Mechanism behind old concept revealed
An international research team led by Bert Weckhuysen  (Utrecht University) and Sara Bals  (University of Antwerp) has shown that a promising catalyst for clearing CO2 becomes significantly more active and selective if its pretreatment is modified. The scientists visualized the mechanism underlying this with unparalleled precision. The results of the study appear in the leading scientific journal Science on 11 May. Matteo Monai, Kellie Jenkinson and Angela Melcherts are first authors.
Cleaning up carbon dioxide or converting it into something useful is becoming increasingly common, for example in the energy and transportation sectors, where huge amounts of the greenhouse gas are emitted. Catalysts are necessary for such a cleanup process to proceed properly and quickly. In the case of CO2 hydrogenation, which is a widely used chemical reaction to clean up CO2, a nickel-supported titanium dioxide catalyst is used.
In this study, the scientists show that the performance of this catalyst is highly dependent on the temperature at which it is prepared. The selectivity and activity of the catalyst were much better during CO2 hydrogenation at a pretreatment temperature of 600 degrees Celsius than at 400 degrees Celsius. Better selectivity is desirable because the catalyst provides fewer unwanted byproducts. Better activity results in a faster progression of the catalytic reaction. The researchers expect the same principle to apply to catalysts with metal oxides other than titanium oxide.
The team of scientists made the discovered principle visible with advanced electron microscopy. They did so with unparalleled precision: at the atomic level. They inserted a special nanoreactor, in which the catalytic process took place, into the microscope and were able to see exactly what happened to the catalyst. "During the process, you see the titanium atoms crawling onto the nickel like layers and coming off again," Bert Weckhuysen describes. "At a high pretreatment temperature, some titanium atoms remained on the nickel. At a low temperature, the layers of titanium atoms disappeared completely."
Two chemical elements, such as nickel and titanium, interacting and forming a catalyst, is an old concept known as strong metal support interaction (SMSI). This study reveals for the first time what happens during this interaction at the atomic level. The sliding layers of titanium have never been seen before. "The new technique has made it possible for us to observe this with our own eyes," said Weckhuysen. "With, moreover, a very high resolution. We can count the titanium atoms on the nickel!"
The international research team that worked on the study is highly interdisciplinary. According to Sara Bals, this was critical to achieving the results obtained. "We used electron microscopy techniques that are generally difficult to combine with experiments in a nanoreactor," she says. "Researchers with different backgrounds and access to a variety of advanced techniques each laid out small pieces of a larger puzzle. Ultimately, these combined led to the complete picture."