Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152

The investigation of Ru/Mn/Cu–Al2O3 oxide catalysts for CO2/H2 methanationin natural gas

A.H. Zamani, R. Ali *, W.A.W.A. Bakar

Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia

A R T I C L E I N F O

Article history:

Received 30 November 2012

Received in revised form 8 April 2013

Accepted 11 April 2013

Available online 23 May 2013

Keywords:

Natural gas

Methanation

Wetness impregnation

Manganese oxide

Copper oxide

A B S T R A C T

Malaysia sour natural gas that containing high contents of carbon dioxide (CO2), hydrogen sulphide

(H2S) and other which can freeze out on exchanger surface, plugging lines and reduce plant efficiency.

Therefore there is need for treat sour to sweet natural gas. In this study, the investigation influences of Ru

over manganese/copper oxides catalysts were conducted and the catalysts were prepared by wet

impregnation method. The Ru/Mn/Cu (RMC) catalysts were activated at different temperatures (100–

1100 8C) for 5 h and at different ratios of Ru. Ru/Mn/Cu(10:30:60)/Al2O3 (RMC10) catalyst calcined at

1000 8C was assigned as the most potential catalyst, which gave 98.5% CO2 conversion and 70% methane

(CH4) formation at reaction temperature of 220 8C. Moreover, the XRD diffractograms showed that the

catalyst calcined at 1000 8C and 1100 8C are highly crystalline phase, while catalysts calcined at 100 8C,

400 8C and 700 8C showed highly amorphous in structure which dominated by Al2O3 support material.

The FESEM analysis revealed that fresh and used catalysts were covered with homogeneously dispersed

and small size surface particles in the range of 0.2–0.3 mm. Furthermore, EDX analysis revealed that

there was 24% reduction of Ru in the used catalyst compared to fresh catalyst. Nitrogen adsorption

analysis showed 9% reduction of surface area over the used catalyst.

� 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

Natural gas at its geological conditions in some deposits containsome complex contaminants such as CO2, H2S, CO and others whichcontribute great environmental hazards when get to the atmosphereand also hindered natural gas processes [1–3]. Liquefaction processwhich is the transformation of natural gas to liquid form involvesoperation at a very low temperature (�161 8C) and as low asatmospheric pressure. At these conditions CO2 can freeze out onexchanger surface, plugging lines and reduce plant efficiency.Therefore removal of CO2 before liquefaction process is necessary.This is done not to overcome the process bottle necks but also tomeet the LNG product specifications (maximum carbon dioxidelimits corresponding to 4 vol% and 50 ppmv), prevent corrosion ofprocess equipments and environmental performance [2].

Among the catalytic reactions, CO2 methanation is the mostpromising method for the removal of CO2. The methane gasproduction is environmental friendly approach for purification ofnatural gas. Many studies have been focused using noble metalcatalysts, e.g., Ru, Pd and Rh as based catalyst for CO2 methanation[4–7] which are very expensive metals. Nevertheless, researchers

* Corresponding author. Tel.: +60 19 7114144; fax: +60 75566162.

E-mail address: rusmidah@kimia.fs.utm.my (R. Ali).

1876-1070/$ – see front matter � 2013 Taiwan Institute of Chemical Engineers. Publis

http://dx.doi.org/10.1016/j.jtice.2013.04.009

still need to explore the use of various inexpensive transitionmetals such as Mn [5], Ni [8–11] and [12], Cu [13,14], La [15] andothers. Among these catalysts, nickel oxide based catalysts havebeen widely used for methanation, however these catalysts aredeactivated in the methanation due to nickel particles are sinteredat high temperature methanation reaction [9–12]. In this work, Mnand Cu oxide was chosen as based catalysts due to their highcatalytic activity and high selectivity for methane [13,14,16]. It wasproposed that the redox couple Cu2+ + Mn3+ = Cu1+ + Mn4+ isresponsible for the activity of the catalyst in CO2 methanation,which Mn oxide acts as oxygen donor (Mn2O3 to MnO2) and Cuoxide as acceptor (CuO to CuO2). However, this catalyst was not apromising catalyst due to lower conversion of CO2 and highreaction temperature (>350 8C) in methanation process [8,9].Therefore, Ru doped was added to Mn/Cu catalyst to overcomethese problems by increasing the percentage conversion of CO2 andachieved at lower reaction temperature (<250 8C). Up to date, Mn/Cu oxide catalyst dopant with Ru has never been reported andexplored towards methanation reaction.

