ORIGINAL PAPER

Redox State Dynamics at the Surface of MoVTe(Sb)NbO M1Phase in Selective Oxidation of Light Alkanes

Benoit Deniau • Thi Thao Nguyen • Pierre Delichere •

Olga Safonova • Jean-Marc M. Millet

� Springer Science+Business Media New York 2013

Abstract The influence of Te or Sb on the catalytic

properties of the M1 phase of MoVTe(Sb)NbO catalysts in

the oxidation of propane or oxidative dehydrogenation of

ethane has been investigated. The results show that Te and

Sb affect the intrinsic activity of the catalyst for ethane and

propane oxidation in the same way and that the Te-con-

taining M1 phase was significantly more active than the

Sb-containing phase, even when they contained the same

amount of vanadium, which is known to be the active

species for the reactions. This feature has been explained

by a strong modification of the relative number of available

V5? sites, which is related to the fact that Sb tend to sta-

bilize vanadium in active site, in a reduced state. Sb and Te

were also shown to affect the selectivity in the case of

propane oxidation but not in the case of ethane oxidative

dehydrogenation. This demonstrated that they should be

involved directly in the further transformation of the alkene

molecules that are formed as an intermediate and pre-

sumably play a role in the a-hydrogen abstraction from

these molecules, as proposed earlier. Blocking problems

related to the substitution of Te by Sb in view of indus-

trialization are also discussed.

Keywords MoVTeNbO oxide catalysts � Selective

oxidation � Ammoxidation � Alkanes � Ethylene � M1 phase

1 Introduction

Among the best catalysts proposed for light alkane oxi-

dation, the multi-component MoVTe(Sb)NbO catalysts

are very promising [1]. The active phase of these cata-

lysts, called M1, is a solid solution containing the four

metallic cations with V/Mo, Nb/Mo and Te(Sb)/Mo ratios

that can vary, but are most frequently close to 0.3, 0.1

and 0.2, respectively [2–8]. Although the presence of Nb

is not essential, all of the other elements are and key

catalytic functions have been attributed to them. A gen-

eral agreement has been obtained on the fact that V is the

paraffin activator, and that Te or Sb is involved in a-

hydrogen abstraction from the olefin that is formed from

the alkane. In their higher oxidation state, Te and Sb have

been proposed to be the O- or NH-inserting element, in

contrast to Mo, which generally plays this role. As pre-

viously pointed out, all of these elements are strategically

placed adjacent to each other in the active phase of the

catalysts called M1 and comprising the multifunctional

catalytic site assembly [9]. Such site assembly has been

proposed to be associated with a specific surface termi-

nation of the M1 phase corresponding to ab plane

[10, 11].

In the structure of the M1 phase, Te and Sb cations are

located in hexagonal channels formed by MO6 octahedra

(M = Mo, V, Nb) sharing corners. They are formed with

oxygen chains running in the [001] direction, but the

oxygen and Te or Sb sub-stoichiometry in the channels are

not linked to each other [10]. Tellurium has been shown to

be Te?4 on the whole, whereas antimony was shown to be

B. Deniau � T. T. Nguyen � P. Delichere � J.-M. M. Millet (&)

Institut de Recherches sur la Catalyse et l’Environnement de

Lyon, IRCELYON UMR5256, CNRS-Universite Claude

Bernard, Lyon I, 2 Avenue A. Einstein,

69626 Villeurbanne Cedex, France

e-mail: jean-marc.millet@ircelyon.univ-lyon1.fr

O. Safonova

Institute for Chemical and Bioengineering, ETH Zurich, Zurich,

Switzerland

123

Top Catal

DOI 10.1007/s11244-013-0132-0

present both as Sb5? and Sb3? [12, 13]. The oxidation state

of both elements at the surface of the catalysts has not been

ascertained, since the position of the Te3d3/2 and Sb3d3/2

peaks in the X-ray photoelectrons spectra were most

commonly observed between those of Te4? and Te6?, and

Sb3? and Sb5?; however, most of the time this was closer

to the higher oxidation state cations. Consequently, several

publications have proposed that the oxidation state of Te at

the surface was ?6, while others have suggested ?4

[6, 14–16].

Te-based catalysts have been shown to be more effi-

cient than Sb-based ones, regardless of the paraffin to be

oxidized [6, 17]. However, there is a major drawback of

using tellurium-containing catalysts, which is its volatility

when reduced up to the metallic state, which can always

append in catalytic reaction conditions. This is why it

would be advantageous to substitute Te with Sb or Bi,

both of which are less volatile and more eco-friendly. The

Sb-containing M1 phase can be prepared relatively easily,

as shown earlier, but bismuth-containing M1 phases have

never been obtained despite many attempts [18]. This

may be due to the much larger size of the Bi3? cations,

which cannot accommodate the hexagonal channels of the

structure.

To better understand the complete reaction mechanism

of paraffin oxidation or ammoxidation of the MoV-

Te(Sb)NbO catalysts, it is necessary to determine the exact

respective functionalities of the various elements, including

Te or Sb, but first their catalytic effect has to be better

evidenced. Most commonly, and because of the existence

of a synergetic effect between the M1 active phase and a

second phase called the M2 phase, the studied catalysts

contain this second phase. Furthermore, other phases may

also be present, like Mo5–x(V,Nb)xO14, TeMo5O16 or

VSbO4. The presence of these phases in most of the cata-

lysts of the published studies enables to precisely define the

effect of a given element in the active M1 phase on its

catalytic properties.

In this paper we report the preparation and study of pure

or almost pure M1 phases containing Te or Sb as catalysts.

