Acid-redox properties of titania-supported 12-molybdophosphates for methanol oxidation

Article abstract of DOI:10.1016/S1381-1169(98)00279-9

Physicochemical and catalytic properties of titania-supported 12- molybdophosphates have been investigated. The samples were characterized by the S(BET) method, Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies, temperature-programmed desorption (TPD) of ammonia and temperature-programmed reduction (TPR). The effect of temperature pretreatment on the catalytic behaviour of the samples in methanol oxidation was investigated. A significant change of selectivity to the main reaction products: dimethyl ether (DME) and formaldehyde (HCHO), as a function of the acid properties of the catalysts was observed. The highest selectivity to DME at 523 K suggested that the undecomposed molybdophosphoric acid (HPMo) on titania behaves as an acid catalyst. Pretreatment of the samples at higher temperatures (up to 723 K) and replacement of the protons in HPMo by cations (Co and Ni) lead to development of the redox properties (HCHO formation) of the catalyst, due to the suppression of the Bronsted acidity. The IR and XPS results provided clear evidence for the preservation of Keggin unit up to 623 K pretreatment after test reaction.

Full text of DOI:10.1016/S1381-1169(98)00279-9

Journal of Molecular Catalysis A: Chemical 142 Ž1999. 85–100
Acid-redox properties of titania-supported
2-molybdophosphates for methanol oxidation
1
a
b
c,)
S. Damyanova , M.L. Cubeiro , J.L.G. Fierro
a
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
b
Facultad de Ciencias, UniÕersidad Central de Venezuela, Apdo. 47102, Caracas, Venezuela
c
Instituto de Cat a´ lisis y Petroleoqu ´ı mica, CSIC, Cantoblanco, 28049 Madrid, Spain
Received 30 April 1998; accepted 4 August 1998
Abstract
Physicochemical and catalytic properties of titania-supported 12-molybdophosphates have been investigated. The samples
were characterized by the SŽBET. method, Fourier-transform infrared ŽFTIR. and X-ray photoelectron ŽXPS. spectroscopies,
temperature-programmed desorption ŽTPD. of ammonia and temperature-programmed reduction ŽTPR.. The effect of
temperature pretreatment on the catalytic behaviour of the samples in methanol oxidation was investigated. A significant
change of selectivity to the main reaction products: dimethyl ether ŽDME. and formaldehyde ŽHCHO., as a function of the
acid properties of the catalysts was observed. The highest selectivity to DME at 523 K suggested that the undecomposed
molybdophosphoric acid ŽHPMo. on titania behaves as an acid catalyst. Pretreatment of the samples at higher temperatures
Žup to 723 K. and replacement of the protons in HPMo by cations ŽCo and Ni. lead to development of the redox properties
ŽHCHO formation. of the catalyst, due to the suppression of the Bronsted acidity. The IR and XPS results provided clear
¨
evidence for the preservation of Keggin unit up to 623 K pretreatment after test reaction. q 1999 Elsevier Science B.V. All
rights reserved.
Keywords: 12-Molybdophosphates; Titania; FTIR; TPD; TPR; XPS; Methanol oxidation
1
. Introduction
concentrated on compounds such as H PW O
3
12 40
or PMo O , which possess a Keggin structure
12
40
Due to their unique combination of acid-base
w3–7x in which 12-edge-sharing octahedra of
oxygen atoms with a central atom of W or Mo,
surround and share atoms with a central tetrahe-
dron. These bulk compounds catalyse many re-
actions much more effectively than the conven-
tional protonic acids, such as sulfuric and nitric
acids. It has been proposed that the high cat-
alytic efficiency of these heteropoly acids is
essentially due to specific properties of the het-
eropoly anion which can be characterized by
very weak basicity and great softness, in addi-
and redox properties the heteropolyoxometal-
lates have been used successfully as catalysts in
their acidic form or in their cationic exchanged
or substituted form for acid and redox catalyzed
reactions in both homogeneous and heteroge-
neous media w1,2x. Many of these studies have
)
Corresponding author. Fax: q34-1-585-4760; E-mail:
jlgfierro@icp.csic.es
1
381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S1381-1169Ž98.00279-9

