Session: 08-02: Solar Chemistry: Thermochemical Fuel Production II

Paper Number: 137449

137449 - Hydrogen Production by Chemical Looping Ammonia Oxidation and Water Splitting Using Iron-Based Oxygen Carriers 

Abstract: 

In response to climate change, it has become imperative to discover and implement efficient energy carriers for renewable energy storage and utilization. Clean hydrogen (H₂) derived from renewable sources holds promise for achieving carbon neutrality in sectors emitting Gt-scale greenhouse gases. Despite its potential, the cost of hydrogen transport hampers widespread utilization. Addressing this, our research focuses on redox-active monometallic and multimetallic oxides for the sustainable conversion of ammonia (NH₃) to hydrogen. Ammonia is widely utilized in agriculture and food industries with established infrastructure worldwide.

Ammonia proves to be an excellent hydrogen carrier due to its high volumetric and gravimetric hydrogen density. Our proposed Chemical Looping Ammonia Oxidation (CLAO) process distinguishes itself by providing two hydrogen streams at different purities, to satisfy different needs. CLAO redox reactions can be an efficient method to produce hydrogen without pollutant by-products. Increasing the selectivity of nitrogen-containing products toward nitrogen gas (N₂) rather than nitrogen oxides, such as nitric oxide, nitrogen dioxide, and nitrous oxide, guarantees a prospective NO_x-free and carbon-free hydrogen production method.

The chemical looping process involves redox reactions where a metal oxide serves as the oxygen carrier. CLAO, a two-step cyclic process, initially oxidizes and decomposes ammonia into hydrogen, water, and nitrogen. Subsequently, steam re-oxidizes the metal oxide, producing additional hydrogen. Solar thermal energy can serve as the thermal energy resource to drive the endothermic water splitting step. Furthermore, In cases where an autothermal process is needed, the metal oxide can undergo further oxidation by air after steam splitting, supplying heat for the NH₃ oxidation step.

Our research focuses on identifying, synthesizing, and testing suitable monometallic and multimetallic oxides that exhibit high NH₃ conversion, steam conversion, selectivity toward N₂, and H₂ yields. Synthesized through a modified wet impregnation method, our metal oxides underwent 20 cycles at 600°C. Optimizing the iron-to-support ratio yielded 93% ammonia conversion, 93% hydrogen yield from ammonia, and 25% hydrogen yield from water. Among multimetallic oxides, iron-based oxides (Fe_xM_1-xO_y, M = Mg, Ni, Zn, etc.) exhibited exceptional outcomes, with the most successful oxide achieving 90% ammonia conversion, 82% hydrogen yield from ammonia, and 31% hydrogen yield from water.

The redox reaction mechanism of the redox active oxides above involves solid-state phase transformation among metal alloy, spinel, and rocksalt phases, demonstrated by x-ray diffraction and electron microscopy of reduced and oxidized oxides that were protected by an inert atmosphere. We have also utilized materials thermodynamic models and materials phase diagrams to explain the oxygen exchange capacities of the redox active oxides. These findings mark significant progress in developing sustainable and efficient processes for hydrogen production from ammonia.

Presenting Author: Amir Arjomand Ohio State University

Presenting Author Biography: Amir Arjomand obtained his B.Sc. in Mechanical Engineering from the University of Tehran in 2022; after graduation, he started his Ph.D. in Mechanical Engineering at the Ohio State University. During his undergraduate research, he worked on the design and extensive thermodynamic analysis of power generation systems. As he began his Ph.D., he has been working on projects regarding green hydrogen production through chemical looping ammonia oxidation and water splitting as well as mixed ionic-electronic conducting membranes.

Authors:

Amir Arjomand Ohio State University
Shang Zhai Ohio State University