A New Understanding of Metal-Carbon Interactions

Researchers from the Vienna University of Technology have successfully measured and described the interaction between metal nanoparticles and a carbon substrate, according to a study published in ACS Catalysis.

Precious metals are vital catalysts in the chemical industry because they enable chemical reactions that would otherwise not occur or would proceed at a much slower rate. Examples of these metals include silver, platinum, palladium, and others.

Tiny nanoparticles of these metals are commonly used in catalytic processes, but the surface on which they are placed also influences their performance. For a long time, the reasons why nanoparticles on a carbon substrate appeared to perform so well remained unclear.

It was discovered that atoms in pure silver are 200 times less active than silver atoms on a carbon support. According to computer simulations, the area where the silver and carbon come into direct contact plays a crucial role. To address this, a new technique was developed to quickly and easily test the efficacy of catalyst supports using hydrogen isotope exchange.

From “Black Art” to Science

For a long time, the use of carbon as a carrier material for catalysis had something almost magical.

Günther Rupprechter, Professor, Institute of Materials Chemistry, Vienna University of Technology

The origin of the carbon used in catalytic processes proved to be significant. Carbon derived from coconut shells, fibers, or specialty woods is employed in some processes, and even patent documents include these “recipes.” However, the provenance of chemical substances is typically thought to be irrelevant.

“It always seemed a bit like black art,” Rupprechter added.

The theory suggests that the carbon may arrange itself differently depending on the manufacturing method, with slight chemical or physical differences potentially arising from varying techniques. It could also contain traces of other chemicals or surface-accumulating functional groups—tiny molecular building blocks—that might interfere with the chemical reaction.

“In the chemical industry, people are naturally often content with the fact that a process works and can be repeated reliably. But we wanted to get to the origin of the effect and understand exactly what is actually going on here at the atomic level,” Rupprechter stated.

The study also involved the Center for Electron Microscopy USTEM at TU Wien and the University of Cadiz in Spain.

Precision Measurements in a Microreactor

The group first created samples that were precisely characterized: thin silver foil free of carbon and silver nanoparticles of known size placed on a carbon substrate.

They then used a chemical reactor to examine both samples:

Silver can be used to split hydrogen molecules into individual hydrogen atoms. This hydrogen can then be used, for example, for the hydrogenation reaction of ethene. In an analogous manner, one can also mix ‘ordinary’ hydrogen molecules with molecules made of heavy hydrogen (deuterium). Both molecules are then dissociated by the silver and recombined.

Thomas Wicht, Study First Author and Project Assistant, Vienna University of Technology

The frequency of exchange between the two hydrogen isotopes increases with catalyst activity, providing highly accurate information about the catalyst's performance.

This enabled the team to precisely measure the difference in activity between silver atoms with and without a carbon support for the first time, yielding astounding results.

Wicht added, “For each silver atom, the carbon background induces a two hundred times higher activity. This is, of course, very important for industrial applications. You only need a two-hundredth of the amount of expensive precious metals to achieve the same activity – and you can do that simply by adding comparatively inexpensive carbon.”

The Exciting Effect Happens Right at the Border

Computer simulations comparing the activation of hydrogen by silver nanoparticles on carbon and pure silver were conducted by Alexander Genest from the TU Wien team. The simulations revealed the critical role of the area between the silver particles and the carbon carrier. The point of contact between the two is where the catalyst effect is strongest.

So, it is not the size of the carbon surface or any foreign atoms or functional groups. An extreme catalytic effect occurs when a reactant molecule comes into contact with both a carbon and a silver atom directly at the interface.

Alexander Genest, Senior Postdoctoral Fellow, Vienna University of Technology

The activity increases with the size of the direct contact area between the silver particles and the carbon carrier.

This insight allows for the easy evaluation of the efficacy of various carbon batches from different sources.

Rupprechter added, “Now that we have understood the mechanism of action, we know exactly what to pay attention to. Our experiment, in which we expose the catalysts to a mixture of ordinary and heavy hydrogen, is relatively easy to carry out and provides very reliable information as to whether this variant of the carbon carrier is also suitable for other chemical reactions or not.”

By explaining processes at the atomic level, it should now be possible to streamline quality assurance, saving both time and money in industrial applications.

Journal Reference:

Wicht, T. et. al. (2024) Role of Interfacial Hydrogen in Ethylene Hydrogenation on Graphite-Supported Ag, Au, and Cu Catalysts. ACS Catalysis. doi.org/10.1021/acscatal.4c05246