Synergetic Behavior of TiO2-Supported Pd(z)Pt(1-z) Catalysts in the Green Synthesis of Methyl Formate

Article abstract of DOI:10.1002/cctc.201501211

Methyl formate (MF) is a valuable platform molecule, the industrial production of which is far from being green. In this contribution, TiO₂-supported Pd(z)Pt(1-z) catalysts were found to be effective in the green synthesis of methyl formate (MF) - at T=323 K and ambient pressure - through methanol (MeOH) oxidation. Two series of catalysts with similar bulk Pd/(Pd+Pt) molar ratios, z, were prepared; one by a water-in-oil microemulsion (MicE) method and the other by an incipient wetness impregnation (IWI). The MicE method led to more efficient catalysts owing to a weak influence of z on particle size distributions and nanoparticles composition. Pd(z)Pt(1-z)-MicE catalysts exhibited strong synergistic effects for MF production but weak synergistic effects for MeOH conversion. The catalytic performance of Pd(z)Pt(1-z)-MicE was superior to that of Pd(z)Pt(1-z)-IWI catalysts despite the latter displaying synergetic effects during the reaction. The catalytic behavior of TiO₂-supported Pd(z)Pt(1-z) catalysts was explained from correlations between XRD, TEM, and X-ray photoelectron spectroscopy characterizations.

Full text of DOI:10.1002/cctc.201501211

DOI: 10.1002/cctc.201501211
Full Papers
Synergetic Behavior of TiO2-Supported Pd(z)Pt(1Àz)
Catalysts in the Green Synthesis of Methyl Formate
Víctor G. Baldovino-Medrano,*[a, e] Glenn Pollefeyt,[b] Vitaliy Bliznuk,[c] Isabel Van Driessche,[b]
Eric M. Gaigneaux,[a] Patricio Ruiz,[a] and Robert Wojcieszak*[d]
Methyl formate (MF) is a valuable platform molecule, the in-
dustrial production of which is far from being green. In this
contribution, TiO2-supported Pd(z)Pt(1Àz) catalysts were found
to be effective in the green synthesis of methyl formate
(MF)—at T=323 K and ambient pressure—through methanol
(MeOH) oxidation. Two series of catalysts with similar bulk Pd/
(Pd+Pt) molar ratios, z, were prepared; one by a water-in-oil
microemulsion (MicE) method and the other by an incipient
wetness impregnation (IWI). The MicE method led to more effi-
cient catalysts owing to a weak influence of z on particle size
distributions and nanoparticles composition. Pd(z)Pt(1Àz)-MicE
catalysts exhibited strong synergistic effects for MF production
but weak synergistic effects for MeOH conversion. The catalytic
performance of Pd(z)Pt(1Àz)-MicE was superior to that of
Pd(z)Pt(1Àz)-IWI catalysts despite the latter displaying syner-
getic effects during the reaction. The catalytic behavior of TiO2-
supported Pd(z)Pt(1Àz) catalysts was explained from correla-
tions between XRD, TEM, and X-ray photoelectron spectrosco-
py characterizations.
Introduction
Methyl formate (MF), an important chemical intermediate, is in-
dustrially produced by reacting liquid methanol (MeOH) with
CO under high pressure, 2–4 MPa, and temperatures between
323–423 K over a potassium/sodium alkoxide catalyst.[1–3] The
efficiency of this process is rather poor. It requires recirculation
of MeOH into the reaction system. In addition, the catalyst
yields undesirable alkali methyl formates that tend to deposit
on pipes and valves, thereby causing plant stops. Viable green
routes to produce MF, that is, at low temperature (T<473 K)
and atmospheric pressure, are the aerobic photocatalytic oxi-
dation of MeOH[4–6] and the aerobic gas-phase oxidation of
MeOH over solid catalysts. Particularly, Kominami et al.[4] report-
ed a MF yield of 91% at a MeOH conversion lower than 10%
for a TiO2 photocatalyst working in flow mode. Other photoca-
talytic studies on the oxidation of MeOH to MF have been de-
voted to the establishment of a reaction mechanism.[5,6] Con-
cerning gas-phase oxidation, supported and unsupported
MoO3, V2O5, H3+nPVnMo12ÀnO40, RuO2, and PdO have been
tested in the reaction.[1,7–9] The challenge for an industrially
green process is to operate the reaction at low temperatures
and pressures while having high MeOH conversion but without
sacrificing MF at the expense of the more thermodynamically
favored COx. Except for noble-metal-based catalysts, the afore-
mentioned oxides perform modestly in the reaction.[1,7] The
performance of noble metals has been related to their reduci-
bility.[8–11] Lichtenberger et al.[9] showed that reduced palladium
species perform better in the reaction than oxidized PdO. Woj-
cieszak et al.[10–13] demonstrated that reduced palladium nano-
particles prepared by a microemulsion method (MicE) are
promising catalysts for the green production of MF, that is, at
ambient pressure and T=353 K. This contribution concerns the
green aerobic oxidation, at ambient pressure and T=323 K, of
MeOH to MF over TiO2-supported Pd(z)Pt(1Àz) catalysts. This
bimetallic system was selected to benefit from the probable
synergetic effects coming from the individual properties of
each metal in oxidation. On the one hand, platinum is known
to be a very efficient catalyst in oxidation reactions, and, on
the other hand, palladium has shown a high selectivity to
MF.[9–13] Several papers report on the synergy between Pd and
Pt for diverse reactions such as the hydrogenation of aromat-
[a] Prof. V. G. Baldovino-Medrano, Prof. E. M. Gaigneaux, Prof. P. Ruiz
Institute of Condensed Matter and Nanosciences – IMCN
Division “MOlecules, Solids and ReactiviTy – MOST”
UniversitØ catholique de Louvain
Croix du Sud 2L7.05.15
B-1348 Louvain-la-Neuve (Belgium)
E-mail: vicbaldo@uis.edu.co
[b] Dr. G. Pollefeyt, Prof. I. Van Driessche
Department of Inorganic and Physical Chemistry
Ghent University
Krijgslaan 281-S3
9000 Gent (Belgium)
[c] Dr. V. Bliznuk
Department of Materials Science and Engineering
Ghent University, Technologiepark Zwijn
9052 Zwijnaarde (Belgium)
[d] Dr. R. Wojcieszak
Univ. Lille, CNRS, Centrale Lille, ENSCL
Univ. Artois, UMR 8181—UCCS—UnitØ de Catalyse et Chimie du Solide
F-59000 Lille (France)
E-mail: robert.wojcieszak@univ-lille1.fr
[e] Prof. V. G. Baldovino-Medrano
Current address: CICAT-UIS (@CICATUIS)
Universidad Industrial de Santander
680006 Bucaramanga (Colombia)
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201501211.
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ics[14–19] and oxygenates,[20–22] and the combustion of hydrocar-
bons.[23] The bulk molar ratio between the metals is often criti-
cal for the catalytic performance. Therefore, the methodology
adopted herein includes a study of the effect of the bulk Pd/
(Pd+Pt) molar ratio (z) on the physicochemical and catalytic
properties of two series of TiO2-supported Pd(z)Pt(1Àz) materi-
als. The first series of catalysts was prepared by an incipient
wetness (co-)impregnation (IWI method) and the second by
the in situ reduction of metal precursors in a water-in-oil mi-
croemulsion (MicE method).[10–13,24,25] Important differences be-
tween these two methods exist. The IWI method favors the
formation of oxidized nanoparticles as it comprises a calcina-
tion step, whereas the MicE method leads to the direct deposi-
tion of reduced nanoparticles onto the support. Nanoparticles
obtained by both methods were considered to serve as precur-
sors for the catalytically active phase because the catalysts
were always reduced under hydrogen flow before testing. In
this manner, it was possible to determine what preparation is
more convenient for a probable industrial application. Catalysts
were characterized at different stages by inductively coupled
plasma atomic emission spectroscopy (ICP-AES), X-ray diffrac-
tion (XRD), transmission electron microscopy–energy dispersive
X-ray analysis (TEM-EDX in STEM or HRTEM modes), and X-ray
photoelectron spectroscopy (XPS).
