Nitrobenzene hydrogenation on Au/TiO2 and Au/SiO2 catalyst: Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1007/s10562-013-1034-2

Recently, gold has been proposed as an active phase for the hydrogenation of nitro-arenes. This metal has been rarely used in hydrogenation reactions because gold does not possess hydrogen chemisorption capacity. However, small gold particles behave differently and they may be able to chemisorb hydrogen to same extent, leading to possible activity in hydrogenation reactions. This may provide an advantage because the reactions catalyzed by highly dispersed gold particles may be better controlled. In this work, TiO₂ and SiO ₂ supported Au catalysts were prepared by the deposition- precipitation method using urea and NaOH to precipitate the metallic component at different temperatures and hydrogen pressures. The metal loading for all the catalysts was 1 wt%. The catalysts were characterized by X-ray diffraction, high resolution transmission electron microscopy among others. The catalysts were then evaluated in the hydrogenation of nitrobenzene in a batch type reactor at 25 C. All the catalysts were active in the hydrogenation reaction and the major obtained product was aniline.

Full text of DOI:10.1007/s10562-013-1034-2

Catal Lett (2013) 143:763–771
DOI 10.1007/s10562-013-1034-2
Nitrobenzene Hydrogenation on Au/TiO and Au/SiO Catalyst:
2
2
Synthesis, Characterization and Catalytic Activity
Cecilia Torres
Jos e´ Luis Garc ´ı a Fierro
Patricio Reyes
•
Cristian Campos
•
•
Marcelo Oportus
•
Received: 24 February 2013 / Accepted: 31 May 2013 / Published online: 13 June 2013
Ó Springer Science+Business Media New York 2013
Abstract Recently, gold has been proposed as an active
phase for the hydrogenation of nitro-arenes. This metal has
been rarely used in hydrogenation reactions because gold
does not possess hydrogen chemisorption capacity. How-
ever, small gold particles behave differently and they may
be able to chemisorb hydrogen to same extent, leading to
possible activity in hydrogenation reactions. This may
provide an advantage because the reactions catalyzed by
highly dispersed gold particles may be better controlled. In
this work, TiO and SiO supported Au catalysts were
1 Introduction
The preparation of compounds with potential biological
activity has received considerable attention at both lab-
oratory and industrial levels. For the production of fine
chemicals to be economically viable, high selectivity and
specificity is an important requirement. In addition,
environmentally benign processes are fundamental in
drug production. A process of particular interest that
embodies these stringent requirements is the catalytic
production of aromatic amines from aromatic nitro
compounds [1, 2].
2
2
prepared by the deposition–precipitation method using urea
and NaOH to precipitate the metallic component at dif-
ferent temperatures and hydrogen pressures. The metal
loading for all the catalysts was 1 wt%. The catalysts were
characterized by X-ray diffraction, high resolution trans-
mission electron microscopy among others. The catalysts
were then evaluated in the hydrogenation of nitrobenzene
in a batch type reactor at 25 °C. All the catalysts were
active in the hydrogenation reaction and the major obtained
product was aniline.
This process has been approached by three different
procedures: (i) by stoichiometric reduction of the corre-
sponding aromatic nitro compounds using iron, tin, zinc or
a metal sulfide such as Na S. However, this method dis-
2
plays low selectivity for nitro group, and large amounts of
waste acids and residues are generated during the reaction.
(ii) By catalytic hydrogenation of nitro compounds with H2
over supported noble metal catalysts, e.g. Pd, Pt, Ru and Ni
catalysts [3–7], etc. Nevertheless, the high hydrogenation
ability of this metals usually procedures the hydrogenation
of other groups existing in the arene substrates leading to
undesirable byproducts. (iii) Another method is the
chemioselective hydrogenation of aromatic nitro com-
pounds catalysed by carbonyl or organic metals in the
Keywords Gold Á Nitrobenzene Á Hydrogenation Á
Heterogeneous
C. Torres (&) Á C. Campos Á M. Oportus Á P. Reyes (&)
Grupo de Cat a´ lisis por Metales, Facultad de Ciencias Qu ´ı micas,
Universidad de Concepci o´ n, Edmundo Larenas 129,
Concepci o´ n, Chile
presence of CO and H O [8, 9]. It is therefore desirable to
2
develop a more efficient and environmental process to
achieve chemioselective hydrogenation to obtain valuable
functionalized anilines.