Depending on the total composition of the catalysts and the

preparation method, Te or Sb can be present in sub-stoi-

chiometry and can exhibit several oxidation states. While

these variations do not alter the crystal structure, they may

affect the activity and selectivity of the catalyst. Thus,

there is a need for a better understanding of the impact of

the presence of these elements, as they will define the

active sites or modify their environment and consequently

their catalytic properties. We have prepared numerous

catalysts but have selected a few that are as pure as possible

in M1 phases, as well as those containing Te or Sb, in order

to study the effect of the nature of the element and its

relative content on the catalytic properties. Furthermore, it

has been shown by Naraschewski et al. [19] that the

initial catalytic rate of propane oxidation is proportional

to the total concentration of vanadium in the structure for

Te-containing M1 phases. Consequently, it was necessary

to compare M1 phases with approximately the same V

content. The catalysts have been studied either in ethane

or propane oxidation to better detail the different poten-

tial roles of Te and Sb and to better understand the

intricacies of the catalytic processes. In the case of eth-

ane, the reaction pathway is simpler with the formation

of only carbon oxides as side products, making it easier

to focus on the effect of the elements on paraffin acti-

vation; in contrast, in the case of propane, the effect of

Te and Sb in the consecutive steps of the reaction, like

the transformation of propene formed intermediately, can

be studied.

Finally in order to try to explain the results obtained, the

M1 phase catalysts have been studied by XANES spec-

troscopy at the V K, Sb and Te L1-edge during reduction

by propane at the catalytic reaction temperature. In a pre-

vious paper on MoVSbNbO catalysts studied by in situ

X-ray absorption spectroscopy, we had shown that the

oxidation state of Sb could change in the reaction condi-

tions and could be involved in the total redox process of the

catalysts, as well as the fact that these changes could be

evidenced by XANES spectroscopy [20]. XPS spectros-

copy has also been used to study the reduction by propane

of a Te containing M1 phase and to confirm the XANES

data obtained.

2 Experimental

2.1 Preparation of the Pure M1 Phase Catalysts

M1 phases containing Te and Sb were prepared using

protocols described elsewhere [21]. Both protocols inclu-

ded the preparation of aqueous slurry containing Mo, V, Sb

or Te precursors in the chosen ratios. In the Sb containing

M1 phase, precursors were dissolved in an aqueous solu-

tion maintained at 80 �C for 2 h. When Sb2O3 was used,

hydrogen peroxide in H2O2/Sb ratio of three was then

added to re-oxidize the V species [12]. Silica Ludox AS40

with an amount corresponding to a Si/Mo = 0.95 was

added to the solution. In parallel, niobic acid was dissolved

at 100 �C in an aqueous solution of oxalic acid and added.

The slurries obtained were dried at 130 �C and heat-treated

in air at 300 �C for 4 h and in N2 at 600 �C for 2 h. The

solids obtained contained the M2 phase, which was

removed by selective dissolution in aqueous hydrogen

peroxide solution at 60 �C. The solids were finally washed

in water, dried at 130 �C and heated in N2 at 600 �C for

2 h.

Top Catal

123

2.2 Characterization of the Catalysts

The metal content of the mixed oxides was determined by

atomic absorption (ICP), and their specific surface areas

were determined using the BET method with nitrogen

adsorption. Powder X-ray diffraction (XRD) patterns were

obtained using a BRUKER D5005 diffractometer with a

Ni-filtered CuKa (0.15418 nm) radiation source.

V K-edge (5.465 keV) and Sb and Te L1-edge (4.9485

and 4.698 keV) XANES spectra were collected at the

BM29 beamline at the European Synchrotron Radiation

Facility (ESRF) in Grenoble, France. Mo spectra were not

recorded since it had been shown that it was not involved in

the redox process or at a level that is difficult to detect [20].

Sb and Te L1-edge XANES spectra were recorded with a

varying energy step of 0.4 eV/20 s in the range

4680–4730 eV and 0.3 eV/20 s in the range 4680–4735.

The V K-edge spectra were collected with a varying

energy step of 0.2 eV/5 s in the range 5420–5560 eV. The

experiments were recorded using a Si(111) double crystal

and in the fluorescence mode using a Camberra Si diode

detector, which was mounted at 90� with respect to the

incident beam. For the in situ spectra, a 20 mL cell

described in a previous work was used [20]. This cell,

which was equipped with a 25 lm Kapton window,

allowed measurement of XAS at the catalytic reaction

temperature under various flowing atmospheres. The study

was performed at atmospheric pressure under a gas flow of

20–30 mL min-1, the composition of which was moni-

tored using Brooks flowmeters. XANES spectra were

recorded at in N2 and O2/N2 (10/90) atmosphere. The

heating from 25 to 380 �C was performed under N2 and the

rate of heating was 5 �C min-1. Spectra were also recorded

in N2 after different pulsing of propane and using a specific

valve allowing 5 mL propane to be sent on the solid to

reduce it. In such cases, we waited 20 min before starting

the recording in order to allow the M1 phase to reach

equilibrium. This length of time was required for the dif-

fusion of oxygen from the bulk to the surface of the M1

crystallites through the hexagonal channels [20].

XANES spectra were normalized and background-cor-

rected using the IFFEFIT program. In order to compare the

different Te and Sb L1-edge XANES spectra, the absorp-

tion background was first determined over the entire range

and the spectra were normalized in the middle of the first

EXAFS oscillation, at ca. 50 eV above the absorption edge.

The relative ratio of SbIII and SbV or of TeII and TeIV in the

compounds could be determined by fitting the L1-edge

XANES spectra considering a combination of two peaks

whose relative areas are proportional to the relative amount

of the two cations [11]. For the V edge spectra, the analysis

was based on the observations of the pre-edge. The position

of the center of mass and the total area of the V pre-edge

peak were calculated using the Peak-Fit program and used

to determine the mean oxidation state of V as previously

described [20]. Ti, V and Sb metal foils were measured for

the different calibrations.

