An FTIR spectroscopic study of the selective oxidation of nitrosobenzne to nitrobenzene by metal oxides

Article abstract of DOI:10.1006/jcat.1996.0117

Catalytic conversion of nitro- into nitrosobenzene by transition metal oxides is of considerable practical and theoretical interest. Therefore, the surface chemistry of nitrosobenzene on various metal oxides has been studied using IR spectroscopy. The main products of surface reactions are nitrobenzene and azoxybenzene. Findings of this study are compared with the results of a mass spectroscopic study carried out with nitrosobenzene on the same oxides. Molecularly adsorbed nitrosobenzene is found to be coordinated to metal cations by σ-N as well as σ-O bonds. Also the cis-dimer of nitrosobenzene is detected. As a reference, the spectra of adsorbed nitrosobenzene were compared with the spectra of monomeric nitrosobenzene dissolved in benzene and dimeric nitrosobenzene dissolved in ethanol. Some IR absorptions not reported earlier are assigned to C-N stretching and ring vibrations of σ-O coordinated and dimeric nitrosobenzene. The coordination modes of nitrosobenzene observed with the different oxides, and the reverse relationship found between ν(N=O) and ν(C-N) are in agreement with the observations made with nitroso compounds coordinated as ligands in organometallic complexes. A link to the catalytic behavior of nitrosobenzene on oxides is indicated.

Full text of DOI:10.1006/jcat.1996.0117

JOURNAL OF CATALYSIS 160, 1–9 (1996)
ARTICLE NO. 0117
An FTIR Spectroscopic Study of the Selective Oxidation of
Nitrosobenzene to Nitrobenzene by Metal Oxides
S. Meijers and V. Ponec
Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
Received July 18, 1995; revised December 18, 1995; accepted January 4, 1996
NO can be transformed to 8-NO2 also by the nontransition
metal oxide Al2O3. The Mars–van Krevelen mechanism
cannot account for the formation of 8-NO2 on alumina,
since the creation and replenishment of oxygen defects in
the catalyst surface require variable valence cations to com-
pensate for the varying degree of surface oxidation. On the
other hand, vanadia, which has shown a substantial activ-
ity in the selective reduction of 8-NO2 (4), is completely
inactive in the oxidation of 8-NO. In this paper the oxides
investigated by Grootendorst and Ponec (10) are studied
by FTIR spectroscopy, in an attempt to gain the lacking
information concerning the relation between redox prop-
erties and activity in the oxidation of 8-NO. Additionally,
the nontransition metal oxides MgO and ZnO were studied.
Since one can expect that 8-NO dimers could be ob-
served in the IR spectra, the spectra of the monomer and
the dimer dissolved in benzene and ethanol, respectively,
weretakenasreferencesforassignment. Inmostsolventsan
equilibrium exists between the monomeric and the dimeric
form of nitroso compounds, but in benzene nitrosoben-
zene is present for almost 100% as a monomer, due to
ꢀ-complexation between the molecule and the solvent (11).
Most dimers of nitroso compounds occur in the cis as well as
in the trans form. In the dimer a conjugated six ꢀ-electron
system is formed by the two NO groups. However, dimer-
ization is less favored with nitrosobenzene than with other
nitroso compounds because of a conjugation between the
nitroso group and the phenyl ring. Therefore, of nitrosoben-
zene only the cis-dimer has been found. The existence of
this dimer can be explained by removal of the conjugation
between the NO group and the ring due to steric hindrance:
in the cis-dimer the rings are twisted out of the plane of the
ON ꢀ NO system (12a–c). According to Dietrich et al. (12c)
twisting occurs round b and b0 by 64.8ꢁ and 111.7ꢁ (Fig. 1).
The torsion angle around a is 17.9ꢁ.
Catalytic conversion of nitro- into nitrosobenzene by transition
metal oxides is of considerable practical and theoretical interest.
