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A highly reactive and stable Ru/Co6LxMgxAl2catalyst for hydrogen production via methanesteam reforming

Doris Homsi a,b,c, Samer Aouad c,*, Cedric Gennequin a,b,Antoine Aboukaıs a,b, Edmond Abi-Aad a,b

aUniv Lille Nord de France, F-59000 Lille, FrancebULCO, LCE, F-59000 Dunkerque, FrancecDepartment of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon

a r t i c l e i n f o

Article history:

Received 7 March 2014

Received in revised form

17 April 2014

Accepted 21 April 2014

Available online 19 May 2014

Keywords:

Methane steam reforming

Ruthenium

Cobalt

Magnesium

EPR

Deactivation

a b s t r a c t

Hydrogen production by methane steam reforming is an important yet challenging pro-

cess. A performing catalyst will favor the thermodynamic equilibrium while ensuring good

hydrogen selectivity. We hereby report the synthesis of a ruthenium based catalyst on a

cobalt, magnesium, and aluminum mixed oxides supports. An interaction between cobalt

and ruthenium favors the formation of smaller, well dispersed cobalt/ruthenium oxide

species. The Ru/Co6Al2 catalyst outmatches the widely used industrial Ru/Al2O3 catalyst.

The catalyst is stable for 100 h on stream. After test characterization shows the formation

of carbon and coke deposits at trace levels. However, this does not affect the catalytic

performance of the catalysts making it good candidates for industrial applications.

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

reserved.

Introduction

Hydrogen is an important raw material for several industrial

applications and is a clean fuel that can be used in fuel cells

and internal combustion engine [1,2]. It provides a solution in

reducing energy consumption and environmental pollution

[3]. One of the main processes for hydrogen production is the

catalytic steam reforming of methane. Two reactions can be

considered in the methane steam reforming (MSR) process:

the first one is the methane steam reforming (Eq. (1)) and the

second is the water gas shift reaction WGS (Eq. (2)) to produce

additional hydrogen:

CH4 þH2O4COþ 3H2 DH+ ¼ þ206 kJ mol�1 (1)

COþH2O4CO2 þH2 DH+ ¼ �41 kJ mol�1 (2)

A high steam/methane ratio in the feed favors high

methane conversions and minimizes carbon deposition pro-

duced by the methane cracking (Eq. (3)), Boudouard reaction

(Eq. (4)) and the reverse carbon gasification reaction (Eq. (5))

that destroy the catalyst structure and deteriorate its activity.

* Corresponding author. Tel.: þ961 6 930250x3840; fax: þ961 6 930277.E-mail address: samer.aouad@balamand.edu.lb (S. Aouad).

Available online at www.sciencedirect.com

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CH44Cþ 2H2 DH+ ¼ þ75 kJ mol�1 (3)

2CO4CO2 þ C DH+ ¼ �172 kJ mol�1 (4)

COþH24H2Oþ C DH+ ¼ �131 kJ mol�1 (5)

However, it is interesting to develop a catalyst that is able

to operate at low steam/methane ratio without the formation

of carbon. Noble metals (such as Ru, Rh, Pd, Pt, Ir .) are used

for the production of synthesis gas at low steam to carbon

ratios and they are resistant to carbon formation [4,5]. In

addition, according to literature, gasification of carbon species

could be reduced by the use of basic promoters like hydro-

talcites [6]. In fact, cobalt was found to have high catalytic

activity in the reforming of methane [7e9]. Moreover, MgO

basic supports resist to coking. This is due to the enhance-

ment in the oxidation rate of CHx fragments adsorbed on the

activemetal [10]. On the other hand, the acidity in the support

(Al2O3) is known to facilitate the decomposition of methane

[11].

In this study, we report the preparation of Ru/CoxMg6�xAl2solids and their catalytic activity in the MSR reaction. The

solids are characterized before and after aging tests to check

their stability and determine a potential deactivation

mechanism.

Materials and methods

Four different “CoxMg6�xAl2HT” (x ¼ 0, 2, 4 and 6) supports are

prepared via the hydrotalcite route [12] and are then calcined

under air at 500 �C. The calcined supports are impregnated

with a solution of ruthenium nitrosyls nitrate according to

Ref. [13] in order to obtain solids containing 1 wt.% metallic

ruthenium after calcination under air at 500 �C. The freshly

calcined catalysts are named Ru/Co6�xMgxAl2.

