Pd/Al2O3 catalysts modified with Mg for catalytic combustion of methane:Effect of Mg/Al mole ratios on the supports and active PdOx formation

ZHAN Ying-ying,KANG Liang,ZHOU Yu-chang,CAI Guo-hui,CHEN Chong-qi,JIANG Li-long

(National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University,Fuzhou 350002,China)

Abstract:Pd/Al2O3 catalysts modified by different amount of magnesium were fabricated for catalytic combustion of methane (CCM).After the introduction of different amount of magnesium,Al2O3,MgAl2O4-like mixed oxide and Mg(Al)Ox solid solution were formed.Owing to the formation of distinguished supports,the supported Pd species,i.e.metallic Pd,PdOx and support-Pd oxide complex were formed,and they were quite different in relative content and Pd?PdO transformation ability.It was found that PdOx was active at low temperature,while metallic Pd particles and support-Pd oxide complex were active at high reaction temperature.The one with Mg/Al mole ratio of 1∶3 was the most easily in Pd?PdO transformation,demonstrating the best catalytic activity towards CCM reaction.

Key words:methane catalytic combustion;Pd-based catalyst;MgAl2O4;Mg(Al)Ox

Catalytic combustion of methane (CCM)has been triggering considerable research interests because methane is a greenhouse gas with a global warming potential about 21 times that of CO2[1],and the CCM reaction that carried out at relatively low temperature (<500 ℃)can reduce the emissions of NOxand CO[2].However,due to the high stability of C-H bond,methane is difficult to be oxidized compared to ethane,propane or other long-chain hydrocarbons[3],which has resulted in a great effort to develop efficient catalysts for CCM reaction operated at low temperature[4,5].

Pd-based catalysts are known among the most active catalysts for CCM reaction[2,6-8].Pd metal atoms (*-*),chemisorbed oxygen sites (O*-O*)and site pairs consisting of Pd2+and lattice O2-ions in bulk PdO are active sites for C-H bond activation depending on the O2/CH4ratios from rich to lean conditions[9].On bare surfaces (*-*),Pd atom inserts into the C-H bond with electron back donation from Pd to C-H antibonding states,forming tight three-center (H3C…Pd…H)transition states.On O*-saturated Pd surfaces (O*-O*),homolytic C-H bond cleave leads to form radical-like CH3· species and O-H bonds (O*…CH3·…*OH).On PdO surfaces,the site pairs (Pdox-Oox)cleave C-H bonds heterolytically viaσ-bond metathesis,with Pd2+adding to the C-H bond and O2-abstracts the H-atom to form a four-center (H3Cδ-…Pdox…Hδ+…Oox)transition states without detectable Pdoxreduction[10].Amongst,PdO clusters are much more effective in activating the C-H bonds than that uncovered or O*saturated Pd0clusters[10].Hence,it is generally accepted that PdO phase is the most active phase for CCM reaction[11,12].Taken this into account,catalysts with suitable Pd-support interaction in stabilizing PdO structures and recovery of PdO from metallic Pd in forming active PdO sites have been studied[4,12-14].

The support upon which Pd species dispersed is of great importance to the formation of active PdOxstructures[15].Due to the unique oxidation kinetics and oxygen storage capacity of CeO2,PdOxformation occurs more readily on its surface compared to other supports[15,16].CeO2modified with ZrO2or incorporating Zr4+into the lattice of CeO2to form CexZr(1-x)O2solid solution,as support for Pd-based catalysts,demonstrates improved catalytic stability (thermal and sulfur tolerance )and activity towards CCM reaction compared to the bare one[4,17,18].Alumina is another critical support for Pd-based catalysts over CCM reaction due to its unique thermal stability and mechanical strength.Pd-Al2O3interaction varies with different crystalline phases of Al2O3(γ-,θ- andα-),and thus alters the crystal sizes of the supported Pd particles,which in turn affects their catalytic activities towards CCM reaction[13].Pd-Al2O3catalyst with enhanced catalytic performance for CCM reaction has also been reported by introducing additives,e.g.Ce,La or Ba[7,19,20].