Table 1. Crystallinity of fresh and reduced Pd(z)Pt(1Àz)/TiO2 catalysts.[a]
Catalyst
Crystallinity (XRD)
Size [nm] Phase
2q
b
Fresh
Pd(1.0)-IWI
Pd(0.82)Pt(0.18)-IWI
33.85
N.D.
0.379 23
PdO(101)
–
–
–
40.06
46.54
39.98
46.56
39.75
46.18
40.09
N.D.
0.446 19
0.420 21
0.289 31
0.263 36
0.286 32
0.303 30
Pt(111)+Pd(111)*
Pt(200)+Pd(200)*
Pt(111)+Pd(111)*
Pt(200)+Pd(200)*
Pt(111)
Pt(200)
Pd(111)
–
Pd(0.65)Pt(0.35)-IWI
Pd(0.48)Pt(0.52)-IWI
Pt(1.0)-IWI
Pd(1.0)-MicE
0.935
9
–
–
–
Pd(0.81)Pt(0.19)-MicE
Pd(0.65)Pt(0.35)-MicE
Pd(0.44)Pt(0.56)-MicE
Pt(1.0)-MicE
–
–
–
N.D.
N.D.
40.14
–
–
0.479 18
Pt(111)
Reduced
Pd(1.0)-IWI
40.35
40.09
40.04
39.98
46.56
39.86
39.78
N.D.
0.573 15
0.585 15
Pd(111)
Pd(0.82)Pt(0.18)-IWI
Pd(0.65)Pt(0.35)-IWI
Pd(0.48)Pt(0.52)-IWI
Pt(111)+Pd(111)*
Pt(111)+Pd(111)*
Pt(111)+Pd(111)*
Pt(200)+Pd(200)*
Pt(111)
Pd(111)
–
Pt(111)+Pd(111)*
–
Pt(111)
0.55
19
0.289 31
0.263 36
0.227 41
0.308 29
Pt(1.0)-IWI
Pd(1.0)-MicE
Pd(0.81)Pt(0.19)-MicE
Pd(0.65)Pt(0.35)-MicE
Pd(0.44)Pt(0.56)-MicE
Pt(1.0)-MicE
–
–
39.69
N.D.
40.14
0.632 14
–
–
0.242 38
Results
[a] b=Full width at half maximum. N.D.=Not detected. *Ambiguous at-
tribution.
Bulk metallic composition of the catalysts
Table S1 (in the Supporting Information) presents the bulk
metal contents of the TiO2-supported Pd(z)Pt(1Àz) catalysts.
The results show that the Pd and Pt contents of the catalysts
prepared by the MicE method are closer to nominal values
than those prepared by the IWI method. A maximum loss of
36% in the (Pd+Pt) loading of Pd(z)Pt(1Àz)-IWI catalysts was
observed for Pd(0.82)Pt(0.18)-IWI, whereas a loss of 5 wt.% was
observed for Pd(0.65)Pt(0.35)-MicE.
duction, only the Pt(111) phase was detected, along with an
increase in crystallite size. For Pd(z)Pt(1Àz)-IWI diffraction
peaks at angles close to those of fresh Pt(1.0)-IWI were identi-
fied. However, these peaks cannot be unambiguously ascribed
to Pt(111) and Pt(200) as they are not sufficiently separated
from the peaks of Pd(111) and Pd(200). Comparing the fresh
and reduced samples, for Pd(0.82)Pt(0.18)-IWI a peak corre-
sponding to Pt(111)+Pd(111) evolved after reduction, where-
as for Pd(0.65)Pt(0.35)-IWI the peak ascribed to Pt(200)+
Pd(200) was no longer observed. For Pd(0.48)Pt(0.52)-IWI, no
changes in the XRD pattern were observed. These trends indi-
cate an evolution of the metallic crystallites during reduction
as a function of z. Considering that the Scherrer equation
mostly provides information on the size of nanoparticles
bigger than approximately 10 nm,[11,25] and supported by TEM
measurements (see the next section), the detection of the
Pt(111)+Pd(111) phase after reduction of Pd(0.82)Pt(0.18)-IWI
can be ascribed to the growth of nanoparticles smaller than
10 nm. These trends indicate an evolution of the size of the
metallic crystallites during reduction as a function of z.
Crystallinity of the catalysts
Table 1 summarizes the crystalline phases of palladium and
platinum as identified by XRD as well as the average size of
the detected crystallites (the XRD patterns are presented in the
Supporting Information: Figures S1–S4).
For the TiO2 support, the typical diffraction pattern (Figure S1)
of a mixture of anatase and rutile was found, which was in
agreement with a previous report.[11] Concerning Pd(1.0)-IWI, the
fresh catalyst, that is, only calcined, exhibited a diffraction peak
at 2q=33.858 that corresponds to PdO(101). This was the only
crystalline oxide phase identified within the whole series of ana-
lyzed materials. After reduction, the PdO(101) phase was not
observed and, instead, a Pd(111) phase was detected. The size
of the crystallites exhibiting this phase, 15 nm, was lower than
that from the PdO(101) phase, 23 nm. For fresh Pt(1.0)-IWI, two
crystalline phases, Pt(111) at 39.758 and Pt(200) at 46.188, were
identified. The crystallite size of these two phases was similar:
32 and 30 nm for Pt(111) and Pt(200), respectively. After re-
Morphology and composition of the metallic phases of the
catalysts
TEM measurements in HRTEM and STEM-EDX modes were per-
formed on selected samples. First, fresh Pd(1.0)-IWI, Pd(1.0)-
MicE, Pt(1.0)-IWI, Pt(1.0)-MicE, and spent Pd(1.0)-IWI and
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Pt(1.0)-IWI were studied. Afterwards, samples of all spent
Pd(z)Pt(1Àz)-IWI and Pd(z)Pt(1Àz)-MicE were analyzed. It must
be mentioned that for Pd(z)Pt(1Àz)-IWI, the amount of nano-
particles observed per grain of catalyst varied significantly. For
example, in some cases none or very few nanoparticles per
grain of TiO2 were observed, whereas a high concentration of
small nanoparticles was also observed in a TiO2 single grain.
Figure 1 shows selected images for fresh Pd(1.0)-IWI (Fig-
ure 1a), Pd(1.0)-MicE (Figure 1b), Pt(1.0)-IWI (Figure 1c), and
Pt(1.0)-MicE (Figure 1d). Mostly, rounded particles were ob-
served for fresh Pd(1.0)-IWI, Pd(1.0)-MicE, and Pt(1.0)-MicE, in
which only a few edged particles were identified (Figure 1a, b,
d, respectively). Fresh Pt(1.0)-IWI displayed a series of faceted
and cornered particles with a polyhedral and tetrahedral ge-
ometry typical of sintered Pt supported catalysts (Fig-
ure 1c).[32,33] Such particles are normally enclosed by (111)
facets.[33,34] Some of the platinum nanoparticles appeared
rounded or exhibited a truncated tetrahedral or cubic-like
shape (Figure 1c). These truncated particles normally comprise
(200) facets.[33]
method leads to highly heterogeneous particle size distribu-
tions (PSDs) with statistical coefficients of variation, CV, higher
than 60%. In contrast, the MicE method led to more homoge-
neous PSD with CVꢀ35%. Comparing fresh catalysts, the Pd
catalysts displayed much lower average particle size (<Dp >)
than the Pt catalysts, regardless of the preparation method. Fi-
nally, spent monometallic catalysts prepared by the IWI
method displayed a decrease in <Dp > compared with the
fresh catalysts.