XPS measurements were performed using a Kratos Axis

Ultra DLD spectrometer. The base pressure in the analysis

chamber was better than 5 9 10-8 Pa. XPS spectra of V2p,

Te 3d, Mo 3d, Nb 3d O1 s, and C1 s levels were measured

at 90� (normal angle with respect to the plane of the sur-

face) using a monochromated AlKa X-ray source with a

pass energy of 20 eV and a spot size aperture of

300 9 700 lm. Binding energies were corrected relative to

the carbon 1 s signal at 284.6 eV. The signal intensities

were measured using integrated areas under the detected

peaks. The experimental precision on XPS quantitative

measurements was considered to lead to precision on the

calculated cationic ratio of 0.15.

Reduction of the samples was conducted in an adjacent

chamber at 380 �C. The sample was heated under Ar flow

(50 mL min-1) and two series of 10 pulses of 5 mL of

propane were sent in the chamber. At the end the sample

was re-oxidize at the same temperature by shifting the Ar

flux to an O2 flux for 1 min.

2.3 Testing of the Catalysts

The oxidative dehydrogenation of ethane and the oxidation

of propane were performed in a fixed bed reactor operating

at atmospheric pressure and isothermal conditions using a

glass plugflow micro reactor. The catalytic properties were

determined between 360 and 400 �C with catalyst amounts

varying from 200 to 3000 mg and total flows varying from

6 to 125 mL min-1. The feedstock compositions were

C2H6/O2/N2 ? He = 30/20/50 and C3H8/O2/N2 ? He =

10/5/45/40, respectively. Ethane (99 %) from commer-

cially available compressed gas cylinders was used without

further purification.

After reaction, alkanes, alkenes, CO and CO2 were

analyzed online using a gas chromatograph equipped with

a TCD detector and either Porapak-Q and CPMolsieve 5A

columns or a Carboxen 1010 column. Organic substrates

were condensed during the reaction and analyzed offline

using a gas chromatograph equipped with an FID detector

and CPWax or a Nukol column. Most of the testing was

performed at 385 �C, and only traces of acetone in the case

of propane oxidation and acetic acid in the case of ethane

oxidative dehydrogenation were detected. These products

were not considered for carbon balance and selectivity

calculations. Intrinsic rates of conversion were calculated

at conversion low enough to be in a chemical regime and

compared to each other. No deactivation of the catalysts

was observed during the reaction times.

Top Catal

123

3 Results

3.1 Characterization of the Fresh Catalysts

The XRD patterns of the samples reveal only one phase,

corresponding to the M1 phase. In some cases better fits

were obtained adding a Mo5O14 type phase, which amount

should not overpass 1 or 2 %. To illustrate this the XRD

pattern of one of the samples with its fitting is shown in

Fig. 1. The cell parameters calculated from the powder

patterns were comparable for catalysts of the same type and

to those of the literature (Table 1). The chemical compo-

sitions of the unit cell of the M1 phases have been deter-

mined from chemical analyses. The results obtained were

in relatively good agreement with the desired stoichiome-

try, except that the Te contents of the catalysts were sys-

tematically lower than expected (Table 2). This may be

explained both by the formation in Te containing catalysts,

before the dissolution step, of a slightly higher amount of

M2 phase, which is richer in Te than the M1 phase and

possibly to a small loss of Te by vaporization during the

heat treatments at 600 �C. However it should be noted that

such loss has been evaluated in several cases and never

exceeds 3 %. The specific surface areas of the solids have

been determined using the BET method; they varied

between 6 and 18 m2 g-1 and seemed to slightly increase

with the vanadium concentration of the samples (Table 2).

3.2 Catalytic Activity

3.2.1 Effect of Sb or Te Content

It has been shown previously that the tellurium content of

the M1 phase had no impact on the catalytic properties

when ranging between 1 and 4 tellurium atoms per unit cell

[22, 23]. It is generally not possible to prepare a pure

M1(Te) phase with lower or higher Te content since other

phases are systematically formed at lower contents and all

of the sites in the hexagonal channels are occupied at

higher content; also an excess of Te led to metallic tellu-

rium or the formation of phases richer in Te like M2 [5,

19]. We have thus focused our attention on the effect of Sb

content in the M1 phase to see whether the same results

could be obtained. It was important to check if it was the

same for the Sb containing M1 phase. For that two M1

phases, M1(Sb)-A and M1(Sb)-B with comparable V

contents but different Sb contents were tested as catalysts

in the oxidation of propane (Table 2). The results are

shown in Table 3. It can be seen that the catalytic prop-

erties of the two M1 phases are comparable.

Fig. 1 X-ray diffraction pattern

of M1(Te)-G with its profile

fitting. The 5–14.5� 2h range is

enlarged and a peak indexation

is given

Table 1 Unit cell parameters of the prepared M1 phase catalysts

Catalyst Unit cell parameters Cell volume

(nm3)a (nm) b (nm) c (nm)

M1(Sb)-A 2.1134 (5) 2.6650 (7) 0.4011 (1) 2.2591

M1(Sb)-B 2.1139 (9) 2.669 (1) 0.4012 (2) 2.2635

M1(Sb)-C 2.1133 (3) 2.6633 (4) 0.40150 (5) 2.2598

M1(Te)-D 2.1124 (2) 2.6575 (3) 0.40167 (4) 2.2548

M1(Te)-E 2.1142 (8) 2.6603 (11) 0.4010 (1) 2.2538

M1(Sb)-F 2.1136 (9) 2.6620 (10) 0.4010 (2) 2.2562

M1(Te)-G 2.1118 (5) 2.6602 (7) 0.4008 (1) 2.2516

Table 2 Unit cell content in atoms, specific and surface area (SSA)

of the prepared M1 phases

Catalyst Unit cell composition in atoms SSA (m2 s-1)