Magnesium was reported to be an effective promoter for the Pd-Al2O3catalyst in CCM reaction[21,22].After introducing magnesium,the crystal features of Al2O3were detected to beγ- orα-Al2O3,and a large amount of Mg (60%)addition resulted in forming MgAl2O4phase[21,22].In our work,a series of Pd/Al2O3catalysts modified with Mg additive were fabricated with different Mg/Al mole ratios.It was shown that MgAl-hydrotalcite precursors were fabricated in the Mg/Al ratios (mol)ranging from 1/4-3/1.After calcination,MgAl2O4-like oxides or Mg(Al)Oxmixed oxides were formed by incorporating Mg2+into Al2O3lattice or Al3+into MgO lattice structures,respectively.For comparison,Pd/Al2O3catalyst without Mg modification was also employed and the support was characterized to beγ-Al2O3.To the best of our knowledge,the effect of different Mg-Al oxides,e.g.MgAl2O4-like oxides or Mg(Al)Oxmixed oxides,supported Pd-based catalysts for CCM reaction has not been reported.Moreover,their structures and catalytic performances of the Pd-based catalysts were evaluated to provide insight into their structure-property relationship using a suite of characterization techniques.

1 Experimental

1.1 Catalyst preparation

A series of Al2O3supported Pd catalysts modified with Mg additive with different Mg/Al mole ratios were prepared by a co-precipitation method.In a typical procedure,calculated Mg(NO3)2·6H2O,Al(NO3)3·9H2O and Pd(NO3)2·6H2O were dissolved in 100 mL deionized water and simultaneously added with NaOH (2 mol/L)aqueous solution into a flask containing 20 mL 0.2 mol/L Na2CO3aqueous solution at room temperature.The pH value was adjusted to 10.0 during the reaction,and after stirring for 40 min,the as-formed slurry was heated to 80 ℃ and aged for 15 h without stirring.The precipitates were filtered,washed with deionized water until a neutral pH value.After drying at 80 ℃ for 10 h,the precursors were calcined in stagnant ambient air at 800 ℃ for 4 h.The as-prepared catalysts (Pd/MgxAlyO)with Mg∶Al mole ratios of 0∶1,1∶4,1∶3,1∶2 and 3∶1 were denoted to Pd/Al2O3,Pd/MgAl4,Pd/MgAl3,Pd/MgAl2and Pd/Mg3Al,respectively.The Pd loading is fixed at 1%.

1.2 Catalyst characterizations

X-ray diffraction patterns (XRD)were recorded on a Panalytical X’Pert Pro diffractometer with a monochromatic CoKα(γ= 0.1790 nm)radiation at 2θranging from 10° to 100°.

The textural characterization was carried out using the nitrogen adsorption/desorption method (N2-physisortpion)operated at 77 K on a Micrometrics ASAP 2020 analyzer.Prior to the measurement,samples were degassed in a vacuum at 250 ℃ for 4 h.

Pd loading content and Mg/Al mole ratios were determined by an inductively coupled plasma optical emission spectrometer (ICP-OES,Optima 8000,Perkin-Elmer).

Transmission electron microscopy (TEM)was performed on a FEI-Tecnai G2 F20 field emission transmission electron microscope operated at 200 kV.

Hydrogen temperature programmed reduction (H2-TPR)and temperature programmed oxidation (TPO)measurements were carried out using a Mciromeritics Autochem 2920 apparatus.For H2-TPR measurement,samples were first pretreated with pure He at 300 ℃ for 1 h.After cooling to room temperature and switching on the 10% H2/Ar reducing gas (30 mL/min),the H2-TPR was performed at a rate of 10 ℃/min from -30 to 500 ℃.The H2consumption was recorded using a TCD detector.

