Abstract
Ammonia synthesis via the catalytic Haber–Bosch process is characterized by its high pressures and low single-pass conversions, as well as by the energy-intensive production of the precursors H2 and N2 and their concomitant greenhouse gas emissions. Alternatively, thermochemical cycles based on metal nitrides stand as a promising pathway to green ammonia production because they can be conducted at moderate pressures without added catalysts and be further driven by concentrated solar energy as the source of high-temperature process heat. The ideal two-step cycle consists of the nitridation of a metal to form a metal nitride, followed by the hydrogenation of the metal nitride to synthetize NH3 and reform the metal. Here, we perform a combined theoretical and experimental screening of mono-metallic nitrides for several candidates, namely for Sr, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W, Li, and Al. For the theoretical screening, Ellingham diagrams and chemical equilibrium compositions are examined with thermodynamic data derived from density function theory computations. For the experimental screening, thermogravimetric runs and mass balances supported by on-line gas analyses are performed for both steps of the cycle at ambient pressure and over the temperature ranges 100–1000 °C for nitridation and 100–500 °C for hydrogenation. The strontium-based cycle is selected as a reference for detailed examination and shown to synthetize NH3 at 1 bar by effecting the nitridation at 407 °C (at peak rate) and the hydrogenation at 339 °C (at peak rate). The co-formation of metal hydrides (SrH2) and metal imides (Sr2HN) are shown to help close the material cycle.
Daniel Notter a, Tiago Elias Abi-Ramia Silva a, María Elena Gálvez b, Brendan Bulfin c and Aldo Steinfeld *a
aDepartment of Mechanical Engineering, ETH Zürich, 8092 Zürich, Switzerland. E-mail: aldo.steinfeld@ethz.ch
bInstituto de Nanociencia y Materiales de Aragon (INMA), CSIC-Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
cSchool of Chemistry & Environmental Research Institute, University College Cork, T12 YN60 Cork, Ireland