8
6
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
tion to a large molecular size. When supported
on a suitable carrier, they also work as active
solid acid catalysts to be comparable, or to
exceed, supported phosphoric acid, silica-
alumina and acidic zeolites.
An essential drawback of these compounds is
their low thermal stability, dehydration of het-
eropoly acid HPA with subsequent destruction
Ž .
of the Keggin’s structure and formation of oxide
compounds occurs during thermal treatment
w8,9x. Accordingly, a number of studies have
focused on the thermal stability of heteropoly
acids by supporting them on porous substrates
w10–18x. It has been reported that silica w10–12x
and carbon w13,14x are good carriers of these
supported 12-molybdophosphates. It has been
observed that a titania-supported heteropoly an-
ion preserved the Keggin unit up to a higher
temperature than that of an unsupported one.
Therefore, it is of great interest to investigate
the influence of the modifications of the struc-
ture and properties of titania-supported het-
eropoly compounds on their behaviour in cat-
alytic reaction. Methanol oxidation reaction was
chosen as a test reaction to characterize the
surface properties of the catalysts w20x. Our
preliminary results w20x showed that the pres-
ence of acidic and basic sites on the surface of
these catalysts is required for the partial oxida-
tion of methanol to dimethyl ether and formal-
dehyde, respectively. In this work, we attempt
to investigate in more detail the influence of the
physicochemical properties and the structure of
the different phases generated upon pretreat-
ments at different temperatures of titania-sup-
ported 12-molybdophosphate catalysts on the
methanol oxidation reaction.
heteropoly compounds. The stability of silica-
supported 12-molybdophosphoric and 12-
molybdosilic acids up to 846 K has been shown
by Kasztelan et al. w15x. Rocchiccioli-Deltcheff
et al. w16x demonstrated that the 12-molybdo-
phosphoric acid
lower temperature for silica-supported HPMo
523 K than for the unsupported one by forma-
HPMo destruction occurs at
Ž .
Ž
.
tion of a mixture of b- and a-MoO phases. In
3
addition, the transformation into b-MoO be-
gins at 523 K and ends at 623 K w11x. Contrary
3
2
. Experimental
¨
w x
to that, Fricke and Ohlman 17 have shown that
the thermal stability of HPMo decreases when
supported on silica; the lower concentration on
the support, the lower stability. Cheng and
Luthra w18x studying HPMo on alumina have
2
.1. Catalyst preparation
Commercially available
Ž
Aldrich, reagent
grade 12-molybdophosphoric acid, abbreviated
.
concluded that the decomposition of polyanion
occurs as a result of the strong interaction be-
tween the support and heteropolyanion, leading
to the formation of the aluminium salt.
as HPMo, was used as a starting material. Ni-
and Co-salts were prepared from aqueous solu-
tions of the parent HPMo, to which the stoichio-
metric amounts of Ni- and Co-nitrates
Ž
Merck,
Taking into account the numerous studies on
the thermal stability or catalytic activity of the
supported heteropoly compounds it would be of
great interest to study the thermal and catalytic
behaviours of these compounds when are sup-
ported on titania support. Our previous work
w19x has shown that the combination of XRD,
reagent grade were added, as described by
Tsigdinos w21x. The TiO support was prepared
by hydrolysis of TiCl followed by dehydration
.
2
4
of Ti
tion at 773 K. The BET area of the powder
ŽOH.4 xerogel at 373 K and then calcina-
2
y1
sample was 70 m g
and phase proportion
resulted in 85% anatase and 15% rutile. The
TDG, IR, Raman and X-ray photoelectron
Ž
XPS
.
titania-supported HPMo
and its Co and Ni salts
Ž
denoted as HPMorT
denoted as CoPMorT
were prepared by
conventional wetness impregnation of TiO
.
spectroscopic techniques can provide informa-
tion regarding the surface intermediates which
are formed upon thermal treatment of titania-
Ž
and NiPMorT, respectively
.
2

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
87
powder with aqueous solutions of purified
HPMo and its corresponding salts, respectively.
The impregnates were then dried at 343 K for 4
h. For characterization studies the samples were
heated in air at different temperatures while
keeping each temperature for 2 h. Subsequently,
they were cooled to room temperature and stored
in a dessicator. The spent catalysts were col-
lected in isoctane in order to prevent any further
contact with air prior to analysis.
2.2.3. Fourier-transform IR spectroscopy (FTIR)
Lattice vibrations and the IR spectra of
chemisorbed molecular probes were recorded on
an FTIR Nicolet 5 ZDX spectrophotometer
y
1
working at a resolution of 4 cm and averaged
over 100 scans. The framework vibration spec-
y
1
tra were recorded in the region 1200–600 cm
from self-supporting wafers prepared by pellet-
ing the samples 1:100 diluted in KBr. The
vibration spectra of chemisorbed pyridine were
y
1
recorded in the region 1700–1300 cm . A
special IR cell fitted with greaseless stopcocks
and KBr windows was used for this purpose.
Self-supporting wafers of the catalysts were out-
2
2
.2. Techniques and procedures
.2.1. Chemical analysis
y
4
The elemental analysis of catalyst con-
gassed at 10
Torr 1 Torrs133.33 Pa at
Ž .
stituents
Ž
Mo, P, and Ni or Co
.
was determined
523, 623 and 723 K for 1 h. After cooling to
room temperature, the samples were exposed to
ca. 1.5 Torr pyridine and the physically ad-
sorbed fraction removed by outgassing at 393 K
for 1 h before the spectra were recorded. The
net spectrum of adsorbed pyridine was obtained
by calculating the difference between the total
and background spectra.
by atomic absorption spectrophotometry using a
Perkin Elmer 3030 instrument. The solid sam-
ples were first acid digested in a microwave
oven for 2 h, and then aliquots of solution were
3
diluted to 50 cm using deionized water
Ž
m-Rho
.
quality . The chemical analysis data of the sam-
ples is listed in Table 1.
2
.2.2. BET area
Specific area of the catalysts was computed
2.2.4. Temperature-programmed desorption
(TPD) of ammonia
according to the BET method from the nitrogen
adsorption isotherms obtained at 77 K, taking a
value of 0.162 nm for the cross-section of the
TPD patterns of chemisorbed ammonia were
2
recorded with a quadrupole mass filter
QMS 100 connected on-line with a microreac-
tor. The samples ca. 0.50 g were placed in a
glass reactor and heated at a rate of 10 K min
Ž
Balzers
adsorbed N molecule at that temperature. The
.
2
nitrogen adsorption isotherms were measured in
an automatic Micromeritics ASAP 2000 appara-
tus, selecting only the range of relative pres-
Ž
.
y
1
3
y1
in an Ar flow 100 cm min for 2 h, and then
cooled to room temperature under the same
atmosphere. Many products were desorbed dur-
ing catalyst pretreatment. The principal mo-
sures 0.05-PrP -0.30 for BET calculation.
o
BET areas of catalysts are summarized in Table
1
.
lecules
by recording the masses mrz of 18, 28, 32 and
4. Other mrz masses were also recorded in
order to monitor destruction of the NH
ŽH O, CO, O , and CO2. were followed
2 2
Table 1
Chemical analysis and SBET of the samples
4
a
3
Sample
MorTi
NiŽCo.rTi
PrTi
S
Žm
BET
2
y1.
molecule
Ž
mrz of 16, 28, 44 and 46 for NH ,
g
3
N , N O and NO molecules, respectively
.
. A
HPMorT
NiPMorT
CoPMorT
0.282
0.296
0.291
–
–
0.002
0.003
0.003
–
52
57
63
70
2
2
2
0.014
0.014
–
flow of 900 ppm NH in Ar was passed over
3
the samples at 313 K until all the masses reached
a constant level. Then, the physically adsorbed
ammonia was desorbed in a flow of pure Ar and
TiO
2
a
Atomic ratios.