Therefore, the surface chemistry of nitrosobenzene on various metal
oxides has been studied using IR spectroscopy. The main products
of surface reactions are nitrobenzene and azoxybenzene. Findings
of this study are compared with the results of a mass spectroscopic
study carried out with nitrosobenzene on the same oxides. Molecu-
larly adsorbed nitrosobenzene is found to be coordinated to metal
cations by ꢀ-N as well as ꢀ-O bonds. Also the cis-dimer of ni-
trosobenzene is detected. As a reference, the spectra of adsorbed
nitrosobenzene were compared with the spectra of monomeric ni-
trosobenzene dissolved in benzene and dimeric nitrosobenzene dis-
solved in ethanol. Some IR absorptions not reported earlier are
–
assigned to C N stretching and ring vibrations of ꢀ-O coordinated
and dimeric nitrosobenzene. The coordination modes of nitrosoben-
zene observed with the different oxides, and the reverse relationship
==
–
found between ꢁ(N O) and ꢁ(C N) are in agreement with the ob-
servations made with nitroso compounds coordinated as ligands
in organometallic complexes. A link to the catalytic behavior of
nitrosobenzene on oxides is indicated. ꢀc 1996 Academic Press, Inc.
INTRODUCTION
The selective reduction of nitrobenzene (8-NO2) to ni-
trosobenzene (8-NO) and the surface chemistry of 8-NO2
have already been studied on many transition metal oxides
(1–8). This reduction, which is believed to proceed via a
reversed Mars–van Krevelen mechanism (9), is of consid-
erable practical importance because 8-NO is used as an in-
termediate in the synthesis of many industrial products. The
conventional way to synthesise 8-NO comprises two steps
(1), each of them producing inorganic salts in stoichiomet-
ric amounts. In contrast, the catalytic selective reduction
of 8-NO2 over transition metal oxides is an almost waste-
free reaction. Since the Mars–van Krevelen mechanism
occurs in many selective oxidation reactions, it is desir-
able to achieve a better understanding of the oxygen trans-
fer between adsorbed molecules and oxidic catalysts. The
selective oxidation of 8-NO has been studied earlier by
Grootendorst and Ponec (10), using mass spectrometry
(MS). A surprising result of his investigation was that 8-
It has to be noted that the IR spectroscopic study pre-
sented here was carried out at temperatures below 0ꢁC,
whereas the MS study mentioned above was carried out
between 0 and 300ꢁC. Furthermore, most oxides used here
have been prepared following the same methods as those
used for the MS study, but in the case of Fe2O3 and Co3O4
1
0021-9517/96 $18.00
c
Copyright ꢂ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

2
MEIJERS AND PONEC
For the IR spectroscopic measurements, about 50 mg of
each sample was pressed to pellets of 2.8 cm diameter us-
ing a pressure of 1 ꢃ 107 Pa. The pellets were placed in an
infrared transmission cell equipped with CaF windows and
connected to a conventional vacuum system. Since 8-NO
is very reactive after adsorption on most metal oxides, the
interaction of the molecule with the oxide samples was fol-
lowed between ꢄ40ꢁC and room temperature. Spectra were
recordedwitharesolutionof2cmꢄ1, between1800and1100
(sometimes 1000) cmꢄ1 on a Mattson Galaxy 3000 FTIR
spectrometer. The samples were activated by outgassing in
vacuum at 350ꢁC. However, Co3O4 (as a p-type semicon-
ductor with a surplus of oxygen) was pretreated by a mild
reduction with H2 in order to obtain an IR transparent sam-
FIG. 1. The cis-dimer of nitrosobenzene.
somewhat different methods had to be used in order to ob-
tain IR transparent samples.
METHODS
The titania used was Eurotitania-1 from TiOxide, UK. ple (14).
MgO and ZnO were from Merck. V2O5 was prepared by de-
To prevent condensation of 8-NO on the cool parts of the
composition at 500ꢁC in air of NH4VO3 from Baker Chem- cell, the compound was admitted from a glass bulb attached
icals. The ꢁ -alumina was Aluminiumoxid C from Degussa. directly to the IR cell, with the outlet placed as near as
Cr2O3 was obtained by decomposition of (NH4)2Cr2O7 possible to the catalyst pellet.
from Merck, in a flow of air. Because of the possible ex-
The path length of the liquid phase IR cell used to record
plosive decomposition of the ammoniumbichromate, it was the spectra of dissolved 8-NO is 0.1 mm. The windows are
slowly heated to 200ꢁC, followed by heating at 300ꢁC for NaCl.
