ORIGINAL PAPER
On the Synergy Effect in MoO
3
–Fe
2
(MoO
4
)
3
Catalysts
for Methanol Oxidation to Formaldehyde
Emma So
¨
derhjelm Æ Matthew P. House Æ Neil Cruise Æ
Johan Holmberg Æ Michael Bowker Æ Jan-Olov Bovin Æ
Arne Andersson
Ó Springer Science+Business Media, LLC 2008
Abstract Methanol oxidation to formaldehyde was
studied over a series of Fe–Mo–O catalysts with various
Mo/Fe atomic ratio and the end compositions Fe
2
O
3
and
MoO
3
. The activity data show that the specific activity
passes through a maximum with increase of the Mo content
and is the highest for Fe
2
(MoO

4
)
3
. The selectivity to
formaldehyde, on the other hand, increases with the Mo
content in the catalyst. A synergy effect is observed in that
a catalyst with the Mo/Fe ratio 2.2 is almost as active as
Fe
2
(MoO
4
)
3
and as selective as MoO
3
. Imaging of a MoO
3
/
Fe
2
(MoO
4
)
3
catalyst by SEM and TEM shows that the two
phases form separate crystals, and HRTEM reveals the
presence of an amorphous overlayer on the Fe
2
(MoO
4

)
3
crystals. EDS line-scan analysis in STEM mode demon-
strates that the Mo/Fe ratio in the amorphous layer is *2.1
in the fresh catalyst and *1.7 in the aged catalyst. The
enrichment of Mo at the catalyst surface is confirmed by
XPS data. Raman spectra give evidence for the Mo in the
amorphous material being in octahedral coordination,
which is in contrast to the crystalline Fe
2
(MoO
4
)
3
bulk
structure where Mo has tetrahedral coordination. X-ray
diffraction (XRD) analysis gives no support for the for-
mation of a defective molybdate bulk structure. The results
presented give strong support for the Mo rich amorphous
structure being observed on the Fe
2
(MoO
4
)
3
crystal sur-
faces being the active phase for methanol oxidation to
formaldehyde.
Keywords Methanol oxidation Á Formaldehyde Á
Fe–Mo–O catalysts Á Iron molybdate Á XRD Á

Electron microscopy Á SEM Á TEM Á HRTEM Á
STEM–EDS Á XPS Á Raman spectroscopy
1 Introduction
Formaldehyde is a reactive intermediate, which is used for the
production of a large number of products [1, 2]. The largest
amounts of formaldehyde are used to produce resins (con-
densates) with urea, phenol and melamine, which are used for
the production of adhesives and impregnating resins. Another
market is the manufacture of molding compounds for surface
coating. Formaldehyde is also used as an intermediate in the
production of a variety of chemicals where the most important
are polyacetals, MDI, 1,4-butanediol and polyols. The world
consumption of formaldehyde in the form of its solution with
water (37% HCHO) was about 28 million tons in the year
2005, and the present growth rate has been estimated to be
around 3–4% per year [2].
Methanol and air are the raw materials for commercial
production of formaldehyde in two competing technologies,
E. So
¨
derhjelm Á N. Cruise
Perstorp Specialty Chemicals AB, Process and Catalyst
Development, 284 80 Perstorp, Sweden
M. P. House Á M. Bowker
School of Chemistry, Main Building, Cardiff University,
Cardiff CF10 3AT, UK
J. Holmberg
Perstorp Specialty Chemicals AB, Perstorp Formox, 284 80
Perstorp, Sweden
J O. Bovin

Division of Polymer and Materials Chemistry, Department of
Chemistry, Lund University, Chemical Center, P.O. Box 124,
221 00 Lund, Sweden
A. Andersson (&)
Department of Chemical Engineering, Lund University,
Chemical Center, P.O. Box 124, 221 00 Lund, Sweden
e-mail:
123
Top Catal
DOI 10.1007/s11244-008-9112-1
which are referred as the silver process and the oxide pro-
cess, respectively. The silver process operates at methanol-
rich conditions with silver as catalyst, while the oxide pro-
cess uses an iron molybdate catalyst under methanol-lean
conditions. The first use of a silver catalyst was patented in
1910 [1]. In 1931, Adkins and Peterson [3]reportedthat
methanol was selectively oxidised to give formaldehyde
over an oxide catalyst with equimolar amounts of molyb-
denum and iron. Although a catalyst with excess Mo gave
lower activity, it was more selective to formaldehyde.
However, it was not until the 1950s that that the molybdate
catalyst got commercial importance.
The basic chemical composition of the oxide catalysts
has practically been the same over the years. It is well
known that the fresh catalyst consists of two crystalline
phases, namely MoO
3
and Fe
2
(MoO