In this research, a series of Ru over Mn/Cu catalyst withdifferent ratio metal contents were prepared by wet impregnationmethod, and they were tested on the CO2/H2 methanation.Therefore, the aim of this research is to investigate the catalyticactivity of Ru over Mn/Cu oxide catalyst on the CO2/H2 methana-tion and physicochemical properties was also investigated.

hed by Elsevier B.V. All rights reserved.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152144

2. Experimental

2.1. Reagents and chemicals

Copper (II) nitrate hexahydrate (Cu(NO3)2�6H2O) from Riedel-de-Haen was used as a based in this research. Meanwhile,ruthenium (III) chloride hydrate (RuCl3�xH2O) was purchasedfrom Sigma Aldrich Chemical and manganese (II) chloride hydrate(MnCl2�2H2O) from MERCK. Aluminium oxide bead (Al2O3)produced by MERCK Eurolab was used as the support materialsfor the preparation of catalysts.

2.2. Preparation of catalysts

All the catalysts were prepared by aqueous incipient wetnessimpregnation method (IWI). The copper loading used was 60 wt%Cu(NO3)2�6H2O which was dissolved in a small amount of distilledwater and was then mixed together with solution of rutheniumchloride salt and manganese salt in a beaker according to thedesired ratio. A homogeneous mixture was obtained by electro-magnetic stirring at room temperature for 30 min. Alumina beadswith diameter of 4–5 mm were used as support material in thisstudy. The support was immersed into the catalysts solution untilthe solution was evenly absorbed on the surface of the support. Thecoating process was repeated three times with drying at ambienttemperature for every coating process. It was then aged inside anoven at 80–90 8C for 24 h followed by calcination using a furnace inthe air atmosphere (stagnant air) at 400 8C, 700 8C, 900 8C and1000 8C for 5 h using a ramp rate of 10 8C/min to remove all themetal counter ions and water presence in the catalyst.

2.3. Catalytic screening of catalyst

The catalytic reaction of CO2/H2 methanation was performedunder atmospheric pressure in a home built micro-reactor coupledwith Fourier Transform Infrared Spectrophotometer (NicoletAvatar 370 DTGS Spectrometer) as displayed in Fig. 1. The samplecatalyst (5.0 g) was put in the middle of the glass tube made ofPyrex glass with diameter 10 mm and length of 360 mm and pre-treatment process at 100 8C for 30 min to activate the sample.

A mixed gas composed of CO2 and H2 was fed through thecatalyst sample that was stored in the reactor furnace. The mixture

Fig. 1. Schematic diagram of home-buil

of CO2 (10 mL/min) and H2 (40 mL/min) gases was introduced intothe reactor system in a stoichiometric ratio of 1:4. Total feed ratewith respect to catalyst weight was maintained at 9600 mL/h g.The FTIR spectra were recorded in a range of 4000–450 cm�1 at theresolution of 4 cm�1 and 5 scans to ensure a better signal-to-noise-ratio. Conversion of CO2 was calculated according to Eq. (1).

% CO2 Conversion

¼½Peak area of CO2�initial � ½Peak area of CO2�experiment

½Peak area of CO2�initial

� 100%

Quantitative CH4 gas analysis of the experiments was done byoff-line gas chromatography using Hewlett Packard 6890 SeriesGas Chromatography System with the injection port temperatureat 150 8C, detection temperature at 200 8C and oven temperature at40 8C. Capillary column (Brand: Ultra 1 932530) with25.0 m � 200 mm � 0.11 mm nominal was used in this system.Helium gas with the flow rate of 20 mL/min with pressure of75 kPa was used as the carrier gas.

2.4. Characterization of catalysts

The characterization was studied by X-ray diffraction (XRD),field emission scanning electron microscope (FESEM), nitrogenadsorption/desorption (NA) and thermogravimetric analysis(TGA). XRD analysis was done by using Diffractometer D5000Siemens Crystalloflex with Cu Ka radiation (l = 1.54060 A). Scanswere performed in step mode of 0.20 s/step. The data obtainedwere analyzed by a PC interfaced to the diffractometer usingsoftware called Diffrac Plus. The results were then compared withthe accumulated powder diffraction file (PDF) data which comeswith the software using in this technique. For FESEM analysis,sample was scanned using Zeiss Supra 35 VP FESEM operating of15 kV, couple with EDX analyzer and 1500� magnification. Thecatalyst sample was bombarded by electron gun with tungstenfilament under 25 kV. N2 adsorption–desorption isotherms for thecatalysts were measured by Micromeritics ASAP 2010 afterdegassing at 100 8C. TGA analysis for the samples was carriedout by TGASDTA 851 Mettler Toledo simultaneous thermalanalyzer up to 800 8C at 15 8C/min. The sample in the form offine powder was placed in an alumina covered crucible, an empty

t micro reactor coupled with FTIR.

