Characterization of molybdenum oxide supported on a-Al2O3

Zi Fenglan,a Yan Jingfeng,b Yang Pengcheng,b Wang Xuekai,b Guo Hongyoua and

Wu Nianzu*b

aApplied Chemistry Department, Beijing University of Chemistry and Technology, Beijing100029, China

bState Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute ofPhysical Chemistry, Peking University, Beijing 100871, China.E-mail: wunz@chem.pku.edu.cn

Received 20th November 2002, Accepted 10th March 2003

First published as an Advance Article on the web 28th March 2003

The monolayer dispersion of molybdenum oxide on the surface of a-Al2O3 has been studied by X-ray

photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). The dispersion

threshold is 4.65 mg MoO3 per g a-Al2O3 or 5.6 Mo atoms nm22. The structure of supported MoO3 is strongly

dependent on the MoO3 loading. When the amount of MoO3 is below the dispersion threshold, the MoO3

species are highly dispersed on the surface of a-Al2O3, with the dispersed phase consisting of discrete [MoO4]

polyhedrons. When the MoO3 loading exceeds the dispersion threshold, crystalline MoO3 appears on the

surface of a-Al2O3. MoO3 exists in both dispersed and crystalline phases. According to the theory of phase

isolation, the dispersed phase remains in the form of discrete [MoO4] polyhedrons. However, the crystalline

phase of MoO3 is octahedral.

Introduction

The interaction of molybdenum oxide with alumina has beenthe subject of many investigations because molybdenum oxidecatalysts supported on alumina are widely used in a greatnumber of reactions, such as isomerization, ethylene polymer-ization and hydrodesulfurization.1,2 However, there have onlybeen a few studies focussed on a-Al2O3 supports. Chappelet al.3 found that tungsten oxide supported on a-Al2O3 is moreefficient for the isomerization of linear alkenes into branchedchain alkenes than tungsten oxide supported on c-Al2O3. Thisindicates that a-Al2O3 is a potential support for the preparationof catalysts. Barath et al.4 studied MoO3/a-Al2O3 catalysts forreforming of hexanes and reached the conclusion that theextent of reduction and the catalytic activity mainly dependson the molybdenum oxide content. Obviously, the catalyticactivity strongly depends on the chemical environment ofmolybdenum. However, their research did not look at thestructure of MoO3 supported onto a-Al2O3.a-Al2O3 is a more stable support than c-Al2O3. Imamura

et al.5 found that when the calcination temperature of MoO3

supported on alumina was increased to about 600 uC,Al2(MoO4)3 was not be formed on the surface of a-Al2O3,although it was produced on c-Al2O3. Furthermore, a-Al2O3

has a close-packed structure which is more suitable to be usedas a model support than c-Al2O3.Xie and Tang6 have demonstrated that many kinds of metal

oxides can disperse spontaneously onto the surface of supportsunder appropriate conditions to form monolayers or sub-monolayers. A great many oxides and salts can disperse ontothe surface of supports, since a monolayer is thermodynami-cally stable. If the melting point of the dispersed compound isnot high, a monolayer dispersion can be achieved by simplyheating a physical mixture. It has been found that crystallineMoO3 can disperse onto c- Al2O3 and TiO2 supports attemperatures of 350–450 uC, i.e. well below its melting point(795 uC). For each system, there is an ultimate dispersioncapacity called the dispersion threshold. Furthermore, there aremany unique effects when compounds disperse on supports.7–9

Monolayer dispersion systems are widely used in the catalyst

industry. However, the supports they employ are mostly ofhigh specific surface area. Monolayer dispersion phenomenararely involve low specific surface area supports.In this paper, we use a-Al2O3 as a support to investigate if

monolayer dispersion phenomena will occur on a support witha low specific surface area. The structure of supported surfacespecies is investigated using extended X-ray absorption finestructure (EXAFS) analysis, which detects the X-ray scatteringof atoms around an absorbing atom and can therefore provideinformation on the coordination structure of the absorbingatom.10,11

Experimental

Sample preparation

An appropriate amount of Al2O3 (99.99 wt.%) was heated at1200 uC in a muffle furnace for 10 h. After being cooled toroom temperature in the air, the Al2O3 and 2M aqueous HNO3

was mixed in a beaker in the Al2O3 :HNO3 volume ratio 1 : 4.The solution was heated to 70 uC, while being stirred for 1 h.After decanting the supernatant, fresh HNO3 solution wasadded. The process of heating and stirring was repeated12 times, then the solid was filtered out and washed with hotdeionized water. The pH of the final filtrate was less than 4.Finally, the Al2O3 was calcined at 700 uC for 5 h. X-Raydiffraction (XRD) analysis confirmed that the product hadthe a-Al2O3 structure. This material has no pore structure;a specific surface area 3.47 m2 g21 was determined via theBrunauer–Emmett–Teller (BET) method.MoO3 dispersions were prepared by thorough mixing of

appropriate quantities of MoO3 (orthogonal crystals) anda-Al2O3 and then hand grinding in an agate mortar for 30 min.Each of the samples was divided into two portions. These werecalcined in the air at 450 uC for 2 and 10 h, respectively. Thesamples were hand ground in an agate mortar again, andprepared for further characterization. The surface area ofthe samples is basically unchanged in comparison with theas-prepared a-Al2O3.

