam in the feed,

the heat generated by oxidation reactions is used to sustain

endothermic steam reforming (SR), aiming at providing an

energy self-sustaining system that does not require external

heat. Consequently, autothermal reformers are simpler,

smaller than steam reformers and faster to start-up than

steam reformers, and particularly adapted as small-scale re-

formers [7].

399.haume).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 0 5773

Hydrocarbon steam reforming (SR) and autothermal

reforming (ATR) are generally catalysed by Ni-based catalysts

that combine cheapness with a reasonably high activity;

however, Ni catalysts are susceptible to performance degra-

dation, essentially through carbon deposition, sulphur

poisoning and sintering [8]. Three type of carbon deposits

have been identified: (i) pyrolytic carbon is formed at high

temperature on hot spots anywhere in the reactor, (ii)

encapsulating carbon is a thin film of amorphous or graphitic

carbon covering the catalyst particles, (iii) carbon whiskers,

that do not hinder access of reactants to the metal active sites

but can lead to catalyst loss and reactor clogging.

Support modification by introducing basic components

such as Mg [9], Ce, La [10] can improve the resistance of Ni to

sintering and carbon deposition. Various Ni-based bime-

tallic compositions have also been studied in SR and ATR of

hydrocarbons, in order to improve the catalyst stability.

Bimetallic NieCe and NieRe catalysts were evaluated for the

SR of gasoline [11]. Ceria addition enhanced H2 productivity,

while NieRe exhibited a high resistance to sulphur

poisoning. A Ni/Fe/MgO/Al2O3 catalyst showed good stabil-

ity for 700 h in ATR of isooctane [12]. NieMn, NieW and

RheCe catalysts supported on alumina mesh were also

tested in the ATR of isooctane using spray-pulsed injection

[13] but the catalyst stability was not studied. Rh-based

catalysts also showed high activity and stability in the

ATR of jet fuels [14] and Diesel surrogates [15,16] in the

presence of sulphur, but a 0.3 wt.%Rh/3 wt.%MgO/20 wt.%

CeO2eZrO2 catalyst formulation was found to deactivate

slowly by sintering and carbon deposition during ATR of S-

containing gasoline [17].

NieSn bimetallic catalysts were found more resistant to

carbon poisoning than monometallic Ni in the SR of methane

and isooctane [18,19]. NieSn alloying was suggested to in-

crease the carbon tolerance by decreasing the rate of CeC

bond formation compared to the rate of CeO bond formation.

During autothermal reforming, oxidation reactions gener-

ally take place at the reactor inlet where they generate the

heat to perform downstream the endothermic steam

reforming reaction. The non-uniform temperature profile can

result in the formation of “hot spots” that should be mini-

mized, since they lead to catalyst deactivation by sintering [6],

while the cold spots favour carbon deposition. Packed-bed

reactors loaded with powder or pellet catalysts have poor

heat transfer performances, while in contrast monoliths can

improve heat transfer by allowing high space velocities with

low pressure drop [6,20]. Although metallic monoliths have

higher thermal conductivities, ceramic monoliths are prefer-

able since they exhibit a higher thermal stability and can be

operated at nearly adiabatic conditions due to their low

thermal conductivity [21,22], whereas the washcoat is more

efficiently anchored on ceramic (due to similar thermal

expansion coefficients) than on metallic surfaces.

We previously optimized the preparation of Ni/Al2O3/

cordierite monolith catalysts [23] and investigated their ac-

tivity for ATR of isooctane, used as gasoline surrogate. Carbon

deposition was evidenced under reaction conditions, essen-

tially as carbon whiskers and amorphous carbon.

In the present study, we comparedmonometallic Ni/Al2O3/

cordierite, Rh/Al2O3/cordierite and bimetallic NieRh/Al2O3/

cordierite monolith formulations, with emphasis on the long-

term stability under reaction conditions and on the resistance

to coking.

2. Experimental

2.1. Washcoating and impregnation of monoliths

Small monoliths (20mm length, 18mmdiameter) with square

channels (400 cpsi) were cut out of cordierite industrial

monoliths (Corning). g-Al2O3 powder (3 mm average particle

size powder, Alpha-Aesar) was used to prepare the alumina

washcoat. The powder was dispersed in an aqueous solution

of HNO3 (g-Al2O3/H2O ¼ 25 wt.%, HNO3/g-Al2O3 ¼ 2 mmol g�1)

and stirred vigorously for 15 h at room temperature. The

monoliths were dipped vertically into the suspension for

2 min, removed and the excess suspension was blown off the

channels with a mild airflow. The monoliths were dried at

100 �C for 1.5 h and weighted. The procedure was repeated

until a 13e15 wt.% increase was obtained. The washcoated

monoliths were calcined in air at 800 �C for 4 h, with a tem-

perature ramp of 0.5 �C min�1.