2 h. ꢂ-Fe2O3 was made by decomposition in a flow of He at
RESULTS
200ꢁC of FeO(OH), which was prepared from iron nitrate
(J. T. Baker Chemicals BV) according to Ref. (13). Co3O4
Nitrosobenzene Dissolved in Benzene and Ethanol
and Mn3O4 were made by decomposition of their hydrox-
ides at 500ꢁC in air. The nitrosobenzene used was from Ega
The spectra of 8-NO dissolved in benzene and ethanol
Chemie. Benzene was from Merck, and ethanol was from are shown in Fig. 2. The spectrum of the monomer in ben-
J. T. Baker Chemicals BV.
zene can be interpreted according to the Wilson notation
FIG. 2. Spectra of monomeric 8-NO dissolved in benzene (about 3 mg in 5 ml) and a mixture of monomeric and dimeric 8-NO dissolved in ethanol
(saturated solution) both at room temperature.

OXIDATION OF NITROSOBENZENE BY METAL OXIDES
3
TABLE 1
Assignments of the IR Absorption Bands (in cmꢄ1) of
the Nitrosobenzene Monomer Dissolved in Benzene and the
Dimer Dissolved in Ethanol (Room Temperature)
Monomer
Monomer +
Gas (15) (benzene) dimer (ethanol)
Assignment
8a or 8b
FIG. 3. ꢃ-N and ꢃ-O bound nitrosobenzene.
ꢅ1615
==
1523
1506
1484
1466
1443
1507
1484
1457
1445
1389, 1412
1313
1294
1270
1191
ꢄN O (ꢃ-O)
==
ꢄN O (ꢃ-N)
(15) in the same way as the gas phase spectrum assigned by
Bradley and Strauss (16). Some bands are shifted in com-
parison with the gas phase and the overall spectrum is very
similar to the spectrum of 8-NO adsorbed on Mn3O4 re-
1477
1455
19a
19b
==
ꢄN O cis-dimer
1314
1290
1310
1290
14
3 monomer
3 dimer
==
ported by Angevaare (3). In both cases the N O stretching
band is shifted to lower frequency by 17 cmꢄ1. This has been
explained by hydrogen bonding between the nitroso group
O atom and a surface hydroxyl group of Mn3O4 (3). This is
in agreement with the observation of Vasopollo et al. (17),
–
13, C N dimer
1176
1159
1112
1176
1160
1112
1178
1160
1117
9a
15
–
13, C N monomer
==
that the N O stretching band of nitroso ligands coordi-
nated by a ꢃ-O bond to a low-valent metal center occurs
around 1500 cmꢄ1 (Fig. 3). Since the solvent used was not is expected between 1290 and 1190 cmꢄ1. If the interaction
absolutely water free, it is likely that a similar kind of in- with a solvent could remove the above mentioned conju-
teraction occurs here with 8-NO dissolved in benzene and gation between the NO group and the phenyl ring, the for-
with 8-NO adsorbed on Mn3O4, namely hydrogen bonding, mation of the trans-dimer might be possible.
to dissolved water molecules in the first case and to surface
OH groups in the second case.
(B) Ring vibration 3 (C–H in-plane bending) of mono-
substituted benzenes, which is found at 1294 cmꢄ1 in the
–
The presumed ꢀ-bonding between 8-NO and benzene caseofthemonomer, isexpectedtoshifttolowerwavenum-
(11) is not reflected by the IR spectrum. No new IR ab- ber with increasing weight of the substituent. The cis
sorptions, in comparison with the gas phase spectrum, are 8-NO dimer can be considered as a system of two heavily
observed in the spectrum of 8-NO dissolved in benzene. monosubstituted phenyl rings and therefore ring vibration
The band at 1484 cmꢄ1 is assigned to another adsorption 3 might occur between 1275 and 1253 cmꢄ1 (19), thus offer-
mode of 8-NO, namely the ꢃ-N coordinated molecule. ing a more likely explanation for the band at 1270 cmꢄ1
Vasopollo et al. (17, 18) locate the N O stretching of N co-
.
==
In the 1200–1100 cmꢄ1 range, the differences observed
between the pure monomeric and the mixed spectrum are
a new band at about 1190 cmꢄ1 and a decrease in intensity
ordinated nitroso ligands in metal complexes between 1505
and 1465 cmꢄ1. Thus, the influence of benzene as a solvent
on the equilibrium between monomeric and dimeric 8-NO
should probably be related to the interaction of benzene
ꢄ1
–
–
of the C N stretching band at 1117 cm . The C N stretch-
ing band (ring vibration 13 of monosubstituted benzenes)
can vary between 1280 and 1100 cmꢄ1 depending on the
weight and nature of the substituent (19). Most likely, the
–
with the phenyl ring, but not with the NO group.
