2 research outputs found
Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides
An integrated experimental–theoretical
approach for the
solid-state NMR investigation of a series of hydrogen-storage materials
is illustrated. Seven experimental room-temperature structures of
groups I and II metal hydrides and borohydrides, namely, NaH, LiH,
NaBH<sub>4</sub>, MgH<sub>2</sub>, CaH<sub>2</sub>, CaÂ(BH<sub>4</sub>)<sub>2</sub>, and LiBH<sub>4</sub>, were computationally optimized.
Periodic lattice calculations were performed by means of the plane-wave
method adopting the density functional theory (DFT) generalized gradient
approximation (GGA) with the Perdew–Burke–Ernzerhof
(PBE) functional as implemented in the Quantum ESPRESSO package. Projector
augmented wave (PAW), including the gauge-including projected augmented-wave
(GIPAW), methods for solid-state NMR calculations were used adopting
both Rappe–Rabe–Kaxiras–Joannopoulos (RRKJ) ultrasoft
pseudopotentials and new developed pseudopotentials. Computed GIPAW
chemical shifts were critically compared with the experimental ones.
A good agreement between experimental and computed multinuclear chemical
shifts was obtained
Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO<sub>2</sub> Reduction
The encapsulation of organometallic complexes into reticular
covalent
organic frameworks (COFs) represents an effective strategy for the
immobilization of molecular electrocatalysts. In particular, well-defined
polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher
selectivity and activity toward electrochemical CO2 reduction
compared to the parent molecular derivative noncovalently immobilized
on carbon electrodes. In situ mechanistic studies revealed that the
electronic and steric features of the reticular framework strongly
affect the redox mechanism of the Mn sites, stabilizing the formation
of a mononuclear Mn(I) radical anion intermediate over the most common
off-cycle Mn0–Mn0 dimer. Herein, we report
the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore
how tuning the structural and electronic properties of the lattice
may affect the catalytic CO2 reduction performance and
the mechanism at the molecular level of the reticular system. The
Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic
catalytic CO2 reduction activity at near-neutral pH compared
to that of the corresponding noncovalently immobilized molecular derivative.
On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion
and spatial dynamics of the phenanthroline linker, were found to limit
its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry
revealed that the higher spatial mobility of the Mn sites does not
completely suppress the Mn0–Mn0 dimerization
upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive
role of the reticular structure of the material in enhancing its catalytic
activity versus that of its molecular counterpart and provides useful
hints for the future design and development of efficient reticular
frameworks for electrocatalytic applications