Effect of the metal-support interaction on the activity and selectivity of methanol oxidation over Au supported on mesoporous oxides

Article abstract of DOI:10.1039/c8cc04295k

To elucidate the factors affecting the catalytic properties of supported Au catalysts on the metal oxide support we investigated Au NPs deposited on crystallized mesoporous transition-metal oxides (m-oxides: Co₃O₄, NiO, and α-Fe₂O₃) prepared using the nanocasting method. The metal-oxide interaction in Au/mesoporous oxides resulted in higher catalytic activity for converting methanol to CO₂ as a full oxidation product than pure m-oxides. Au/m-Fe₂O₃ exhibited high activity and low selectivity for methyl formate as a partial oxidative coupling product. We correlate the change in activity and selectivity with the interface between the Au and m-oxides.

Full text of DOI:10.1039/c8cc04295k

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Effect of the metal–support interaction on the activity and
selectivity of methanol oxidation over Au supported on
mesoporous oxides
Received 00th January 20xx,
Accepted 00th January 20xx
Sunyoung Oh,1ac You Kyung Kim,1bc Chan Ho Jung,c Won Hui Doh,c and Jeong Young Park*abc
DOI: 10.1039/x0xx00000x
www.rsc.org/
To elucidate the factors affecting the catalytic properties of important transformation processes for energy conversion and
supported Au catalysts depending on the metal oxide support, we chemical synthesis of fine chemicals via green chemistry.9, 12-14
investigated Au NPs deposited on crystallized mesoporous Recently, novel unsupported transition metal materials have
transition-metal oxides (m-oxides: Co3O4, NiO, and α-Fe2O3) gained attention for addressing the intrinsic origin of the catalytic
prepared using the nanocasting method. The metal–oxide activity for selective oxidation of methanol. Studies on the size
interaction in Au/mesoporous oxides resulted in higher catalytic effect of Pt NPs have suggested that the production of valuable
activity for converting methanol to CO2 as a full oxidation product chemicals, such as formaldehyde and carbon dioxide, can be
than pure m-oxides. Au/m-Fe2O3 exhibited high activity and low obtained via the oxidative reaction of methanol under ambient
selectivity for methyl formate as a partial oxidative coupling pressure at low temperature.15 As the size of the particles
product. We correlate the change in activity and selectivity with the decreased, they showed higher selectivity toward partial
interface between the Au and m-oxides.
oxidation of methanol to formaldehyde, although the catalytic
activity was very low. In particular, unsupported nanoporous Au
(np-Au) is catalytically more active for the selective oxidative
coupling of methanol to methyl formate at temperatures below
80 °C.16 In oxidation reactions, the activation of molecular
oxygen is essential and the support should be involved in
creating dissociated oxygen at the perimeter sites of the metal
particles. Since heterogeneous catalysts can be prepared by
depositing NPs on oxide supports, the metal–oxide interface
plays a crucial role in affecting the catalytic properties of
oxidation reactions because of geometric changes and charge
transfer between the metal and the oxide support.17-20 Despite the
various studies, there are limitations for utilizing Au catalysts
because of the low dissociation probability on single-crystal Au
surfaces and the poor stability of supported Au catalysts. To
overcome these problems and to address the intrinsic catalytic
origins of supported Au catalysts, porous oxides have been used
recently as suitable oxide supports because of their thermal
stability and high surface area.21-23
To elucidate the factors affecting the catalytic properties of
supported Au catalysts depending on the different metal oxide
supports, we investigated Au NPs deposited on crystallized
mesoporous transition-metal oxides (m-oxides: Co3O4, NiO, and
α-Fe2O3) prepared using the nanocasting method. Methanol
oxidation was carried out on all the Au supported on m-oxide
catalysts, which serve as promising model catalysts to probe the
catalytic origin of supported Au catalysts without the propensity
for sintering the NPs. We found a comparable catalytic
Gold-based catalysts have attracted much attention since the
discovery of the origin of the catalytic activity of supported Au
nanoparticles (NPs) for low-temperature CO oxidation.1 Even
though extensive experimental and theoretical studies have been
performed, the intrinsic origin of the catalytic activity of Au is
still controversial. For oxidation catalysis, O2 dissociation is a
critical step because single-crystal Au surfaces have an
extremely low oxygen dissociation probability (<<10−6) and
weak interaction. Mechanistic studies have thus relied on more
reactive oxygen sources, such as ozone or oxygen atoms.2-4 Yet,
supported Au catalysts with diameters of 2 to 5 nm were quite
active on suitable oxide supports, such as titania and ceria.5, 6
Moreover, Au-based catalysts have been recently reported for a
wide range of catalytic reactions, including low-temperature CO
oxidation, H2 oxidation, and the selective oxidation of alcohols
and hydrocarbons.7-11
Methanol (CH3OH) is one of the most important industrial
chemicals and selective oxidation of alcohols is a key issue in
a.Department of Chemistry, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea. E-mail: jeongypark@kaist.ac.kr
b.Graduate School of EEWS, Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 34141, Republic of Korea.
