Dehydrocyclization of n-heptane over Pt catalysts supported on Al- and Si-promoted TiO2

Article abstract of DOI:10.1016/j.cattod.2013.10.019

The effect of content of Si- and Al-promoted TiO₂ catalysts in the dehydrocyclization of n-heptane was studied. The sol-gel method was used to prepare pure TiO₂ and mixed oxides with 1%, 3% and 10% of Al ₂O₃ and SiO₂. The structure and surface characterization (XRD, N₂ sorption, H₂ chemisorption, NH₃ uptake) revealed that the promoted cations were dispersed in the TiO₂ lattice, modifying structural and surface properties of catalysts. Dehydrocyclization activity and toluene yields were useful test reaction to detect the effect of content of cations in both acidic and catalytic properties of promoted catalysts. Increase in the TOF was observed as a function of cation content, hand in hand with the increase of overall reaction rate and supported Pt dispersion. The effects in both catalytic activity and deactivation resistance could be explained by the metal-support contact theory, which relates the atomic coordination of promoters with changes in catalytic activity and chemisorption properties of Pt supported on TiO₂.

Full text of DOI:10.1016/j.cattod.2013.10.019

Catalysis Today 220–222 (2014) 61–65
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Dehydrocyclization of n-heptane over Pt catalysts supported on
Al- and Si-promoted TiO2
Gustavo Pérez Lópeza,∗, Román Ramírez Lópeza, Tomás Viverosb
a Departamento de Ingenieria Química Industrial, Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, Mexico
b Area de Ingeniería Química, Departamento de Ingeniería de Procesos e Hidráulica, Universidad Autónoma Metropolitana-Iztapalapa, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 March 2013
Received in revised form 7 October 2013
Accepted 8 October 2013
Available online 5 November 2013
The effect of content of Si- and Al-promoted TiO2 catalysts in the dehydrocyclization of n-heptane was
studied. The sol–gel method was used to prepare pure TiO2 and mixed oxides with 1%, 3% and 10% of Al2O3
and SiO2. The structure and surface characterization (XRD, N2 sorption, H2 chemisorption, NH3 uptake)
revealed that the promoted cations were dispersed in the TiO2 lattice, modifying structural and surface
properties of catalysts. Dehydrocyclization activity and toluene yields were useful test reaction to detect
the effect of content of cations in both acidic and catalytic properties of promoted catalysts. Increase in
the TOF was observed as a function of cation content, hand in hand with the increase of overall reaction
rate and supported Pt dispersion. The effects in both catalytic activity and deactivation resistance could
be explained by the metal-support contact theory, which relates the atomic coordination of promoters
with changes in catalytic activity and chemisorption properties of Pt supported on TiO2.
© 2013 Published by Elsevier B.V.
Keywords:
Titania–silica
Titania–alumina
Mixed oxides
n-Heptane conversion
1. Introduction
of catalyst higher than the number of acidic sites for the respec-
tive components as pure oxides (TiO2 and SiO2). This phenomenon
It has been demonstrated [1,2] the effects on the strength and
distribution of the number of acid sites to promote TiO2 with Al and
Si, modifying its catalytic performance in several reaction systems.
Furthermore, Pt/TiO2 catalyst is a system which exhibits proper-
ties of strong metal-support interaction (SMSI), which could be
modified to promote cations on the support. It is interesting to
explore the application of Pt/TiO2 catalysts promoted with different
contents of Al and Si in reaction systems that are sensitive to both
the performance of the metallic phase as the acidic properties of
TiO2 promoted with Al and Si. The general mechanism of n-alkanes
dehydrocyclization functionality involves simultaneous metal and
acid sites [3–5]. Therefore, the conversion of n-heptane is a sen-
sitive reaction to the morphological properties of the supported
metal phase and the acidic properties of the support.
is similar to that showed in the combination of different pairs of
cations as mixed oxides, and it is relate to the formation of an effec-
tive charge by the mutual interaction between the charges of the
cations which is distributed according to the coordination num-
ber [11]. In particular for the mixed oxide of TiO2–SiO2, TiO2 being
the major component, the hypothesis predicts forming Brønsted
and Lewis acid sites for various combinations of coordination het-
eroatom bonds. Thus, the catalytic properties of TiO2–SiO2 mixed
oxide have been exploited in reactions which require the presence
of Lewis or Brønsted acidity [6–9,12–15]. In these studies, it was
reported that site density (number of acid sites per m2) decreased
due to the simultaneous increase of both acidity and BET surface
area as Si was added. At the same time, with the addition of low con-
tent of Si into TiO2, a decrease in the anatase crystal size is detected
by X-ray diffraction, involving to crystal edge effects in generation
of acid sites.