2.1.1. Ni impregnationThe Al2O3-washcoated monoliths were dipped in a stirred

aqueous solution of 0.5 M Ni(NO3)2$6H2O for 2 h. After

removal, the solution remaining in the channels was blown

off with a mild airflow. The wet impregnated monoliths were

dried in a microwave oven operating at 200 W for 50 min. The

monoliths were then calcined at 550 �C for 4 h in air, with a

temperature ramp of 1 �C min�1.

2.1.2. Rh and NieRh impregnationRh/Al2O3 and NieRh/Al2O3 monoliths were prepared by dip-

ping Al2O3/cordierite or Ni/Al2O3/cordierite monoliths in a

saturated aqueous solution (z0.001 M) of Rh(NO3)3$2H2O for

2 h under stirring, followed by the same drying/calcination

procedure as for Ni-impregnation. For the Rh/Al2O3 composi-

tion, the complete impregnation/drying/calcination sequence

was repeated a second time to increase the Rh loading.

For each catalyst composition, a series of 5 monoliths was

prepared in order to check the reproducibility of the

preparation.

2.2. Catalyst characterization

Specific surface areas were measured by nitrogen adsorption

at 77 K using the multipoint method, using whole monoliths

placed in a specifically designed cell. The samples were pre-

viously desorbed at 623 K under vacuum (2.10�3 Pa) for 2 h.

Chemical analyses of Ni and Rh were performed by ICP-

OES on crushed monolith samples.

Scanning Electron Microscopy (SEM) micrographs were

performed on small pieces of monolith using a Hitachi S800

electron microscope operating at 15 kV, after metallization of

the samples under an AuePd film. EDS analyseswere obtained

on a JEOL JMS840A microscope equipped with a Princeton

Gamma-Tech detector and operating at 15 kV, after covering

the samples with a carbon film. For some samples, the metals

i n t e rn 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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 05774

distribution profiles in the alumina washcoat were also ana-

lysed using a CAMECA SU30 electron microprobe analyser.

Transmission Electron Microscopy (TEM) micrographs

were obtained with a JEOL 2010 microscope equipped for EDS

analysis. Amonolith piece was cut and placed in an ultrasonic

bath in ethanol to remove the washcoat from the cordierite

support. A drop of washcoat suspension was deposited on the

grid for examination.

X-ray photoelectron spectroscopy analyses were per-

formed on monolith pieces taken at different positions in the

channels length, and placed directly in the sample holder of a

VG Scientific ESCALAB 200R spectrophotometer. The analysed

areas werez1 mm2, which approximately corresponds to the

channel width in a 400 cpsi monolith.

TPO experiments were performed in a quartz microreactor

under 20% O2 in Ar, using a calibrated mass spectrometer

to quantify the formation of CO2. The total gas flow was

35 mL/min.

2.3. Catalytic activity measurements

Activity measurements were carried out at atmospheric

pressure in laboratory-scale fixed-bed tubular quartz reactors

adapted to host small monoliths with diameter 18 or 7 mm.

Themonoliths were held using ceramic and the dead volumes

in the reactor were filled with quartz beads. A thermocouple

(diameter 0.5 mm) was placed in a channel at the centre of the

monolith. The catalysts were reduced at 650 �C for 1 h

(25 vol.% H2 in Ar, total flow rate 130 mL/min) prior to activity

measurements. In standard activity measurements, the feed

gas was composed of 1 vol.% isooctane, 4 vol.% O2, 16 vol.%

steam, 1 vol.%N2 as internal standard and balance argon, with

a total gas flow rate of 200 mL/min corresponding to a gas

hourly space velocity (GHSV) of 2360 h�1, relative to the

monolith volume. This composition corresponds to the molar

ratios O/C ¼ 1 and H2O/C ¼ 2. In some experiments, these

ratios were modified, keeping the GHSV constant. In experi-

ments performed at higher spaces velocities, the monolith

diameter was reduced to 5 channels (z6.5 mm) and the total

flow rate was adjusted between 100 and 300 mL/min to obtain

GHSVs between 7800 and 23,500 h�1, keeping the same

composition. The isooctane and water feeds were adjusted by

liquid flow controllers and were vaporized separately, then

mixedwith the other gaseous components in heated gas lines.