8
8
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
subsequently the TPD profile was recorded upon
heating the sample at a rate of 10 K min up
to 843 K.
steel tubular reactor
of catalyst held between two layers of quartz
wool. A CH OH:O :Hes5:10:85 molar ratio
mixture was fed into the reactor at a rate of 100
6.2 mm i.d. using 0.05 g
Ž .
y
1
Ž
.
3
2
2
.2.5. X-ray photoelectron spectroscopy (XPS)
3
y1
cm min
methanol was fed by means of a liquid pump
Becton-Dickinson and vaporized in a pre-
at atmospheric pressure. Liquid
XP spectra were acquired with a VG ES-
CALAB 200R spectrometer equipped with a
Ž
.
hemispherical electron analyzer and Mg Ka
heater prior to the reactor inlet. The products
y
19
Ž
hns1253.6 eV, 1 eVs1.6302=10
J X-
.
were analyzed by an on-line gas chromatograph
ray source. The powder samples were pressed
into small Inox cylinders and then mounted on a
heater placed in a pretreatment chamber. Prior
to being moved into the analysis chamber the
fresh samples were evacuated in the pretreat-
ment chamber of the instrument from 523 to
Ž
GC, Varian 3400 CX provided with a TCD
.
and a Porapack N packed column. In order to
avoid condensation of the reactor effluents, the
outlet of the reactor-GC inlet was heated to 393
K. Blank runs were conducted with an empty
reactor without any detectable conversion. Pre-
treatment of the samples was done at the corre-
sponding calcination temperature for 1 h in an
O :Hes10:90
each run. Catalytic runs were obtained for 12–15
h, beginning 10 min after methanol was injected
through the reactor, on each sample at 483 and
7
23 K for 2 h, while the used samples were
evacuated at 333 K. The residual pressure in the
ion-pumped analysis chamber was maintained
molar ratio mixture prior to
Ž .
2
y
9
below 5=10
Torr during data acquisition.
The intensities of Mo3d, Co2p, Ni2p, P2p and
Ti2p3 peaks were estimated by calculating the
r2
integral of each peak after smoothing and sub-
traction of the ‘S-shaped’ background and fit-
ting the experimental curve to a combination of
Gaussian and Lorentzian lines of variable pro-
5
03 K, always in increasing order of tempera-
ture, to obtain conversion and selectivity data.
portion. The binding energies
Ž
BE
.
were refer-
3. Results
enced to the C1s peak at 284.9 eV giving BE
values with an accuracy of "0.1 eV.
3
.1. Framework Õibrations
The FTIR spectra of HPMorT and
2
(
.2.6. Temperature-programmed reduction
TPR)
TPR experiments were recorded on a Mi-
cromeritics instrument model TPRrTPD 2900
fitted with a thermo conductivity detector TCD
and controlled by a computer. The samples
0.025 g were pretreated at 423 K in a He
Co Ni PMorT catalysts before and after on-
Ž .
stream reaction are shown in Figs. 1 and 2,
respectively. For comparative purposes, the in-
frared spectrum of fresh bulk HPMo is also
included in Fig. 1. The IR spectra of bulk
Ž
.
HPMo
73 and 787 cm assigned to stretching vibra-
tions nas P–Od , nas Mo–Ot , nas Mo–O –Mo
and nas Mo–O –Mo , respectively, characteriz-
ing the Keggin unit w22x. As for the titania-sup-
ported HPMo and Co Ni PMo catalysts there
are strong framework vibration bands of the
Fig. 1A shows the bands at 1065, 962,
Ž .
Ž
.
y
1
8
stream for 1 h in order to remove surface con-
Ž
.
Ž
.
.
Ž
.
b
taminants. After cooling to room temperature, a
flow of 10% H in Ar was passed across the
samples and the temperature increased at a rate
of 10 K min
Ž
c
2
Ž
.
y
1
up to 873 K while the TCD
signal was recorded.
y
1
TiO2 carrier below 900 cm
which over-
shadow that coming from the Keggin structure,
no further attention will be paid to this region
for the supported HPMo and its salts. Conse-
quently, the Keggin unit on titania can be char-
2
.3. ActiÕity measurements
Catalytic experiments were performed in a
continuous flow system provided by a stainless

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
89
Fig. 1. IR spectra of fresh ŽA. and used ŽB. TiO -supported HPMo catalysts subjected to different pretreatment temperatures: Ža. 523 K, Žb.
2
6
23 K, and Žc. 723 K.
acterized only by its two highest frequencies
heating at 623 K, only small changes in the
spectrum of HPMorT sample are observed as a
consequence of a partial degradation of the
y
1
Ž
1065 and 962 cm
.
, which are identical to the
Figs. 1 and 2 . Upon
bulk HPMo up to 523 K
Ž
.
heteropoly anion w19x, a small band of MoO at
3
y
1
9
93 cm on the higher wavenumber side of the
stretching vibration nas Mo–Ot begins to ap-
Ž .
pear. However, a significant changes are ob-
served in the spectrum of the sample heated at
7
23 K. The HPMo is decomposed into molyb-
denum oxide phases a-MoO3 and b-MoO3
w19x
with preserved Keggin unit, as revealed by
the appearance of a visible band at 990 cm
and the simultaneous decrease of the main bands
Ž
.
y
1
y
1
at 1065 and 962 cm .
Structural changes brought about by catalytic
reaction are more clear, specifically for the cata-
lyst pretreated at higher temperatures
and 2 . The spectra of the spent samples previ-
Ž
Figs. 1B
.
ously calcined at 523 K differ only slightly from
that of the fresh calcined counterparts. Some
increase of the band width and a simultaneous
decrease in band intensity becomes apparent,
which can be related to some dehydration
andror a certain reduction during on-stream
Fig. 2. IR spectra of fresh Ža,b. and used Žc,d. TiO -supported
MPMo salt catalysts pretreated at 523 K: Ža,c. and Žb,d. corre-
spond to NiPMorT and CoPMorT, respectively.
2
Figs. 1B and 2 . A significant
operation w4x Ž .