The addition of the Si+4 on TiO2 modifies both structural prop-
erties and porosity, enhancing surface area as a function of added
Si content [6–10]. The TiO2–SiO2 mixed oxide has been studied
with various molar ratios in its formulation, and effect of Si con-
tent on acid–base properties of surface was observed. It has been
found that the combination of SiO2 and TiO2 in the synthesis of
TiO2–SiO2 mixed oxides generates a number of acidic sites per gram
The addition of Al+3 in the TiO2 matrix for making TiO2–Al2O3
mixed oxides modifies both structural and surface properties,
enhancing specific surface area compared with pure components.
Addition of Al+3 into TiO2 increase the number of acidic sites per
gram greater than the pure components [16]. Unlike to TiO2–SiO2
mixed oxide, addition of Al+3 generated Lewis acid sites exclu-
sively [15], although it has been reported the presence of Brønsted
acidity on TiO2–Al2O3 mixed oxides [17,18] using the adsorption
of 2,6-dimethylpyridine (lutidine) as specific molecule to detect
Brønsted acidity. There are several research related to study of
TiO2–Al2O3 mixed oxides as carriers for catalytic phase, reporting
∗
Corresponding author at: Edificio-6, ESIQIE-IPN, Unidad Profesional Adolfo
López Mateos, Zacatenco, Delegación Gustavo A. Madero, D.F., México.
Tel.: +52 55 5568 6329.
E-mail address: gmperez@ipn.mx (G.P. López).
0920-5861/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.cattod.2013.10.019

62
G.P. López et al. / Catalysis Today 220–222 (2014) 61–65
that the addition of Al+3 into TiO2 not only improves several prop-
erties of catalytic support but also modifies the activity of catalytic
supported metal [19].
The effect of Al and Si content in acidic properties were analyzed
by ammonia uptake measurements, according to the procedure in
a flow analyzer set-up: a sample of 100 mg of catalyst was dried in
situ in 50 cc/min of helium flow at 400 ◦C for 1 h. Then, it was cooled
at 100 ◦C and saturated with a calibrated ammonia-helium gas mix
(5% mol NH3 95% mol He, PraxAir) using a gas flow of 20 cc/min
for 1 h, followed by gas flushing with helium at the same temper-
ature and flow. The ammonia desorption was carried out heating
the sample at 10 ◦C/min from 100 ◦C to 500 ◦C with a helium flow
of 100 cc/min. The ammonia uptake was measured by thermal con-
ductivity detector (Gow-Mac).
Several researches related to the application of TiO2 and
TiO2–SiO2 and TiO2–Al2O3 mixed oxides as catalytic supports of
metals have been reported [10,19–24], being notably greater the
number of researches related to M/TiO2 catalytic system com-
pared to the number of researches related to both M/TiO2–SiO2
and M/TiO2–Al2O3, emphasizing the study of the phenomenon
of metal-support interaction (SMSI) in M/TiO2 catalytic system
[25–27]. In researches related to M/TiO2–SiO2 or M/TiO2–Al2O3,
it has been reported that the catalytic performance of supported
metal responds to the type and content of added cation and it has
been attributed to changes the interaction between the support
and the active phase supported [10,24,28–30], resulting a catalyst
with enhanced activity in respect to catalysts supported over sin-
gle oxides. For TiO2 supported catalysts, the addition of a cation
into the TiO2 lattice may cause changes in the electronic properties
of the support, whereas the electronic charge balance between the
metal and the support is sensitive to the semiconducting properties
of TiO2 [31]. Thus, electronic interaction of transition metals sup-
ported on TiO2 can be modified by the incorporation of an element
that interferes with the original semi conduction properties of pure
TiO2. But in addition to the effects on the interaction between the
metal and the support, it also must consider the effects of the addi-
tion of Al+3 or Si+4 in the structural properties, porosity, specific
area and surface acidity of TiO2 [32].
