Journal of Industrial and Engineering Chemistry 20 (2014) 2858–2863

Effect of alkaline earth promoters (MgO, CaO, and BaO) on the activityand coke formation of Ni catalysts supported on nanocrystalline Al2O3

in dry reforming of methane

Zahra Alipour a, Mehran Rezaei a,b,*, Fereshteh Meshkani a

a Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iranb Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran

A R T I C L E I N F O

Article history:

Received 25 July 2013

Accepted 4 November 2013

Available online 13 November 2013

Keywords:

Nickel

Alkaline earth promoter

Syngas

Dry reforming of methane

Coke formation

A B S T R A C T

Ni/Al2O3 promoted catalysts with alkaline earth metal oxides (MgO, CaO, and BaO) were prepared and

employed in dry reforming of methane (DRM). The catalysts were prepared by impregnation method and

characterized by XRD, BET, TPR, TPO, and SEM techniques. The obtained results showed that the addition

of MgO, CaO, and BaO as promoter decreased the surface area of catalysts (SBET). The catalysis results

exhibited that adding alkaline earth promoters (MgO, CaO, and BaO) enhanced the catalytic activity and

the highest activity was observed for the MgO promoted catalyst. TPR analysis showed that addition of

MgO increased the reducibility of nickel catalyst and decreased the reduction temperature of NiO

species. The TPO analysis revealed that addition of promoters decreased the amount of deposited coke;

and among the studied promoters, MgO has the most promotional effect for suppressing the carbon

formation. SEM analysis confirmed the formation of whisker type carbon over the spent catalysts.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Production of synthesis gas (H2 and CO) by the catalytic reactionof carbon dioxide reforming of methane has attracted muchattention due to its environmental and industrial importance. Twoimportant greenhouse gases (CH4 and CO2) are simultaneouslyconverted into useful synthesis gases (CO and H2) during the DRMreaction. In addition, this process gives a lower H2/CO ratio, whichis desirable for synthetic reactions of oxygenated compounds, suchas the Fischer–Tropsch and hydroformylation reactions [1–6].

The major drawback of the dry reforming process is the rapidcoke deposition that causes catalyst deactivation, catalyst destruc-tion, and reactor blockage. The coke originates mainly from tworeactions:

The methane cracking reaction

CH4$ C þ 2H2 DH0298 ¼ 75 kJ=mol (1)

* Corresponding author at: University of Kashan, Chemical Engineering Depart-

ment, Catalyst and Advanced Materials Research Laboratory, Km 6 Ravand Road,

Kashan, Iran. Tel.: +98 361 5912469, fax: +98 361 5559930.

E-mail address: rezaei@kashanu.ac.ir (M. Rezaei).

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.11.018

and, CO decomposition reaction:

2CO $ C þ CO2 DH0298¼ �172 kJ=mol½9�11; 17�: (2)

The deposited carbon could cover the active center andconsequently, result in the deactivation of the catalysts. Untilnow, more than 10 kinds of single metals and a large number ofalloys have been reported for the dry reforming reactions [5].Among them, Ni, Co, Rh, and Pt were the most reported catalysts.Due to its higher reactivity, availability, and lower price comparedwith noble metals, nickel is the most frequently reported metal indry reforming of methane. However, Ni-based catalysts deactivateeasily due to carbon deposition and/or metal sintering. Therefore,much effort has focused on improving the activity and stability ofNi in the following aspects: changing the nature of the support,selecting the support, and catalyst preparation method and theaddition of promoters [3,5,11,12]. Al2O3 and SiO2 supported Nicatalysts have been widely investigated for dry reforming ofmethane [5,8,12,13]. Ni/Al2O3 and Ni/SiO2 prepared by impregna-tion had high initial conversions but deactivated with time due tocarbon deposition or sintering. Many modification methods havebeen studied to improve the performance of Ni/Al2O3 and Ni/SiO2

catalysts, which include different preparation methods, pretreat-ments, and precursors or adding metal or oxide promoters toinfluence the acidity and basicity, the dispersion of Ni, and/or themetal support interaction [12]. As carbon formation occurs easily

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. XRD patterns of: (1) 5%Ni/Al2O3, (2) 5%Ni–3%MgO/Al2O3, (3) 5%Ni–3%CaO/

Al2O3, (4) 5%Ni–3%BaO/Al2O3.