In ethanol an equilibrium exists between monomeric and
dimeric 8-NO, giving rise to an IR spectrum containing
new band at 1190 cmꢄ1 arises from the C N stretching of
–
==
bands of both species. The N O stretching bands at 1507
the 8-NO dimer at the expense of some intensity of the
monomeric C N stretching band. The assignments of the
bands observed in the spectra of 8-NO dissolved in ben-
and 1484 cmꢄ1 show the presence of ꢃ-O as well as ꢃ-N co-
ordinated 8-NO. In ethanol hydrogen bonding might occur
eitherwith the solvent orwithdissolvedwater. Themostob-
viousdifferencebetweenthepuremonomerinbenzeneand
the mixture of monomer and dimer in ethanol is a strong
doublet in the latter spectrum with absorption maxima at
1412 and 1389 cmꢄ1. This absorption has been assigned by
Gowenlock and Lu¨ttke (12a) to the symmetric and antisym-
–
zene and ethanol are given in Table 1.
Nitrosobenzene Adsorbed on Metal Oxides
The spectra of 8-NO adsorbed on various oxides are
shown in Figs. 4–7. The assignments of the bands observed
are given in Table 2. Band intensities are not presented
since they change upon heating of the sample and repre-
sentative spectra obtained with various oxides are included
in this paper. Considerable variation in the quality of the
==
metric N O stretching frequencies of cis-dimeric 8-NO.
Another new, weak band is observed at about 1270 cmꢄ1
.
Two interpretations could be suggested for the 1270 band:
==
(A) The N O stretching band of the 8-NO trans-dimer spectra is due to different oxide transparencies, to different

4
MEIJERS AND PONEC
FIG. 4. Spectra of 8-NO adsorbed on TiO2 at ꢄ30 and ꢄ10ꢁC and 10ꢄ4 Torr.
strengths of 8-NO adsorption, and to a strong temperature ꢃ-O and ꢃ-N coordinated 8-NO monomer, the cis-dimer,
dependence of some of the oxidic background IR spec- and the products reported already by Angevaare upon
tra, leading to artifacts in the spectra generated by sub- 8-NO adsorption on Mn3O4, namely 8-NO2 and azoxyben-
traction.
zene (8-N(→O)N-8). Most 8-NO bands are only weakly
For the transition as well as the nontransition metal ox- perturbed by adsorption. The only band that shows con-
ides most bands observed by this study are assignable to the siderable shifts, compared with the spectra of gaseous and
FIG. 5. Spectra of 8-NO adsorbed on Mn3O4 at ꢄ30ꢁC and on Co3O4, at ꢄ40ꢁC at 10ꢄ4 Torr.

OXIDATION OF NITROSOBENZENE BY METAL OXIDES
5
FIG. 6. Spectra of 8-NO adsorbed on Cr2O3 and Fe2O3 at ꢄ30ꢁC and 10ꢄ4 Torr.
–
dissolved 8-NO, is the ꢄ(C N) (ring vibration 13) of the Fe2O3, weak on Al2O3, Mn3O4, and Co3O4, and absent on
monomer, which is found around 1130 cmꢄ1 on the tran- ZnO, MgO, and Cr2O3. The assignment of this band is not
sition metal oxides and 1112 cmꢄ1 in the vapor. The shift straightforward and will be discussed below. Adsorption
to higher frequency is possibly caused by a partial delo- of 8-NO could not be detected by IR spectroscopy with
calization of the nonbonding electrons on nitrogen. A new the two lead oxides and with vanadia. This would explain
band is found at 1240 cmꢄ1. This band is strong on TiO2 and the lack of 8-NO2 formation from 8-NO by these oxides.
FIG. 7. Spectra of 8-NO adsorbed on MgO, ZnO, and Al2O3 at ꢄ30ꢁC and 10ꢄ4 Torr.