9
0
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
Br o¨ nsted acidity
Ž .
, visible as a 1540 cmy band
1
decrease of the Keggin bands takes place on the
spent catalyst previously calcined at 623 K Fig.
with simultaneously appearance of a-MoO
Ž
of pyridinium ion. The observed drop in the
intensity of this band after outgassing at higher
temperatures can be assigned to the lack of
protons within the heteropolyanion structure
with which a strong interaction can occur. On
the one hand, some trapping of the Keggin
protons by OH groups of titania can occur with
increasing of the temperature, leading to the
1B
.
3
y
1
band at 993 cm
which is more discernible
compared to that of the fresh one. This can be
related to a certain reduction w4x during on-
stream operation, i.e., the appearance of a mix-
ture of oxidized and reduced form of 12-
molybdophosphoric acid. Finally, the Keggin
unit almost completely disappeared in the used
catalyst, calcined at 723 K, and the small band
q
formation of TiOH2 groups. This is similar to
the observation for silica-supported heteropoly
of MoO becomes significantly strong.
compounds w23,24x. On the other hand, the loss
3
of acid protons at higher temperatures can be
derived from their reaction with the lattice oxy-
gen atoms to produce water w25x. After out-
3
.2. Acid properties
Information about the mode of the acid cen-
gassing at 623 K, a band of Lewis acidity at
y
1
ters on the surface of supported catalysts was
1450 cm appears, which is related to isolated
6
q
previously provided w20x by an IR study of
Mo
cations of molybdenum oxide phase s
Ž .
pyridine adsorption. The IR spectra of HPMorT
formed at higher calcination temperature. Both
Lewis and Br o¨ nsted acidity is observed in
CoPMorT and NiPMorT samples. The strong
band of Lewis acidity in these samples may be
and Co Ni PMorT catalysts outgassed at differ-
Ž .
ent temperatures are presented in Fig. 3. As can
be seen pyridine on HPMorT at 523 K is
predominantly adsorbed in a protonated form
2
q
2q
assigned to Co
and Ni
cations. The in-
crease of Lewis acidity with increasing tempera-
ture could be related to the presence of some
d
q
unsaturated molybdenum ions, Mo
w26x. The Br o¨ nsted acidity for the supported
Ž
6FdF
4
.
Co and Ni salts of HPMo can be caused by the
incomplete stoichiometry of cationrpolyanion
andror some hydrolysis of the molybdophos-
phate anion during its preparation w6x. It has
been proposed w27x that protons can be gener-
ated by dissociation of water as a function of
electronegativity of the metal cations. Forma-
tion of Br o¨ nsted acidity in Co
Ni PMorT sam-
Ž .
ples Fig. 3 could result from this phenomenon
Ž
.
since water is removed during vacuum heating.
In to measure the catalyst acid strength distri-
bution, the ammonia TPD profiles of HPMorT
catalyst have been recorded and are displayed in
Fig. 4A. The broad ammonia desorption peaks
indicate the presence of centers of different acid
strength. The tendency to decrease the overall
acidity with increasing of the temperature pre-
treatment from 523 to 723 K is accompanied by
a shift of the maximum ammonia desorption
Fig. 3. IR spectra of adsorbed pyridine on the titania-supported
heteropolymolybdate catalysts: Ža. HPMorT pretreated at 523 K,
Žb. HPMorT pretreated at 623 K, Žc. HPMorT pretreated at 723
K, Žd. CoPMorT pretreated at 523 K, and Že. NiPMorT pre-
treated at 523 K.

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
91
Fig. 4. TPD profiles of adsorbed NH3 on HPMorT catalysts ŽA. subjected to different pretreatment temperatures: Ža. 523 K, Žb. 623 K, and
Žc. 723 K; and on MPMorT salt catalysts pretreated at 523 K ŽB.: Ža. HPMorT, Žb. CoPMorT and Žc. NiPMorT.
peak toward lower desorption temperatures, in-
dicating relatively weaker acid sites. A distinct
desorption peak situated at about 750–800 K
indicates the presence of strong acid centers that
disappear when the sample is pretreated at higher
temperatures. The maximum of this peak is
close to that of crystalline HPMo O w25x.
1
2
40
However, this desorption peak is broader than
Table 2
Binding energies ŽeV. of core electrons of XPMorT catalysts before and after reaction
a
Catalysts Žpretr.
O1s
Mo3d5
Ti2p
Ni2p
3r2
ŽCo2p3r2
P2p
r2
3r2
.
HPMorT-f Ž523 K.
HPMorT-u Ž523 K.
HPMorT-f Ž623 K.
HPMorT-u Ž623 K.
HPMorT-f Ž723 K.
HPMorT-u Ž723 K.
NiPMorT-f Ž523 K.
NiPMorT-u Ž523 K.
CoPMorT-f Ž523 K.
CoPMorT-u Ž523 K.
530.0 Ž80.
232.1
458.5
458.5
458.5
458.5
458.5
458.5
458.5
458.5
458.5
458.5
–
133.0
132.7
133.4
132.9
133.7
133.0
132.8
132.8
132.8
133.3
5
31.1 Ž20.
529.9 Ž70.
31.0 Ž30.
530.0 Ž83.
31.2 Ž17.
529.9 Ž77.
30.8 Ž23.
529.9 Ž86.
31.1 Ž14.
530.0 Ž81.
31.1 Ž19.
530.0 Ž82.
31.1 Ž18.
529.8 Ž75.
30.8 Ž25.
529.9 Ž78.
31.2 Ž22.
529.9 Ž77.
31.0 Ž23.
231.2 Ž22.
232.1 Ž78.
232.2
–
–
5
5
231.1 Ž20.
232.2 Ž80.
232.2
–
5
–
5
231.1 Ž22.
232.2 Ž78.
232.2
–
5
854.6
854.3
781.3
781.0
5
231.1 Ž26.
232.2 Ž74.
232.2
5
5
231.0 Ž28.
232.2 Ž72.
5
a
fsFresh; usUsed.