Z. Alipour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2858–2863 2859

during dry reforming process, it was suggested that increase inadsorption and activation of CO2 would accelerate the gasificationof the surface carbons, and retard the formation of inactive carbon[5,9]. In order to increase the alkalinity of the catalyst, a lot of basicmaterials were selected to support Ni catalysts. At the same time,alkali-, alkaline-, and rare-earth metals promoted Ni catalysts werealso reported extensively. Certain amount of alkaline-earth metaloxide MgO and CaO could improve the catalytic performance of Ni/Al2O3 and Ni/SiO2 because of the strong interaction between Ni andsupport; and the basic property of metal oxide could depresscarbon formation in the dry reforming process [3,12].

The objective of this study is to prepare supported Ni-basedcatalysts that maintain high activity and stability while minimiz-ing the formation of coke during the dry reforming of methane. Weinvestigated the influence of MgO, CaO, and BaO as promoters onthe activity, stability, and coke formation of Ni/g–Al2O3 catalysts inCO2 reforming of methane. In addition, the effect of feed ratio andGHSV on the catalytic performance of DRM over the catalysts wasstudied.

2. Experimental

2.1. Catalysts preparation

The catalysts employed in the present work were prepared by awet impregnation method. Nickel nitrate (NiN2O6.6H2O),MgN2O6.6H2O, CaN2O6.4H2O, and BaN2O6 were used as Ni, Mg,Ca, and Ba precursors, respectively. g-Al2O3 as catalyst support wasprepared by sol–gel method. For this purpose, aluminum triisopropylate (98% purity, Merck) was first hydrolyzed in distillatedwater by stirring for 1 h at 80–85 8C. Subsequently, HNO3 wasadded drop wise with HNO3 to Al molar ratio of 1:1 and refluxed for12 h at 98 8C. After that, the sol was kept for 2 h at 98 8C in air. Afterthis step the sol was became so viscous. The formed gel was driedovernight at 80 8C and calcined at 600 8C for 4 h. Then, prepared g-Al2O3 was impregnated with an aqueous solution of salt promoterwith appropriate concentration to obtain 3 wt.% of promoter. Afterimpregnation, the promoted support was dried at 80 8C overnightand calcined at 500 8C for 4 h. After that, the promoted g-Al2O3 wasimpregnated with nickel nitrate with the same procedure asdescribed above.

2.2. Characterization

X-ray diffraction (XRD) analysis were performed using an X-raydiffractometer (PAN alytical X’Pert-Pro) with a Cu-Ka monochro-matized radiation source and an Ni filter in the range 2u = 10–808to examine the crystallinity of the prepared samples. The N2

adsorption/desorption analysis was carried out at boiling temper-ature of nitrogen (�196 8C) using an automated gas adsorptionanalyzer (Tristar 3020, Micromeritics). The samples were purgedwith nitrogen gas for 2 h at 200 8C using VacPrep 061 degas system(Micromeritics). Temperature programmed reduction (TPR) anal-ysis was used for evaluating the reduction properties of preparedcatalysts with Micromeritics chemisorb 2750 gas-adsorptionequipment. In the TPR measurement, 50 mg catalyst was subjectedto a heat treatment (10 8C/min) in a gas flow (30 ml/min)containing a mixture of H2:Ar (10:90). Prior to TPR experiment,the samples were heat treated under an inert atmosphere (Ar) at200 8C for 1 h. The H2 uptake amount during the reduction wasmeasured using a thermal conductivity detector (TCD). Tempera-ture-programmed oxidation (TPO) of the spent catalysts wascarried out using a similar apparatus by introducing a gas flow(30 mL/min) containing a mixture of O2:He (5:95) and thetemperature was increased upto 800 8C at a heating rate of

10 8C/min. Scanning electron microscopy (SEM) analysis wasperformed with VEGA TESCAN that was operated at 30 kV.