The reformate composition was analysed with a micro gas

chromatograph (Agilent) equipped with 3 analytic modules

(molecular sieve and Poraplot U columns with backflush, OV1

Table 1 e Comparison of Ni and Rh analyses in the bulk, top ladifferent positions of the monolith channels.

Atomic ratios Bulk chemical analyses

Ni/Al Rh/Al

Ni/Al2O3 End 0.048 e

Middle e

Rh/Al2O3 End e 0.001

Middle e

NieRh/Al2O3 End 0.047 0.0006

Middle

column without backflush) allowing complete analysis in

160 s. A mass spectrometer (Inficon) was also used for a

continuous monitoring of the reaction.

The HSC-Chemistry version 4.0 software (OUTOKUMPU,

Finland) was used to calculate the equilibrium compositions

using the Gibbs energy minimization method.

3. Results and discussion

3.1. Monoliths characterization

The procedure developed previously for the preparation of Ni/

Al2O3/cordierite lab-scale monoliths [23] was applied in the

present study to prepare Rh and NieRh formulations. The

amount of alumina washcoat deposited on all monoliths was

z15 wt.%, with a washcoat thickness of z10 mm that could

reach 80 mm in the corners of the channels [23]. The surface

area (referred to the alumina washcoat) ranged between 50

and 55 m2 g�1 for all monoliths, after calcination of the

washcoat at 800 �C followed after impregnation by calcination

at 550 �C to decompose the metal salts.

The bulk chemical composition of the washcoat, deter-

mined by chemical analysis, was 5.2 wt.% Ni/Al2O3, 0.23 wt.%

Rh/Al2O3 or 5.1 wt.% Nie0.12 wt.% Rh/Al2O3 for Ni, Rh and

NieRh formulations, respectively. We have shown previously

that microwave drying after wet impregnation gives a ho-

mogeneous distribution of Ni along the channels length [23].

This was confirmed with the present monoliths, using XPS

and SEM/EDS analyses performed at different positions of the

monoliths (compare in Table 1 the analyses performed at the

ends and in themiddle of channels). Themetals distributions,

however, are different and not homogeneous in the washcoat

thickness. For the Ni/Al2O3 monolith, Ni particles are found

uniformly in the whole washcoat thickness, and the Ni con-

centration in the top-layer (5 mm thickness) is similar to that in

the bulk. The alumina surface (5 nm thickness), however,

appears enriched in Ni, with a surface Ni/Al atomic ratio about

twice that of the bulk.

In the Rh/Al2O3 monolith, Rh microprobe profiling (not

shown) shows that Rh is found essentially in the first 5 mm of

the alumina washcoat. Consequently, the Rh/Al atomic ratio

is higher in the top-layer than in the bulk, as confirmed by EDS

analyses. In addition, XPS analyses reveal a considerable Rh

surface enrichment (surface Rh/Al ratio is about 1000 times

higher that the bulk ratio), which suggests that during the

impregnation, the rhodium nitrate precipitates rapidly on the

yer (5 mm) and surface (5 nm) of the alumina washcoat at

Top layer (5 mm)EDS analyses

Surface layer (5 nm)XPS analyses

Ni/Al Rh/Al Ni/Al Rh/Al

0.04 e 0.11 e

0.07 e 0.13 e

e 0.008 e 0.12

e 0.007 e 0.09

0.03 0.006 0.15 0.033

0.07 0.005 0.07 0.026

Fig. 1 e Activity of monolithic catalysts for the autothermal

reforming of isooctane vs. temperature. (A) Conversion of

isooctane over Ni/Al2O3, Rh/Al2O3 and NieRh/Al2O3; (B)

Product stream composition over Ni/Al2O3; (C) Product

stream composition over Rh/Al2O3; (D) Product stream

composition over NieRh/Al2O3 (dotted lines: calculated

equilibrium composition). Conditions: 1 vol.% isooctane,

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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 0 5775

alumina surface hydroxyls, due to the low solubility of this

salt in water. Moreover, Rh impregnation was performed two

times on this monolith series.

Since the NieRh/Al2O3 monolith was prepared by subse-

quent Rh-impregnation of a Ni-impregnated monolith, the Ni

distribution is very similar as that in the Ni/Al2O3 monolith.

