Title: Nanoparticle stabilization via scalable ALD/MLD coating process
Participants: Miao Yu and John R. Regalbuto
Motivation: Heterogeneous catalysts are used to manufacture most chemicals (e.g., making
gasoline from petroleum1, producing fertilizers2, controlling automotive exhaust pollution via the
catalytic muffler3). Many catalysts used on a large scale consist of small metal particles
dispersed on a high surface area support, which helps maximize the number of metal atoms on
the surface; only the top atomic layer of metal atom is involved in catalytic reactions. Since the
metals that are good catalysts (Pt, Ru, Pd, etc.) are also expensive, a large fraction of the metal
atoms must be surface atoms for their economical use. The metal particles, therefore, are small
(<10 nm), so that a large fraction of the metal atoms are on the surface and thus available to
adsorb reactant molecules and catalyze reactions. A significant problem with using small metal
particles as catalysts is they lose their activity over time. Sintering of nanoparticles at elevated
temperatures and under various environments is one of the most important reasons of activity
loss.
Recent studies demonstrated that sintering of unsupported metal particles can be
dramatically reduced by an oxide coating. Joo et al.4 coated individual, unsupported metal
nanoparticles of platinum (Pt) with a porous oxide layer. They reported that 14-nm Pt
nanoparticles, when coated with a 17-nm thick, mesoporous silica shell by a sol-gel process,
sintered much slower. Because their silica layer was porous, reacting molecules could access
the Pt surface, but the silica layer prevented Pt particles from contacting each other. The oxide
coating inhibited sintering so that the Pt core/silica shell particles could be heated to 750oC.
Without the silica coating, the Pt particles could only be heated to 300oC. This is a dramatic
increase in stability. Similarly, Arnal et al.5 encapsulated unsupported Au nanoparticles in
hollow, porous zirconia spheres by first coating Au nanoparticles with dense silica, coating this
composite with zirconia nanoparticles that formed 3-nm pores, and then leaching out the silica
so that the Au nanoparticles were released. These catalysts had similar CO oxidation activity
after treatment at 800oC, indicating that the porous zirconia spheres prevented sintering of the
Au nanoparticles. These two studies show that an oxide coating dramatically decreased the rate
of catalyst particle sintering for unsupported metal particles, but such particles are not used for
commercial catalytic reactions because:
 Although 14-nm Pt particles are small, they are larger than the metal particles supported
on porous oxides of actual catalysts.
 Supported catalysts are typically used in packed bed reactors, and catalyst pellets are
0.5 to 1.0 cm in diameter so that the pressure drop across the bed is not too large.
 Unsupported metal particles are much less stable than supported ones and cannot be
readily
utilized on a large scale.
Porous
coating
Moreover,
methods used to coat unsupported
metal particles cannot be used to coat high
surface area catalysts. Because the metal
nanopartiles are not porous, the methods only
were required to coat the external surface. For
a porous catalyst pellet, such methods would
Catalyst
block
the pores of the supports and drastically
reduce
the catalyst surface area.
particle
Molecula
Porous coating
OH
OH
O
Based on these previous studies, a
Support
porous oxide coating on supported metal
nanoparticles, as shown in Figure 1, Figure 1: Cross-section of supported metal
therefore, may have great potential to prevent nanoparticles with a thin porous coating to
(a)
(b)
them from sintering while allowing reactants to prevent sintering.
OH
OH
A:
OH* + Al(CH3
CH3
CH3 CH3 CH3 CH3
Al
O
B:
Al
O
O
Al
AlCH3* + OHCH
reach metal surface. Molecular layer deposition (MLD) is a gas phase deposition process and
can be used to prepare ultrathin (<10 nm), conformal porous metal oxide coatings (Al2O3, TiO2,
SiO2, etc.) on various substrates6-8. It may have great potential to deposit porous coatings with
controllable composition, thickness, and pore sizes on a wide range of metal nanoparticles in a
scalable way, if devised appropriately, to prevent them from sintering. We propose to create a
<10 nm thick, porous oxide coating on the entire catalyst surface (both the metal and the oxide
support, Fig. 1) using MLD. Molecular layer deposition can deposit conformal coatings whose
thicknesses are precisely controlled because two self-limiting reactions are used (see below). A
nonporous metal-polymer layer will be deposited by MLD, and subsequent treatment will
convert it to a porous oxide layer.
Hypothesis: Our hypothesis is that such a thin coating will isolate the metal particles from each
other so that they will not readily be able to contact neighboring metal particles and sinter. We
also hypothesize that the oxide layer will not significantly affect the overall rate of catalytic
reaction because it is so thin, and because it would have high porosity.
Objective: Our goal is to dramatically decrease the rate of sintering so that the catalyst could
be used for much longer times before being
CH
CH
CH
CH
CH CH CH
CH
Al
Al
replaced, and/or they could be used at higher
Al
Al
O
O
OH OH
OH OH
O
O
temperatures. We propose to use MLD to
+ Al(CH )
mimic the type of catalysts that were formed
A:
OH* + Al(CH3)3
OAl(CH3)2* + CH4
by Joo et al., but to form thinner oxide layers
on supported metal nanoparticles instead of
on unsupported nanoparticles. We will use
MLD to create a thin metal-organic layer on
CH
CH
CH
CH
CH CH CH
CH
Al
Al
the entire surface of a high surface area
Al
Al
Al
Al
Al
Al
O
O
O
O
O
O
O
O
supported catalyst, isolating the metal
+ OHCH CH OH
nanoparticles from each other. This metalorganic layer will then be converted to a
B: AlCH3* + OHCH2CH2OH
AlOCH2CH2OH* + CH4
porous oxide layer by subsequent treatment.
The MLD technique is a recent variation of Figure 2: An example using molecular layer
atomic layer deposition (ALD) 6 7 8. The ALD deposition (MLD) to form an aluminum hybrid film
by repeat A-B self-limiting surface reactions
and MLD layers are produced by sequentially
conducting two half-reactions at the surface of a substrate; each half reaction stops when all the
surface sites have reacted; they are self-limiting. Each set of half-reactions deposits a layer that
conforms to the surface of the underlying substrate. By repeating the reaction sequence, a
layer of any thickness can be deposited. This is a significant advantage over other methods.
Figure 2 shows two half reactions to deposit aluminum hybrid coating that can be subsequently
converted to microporous alumina, for example, by calcination in air.
3
3
3
3
3
3
3
3
3
3
3
3
3
2
OCH 2CH 2OH
3
OCH2 CH OH
2
OCH C
2 H OH
2
OC
HC
2 H
2 OH
O CH
2 CH O
2 H
OCH2CH2OH
3
OCH C
2 H2 OH
3
H
CH 2O
OCH 2
3 3
2
Volume, cc/Å/g
We will use MLD to deposit a <10 nm thick, metal-organic coating on supported metal
nanoparticles. The resulting layer would then be oxidized
to create a uniform porous coating that we hypothesize
will completely coat the catalyst surface. We have used
MLD to prepare ultra-thin, porous metal oxide films onto
non-porous silica nanoparticles8.
The metal-organic
coating had a regular structure, and its oxidation produced
a uniform porous structure with BET surface areas greater
than 1,000 m2/g8. The sequential reactions of trimethyl
aluminum (TMA) and ethylene glycol at 375 K (Fig. 2)
formed the metal-polymer layers on dense SiO2 particles
(Fig. 3). These layers were oxidized to form a Figure 3: Using MLD to form an
microporous-mesoporous alumina coating with pore-size aluminum hybrid film by repeat A-B
self-limiting reactions
distributions centered at approximately 0.6 and 3.8 nm.
The MLD method has significant advantages:
0.03
0.02
0.01
0
1

