Solar Thermochemical Hydrolysis of Metal Nitrides for H2 Production and Integrated Storage In Form of Ammonia

Solar thermochemical hydrolysis of metal nitrides for H₂ production and integrated storage in form of ammonia

Ronald Michalsky and Peter H. Pfromm

 Department of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA

 

Conversion of abundant but intermittent solar energy at high temperatures to high value chemical energy stored in the products of a solar thermochemical reaction cycle, such as H₂ or syngas has been studied widely. Ammonia (NH₃) has been proposed more recently as hydrogen carrier for sustainable transportation fuel. Easily liquefied and transported, NH₃ exceeds the H₂ storage requirements of the Department of Energy. Modified diesel engines can combust NH₃ releasing mainly H₂O and N₂. H₂ can also be recovered catalytically on board a vehicle from NH₃ for subsequent combustion.

Solar thermochemical NH₃ synthesis from steam and nitrogen produces a sustainable solar fuel and avoids the storage problem for H₂. Production of metallic nitrides by reduction of their metal oxides is a high-temperature and energy-intensive process that may take advantage of concentrated solar radiation as inexpensive and sustainable source for process heat. This work presents a solar thermochemical NH₃ synthesis process sequence of nitride hydrolysis splitting H₂O and absorbing protons released in the formation of NH₃ at ambient pressure, and endothermic metal oxide reduction and nitridation driven by concentrated solar energy.

A thermodynamic rationale is presented. Various characteristic metals are selected and experimentally explored studying yield and kinetics of breaking the N₂ triple bond during nitridation and liberating NH₃ when hydrolyzing the nitride. Experimental data will focus transition metal interactions on N₂ uptake from the gas phase and nitrogen distribution in the solid bulk material studied using various solid-state analytical techniques. Protonation of the nitrogen liberated during steam hydrolysis of various ionic, covalent, intermediate and interstitial nitrides is examined establishing a mass balance on the nitrogen atom. The yield of NH₃ will be correlated with the nitride ionicity and with the activation energy of nitride hydrolysis to thus discuss the trade-off between desirable high yields of metal nitridation and nitride hydrolysis on one side and undesirable strong bonds between the metallic component and oxygen formed during hydrolysis.