Operando imaging of a methane-reforming catalyst bed

05-02-2019

A complex Ni-Pd/CeO2-ZrO2/Al2O3 solid catalyst was investigated for the first time with 3D-XRD-CT under various operating conditions. This first 5D operando tomographic diffraction imaging experiment allowed the evolving solid-state chemistry of a catalyst bed to be tracked and rationalised.

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Heterogeneous functional materials and devices, such as catalytic solids, batteries and fuel cells tend to possess complex structures where the 3D spatial distribution of the various components is rarely uniform. Such materials are known to change with time under operating conditions. Therefore, it is highly desirable to study them in situ with spatially-resolved techniques to gain an insight into the structure-function relationships [1].

Synchrotron X-ray diffraction computed tomography (XRD-CT) can be used to study complex materials system in 3D and under real process conditions. This is demonstrated by the investigation described here of a complex multi-component Ni-Pd/CeO2-ZrO2/Al2O3 solid catalyst under different operating conditions. This catalyst belongs to a family of catalysts used for methane reforming reactions to produce CO and H2, a mixture also known as ‘synthesis gas’ and used in gas-to-liquid (GTL) industrial plants to produce synthetic fuels.

As shown in Figure 1, XRD-CT is able to discriminate between different crystalline phases present in this multi-component sample and the Rietveld analysis of the obtained diffraction data allows the distribution of these phases to be mapped. These results are derived from a high resolution XRD-CT dataset of a single catalyst particle collected  at ID15A (1 μm pixel size; 10 ms per point acquisition time; X-ray energy of 50 keV).

 Phase distribution maps of Al2O3, CeO2, ZrO2, NiO and PdO

Figure 1. Phase distribution maps of Al2O3, CeO2, ZrO2, NiO and PdO. The maps were derived from the Rietveld analysis of the single catalyst particle XRD-CT data. The scale bar corresponds to 0.1 mm.

These heterogeneities could also be captured at the reactor level with 3D-XRD-CT, as shown in Figure 2. It should be emphasised that such spatially-resolved chemical information cannot be obtained with conventional material characterisation techniques such as X-ray absorption-contrast CT (also known as micro-CT) or bulk XRD. These data were collected at ID31, where we investigated the behaviour of this catalyst bed under different operating conditions.

3D maps obtained from the Rietveld analysis of the 3D-XRD-CT data collected at room temperature

Figure 2. 3D maps obtained from the Rietveld analysis of the 3D-XRD-CT data collected at room temperature. Volume rendering of the normalised scale factors data volume (phase distribution volumes). The values in the colour bar axes have been chosen to achieve the best possible contrast.

By applying the XRD-CT technique, a non-destructive technique that allows intact reactors to be studied, it was possible 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. This chemical evolution of the Ni-containing species is presented in Figure 3.

5D chemical evolution of the Ni-containing species in the catalyst bed

Figure 3. 5D chemical evolution of the Ni-containing species in the catalyst bed. Phase distribution volumes of NiO, NiAl2O4 and Ni as obtained from the Rietveld analysis of the 3D-XRD-CT data collected at the four different operating conditions. The values in the colour bar axes have been chosen to achieve the best possible contrast.

These results show that real-time 3D chemical imaging can provide a better understanding of how the catalyst bed behaves under real process conditions and indeed of the complex structure-function relationships. Chemical tomography techniques are set to be further improved with the continuous advancements in synchrotron brightness, detector performance, sample environment (new reactor cells) and data analysis. Such techniques have potential to provide insight into some of the technical challenges concerning functional materials and device performance in fields such as catalysis, energy storage and transport.

 

Principal publication and authors
5D tomographic operando diffraction imaging of a catalyst bed, A. Vamvakeros (a,b,c,d), S.D.M. Jacques (c), M. Di Michiel (d), D. Matras (b,e), V. Middelkoop (f), I.Z. Ismagilov (g), E.V. Matus (g), V. V. Kuznetsov (g), J. Drnec (d), P. Senecal (a,b), A.M. Beale (a,b,c), Nature Communications 9, 4751 (2018); doi: 10.1038/s41467-018-07046-8.
(a) Department of Chemistry, University College London (UK)
(b) Research Complex at Harwell, Rutherford Appleton Laboratory, Didcot (UK)
(c) Finden, Harwell (UK)
(d) ESRF
(e) School of Materials, University of Manchester (UK)
(f) Flemish Institute for Technological Research, VITO NV, Mol (Belgium)
(g) Boreskov Institute of Catalysis SB RAS, Novosibirsk (Russia)

 

References
[1] A.M. Beale et al., Coordination Chemistry Reviews 277-278, 208-223 (2014).