Theoretical study of the interaction of simple molecules
such as H2, C2H2, and C2H4 with Pd–Pb catalysts

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Theoretical study of the interaction of simple molecules such as H2, C2H2, and C2H4 with Pd–Pb catalysts

Introduction

Palladium is widely used as a catalyst of selective hydrogenation reactions [1–۳].
The selective hydrogenation of alkynes to mono-olefins, that is, the so-called
semihydrogenation of alkynes, is a crucial step in many laboratory and industrial
processes [2]. In particular, the semihydrogenation of acetylene to ethylene is
required in the production of polyethylene [4, 5]. In the polymerization of C2H4, the complete elimination of C2H2 from ethylene feedstocks via the semihydrogenation
is to avoid deactivation of polymerization catalysts.
According to the classical interpretation, the high selectivity of Pd toward the
semihydrogenation of alkynes is attributed to the stronger adsorption of alkynes on
the Pd surface compared to that of alkenes formed in the semihydrogenation
reaction [2]. Since the interaction between alkynes and the catalyst surface is
thermodynamically preferred, the formed alkenes are displaced from the surface and
their readsorption is blocked. As a consequence, alkynes are hydrogenated to
alkenes and the parallel undesirable hydrogenation of alkenes to alkanes is
suppressed. It should be noted that recently the interpretation of the high selectivity
of Pd has been revisited by Teschner et al. [6, 7] who have related the selectivity to
the formation of a Pd/C phase on the catalyst surface. Carbon in the Pd/C phase
hinders the participation of bulk-dissolved and subsurface hydrogen in the
hydrogenation of alkynes [6–۹]. This hydrogen is thought to be very reactive and
unselective. Then alkynes are hydrogenated selectively to alkenes only by hydrogen
present on the catalyst surface

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