2.3. Catalytic evaluation

The catalytic evaluation was performed in a tubular fixed-bedcontinuous-flow reactor of quartz under atmospheric pressure.The reactor was charged with 200 mg of the crushed and sieved(35–60 mesh) catalyst. Prior to the reaction, the catalysts werereduced in situ at 600 8C for 4 h in flowing H2 (20 ml/min). Afterthat, the reactant gas feed containing a mixture of CH4 and CO2 wasintroduced into the reactor, and the activity tests were carried outat different temperatures ranging from 550 to700 8C in steps of50 8C that were kept for 30 min at each temperature. The gascomposition of reactants and products were analyzed using a gaschromatograph equipped with a TCD and a carboxen 1000 column.

3. Results and discussion

3.1. Structural properties of the catalysts

Fig. 1 shows the XRD patterns of the promoted and unpromotedcatalysts. It is seen that all catalysts have crystalline structure andas shown in Fig. 1 with addition of alkaline earth promoters (MgO,CaO, and BaO) into the catalysts, no additional diffraction peakcould be clearly observed. This result implies that all of thesepromoters were highly dispersed as small particles on support.Also all XRD patterns did not show the diffraction peaks of NiO; andthis indicates that Ni was highly dispersed on the catalyst supportand hence could not be obviously detected by XRD analysis.Nevertheless, three peaks with high intensity were appeared in allthe catalysts. These peaks were related to Al2O3 (JCPDS Card No.01-130). Crystallite size of Ni particles at 2u = 468 and 2u = 668 wascalculated according to the Scherrer equation, and presented inTable 1.

The structural properties of catalysts are summarized inTable 1. It is seen that addition of promoters reduced the surfaceareas due to pore filling of catalyst support. Furthermore, additionof promoters decreased the pore volume as indicated in Table 1.

The N2 adsorption/desorption isotherm of prepared samples aredisplayed in Fig. 2a. For all the samples, all the isotherms belongedto IV type isotherms with H1 shaped hysteresis loops, which were

Table 1Structural properties of the catalysts.

Catalyst SBETa (m2 g�1) Pore volumeb (cm3 g�1) Pore widthc (nm) Crystallite sized (nm)

2u = 468 2u = 668

5%Ni/Al2O3 173 0.47 8.7 9 5

5%Ni–3%MgO/Al2O3 155 0.42 8.4 10 4

5%Ni–3%CaO/Al2O3 154 0.43 8.3 9 8

5%Ni–3%BaO/Al2O3 169 0.46 8.3 12 10

a Calculated by the BET equation.b BJH desorption pore volume.c BJH desorption average pore diameter.d Average NiO crystalline diameter using Scherer equation from XRD.

Z. Alipour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2858–28632860

the significant features for ordered mesoporous materials. The H1shaped hysteresis loops also illuminated that all the mesoporeswere cylindrical-shaped channels. Furthermore, the pore sizedistributions of these materials are exhibited in Fig. 2b and theposition of the peaks of the curves was all located in the range of5.5–16 nm. It is seen that the addition of promoters shifted thepore size distribution to smaller sizes.

Fig. 2. (a) N2 adsorption/desorption isotherms of (1) 5%Ni/Al2O3, (2) 5%Ni–3%MgO/

Al2O3, (3) 5%Ni–3%CaO/Al2O3, (4) 5%Ni–3%BaO/Al2O3 catalysts and (b) pore size

distributions of calcined catalysts.

3.2. Temperature-programmed reduction (TPR)

TPR profiles of Ni/g-Al2O3 catalysts with and without promo-ters are shown in Fig. 3. All the patterns demonstrated threereduction peaks. It is seen that addition of promoters increased theintensity of reduction peaks, indicating the increasing in reduc-ibility of NiO species in promoted catalysts. The first peak at 550 8Cis ascribed to NiO species with weak interaction with catalystsupport [3,7]. It is seen that with adding CaO and BaO, the intensityof this peak increased. But, by addition of MgO, the intensity of thispeak slightly decreased. The second peak is assigned to NiO speciesstrongly interacting with support [3]. As can be seen, the secondpeak is shifted into the lower temperature by adding MgO. Itmeans that MgO causes the weak interaction between support andactive phase. The last peak located at 850 8C is attributed toNiAl2O4 reduction [7,14]. It is clear that addition of MgO reducedthe NiAl2O4 reduction temperature compared to CaO, BaO, andunpromoted catalysts. Also, the intensity of this reduction peak inCaO and BaO promoted catalysts is more than unpromotedcatalysts. Adding CaO and BaO cause the strong interactionbetween active phase and the support; hence, hardly to be reducedbut addition of MgO causes the weak interaction between activephase and support.