Figure 2 presents TEM particle size distributions for the
spent bimetallic catalysts. The trends in particle size distribu-
tion for spent Pd(z)Pt(1Àz)-IWI and Pd(z)Pt(1Àz)-MicE were dif-
ferent. Spent catalysts prepared with the IWI method showed
average <Dp > values smaller than the spent catalysts pre-
pared with the MicE method. Moreover, PSDs for Pd(z)Pt(1Àz)-
IWI were skewed to Dp values lower than 11.8 nm. Further-
more, the IWI method led to catalysts with more heterogene-
ous PSDs compared with the MicE method. This fact is evident
from the high variability of the coefficients of variation in Fig-
ure 2a. It is apparent that PSDs for both groups of TiO2-sup-
ported Pd(z)Pt(1Àz) catalysts are affected by z. To be sure that
In general, TEM results for the monometallic catalysts (Fig-
ure S5 in the Supporting Information) showed that the IWI
Figure 2. Particle size distributions for spent a) Pd(z)Pt(1Àz)-IWI and
b) Pd(z)Pt(1Àz)-MicE catalysts and summary of statistics as derived from TEM
measurements.
Figure 1. Selected TEM images of the fresh monometallic samples of TiO2-
supported Pd(z)Pt(1Àz) as acquired in STEM mode.
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such an effect was statistically meaningful, we performed
a single factor ANOVA test for both series of prepared cata-
lysts. Results of this test showed a p-value=9.97”10À15 for
Pd(z)Pt(1Àz)-IWI and a p-value=7.12”10À3 for Pd(z)Pt(1Àz)-
MicE. These p-values allow us to ascertain that indeed PSDs
depend on z to a confidence level of 95%. On the other hand,
by comparing both p-values, it can be said that the effect of z
on PSDs is stronger for catalysts prepared with the IWI
method.
For the spent Pd(z)Pt(1Àz)-IWI catalysts, a p-value=1.85”
10À27 was obtained, which implies that the Pd/(Pd+Pt) molar
ratio employed for catalyst preparation has a strong effect on
the composition of the metallic nanoparticles formed on the
catalysts. In the case of the MicE series of catalysts, a p-value=
2.7”10À3 was obtained. Such a value is very close to the
a value of the ANOVA test (a=0.05), which implies that al-
though the molar ratio of the metals used for preparing the
catalysts has an effect on the final composition of its metallic
nanoparticles, such an effect is rather weak. Therefore, al-
though for the IWI method, z has a deep impact on the final
composition of the metallic nanoparticles of the catalysts, this
is not the case for the MicE preparation method.
Figures 3 and 4 show the statistical distribution of the molar
Pd/(Pd+Pt) ratio of the metallic nanoparticles analyzed by
HRTEM-EDX (zEDX
)
for the spent Pd(z)Pt(1Àz)-IWI and
Pd(z)Pt(1Àz)-MicE catalysts. Regardless of the bulk metallic
molar ratio and the preparation method, a mixture of mono-
and bimetallic nanoparticles was observed (HRTEM images en-
closed in Figures 3 and 4). To determine if the bulk metallic
molar ratio of the catalysts had a statistically meaningful effect
on the composition of the nanoparticles, a single factor
ANOVA test was also performed, in this case giving a confi-
dence level of 95%.
On the other hand, it is important to notice that the MicE
method led to a higher relative fraction of Pd-enriched nano-
particles, zTEM =0.8–0.9, and monometallic Pd nanoparticles
compared with the IWI method. In addition to the above ob-
servations, both the mono- and bimetallic nanoparticles be-
longing to spent Pd(0.65)Pt(0.35)-IWI (Figure 3) exhibited typi-
cal lattice fringes in the orientations (111) and (200). However,
the nanoparticles belonging to spent Pd(0.65)Pt(0.35)-MicE
showed lattice fringes only in the (111) orientation. In any
case, the acquired HRTEM images did not show the existence
of core–shell Pd–Pt bimetallic nanoparticles.
Composition and chemical state of the surface of the
catalysts
The surface XPS composition for both the fresh and spent cat-
alysts is presented in Table S2 (in the Supporting Information)
in reference to the element/Ti molar ratios. The presence of
several impurities belonging to the commercial TiO2 support
was unexpectedly revealed; namely, potassium, phosphorus,
and calcium. Of these impurities, only potassium was not
found on Pd(z)Pt(1Àz)-MicE, whereas phosphorous and calci-
um remained at similar concentration levels for all of the
catalysts.
Figure 3. Distribution of the composition of metallic nanoparticles for spent
TiO2-supported Pd(z)Pt(1Àz)-IWI catalysts as calculated from HRTEM-EDX.
Probe diameter=0.5–1.0 nm.
Comparing the C/Ti ratios, two trends can be remarked
upon: first, Pd(z)Pt(1Àz)-MicE catalysts presented more surface
carbon, which can be ascribed to carbonaceous residua from
the preparation method; second, C/Ti ratios did not increase in
the spent catalysts, which suggests that no coke deposition
occurred during the catalytic tests.
XPS results showed that the relative concentration of palla-
dium on the surface is higher than the relative concentration
of platinum, regardless of the preparation method and wheth-
er the catalysts were fresh or spent. This is in good agreement
with the TEM results mentioned earlier.
Table S3 (in the Supporting Information) presents the results
obtained for the decomposition of the Pd3d core level for the
prepared catalysts. The following general observations can be
made: (i) regardless of the preparation method, there was
always a combination of Pd0 and Pdd+ before and after the
catalytic tests; (ii) the relative percentages of Pdd+ or Pd0 re-
mained more or less constant with z; (iii) Pd(z)Pt(1Àz)-MicE cat-
alysts clearly distinguished themselves from Pd(z)Pt(1Àz)-IWI
because a majority of Pd0 species was always present on their
Figure 4. Distribution of the composition of metallic nanoparticles for spent
TiO2-supported Pd(z)Pt(1Àz)-MicE catalysts as calculated from HRTEM-EDX.
Probe diameter=0.5–1.0 nm.
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surface; (iv) a high fraction of palladium present in fresh
Pd(z)Pt(1Àz)-IWI was Pdd+, but almost 80% of palladium in
spent Pd(z)Pt(1Àz)-IWI was Pd0. This percentage was similar to
the one determined for Pd(z)Pt(1Àz)-MicE. Concerning plati-
num, Table S4 (in the Supporting Information) shows the iden-
tified binding energies (BE) for the Pt4f7/2 core level as well as
the FWHM values of the peaks. The results allow us to ascer-
tain that a high fraction of platinum was present in fresh
Pd(z)Pt(1Àz)-IWI as Ptd+ and then further reduced to Pt0
during the catalytic tests. Conversely, in Pd(z)Pt(1Àz)-MicE
a higher fraction of platinum was always as Pt0.
Stability test
Considering the superior performance of Pd(0.65)Pd(0.35)-MicE
in the reaction, a stability test was conducted for this catalyst.
