CO 2reforming of CH 4over nanocrystalline
zirconia-supported nickel catalysts
M.Rezaei a ,d ,*,S.M.Alavi b ,S.Sahebdelfar c ,Peng Bai d ,
Xinmei Liu d ,Zi-Feng Yan d ,*
a
University of Kashan,Faculty of Engineering,Chemical Engineering Department,Iran
b
Chemical Engineering Department,Iran University of Science and Technology,P .O.Box 16315-67,Tehran,Iran
c
Petrochemical Research &Technology Company (NPC-RT),Tehran,Iran
d
State Key Laboratory for Heavy Oil Processing,Key Laboratory of Catalysis,CNPC,
China University of Petroleum,Dongying 257061,China
Received 9June 2006;received in revised form 2July 2007;accepted 7August 2007
Available online 15August 2007
Abstract
Mesoporous nanocrystalline zirconia with high-surface area and pure tetragonal crystalline phase has been prepared by the surfactant-assisted route,using Pluronic P123block copolymer surfactant.The synthesized zirconia showed a surface area of 174m 2g À1after calcination at 7008C for 4h.The prepared zirconia was employed as a support for nickel catalysts in dry reforming reaction.It was found that these catalysts possessed a mesoporous structure and even high-surface area.The activity results indicated that the nickel catalyst showed stable activity for syngas production with a decrease of about 4%in methane conversion after 50h of reaction.Addition of promoters (CeO 2,La 2O 3and K 2O)to the catalyst improved both the activity and stability of the nickel catalyst,without any decrease in methane conversion after 50h of reaction.#2007Elsevier B.V .All rights reserved.
Keywords:Zirconia;Nanocrystal;Ni catalysts;Carbon dioxide reforming
1.Introduction
CO 2reforming of methane shows a growing interest from both industrial and environmental viewpoint.From an environmental viewpoint,CO 2and CH 4are undesirable greenhouse gases and both are consumed by this reaction.One potential advantage of dry reforming that would have an impact on the industrial sector is the lower H 2:CO product ratio that can be ,1:1or less.A lower H 2:CO ratio is preferred for the production of oxygenated compounds [1,2],and it also introduces the possibility of combining the steam reforming,partial oxidation,and dry reforming reactions to get the desired H 2:CO ratio [3,4]for different applications.The major drawback of this reaction,however,is the rapid deactivation caused by carbon deposition via the Boudouard
reaction (2CO $C +CO 2)and/or CH 4decomposition.Many efforts have focused on development of metal catalysts,which bear high-catalytic performances towards synthesis gas formation,and are also resistant to carbon deposition,thus displaying stable long-term operation.Noble metal catalysts are less sensitive to carbon deposition [5–7].However,transition metals,such as Ni,Fe and Co,are often preferred considering the high cost and limited availability of noble metals.Among these metals nickel might be the optimum active component of the potential catalyst designed [7].Usually,Ni has been supported on different carriers such as MgO,Al 2O 3,promoted Al 2O 3,TiO 2,CeO 2,etc.However,it ten
ds to deactivate by coke formation and sintering of nickel particles [7,8],which is closely related to the catalyst structure and composition [9].Among these supports,ZrO 2has a high-thermal stability as a catalyst support [10–12].ZrO 2has three polymorphs:monoclinic (m-phase,below 11708C),tetragonal (t-phase,between 1170and 23708C)and cubic (c-phase,above 23708C)[13].Among them,the tetragonal phase,(t-ZrO 2),has both acid and basic properties [14],and the t-ZrO 2phase is
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Applied Catalysis B:Environmental 77(2008)346–354
*Corresponding authors.Tel.:+983615555333;fax:+983615559930.E-mail addresses:rezaei@kashanu.ac.ir (M.Rezaei),zfyancat@hdpu.edu (Z.-F.Yan).
0926-3373/$–see front matter #2007Elsevier B.V .All rights reserved.doi:10.1016/j.apcatb.2007.08.004
active for some reactions[15].The use of zirconia requires a high-specific surface area and suitable por
e structure for catalysis applications.However,zirconium oxides generally have surface areas of50m2gÀ1or less,which is rather low compared with conventional supports such as SiO2,Al2O3or TiO2.Higher surface areas are attainable with amorphous zirconia(200–300m2gÀ1),but this was usually achieved at the expense of much lower thermal stability[16].Recently many methods have been explored in order to get nanocrystalline ZrO2powders with high-surface area,such as glycothermal process[17],alcohothermal-SCFD(supercriticalfluid drying) process[18],CO2supercritical drying[19],sol–gel method [20],solid-state reaction method[21],etc.Surfactant-assisted synthesis of nano-inorganic materials has also attracted considerable interest because of its effective soft template effect,reproducibility,and simple maneuverability[22,23].
