Catalytic oxidation of methane on MoO3-SiO2 : mechanism of oxidation with O2 and N2O studied by surface potential measurements

147

Catalysis Today, 1(1987) 147-156 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

CATALYTIC

OXIDATION

AND N20 STUDIED

ON MOOS-SiO2

OF METHANE

BY SURFACE

POTENTIAL

Y. BARBAUX,

A. ELAMRANI

and J.P. BONNELLE

Laboratoire

de Catalyse

Heterogene

des Sciences Cedex

et Techniques

OF OXIDATION

: MECHANISM

WITH

02

MEASUREMENTS

et Homogene,

no 402, Universite

U.A. C.N.R.S.

Lille Flandres-Artois, 59655 Villeneuve d'Ascq

de

(France)

ABSTRACT

The catalytic oxidation of methane at temperatures higher than 5OO'C leads to carbon monoxide when using O2 ; whereas formaldehyde is produced with a good selectivity when N 0 is used. Surface potenti 3 1 measurements have been performed to determine the nature of the oxygen species involved during the methane oxidation. Both 0 and N 0 give O- which is localized at the catalyst surface. At temperatures hf?gher thgn 400°C,0- is transformed into O--. Depending on the temperature, methane reacts with these two negative oxygen species. Surface potential measurements under catalytic conditions show that the catalyst is more oxidized in the case of 0 than it is when N 0 is used. The

variations of the stationary potential. &served when varyisg the partial pressures of CH and 0 (or N O),are in accordance with the attack by methane of adsorbed oxy8en speE. ies (i$bably O-! in the case of 02, while in the case of N20 methane reacts with 0 species.

INTRODUCTION Natural

gas

is the

source of methane, containing 40 to 95 volume

major

percent of CH4. The use of methane limited because of the difficulty fore

it

would be extremely

handle and more valuable

for other

purposes

than

to store and transport

valuable

to convert methane

for the chemical

industry,

heat production

this compound.

into products

like methanol

easier

or

is

Thereto

formalde-

hyde. The direct partial

oxidation

of methane

by air has been extensively

in the past (I) ; conversions and selectivities measured So far the

research

on methane

conversion

were

by heterogeneous

always

studied very

catalysis

low.

has been

developed along two lines : the oxidative coupling of methane to ethane and ethylene on catalysts such as doped MgO or rare earths oxides (2-4) and the partial oxide

oxidation

of methane

to methanol

(5-8). In the later reaction,

v205 supported

on silica. Although

oxide has not been studied,

0920-5861/87/$03.50

and

formaldehyde

the most selective the mechanism

an O- species

of the

using are

oxidation

has been claimed

0 1987 Elsevier Science Publishers B.V.

by

catalysts

to

be

nitrous Moo8

by

the

or

nitrous active

l&3 intermediate. The aim of the present work methane

on MoO3 supported

is the study of the difference

of reactivity

of

when oxygen or nitrous oxide are

on silica catalysts,

used as oxidizing agents using surface potential measurements. This technique has been shown to be able to provide information species

involved

in the catalytic

oxidation

about the nature of the oxygen

reactions

(9).

EXPERIMENTAL Catalysts The samples of MoO3 supported on SiO2. were prepared by impregnation of silica (Aerosil DEGUSSA 130 m2.g-') by an ammonium heptamolybdate solution, followed

by evaporation

under continuous

stirring,

and drying

at llO°C

for

16

hours. The catalyst was then calcined at 5OO*C under dry air for 16 hours. The concentration of ~lybdenum is expressed in weight percent of atomic molybdenum. Catalytic activity measurements The catalytic

activity

measurements

were performed

in a fixed

reactor,

bed

under continuous flow at atmospheric pressure. The mass of the catalyst was lg and the total flow rate of the gas was 3 1-h-l. The partial pressures (in atm.) of the gases were : CH, : 0.11, 02 or N20 : 0.33, H20 : 0.22, He : 0.34,for experiments

conducted

with water:

and CH

4

: 0.13, 02 or N20 : 0.42,

He

: 0.45,

for experiments without water. The chromatographicanalysis of the products was perfo~ed

on two columns : carbosieve and Porapak Q, with a cathar~eter

detector. Surface potential ~asurements The surface potential of the samples was measured by the vibrating capacitor method. The reference electrode was made of graphite. The potential measuring cell was connected to a gas flow system which allowed controlled streams of oxygen or nitrous oxide, methane, argon and their mixtures to pass through the apparatus. All the surface potential values presented in this work are relative to the graphite electrode potential : V = Vtgraphitel - Vtsamplef. Hence, an increase of the value of the potential difference indicates that the surface becomes more negative. RESULTS AND DISCUSSION