Mo V Nb Te Sb

M1(Sb)-A 24.2 7.4 3.8 2.8 19.1 ± 0.2

M1(Sb)-B 28.6 7.6 3.8 4.0 15.4 ± 0.1

M1(Sb)-C 29.2 6.2 4.6 3.3 28.8 ± 0.3

M1(Te)-D 29.8 6.5 3.7 2.3 23.8 ± 0.3

M1(Te)-E 28.4 7.4 4.0 2.6 22.3 ± 0.1

M1(Sb)-F 28.8 7.4 3.8 4.0 19.3 ± 0.2

M1(Te)-G 29.2 6.6 4.2 3.7 24.2 ± 0.1

Top Catal

123

3.2.2 Effects of Te and Sb Nature

In order to show the influence of the nature of Sb or Te

cations, the catalytic properties of M1(Sb)-C and M1(Te)-

D catalysts with again approximately the same V content

were compared with regard to the oxidation of ethane and

propane. For the oxidation of ethane, the results showed

that the presence of Te or Sb has almost no influence on the

selectivity in ethylene (Fig. 2). However a marked effect

on the activity was observed. This clearly appeared when

intrinsic rates of ethane conversion were compared at the

same contact time (Table 4). The apparent activation

energies that were calculated were similar, although

slightly higher for the M1(Te) phase. When the solids were

tested in propane oxidation, the same effect on the rate of

propane conversion was observed (Table 5). Interestingly,

the ratios between the rates measured in both reactions

were comparable. However, a higher selectivity to acrylic

acid on Te containing M1 phase was observed with pro-

pane. This result is consistent with previous studies also

conducted on pure or almost pure M1 phase [24]. We again

observed higher apparent activation energy for the Te

containing phase than for the Sb containing one with regard

to the transformation of propane.

3.2.3 In Situ Characterization by XANES Upon

Reduction by Propane

It is known that Sb or Te can be involved in redox reactions

of the M1 phase [20]. It was therefore interesting to study

the reducibility of these elements in situ by an alkane to see

whether it could explain the catalytic data obtained. We

recorded Te and Sb L1-edge XANES spectra of the M1

phase at the catalytic reaction temperature when propane

was contacted with the phase. Furthermore and because it

is also known that V is the element activating the alkane

molecules, V K-edge XANES spectra were also recorded.

Two M1 phase catalysts, M1(Te)-E and M1(Sb)-F, again

with comparable V contents, were used for this study

(Table 2).

The Te L1-edge and V K-edge XANES spectra of the

M1(Te)-E sample were recorded at 380 �C in N2 and after

10 pulses of propane (5 mL) or after shifting the N2 flux to

and O2/N2 flux (10/90). These spectra are respectively

shown in Figs. 3 and 4 respectively. The V K-edge spec-

trum showed a pre-edge peak, which center of mass cor-

responded to a mean oxidation state of about 4.1, whereas

the Te L1-edge spectrum was typical of Te4? cations. A

slight modification of the V pre-edge was observed under

oxidative atmosphere with a small shoulder appearing

reversibly on the right side, this indicated the partial oxi-

dation of about 3 % of V41 into V51. When the flux is

shifted back to pure N2, the initial V spectrum is observed.

Concomitantly, no change in the Te spectrum could be

Table 3 Catalytic properties of M1(Sb) catalysts in propane oxidation at 385 �C

Catalyst Mass (mg) Conv. (%) Rate (mol s-1 m-2) Selectivity (%)

COx C3H6 AceA ProA. AcrA

M1(Sb)-A 340 26.3 4.8 9 10-8 19 15 6 1 58

M1(Sb)-B 300 25.0 5.4 9 10-8 24 13 7 0 56

AceA acetic acid, ProA. propionic acid, AcrA acrylic acid

50

60

70

80

90

100

0 10 20 30 40 50 60 70

Sélé

ctiv

ité

C2H

4(%

)

Conversion C2H6 (%)

M1(Sb)M1(Te)

Fig. 2 Variation of the selectivity in ethylene as a function of the

conversion on M1(Sb)-C and M1(Te)-D catalysts

Table 4 Intrinsic rate of ethane conversion on M1(Te) and M1(Sb)

catalysts with apparent activation energy (Ea)

Catalyst Ea (kJ mol-1) Rate (mol s-1 m-2)

M1(Sb)-C 68 5.3 9 10-8

M1(Te)-D 76 13.0 9 10-8

Table 5 Intrinsic rate of propane conversion on M1(Te) and M1(Sb)

catalysts with apparent activation energy (Ea)

Catalyst Ea (kJ mol-1) Rate (mol s-1 m-2)

M1(Sb)-C 54 5.5 9 10-8

M1(Te)-D 61 14.5 9 10-8

Top Catal

123

detected. After reduction by propane, no difference in the

position or shape of the pre-peak was observed in the V

spectrum, whereas the Te L1-edge spectrum showed a

shoulder at 4941 eV that was attributed to the presence of

Te2?. The evaluation of the relative area of the new peak

allowed the evaluation of the reduction of Te cations to

5 at.%.

For the M1(Sb)-F sample, the spectra recorded at the

V K-edge and Sb L1-edge are shown in Figs. 5 and 6. The

V K-edge spectrum of the sample was very similar to that

observed for M1(Te)-E and the same mean oxidation state

of 4.1 for the vanadium was calculated. The in situ study of

the oxidation of an Sb-containing M1 phase has previously

been published [20]. It was observed that Sb and V were

a b

Fig. 3 a XANES spectra of M1(Te)-E at Te L1-edge recorded at 380 �C (a) before and (b) after oxidation. b XANES spectra of M1(Te)-E at

V K-edge recorded at 380 �C (a) in N2 and (b) after oxidation

a b

Fig. 4 a XANES spectra of M1(Te)-E at Te L1-edge recorded at 380 �C (a) before and (b) after reduction by propane. b XANES spectra of

M1(Te)-E at V K-edge recorded at 380 �C (a) under N2 and (b) after reduction by propane

Top Catal

123

oxidized in parallel. We have not reproduced similar

experiments and we have focused our attention on the

in situ reduction of the M1 phase by propane.