The Rh distribution, however, appears modified by the pres-

ence of Ni since Rh is found deeper in the washcoat thickness

(up to 50 mm in corners, where the washcoat is the thickest).

Consequently, although a strong surface Rh enrichment is

again evidenced by XPS, the single Rh impregnation appears

to result in a better distribution of Rh in the alumina

washcoat.

For all monoliths, the preparation by wet impregnation

leads to a strong alumina surface enrichment in both metals,

probably due to the capillary forces that drive the diffusion of

the precursors solution from the large cordierite pores to-

wards the smaller pores of the alumina washcoat upon dry-

ing. The catalytic activity should benefit from this surface

enrichment, and the amount of Ni and Rh metals might be

optimized and probably reduced, since the metals concen-

tration in the alumina surface layer is actually very high.

3.2. Performances of monolithic catalysts for syngasproduction

Based on thermodynamic calculations, we showed in a pre-

vious study [24] that optimal reaction conditions for the

autothermal reforming of isooctane correspond to O/C¼ 1 and

H2O/C¼ 2. This compositionwas used for a first assessment of

catalysts performances.

The activity of Ni/Al2O3, Rh/Al2O3 and NieRh/Al2O3

monoliths vs. temperature is plotted in Fig. 1. With Ni/Al2O3,

the isooctane conversion (Fig. 1(A)) rises slowly up to 50%

between 350 and 570 �C, then abruptly reaches completion at

580 �C. The products stream composition displayed in Fig. 1(B)

reveals that up to 570 �C, the main reactions are the cracking

of isooctane in butene at low temperature, while the total

oxidation in CO2 becomes predominant up to 570 �C, tem-

perature at which oxygen becomes fully consumed (for the

sake of clarity, only the products formed are depicted on the

figure, the oxygen concentration profile is not shown). Steam

reforming and WGS reactions start as soon as oxygen is fully

depleted and the conversion of isooctane reaches completion

very rapidly, while H2 and CO are simultaneously produced.

H2 production, however, reaches the thermodynamic equi-

librium only at 750 �C.With the Rh/Al2O3 monolith, the combustion of isooctane

is clearly predominant in the 350e570 �C temperature range

(Fig. 1(C)), whereas the oxygen conversion (not shown) is total

from 350 �C. Hydrogen production starts at 500 �C and in-

creases progressively up to 650 �C, but decreases when the

temperature is further raised and never reaches the equilib-

rium composition.

The NieRh/Al2O3 monolith exhibits a very different

behaviour (Fig. 1(D)) since H2 production is significant from

4 vol.% O2, 16 vol.% H2O, 1 vol.% N2, balance Ar, heat-up

ramp 2 �C/min, GHSV [ 2360 hL1.

i n t e rn 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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 05776

the beginning of the temperature ramp and follows the

calculated equilibrium composition in the whole 450e740 �Ctemperature range. H2 production is maximum around

640 �C, as expected by thermodynamic calculations, and

slightly decreases at higher temperature due to the reverse-

WGS reaction. Rh addition to Ni clearly creates a synergy

effect in the reaction pathway and promotes an efficient

utilization of isooctane towards H2 in the 400e550 �C tem-

perature range, at which the monometallic catalysts are

essentially active for isooctane cracking or total oxidation.

The role of Rh in this synergy might be to activate steam at

lower temperature than Ni, and therefore to promote the

reactions involving steam (WGS and steam reforming) in

this temperature range.

Following the temperature ramp, the catalysts were eval-

uated in isothermal conditions at 740 �C for 12 h (Fig. 2). The

rate of H2 formation (Fig. 2(A)) tends to decrease slowly with

the Ni/Al2O3 monolith, whereas it shows the reverse trend

with the Rh/Al2O3 catalyst and remains stable, and very close

to the equilibrium composition, with the NieRh/Al2O3 cata-

lyst. The situation is opposite as regards the rate of CH4 for-

mation (Fig. 2(B)), methane being formed in relatively large

amounts at the beginning of the temperature dwell over Rh/

Fig. 2 e Products formation rates over Ni/Al2O3, Rh/Al2O3

and NieRh/Al2O3 monoliths at 740 �C vs. time. (A)

Hydrogen production; (B) CH4 production. The full lines

show the data trend with time on stream. Conditions:

1 vol.% isooctane, 4 vol.% O2, 16 vol.% H2O, 1 vol.% N2,

balance Ar, GHSV [ 2360 hL1.