X-ray diffraction (XRD) experiments are performed at

ambient temperature on a BRUKERD8Advance diffractometer

using CuKa radiation (1.5405 �A). Diffraction patterns are

recorded over a 2q range of 20e80� using a step size of 0.02�.The diffraction patterns are indexed by comparison with the

JCPDS files.

Temperature programmed reduction/oxidation experi-

ments (TPR/TPO) are carried out on Altamira AMI-200 appa-

ratus. In the TPR analysis, the hydrogen flow (5 vol.% in Ar) is

30 mL min�1, while in the TPO experiments, oxygen flow

(10 vol.% in He) is 30 mL min�1. Gases passed through a U-

shaped reactor containing the catalyst under atmospheric

pressure and the amount of H2 or O2 consumed is monitored

with a thermal conductivity detector (TCD).

The electron paramagnetic resonance (EPR) measurements

are performedwith an EMX Bruker spectrometer with a cavity

operating at a frequency of w9.5 GHz (X band). The magnetic

field is modulated at 100 KHz and the power supply is suffi-

ciently small to avoid saturation effect. The measurements

are performed at room temperature. The g values are deter-

mined from precise frequency and magnetic field values.

Catalysts performances are evaluated in the MSR reaction

which is carried out under atmospheric pressure in a fixed

catalytic bed reactor coupled to a micro-GC (Varian CP-4900)

equipped with a TCD. Two hundred milligrams of the cata-

lyst are introduced into the reactor. The catalytic reactivity is

studied in the 400e800 �C temperature range. The reactant gas

flow consisted of a 20 mL min�1 steam and methane mixture

with H2O/CH4 ¼ 3 and 30 mL min�1 of argon used for

balancing. A test using carborundum (SiC) is done as a refer-

ence to evaluate the uncatalyzed MSR reaction.

Results and discussion

Fresh catalysts characterization

Fig. 1 (a) shows the diffraction peaks of the freshly calcined

Ru/Co6�xMgxAl2 solids. All cobalt containing solids give

diffraction lines corresponding to the four cobalt oxide spinel

Fig. 1 e (a) XRD patterns (“s”: Co3O4/CoAl2O4/Co2AlO4; “B”:MgAl2O4; “w”: MgO; “*”: RuO2; “¤”:Co2RuO4) and (b) H2

consumption profiles for Ru/CoxMg6LxAl2 calcined catalysts.

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phases which are difficult to differentiate by XRD: Co3O4

(JCPDS N�42-1467), CoAl2O4 (JCPDS N�44-0160) and Co2AlO4

(JCPDS N�38-0814). The MgO periclase type lines (JCPDS N�45-0946) and MgAl2O4 spinel mixed oxides (JCPDS N�73-1959) areonly present for Ru/Mg6Al2. The diffraction spectra of the Mg

containing catalysts present RuO2 in tetragonal phase (JCPDS

N�40-1290). Moreover, solids with high Mg content showed

more intense RuO2 diffraction lines. Table 1 shows RuO2

crystallite size that is calculated from the line broadening of

themost intense reflection of ruthenium oxide (2q¼ 28�) peak.It is observed that when magnesium content in the support

increases, the RuO2 particles size increases. This is due to the

agglomerate formation on Mg rich support. Therefore, the

interaction of Ru with Mg rich supports is different from the

interaction with Co rich supports. The presence of high Co

loading leads to the formation of smaller ruthenium and co-

balt/ruthenium oxide species that are well dispersed on the

supportmaking it non detectable by the XRD technique. These

ruthenium oxide species form agglomerates on Mg rich sup-

ports making its detection by XRD technique possible. The

diffraction line at 37.8� attributed to the presence of Co2RuO4

phase (JCPDS N�73-1048) is just observed for Ru/Co6Al2, indi-

cating that ruthenium integrated the supportmatrix following

the impregnation and calcination steps.

