The world is facing a variety of serious challenges in securing more
efficient and sustainable chemical and energy production. Catalysis already
plays a central role in such technologies, but novel and inexpensive catalysts
are urgently required if we are to meet the global challenges. Scanning
Tunneling Microscopy (STM) is a unique real-space technique of catalyst model
systems that can provide new insights into industrial catalytic systems
identifying the active sites, the importance of defect sites, and support
effects. We are approaching an era where fundamental atomic-scale insights into
surface structure and reactivity may lead to the design of a new superior
catalyst operating under technically relevant conditions.
Step-by-Step Surface-Catalytic Reactions
Our first example deals with the design of a new steam-reforming catalyst.
While gold and nickel are immiscible in the bulk, Professor Besenbacher
and Dr. Thostrup
at the Interdisciplinary
Nanoscience Center (iNANO) have discovered that the two do in fact form a
surface alloy. Taken together with the fact that Ni, when used as a
steam-reforming catalyst, is quickly passivated by graphite formation, Professor Besenbacher
and Dr. Thostrup
investigated whether the Au-Ni surface alloy is more resistant.
From high-resolution STM images it was revealed that the Au atoms alloyed
into the Ni surface layer perturb the electronic structure of the nearby Ni
atoms, in the sense that the Ni atoms with a neighboring Au atom are imaged
brighter by STM (see figure below). This has the surprising effect of lowering
the tendency of the surface to bind carbon and form graphite. These fundamental
findings inspired the synthesis of a high surface area,
MgAl2O4-supported Au-Ni steam-reforming catalyst.
As a second example, Professor Besenbacher and Dr. Thostrup
successfully resolved a reaction intermediate in the hydrodesulphurization (HDS)
process, which is employed on practically all combustion products derived from
crude oil and is as such a very important factor in abating sulphur emissions.
Our atomically resolved images revealed a hitherto unknown electronic "brim"
state with metallic character at the edges of catalytically active
MoS2 nanoclusters. It turns out that this brim state imparts unusual
chemical characteristics to the MoS2 clusters.
The figure below shows the reaction intermediate cis-but-2-ene-thiolates
(C4H7S-) coordinated through the terminal sulfur atom to
the metallic brim formed upon partial hydrogenation of thiophene
(C4H4S). Based on insight gained from the STM studies
combined with detailed theoretical DFT calculations, our collaborators at Haldor
Topsøe A/S recently managed to synthesize a new type of superior HDS catalysts,
the BRIM hydrotreating catalysts.
Besenbacher and Dr. Thostrup dealt with the surprising finding that the
catalytic activity of finely dispersed Au nanoparticles on rutile
TiO2 supports exceeds those of commonly used transition-metal
catalysts such as Pt, Rh, and Pd. At present, the most serious problem
associated with Au nanocatalysts is their long-term stability, since when the Au
nanoclusters sinter, they deactivate.
From interplay between STM and DFT results, Professor Besenbacher
and Dr. Thostrup
have revealed that not oxygen vacancies, but rather O-rich Au-support interfaces
are important to stabilize Au nanoclusters under real reaction conditions. These
results show that the oxidation state of the supporting oxide is highly relevant
and may also indicate that the perimeter of the Au nanoclusters is of special
interest to catalytic reactions.
By means of time-lapsed STM movies, Professor Besenbacher and Dr. Thostrup can
visualize the Au nanoclusters and resolve the dynamic behavior of the chemical
species present on the TiO2 surface. This gives the opportunity of
realizing one of the "Holy Grails" within the area, that is to directly "watch"
chemical reactions at the atomic scale, step-by-step, as they happen. For
Besenbacher and Dr. Thostrup have revealed that defects such as vacancies and
Ti interstitials play a key role in and may dictate the ensuing surface
oxidation chemistry, such as providing the electronic charge required for O2
adsorption and dissociation.
for STM movies showing the dynamics of water-mediated hydroxyl movement on
Benefits to Society
Industrial catalysts are invariably structurally complex and generally
unamenable to atomic-scale scrutiny with today's surface-sensitive techniques.
In the so-called "surface-science approach", we therefore resort to idealized
system such as those presented above. These gross simplifications
notwithstanding, we still run fruitful research collaborations with our
To researchers in industry and academia alike, real-space visualization in
itself is a great inspirational factor, but the elucidation of fundamental
phenomena or critical reaction parameters help us approach the goal of designing
new catalysts from first principles.
F. Besenbacher et al., Science 279, 1913-1914 (1998).
J.V. Lauritsen et
al., J. Catal. 224, 94-106 (2004).
H. Topsøe et al., Catal. Today 107-08,
J.V. Lauritsen et al., Nature Nanotechnol. 2, 53 (2007)
Matthey et al., Science 315, 1692-1696 (2007).
S. Wendt et al., Science 320,
J. Matthiesen et al., ACS Nano 3, 517 (2009).
et al., Surf. Sci. 603, 1325 (2009).
Copyright AZoNano.com, Professor Flemming Besenbacher and Dr.
Peter Thostrup (Interdisciplinary Nanoscience Center (iNANO), Aarhus University)