A catalyst is a substance that is used to drive a chemical reaction in a particular direction with greater efficiency and control. Catalysts are essential in many industrial processes, offering direct economic and environmental benefits (such as fewer by-products and lower energy costs), as well as indirect societal benefits (for example, reduced pollutants).
Given the widespread use of industrial catalysts, there is a drive to optimise catalyst properties to enhance their performance. To do so, it is helpful to understand how a catalyst functions at the microscopic level while under realistic reaction conditions. Yet, there are relatively few methods to observe what is happening inside a reactor in real time.
A collaboration, including researchers from ISIS, the University of Glasgow, and industrial partners Johnson Matthey and Evonik Technology & Infrastructure GmbH, used neutron imaging to observe the activity of an important hydrogenation catalyst, palladium on carbon (Pd/C), in situ.
Unlike X-ray imaging, neutron imaging can be used to study light atoms (such as hydrogen, carbon, nitrogen and oxygen) that typically make up reactants and products, in the presence of heavier elements. Exploiting this advantage, the team used neutron imaging to follow the adsorption/absorption of hydrogen and then its replacement by deuterium (H2 and D2) on a Pd/C catalyst contained within an aluminium sample can.
Neutron radiographs were collected over time as H2 and then D2 was flowed into the packed catalyst bed. Since neutrons are scattered more strongly by hydrogen than deuterium, as hydrogen was displaced by deuterium there was an increase in the signal as fewer neutrons are removed from the beam.
Video depicting the absorption process using difference images. We see an initial disruption of the catalyst bed, then, progressive darkening of the central region as the Pd is converted to PdH.
Neutron imaging shows that H2 uptake begins in the lower-left corner of the catalyst bed and advances in the form of a gas uptake front until the entire sample is saturated. Regions of the catalyst bed that are more accessible, for example, due to cracking, show faster uptake of H2. The expected product is palladium hydride (β-PdHx) and this was confirmed by post-reaction inelastic neutron scattering (INS) studies.
As well as highlighting the importance of catalyst packing within a reactor, the data shows that it is possible to follow the progress of a reaction in real time in situ. Although not shown in the video, when H2 was exchanged for D2 the signal clearly increased, demonstrating that this technique can directly capture H2/D2 exchange on a catalyst.
“The availability of both neutron imaging and INS at the same institution is a major advantage of carrying out these studies at ISIS” Stewart Parker, ISIS Catalysis Scientist.
Fundamentally, the experiments demonstrate the immense potential of neutron imaging for future catalyst studies. Based on the initial findings, there is scope to vary catalyst composition, reactor material, reaction conditions and ultimately observe working catalysts under real reaction conditions.