Table 1Percentage conversion of CO2 from methanation reaction over alumina supported

Cu/Mn as based catalysts at calcination 1000 8C for 5 h.

Transition metals

based catalysts

% Conversion of CO2

Reaction temperature

100 8C 200 8C 300 8C 400 8C

Cu/Al2O3 0.5 1.1 2.5 5.7

Mn/Al2O3 0.3 0.9 2.1 4.8

Mn/Cu (10:90)/Al2O3 5.2 11.7 14.7 27.9

Mn/Cu (40:60)/Al2O3 7.8 17.5 25.6 38.4

Mn/Cu (60:40)/Al2O3 4.1 12.3 16.2 19.9

Mn/Cu (90:10)/Al2O3 5.3 10.2 13.4 15.3

Fig. 2. Catalytic performance of CO2 conversion from methanation reaction over Ru/

Mn/Cu/Al2O3 catalyst calcined at 1000 8C for 5 h with various loading of ruthenium:

(i) 1 wt% RMC1, (ii) 5 wt% RMC5, (iii) 10 wt% RMC10, (iv) 15 wt% RMC15, and (v)

20 wt% RMC20.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152 145

crucible being the reference. Nitrogen gas with the flow rate of50 mL/min was used as the atmosphere.

3. Results and discussion

3.1. Catalytic performance over prepared catalysts for CO2

methanation reaction

Table 1 illustrates the catalytic performance of single andbimetallic catalysts which were calcined at 1000 8C for 5 h. Thecatalysts were calcined at 1000 8C according to previous workconducted by Wan Azelee et al. [6] and also suggested by Oh et al.

[18] who found that when the calcination temperature wasincreased to 1000 8C, the surface areas with pore are increasesand it will help to increase the catalytic activity towards CO2

conversion. It can be seen that, the CO2 conversion of Cu/Al2O3

catalyst was only 5.7% compared to Mn/Al2O3 catalyst with gave4.8% CO2 conversion at reaction temperature of 400 8C. Theperformance of this catalyst was improved when manganese(10% and 40%) were added to Cu/Al2O3 catalyst which is able toconvert around 38.4% of CO2 at reaction temperature of 400 8C toMn/Cu(40:60)/Al2O3 catalyst. Further increased of manganeseloading into the Cu/Al2O3 from 40% to 60% and 90% showed thedecreased CO2 conversion, which achieved 19.9% and 15.3%respectively. The performance of the prepared catalysts withvarious ratio follows the sequence of Mn/Cu(40:60)/Al2O3 > Mn/Cu(10:90)/Al2O3 > Mn/Cu(60:40)/Al2O3 > Mn/Cu(90:10)/Al2O3.

The increased of Mn loading into the catalyst showed thedecreased of CO2 conversion due to Mn oxide catalyst is oxygendonor in nature. When the amount of Mn was increased in thematrix catalyst, more oxygen will be donated and decreased theperformance of CO2 conversion.

3.2. Effect of ruthenium loading

Fig. 2 compares the amount of Ru loading towards thepercentage CO2 conversion by the RMC catalyst calcined at1000 8C. The Ru loading used were 1 wt%; Ru/Mn/Cu(1:39:60),5 wt%; Ru/Mn/Cu(5:35:60), 10 wt%; Ru/Mn/Cu(10:30:60),15 wt%; Ru/Mn/Cu(15:25:60) and 20 wt%; Ru/Mn/Cu(20:20:60). The addition of 5 wt% Ru, into catalyst showedan increased of catalytic activity for the conversion of CO2

compared to the 1 wt% Ru loading which was 100% of CO2

conversion at 250 8C. Further the increased of Ru loading to the10 wt% showed high significant enhancement reduce reactiontemperature from 250 8C to 220 8C with 98.5% CO2 conversion.Based on the obtained results of 15% and 20% of catalyst dosage,no significant reduction observed on the reaction temperatureand CO2 conversion. This is in a good agreement with Sudhanshuet al. [19] who found that a small amount of noble metalspromoted catalysts to achieve greater activity for methaneformation. These results were also supported by Kowalczyk et al.

