6 research outputs found
Distinguishing Active Site Identity in Sn-Beta Zeolites Using <sup>31</sup>P MAS NMR of Adsorbed Trimethylphosphine Oxide
The
identity of the active sites of hydrophobic Lewis acid zeolites
was elucidated by solid-state <sup>31</sup>P nuclear magnetic resonance
(NMR) spectroscopy following adsorption of trimethylphosphine oxide
(TMPO) probe molecules. The adsorption of TMPO on these materials
resulted in distinct <sup>31</sup>P NMR resonances between ÎŽ<sub>iso</sub> = 59.9 and 54.9 ppm that correspond to unique chemical
environments of the Lewis acidic heteroatoms in the Beta zeolite framework.
The <sup>31</sup>P NMR resonances were assigned to sites in Sn-Beta
zeolites by correlating the variation of <sup>31</sup>P NMR spectra
during TMPO titration experiments with the corresponding changes in
the <sup>119</sup>Sn NMR spectra. This method allowed us to establish
quantitative relationships between the assignments for each site and
the catalytic activity for the glucose isomerization and aldol condensation
reactions. The rate of glucose isomerization directly correlated with
the combined integrated intensities of <sup>31</sup>P MAS NMR resonances
at ÎŽ<sub>iso</sub> = 55.8 and 54.9 ppm, which amounted to 12â33%
of total Sn sites. In contrast, the integrated peak area of a different
site at ÎŽ<sub>iso</sub> = 58.6 ppm was shown to correlate with
aldol condensation activity. The probing method used to identify and
quantify distinct active sites within the framework of low-defect
Lewis acid zeolites developed in this work is applicable to a wide
range of microporous materials, regardless of heteroatom identity
Mechanochemical Synthesis of Methylammonium Lead MixedâHalide Perovskites: Unraveling the Solid-Solution Behavior Using Solid-State NMR
Mixed-halide
lead perovskite (MHP) materials are rapidly advancing
as next-generation high-efficiency perovskite solar cells due to enhanced
stability and bandgap tunability. In this work, we demonstrate the
ability to readily and stoichiometrically tune the halide composition
in methylammonium-based MHPs using a mechanochemical synthesis approach.
Using this solvent-free protocol we are able to prepare domain-free
MHP solid solutions with randomly distributed halide ions about the
Pb center. Up to seven distinct [PbCl<sub><i>x</i></sub>Br<sub>6âx</sub>]<sup>4â</sup> environments are identified,
based on the <sup>207</sup>Pb NMR chemical shifts, which are also
sensitive to the changes in the unit cell dimensions resulting from
the substitution of Br by Cl, obeying Vegardâs law. We demonstrate
a straightforward and rapid synthetic approach to forming highly tunable
stoichiometric MHP solid solutions while avoiding the traditional
solution synthesis method by redirecting the thermodynamically driven
compositions. Moreover, we illustrate the importance of complementary
characterization methods, obtaining atomic-scale structural information
from multinuclear, multifield, and multidimensional solid-state magnetic
resonance spectroscopy, as well as from quantum chemical calculations
and long-range structural details using powder X-ray diffraction.
The solvent-free mechanochemical synthesis approach is also compared
to traditional solvent synthesis, revealing identical solid-solution
behavior; however, the mechanochemical approach offers superior control
over the stoichiometry of the final mixed-halide composition, which
is essential for device engineering
Composition-Tunable Formamidinium Lead Mixed Halide Perovskites via Solvent-Free Mechanochemical Synthesis: Decoding the Pb Environments Using Solid-State NMR Spectroscopy
Mixed-halide lead
perovskites are becoming of paramount interest
in the optoelectronic and photovoltaic research fields, offering band
gap tunability, improved efficiency, and enhanced stability compared
to their single halide counterparts. Formamidinium-based mixed halide
perovskites (FA-MHPs) are critical to obtaining optimum solar cell
performance. Here, we report a solvent-free mechanochemical synthesis
(MCS) method to prepare FA-MHPs, starting with their parent compounds
(FAPbX<sub>3</sub>; X = Cl, Br, I), achieving compositions not previously
accessible through the solvent synthesis (SS) technique. By probing
local Pb environments in MCS FA-MHPs using solid-state nuclear magnetic
resonance spectroscopy, along with powder X-ray diffraction for long-range
crystallinity and reflectance measurements to determine the optical
band gap, we show that MCS FA-MHPs form atomic-level solid solutions
between Cl/Br and Br/I MHPs. Our results pave the way for advanced
methods in atomic-level structural understanding while offering a
one-pot synthetic approach to prepare MHPs with superior control of
stoichiometry