Catalytic activity measurements The conversions state are reported

and selectivities

measured

in table 1. The catalytic

after establishment oxidation

SiO*. 1 wt% /Mo/Si02 and 4.3 wtX No/'Si02 catalysts

of methane

leads to total

of the steady with

02

oxidation

on to

carbon oxides,CQ ~~nv~rs~~n

and

coy

and changes

The presence of water in the flowing gas Iowers the

the selectivity

with an enhan~~@nt

of the CO production

at the expense of C02. When N2Q is used as the oxidizing agent, the reaction produces formaldehyde only on MOO3 su~p~rt~ an SiO2. During the first minutes of the reaction, the conversion is high (up to 30%) and subsequently drops rapidly. The results reported in table 1 have been obtained after stab~~~~ati~nof the conversion,

Oxidizing agent

Catalyst

Temperature

87

60

40

9

fOO

600

2

100

580

50

81

600 600

N2~~ate~ 02+water

02+water N20

Selectivities co2 CH2O C2 50

9a

02

CO 50

600

SiO2

1% MO

Conversion

14

4

11

16

N20+water

600

6

73

O2

550

43

95

3

O2

580

67

68

24

-

8

O2

610

92

39

50

-

1f

02~water

580

59

70

23

4.3% Mo

-

I

2

-

2

4 4

02+water

630

82

57

39

-

N20~water

540

4

5

-

75

'20

N2~~ate~

590

8

60

-

30

10

In the literature, Liu et al, (5,6) reported that the reaction leads ta ethanol

and f~~aldehyde- Any f~~ati~~

of metha~~i was not observed: the

only partial oxidation Proust was f~~a~dehyde. ~~re~v~r~ for the same ex~erjment~~ conditions* our conversions are Disagreeing conversions and selectivities

can

differences

catalysts (e.g. impregnation

in

the

preparation

solutions

of different

catalytic

test manifold,

of

the

pH);'in the surface

result

from

area of the support

with the possibility

of B homogeneous

several

Tower.

sources: wSth

used; or in

the

reaction

the

in

150 post-reactor

area. Liu et al. have not mentioned the surface area of their

support. Potential ~asure~nts

in oxygen at~sphere

The surface potential values of the MoO3/SiO2 samples treated at 500°C in pure oxygen, and in an oxygen-argon mixture (PO

= 0.05 atm.) have been

measured in the temperature range 200-500°C and deported in figure 1. The curves for the molybdena-covered silica show three temperature regions where potential values are constant or vary slowly : 240-3OO"C, 340-420°C and > 45O'C. Between these regions, potential values rise rapidly, the

surface

becoming more negative when the temperature increases.

1.6

1.01

200

300

400

T(Y)

!‘n(p-1

02

Fig. 1. Surface potential variations versus temperature under a 0.05 atm. oxygen partial pressure. * SiO ; (II) : 1 wt% MofSi02 (1) I 4.3&% !?o/Si02 (II)

Fig, 2. Surface potential variations versus In PO of the lwt% Mo/SiO2 catalyst. (I) : T = 2:OY ; (II) : T = 350°C (III): T = 46OY

In these three temperature ranges, the variations of the potential value when the partial pressure of oxygen is changed have been interpreted on the

151

basis of a method previously described (9). This method allows the determine tion of the nature of the negative oxygea-species adsorbed on the surface and in equilibrium with gaseous oxygen. The dependence of the potential on the partial oxygen pressure is : V = g

In PO2 + constant

with k the Boltzman

constant,

T the absolute

temperature

and e

the

charge

of

the electron. The

value of n depends on the equilibrium between gaseous and adsorbed

oxygen species. The possibilities are : n=l

O2 + e2

n=

n=4

O;

02 t 2e-

2

20-

02i4e-

=

202-

For the three temperature ranges previously distinguished,the variation of the potential value of the Mo03/Si02 catalysts with the partial pressure of oxygen is shown in figure 2. The slopes of the straight lines obtained are equal to kT/ne and lead to values of n indicated in table 2. TABLE 2 Nature of the adsorbed oxygen species on Mo031Si02 catalysts under an oxygen gas phase and at different temperatures n