9
2
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
that of the crystalline sample. This indicates a
wider distribution of the strong acid sites as a
result of the interaction between the HPMo and
the TiO carrier.
2
The TPD profiles of ammonia from NiPMor
T and CoPMorT after pretreatment at 523 K
are presented in Fig. 4B. The relative qualitative
difference of these profiles when compared to
that of the parent HPMorT sample at the same
temperature, lies in their much more regular
desorption profiles. The maxima of ammonia
desorption occur approximately at the same
temperature ranges, which means that similar
strengths are involved, but in the lower tempera-
ture range their proportion is higher for
Ni Co PMorT samples. The peak at about 780
Ž .
K also remains in these samples, indicative of
the presence of relatively strong acid sites at the
surface. The participation of weak acid sites
coming from the TiO support would not be
2
ruled out as IR spectra revealed some Lewis
acidity on the titania surface
here
Ž
spectra not shown
.
.
3
.3. Surface analysis
Photoelectron spectroscopy has been used to
reveal not only the chemical state of the ele-
ments but also the changes in concentration
brought about by the catalytic reaction. The
binding energies
Ž
Ž
BE
.
of various core-levels
are
P2p, Mo3d5r2 and Co2p3r2 or Ni2p3r2
.
summarized in Table 2. The XP spectra of the
used HPMorT sample, pretreated at different
temperatures, are shown in Fig. 5. The BE of
the Mo3d5r2 core level for the fresh samples at
Fig. 5. Mo3d core level spectra of catalysts pretreated at different
6
q
2
32.2 eV is indicative of Mo ions Table 2 .
Ž .
temperatures and then used in the CH OH partial oxidation
3
However, the spectra of the catalysts subjected
to on-stream operation show the overlapping
reaction.
6
q
peaks of Mo
ions and another less intense
one at a BE of 231.1 eV associated with a lower
of HPMorT at the same temperature. The re-
5
q
oxidation state of Mo
Ž
Mo
ions
.
w26x. The
sults suggest that the surface molybdenum
fraction of reduced molybdenum slightly
changes for the used HPMorT samples with the
pretreatment temperature, but it increases for
NiPMorT and CoPMorT with respect to that
species, involved in the CH OH oxidation, in-
3
clude more than one species. The BE for
Ti2p3r2 , Co2p3r2 and Ni2p3r2 core-levels re-
main virtually unchanged before and after reac-

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
93
Table 3
Ž
creases with increasing of the temperature Ta-
ble 3 . A similar tendency is observed for the
Surface atomic ratios of XPMorT catalysts Žsubjected to different
.
temperatures of pretreatment. before and after reaction
fresh catalysts, indicating that breaking of the
large oxo-anions into smaller structures yields
well dispersed molybdenum oxide phases at 623
K. However, at higher pretreatment tempera-
tures there is some aggregation, caused by the
Catalystsa
ŽMorTi.at
ŽNirTi.
ŽPrTi.at
at
ŽCorTi.
at
HPMorT-f Ž523 K.
HPMorT-u Ž523 K.
HPMorT-f Ž623 K.
HPMorT-u Ž623 K.
HPMorT-f Ž723 K.
HPMorT-u Ž723 K.
NiPMorT-f Ž523 K.
NiPMorT-u Ž523 K.
CoPMorT-f Ž523 K.
CoPMorT-u Ž523 K.
0.317
0.657
0.433
0.456
0.413
0.447
0.474
0.770
0.375
0.579
–
–
–
–
–
–
0.050
0.059
0.064
0.071
0.066
0.047
0.046
0.065
0.045
0.077
formation of bulk MoO , as already revealed by
3
IR spectra. Therefore, it appears that thermal
0.077
0.143
0.067
0.070
decomposition of the HPA into MoO under-
3
goes some rearrangement of the heteropoly an-
ion forming an intermediate oxide compound
with preserved Keggin unit although the molyb-
denum ions are placed in a distorted octahedral
environment of oxide ions. This transformation
of HPA is favored during the methanol oxida-
tion reaction due to some reduction of molybde-
num species.
a
fsFresh; usUsed.
4
q
2q
tion
and Ni
of P2p for the used HPMorT catalyst decreases
with respect to its fresh counterpart Table 2
Ž
Table 2
.
and correspond to Ti , Co
2
q
ions, respectively. However, the BE
Ž
.
,
suggesting that the chemical environment of the
phosphorus changes under reaction conditions.
The O1s peak displays two components with
3.4. Redox properties
The redox properties of the catalysts have
been revealed by TPR in a hydrogen flow. The
Ž .
TPR profiles of the samples Fig. 6 display a
binding energies of 529.9 and 531.0 eV
: the former is related to oxo-molybdenum
Ž
Table
2
.
species, while the latter is probably due to the
OH groups on titania. This assignment is sup-
ported by the fact that the electron density in the
OH group is substantially lower than in the
lattice oxygen, and also because the intensity of
the component at ca. 531.0 eV decreases with
increasing of the pretreatment temperature.
From the peak intensities and atomic sensitiv-
single TPR peak with a maximum at 789, 762
ity factors, surface XPS atomic ratios have
Ž .
been computed w28x. Variations of the XPS
MorTi atomic ratios of the catalysts, subjected
to different pretreatment temperatures before and
after reaction are listed in Table 3. The values
of XPS MorTi ratios of the used catalysts are
higher than those of the fresh ones. The XPS
ratio for HPMorT at 523 K is the highest,
indicating that the exposure of HPMo is pre-
served during reaction. In addition, the higher
XPS MorTi ratios than the MorTi ratios de-
rived from chemical analysis Table 1 suggest
Ž .
that the components are well dispersed on the
Fig. 6. TPR profiles of TiO -supported molybdophosphate cata-
2
support surface. The XPS MorTi ratio de-
lysts: Ža. CoPMorT, Žb. HPMorT, and Žc. NiPMorT.

