Activation, Steady-State and Passivation Regimes for Ethene Hydrogenation over a Pd/Al <b> ₂ </b> O <b> ₃ </b> Catalyst: An <i>Operando</i> Neutron Imaging Study

Neutron imaging combined with in-line mass spectrometry is used to perform an operando study of hydrogen adsorption and ethene hydrogenation over a powdered 5 wt % Pd/Al2O3 catalyst contained within a stainless-steel reactor. The approach adopted enables the partitioning of hydrogen throughout the catalyst bed to be examined as reaction conditions are varied. Aspects of the catalyst activation procedure are examined including drying and reduction stages. Spatially resolved temporal profiles indicate how hydrogen is partitioning throughout the length of the reactor during these catalyst pretreatments. For the case of hydrogen exposure to the as-received catalyst at 293 K, while the mass spectrometer detects hydrogen breakthrough after 20 min, the neutron intensity profiles for 5 spatially distinct regions along the catalyst bed map out the progression of hydrogen along the length of the bed throughout and beyond the 20-min period, allowing the spatially resolved varying rates of adsorption to be assessed, and showing that hydrogen adsorption is fastest at the top of the catalyst bed. These neutron imaging profiles are discussed in terms of coincident reduction and drying events. Addressing matters of reaction engineering, the ethene hydrogenation experiments performed at 333 K examine how hydrogen supply affects the reactor/catalyst combination. Operational conditions are varied from hydrogen-excess to hydrogen-lean regimes. Hydrogen starvation experiments are used to assess the durability of the activated catalyst. The complete recovery of catalytic activity following reinstatement of hydrogen supply following a period of ethene-only feed demonstrates the reversibility of the hydrogenation process over this catalyst. The operando neutron imaging approach outlined here provides a new perspective on how hydrogenous entities pass through an extended catalyst bed.