4
)
3
[4–9]. In spite of
this fact, there have been several process improvements
since the early 1960s as shown in Fig. 1 [2]. The intro-
duction of gas recirculation from the absorber allowed the
methanol inlet concentration to be increased from 6.5 to 7.5
vol.%. A few years later exhaust catalyst systems (ECS)
became common, leading to increased steam production
but no increase in productivity. Replacement of the gran-
ular catalyst with ring-shaped catalysts allowed the gas
flow rate to be increased and so the productivity. Dilution
of the first part of the catalyst with inert rings (mixed layer)
gave better temperature control and allowed increase of
both the gas velocity and the methanol concentration up to
8.5 vol.%. A further development was pressurisation of the
plants (0.3 bar g). More recently a refined catalyst activity
profile (CAP) is used in new plants, allowing the heat of
reaction to be properly distributed along the length of the
reactor tube and making possible operation with a metha-
nol concentration at the inlet of *10 vol.%.
The durability of the catalyst depends on several factors
including the methanol and oxygen concentrations, the
temperature and the pellet diameter [10]. In practice it
varies between 8 and 18 months depending on the oper-
ating conditions and tolerances. By now, it is well
established that the catalyst deactivates because it looses
Mo during operation due to the formation of volatile spe-
cies [9–13], causing lower activity and selectivity as well

as increased pressure drop as molybdena needles condense
in the lower part of the reactor. Another deactivation cause
is sintering of the catalyst in the hot spot [11, 14, 15]. The
deactivated catalyst consists mainly of Fe
2
(MoO
4
)
3
, but
contains as well the worse performing Fe
2
O
3
and FeMoO
4
phases [9, 11, 13, 16, 17].
It has been reported that the catalyst must contain both
MoO
3
and Fe
2
(MoO
4
)
3
for being active, selective and long-
lasting [8, 9, 12, 13, 18, 19]. Different explanations have
been given in the literature for the observation that both
phases are required. One explanation is that Fe

2
(MoO
4
)
3
is
the active phase [6, 7, 20] and MoO
3
is required to secure
that no iron rich phase is formed [12, 13, 21–23], which is
also the opinion expressed in a comprehensive review to
summarize the present knowledge about the active phase
[4]. Other investigators have proposed that the active phase
is Fe
2
(MoO
4
)
3
with excess Mo in the structure. Fagherazzi
and Pernicone [24] suggested from X-ray diffraction (XRD)
data that the active ferric molybdate is Fe-defective due to
Mo
6?
substitutes for some of the Fe
3?
ions in octahedral
coordination and additional oxygen goes into interstitial
positions. A similar conclusion was drawn by Leroy et al.
[25] considering combined XRD and Raman results.

According to Pernicone [8] the active structure for methanol
oxidation has the composition Fe
2-3x
(Mo
1?x
O
4?1.5x
)
3
with
x B 1/9 and usually below 0.05, consisting of nanoregions
of MoO
6
octahedra included in the Fe
2
(MoO
4
)
3
structure.
Some researchers have concluded that the active phase is a
solid interstitial solution of MoO
3
in the Fe
2
(MoO
4
)
3
lattice

[5, 26, 27], rather than being a substitution compound. Here
it is also worth pointing out that Massarotti et al. [28]
excluded any solubility of MoO
3
in Fe
2
(MoO
4
)
3
, because
they obtained very similar lattice-constant values for the
stoichiometric molybdate and the molybdate in non-stoi-
chiometric samples prepared by solid-state synthesis.
Moreover, considering IR spectra, Sun-Kuo et al. [19] draw
the conclusion that the active material consists of a dis-
persed polymeric amorphous structure, and Aruanno and
Wanke [14] have proposed that the activity is due to for-
mation of a Mo rich surface on the Fe
2
(MoO
4
)
3
crystals.
Briand et al. [29] observed that bulk molybdates and
supported molybdenum oxide catalysts possess similar
activity and turnover frequency, indicating that the surfaces
0
5

10
15
20
25
Time
Productivity (kg 37% HCHO/tube/day)
Gas recirculation
Ring-shaped
catalyst
Mixed layer
Pressurization
Refined activity
profile
Fig. 1 Gains in the productivity of formaldehyde per reactor tube and
day since the early 1960s
Top Catal
123
of bulk molybdates may be composed only by molybdenum
oxide species in a two-dimensional overlayer. Besides
being a dopant entering in small amount the interstial
positions in the Fe
2
(MoO
4
)
3
structure, Trifiro
`
et al. [17]
have pointed out that Mo in excess is necessary for

improvement of the mechanical properties of the catalyst
and in the process for securing efficient reoxidation of
formed ferrous molybdate.
In light of the disagreement in previous literature on the
role of excess Mo for the performance of iron molybdate in
methanol oxidation, the purpose of our work is to present
new findings to clarify the origin of the observed synergy
between the two catalyst constituents MoO
3
and
Fe
2
(MoO
4
)
3
.
2 Experimental
2.1 Catalyst Preparation
A series of Fe–Mo–O catalysts was prepared with the
atomic Mo/Fe ratios equal to 0.2, 0.5, 1.0, 1.5 and 2.2
starting from solutions of 100 ml ammonium heptamo-
lybdate (BDH C 99%, 0.254 M for the Mo/Fe 1.5 catalyst)
and 50 ml iron nitrate (BDH C 98%, 0.338 M). The
molybdate solutions were acidified to *pH 2 using nitric
acid (Fisher, Laboratory Grade), before drop wise addition
of iron nitrate with stirring at 60 °C giving canary yellow
precipitates, which were evaporated to near dryness at
90 °C. The resulting solids were then dried at 120 °C
overnight before being calcined in air at 500 °C for 48 h.