9
4
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
Table 4
Methanol oxidation over titania-supported 12-molybdophosphates subjected to different calcination temperatures at Treac.s503 K
Sample Žpretr.
Conversion Ž%.
Yields Ž%.
CH OCH
HCHO
HCOOCH3
CO2
3
3
HPMorT Ž523 K.
HPMorT Ž623 K.
HPMorT Ž723 K.
NiPMorT Ž523 K.
CoPMorT Ž523 K.
27.4 Ž18.5.
15.6 Ž5.0.
14.2 Ž6.0.
12.0 Ž4.4.
13.5 Ž5.5.
11.7 Ž12.4.
3.2 Ž2.1.
3.0 Ž2.2.
2.0 Ž1.7.
3.2 Ž2.4.
10.0 Ž3.6.
8.6 Ž2.9.
7.7 Ž3.8.
5.7 Ž2.7.
7.9 Ž3.1.
5.6
3.7
3.3
4.3
2.4
0.1
0.2
0.3
–
0.1
Values in parenthesis are data obtained at 483 K.
and 811 K for catalysts HPMorT, NiPMorT
and CoPMorT, respectively. The reduction
temperature maximum of the samples can be
attributed to the reduction of octahedrally coor-
sumption in catalyst HPMorT. This means that
the molybdenum in catalyst HPMorT is easily
reduced than in CoPMorT sample.
6
q
dinated Mo
valence state
of polymolybdates to a lower
3.5. Catalyst performance
5
q
4q
Ž
Mo
andror Mo
. w29x. The
highest hydrogen consumption is observed for
NiPMorT, probably as a consequence of the
hydrogen spillover produced by the reduced
nickel. The comparison of HPMorT and
CoPMorT catalysts reveals not only that the
former is reduced at somewhat lower tempera-
The effect of temperature pretreatment on the
catalytic behaviour of the samples was investi-
gated. The methanol conversion and the yields
of different products at reaction temperatures of
483 and 503 K are presented in Table 4. The
catalytic behaviour data of the samples at 503 K
are taken from Ref. w20x. The main products
ture than the latter, but also a higher H con-
2
Fig. 7. Dependence of DME ŽA. and HCHO ŽB. selectivity of HPMorT catalyst on pretreatment temperature at reaction temperatures of Ža.
83 K, and Žb. 503 K.
4

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
95
Fig. 8. Dependence of DME ŽA. and HCHO ŽB. selectivity of TiO -supported molybdophosphate catalysts on the cation at reaction
2
temperatures of Ža. 483 K, and Žb. 503 K.
observed from methanol oxidation are dime-
thyl ether CH OCH , DME , formaldehyde
HCHO and methyl formate HOOCCH3
Some traces of CO2 are observed at higher
reaction temperature. Changes in the selectivity
to DME and HCHO selectivity over titania-sup-
ported HPMo catalyst as a function of the pre-
treatment temperature are shown in Fig. 7. The
Ž
.
3
3
Ž
.
Ž
.
.
Fig. 9. Dependence of the dehydrogenation selectivity ŽDHG. of HPMorT on the pretreatment temperature ŽA. and on the cation ŽB. at
reaction temperatures of Ža. 483 K, and Žb. 503 K.

