3 research outputs found
Hierarchically Structured Porous Spinels via an Epoxide-Mediated Sol–Gel Process Accompanied by Polymerization-Induced Phase Separation
Enhancing the activity
and stability of catalysts is a major challenge
in scientific research nowadays. Previous studies showed that the
generation of an additional pore system can influence the catalytic
performance of porous catalysts regarding activity, selectivity, and
stability. This study focuses on the epoxide-mediated sol–gel
synthesis of mixed metal oxides, NiAl<sub>2</sub>O<sub>4</sub> and
CoAl<sub>2</sub>O<sub>4</sub>, with a spinel phase structure, a hierarchical
pore structure, and Ni and Co contents of 3 to 33 mol % with respect
to the total metal content. The sol–gel process is accompanied
by a polymerization-induced phase separation to introduce an additional
pore system. The obtained mixed metal oxides were characterized with
regard to pore morphology, surface area, and formation of the spinel
phase. The Brunauer–Emmett–Teller surface area ranges
from 74 to 138 m<sup>2</sup>·g<sup>–1</sup> and 25 to
94 m<sup>2</sup>·g<sup>–1</sup> for Ni and Co, respectively.
Diameters of the phase separation-based macropores were between 500
and 2000 nm, and the mesopore diameters were 10 nm for the Ni-based
system and between 20 and 25 nm for the cobalt spinels. Furthermore,
Ni–Al spinels with 4, 5, and 6 mol % Ni were investigated in
the dry reforming of CH<sub>4</sub> (DRM) with CO<sub>2</sub> to produce
H<sub>2</sub> and CO. CH<sub>4</sub> conversions near the thermodynamic
equilibrium were observed depending on the Ni content and reaction
temperature. The Ni catalysts were further compared to a noble metal-containing
catalyst based on a spinel system showing comparable CH<sub>4</sub> conversion and carbon selectivity in the DRM
Morphology of Porous Hosts Directs Preferred Polymorph Formation and Influences Kinetics of Solid/Solid Transitions of Confined Pharmaceuticals
The pore morphology of a porous host
may determine which polymorph
a crystallizable guest preferentially forms and may influence the
kinetics of solid/solid transitions. Slow cooling of the drug acetaminophen
(ACE) inside the straight cylindrical pores of anodic aluminum oxide
(AAO, tortuosity = 1) in contact with a bulk ACE surface film preferentially
yields uniformly oriented form II and/or form III crystals. The occurring
orientations of form II and form III crystals are characterized by
high structural registry along the AAO pores. The uniformly oriented
form III crystals inside the AAO pores were readily converted into
likewise uniformly oriented form II crystals by a solid/solid transition.
Thus, we obtained uniformly oriented form II crystals in AAO at high
yields. We suggest that sporadic heterogeneous nucleation at bulk
crystals formed in the bulk surface film on top of the AAO coupled
with kinetic selection of crystal orientations results in fast growth
of properly oriented crystals along the 100 μm deep AAO pores.
This mechanism is suppressed in controlled porous glass (CPG) having
isotropic spongelike pores (tortuosity > 1.5) with free growth
paths
on the order of 100 nm, where form I formed instead. Moreover, the
transition from form III to form II is suppressed in CPG. Possible
reasons may include impingement of the propagation front of the solid/solid
transition on the CPG pore walls after short propagation paths and
inevitable formation of form II grains with different orientations
separated by energetically disadvantageous grain boundaries. The results
reported here are relevant to mesoscopic crystal engineering aimed
at controlled drug release from nanoscale delivery systems. Polymorphs
not accessible otherwise in nanoscale containers may be produced at
high yields. The principles reported here may be transferred to areas
such as nanowire-based organic electronics
Reactive Additive Capillary Stamping with Double Network Hydrogel-Derived Aerogel Stamps under Solvothermal Conditions
Integration of solvothermal reaction products into complex
thin-layer
architectures is frequently achieved by combinations of layer transfer
and subtractive lithography, whereas direct additive substrate patterning
with solvothermal reaction products has remained challenging. We report
reactive additive capillary stamping under solvothermal conditions
as a parallel contact-lithographic access to patterns of solvothermal
reaction products in thin-layer configurations. To this end, corresponding
precursor inks are infiltrated into mechanically robust mesoporous
aerogel stamps derived from double-network hydrogels. The stamp is
then brought into contact with a substrate to be patterned under solvothermal
reaction conditions inside an autoclave. The precursor ink forms liquid
bridges between the topographic surface pattern of the stamp and the
substrate. Evaporation-driven enrichment of the precursors in these
liquid bridges, along with their liquid-bridge-guided conversion into
the solvothermal reaction products, yields large-area submicron patterns
of the solvothermal reaction products replicating the stamp topography.
For example, we prepared thin hybrid films, which contained ordered
monolayers of superparamagnetic submicron nickel ferrite dots prepared
by solvothermal capillary stamping surrounded by nickel electrodeposited
in a second orthogonal substrate functionalization step. The submicron
nickel ferrite dots acted as a magnetic hardener, halving the remanence
of the ferromagnetic nickel layer. In this way, thin-layer electromechanical
systems, transformers, and positioning systems may be customized