Arne Wittstock1, Gunther Wittstock2, Marcus Bäumer1,*
Nanoporous gold is a relatively new catalyst with great potential in heterogeneous gas and liquid phase catalysis and electrocatalysis. It is a sponge-like material with ligaments and pores in the range of a few 10 nm. Its catalytic properties are influenced by traces of a second metal and the nanostructure.
The DFG research unit NaGoCat aims at elucidating these factors on different length scales from the atomistic level (active sites for adsorption and reaction) up to the mesoscopic level (transport of the reactants by diffusion). This interdisciplinary effort is only possible by bringing together the expertise of different groups from chemistry, physics and material science working collaboratively together.
During the last decade, highly porous bulk-nanostructured metals have been investigated and explored for a large variety of applications, such as catalysis, actuation, energy storage, and energy conversion (e.g. fuel cells). One of the grand challenges is the preparation of such materials with a well-defined structure and composition, allowing a fundamental understanding of the complex interplay between structure and properties and, based thereon, a rationally guided optimization and tailoring of functionalities. Among the few examples showing a large potential in this respect, is a mesoporous Au material, usually called nanoporous gold (npAu) in the literature. Today, npAu is employed and investigated in almost all the application areas that can benefit from porous bulk-structured materials, e.g. catalysis, sensors, actuators, optics, energy harvesting and storage, and many more areas (Fig. 1).
A particularly interesting field of application is catalysis, where not only the special physical properties but also its surface chemistry is of advantage . The combination of a high specific surface area, a controllable and defined porosity as well as high heat and electronic conductivity makes npAu a fascinating heterogeneous catalyst which allows understanding and optimization of catalysis on various scales ranging from reaction sites on the surface over the transport of reactants by diffusion finally to the macroscopic performance at the scale of a reactor.
Preparation of npAu
The material is generated by a dealloying process of a suitable alloy of Au and a less noble constituent (Ag, Cu, Al).
For example, submersing an Au-Ag alloy into acidic media initiates the bulk corrosion of the less noble metal (here Ag) at open circuit potential. This process can also be driven by actively applying an external potential. Key to the formation of an absolutely homogenous porous structure with pores and ligaments in the range of ~ 40 nm is a self-organization process of the Au atoms during the dissolution of the Ag. While the edge front is propagating towards the bulk of the alloy sample, the gold atoms at the surface diffuse and cluster forming initial islands. Later on they form the pinnacles resulting in the respective ligaments of the open cell foam structure. Owing to this self-organization of the residing Au atoms, the size distribution of pores and ligaments of the resulting foam is completely homogeneous even on centimeter scale. This facilitates preparation of various shapes and geometries of macroscopic npAu samples (Fig. 1).
Gas Phase Catalysis
In 2006, it was shown for the first time that npAu is catalytically active for CO oxidation at and below room temperature with surprisingly high activity. More recent experiments revealed that npAu exhibits highly interesting catalytic properties in the context of green catalysis. It could be shown that npAu catalyzes the oxidation of primary alcohols such as methanol and ethanol at low temperatures without additional bases with selectivity close to 100 %.
Two factors have been speculated to play a role for its catalytic activity, low coordinated surface atoms and the presence of a second metal. TEM studies by Fujita et al. revealed that the curved parts of the ligaments exhibit a surprisingly large number of low-coordinated sites, mimicking the number density on small Au nanoparticles (2). Secondly, the residual content of ad-metals, such as Ag or Cu, is crucial especially for the activation of molecular oxygen. Indeed, the activity and selectivity of the npAu catalyst can be controlled by variation of the Ag content, equivalent to a tunable “oxidation” power of the catalyst (Fig. 2) . New functions can be added to the intrinsic properties of npAu by adding a second component (such as a metal or oxide, (Fig. 1)). However, the grand challenge for a further development of this concept consists in unraveling the contribution and role of these various partners on an atomistic scale paving the way to a rational catalyst design.
Liquid Phase Catalysis
Since about the last four years considerable progress in the study of npAu for liquid phase reactions has been made. Besides important oxidation reactions of secondary alcohols, glucose, and organo silanes, the reported reactions include the reduction reactions of alkynes and quinolones, also C-C ring formation reactions ([4+2] benzannulation) and azo compound decomposition reactions . Gold – also in its unsupported form – has become an interesting material in chemical synthesis offering unique catalytic performance. For example, gold occupies a niche in the hydrogenation of organic compounds, in particular alkynes. Compared to more active hydrogenation catalysts such as Pd it provides a higher selectivity in terms of steroselectivity (E/Z isomere) and overreduction (alkene vs. alkane).
