Use of methanol as a probe to study the adsorption sites of different MgO samples

Materials

Chemistry

and Physics,

USE OF METHANOL DIFFERENT

28 (1991)

309-320

AS A PROBE

309

TO STUDY

THE

ADSORPTION

SITES

OF

Ma0 SAMPLES

M. BENSITELl,

0. SAUR and J.C. LAVALLEYZ

URA.CNRS.0414

“Catalyse

ISMRA - 6, Boulevard

et Spectrochimie”

du Markhal

Juin - 14050 Caen Cedex (France)

Received January 30, 1991, accepted

March 4, 1991

ABSTRACT Methanol

adsorption

spectroscopy. temperature

MgO has been studied

(species

H) coordinated

cm-1 (species

by gravimetry

: i) one reversibly

II) resulting

exposed,

regular faces, while species

II formation

cations

in a low coordination

surface

of which varies

that the proportion of the total

state.

species involves

of methanol

the presence

in

to be formed on

of defects, such as Mg*+ (ex-carbonate),

with the surface area, reaching

adsorption

to

that faces of the

of the study to five MgO samples,

from 12 m* g-1 (smoke) to 565 m* g-l

of defects increases

amount

H and I are postulated

Extension

H-bonded

both from the dissociation

of the O-H bond as shown by use of CH 31sOH. Considering

(100) type are preferentially

at room

by a v(O-C) band at 111.5

species characterized

I) and 1090 cm-1 (species

and FT-IR

adsorbed

to Mg*+ sites but also strongly

02. sites ; ii) two methoxy

unsaturated

methanol

on activated

It gives rise to three kinds of species

sites in the case

the

shows

more than 25 %

of high surface

area

samples.

INTRODUCTION The local arrangement from the study of the very sensitive association

VOH

of the surface sites of a divided vibration

to their environment

state of the hydroxyl

of surface [l].

hydroxyl

However,

hydrogen

metal oxide can be deduced

groups,

this frequency

their frequency depends

atom than on the coordination

atom, which means that only free OH groups can be used as probes. only

appear

when

1Present address

the surface

is partly

dehydroxylated.

: DBpartement de Chimie Facultb des Sciences

By heating

being

more on the of the oxygen

Such free groups under

vacuum,

El Jadida (Maroc)

2To whom corespondance should be addressed

0254-0584/91/$3.50

0 Elsevier

Sequoia/Printed

in The

Netherlands

310

however, totally

some types

eliminated,

of OH groups,

preventing

not very thermally

their detection. (VOH =

673 K. For these reasons,

we preferred

of methoxy

multidentate

using methanol

depending

only

[2 - 41, i.e. the local arrangement

Previous formation

species

results

have

shown

that

CHs

idealized

adsorption

The adsorption

on their

methanol

coordinated

of molecules

structure,

adsorption

leads

frequencies

to methoxy

or

to the at 1030,

species, postulated

of CHsOH, via rupture of the H-OCHs and HOsites were

supposed

However,

to be exposed

on the

in recent papers devoted

to

like H2 [6, 71, NH3 [8] or Hz0 [9], it was shown that 3- and 4-

Mga+ and Oa- ions existed on the surface, for instance on corners or edges,

and that they were more reactive

than the Scoordinated

sites on the idealized

The present paper reports novel results on the IR study of the adsorption MgO. Use of CHstsOH

in particular

allows one to discriminate

the O-H and C-O bonds. Gravimetric five

at

the voc

monodentate

on MgO

by voc stretching

(100) face of a cubic MgO monocrystal.

the adsorption

by evacuation

on the surface.

of four types of species, characterized

bond respectively.

in the case of

as a probe molecule,

1060, 1114 and 1080 cm-l [5]. The two latter correspond to result from the dissociative

can be partly or even

for instance

3745 cm-l) being almost eliminated

ThOs [2], type I OH groups

vibration

stable,

This appears

different

MgO

samples

measurements

presenting

very

complete

different

surface.

of methanol

on

dissociations

of

the results extended

to

between

surface areas, from 12 to 565

ma g-1.

EXPERIMENTAL Five different

- by combustion

MgO samples were used. They were obtained of a ribbon of Mg in air (sample noted MgO-I)

- by thermal decomposition

of

- a monocrystal

of magnesium

hydroxide

(brucite) : MgO-II

- a monocrystal

of magnesium

carbonate

: MgO-Ill

- a commercial

magnesium

- from Carlo-Erba Their thermal mass during

hydroxide

(Merck) : MgO-IV

: MgO-V. decomposition

evacuation

was studied with a McBain thermobalance.

from room temperature

Table I, with the surface activated

:

area,

determined

to 873 K and 1023 K is reported

by the BET method,

the samples

in

being

at 1023 K.