1206 J. Mater. Chem., 2003, 13, 1206–1209 DOI: 10.1039/b211494a

This journal is # The Royal Society of Chemistry 2003

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Sample characterization

X-Ray photoelectron spectroscopy (XPS) was carried out witha VG-ESCALAB5 spectrometer. The system was operated at9 kV and 18.5mAusing anMg-Ka anode. The chamber pressurewas kept below 1028 Torr. A binding energy (Eb) of 284.8 eVfor the C1s level was used as an internal reference. The ratioof the intensities of the XPS peaks due to Mo3d and Al2p wastaken as a measure of the relative content of MoO3 on thea-Al2O3.EXAFS spectra of the K edge of the Mo absorption

were collected on the X181B beam of Brookhaven NationalLaboratory National Synchrotron Light Source at 80 mAand 2.5 GeV. A Si(111) double-crystal monochromator wasemployed to produce a monochromatic beam. The harmonicswere minimized by detuning the monochromator. The energyscan ranged from 200 eV prior to the absorption edge to 900 eVbeyond it. Fine powder samples were prepared by mixing withparaffin and filled in an aluminum frame of 1 mm thickness.The samples were measured at room temperature in fluores-cence mode, crystalline MoO3 with a known structure was usedas a reference compound12 and measured in transmissionmode. The phase and amplitude functions used for the fittingwere obtained using the UWXAFS3.0 computer program fromWashington University. The paths feff0001.dat, feff0003.datand feff0004.dat were used to fit the first shell and the pathsfeff0013.dat, feff0019.dat and feff0028.dat were used to fitthe second shell. For each sample analyzed, the k range is 2.8–13.9 A21 and the k weight is 3. The R range is 0.5–2.0 A for thefirst shell and 2.5–4.0 A for the second shell. According to thek and R ranges, the maximum number of free parameters forthe two shells was 10. There were 6 free parameters to be fitted:Ni (i ~ 1, 2, 3), DR, DE, and the Debye–Waller factor, s2.

Results and discussion

Since XPS is a surface-sensitive technique, the analytical depthis three times the attenuation length and changes with thematerial and the kinetic energy. The XPS peak intensity ratioIMo3d

/IAl2pas a function of MoO3 content can be used to identify

the state of MoO3 in MoO3/a-Al2O3 (Fig. 1).13 For the samples

calcined for 2 h, the changing tendency is made up of two lineswith different slopes. The point of intersection of the two linescan be regarded as a critical value, i.e. the dispersion threshold.The dispersion threshold of MoO3 on the surface of a-Al2O3 is4.62 mgMoO3 per g a-Al2O3. When the MoO3 content is belowthis value, the slope of the straight line is steep. This means thatMoO3 is dispersed onto the surface of the a-Al2O3 support, soIMo3d

/IAl2pincreases rapidly as the amount of dispersed MoO3

increases. When the MoO3 content is above the dispersionthreshold, the slope of the plot becomes much more shallow.This shows that crystalline MoO3 is beginning to appear on thea-Al2O3 surface. According to XPS theory,14 for the same

amount of MoO3, the contribution of crystalline MoO3 to IMo3d

is much smaller than that of dispersed MoO3. The samephenomenon occurs for the samples calcined for 10 h. Fromthe XPS results, the dispersion threshold for these samples is4.67 mg MoO3 per g a-Al2O3, very close to the value found forthe samples calcined for 2 h. This indicates that the samplesreach dispersion equilibrium after being calcined for 2 h. Thedispersion threshold of MoO3 on the surface of a-Al2O3 istherefore 4.65 mg MoO3 per g a-Al2O3 or 5.6 Mo atoms nm22.This value is much smaller than the number of Mo atomsin the (010) plane of orthorhombic MoO3 because, onthe surface of a-Al2O3, MoO3 cannot arrange as it does inthe crystalline state due to the principle of least energy. Thedistance between MoO3 molecules on a-Al2O3 is longer thanthat in crystalline MoO3. Therefore, some of the a-Al2O3

surface always remains bare, even the MoO3 loading exceedsthe dispersion threshold.The FTs of the Mo K-edge EXAFS spectra of the samples