In this paper,the surfactant-assisted route was employed to prepare mesoporous nanocrystalline zirconia with high-surface area as a support for nickel catalyst in dry reforming reaction and the effect of basic promoters on the catalytic activity and structural properties of the catalysts were investigated.
2.Experimental
2.1.Materials
The starting materials were ZrO(NO3)2Á6H2O,Ce(N-O3)3Á6H2O,La(NO3)3Á6H2O,KNO3,and Ni(NO
3)2Á6H2O. Pluronic P123block copolymer and ammonium hydroxide were used as the surfactant and precipitation agent,respectively.
2.2.Zirconia preparation
In the surfactant-assisted route,aqueous ammonia(25wt.%) was added dropwise at room temperature to an aqueous solution containing zirconium precursor(0.03M)and Pluronic P123 block copolymer surfactant under rapid stirring by careful pH adjustment to11.The surfactant to zirconia molar ratio was chosen as0.03.After precipitation,the slurry was stirred for another30min and then refluxed at888C for24h under continuous stirring.The mixture was cooled to room temperature,filtered and washed,first with deionized water andfinally with acetone for an effective removal of the surfactant.Thefinal product was dried at1108C for24h and calcined at different temperatures.A diagram of the preparation is shown in Fig.1.
2.3.Catalyst preparation
Supported nickel catalysts were prepared by excess-solution impregnation using zirconia powder and an aqueous solution of Ni(NO3)2Á6H2O of the appropriate concentration to obtain a 5wt.%nickel content.After impregnation the powder was dried at808C and calcined at7008C for2h.The Ni-promoted
catalyst was prepared by subsequent impregnation of nitrate salt of promoters and nickel nitrate in the same procedure as described above.2.4.Characterization
The surface areas(BET)were determined by nitrogen adsorption atÀ1968C using an automated gas adsorption analyzer(Tristar3000,Micromeritics).The pore size distribution was calculated from the desorption branch of the isotherm by the Barrett,Joyner and Halenda(BJH)method.The XRD patterns were recorded on an X-ray diffractometer(PANalytical X’Pert-Pro)using a Cu K a monochromatized radiation source and a Ni filter in the range2u=10–808.The relative amounts of tetragonal phase present in samples and the crystallite sizes were estimated as described elsewhere[18].Temperature-programmed reduc-tion(TPR–H2)was performed in an automatic apparatus (ChemBET-3000TPR/TPD,Quantachrome)equipped with a thermal conductivity detector.The fresh catalyst(200mg)was submitted to a heat treatment(108C minÀ1up to8258C)in a gas flow(30ml minÀ1)of the mixture H2:Ar(10:90).Temperature-programmed oxidation(TPO)of spent catalysts were performed in a similar apparatus by introducing a gasflow(30ml minÀ1)of the mixture O2/He(5:95)over50mg of spent catalysts,and the temperature was increased with a heating rate of108C minÀ1up to8008C.Temperature-programmed hydrogenation(TPH)of spent catalysts were performed in the same apparatus as described for TPR.The spent catalyst(about25mg)was submitted t
o a heat treatment(108C minÀ1up to8008C)in a gas flow(30ml/min)of the mixture H2:Ar(10:90).Prior to the
TPR Fig.1.The preparation diagram of zirconium oxide powders.
M.Rezaei et al./Applied Catalysis B:Environmental77(2008)346–354347
and TPO experiments,the samples were outgassed under inert atmosphere,at3008C for3h.In CO2–TPD experiments,the sample was pretreated at3008C for3h in a He atmosphere,after cooled down to room temperature,the pretreated sample was exposed in CO2for30min.Then the sample was purged with He at room temperature for30min.CO2–TPD was carried out with a ramp of108C minÀ1from room temperature to a needed temperature under He stream.
The nickel dispersion and Ni metal surface area was measured by H2chemisorption at408C,assuming that chemisorption stoichiometry is H/Ni=1and that a Ni atom occupies0.065nm2on a Ni particle.Scanning electron microscopy(SEM)and transmission electron microscopy (TEM)investigations were performed with JEOL JSM-5600LV scanning electron microscope(SEM)operated at 15kV and JEOL2010,operated at200kV,respectively.