Al2O3, then decreasing slowly with time on stream, while the

reverse is observed with Ni/Al2O3 and in a lesser extent with

NieRh/Al2O3.

Methane formation by hydrogenation of CO, CO2 or C is not

thermodynamically favoured at 740 �C and atmospheric

pressure [25]: as shown in Fig. 1(D) (dotted line), CH4 formation

should be maximum at 350 �C, decrease up to 600 �C and be

marginal at 750 �C. Therefore, methane formation is more

likely related to the direct cracking of isooctane on the acidic

sites of the alumina support, which suggests that isooctane

starts to react on the support as the metal sites are slowly

poisoned by carbon [23]. The NieRh composition appears to

slow down this poisoning and to limit the extent of catalyst

deactivation, since the CH4 production rate increases very

slowly for z7 h and stabilizes afterwards at the steady-state

value of 0.35 mL/min.

3.3. Activity and long-term stability of NieRh/Al2O3

monolithic catalyst

Long-term activity tests of the NieRh/Al2O3 monolith catalyst

were carried out for 140 h at 700 �C. In Fig. 3, the data points

displayed were analysed by GC, whereas the lines show the

calculated trends fitting the experimental data.

Isooctane conversion is total over 140 h time on stream, the

overall composition is at the thermodynamic equilibrium and

the H2 production is constant atz27mL/min,which represent

a hydrogen yield of 27 mol/mol isooctane. The reaction

selectivity towards syngas (H2 þ CO) reaches 79%, whereas the

carbon balance is 100 � 2% during all the experiment. This

figure, however, shows only an apparent catalyst stability over

140 h, since the total conversion of isooctane does not allow

assess how the intrinsic activity might change with time on

stream.

In order to simulate a shortage in the fuel supply, which

will leave the catalyst under a mixture of O2 and steam, the

isooctane supply was interrupted for 30 min then put in again

(Fig. 4).

Fig. 3 e Products formation rates over NieRh/Al2O3

monolith at 700 �C during 140 h of operation. Conditions:

1 vol.% isooctane, 4 vol.% O2, 16 vol.% H2O, 1 vol.% N2,

balance Ar, GHSV [ 2360 hL1. The lines show the data

trends.

Fig. 4 e Behaviour of NieRh/Al2O3 monolith at 700 �Cduring isooctane feed cut off. Conditions: 1 vol.% isooctane,

4 vol.% O2, 16 vol.% H2O, 1 vol.% N2, balance Ar,

GHSV [ 15,600 hL1.

Fig. 6 e Calculated reaction enthalpy as a function of O/C

ratio, for H2O/C [ 2.

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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 0 5777

The catalyst is clearly unaffected by oxidizing conditions at

700 �C for 30 min, which suggests that the initial catalyst pre-

reduction under H2 before catalytic activity measurements is

probably unnecessary.

The influence of the gas hourly space velocity (GHSV) on

the catalytic activity is shown in Fig. 5. The GHSV was varied

between 2360 and 23,400 h�1 by decreasing the monolith

diameter and adjusting the reactants and gas flow rates,

keeping a similar inlet composition.

In the range of GHSVs investigated, the reactor exit stream

exhibits the same composition, which corresponds to the

equilibrium composition. This means that the catalyst oper-

ates far from the kinetic regime and that only a part of the

monolith is efficiently involved in the reaction.

The O/C ratio has a strong influence on the H2 yield and on

the enthalpy of reaction. Fig. 6 shows how the calculated re-

action enthalpy changes from endothermic to exothermic as a

function of the O/C ratio in the feed stream. Autothermal

Fig. 5 e Influence of gas hourly space velocity on the

reformate composition at 700 �C. Conditions: 1 vol.%

isooctane, 4 vol.% O2, 16 vol.% H2O, 1 vol.% N2, balance Ar.

Total flow rate 100e300 mL/min, monolith dimensions:

diameter 17 or 7 mm, length 20 mm.

conditions, in which exothermic reactions (isooctane com-

bustion, WGS) compensate exactly the endothermic reform-

ing reactions, should be achieved at O/C ¼ 0.66.