6
MEIJERS AND PONEC
TABLE 2
Assignments of the IR Absorption Bands (in cmꢄ1) of Nitrosobenzene and Its Products Adsorbed on Various
Oxides at ꢄ40ꢁC and 10ꢄ4 Torr
TiO2
Fe2O3
Co3O4
Mn3O4
Al2O3
ZnO
MgO
1592
Cr2O3
1590
Assignment
1591
1518
1503
1485
1477
1465
1444
ꢅ1420
1356
1317
1302
1600, 1590
1524
1588
1523
1502
1487
1478
1464
1441sh
1411
1346
1312
1296
1600, 1590
1522
1503
1483
1475
1461
1441
1395, 1365
1347
1316
1592
8a
ꢄasNOa
ꢄNO(ꢃ-O)
ꢄNO(ꢃ-N)
19bb
1501
1481
1503
1487
1503
1484
1503
1505
1485
1465
1439
1466
1441
1407, 1373sh
1355
1318
1464
1443
1410, 1375
1349
1317
1463
1439
1405
1461
1444
1389, 1375
1346
1311
19a
19b ꢄN N
b
==
1408, 1394, 1371
1352
ꢄNO dimer
ꢄsNOa
14
1313
1296
1312
c
c
1296
1295
1293
3
1270
1242
1259
1240
1260
1215
1180
1120
1264
3 dimer
ꢄCN(ꢃ-N)
1244
1189
1239
1220
1200
1185
1174
1136
1237
1200
1187
1165
1131
1192
1176
1160
1115
ꢄCN dimer
9a
15
1187
1168
1129
1185
1166
1129
1181
1122
1160
1131
ꢄCN(ꢃ-O)
a Nitrobenzene.
b Azoxybenzene.
c Nitrosobenzene bands can not be identified here due to spectral artifacts resulting from temperature dependent changes in the
oxidic background spectra.
(NB: These oxides are, however, active in the oxidation 8-NO nor combustion products like carbonates have been
of the phenyl ring (4)). On MgO and ZnO the dimer and observed. Obviously 8-NO is on vanadia not adsorbed in
traces of 8-NO2 are observed.
detectable amounts, perhaps for a similar reason as with
A comparison of the data presented here with the find- the lead oxides. Vanadium (V) bears a doubly bonded oxy-
ings of Ref. (10) reveals a good agreement between the IR gen and may be inaccessible for 8-NO. Unlike the lead
spectroscopic and the MS study. The only exception is TiO2, oxides, V2O5 has a considerable activity in the selective
where a small amount of (chemisorbed) 8-NO2 was found reduction of nitrobenzene. This might be explained by a
in the IR spectrum, in contrast to the mass spectrum (of the positive electrostatic potential (attractive toward negative
gas phase).
charges) above and below the ring plane of 8-NO2 (20).
This could enhance the adsorption of the molecule on the
vanadia surface, whereas 8-NO has a negative potential
above and below the ring (21), which is probably repelled
DISCUSSION
==
First, it is remarkable that similar positions of the N O by the vanadia surface. A definitive explanation of all phe-
stretching band are found with 8-NO dissolved in benzene, nomena observed is not yet available, but the following
adsorbed on metal oxides, and coordinated as a ligand in consistent picture of 8-NO can be suggested.
==
metal complexes. This suggests that ꢄ(N O) depends only
8-NO can be ꢃ-N or ꢃ-O bound, either to a surface
on the bonding mode of 8-NO (about 1505 cmꢄ1 for ꢃ-O
OH group or to a cation. Lead oxides and vanadia induce
–
and about 1485 cmꢄ1 for ꢃ-N) and not on the nature of the reduction of 8-NO to aniline (albeit in a small extent)
the interacting center. The latter may be a dissolved water and it is known that this reaction consumes hydrogen from
–
molecule, a Lewis acidic surface ion, or a metal center in an
organometallic complex.
OH groups (10). In this situation one can assume that on
–
PbO and V2O5 the 8-NO is only very weakly bound to OH
The absence of 8-NO adsorption on the lead oxides has groups.
been predicted by Grootendorst and Ponec (10) and at-
However, with oxides like Mn3O4 or Co3O4 the adsorp-
tributed to the presence of a nonbonding electron pair on tion of 8-NO can be detected by IR spectroscopy and they
the Pb2 + ions. On vanadia the absence of 8-NO2 produc- are active in the selective oxidation of 8-NO to 8-NO2. We
tion has been explained by the immediate combustion of suggest that this is due to the coordination of 8-NO to a
8-NO to CO and CO2 on the highly reactive V2O5 surface metal cation, which facilitates the transfer of oxygen in the
(10). However, with IR spectroscopy neither adsorption of 8-NO ↔ 8-NO2 couple.