Figure 5 shows the evolution of the catalytic performance with
time on stream. After 1800 min, the catalyst lost 7% of its ini-
tial MeOH conversion and 25% of its initial MF production.
These results demonstrate that even though the catalyst does
not rapidly deactivate, its selectivity can be affected in the
long term.
Catalytic performance
As reported previously,[11,13] the TiO2 support was not active
under the reaction conditions employed herein. Table 2 pres-
ents the results for the performance of the catalysts in the
reaction.
Pd(z)Pt(1Àz)-MicE displayed the best catalytic performance
and bimetallic Pd(z)Pt(1Àz)-MicE were always more effective in
the reaction than the monometallic catalysts.
A maximum in MF selectivity, SMF =0.67, was obtained for
Pd(0.65)Pd(0.35)-MicE at XMeOH =0.78. Pt(1.0)-MicE displayed the
highest MeOH conversion for the series (XMeOH =0.87), but with
Figure 5. Evolution of the catalytic performance, methanol conversion, and
methyl formate yield for Pd(0.65)Pt(0.35)-MicE during a stability test at
323 K, patm, 100 cm3 minÀ1 of 5 vol.% MeOH and 2.5 vol.% of O2 in He.
a
SMF value about three times lower, SMF =0.23, than
Pd(0.65)Pd(0.35)-MicE. Pd(1.0)-MicE displayed lower MeOH con-
version (XMeOH =0.68), but a slightly better SMF value, SMF =0.29,
than Pt(1.0)-MicE. The best Pd(z)Pt(1Àz)-IWI catalyst was
Pd(0.65)Pd(0.35)-IWI. It is important to note that this is indeed
the same z value as for the best catalyst in the reaction. The
conversion for Pd(0.65)Pd(0.35)-IWI was XMeOH =0.56 with a cor-
responding selectivity SMF =0.48. Pt(1.0)-IWI displayed higher
Discussion
Establishment of synergetic effects in TiO2-supported
Pd(z)Pt(1Àz)
To establish the existence of synergetic effects between palla-
dium and platinum, we prepared the plots presented in
Figure 6 according to the definitions given in the Supporting
Information. The figure shows the observed XMeOH and yield of
MF (YMF) by bulk mmol of (Pd+Pt) as a function of the corre-
sponding calculated XMeOH and YMF by bulk mmol of (Pd+Pt)
for all of the TiO2-supported Pd(z)Pt(1Àz) catalysts. For the
sake of clarity, a 458 line representing the absence of synerget-
ic effects was traced. The behaviors of monometallic catalysts
naturally fall within this line. For bimetallic catalysts, points
above the 458 line imply synergy. The farther the values above
this line are, the stronger the apparent synergetic effect is. Ac-
cordingly, Pd(z)Pt(1Àz)-IWI catalysts exhibited synergetic ef-
fects for both the conversion of MeOH and for the production
of MF, Figure 6a and c, respectively. For Pd(z)Pt(1Àz)-MicE the
synergetic effects were very strong for MF production but very
weak for the conversion of MeOH, Figure 6b and d, respective-
ly. The bottom line is, thus, that the preparation method plays
a central role on the synergetic behavior of platinum and
palladium.
methanol conversion and selectivity, XMeOH =0.38 and SMF
=
0.44, than Pd(1.0)-IWI (XMeOH =0.16, SMF =0.13). One fact worth
noticing is that Pd(1.0)-IWI produced much less MF than
Pt(1.0)-IWI, whereas the contrary was true for Pd(1.0)-MicE. Fi-
nally, a comparison between the two series of catalysts leads
to the general conclusion that the MicE method is of much
more interest for a probable industrial application (STYMF
values in Table 1).
Table 2. Catalytic performance of TiO2-supported Pd(z)Pt(1Àz) catalysts
in methanol partial oxidation.[a]
Catalyst
XMeOH
[%]
YMF
[%]
SMF
[%]
SCO
[%]
STYMF
[kgMF ”kgcatÀ1 hÀ1
]
2
Pd(1.0)-IWI
0.16
0.29
0.56
0.32
0.38
0.02
0.08
0.29
0.06
0.16
0.13
0.28
0.48
0.19
0.44
0.87
0.72
0.52
0.81
0.56
29.4
117.6
426.3
88.2
Pd(0.82)Pt(0.18)-IWI
Pd(0.65)Pt(0.35)-IWI
Pd(0.48)Pt(0.52)-IWI
Pt(1.0)-IWI
235.2
Pd(1.0)-MicE
0.68
0.72
0.78
0.85
0.87
0.25
0.27
0.52
0.45
0.20
0.29
0.37
0.67
0.53
0.23
0.71
0.63
0.33
0.47
0.77
367.5
396.9
764.4
661.5
294.0
Pd(0.81)Pt(0.19)-MicE
Pd(0.65)Pt(0.35)-MicE
Pd(0.44)Pt(0.56)-MicE
Pt(1.0)-MicE
Particle size effects on the catalytic performance
[a] Conditions: T=323 K, ambient pressure, total feed flow=
100 cm3 minÀ1 with 5 vol.% MeOH and 2.5 vol.% O2.
The preparation method clearly influenced the particle size dis-
tribution of the TiO2-supported Pd(z)Pt(1Àz) catalysts. Particle
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Figure 6. Synergy curves for TiO2-supported Pd(z)Pt(1Àz) catalysts in the methanol oxidation to methyl formate. a) and b) MeOH conversion for Pd(z)Pt(1Àz)-
IWI and Pd(z)Pt(1Àz)-MicE, respectively; c) and d) MF production for Pd(z)Pt(1Àz)-IWI and Pd(z)Pt(1Àz)-MicE, respectively. T=323 K and atmospheric pressure,
methanol to oxygen molar ratio=2.
size distributions of Pd(z)Pt(1Àz)-IWI were strongly influenced
by the molar ratio of the metals employed for catalyst prepara-
tion, whereas those for Pd(z)Pt(1Àz)-MicE were weakly affected
by z. At the same time, Pd(z)Pt(1Àz)-IWI showed stronger syn-
ergetic effects regarding XMeOH than Pd(z)Pt(1Àz)-IWI. This sug-
gests that synergetic effects between palladium and platinum
in the oxidation of methanol to methyl formate are influenced
by the degree of heterogeneity in particle size distributions.
As early as 1975, Gómez et al.[14] reported no synergetic effects
on benzene hydrogenation over Pd(z)Pt(1Àz)/Al2O3 catalysts,
which exhibited a narrow, almost homogeneous, particle size
distribution. Rousset et al.[27,28] prepared Pd(z)Pt(1Àz)/a-Al2O3 cat-
alysts with homogeneous particle size distributions, which did
not exhibit synergetic effects in aromatic hydrogenation.
