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Nanocrystal Bilayer for Tandem Catalysis
Yusuke Yamada
1,3)
, Chia-Kuang Tsung
1,2,4)
, Wenyu Huang
1,2)
, Ziyang Huo
1)
, Susan E. Habas
1,2)
, Tetsuro Soejima
1)
, Cesar
E Aliaga
2)
, Gabor A. Somorjai
1,2)
, Peidong Yang
1,2)
*
1. Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, USA
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720,
USA
3. Current Address: Department of Material and Life Science, Graduate School of Engineering, Osaka University 2-1
Yamada-oka, Suita, Osaka 565-0871
4. Current Address: Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut
Hill, Massachusetts 02467, USA
*e-mail: p_yang@berkeley.edu
Supported catalysts have been widely used in industries and can be optimized by tuning the composition and
interface of the metal nanoparticles and oxide supports. Rational design of metal-metal oxide interfaces in
nanostructured catalysts is critical for achieving better reaction activities and selectivities. We introduce here a
new class of nanocrystal tandem catalysts having multiple metal-metal oxide interfaces for the catalysis of
sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling sub-10 nm platinum and
cerium oxide
nanocube monolayers on a silica substrate. The two distinct metal-metal oxide interfaces, CeO
2
-Pt
and Pt-SiO
2
, can be used to catalyze two distinct sequential reactions. The CeO
2
-Pt interface catalyzed methanol
decomposition to produce CO and H
2
, which were then subsequently used for ethylene hydroformylation
catalyzed by the nearby Pt-SiO
2
interface. Consequently, propanal was selectively produced on this nanocrystal
bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach
towards designing high performance, multi-functional nanostructured catalysts.
High performance catalysts are central for the development of new generation energy conversion and storage
technologies.
1,2
While industrial catalysts can be optimized empirically by tuning the elemental composition, changing the
supports, or altering preparation conditions in order to achieve higher activity and selectivity, these conventional catalysts
are typically not uniform in composition and/or surface structure at the nano- to micro-scale. In order to significantly
improve our capability of designing better catalysts, new concepts for the rational design and assembly of metal-metal
oxide interfaces are desired. Metal nanocrystals with well-controlled shape and size are interesting materials for catalyst
design from both electronic structure and surface structure aspects.
3,4,5
From the electronic structure point of view, small
metal nanoclusters have size-dependent electronic states, which make them fundamentally different from the bulk. From
the surface structure point of view, the shaped nanocrystals have surfaces with well-defined atomic arrangements. It has
been clearly demonstrated by surface science studies in recent decades that the atomic arrangement on the crystal
surface can affect catalytic phenomena in terms of activity, selectivity, and durability.
1
The application of shape- and size-controlled metal oxide nanocrystals as catalyst supports has even greater potential for
innovative catalyst design.
6,7
It is well known that catalysis can be modulated by using different metal oxide supports, or
metal oxide supports with different crystal surfaces.
8
For example, platinum loaded molybdenum oxide and silica
catalysts, both showed similar activation energies for ethylene hydrogenation.
9
On the other hand, the activation energy
for ethane hydrogenolysis over platinum-silica was lower than that over platinum-molybdenum oxide.
10
It is believed that
the metal oxides not only work as supports, but also function as electronic modulators, in addition to contributing spillover
and adsorption sites. The precise selection and control of metal-oxide interfaces could lead to better activity and
selectivity for a desired reaction.
11
The integration of multiple types of metal-metal oxide interfaces on the surface of a single active metal nanocrystal could
in principle yield a novel tandem catalyst for multi-step reactions. The catalytic activity and selectivity of such a tandem
catalyst can be optimized by establishing suitable metal oxide interfaces for each reaction step. However, it is almost
impossible to control the composition of multiple interfaces on an atomic level using traditional catalyst synthesis.
Integrating binary nanocrystals to form highly ordered superlattice represents a new way to form multiple interfaces with
new functionalities.
12,13
Here we utilized a nanocrystal bilayer structure formed by assembling sub-10 nm platinum and
cerium oxide
nanocube monolayers on a silica substrate. The two distinct metal-metal oxide interfaces in the catalyst,
CeO
2
-Pt and Pt-SiO
2
, were used to catalyze two separate and sequential reactions. The CeO
2
-Pt interface catalyzed
methanol decomposition to produce CO and H
2
which were subsequently used for ethylene hydroformylation catalyzed
by the nearby Pt-SiO
2
interface. Consequently, propanal was selectively produced on this nanocrystal bilayer tandem
catalyst.