The effects of Al and Si addition in both acidic and catalytic
properties of Pt supported on TiO2 and TiO2–Al2O3 and TiO2–SiO2
mixed oxides are studied in this paper. The dehydrocyclization of
n-heptane is used as a model test reaction, taking advantage of
its reaction scheme that requires the alternating involvement of
both catalytic metal activity and the acidic activity to carry out the
transformation of the linear alkane. Also, the distribution of the
dehydrocyclization reaction products may determine the predom-
inant mechanism, cyclization or isomerization, and point out the
influence between metal and acidic functions in catalytic perfor-
mance.
The catalytic properties of these materials were tested by
the dehydrocyclization of n-heptane. The reaction was con-
ducted in an experimental set-up consisting of continuous gas
flow microreactor mounted in a tubular furnace with control
of temperature (Omega).
A Pyrex tubular fixed bed reactor
with 7 mm internal diameter was used. Reaction experimen-
tal runs were developed as follows: first step consist in the
charge of 50 mg of 1% Pt catalyst (100–120 mesh) and reduc-
tion at 500 ◦C for 12 h in a flow of 30 cc/min of H2 (PraxAir).
Second step consist in the feed of a gas current of hydrogen
saturated with n-heptane (Aldrich) to the reactor. Operating con-
dition during the reaction experiments were as follows: gas
flow = 50 cc/min, temperature = 500 ◦C, pressure = 585 Torr, hydro-
carbon partial pressure = 10.7 Torr, hydrogen–hydrocarbon molar
ratio H2/HC = 53.5, WHSV = 345 h−1. The analysis of reaction prod-
ucts stream was made by on-line sampling, which was injected to
a gas chromatograph GC (HP5790) with a SE30 Carbowax column
(10% weight) and equipped with a thermal conductivity detector
(TCD). Differential reactor operation was observed at these condi-
tions, with 10% of conversion or lower, and the reaction rate was
calculated according to Eq. (1):
FA · X
r =
(1)
W
where r is the reaction rate (␮mol g−1 s−1), W is the weight
of catalyst (g), FA is the n-heptane molar flow (mol s−1), and X is
the conversion. All reaction experimental runs were conducted on
stream for 3 h of continuous time, detecting the deactivation of cat-
alyst. Basic kinetics parameters were calculated according to Eq. (2)
for a flow differential reactor with pseudo first order reaction rate
and first order deactivation:
2. Materials and methods
W
X
k
Pure TiO2 and TiO2–Al2O3 and TiO2–SiO2 mixed oxides were
synthesized by sol–gel method, with 1%, 3% and 10% of Al2O3 and
1%, 3% and 10% of SiO2. The synthesis consisted in the simultaneous
hydrolysis and condensation of titanium isopropoxide (Aldrich),
tri-sec-butoxide (Aldrich) or tetraethyl ortho silicate (Aldrich) in
isopropyl alcohol (Baker) as solvent. The hydrolyzing agent was a
0.14 M aqueous solution of HNO3 (Baker). Excess of solvent was
removed by evaporation at room temperature. The xerogel was
dried at 110 ◦C for 12 h, followed by calcination at 500 ◦C for 4 h in
air flow. These oxides were used as a support to prepare 1% weight
of Pt catalysts by incipient wetness method, using an aqueous solu-
tion of hexachloroplatinic acid (Aldrich).
The BET specific surface area and pore volume of catalyst were
characterized by N2 physisorption at 77 K (Quantachrome), crystal
structure was analyzed by X-ray diffraction (D-500, Siemens). The
measurement of H2 uptake was carry out by volumetric method
(Micromeritics) for 50 mg samples reduced at 500 ◦C in 50 cc/min
H2 flow, flushing degasification by high vacuum at 500 ◦C for 12 h.
