Dynamics of Nanocatalytical Model Systems

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.

Third, Professor 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 instance, Professor 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.

See http://phys.au.dk/forskning/condensed-matter-physics/spm/stm-movies/azonano for STM movies showing the dynamics of water-mediated hydroxyl movement on TiO₂.

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 industrial partners.

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.

References

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, 12-22 (2005).
J.V. Lauritsen et al., Nature Nanotechnol. 2, 53 (2007)
D. Matthey et al., Science 315, 1692-1696 (2007).
S. Wendt et al., Science 320, 1755 (2008).
J. Matthiesen et al., ACS Nano 3, 517 (2009).
F. Besenbacher et al., Surf. Sci. 603, 1325 (2009).

Copyright AZoNano.com, Professor Flemming Besenbacher and Dr. Peter Thostrup (Interdisciplinary Nanoscience Center (iNANO), Aarhus University)