Bi-metallic catalysts of mesoporous Al2O3supported on Fe,Ni and Mn for methane decomposition:Effect of activation temperature

Anis H.Fakeeha,Ahmed S.Al-Fatesh,*,Biswajit Chowdhury,Ahmed A.Ibrahim,*,Wasim U.Khan,Shahid Hassan,Kasim Sasudeen,Ahmed Elhag Abasaeed

1Chemical Engineering Department,College of Engineering,King Saud University,P.O.Box 800,Riyadh 11421,Saudi Arabia

2Department of Applied Chemistry,Indian School of Mines,Dhanbad,India

Keywords:Carbon nanotube Hydrogen production Methane decomposition Manganese promoter Nickel promoter

A B S T R A C T Methane decomposition reaction has been studied at three different activation temperatures(500 °C,800 °C and 950 °C)over mesoporous alumina supported Ni–Fe and Mn–Fe based bimetallic catalysts.On co-impregnation of Ni on Fe/Al2O3the activity of the catalyst was retained even at the high activation temperature at 950°C and up to 180 min.The Ni promotion enhanced the reducibility of Fe/Al2O3oxides showing higher catalytic activity with a hydrogen yield of 69%.The reactivity of bimetallic Mn and Fe over Al2O3catalyst decreased at 800 °C and 950 °C activation temperatures.Regeneration studies revealed that the catalyst could be effectively recycled up to 9 times.The addition of O2(1ml,2ml,4 ml)in the feed enhanced substantially CH4conversion,the yield of hydrogen and the stability of the catalyst.

1.Introduction

The declining nature of non-renewable fossil fuels and global warming stimulated the fundamental research in promoting better energy options with low emission,and therefore renewable alternative energy sources are gaining considerable attention[1].In the recent era,hydrogen is emerging as one of the most promising clean and renewable sources of energy[2,3].Increasing demand for CO and CO2free hydrogen has prompted research to develop a cleaner and environmentally friendly catalytic process[4,5].Among several processes,hydrogen may be produced through a dry reforming process,steam reforming process or partial oxidation of petroleum and natural gas[6–8].The absence of CO and CO2favors the direct thermal decomposition of methane process from the application point of view[9].Pudukudy et al.[10]reported that promoting Ni/MgAl2O4catalysts with Pd,increased the surface area of the catalyst and lowered the reduction temperature and thus provided high catalytic activity and stability for methane decomposition.A hydrogen yield of about 57%was attained at 700°C reaction temperature and no deactivation was observed for 420 min of time on stream.

Fe,Cu,Co,and Ni-based catalysts are very promising catalysts for methane decomposition reaction [11,12].The utilization of unsupported porous NiO and Fe2O3catalysts for non-oxidative thermal decomposition of methane exhibited high activity and stability at several reaction temperatures of 600 °C,700 °C and 800 °C[13].For instance,the NiO and Fe2O3catalysts generated maximum hydrogen yield of 66%and 53%respectively at 800°C.At the end of 6 h of time on stream, the hydrogen yield assumed steady values of 49%and46%respectively.There are several reports for the use of acidic oxide-supported metal nanoparticle catalyst,however,optimization of the process by varying activation temperature,reaction temperature,and recyclability needs to be investigated for development of suitable catalysts[14].Pudukudy et al.[15]investigated the use of unsupported mesoporous nickel ferrite catalysts for the CH4decomposition.The catalytic performance was assessed at various reaction temperatures,and the dependence of the properties of the nanocarbon on reaction temperature was investigated.The NiFe2O4catalyst was found to be highly phased pure and porous.H2yields and nanocarbon were found to increase considerably upon increasing the temperature from 700°C to 900°C;a maximum H2yield of 68%was obtained.No deactivation was observed for the catalyst due to the formation of NiFe bimetallic alloys,which in turn increased the carbon diffusion rate and prevented deactivation of the catalyst. Also, the same authors examined Nickel,Cobalt,and Iron-based monometallic catalysts supported over sol–gel derived silica micro flakes for direct thermal decomposition of methane into COx-free hydrogen and nanocarbon.They found NiO,Co3O4,and Fe2O3are the active phase of the fresh catalysts.A maximum H2yield of 74%was recovered for the Ni-based catalyst,while the Co and Fe-based monometallic catalysts were found to be less active but more stable than the Ni catalyst.Ni,Co,and Fe catalysts produced multi-walled carbon nanotubes,carbon particles with a fruit like morphology and multilayer graphene sheets respectively.In earlier work,Ni,Co,and Fe based bimetallic catalysts supported over mesoporous SBA-15 were employed in the CH4decomposition[16].The experimental result depicted that allof the bimetallic catalysts were highly active and stable for the reaction at 700°C.The maximum H2yield was 56%for the NiCo/SBA-15 catalyst.While a high catalytic stability was presented by the CoFe/SBA-15 catalyst with 51%of H2yield.The catalysts were stable due to the formation of bimetallic alloys.The innovation of the presented work regards the fact that the combination of Mn–Fe and Ni–Fe bimetallic catalysts was considered,the effect on the activation temperatures of 500 °C,800 °C and 950 °C for the chosen bimetallic catalyst was not performed either before.Moreover,no one has also explored the possibility of regeneration and its impact of the recovery of the spent catalyst. In this work,we have studied the effect of activation temperature on the performance of the unpromoted and Ni,Mn promoted Fe/Al2O3catalysts for methane decomposition reaction.All catalysts were prepared by the impregnation method and calcined at 450 °C temperature.The catalysts were being characterized by the N2physisorption,XRD,and H2-TPR techniques.The catalytic activities for a methane decomposition reaction were tested in the fixed-bed reactor at atmospheric pressure.TGA technique was used to characterize the spent catalysts.

