We address the fundamental question of which size a metallic nano-particle needs to have before its surface chemical properties can be considered to be those of a solid, rather than those of a large molecule. Calculations of adsorption energies for carbon monoxide and oxygen on a series of gold nanoparticles ranging from 13 to 1,415 atoms, or 0.8-3.7 nm, have been made possible by exploiting massively parallel computing on up to 32,768 cores on the Blue Gene/P computer at Argonne National Laboratory. We show that bulk surface properties are obtained for clusters larger than ca. 560 atoms (2.7 nm). Below that critical size, finite-size effects can be observed, and we show those to be related to variations in the local atomic structure augmented by quantum size effects for the smallest clusters.
Photocatalysis is
a potentially promising approach to harvest aromatic
compounds from lignin. However, the development of an active and selective
solid photocatalyst is still challenging for lignin transformation
under ambient conditions. We herein report a mild photocatalytic oxidative
strategy for C–C bond cleavage of lignin β-O-4 and β-1
linkages using a mesoporous graphitic carbon nitride catalyst. Identifications
by solid-state NMR techniques and density functional theory (DFT)
calculations indicate that π–π stacking interactions
are most likely present between the flexible carbon nitride surface
and lignin model molecule. Besides, low charge recombination efficiency
and high specific surface area (206.5 m2 g–1) of the catalyst also contribute to its high catalytic activity.
Mechanistic investigations reveal that photogenerated holes, as the
main active species, trigger the oxidation and C–C bond cleavage
of lignin models. This study sheds light on the interaction between
complex lignin structures and the catalyst surface and provides a
new strategy of photocatalytic cleavage of lignin models with heterogeneous
photocatalysts.
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