D2o and CO2 Adsorption On Na-Precovered MnO(001) and NaMnO2-Like Surfaces

A catalytic route to hydrogen production via thermochemical water splitting is highly desirable because it directly converts thermal energy into stored chemical energy in the form of hydrogen and oxygen.  Recently, the Davis group at Caltech reported an innovative low-temperature (max 850°C) catalytic cycle for thermochemical water splitting using a non-toxic and non-corrosive heterogeneous system based on sodium and manganese oxides.¹  The key steps are thought to be hydrogen evolution from a Na₂CO₃/MnO mixture, and oxygen evolution by thermal reduction of solids formed by Na⁺ extraction from NaMnO₂.  Our work is aimed at understanding the fundamental chemical processes involved in the catalytic cycle.

In this work, we start with a well-defined MnO(001) single crystal surface in ultra-high vacuum (UHV), then modify the system by the deposition of Na metal.  Surface compounds generated by thermal and oxidative treatments of the clean and Na-covered MnO(001) have been examined by X-ray photoelectron spectroscopy (XPS).  Oxidation and UHV annealing of the Na-precovered MnO(001) surface yields a surface compound with a composition close to NaMnO₂.  Temperature programmed desorption (TPD) of D₂O, CO₂ and co-adsorbed D₂O and CO₂ were investigated on the MnO(001) surface, a NaMnO₂-like surface and a Na-precovered MnO(001) surface.  On MnO(001) and NaMnO₂-like surfaces, water adsorption is predominantly molecular, and no hydrogen evolution (D₂) from D₂O is observed.  On the Na-precovered MnO(001) surface, CO₂ TPD suggests the decomposition of a sodium carbonate compound, and D₂O TPD shows evidence for hydrogen (D₂) evolution from the reaction with sodium metal.  In contrast, after pretreating the Na-precovered MnO(001) surface by CO₂ pre-adsorption followed by controlled heating to form a Na₂CO₃/MnO mixture, Na metal is no longer present on the surface.  Interestingly, on this Na₂CO₃/MnO surface, hydrogen evolution is significantly attenuated.  The results suggest that Na metal is the primary active component for hydrogen production under the conditions of our study.  For the working catalyst at elevated temperatures, other forms of sodium may be active as it shuttles between Na₂CO₃ and NaMnO₂ during the catalytic cycle.

1.  Xu, B.; Bhawe, Y.; Davis, M. E., Low-temperature, manganese oxide-based, thermochemical water splitting cycle. Proceedings of the National Academy of Sciences 2012, 109 (24), 9260-9264.