The single oxides of Fe
2
O
3
(Aldrich C 99%) and MoO
3
(BDH C 99.5%) used were commercially sourced.
For comparative purposes the pure Fe
2
(MoO
4
)
3
phase
was prepared by solid-state reaction comprising mixing
and grinding stoichiometric amounts of Fe
2
O
3
and MoO
3
,
heating at 600 °C for 16 h, followed by a second grinding
and heating for another 16 h, now at 650 °C.
Some of the samples being characterized in this work
have been supplied by Perstorp Formox.
2.2 Catalyst Characterisation
The surface areas of the catalysts were measured either
using a Micromeritics Gemini 2360 or a CE instruments
QSurf M1. A five or six point BET method was used with

adsorption of nitrogen at liquid nitrogen temperature and
subsequent desorption at room temperature. All samples
were degassed at 150 °C for 1 h.
XRD was performed using either an Enraf Nonus FR590
fitted with a hemispherical analyser, or, a Seifert XRD
3000 TT diffractometer. In both cases Ni-filtered Cu Ka
radiation was used.
Fourier transform Raman (FT-Raman) spectra were
recorded on a Bruker IFS66 FTIR spectrometer equipped
with a Bruker FRA106 FT-Raman device, a Nd:YAG-laser
and a germanium diode detector. The laser power was 100
mW, the resolution was 4 cm
-1
and 400 scans were col-
lected for each spectrum.
XPS analysis was performed on a Kratos XSAM 800
spectrometer using Al Ka X-ray radiation (1486.6 eV).
Quantifications were made using a linear background and
instrumental sensitivity factors. Charging effects were
corrected for by adjusting the main C 1s peak to a position
of 285.0 eV. The anode was operated at an accelerating
voltage of 13 kV and a current of 19 mA. The pass energy
was 80 eV and the residual pressure in the spectrometer was
10
-8
torr, or, lower.
SEM images were recorded on a JEOL 840A micro-
scope using a tungsten filament and a voltage of 20 keV.
EDS analyses were made using an Oxford instruments
analyser with the INCA software.

TEM imaging was performed using a JEOL 3000F
microscope, operating at an acceleration voltage of 300
keV. The used EDS analyser from Oxford instruments was
equipped with the INCA software. Linescanning was per-
formed using EDS in STEM mode. Several dots in a line
were analysed on each crystal, starting in the vacuum and
then gradually moving onto the crystal. The typical dis-
tance between the dots was between 20 and 30 nm.
2.3 Activity Measurements
Activity and selectivity measurements on the laboratory
prepared samples were made on a pulse flow micro reactor
system [23]. The reactor basically consists of a U-tube
mounted vertically within a Phillips PU 4500 GC oven with
gas continuously flowing over the bed. A small amount of
the outlet gas stream is monitored by a Hiden Analytical Hal
201 quadruple mass spectrometer, while the rest of the gas
is vented via a Leybold Heraeus Trivac rotary pump. In this
work, a heated gas with 10 vol.% O
2
in He (BOC) was
flowed over 0.5 g catalyst at a rate of 30 ml/min (STP) with
1 ll methanol injections being made every 2 min, while the
temperature in the furnace was ramped from *150 to
*380 °C. Before each run, methanol injections were made
over a bypass to account for the daily drift in the mass
spectrometer.
Comparative activity measurements on fresh and used
commercial samples were made under adiabatic conditions
in a stainless-steel micro reactor with a diameter of 21 mm. A
flow with 1.75 vol.% methanol in dry 25 l/min (STP) air with

an inlet temperature of 260 °C was passed over 13 g of 1 mm
particles of crushed and sieved catalyst. The conversion of
methanol was calculated comparing the signal from a Ber-
nath Atomic 3006 FID-analyser before and after the reactor.
Top Catal
123
3 Results
3.1 Influence of the Phase Composition on the
Catalytic Performance
The laboratory prepared Fe–Mo–O samples were tested in
a pulse flow micro reactor system for their performance in
methanol oxidation. In Table 1 are the metal compositions
and the specific surface areas of the prepared samples lis-
ted. XRD and Raman spectroscopy showed that the two
single cation catalysts are single phase haematite Fe
2
O
3
(JCPDS file no. 33-664) and orthorhombic MoO
3
(JCPDS
file no. 35-609), respectively [30]. The catalysts with the
Mo/Fe ratios 0.2 and 0.5 consist of Fe
2
O
3
and Fe
2
(MoO
4

)
3
(JCPDS file no. 31-642) [30]. In the samples with the Mo/
Fe ratios 1.0 and 1.5 only diffraction peaks from mono-
clinic Fe
2
(MoO
4
)
3
were observed. The stoichiometry of the
former sample, however, dictates that there must be an
additional iron phase, presumably Fe
2
O
3
, which is either
amorphous or highly dispersed. The catalyst with Mo/Fe
equal to 2.2 is a mixture of Fe
2
(MoO
4
)
3
and MoO
3
. The
catalysts below the stoichiometric level (Mo/Fe \ 1.5)
presented a brown colour, the Mo/Fe 1.5 catalyst presented
a yellow colour, while the Mo/Fe 2.2 catalyst was light

