Optimization of manganese content by high-throughput experimentation of Pt/WOx-ZrO2-Mn catalysts

Article abstract of DOI:10.1016/j.catcom.2009.11.010

A library of Pt/WO_x-ZrO₂-Mn catalysts was developed in order to optimize the manganese content in this catalytic system for the isomerization of n-hexane. The catalysts were synthesized, characterized and screened using high-throughput experimentation (HTE) techniques. The catalysts were prepared by surfactant-assisted coprecipitation whereas the characterization was done by X-ray diffraction (XRD), Raman and UV-vis spectroscopy. For this second screening, several catalysts with different manganese contents were prepared; it was found that the incorporation of Mn modifies the anchorage of tungsten on the zirconia surface, thus improving its catalytic properties, in terms of the n-hexane conversion and selectivity, depending on the catalyst composition. These results suggest that this methodology allows the optimization of manganese and tungsten contents on these solid catalysts.

Full text of DOI:10.1016/j.catcom.2009.11.010

Catalysis Communications 11 (2010) 408–413
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Optimization of manganese content by high-throughput experimentation
of Pt/WOx–ZrO2–Mn catalysts
b
b
M.L. Hernandez-Pichardo a, J.A. Montoya de la Fuente b, , P. del Angel , A. Vargas ,
*
I. Hernández c, M. González-Brambila c
a Instituto Politécnico Nacional, ESIQIE, Laboratorio de Catálisis y Materiales, Av. IPN S/N Zacatenco, 07738 México, Mexico
b Instituto Mexicano del Petróleo, DIyP, Eje Central Lázaro Cárdenas Norte 152, 07730 México D.F., México
c Universidad Autónoma Metropolitana, División de CBI, Av. San Pablo 180 Col. Reynosa, 02200 México D.F., México
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2009
Received in revised form 6 November 2009
Accepted 11 November 2009
Available online 16 November 2009
A library of Pt/WOx–ZrO2–Mn catalysts was developed in order to optimize the manganese content in this
catalytic system for the isomerization of n-hexane. The catalysts were synthesized, characterized and
screened using high-throughput experimentation (HTE) techniques. The catalysts were prepared by sur-
factant-assisted coprecipitation whereas the characterization was done by X-ray diffraction (XRD),
Raman and UV–vis spectroscopy. For this second screening, several catalysts with different manganese
contents were prepared; it was found that the incorporation of Mn modifies the anchorage of tungsten
on the zirconia surface, thus improving its catalytic properties, in terms of the n-hexane conversion
and selectivity, depending on the catalyst composition. These results suggest that this methodology
allows the optimization of manganese and tungsten contents on these solid catalysts.
Keywords:
Tungstated zirconia
Manganese
n-Hexane hydroisomerization
High-throughput experimentation
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
decoration of platinum with oxides, favoring the migration of the
activated species, modifying the reducibility of the tungsten phase
Solid acids, such as sulfated zirconia (SZ) and tungstated zirco-
nia (WZ) catalysts have been thoroughly studied as an environ-
mentally friendly alternative for the current isomerization
processes. The catalytic properties of these solids can be improved
by the addition of some promoters such as Fe, Mn, and/or Cr [1,2],
Al [3,4], or Ce [5]. Particularly, in the case of tungstated zirconia,
several authors have studied the effect of doping these catalysts
trying some others metals like Ga or Al [6–8], and have pointed
out the importance of adding promoters to this system in order
to improve the catalytic performance of these materials. However,
it was found that although Fe and Mn promote sulfated zirconia for
n-alkenes isomerization, they do not promote tungstated zirconia
due to the formation of Mn or Fe tungstates that hinders the pro-
moter effect of these components [9].
