H. Al-Kandari et al. / Applied Catalysis A: General 417–418 (2012) 298–305
301
following the reduction of MoTi at 673 K. Considerable decrease in
the relative intensities of these two absorptions is observed in the
spectrum taken of adsorbed Py on reduced KMoTi catalyst (Fig. 5).
This relative decrease in intensity may be attributed to the fact
that potassium-alkalization seems to convert acidic Mo OH into
Mo OK species. Research is under way in order to evaluate the
changes in the relative intensities of these two absorptions as a
function of K and Cs relative concentrations.
3.2. Catalytic measurements
(b)
The effect of platinum and cesium additions on the catalytic
properties of MoO2 (OH)y catalyst generated on reduced MoTi
−x
surface is assessed towards the different reactions of 1-heptene, n-
heptane and methyl cyclohexane. The choice of these compounds
resides in the fact that specific catalytic functions (viz., metal-
lic, acidic and the bifunctional metallic-acidic) are required for
well established hydrogenation/dehydrogenation and isomeriza-
tion reactions. Consequently, we should be able to obtain valuable
information concerning the active species on the basis of the prod-
uct distribution of each reaction. Characterization of these systems
by the different surface techniques presented above will be taken
into consideration in order to assign the possible active site(s)
responsible for a given catalytic behavior. In the following, we
present the catalytic data obtained for each system following their
activation (reduction) in a stream of hydrogen at 673 K for 12 h.
This treatment corresponds to the formation of a stable bifunctional
MoO2−x(OH)y structure on TiO2 [12,29].
(
a)
2
26
228
230
232
234
236
238
240
Binding Energy (eV)
Fig. 4. The XPS of the Mo(3d) energy region of 5% Cs–MoO3/TiO2 (a) before reduction
b) after reduction by hydrogen at 673 K for 12 h.
(
3.2.1. 1-Heptene reactions
cesium-alkalization of the MoTi catalyst may be considered to lead
to partial depolymerization of the polymolybbdates present on the
titania surface [24]. In this structure, the metallic function in terms
of density of states at the Fermi level was clearly observed in XPS
obtained over the VB energy region. Generation of reduced Mo4
state in an oxide mixture containing Mo, V and W was previously
found to catalyze the oxidation of methanol [25]. These results
clearly indicate that the reduction process of MoO3 by hydrogen to
lower valency state(s) at 673 K for 12 h: MoO → Mo O5 → MoO
Fig. 6 shows that complete conversion of 1-heptene molecules
takes place on the surface of reduced MoTi (i.e., MoO2−x(OH)y/TiO2)
at reaction temperatures between 423 and 673 K. Hydrogena-
tion of 1-heptene to n-heptane is the dominant transformation
with 94.7% at 423 K. This concentration decreases to 71.6% at
473 K and to 40.3% at 523 K. The branched iC7, mainly 2 and 3 –
methylhexanes products present at thermodynamic equilibrium
of 2MH/3MH = 1.0, increased from 5.3% at 423 K to 59.6% at 573 K.
Beyond 573 K, different hydrocracking processes become domi-
nant.
Similar catalytic behaviors were observed on reduced Pt-
modified MoTi catalyst samples of Pt. Afar from a slight increase
to 96.6% of hydrogenation to n-heptane at 423 K on the 0.5-PtMoTi
catalyst sample, product distribution as a function of reaction tem-
perature resembled what is observed on in the unmodified MoTi
catalyst (Fig. 6). These results may be attributed to Pt particle
formation on the PtMoTi catalyst surface without causing any mod-
+
3
2
2
is more efficient in the presence of Cs and to lesser extent with Pt
Table 1).
(
3.1.3. In situ IR spectra of adsorbed Py
In order to demonstrate the presence of acidic function(s) on
MoTi and the effect of K or Cs additives on this function, we present
in Fig. 5 in situ IR spectra of irreversibly adsorbed pyridine on MoTi
and KMoTi catalyst samples following partial reduction of the sam-
ples at 673 K [26,27]. The spectrum taken of pyridine adsorbed on
pure TiO2 (Py/Ti) is also presented for comparison purposes. As
ification to the chemical structure of the active MoO
surface species. The slight increase in the hydrogenation capac-
ity of this system is simply due to the enhancement of the
(OH)y/TiO2
2−x
−
1
could be observed in Fig. 5, two absorptions at 1636 and 1536 cm
metallic character of the bifunctional MoO
tem.
(OH)y/TiO2 sys-
are uniquely monitored for Py/MoTi and Py/KMoTi. These two
absorptions are diagnostic of pyridinum ion formation (i.e., proto-
nated pyridine on BrØnsted acid sites) [28]. Formation of pyridinum
ions (BPy species, Fig. 5) is indicative of the generation of BrØnsted
2−x
A different catalytic behavior takes place on the cesium-
modified CsMoTi catalyst samples (Fig. 7). Regardless of the
reaction temperature up to 673 K, hydrogenation to n-heptane
constitutes the dominant catalytic process. Negligible concentra-
tions of iC7 branched products were observed as compared to
cracking products at higher temperatures (Fig. 7). Taking into
consideration that the hydrogenation capacity of this CsMoTi
catalyst, is due to the metallic function, the BrØnsted Mo OH
acid sites (viz., Mo OH representing the MoO2 (OH)y structure)
−x
Table 1
XP spectra relative concentrations of different Mo oxides species produced following
the exposure of each of MoTi, 5-PtMoTi and 5-CsMoTi systems to hydrogen at 673 K
for 12 h.
acidic sites of the MoO2 (OH)y/TiO2 active species seems to be
−x
Catalyst
XPS relative concentrations %
either absent or deactivated. Most probably, this is due to the
conversion of Mo OH into Mo OCs moiety during the reduc-
tion process of MoO3 to MoO2 on the catalyst surface. This
implies that the dissociation process of H2 to active H atoms does
not restore the BrØnsted Mo OH acidic groups. Consequently,
MoO2
Mo2O5
MoO3
MoTi
58.1
61.7
91.3
23.3
32.2
6.5
18.6
6.1
2.2
5-PtMoTi
-CsMoTi
5