Fig. 1(b) shows the TPR profiles of Ru/CoxMg6�xAl2 catalysts

calcined at 500 �C. Peak I (280e420 �C) on the Ru/Mg6Al2reduction profile is a composite peak that can be attributed to

the reduction of Ru4þ into Ru0 in a stepwisemanner, asMg6Al2support alone does not show any reduction peak in the

studied conditions because of the stability of its oxides [12].

Cobalt containing catalysts present two reduction peaks: peak

II in the range between 100 and 280 �C that is partially due to

the reduction of ruthenium oxide species [14] and peak III at

higher temperatures attributed to the reduction Co3þeAl3þ or

Co2þeAl3þ species [15].

Table 1 represents the experimental (peak I, II and III) and

theoretical (RuO2 / Ru) hydrogen consumptions for Ru/

Co6�xMgxAl2 catalysts. Experimental hydrogen consumptions

are much higher than the theoretical ones calculated for the

reduction of RuO2 into Ru except for Ru/Mg6Al2 catalyst. This

indicates that cobalt is reduced simultaneously with ruthe-

nium [16] and confirms the interaction between the two

metals as shown in XRD results (presence of Co2RuO4 phase).

In the case of Ru/Mg6Al2, experimental hydrogen consump-

tion is lower than the theoretical one required for the reduc-

tion of RuO2 to Ru indicating that Ru4þ has not been

completely reduced.

Methane steam reforming reaction and stability of thecatalysts

Methane conversion (%), H2 and CO molar concentrations

obtained from the catalyzed and uncatalyzed reactions are

displayed in Fig. 2. The uncatalyzed reaction is very slow and

yields negligible product amounts in the studied temperature

range. For instance, methane conversion barely reached 5%

even at 800 �C. H2 and CO molar concentrations in the pres-

ence of SiC are not represented as they are negligible. In the

presence of the catalysts, the MSR reaction proceeds at faster

rates even at the lowest studied temperature. The increase of

Table 1 e RuO2 crystallite size and hydrogen consumption for the different catalysts.

Sample RuO2 crystallite size from XRD (nm) H2 consumption [mmol H2 g�1 catalyst]

Experimental Theoretical

I II III Co3O4/Co RuO2/Ru

Ru/Mg6Al2 79.3 64 e e e

Ru/Co2Mg4Al2 62.1 e 908 4018 6294 198

Ru/Co4Mg2Al2 27.9 e 1753 4125 10,590

Ru/Co6Al2 e e 1936 8344 13,709

Fig. 2 e (a) CH4 conversion (%) and (b) molar concentrations of H2 and CO as a function of the reaction temperature for Ru/

CoxMg6LxAl2 catalysts and for the uncatalyzed reaction.

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the reaction temperature led to an improved methane con-

version and a greater hydrogen quantity. The reactivity in-

creases with the cobalt content, which is the result of

improved surface properties of the support. In fact, the Ru/

Co6Al2 catalyst showed the closest CH4 conversion to the

thermodynamic curve with the highest H2 molar concentra-

tion produced and the lowest CO concentration, indicating

that it favors the WGS reaction. This is confirmed by the in-

crease in the CO2 and the decrease in CO quantities obtained

with increasing cobalt content in the solids (result not shown).

An opposite trend is observed for the Ru/Mg6Al2 catalyzed

reaction indicating the operation of the reversewater gas shift

reaction during the experiment to consume H2 and CO2 and

produce CO. This is due to the presence of RuO2 agglomerates

species as seen in Fig. 1(a). These agglomerates led to a

decrease in the active phase dispersion thus affecting the

contact between the reactant and the active catalytic sites and

leading to a decreased catalytic reactivity. An intermediate

behavior is observed for Ru/Co4Mg2Al2 and Ru/Co2Mg4Al2catalysts. The catalytic performance of Ru/Co6Al2 can be

related to the well-dispersed catalytic active sites (Fig. 1(a))

and it is well-known that a better dispersion of the active

metal on the high surface area support improves the stability

and activity of the catalyst [17,18]. It must be noted that

methane conversion did not exceed 6% in the case of the non-

impregnated solid Co6Al2 indicating the essential role played

by the active phase, “ruthenium oxides” [13].