It is important to notice that despite the weak synergetic ef-
fects in MeOH conversion displayed by Pd(z)Pt(1Àz)-MicE,
these catalysts were more active in the reaction than
Pd(z)Pt(1Àz)-IWI and that synergetic effects for the production
of MF were indeed present for both series of catalysts. The lit-
erature also indicates that one of the characteristics that make
Pd nanoparticles very active and selective in the reaction is
a narrow particle size distribution.[10,11] This characteristic is
present for Pd(z)Pt(1Àz)-MicE.
ing,[52–54] because, as Miller et al. puts it,[54] the method de-
pends on many different variables at the same time: “support
composition, metal salt, method of metal addition, pH, metal
loading, calcination temperature, etc.” On the other hand, the
literature shows that MeOH oxidation is strongly influenced by
particle size effects. Louis et al.[52] prepared a series of MoO3/
SiO2 catalysts with different molybdenum loadings by impreg-
nation and grafting for this reaction. They found a strong
effect of the Mo loading on the particle size distribution of the
impregnated catalysts. Because of this fact, it was not possible
to establish a correlation between the dispersion of MoO3 and
the catalytic performance. Conversely, a straight correlation be-
tween MeOH turnover numbers and MoO3 dispersion for the
grafted catalysts, the dispersion of which did not depend on
the Mo loading, was established. A similar correlation was not
found for SMF, however. One may notice the resemblance be-
tween these results and the ones presented herein. Regarding
an explanation for this effect, the relationship between the
synergy in the catalytic activity and the heterogeneity in parti-
cle size distribution of Pd(z)Pt(1Àz)-IWI could be associated
with some sort of cooperative effect between small and bigger
nanoparticles. For example, very recently, Fernµndez et al.[55]
evoked hydrogen spillover from big to small nanoparticles in
mechanical mixtures to explain the catalytic behavior of Ru/
Al2O3 in ammonia synthesis.
Concerning the effect of the preparation method, despite its
wide use, impregnation methods for catalyst preparation yield
particle size distributions that strongly depend on metal load-
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Effect of nanoparticles composition and structure on the
catalytic performance
The figure shows a correlation between the composition of
the nanoparticles and the catalytic performance. The trend ob-
served in Figure 7b demonstrates that the increase in the con-
centration of bimetallic nanoparticles whose molar Pd/(Pd+Pt)
ratio is above 0.6 promotes the catalytic performance of TiO2
supported Pd(z)Pt(1Àz).
Regardless of the preparation method, the strongest synergetic
effect was observed for catalysts prepared with z=0.65. TEM-
EDX results showed that preparing bimetallic catalysts with
a given bulk molar composition, z, does not imply that the
composition of the nanoparticles of the synthesized catalyst
will have such a composition. Furthermore, a mixture of mono-
metallic and bimetallic nanoparticles of different compositions
is present in the TiO2-supported Pd(z)Pt(1Àz) catalysts studied
herein. For catalysts prepared with z=0.65, as already men-
tioned, most of the analyzed bimetallic nanoparticles had
a Pd/(Pd+Pt) molar ratio within the range 0.7<zTEM <1.0.
Figure 7 plots the apparent rates of reaction as a function of
the composition of the metallic nanoparticles as measured by
HRTEM-EDX.
One may notice that the presence of monometallic nanopar-
ticles and bimetallic nanoparticles whose composition is within
the range 0<zTEM ꢁ0.6 has a negative impact on the catalytic
performance (Figure 7a and b). Most literature works report
synergetic effects for Pd(z)Pt(1Àz) catalysts with z=0.6–
0.8.[16,56,57] At present, there is not a satisfactory explanation for
why such a composition leads to these effects. In the case of
hydrotreatment reactions, Jiang et al.[57] found that a substitu-
tion of Pd atoms on Pt(111) crystallites changes the dynamics
of the competitive adsorption and dissociation of sulfur, H2,
and H2S. The geometric structure of the Pd-replaced bimetallic
surface is more favorable for the adsorption of H2 compared
with H2S and S atoms and this, in addition, increases the ad-
sorption energy of H2 on the surface. One may speculate that
a similar structural configuration of the bimetallic Pd–Pt nano-
particles could be behind the trends in catalytic performance
for TiO2-supported Pd(z)Pt(1Àz). By considering that Pauling’s
electronegativity for oxygen (3.44) is higher than that for sulfur
(2.58),[31] one may postulate that if a given surface has a lower
affinity for sulfur it would likely also have a lower affinity for
oxygen. In this sense, a weakening of the oxygen–metal bond
during the oxidation reaction could be expected. Such weak-
ening would facilitate the surface reaction of adsorbed me-
thoxy species (CH3Oads), which are responsible for methyl for-
mate production,[58–60] and would inhibit the oxidation of these
intermediates to COads and therefore to gaseous COx. However,
testing the above hypothesis is out of the scope of this work.
Role of palladium and platinum on the catalytic performance
The XPS results showed that metallic Pd0 and Pt0 species
should be prevalent on the active phase of TiO2-supported
Pd(z)Pd(1Àz). On the other hand, the XRD and TEM results
demonstrated that the crystalline structure of the nanoparticles
of the reduced catalysts corresponds to Pd(111) and/or
Pt(111). As previously mentioned, the resolution of the XRD
peaks obtained during these tests was not enough as to clear-
ly distinguish Pd(111) from Pt(111) in the bimetallic catalysts
(Figure S1). Therefore, a fully satisfactory explanation of the
role of these phases in the reaction cannot be obtained from
the present work. From the literature, one might analyze some
key factors on the subject and thus try to correlate them with
the behavior of the TiO2-supported Pd(z)Pd(1Àz) catalysts
nonetheless. Particularly, DFT calculations have shown that the
energy of adsorption of CH3O over Pt(111) is lower, À1.80 eV,
compared with that over Pd(111), À1.99 eV.[61] This implies that
platinum is more effective than palladium for CH3Oads decom-
position; a fact that is in agreement with the catalytic trends
presented herein. In addition, Ren et al.[61] also showed that
CH3O is preferentially adsorbed on a face-centered cubic (fcc)-
h2(C,O) configuration on Pt(111), that is, with the oxygen
Figure 7. Steady-state apparent reaction rates as a function of the relative
fraction of nanoparticles (N), the composition (zTEM) of which was deter-
mined by HRTEM-EDX measurements performed on the spent catalysts.
a) Bimetallic nanoparticles with 0<zTEM ꢁ0.6; b) bimetallic nanoparticles
with 0.6<zTEM <1; c) monometallic Pd (zTEM =1) and Pt (zTEM =0) nanoparti-
cles (NMonometallic). Unfilled marks correspond to average values.
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bound to one Pt atom, whereas an h1(O) configuration, in
which the oxygen atom of CH3O is triply bound to the surface
and at shorter distance compared with the fcc-h2(C,O) configu-
ration, is favored for CH3O adsorption on Pd(111). The reactivi-
ty of methanol over the bimetallic catalysts would be given by
a competition between both mechanisms and by a competitive
adsorption and transformation of the reaction intermediates
over the available surface palladium and platinum atoms. Lich-
tenberger et al.[9] described three possible mechanisms for MF
formation: (i) condensation of CH3Oads with h2(C,O)–formalde-
hyde species to form CH3OCH2OH (methoxymethanol) inter-
mediates, which then dehydrogenate to methyl formate; (ii) es-
terification of formic acid (HCOOH) intermediates formed by
CH2O oxidation; or (iii) CH2O dimerization through a Tischenko-
type reaction. In addition, Levis et al.[58–60] proposed that stable
CH3ads species from the scission of CH3OHads could also be im-
lysts prepared by incipient wetness (co)-impregnation, IWI,
were less efficient in the reaction despite also displaying syner-
gy between palladium and platinum. The key factors behind
such trends were found to be: first, narrower particle size dis-
tributions and less dependence on the metallic palladium to
platinum molar ratio used for catalysts preparation in the
water-in-oil microemulsion method compared with incipient
wetness (co)-impregnation; second, the presence of an impor-
tant amount of bimetallic nanoparticles with a palladium to
platinum molar higher than 0.6; third, the constitution of an
active phase composed of palladium and platinum species of
metallic character, particularly, zerovalent palladium and plati-
num with crystalline phases corresponding to Pd(111) and
Pt(111). It was postulated that the role of platinum in the bi-
metallic species would be to promote the activation of the re-
actants whereas palladium would act a selectivity modifier that
promotes MF formation. The ensemble of the results presented
in this paper contribute to the development of green synthesis
processes for methyl formate production.
portant for oxidation mechanism by their reaction with Oads
.