[20] who found that ruthenium catalysts are highly selectivetowards methane. Another research by Takeishi and Aika whostudied on Ru catalysts found that a small amount of methanolwas produced on supported Ru catalyst but the methane gas wasproduced thousands of times more [26].

From this observation, it can be concluded that the catalyticactivity of Ru loading follows the trend in the order ofRMC20 > RMC15 > RMC10 > RMC5 > RMC1. From these results,RMC10 catalyst was chosen as the potential catalyst which is moreeconomical using only a small amount of Ru but showed asignificant increased and similar performance was observed withusing high amount of Ru loading of 15 wt% and 20 wt% at lowerreaction temperature 220 8C. Generally, the conversion rate of CO2

for all the prepared catalysts increased with the increasing ofreaction temperature.

3.3. Effect of calcination temperatures

This parameter was investigated to determine the effect ofcalcination temperature on alumina supported catalyst towardsCO2 conversion. RMC10 catalyst was coated on alumina and agedin an oven for 24 h before the catalyst was calcined at five differenttemperatures of 400 8C, 700 8C, 900 8C, 1000 8C and 1100 8C. Fig. 3indicates the trend plot of catalytic activity over RMC10 at variouscalcination temperatures. It can be observed that the highest CO2

conversion was obtained from RMC10 catalyst calcined at 1000 8Cwith 98.5% conversion, followed by catalyst calcined at 1100 8C,900 8C, 700 8C and 400 8C which achieved only 70.4%, 45.9%, 29.9%and 25.9% of CO2 conversion at 220 8C, respectively. The highcatalytic activities of the catalysts are possibly attributed bythe presence of small particles size (<300 nm) that indicateshigh surface area [25] and agglomeration due to calcinationtemperature.

These results suggested that the high calcination temperatureactivates the catalytic centre of the catalyst, thus enhancing theactivity. In addition, the catalysts exhibit larger surface area at thecalcination temperature of 1000 8C compared to those calcined at700 8C (support by results from surface area and FESEM).According to Wan Azelee et al. [11], during calcination, severalprocesses will occur such as the loss of chemically bounded wateror precursors, modification of the texture through sintering,modification of the structure, active phase generation andstabilization of mechanical properties, then increased the surfacearea of those high activity catalyst. It is interesting to note thatsamples calcination at high temperatures 1000 8C (RMC10 sample)showed big curve hysteresis loop and possess a high amount ofmesoporous (Fig. 4(a)).

400 °C700 °C900 °C1000 °C1100 °C

Fig. 3. Catalytic performance of CO2 conversion from methanation reaction over RMC10 catalyst calcined for 5 h at different calcination temperatures: (i) 400 8C, (ii) 700 8C,

(iii) 900 8C, (iv) 1000 8C and (v) 1100 8C.

Fig. 4. Stability test for catalyst: (i) MC40, (ii) RMC5, (iii) RMC10 and (iv) RMC15.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152146

At lower calcination temperature of 400 8C and 700 8C showedonly 73.1% and 80.1% of CO2 conversion at maximum temperatureof 300 8C, respectively. As expected, further increased of thecalcination temperature to 1100 8C induces the decreasing ofcatalytic activity over this RMC10 catalyst. It may be due toformation of large particle size, thus reducing surface area ofcatalyst that leads to low catalytic performance. In other words, theconversion of CO2 over RMC10 catalyst in this research withdifferent calcination temperatures is in the increasing order asfollows: 1000 8C > 1100 8C > 900 8C > 700 8C > 400 8C.

3.4. Stability test

Fig. 4 illustrates the results of the stability measurement of thefour samples. It should be noted that the reaction temperatureswere different (RMC5, RMC10, RMC15 reaction temperature of220 8C and MC40 reaction temperature of 400 8C) and weredecided according to the temperature at which the apparent

Table 2The product and by-product of CO2 methanation reaction over RMC10 catalyst detecte

Catalyst Reactant Reaction temp. (8C) CO2 conve

Product CH

RMC10 CO2 + H2 100 0.0

150 3.3

200 10.8

210 31.1

220 70.0

maximum conversion. The method use for stability testing basedon number of catalytic testing giving similar results. All thesamples showed the same pattern, whereby the CO2 conversiondecreased slowly except for MC40 catalyst which is unstable.Addition of Ru into matrix catalyst showed increased the stabilityfor CO2 conversion. Sample of RMC10 catalyst showed more stablethan sample of RMC5 and RMC15.