Developing a stable catalyst is one of the most important

concerns when addressing catalytic reactions. Thus, ten suc-

cessive cycles are done on the best catalytic system Ru/Co6Al2to evaluate its stability in the methane steam reforming re-

action under the following conditions: H2O/CH4 ratio equal to

1:1 with a 50 mg mass of the catalyst diluted with 150 mg of

carborundum SiC. The low molar steam/methane ratio is

adopted to provide harsh reaction conditions to favor deacti-

vation and potential coke deposition. Each cycle consists of a

temperature increase from 400 to 800 �C under the gaseous

mixture flow followed by a cooling in the absence of the

gaseous mixture flow. Fig. 3 (a) shows that the catalyst dis-

plays a quasi-constant stability during the 10 cycles with no

detectable deactivation. It is noticed that, for the second cycle,

the CH4 conversion is equal to 20 and 36% at 400 and 450 �Crespectively while it was just 4 and 16% at the same temper-

atures during the first cycle. The remaining cycles exhibited

profiles similar to the one recorded for the second cycle. It is

the hydrogen produced from the reaction after the 1st cycle

(produced by the methane steam reforming, water gas shift

and methane decomposition reactions) that initiates an in-

situ reduction of the catalyst leading to the improved activ-

ity at low temperatures which is clearly observed in the sec-

ond cycle.

The same catalyst is tested for its stability with time at a

constant temperature of 550 �C over a period of 100 h. The

ratio H2O/CH4 is equal to 1:1, and 10 mg of the catalyst are

Fig. 3 e (a) Evolution of themethane conversion (%) for Ru/Co6Al2 catalyst as a function of the reaction temperature during 10

successive cycles and (b) as a function of time (100 h at 550 �C).

Fig. 4 e Methane conversion in the presence of the Ru/

Co6Al2 and the 5Ru/Al2O3 catalysts at different

temperatures.

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diluted in 190mg SiC. The static deactivation test temperature

is chosen because at higher temperatures, thermal effects

dominate on the catalytic ones. Fig. 3(b) shows that the ac-

tivity remains stable at about 70% methane conversion with

no significant deactivation during 100 h on stream. Thus, the

Ru/Co6Al2 catalyst is stable in the methane steam reforming

reaction even when the conditions, H2O/CH4 ratio and cata-

lytic bed volume, are non favorable.

In order to compare the prepared catalyst with the indus-

trial one, a widely used industrial catalyst 5Ru/Al2O3 (Sigma-

eAldrich) is used. The MSR reaction over this catalyst is

performed under the same conditions detailed in the

Materials and Methods Section. Fig. 4 shows methane con-

version (%) for the prepared Ru/Co6Al2 and the industrial 5Ru/

Al2O3 catalysts. The commercial catalyst presents a lower CH4

conversion even though it has a five times greater amount of

active phase. For example, at 600 �C, methane conversion

reached 83% for the industrial catalyst whereas it is 93% for

the Ru/Co6Al2 catalyst. The lower performance of the indus-

trial catalyst is due to the formation of RuO2 agglomerates on

the freshly calcined industrial catalyst and the formation of

coke (detected by TPO) during the catalytic reaction [19].

Used catalysts characterization

In order to check for carbon deposits and the redox state of the

catalysts after usage, an XRD and a temperature programmed

oxidation (TPO) analyses are carried out after the MSR reac-

tion. Fig. 5 (a) shows the XRD patterns of Ru/CoxMg6�xAl2catalysts after the MSR reaction. Lines corresponding to

reduced metallic cobalt (JCPDS N�15-0806) are present for Ru/

Co6Al2 and Ru/Co4Mg2Al2. Metallic ruthenium diffraction lines

(JCPDS N�06-0663) are only observed for Ru/Co6Al2. The cobalt

oxide spinel phase is present indicating that not all cobalt

species are reduced after the MSR reaction. It is important to

note that for the Ru/Co2Mg4Al2 and Ru/Mg6Al2 metallic cobalt

and ruthenium diffraction lines are absent, and the hydro-

talcite phase (JCPDS N�22-0700) reappears. This is due to the

high affinity of magnesium in the support to the water in the

reactants gaseous stream which leads to a reconstruction of

the hydrotalcite phase. Other phases;MgO,MgAl2O4 and RuO2;

are also present on the XRD patterns of Ru/Co2Mg4Al2 and Ru/

Mg6Al2 catalysts. The oxygen consumptions of the different

used Ru/CoxMg6�xAl2 catalysts are represented on Fig. 5(b).