The cubic shape of nanocrystals is ideal for assembling metal-metal oxide interfaces with a large contact area. Figure 1
shows our tactic to achieve the “tandem” bilayer structure with nanocubes of metal and metal oxide. First, a two
dimensional metal (Pt) nanocube array was assembled onto flat metal oxide substrate (SiO
2
) by using Langmuir-Blodgett
(LB) method to make the first metal-metal oxide interface. The second metal oxide (CeO
2
) nanocube LB array was then
assembled on top of the metal nanocube monolayer, which provides the second metal-metal oxide interface. The
capping agents of the nanocrystals were removed by UV/ozone treatment to form clean metal-metal oxide interfaces.
20
After the capping agent removal, the vertical clefts between the nanocrystals assure access to both catalytic interfaces,
while providing high surface area in the close packed array.
Olefin hydroformylation is an important reaction for the production of aldehydes from olefins, carbon monoxide, and
hydrogen.
14
Usually the reaction is carried out with homogeneous catalysts, such as Rh complexes. The disadvantages
of this process include the use of toxic CO and explosive H
2
gas. This process would also typically employ high pressure
conditions and purification processes. Therefore, it would be advantageous to carry out olefin hydroformylation via
heterogeneous catalysis with CO and H
2
produced in situ from the decomposition of a benign chemical, such as
2
methanol. It is known that Pt loaded on CeO
2
shows high activity toward methanol decomposition to provide CO and
H
2
.
15,16
In addition, Naito and Tanimoto reported that Pt loaded SiO
2
catalyzed propene hydroformylation and produced
aldehydes with a high conversion rate. However, the selectivity for this reaction was poor.
17
Here, we demonstrate that
our nanocrystal bilayer, made of a CeO
2
nanocube monolayer and Pt nanocube monolayer on a SiO
2
substrate,
effectively catalyzes ethylene hydroformylation with methanol to produce propanal selectively.
Results and Discussions
Preparation of tandem catalyst Platinum and ceria nanocubes with edge lengths of 6-8 nm were prepared by literature
methods with minor modifications.
18,19
A monolayer of platinum nanocubes was prepared by LB, and then transferred
onto a Si wafer substrate with a native oxide layer on the surface. The original capping agent on the platinum nanocubes,
tetradecyltrimethylammonium bromide (TTAB), was exchanged for oleylamine to facilitate LB assembly and deposition.
The high resolution transmission electron microscopy (HRTEM) image of a single Pt nanocube and low magnification
TEM images of a Pt LB film are shown in Figure 2a. The Pt nanocubes are single crystalline and enclosed by six (100)
facets. The domain size of the monolayer film is over one micron by one micron, and the total coverage of the film was
more than 80%. The gaps between the nanocrystals were about 2-3 nm, which are sufficient for diffusion of small
molecules. The oleic acid capped CeO
2
nanocube monolayer film was prepared by drop casting or LB. Figure 2b shows
a film prepared by drop casting. The gaps between the nanocrystals are 4-5 nm, which is close to the thickness of the
oleic acid bilayer. The dropcast CeO
2
film showed long range ordering. For catalytic samples, the CeO
2
film was
prepared by LB to give a large film area. The double-layered film was obtained by depositing a CeO
2
film onto a Pt film. A
TEM image of a large area bilayered film of CeO
2
on Pt is shown in Figure 2c. Although the CeO
2
nanocrystals above the
Pt nanocrystals cannot be clearly observed over most of the area due to their lower contrast, the CeO
2
nanocrystals were
visible at some defect areas on the Pt film (Figure 3a). The presence of CeO
2
nanocubes on the Pt nanocube film was
confirmed by HRTEM and by performing an energy dispersive X-ray (EDX) spectroscopy line scan as shown in Figure 3b
and c. The Pt and CeO
2
lattice were both observed on the bilayer film
by HRTEM (Figure 3b). The EDX line scan over
the defect area of the bilayer film is shown in Figure 3c. It shows the intensity change of Pt and Ce along the line on the
film where the Pt film is discontinuous. The Pt intensity decreases at the gap between Pt nanocrystals while the Ce
intensity was nearly constant.
