Infrared studies of the diatomic molecules O2, N2, NO and H2 adsorbed on Fe2O3

Journal of Molecular Structure, 80 (1982) 181-186 Elsevier Scientific Publishing Company, Amsterdam

INFRARED STUDIES OF THE DIATOMIC V. LORENZELLI

181 - Printed in The Netherlands

MOLECULES

02, N2, NO AND H2 ADSORBED

ON Fe203

and G. BUSCA

Laboratorio

di Chimica,

N. SHEPPARD

and F. AL-MASHTA

School of Chemical

Facolta

di Ingegneria,

University

Sciences,

Universita,

of East Anglia,

Genova

Norwich

(Italy)

(Gt. Britain)

ABSTRACT An infrared adsorbed

spectroscopic

under different

Complex

patterns

high temperature

study of the diatomic

conditions

of absorption

are assigned

molecules

02, N2, NO and H2

on Fe203 has been performed.

on both a-Fe203

to vibrations

and y-Fe203

of two different

activated

in 02 at

chemisorbed

02

species. N2 molecules do not interact with "oxygen rich" a-Fe203 surfaces, 2species when chemisorbed on evacuated surfaces. N20- and N202 NO molecules

give complex

Three different and cis-N202 contact

patterns

types of nitrate

chemisorbed

of absorption,

structures

species.

can be identified,

Chemisorbed

of H2 with Fe203 surfaces

depending

water molecules

but give

on the gas pressure as well as NO, NOare formed

by

even at room temperature.

INTRODUCTION An infrared different

spectroscopic

conditions

of interaction

on Fe203 has been performed

of such simple species,

is well characterized

whose

molecules

strength

adsorbed

in order to identify

spectroscopic

behaviour

(ref. l),with an oxide surface which

with acid sites of medium properties

study of some diatomic

under the types

as ligands

has a complex

structure

(refs. 2,3), basic sites (ref. 4) and oxidative

(refs. 2,4).

EXPERIMENTAL a-Fe203

samples were obtained

discs of a-FeOOH y-Fe203

(20-50 mg/cm';

were supplied

Treatment

by Magnex

of the discs at 400°C

the starting

by vacuum decomposition 3 tonnlcm') (Milano,

(180-250°C)

in the infrared

Italy) and directly

in dry 02, followed

cell.

0 1982 Elsevier Scientific

Publishing Company

of

into discs.

in dry 02, gave

"oxygen rich" surfaces.

0022-2860/82/0000-0000/$02.75

The samples

pressed

by cooling

of pressed

182 Infrared spectra were recorded with a Digilab Interferometer

FTS-14 Fourier Transform

Infrarel

or with a Perkin Elmer mod. 521 double beam spectrophotometer.

The results, where no mention

is made otherwise,

were perfectly

reproducible

in

both conditions. All gases, of commercial and drying

by classical

high-purity

quality,

were used after purification

methods.

RESULTS AND DISCUSSION Oxygen Fig. 1 shows the evolution cr-Fe203 (Fig. 1,a) caused

of the spectrum

by hydration

of an "oxygen rich" surface

and by heating

of

under vacuum.

\

c

I

L I

1600

I

1200cm"

600

Fig. 1. Infrared spectra of an a-Fe20 disc: a) oxygen rich surface prepared by heating in oxygen for 1 hr at 400°C; 8) after treatment with water vapour at room temperature (an absorption from adsorbed water occurs between 1640 and 1620 cm-l); b') ratioed spectra b/a; c) after a second stage of water adsorption; c') ratioed spectra c/a; d) after further evacuation for 12 hr at 400°C (the maximum TRANSMISSION in spectrum d is approximately 4 times less than in spectra a to c).

183 Features

common

to all these spectra are a strong blackout absorption between -1 -1 and a broad weaker feature centred near 1540 cm . The ca. 720 and 480 cm strong feature particles,

undoubtedly

band involving

lower frequency

there are complex relatively

corresponds

to the lattice modes of the a-Fe203

and the 1540 cm-' band is presumed

weak

patterns

intensity

to be a multiphonon

lattice fundamentals.

of absorption compared

between

or combination

In the intervening

1350 cm

-1

region

and ca. 1250 cm-' of

with the lattice absorptions

(type A bands)

and between -ca. 1100 and 900 cm-' of moderate

intensity (type 6 bands). As -1 O2 species are known to absorb at 1552 cm -' (0;) and (02), cd. 1140 cm

adsorbed

at -ca. 850 cm-' chemisorbed

(O$-) (ref. I), type A bands have been assigned

on the "oxygen

rich" surface,

and O;, and type B bands to species Perturbations molecules spectra oxygen

Adsorption

eliminates

remains very similar

Evacuation

presumably

transmission

non-stoichiometry

that of magnetite

is fully restored

manner

displaces

of O2

of water

water,

the adsorbed

of the higher

involving

elsewhere

(ref. 5).

some readsorption Evacuation

but the infrared

to that of Fig. 1, b: the original

by re-heating

at higher temperatures

spectral

increasing towards

in a complex

the chemisorbed

surface can only be restored

overall

of water

sites, as has been discussed

room temperature otherwise

adsorption

from the less stable Lewis acid sites,because

of the new adsorbate,

on different

between

the extremes 202 and O2 .

(Fig. 1, b-c), which are better shown in the ratio-mode

(Fig. I, b'-c').

basicity

intermediate

(ref. 5) to specie

between

of these bands are induced by progressive

on the surface

species

intermediate

"oxygen rich"

the sample in O2 to above 300°C.

leads, by 400X,

(Fig. I,d).

to a major

reduction

This is to be attributed

Fe304

(ref. 5).

in

to the

of the Fe203 sample, as its composition

only by re-heating

at

spectrum

changes

Even in this case, the original

sample

in 02.