3.5. Methane gas formation measurement via gas chromatography

The reactor gas product from FTIR cell was collected and analyzedfor CH4 formation via gas chromatography. Table 2 shows thetesting results of CO2/H2 methanation over RMC10 catalyst detectedvia gas chromatography. There are two main products obtainedduring the CO2 methanation reaction namely water and methane.From Table 2, it can be seen that the percentage of unreacted CO2

decreases as the CO2 was converted into CH4 and H2O, while, CH4

content was increased as temperature increased. Even at reactiontemperature of 220 8C, instead of fully conversion of CO2 to CH4, itconverts to H2O, methanol and other intermediated product [22].Only 70% of methane was formed over this catalyst at maximumstudied temperature.

3.6. Texture and morphology

Fig. 5 gives the N2 adsorption/desorption isotherm plots. It isinteresting to note that samples RMC10 and RMC10U exhibit typeIV isotherms with hysteresis loop of type H3 and possess asignificant amount of mesoporous [25]. In addition, a big cursehysteresis loop is observed for fresh catalyst than for used catalyst,suggesting a high surface area associated with high mesoporousproperty in fresh catalyst compared after used catalyst with smallcurse hysteresis loop indicating low mesoporous property. Theexistence of the mesoporous structure give the optimum pore size

d via gas chromatography.

rsion (%) Unreacted CO2 (%)

4 Other products (e.g., H2O, CH3OH)

1.6 98.4

1.2 95.5

19.7 69.5

10.2 58.7

28.5 1.5

Fig. 5. Isotherm plots of (a) RMC10 catalyst and (b) RMC10U catalyst calcined at 1000 8C for 5 h.

Table 3Composition and texture data of samples (calcined at 1000 8C).

Samples Catalysts (ratio) SBET (m2/g)

Al2O3 Al2O3 192.0

C100 Cu/Al2O3 98.0

MC10 Mn/Cu (10:90)/Al2O3 120.2

MC40 Mn/Cu (40:60)/Al2O3 113.2

RMC5 Ru/Mn/Cu(5:35:60)/Al2O3 224.5

RMC10 Ru/Mn/Cu(10:30:60)/Al2O3 270.6

RMC10a Ru/Mn/Cu(10:30:60)/Al2O3 221.4

RMC10U Ru/Mn/Cu(10:30:60)/Al2O3-U 246.2

RMC15 Ru/Mn/Cu(15:25:60)/Al2O3 265.8

U, used catalyst.a Calcined at 700 8C.

Table 4EDX analysis of fresh and used RMC10 catalysts.

Element Composition of RMC (wt%)

From calculation (expected) From EDX analysis

RMC10a, RMC10, RMC10U RMC10a RMC10 RMC10U

Cu 60.00 64.77 62.79 64.48

Mn 30.00 26.60 28.76 29.10

Ru 10.00 9.63 8.45 6.42

a As-synthesized.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152 147

in helping to adsorb the reactant gases on the surface of thecatalyst itself [23].

Moreover, the corresponding texture data of the samples aresummarized in Table 3. In this research, alumina has 192 m2/g ofsurface area. After impregnation process, some alumina pores willbe blocked which may contribute to the decreasing surface areaand pore diameter of the catalysts. From the SBET analysis, thesurface area increased when metal loading from single tobimetallic catalyst. From the SBET analysis, the surface areaincreased from mono to bimetallic catalyst. Doping catalyst withRu further improved the texture data of Mn/Cu with high surfacearea and this is likely due to the existence of the mesoporousmaterial. Increased the ratio of Ru from 5 wt% to 10 wt% hadincreased the surface area accordingly. Further increased the ratioof Ru to 15 wt% reduced the surface area to is 265.8 m2/g.

From surface area analysis of sample RMC10, it can be seen thatsurface area of fresh catalyst is 270.6 m2/g which was 9.0% higherthan its used catalyst RMC10U which is 246.2 m2/g. According toWan Abu Bakar et al. [11] the BET surface area is presumed to bereduced when there is no generation of new active sites and notransformation of active species occurred during the catalyticreaction. However, the surface area of sample RMC10 catalyst ishigher than RMC5 and RMC15. The high surface area of catalystdenoted the increase of catalyst active sites [25].