9
6
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
HPMorT sample calcined at 523 K exhibits the
highest DME selectivity and yield as well as the
catalytic behavior of these catalysts in CH OH
oxidation.
3
largest methanol conversion
Ž
Table 4
.
. After
The present IR results provide clear evidence
on the preservation of the Keggin unit up to 623
K pretreatment after test reaction, as revealed
increasing of the temperature from 523 to 623 K
for the HPMorT, a significant decrease of both
the selectivity and the yield of CH OCH are
by the XPS MorTi ratio
Table 3 , although
Ž .
3
3
observed
creasing the temperature up to 623 K leads to a
significant increase in HCHO selectivity Fig.
. A further increase of pretreatment tempera-
Ž
Fig. 7A and Table 4
.
. However, in-
with some partial decomposition. This fact is
not surprising, because under the conditions of
methanol oxidation ‘heteropolymolybdate blue’
w30x could be formed, and also there is water
Ž
7B
.
ture up to 723 K does not produce great changes
in the catalytic performance. The dependence of
the DME and HCHO selectivity on the change
of the protons in the HPMo acid by Co and Ni
cations is shown in Fig. 8. Replacement of the
protons by Co and Ni leads to a decreased DME
selectivity and an increased dehydrogenation
selectivity. Dehydrogenation selectivity of
vapor in the catalyst environment. These two
factors are known to increase the stability of
HPMo with a Keggin structure. These mixed-
6
q
5q
valence compounds contain Mo
and Mo
formed by the rapid electron trapping among the
12 equivalent Mo atoms in a Keggin anion and
the anion structure is maintained w31–33x. The
presence of the MoO band only in the IR
3
HPMorT and Ni
sum of the mild oxidation products
CO2
w10x, is shown in Fig. 9. The dehydro-
Ž
Co
.
PMorT, presented as a
spectra of the used HPMorT pretreated at 723
Ž
HCHOq
K indicates that in the conditions of the CH OH
3
.
oxidation reaction the HPMo is almost de-
stroyed at temperature lower than that of the
fresh catalyst. Rocchiccioli-Deltcheff et al. w11x
have also reported that after different thermal
treatments from 523 to 773 K, the silica-sup-
ported 12-HPMo catalysts exhibit very poor
infrared spectra after catalytic reaction. How-
ever, the authors showed that silica support
decreases the thermal stability of supported
genation selectivity, which increases for all the
samples with increasing of the calcination tem-
perature, is always higher at a reaction tempera-
ture of 503 K.
4
. Discussion
4
.1. Structural features and thermal stability
In our previous work w19x, it has been shown
HPMo and the transformation into b-MoO be-
3
gins at a temperature lower than that of the
bulk. Taking this into consideration, a lower
stability of the silica-supported HPMo should be
expected under reaction conditions compared to
titania-supported HPMo. It has been concluded
w19x that the stability of the HPMo supported on
that the thermal decomposition of the supported
HPMo proceeds in two steps: gradual dehy-
dration of the heteropoly compound with forma-
tion of anhydrous phase, the Keggin unit being
preserved, and
anion into molybdenum oxides with different
morphological phases and phosphates, which is
completed at temperature above 773 K. A simi-
lar characterization has been carried out on the
used samples in order to know if the catalysts
remain unchanged or not during the catalytic
reaction for a given pretreatment temperature.
This may provide an explanation for the marked
influence of the pretreatment conditions on the
i
Ž .
Ž
ii
.
transformation of heteropoly
titania increases due to the titania-support inter-
action.
The most important affect on Mo dispersion
occurs after catalyst pretreatment at 523 K, at
which a significant increase of the XPS MorTi
ratios Table 3 is observed. Probably, the pre-
Ž .
liminary clusters of heteropolymolybdate anions
are broken into smaller oxoanions with a pre-
served Keggin unit which remain well dispersed
even after prolonged on-stream operation. The

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
97
dispersion of molybdenum species after reaction
is less affected for samples at higher pretreat-
ment temperatures, probably, because of the
presence of thermally more stable b- andror
weaker sites. The sample which is pretreated at
a lower temperature and therefore retained more
constitutional protons contains a significant
amount of acidic centers, whereas the sample
pretreated at a higher temperature lack such
acidic sites and the Lewis type sites should be
a-MoO phases w19x.
3
4
.2. Acidity
dominant, being seen from IR Fig. 3 .
Ž .
The IR data of pyridine adsorption and the
ammonia TPD experiments consider that the
acidic centers in the titania-supported HPMo are
Ž .
of Br o¨ nsted protonic and Lewis type and the
acidic properties of the samples depend on their
hydration degree. According to literature data
w25,30x two types of protons: nonlocalized hy-
drated protons, and nonhydrated, less mobile
protons has been distinguished. It should be
noted, that most of the constitutional protons
present weak acid centers, rather firmly bound
to the Keggin anion, whose amount decreases
upon thermal treatment. It has been shown w34x
that silica-supported HPMo calcined at 323 K
only absorbed 1.5 pyridine molecules per Keg-
gin anion. The amount absorbed by the same
sample pretreated at 623 K was half this value
and only 25% of the constitutional protons were
contributed to the strong acidity of the samples.
The large ammonia desorption peak in the
4
.3. ActiÕity
According to the mechanism of the methanol
dehydration on the oxide catalysts based on an
overview of many data w35–38x, the main step
consists of a formation of the methoxy interma-
diate on the acid sites of the catalyst. This
intermediate can either, react with another
methanol molecule to form dimethyl ether
Ž
acid-base pathway
attack of surface oxide ion to form a precursor
of formaldehyde redox pathway . Therefore, it
.
, or undergo a nucleophilic
Ž
.
could be expected that the different structures
present on the surface of the catalysts w19x would
influence in a different way the various elemen-
tary steps of the transformation of methanol and
hence the selectivity of the reaction.
The highest selectivity to DME at 523 K
Ž
about 70%. ŽFig. 7A.
suggests that the unde-
lower temperature range of the spectra
Ž
Fig. 4
.
composed HPMo acid on the titania surface
behaves as an acid catalyst, containing protons
available to protonate methanol w35,38x. Pre-
can be assigned to acidic sites of weak andror
moderate strength due to the presence of consti-
tutional protons as well as of Lewis acid sites,
caused from the positively charged cations
treatment of the catalyst at a higher temperature
suppresses the Br o¨ nsted acidity, being seen from
the Fig. 3. The latter leads to an increase of the
6
q
2q
2q
Ž
Mo andror Co and Ni
higher temperature peak revealed in the spectra
of the samples pretreated at 523 K Fig. 4
.
. However, the
redox selectivity, HCHO formation,
Ž
.
Fig. 7B
. Replace-
ment of the protons in HPMo acid by cations
Ni and Co also causes a substantial change of
.
at
Ž
.
the expense of CH OCH Fig. 7A
Ž
3
3
shows the presence of strong acid sites, which
could be mainly associated with some hydrated
protons retained by the bulk of the acid phase,
which disappear with increasing of the tempera-
ture pretreatment. As can be seen from ammo-
nia desorption profiles of the HPMorT sample
Ž
.
the proton donor capacity of the acid centers
and hence the selectivity to DME decreases
Ž .
compared to that of supported HPMo Fig. 8A .
The role of acidic protons in a dehydration
reaction has been investigated by many authors
w39–41x and has been found that the overall
methanol oxidation rate on HPVMorKPMo cat-
alysts depends on the number of protons per
pretreated at 523 and 723 K Fig. 4A , the
Ž .
dehydration process reduces both, the number
of the acidic centers able to absorb NH and
3
their strength causing shift towards relatively