c. Center for Nanomaterials and Chemical Reactions, Institute for Basic Science
(IBS), Daejeon 34141, Republic of Korea.
1 Authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: Experimental details,
supplementary figures, table, and references. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C8CC04295K
Fig.
1 Schematic diagram of the hard-templating (nanocasting)
approach for the preparation of mesoporous oxide supported Au
nanoparticle catalysts
activity and selectivity of Au/m-oxide catalysts for methanol
oxidation to carbon dioxide (i.e., the full oxidation product) and
methyl formate (the partial oxidation and coupling product).
From this study, we varied the different m-oxides to engineer the
metal–oxide interface and evaluate its role in altering the
catalytic activity and selectivity.
Fig. 2 TEM images of (a) the mesoporous silica template KIT-6, (b) m-
Co3O4, (c) m-NiO, (d) m-Fe2O3, (e) Au/KIT-6, (f) Au/m-Co3O4, (g) Au/m-
NiO, and (h) Au/m-Fe2O3.
AES) (see Table S1, ESI†). The surface areas and pore size
distributions of the prepared supported Au catalysts, obtained
using the Brunauer–Emmett–Teller (BET) method with the
adsorption of N2 at the temperature of liquid nitrogen, are quite
high compared with other normal oxides (Fig. S4 and Table S1,
ESI†).28, 29
The oxidation of methanol (O2 : CH3OH = 1 : 1 v/v) was
carried out on the pure mesoporous oxides and the Au-deposited
mesoporous oxides at temperatures of 80–220 °C, following a
reductive pre-treatment. The measured CO2 evolution is plotted
in Fig. 3. The CO2 obtained was dependent upon the pure m-
oxide (Fig. 3a). The mesoporous oxides have shown notable
catalytic activity in the absence of noble metal particles,30-32
which indicates that the higher surface area of their structures can
provide more active sites and that the porous structure promotes
the adsorption and diffusion of reactant molecules compared
with bulk metal oxides.33 Recent experiments on mesoporous
structures, such as chromium oxide and cobalt oxide, have
The hard-templating approach using high-quality large
mesoporous silica with cubic Ia3d symmetry (KIT-6) was
chosen to synthesize highly crystallized mesoporous oxides of
Co3O4, NiO, and Fe2O3. Fig. 1 shows the procedure for preparing
the mesoporous oxides and their supported Au catalysts. Fig. 2
shows representative transmission electron microscopy (TEM)
images of the m-oxides as well as the supported Au catalysts.
Such m-oxides exhibit ordered porosity and single crystallinity
in larger domains (Fig. 2b–d). In addition, their structures have
large pore diameters and high surface areas (see Table S1, ESI†).
To demonstrate the role of the interface between the Au and the
oxides during the methanol oxidation reaction, we deposited Au
NPs on m-oxides using the urea reduction method (Fig. 2e–h).