2.5.Catalytic performance evaluation
Activity measurements were carried out in afixed-bed continuous-flow reactor made of a7mm i.d.quart
z tube at atmospheric pressure.The reactor was charged with200mg of the prepared catalyst.Prior to the reaction,the catalyst was reduced in situ at6508C for4h inflowing H2(30ml minÀ1) and cooled down to5008C in aflow of Ar(30ml/min).After that a reactant gas feed consisting of a mixture of CH4and CO2 (CH4/CO2=50/50vol.%)was introduced into the reactor,and the activity tests were performed at different temperatures, ranging from500to7008C in steps of508C that were kept for 30min at each temperature.The loss in catalyst activity at 7008C was monitored up to50h on stream.The gas composition of reactants and products were analysed using a gas chromatograph equipped with a TCD and a Carbosphere
column.
3.Results and discussion
3.1.Structural properties of the zirconia samples
The XRD patterns and the structural properties of the zirconia samples calcined at different temperatures are shown in Fig.2a and Table1,respectively.The XRD patterns illustrated that the samples calcined at350and4508C are amorphous.Increasing the calcination temperature,the crystal-line tetragonal phase was obtained.The results showed that the tetragonal crystalline phase was stabl
e towards higher temperatures.Increasing the calcination temperature,the crystallite sizes were increased,but the specific surface areas decreased.Of interest is that the tetragonal phase was stabilized at room temperature without addition of any dopants,which can be explained by nano-size effect,which affects the crystalline phase composition and stabilizes of tetragonal phase at room temperature[24,25].The thermodynamically most stable ZrO2 phase at room temperature is the m-phase.Probably,the nano-size effect of the ZrO2crystallites leads to the thermal stabilization of t-phase[24,26].Garvie and Goss suggested that the difference in the surface energy between the tetragonal and monoclinic phases could cause the tetragonal phase to be thermodynamically stable for very small crystals[26].
The pore size distributions and N2adsorption/desorption isotherms of the samples calcined at different temperatures show that all the samples have mesoporous structure(Fig.2b). The nitrogen adsorption/desorption isotherms can be classified as a type IV isotherm,typical of mesoporous materials and the H2hysteresis loop,which means solids consist of particles crossed by nearly cylindrical channels or made by aggregates (consolidated)or agglomerates(unconsolidated)of spherical particles.Of special is that the synthesized zirconia has a high-thermal stability and the mesoporous structure remained towards higher temperatures.
The TEM analysis(Fig.3)shows that the particles of the zirconia calcined at7008C are closely sintered together and most of the particles have a slightly irregular,rounded shape. The TEM analysis of this sample showed a particle size from5 to10nm in diameter.
3.2.Structural properties of the catalysts
Fig.4shows the XRD patterns of the calcined(Fig.4a)and reduced catalysts(Fig.4b)prepared by impregnation of zirconia powder calcined at7008C for4h.Although it
is Fig.2.(a)XRD patterns and(b)pore size distributions and N2adsorption/ desorption isotherms of the zirconia calcined at different temperatures.
M.Rezaei et al./Applied Catalysis B:Environmental77(2008)346–354 348
difficult to identify the difference between the intensities of the peak,corresponding to the NiO,due to the high dispersion of nickel but it seems the NiO peak has a lower intensity on the promoted catalysts than on the unpromoted catalyst (Fig.4a).The same trend was observed for the Ni peak in reduced catalysts (Fig.4b),which means smaller nickel crystallite size or higher nickel dispersion achieved in the promoted catalysts.The H 2chemisorption analysis was performed for the catalysts and the obtained results are reported in Table 2.As it can be seen,the addition of promoters except of the potassium oxide,decreased the nickel crystallite size and led to an increase in nickel dispersion.These results confirmed the XRD results,as described above.The decrease in nickel dispersion for the potassium-promoted catalyst could be due to the lower specific surface area and pore volume of this catalyst,as shown in Table 3.
Fig.5indicates that the catalysts bear a mesoporous structure with a narrow pore size distribution.By addition of promoters,the mesoporosity is positively improved.Table 3shows a decrease in the specific
surface areas of the reduced and calcined catalysts (5%Ni/ZrO 2)after addition of promoters,especially for the potassium-promoted catalyst.The results also show an increase in the pore diameter of the calcined catalyst (5%Ni/ZrO 2)after addition of promoters.
