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In situ studies
tron
gfu
r el
S was
sis. Th
ience,
nt env
a hear
equipment was exploited, that relevant techniques and methods
were implemented and that their application bene\ufb01tted the com-
pany\u2019s research and development activities with most impact.
With this entrepreneurship, he supported the introduction of yet
more and more advanced and sophisticated tools for catalyst
characterization.
The characterization tools were in the beginning primarily pho-
ton-based techniques operating at different wavelength (e.g., visi-
ble light, infrared light, X-rays, and gamma rays), and their
advancement made atomic-scale insight into the structural and
experimental and theoretical surface science studies (made in-
acroscopic scale
uctures. Comple-
ut the mor
face struct
the catalysts under similar conditions was inaccessible. Such
tural information is in particular important because cataly
surface phenomenon and gas\u2013surface interactions generall
bit site-dependency.
During the 1990s, advancements in transmission electron
microscopy (TEM) and scanning transmission electron microscopy
(STEM) made imaging of catalysts with a spatial resolution down
to ca. 1 Å more widely accessible [3]. The high-resolution electron
microscopy provides images that re\ufb02ect a two-dimensional projec-
tion of the three-dimensional physical structure and thereby
Journal of Catalysis 328 (2015) 102\u2013110
lab
f
w.e
prenatal as well as postmortem analyses of the catalyst materials.
However, Dr. Haldor Topsøe continuously kept striving toward
new fundamental understanding in catalysis. He engaged himself
actively in facilitating that the newest and best experimental
techniques provide information averaged over a m
being far larger than the catalytic active nanostr
mentary atomic-level, real-space information abo
gies as well as the exposed surface sites and inter
http://dx.doi.org/10.1016/j.jcat.2014.12.017
0021-9517/\ufffd 2015 Elsevier Inc. All rights reserved.
pholo-
ures in
struc-
sis is a
y exhi-
research; an approach that he catch-phrased \u2018\u2018science to dollars\u2019\u2019 [1].
For long, the developments in catalysis were based on trial-and-
error methodologies, on measurements of reaction kinetics, and on
house and through external collaborations) helped to further
include detailed information about gas\u2013surface interactions into
the description of the catalysis. However, the in situ photon-based
a determination of basing its practical work on fundamental
mental understanding of atomic-scale phenomena in surface sci-
ence and catalysis would open up a basis for engineering
catalysts for any process. He therefore formed the company with
unambiguous atomic-level insight into the active state and
dynamic phenomena has been obtained for many catalysts in the
company\u2019s portfolio. Moreover, interplay with information from
Transmission electron microscopy
1. Introduction
The company Haldor Topsoe A/
enterprise in heterogeneous cataly
Topsøe, had a strong passion for sc
and personal relations in the vibra
Bohr\u2019s school in Copenhagen. He had
formed in 1940 as an
e founder, Dr. Haldor
spurred by his studies
ironment around Niels
tfelt belief that a funda-
chemical state of catalysts accessible [2]. A particular bene\ufb01t was
the introduction of instrumentation for characterizing catalysts
during exposure to single gasses or gas mixtures at elevated tem-
peratures and to conditions under which catalysis is taking place.
Such experiments are often referred to as in situ (or operando) stud-
ies and allow information about the catalyst structure and its prop-
erties to be simultaneously acquired. Hereby, new and
Heterogeneous catalysis
Nanoparticles
An industrial perspective of the impact o
electron microscopy in catalysis
Stig Helveg
Haldor Topsoe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark
a r t i c l e i n f o
Article history:
Received 22 October 2014
Revised 25 November 2014
Accepted 9 December 2014
Keywords:
a b s t r a c t
With recent advances, elec
and under catalytic meanin
dor Topsøe\u2019s enthusiasm fo
in catalysis.
Contents lists avai
Journal o
journal homepage: ww
Haldor Topsøe on (in situ)
microscopy can now visualize heterogeneous catalysts at the atomic scale
l conditions. A selection of the advancements is outlined to re\ufb02ect Dr. Hal-
ectron microscopy as means for improving the fundamental understanding
\ufffd 2015 Elsevier Inc. All rights reserved.
le at ScienceDirect
Catalysis
lsevier .com/locate / jcat
Since the invention of the transmission electron microscope,
3. Surface structure and dynamics in catalysts
talys
instrument developments have focused on improving the imaging
resolution and sensitivity and on integrating complementary spec-
troscopic techniques. In parallel, developments also pursued the
introduction of gas or liquid environments into electron micro-
scopes [6,7]. Such developments are challenging due to the small
mean-free path of electrons in dense gasses and the need for high
electron acceleration voltages. A con\ufb01nement of the gas environ-
ment to the vicinity of the catalyst specimen is therefore required
in order to limit electron scattering by the gas molecules and to
avoid degradation of the imaging resolution and sensitivity.
For several decades, methods for gas con\ufb01nement were under
development employing either a differentially pumped microscope
vacuum column or a window cell [7,8]. In a differentially pumped
microscope, the vacuum system is divided into separate volumes
by incorporation of apertures along the microscope column. Gas
can be introduced into the volume hosting the catalyst sample
and leaks out into the adjacent volumes that are connected to vac-
uum pumps. Thereby, a gas phase is con\ufb01ned near the catalyst
sample, while the density of gas molecules is kept at a suf\ufb01ciently
low level elsewhere in the microscope to maintain electron scatter-
ing at a negligible level. The window cell approach employs a
closed volume equipped with electron-transparent windows and
separates the specimen and gas phase from the microscope vac-
uum system. In this setup, the electron beam is disturbed by elec-
tron scattering on both the gas phase and the electron-transparent
windows. In both approaches, catalyst samples are mounted on
devices that enable heating in the gas environments and during
in situ observations.
In the 1970s, the pioneering applications of electron microscopy
enables pro\ufb01le views of the morphologies and surface structures in
catalysts at the atomic scale. Typically, the electron microscopy has
been applied for ex situ characterization of catalysts before or after
various gas treatments. The complementary use of electron
microscopy for in situ characterization of catalysts was rare, and
images of catalysts at the atomic scale during exposure to elevated
pressures and temperatures were therefore scarce at the end of the
1990s [4,5]. The situation incited Dr. Haldor Topsøe\u2019s entrepre-
neurial attitude, and he took the initiative to establish an electron
microscopy facility for in situ studies in 1999 to complement the
arsenal of other in situ techniques used in the company\u2019s research
and development activities.
In the following time, I have had the privilege of discussing the
applications, instrumentation, and methodologies of electron
microscopy with Dr. Haldor Topsøe on a regular basis. Typically,
the discussions touched upon experimental results and their impli-
cations for catalysis as well as on technical limitations and future
advancements in electron microscopy. Often, we exchanged ambi-
tions and dreams; some of which were practically impossible and
seemed like daunting tasks at the time. The challenges included,
e.g., capabilities for pinpointing each and every single atom at
the catalytic active sites or for monitoring catalytic reactions
site-by-site