Experimental and
Theoretical Evidence for Distorted
Tetrahedral Ti–OH Sites Supported on Amorphous Silica
and Their Effect on the Adsorption of Polar Molecules
posted on 2023-12-27, 15:04authored byBranden
E. Leonhardt, Hengyuan Shen, Martin Head-Gordon, Alexis T. Bell
This study presents experimental and theoretical evidence
for distorted
Ti–OH groups supported on amorphous SiO2 and examines the influence of distortion on the strength of adsorption
of polar molecules. For the theoretical part of this effort, we developed
a model for isolated Si–OH or Ti–OH
groups on the surface of amorphous silica. The M–OH
group is represented by a T8–11 cluster surrounded by a T747
cluster representing the surrounding amorphous silica. The properties
of the small cluster are described by high-level density functional
theory (DFT) (i.e., the quantum mechanical (QM) region), whereas the
large surroundings are represented by molecular mechanics (MM). The
QM/MM model was validated by demonstrating that the predicted enthalpy
of adsorption for seven polar molecules on Si–OH groups
agrees satisfactorily with experimentally measured values determined
by microcalorimetry. We also found that enthalpies of adsorption on
isolated Si–OH groups determined from isotherms obtained
by infrared (IR) spectroscopy agree very well with the microcalorimetric
values. IR spectroscopy was then used to measure isotherms for pyridine
adsorption to Lewis acidic Ti isolated Ti–OH groups
grafted to amorphous silica. The isosteric enthalpy of adsorption
decreased in magnitude with increasing pyridine coverages up to a
coverage of 15% and then remained relatively constant for higher coverages.
QM/MM calculations made with our model revealed that the enthalpy
of adsorption is proportional to the area of the triangular O–Ti–O
facets of the Ti–OH group to which pyridine is bound.
Ti–OH sites exhibiting facet areas consistent with
those deduced from X-ray absorption spectroscopy (XAS/EXAFS) measurements
bind pyridine with enthalpies of adsorption consistent with the observed
plateau for pyridine coverage above 15%. Larger tetrahedral facet
areas are required to explain the coverage-dependent enthalpies of
adsorption below 15% coverage, evidencing distorted Ti–OH
structures. Distorted Ti–OH sites are also qualitatively
consistent with reduced-intensity pre-edge X-ray absorption near-edge
structure (XANES) measurements reported in this study. Energy decomposition
analysis (EDA) calculations were carried out to understand the underlying
physical phenomenon governing the change in the enthalpies of adsorption
pyridine as a function of tetrahedral facet area.