green.
For comparing the catalytic activity of the samples,
methanol conversions at 180 °C are given in Table 1.Itis
seen that the samples with 0.2 and 0.5 Mo/Fe ratios give
the highest conversions, mainly due to their high surface
areas. For a better comparison, the conversions should be
normalised with respect to surface area. The normalised
values in Table 1 indicate that the two samples with the
Mo/Fe ratios 1.5 and 2.2 are the most active preparations
per unit surface area. To better account for the differences
in methanol conversion, first order surface area normalised
rate constants were calculated. The values clearly show
that the activity increases when the Mo/Fe ratio is
increased from 0 (Fe
2
O
3
) up to 1.5 (Fe
2
(MoO
4
)
3
). With
further increase of the Mo/Fe ratio, the activity declines.
Concerning the selectivity to formaldehyde at high meth-
anol conversion the data in Table 1 for 90% conversion,
show that the selectivity steadily increases with the Mo
content in the catalyst. Of the pure phases, Fe
2

O
3
does not
produce any formaldehyde, whereas the selectivity on
Fe
2
(MoO
4
)
3
and MoO
3
is 73% and 90%, respectively.
Considering both activity and selectivity, the sample with
Mo/Fe = 2.2 is outstanding, being almost as active as the
pure Fe
2
(MoO
4
)
3
and as selective as the pure MoO
3
.
Also in industrial operation of Fe–Mo–O catalysts there
is an optimal Mo/Fe ratio for the catalyst to perform well
with good activity, selectivity and durability [9]. In Fig. 2
are the activities of used commercial catalysts compared
with data for the corresponding freshly prepared catalysts.
The used catalyst samples had been collected from the

Table 1 Specific surface area, activity and selectivity of prepared Fe–Mo–O catalysts
a
Catalyst
Mo/Fe ratio
Specific surface
area (m
2
/g)
Conversion at
180 °C (%)
Activity (conversion/m
2
surface area)
First order rate
constant k (cm
3
/min/m
2
)
Selectivity (%) to formaldehyde
at 90% conversion
b
0 (Fe
2
O
3
) 2.1 *2 1.90 0.577 0 (322 °C)
0.2 55.4 55 1.99 0.865 18 (204 °C)
0.5 38.7 50 2.58 1.075 27 (210 °C)
1.0 16.3 38 4.66 1.760 47 (244 °C)

1.5 7.8 35 8.97 3.314 73 (249 °C)
2.2 6.7 29 8.66 3.067 90 (256 °C)
? (MoO
3
) 1.0 *2 4.00 1.212 90 (377 °C)
a
Activity and selectivity measured in a pulse flow reactor injecting 1 ll liquid methanol every 2 min into a 30 ml/min gas flow with 10 vol.% O
2
in He, passing over 0.5 g catalyst while ramping the temperature from *150 to *380 °C (see Section ‘‘Experimental’’ )
b
The temperature giving 90% methanol conversion on 0.5 g catalyst is given within brackets
0
5
10
15
20
25
30
35
40
45
Methanol conversion (%)
Inlet layer
Fresh catalyst
Outlet layer
Fresh catalyst
Outlet layer
Used catalyst
Inlet layer
Used catalyst

Fig. 2 Conversion of methanol as measured in an adiabatic micro
reactor (21 mm in diameter) over freshly prepared commercial
catalysts and the corresponding used samples. The used samples had
been collected from the inlet and outlet parts of an industrial
multitube reactor after a full lifetime cycle. Reaction conditions: 1.75
vol.% methanol in dry 25 Nl/min air with an inlet temperature of
260 °C. The amount of catalyst in the micro reactor was 13 g of 1 mm
particles of the crushed and sieved catalyst
Top Catal
123
upper and lower part of the reactor, respectively, after the
operation of a catalyst load in an industrial reactor had
been terminated due to normal ageing. According to XRD
the inlet fraction consisted mainly of Fe
2
(MoO
4
)
3
, and
elemental analysis indicated the presence of another 0.55
wt.% of Fe
2
O
3
in agreement with the catalyst surface being
covered with a thin reddish brown layer. The fraction from
the outlet consisted of the MoO
3
and Fe

2
(MoO
4
)
3
phases,
however, the Mo/Fe ratio was higher than in the unused
catalyst and needle-like MoO
3
crystals were observed on
the catalyst rings. As previously has been explained, Mo in
the upper part of the catalyst forms volatile species with
methanol [9, 10], which species decompose and condense
as needles in the lower part of the bed [9, 11]. Thus, the
activity data in Fig. 2 for industrial catalysts are in general
agreement with the data in Table 1 for the laboratory
prepared samples. The deactivation of the catalyst at inlet
conditions is due to the surface being coated with a thin
layer of Fe
2
O
3
, which according to Table 1 has lower
activity and selectivity for methanol oxidation. A deacti-
vation cause for the catalyst at the outlet of the reactor is
that it contains more MoO
3
compared to the fresh catalyst.
The data in Table 1 shows that the pure MoO
3

is less active
than the composition Mo/Fe = 2.2 with both MoO
3
and
Fe
2
(MoO
4
)
3
.
3.2 Catalyst Characterisation with Electron
Microscopy and XPS
MoO
3
/Fe
2
(MoO
4
)
3
catalysts were characterised with SEM,
TEM, HRTEM and STEM–EDS before and after use in an
industrial reactor. SEM shows the presence of large plate-
like MoO
3
crystals being about 10 lm in size (Fig. 3). The
crystal composition was verified by EDS analysis and,
moreover, the observed crystal habit is typical of
orthorhombic MoO