In this case the reduction was conducted in two steps,

with the first 10 pulses of propane sent to the solid

(reduction 1) and then, after recording of spectra, five more

pulses were sent (reduction 2). We can see that V and Sb

were reduced after the two treatments. Calculation showed

that about 4 % of vanadium and 15 % of Sb were reduced

(1 % and 10 % after the first reduction). It is interesting to

note that Sb contributed for about four times more to the

reduction even though contents were less than half of V.

Such a phenomenon, which was already observed in the

previous study for oxidation of both cations, confirms the

extremely high oxygen mobility in these channels and the

role of the latters as an oxygen reservoir.

3.2.4 In Situ Characterization by XPS Upon Reduction

by Propane

The M1 phase has been characterized by XPS before and

after reduction by propane. Mo, V, Te, Nb and O have been

analyzed. The results obtained are presented on Table 6.

The surface composition of the fresh sample corresponded

to that of the bulk. Contrarily to what was observed before

no surface enrichment in Te was observed and this content

was even lower than the bulk one (3.1 instead of 3.7 Te per

unit cell) [12, 17]. Both V4? and V5? were detected at the

surface leading to as surface mean oxidation state of V

slightly higher than the bulk one (4.3 instead of 4.1). For

Te region only a single Te 3d5/2 peak assigned to Te4? was

observed. The analysis of the literature showed that this

peak has also been in certain cases assigned to Te6? [15,

16]. Such mismatch was related to the fact that the binding

energy of Te 3d levels vary over a large range depending

upon the configuration of the species present (TeO3E or

TeO4E) [25].

After the first series of pulse of propane, a strong

reduction of Te was observed (Fig. 7). In addition to the Te

3d5/2 main peak at 576.7 eV, an additional peak at

574.1 eV was observed. The latter peak has been attributed

to Te2? state [26, 27]. The reduction of tellurium was much

more important that what was observed by XANES (40

instead of 5 %). However the reduction conditions were

different in terms of reaction cell geometry and gas flux. In

the same time, a reduction of Mo and V was detected but to

a much less extend. These results confirmed well those

obtained by XANES spectroscopy. In parallel the relative

amount of surface Te has decreased whereas those of the

other elements remained similar. When the solid was fur-

ther reduced with a second series of 10 pulses of propane,

no further reduction of Te was observed and almost the

same Te4?/Te2? ratio was observed. The surface compo-

sition also remained the same. After the second reduction,

the solid was treated under O2 at 380 �C. The spectrum

recorded after this treatment showed that Te was entirely

re-oxidized to Te4?. In the same time the Te content

increased to reach its starting value. V was also re-oxidized

to reach an oxidation state even slightly higher than the

starting one similarly to the sample studied by XANES

when treated under oxidative atmosphere (Fig. 7).

Fig. 5 XANES spectra of M1(Sb)-F at Sb L1-edge recorded at

380 �C (a) before and (b) after different successive reductions by

propane

Fig. 6 XANES spectra of M1(Sb)-F at V K-edge recorded at 380 �C

(a) before and (b) after different successive reductions by propane

Top Catal

123

4 Discussion

Since the results of the in situ study may help to understand

the catalytic results it appears more fruitful to discuss them

first. We have seen that the oxidation state of V and Te or

Sb in the studied M1 phase corresponded well to those

determined before in various studies using similar tech-

niques. XANES data showed that the mean oxidation state

of V was slightly higher than ?4, and that all of the Te

cations were ?4, whereas Sb cations were both ?3 and ?5,

but mainly ?3 (Sb5?/Sb3? = 30/70). There is a general

agreement that the active species for the light alkane oxi-

dation reaction on these catalysts are vanadium species and

that the V51/V41 redox couple would reasonably be the

first redox couple that would be affected by a reduction by

propane [4]. This was observed for Sb-containing catalysts,

for which we observed that antimony was concomitantly

reduced [20]. However, a different behavior was observed

in the case of Te-containing phase. Tellurium was partially

reduced but vanadium was not. An hypothesis to explain

these results is to consider an oxido-reduction process

between surface Te4? and V4?:

Te4þ þ 2V4þ�Te2þ þ 2V5þ ð1Þ

Furthermore, in the catalytic reaction mechanism pro-

posed by Grasselli et al. [9], the dehydrogenation of pro-

pane was proposed to take place with the transitory

reduction of a V5? cation to V4? and that of a Te4? cation

to Te3?. The charge transfer could then take place between

these cations leading to the stabilization of Te2?cations:

Te4þ þ V4þ� ðTe3þ þ V4þÞ�Te2þ þ V5þ ð2Þ

Surface Te4? can be further reduced to Te0 by undergoing

a second charge transfer or by being reduced by the propene

formed. The same type of transfer does not exist in the Sb-

containing compounds since the Sb5?/Sb3? redox potential

is much lower than the V5?/V4? one (Sb5?/Sb3?:

0.649 eV - V5?/V4?:1.00 eV). This feature is clearly

shown by the stabilization in air and at temperatures as

high as 700 �C of V4? and V3? with Sb5? species in the

VSbO4 mixed oxide [28]. Concerning tellurium, it can be

noted that a similar effect of tellurium had already been

reported in a study of Te-doped VPO used for the oxidative

dehydrogenation of ethane with a positive effect on the re-

oxidation of V4? species. It has to be pointed that tellurium

and vanadium can be reduced independently from one

another and the proposed oxido-reduction may not be so

important in the reaction conditions.

