30 August 2016

Publication by Meirer and Weckhuysen in Nature Communications

What does a clogged road network in a catalyst particle look like?

Every catalyst particle contains a kind of “road network” over which molecules travel to the active sites in the catalyst. Metals can clog this road network, making the active sites unreachable. A research team from Utrecht University and Stanford has used a new imaging methodology to create a detailed road map of one catalyst particle. This new knowledge will eventually lead to the design of better catalysts for existing or new chemical production processes. The researchers published their results on 30 August 2016 in the open access journal Nature Communications.

The large number of pore channels in a catalyst particle makes it very difficult to study how the pores become blocked by metals. Florian Meirer (Utrecht University) explains: “When looking at a catalyst particle, which is about half as wide as a human hair, we measured the total length of the pore channels inside to be about 1.3 meters. By comparison, if the catalyst particle were as big as an average apple, the total length of the pore channels would be approximately two kilometers.”

Catalysis - Combined Wireball Resistance
A catalyst particle composed of channels that can be regarded as electrical wires with different resistances. Molecules can travel to active sites more easily through channels with low resistance than through those with high resistance.

Electrical resistances

Because these channels are strongly folded inside the catalyst particle, they intersect with each other hundreds of thousands of times. “This means that there are billions of different routes that a molecule can take,” Meirer explains. “There are many roads to Rome, even inside a catalyst particle. Identifying all those paths would take months of computer calculations.” Together with researchers from the universities of Utrecht and Stanford, Meirer developed a method to experimentally measure the pore network and then model it as an electrical resistor network. This makes it much easier to identify possible routes, and enables the researchers to calculate the accessibility of the most active regions of the catalyst particle.

Catalysis - Combined Wireball Resistance
The amount of resistance on the route from point A to point B depends on the nature and quantity of the impurities, such as iron (Fe), nickel (Ni), calcium (Ca) and vanadium (V).

Simulating roadblocks

“It is very difficult to measure the concentration of metal that will completely clog a pore,” says Meirer. “But we can now simulate those blockages and see how they affect transport through the catalyst.” The research team found that a catalyst particle with a great number of blockages may still be functional, because of the large amount of possible routes in the road network of the catalyst. But if too many roads are blocked, there is no way for a molecule to reach the active regions of the catalyst, which is then considered deactivated.

Creating a road map

“The high-resolution microscopy data provided a map of the pores, and the high sensitivity of X-ray fluorescence showed us where metals in the refining fluids were poisoning the catalyst, which appeared as a colored fog in our visualization,” said Yijin Liu, staff scientist at SLAC's Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility.

Method for creating a road map of a catalyst particle, which includes determining the positions of the road blocks and simulating the transport of a molecule through the road network, which may or may not be clogged by impurities.

More sustainable production processes

“Our findings are very valuable for a better understanding of the chemical processes that take place within a catalyst,” explains Prof. Bert Weckhuysen, co-author of the publication. “Ultimately, this will lead to catalysts with less sensitive traffic networks, and thus more sustainable chemical processes based on new or improved catalysts. In addition, the developed method – which is similar to a navigation system not for cars, but for molecules – is more generally applicable in the study of dynamic changes in pore networks of functional materials, such as batteries and fuel cells.”

Publication

Relating Struture and Composition with Accessibiity of a Single Particle Catalyst using Correlative 3-Dimensional Micro-Spectroscopy
Yijin Liu, Florian Meirer*, Courtney M. Krest, Samuel Webb & Bert M. Weckhuysen*
Nature Communications, 30 August 2016, DOI 10.1038 / ncomms12634

* Connected to Utrecht University

Contact

Nieske Vergunst, Faculty of Science press officer, N.L.Vergunst@uu.nl, 06 2490 2801
Utrecht University press office, news@uu.nl, (030) 253 3550.

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