3
[31]. Generally it was observed that
the MoO
3
crystals are more frequent in fresh catalyst than
in used catalyst. Besides the plate-like crystals, the major
part of the catalysts consists of smaller crystals, which
appear to be randomly close packed as Fig. 3 shows.
To gain information about the small crystals, samples
were investigated by TEM and EDS. Most of the observed
crystals are Fe
2
(MoO
4
)
3
but also some are MoO
3
. The two
types of crystals are in the same size range as Fig. 4 shows.
Fe
2
(MoO
4
)
3
presents crystals which mostly are rectangular/
elliptic, while the MoO
3
crystals are thin and plate-like.

HRTEM imaging of the Fe
2
(MoO
4
)
3
crystals revealed a
5–10 nm thick amorphous surface structure, extending
around the crystals. The amorphous structure, which was
observed both in fresh and aged samples, can be seen on
the edges of the crystal in Fig. 5.
Fig. 3 SEM image of a fresh MoO
3
/Fe
2
(MoO
4
)
3
catalyst
Fig. 4 TEM image of a fresh MoO
3
/Fe
2
(MoO
4
)
3
catalyst
Fig. 5 A HRTEM image of a typical Fe

2
(MoO
4
)
3
crystal with an
amorphous layer extending around the edge of the crystal
Top Catal
123
The very surface of the Fe
2
(MoO
4
)
3
crystals in a freshly
prepared and an aged fraction of the same catalyst were
analysed using EDS in STEM mode. Line scans were col-
lected starting in the vacuum outside the edge of the crystal
and then gradually moving in onto the bulkier parts of the
material. By using this technique, it is expected to get
information about any difference in elemental composition
between the surface and the bulk of the crystal. For the
wedge-like crystal terminations, the surface composition
should be analysed at the edge of the crystal and with
increasing distance from the edge, the analyses should show
an increasing contribution from the bulk as the thickness of
the amorphous layer being only about 5–10 nm. The ana-
lysed Mo/Fe ratios are plotted in Fig. 6 against the distance
from the edge of the analysed crystal. For the unused cat-

alyst the Mo/Fe ratio is the highest at the edge, where the
ratio is *2.1. With increase of the distance from the edge,
the Mo/Fe ratio drops to a value of about 1.5. Compared
with the fresh catalyst, the data for the corresponding aged
sample shows a considerably lower Mo/Fe ratio on the edge
(*1.7) and an identical value 1.5 further from the edge. The
measured ratio 1.5 for the bulk is in perfect agreement with
the stoichiometry of the Fe
2
(MoO
4
)
3
crystal.
As a complement to the EDS point analyses performed in
STEM mode on separate Fe
2
(MoO
4
)
3
crystals, XPS analy-
ses were performed on a number of MoO
3
/Fe
2
(MoO
4
)
3

preparations to give information about the average Mo/Fe
ratio in the surface region. Figure 7 shows a comparison of
Fresh Catalyst
1
1,2
1,4
1,6
1,8
2
2,2
2,4
0,0E+00 5,0E-08 1,0E-07 1,5E-07 2,0E-07 2,5E-07
Distance from crystal edge (m)
Mo:Fe ratio
crystal 1
crystal 2
crystal 3
crystal 4
crystal 5
crystal 6
crystal 7
crystal 8
Aged Catalyst
1,00
1,20
1,40
1,60
1,80
2,00
2,20

2,40
0,00E+00 5,00E-08 1,00E-07 1,50E-07 2,00E-07 2,50E-07
Distance from crystal edge (m)
Mo:Fe ratio
crystal 1
crystal 2
crystal 3
crystal 4
crystal 5
crystal 6
crystal 7
Fig. 6 The Mo/Fe ratios as
determined by STEM–EDS line
scan analysis on a Fe
2
(MoO
4
)
3
crystal in (upper figure) a
freshly prepared MoO
3
/
Fe
2
(MoO
4
)
3
catalyst and (lower

figure) the corresponding
catalyst after ageing in an
industrial reactor
Top Catal
123
the bulk ratio with the corresponding surface ratio as
determined by XPS. In agreement with the EDS analyses,
the XPS data clearly shows a general trend, namely that the
catalyst surface is richer than the bulk in Mo.
3.3 XRD and Raman Characterisation of the
Fe
2
(MoO
4
)
3
phase
Figure 8a shows an overlay of the XRD patterns of a
MoO
3
/Fe
2
(MoO
4
)
3
catalyst and a phase pure Fe
2
(MoO
4