For TPO experiments,samples were first heated to 400 ℃ in flowing 2% O2/He.After cooling to 300 ℃,the TPO was performed by heating to 900 ℃ at a ramping rate of 10 ℃/min and holding at 900 ℃ for 10 min,followed by cooling to 300 ℃ at the same ramping rate.

For H2-O2titration experiment,100 mg sample was first pretreated under 300 ℃ with 10% H2/Ar for 1 h,and then purged with pure Ar for another 1 h until the temperature down to 110 ℃.H2absorbed on the surface of catalyst was titrated by O2pulses until full saturation,and then the resulting monolayer of adsorbed O2was titrated using H2pulses until observing the equal areas of eluted peaks.The Pd surface area was calculated from the volume of H2used for titration of O2by the following equation[23]:

Pd surface area:

(1)

1.3 Catalytic activity tests

Catalytic combustion of methane was carried out by loading 45 mg catalysts diluted with an identical weight of quartz sand with the same particle sizes (40-60 mesh)into a quartz micro-reactor on a quartz wool bed.The tests were carried out at the temperature ranging from 300 to 650 ℃ at a space velocity of 100000 mL/(h·gcat).Prior to each test,the catalyst was exposed to pure N2with a flow rate of 50 mL/min from room temperature to 300 ℃.The feed gas composition was 2% CH4,4% O2,20% CO2and balance with N2.After passing through a condenser to remove the residual water,the effluents entered a gas chromatograph (Shimadzu,GC-2014)equipped with TCD and FID to evaluate the concentration of CH4,N2,O2,and CO2,from which the methane conversion was calculated.

2 Results and discussion

2.1 Crystal and textural structures

XRD patterns of the support Pd catalysis are shown in Figure 1.A distinct peak (2θ=39.6°)and two small peaks (2θ=64.6° and 85.4°),marked with “*”,are assigned to PdO (JCPDS 43-1024).This indicates that a portion of Pd species are existed as PdO structures,while a portion of them are detected to be metallic Pd and/or Pd-support oxide complex,which will be depicted in the following H2-TPR characterization results.Pd loadings for all the catalysts,determined by the ICP-OES measurement,are close to the nominal ones (1%)as shown in Table 1.It also can be found that Mg/Al ratios for all the catalysts are also close to the theoretical ones when different amounts of Mg are introduced.

From Figure 1,γ-Al2O3(JCPDS 29-0063)with poor crystalline is detected in the catalyst without Mg addition,observing small and broad characteristic diffraction peaks,while Mg-Al mixed oxides are detected in the Mg-containing catalysts (Figure 1b-1e).For those with Mg/Al mole ratios lower than 1 (Figure 1b-1d),MgAl2O4-like mixed oxides (JCPDS 70-5187)are formed due to the incorporation of Mg2+into the lattice of Al2O3.With increasing Mg/Al ratios from 1/4 to 1/2,the characteristic diffraction peaks shift to lower 2θvalues,indicating more Mg2+incorporates into Al2O3,and for Mg/Al=1/2 (Figure 1d),MgAl2O4spinel oxide is formed.A blue shift is also observed in XRD patterns forγ-Al2O3supported Pd catalysts modified with Mg prepared by IWI method[21].It is attributed to the enrichment of MgO species on the surface of catalysts,resulting in the structural distortion inγ-Al2O3because MgO species are detected with a Mg addition of 10%.In our cases,all the catalysts are prepared by co-precipitation method,and no diffraction peaks corresponding to MgO species are observed with a high Mg content (9.8%-16.9% in catalysts with Mg/Al mole ratios of 1/4-1/2),therefore,the blue shift is ascribed to the incorporation of Mg2+.When Mg/Al=3 is adopted,a periclase-like phase is observed,which can be assigned to the Mg(Al)Oxsolid solution from Mg-Al hydrotalcite decomposition,attributing to the incorporation of Al3+into the lattice of MgO[24].Crystal sizes of PdO calculated from Pd(101)diffraction peak for each catalyst are depicted in Table 1.After the addition of Mg,PdO with smaller crystal sizes is obtained,and the one with Mg/Al=3 shows the smallest size of 11.9 nm.