Tentatively, the behavior of Pd(z)Pd(1Àz) in the reaction can be
explained by considering a mechanism in which platinum
atoms would rapidly activate MeOH molecules in an fcc-
h2(C,O) mode whereas palladium would play the role of a stabil-
izer for the corresponding reaction intermediates by adsorbing
them in the h1(O) configuration. Moreover, the MeOH mole-
cules activated by palladium atoms would also present a more
stable surface configuration. This hypothesis is in better agree-
ment with mechanism (i). Therefore, the synergetic effects ob-
served in the production of methyl formate with Pd(z)Pt(1Àz)
could be ascribed to the stabilization of different species:
CH3Oads, h2(C,O)–formaldehyde, CH2Oads, or HCOOH, on the
metallic active phase and further condensation to form MF. Fi-
nally, if, as proposed by Levis et al.,[58–60] stable CH3ads species
are participating in the mechanism of reaction, the surface oxi-
dative dehydrogenation of such species could promote Cads
species, which are known coke precursors. The relationship be-
tween the latter phenomenon and the slight deactivation ob-
served for Pd(0.65)Pt(0.35)-MicE during the long duration test
deserves future studies. In addition, a correlation between the
XRD, TEM, and XPS results indicated that both the small and
big nanoparticles of the catalysts grew during the catalytic
tests. The losses in catalytic activity and MF productivity during
the stability test could be associated with particle size growth.
Although some pieces of evidence are still lacking to gain fur-
ther insight into these phenomena, it can be concluded with
certainty that particle size distribution is a central player in the
existence of synergetic effects in the reaction. As MeOH oxida-
tion is a surface sensitive reaction,[1] the explanation behind
this trend should be related to the structure and to the chemi-
cal state of the active phase of the catalysts.
Experimental Section
Catalysts preparation
Commercial grade TiO2 (Merck) was employed as the support with-
out further treatment. The Brunauer–Emmett–Teller (BET) surface
area of this material is 8 m2 gÀ1 [10,11,24,25]
The selection of such a low
.
area support was based on previous works, where their advantages
for partial oxidation reactions were demonstrated owing to the
deposition of the catalytic active phase on the external surface of
the support.[10–13,24,25] For both series of catalysts, a nominal Pd+Pt
metallic content of 1 wt.% was fixed. Appropriate amounts of am-
monium tetrachloroplatinate(II) [(NH4)2PtCl4, Johnson Matthey] and
ammonium tetrachloropalladate(II) [(NH4)2PdCl4, 97%, Sigma–Al-
drich] were used as the metallic precursors. For the IWI method,
aqueous solutions of these precursors were poured drop by drop
onto the TiO2 support. As the support became progressively
wetted, a slurry was formed. This slurry was left to dry at ambient
conditions and then calcined for 4 h at 773 K in a static oven. The
MicE method was adapted from previous works.[10–13,24,25] Briefly, it
consists on the in situ reduction of an aqueous phase of the metal-
lic precursors, which form a microemulsion with an oil phase by
means of an amphiphile surfactant. Herein, cyclohexane (commer-
cial grade, Sigma–Aldrich) was the oil phase, AOT [sodium bis(2-
ethylhexyl)sulfosuccinate, 96%, Sigma–Aldrich] was the surfactant,
and hydrazine (80 wt.% in water, Fluka) was the reducing agent.
An appropriate amount of TiO2 was added to the reactant flask
containing the microemulsion at 323 K and under magnetic stir-
ring. After 30 min, hydrazine (3 mL) was injected into the reactant
flask while keeping the temperature constant. Reduction, under an
inert N2 atmosphere, was allowed to proceed until the suspension
in the flask turned black. Upon reduction completion, a slurry was
recovered from the suspension by filtering and washing with ace-
tone (99%, VWR) and hot distilled water. The catalysts thus synthe-
sized were afterwards dried at 373 K for 1 h and used as such.
Conclusions
The green production of methyl formate through methanol ox-
idation at industrially relevant reaction rates was achieved over
TiO2-supported Pd(z)Pt(1Àz) catalysts. It was determined that
the water-in-oil microemulsion preparation method, MicE,
leads to highly efficient catalysts that exhibit strong synergetic
effects for the production of methyl formate. Conversely, cata-
Bulk Pd and Pt contents and catalyst nomenclature
Bulk Pd and Pt contents were determined by ICP-AES with
a Thermo Jarrell ASH IRIS Advantage apparatus. Bulk metallic con-
tents (reported in Table S1) were converted to bulk molar ratios, z
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for Pd, [Eq. (1)], and (1Àz) for Pt, and these values were employed
in the catalyst nomenclature: Pd(z)Pt(1Àz). The suffixes -MicE, and
-IWI indicate the preparation method. For example, Pd(1.0)-MicE
stands for monometallic palladium prepared by the MicE method.
the photoelectron kinetic energy. The spectra were decomposed
with CasaXPS employing a Gaussian/Lorentzian (85/15) product
function after subtraction of a Shirley nonlinear sigmoid-type base-
line. The binding energy (BE) scale was corrected by using the C-
[C,H] component of the adventitious carbon peak at 284.8 eV as
a reference.[30] To determine the surface chemical state of palladi-
um, a contribution from metallic palladium (Pd0), Pd3d5/2 BEꢁ
335.0 eV,[25,37,40] and another from oxidized palladium (Pdd+),
Pd3d5/2 BE>335 eV,[18,25,36–37,40] were considered (Table S3). Peak de-
composition was performed with the following constraints: a) The
FWHM of all Pd components was fixed within the range 1.2–2.0 eV;
b) FWHM of Pd3d3/2 peaks=FWHM of Pd3d5/2 peaks, for the same
molPd
z ¼
ð1Þ
molðPd þ PtÞ
Crystallinity by XRD
An X-ray diffraction study was carried out with a Siemens D5000
diffractometer using CuKa radiation (l=1.5418 Š). The 2q range
was scanned between 2 and 708 at a rate of 0.01 8sÀ1. The identifi-
cation of the crystalline phases was made with the ICDD-JCPDS da-
tabase after analyzing the recorded spectra with CasaXPS (Casa
Software Ltd.) where the position of the peaks and their full width
at half maximum (FWHM) was determined. A Gaussian/Lorentzian
(70/30) product function and a linear background were employed
for peak fitting. Catalysts were examined after preparation and
after reduction under the same conditions as the reaction tests.
XRD spectra were recorded at least twice to confirm the presence
of the identified crystalline phases. The Scherrer equation[26] was
employed for estimating the crystallite size of the detected phases.
considered species; c) area of Pd3d3/2 =2/3”Pd3d5/2; d) a separa-
[36]
tion of 5.25 eV between the peaks Pd3d5/2 and Pd3d3/2
.