Samples were cooled and analyzed at 25 ◦C. The H2 uptake is
reported in ␮moles of H2 per gram of catalyst, and metal sup-
ported dispersion (%D) is calculated according to the relation of
atoms of platinum exposed in the metal crystallite surface (double
of H2 uptake) respect to the total platinum loaded. Samples were
reduced at the same temperature of reaction (500 ◦C)
=
exp[−kDt](1 − X)
(2)
v0
where v0 is the volumetric flow (L s−1), t is the time on stream (s), k is
the first order kinetic constant (L g−1 s−1), and kD is the deactivation
constant (s−1). This equation can be written like:
ꢀ
ꢁ
ꢀ
ꢁ
X
Wk
− ln
= kDt − ln
(3)
1 − X
v0
Eq. (3) corresponds to the linear correlation y = mx + b. Then, the
kinetic constant and deactivation constant can be estimated for
each catalyst:
v0 exp(−b)
kD = m, k =
(4)
W
Turnover frequency is calculated according to the relation of ini-
tial reaction rate and the number of metallic active sites, estimated
by H2 uptake and considering a sorption atomic ratio of 1:1 for H:Pt.
3. Results and discussion
Table 1 shows the results in the characterization of 1% Pt cata-
lysts calcined at 500 ◦C by X-ray diffraction, N2-physisorption and
NH3 uptake. Table 2 lists the results of characterization by H2
uptake, overall initial reaction rate and turnover frequency.

G.P. López et al. / Catalysis Today 220–222 (2014) 61–65
63
Table 1
Effect of content of Al2O3 and SiO2 in anatase crystal size, BET surface area, pore size properties and specific ammonia uptake for catalysts calcined at 500 ◦C.
Catalysts
%
XRD anatase crystal size (Å)
SBET (m2 g−1
)
Pore volume (cm3 g−1
)
Pore diameter (Å)
NH3 uptake (␮mol m−2
)
Pt/TiO2
–
1
3
10
1
3
114
92
82
43
101
94
112
154
216
287
163
217
290
0.12
0.12
0.21
0.42
0.22
0.23
0.28
43
31
39
59
54
42
38
3.10
2.71
2.10
1.68
2.54
2.44
2.14
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–SiO2
Pt/TiO2–SiO2
Pt/TiO2–SiO2
10
55
Table 2
Effect of content of Al2O3 and SiO2 in H2 uptake per gram of catalyst, metal supported dispersion, overall initial reaction rate and TOF for catalyst reduced at 500 ◦C.
Catalysts
%
H2 uptake (␮mol g−1
)
%D
Initial reaction rate (␮mol g−1 s−1
)
TOF (s−1
)
Pt/TiO2
–
1
3
10
1
3
6.8
11.5
15.0
13.4
7.3
27
41
43
48
34
41
49
3.10
2.71
2.10
1.68
2.54
2.44
2.14
0.017
0.024
0.022
0.028
0.032
0.036
0.022
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–SiO2
Pt/TiO2–SiO2
Pt/TiO2–SiO2
12.6
16.8
10
The effect of content of Al and Si in the anatase crystal size of
TiO2 indicates that the addition of cation into the TiO2 crystal lat-
tice modify the structural conformation of anatase, diminishing the
mean crystal size as the cation content increase. At the same time,
an increase in the BET surface area is observed as a consequence of
the cation addition, and more sensitive than the changes observed
in pore size properties (pore volume and pore diameter). These
results indicate that the addition of Al or Si in the synthesis of TiO2
correspond to the synthesis of mixed oxide with cations disperse
into the crystal lattice, modifying the structural and textural prop-
erties of these materials. The H2 chemisorption is also modified by
the addition of Al and Si, showing a noticeable increase in the H2
uptake per gram of catalyst as a function of Al and Si content. It is an
important fact indicating that the addition of cations to the support
not only modifies the structural properties of anatase but also the
H2 uptake of supported metal is increased as a function of cation
content in support, and therefore, its catalytic performance. It is
expected that hand in hand with the change in H2 uptake prop-
erties, the hydrogenation and dehydrogenation activities can be
modifying by the addition of cations to the support. These effects
on H2 uptake can be related with changing in the Pt crystallite size,
associated with the increase in the metal supported dispersion (%D)
according to the increase in BET surface area. However, additional
to the increase in Pt dispersion, change in H2 chemisorption could
be associated with an effect in the inherent semiconductor proper-
ties of TiO2 by addition of cations, changing the electronic exchange
in the metal-support interface as a function of Al or Si content. An
interesting results has been reported by Escobar et al. [32] related
to the effect of Al2O3–TiO2 mixed oxides supports in the metal sup-
ported dispersion, showing that the SMSI state can be extensively
modified by the interaction between Al and Ti.
