How effective a catalyst is often relies heavily on the surface it is applied to. It has been established for quite some time that carbon substrates work effectively with precious metal catalysts, though the reason behind this was not well understood. Recently, researchers have uncovered the underlying reasons for this phenomenon, revealing some impressive findings: Metal atoms positioned right next to carbon demonstrate two hundred times greater catalyst efficiency.
Precious metals are crucial in the chemical industry as catalysts. Silver, platinum, palladium, and similar elements facilitate chemical reactions that typically would not occur or would do so at significantly lower rates. Frequently, these metals are utilized in the form of tiny nanoparticles. However, their performance is also influenced by the surface on which they are deposited. It appears that nanoparticles on a carbon base exhibit particularly superior efficacy, a fact that remained shrouded in mystery for many years.
Researchers at TU Wien have made significant strides by accurately measuring and elucidating the interaction between metal nanoparticles and carbon substrates. They discovered that silver atoms on a carbon support demonstrated two hundred times more activity compared to those found in pure silver. Simulations conducted on computers suggest that the area where silver is in direct contact with carbon is vital. A new method involving hydrogen isotope exchange was developed to quickly and efficiently evaluate the effectiveness of catalyst supports.
From “black art” to science
According to Prof. Günther Rupprechter from the Institute of Materials Chemistry at TU Wien, carbon’s role as a support material in catalysis has long had an air of mystery. The source of the carbon was revealed to be crucial. Various processes employ carbon derived from coconut shells, fibrous materials, or unique types of wood. These “recipes” are even referenced in patent documents, even though the origin of chemicals should ideally be relatively inconsequential. “It always felt like a bit of a black art,” remarks Günther Rupprechter.
The underlying notion was that differing production techniques might create slight chemical or physical variations: perhaps the carbon structures itself differently based on the production method, contains small amounts of additional chemical elements, or has functional groups on its surface—tiny molecular units that participate in the chemical reaction.
“In the chemical industry, practitioners often settle for knowing that a process is effective and reproducible,” Rupprechter notes. “But we aimed to trace the root of the effect and comprehend exactly what transpires at the atomic level.” Collaborations included the University of Cádiz (Spain) and the Center for Electrone Microscopy USTEM at TU Wien.
Precision measurements in a microreactor
The research team first created samples that could be analyzed with great precision: silver nanoparticles of a known size were created on a carbon substrate alongside a thin silver foil that lacked carbon.
The team then investigated both samples in a chemical reactor: “Silver can effectively break down hydrogen molecules into separate hydrogen atoms,” explains Thomas Wicht, the lead author of the study. “This hydrogen is then available for processes like the hydrogenation of ethene. In a similar fashion, regular hydrogen molecules can be blended with heavier hydrogen (deuterium). Both types are then broken down by silver and recombined.” The frequency of exchange between the two hydrogen isotopes directly reflects the activity of the catalyst.
This approach allowed for the first precise quantification of the activity variance between silver atoms with and without a carbon substrate—isolating some remarkable outcomes: “The presence of carbon results in a two hundred times increase in activity per silver atom,” states Thomas Wicht. “This is incredibly significant for industrial use, as you would only require one two-hundredth of the amount of expensive noble metals to achieve the same catalytic effect by simply adding relatively inexpensive carbon.”
The exciting effect occurs right at the interface
Alexander Genest from the TU Wien team executed computer simulations evaluating the hydrogen activation by silver nanoparticles on carbon versus that of pure silver. The findings indicated that the crucial element lies in the interface region between the silver particles and the carbon support. The greatest catalytic effect occurs precisely where the two materials connect. “It’s not the size of the carbon surface or the presence of foreign atoms or functional groups that’s important. An extreme catalytic effect arises when a reactant interacts with both a carbon and a silver atom directly at their interface,” explains Alexander Genest. The larger this interface is, the more significant the catalyst’s activity.
With this understanding, it’s now feasible to easily test various carbon sources for their catalytic effectiveness. “Having unraveled the mechanics at play, we know what to focus on,” says Günther Rupprechter. “Our experiments involving a mix of regular and heavy hydrogen are straightforward to perform and yield dependable data on whether a specific carbon carrier variant is effective for additional chemical reactions.” Understanding processes down to the atomic level promises to save both time and money in industrial applications and enhance quality assurance.