3.3. Catalytic performance

The conversions of CH4 and CO2 are shown in Fig. 4a and b,respectively. With increasing reaction temperature, the conversions

Fig. 3. TPR patterns of (1) 5%Ni/Al2O3, (2) 5%Ni–3%MgO/Al2O3, (3) 5%Ni–3%CaO/

Al2O3, (4) 5%Ni–3%BaO/Al2O3 catalysts.

Fig. 4. (a) CH4 conversion, (b) CO2 conversion and (c) H2/CO ratio. Reaction

conditions: CH4/CO2 = 1/1, GHSV = 12,000(ml/h gcat).

Fig. 5. (a) short time stability of the Ni/Al2O3, Ni–Mg/Al2O3, Ni–Ca/Al2O3 and Ni–Ba/

Al2O3catlystst and (b) long time stability of Ni–Mg/Al2O3 catalyst. Reaction

condition: T = 7008C, CH4:CO2 = 1/1, GHSV = 12,000(ml/h gcat).

Z. Alipour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2858–2863 2861

of CO2 and CH4 increased. It is also obvious that the conversionof CO2 is higher than that of CH4 on the unpromoted andpromoted catalysts due to the reverse water-gas shift reaction(CO2 + H2$ CO + H2O). The H2/CO molar ratio on all the catalysts isdisplayed in Fig. 4c. It is clear that the H2/CO molar ratio for all

catalysts is less than one because of reverse water gas shift reactionaccruing in CO2 reforming of methane. As can be seen in Fig. 4a and b,MgO can improve catalyst activity due to formation of MgAl2O4

phase [16].Xu et al. [3] reported that NiO–MgO–Al2O3 catalyst was

displaying high catalytic activity and long catalytic stability.Monica Garcıa-Dieguez et al. [15] expressed that the Mgincorporation to Ni/Al2O3 catalysts improves the Ni dispersionby surface rearrangement and further reduces the carbonformation.

Helvio Silvester A. Sousa et al. [16] reported that better catalyticperformance was found in case of Ni-containing MgAl2O4 andNiAl2O4 phases due to the increased resistance against physicaldegradation, but the coking was found to decrease the perfor-mance to a minor extent.

The results showed that the addition of CaO and BaO caused adecrease in catalytic activity.

A short time stability test was carried out at 700 8C and theresults showed that all the catalysts had good stability during thetest period of 300 min (Fig. 5a). Also, the long term stability of

Fig. 6. Effect of GHSV on the catalytic activity. Reaction conditions: CH4/CO2 = 1:1,

T = 6508C.

Z. Alipour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2858–28632862

5%Ni–3%Mg/Al2O3 catalyst exhibited very high stability during15 h time on stream (Fig. 5b)

Fig. 6 shows the effect of gas hour space velocity (GHSV) overthe prepared catalysts at 650 8C and with constant feed ratio (CH4/CO2 = 1). As it is obvious, for all the catalysts, both CH4 and CO2

conversion decreased with increasing GHSV because of reducedcontact time, and the amount of adsorbed reactant.

The effect of feed ratio on the catalytic performance of5 wt.%Ni–3 wt.%Mg/Al2O3 catalyst at 650 8C is plotted in Fig. 7.The acquired results show that with increase in the CH4/CO2 from1:2 to 4:1, the CH4 conversion decreased; whereas, the CO2

conversion increased, as a result of the presence of excess CO2 inthe main reaction of dry reforming (CH4 + CO2$ 2CO + 2H2). TheH2/CO ratio also decreased with the increase of CO2/CH4 ratio;since the RWGS reaction took place seriously at a higher ratio ofCO2/CH4.