Temperature range

Oxygen species

240-300°C

1

340-42O'C

2

O; o-

> 450°C

4

02-

Within the t~perature range considered, O;, O- and 02- species are adsorbed on an oxidized Mo03fSi02 catalyst, 02A being the dominant species at the temperatures higher than 450°C. The same measurements done on the Si02 support show two temperature regions (figure 1). The n values calculated from the slope of the V = f(lnPo ) straight lines as indicated

in table 3, show that now the O- is the dominant

& ecies

at

high temperature. When the Mo03/Si02 sample is reduced-in a hydrogen-argon flow (P,.,= 0.1 atm.) at 500~ for 30 minutes, the potential value falls down, the%urface

152 becoming less negative. After cooling at lOO*C, an oxygen flow was

been

in~rodu~~d~~~nd the varfations of the pcrtentiai,Yrhenjincreasing the temperature, have been recorded and reported in figure 3. The three temperature regions observed before can be seen again,but the potential variation between each plateau

is now larger than in the case of the well

the oxygen

species

adsorbed

an the surface

with

sample.

However,

gaseous,oxygen

as determined by the Y = f(lnP0 ) variations. Reoxidation

is still the same,

of the bulk of MOO3 occurs at TABLE 3

oxidized

in equilibrium

.i& are formed. 400°C when O*- specs

a

Nature of the adsorbed oxygen species on SiO2 under an oxygen gas phase and at

different temperatures. Temperature range

n

160-32o*c

7

> 32V'C

2

Oxygen species 0; 0-

Potential measurements in N20 atmosphere The potential variations measured in the course of the reoxidation of the

reduced Mo03/Si02 sample by a N20-argon flow fPN o = 0.1 atm.1 are shown in 3 and can be compared with the reox~dation~y Oz.

figure

At temperatures lower than 24O"C, surface potential values are lower than the corresponding values obtained under oxygen atmosphere. In the 240-28OOC temperature range, potential values under O2 and N20 are equal. Between 280 and 480°C, the potential value in 120 atmosphere is constant,and it rises rapidly between 490 and SOO*C. At this temperature, the potential value under N20 reaches the value obtained in oxygen atmosphere. When the temperature is lowered,

the surface potential

decreases

slowly,

as can

been seen in figure 3.

In the temperature range 140-280°C, the surface potential variations obtained in N20 atmosphere can be explained by the adsorption of 0; and O* species.

created by the decompasition of N20 on the surface, folfowing:

N20 + e- -

N2 t O-

2 o-

O;te-

=

At t~peratu~s

higher than 280%

under oxygen atmosphere, O- species are

in equilibrium with gaseous 02 molecules. This is not the case in N20 atmosphere ; the 0' species created by the N20 adsorption desorb to give gaseous

153 oxygen: the surface potential value is then constant. In the temperature range 360-42O"C, under oxygen,the potential rises, as the O- species transform into 02-,which reoxidizes N20,

the

O-

species

can

either

desorb

or

the

transform

reduced

into

MOOS.

Under

The stable

02-_

potential value observed up to 48O'C shows that desorption is faster than reoxidation. Reoxidation

only occurs at temperatures higher than 49O*C.

= 0.1 atm.

po2 pN C

0.1 atm.

q

2

= PO2

0.15 PCH = 0.6atm.

1.25 ..

0.85

Fig. 3. Surface potential variations versus temperature after H reduction of the 4.3 wt.%MolSiO catglyst at 500°C. (I) : P = 0.05 atm.2; (II) PN o = 0.1 atm. O2 2

= 0.15 PCH = 0.6atm. 4 = 0.1 atm.

‘N20 PCH

J

'4

4

Fig. 4, Stationary surface potential values of the 4.3 wt% MoiSiO catalyst under different g$s phases at 5OO'C.

In the catalysis temperature range (T > 550°C), 02- can be adsorbed on the surface of Mo03/Si02 catalysts, in O2 as well as in N 0 atmosphere, being

an

intermediate

2

state under CH4 and O2 or N20 gaseous mixture, the two species coexist

on the surface.

the O-

species giving 02- adsorbed ions. In the stationary The reaction

creating

them constitutes

O- and 02- can the

reoxidation

step of the redox reaction. Steady state surface potential measurements In general,

the values of the surface

are exposed to the reaction Such steady state surface in figure 4.

mixture

potentials

potential

decrease

and reach stable values for different

reactant

when the in

a

catalysts few

flows are

hours. reported

164 When MoO,/SiO2 catalysts are placed in CH4-02 or CH4-N20 gaseous mixtures at temperatures higher than 500°C, the potential value is intermediate between the oxidized state (02 or N20) and the reduced state (CH4) (figure 4). This indicates that the surface of the catalyst is partially reduced during the reaction. Interestingly,the value measured in CH4-02 flow is higher than in CH4-N20 flow,implyingthat the surface layers are more oxidized in the former case. At constant temperature, the stationary potential values vary reversibly when increasing and decreasing the partial pressures of 02 or N20 and CH4_ The observed variations are in accordance with the following laws :

V=gln$

pO

in case of oxygen 4 in case of nitrous oxide

variations

Plots of the surface potential partia't pressures

of reactants

are

shown

in

versus

the

figure 5.

logarithms

The

of

the

slopes of

the

straight lines obtained equal kTI2e in both O2 and N20 cases. V volt)

V (volt)

.

