Using Individual Atoms To Achieve Fossil-free Chemistry

Every chemical reaction faces a barrier: for substances to react with one another, it is first necessary to supply energy. In many cases, this energy barrier is low - such as when striking a match. For many key reactions in industry, however, it is much larger - and increased energy requirements drive up production costs. To lower this barrier, chemists use "reaction helpers" known as catalysts. The best of these substances contain metals - including, in some cases, rare metals.

Better, more efficient and leaving nothing to chance

Now, chemists from ETH Zurich have achieved a breakthrough in catalysis research on multiple levels:

• They have developed a catalyst that significantly lowers the energy barrier for the production of methanol - an alcohol - from the greenhouse gas CO₂ and hydrogen.
• In their catalyst, the researchers use the metal indium in an extremely efficient manner - in the sense that each individual indium atom behaves as an active site.
• In the past, catalysis research often followed a "hit or miss" approach. The newly discovered catalyst allows more precise analysis of the mechanisms taking place on its surface, paving the way for rational catalyst design.

The Swiss army knife of green chemistry

"Methanol is a universal precursor for the production of a wide range of chemicals and materials, such as plastics - the Swiss army knife of chemistry, so to speak," says Javier Pérez-Ramírez, Professor of Catalysis Engineering at ETH Zurich. The liquid therefore plays a vital role in the transition to sustainable and fossil-free production of chemical products and fuels.

If the energy used to produce the hydrogen and for catalysis is generated sustainably, methanol can ultimately even be produced in a climate-neutral manner. This provides a way of using CO₂ from the atmosphere as a raw material instead of merely releasing it as we do today.

Maximum use of the metals

"Our new catalyst has a single atom architecture, in which isolated active metal atoms are anchored on the surface of a specially developed support material," Pérez-Ramírez explains. In conventional catalysts, on the other hand, metals are usually present as aggregates, usually small particles. Although these particles are tiny, they often contain between a hundred and several thousand metal atoms.

It is no wonder that single-atom catalysts are currently a hot topic in catalysis research. They represent the pinnacle of efficiency when it comes to the use of expensive and scarce chemical elements. If metals are used as individual atoms, it can even be possible to use precious metals in an economically viable manner.

If the atoms can work in isolation, their catalytic properties also frequently change. "Indium has already been used in this catalyst for over a decade," says Pérez-Ramírez. "In our study, we show that isolated indium atoms on hafnium oxide allow more efficient CO₂-based methanol synthesis than indium in the form of nanoparticles containing large numbers of atoms."

Single atoms in the right place

In order to anchor single indium atoms to the hafnium oxide surface in a targeted manner, the interdisciplinary ETH team developed various synthetic pathways in collaboration with colleagues from other research institutions. One key part of this development was the specific structure of the support material, which provides the atoms with a stable and, at the same time, reactive environment.

In one tested production process, the starting materials are combusted in a flame at 2,000 to 3,000°C and then rapidly cooled. Under these conditions, the indium tends to remain on the surface, where it is stably incorporated.

With the incorporation of the catalyst atoms into a heat-resistant hafnium oxide support, the ETH chemists show that single-atom catalysts can remain stable even in extreme conditions. Reactions that require high temperatures and pressures are therefore also within reach. For example, the synthesis of methanol from CO₂ and hydrogen gas requires temperatures of up to 300°C and pressures of up to 50 times normal atmospheric pressure.

Interaction between catalyst metal and matrix

Moreover, the existing nanoparticles used for analysis were a black box. While the catalytic processes only took place at the small number of atoms on the surface, many measurement signals originated from inside the particles, from atoms that were not even involved in the reaction. This made interpretation more difficult. In catalysts with isolated atoms, however, the reaction mechanisms can be analysed with far fewer interfering signals.

Pérez-Ramírez has not only been researching better catalysts for methanol production from CO₂ at ETH since 2010 but also works closely with industry and holds several patents in this area. One key factor in the development of the new single-atom catalyst method was the large network that has emerged in terms of catalysis research in Switzerland in recent years, says Pérez-Ramírez: "The development of the methanol catalyst and the detailed analysis of the mechanism would not have been possible without this interdisciplinary expertise."