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Abstract:
Intermetallic compounds are a promising class of materials as stable and
selective heterogeneous catalysts. Here, the (111) and (-1-1-1) single
crystal surfaces of the PdGa intermetallic compound were studied as
model catalysts with regard to the selective hydrogenation of acetylene
(C2H2) to ethylene (C2H4). The distinct atomic surface structures
exhibit isolated active centers of single atomic and three atomic Pd
ensembles, respectively. For the two prototypal model catalyst surfaces,
the adsorption sites and configurations for hydrogen (H-2), acetylene,
and ethylene were investigated by combining scanning tunneling
microscopy, temperature-programmed desorption, and ab initio modeling.
The topmost Pd surface atoms provide the preferred adsorption sites for
all studied molecules. The structural difference of the Pd ensembles has
a significant influence on the adsorption energy and configuration of
C2H2, while the influence of the ensemble structure is weak for C2H4 and
H-2 adsorption. To approach the question of catalytic performance, we
simulated the reaction pathways for the heterogeneous catalytic
hydrogenation of acetylene on the two surfaces by means of density
functional theory. Due to the geometrical separation of the Pd sites on
the surfaces, the steric approach of the reactants (H and C2Hx) was
found to be of importance to the energetics of the reaction. The
presented study gives a direct comparison of binding properties of
catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore
of major relevance to the knowledge-based design of highly selective
hydrogenation catalysts.