Fig. 1. Effect of Al addition on the reaction rate on 1% Pt/TiO2 catalysts. (♦) 0%, (ꢀ)
1%, (ꢀ) 3%, (ꢀ) 10% Al2O3.
of mixed oxides, and therefore, changing their acidic properties.
These results in acidic density can be also explained according to
the effect of coordination Al and Si on the catalytic surface, and
also it would be possible an associated effect with changes in the
semiconductor properties of TiO2.
In Table 2, the overall initial reaction rate per gram of catalyst
to n-heptane dehydrocyclization and the catalytic activity per site
(TOF) are listed. These results indicate that there is a noticeable
improvement in the overall initial reaction rate as the Al and Si con-
tent is increased, achieving threefold activity for catalyst with 10%
Al2O3 and 10% SiO2, respect to the catalyst supported on pure TiO2.
In contrast, the addition of Al and Si generates moderate increase in
TOF according to the low structure sensitivity of hydrogenation and
dehydrogenation reactions. However, it is significant to observe
that the increase in TOF is lower for Al-promoted catalysts than
Si-promoted ones, achieving a twofold increase for 3% SiO2 cata-
lyst, showing that the increase in catalytic activity responds to the
type of promoted cation. Although the moderate TOF increase, it is
important to emphasize that these results demonstrate that is pos-
sible to detect a modification in the metallic activity by the addition
of Al and Si into TiO2. In others words, adding of cations into the
TiO2 support not only modify the structural and textural proper-
ties, but also has effect in the catalytic performance of supported
metal [32].
Another interesting result is the decrease in the specific uptake
of NH3 (␮mol m−2) by the content of cations in support, showing
that the acidic density is also affected by addition of cations. This
behavior can be explained according to the action of two simulta-
neous effects: decreasing of anatase crystal size and increasing of
specific surface area. The increase of surface acidity (NH3 per gram)
by addition of cations has their origin mainly by two contributions:
anatase crystal edge effects and heteroatom bond formation on
oxide surface (Ti
O Al, Ti O Si). On the other hand, increase of
surface area can be related to interaction of cations into anatase
crystal structure. It results in a decrease of specific number of acid
sites as a function of Al and Si content, showing that these cations
have the capacity to modify the electronic balance on the surface
Figs. 1 and 2 illustrate the effects of the content of Al and Si in
the overall reaction rate of n-heptane dehydrocyclization, showing

64
G.P. López et al. / Catalysis Today 220–222 (2014) 61–65
Table 3
Effect of content of Al2O3 and SiO2 in first order kinetic constant (k) and deactivation constant (kD) for catalyst reduced at 500 ◦C.
Catalysts
%
Kinetic constant k ×10−3 (L g−1 s−1
)
Deactivation constant kD ×10−3 (s−1
)
Pt/TiO2
–
1
3
10
1
3
0.357
0.742
0.979
0.870
0.556
1.027
1.324
0.070
0.089
0.075
0.074
0.077
0.164
0.113
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–SiO2
Pt/TiO2–SiO2
Pt/TiO2–SiO2
10
Table 4
Selectivity of n-heptane dehydrocyclization products on 1% Pt/TiO2 catalysts, TREAC = 500 ◦C, at time on stream t = 0.
Catalysts
%
C1–C6 (%)
Cyclo alienes (%)
Benzene (%)
iC7 (%)
C7 = (%)
Toluene (%)
Pt/TiO2
–
1
3
10
1
3
5.2
13.5
11.3
4.3
3.6
7.7
–
–
–
4.0
9.6
11.0
9.1
11.8
5.7
10.3
90.8
62.8
66.2
73.2
78.2
69.0
67.1
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–SiO2
Pt/TiO2–SiO2
Pt/TiO2–SiO2
3.0
2.3
6.9
2.4
10.0
3.3
3.7
3.6
1.6
–
7.6
–
7.4
5.6
4.9
4.1
–
10
14.3
5.0
Table 5
Selectivity of n-heptane dehydrocyclization products on 1% Pt/TiO2 catalysts, TREAC = 500 ◦C, at time on stream t = 3 h.