In the case of sulfated zirconia, the promoting effect of Mn, Fe
and others ions has been attributed to many factors. Some of these
factors include the stabilization of carbenium ion and alkene inter-
mediates on the surface, the increase in the dehydrogenation abil-
ity, the improvement of the dispersion of the active species on the
surface or the preferential incorporation of some cations into the
zirconia lattice [10]. It has also been suggested that the promoting
effect of these metals is a combination of several factors, including
[8], increasing the stability of the tetragonal zirconia or modifying
the domain size of the WOx species on the surface [11]. The pro-
moting effect of Al or Ga for instance, was also suggested to be a
combination of several factors, such as the stability of the tetrago-
nal zirconia structure, the modification of the crystallite size of
WO3 on the surface of WZ or the improvement of the redox prop-
erties of W6+ [6].
Despite of all the efforts focused to the understanding of the pro-
moter role, there is still some controversy about the catalytic effect
of these metals. Particularly for Fe and Mn, some research about the
individual role of these promoters is necessary. In this work, the
influence of the manganese content on the nanostructure and reac-
tivity of WOx–ZrO2 materials was studied. This study is a continua-
tion of previous works on Pt/WOx–ZrO2–Mn catalysts [11,12] where
we developed a primary study for the leads discovery in tungsten
composition, pre-treatments and precursors. We reported that
the catalytic properties depend on the dispersion of the tungstated
phase and those results suggested that the incorporation of Mn
modifies the properties of the surface WOx species. However, the
specific effect of the Mn addition was not very clear. We therefore
prepared a new series of Pt/WOx–ZrO2–Mn materials with different
manganese contents in order to evaluate the effects of Mn over the
catalytic performance, and to optimize the content of this ion by
using HTE techniques, since these approaches allow the comparison
of the catalytic behavior under identical conditions.
* Corresponding author. Tel.: +52 55 91758375.
E-mail address: amontoya@imp.mx (J.A. Montoya de la Fuente).
1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.010

M.L. Hernandez-Pichardo et al. / Catalysis Communications 11 (2010) 408–413
409
(General Area Detector Diffraction Systems, two-dimensional
detector) diffractometer fitted with a Cu tube (40 kV, 40 mA) using
HTE approaches for both, measurements and patterns evaluation.
Ultraviolet–visible (UV–vis) spectra were obtained using a Varian
(Cary 1G) spectrophotometer for the data handling with an inte-
gration sphere accessory. Finally, Raman spectra were recorded
in the 100–1200 cmÀ1 wave number range using a ThermoNicolet
Raman apparatus (Almega model) equipped with a Nd:YVO4DPSS
laser source. The excitation line of the laser was 532 nm and the la-
ser power was of 25 mW.
2. Experimental
2.1. Catalysts preparation
High-throughput experimentation techniques were applied to
the synthesis of a library of 24 Pt/WOx–ZrO2–Mn catalysts, with dif-
ferent manganese (0, 0.5, 1 and 2 wt.%), tungsten (10, 15 and 20
wt.%) and surfactant contents. Catalysts were prepared by
surfactant-assisted coprecipitation method, as it was previously re-
ported [11]. Briefly, the Multicomponent Pt/WOx–ZrO2–Mn catalyst
library has been synthesized using Cavro robots (model MSP 9500
by Symyx) in an automated parallel synthesis. Hydrous zirconia
was coprecipitated with tungsten, Mn and surfactant solutions that
were prepared from zirconyl chloride, ammonium metatungstate,
manganese nitrate, and cetyl-trimethylammonium bromide
(CTAB), respectively, all from Aldrich. The precipitates were washed
several times with deionized water, and the resulting paste was
dried at 110 °C overnight, and then calcined at 800 °C for 4 h.