OXIDATION OF NITROSOBENZENE BY METAL OXIDES
7
TABLE 3
Absolute Intensities of the Strongest Bands, Specific Surface Area of the Oxides, Relative Amounts
of N Coordinated and O Coordinated Nitrosobenzene, Relative Intensities of the Band Assigned as
ꢁCN(ꢀ-N), Spectral Position (in cmꢄ1) of ꢁCN(ꢀ-O), and Electronic Configuration of the Oxides.
Specific surface
area (m2/g)
I (strongest
band)
IꢄNO(ꢃN)/
IꢄNO(ꢃ-O)
Irel
ꢄCN(ꢃ-N)
ꢄCN(ꢃ-O)
(cmꢄ1
Cation
el. conf.
Oxide
TiO2
)
47
0.2
>1
very
strong
strong
weak
weak
weak
—
1131
1140 (RT)
1129
1136
1129
1131
1122
1120
1115
d0
Fe2O3
Co3O4
Mn3O4
Al2O3
ZnO
0.15
0.1
0.05
0.1
0.02
0.01
0.2
>1
>1
>1
<1
<1
0
d5
d6 a
d4 a
p6
18
25
95
10
d10
p6
MgO
Cr2O3
—
—
36
<1
d3
Note. Nitrosobenzene adsorbed at ꢄ30ꢁC and 10ꢄ4 Torr.
a Based on results presented in (1–7) it is assumed that only Co3 + and Mn3 + are exposed in the surface of Co3O4
and Mn3O4, respectively.
–
The location of ꢄ(C N) of the ꢃ-N coordinated 8-NO
Several assignments can be proposed for the band at
1240 cmꢄ1 found in the spectra of 8-NO adsorbed on at 1240 cmꢄ1 agrees with an empirical relationship estab-
TiO2, Fe2O3, Al2O3, Mn3O4, and Co3O4. Azobenzene lished by Talberg (25) and Cameron et al. (26). It relates
==
==
–
–
(8-N N-8), azoxybenzene, and various other possible re- the N O stretching frequency and the r(C N) and r(N O)
action products of adsorbed 8-NO like diphenylhydrazine bond lengths in nitroso compounds:
(8-NH NH-8) or the NOꢄ2 1 ion have IR adsorption bands
–
ꢄ1
˚
==
==
near 1240 cmꢄ1 (22–24). However, adsorption of these com-
pounds on TiO2, which showed the strongest 1240 cmꢄ1
band, did not produce the same spectral feature. We know
r(N O) [A] = k1 ꢄ k2ꢄ(N O) [cm
]
˚
˚
==
–
r(N O) [A] = k3 + k4/[r(C N) [A] ꢄ k5].
that the band belongs to a stable species, since it remains Although these relationships have been obtained with solid
visible upon heating the oxide to room temperature and in nitroso compounds, Cameron et al. (26) have shown re-
the case of TiO2 it even increases in intensity with increas- cently that they hold for dimers and for ꢃ-N and ꢃ-O co-
ing temperature (Fig. 4). Thus, it can be excluded that a ordinated nitroso compounds too, provided that the com-
==
trans-dimer is responsible for the 1240 cmꢄ1 band.
The high intensity of this band indicates that it is a C N be accompanied by a decrease in the C N bond length
pounds are aromatic. Thus, a decrease in ꢄ(N O) should
–
–
==
–
–
or N O stretching absorption. A C N stretching mode and, accordingly, an increase in ꢄ(C N). Compared with the
==
–
seems to be the most likely for the following reasons. ꢃ-O coordinated molecule, ꢄ(N O) is lower, and ꢄ(C N)
–
First, according to Varsanyi (19) the position of ꢄ(C N) is higher in ꢃ-N coordinated 8-NO on metal oxides.