The XPS study confirmed that upon reduction by pro-

pane Te2? was formed whereas the V5? content was

slightly modified. Interestingly this change in oxidation

state was coupled with a decrease in relative content. Both

modifications appeared to be totally reversible upon re-

oxidation by O2. It should be noticed that no significant

change in Mo and V relative contents were observed in

parallel upon oxidation or reduction (Table 6). Two

hypotheses may be considered to explain the decrease of the

Te surface content. Firstly we have seen that upon reduc-

tion, Te was reduced to Te2?, which may itself easily be

further reduced to Te0, which is volatilized. Secondly the

equilibration of charge balance at the surface may take

place not only by reduction of metallic elements but also by

diffusion of Te from the surface into the bulk. In both cases

the replenishment of the surface in Te upon oxidation could

only be explained by diffusion of Te from the bulk to the

surface. It is interesting to note that such diffusion of Te

from the bulk to the surface explained why the bulk Te

content of the M1 phases has, in a certain range of com-

position, a limited impact on the catalytic properties, since

the surface Te content seems to depend mainly from the

Table 6 XPS analyses of the M1(Te) sample before, after reduction

by propane and re-oxidation by O2 at 380 �C

Solid Binding energy (eV) Te/Mo V/Mo

M1(Te)-G before reduction

Mo 3d5/2 Mo6?: 232.9 0.104 0.137

V 2p3/2 V5?: 517.5 (31 %)

Te 3d5/2 V4?: 516.6 (69 %)

Nb 3d5/2 Te4?: 576.7

Nb5?: 207.0

M1(Te)-G after reduction 1

Mo 3d5/2 Mo6?: 232.8 (85 %) 0.037 0.118

Mo5?: 231.6 (15 %)

V 2p3/2 V5?: 517.4 (26 %)

V4?: 516.5 (74 %)

Te 3d5/2 Te4?: 576.7 (62 %)

Te2?: 574.1 (38 %)

Nb 3d5/2 Nb5?: 207.0

M1(Te)-G after reduction 2

Mo 3d5/2 Mo6?: 232.7 (86 %) 0.035 0.131

Mo5?: 231.6 (14 %)

V 2p3/2 V5?: 517.4 (24 %)

V4?: 516.5 (76 %)

Te 3d5/2 Te4?: 576.7 (60 %)

Te2?: 574.1 (40 %)

Nb 3d5/2 Nb5?: 207.0

M1(Te)-G after re-oxidation

Mo 3d5/2 Mo6?: 232.9 0.095 0.127

V 2p3/2 V5?: 517.6 (35 %)

V4?: 516.6 (65 %)

Te 3d5/2 Te4?: 576.7

Nb 3d5/2 Nb5?: 207.0

Top Catal

123

redox atmosphere in equilibrium with the phase. It can be

observed that the surface reduction increased only slightly

after the first 10 pulses. The same behavior was observed by

Kubo et al. [29] in a previously study of the reduction

behavior of MoVTeNbO catalysts by pulse of propane

followed by gas chromatography. It was also shown in the

same study that up to a calculated surface reduction of 33 %

the reduction was reversible. In our case the analysis of the

O1s signal showed that the reduction was about 25 %.

The reduction of antimony by itself has no effect on the

stability of the catalyst. This should not be the case for

tellurium. Indeed, Te2? cations are not stable species and

may easily be further reduced to Te0 and be volatized. If as

shown here, the surface Te content can be replenished, over

long periods of time, this could have an effect on the

selectivity and on the stability of the M1 structure itself.

However it should be recalled that no such loss of selec-

tivity and catalyst deactivation has yet been reported in

catalytic reaction studies performed at the laboratory scale.

This study tend to prove that tellurium may be lost under

strong reducing atmosphere, it also showed that Te is

mobile in the channels of the structure and can easily

replenish the Te surface content. We had already shown

some years ago that Te was mobile at the surface of the

M1(Te) phase when mixtures of M1(Te) and M2(Sb) phases

were studied with the evidence of diffusion of Te at the

surface of M2(Sb) [17]. This confirms the dynamic behavior

of this type of oxidation catalyst and open the door to a

possible additional supra-surface role of tellurium [30].

Before discussing the influence of the nature of Sb or Te

on the catalytic properties, it is important to focus on the

effect of the Sb or Te contents in the M1 phase. The Sb or

Te content apparently has no significant effect for a given

composition range, on either the activity or selectivity of

the phase. The surface Te or Sb content presumably

remained the same whatever the bulk composition was and

the loss of these elements could be compensated by their

migration from the bulk as shown in the XPS study. Other

XPS analyses performed on various catalysts tend to con-

firm this interpretation since they always show a rather

similar content [27]. It can be recalled that, at low content

the M1 phase cannot be prepared alone or in a mixture with

M2, and that other phases like Mo5O14, which cannot be

removed are generally formed. At high content the for-

mation of M2 is preferential and, the Te and Sb content of

the M1 phase cannot be increased since the M2 phase

contains more Te or Sb than the M1 phase.