2.Experimental

2.1.Catalyst preparation

Supported Fe catalysts used in this study were prepared by an incipient wet-impregnation method.The pellets of alumina(γ-Al2O3;SA6175)were supplied by(Norton,USA).The ferric nitrate[Fe(NO3)3·9H2O;99%](Loba Chemical,India)was used as an active metal precursor.The solution formed from the stoichiometric amount of[Fe(NO3)3·9H2O]and 100 ml double distilled water placed in a crucible,then γ-Al2O3support was added to the solution and stirred at 80 °C for 3 h.Afterwards,the catalysts were dried overnight at 120°C and followed by calcination at a temperature of 450°C for 3 h.30 wt%Fe/Al2O3catalyst promoted by different Ni and Mn loadings(6,15 and 30 wt%)was prepared from the co-impregnation of nitrate salts i.e.Ni(NO3)2·6H2O and Mn(NO3)2·4H2O with Al2O3support using the same procedure as described above.

2.2.Catalyst activity test

Catalytic methane decomposition(CMD)experiments over Fe based catalysts were performed at atmospheric pressure in a vertical stainless steel fixed-bed tubular(9.1 mmi.d.and 13 cm long)micro-reactor(PID Eng&Tech micro activity reference). Activity tests were conducted over a fixed mass (0.3 g) of the catalyst placed into a quartz wool bed and the actual temperature in the reactor was monitored by,a K-type stainless steel sheathed thermocouple placed axially at the center of the catalyst bed.The activity test time used in this study was 180 min.Prior to the tests,the catalysts were subjected to a reduction treatment under a continuous flow of H2(40 ml·min-1)for 90 min at the desired activation temperature.The test volume ratio of the feed gas mixture(CH4and N2)was 1.5/1.0,whereas the total flow rate was 25 ml·min-1with a space velocity of 5000 ml·h-1·gcat.The composition of the outlet gas was analyzed by on-line gas chromatography(Alpha Mos PR2100)equipped with a thermal conductivity detector(TCD).Methane conversion,hydrogen and carbon yields were calculated using the following formulae:

2.3.Catalyst characterization

2.3.1.XRD characterization

A Rigaku(Mini flex)diffractometer,with a CuKαX-ray radiation operated at 40 kV and 40 mA,was used to study the structure of the catalysts before and after the reaction. The scanning 2θ range and steps were at 10°–85°and 0.02°respectively.The raw data file of the instrument was analyzed by X'pert high score plus software.Different phases with their scores were matched with the JCPDS data bank.

2.3.2.N2physisorption study

The specific surface area and distribution of poresize of the catalysts were determined using a Micromeritics Tristar II 3020 surface area and porosity analyzer.The pore size distribution was calculated by the BJH method.

2.3.3.Temperature programmed reduction(TPR)

The TPR measurements were carried out using a Micromeritics Auto Chem II apparatus.Seventy milligrams of sample was taken in the TPR cell and flushed with argon at 150°C for 30 min.Then the sample was cooled down to room temperature.Finally,the furnace temperature was raised to 1000 °C at 10 °C·min-1ramp in 40 ml·min-1flow rate with the H2/Ar mixture(10:90 ratio).The signals of H2consumption were monitored by a thermal conductivity detector(TCD).