e-mail: cectorres@udec.cl
P. Reyes
e-mail: preyes@udec.cl
The hydrogenation of nitrobenzene, which is used to
produce aniline, has been carried out either in gas or liquid
phase, using supported metal catalysts and organic solvents
such as alcohols, acetone, benzene, ethyl acetate, or
aqueous acidic solutions [10–13] and CO2 under
J. L. G. Fierro
Grupo de Energ ´ı a y Qu ´ı mica Sostenible (EQS),
Instituto de Cat a´ lisis y Petroleoqu ´ı mica (ICP-CSIC),
c/Marie Curie s/n Cantoblanco, Madrid, Spain
123

7
64
C. Torres et al.
supercritical conditions [14]. The use of these solvents has
some drawbacks owing to their toxicity, flammability, or
environmental hazards. In addition, the solvent may play a
crucial role in the stabilization of reactive intermediates
and have a decisive influence on chemical reactions.
Though many studies have been carried out, the efficient
hydrogenations with high selectivity, as well as with high
activity, for nitro compounds have not so far been reported.
The most frequently used nitro-group hydrogenation cata-
lysts are palladium or platinum on carbon and Raney nickel
[
15–17]. The hydrogenation of aromatic nitro groups can
Scheme 1 Hydrogenation mechanism of nitrobenzene proposed by
Haber in 1898
be performed easily under mild conditions, while hydro-
genation of aliphatic nitro groups is usually slower,
requiring slightly elevated temperatures and pressures. In
the hydrogenation of the former reactions, the nitroso and
the hydroxylamine compounds have been observed as
intermediates, while a series of side reactions might lead to
the formation of hydrazo compounds, which can be
sometimes isolated.
As was mentioned previously, the experimental condi-
tions affect significantly, the catalytic behavior. If the
reaction is performed in liquid phase batch type reactors,
usually the reaction temperature for this kind of reactions
are in the range 80–120 °C, using mainly THF or ethanol
as solvent and 10 bar of hydrogen pressure [8, 9], whereas
in gas phase the reactions are studied at temperature close
to 250 °C and 1 bar of hydrogen [26–30].
Gold has been proposed as an active phase for hetero-
geneous catalyst [31, 32]; it displays a lower hydrogenating
ability in contrast to other noble metals such as Pt, Rh or
Pd, leading to particular advantages as direct selectivity
towards the functional group of interest.
The activity and selectivity of this reaction depends on
the catalysts as well as the reaction conditions [18]. Thus,
one of the intermediates or side products may become the
dominant product, for instance, azoxy, hydrazo and hy-
droxylamines can be in high yields [15, 17, 19]. The
accumulation of hydroxylamines is [20] thermodynami-
cally unstable. The accumulation may be diminished using
iron or vanadium-modifies catalysts [21, 22]. Recently, the
reaction was studied using gold as hydrogenation catalyst
More recently, it has been established that Au [20]
catalysts exhibit high selectivity in hydrogenation reactions
of nitro compounds, especially when Au metal particles are
smaller than 5 nm.
[
23] that gave high selectivity to substituted anilines, but
showing lower activity. Nevertheless a catalyst with the
same selectivity as gold but with a higher activity would be
desired for industrial application [24]. Thus, there is a
scientific and technological challenge to transform active
but not selective noble metals (Pt, Pd, Ru) and a non-noble
metal (Ni) into highly active and chemioselective catalysts.
Some recent studies have shown that the addition of cat-
ionic species or salts and presence of partially reduced
species may contribute to enhance the selectivity in these
systems. Additionally, in principle the behavior of gold
catalysts, mainly in terms of activity may also be improved
with appropriate preparation conditions.
In general these reactions proceed via intermediaries such
as the azoxy, hydrazo, and hydroxylamines (Scheme 1) [22,
33]. The hydrogenation of these compounds is considered
high risk due to the production of intermediaries such as
hydroxylamines which are highly unstable and can reach
disproportionation with an increase of temperature, conse-
quently leading to an explosion [34]. The accumulation of
these compounds is diminished by using catalysts modified
by Fe or V, or by using additives based on vanadium, or by
optimizing the reaction conditions [35]. Due to the large
amount of secondary product it is of great importance to find
the most direct possible route in order to avoid the formation
of these compounds.
In most of the researches published before 2000, the
used support for these kind of reactions was activated
carbon and in minor extent alumina, silica, and MCM-41.