Porous Au structures play an important role in electrocatalytic applications. First approaches for generation of such structures used templating techniques such as currentless or galvanic coating of track-etch membrane channels. Already these first examples revealed a number of unique phenomena that can be exploited using such structures, including selective transport of ions in pores with charged walls or direct electron transfer from redox proteins and redox enzymes. Electrochemical applications of npAu described in the literature include the electrocatalytic oxidation of small molecules. As in the case of gas phase catalysis, the reactivity is often dependent on the presence of remaining atoms of the second metal of the original alloy or on the presence of under potential deposition (UPD) layers of another metal. Of potential importance with respect to applications, is the use of npAu as gas diffusion electrodes in lithium-air batteries because they can avoid the problem of carbon corrosion at the high potential required for recharging this cell . This approach also exploits npAu as a conducting matrix for redox active materials with rather low electronic conductivity, i.e. the Li2O2 formed during discharge.
Based on these application areas, the research unit follows two directions in order to modify (chemically and structurally) and optimize the material for catalytic applications:
Variation of composition: Bimetallic npAu foams
By electrochemical corrosion an almost pure gold material can be obtained. Variation of the preparation procedure, however, allows for adjusting the content of the residual alloy metal which plays an important role for the catalytic properties. When dealloying bimetallic alloys, such as Au-Ag, Au-Cu, or intermetallics of Au-Al, npAu materials with well adjustable amounts of a second metal can be prepared; for example, Ag contents from about 0.1 at% all the way up to several percent can be realized without changing the porosity and size of the ligaments considerably. Also, adding a second or third component to the existing porous structure is possible by UPD or chemical vapor deposition (CVD).
Variation of structure: npAu with tailored size
Nanoporous gold offers the unique possibility to change the size of structures (ligaments and pores) from a few nanometers all the way up to micrometers without changing its morphology (Fig. 3). Such a microstructural tailoring becomes possible by taking advantage of the intrinsic instability of the evolving nanostructure due to the curvature driven surface diffusion of atoms. Variation of the structure size has an impact on the mass transport within the pores, the surface to volume ratio, and, importantly, on the local coordination of surface atoms. One needs to balance the mass transport (large pores) and the number of low coordinated sites (small ligaments) to find an optimal structure size.
Nanoporous gold is an active and selective catalytic material with potentials for industrial applications. Still, the influence of the chemical (surface) composition and nanostructure on the catalytic performance has to be elucidated in more detail. The DFG research unit NaGoCat is aiming at contributing to an understanding of the processes on different length scales by bringing together the expertise of chemists, physicists and material scientists. This interdisciplinary effort made possible by funding of the German Science Foundation DFG within the “Forschergruppen”-framework has the potential to provide guiding principles on the way to less empirically and more rationally designed heterogeneous catalysts.
1 University Bremen, Institute of Applied and Physical Chemsitry & Center for Environmental Research and Sustainable Technology, Center for Materials and Processes, Leobener Strasse UFT, Bremen, Germany
2 Carl von Ossietzky University of Oldenburg, Institute of Chemistry, Oldenburg, Germany, vice speaker of the Research Unit
Prof. Dr. Marcus Bäumer
Speaker of the Research Unit
Leobener Strasse UFT
Articles on catalysis: http://www.laboratory-journal.com/search/gitsearch/Catalysis%20type:topstories
DFG Project nagocat: http://www.nagocat.uni-bremen.de/typo3_src-6.2.14/index.php?id=2/
 Arne Wittstock, Marcus Baumer: Catalysis by Unsupported Skeletal Gold Catalysts, Accounts of Chemcal Research, 47, 731-739, (2014), DOI:10.1021/ar400202p.
 Takeshi Fujita, Pengfei Guan, Keith McKenna, Xingyou Lang, Akihiko Hirata, Ling Zhang, Tomoharu Tokunaga, Shigeo Arai, Yuta Yamamoto, Nobuo Tanaka, Yoshifumi Ishikawa, Naoki Asao, Yoshinori Yamamoto, Jonah Erlebacher, Mingwei Chen: Atomic origins of the high catalytic activity of nanoporous gold, Nature Materials, 11, 775-780, (2012) DOI:10.1038/nmat3391.
 A. Wittstock, V. Zielasek, J. Biener, C. M. Friend, M. Bäumer: Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature, Science, 327, 319-322, (2010), DOI:10.1126/science.1183591.
 Zhangquan Peng, Stefan A. Freunberger, Yuhui Chen, Peter G. Bruce: A Reversible and Higher-Rate Li-O2 Battery, Science, 337, 563-566, (2012), DOI:10.1126/science.1223985.