2.3.4.Thermo-gravimetric analysis(TGA)

The quantitative analysis of carbonaceous material deposition after reaction over the catalyst's surface was carried out in a TGA-S1 SHIMADZU analyzer under aerial atmosphere.10–15 mg of the used catalyst was heated from room temperature to 900°C at a heating rate of 20 °C·min-1and the loss of weight was measured.

2.3.5.TEM

TEM measurements of spent samples were accomplished on a JEOL JEM-2100F transmission electron microscope operated at 120 kV accelerating voltages to examine the morphology of the deposited carbon.

3.Results and Discussion

3.1.X-ray diffraction(XRD)

XRD image and phase distribution of different catalyst samples are shown in Fig.1(A)and(B).The hematite(Fe2O3)(JCPDS card no.33-0664),maghemite Fe2O3(JCPDS card no.00-039-1346),hercynite(FeAl2O4,JCPDS card no.00-034-0192)and corundum(Al2O3,JCPDS card no.00-046-01212)phases were obtained in 30%Fe/Al2O3catalyst.While magnetite(Fe3O4,JCPDS card no.00-019-0629),hematite(Fe2O3),maghemite(Fe2O3)and trevorite(NiFe2O4,JCPDS card no.00-044-1485)phases were obtained in the XRD pattern of 6%Ni–30%Fe/Al2O3catalyst.On the other hand,the XRD profile of 30%Ni–30%Fe/Al2O3displayed bunsenite(NiO,JCPDS cardno.00-047-1049),hercynite(FeAl2O4,JCPDS card no.00-034-0192)and corundum(Al2O3,JCPDS card no.00-046-01212)phases.In the 6%Mn–30%Fe/Al2O3catalyst,hematite,maghemite,magnetite,AlFeO3,bixbyite(Mn2O3JCPDS card no.00-041-1442),Mn2AlO4(JCPDS card no.00-029-0881),ramsdellite(MnO2,JCPDS card no.00-043-1455),and MnFe2O4(JCPDS card no.00-10-0319)phases were obtained.While the 15%Mn–30%Fe/Al2O3catalyst exhibited hausmannite(Mn3O4,JCPDS card no.00-024-0734),and galaxite(MnAl2O4,JCPDS card no.00-010-0310)phases in addition to the phases of 6%Mn catalyst.

Fig.1.(A)-XRD pattern of(a)30%Fe/Al2O3,(b)6%Ni–30%Fe/Al2O3,(c)15%Ni–30%Fe/Al2O3,and(d)30%Ni–30%Fe/Al2O3;(B)-XRD pattern of(a)30%Fe/Al2O3,(b)6%Mn–30%Fe/Al2O3,(c)15%Mn–30%Fe/Al2O3,and(d)30%Mn–30%Fe/Al2O3;and(C)XRD pattern of(a)30%Fe/Al2O3,(b)15%Ni–30%Fe/Al2O3,and(c)15%Mn–30%Fe/Al2O3catalysts activated at 800 °C.

XRD image and phase distribution of three catalysts(30%Fe/Al2O3,15%Ni–30%Fe/Al2O3and 15%Mn–30%Fe/Al2O3)which have been activated at 800°C are shown in Fig.1(C).The effect of activation temperature could be noticed from the comparison of fresh and activated catalysts.The crystallinity of the three catalysts has increased after heating at 800°C[17].In the case of 30%Fe/Al2O3,in addition of phases present in the fresh 30%Fe/Al2O3sample,the activated 30%Fe/Al2O3sample has new phases of AlFeO3and Fe3O4.From the XRD pattern of activated 15%Ni–30%Fe/Al2O3we observe that it has additional phases of AlFeO3and NiO in comparison with the fresh 15%Ni–30%Fe/Al2O3sample.While comparing the XRD pattern of fresh 15%Mn–30%Fe/Al2O3with the activated 15%Mn–30%Fe/Al2O3sample we may observe that the Mn2O3,and MnFe2O4phases which are present in the fresh sample,have disappeared after activation.The activated 15%Mn–30%Fe/Al2O3sample includes the new phases of Al2O3,Mn2AlO4,FeAl2O4(hercynite),Mn3O4,and MnO2(ramsdellite).