In addi-
tion, an asymmetry factor for Pd0 was introduced in the mathemat-
ical decomposition so as to reflect the asymmetry of the peak of
this species as observed in palladium foil samples.[25] Such a factor
corresponded to
a modified Gaussian/Lorentzian line shape:
A(0.25,0.25,0)GL(15) in CasaXPS. Finally, the contribution of elec-
trons coming from the Pt4d3/2 core level, BE=330–335 eV,[37] to the
Pd3d signal was not taken into account owing the very low inten-
sity of Pt4d3/2. Concerning platinum, the following constraints
were imposed for peak decomposition: a) the FWHM of the main
Pt4f7/2 peak was fixed within the range 1.5–2.0 eV; b) the FWHM of
the Pt4f5/2 peaks=FWHM of the Pt4f7/2 peaks; c) the area of the
Pt4f5/2 peaks=0.75”area of the Pt4f7/2 peaks; d) a separation of
[25]
Nanoparticle identification, morphology, size, and composi-
tion by TEM
3.35 eV between Pt4f5/2 and Pt4f7/2
.
Although, as, in the case of
palladium, two species of platinum could be present on the sur-
face of the catalysts; namely, one corresponding to Pt0 with BE for
Pt4f7/2 <71 eV,[36,40] and another corresponding to Ptd+ with BE for
Pt4f7/2 >72,[36,40] it was considered reasonable to use only one
component owing to the low intensity of the signal and to peak
overlapping with the Ti 3s plasmon loss peak for the TiO2 support.
In the case of Pd(0.82)Pt(0.18)-IWI, it was not possible to add com-
ponents corresponding to platinum into the peak because the
signal from platinum was completely masked by the Ti 3s plasmon
loss peak. Selected XPS spectra of the Pd3d and Pt4f core levels
are presented in the Supporting Information (Figures S6–S9).
TEM measurements were performed with a JEOL JEM-2200FS/Cs-
corrected FEG TEM instrument operated at 200 kV and provided
with an in-column omega filter. Two modes or measurement were
employed: namely, high resolution (HRTEM) and scanning (STEM)
mode. The probe diameter was 0.5–1.0 nm. By combining STEM
with energy dispersive X-ray analysis (EDX), we performed a system-
atic measurement of the composition of the observed individual
metallic nanoparticles. On the basis of these measurements, Pd/
(Pd+Pt) molar ratios (zTEM) for the individual nanoparticles were
defined in the same fashion as in Equation (1). Similar approaches
have been previously presented in the literature.[23,27,28]
Catalytic tests
Chemical composition and oxidation state by XPS
The aerobic gas-phase oxidation of methanol was performed at at-
mospheric pressure in a metallic fixed-bed microreactor made of
an inconel tube of 1 cm internal diameter (PID ENG&Tech). The cat-
alytic bed (ꢀ0.5 cm3) was composed of 100 mg of catalyst powder
selected within the granular fraction 200–315 mm and diluted in
600 mg glass spheres that were previously confirmed inactive.
Before testing, all catalysts were treated under a 50 cm3 minÀ1 flow
of pure hydrogen (99.999%, Praxair) at 573 K (heating ramp=
10 K”minÀ1) for 1 h, to reduce them under the same conditions.
After this treatment, the reactor was cooled down to the reaction
temperature: 323 K. The reactor feed consisted in 100 cm3 minÀ1 of
a gas mixture of 5 vol.% MeOH (anhydrous, 99.995%, Sigma–Al-
drich) and 2.5 vol.% O2 (99.999%, Praxair) diluted in He (99.999%,
Praxair). The gas mixture was prepared by passing the stream of
He through a container with liquid methanol, which was, in turn,
placed in a saturator maintained at 278 K. Once the flows of MeOH
and O2 were stable, as verified by carrying out several injections of
this stream into the online GC system, the reactants were allowed
into the reactor and the catalytic test was begun. Reaction prod-
ucts were analyzed each 30 min by using a CP3800 Varian GC pro-
vided with TCD detector. CP-Pora-PLOT Q (25 m; 0.53 mm) and CP-
Molsieve 5 Š (25 m; 0.53 mm) columns were used for the separa-
XPS measurements were carried out with an SSI-X-probe spectrom-
eter (SSX-100/206, Surface Science Instruments) equipped with
a monochromatic microfocused AlKa (1486.6 eV) X-ray source
(10 kV; 22 mA). Both the fresh and spent catalysts were analyzed.
At least three samples of the fresh catalysts were analyzed to test
the reproducibility of the measurements. Catalyst particles were
pressed into small stainless steel troughs mounted on a multi-
specimen ceramic sample holder. The analysis chamber was oper-
ated under ultrahigh vacuum with a pressure close to 5”10À7 Pa.
Charge stabilization was achieved by using an electron flood gun
adjusted at 8 eV and by placing a nickel grid 3 mm above the sam-
ples. The following sequence of spectra was recorded: general
spectrum, O1s, C1s+K2p, Ti 2p, P2p, Ca2p, Pd3d, Pt4f, and
C1s+K2p again to check the stability of charge compensation as
a function of time. General spectra were recorded at a pass energy
of 150 eV whereas the elements spectra were recorded at a pass
energy of 50 eV. The analyzed area was approximately 1.4 mm2 as
estimated from an elliptic spot of 1000 mm”1700 mm. Surface
molar concentrations were calculated by correcting peak intensi-
ties with theoretical sensitivity factors based on Scofield cross sec-
tions[29] and the mean free path varying according to 0.7 power of
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[24] R. Wojcieszak, M. J. Genet, P. Eloy, E. M. Gaigneaux, P. Ruiz, Stud. Surf.
Sci. Catal. 2010, 175, 789–792.
[25] R. Wojcieszak, M. J. Genet, P. Eloy, P. Ruiz, E. M. Gaigneaux, J. Phys.
Chem. C 2010, 114, 16677–16684.
[26] A. L. Patterson, Phys. Rev. 1939, 56, 978–982.
[27] J. L. Rousset, L. Stievano, F. J. C. S. Aires, C. Geantet, A. J. Renouprez, M.
Pellarin, J. Catal. 2001, 197, 335–343.
[28] A. Renouprez, J. L. Rousset, A. M. Cadrot, Y. Soldo, L. Stievano, J. Alloys
Compd. 2001, 328, 50–56.
[29] J. H. Scofield, J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129–137.
[30] P. G. Rouxhet, M. J. Genet, Surf. Interface Anal. 2011, 43, 1453–1470.
[31] M. Dayah, Dynamic Periodic Table, 1997, http://www.ptable.com (ac-
cessed September 2013).
tion of the reaction products. The concentrations of the following
molecules were monitored during the reaction: H2, O2, CH4, CO,
CO2, MeOH, MF, formaldehyde, formic acid, and H2O. Only O2,
MeOH, MF, H2O, and CO2 were detected at the reactor exit. The cat-
alytic performance was expressed in terms of the methanol molar
conversion (XMeOH), the molar yield to methyl formate (YMF), the se-
lectivities to MF (SMF) and to CO2 (SCO2), and of the space time yield
to methyl formate (STYMF), which were calculated as shown in the
Supporting Information. A stability test was performed under the
same reaction conditions of a conventional reaction test and after
1800 min.
[32] P. J. F. Harris, Appl. Catal. 1985, 16, 439–442.
[33] Z. L. Wang, T. S. Ahmad, M. A. El-Sayed, Surf. Sci. 1997, 380, 302–310.
[34] M. H. Yao, D. J. Smith, J. Microsc. 1994, 175, 252–265.
[35] I. Capek, Adv. Colloid Interface Sci. 2004, 110, 49–74.
[36] C. D. Wanger, W. M. Riggs, L. E. Davis, J. F. Moulder, G. Muilenberg,
Handbook of X-ray Photoelectron Spectroscopy, 1st ed., PerkinElmer, Eden
Prairie, 1979.
[37] NIST X-ray Photoelectron Spectroscopy Database, version 3.5, National
Institute of Standards and Technology, Gaithersburg, MD, 2003, http://
srdata.nist.gov/xps/ (accessed September 2013).