These samples were labeled as Mn(x)W(y)Z–S, where x and y
are the Mn and W concentrations in wt.% respectively, and S = Sur-
factant/ZrO2 molar ratio employed (0 or 2). For platinum impreg-
nation (0.3 wt.%), the vials of the library plate were filled with
the WOx–ZrO2–Mn powders, and a H2PtCl6Á6H2O (Aldrich) aqueous
solution was dispensed onto the samples by the pipetting robot
system. Then the solvent was evaporated and the remaining coat-
ing on the samples was dried at 110 °C overnight and finally cal-
cined at 400 °C for 3 h. In analogy with previous nomenclature,
the resulting materials were denoted as Pt/Mn(x)W(y)Z–S, indicat-
ing that the materials were impregnated with platinum.
3. Results and discussion
In previous works dealing with the improvement of catalytic
performance of Pt supported on tungstated zirconia materials dop-
ped with manganese [11,12], we have studied the effects of the
main variables that affect the catalytic activity of these materials
on isomerization processes. We found the method, composition,
precursor and pre-treatments that lead to a higher catalytic perfor-
mance. However, as far as the manganese doping is concerned, we
found that the incorporation of 1% of Mn yields different behaviors,
depending on the tungsten dispersion on the zirconia surface [11].
Thus, in this secondary screening we have focused our study on the
optimization of the manganese content by allowing parallel testing
to obtain high conversions and selectivity towards the bi-ramified
isomers.
3.1. Catalytic activity
The catalytic activity of these materials was evaluated in the n-
hexane hydroisomerization reaction at 260 °C; the general results
are shown in Fig. 1. These results show the influence of the Mn
content on the n-hexane conversion of Pt/WOx–ZrO2–Mn catalysts
as a function of tungsten loadings and surfactant content. It is ob-
served that the catalytic activity of samples without Mn increases
according with the increment of tungsten content from 10% to 20%
in both cases: without surfactant (S = 0) and with surfactant (S = 2).
In general, it is observed that the surfactant improves the catalytic
activity of these materials. On the other hand, when Mn is incorpo-
rated into the catalytic system it is observed as general tendency
that as the tungsten concentration increases the catalytic activity
decreases for every individual concentration of Mn, being the high-
est conversions at low Mn concentrations. One possible explana-
tion is that at low Mn concentrations (0.5% Mn) this ion acts as a
dopant and it is highly dispersed, thus it can be located on the sup-
port but also on the WOx clusters modifying the WOx anchorage
and therefore the catalytic activity. As the Mn concentration in-
creases, it begins to form MnOx nanocrystals that go preferentially
to the zirconia support and the effect over the WOx active sites is
less intense. From these results, it is clear that the behavior of cat-
alysts doped with Mn is very different from those catalysts without
Mn, since in the last ones the catalytic activity increases with the
W content, whereas the doped catalysts show an inverse behavior.
It is also possible that the small differences between real and nom-
inal compositions, as well as differences in surface area are chang-
ing the catalytic results; however, the tendencies indicate that the
variations are similar for all the catalysts. It is also observed that
the addition of this cation promotes mainly the activity of samples
with low tungsten content (10% W). In the case of samples without
surfactant (S = 0), the manganese presents a promoter effect only
in samples with low tungsten content (10–15% W). This result sug-
gests that the incorporation of manganese to this catalytic system
allows the optimization of the tungsten content, since the addition
of only 0.5% Mn increases the conversion catalyst with low tung-
sten content by 50% mol. It is important to mention that in this
2.2. Catalytic testing
Catalytic activity was measured in the n-hexane hydroisomer-
ization reaction over the Pt/WOx–ZrO2–Mn catalysts. The evalua-
tion was carried out in a Combinatorial Multi Channel Fixed Bed
Reactor (MCFBR) (Symyx, Tech) fully automated, evaluating 48
samples in parallel. A catalyst sample of 100 mg diluted with
200 mg of inert silicon carbide was loaded in each well and fixed
into the reactor heads, containing a set of eight micro-reactors of
approximately 4 mm of inner diameter and 47 mm in length. The
six reactor heads are connected independently to six chromato-
graphs (Agilent, 6850 Series) each one equipped with a SPB-1 cap-
illary column (Supelco) with a length of 100 m, and a flame
ionization detector (FID) for the analysis of products. The injection
´
of the reactant and products to the GCs was carried out at the reac-
tion pressure. The reliability of the quantitative method was evalu-
ated with an uncertainty of the conversion 2% measurable through
the 48 wells. The pretreatment of the catalysts was carried out
in situ prior to the activity test. It comprised a drying-reduction
program, drying the samples at 260 °C for 2 h in helium (200 cm3
minÀ1) followed by reduction in a hydrogen flow (200 cm3 minÀ1
)
at 450 °C for 3 h. Hydrogen and n-hexane flows were adjusted to
give a H2/n-C6 = 1.47 M ratio. The reaction was conducted at
260 °C, 0.689 MPa, 3.7 hÀ1 WHSV using a mixture of 100 cm3 minÀ1
H2 and 0.4 cm3 minÀ1 of n-hexane fed with an HPLC pump.