The results obtained clearly show that the nature of the

Sb or Te element has an important effect on the activity of

the catalyst and this is observed for both ethane and pro-

pane oxidation. Sb and Te also affect the selectivity, but

only in the case of propane oxidation; this second effect

will be discussed later. It has been clearly shown in pre-

vious studies that the oxidative dehydrogenation of the

alkane molecule was the slowest step in the reaction path

[30, 31] and that the alkene was often the only primary

product, while the direct oxidation of propane to carbon

oxides was rarely observed (<1 %). The same conclusion

could be reached for the oxidative dehydrogenation of

ethane, for which the initial selectivity to carbon oxides

was generally lower than 3 %. As stated before, there is a

general agreement on the role of vanadium in this first

reaction step. The alkane activating and methylene

hydrogen abstracting element is V5?, which is able to

Fig. 7 XPS spectra for The M1

phase (M1(Te)-G) of the Te 3d5/2

region before reduction (a) after

reduction with 10 pulses of

propane at 380 �C (b) and after

re-oxidation by O2 at the same

temperature (c)

Top Catal

123

undergo an electron transfer within the vanadyl cation that

imparts a radical character to its oxygen that is capable of

attacking the methylene–H and abstracting it as an H. In

view of this mechanism it was not so easy to understand the

effect of Sb and Te on the reaction rate, which was more

than two times lower for the Sb-containing M1 phase. On

another hand, the apparent activation energy for ethane

oxidative dehydrogenation is lower for the Sb-containing

M1 phase than for the Te-containing phase. If we look to

the variation of Ea with the logarithm of the rate when

moving from Sb- to Te-containing M1 phases we observed

that both increased although Ea increased more gently. This

feature can only be explained by the presence of intrinsi-

cally more active catalytic sites on M1(Sb) than on

M1(Te). The M1(Sb)-C phase has a slightly higher content

in V than the M1(Te)-D phase. Such a difference should

imply a higher rate of conversion of the alkane for the Sb-

containing phase if it was the most determinant parameter,

but this was not the case. One may postulate that the dis-

tribution of vanadium over the different sites and conse-

quently on the active sites is different between the Sb- and

Te-containing M1 phases. However structural determina-

tions made on both types of phases do not show significant

differences in vanadium distribution over the sites that

could account for the difference in activity. Furthermore

both the bulk and surface V content are comparable in both

types of compounds. It can be proposed from the reduction

experiments that the reduction level of V cations in the

M1(Sb) phases in steady state conditions under a reaction

gas mixture is higher than in the M1(Te) phases. Thus, part

of the active sites would be reduced and unavailable for the

reaction. V5?/V4? redox potential lies in between those of

Sb5?/Sb3? and Te4?/Te2? and reduced V species could

rapidly re-oxidized in M1(Te), whereas they are stabilized

in M1(Sb). This leads to a strong difference in number of

active sites in steady state reaction conditions, which

explained the important difference in the initial rate of

alkane conversion, which appeared almost independent of

the type of alkanes considered. Such interpretation stresses

the importance of having the active V5? oxo-species in

close vicinity to Te or Sb sites. One may note that such

requirement is fulfilled if the active site is the M7 site

occupied by V5? as proposed in several studies [4, 32]. It is

also fulfilled if the hypothesis of ensembles of V-Te(Sb)

oxo-species support on the M1 phase is considered [33].

Furthermore, the M1 phase has to be able to undergo

oxidation and reduction to a certain degree without changes

to its structure. However, a structure built from corner-

sharing octahedra should not support a high concentration

of oxygen vacancies. It is important to recall the key-role

played by hexagonal channels as oxygen reservoirs in the

M1 structure and the important role played by Te and Sb

occupying these channels [12, 20]. The surface reduction

level of Te4? cations appeared in this study much higher

than that of possible V5? active sites that are less numer-

ous. This confirms the importance of the former cations in

the re-oxidation of the catalysts in the reaction atmosphere

and in the buffering of oxygen content at the surface of the

catalysts.

Sb and Te also affect the selectivity but only in the case

of propane oxidation. This shows that they may not be

involved directly in the dehydrogenation of the alkanes

molecules but should be involved in the further transfor-

mation of the alkene molecules that are formed.

It was shown a long time ago that elements like Te or Sb

were key to enhancing the selectivity of desired partial

oxidation products [34]. It was proposed that the a-

hydrogen abstracting site of the olefin oxidation or am-

moxidation catalyst is an element possessing a lone pair of

electrons, which imparts a partial radical character to the

oxygen associated with it, thereby causing the abstraction

of the a-H as a H� [4]. In bi-molybdates the a-hydrogen-

abstracting element is Bi and it is reasonable to consider

that Te or Sb play the same role in the M1 phases. Sb and

Te would thus directly play a determinant role in a crucial

step of the reaction and the intrinsic electronic properties of

the two elements could explain the differences in selec-

tivity observed between the two types of M1 phases. The

presence of both types of cation (Sb5? and Sb3?) might

also explain them. Similar effects have been evidenced and

similarly explained in catalysts containing Te that were

used for the oxidation of other alkanes. For example, this

was the case for Keggin-type phosphomolybdic acid and

cesium salts used as catalysts in the oxidation of isobutane

into methacrolein or methacrylic acid, with protons that

were partially substituted by tellurium [35]. In this case,

tellurium induced a strong positive effect on the selectivity

to methacrylic acid and methacrolein when introduced as

counter-cation without any significant effect on the activ-

ity. A similar effect was also observed on Te doped Ni-

MoO4 catalysts [36].

5 Conclusions

This study is part of a general study undertaken to pinpoint

the respective functionalities associated with the various

metals of the active catalytic centers in the M1 phase. In

this study we have tried to determine the impact of Te or

Sb cations on the activity and selectivity either in propane

oxidation or ethane oxidative dehydrogenation and to

understand the redox state dynamics occurring at the sur-

face. We have shown that Te and Sb affect the intrinsic

activity of the catalyst for ethane and propane oxidation in

the same way and that the M1(Te) phases were much more

active than the M1(Sb) ones. These elements should not be

Top Catal

123

directly involved in the first dehydrogenation of the alkane

molecules, which is the rate-limiting step and their effect

on activity has been explained by an indirect effect on the

relative number of V5? active sites in steady state condi-

tions. A study of catalyst reduction at catalytic reaction

temperature by XANES spectroscopy allowed to show that

Te contrarily to Sb participates in the re-oxidation of the

vanadium species by local electron transfer. It leads any-

way to a higher active site density at the surface of the

catalyst than Sb. Such a local effect would be facilitated by

the strategic localization of Te or Sb and V active sites in

adjacent positions to each other in the M1 structure.