It has been pointed out that activated carbons seems to be
the most appropriate support for this type of reactions
because it can provide more sites to produce a high dis-
persion of the metal component. Water produced in the
hydrogenation reaction may also affects the activity of the
catalysts supported on oxides [14, 15]. Only few studies
have been devoted to other oxides such as TiO , Fe O [17,
In this work we studied the hydrogenation of nitroben-
zene on Au catalysts supported on TiO and SiO and
2 2
determined how the catalytic activity is affected by the
type of support. All catalysts were synthesized with a metal
loading of 1 wt% using different preparation methods for
active phase deposition, in order to obtain catalysts with
metal particle size below 15 nm where Au is capable of
2
2 3
1
8], ZrO [25] which have shown interesting behavior.
2
chemisorbing H .
2
1
23

Nitrobenzene Hydrogenation on Au/TiO
2
and Au/SiO
2
Catalyst
765
2
Experimental
2.3 Characterization
2
.1 Reagents
The characterization of the catalytic systems was con-
ducted using the several techniques. X-ray diffraction
(XRD) was performed on a Rigaku X-ray Geigerflex using
Nitrobenzene (Fluka), absolute ethanol (Merck), TiO
2
2
SBET = 168 m g , 99.7 % anatase, Alfa Aesar), SiO2
2 -1
SBET = 298 m g ), gold(III) chloride trihydrate Sigma-
-1
(
(
a Ni filter and Cu K a radiation within 2–808 2h range. N
2
adsorption–desorption isotherms were carried out in an
ASAP 2010 Micromeitics apparatus. HR-TEM was per-
formed using a Philips CM200 model high resolution
electron microscope with energy dispersive analyzer and
digital camera coupled to a high speed TVIPS FastScan
F-114 model of 1,024 9 1,024 pixels and 12 bits. UV–Vis
spectra of diffuse reflectance of solid state was studied in
the range of 200–900 nm on a Varian Cary 3 UV–Vis
spectrophotometer equipped with an area of 150 mm in
diameter covered with poly integration tetra-fluoroethylene
(PTFE). The dust samples were mounted in a quartz cell,
which provided a sample thickness greater than 3 mm and
thus guaranteed ‘‘infinite’’ sample thickness. Chemical
analysis was performed by inductively coupled plasma-
mass spectrometry on an ICP-MS Spectrometer Perkin
Elmer Elas 6000. X-ray photoelectron spectroscopy (XPS)
were recorded using an Escalab 200R spectrometer pro-
vided with a hemispherical analyzer, operated in a constant
pass energy mode and Mg Ka X-ray radiation
Aldrich (99.9 %) and H by AGA, Chile.
2
2
.2 Catalyst Preparation
2
.2.1 Deposition–Precipitation with NaOH at 100 °C
The appropriate amount of support (TiO or SiO ) is placed
2
2
in a round bottom flask together with 50 mL of water. The
solution containing the required amount of the precursor,
gold(III) chloride trihydrate, needed to obtain the desired
metal loading is also added to the support suspension. An
stoichiometric amount of an aqueous solution of NaOH is
added and a flow of H at atmospheric pressure is fed to the
2
round bottom flask containing the mixture; the system is
then heated to reflux with stirring at 100 °C until the color
changes from yellow to violet. The obtained solid was
filtered and washed with deionized water until constant
conductivity. These are denoted Au/Support-T.
(
hm1,253.6 eV) operated at 10 mA and 12 kV.
2
.2.2 Deposition–Precipitation with NaOH at High H2
Pressure
2.4 Hydrogenation Reaction
The required amount of support, 50 mL of water and the
metal precursor was placed in an autoclave. Stoichiometric
amount of NaOH was then used to neutralize the acidic
environment generated by the precursor; finally, the auto-
clave was closed and it was flashed with hydrogen and then
pressurized up to 40 bar of hydrogen [36], and kept under
constant stirring for 2 h at room temperature. The obtained
solid was filtered and washed with deionized water until
constant conductivity. These samples are denoted Au/
Support-H2.
Catalytic evaluation was carried out in liquid phase in a
Parr stainless steel batch reactor at 298 K using 100 mg of
-
1
catalyst and 0.02 mol L of nitrobenzene, under 40 bar
hydrogen pressure. All the components (catalyst, solvent
and substrate) were fed to the reactor and stirring at
800 rpm. Liquid samples were taken periodically from the
reactor and analyzed in a GC–MS Shimadzu GCMS-
QP5050 with a capillary column b-Dex 225 (Supelco).
