Co6−xMgxAl2 catalyst for hydrogen production via methane steam reforming

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A highly reactive and stable Ru/Co6LxMgxAl2 catalyst for hydrogen production via methane steam reforming Doris Homsi a,b,c, Samer Aouad c,*, Ce´dric Gennequin a,b, Antoine Aboukaı¨s a,b, Edmond Abi-Aad a,b a

Univ Lille Nord de France, F-59000 Lille, France ULCO, LCE, F-59000 Dunkerque, France c Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon b

article info

abstract

Article history:

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

Received 7 March 2014

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

Received in revised form

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

17 April 2014

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

Accepted 21 April 2014

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

Available online 19 May 2014

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

Keywords:

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

Methane steam reforming

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

Ruthenium

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

Cobalt Magnesium EPR Deactivation

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 þ H2 O4CO þ 3H2 CO þ H2 O4CO2 þ H2

1

DH+ ¼ þ206 kJ mol 1

DH+ ¼ 41 kJ mol

(1) (2)

A high steam/methane ratio in the feed favors high methane conversions and minimizes carbon deposition produced 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: [email protected] (S. Aouad). http://dx.doi.org/10.1016/j.ijhydene.2014.04.151 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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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.

CH4 4C þ 2H2

1

DH+ ¼ þ75 kJ mol

(3)

1

2CO4CO2 þ C DH+ ¼ 172 kJ mol

(4) 1

CO þ H2 4H2 O þ C DH+ ¼ 131 kJ mol

(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 hydrotalcites [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 enhancement in the oxidation rate of CHx fragments adsorbed on the active metal [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/CoxMg6xAl2 solids 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 “CoxMg6xAl2HT” (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/Co6xMgxAl2. X-ray diffraction (XRD) experiments are performed at ambient temperature on a BRUKER D8 Advance 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 experiments (TPR/TPO) are carried out on Altamira AMI-200 apparatus. In the TPR analysis, the hydrogen flow (5 vol.% in Ar) is 30 mL min1, while in the TPO experiments, oxygen flow (10 vol.% in He) is 30 mL min1. Gases passed through a Ushaped 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 performed with 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 sufficiently small to avoid saturation effect. The measurements are performed at room temperature. The g values are determined 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 catalyst 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 min1 steam and methane mixture with H2O/CH4 ¼ 3 and 30 mL min1 of argon used for balancing. A test using carborundum (SiC) is done as a reference to evaluate the uncatalyzed MSR reaction.

Results and discussion Fresh catalysts characterization Fig. 1 (a) shows the diffraction peaks of the freshly calcined Ru/Co6xMgxAl2 solids. All cobalt containing solids give diffraction lines corresponding to the four cobalt oxide spinel

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

H2 consumption [mmol H2 g1 catalyst]

RuO2 crystallite size from XRD (nm)

Experimental

Ru/Mg6Al2 Ru/Co2Mg4Al2 Ru/Co4Mg2Al2 Ru/Co6Al2

79.3 62.1 27.9 e

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 450946) and MgAl2O4 spinel mixed oxides (JCPDS N 73-1959) are only 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 the most intense reflection of ruthenium oxide (2q ¼ 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 cobalt/ruthenium oxide species that are well dispersed on the support making it non detectable by the XRD technique. These ruthenium oxide species form agglomerates on Mg rich supports 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, indicating that ruthenium integrated the support matrix following the impregnation and calcination steps. Fig. 1(b) shows the TPR profiles of Ru/CoxMg6xAl2 catalysts calcined at 500  C. Peak I (280e420  C) on the Ru/Mg6Al2 reduction profile is a composite peak that can be attributed to the reduction of Ru4þ into Ru0 in a stepwise manner, as Mg6Al2 support alone does not show any reduction peak in the studied conditions because of the stability of its oxides [12].

Theoretical

I

II

III

Co3O4/Co

64 e e e

e 908 1753 1936

e 4018 4125 8344

e 6294 10,590 13,709

RuO2/Ru 198

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/ Co6xMgxAl2 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 ruthenium [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 consumption is lower than the theoretical one required for the reduction of RuO2 to Ru indicating that Ru4þ has not been completely reduced.

Methane steam reforming reaction and stability of the catalysts 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 presence 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

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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Fig. 3 e (a) Evolution of the methane 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).

the reaction temperature led to an improved methane conversion and a greater hydrogen quantity. The reactivity increases 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 concentration produced and the lowest CO concentration, indicating that it favors the WGS reaction. This is confirmed by the increase 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 reverse water 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/Co2Mg4Al2 catalysts. 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 nonimpregnated 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 successive cycles are done on the best catalytic system Ru/Co6Al2 to evaluate its stability in the methane steam reforming reaction 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 deactivation 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 displays 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  C respectively while it was just 4 and 16% at the same temperatures 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 insitu reduction of the catalyst leading to the improved activity at low temperatures which is clearly observed in the second 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. 4 e Methane conversion in the presence of the Ru/ Co6Al2 and the 5Ru/Al2O3 catalysts at different temperatures.

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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.

diluted in 190 mg 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 activity 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 catalytic bed volume, are non favorable. In order to compare the prepared catalyst with the industrial one, a widely used industrial catalyst 5Ru/Al2O3 (SigmaeAldrich) 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 conversion (%) 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 industrial 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].

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/CoxMg6xAl2 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

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 reaction. Fig. 5 (a) shows the XRD patterns of Ru/CoxMg6xAl2 catalysts 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 hydrotalcite phase (JCPDS N 22-0700) reappears. This is due to the high affinity of magnesium in the support to the water in the

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 temperature 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 temperature 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  C and 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 decomposition, 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/Co6xMgxAl2 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 conversion 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 conditions. 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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