Active phase evolution in single Ni/Al2O3 methanation catalyst bodies studied in real time using combined μ-XRD-CT and μ-absorption-CT

Abstract

A combination of synchrotron μ-XRD-CT and μ-absorption-CT (CT = computed tomography) is demonstrated, providing a unique insight into the solid state changes occurring from within crystalline materials. Specifically, we examine here the solid state changes that occur in a millimetre-sized Ni/γ-Al₂O₃ catalyst body in both 2D and 3D during calcination and CO methanation for the first time. The combination provides a unique insight into the spatial phase distribution of these materials and how these evolve via a series of solid state transformation processes. For example, initially, two Ni-ethylenediamine (en) complexes were observed on the impregnated and dried body; a hydrated and non-hydrated form, which 2D scans reveal possess an egg-shell and egg-yolk distribution, respectively. Furthermore, the μ-XRD data were of sufficient quality so as to be able to reveal that the particles within the ‘egg-shell’ were larger (∼35 nm) than those of the ‘egg-yolk’ (∼19 nm) and that there were more of them. On calcination, both precursors collapsed, yielding metallic fcc Ni particles with a surprisingly uniform average size distribution over the catalyst (∼4 nm). However, a comparison of the scattering at different stages of the experiment suggested that the crystalline structure of some of the Ni remained diffraction ‘silent’. Calcination in oxygen lead to both Ni oxidation and particle sintering, mainly at the exterior, which on pre-reaction reduction (in H₂) yielded again fcc Ni particles (∼4 nm interior, ∼6 nm exterior) with a significant reduction in the amorphous Ni component. The catalyst proved active for CO methanation and, during 2 h time on-stream, no change in the structure composition or shape was observed, leading us to conclude that nano-sized fcc Ni particles on γ-Al₂O₃ are the active component in CO methanation. This work therefore demonstrates both the power of spatially resolved μ-XRD-CT/μ-absorption-CT measurement of catalytic systems and its advantage over more ‘traditional’ single point studies on small sieve fractions.