jp410478c_si_001.pdf (1.9 MB)
Molecular-Level Understanding of CeO2 as a Catalyst for Partial Alkyne Hydrogenation
journal contribution
posted on 2014-03-13, 00:00 authored by Javier Carrasco, Gianvito Vilé, Delia Fernández-Torre, Rubén Pérez, Javier Pérez-Ramírez, M. Verónica Ganduglia-PirovanoThe unique catalytic properties of
ceria for the partial hydrogenation
of alkynes are examined for acetylene hydrogenation. Catalytic tests
over polycrystalline CeO2 at different temperatures and
H2/C2H2 ratios reveal ethylene selectivities
in the range of 75–85% at high degrees of acetylene conversion
and hint at the crucial role of hydrogen dissociation on the overall
process. Density-functional theory is applied to CeO2(111)
in order to investigate reaction intermediates and to calculate the
enthalpy and energy barrier for each elementary step, taking into
account different adsorption geometries and the presence of potential
isomers of the intermediates. At a high hydrogen coverage, β-C2H2 radicals adsorbed on-top of surface oxygen atoms
are the initial reactive species forming C2H3 species effectively barrierless. The high alkene selectivity is
owed to the lower activation barrier for subsequent hydrogenation
leading to gas-phase C2H4 compared to that for
the formation of β-C2H4 radical species.
Moreover, hydrogenation of C2H5 species, if
formed, must overcome significantly large barriers. Oligomers are
the most important byproduct of the reaction and they result from
the recombination of chemisorbed C2Hx species. These findings rationalize for the first time the
applicability of CeO2 as a catalyst for olefin production
and potentially broaden its use for the hydrogenation of polyunsaturated
and polyfunctionalized substrates containing triple bonds.