3-D scans of single atoms show why catalyst loses power
Aluminium lumps sabotage ‘assembly line’ in catalyst
For decades, zeolites have been used as a catalyst in the conversion of crude oil to fuel, but until recently, it was largely unknown why these zeolites are such good catalysts. An international team of researchers, among which prof.dr.ir. Bert Weckhuysen, professor of Anorganic Chemistry and Catalysis at Utrecht University, has come a step closer to explaining how zeolite catalysts work. The researchers have found an innovative way to locate – with atomic precision – active regions within the material where chemical reactions take place and other regions where the material is dead. With this knowledge, scientists can improve catalysts that help produce fuel and other important chemicals.
The team, including scientists from Pacific Northwest National Laboratory, petroleum refining technology company UOP LLC and Utrecht University, was the first to make a detailed 3-D scan of individual aluminium atoms in a zeolite crystal. These atoms, also called active sites, provide the zeolite’s catalytic power. When molecules pass along these active sites in the zeolite, the aluminium atoms help rip apart and rearrange the molecules, like an assembly line in a factory. A process called steaming causes these active sites to cluster. Like workers crowded around one spot on the assembly line, this clustering effectively shuts down the catalytic factory. The researchers describe their results in a paper that is published this week in the scientific journal Nature Communications.
Most of the world’s gasoline is produced using a conversion process with zeolites. A pretreatment process called steaming uses heat and water to awaken active sites within zeolites. But too much steaming somehow switches the sites off. Scientists have long suspected that steaming causes aluminium to move around within the material, thus changing its properties, but until now aluminium has evaded detailed analysis.
Earlier studies of zeolite structure have used electron microscopes, which cannot easily distinguish aluminium from silicon because of their similar masses. Worse, the instrument’s intense electron beam tends to damage the material, changing its inherent structure before it can be seen. The team of scientists used atom probe tomography instead. This technique works by zapping a sample with a pulsing laser, providing just enough energy to knock off one atom at a time. Sensitive time-of-flight mass spectrometers analyze each atom, at a rate of about 1,000 atoms per second. Unlike an electron microscope, this technique can distinguish aluminium from silicon.
3-D atomic map
By ‘peeling off’ and ‘reading’ tens-of-millions of atoms with this innovative technique, the researchers reconstructed a three-dimensional atomic map of a sample about a thousand times smaller than the width of a human hair. After a number of failed attempts, when electromagnetic forces within the instrument vaporized the entire sample, the team managed to find the right circumstances for the analysis. They coated a pristine specimen with a layer of metal to help provide conductivity and strength to withstand the huge stress during the analysis. This enabled the researchers to make a successful first of its kind scan of an industrially very important zeolite material, better known as ZSM-5.
Determining the Location and Nearest Neighbors of Aluminum in Zeolites with Atom Probe Tomography
Daniel E. Perea, Ilke Arslan, Jia Liu, Zoran Ristanović*, Libor Kovarik, Bruce W. Arey, Johannes A. Lercher, Simon R. Bare en Bert M. Weckhuysen*
Nature Communications, 2015, 6, 7589, published online on 2 July 2015.
* researchers at Utrecht University.