Tuneable catalysis: Solving the particle size puzzle

Date:
October 27, 2021
Source:
Vienna University of Technology
Summary:
Chemical reactions can be studied at different levels: At the
level of individual atoms and molecules, new compounds can
be designed. At the level of tiny particles on the nano and
micrometer scale, one can understand how catalyst materials
influence chemical reactions. Now it is possible to connect all
levels from the microscopic to the macroscopic level in order
to describe a technologically important chemical reaction under
realistic conditions.



FULL STORY ========================================================================== Chemical reactions can be studied at different levels: At the level
of individual atoms and molecules, new compounds can be designed. At
the level of tiny particles on the nano and micrometre scale, one can understand how catalyst materials influence chemical reactions. And in
order to use chemical reactions in industry, it is necessary to look at
the macroscopic scale.


========================================================================== Typically, different approaches are used for each area. But this is
not sufficient for complex chemical reactions on catalyst surfaces. At
TU Wien (Vienna), an important step has now been taken: for the first
time, it was possible to connect all levels from the microscopic to
the macroscopic level in order to describe a technologically important
chemical reaction under realistic conditions. This allows to understand
why the size of catalyst particles plays a decisive role. The results
have now been published in the scientific journal Nature Communications.

Isomers: Same composition, different molecules Many molecules come in
different variants: The same set of atoms can be arranged in different
ways, which are then referred to as "isomers." It is important to
distinguish between these isomers -- for example, a certain isomer of
the hydrocarbon butene is favourable for fuel production, but another
butene variant is preferred for polymer manufacturing. Producing exactly
the desired isomers or converting one isomer into another is a tricky
task that can be achieved with very specific catalysts.

"A particularly important catalyst for such processes is palladium,"
says Prof.

Gu"nther Rupprechter from the Institute of Materials Chemistry at TU Wien.

"Normally, palladium is placed on a surface in the form of tiny
nanocrystals.

Certain molecules then bind to these granules, and this enables the
chemical reaction." It is a well-known fact that the particle size is
often crucial for a specific catalytic function, but mostly there has
been no detailed rationalisation of how this works. "It is impossible
to create a full-scale quantum-chemical model of these particles on
a computer, because they simply consist of too many atoms," says Dr
Alexander Genest, the first author of the current study. "We therefore
have to find alternatives to combine the different methods to study
chemical catalysis." Realistic conditions instead of idealised systems


==========================================================================
The research team at TU Wien and its cooperation partners from Singapore, Alicante and Munich chose a complex but important reaction for their investigations: The isomerisation of alkenes. "This is particularly
challenging because there are several reaction pathways that play a
role at the same time," says Gu"nther Rupprechter. "It was important
for us to study the reaction under realistic conditions: In previous
basic research, reactions were often analysed in (ultra-)high vacuum,
at low temperatures. But in an industrial setting, you have to deal with completely different parameters. We therefore wanted to find out how
this isomerisation takes place at atmospheric pressure and 100DEGC."
The team started at the level of atoms and molecules: "With the help
of density functional theory, we can model elementary reaction steps of
the molecules that attach to various facets of the palladium crystals,"
says Alexander Genest.

These calculations yield parameters for so-called microkinetic models,
which can be used to predict the dynamics of chemical reactions on a
much larger time scale on a computer. And from these results, in turn,
it is then possible to infer the total amount of desired chemical products
that will be present after a certain time at certain parameters.

"The model calculations agree very well with our experimental
measurements, not only qualitatively but also quantitatively," emphasises
Prof. Gu"nther Rupprechter. "This is an important breakthrough -- such agreement was not possible like this before." Now it can be explained in
detail why various sizes of palladium particles have different effects
on the chemical processes: Large particles have smooth surfaces, while
smaller ones are more round and stepped.

The arrangement of the palladium atoms in alternative geometries
influences the reaction energy and thus the catalytic behaviour.

Optimal results instead of just trial and error "When you optimise
a chemical process in industry, you often have to rely on trial and
error," says Gu"nther Rupprechter. "Which external parameters should
be chosen? Which catalysts do you use -- and in what form? These are
questions that could hardly be answered on a theoretical level until
now." Usually multiple variants are tested and then the most successful
one is chosen. But if a process is then supposed to be scaled up from laboratory scale to industrial scale, completely different parameters
may be required.

"We have now shown that you can comprehensively understand such
processes if you link several time- and length scales," says Alexander
Genest. "This approach is of course also applicable to many other
catalytic reactions." In the chemical industry, it should thus become
possible to optimise chemical manufacturing processes through computer modelling and at the same time reduce expensive and time-consuming
benchmarking to a minimum.

========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Alexander Genest, Joaqui'n Silvestre-Albero, Wen-Qing Li, Notker
Ro"sch,
Gu"nther Rupprechter. The origin of the particle-size-dependent
selectivity in 1-butene isomerization and hydrogenation on
Pd/Al2O3 catalysts. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021- 26411-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211027122059.htm

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