Two oxidation peaks are observed at 115 and 219 �C for the Ru/

Co6Al2. These latter correspond to the simultaneous oxidation

of metallic ruthenium and cobalt. The two oxygen

Fig. 5 e (a) XRD analysis (“s”: Co3O4, CoAl2O4, Co2AlO4; “^”: metallic Co; “d”: metallic Ru; “*”: tetragonal RuO2; “�” MgAl2O4;

“w” MgO and “#”: hydrotalcite phase) and (b) Oxygen consumption for Ru/CoxMg6LxAl2 catalysts after MSR reaction.

Fig. 6 e EPR spectra for Ru/Mg6Al2 recorded at ambient

temperature after static methane steam reforming catalytic

test.

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consumption peaks are less intense for Mg containing solids

with the absence of any TPO feature for Ru/Mg6Al2. These

results correlate well with the obtained XRD patterns that

confirmed the presence of reduced cobalt and ruthenium

species after the MSR reaction. It is important to note that no

oxygen consumption is recorded in the coke oxidation tem-

perature range (400e600 �C). In addition, no diffraction lines

due to graphitic carbon are observed in the XRD patterns of

any of the catalysts.

Fig. 6 presents the EPR spectra recorded at ambient tem-

perature of the Ru/Mg6Al2 after three isothermal on stream

hours at different temperatures. An isotropic signal, “S1”,

centered at g ¼ 2.0029 is observed for samples tested at 650 �Cand above. The S1 signal is attributed to the presence of a

small amount of carbon species at the surface of the catalyst.

Keeping in mind that the EPR technique is very sensitive to

the presence of trace amounts of paramagnetic species [20], it

is evident that these species were not detected in the TPO

analysis. The intensity of the S1 signal increases and reaches

a maximum at 750 �C related to the highest amount of

formed carbon at this temperature. At 800 �C, a new EPR

signal, “S2”, centered at g ¼ 2.0032 appears. This latter might

be due to coke deposition on the surface of the catalyst at

these temperatures. In fact, carbon one of the products of the

Boudouard reaction (Eq. (4)) which is thermodynamically

favored above 600 �C, while coke is produced by the decom-

position, cracking or condensation of hydrocarbons on the

catalyst surface at higher temperatures (�800 �C) [21]. All

isothermal tests for the cobalt containing catalysts (results

are not shown) revealed a large EPR signal. It is attributed to

the presence of an important quantity of paramagnetic cobalt

species which hinders the detection of deposited carbon and

coke.

However, despite the confirmation that traces of carbon

species in addition to some traces of coke are deposited on the

surface of the catalysts during the MSR reaction, the stability

and the performance of these latter aren’t affected over a

100 h time period.

Conclusions

The prepared Ru/Co6�xMgxAl2 catalysts are tested in the MSR

reaction. The TPR and XRD showed that RuO2 species exist as

agglomerates in magnesium containing catalysts whereas

they are well dispersed at the surface in magnesium free

solids. It proves that higher cobalt content leads to better

active phase dispersion. The Ru/Co6Al2 exhibits an excellent

catalytic performance, where experimental methane conver-

sion matched the theoretical thermodynamic equilibrium. In

addition, H2 production is the highest accompanied with the

lowest CO production over the Ru/Co6Al2 solid. Moreover, the

Ru/Co6Al2 catalyst is more active than the industrial catalyst

with proven stability for 100 h on stream under harsh condi-

tions. The EPR technique revealed the formation of traces of

carbon species by the Boudouard reaction and traces of coke

deposited as a result of the methane decomposition reaction.

The formation of carbon species and coke is temperature

dependant but it does not hinder the catalysts performance

during practical operation periods.

Acknowledgments

The authors thank the CEDRE 2009 program, grant “09 Sci F 7/L

22”, the AUF-CNRS-L and the BRG 8/2009 for financial support.

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