The mean diameter of the Au NPs for the prepared catalysts is
quite narrow at 5 nm (Fig. S1, ESI†). Moreover, the structural
information for the bare m-oxides and Au/m-oxides is shown in
the X-ray diffraction patterns (Rigaku D/MAX-2500 at 40 kV,
300 mA) (see Fig. S2, ESI†), which indicates that the metal
nitrate precursors were completely converted to crystallized
metal oxide structures after the calcination process.24-26 The
chemical bonding states of the Au/m-oxide catalysts were further
examined using XPS analysis (Fig. S3, ESI†). The Au NPs were
also deposited on the m-oxides.27 The Au compositions were
2.5%, 3.6%, 2%, and 2.7% for the Au/KIT-6, Au/m-Co3O4,
Au/m-NiO, and Au/m-Fe2O3, respectively, measured by
inductively coupled plasma atomic emission spectroscopy (ICP-
suggested that the high catalytic activity can be ascribed to the
35
complete oxidation of toluene and propane.34,
Also, CO
oxidation results using different kinds of oxide supports are in
agreement with our catalytic results, depending on the pure
mesoporous oxides.36 This demonstrates that pure mesoporous
oxides for methanol oxidation exhibit a redox capability
consistent with previous results for CO oxidation.
When Au NPs with an average diameter of 5 nm are
incorporated into the mesoporous oxides, the observed catalytic
-10
(c)
(a)
(b)
m-Co3O4
m-NiO
m-Fe2O3
100
80
60
40
20
0
100
80
60
40
20
0
Au/m-Co3O4
Au/m-NiO
Au/m-Fe2O3
Ea = 55.7 kcal/mol
-12
Au/m-Co3O4
-14
-16
Ea = 12.1 kcal/mol
Au/m-NiO
Au/m-Fe2O3
Au/m-SiO2
Ea = 58.2 kcal/mol
80
120
160
200
80
120
160
200
2.2
2.4
2.6
2.8
1000/T (K-1)
Temperature (oC)
Temperature (oC)
Fig. 3 Catalytic activity of full-oxidation of methanol to CO2 as a function supported on mesoporous oxides. (c) Arrhenius plots of the rate of CO
of temperature for (a) pure mesoporous oxides and (b) Au nanoparticles production on the Au/m-oxide catalysts
2 | J. Name., 2012, 00, 1-3
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activities for full methanol oxidation are considerably higher dissociation should be possible on the Au surface.43 The
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than for pure m-oxides (Fig. 3). Small Au NPs supported on activation energies for Au/m-Co3O4, ADuO/mI: 1-0N.1i0O3,9/Can8dCCA04u2/9m5K-
porous metal oxides lead to an enhancement of the catalytic Fe2O3 obtained from Arrhenius plots for the rate of CO2
activity, and the metal–oxide interface provides active sites.10 As production are 58.2, 55.7, and 12.1 kcal mol−1, respectively (Fig.
reported by Leppelt et al., this can be justified by the fact that 3c). Au/m-oxide catalysts display
a dependence on the
catalytic active perimeter sites should exist at the interface mesoporous oxides for the catalytic activity for CO2 formation,
between the gold and the support.37 Additionally, it is believed implying that each oxide has a different interaction mechanism
that the support is critical for catalytic activity because pure Au for the full methanol oxidation reaction.