Catalysts
%
C1–C6 (%)
Cyclo alienes (%)
Benzene (%)
iC7 (%)
C7= (%)
Toluene (%)
Pt/TiO2
–
1
3
10
1
3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
15.8
15.2
14.1
11.7
15.3
21.8
13.2
84.2
73.1
77.9
88.3
84.7
78.2
75.4
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–Al2O3
Pt/TiO2–SiO2
Pt/TiO2–SiO2
Pt/TiO2–SiO2
3.8
2.8
–
–
–
7.9
5.2
–
–
–
10
5.3
6.1
both deactivation by the time on stream, and improving of reac-
tion rate as a function on cations content. Kinetic and deactivation
constants are listed in Table 3. It is noticeable to observe that adding
of cations has an improvement in the catalytic activity, showing an
increase in kinetic constant as a function of Al and Si content, but
also the resistance to deactivation responds according to the type
of cation. It is important to observe that the Al-promoted catalysts
have no significant change in deactivation constant, in contrast to
the Si-promoted ones, showing a noticeable increase in kD with the
Si content. The deactivation of these catalysts is usually associated
with the acidic density, according to the fact that the deactivation
by carbon deposition is promoted by acid sites of catalyst. How-
ever, in this case, there is not a direct relationship between the
change in acidic density observed for these catalysts and deactiva-
tion resistance by promotion of cation, showing that deactivation
resistance is noticeable different according to the type of promoted
cation. In other words, these results show that deactivation resis-
tance in n-heptane dehydrocyclization is sensitive to the type of
promoted cation because the difference in atomic coordination in
the catalytic surface. Furthermore, the catalytic effects observed
by addition of cations can be associated with electronic transfer
effects, according to the results in the selectivity (Tables 4 and 5).
The main product is toluene for all samples, followed by hydro-
cracking production (C1–C6), isomerization (iC7), cyclization and
benzene selectivities. These results indicate that dehydrocycliza-
tion is the predominant reaction mechanism. At time on stream
t = 0, hydrocracking reaction has an important activity, it associated
with initial high activity of Pt, which is diminished by deactiva-
tion by coke deposition (Table 5). The same way happened with
cycloalkane, benzene and isomerization selectivity.
Changes in the activity of dehydrogenation, cyclization and
aromatization can be associated to the effect of the support func-
tionality in the metallic phase [5], without ruling out the conversion
of n-heptane depends on the combination of both metal and acid
sites [3,4]. This results in selectivity are indicative of moderate
acidic strength that promotes toluene selectivity by the dehydro-
genation of n-heptane with the alkene formation as a first step
(metallic function), and cyclization by the action of moderate acidic
function as a second step. A third step by the metallic function is
needed to complete the toluene formation. Then, the addition of
cations has an important role in the metallic role of the catalyst,
not only increasing the TOF as a function of cation content but also
promoting the toluene selectivity.
The model of metal-semiconductor contact theory has been pro-
posed to explain the changes in the TOF of the Pt supported on Al-
and Si-promoted TiO2. Ioannides and Verykios [31] have suggested
that the catalytic activity of metal supported on TiO2 depends on
the electronic properties of the support, in particular of electronic
Fig. 2. Effect of Si addition on the reaction rate on 1% Pt/TiO2 catalysts. (♦) 0%, (ꢀ)
1%, (ꢀ) 3%, (ꢀ) 10% SiO2.

