10.1002/anie.201807450
Angewandte Chemie International Edition
COMMUNICATION
Colloids for Catalysts: A concept for the preparation of superior
catalysts of industrial relevance
J. Quinson,*[b] S. Neumann,[a] T. Wannmacher,[a] L. Kacenauskaite,[b] M. Inaba,[b] J. Bucher,[c] , F.
Bizzotto,[c] S. B. Simonsen,[d] L. Theil Kuhn,[d] D. Bujak,[a] A. Zana,[c] M. Arenz* [c] and S. Kunz*[a]
support by impregnation, adsorption, incipient wetness etc.[2]
Second, calcination and reduction treatments are applied that
Abstract: Compared to conventional preparation methods for
supported heterogeneous catalysts, the use of colloidal
nanoparticles (NPs) allows for a precise control over size, size
distribution, and distribution/location of the NPs on the support.
lead to the formation of active NPs. The fact that the NPs form
on the support leads to various limitations regarding the control
and optimization of the catalysts’ physical properties. Due to
capillary forces, NPs are formed inside small pores of the
support, thus reducing their accessibility.[3] Furthermore, the
chemical and physical properties of the support surface
influence strongly the distribution of the NPs on the support. This
is a particular problem when higher loadings are required,
because the formation of aggregates and the probability of
sintering increase (see also Fig. S1 and discussion in SI),
lowering the precious metal-related activity.[2]
The use of colloids is an approach that overcomes these limits
of conventional catalyst preparation methods. NPs are formed in
a solvent by reduction of dissolved metal precursors and are
subsequently deposited onto the desired support. Alternatively,
they may be used directly without a support to prepare bulk
catalysts. The separation of NP preparation and deposition into
individual steps diminishes any contribution of the support on the
NP formation. This allows for optimized control over NP size[4].
NPs cannot form in small support pores, but particle deposition
occurs preferably at the external surface, which improves their
accessible to reactants. As a result, the physical properties of
supported catalysts can be optimized to achieve a more efficient
use of the expensive precious metal.
However, colloidal methods bear also challenges, recently
highlighted by BASF,[5] that hamper their use for industrial
catalyst preparation. These are (i) the need of surfactants (ii) the
use of high-boiling solvents[6], (iii) restrictions to low metal
concentrations.
In most colloidal approaches, sintering of precious NPs is a
fundamental problem. For this reason, often surfactants (e.g.
PVP) are used that cover the NP surface to prevent sintering.
However, surfactants are detrimental for catalytic applications,
as they block the NP surface. Their removal after NP deposition
on the support is hence an essential step.[7] Several surfactant
removal procedures have been demonstrated on lab-scale, but
their technological feasibility has not been proven, yet.
However, common colloidal syntheses have restrictions that limit
their applicability for industrial catalyst preparation. We present a
simple, surfactant-free and scalable preparation method for colloidal
NPs to overcome these restrictions. We demonstrate how precious
metal NPs are prepared in alkaline methanol, how the particle size
can be tuned and how supported catalysts are obtained. The
potential of these colloids to prepare improved catalysts is
demonstrated by two examples from heterogeneous and
electrocatalysis.
Supported precious nanoparticles (NPs) are among the
industrially most relevant catalysts. They are applied in refinery,
as exhaust gas catalysts, for bulk and fine chemicals production,
and in electrocatalysis.[1] Due to the high costs of precious
metals, even small performance enhancements lead to
significant ecologic and economic improvements. For this
reason, novel but simple ways to prepare supported catalysts
with improved properties are highly desirable.
For a given metal the performance of a supported catalyst
depends mainly on the physical properties: i) NP size, ii) size
distribution, and iii) distribution and location of the NPs on the
support. As the surface to volume ratio increases with
decreasing particle size, smaller NPs are usually beneficial in
terms of activity per mass of metal. Depending on the reaction,
selectivity can also strongly depend on the NP size. Thus,
accurate size control is highly desirable. In addition, supported
NPs must be well accessible for the reactants to utilize
effectively the expensive precious metal. This means they
should be evenly distributed over the external surface of the
support, but not be located inside small pores where
accessibility is hampered.
The typical industrial approach to prepare supported catalysts
follows two steps. First, the active metal is spread onto the
[a]
[b]
[c]
[d]
S. Neumann, T. Wannmacher, D. Bujak, Dr. S. Kunz
Institute of Applied and Physical Chemistry, University of Bremen
Leobenerstraße, 28359 Bremen, Germany
Dr. J. Quinson, L. Kacenauskaite, M. Inaba
Nano-science center, Universitetsparken, University of Copenhagen
5, 2100 Copenhagen Ø, Denmark
For the synthesis of precious metal colloids, usually solvents
with high boiling points like ethylene glycol (EG) or oleylamine
are applied. Alkaline EG functions as solvent and reducing agent
and the reaction products stabilize the NPs,[8] whereas water as
solvent requires surfactants, e.g. glucose, to stabilize the NPs[9]
To deposit the NPs onto a support, ideally one may add only the
support to a surfactant-free colloidal NP dispersion and then
remove the solvent at reduced pressure. Certainly, this is not
feasible with a solvent such as EG due to its high boiling point.
Therefore, the NPs must be precipitated, cleaned from EG, and
re-dispersed in a low boiling point solvent prior to deposition.
This complicates the preparation and additional chemicals like
J. Bucher, F. Bizzotto, Dr. A. Zana, Prof. Dr. M. Arenz
Department of Chemistry and Biochemistry, University of Bern
Freiestrasse 3 CH-3012 Bern, Switzerland
Dr. S. B. Simonsen, Dr. L. T. Kuhn
Department of Energy Conversion and Storage, Technical
University of Denmark, Frederiksborgvej 399, 4000 Roskilde,
Denmark
Supporting information (SI) for this article is given via a link at the end of
the document.
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