)
3
prepared by solid-state reaction from stoichiometric
amounts of MoO
3
and Fe
2
O
3
. The difference pattern in
Fig. 8b clearly exhibit peaks only from orthorhombic
MoO
3
, indicating no difference in unit cell between the
phase pure Fe
2
(MoO
4
)
3
and the molybdate in the catalyst
with excess MoO
3
. Included in the figure is a spectrum
recorded for a pure MoO
3
sample, showing very intense
(0 k 0) peaks due to the preferred orientation of the plate-
like crystals in the sample holder. The most intense peak
is (040), while in the difference pattern the (021) peak is

the most intense for the MoO
3
in the catalyst matrix.
According to data calculated from the unit cell (JPDS file
no. 35-609) [30], the (021) peak should be the most intense
peak for a non-oriented sample, which obviously is the case
for the catalyst sample where the MoO
3
crystals have no
preferential orientation in a matrix of Fe
2
(MoO
4
)
3
.
The Raman spectrum of a deactivated molybdate catalyst
from a full-scale reactor is shown in Fig. 9 together with the
spectrum of pure MoO
3
. According to XRD and atomic
absorption the used catalyst consists of Fe
2
(MoO
4
), and the
recorded Raman spectrum is in perfect agreement with
spectra reported in the literature for the pure Fe
2
(MoO

4
)
3
phase [29, 32, 33]. The spectrum in Fig. 9 of the pure MoO
3
shows strong bands at 996, 818, 666 and 284 cm
-1
together
with a number of less intense bands in the region below
500 cm
-1
. In the spectrum of the Fe
2
(MoO
4
)
3
phase, bands
are seen at 988, 967, 934, 818, 781 and 348 cm
-1
. Thus, it is
clearly seen that the molybdate spectrum does not include
any contribution from crystalline MoO
3
because there are
no bands at 666 and 284 cm
-1
and, moreover, the bands at
988 and 818 cm
-1

are of equal size, whereas in MoO
3
the
band at 818 cm
-1
is more than twice as large as the band at
996 cm
-1
. It has been suggested for Fe
2
(MoO
4
)
3
that the
two bands at 988 and 818 cm
-1
can be from some minor
octahedrally coordinated Mo species [32] in contrast to the
dominant Mo species, which is tetrahedral in a perfect
Fe
2
(MoO
4
)
3
lattice [24].
4 Discussion
The catalytic data in Table 1 reveal a pronounced synergy
effect in that an atomic Mo/Fe ratio above the stoichiom-

etric ratio for Fe
2
(MoO
4
)
3
is needed for a catalyst to be
0
0.5
1
1.5
2
2.5
3
3.5
123456789101112
Catalyst #
Mo/Fe ratio
Bulk
Surface
Fig. 7 Comparison of the Mo/Fe bulk ratios with the corresponding
ratios determined by XPS for a number of MoO
3
/Fe
2
(MoO
4
)
3
preparations

0
20
40
60
80
100
120
0 5 10 15 20 25 30
Theta (
˚
)
XRD intensity
Pure iron molybdate
Commercial fresh catalyst
*
*
*
*
MoO
3
[0
k
0] peaks
(a)
-20
0
20
40
60
80

100
120
0 5 10 15 20 25 30
Theta (
˚
)
Difference in XRD intensity
Difference
MoO3
(020) (110)
(040)
(021)
(111)
(060)
(200)
(061)
(002)
(081)
(112)
(211)
(b)
Fig. 8 A comparison of the XRD patterns recorded for a MoO
3
/
Fe
2
(MoO
4
)
3

catalyst and the pure Fe
2
(MoO
4
)
3
phase prepared by
solid state reaction (see Section ‘‘Experimental’’). The upper figure
(a) shows an overlay of the two diffractograms. In the lower figure (b)
is shown the resulting difference pattern obtained by subtracting the
XRD for the pure phase from that of the catalyst. Here it is seen that
the remaining peaks in the difference pattern correspond well with the
XRD recorded for orthorhombic MoO
3
. The difference peaks have
been indexed according to the JPDS file no. 35-609 [30]
Top Catal
123
both active and selective to formaldehyde formation. This
result is in general agreement with previous reports,
although these in most cases have not considered both
activity and the selectivity at high methanol conversion in
the whole range of compositions from iron to molybdenum
oxide. A surface area normalised activity maximum for
methanol oxidation has been reported for the ratio Mo/
Fe = 1.7 [18, 19], which is not in conflict with the data in
Table 1 showing that the catalysts with the Mo/Fe ratios
1.5 and 2.2 present similar activity. Moreover, in another
report [22] is described that the highest specific activity
was observed for the stoichiometric composition

Fe
2
(MoO
4
)
3
. It has also been reported that too high Mo
content in the Fe–Mo–O catalyst results in lower activity
[11], which is in agreement with the data in Fig. 2 for a
catalyst collected from the outlet of an industrial reactor.
This catalyst with excess molybdena needles on the surface
is less active than the corresponding unused catalyst. The
observation in Table 1 that the normalised activity
(k value) of the pure MoO
3
is almost a factor three lower
than that for the sample with the Mo/Fe ratio 2.2 is in
perfect agreement with a previous comparison of a Har-
shaw MoO
3
/Fe
2
(MoO
4
)
3
catalyst with a MoO
3
catalyst
[34]. Concerning the selectivity to formaldehyde, the val-