3.3.Temperature-programmed reduction (TPR)
Fig.6presents the results of temperature-programmed reduction (TPR)of the catalysts.For all the samples two major peaks were observed.The first peak at low temperatures could be due to the reduction of bulk nickel,which has a low interaction with the support,and the second peak at higher temperatures,which are related to reduction of nickel with higher interaction with the support.
The TPR profiles of the promoted catalysts with cerium and lanthanum oxides indicate that the addition of these promoters strengthen the metal–support interaction,as the temperatures of the highest TPR intensity were shifted to higher temperatures in comparison to 5%Ni/ZrO 2catalyst.For the 5%Ni/ZrO 2,the
Table 1
Structural properties of the samples Calcination conditions Tetragonal (wt.%)BET area (m 2g À1)Pore
volume (cm 3g À1)D XRD (nm)t(101)t(110)t(112)t(211)3508C/5h Amorphous 3370.3501*–––4508C/5h Amorphous 2810.282  1.2*–––6008C/4h 1002610.253  5.3  6.1  5.4  5.17008C/4h
100
174
0.234
6.4
6
7
4.7
*
Corresponding to the peak at 2u =308
.
Fig.3.TEM picture of the zirconia calcined at 7008C for 4
h.Fig.4.XRD patterns of the (a)calcined and (b)reduced catalysts.
M.Rezaei et al./Applied Catalysis B:Environmental 77(2008)346–354
349
temperatures of the highest TPR intensity were located at about 450and6708C,respectively.Addition of promoters shifted these temperatures to455and7158C for cerium and460and 7308C for lanthanum-promoted catalysts,while the catalyst promoted with potassium oxide showed a different profile, which was dominantly a small peak at4058C and a second one at6708C,although a third H2consumption zone presents at 5258C.It means that in the potassium containing catalyst, nickel presents as several species with different reducibility. The TPR data reveals that potassium accelerates the reduction of nickel oxide,which is in agreement with XRD and H2 chemisorption results.The catalysts with higher nickel dispersion show a higher metal–support interaction.
3.4.Catalytic performance
Fig.7shows that all the promoted catalysts have a higher conversion for CH4and CO2than that of the unpromoted catalyst.Of interest is that the lanthanum-and potassium-promoted samples show even hi
gher activity than that of the cerium-promoted catalyst.It indicates that the potassium and lanthanum oxide has similar promotion effect for the activity of the nickel catalyst.Fig.7also shows that the CO2conversion was higher than of the CH4conversion due to the reverse water gas shift(RWGS)reaction(CO2+H2!CO+H2O).
Fig.8shows the long-term stability of the catalysts after50h of reaction.As it can be seen,the unpromoted catalyst showed a
Table2
H2chemisorption of the catalysts
Catalyst H2uptake(m mol g-1
cat
)Ni area(m2g-1cat)Ni area(m2g-1Ni)Ni size(nm)Dispersion(%) 5%Ni/ZrO2  1.950.076  1.9114.117.08
5%Ni–3%CeO2/ZrO2  2.070.081  2.0213.287.52
5%Ni–3%La2O3/ZrO2  2.260.088  2.2112.198.20
5%Ni–3%K2O/ZrO2  1.760.069  1.7215.59  6.41
Table3
Structural properties of the catalysts
reactor technologySample BET area(m2gÀ1)Pore volume(cm3gÀ1)Pore diameter(nm)
Calcined Reduced Spent Calcined Reduced Spent Calcined Reduced Spent 5%Ni/ZrO2134.6130.7122.60.1660.1710.215  3.78  4.017.01 5%Ni–3%CeO2/ZrO2133.1127.4129.50.1890.1670.231  4.22  4.02  5.97 5%Ni–3%La2O3/ZrO2134.4130.7118.10.1810.1690.238  4.07  3.97  6.50
5%Ni–3%K2O/ZrO2111.34109.6108.50.1660.1640.194  4.19  4.26
5.22
Fig.5.Pore size distribution of the(a)calcined and(b)reduced
catalysts.
Fig.6.TPR profiles of the catalysts,(a)5%Ni/ZrO2,(b)5%Ni–3%CeO2/
ZrO2,(c)5%Ni–3%K2O/ZrO2and(d)5%Ni–3%La2O3/ZrO2.
M.Rezaei et al./Applied Catalysis B:Environmental77(2008)346–354 350

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