(I)

1;21:,

I!

y/

8

c

-2

-1

*

ln(pO,/PR)

,

-2

. -1

ln(poxh,)

Fig. 5. Surface potential variations of the 4.3 wt% Mo/SiO catalyst versus : (I) : In Po2/PCH4 at T = 51O'C ; (II) : in PN20/PCH4 at 54GC Such variations could be explained by a ~chanism of reduction and oxidation of the surface (lO,tt), The oxidation reaction step involves the adsorption of negative oxygen species; the reduction step is the attack /on these negative species by methane. In the oxidation step, two electrons are given by the catalyst for one oxygen or nitrous oxide molecule. That could mean that the active oxygen species are O-(in the case of catalytic oxidation of methane by oxygen)and 02-(in the case of N20), The reduction-oxidationmechanisms proposed on the basis of the surface potential ~asure~nts

are :

155 02 f 2 e- -

2 0" or

in case of oxygen : CH4 + O-

in case of nitrous oxide:

-

O2 + e-

=

0;

0; + e-

-

2 O-

products + e-

N20 + 2 e-

-

02- + N2

CH4 + 02-

-

products + 2 e-

The proposed mechanism is in accordance with the classification of oxygen species proposed in the literature (9,12) for the catalytic oxidation of hydrocarbons. The O- species lead to total oxidation while the 02- species give partial oxidation products. However recently Liu et al. (6) have concluded to the contrary for the oxidation of methane. Their EPR results obtained at low temperature have clearly demonstrated the generation of O- species and of methyl radical on these catalysts but these results were obtained the actual reaction

conditions.

oxidation

from

The results obtained in situ, reported in this

work, complete Liu's et.al. results by demonstrating the selective

far

the role of 0

2-

species

in

process.

CONCLUSION

The adsorption of gaseous oxygen or nitrous oxide on Mo03/Si02 catalysts creates 02, O- and 02- species within the range of temperature 160-5OOY. In the course of the catalytic oxidation of methane, the surface of the catalyst is partially reduced, out to a greater extent in the case of N20

than

in the case oi‘Oz. The catalytic oxidation of methane can be explained by a reduction-oxidation mechanism of the catalyst surface. The negative oxygen species involved in the catalytic oxidation by oxygen is an O-; the main products formed are CO and C02. In the case of N20, the negative species taking part in the reaction are 02- species and formaldehyde is formed with a good selectivity. REFERENCES N.R. Foster, Applied Catalysis, 19 (1985) 1. G.E. Keller and M.M. Bhasin, J. Catalysis, 73 (1982) 9. T. Ito and J.H. Lunsford, Nature, 314 (1985) 721. K. Otsuka, K. Jinno and A. Morikawa, Chem. Lett., (1985) 499. yiS. Liu, M. Iwamoto and J.H. Lunsford, J. Chem. Sot. Chem. Cornnun.,(1982) H-i. Liu, R.S. Liu, K.Y. Liew, R.E. Johnson and J.H. Lunsford, J. Am. Chem. sot., 106 ($984) 4117. K.J. Zhen, M.M. Khan, C.H. Mak, K.B. Lewis and G.A. Samorjai, J. Catalysis, 94 (1985) 501.

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J.R. Anderson and P. Tsai, Applied Catalysis, 19 (1985) 141. 3-M. Libre, Y. Barbaux, B. Grzybowska and J.P. Bonnelle, React. Kinet. Catal. Lett., 30 (1982) 249. 10 Y. Barbaux, J.P. Bonnelle and J.P. Beaufils, J. Chem. Research, (S) (1979) 48, M (1979) 556. 11 B. Grzybowska, Y. Barbaux and J.P. Bonnelle, J. Chem. Research, S (1981) 48. M (1981) 650. 12 A.'Bieianski and J. Haber, Catal. Rev. Sci. Eng., 19 (1979) 1.