The FESEM images of sample RMC10 calcined at 700 8C and1000 8C are presented in Fig. 6. All the micrographs of thesupported catalysts (Fig. 6) showed rough surface morphology. Ingeneral, there was significant change in morphology and elementaldistribution for the entire sample after calcination due to themigration of particle into bulk matrixes. From the Fig. 6(a), RMC10catalyst calcined at 700 8C showed rough surface morphology withspherical shape and more aggregation. As the calcinationtemperature was increased at 1000 8C, the particles showedincrement in size, due to at high calcinations temperature theparticles agglomerate to produce larger particles in size. Althoughthe particles increased in size, the dispersion of smaller particle canstill be observed at higher calcinations temperature.

Fig. 6(b) and (c) shows the FESEM micrographs of RMC10 andRMC10U catalysts, respectively calcined at 1000 8C for 5 h withmagnification of 10,000�. The fresh catalyst showed rough surfacemorphology with homogeneous spherical shape, more aggregationcompared agglomeration and comes with small particles sizes.This smaller particle size is presumed to contribute in theincrement of the surface area. This result agrees well with theparticle size of RMC10 catalyst obtained from FESEM micrograph,where the catalyst showed small particles size (<300 nm). Themorphology of fresh catalyst changed significantly after thehydrogenation reaction which showed the formation of aggregatedand high agglomerated undefined shape on the surface of usedcatalyst (Fig. 6(c)). This observation was possibly due to the heatgenerated during the catalytic reaction which caused the catalystto agglomerate thus decreasing the activity. In this work, it wasfound that the particle size of fresh and used RMC10 catalysts wascategorized in micro level which varies from 0.2 to 0.3 mm. Thesmaller particles size plays an important role to exhibit highercatalytic activity. As the calcination temperature was increased athigh temperature, the particles showed increment in size, becauseat higher calcinations temperature the particles agglomerate toproduce larger particles in size. Although the particles increased insize, the dispersion of smaller particle can still be observed athigher calcinations temperature.

Table 4 shows the EDX analysis for the fresh and used RMC10catalysts. The elemental analysis performed by EDX confirmed thepresence of Cu, Ru, Mn, Al, Au, Cl and O in the potential catalyst. Ascan be seen in the Fig. 7, the Al peak is the highest peak whichsuggested that the catalyst surface is dominated by the Al from thesupport and the catalyst is not well dispersed on the aluminasupport. This may be due to Cu, Ru and Mn being adsorbed into thehigh porosity and high surface area of alumina beads as claimed byNurunnabi et al. [6] and Safariamin et al. [7]. Another explanationfor high weight percentage of Al is because Al2O3 is majoritycompound in catalyst since it is the catalyst support.

From the ratio given by the EDX analysis (Table 4), it seems thatthe elemental ratio comes nearly to the expected weightpercentage. These results have proven that the elements weresuccessfully according ratio preparation. It can also be observedthat the composition of Ru was higher on the as synthesized

Fig. 6. FESEM micrographs of Ru/Mn/Cu (10:30:60)/Al2O3 catalysts, (a) RMC10 calcined at 700 8C, (b) RMC10 and (c) RMC10U calcined at 1000 8C for 5 h, magnification

10,000�, scale bar 1 cm: 1 mm.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152148

catalysts compared to fresh and used catalysts. This phenomenoncan be explained due to the incorporation of Ru into support afterbeing calcined at 1000 8C. Moreover, the used RMC10U catalystshowed decreased weight percentage of Ru from 8.45% to 6.42%(Table 4) compared the fresh catalyst due to the migration of Ruparticles into the porous support during the reaction.

One of the most efficient ways to improve the reactivity of thecatalyst for CO2 methanation is to use materials with large surfacearea and high dispersion as had been explained by Kodama et al.

[17]. The distribution of the elements on the surface of RMC10catalyst can be seen in Fig. 8. Each element (Ru, Mn and Cu) is welldistributed on the surface of the catalyst representing that thecatalyst has higher degree of dispersion.

0.00 1. 00 2. 00 3. 00 4. 00 5. 00 6. 00 7. 00 8. 00 9. 00 10 .00

keV

005

04008001200160020002400280032003600400044004800

Counts

O

Al

ClClMn

Mn Mn Mn

CuCu

Cu CuRu

RuRu

AuAu

Au

Au Au

Fig. 7. EDX spectrum of RMC10 catalyst.

