(583ef) 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.