[38] F. Ahimou, C. J. P. Boonaert, Y. Adriaensen, P. Jacques, P. Thonart, M.
Paquot, P. G. Rouxhet, J. Colloid Interface Sci. 2007, 309, 49–55.
[39] V. G. Baldovino-Medrano, B. Farin, E. M. Gaigneaux, ACS Catal. 2012, 2,
322–336.
[40] V. G. Baldovino-Medrano, P. Eloy, E. M. Gaigneaux, S. A. Giraldo, A. Cen-
teno, Catal. Today 2010, 150, 186–195.
[41] E. A. Kozlova, T. P. Lyubina, M. A. Nasalevich, A. V. Vorontsov, A. V. Miller,
V. V. Kaichev, V. N. Parmon, Catal. Commun. 2011, 12, 597–601.
[42] S. P. Tonner, D. L. Trimm, M. S. Wainwright, N. W. Cant, Ind. Eng. Chem.
Prod. Res. Dev. 1984, 23, 384–388.
[43] M. Ai, Appl. Catal. 1984, 11, 259–270.
[44] L. Domokos, T. Katona, . Molnµr, A. Lovas, Appl. Catal. A 1996, 142,
151–158.
[45] X. Huang, N. W. Cant, M. S. Wainwright, L. Ma, Chem. Eng. Process. 2005,
44, 393–402.
[46] M. Ai, J. Catal. 1982, 77, 279–288.
[47] E. Tronconi, A. S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca, P. Tittarelli, Ind.
Eng. Chem. Res. 1987, 26, 1269–1275.
[48] L. Briand, L. Gambaro, H. Thomas, React. Kinet. Catal. Lett. 1994, 52,
169–174.
[49] M. A. BaÇares, N. D. Spencer, M. D. Jones, I. E. Wachs, J. Catal. 1994, 146,
204–210.
[50] F. Solymosi, A. Berkó, Z. Tóth, Surf. Sci. 1993, 285, 197–208.
[51] J. Cai, Y. Fu, Q. Sun, M. Jia, J. Shen, Chin. J. Catal. 2013, 34, 2110–2117.
[52] C. Louis, J.-M. Tatibout, M. Che, J. Catal. 1988, 109, 354–366.
[53] B. C. Gates, Chem. Rev. 1995, 95, 511 –522.
Acknowledgements
V.G.B.-M. thanks “Fonds spØciaux de la recherchØ” and Marie
Curie Actions of the European Union for funding during his post-
doctoral stay at UCLouvain.
Keywords: green chemistry · methanol oxidation · methyl
formate · supported catalysts · synergy
[1] J. M. Tatibout, Appl. Catal. A 1997, 148, 213–252.
[2] Wikipedia contribution; methyl formate, Wikipedia, The Free Encyclope-
dia: 2013; http://en.wikipedia.org/w/index.php?title=Methyl_forma-
te&oldid=579144212 (accessed September 2013).
[3] C. Adami, M. Slany, J. Karl, G. Kaibel, M. Schäfer, P. Zehner, M. Rçper U.S.
Patent No. 7,053,239; BASF Aktiengesellschaft, Ludwigshafen, DE, 2006.
[4] H. Kominami, H. Sugahara, K. Hashimoto, Catal. Commun. 2010, 11,
426–429.
[5] K. R. Phillips, S. C. Jensen, M. Baron, S.-C. Li, C. M. Friend, J. Am. Chem.
Soc. 2013, 135, 574–577.
[6] Q. Guo, C. Xu, W. Yang, Z. Ren, Z. Ma, D. Dai, T. K. Minton, X. Yang, J.
Phys. Chem. C 2013, 117, 5293–5300.
[7] H. Liu, E. Iglesia, J. Catal. 2004, 223, 161–169.
[8] W. Li, H. Liu, E. Iglesia, J. Phys. Chem. B 2006, 110, 23337–23342.
[9] J. Lichtenberger, D. Lee, E. Iglesia, Phys. Chem. Chem. Phys. 2007, 9,
4902–4906.
[10] R. Wojcieszak, E. M. Gaigneaux, P. Ruiz, ChemCatChem 2012, 4, 72–75.
[11] R. Wojcieszak, E. M. Gaigneaux, P. Ruiz, ChemCatChem 2013, 5, 339–
348.
[12] R. Wojcieszak, M. N. Ghazzal, E. M. Gaigneaux, P. Ruiz, Catal. Sci. Technol.
2014, 4, 738–745.
[13] R. Wojcieszak, R. Mateos-Blanco, D. Hauwaert, S. M. Carrazan, E. M.
Gaigneaux, P. Ruiz, Curr. Catal. 2013, 2, 27–34.
[14] R. Gómez, S. Fuentes, F. J. F. del Valle, A. Campero, J. M. Ferreira, J. Catal.
1975, 38, 47–53.
[54] J. T. Miller, M. Schreier, A. J. Kropf, J. R. Regalbuto, J. Catal. 2004, 225,
203–212.
[15] H. R. Reinhoudt, R. Troost, A. D. van Langeveld, S. T. Sie, J. A. R.
van Veen, J. A. Moulijn, Fuel Process. Technol. 1999, 61, 133–147.
[16] Y. Yoshimura, M. Toba, T. Matsui, M. Harada, Y. Ichihashi, K. Bando, H.
Yasuda, H. Ishihara, Y. Morita, T. Kameoka, Appl. Catal. A 2007, 322, 152–
171.
[17] A. Niquille-Rçthlisberger, R. Prins, J. Catal. 2006, 242, 207–216.
[18] V. G. Baldovino-Medrano, P. Eloy, E. M. Gaigneaux, S. A. Giraldo, A. Cen-
teno, J. Catal. 2009, 267, 129–139.
[55] C. Fernµndez, C. Sassoye, D. P. Debecker, C. Sanchez, P. Ruiz, Appl. Catal.
A 2014, 474, 194–202.
´
´
[56] Z. Karpinski, T. Koscielski, J. Catal. 1980, 63, 313–323.
[57] H. Jiang, H. Yang, R. Hawkins, Z. Ring, Catal. Today 2007, 125, 282–290.
[58] R. Levis, J. Zhicheng, N. Winograd, S. Akhter, J. M. White, Catal. Lett.
1988, 1, 385–389.
[59] R. J. Levis, J. Zhicheng, N. Winograd, J. Am. Chem. Soc. 1988, 110, 4431–
4432.
ˇ
[19] Z. Vít, D. Gulkovµ, L. Kaluza, M. Boaro, Appl. Catal. B 2014, 146, 213–
[60] R. J. Levis, J. Zhicheng, N. Winograd, J. Am. Chem. Soc. 1989, 111, 4605–
220.
4612.
[20] S. Eriksson, U. NylØn, S. Rojas, M. Boutonnet, Appl. Catal. A 2004, 265,
[61] R. Ren, C. Niu, S. Bu, Y. Zhou, Y. Lv, G. Wang, J. Nat. Gas Chem. 2011, 20,
207–219.
90–98.
˝
[21] N. Gyorffy, Z. Paµl, J. Mol. Catal. A 2008, 295, 24–28.
[22] J. Xu, L. Ouyang, G.-J. Da, Q.-Q. Song, X.-J. Yang, Y.-F. Han, J. Catal. 2012,
285, 74–82.
Received: November 3, 2015
[23] K. Persson, A. Ersson, K. Jansson, J. L. G. Fierro, S. G. Järås, J. Catal. 2006,
243, 14–24.
Published online on February 17, 2016
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