Fig. 9 shows the TGA thermogram of RMC10 catalyst. It showedtwo significant weight lost curves which occurred at 60 8C and280 8C. The total weight loss was 24.6%. Starting from temperatureof 60 8C until 280 8C, free water molecule and nitrate compoundfrom the supported catalysts were removed, while, at 280 8Conwards, nitrate and surface hydroxyl were decomposed from thesamples. Meanwhile, the decomposition of nitrate occurred attemperature range of 190–360 8C. Finally, the observed weight lossat temperature higher than 360 8C should be attributed to theremoval of hydroxyl molecular in the sample. In addition, thephase transformation of cubic alumina to hexagonal alumina wasobserved. This result is further supported by result XRD (Fig. 10)which the Al2O3 cubic phase which was observed in catalysts atlow calcination temperature (<700 8C) had transformed into aAl2O3 hexagonal phase at high temperatures of 1000 8C and1100 8C.

3.7. Structure and Redox behaviour

Fig. 10 depicts the XRD patterns of RMC10 sample calcined atdifferent temperatures. A high crystallinity phases was onlyobserved for catalysts calcined at 1000 8C and 1100 8C, wherebythe catalysts calcined at as-synthesized, 400 8C and 700 8C showedan amorphous phase. At a as-synthesized and 400 8C calcinationtemperatures, the phase is highly amorphous and dominated bythe alumina support. This is in a good agreement with Wang et al.

[12] who found that no crystalline phases was detected in the Ni/Al2O3 catalyst that was calcined at as-synthesized and 400 8C byXRD. This is possibly due to the active components of metal oxides

Fig. 8. EDX mapping profile over RMC10 catalyst calcined at 1000 8C for 5 h.

Fig. 9. TGA thermogram of RMC10 catalyst after ageing in an oven for 24 h at 80–90 8C.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152 149

are highly dispersed in the alumina support which is highlyamorphous. This result is further supported by Tang et al. [16] whoclaimed that the alumina support only exhibits broad diffractionpeaks when calcined below 500 8C.

The Al2O3 cubic phase at 2u values of 46.048 (I100), 67.038 (I93)and 37.648 (I47), with d�obs values of 1.97, 1.39 and 2.39 A (d�ref

values (A): 1.98, 1.39, and 2.38), which was observed in catalystscalcined at 700 8C, had transformed into a Al2O3 hexagonal phase

Fig. 10. XRD diffractograms of RMC10 catalyst calcined at (a) as-synthesized, (b) 400 8C, (c) 700 8C, (d) 1000 8C and (e) 1100 8C for 5 h.

Fig. 11. Proposed mechanism of CO2 methanation reaction over RMC10 catalyst.

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152150

that was detected at 2u values of 35.498 (I100), and 58.048 (I36) withd�obs values of 2.53 and 1.59 A (d�ref values (A): 2.52 and 1.59) aftercalcined at temperatures of 1000 8C and 1100 8C.

Interestingly, new peaks attributed to the RuO2 tetragonalphase species were observed at calcination temperature of1000 8C at 2u values of 28.128 (I100) and 54.398 (I54), with d�obs

values of 3.17 and 1.69 A (d�ref values (A): 3.17 and 1.69) but wasnot observed at calcination temperature of 1100 8C and below700 8C. Our finding was in a good agreement with Tang et al. [15]who claimed that no diffraction peaks from Mn or Ru specieswere observed when calcined below 500 8C. This was the mainreason for the reduction in the catalytic activity of the catalyst

A.H. Zamani et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 143–152 151

towards CO2 conversion at calcination temperature below1000 8C (see discussion catalytic performance). Moreover, thepeaks due to the Mn3O4 tetragonal phase at 2u values of 36.138(I100) and 32.498 (I76), with d�obs values of 2.48 and 2.75 A (d�ref

values (A): 2.48 and 2.76) were observed in catalysts calcined at1000 8C and 1100 8C, but no diffraction peaks of Mn3O4 wasobserved in catalysts calcined at as-synthesized, 400 8C and700 8C.

These results were confirmed by EDX analysis which showedthe existence of Cu, Ru, Mn, Al and O elements in the RMC10catalyst itself (Fig. 8). Therefore, it can be concluded that thesespecies were responsible to the higher conversion of CO2 atreaction temperature of 220 8C (see discussion catalytic perfor-mance). The highest CO2 conversion was obtained for RMC10catalyst calcined at 1000 8C followed by calcined at 1100 8C, 700 8C,400 8C and as-synthesized. The formation of active species such astetragonal phase of RuO2, Mn3O4 and CuO also leads to highercatalytic activity towards CO2 methanation reaction especially thepresent of RuO2 on the catalyst surface.

