  The measurement of hydrogen induced changes on the electronic structure of
transition metal oxides by X-ray photoelectron spectroscopy is a challenging
endeavor, since the origin of the photoelectron cannot be unambiguously
assigned to hydrogen. The H-induced electronic structure changes in tungsten
trioxide have been known for more than 100 years, but are still being
controversially debated. The controversy stems from the difficulty in
disentangling effects due to hydrogenation from the effects of oxygen
deficiencies. Using a membrane approach to X-ray photoelectron spectroscopy, in
combination with tuneable synchrotron radiation we measure simultaneously core
levels and valence band up to a hydrogen pressure of 1000 mbar. Upon
hydrogenation, the intensities of the W$^{5+}$ core level and a state close to
the Fermi level increase following the pressure-composition isotherm curve of
bulk H$_x$WO$_3$. Combining experimental data and density-functional theory the
description of the hydrogen induced coloration by a proton polaron model is
corroborated. Although hydrogen is the origin of the electronic structure
changes near the Fermi edge, the valence band edge is now dominated by tungsten
orbitals instead of oxygen as is the case for the pristine oxide having wider
implication for its use as (photo-electrochemical) catalyst.