2
.2.3 Deposition–Precipitation with Urea
The procedure is similar to that proposed by Zanella [37]. An
Table 1 Chemical analysis, ICP-MS for Au/support
aqueous solution of the precursor, HAuCl , is added to urea
4
Catalyst
Au (wt%)
Ti (wt%)
Si (wt%)
in a 1:100 mol ratio and heated to 80 °C for 2 h under vig-
orous stirring. Urea decomposition causes the increase of pH
due to the formation of hydroxyl ions. This preparation must
be performed in an amber flask to avoid decomposition
caused by light. The solid was filtrated and dried under
vacuum for 2 h at 100 °C. The final solid was calcined with
synthetic air from room temperature to 300 °C for 2 h and
kept for 4 h. These are labeled Au/Support-U.
Au/TiO
Au/TiO
Au/TiO
2
–T
–H
–U
0.81
0.80
0.88
0.46
0.72
0.77
58.9
58.4
60.2
–
–
2
2
2
–
–
Au/SiO
Au/SiO
Au/SiO
2
–C–T
–C–H
–U
43.7
43.1
48.2
2
2
2
–
–
123

7
66
C. Torres et al.
Fig. 1 XRD patterns of catalyst
supported on a TiO and b SiO
2
2
Table 2 Specific surface area, metal particle size and metal disper-
sion of Au catalysts
2
-1
Catalyst
S
B.E.T. (m g
)
dHR-TEM (nm)
D (%)
Au/TiO
Au/TiO
Au/TiO
2
–T
–H
–U
159
172
143
293
287
284
5
6
7
5
5
22
18
16
22
22
11
2
2
2
Au/SiO
Au/SiO
Au/SiO
2
–C–T
–C–H
–U
2
2
2
10
support interaction generated during deposition of Au
which occurs due to the precipitation of the metallic phase.
It is observed that for the three used methods, the one
that favors the deposition of colloidal gold is the method
2 2
Fig. 2 UV–Vis spectra of diffuse reflectance of TiO - and SiO -
supported catalysts
using urea. For those supported on TiO no significant
2
difference in the amount deposited was found despite the
different methods of preparation.
3
Results and Discussion
Figure 1 shows the X-ray patterns of the prepared cat-
alysts. It can be clearly observed that in the Au/TiO cat-
2
3
.1 Characterization of Supports and Catalyst
alyst series various diffraction lines appear which are
assigned to the anatase phase. The most significant peak of
Au corresponding to the (111) hkl plane overlaps with one
The metal loading of the catalysts was determined using
ICP-MS and summarized in Table 1. It can be observed
that the experimental loadings were generally lower com-
pared to the nominal ones, especially for the silica-sup-
ported catalysts. For silica supported catalysts, this can be
attributed to the similarity of the isoelectric point between
the metal precursor solution and the silica support, mini-
mizing the metal/support interaction which leads to metal
°
°
of the titania approximately at 38 2h; however, at 45 2h a
broad and less intense peak corresponding to the plane
(200) of gold can be observed. This is characteristics of
species with low crystallinity. The XRD pattern of SiO
supported catalysts shows all the diffraction lines of Au
due to the amorphous nature of support. For the Au/SiO –T
-
2
2
catalyst, no diffraction lines were detected which suggests
that Au particles were below the instrument detection limit
of 5 nm; furthermore, this catalyst was the one with the
least amount of Au deposited.
loss [13]. In the case of the TiO -supported catalyst, the
2
obtained Au loading only differed slightly from the theo-
retical loading which can be related to the higher metal/
1
23

Nitrobenzene Hydrogenation on Au/TiO
2
and Au/SiO
2
Catalyst
767
2 2
Fig. 3 HR-TEM for a Au/TiO –U, b Au/SiO –T
centered at 250 nm were observed for the silica-supported
catalysts which are attributed to silica species. The plas-
mon absorption band also depends on the size of the par-
ticles [41–44]. Generally, formation of metal nanoparticles
involves two processes: the nucleation and growth of
nuclei. There is a competition between the nucleation and
growth. In the general field of colloid chemistry, it is
known that fast nucleation relative to the growth results in
small particle size [44]. In the case of silica series the three
catalysts showed significant differences in intensity due to
the preparation method. Au/SiO –U shows the highest
2
intensity indicating that this catalyst contains the largest
metal crystallites: which may be attributed to partial
agglomeration. This can be explaining by the lower inter-
action of the support with the metallic phase due to the
repulsive interaction between them, which leads to
agglomeration of Au NPs in the precipitation stage. Au/
SiO –T and Au/SiO –H shown same intensities indicating
2
2
2
similar particles size which is verified by HR-TEM (see
Table 2).
