Ceria
has been used as a hydrogenation catalyst especially in selective
alkyne hydrogenation, but the reaction mechanism regarding the role
of different surface hydrogen species remains unclear. In this work,
we utilized in situ neutron and infrared vibration
spectroscopy to show the catalytic role of cerium hydride (Ce–H)
and hydroxyl (OH) groups in acetylene hydrogenation over ceria surfaces
with different degree of reduction. In situ inelastic
neutron scattering spectroscopy (INS) proved that not only Ce–H
but also surface atomic hydrogen species on the reduced ceria surface
can participate in acetylene semihydrogenation. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)
results implied that bridging OH groups both on the oxidized and reduced
ceria are active in the selective hydrogenation of acetylene. It appears
that surface Ce–H is more reactive than the coexisting OH species
on the reduced ceria surface, but over-reduction of ceria also results
in strongly bound species that may lead to catalyst deactivation.
These spectroscopic results clearly explain the reaction mechanism
including not only the surface chemistry but also the nature of the
active hydrogen species for selective hydrogenation over ceria, providing
insights into the design of more active and stable ceria-based catalysts
for hydrogenation reactions.