9
8
S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
Keggin unit. The lack of protons decreases the
concentration of protonated methanol and the
amount of the common intermediate between
DME and dehydrogenation products w40x, which
centration of the constitutional protons is in-
significant and the selectivity to DME begins to
decrease with parallel increase of the selectivity
to HCHO Fig. 7 . The increase of the HCHO
Ž .
lead to a decreased conversion
The lower DME selectivity for the NiPMorT
sample compared to that of CoPMorT Fig.
would be an indication of a higher degree
Ž
Table 4
.
.
formation with increasing of the temperature
pretreatment has been explained on the basis of
a mechanism which requires the presence of
two adjacent surface dioxo units existing on the
Ž
8A
.
of substitution of protons with Ni atoms. On the
Ž
010
MoO3
tion 46,47
the redox properties of the catalysts with the
temperature Fig. 7 is caused mainly from the
.
surface of a well dispersed MoO , whereas
3
other hand, NiPMorT and CoPMorT show a
Ž
100
. Therefore, the observed increase in
.
leads to a methyl formate forma-
significant selectivity to DME
Ž
Fig. 8A
.
, due to
Ž
.
the acidic protons, as revealed by TPD of NH3
and IR of pyridine. The formation of protonic
acidity during the reaction is also possible as a
result of the exposure of the salts to water
vapor, produced during methanol transforma-
tion, and its dissociation on the metal cations
w27x. Since for dehydration of methanol moder-
ate and strong acid sites are required w41x, it
Ž
.
transformation of HPMo into molybdenum ox-
ide phases.
The TPR and XPS results suggest that the
surface Mo in supported catalysts tend to be
slightly reduced during methanol oxidation reac-
tion. Variation of the intensity of the IR bands
of the used catalysts with increasing of the
may be assumed that the cation substituted sam-
ples contain, besides strong acid sites, a large
distribution of weak and moderate acid strength
pretreatment temperature Fig. 1BFig. 2B also
Ž .
consists with the presence of reduced molybde-
num sites under reaction conditions. Conse-
quently, the presence of both, the bulk MoO3
produced during on-stream, as observed by IR
Ž
Fig. 4b,c
.
, sufficient to protonate methanol.
Ni
However, the catalytic properties of Co
Ž
.
-
PMorT catalysts do not correlate with the bulk
acidity of these samples. The selectivity to DME
should vary with the proton content, which is
not the case, the HPMorT sample is more
Ž
Fig. 1 , and the reduced molybdenum sites
.
would retard the production of HCHO; approxi-
mately the same selectivity to HCHO after pre-
active than Co
Ž
Ni
.
PMorT samples. Ammonia,
treatment at 623 and 723 K is observed
7B . However, the presence of oxygen in the
Ž
Fig.
as a polar molecule, can enter into the whole
crystallites and neutralize all the bulk protons.
Probably, NiPMorT and CoPMorT contain su-
perficial sites of moderate strength which con-
tribute to the high activity in methanol dehydra-
tion. This is supported by the higher values of
the surface area and XPS MorTi atomic ratio of
.
gas phase maintains the high oxidation state of
the surface molybdenum atoms, inhibiting their
deep reduction. Reoxidation of the reduced
molybdenum by gaseous oxygen would be oc-
curred during reaction conditions. The latter can
be confirmed by the constancy of the XPS
MorTi ratio and of the fraction of reduced
these samples
The catalysts have a complex composition in
the temperature range of 623–723 K w19x:
not yet totally destroyed HPA with preserved
Keggin unit and ii the presence of b- and
a-MoO phases, the latter dominates at higher
Tables 1 and 3 .
Ž .
molybdenum Tables 2 and 3 . Consequently,
Ž .
Ž
i
.
the higher selectivity to HCHO in the tempera-
ture range of 623–723 K is maintained by the
higher electron density of the molybdenum
bridging oxygen atoms according to the mecha-
nism of HCHO formation w37,42x.
Ž
.
3
temperature 723 K . Therefore, in the region
Ž .
corresponding to the appearance of the anhy-
dride phase and molybdenum oxide phases with
different morphological composition, the con-
The reducibility trends derived from TPR
experiments do not correlate with methanol ac-
tivity. Since the catalytic reactivity is only con-

S. DamyanoÕa et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 85–100
99
trolled by the surface reduction process, the
Acknowledgements
contribution of the bulk reduction may result
negligible for this study. The Tmax temperature
presented in Fig. 6 reflects the ease of reduction
by oxygen removal from the catalyst. However,
the difference in the selectivity to HCHO at
reaction temperatures of 483 and 503 K may
originate from the difference in the amount of
reduced molybdenum. Less reduced molybde-
num species would increase the number of
molybdenum bridging oxygen sites, which can
activate the formation of HCHO. As can be
seen in Fig. 9, there is a difference among the
catalysts with respect to the change of their
dehydrogenation selectivity at both reaction
temperatures. According to Ref. w43x, the forma-
S.D. thanks the Ministerio de Educacion y
Cultura for a NATO grant. The authors would
like to thank Dr. M. Lopez Granados for help-
ing in the ammonia TPD measurements. Finan-
cial support from CICYT, Spain, under project
MAT95-0894 is also acknowledged.
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.
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