We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

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Widespread adoption of solar-thermochemical fuel production depends on its economic viability, largely driven by the efficiency of use of the available solar resource. Herein, we analyze the efficiency of two-step cycles for thermochemical hydrogen production, with emphasis on efficiency. Owing to water thermodynamics, isothermal H2 production is shown to be impractical and inefficient, irrespective of reactor design or reactive oxide properties, but an optimal temperature difference between cycle steps, for which efficiency is the highest, can be determined for a wide range of other operating parameters. A combination of well-targeted pressure and temperature swing, rather than either individually, emerges as the most efficient mode of operation of a two-step thermochemical cycle for solar fuel production.

The adsorption and decomposition of ammonia and hydrogen have been studied on surfaces of clean planar Ir(210) and clean nanoscale-faceted Ir(210), which are prepared from the same crystal in situ. We find evidence for structure sensitivity in recombination and desorption of H2 and in thermal decomposition of NH3 on clean planar Ir(210) versus clean faceted Ir(210). Moreover, the decomposition kinetics of NH3 on faceted Ir(210) exhibit size effects on the nanometer scale, which is the first observation of size effects in surface chemistry on an unsupported monometallic catalyst with controlled and well-defined structure and size.