Preferential
oxidation of CO (COPrOx) is a catalytic reaction targeting
the removal of trace amounts of CO from hydrogen-rich gas mixtures.
Non-noble metal catalysts, such as Cu and Co, can be equally active
to Pt for the reaction; however, their commercialization is limited
by their poor stability. We have recently shown that CoO is the most
active state of cobalt for COPrOx, but under certain reaction conditions,
it is readily oxidized to Co3O4 and deactivates.
Here, we report a simple method to stabilize the Co2+ state
by vanadium addition. The V-promoted cobalt catalyst exhibits considerably
higher activity and stability than pure cobalt. The nature of the
catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co2+ state
on the surface was verified by in situ NEXAFS at
1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate
surface layer, containing tetrahedral V5+ and octahedral
Co2+ cations, with an electronic and geometric structure
that is deviating from the standard mixed bulk oxides. In addition,
V addition helps to maintain the population of Co2+ species
involved in the reaction, inhibiting carbonate species formation that
are responsible for the deactivation. The promoting effect of V is
discussed in terms of enhancement of CoO redox stability on the surface
induced by electronic and structural modifications. These results
demonstrate that V-promoted cobalt is a promising COPrOx catalyst
and validate the application of in situ spectroscopy
to provide the concept for designing better performing catalysts.