To learn more,XRD patterns of the precursors for those catalysts modified with Mg additives before calcination are collected as shown in Figure 2.As for the precursors with low Mg/Al ratios (<1),both Mg-Al layered double hydroxides (LDHs)Mg4Al2(OH)12CO3·3H2O (JCPDS 51-1525,denoted with “●”)and Al(OH)3(JCPDS 03-0145,denoted with “*”)are detected.After calcined at 800 ℃,which is higher than the decomposition temperature of Mg-Al LDHs precursors (<500 ℃,obtained from TG analysis),Mg4Al2(OH)12CO3·3H2O LDHs and Al(OH)3decarbonated and dehydroxylated to form poorly crystalline Mg(Al)Oxsolid solution and Al2O3.With further increasing the calcination temperature,Al3+cations,a homogeneous structure in Mg(Al)Oxsolid solution by occupying octahedral positions,could convert to tetrahedral coordination by partially forming MgAl2O4spinel[25].Mg(Ni)Al2O4spinel species were also detected by Ohi et al[26]in catalysts with high Al contents (0.5≤(Mg+Ni)/Al ≤2).For Pd/Mg3Al catalyst,the Mg2+/Al3+cation ratios are in the range of 2-4,which leads to feasible formation of Mg-Al LDHs Mg6Al2(OH)18·4.5H2O (JCPDS 35-0965,denoted with “◆” in Figure 2),and subsequently decomposes to Mg(Al)Oxsolid solution after calcination[24,27].

Table 1 Physical-chemical properties of the Pd/MgxAlyO catalysts with different Mg/Al mole ratios

a:determined by the ICP-OES analysis;
b:estimated from PdO (101)diffraction peak at ca.39.5° by Scherrer equation;
c:BET surface area calculated from N2physisorption isotherm;
d:temperature hysteresis (thystersis)between the heating and cooling cycles from TPO profiles in Figure 4，for example,in Pd/Al2O3catalyst the decomposition of PdOxstarts at 710 ℃ during the heating process,while the oxidation of Pd starts at 574 ℃ during the cooling process,sothystersis=(710-574)℃ =136 ℃;
e:H2consumption calculated from H2-TPR profiles (sum of peakαandβ);
f:H2consumption calculated from H2-TPR profiles (subtracting H2consumption of peakαandβto that of peakγ);
g:Pd surface area derived from the H2-O2titration experiment

Morphologies of the supported Pd catalysts are depicted in Figure 3.Small particle size is observed for Pd/Al2O3,while larger particles are formed after a small amount of Mg is added (Figure 3(b)-3(d))due to the formation of MgAl2O4-like mixed oxides.For Pd/Mg3Al catalyst (Figure 3(e)),plate-like morphology (rectangle with black line)is preserved after decomposition of MgAl-LDHs precursor into Mg(Al)Oxsolid solution shaped by small particles.These are in accordance with the above XRD results.

Texture properties are investigated by a N2-physisorption technique.As seen in Table 1,a highABET(168 m2/g)is observed for Pd/Al2O3.This is ascribed to their poor crystalline,broad diffraction peaks of Al2O3,and/or small particle sizes of Al2O3support shown in Figure 3(a).The highestABETis found for the Pd/Mg3Al catalyst,which should be attributed to the formation of Mg-Al LDHs precursor[28].For catalysts supported on MgAl2O4-like oxides,ABETis higher than 100 m2/g,which is larger than those reported in literatures for MgAl2O4mixed oxides prepared by combustion or flame pyrolysis methods[29,30],confirming the effect of LDHs-containing precursors in obtaining mixed oxides with high surface areas.Pore size distribution suggests that the mesoporous pores are formed in all the catalysts.Pd/Al2O3catalyst shows uniform pores with diameters from 4 to 10 nm.After the introduction of Mg,larger pores are formed with most of them sizing from 10 to 20 nm,while for the Pd/MgAl3catalyst parts of them located at 20-30 nm.