Fig. 7. Effect of feed ratio on the catalytic performance of 5%Ni–3%Mg/Al2O3 at

650 8C and GHSV = 12,000(ml/h gcat).

Fig. 8. TPO profiles of (a) the spent catalysts with and without promoters (1) Ni/

Al2O3, (2) Ni–Mg/Al2O3, (3) Ni–Ca/Al2O3 and (4) Ni–Ba/Al2O3catlyststs, (b) Ni–Mg/

Al2O3 spent catalyst after running the reaction for 15 h at 700 8C.

3.4. Coke formation

3.4.1. Temperature-programmed oxidation (TPO)

TPO profiles of carbon deposition on the spent catalysts areillustrated in Fig. 8a. The TPO profiles of all the catalysts show threepeaks, indicative of three kinds of carbonaceous species formed onthe catalysts. The first observed peaks on 5%Ni/g-Al2O3 and 5%Ni–3%Ca/-Al2O3 occur around 180 8C while they are appearing at210 8C for 5%Ni–3%Mg/g-Al2O3 and 5%Ni–3%Ba/g-Al2O3 catalysts,attributed to active carbon species or reaction intermediates thatreacted with oxygen at low temperatures. Among these catalysts,MgO promoted catalyst has the lower active carbon species. Thesecond peak occurred at 350–400 8C, ascribed to amorphous and/or graphite forms of carbon. The high temperature peak around680 8C could be assigned to filamentous form of carbon [2]. As it isobvious, the highest amount of coke deposition was correspondingto filamentous carbon. It is seen that by adding promoters (Mg, Ca,

Fig. 9. SEM images of spent (a) Ni/Al2O3, (b) Ni–Mg/Al2O3 catalysts. Reaction

condition: CH4/CO2 = 1, GHSV = 12,000 ml/g h, time on the stream = 300 min.

Z. Alipour et al. / Journal of Industrial and Engineering Chemistry 20 (2014) 2858–2863 2863

and Ba) into the Ni/Al2O3 catalysts, carbon formation is decreased.The decrease in carbon formation is related to enhance basicproperties of the catalysts and CO2 adsorption, resulting in cokeresistance [14]. Based on Fig. 4, MgO promoted catalyst had thehighest activity compared with Ca and Ba promoted catalysts;hence, these catalysts have higher degree of coke formation.

Temperature-programmed oxidation (TPO) was carried outafter 15 h of reaction on the 5%Ni–3%Mg/Al2O3 catalyst (Fig. 8b).The TPO profile of this catalyst clearly demonstrates that threekinds of carbon species deposited on the catalyst surface duringDRM reaction. One small peak observed at a 200 8C could beascribed to active carbon species or reaction intermediates thatreacted with oxygen at low temperatures. The peak which isobserved between 400–500 8C is related to amorphous type of

carbon, and the last peak at 900 8C can be assigned to filamentouscarbon. As it is shown, the major amount of coke deposition wascorresponding to amorphous and filamentous carbons.

3.4.2. SEM images

Fig. 9a and b shows the SEM images of spent Ni/Al2O3 andpromoted nickel catalyst with MgO, respectively. As can be seen, inboth catalysts, filamentous carbon was observed. The SEM resultsshowed that the addition of MgO promoter significantly decreasedthe amount of deposited carbon. These results are in agreementwith those observed from TPO analysis.

4. Conclusions

The Ni/Al2O3 without and with MgO, CaO, and BaO promoterswere prepared and tested in dry reforming of methane. The BETresults showed that the addition of these promoters decreasedSBET. From TPR profile, it can be understood that an addition of MgOinto the Ni/Al2O3 catalyst decreased the reduction temperature ofNiO species; and the catalyst reduced well in reaction conditioncompared to other studied catalysts. Based on activity tests andTPO results, it can found that adding MgO into the Ni/Al2O3 catalystenhanced methane conversion and decreased the coke formationcompared with unpromoted catalyst. Adding CaO and BaO into theNi/Al2O3 reduced the methane conversion, but decreased the cokeformation in CO2 reforming of methane.

Acknowledgment

The authors are grateful to University of Kashan for supportingthis work by Grant No. 158426/16),.

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