However it should be pointed that the observation of this

transfer does not mean that it necessarily takes place in the

reaction conditions.

Sb and Te were shown to affect the selectivity in the

case of propane oxidation but not in the case of ethane

oxidative dehydrogenation. This demonstrates that they

may not be involved directly in the dehydrogenation of the

alkane molecules, but should be involved directly in the

further transformation of the alkene molecules that are

formed. Such involvement has already been assessed in

numerous studies, showing that Te or Sb play the role of a-

hydrogen abstraction from the alkene molecules. The

substitution of Te by Sb that would be beneficial to several

points of view for future industrialization did not lead to

catalysts similarly efficient. Specifically for propane oxi-

dation, the difference in selectivity to acrylic acid (or

acrylonitrile in the case of ammoxidation) should be related

either to the intrinsic property of the Sb or Te element or to

the oxidation state of Sb. Only in the second case could a

solution be envisaged if the oxidation state could be

modified by a judicious surface or bulk doping that should

concomitantly not affect the active V sites. Finally, the

problem of the synergetic effect between M1 and M2,

which is not observed in the case of Sb catalysts remains to

be solved.

Acknowledgments We gratefully acknowledge ESRF for providing

the facilities at ID29 and we thank Dr. Carmelo Prestipino, the local

contact of beamline BM29, for help in preparing of the in situ setup

and XAS spectra recording.

References

1. Ushikubo T, Oshima K, Kayo A, Umezawa T, Kiyono K, Sawaki

I (1992) European Patent No. 529,853

2. De Santo P, Buttrey J, Grasselli RK, Lugmair CG, Volpe AF,

Togy BH, Vogt T (2004) Z Kristallogr 219:152

3. Ueda W, Vitry D, Katou T (2004) Catal Today 96:235

4. Grasselli RK, Buttrey DJ, Burrington JD, Andersson A, Holm-

berg J, Ueda W, Kubo J, Lugmair CG, Volpe AF Jr (2006) Top

Catal 38:7

5. Lopez Nieto JM, Botella P, Concepcion P, Dejoz A, Vazquez MI

(2004) Catal Today 91–92:241

6. Botella P, Dejoz A, Lopez Nieto JM, Concepcion P, Vazquez MI

(2006) Appl Catal A 298:16

7. Baca M, Pigamo A, Dubois JL, Millet JMM (2003) Top Catal

23:39

8. Botella P, Nieto JML, Solsona B, Mifsud A, Marquez F (2002) J

Catal 209:445

9. Grasselli RK, Burrington JD, Buttrey DJ, DeSanto P Jr, Lugmaird

CG, Volpe AF Jr, Weingand T (2003) Top Catal 23:5

10. Guliants VV, Bhandari R, Brongersma HH, Knoester A, Gaffney

AM, Han S (2005) J Phys Chem B 109:10234

11. Guliants VV, Bhandari R, Swaminathan B, Vasudevan VK,

Brongersma HH, Knoester A, Gaffney AM, Han S (2005) J Phys

Chem B 109:24046

12. Baca M, Millet JMM (2005) Appl Catal A 279:67

13. Millet JMM, Baca M, Pigamo A, Vitry D, Ueda W, Dubois JL

(2003) Appl Catal A 244:359

14. Oliver JM, Lopez Nieto JM, Botella P, Mifsud A (2004) Appl

Catal A 257:67

15. Haggblad R, Wagner JB, Deniau B, Millet JMM, Holmberg J,

Grasselli RK, Hansen S, Andersson A (2008) Top Catal 50:52

16. Millet JMM, Roussel H, Pigamo A, Dubois JL, Jumas JC (2001)

Appl Catal A 232:77

17. Baca M, Aouine M, Dubois JL, Millet JMM (2005) J Catal

233:234

18. Nguyen TT, Deniau B, Baca M, Millet JMM (2011) Top Catal

54:650

19. Naraschewski FN, Jentys A, Lercher JA (2011) Top Catal 54:639

20. Safonova O, Deniau B, Millet JMM (2006) J Phys Chem B

110:23962

21. Nguyen TT, Aouine M, Millet JMM (2012) Catal Commun 21:22

22. Naraschewski FN, Kumar CP, Jentys A, Lercher JA (2011) Appl

Catal A 391:63

23. Deniau B (2007) Ph.D. Thesis 315

24. Baca M, Pigamo A, Dubois JL, Millet JMM (2005) Catal Com-

mun 6:215

25. Mekkia A, Khattaka GD, Wengerb LE (2009) J Electron Spec-

trosc Relat Phenom 175:21

26. Zhou WP, Kibler LA, Kolb DM (2002) Electrochim Acta

47:4501

27. Detty MR, Lenhart WC (1989) Organometallics 8:861

28. Birchall T, Sleight AW (1976) Inorg Chem 15:868

29. Kubo J, Watanabe N, Ueda W (2008) Chem Eng Sci 63:1648

30. Schloegl R (2011) Top Catal 54:10

31. Balcells E, Borgmeier F, Grisstede I, Lintz HG, Rosowski F

(2004) Appl Catal A 266:211

32. Oshihara K, Hisano T, Ueda W (2001) Top Catal 15:153

33. Havecker M, Wrabetz S, Krohnert J, Csepei L, Naumann R,

d’Alnoncourt R, Kolen’ko Y, Girgsdies F, Schlogl R, Trunschke

A (2012) J Catal 285:48

34. Grasselli RK, Centi G, Trifiro F (1990) Appl Catal A 57:149

35. Huynh Q, Shuurmann Y, Delichere P, Loridant S, Millet JMM

(2009) J Catal 261:166

36. Kaddouri A, Mazzocchi A, Tempesti E (1999) Appl Catal A

180:271

Top Catal

123