3.2.Temperature programmed reduction(TPR)

Fig.2(A)and(B)shows the TPR profiles.The three well-defined TPR peaks suggest the reduction of Fe2O3(hematite and maghemite),→Fe3O4(magnetite)→ FeO→ Fe.The first peak at around 340–470°C,could be attributed to the reduction of Fe2O3→Fe3O4.The second peak at around 500–780 °C could be assigned to the transition from Fe3O4→FeO.Whereas the third peak in the range 800°C to 950°C temperature could be ascribed to the reduction of FeO→Fe(0)[18–21].

Fig.2.(A)-H2-TPR profiles of catalysts(a)30%Fe/Al2O3,(b)6%Ni–30%Fe/Al2O3,(c)15%Ni–30%Fe/Al2O3and(d)30%Ni–30%Fe/Al2O3;(B)-H2-TPR profiles of catalysts(a)30%Fe/Al2O3,(b)6%Mn–30%Fe/Al2O3,(c)15%Mn–30%Fe/Al2O3and(d)30%Mn–30%Fe/Al2O3;and(C)-hydrogen consumption of different catalysts.

Table 1N2physisorption results for different catalysts

For Ni–Fe/Al2O3series of catalyst,peaks are shifted to the low-temperature region relative to Fe/Al2O3catalyst.The low-temperature reduction peak centered at 370°C was attributed to the bulk NiO.The peak centered at 540°C was assigned to the reduction of Ni ferrite(NiFe2O4)[15].The change in the shape of the reduction curve may be attributed to the reduction of stoichiometric NiAl2O4and non-stoichiometric NiAl2O4phases[22,23].

For Mn–Fe/Al2O3series of catalysts,the reduction peaks were found near 320°C;they intensified with the increase in the loading of manganese from 6%to 30%.The peak centered at around 420°C was attributed to the reduction of Mn2O3→Mn3O4and Fe2O3→Fe3O4phases.The broad reduction peak near 680°C for the higher Mn loading may correspond to the reduction of Mn3O4→MnO and Fe3O4→FeO→Fe phases[24].The 6%Mn loading presented two peaks at the higher temperature between 600 and 1000°C.However,the increase in the percentage of manganese in the present study decreased the catalytic performance.This might be due to poor accessibility of Fe-active centers[25,26].Fig.2(C)displays the hydrogen consumption of all the catalysts.Fig.2(C)also reveals that nickel impregnation increased the reducibility of the 30%Fe/Al2O3catalyst more than the manganese impregnation[18].

3.3.N2-physisorption analysis

Results obtained from nitrogen physisorption analysis of different catalysts are shown in Table 1.The adsorption isotherm and pore width distribution are presented in Fig.3(A)and(B).The catalysts were characterized by the presence of Type-IV isotherms with H3-type hysteresis loop[27,28].The hysteresis loops do not exhibit any limiting adsorption at high P/Po which indicates either non-rigid aggregates of plate-like particle or assemblages of slit-shaped pores[28].The specific surface area was found to be reduced after the Ni and Mn impregnation.This can be attributed to the partial blocking of mesopores by the metal clusters[16].

Fig.4displays the EDS results for(a)30%Fe/Al2O3;(b)15%Ni+30%Fe/Al2O3;and(c)15%Mn+30%Fe/Al2O3and established the consistency and similarity of the actual catalyst concentration to that of its EDS output as demonstrated in the embedded tables of Fig.4.

3.4.Conversion of methane over Mn–Fe/Al2O3catalysts

Fig.3.BET isotherm and pore-size distribution of(A)Ni–Fe/Al2O3and(B)Mn–Fe/Al2O3catalyst calcined at 450 °C temperature.

Fig.4.EDS results of(a)30%Fe/Al2O3;(b)15%Ni+30%Fe/Al2O3;and(c)15%Mn+30%Fe/Al2O3.

Fig.5.(A)-CH4conversion curves for Mn–Fe/Al2O3at Treaction700°C and Tactivation500 °C(a)15%Mn–30%Fe/Al2O3,(b)6%Mn–30%Fe/Al2O3,(c)30%Mn–30%Fe/Al2O3and(d)30%Fe/Al2O3;(B)-CH4conversion curves for Mn–Fe/Al2O3at Treaction700 °C and Tactivation800 °C(a)30%Mn–30%Fe/Al2O3,(b)15%Mn–30%Fe/Al2O3,(c)6%Mn–30%Fe/Al2O3and(d)30%Fe/Al2O3;and(C)-CH4conversion curves for Mn–Fe/Al2O3at Treaction700 °C and Tactivation950 °C(a)30%Mn–30%Fe/Al2O3,(b)15%Mn–30%Fe/Al2O3,(c)30%Fe/Al2O3and(d)6%Mn–30%Fe/Al2O3.