ues in Table 1 show a steady increase with the Mo content
up to the Mo/Fe ratio 2.2 with a selectivity of 90% at 90%
methanol conversion, which is identical with the value for
the pure MoO
3
. A similar trend has been reported previ-
ously [18, 23]. For instance, Kolovertnov et al. [18]
reported that the selectivity was low in Fe rich samples,
whereas samples with Mo/Fe ratios above 1.5 and the pure
MoO
3
are highly selective to formaldehyde at high meth-
anol conversion.
In our work, we have chosen first order rate constants as
a means to account for the differences in surface area and
conversion among the catalysts (see Table 1). The same
method has been adopted by others [18]. According to
Machiels and Sleight [35], the kinetics for a large number
of molybdates and MoO
3
follows a power-law rate model
with about 0.5 order in the methanol concentration.
Therefore, we also calculated the half order rate constants
for the catalysts in Table 1. The values, however, are not
shown as they perfectly confirmed the trend indicated by
the first order rate constants.
In general, several possibilities have to be considered to
explain an observed synergy between phases. One of the
possibilities is dual phase catalysis, where some of the
reaction steps occur on one phase while following steps

take part on another phase. An example here is the
ammoxidation of propane over the Mo–V–Nb–Te–O system
with the two phases designated M1 and M2, where propane
reacts to give propene on M1 and the formed propene then
readsorbs on M2, which is more selective than M1 for the
transformation of propene to acrylonitrile [36]. In the case of
methanol oxidation on MoO
3
/Fe
2
(MoO
4
)
3
, such type of
mechanism is unlikely since the reaction pathway to form-
aldehyde involves no intermediate gaseous product and,
moreover, the methoxy intermediate does not migrate over
the surface as it is relatively strongly bound [23].
Another alternative to be considered is that the catalysis
may occur on the grain boundaries between the phases as
has been suggested for 3-picoline ammoxidation on vanadia
phases [37]. However, the electron microscopy results in
Figs. 3 and 4 show separate crystals of the MoO
3
and
Fe
2
(MoO
4

)
3
phases. Although in contact with each other, no
intergrowth is observed of the crystals from the two phases.
In a study of the reduction behaviours of MoO
3
,
Fe
2
(MoO
4
)
3
and their mixtures by in situ electron micros-
copy in a CH
3
OH/He atmosphere [38], it was noted that the
crystals in the mixtures remain as distinct phases with the
same reduction behaviour as the individual phases. In view
of the above results, it seems that the observed synergy
effect is not primarily related to the grain boundaries. In
other cases an observed synergy between two phases has
been explained by oriented growth of one phase on another,
e.g. in the case of Sb–Sn–O mixed oxides for propene
oxidation [39]. However, our microscopic investigation
showed no oriented growth of MoO
3
on Fe
2
(MoO

4
)
3
, see
Figs. 3 and 4.
In some catalyst systems an activation process occurs
under influence of the catalytic reaction leading to the for-
mation of new phases, which may be more active and
selective than the original composition [40]. This occur-
rence is not the case in methanol oxidation on MoO
3
/
1003005007009001100
Wavenumber (cm
-1
)
Raman intensity (a. u.)
Fe
2
(MoO
4
)
3
MoO
3
Fig. 9 The Raman spectra recorded for MoO
3
and a used catalyst
sample consisting of Fe
2

(MoO
4
)
3
Top Catal
123
Fe
2
(MoO
4
)
3
catalysts. On the contrary, in practice a steady
deactivation of the catalyst is observed with time-on-stream
as the data in Fig. 2 confirms, which also concurs with
previous results [9, 11]. During the deactivation, Fe
2
O
3
and
FeMoO
4
may form [6, 9, 11]. The iron oxide has low
activity and is unselective (Table 1), and FeMoO
4
has been
reported to be selective with an activity comparable to that
of MoO
3
[35].

HRTEM imaging of the Fe
2
(MoO
4
)
3
phase in the catalysts
revealed an amorphous surface structure on the edges of the
crystals as shown in Fig. 5. In fact the same type of structure
was observed by Gai and Labun [38], although they just
briefly mention it as a note in their article when referring to a
small area in one of the images. The investigators focused in
their work on the bulk structures and their reduction. The
EDS data in Fig. 6 shows that the amorphous structure on the
fresh catalyst is rich in Mo (Mo/Fe *2.1) while the amor-
phous material on the aged catalyst has a lower content
(Mo/Fe *1.7). Considering there being a link between the
ageing and the composition and the performance of
the catalyst (Fig. 2), we believe that the active material is the
amorphous structure with excess Mo as compared to the bulk
Fe
2
(MoO
4
)
3
structure. The fact that the Mo/Fe ratio is higher
on the molybdate surface than in the bulk, moreover, is
confirmed by the XPS analyses in Fig. 7. A similar trend has
been observed by other investigators using EDS and XPS

[12, 13, 27]. The Raman spectrum of the Fe
2
(MoO
4
)
3
phase
in Fig. 9 confirms the existence of an additional structure on
the molybdate. In agreement with published spectra of the
pure phase [29, 32, 33], Fig. 9 shows two bands at 988 and
818 cm
-1
, respectively, appearing as shoulders on the strong
bands at 967 and 781 cm
-1
from the tetrahedrally coordi-
nated Mo in the bulk [33]. In agreement with a previous
assignment [32], the two shoulder bands can be from Mo in
octahedral environment considering that MoO
3
with octa-
hedrally coordinated Mo gives two bands at similar
wavenumbers i.e. 996 and 818 cm
-1
(Fig. 9). Also in IR,
Fe
2
(MoO
4
)

