X-Ray Elemental and Crystalline Phase Distribution Analysis of the Process to Produce Hydrogen and Ammonia from Iron and Carbonated Water

Currently, hydrogen energy is attracting much attention as a clean and sustainable energy alternative to fossil fuels, and the demand for hydrogen fuel used in fuel cell vehicles is expected to increase. We focused on the hydrogen (H₂) production through the reaction of Fe metal obtained from scrap iron with carbonate water (CO₂ and H₂O) [1]. Ammonia (NH₃) is also an important raw material for nitrogen fertilizers used in agriculture, and it has been identified as a potential hydrogen carrier in recent years. However, NH₃ is mainly synthesized by the Haber–Bosch process, which requires high temperature and pressure. To lower the environmental burden, it is necessary to achieve NH₃ synthesis under milder conditions. Therefore, we investigated the production of NH₃ ammonia from iron nitride (Fe₄N) and carbonated water at ambient temperature and pressure [2].<br/><br/>The reaction mechanism was understood to be a redox reaction of hydrogen (H) derived from water molecules carried by carbonic acid (H₂CO₃ and HCO₃⁻) with Fe to ionize Fe and produce H₂. When iron nitride was used as the raw material, NH₃ was formed by reaction with N. CO₂ promoted the reactions, allowing them to proceed at lower temperatures, and was absorbed and immobilized as iron carbonate (FeCO₃). Since the reaction involved solid, liquid, and vapor phases, analysis of each phase was conducted. In particular, we analyzed the distribution of elements and crystal structures, considering that it is important to observe the electrochemical reaction on the solid phase surface, i.e., the corrosion phenomenon of iron material.<br/><br/>We have developed a non-scanning X-ray fluorescence (XRF) imaging setup that enables rapid observation of elemental distribution using a projection-type configuration and a two-dimensional detector [3]. 2D spatial XRF image is acquired in a single shot, and temporal changes in the distribution can be obtained by repeating the imaging step.<br/><br/>For the analysis of specific mini-regions, a confocal X-ray setup was constructed [4]. The first polycapillary focusing optic was used as the X-ray incident channel and the second polycapillary focusing optic was used as the X-ray detection channel. The focal points of both overlap, and the X-ray signal from the confocal can be observed. Using this confocal X-ray optics, the measurement was repeated while scanning the sample on the XYZ stage. A 3D scan of the sample through the confocal provides a spatial distribution of the components that make up the sample. Both the projection and confocal setups are not limited to the XRF analysis, but can be applied to other X-ray analyses as well: by detecting X-ray diffraction (XRD) signals, distribution images of crystalline phases can be obtained. In the presentation, we will report the results of the observation of the reaction processes as changes in XRF and XRD distributions using these setups.<br/><br/>[1] Hiromi Eba et al., <i>Int. J. Hydrogen Energy </i>45(2020)13832-13840.<br/>[2] Hiromi Eba et al., <i>Int. J. Hydrogen Energy</i> 46(2021)10642-10652.<br/>[3] Hiromi Eba et al., <i>J. Anal. At. Spectrom.</i> 31(2016)1105-1111.<br/>[4] Hiromi Eba et al., <i>Chem. Lett.</i> 47(2018)1545-1548.