3.8. Proposed mechanism of RMC10 catalyst

Mechanism of methanation reaction has been studied a longtime ago. Many researchers agreed that the methanation processinvolves Langmuir–Hinshelwood mechanism to support thereaction process between active species and catalyst surface.

A study by Jacquemin and co-workers [24], suggested themechanism of methanation reaction over Rh/Al2O3 catalystinvolves three steps. First step could be the chemisorptions ofCO2 on the surface catalyst. Secondly, the adsorbed CO2 woulddisassociate to form CO(ads) and O(ads) species on the surfacecatalyst. Third step is the reaction of dissociated species withhydrogen. The CO2 methanation over RMC10 catalyst isproposed in Fig. 11. From the Fig. 11, seven step mechanismof CO2/H2 methanation at the surface catalyst are involvedwhich the first step is CO2 and H2 adsorption on surface, followby rearrangement of CO2 and H2 on the surface. The third step isthe dissociation of CO2 into CO and O adsorbed on the surfaceand dissociated species with H2 into H. From the research doneby Sehested et al. [21], they state that at very low COconcentration, CO molecules and H atoms compete for adsorp-tion at the active sites of surface.

Next step is the reaction of two species H+ attached to oxygenO2� and then, the water was formed. After the water in removed,two species H+ attached to C and the last step is the CH4 formation.Fig. 11, has shown that metal based oxide and dopant playdifferent roles. Metal based oxide initiates the reaction by bindingwith CO2 molecule, forming a carbonate species on the surface anddopant allows the reaction to proceed by supplying H atoms that isneeded for further hydrogenation of carbonate to form methane.This catalytic cycle continuously occur as new molecules areattracted to the surface catalyst.

4. Conclusion

Overall performance from the catalytic activity studies oninfluence of Ru over Mn/Cu oxide catalyst, RMC10 catalyst at1000 8C gives 100% conversion of CO2 and 70% methane (CH4)formation at reaction temperature of 220 8C. The XRD diffracto-grams showed that the catalyst calcined at 1000 8C and 1100 8Care highly crystalline phase while, catalysts calcined below700 8C showed highly amorphous in structure which dominatedby Al2O3 support material. The FESEM analysis revealed thatfresh and used catalyst were covered with homogeneouslydispersed small size surface particles in the range of 0.2–0.3 mm.Furthermore, EDX analysis revealed that there was small

reduction of Ru in the used catalyst compared to fresh catalyst.Nitrogen adsorption analysis showed RMC10 exhibit type IVisotherms with hysteresis loop of type H3 and possesses asignificant amount of mesoporous property with high surfacearea. Addition of Ru into matrix catalyst showed more stableconversion of CO2 and high catalytic activity with lowertemperature reaction.

Acknowledgements

The authors would like to thank the Ministry of HigherEducation (MOHE), Malaysia for the financial support given underthe Research University Grant, Vot No 01H57 and UniversitiTeknologi Malaysia for the financial support.

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Rusmidah Ali was born on July 17, 1957 at Klang, Selangor (Malaysia). She finished herdegree in chemistry in 1980 at Universiti Kebangsaan Malaysia (UKM). Later, shefinished her MPhl studies at the University of Southampton in the United Kingdom in1983 and PhD Duraham University in 1987. She joined Universiti Teknologi Malaysia(UTM) 1980 and presently is an Associate Professor of Inorganic Chemistry. Her majorresearch works is on photocatalysis of wastewater treatment. Dr. Rusmidah Ali is the

author of more than 50 papers published in national and international journals. Shealso had published 11 books.

Wan Azelee Wan Abu Bakar was born on May 11, 1959 at Kelantan (Malaysia). Hefinished his degree in chemistry from Universiti Kebangsaan Malaysia (UKM) in 1983.Later, he continued his studies and majoring in heterogeneous catalysis at NottinghamUniversity, England and received his PhD in 1995. He then joined Universiti TeknologiMalaysia (UTM) and presently is a Professor of Inorganic Chemistry at this university.His field of research is concentrating on catalyst synthesize and its applications.Professor Dr. Wan Azelee Wan Abu Bakar is the author of 100 papers published innational and international journals and had published four chemistry books.

Ahmad Zamani Ab Halim was born on November 10, 1982 at Terengganu (Malaysia).He obtained his first degree in Science (Industrial Chemistry) from Universiti TeknologiMalaysia (UTM) in 2005 and master in Chemistry (Research) in 2008. Currently, he ispursuing his PhD in Chemistry at UTM, conducting the research in the field ofmethanation and catalyst.