2.2 Pd?PdO transformation and Pd chemical states

Although both metallic Pd (saturated with chemisorbed oxygen or not)and PdO are active for CCM reaction,PdO species are reported more active especially at low reaction temperature[12].Therefore,for Pd-based catalysts,Pd?PdO transformation capacity is reported to be crucial[4],which is commonly investigated by TPO measurements.In Figure 4,oxygen release and uptake profiles measured by TPO characterization for the as prepared Pd/MgxAlyO catalysts are present.

One broad oxygen release peak at ca.505 ℃ is detected,including the MgAl3support (Figure 4(f)).This is desorption of chemisorbed O2because the catalysts are pretreated with 2% O2/He at 400 ℃ prior to TPO measurement.No such a peak is observed for the Pd/Al2O3catalyst when no pre-oxidation step is involved in the TPO measurement[4,20].At a temperature higher than 700 ℃,those oxygen release peaks are assigned to the decomposition of PdOxspecies,and the number of peaks is dissimilar while PdO supports on different supports.According to the previous studies[4,31],this suggests different PdO species and/or PdO-support interaction in the Pd/MgxAlyO catalysts.

For Pd/Al2O3catalyst,a magnitude peak at 780 ℃ with a shoulder at 740 ℃ and a small peak at 879 ℃ are recorded.The shoulder peak is associated with amorphous PdOxdispersed on bulk Pd metal (PdOx-Pd)suggested by Farrauto et al[32]and the peak at 780 ℃ is ascribed to more stable PdO crystals[33].The one located at the highest temperature (t=879 ℃)is assigned to the PdO strongly interacts with Al2O3support,possibly consisting of a support-Pd oxide complex[31].After modified with Mg,only one or two PdO decomposition peaks are observed without the presence of amorphous PdOx.In Pd/MgAl4catalyst,merely PdO crystals are formed.With increasing doping amount of Mg from 1/4 to 1/2 (Pd/MgAl4to Pd/MgAl2),the decomposition of PdO interacted strongly with supports emerges,and the relative ratio of support-Pd oxide complex to PdO crystals increases.As for Pd/Mg3Al catalyst,only one decomposition peak corresponding to support-Pd oxide complex is detected.It is suggested that the interaction between PdO and supports become stronger after Mg modification,and the one with more Mg addition shows stronger Pd-support interaction.

When cooling the catalysts in 2% O2/He from 900 to 300 ℃,re-oxidation of Pd metals occurs.No O2uptake due to the re-oxidation is observed until about 650 ℃,which is lower than the decomposition of PdO species (>700 ℃).This temperature hysteresis (thystersis)between the decomposition (tde)and reformation (tre)of PdO was first reported by Farrauto et al[32],and the reformation percentage of PdO is about 1/3 of the decomposition ones.Smallerthystersisis reported to indicate a larger temperature stability range for PdO[15],which is quite favorable for catalytic combustion applications because PdO is the most active site.thystersisof the Pd/MgxAlyO catalysts is summarized in Table 1,indicating that only appropriate amount of Mg addition (Mg/Al=1∶3)can narrow thethystersis,and the Pd/MgAl3catalyst shows the smallestthystersisof 119 ℃.For the other Mg modified catalysts,they show even higherthystersisthan the undoped one.

The presence of different PdO species is also found on the Pd/MgxAlyO catalysts,observing from H2-TPR profiles (Figure 5).At high reduction temperature (peakδ,above 280 ℃),peaks attributed to the reduction of support-Pd oxide complex are detected.The reduction peak areas increase with increasing amount of Mg additive,which is in good agreement with the trends of variation of oxygen release peak area for support-Pd oxide complex species in Figure 5.