We examined experimentally the dependence of the

gaseous products stream composition on the O/C ratio in

the feed-stream (Fig. 7(A)). The conversion of isooctane is

always total. H2 production decreases monotonously as the

Fig. 7 e (A) Reformate composition at 700 �C as a function of

O/C ratio in the feed. Conditions: 1 vol.% isooctane,

0e8 vol.% O2, 16 vol.% H2O, 1 vol.% N2, balance Ar,

GHSV [ 15,600 hL1. The dotted lines show the calculated

equilibrium compositions under similar conditions; (B)

Temperature deviation inside the NieRh/Al2O3 monolith

between Ar flow and reaction gas feed at different O/C

ratios.

Fig. 8 e Reformate composition at 700 �C as a function of

H2O/C ratio in the feed. Conditions: 1 vol.% isooctane,

4 vol.% O2, 0e24 vol.% H2O, 1 vol.% N2, balance Ar,

GHSV [ 15,600 hL1. The dotted lines show the calculated

equilibrium compositions under similar conditions.

i n t e rn 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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 05778

O/C ratio increases from zero to two, as the reaction

pathway changes from pure steam reforming to a combi-

nation of oxidation and reforming reactions, equilibrated

through the WGS reaction. The experimental data are very

similar to the calculated equilibrium composition (dotted

lines) in the whole O/C range.

Although our reactor is heated in a furnace and not adia-

batic, we tried to estimate the endo- or exothermicity of the

reaction by measuring the temperature inside the monolith

under reaction conditions, and to compare it to the tempera-

ture measured under similar heating conditions when the

reaction feed stream is replaced by pure argon at a similar

flow rate (DT ¼ Treaction � TAr). The results are depicted in

Fig. 7(B).

At O/C ¼ 0, the DT in the monolith is �37 �C, it increaseswith O/C and becomes positive at O/C> 0.7. The conditions for

DT ¼ 0 extrapolated from the curve are O/C ¼ 0.66 and should

correspond to autothermal conditions, in perfect agreement

with the calculated reaction enthalpy.

Fig. 9 e SEM micrographs of a NieRh/Al2O3 monolith after a sta

inlet, (B) middle.

The effect of the H2O/C ratio on the products stream

composition is shown in Fig. 8. The conversion of isooctane is

total for all ratios except at H2O/C ¼ 0 where it is 99%. This

composition was the last tested because it leads to coke de-

posits and catalyst deactivation.

The stoichiometric H2O/C ratio for the SR of isooctane into

CO þ H2 is 1. Increasing the H2O/C above 1 improves the

production of hydrogen, but the H2 þ CO selectivity is not

modified, which suggests that the WGS equilibrium is dis-

placed towards the formation of H2 and CO2 when the steam

content increases. The experimental data are very similar to

the calculated equilibrium composition (dotted lines) in the

whole H2O/C range. The calculated H2 production rate in-

creases from 24.3 mL/min at H2O/C ¼ 1 to 29 mL/min at H2O/

C¼ 3, while simultaneously the CO production decreases from

9.5 to 5 mL/min, but this requests a threefold increase in the

amount of steam in the feed, and water vaporization is

energy-intensive. A small DT of þ2.5 to þ3 �C is measured in

the monolith when the H2O/C ratio changes from 1 to 3,

probably resulting from the exothermic WGS reaction.

3.4. Post-reaction characterization of NieRh/Al2O3

monolithic catalyst

We showed previously that carbon deposition takes place

during ATR of isooctane over Ni/Al2O3 monolithic catalysts.

SEM and TPO analyses evidenced two types of carbon de-

posits, amorphous carbon and carbon whiskers, the whiskers

being observed only at the inlet of the monolith channels [23].

SEM images recorded at the inlet and in the middle of

channels are shown in Fig. 9.

No carbon whiskers are observed on the monolith wash-

coat (Fig. 9), but SEM cannot grant for the absence of an

amorphous carbon layer. The absence of carbon whiskers

might be related to the role of Rh in promoting the gasification

of adsorbed carbon before it can grow into whiskers [4,15],

and/or to the fact that Ni particles are very small. Carbon

whiskers grow by reaction of adsorbed atomic carbon or CO on

the step sites of Ni particles to form surface carbides, migra-

tion and nucleation of graphene layers over Ni (111) facets [8].

Carbon whiskers do not form on very small Ni particles,

ndard catalytic test at different positions of a channel. (A)

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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 0 5779

because the critical nucleus of graphene is large and nucle-

ation into whiskers cannot proceed if the facets or step edges

of the Ni particles are too small [26]. A critical size for whiskers

formation has been diversely reported to be >5 nm [27] or

>10 nm [28].