In order to facilitate interface formation between the SiO
2
, Pt, and CeO
2
layers, the various capping agents, oleylamine
on Pt and oleic acid on CeO
2
, need to be removed. Although CeO
2
crystals are stable under high temperature treatment
for capping agent removal, Pt nanocubes are not stable under such conditions. When Pt nanocrystal loaded samples
were heated at 250°C in air, the shape of the Pt nanocrystals was lost. Thus, we applied a room temperature
UV-irradiation process to remove surface capping agents. Previously, it was found that UV/ozone treatment is effective
for removing organic capping agents from Pt nanoparticles.
17
Here, removal of the capping agent was monitored by sum
frequency generation vibrational spectroscopy as shown in Figure S1 (Supporting information). Before the UV/ozone
treatment, three peaks assigned to symmetric CH
2
(2853 cm
-1
), symmetric CH
3
(2879 cm
-1
), and asymmetric CH
2
(2929
3
cm
-1
) stretches, were observed. After treatment, the intensity of the peaks was significantly decreased. TEM observations
of the sample before and after the UV/ozone treatment indicated that the crystal shapes remained unchanged. Oxidation
of CO is commonly employed to examine the interaction between Pt and metal oxides because the activation energy of
Pt loaded on a metal oxide is highly dependent on the nature of metal oxide support.
21,22,23
The strong Pt-metal oxide
interaction decreases electron donation from Pt to adsorbed CO weakening the CO bond. As the result, the interaction
between Pt and the metal oxide increases the activation energy for CO oxidation. Arrhenius plots for CO oxidation over
our CeO
2
-Pt bilayers on SiO
2
substrates before and after UV/ozone treatment are shown in Figure S2. The as-prepared
bilayers showed an apparent activation energy of 19.7 kcal/mol which is comparable to the reported value of Pt
nanocubes.
22
The UV/ozone treated samples showed an apparent activation energy of 30.1 kcal/mol. The increase in
activation energy indicates the formation of two metal-metal oxide interfaces of CeO
2
-Pt and Pt-SiO
2
following capping
agent removal.
Ethylene hydroformylation over tandem catalyst with MeOH The assembly of CeO
2
-Pt-SiO
2
bilayers with
two
different metal-metal oxide interfaces is an ideal catalyst design for olefin hydroformylation with CO and H
2
formed in situ
by the decomposition of MeOH. It was reported previously that Pt/CeO
2
can selectively catalyze MeOH decomposition to
CO and H
2
, while Pt/SiO
2
catalyzes olefin hydroformylation. Prior to examining the two-step tandem reaction, control
experiments were performed to monitor each step over each interface individually: MeOH decomposition over the
Pt/CeO
2
interface, and then separately ethylene hydroformylation with CO and H
2
gas input over the Pt/SiO
2
interface.
The decomposition of MeOH over the Pt/CeO
2
interface was examined at 190 °C over Pt-CeO
2
-SiO
2
catalyst, which
contains only of Pt/CeO
2
- metal/oxide interface. The as-prepared catalyst showed no catalytic activity for the reaction
due to the lack of clean metal-metal oxide interfaces (Figure S3). After UV/ozone treatment, the Pt-CeO
2
-SiO
2
tandem
catalyst showed MeOH decomposition activity as shown in Figure 4a. The concentration of formed hydrogen and
decomposed MeOH in the batch reactor changed in proportion to the reaction time and the ratio of formed H
2
to
decomposed MeOH is 1:2, which confirms the formation of H
2
and CO. The turn over frequency, TOF, in terms of H
2
was
1.8x10
-3
s
-1
per Pt atom. Separately, ethylene hydroformylation with CO and H
2
gas was carried out over the Pt-SiO
2
catalyst also at 190 °C. Figure 4b shows the concentration change of propanal and MeOH in a batch rector as a function
of the reaction time. The propanal formation was clearly observed. The production of MeOH was due to the
hydrogenation of CO, which was confirmed by CO hydrogenation with only CO and H
2
without ethylene (Figure S4). The
TOF in terms of MeOH was 5.8x10
-2
s
-1
per Pt atom and the TOF in terms of propanal was 2.7x10
-3
s
-1
per Pt atom. On a
bare Pt surface, the formation of MeOH by CO hydrogenation is much faster than propanal formation by
hydroformylation.
Figure 5a shows time dependent propanal formation from ethylene hydroformylation with in situ MeOH decomposition
over the CeO
2
-Pt-SiO
2
tandem catalyst at 190 °C. The as-prepared sample produced a negligible amount of propanal
even after a longer reaction time (Figure S3). On the other hand, the formation of propanal was clearly observed over the
UV/ozone treated catalyst as shown in Figure 5a. Propanal formation over the UV/ozone treated catalyst was further
4
