ORNL Researchers Develop Gas Flow Cell for Studying the Atomic Structure of Catalysts

Access to a cutting-edge capability for studying catalysts has now been obtained by Researchers at the US Department of Energy’s Oak Ridge National Laboratory (ORNL).

Catalysts are specific materials that facilitate chemical reactions, starting from refining petrochemical products and purifying gasses to preparing food and processing fuel. The North American Catalysis Society points out that catalysts contribute to more than 35% of the worldwide GDP and represent a $12 billion market in just the United States. As a result, the scientific community considers research on understanding the material properties and optimizing the performance of integral catalysts as a high priority.

Under conventional research approaches, the catalyst and other products are just examined before or after the reaction. However, a gas flow cell that has the potential to study the atomic structure of these materials in real time was recently developed by a team of Scientists from ORNL and Colorado State University. Experiments using total scattering and neutron diffraction techniques will be able to imitate real-world conditions with industrial relevance — such as, catalytic converters in vehicles — in order to provide new insights into the temporary relationship between the reaction products and the catalyst.

If we want to understand the limits of current technologies and help design new materials, better materials, we have to understand why they work.

Daniel Olds, a Postdoctoral Researcher, ORNL’s Spallation Neutron Source (SNS)

Contributors from SNS and ORNL’s Center for Nanophase Materials Sciences (CNMS) included Sample Environment Experts, Data Reduction Specialists, Instrument Scientists and Chemists. The project made use of Laboratory Directed Research and Development (LDRD) funds, and both users and staff have already taken advantage of this new capability.

It’s one of those pieces that was immediately adopted by the community, which is really exciting for our instrument team.

Katharine Page, NOMAD Instrument Scientist

The Researchers installed the gas flow cell at the high-intensity NOMAD diffractometer, called SNS beamline 1B, in order to develop a new sample environment where users can analyze catalytic reactions under genuine operating conditions. The neutron’s potential to differentiate between isotopes was considered to be the key to efficiently study gas-solid interfaces between a material sample and a catalyst.

“Diffraction techniques can often probe changes to the catalyst itself, but the interaction of the catalyst with the entity that you’re catalyzing is often very difficult to probe,” Page said.

Since all the isotopes of a parent element comprise of the same number of protons, several analytical methods cannot in fact tell them apart. However, neutron diffraction techniques are capable of differentiating between isotopes since each separate atom contains a different number of neutrons. By simultaneously using neutron diffraction and the steady-state isotope transient kinetic analysis (SSITKA) technique, the Researchers were able to study the collaboration of an adsorbing gas with a tubular reactor sample filled with solid particles of the mineral zeolite-X, which is a common commercial catalyst.

“The techniques we use are uniquely sensitive to the amorphous and transient interfaces in these catalyst materials,” Page explained.

The team, alternating between different isotopes of nitrogen, detected parts of the sample on which to observe gas flow and adsorption via powder diffraction. An ongoing flow of nitrogen was established in order to help the sample arrive at a constant reaction state, required for taking SSITKA measurements.

A valve in the flow cell permits switching between varied gases such that their impacts on the reaction can be detected while a residual gas analyzer measures gas released by the sample. These data, combined with results from the SSITKA and diffraction methods, helped the team trace areas of interest in their sample while getting rid of unnecessary information.

“We were able to see this signal that you would be hard pressed to find any other way, and it was not easy,” Olds said.

In order to make future research easier, Olds produced combinatory appraisal of transition states (CATS), a software program enabling researchers to upload hundreds or thousands of data sets at once. This is then followed by the algorithm providing graphical representations of reactions that take place and also helping to detect any potential problems at the beamline.

Initially, the Researchers built a complex gas flow cell, however, their ultimate design of a simple U-tube shape helps avoid the engineering issues that can in fact plague more complicated equipment.

Nothing here came out of a box. It was all custom and had to be integrated together.

Daniel Olds, a Postdoctoral Researcher, ORNL’s Spallation Neutron Source (SNS)

The Researchers explain their work in a study titled “A high precision gas flow cell for performing in situ neutron studies of local atomic structure in catalytic materials.”

“The gas flow cell LDRD project really generated a whole new class of sample environment capabilities,” Page said.

The team also included Peter F. Peterson, Jue Liu, Gerald Rucker, Mariano Ruiz-Rodriguez, Michelle Pawel, and Steven H. Overbury from ORNL and Arnold Paecklar, Michael Olsen, and James R. Neilson from Colorado State University.

“As always, it was terrific working with the fantastic ORNL Researchers to bring a new idea to fruition through design, build, testing, and use. The LDRD program was a terrific opportunity for us as external users and collaborators,” Neilson said.

DOE’s Office of Science and ORNL’s LDRD Program supported the research.