G.P. López et al. / Catalysis Today 220–222 (2014) 61–65
65
interaction in the metal-support inter-phase, which is feasible to
modify by addition of cations inside the crystal lattice of TiO2
(cation doping) in order to handle the electronic transfer between
the metal and the support. According to this theory, the valence of
the cation added to the support plays a major role in electronic bal-
ance between metal and support, since both the value of the valence
and cation content modified potential electronic charge transfer
(work function the support, ˚S), being the difference between the
charge transfer potential of the metal and the support (work func-
tion metal and support ˚M − ˚S) the driving force for the electronic
transfer from the metal to support or vice versa.
toluene as a main product, showing that the metallic function is
promoted over the acidic function. The increase in dehydrogenation
and aromatization activity by the addition of Al and Si is asso-
ciated with a moderate acidic activity of support since the low
hydrogenolysis and isomerization selectivity. Moderate increase in
the TOF is observed as a function of cation content, hand in hand
with the increase of the H2 uptake (Pt dispersion) and a notice-
able increase in overall reaction rate. This effect in TOF could be
explained by the metal-support contact theory, which explains the
effects in the catalytic activity and H2 uptake properties of Pt sup-
ported on TiO2 by changes in the electronic transfer between metal
and support due the addition of cation into the TiO2 crystal lattice.
Also, the distinctive effect of promoters in deactivation resistance
can be associated with the difference in atomic coordination of Al
and Si on the catalytic surface.
The addition of cation into the network of TiO2, with different
valence to +4 (Al+3, W+6) means an aggregate number of species in
the donor or recipient charging, creating donors or recipients levels
of potential between the conduction bands and valence (band gap)
and causing a shift in the electrochemical potential of the support
(energy or Fermi level, EF). The charge transfer potential of a solid
(˚M − ˚S) is determined as the difference between the electrostatic
potential (potential energy of a free electron) and the electrochem-
ical potential of the electrons inside the solid (Fermi level) [33]
therefore ˚M − ˚S is the measure of the energy required to trans-
port charge of the solid (or metal support) and transported in the
form of free electrons. By modifying the value of the work function
of support ˚S with the addition of cations, the transfer potential
between the pair of metal support ˚M − ˚S is also changed, result-
ing in an offset charge from the metal to the support or vice versa,
depending on the valence of the cation content and added. The the-
ory predicts that the addition of cations into the support of M/TiO2
References
[1] H. Nakabayashi, Bull. Chem. Soc. Jpn. 65 (1992) 914–916.
[2] C. Beck, T. Mallat, T. Bürgi, A. Baiker, J. Catal. 204 (2001) 428–439.
[3] K. Tanabe, M. Misono, Y. Ono, H. Hattori, Stud. Surf. Sci. Catal. 51 (1989)
303–308.
[4] J. Fung, I. Wang, J. Catal. 130 (1991) 577–587.
[5] G. Del Angel, G. Torres, V. Bertin, Stud. Surf. Sci. Catal. 130 (2000) 2531–2536.
[6] S.M. Jung, P. Grange, Appl. Catal. A: General 228 (2002) 65–73.
[7] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233.
[8] Z. Liu, J. Tabora, R.J. Davis, J. Catal. 149 (1994) 117.
[9] Z. Liu, G.M. Crumbaugh, R.J. Davis, J. Catal. 159 (1996) 83.
[10] M.S. Rana, S.K. Maity, J. Ancheyta, G. Murali Dhar, T.S.R. Prasada Rao, Appl. Catal.
A: General 258 (2004) 215–225.
[11] K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solids Acids and Bases, Amster-
dam, Elsevier, 1989.
[12] C. Beck, T. Mallat, T. Bürgi, A. Baiker, J. Catal. 204 (2001) 428.
[13] J.M. Fraile, J.I. García, J.A. Mayoral, E. Vispe, J. Catal. 189 (2000) 40–51.
[14] M. Buecher-Skoda, R. Gill, D. Vu, C. Nguyen, G. Larsen, Appl. Catal. A 185 (1999)
301–310.
[15] H. Nakabayashi, Bull. Chem. Soc. Jpn. 65 (1992) 914.
[16] E. Rodenas, T. Yamaguchi, H. Hattori, K. Tanabe, J. Catal. 69 (1981) 434.
[17] C. Lahousse, F. Maugé, J. Bachellier, J.-C. Lavalley, J. Chem. Soc.: Faraday Trans.
91 (1995) 2907–2912.
[18] K. Tanabe, W.F. Holderich, Appl. Catal. A 181 (1999) 399–434.