Specific surface area of the Au catalyst shows values
slightly smaller than the corresponding supports which are
explained in term of the partial blockage of the porous of
the support.
2 2
Fig. 4 XPS spectra of Au catalysts supported on a TiO and b SiO
High resolution transmission electron micrograph was
used to determine the average metal particle size of the
catalysts and the results are summarized in Table 2 and
representative micrographs showed in Fig. 3. Metal dis-
persion was estimated from HR-TEM values assuming that
the crystals possess spherical shapes.
Figure 2 shows the UV–Vis diffuse reflectance spectra
of the supports and catalysts. The bands displayed between
2
00 and 400 nm are ascribed to TiO while the peak cen-
2
tered at 550 nm belongs to the gold species present in the
catalysts. The silica-supported catalysts also displayed the
same type of peak indicating that the gold species were
similar regardless of the type of support. The position in
the spectra attributed to the Au species suggests the pres-
ence of spherical gold particles [38]. This shape of gold
nanoparticles show a strong surface plasmon absorption
band in the visible region [39, 40]. Additionally, peaks
For Au/TiO catalyst the analysis of Au 4f core level
2 7/2
spectra reveals the presence of two types of gold, metallic
Au at 83.8 eV and partially oxidizing species at 85.5 eV
(See Fig. 4). Conversely, Au/SiO catalyst only exhibits
2
0
the presence of Au , in line with the differences in inter-
action between gold and the support.
123

7
68
C. Torres et al.
Au/Si at
Table 3 Binding energies [eV] of inner core electrons and atomic surface ratio of supported gold catalysts
Catalizador Binding energies, eV
Au/Ti at
Ti2p3/2
458.6
Si2p
O1 s
Au4f7/2
Au/TiO
Au/TiO
Au/TiO
2
2
2
–T
–H
–U
–T
–
–
–
530.0 (87)
83.8 (70)
85.6 (30)
83.9 (80)
85.5 (20)
83.9 (81)
85.6 (19)
84.1
0.0107
0.0030
0.0033
–
–
–
5
31.5 (13)
529.9 (90)
31.4 (10)
530.0 (88)
31.4 (12)
2
458.6
458.6
5
5
Au/SiO
Au/SiO
Au/SiO
2
2
2
–
–
–
103.4
103.4
103.4
532.8
532.8
532.8
–
–
–
0.0013
0.0011
0.0033
–H
–U
2
84.0
84.1
O
O
N
Table 4 Catalytic data for NB hydrogenation at 25 °C on TiO
supported catalyst at a reaction time of 7 h
2
-
+
N
-
NH OH
k3
NH2
O
k2
k1
-
(h
1
-1 a
)
Catalyst
k
g
g
Conversion (%) Yield An15 % (%)
Au/TiO
Au/TiO
Au/TiO
2
–T
–H
–U
1.52
1.20
1.05
1.46
1.19
0.50
70.8
59.9
58.5
67.4
59.8
31.4
93.3
100.0
88.7
93.2
98.8
98.8
Fig. 5 Reaction scheme of the hydrogenation of nitrobenzene
2
2
2
Au/SiO
Au/SiO
Au/SiO
a
2
–T
–H
–U
2
2
2
k
g
= k
1
? k
2
3
? k (see Fig. 5)
interacting with Au species on the support. A summary of
the obtained results by XPS are compiled in Table 3.
3.2 Catalytic Activity
A scheme of the hydrogenation of nitrobenzene is shown in
Fig. 5. It is highly desirable that the first step be reaction
limiting step, to avoid undesirable intermediates products.
Figure 6 shows the evolution of conversion on time
during the hydrogenation of NB over the Au/TiO cata-
2
lysts: all the catalysts were active showing a pseudo first
order dependence to NB. The Au/TiO –T showed a
2
behavior like an induction time during approximately 3 h,
despite that it showed the most active behavior of all the
catalysts reaching a conversion of 70 % after 7 h. (see
Table 4). Au/TiO –H and Au/TiO –U also showed a
2
2
2
similar trends by this induction period is less marked than
that displayed by Au/TiO –T.