calcined for 10 h are displayed in Fig. 2 (uncorrected phaseshift). Crystalline MoO3 shows two very intense characteristicpeaks due to the periodic lattice. Table 1 lists the coordinationatoms and coordination distances for crystalline MoO3.According to Table 1, the first peaks (v2.0 A) are due tothe nearest oxygen scatters. Not only molybdenum atoms, butalso oxygen atoms contribute to the second peak (2.5–4.0 A).The strength of the peak at 1.2 A is basically the same foreach sample. When the MoO3 loading is below the dispersionthreshold of 4.67 mg MoO3 per g a-Al2O3, two unique featuresappear in the spectra of the samples as compared with thatof pure MoO3. These are the disappearance of the Mo–Mocoordination shell at 3.2 A and the peak at 1.6 A becomingweaker than in the spectrum of pure MoO3. When the MoO3

content is increased above the dispersion threshold, thecharacteristic peak of crystalline MoO3 at 3.2 A graduallybegins to appear. This indicates that crystalline MoO3 isstarting to form on the surface of a-Al2O3. The peaks at 1.6 and3.2 A become more and more intense with increasing MoO3

loading.

Fig. 1 Plot of XPS peak intensity ratio IMo3d/IAl2p

versus MoO3 contentfor MoO3/a-Al2O3 samples calcined for 2 and 10 h.

Fig. 2 Fourier transforms of Mo K-edge EXAFS spectra of samplescalcined for 10 h: (A) 2.78 mg MoO3 per g a-Al2O3; (B) 3.35 mgMoO3 per g a-Al2O3; (C) 4.49 mg MoO3 per g a-Al2O3; (D) 6.50 mgMoO3 per g a-Al2O3; (E) 13.0 mg MoO3 per g a-Al2O3; (F) crystallineMoO3.

Table 1 The parameters of crystalline MoO3

First shell Second shell

N R/A N R/A

Mo–O 1 1.6717 Mo–Mo 2 3.4378Mo–O 1 1.7344 Mo–O 4 3.5528Mo–O 2 1.9479 Mo–Mo 2 3.6970Mo–O 1 2.2507 Mo–O 2 3.7648Mo–O 1 2.3327 Mo–Mo 2 3.9630

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For crystalline MoO3 in Fig. 2, the first shell consists ofthree sub-peaks between 0.5–2.0 A. Using the UWXAFS3.0computer program, five scatter path functions of Mo–Oare obtained (Fig. 3). The first shell is the contributionsof the oxygen atoms. The R range of fitting for the first shellis 0.5–2.0 A. According to the structure of MoO3, not onlymolybdenum atoms, but also oxygen atoms contribute tothe second peak. The contributions of the Mo–Mo andMo–O scattering to the second peak are presented in Fig. 4.However in comparison with Mo–Mo, the contribution ofMo–O is so weak that it can be neglected in the fitting of secondpeak.For the first and second shell in Fig. 2, according to the k and

R ranges, the maximum number of free parameters is 10 due tothe limit of the independent data points. The path functions offeff0001.dat, feff0003.dat and feff0004.dat were used to makethe first shell fitting, because feff0002 and feff0005 are primarilythe same as feff0001 and feff0004, respectively, as shown inFig. 3. There are 6 free parameters to be fitted, they are Ni (i~1, 2, 3), DR, DE and the Debye–Waller factor, s2; DR is a publicvariable. As the contribution of Mo–O to the second peakwas neglected, the three Mo–Mo functions of feff0013.dat,feff0019.dat and feff0028.dat were used to fit the second peak.The free parameters to be fitted are Ni (i~ 1, 2, 3), DR, DE ands2.Using the paths selected to fit the experimental functionof pure MoO3, S0

2 data are obtained; S02 ~ 0.95 for the two

peaks to be fitted.Table 2 presents the fitting result of the samples, Na is

the apparent coordination number, where Na ~ SNi; theparameter R is the goodness-of-fit. When the loading of MoO3

is below the dispersion threshold of 4.67 mg MoO3 per ga-Al2O3,Na ~ 4 for the Mo–O shell, whereas the Mo–Mo shelldoes not exist. When the loading of MoO3 is above thedispersion threshold, Na increases with increasing of MoO3

loading for both the Mo–O and Mo–Mo shells. The fittingof the radial distribution and x(k) functions of the samplecontaining 4.49 mg MoO3 per g a-Al2O3 (sample C) are shownin Fig. 5 and 6, respectively.In accordance with Lu’s theory,15 if the absorption atoms

in the samples are present in mixed phases and the peaks in thetwo phases do not split for one compound, we can make anapproximation using the equation Na ~ fcNc 1 fdfd, where Na

is the apparent coordination number determined by experi-ment; Nc and Nd are coordination numbers of the crystallineand dispersed phases, respectively, and fc and fd are the weight

Fig. 3 Fourier transforms for each Mo–O path function.

Fig. 4 Fourier transforms of the Mo–Mo and Mo–O path functions.