[19] E. Romero-Pascual, G. Pérez, A. Montoya, C. Guimon, T. Viveros, A. Monzón,
Catalyst deactivation 1999, Stud. Surf. Sci. Catal. 126 (1999) 477.
[20] M. Kantcheva, M.U. Kucukkal, S. Suzer, J. Catal. 190 (2000) 144–156.
[21] S. Won Ahn, L. Kevan, J. Chem. Soc.: Faraday Trans. 94 (1998) 3147–3153.
[22] T.M. Salama, H. Hattori, H. Kita, K. Evitan, T. Tanaka, J. Chem. Soc.: Faraday Trans
89 (1993) 2067–2073.
system with a valence higher than the valence of Ti+4 (like W+6
)
results in a greater difference between ˚M − ˚S in comparison with
the addition of a cation equal or lower valence (Si+4, Al+3) to the
valence of Ti+4, so that the effects of charge shift between the metal-
support inter-phase. In support of this theory, the authors report
the results for Pt catalysts supported on TiO2 which is observed in
the decline in both catalytic activity and hydrogen chemisorption
using a promoter with higher valencies than +4, while these prop-
erties are concerned with the addition of promoters with lower
valences than +4. It means that the charge transfer at the metal-
support inter phase can be changed by modifying the electronic
structure of supported metal crystallites, and thereby promote or
inhibit both the catalytic properties and hydrogen chemisorption.
Then, the effects of Al and Si addition in both H2 uptake and the
catalytic activity of Pt/TiO2 catalysts could be explained by the
metal-semiconductor contact theory, showing that the changes in
the toluene yield and TOF are promoted by changes in the electronic
interaction between metal and support inter phase. Additionally
and according to the effect in supported Pt pointed out above, the
differences in deactivation resistance observed for these catalysts
can be related to the type of atomic coordination of promoters in
the TiO2, showing that the deactivation constant is more sensitive
for Si-promoted catalyst than the Al-promoted ones [34].
[23] V. Harlé, M. Vrinat, J.P. Scharff, B. Durand, J.P. Deloume, Appl. Catal. 196 (2000)
261–269.
[24] G.M. Dhar, B.N. Srinivas, M.S. Rana, M. Kumar, S.K. Maity, Catal. Today 86 (2003)
45–60.
[25] R. Burch, in: Z. Paál, P.G. Menon (Eds.), Strong Metal-Support Interactions in
Hydrogen Effects in Catalysis, Dekker, New York, 1988, pp. 347–372.
[26] G.L. Haller, D.E. Resasco, Adv. Catal. 36 (1989) 173–235.
[27] P. Li, P.A. Crozier, N. Nag, J. Liu, Extended Abstracts of 13th International
Congress on Catalysis, O1-049, Paris, France, 2004.
[28] G. Wan, A. Duan, Z. Zhao, G. Jian, D. Zhian, R. Li, T. Dou, K.H. Chung, Energy Fuels
23 (1) (2009) 81–85.
[29] S. Nun˜ez, J. Escobar, A. Vazquez, J.A. De Los Reyes, M. Hernandez-Barrera, Mater.
Chem. Phys. 126 (2011) 237–247.
[30] C.-M. Lu, Y.-M. Lin, I. Wang, Appl. Catal. A 198 (1–2) (2000) 223–234.
[31] T. Ioannides, X. Verykios, J. Catal. 161 (1996) 560–569.
[32] J. Escobar, J.A. De Los Reyes, T. Viveros, M.C. Barrera, Ind. Eng. Chem. Res. 45
(2006) 5693–5700.
[33] S.R. Morrison, The Chemical Physics of Surfaces, 2nd ed., Plenum Press, New
York, 1990.
[34] G.F. Froment, Catalyst Deactivation 1997, in: C.H. Bartholomew, G.A. Fuentes
(Eds.), Studies in Surface Science and Catalysis 111, Elsevier Science B.V., 1997,
pp. 53–68.
4. Conclusions
The addition of Al+3 and Si+4 into TiO2 lattice increase the
overall catalytic activity of n-heptane dehydrocyclization for Pt cat-
alysts supported on TiO2–Al2O3 and TiO2–SiO2 mixed oxides, with