2.3. Catalyst characterization
WOx–ZrO2–Mn nanostructured mixed oxides were character-
ized by X-ray powder diffraction (XRD), UV–vis, and Raman spec-
troscopy. The WOx–ZrO2–Mn materials were characterized
without Pt since in this study only the quantity of sample needed
for the catalytic evaluation (100 mg) was impregnated with Pt with
the aim of not wasting this expensive metal. X-ray diffraction pat-
terns were obtained in a Bruker-Axs D8 Discover with GADDS

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M.L. Hernandez-Pichardo et al. / Catalysis Communications 11 (2010) 408–413
0%Mn
0.5%Mn
1%Mn
2%Mn
70%
60%
50%
40%
30%
20%
10%
0%
2%Mn
1%Mn
0.5%Mn
0%Mn
10 % W
(S=0)
15 % W
(S=0)
20 % W
(S=0)
10 % W
(S=2)
15 % W
(S=2)
20 % W
(S=2)
% W (S=Surfactant content)
Fig. 1. Secondary screening for the catalytic performance in the n-hexane isomerization reaction at 260 °C of Pt/WOx–ZrO2–Mn catalysts at different manganese and tungsten
loadings.
work, the Pt content was maintained constant (0.3 wt.%) keeping
the amount of Pt used industrially since the most important role
of platinum is to stabilize the catalytic activity [13].
with surfactant. It is also possible to observe that all the catalysts
presented the formation of the four isomers: 2-methyl-pentane
(2 MP), 3-methyl-pentane (3 MP), 2,2-dimethyl butane (2,2 DMB)
and 2,3-dimethyl butane (2,3 DMB). It was found that the addition
of Mn improves the selectivity to the secondary bi-ramified iso-
mers, of higher octane number, in catalysts with low tungsten con-
tent (10% W). Particularly, the increase in the 2,2-dimethyl butane
product which is a secondary product indicates an increase in the
activity of the Mn-containing catalysts. The explanation of this
behavior is the reaction mechanism; according to most accepted
mechanism (Scheme 1) both methylpentanes (2 MP and 3 MP)
are primary products, while 2,2 DMB and 2,3 DMB are secondary
products, but 2,3 DMB approaches rapidly to equilibrium.
Some authors have suggested that the role of manganese as
dopant in Fe and Mn sulfated zirconia catalysts (FMSZ) might be
to raise or stabilize the dispersion of the Fe [10,15]. According to
the results obtained in this work, we suggest that the promoter ef-
fect of manganese is generated when the Mn is incorporated into
the zirconia and modifies the interactions between the polytung-
state species and the zirconia. It allows the formation of well-dis-
persed WOx species on suitable sizes to form the active sites for the
isomerization of the alkane. However, when the tungsten content
is higher, the addition of a higher content of Mn (2% for instance)
Moreover, as we expected, the preparation of the catalysts using
surfactant produces an increase in the catalytic activity of these
materials. As we have previously reported [14], the rise on the sur-
factant content on this type of samples generates modifications in
the mesoporosity, as well as an increase in its specific area, which
produces modifications on the WOx dispersion and thus on the cat-
alytic performance of these materials. Some N2 physisorption anal-
ysis were performed on the Mn(1)W(15)Z-0 and Mn(1)W(15)Z-1
catalysts, in order to confirm these trends. From these analysis, iso-
therms type IV (not showed) with the hysteresis loop characteristic
of mesoporous materials with open pores systems were obtained
for both catalysts, but the surface area for the catalysts prepared
with surfactant (Sg = 90.3 m2/g) was higher than the catalysts
without surfactant (Sg = 70.6 m2/g).