For IR studies, the samples were pressed into the form of self-supporting 30 mg) and prepared

The loss of

evacuated

by hydrolysis

were recorded

in situ at 823 K. CHstsOH of trimethyl

phosphate

on a Nicolet MX-1 spectrometer

by Ha’s0

(isotopic

purity

m.

(CEA) at 378 K (lo].

at room temperature.

disks (1592 %) was IR spectra

311

Table I. Characteristics

of the different MgO samples. Surface

Sample

m2

MgO-I

area

Loss of mass after evacuation

g-l

823 K

1023 K

12

1.0 %

1.1 %

30.5 %

32.7 %

MgO-II

175

MgO-Ill

565

MgO-IV

290

30.5 %

33 %

MgO-V

30

2.5 %

2.6 %

at

53.3 %

RESULTS and INTERPRETATION Gravimetric

measurements

On the samples at room

activated

temperature

at 1023 K, successive

(r.t.) until the equilibrium

samples were evacuated

at increasing

doses of CHsOH were introduced

pressure

temperatures,

of mass is similar for the different samples and is reported IV. It shows different desorption

- species

I, slightly

bonded

- species II, desorbed

steps corresponding

reached

2 Torr. Then the

from r.t. to Q& 873 K. The variation in Fig. 1 in the case of MgO-

to the evolution

to the surface and first desorbed

of different species

(T < 430 K)

when T z 590 K.

4 0

*I

10

*

20.

30.

40.

50.

I 473

Fig. 1 previously

Variation contacted

573

with the evacuation

673

T/k

temperature

with CHsOH (Pe = 2 Torr).

of the mass of sample

MgO-IV

312

The amount of chemisorbed is reported

species after evacuation

at r.t. and at 590 K (species II)

in Table I I for the different samples studied.

Table I I. Amounts of methanol In presence

adsorbed

on the activated

After evacuation

of a gas

590 K pm01 m-*

7.1

62

5.2

15

1.3

7.8

490

2.8

pm01 m-2

85

pm01 g-l

pm01 m-2

pm01 g-l

pmol g-1 MgO-I

at

r.t.

phase (Pe = 2 Torr)

Sample

samples,

MgO-II

1645

9.4

1370

MgO-Ill

4790

8.5

3935

7.0

1500

2.65

MgO-IV

2740

9.4

2240

7.7

910

3.1

MgO-V

233

7.8

183

6.1

46

1.5

Different addition

tvpes

of several

of adsorbed

soecies.

In their study [5], Tenth

aliquots of CHsOH first led to the appearance

1 t 14 at 1060 cm-f, the latter being largely removed an undissociated oxygen

methanol

species

interacts with a magnesium

adsorbed

&&

showed

that

of two strong bands at

by pumping at r.t. It was assigned to

in such a way that a lone pair of the

ion on the surface :

CH3

‘.O - H f&J

(called, in the present paper, species H)

“‘0 That characterized

by the 1114 cm-t band was assigned to a methoxy

species

FH3

0

(species I)

Y 0

hg

resulting from the dissociation Our results obtained

on MgO-IV

evacuation

of the methanol

are in agreement either

at 400 K (Fig. 2) allows to species

wavenumber

of the latter confirms

hydrogen

Such wavenumbers bond between

of CHsOH

us to complete

I, vct.13 bands

groups formed from the dissociative

O-H bond.

such assignments.

in presence

corresponds

920 cm-l.

with

Comparison

the assignments

adsorption

of a methoxy

of spectra

(Pe = 2 Torr) or after

at 2915 and 2785 cm-’

the formation

suggest,

vapor

of the bands.

It

(Fig. 2b) ; the low

species.

The new OH

give rise to broad bands near 3500 and

as in the case of ThO2 [2], an interaction

these OH and the oxygen atom of the methoxy

groups.

by a

313

0.25

I 3800

Fig. 2

Infrared

/

I

I

3200

2600

spectra

I

of species

I 2000

I

I

adsorbed

/

1 800

on activated

of CH30H gas (Pe = 2 Torr) ; b) after evacuation

presence

1

I 1400

MgO-IV

sample

; a) in

at 400 K ; c) after evacuation

at 590 K.

a

J b

2800

Fig. 3

Infrared

activated

MgO-IV.

spectrum

2000

of adsorbed

1100

1600

species

L

given

by CD3OH

-1 800

(Pe = 2 Torr) on

314

Species broad

band

observed window

H leads to a voc

band at 1060 cm-l.

near

with a transmittance

1500 cm-l

in the case

of methanol

does not appear

between

the

6~~3

Tenth species cm-t.

by the appearance

hydrogen

bonded

species.