Table 2 Fitting results for MoO3/a-Al2O3 samples

Sample

MoO3

content/mg g(Al2O3)

21

Mo–O Mo–Mo

Ni R/A Na,b s2/A2 DE/eV R Ni R/A Na s2/A2 DE/eV R

A 2.78 N1, N2, N3 2.64 1.7183 4.22 0.0050 4.21 0.037N2 1.58 2.0032

C 4.49 N1, N2, N3 2.65 1.6929 4.09 0.0051 21.07 0.042N2 1.44 1.9736

D 6.50 N1, N2, N3 2.56 1.7069 4.58 0.0050 21.01 0.031 N1, N2, N3 0.41 3.5158 1.59 0.0062 23.78 0.045N2 1.46 1.9899 N2 0.62 3.7790N3 0.56 2.3828 N3 0.56 4.0530

E 13.01 N1, N2, N3 2.58 1.6648 5.37 0.0036 6.01 0.040 N1, N2, N3 1.60 3.4769 3.70 0.0045 6.47 0.025N2 1.53 1.9408 N2 1.04 3.7390N3 1.26 2.3241 N3 1.06 4.0080

Fig. 5 Fitting of Fourier function for sample C.

1208 J. Mater. Chem., 2003, 13, 1206–1209

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percentages of the crystalline and dispersed phases, respec-tively, which can determined from the dispersion thresholdand the loading of the samples. For crystalline MoO3, thecoordination numbers of the Mo–O and Mo–Mo shells areboth 6. Therefore, we can obtain the coordination number ofthe dispersed phase. Table 3 presents the results calculatedusing the above equation.According to the data presented in Table 3, when the MoO3

loading is below the dispersion threshold, the Mo–O coordina-tion number of dispersed MoO3 is the same as the apparentcoordination number and the Mo–Mo coordination numberis zero. This means there is no connection between [MoO4]units in dispersed MoO3. When the MoO3 loading is abovethe dispersion threshold, the apparent coordination number ofthe two shells increases with the MoO3 loading; however, forthe dispersed phase of MoO3, the Mo–O coordination numberis always 4 and the Mo–Mo coordination number is alwayszero. This shows that the structure of dispersed MoO3 isunaffected by the MoO3 loading.Kisfaludi et al.16 reported the spreading of MoO3 over the

surface of c-Al2O3 and proposed that [Mo7O24]62 was formed;

however, Shi et al.17 showed the dispersed MoO3 on the surfaceof c-Al2O3 exists in isolated [MoO4] polyhedrons. Barath et al.4

suggested that on the surface of c-Al2O3, only isolatedmolybdate species with tetrahedral coordination are presentat MoO3 contents well below that for a monolayer, isolatedmolybdate species with tetrahedral coordination and well-dispersed polymolybdate moieties with octahedral coordina-tion are present at MoO3 contents close to that for amonolayer, and that crystalline MoO3 appears at contentsgreater than that for a monolayer. However, they could notexplain why the structures for samples where theMoO3 loadingis less than that for a monolayer have many more differencesbetween them. In our experiment, we could not obtain twodifferent dispersed phases when the loading of MoO3 wasbelow that required for a monolayer. Our results on a-Al2O3

are consistent with the conclusion drawn from c-Al2O3 by Shiet al.17

Conclusion

For MoO3/a-Al2O3 samples, MoO3 can be highly dispersedon the surface of a-Al2O3 by being calcined at 450 uC. Thedispersion threshold is 4.65 mg MoO3 per g a-Al2O3 or 5.6 Moatoms nm22. When the MoO3 loading is below the dispersionthreshold, the structure of the dispersed MoO3 is dramaticallychanged in comparison with crystalline MoO3. The dispersedpahse has no Mo–Mo coordination structure and consists of[MoO4] polyhedrons which are not connected to each other.When the MoO3 loading is above the dispersion threshold,crystalline MoO3 appears on the surface of a-Al2O3; the MoO3

consists of two phases, i.e. a crystalline phase and a dispersedphase. However, the dispersed MoO3 still exists as isolated[MoO4] polyhedrons.

Acknowledgements

This work was supported by the Brookhaven NationalLaboratory (USA), the National Natural Science Foundationof China (Project No. 29733080) and the Major State BasicResearch Development Program (Grant No. G2000077503).

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Fig. 6 Fitting of x(k) function for sample C.

Table 3 Coordination number in MoO3/a-Al2O3 samples

Sample

MoO3

content/mg g(Al2O3)

21 fd

Mo–O Mo–Mo

Na Nd Na Nd

A 2.78 1 4.22 4.22 0.00 0.00C 4.49 1 4.09 4.09 0.00 0.00D 6.50 0.71 4.58 4.00 1.59 20.21E 13.01 0.36 5.37 4.25 3.70 20.38

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