Effect of regeneration temperatures on conversion efficiency of an ortho-to-parahydrogen catalyst

Abstract

At the European Spallation Source (ESS), two cold hydrogen moderator vessels were designed to enhance neutronic performance, particularly for generating high-brightness pulsed neutron beams used in neutron scattering experiments. This was achieved by optimizing the neutron scattering characteristics of parahydrogen, ensuring a fraction exceeding 99.5 %. Under intense neutron irradiation, para-to-orthohydrogen back conversion occurs due to energy transfer from neutron collisions. To meet the hydrogen moderator requirements, hydrous ferric oxide, commercially known as IONEX® Type OP, was selected as the ortho-to-parahydrogen catalyst. In this study, water was first identified as the primary adsorbate on the catalyst through qualitative analysis. A subsequent quantitative assessment of the adsorbed water content revealed that at each activation temperature (T_R), there was a distinct limit to the amount of water that could be removed. Heating the catalyst to 250 °C enabled the removal of the vast majority of the adsorbed water. To investigate the influence of the residual water on the catalyst conversion performance, the ortho-to-parahydrogen fractions of hydrogen passing through the activated catalyst immersed in saturated liquid nitrogen were measured using Raman spectroscopy. The catalyst’s conversion performance improved as T_R increased to 160 °C, attributed to the removal of water. However, at a higher T_R of 230 °C, catalyst performance deteriorated to a level comparable to that observed at T_R = 130 °C. Once catalyst degradation occurred due to overheating, performance could not be recovered. When the residual water content (W_R) after activation exceeded 1.4 wt%, corresponding to T_R = 160 °C, a linear relationship was identified between W_R and both the efficiency degradation and the rate constant, with both improving as W_R decreased. These findings suggested the catalyst conversion performances could be indirectly inferred from W_R, thereby eliminating the need for cryogenic temperature measurements.