Stockenhuber, Sebastian; (2025) Determining failure mechanisms of CuO/ZnO/Al₂O₃-Cs₂O Low Temperature Water Gas Shift catalysts during activation using X-ray spectroscopy and diffraction techniques. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A major cost contributor to the process of ammonia production is the water-gas shift reaction. Specifically, the low-temperature water gas shift process (LT-WGS) in industry requires a copper-based catalyst of 60:30:10 mol. % Cu:Zn:Al. Inadvertently, similar conditions and catalyst formulation are also used in methanol synthesis. Considered a side-reaction in the LT-WGS process, methods to supress this resulted in promoting the catalyst with 1 wt. % Cs₂O. This has recently been suggested to dramatically decrease the mechanical strength during the activation phase via an increase in chemical sintering. In-situ X-ray absorption fine structure (XAFS) and ex situ X-ray diffraction (XRD) analysis of the reduction process under ideal (4 % H₂/Inert) and industrially relevant wet gas conditions were used on 0, 1 and 5 wt. % Cs₂O Cu:Zn:Al catalysts. These showed that as the Cs₂O concentration increased, reduction onset temperature also increased, with time taken to achieve the same active phase also increasing. This, accompanied by an increase in crystallite size for Cu⁰, suggested that the catalyst was sintering. This was also true for the wet gas; elevated temperatures being required for CuO reduction into Cu⁰. Pellets (5 x 4 mm) in a large (12 x 160 mm) reactor were studied under pseudo operando XRD-Computed Tomography (CT) measurements, highlighting how water (20 vol. %) and Cs₂O concentration increased Cu0 crystallites formed. Water was suggested to condense into catalyst pores, causing speciation to CuO and Cu₂O, with higher pressure in industrial conditions exacerbating these effects. Using reaction pressures of 1 and 20 bar, XRD-CT suggested that water would prevent CuO reduction via capillary condensation on sample periphery while Cu₂O would be formed by water adsorbing and interacting with the framework of the copper. The Cs2O facilitated capillary condensation, shown by Cu₂O:CuO ratios increasing with Cs₂O where 20 bar allowed for easier intrusion.

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