In addition, Table 1 shows the conversion and selectivity ob-
tained in the n-hexane isomerization reaction for blank catalysts
(manganese and surfactant free) as well as the lead catalysts. Com-
parison of the conversions and selectivity of the lead samples
shows that the most active catalysts were materials prepared with
manganese and low contents of tungsten, or with 20% W prepared
Table 1
Conversions and products distribution for blank and lead catalysts in the n-hexane isomerization reaction at 260 °C.
Catalyst
XA (% mol)
2,2 DMB
2,3 DMB
2 MP
3 MP
Cracking
Pt/W(10)Z
Pt/W(15)Z
Pt/W(20)Z
41.4
47.9
54.0
64.2
55.0
64.4
60.6
60.3
59.7
63.0
66.7
56.9
63.6
61.2
3.2
3.6
4.3
5.1
4.3
5.2
4.9
4.5
4.5
5.0
6.4
4.7
5.2
4.5
12.1
12.8
12.0
12.2
12.05
12.9
12.5
12.1
12.4
11.8
11.6
12.4
11.7
12.9
49.7
48.9
49.6
49.1
49.7
48.8
48.4
49.1
48.8
49.5
48.7
48.9
49.3
47.7
32.9
32.0
32.9
32.4
33.5
31.6
31.3
32.7
32.2
32.7
31.9
32.1
32.5
31.0
1.3
2.0
0.7
0.7
0.5
1.5
2.4
1.1
2.1
1.0
1.0
1.9
0.8
3.7
Pt/Mn(0.5)W(10)Z-0
Pt/Mn(1)W(10)Z-0
Pt/Mn(2)W(10)Z-0
Pt/Mn(0)W(15)Z-2
Pt/Mn(0)W(20)Z-2
Pt/Mn(0.5)W(10)Z-2
Pt/Mn(1)W(10)Z-2
Pt/Mn(2)W(10)Z-2
Pt/Mn(1)W(15)Z-2
Pt/Mn(2)W(15)Z-2
Pt/Mn(2)W(20)Z-2

M.L. Hernandez-Pichardo et al. / Catalysis Communications 11 (2010) 408–413
411
2 − MP
⇔ 2,3DMB ⇔ 2,2DMB
3 − MP
n − C6 ⇔
Scheme 1. n-Hexane isomerization mechanism.
is necessary. In this regard, Iglesia and his group [16] have sug-
gested that the activity of WOx–ZrO2 catalysts depends on the size
of the WOx species developed on the zirconia surface. If the tung-
sten surface density is lower than 4 W nmÀ2, they will generate a
strong interaction with the support, while in larger crystals most
of the active sites are founded in the bulk of the cluster. This could
also explain the difference between the catalytic activity of mate-
rials with different compositions, since, if there is an excess or a
deficiency of manganese, depending on the tungsten content, the
promoting effect of this cation is not generated. This effect is clear
on catalysts with 10% of W, since the activity of these samples was
promoted by the addition of manganese, even with low manganese
content.
