2 research outputs found
Predicting Promoter-Induced Bond Activation on Solid Catalysts Using Elementary Bond Orders
In this Letter, we examine bond activation
induced by nonmetal
surface promoters in the context of dehydrogenation reactions. We
use C–H bond activation in methane dehydrogenation on transition
metals as an example to understand the origin of the promoting or
poisoning effect of nonmetals. The electronic structure of the surface
and the bond order of the promoter are found to establish all trends
in bond activation. On the basis of these results, we develop a predictive
model that successfully describes the energetics of C–H, O–H,
and N–H bond activation across a range of reactions. For a
given reaction step, a single data point determines whether a nonmetal
will promote bond activation or poison the surface and by how much.
We show how our model leads to general insights that can be directly
used to predict bond activation energetics on transition metal sulfides
and oxides, which can be perceived as promoted surfaces. These results
can then be directly used in studies on full catalytic pathways
Nature of Lone-Pair–Surface Bonds and Their Scaling Relations
We investigate the (surface) bonding
of a class of industrially and biologically important molecules in
which the chemically active orbital is a 2<i>p</i> electron
lone pair located on an N or O atom bound via single bonds to H or
alkyl groups. This class includes water, ammonia, alcohols, ethers,
and amines. Using extensive density functional theory (DFT) calculations,
we discover scaling relations (correlations) among molecular binding
energies of different members of this class: the bonding energetics
of a single member can be used as a descriptor for other members.
We investigate the bonding mechanism for a representative (H<sub>2</sub>O) and find the most important physical surface properties that dictate
the strength and nature of the bonding through a combination of covalent
and noncovalent electrostatic effects. We describe the importance
of surface intrinsic electrostatic, geometric, and mechanical properties
in determining the extent of the lone-pair–surface interactions.
We study systems including ionic materials in which the surface positive
and negative centers create strong local surface electric fields,
which polarize the dangling lone pair and lead to a strong “electrostatically
driven bond”. We emphasize the importance of noncovalent electrostatic
effects and discuss why a fully covalent picture, common in the current
first-principles literature on surface bonding of these molecules,
is not adequate to correctly describe the bonding mechanism and energy
trends. By pointing out a completely different mechanism (charge transfer)
as the major factor for binding N- and O-containing unsaturated (radical)
adsorbates, we explain why their binding energies can be tuned independently
from those of the aforementioned species, having potential implications
in scaling-driven catalyst discovery