(ring vibration 13) in 8-NO may vary between 1280 and
The observation of the ꢃ-O coordinated 8-NO on so
1100 cmꢄ1. Second, there is a correlation between the in- many metal oxides is remarkable, since 8-NO ligands in
tensity of the 1240 cmꢄ1 band and the relative intensities organometallic complexes are only known to be coordi-
ꢄ1
==
==
of ꢄ(N O) (ꢃ-N) at 1485 cm and ꢄ(N O) (ꢃ-O) at 1505 nated by ꢃ-N bonding. The ꢃ-O coordinated nitroso ligands
cmꢄ1 (see Table 3). On TiO2 and Fe2O3 the 1240 band is (not 8-NO) have been found in Sn(Me)2Cl2 (p-nitrosodi-
==
==
strong and ꢄ(N O) (ꢃ-N) is stronger than ꢄ(N O)(ꢃ-O). methylaniline)2 and ZnCl2(p-nitroso-dimethylaniline)2,
On MgO, ZnO, and Cr2O3 where the 1240 cmꢄ1 band is both complexes of d10 metal ions (26). However, the be-
==
==
absent, ꢄ(N O) (ꢃ-N) is weaker than ꢄ(N O) (ꢃ-O) or, havior of 8-NO on metal oxides agrees very well with this:
in case of MgO it is not observed at all. Thus, a reasonable the IR absorption bands assigned by us to the ꢃ-O coordi-
assignment for the 1240 cmꢄ1 band is the C N stretching nated 8-NO are prevailing on ZnO (d10), MgO (p6d0), and
–
mode of ꢃ-N coordinated 8-NO. In Table 3 an indication Al2O3 (p6d0). The bands assigned to the ꢃ-N coordinated 8-
of the intensity ratio between ꢄ(C N) (ꢃ-N) and ꢄ(C N) NO have maximal intensity on TiO2 (d0). The oxides Fe2O3,
(ꢃ-O) is given for each oxide. It is not possible to establish Co3O4, and Mn3O4 with partly filled d-orbitals are between
an exact value for this ratio since the two bands overlap of- the extremes. An exception is formed by Cr2O3, that will be
ten with each other and with neighboring bands. However, discussed separately. Although it is sometimes difficult to
–
–
the trend is obvious.
discriminate between the ꢃ-O and ꢃ-N coordinated 8-NO

8
MEIJERS AND PONEC
FIG. 8. The reaction of a nitrosobenzene molecule with a nitrosobenzene dimer to nitrobenzene and azoxybenzene.
in organometallic complexes (26), the simultaneous obser- ZnO can be explained by this mechanism. Since these ox-
vation of IR bands from both species on various metal ox- ides have not been investigated in the MS study (10), their
ides supports our assignment of the 1503 cmꢄ1 absorption activities in the oxidation of 8-NO are not known, but they
to ꢃ-O coordinated, and the 1485 cmꢄ1 absorption to the are probably less active than Al2O3.
ꢃ-N coordinated 8-NO.
The deviating behavior of TiO2 (large amounts of ꢃ-N
–
Let us now turn to the C N stretching mode of the ꢃ- coordinated 8-NO, increasing with time; no substantial 8-
O coordinated 8-NO. The position of this band varies NO2 production) can be explained by a strong bonding of
between 1140 cmꢄ1 on TiO2 (at room temperature) and 8-NO to the surface. This could prevent both the reaction
1115 cmꢄ1 on Cr2O3. The latter is almost the gas phase fre- of absorbed 8-NO to 8-NO2 and the formation of sufficient
quency of this band (1112 cmꢄ1). In Table 3 the spectral amounts of the dimer, which might be the required source
–
positions of ꢄ(C N) (ꢃ-O) are shown. The specific surface of oxygen with this oxide: due to the strong Ti–O bond,
area of the oxides and an indication of the absolute inten- the Mars–van Krevelen mechanism is probably not oper-
sities of each spectrum are also included to show that the ating on TiO2 (10). The observation of a small amount of
–
frequency of ꢄ(C N) does not change due to the increasing 8-NO2 in the IR spectrum can be explained by some initial
amount of 8-NO adsorbed. The trends shown in Table 3 activity according to reaction (I), which is later poisoned by
–
are that both the frequency of ꢄ(C N) (ꢃ-O) and the rela- irreversibly adsorbed 8-NO.
tive amount of 8-NO(ꢃ-N), indicated by the intensity ratio
==
==
of ꢄ(N O) (ꢃ-N) and the ꢄ(N O) (ꢃ-O) and by the in-
CONCLUSIONS
–
tensity of ꢄ(C N) of the ꢃ bonded molecule, increase, go-
ing from the basic MgO to the strongly Lewis acidic TiO2.
1. Two IR absorption bands observed in the IR spec-
–
The ꢄ(C N) (ꢃ-O) absorption seems to be the only spectral trum of nitrosobenzene dissolved in ethanol at 1270 and
band that is not only sensitive to the coordination mode, but 1190 cmꢄ1, not reported earlier, are assigned to the ring
– –
vibrations 3 (C H in-plane bending) and 13 (mainly C N
also to the nature of the oxide surface.
