Gold for silver: A chemical barter

June 20, 2019

From effective medicines to molecular sensors to fuel cells, metal clusters are becoming fundamentally useful in the health, environment, and energy sectors. This diverse functionality of clusters arises from the variability in size and type. Now, scientists led by Professor Yuichi Negishi, of the Department of Applied Chemistry at Tokyo University of Science, add to this ongoing tale by explaining the dynamics of the metal cluster, thiolate-protected gold-silver alloy, in solution; this helps in understanding the stability, geometry, and tenability of these clusters for their applications.

Metal clusters are formed when a bunch of metal atoms come together to form clumps, somewhere between the size of a molecule and that of a bulk solid. Recently, these clusters have gained a lot of attention owing to their diverse chemical capabilities that depend on their size and composition. Unlike the closed, set, and stable packing observed in bulk metal lattices, the geometry of these clusters, which often governs their chemical reactivity too, is based on special atomic arrangements that minimize the energy. Furthermore, their functionalities vary depending on the number of constituent atoms in the cluster. Because these micro-level factors govern the ultimate macro-level activity of the clusters, understanding the cluster dynamics at the atomic scale is essential. Recent exploration in the field of such metal clusters has enabled the cataloging of these clumps as compounds of defined chemical compositions.

One such interesting metal cluster with catalytic properties and luminescence is the thiolate-protected gold-silver alloy cluster. These metal clusters are formed when thiolate-protected individual gold and silver clusters are kept together in a solution. The individual pure clusters undergo metal exchange, like a chemical "barter": a gold for a silver atom. While the cluster-metal complex reaction (CMCR) method is widely used, the actual dynamics of it and the energy incentive driving such processes are not understood. This became the seed of curiosity for Prof. Negishi's team, as they state, "the dynamic behavior of these clusters in solution must be taken into consideration to understand the origins of the catalytic activity and luminescence properties of gold-silver alloys clusters in addition to the geometric structure."

To illuminate the metal exchange behavior between the pure clusters after synthesis, the team devised an experiment based on reverse-phase chromatography. They identified this setup because it differentiates molecules based on electronic features, i.e., whether the molecule is polar (with a simultaneous positive and negative center) or non-polar (without separation of charge).

Using this setup proved useful as the team reported that, in fact, the individual structural isomers (different spatial and geometrical distribution for a given cluster) change in solution even though the mass of the cluster remains unchanged. This indicated that there was intra-cluster exchange of metal atoms, which changed the electronic state of the cluster even though the mass remained the same. They also reported that after the synthesis, with the passage of time, the concentration of different structural types of gold-silver alloys in the solution changed. This indicated that there was also an inter-cluster metal exchange at play. Lastly, the researchers also observed that the inter-cluster metal exchange occurs much more frequently after synthesis and eventually slows down after standing for a long time. They assigned this to the difference in stability and energy among the different structures. "The metastable geometries formed initially likely convert to thermodynamically stable geometries through inter-cluster (and intra-cluster) metal exchange in solution," explains Prof. Negishi.

The scientists verified their claims about the observed dynamics of the cluster?metal complex reaction (CMCR) by carrying out a comparative study with the alternate synthesis procedure. Since, traditional procedures (Co-Reduction of Metal Ions) produces alloys under severe conditions, only the thermodynamically and energetically favorable structures see the light of day. Thus, predominantly stable structures are formed, indicating that metal exchange is relatively suppressed. This stood in opposition to the clusters formed by the CMCR where signatures for various species are initially observed. As time passes, like all things in nature, the unstable species try to rearrange themselves into stable ones. How? Through metal exchange, of course!

To summarize, Prof. Negishi states, "These results demonstrate that gold-silver alloy clusters have different geometric structures (and distributions) immediately after synthesis, depending on the synthesis method. Thereby, their dynamic behavior in solution also depends on the synthesis method."

The study of clusters with varying core sizes and compositions is exciting as it offers exciting opportunities to harness novel physical and chemical properties. But, that's not all. It also provides an insight into their structure-property relationships, almost like peeping into the "social life" of atoms!
-end-


Tokyo University of Science

Related Fuel Cells Articles from Brightsurf:

Fuel cells for hydrogen vehicles are becoming longer lasting
An international research team led by the University of Bern has succeeded in developing an electrocatalyst for hydrogen fuel cells which, in contrast to the catalysts commonly used today, does not require a carbon carrier and is therefore much more stable.

Scientists develop new material for longer-lasting fuel cells
New research suggests that graphene -- made in a specific way -- could be used to make more durable hydrogen fuel cells for cars

AI could help improve performance of lithium-ion batteries and fuel cells
Imperial College London researchers have demonstrated how machine learning could help design lithium-ion batteries and fuel cells with better performance.

Engineers develop new fuel cells with twice the operating voltage as hydrogen
Engineers at the McKelvey School of Engineering at Washington University in St.

Iodide salts stabilise biocatalysts for fuel cells
Contrary to theoretical predictions, oxygen inactivates biocatalysts for energy conversion within a short time, even under a protective film.

Instant hydrogen production for powering fuel cells
Researchers from the Chinese Academy of Sciences, Beijing and Tsinghua University, Beijing investigate real-time, on-demand hydrogen generation for use in fuel cells, which are a quiet and clean form of energy.

Ammonia for fuel cells
Researchers at the University of Delaware have identified ammonia as a source for engineering fuel cells that can provide a cheap and powerful source for fueling cars, trucks and buses with a reduced carbon footprint.

Microorganisms build the best fuel efficient hydrogen cells
With billions of years of practice, nature has created the most energy efficient machines.

Atomically precise models improve understanding of fuel cells
Simulations from researchers in Japan provide new insights into the reactions occurring in solid-oxide fuel cells by using realistic atomic-scale models of the electrode active site based on microscope observations instead of the simplified and idealized atomic structures employed in previous studies.

New core-shell catalyst for ethanol fuel cells
Scientists at Brookhaven Lab and the University of Arkansas have developed a highly efficient catalyst for extracting electrical energy from ethanol, an easy-to-store liquid fuel that can be generated from renewable resources.

Read More: Fuel Cells News and Fuel Cells Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.