This type of

VOH

the first Overtone 280~ and the fundamental

it appears

substitution

wavenumbers

showing

called

thereafter

at 400 K (Fig. 2b). It is the only species vc~3

bands at 2918, 2850

of the latter band is also in favor of the formation experiments

using CHslsOH

(1115 and 1092 cm-l)

that both methoxy

O-H bond. By analogy

and ZrOs [4], we propose

species,

at 590 K (Fig. 2~). It leads to

Comparative

show that both voc

of another

band at 1080 cm-t. In this study, we find it at 1092

on evacuation

after evacuation

species.

the formation

by a voc

and 2800 cm-l. The wavenumber

methanol

This transmittance

of two broad bands at 2600 and 3100 cm-1 (Fig.

between

[5] also reported

On MgO-IV,

which remains

‘80

[ll].

as already

groups of species H. et.

II, characterized

methoxy

(Fig. 2a) by a

at 1445 cm-t,

on alumina

and 60~ modes of strongly

bond is confirmed

window

using CDsOH (Fig. 3), showing that it is due to an interaction

3) resulting from a resonance of hydroxyl

adsorption

It is also characterized

species

with previous

are sensitive

of a

(Fig. 4)

to the 160

result from the dissociation

results reported

that the 1092 cm-t

and CHslsOH

+

of the

on ThOs [2], CeO2 [3]

band characterizes

bridged

methoxy

groups : CH3 I

0

/

(species II)

\

Mg

Mg

WOVENUMBERS Fig. 4

voc vibration

CHalsOH

followed

of methoxy

by evacuation

species formed at 450 K.

by adsorption

of : a) CHstsOH

or b)

315

This assignment species towards

is confirmed

methylcarbonate-like

species

the first step of the reaction oxygen

lone pairs acting

interaction

by the comparative

study of the reactivity

of methoxy

CO2 : as in the case on other oxides [3, 4, 121, only species I leads to

is easier

[13]. This specificity is an electron

as donor

can be explained

donor-acceptor

and the CO2 carbon

in the case of monodentate

if we consider

interaction,

atom as acceptor.

species

that

the methoxy Such an

than in the case of bridged

ones. To specify the formation Fig. 5 the spectra of adsorbed r-t or at 473 K, without partially

prevents

much weaker

of species

II, from species

H or species

species given by an excess of methanol

evacuation.

It appears

that after heating,

I, we compare

formation

that of species H, the 1060 cm-1 band characteristic

in

(Fe = 2 Torr) at of species II

of the latter being

while the 1092 cm-1 one (species II) is well apparent.

25

1

1200 Fig. 5 : infrared

I

I

1100 spectra

1000 of adsorbed

methanol

species

on MgO-IV

in presence

of

CHsOH gas ; a) species at r.t. ; b) after heating at 473 K.

Effect of the oriain of the Ma0 samole. cm-j range, to obtain information

It is possible from the study of the 1200-900

not only on the nature and the structure of the species

316

formed

but also

different

voc

on their

relative

number

according

bands. Figure 6 allows one to compare

samples either in presence

of methanol

to the relative

intensity

the results on the different

of the MgO

(Fig. 6a), or after evacuation

at r.t. (Fig. 6b) or at

470 K (Fig. 6~). The absorbance

scale chosen takes into account

the surface area of

each sample

a direct comparison.

and allows therefore

i ~

I

A

i

/

\1-t

,;dc ‘i,

I

1200

1000

800

b) A

1200

Fig. 6 . Infrared samples

spectra

of methanol

I

adsorbed

I

1000

species

: a) in presence of CH3OH gas ; b) after evacuation

on the 1, V or II, III, IV MgO at i-2.

317

A

pIyi-el

1000

I200

Fig. 6

Infrared

samples

: c) after evacuation

spectra

of methanol

Figure 6 shows a different

adsorbed

species

on the I, V or II, III, IV MgO

of the samples

MgO-I and MgO-V, e.g. those

at 470 K.

comportment

with the lower surface area : i) the overall very

close

area of the voc

from one sample

measurements species

I (voc

performed

bands observed

to the other,

under pressure

which

of CHsOH (Fig. 6a) is

is in accordance

in the same conditions

(Table

with gravimetric

I). However

the number

of

: 11 15 cm-‘) relative to that of species H (voc = 1060 cm-t) is markedly

less in the case of samples MgO-I and MgO-V ii) the number of species present after evacuation V, in agreement

with gravimetric

measurements.

at r.t. is less on MgO-I and MgO-

However,

on these samples,

species II

(voc = 1090 cm-l) is already partly formed at r.t. iii) after evacuation

at 470 K, the number of species

MgO-V. For the other samples,

it increases

in the following

II is clearly

less on MgO-I and

order :

MgO-IV > MgO-II > MgO-Ill again in nice agreement

with gravimetric

measurements.