In this series of oxides, Cr2O3 behaves differently than stretching) of dimeric nitrosobenzene.
expected for an oxide having partly filled d orbitals and
2. For the ꢃ-N and ꢃ-O coordinated nitrosobenzene, the
being also acidic (27). Its behavior is similar to that of same IR bands are obtained when nitrosobenzene is dis-
MgO. This might be due to the preparation method used, solved in benzene (hydrogen bonding with H2O), adsorbed
namely the decomposition of ammonium bichromate. The on metal oxides (coordination to surface cations), and co-
oxide obtained in this way differs in color (greyish green) ordinated in organometallic complexes. The IR spectrum
and structure (Aerosil-like) from commercially available is determined by the coordination mode rather than by the
chromic oxide and probably its surface composition differs nature of the coordination center. Both species (ꢃ-N and
from other forms of Cr2O3 too.
ꢃ-O bound nitrosobenzene) seem to participate in catalytic
Possibly, both types of coordinated 8-NO are interme- reactions.
diates in the formation of 8-NO2, since no correlation has
3. The strong absorption at 1240 cmꢄ1 in the spectrum
been found between the individual adsorption modes of of nitrosobenzene adsorbed on TiO2 and other oxides con-
8-NO observed on metal oxides and their activity in 8-NO taining Lewis acidic sites has been assigned to the ring vi-
–
oxidation. Concerning the mechanism of 8-NO2 formation, bration 13 (mainly C N stretching) of the ꢃ-N coordinated
it is likely that on the most active transition metal oxides molecule.
8-NO is oxidized by lattice oxygen, i.e., by the Mars–van
4. In agreement with the findings in organometallic com-
Krevelen mechanism, whereas on alumina the only source plexes, nitrosobenzene is ꢃ-O coordinated to oxide sur-
of oxygen is 8-NO itself. Coordinated monomeric 8-NO faces containing metal ions with completely filled valence
could, for example, react with a dimer to yield 8-NO2 and orbitals. A large amounts of ꢃ-N coordinated nitrosoben-
azoxybenzene as shown in Fig. 8 (12). A reaction of this type zene is found on TiO2, which has d0 ions at the surface.
might take place on transition metal oxides too, resulting Both types of coordinated nitrosobenzene are found on
in an additional 8-NO2 production. Also, the observation transition metal oxides containing cations with partly filled
made by IR spectroscopy of traces of 8-NO2 on MgO and d orbitals.

OXIDATION OF NITROSOBENZENE BY METAL OXIDES
9
5. In agreement with the empirical relationships estab- 11. Mijs, W. J., Ph.D. Thesis, Leiden University, 1959.
12. (a) Gowenlock, B. G., and Lu¨ttke, W., Quart. Rev. 12, 321 (1958);
(b) Allman, R., in “The Chemistry of the Hydrazo, Azo and Azoxy
Groups” (S. Patai, Ed.), Vol. 1, p. 50. Wiley, New York, 1975;
(c) Dietrich, D. A., Paul, I. C., and Curtin, D. Y., Chem. Commun.
1710 (1970).
lished by Talberg (25) and Cameron et al. (26) for nitroso
–
==
compounds, ꢄ(C N) increases when ꢄ(N O) decreases.
6. PbO (red and yellow) and V2O5 do not catalyze the
selective oxidation of nitrosobenzene to nitrobenzene be-
cause nitrosobenzene is not adsorbed on these oxides in 13. Rochester, C. H., and Topham, S. A., J. Chem. Soc. Faraday Trans. 75,
591 (1979).
sufficient amounts.
7. TiO2 does not catalyze the oxidation of nitrosoben-
zene to nitrobenzene due to poisoning of the surface by
nitrosobenzene irreversibly bound to Lewis acidic sites.
14. Busca, G., Giudetti, R., andLorenzelli, V., J. Chem. Soc. FaradayTrans.
86, 989 (1990).
15. Wilson, E. B., Phys Rev. 45, 706 (1934).
16. Bradley, G. M., and Strauss, H. L., J. Phys. Chem. 79, 1953 (1975).
17. Vasopollo, G., Giannoccaro, P., Nobile, C. F., and Allegretta, F., J.
Organometall. Chem. 270, 109 (1984).
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