Seeing inside a working catalyst

Industrial catalysis typically uses mm-sized catalyst bodies in fixed bed reactors. The efficiency of the final catalytic system depends on the nature and distribution of metal/metal oxide ‘active species’ within the catalyst body. However, this can be affected during the multi-step preparation process and the evolution of the active species from an initial precursor complex, then delivered via liquid phase impregnation, is often a highly convoluted process. In recent times, several analytical techniques have been used to obtain real time 2D and 3D spatial information on the nature of chemical species present on the catalyst during preparation in order to identify the key physicochemical processes that govern both their spatial distribution and final properties. Such information would allow better control over these properties and lead to improvements in catalyst performance [1]. Recently we employed the novel technique of synchrotron X-ray diffraction computed tomography (XRD-CT) [2] and demonstrated its capability to image a catalyst body as a function of time to follow the preparation process and to study the catalyst as a reaction (CO methanation) takes place (Figure 1).

Figure 1. X-ray diffraction computed tomography requires that the sample/subject is exposed to X-ray radiation with a series of transmission ‘projection’ measurements being made at several angles. A 2D image slice is then reconstructed from these measurements via a suitable algorithm based on the differences in the diffraction signals for the crystalline phases present. Several 2D slices are then ‘stitched’ together to create a 3D image.

Catalyst preparation normally involves two or three steps: impregnation of the catalyst body with the active metal in the form of a metal complex, calcination at high temperatures and, in some cases, ‘activation’ in a pre-conditioning gas. XRD-CT was employed to examine this second step (calcination in N₂) of a Ni supported γ-Al₂O₃ catalyst body starting from a NiCl₂ source.

The most important observation was the formation of the cubic (fcc) metallic Ni phases via a number of complex intermediate phases.  2D XRD-CT scans revealed the formation of two different crystalline phases located at the periphery and in the centre of the body accordingly.  Although both phases yielded fcc Ni, the precursor at the periphery did so in one step, while the phase in the centre did so via a 3-4 step process (Figure 2). This appeared to affect the physical properties, in particular the particle size of the fcc Ni phase with the fcc Ni particles in the centre of the pellet being larger than those seen at the periphery. This of course has important implications for the activity/selectivity of these catalyst bodies in a catalytic reaction.

Figure 2. Reconstructed 2D images of crystalline components observed in the diffraction patterns as a function of time/temperature during thermal activation of a γ-Al2O3-supported NiCl2 catalyst precursor. The colour maps indicate the distribution of various solid-state phases. In the lower panel, thermal colour maps show the variation in crystallite size (nm).  When plotted with a common color axis (top) these indicate the growth of the fcc Ni crystallite size as the formation of the phase proceeds.  For slices, 18 and 22, the data have been re-plotted with bespoke colours axes (shown beneath).

In a second recent study, we combined XRD-CT with absorption-CT to study the phase evolution process in NiNO₃ impregnated Ni/γ-Al₂O₃ catalyst bodies during calcination and activation, but more importantly, under real catalytic conditions. As with NiCl₂ impregnated catalyst bodies, two different, inhomogenously distributed precursors result from the impregnation process although the breakdown of these precursors resulted in the formation of more uniformly-sized fcc Ni nanoparticles. Particles located towards the periphery undergo uncontrolled sintering on exposure to O₂ although this appears to be dependent on both the concentration/distribution of the Ni and its form (it is expected that Ni is present in the sample in both crystalline and diffraction-silent forms). Reduction in H₂ did not induce further sintering but results in an active fcc Ni containing catalyst for CO methanation. In operando catalytic measurements performed during CO methanation, demonstrated the crystalline metallic fcc Ni component to be remarkably stable during the duration of reaction (~ 3 hours), undergoing no change in phase constitution, spatial distribution or average particle size, leading us to conclude that metallic fcc Ni is the active component of the catalyst for CO methanation.

Principal publication and authors
Dynamic X-ray diffraction computed tomography reveals real-time insight into catalyst active phase evolution, S.D.M. Jacques (a,b), M. di Michiel (c), A.M. Beale (a), T. Sochi (b), M.G. O’Brien (a), L. Espinosa-Alonso (a), B.M. Weckhuysen (a) and P. Barnes (b), Angew. Chem. Int. Ed. 50, 10148 (2011); Active phase evolution in single Ni/Al₂O₃ methanation catalyst bodies studied in real time using combined μ-XRD-CT and μ-absorption-CT, M.G. O’Brien (a), S.D.M. Jacques (a,b), M. di Michiel (c), P. Barnes (b), B.M. Weckhuysen (a) and A.M. Beale (a) Chem. Sci. (2011); DOI: 10.1039/C1SC00637A.
(a) Inorganic Chemistry and Catalysis, Debye Institute for NanoMaterials Science, University of Utrecht (The Netherlands)
(b) UCL Chemistry London/ Dept. Crystallography, Birkbeck College, London (UK)
(c) ESRF

References
[1] L. Espinosa-Alonso, A.M. Beale and B.M. Weckhuysen Acc. Chem. Res. 43, 1279 (2010).
[2] P. Bleuet, E. Welcomme, E. Dooryhee, J. Susini, J.L. Hodeau, P. Walter, Nat. Mater. 7, 468 (2008).

Top image: X-ray diffraction computed tomography of a metal/metal oxide catalyst during its preparation.