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Critical Role of Oxygen in Silver-Catalyzed Glaser–Hay Coupling on Ag(100) under Vacuum and in Solution on Ag Particles
journal contribution
posted on 2017-03-22, 00:00 authored by Noé Orozco, Georgios Kyriakou, Simon K. Beaumont, Javier Fernandez Sanz, Juan P. Holgado, Martin J. Taylor, Juan P. Espinós, Antonio M. Márquez, David J. Watson, Agustin R. Gonzalez-Elipe, Richard M. LambertThe
essential role of oxygen in enabling heterogeneously catalyzed
Glaser–Hay coupling of phenylacetylene on Ag(100) was elucidated
by STM, laboratory and synchrotron photoemission, and DFT calculations.
In the absence of coadsorbed oxygen, phenylacetylene formed well-ordered
dense overlayers which, with increasing temperature, desorbed without
reaction. In striking contrast, even at 120 K, the presence of oxygen
led to immediate and complete disruption of the organic layer due
to abstraction of acetylenic hydrogen with formation of a disordered
mixed layer containing immobile adsorbed phenylacetylide. At higher
temperatures phenylacetylide underwent Glaser–Hay coupling
to form highly ordered domains of diphenyldiacetylene that eventually
desorbed without decomposition, leaving the bare metal surface. DFT
calculations showed that, while acetylenic H abstraction was otherwise
an endothermic process, oxygen adatoms triggered a reaction-initiating
exothermic pathway leading to OH(a) + phenylacetylide, consistent
with the experimental observations. Moreover, it was found that, with
a solution of phenylacetylene in nonane and in the presence of O2, Ag particles catalyzed Glaser–Hay coupling with high
selectivity. Rigorous exclusion of oxygen from the reactor strongly
suppressed the catalytic reaction. Interestingly, too much oxygen
lowers the selectivity toward diphenyldiacetylene. Thus, vacuum studies
and theoretical calculations revealed the key role of oxygen in the
reaction mechanism, subsequently borne out by catalytic studies with
Ag particles that confirmed the presence of oxygen as a necessary
and sufficient condition for the coupling reaction to occur. The direct
relevance of model studies to a mechanistic understanding of coupling
reactions under conditions of practical catalysis was reaffirmed.