3
gives a weak band at 990 cm
-1
[27, 32], which
has been assigned to Mo in octahedral coordination [32].
Thus, from these facts it can be proposed that the Mo in the
amorphous layer may be in octahedral coordination sur-
rounded by six oxygen atoms.
The formation of an amorphous surface layer on
Fe
2
(MoO
4
)
3
can be understood considering its crystalline
bulk structure, which in idealized form is illustrated in
Fig. 10 as built up by regular tetrahedra and octahedra.
Looking at the structure it is seen that it is very open.
Therefore, the formation of an amorphous overlayer can be
a means for stabilizing the surface. It is not clear whether
excess Mo is needed in the synthesis only to give an
amorphous layer with a ratio Mo/Fe[2 in the catalyst, or, if
the MoO
3
crystalline phase in the finished catalyst has an
additional role to sustain the desired Mo/Fe ratio in the
active structure during operation in methanol oxidation.
Fig. 10 The ferric molybdate
structure with Mo in tetrahedral

and Fe in octahedral
coordination shown in idealized
form as built up by regular
polyhedra. The red and yellow
polyhedra have Mo and Fe,
respectively, in the center, and
oxygen in the corner positions.
Each oxygen is shared by two
polyhedra and therefore
2-coordinated. As indicated in
the three figures, the structure is
viewed along the [100], [010]
and [001] directions,
respectively
Top Catal
123
Previously, it has been proposed that surplus Mo in the
catalyst is needed for avoiding the formation of Fe
2
O
3
in the
reoxidation of formed ferrous molybdate FeMoO
4
to the
desired ferric molybdate Fe
2
(MoO
4
)

3
with a higher Mo/Fe
ratio [17, 21]. In view of the fact that SEM and TEM
imaging shows separate MoO
3
and Fe
2
(MoO
4
)
3
crystals
with only physical contact (Figs. 3 and 4), it seems doubtful
whether MoO
3
should have such a role. It is true that iron
oxide is not formed until the Mo/Fe ratio in the catalyst
approaches the value 1.5 and the catalyst is free from any
crystalline MoO
3
[9]. However, a kinetic study of the Mo
loss from MoO
3
/Fe
2
(MoO
4
)
3
preparations has shown that

the loss from the MoO
3
crystals occurs considerably faster
than from the remaining Fe
2
(MoO
4
)
3
[41], which is sup-
ported by results in our previous deactivation study [9].
In several cases, it has been concluded that Fe
2
(MoO
4
)
3
is the active phase in MoO
3
/Fe
2
(MoO
4
)
3
catalysts [4, 6, 7,
20–22]. According to our results this is correct though
incomplete. A better description is that the molybdate is a
support for the active structure, which is an amorphous
surface layer with a higher Mo/Fe ratio compared to the

bulk and with Mo in octahedral coordination. This finding
is in partial agreement with some of the earlier proposals,
which however were based on indirect evidence. In these
works the active material has been described as a Mo rich
molybdate surface [14], a dispersed amorphous structure
[19] and molybdenum oxide species forming a monolayer
[29]. Concerning the previous indications of a defective
molybdate bulk structure being formed when prepared in
excess of molybdenum, we have found no evidence for the
formation of either a solid interstial solution of MoO
3
in
the Fe
2
(MoO
4
)
3
lattice [5]orMo
6?
substituting for some
Fe
3?
and extra oxygen entering the interstial positions [24].
The XRD patterns in Fig. 8 of a MoO
3
/Fe
2
(MoO
4

)
3
cata-
lyst and the stoichiometric Fe
2
(MoO
4
)
3
phase, indicate no
difference between the crystalline bulk structure of the
ferric molybdate in the catalyst and the corresponding
phase prepared without excess molybdenum. This finding
is in agreement with previous reports [6, 7, 21, 28].
5 Conclusions
The present study of methanol oxidation to formaldehyde
on Fe–Mo–O catalysts has demonstrated that the best
performing catalyst is a mixture of the crystalline phases
MoO
3
and Fe
2
(MoO
4
)
3
. A synergy effect is observed in
that the catalyst is almost as active as the pure Fe
2
(MoO

4
)
3
,
which is less selective, and as selective as the pure MoO
3
,
which is less active.
HRTEM imaging of a fresh MoO
3
/Fe
2
(MoO
4
)
3
catalyst
and a corresponding aged catalyst discloses that the active
structure may be an amorphous overlayer on the surface of
the crystalline ferric molybdate. Line-scan EDS analysis
reveals that the Mo/Fe ratio in the amorphous surface is
higher in the fresh catalyst than in the used catalyst. The
Mo/Fe surface ratio for the latter approaches the value 1.5,
although the surface still is amorphous. The Raman spec-
trum of a catalyst consisting of Fe
2
(MoO
4
)
3

shows bands
indicating that in the amorphous layer the Mo is in octa-
hedral coordination.
A comparison of the XRD pattern of a MoO
3
/
Fe
2
(MoO
4
)
3
catalyst with that for the pure Fe
2
(MoO
4
)
3
phase evidences that the crystalline bulk structure of ferric
molybdate is the same in both samples, ruling out previous
proposals of the formation of a defective molybdate with
excess Mo in the crystal lattice.
Acknowledgements Perstorp Specialty Chemicals AB and the
EPSRC in the UK are acknowledged for support of a studentship to
M.P.H.
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