lacks high activity for molecular oxygen dissociation so that
In this experiment, the volume ratio of oxygen to methanol
oxygen molecules can be activated by the presence of the Au (1 : 1 v/v) allows the dominant coupling reaction and produces
metal that is in contact with the metal oxide supports.38 Recently, methyl formate as a partial oxidation product with comparable
the interface between the Au support and metal oxide particles selectivity on the Au/m-oxide catalysts at 100 °C, as shown in
serves as an absorption site for CO reactants as well as allowing Fig. 4. The selective oxidation of methanol resulted in 89.3% and
for efficient activation of molecular oxygen.39 Abad et al. 37.3% methyl formate production for the Au/m-Co3O4 and
reported that Au-deposited nanocrystalline ceria showed Au/m-Fe2O3 catalysts, respectively, while the Au/m-NiO
remarkable catalytic activity and selectivity for the oxidation of catalyst only produced methyl formate. The apparent difference
3-octanol compared with Au-deposited carbon.8 The higher in selectivity dependent on the mesoporous oxide could be
catalytic activity of the Au/m-oxides indicates that the influenced by interactions between the Au NPs and the metal
mesoporous oxides increase the interface between the metal and oxide supports. Liu et al. may provide a clarification that the
the oxide compared with non-porous oxides. Among the Au initial adsorption of methanol on the Au sites induces the
nanoparticles supported on mesoporous oxide catalysts, Au/m- formation of a chemisorbed methoxy species, which facilitates
Fe2O3 shows the highest catalytic activity. It is suggested that a the coupling reaction with a neighbouring methoxy species, thus
large amount of oxygen can adsorb onto the support as a generating methyl formate.44 Moreover, the oxide support of the
reducible oxide, such as TiO2 and Fe3O4 (or at the metal–support supported Au catalysts can play a major role in supplying oxygen
interface), possibly on oxygen vacancies.40, 41 Iron oxide can to the Au active center, leading to the oxidative coupling of
adsorb a large quantity of highly mobile oxygen, leading to a methanol to produce methyl formate.45, 46 The methoxy species
higher catalytic activity. Moreover, such support oxides can is an intermediate of methanol oxidation that adsorbs on Ce4+
stabilize the Au nanoparticles and oxygen can transfer to the cations and is able to oxidize to the formate species when a gold
metal surface.42 The oxidation states of Au 4f for the Au/m-Fe2O3 metal is present with the CeO2 support; furthermore the Ce4+
catalysts before and after the methanol oxidation reaction are cations are thus reduced to Ce3+ 47, 48
A suggested role of the Au
.
shown in Fig. S5. After the methanol oxidation reaction, Au3+ is for Ce3+ formate generation from the ceria surface oxygen
species are present, indicating that molecular oxygen can be species, which further creates surface vacancies acting as oxygen
activated by the Au supported on iron oxide as a reducible oxide. pumps (the so-called “reverse spillover effect”). The reaction
Despite the increasing number of studies, the mechanisms for pathway for methyl formate production is still debated. Further
oxygen adsorption and activation are highly controversial, but investigation is needed to understand the detailed mechanism,
they are needed to explain the activity for oxidative reactions. To but these results show that we can control the catalytic activity
discover the contribution of just the Au nanoparticles to catalytic and selectivity for methanol oxidation by altering the oxide
activity, Au supported on SiO2 (i.e., KIT-6, an inert support) was support.
prepared. It shows the lowest CO2 evolution for methanol
When using supported metal catalysts, a negative factor that
oxidation even up to 220 °C compared with Au supported on may be important is the tendency for nanoparticles to sinter
reducible oxides (Fig. 3b). In the case of inert support materials, under reaction conditions. This leads to nanoparticle growth, and
it is a logical consequence that oxygen adsorption and
thus results in lower catalytic activity.49 A TEM image of Au/m-
oxide after the methanol oxidation reaction reveals good thermal
stability while retaining the original nanoparticle size and
morphology (see Fig. S6, ESI†), which can be observed in the
repetitive catalytic performance on Au/m-oxide (see Fig. S7,
ESI†). This also demonstrates that the geometric properties and
large surface area of the mesoporous oxide can prevent the
sintering of Au NPs. Its thermal stability is also influenced by
the strong interaction between the Au particles and the metal
oxide support, which provides insight into the catalytic
performance during the reaction.
In conclusion, we found the intrinsic methanol oxidation
activity and selectivity of Au nanoparticles supported on
crystallized mesoporous oxides (Co3O4, NiO, and α-Fe2O3). The
catalytic activity for CO2 formation as a full oxidation product
over Au/m-oxide catalysts is higher than that for pure
Fig. 4 Selectivity showing the fraction of methanol converted to carbon
dioxide and methyl formate at 100 oC for Au-supported mesoporous
oxides (Co3O4, NiO, and Fe2O3).
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24. P. Dutta, M. S. Seehra, S. Thota and J. Kumar, J. Phys.: Condens.
mesoporous oxides, which can be attributed to the mesoporous
oxides providing an increased interface between the Au and the
oxide support compared with non-porous oxides. This approach
reveals a promising route for controlling the activity as well as
the selectivity by altering the oxide support, which governs the
interface between the metal and the support.
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DOI: 10.1039/C8CC04295K
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