3.2. Catalysts characterization
In regards to the characterization of the WOx–ZrO2–Mn samples
by XRD (Fig. 2), we found that these samples are formed mainly by
the tetragonal zirconia phase and monoclinic tungsten oxide
(WO3). These diffraction patterns were measured simultaneously
under the same conditions using a HTE approach; therefore, the
patterns can be directly compared, as well as the reflection posi-
tions and intensity. The diffraction patterns for samples prepared
without surfactant (S = 0) are shown in Fig. 2a. These patterns indi-
cate that the tungsten oxide is well dispersed, since the WO3 crys-
talline phase (2h = 23.3–24.4) is only observed in samples with
high tungsten content and low manganese content. The overall
analysis of these patterns indicates that the increase in the manga-
nese content improves the dispersion of the WOx phase, since at
higher Mn contents the diffractions corresponding to the WO3
diminishes or they are not detected. This result reveals that the
polytungstate species developed on the surface are smaller than
WO3 crystals and the fact that they are not observed by XRD indi-
cates that these nanostructures have a domain size smaller than
3 nm with a tungsten surface density lower than 10 W nmÀ2. It
also suggests that the inclusion of Mn to the tungstated zirconia
modifies the anchorage of the tungstated species and the mecha-
nism by which these structures are developed on the zirconia
surface.
On the other hand, in samples prepared with surfactant (Fig. 2b)
most of the samples with 15% and 20% of tungsten, show diffrac-
tion peaks corresponding to the formation of the crystalline mono-
clinic WO3 phase, with low intensity due to the higher dispersion
produced by the effect of surfactant, which generates an increase
in the surface area. The effect of the incorporation of Mn on the dis-
persion of the WOx clusters is less evident due to the addition of
surfactant; however, from the diffractograms of Fig. 2b is also pos-
sible to observe a slower growth of these species to the formation
of the crystalline phase of WO3 with the increase in the Mn con-
centration. These results along with the catalytic activity suggest
that a high activity requires the formation of well-dispersed WOx
species.
Fig. 2. XRD spectra for WOx–ZrO2–Mn samples: T = tetragonal zirconia, W = WO3.
(a) Effect of the Mn incorporation, and (b) effect of the surfactant and manganese.
This figure shows as an example, the Raman spectra of samples
with 15% W at different manganese contents, prepared without
surfactant. These spectra present the characteristics bands of crys-
talline WO3 with low intensity and a slight shift to lower frequen-
cies at 260 and 312 cmÀ1, that correspond to W–O–W deformation
modes, and 706 and 798 cmÀ1 corresponding to W–O bending and
stretching modes, respectively [16]. The spectra also show the
presence of a band at about 970 cmÀ1 that is characteristic of the
octahedrally coordinated polytungstate species with terminal
W@O bonds [17]. The presence of these bands indicates that differ-
ent tungstated species are formed in the zirconia surface, from
small oligomeric WOx clusters to the formation of WO3 crystallites.
Moreover, it is observed that at the manganese content increases,
the bands become less intense, with the same tungsten content,
suggesting that the surface WOx density decreases in agreement
with the XRD results. It is important to mention that these spectra
correspond to samples prepared without surfactant, so that the ef-
fect shown is attributed only to the manganese incorporation. As it
has been previously reported [18], Raman scattering cross sections
for crystalline WO3 are much greater than for surface polytung-
state species; thus, the bands corresponding to WO3 dominate
the spectra, even when the polytungstate clusters are the most
abundant surface structures.
However, the activity of these catalysts cannot be explained
considering only the stabilization of the different phases observed
by XRD, therefore, spectroscopic techniques were used to identify
not segregated tungsten species on the zirconia surface. The effect
of the addition of manganese to these catalysts is shown in Fig. 3.
Finally, the UV–vis spectra recorded in air also suggest that
manganese produces a modification on the surface WOx nanostruc-
ture. Fig. 4 compares the UV–vis spectra for samples with 10% of

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M.L. Hernandez-Pichardo et al. / Catalysis Communications 11 (2010) 408–413
mechanism proposed by Barton et al. [16] for the formation of
these sites on ZW catalysts.