In the case of an "oxygen rich" y-Fe203 sample (Fig. 2, a) type A (1340 and 1280 cm-') and type 6 (1070 and 1020 cm-I) bands of surface oxygen once again but with considerably due to surface carbonates 1420 cm-'

attributed

can be eliminated ratio-mode further

lower intensities

(1618 and 1223 cm-'), as well as an additional

to low-perturbed

from the original

Fig. 2,b').

Adsorption

oxygen molecules

sample by heating of water

removes these bands, particularly

displacements

species

than with a-Fe203.

adsorbed

(Fig. 2,~; ratio-mode

band at

on the surface,

under vacuum

that absorbing

occur

Two bands

(Fig. 2,b; Fig. 2,~') -I , and causes

at 1420 cm

and shifts of some of the type A and B absorptions.

Nitrogen No changes

are detected

room temperature were obtained

in the i.r. spectra after admission

and 400°C to an "oxygen rich" hematite

when heating

in N2 an evacuated

of N2 between

surface but new bands

sample or one which

had been

184

12oocld

1600 Fig. 2. Infrared heating in oxygen 1 hr; b') ratioed temperature; c')

pre-heated

spectra of a y-Fe 03 disc: a) oxygen rich surface prepared by for 1 hr at 400 06; b) after heating under vacuum at 400°C for spectra b/a; c) after treatment with water vapour at room ratioed spectra c/a.

with water.

Elmer spectrometer, an "evacuated molecularly

800

Fig. 3,b shows the spectrum,

in a nitrogen

surface"

adsorbed

flow between

of a-Fe203.

conditions different

interaction

previously

probably N2

+ 0

Np + 2 0

n

= N20;-

surface

(ref. 3).

rich surface

O2 species,

with Lewis acid sites

= N20

intensities

followed

but probably

are very

to N20 under similar

(ref. 6) to surface N20- species chemisorbed

can be found with an oxygen

as below:

and relative

for samples exposed

sites on the a-Fe203

does not involve chemisorbed molecules

and 200°C over

no bands could be detected due to -1 strong bands appear at 1570 cm and -ca. 1540 cm-

nitrogen, -1 , whose wavenumbers

and assigned coordination

with the Perkin

Although

and at 1380 and 1350 cm close to those obtained

obtained

beam temperature

indicates requires

on the two

The fact that no that the reaction interaction

by reaction with surface oxygen

of N2 ions,

185

Fig. 3. Infrared spectra of an a-Fe203 disc: a) evacuated surface obtained after elimination of water molecules chemlsorbed on an oxygen rich surface by prolonged evacuation at room temperature; b) after exposure to pure N2 flow at 200°C (I hr), using the Perkin Elmer spectrometer; c) same conditions as b) but the spectrum was measured at room temperature using the Digilab FTS-14 interferometer.

Indeed, the additional

double band measured

at 1450 and 1425 cm

recorded

using the "cold" beam of the Digilab

Interferometer

assigned

to N20;- hyponitrite

or photochemically

"hot" beam of the dispersive All these additional

ions, thermally

-1

in the spectra

(Fig. 3,~) can be unstable

in the

spectrophotometer.

bands disappear

by heating above 250°C.

Nitric oxide Contact causes

of NO with an oxygen-rich

surface of a-Fe203

(Fig. 4,b) immediately

the disappearance of type A bands, and formation of strong bands with -1 -1 at 1540 cm and 1220 cm , assigned to nitrate structures of three

main maxima different

types, as shown by a careful

vacuum treatment

(ref. 6) (Fig. 4,d).

weaker maxima were measured due to slightly

perturbed

the other two can possibly The formation

NO molecules be assigned

of the latter chemisorbed

can however

responsible

At higher

under heat and/or

NO pressures (Fig. 4,~) new -1 ; the first one is probably

at 1858, 1830 and 1735 cm

a band at 1595 cm -I, assigned situation

study of their behaviour

(molecular

NO absorbs

to adsorbed

cis-N202

species

(ref. 7).

species occurs with the disappearance

(ref. 6) to NO- species

be reversibly

at 1876 cm-'), while

restored

(Fig. 4,~).

by evacuation.

for the 1595 cm -' band also disappear

The former

Such species

by degassing

at 150°C.

of

\ -. ., ‘1

\ ._-’ ,

1400 cd

l200

Fig. 4. Infrared spectra of an a-Fe20 disc: a) oxygen rich surface prepared by heating in oxygen for 1 hr at 400°C; 2) after contact with NO (5 Tort-; 30 rain at beam temperature); c) after contact with NO (300 Torr; 30 min at beam temperature); d) after degassing at 150°C for 1 hr. Hydrogen Hydrogen molecules

absorption

(identified

on chemisorbed

oxygen

on an oxygen through

rich surface gives immediate

their deformation

species has been discussed

band at 2.

formation

of water

1600 cm-') whose effect

above.

REFERENCES 1 2 3 4 5 6 7

N. Sheppard, in R.F. Willis (Ed.), Vibration Properties of Adsorbates, Springer-Verlag, Berlin, 1980. V. Lorenzelli, G. Busca and N. Sheppard, 3. Cat., 66(1980)28. G. Busca and V. Lorenzelli, Mater. Chem., 6(1981)173. G. Busca and V. Lorenzelli, J. Cat., 66(1980)155. F. Al-Mashta, G. Busca, V. Lorenzelli and N. Sheppard, J.C.S. Faraday I, in press. G. Busca and V. Lorenzelli, J. Cat., in press. W.G. Fateley, H.A. Bent, B.L. Crawford, J. Chem. Phys., 31(1959)204.