In the low-temperature region (below 50 ℃),two hydrogen consumption peaks can be observed (peakαandβ),and the H2consumptions are summarized in Table 1.The H2consumptions are lower than that of theoretical one (94 μmol/gcatfor 1% Pd loading)for PdO→Pd reduction,suggesting a portion of Pd species exists in the form of metallic state (Pd0)and/or support-Pd oxide complex reduced at high temperature.

In order to specify the peak assignments,H2-TPR profile of the Pd/MgAl3catalyst after oxidizing by 2% O2/He at 500 ℃ is recorded,and the one without oxidization is also performed for comparison as shown in Figure 6.

After oxidation,peakαvanishes while peakβbecomes more intense.Groppi et al[31]found that amorphous PdOxand PdO crystals were correlated,and suggested that amorphous PdOxwas intermediate to the formation of PdO crystals.PdOxcan convert to PdO crystals in different partial pressure of O2or different ramping rates during TPO measurements[31].Both decomposition of PdO and re-oxidation of Pd took place via the formation of this intermediate,and the Pd re-oxidation was strongly affected by its environment[20],i.e.Pd supported on different supports would alter the ratio of amorphous PdOxto PdO crystals[34].Hence,it is reasonable to assign the peakαto amorphous PdOxspecies and peakβto PdO crystals.

Beside these contributions,a hydrogen release peak at about 78 ℃ can be detected due to the decomposition of palladium hydrides,indicating that the H2consumption of peakαandβfor each sample is ascribed to the reduction of PdO to Pd,subsequently partially hydrogenated to PdHx.After subtracting H2consumption of peakαandβto that of peakγ,H2consumption for PdOxcan be obtained.As seen in Table 1,MgAl2O4-like mixed oxides supported Pd catalysts show a larger amount of PdOxspecies than those of Pd/Al2O3and Pd/Mg3Al.

For Pd/Al2O3catalyst,extremely small peaks at a temperature lower than 50 ℃ and another one at about 289 ℃ are observed (Figure 5a).Correlating to its TPO results shown in Figure 4,the PdO decomposition temperature is lower than 800 ℃,which is utilized for catalyst calcination.Therefore,after calcined at 800 ℃ in the stagnant ambient air,most of the PdO would decompose to metallic Pd.Hence,a small amount of H2consumption (4.0 μmol/g,Table 1)for PdO reduction in Pd/Al2O3catalyst is observed.As for Pd/Mg3Al catalyst (Figure 5e),TPO result suggests a strong interaction between Pd species and Mg3Al support is formed,which is further confirmed by observing a large peak in H2-TPR profile at 360 ℃.No obvious H2consumption is detected at low temperature (below 50 ℃),therefore,most Pd present in the form of support-Pd oxide complex and metallic state in Pd/Mg3Al sample.For those catalysts supported on MgAl2O4-like mixed oxides (Figure 5b-5d),there are large reduction peaks at low temperature (<50 ℃),suggesting more PdO species are present with H2consumption higher than 58 μmol/g.This is inconsistent with the PdO decomposition temperature,which locates between that of Pd/Al2O3and Pd/Mg3Al catalysts in TPO characterization results.

It is notable to point out that the reduction peakβof Pd/MgAl3catalyst shows the largest area among all the catalysts,which should be ascribed to the facile transformation of PdOx?Pd.It is coincident with the smallestthystersisdetected for the Pd/MgAl3catalyst in TPO characterization result.

2.3 Activities for catalytic combustion of methane

Results of catalytic combustion of methane carried out for the Pd/MgxAlyO catalysts are shown in Figure 7.Light-off curves show the typical shape of Pd-based catalyst,in which with increasing reaction temperature from 300-600 ℃ the methane conversion increases,followed by slightly decreasing at temperature over 550 ℃,except the Pd/Al2O3and Pd/Mg3Al catalyst due to the exothermic nature of CCM reaction.t10,t50,andt90are temperature corresponding to methane conversion of 10%,50% and 90% in CCM activity test.