TEM examination was performed on a NieRh/Al2O3 cata-

lyst powder sample removed from the cordierite support, after

a standard catalytic test including a temperature ramp from

Fig. 10 e TEM micrographs of a NieRh/Al2O3 powder

catalyst removed from cordierite after a standard catalytic

test (images: L. Burel).

Fig. 11 e TPO analysis of the carbon deposited on NieRh/

Al2O3 and Ni/Al2O3 monoliths after a standard catalytic

test.

350 to 740 �C followed by 12 h at 740 �C. Indeed, most of the Ni

particle sizes are in the 2e3 nm range (Fig. 10(A) and (B)),

although some larger particles with sizes up to 15 nm can also

be observed (Fig. 10(C)). Rh particles could not be visually

found, but Rh was analysed by EDS in various sample areas,

always simultaneously with Ni, whereas some areas con-

tained Ni but no detectable Rh. No carbon film covering the Ni

particles could be clearly evidenced.

After 12 h at 740 �C under autothermal reforming condi-

tions, the Ni particles remain essentially well dispersed on the

alumina washcoat, and the small Ni particle sizes should

prevent the formation of carbon whiskers. However, we pre-

viously evidenced extensive carbon whiskers formation over

Ni/Al2O3 monoliths prepared under similar conditions [23],

and therefore Rh is probably active to minimize carbon

deposition, either as film or as whiskers, by promoting the

gasification rate of adsorbed carbon.

In order to evaluate the catalyst resistance to coking, a TPO

analysis was carried out on the monoliths after a standard

catalytic test (gas feed: 1 vol.% isooctane, 4 vol.% O2, 16 vol.%

H2O, 1 vol.% N2, balance Ar, temperature ramp between 350

and 740 �C followed by 12 h at 740 �C). Fig. 11 shows the CO2

formation during TPO of a NieRh/Al2O3 monolith, and for

comparison the TPO of a Ni/Al2O3 monolith after the same

catalytic test has been included.

According to our previous studies on Ni/Al2O3 monoliths

[23], the strong CO2 peak at high temperature (z680 �C) hasbeen attributed to the oxidation of graphitic carbon whiskers

and the shoulder at z600 �C to amorphous carbon. In

contrast, essentially amorphous carbon is found on the

NieRh/Al2O3 monolith with only a very small amount of car-

bon whiskers (small CO2 peak at 700 �C). Some highly reactive

carbon, probably atomic carbon [29], is also gasified at low

temperature (z260 �C). This species is not observed on the Ni/

Al2O3 formulation. The carbon species deposited on the

NieRh/Al2O3 monolith appear more reactive than those

deposited on Ni/Al2O3, and in much smaller amounts (the

overall area of the CO2 curve for NieRh/Al2O3 is half that of Ni/

Al2O3). The formation of filamentous carbon is effectively

inhibited in the presence of Rh. The total amount of CO2

i n t e rn 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 3 9 ( 2 0 1 4 ) 5 7 7 2e5 7 8 05780

formed during TPO, calculated by integration of the CO2 pro-

file, is 1.16 � 10�4 mol (1.4 mg). For comparison, the amount of

isooctane transformed on the catalyst during ATR corre-

sponds to 0.3 mol of carbon. Consequently, the carbon depo-

sition (<0.04%) on NieRh/Al2O3 monolith appears marginal.

4. Conclusions

� Monometallic Ni/Al2O3/cordierite, Rh/Al2O3/cordierite and

bimetallic NieRh/Al2O3/cordierite monolithic catalysts

were prepared, characterized and tested in ATR of isooc-

tane for syngas production.

� The bimetallic NieRh/Al2O3 catalyst exhibited a strong

synergy effect in 400e550 �C temperature range where H2

production already reached the equilibrium composition.

� The NieRh/Al2O3 monolith exhibited stable performances

for 140 h in ATR of isooctane at 700 �C. The catalyst was not

affected by oxidizing conditions at 700 �C.� The thermoneutral reactions conditions at H2O/C ¼ 2 were

obtained with O/C ¼ 0.66.

� Carbon deposition was marginal during ATR of isooctane

and no carbons whiskers were detected on Ni, suggesting

that Rh enhances the gasification of adsorbed C with steam.

Acknowledgements

The financial support of PSA Peugeot Citroen is gratefully

acknowledged. The authors thank Laurence Burel for TEM

investigations.

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