Solid catalysts are used in almost every field of the chemical industry, ranging from pharmaceuticals to petrochemicals as well as the automotive industry, to produce the desired products. These catalytic solids usually comprise complex 3D structures which can be inhomogeneous. In recent years, it has been realised that it is crucial to investigate these materials using characterisation techniques that provide spatially-resolved information as these heterogeneities can play a crucial role in the catalyst performance.
In our recent paper in Nature Communications we show that synchrotron X-ray diffraction computed tomography (XRD-CT) can be used to study the evolution in solid-state composition in complex materials in 3D under real process conditions and as a function of time; specifically a complex multi-component Ni-Pd/CeO2-ZrO2/Al2O3 solid catalyst under operating conditions. An example is provided in the image below which contains phase distribution maps of the various crystalline phases present in the fresh catalyst. It can be seen that 3D XRD-CT allows us to identify different crystalline phases present in the catalyst particles; such spatially-resolved chemical information cannot be obtained with conventional material characterisation techniques like X-ray absorption-contrast CT (also known as micro-CT) or bulk XRD.
This Ni-Pd/CeO2-ZrO2/Al2O3 solid catalyst is used for methane reforming in order to yield CO and H2. This gas mixture is also known as ‘synthesis gas’ and it is used in gas-to-liquid (GTL) industrial plants to produce synthetic fuels. By applying the XRD-CT technique, which is non-destructive and allows us to study intact samples, we were able to follow the evolving solid-state chemistry in this complex system and relate these changes to the various applied chemical environments. For example, we showed that the Ni-containing species, the main active catalyst components, can take the form of NiO, NiAl2O4 or metallic Ni depending on the operating temperature and gas environment.
We also studied this catalyst during the partial oxidation of methane reaction and we observed the formation of crystalline graphite. The spatially-resolved signals obtained from XRD-CT allowed to correlate the distributions of the various catalyst components and also gain a thorough insight into the several roles of the CeO2-ZrO2 promoters. For example, we observed that one of their role is to supress the formation of the undesired NiAl2O4 phase and enhance the conversion of the Ni oxide species to the catalytically active metallic Ni.
There are always going to be chemical (i.e. in terms of reactant/product composition) and temperature gradients both radially and longitudinally in fixed bed reactors. However, we show that real-time 3D chemical imaging can provide a better understanding of how the bed behaves under process conditions and the complex structure-function relationships. One can readily foresee that with the continuous advancements in synchrotron brightness, detector performance, sample environment (new reactor cells) and data analysis, chemical tomography techniques are bound to become increasingly easier to perform and that they will eventually replace conventional in situ XRD and X-ray imaging as the preferred method for characterising functional materials and devices (e.g. catalytic reactors, batteries and fuel cells).