Thin layers (<10 nm) can be prepared.

Coverage is conformal so the surface will be uniformly coated.

Supports can be used with 30-nm pores or larger without significantly increasing the
diffusion resistance. A catalyst with a surface area of 100 m2/g has 30-nm pores.
Research Plan: We will use MLD to form thin (<10 nm) porous oxide layers on supported metal
nanoparticles (Pt, Au, Ru, Pd, etc.) and study their effects on preventing metal nanoparticles
from sintering. The MLD coating pores may also be closed by appropriate surface treatments of
pore wall. The proposed research includes:
1) Characterize supported metal nanoparticles by SEM, TEM, ICP, N2 adsorption, and
chemisorption before and after deposition of porous oxide layers to determine the catalyst
composition, the oxide layer thickness, pore size, and surface area, and the fraction of
the metal sites available for adsorption. Because the oxide layer deposits on both the
metal surface and the support, the number of catalytic sites available for adsorption may
decrease.
2) Measure how resistant these coated catalysts are to sintering. Catalysts without coatings
will be treated at 700-800 oC until sintering is observed, as verified by TEM. The same
conditions will be used for catalysts with MLD oxide coatings to determine how stable
they are to sintering. We will also study the anti-sintering capability of MLD coated metal
nanoparticles in different environments. For example, we have found significant sintering
of Au nanoparticles supported on activated carbon when exposed to HCl during its
reaction with acetylene to produce polyvinyl chloride (PVC), which is the third-most widely
produced polymer.
3) The MLD conditions and the subsequent treatment to form the oxide layer will be varied
to prepare catalysts with different oxide thicknesses. The objective is to make the oxide
layer thin enough so that catalytic rate is the same, but thick enough to decrease
sintering.
4) MLD layers of Al2O3, TiO2, ZnO, and SiO2 will be deposited on catalysts using various
precursors to determine which is most effective in decreasing the sintering rate.
5) Study reaction activity and stability of metal nanoparticles with and without MLD, using
industrially important reactions, such as PVC production from HCl and acetylene. other
identified interested reactions will also be tested.
First Year Deliverable: Successful demonstration of MLD coatings to prevent metal
nanoparticles from sintering; identification of effective MLD coatings for maintaining catalyst
reactivity and long term stability.
First Year Milestone(s) 1) Months 1-6: Characterization of metal nanoparticles with and without
MLD coatings after various treatments; demonstration of robust MLD coating for preventing
sintering; 2) Months 7-12: measurements of reactivity and stability of MLD coated nanoparticles.
Cost: $60,000 for one year. Funds are requested for one graduate student to perform
experiments as outlined in the research plan and to assist in the collection and analysis of the
data under the direct supervision of the PIs. Funds are also requested on materials, supplies
and services related to this project in the first year. These include chemicals, glassware, gases,
safety gloves, and other items used for conducting the experiments and characterization
services (SEM, TEM, ICP, XRD, etc.).
References
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gasoline. Appl. Catal. A-Gen 2001, 208, (1-2), 47-53.
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available methods of regeneration, recovery and disposal. J Hazard Mater. 2009, 167, (1-3),
24-37.
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some perspectives. Catal. Today 2003, 77, (4), 419-449.
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stable Pt/mesoporous silica core-shell nanocatalysts for high-temperature reactions. Nat.
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