The methane conversion profiles for Mn–Fe/Al2O3catalysts are displayed in Fig.5.At the 500°C activation temperature,the reactivity of the Mn–Fe/Al2O3catalyst was less than the 30%Fe/Al2O3catalyst.This reduction of MnFe2O4occurs at higher temperatures as observed in the TPR study[24].It is also observed from N2physisorption studies that pore width and pore volumes decreased for manganese promoted 30%Fe/Al2O3catalyst.This may also be the reason for the poor activity of Mn impregnated Fe/Al2O3catalysts compared to Fe/Al2O3catalyst.It is suggested that manganese prevents the movement of Fe cations to the surface of the catalyst during activation inhibiting the reduction of Fe.In the activation temperature of 800°C,manganese impregnated catalysts showed lower activity than the catalyst activated at 500°C,while catalyst activated at 950°C indicated the lowest activity.According to previous reports,the low conversion at the activation temperature 900°C may be due to the sintering of iron particles[29].Moreover,alloy formation as a result of synergy between Fe and Mn could also contribute to lower activity as evidenced by XRD.The presence of Fe and Mn aluminates might influence the activity results as well.It can be seen from XRD patterns,Mn addition generated Mn aluminate and no Fe aluminate was detected for 6 and 15%Mn loading.The higher activation temperature would lead to particle aggregation as well as the formation of species which are less active for methane decomposition reaction.The formation of carbon and its interaction with metal particles could lead to metal carbide formation which may inhibit catalytic activity as well.

3.5.Conversion of methane using Ni–Fe/Al2O3catalysts

After nickel impregnation,the catalytic activity showed～61%methane conversion which is quite high with respect to Fe/Al2O3catalyst as shown in Fig.6.The results obtained from the N2physisorption analysis showed that the 30%Ni–30%Fe/Al2O3catalyst has the largest pore size,as well as biggest pore volume among all Ni impregnated catalysts.The TPR profile showed that Ni–Fe/Al2O3catalysts had the highest hydrogen uptake compared to other catalysts.The better reducibility may also be the reason for better activity data of Ni impregnated catalysts.At higher activation temperature,conversion of methane decreased in comparison with lower activation temperature.Sintering of iron particles is expected at the higher temperature which lowers catalytic activity[29].

The influence of doping 30%Fe/Al2O3catalyst with Mn or Ni can be easily seen.Addition of a second metal significantly affected structural properties of the base catalyst as shown in surface area and pore volume results(Table 1 and Fig.3).In each metal doping,a decrease in surface area is observed showing the impact of doping on the base catalyst.Moreover,addition of a second metal improved reducibility,metal–support interaction and amount of hydrogen uptake.The crystalline nature also changed as the interaction of the second metal with the base catalyst generated alloy as well as crystalline species resulting from synergistic interaction between two metals.All these modifications in structural,reduction and crystalline nature of base catalyst with a second metal loading played a significant role in improving the catalytic performance of doped catalysts.

The catalytic results of the present work can also be compared with the already published articles to elaborate the significance of this work.Carrillo and his co-workers[30]studied the role of Fe catalysts in a fluidized bed reactor for methane decomposition reaction.They used 53%Fe/Al2O3catalyst prepared by fusion method tested at 700°C.They found that the 53%Fe/Al2O3catalyst presented only 18%methane conversion.The role of the co-precipitated Fe/CeO2catalyst was investigated for catalytic decomposition of methane in a fixed bed reactor.The catalytic activity tests at 750°C using 30%methane in argon showed a methane conversion of 25%for 150 min time on stream[31].

Wang et al.[6]reported the performance of 42%Ni–20%Fe/Al2O3catalyst for methane decomposition reaction. Itwas found that the catalyst tested at 650°C exhibited methane conversion of 40%for 150 h time on stream.Based on the literature results, it can be inferred that the present catalytic system performed much better than previously reported results.