2
The highest activity displayed by the Au/TiO –T cata-
2
lyst can be related to the smaller metal particle size as
revealed by XRD, UV–Vis and HR-TEM characterizations.
The Au/Ti surface atomic ratio shows that this catalyst was
Fig. 6 Nitrobenzene hydrogenation over a 1 % Au/TiO
SiO . Reaction conditions: NB concentration: 0.02 mol L , PH2:
0 bar, stirring speed: 700 rpm
2
y b 1 % Au/
-
1
2
4
the most highly dispersed of the TiO series.
2
In addition, the O 1 s core level spectra of Au/TiO2
For all studied catalysts, the main observed products
were aniline and hydroxylamine (NBNH-OH); however,
catalyst indicate the presence of two types of oxygen
species, assigned to Ti–O–Ti and surface oxygen groups
for the Au/TiO –U catalyst nitrosobenzene (NB-NO) was
2
1
23

Nitrobenzene Hydrogenation on Au/TiO
2
and Au/SiO
2
Catalyst
769
also detected as an intermediate product. This indicates that
the rate of the second step is very high.
X-ray photoelectron spectroscopy results for all the cata-
lysts indicated the presence of Aud? species which explain
the induction time observed in Fig. 6. We attribute the
induction time of all the series to the restructuration of the
d?
0
active phase, due to the reduction of the Au to Au and the
0 d?
competition between Au and Au for adsorbing the NB.
Also this effect retards the dissociation of H which delayed
2
the start of the reduction of -NO group . The larger Au
2
particle of Au/TiO2–H and Au/TiO –U catalysts leads to a
2
2
low catalytic activity due to the lower hydrogen chemisorp-
tion ability. In spite of Au/TiO –U catalyst contains smaller
2
Fig. 7 Evolution of products as a function of the time for Au/TiO
catalyst
2
–U
average Au particles compared to the Au/TiO –H catalyst
2
2
determined by HR-TEM, the higher intensity of the Au UV–
Vis spectrum suggests a higher contribution of species of
crystallite sizes greater than 10 nm. This explains why the
former catalyst exhibited lower catalytic performance and the
production of small amounts of intermediate products such as
NB-NO and NB-OH (see Fig. 7 and Table 4).
SiO supported catalysts also show pseudo first order
2
kinetic behavior, similar to the TiO supported catalysts;
2
however, the SiO series did not show any induction time. In
2
line with XPS results which indicate the presence of only
0
Au species in the SiO -supported catalysts. On the other
2
hand, HR-TEM results displayed comparable metal particle
sizes (5 and 6 nm for the Au/SiO –T catalyst and Au/SiO –
2
2
H catalyst, respectively). Notwithstanding, these catalysts
2
presented different activities: this can be explained by dif-
ferences in metal loading as a consequence of the amount
of Au deposited during catalyst preparation, derived from
ICP-MS results. The Au/SiO –T catalyst contained lower
2
2
Fig. 8 Product conversion versus total conversion for Au/SiO –U
amount of Au in the catalyst in spite of it showed higher
catalyst
Fig. 9 Schematic deactivation
for Au/SiO –U catalyst
2
H2
-
H O
2
H2
H2
-
H O
2
123

7
70
C. Torres et al.
catalytic performance; this is due to the better distribution of
the active phase on the support which indicate that even
though the different size particle the Au NPs behave in
similar way. The lower catalytic performance of the Au/
SiO –H catalyst is attributed to the possible loss of active
grateful to CONICYT for their doctoral Fellowship. We also thanks
to REDOC.CTA Universidad de Concepci o´ n.
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Conclusions
All the catalytic systems were active in the hydrogenation of
nitrobenzene being the most active catalyst those prepared
by deposition–precipitation with NaOH at 100 °C, due to
these catalysts contain the smallest metallic crystals. The
TiO supported catalysts performed better than the SiO -
4
:17
2
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stronger metal/support interaction for the TiO -supported
2
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Reaction intermediates were found as products for all
the catalytic systems but it is important to mention that the
second step (formation of nitrosobenzene) is very fast and
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Acknowledgments The authors thank the Project FONDECYT
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100259 for funding this research. C. Torres and C. Campos are
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