From Figure 7,the light-off temperature (t10)of Pd/MgxAlyO catalysts,ca.400 ℃,is similar except for the Pd/Al2O3at 450 ℃ and Pd/Mg3Al at ca.475 ℃,suggesting that the Pd0and support-Pd oxide complex is not as effective as PdO in methane conversion.Pd/Al2O3catalyst shows low catalytic activity in the testing temperature range.Murata et al[13]found that interaction between Pd andγ-Al2O3hindered the formation of spherical Pd particles and leads to a low fraction of step sizes,showing lower activity towards methane combustion compared to Pd/θ-Al2O3and Pd/α-Al2O3catalysts.For Pd/Al2O3catalyst,substantial parts of Pd species are present in the form of metallic Pd suggested by the above H2-TPR measurement.Therefore,the low methane conversion is attributed to the distorted shape of Pd0particles.During the whole testing temperature region,the light-off curve of Pd/Mg3Al catalyst shifts to higher temperature compared to the others should be attributed to the unique active site,support-Pd oxide complex,which is active at the high reaction temperature.

The temperature for catalytically conversion of 50% methane (t50)is slightly different from thet10.t50for Pd/MgAl2and Pd/MgAl4shifts to higher reaction temperature than that of Pd/MgAl3,which is consistent with the order ofthysteresisshown in Table 1,i.e.,the largerthysteresisleads to highert50.This confirms the idea that the feasible transformation of Pd?PdO is crucial to Pd-based catalyst showing high catalytic activity for CCM reaction,and the PdO is more active than that of metallic Pd0under lean condition (CH4∶O2is 1∶10 during the activity test)[14]at low reaction temperature.With further increasing reaction temperature,the methane conversions of the Pd/MgxAlyO catalysts vary much more significantly.

Pd0is active for CCM reaction though it is not as active as PdO species.Pd0surface area are obtained from H2-O2titration experiments and the results are shown in Table 1.With increasing content of Mg additives,Pd surface areas increase accordingly and the largest one is found for the Pd/MgAl2catalyst with 67.2 m2/g.This should be the reason why the Pd/MgAl2catalyst indicates largerthysteresis(154 ℃)than that of the Pd/MgAl4catalyst (144 ℃),whereas it shows higher methane conversion at the high reaction temperature (>480 ℃).

As illustrated in the results of XRD and N2-physisorption measurements,supports with different crystal structures are fabricated with alternative amounts of Mg additions,and the formation of Mg-Al LDHs precursors leads to high BET surface area.However,thethysteresiscan only be lowered when an appropriate amount of Mg is introduced (Mg/Al=1/3),which is reported to be a crucial criterion in determining the catalytic performance of Pd-base catalyst for CCM reaction.Apart from the active PdO sites,the effect of metallic Pd species should be taken into account at the high reaction temperature.Pd particles with a higher surface area are found to be more active than the catalyst with a lower one.Moreover,the support-Pd oxide complex from the catalyst with strong Pd-support interaction is also active for CCM reaction,which is found to be active at a higher temperature than that of Pd0and PdO species.

3 Conclusions

A series of Pd-based catalysts supported on Al2O3were fabricated by a co-precipitation method with modification by different amounts of Mg additives.γ-Al2O3is detected in the Mg-free catalyst and MgAl2O4-like mixed oxide or Mg(Al)Oxis detected in Mg-containing ones.Pd?PdO transformation abilities of the Pd/MgxAlyO catalysts are regulated by the Mg addition.The one with a mole ratio of 1∶3 shows the highest Pd?PdO transformation capacity,thus demonstrating the best catalytic activity towards CCM reaction.Moreover,metallic Pd particles (Pd0)and support-Pd oxide complex original from strong Pd-strong interaction are active for CCM reaction,which suppose to be active at the high reaction temperature.