In summary, we observed in this work that the use of Mn as
promoter modifies the nanostructure and WOx–ZrO2 interactions
of tungstated zirconia catalysts. These nanocrystalline structures
exhibit properties different from those observed in microcrystal-
line structures, facilitating the reduction of the WOx clusters and
the formation of the acid sites mainly in catalysts with low tung-
sten content. This result indicates that by means of the incorpora-
tion of a low quantity of Mn, it is possible to optimize the tungsten
loading on Pt/WOx–ZrO2 catalysts. On the other hand, the reactivity
depends not only on the reducibility but also on the accessibility of
the WOx species on the zirconia surface, because the tungsten spe-
cies must be first expelled from the bulk before they form the ac-
tive WOx sites, being the main effect of the surfactant on the high
performance of these catalysts.
These results show the importance of the high-throughput
experimentation because the catalytic activity is not a linear func-
tion of the tungsten content against the Mn concentration, and
only using HTE is possible to measure many different concentra-
tions, which would be very difficult to do by traditional
experimentation.
Fig. 3. Raman spectra of some WOx–ZrO2–Mn samples with 15% W and different
manganese contents.
4. Conclusions
A secondary screening for the study of Pt/WOx–ZrO2–Mn
catalysts was performed in order to optimize the manganese and
tungsten contents in this catalytic system. It was found that the
incorporation of Mn to tungstated zirconia catalysts modifies
the anchorage of the tungstated species on the zirconia surface.
The promoting effect of manganese on this system is generated
when this cation is incorporated into the zirconia lattice and mod-
ifies the interaction between tungsten and zirconia, thus forming
WOx clusters of suitable domain size to be easily reduced to form
active sites. In addition, the use of a surfactant produces a higher
expel of the WOx species from the bulk to generate more active
sites, as well as an increase in the surface area that modify the
tungsten surface density and the interaction with the zirconia
surface developing an acid character. These results show the
importance of the high-throughput experimentation because in
some catalytic systems the catalytic behavior is not a linear func-
tion of the compositions, and only using HTE is possible to measure
many different concentrations, which would be very difficult to do
by traditional experimentation.
Fig. 4. Diffuse reflectance UV–vis absorption spectra of WOx–ZrO2–Mn samples
with 10% W and different manganese contents.
tungsten loading and different manganese contents. These spectra
present a broad absorption band between 3 to 5 eV, corresponding
to different O2À ? W6+ electronic transitions, according to litera-
ture [19], overlapped with the absorption corresponding to the
ZrO2 at about 4–5 eV with almost the same intensity. It is observed
that as the manganese content increases, the intensity of the
absorption band decreases, which suggests a slight reduction of
the tungsten surface density in catalysts with the same tungsten
concentration. This decrease in the absorption band intensity is
similar to that produced for the decrease of the tungsten loading
in tungstated zirconia catalysts [16], with a slight reduction of
the edge energy (E0) towards lower energy values, from 2.7 to
2.4 eV. Therefore, this behavior in catalysts with the same tungsten
content could indicate that the Mn incorporation facilitates the li-
gand-to-metal charge transfers of tungsten species. This could ex-
plain the improved performance in catalysts prepared with Mn in
samples with low tungsten content, since in these samples the
tungsten surface density is low (according to XRD results). These
isolated WOx species present a strong interaction with the zirconia,
and they are difficult to be reduced. Further, the addition of Mn
makes weaker this interaction facilitating the reduction of the clus-
ters and the formation of the Brönsted acid sites according to the
Acknowledgements
M.L.H.P. would like to acknowledge to the Instituto Politécnico
Nacional (IPN) for the financial support received. We thank the
facilities of the Combinatorial Catalysis and Electron Microscopy
Laboratories of the Instituto Mexicano del Petroleo. The authors
also thank Prof. Ricardo Macias Salinas for the revision of the
Manuscript.
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