DISCUSSION The present CHsO-H

bond

methoxy

species

study clearly which

also been observed

shows that species

therefore

respectively.

leads

to assign

Formation

I and II result from the rupture of the them

to monodentate

of monodentate

and bidentate

and bridged species

has

on other metal oxides like CeOa [3], Zr02 [4] and ThOs [2] and their

structure

has been related to the local arrangement

of the surface adsorption

coupling

results obtained

electron

by scanning

transmission

microscopy

sites. By

and by FT-IR

318

spectroscopy

on CHsOH adsorption

monodentate

species

on ThOs, it has been assumed

may be formed

on the (110) faces whereas

may occur on the (211) faces [2]. On MgO, particularly found that the (100) faces are largely valences

species

other adsorption &

by rupture

on MgO smoke,

it has been of the free

HsO on MgO, reported

characterized

by IR absorption

by outgassing

stable and therefore the 3450-3650

the existence

at a temperature

and edges. Accordingly, ions on idealized

on low coordinated

to involve

Recently,

Coluccia eJ of hydroxyl

cm-1 and 3740 cm-l.

higher than 373 K. It is thermally

it could be related to the methoxy

OH groups on corners

of monodentate

necessary

of two types

bands at 3450-3650

formed adsorbed

of bridged species.

The very

II species. The authors assigned

cm-1 bands to OH groups on extended

on the Mg*+

the formation

bond. It is therefore

sites to explain the formation

[9, 151, adsorbing

latter appears

of the CHsO-H

that

species

[14]. The orientation

of the Mga+ ions on such faces only allows

methoxy

groups

predominant

for instance bidentate

planes and that at 3740 cm-1 to

we propose

(100) faces

that methoxy

and that bridged

cations (3 and 4-coordinated

species species

I are II are

Mg2+).

On Al203 [l l] and TiOs [16] which present a Lewis acidity much stronger than MgO, chemisorption

of

methanol

chemisorbed

methoxy

acid sites. Mild heating basicity

species

leads,

of species-H

H-bonded.

room

temperature,

I, to undissociated

species

causes their transformation

is such that the undissociated

but strongly

at

species are expected

species.

to be weakly

of species

II partially

that a part of the latter are adsorbed

corners (Fig. 7).

CH30”

b

/ -Mg-O, /

C”3 ‘04 : 0” ‘, Mg i

/

0 ,

‘I oc.

Mg’_

Fig. 7

Proposed

scheme for the formation

irreversibly

mainly coordinated

into bridged

Taking account that formation

(Fig. 5) we propose

besides

of bridged methoxy

Mg- P MS’ 0’

species.

to Lewis

MgO acidocoordinated poisons that on edges or

319

Considering

12 Mg *+ 02- pain&m2

that the (100) planes expose

results show that a great part of these pairs are involved This indicates

that methanol

the observation

of IR bands characterizing

number

of methoxy

number

of defects.

idealized

may probe different

species

remaining

It amounts

(100) faces which agrees

these sites are propo~ionaliy agrees with generally more highly difficult

to explain

samples

whereas

why bridged

adsorption

kinds of adsorbed

at r.t. by

species.

at 593 K accounts

with the results relative

The

for the

Mga+ 02- pairs on the

to the number

of hydroxyi

by Coluccia et al. 193.Our results further show that on samples

data : ex-hydroxy

surfaces

heating

different

after evacuation

less numerous

reported

defective

in methanol

types of sites, which is confirmed

to about 25 % of the expected

groups on edges or corners obtained

[15], gravimatric

than MgO smoke species

with a low surface area. This

or ex-carbonate

appear

MgO samples

[14 and ref. therein].

present

However

even at r.t. on the low surface

at 373 K or more is necessary

to observe

it is area

them on the other

samples (Fig. 6). It has been reported increases

that the number of 3 fold coordinated

after exposure

to water vapor for several

study this effect using CHsOH adsorption

sites on the MgO smoke

hours [17]. It could be interesting

as a means of site investigation

to

and to extend

it to other MgO samples.

CONCLUSION This study clearly shows that : i) the two types of methoxy

species formed by the dissociative

MgO result from the dissociation ii) the formation coordination

adsorption

of methanol

on

of the OH bond ;

of one of these species involves

defective

sites such as Mg2+ in a low

state ;

iii) gravimetric

results combined

such defects depends

with IR ones allows us to show that the proportion

of

on the origin of the MgO samples.

We thank Professor

Beruto for supplying

us with monocrystal

precursors

of MgO-Ii

and MgO-Ill samples.

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