Fig.6.(A)-CH4conversion curvesforNi–Fe/Al2O3atTreaction700 °CandTactivation500 °C(a)30%Fe/Al2O3,(b)30%Ni–30%Fe/Al2O3,(c)6%Ni–30%Fe/Al2O3and(d)15%Ni–30%Fe/Al2O3;(B)-CH4conversion curves for Ni–Fe/Al2O3at Treaction700 °C and Tactivation800 °C(a)30%Fe/Al2O3,(b)6%Ni–30%Fe/Al2O3,(c)30%Ni–30%Fe/Al2O3and(d)15%Ni–30%Fe/Al2O3;and(C)-CH4 conversion curves for Ni–Fe/Al2O3at Treaction700 °C and Tactivation950 °C(a)30%Ni–30%Fe/Al2O3,(b)15%Ni–30%Fe/Al2O3,(c)6%Ni–30%Fe/Al2O3and(d)30%Fe/Al2O3.

Fig.7.Regeneration profile of 15%Ni–30%Fe/Al2O3catalyst Treaction700 °C and Tactivation500 °C.

Fig.8.TGA profile of 15%Ni–30%Fe/Al2O3catalyst Treaction700 °C and Tactivation500 °C.

3.6.Regeneration study and characterization of spent catalyst

The used catalyst was regenerated in the presence of oxygen at 800°C temperature.It is noteworthy that the regeneration time was set to 180 min.The regenerated catalyst showed good activity for 9 continuous cycles as shown in Fig.7.The TGA profile of the used catalyst is shown in Fig.8.After 180 min the sample showed 89%weight loss.After 1000 min(after 9th regeneration)the weight loss became 59.4%.The morphology of the regenerated 15%Ni–30%Fe/Al2O3catalyst was investigated using TEM.From Fig.9 it could be observed that the filamentous carbon did form during the catalytic methane decomposition reaction.

To further elaborate on the stability of the 15%Ni–30%/FeAl2O3catalyst,the experimental activity for methane decomposition is repeated with the addition of small amounts of oxygen(0 ml·min-1,1 ml·min-1,2 ml·min-1and 4 ml·min-1).Fig.10 displays the carbon yield versus the volume flow rate of injected oxygen.The 0 ml·min-1O2gave the highest carbon yield of about 40.4 formed on the catalyst surface,while the addition of 1 ml·min-1,2 ml·min-1,and 4 ml·min-1O2reduced the carbon yield to 21.3,11.7 and 5.4 respectively.The small addition of oxygen(4 ml)is essential in regenerating the catalyst by decreasing significantly the carbon,which blocks the reactor.

The study of the methane conversion profile for the 15%Ni–30%/FeAl2O3catalyst activated at 500°C is shown in Fig.11.Without feeding O2,the catalyst provided the lowest conversion and the least stability.When small amounts of O2were fed(1 ml·min-1,2 ml·min-1,4 ml·min-1)the conversion and the stability markedly improved.For 4 ml·min-1O2in the feed,the initial conversion increased by 17%and the final conversion increased by 6%.Fig.12 exhibits the%yield hydrogen obtained when different amounts of O2were added to the feed stream.It is evident that yield of hydrogen had increased by the addition of the O2.The regeneration of the catalyst by the O2availed the sites for the reaction and hence upgraded the conversion and the stability.The O2addition at various flow rates(0ml·min-1,1ml·min-1,2ml·min-1and4ml·min-1)presented hydrogen yield of 69.3,74.4,76.2and77.5%respectively.

Fig.9.TEM result of spent 15%Ni–30%Fe/Al2O3catalyst prepared by impregnation method.

Fig.10.Carbon yield obtained after 180 min time on stream for different amounts of O2.

Fig.12.H2yieldcurvesfor 15%Ni–30%/FeAl2O3atTreaction700 °C and Tactivation500 °C,with different amounts of O2.

4.Conclusions

In summary,30 wt%Fe/Al2O3,X wt%Ni–30%Fe/Al2O3and X wt%Mn–30%Fe/Al2O3(X=6,15,30)catalysts were examined for methane decomposition to produce hydrogen at 700°C reaction temperature and at different activation temperatures(500 °C,800 °C,and 950°C).In accordance with TPR results,we found that 30%Fe/Al2O3catalyst was most active at 500°C activation temperature.The activity of this catalyst almost ceased at 950°C activation temperature.The Mn impregnation showed a clear poisoning effect for 800°C activation temperature.Ni impregnation provided a good activity to the catalyst even at 950°C activation temperature.The addition of small amounts of O2(1 ml·min-1,2 ml·min-1,4 ml·min-1)in the feed gases improved significantly the CH4conversion,yield of hydrogen and the stability of the catalyst.

Acknowledgments

The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding this research group No.(RG-1436-119).

Fig.11.CH4conversion curves for 15%Ni–30%/FeAl2O3at Treaction700 °C and Tactivation500°C,with different amounts of O2.