2 research outputs found
Core–Shell Metal–Ceramic Microstructures: Mechanism of Hydrothermal Formation and Properties as Catalyst Materials
Unique metal–ceramic composites
with core–shell microarchitecture
(γ-Al<sub>2</sub>O<sub>3</sub>@Al and spinel-MeAl<sub>2</sub>O<sub>4</sub>@Al, Me = Zn, Ni, Co, Mn, and Mg) were obtained by a
simple hydrothermal surface oxidation (HTSO) of Al metal particles
in an aqueous solution of heterometal ions at elevated temperatures
(393–473 K). The reactions afforded self-constructed core–shell
microarchitecture with Al core encapsulated by the high-surface-area
γ-Al<sub>2</sub>O<sub>3</sub> or spinel metal aluminates (MeAl<sub>2</sub>O<sub>4</sub>) shell with various surface morphologies, compositions,
and excellent physicochemical properties. Extensive experimental and
theoretical investigation with period 3–6 metal elements (Na<sup>+</sup>, Ca<sup>2+</sup>, Sr<sup>2+</sup>, Ba<sup>2+</sup>, K<sup>+</sup>, Fe<sup>3+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Ni<sup>2+</sup>, Co<sup>2+</sup>, Mn<sup>2+</sup>, and Mg<sup>2+</sup>)
at various metal concentrations and temperatures revealed that the
heterogeneous self-construction of spinel-MeAl<sub>2</sub>O<sub>4</sub>@Al primarily depends on two intrinsic properties of the additive
metal ions: (i) thermodynamic stability constant of the metal hydroxide
complex and (ii) size of the metal ion. The spinel-MeAl<sub>2</sub>O<sub>4</sub>@Al microstructures formed with a limited number of
hetero metal ions (Me = Zn<sup>2+</sup>, Ni<sup>2+</sup>, Co<sup>2+</sup>, Mn<sup>2+</sup>, and Mg<sup>2+</sup>) with (i) moderate rates of
the hydroxide formation with compatible kinetics to the hydrolysis
of aluminum on the Al surface and (ii) small size of additive metal
ions enough for diffusion through the shell layer. As heterogeneous
catalyst substrates, these metal–ceramic composites delivered
markedly enhanced catalytic performance at intensive reaction conditions
because of their facile heat transfer and superior physicochemical
surface properties. The performance and effects of the core–shell
metal–ceramic composites were demonstrated using Rh catalysts
supported on MgAl<sub>2</sub>O<sub>4</sub>@Al. The Rh/MgAl<sub>2</sub>O<sub>4</sub>@Al catalyst was utilized for the endothermic glycerol
stream reforming (C<sub>3</sub>H<sub>8</sub>O<sub>3</sub> + 3H<sub>2</sub>O ⇄ 3CO<sub>2</sub> + 7H<sub>2</sub>, Δ<i>H</i><sub>0</sub><sup>298</sup> = 128 kJ mol<sup>–1</sup>), exhibiting markedly greater catalytic performance than the conventional
Rh/MgAl<sub>2</sub>O<sub>4</sub> under intensive reaction conditions
attributed to significantly facilitated heat transport through the
core–shell metal–ceramic microstructures
Zeolitic Imidazolate Framework Membrane with Marked Thermochemical Stability for High-Temperature Catalytic Processes
The thermochemical
stability of metal organic framework (MOF) membranes
is vital for the application in chemical-reaction and -separation
processes, but understanding the stability of MOF membranes and structure–property
relationships under antagonistic chemical atmosphere is still required.
In this work, a supported zeolitic imidazolate framework (ZIF) membrane,
ZIF-7/MgO-Al<sub>2</sub>O<sub>3</sub>, of unprecedented hydrothermal
stability is obtained by a modulation of the acid–base chemistry
at the membrane/support interface. The solid/solid interface acidity
that has been overlooked in the fields turns out to have paramount
inducing effects on the thermochemical stability of ZIF membranes,
resulting in the catastrophic acid-catalyzed decomposition of ZIF
frameworks at atomic level. The ZIF-7/MgO-Al<sub>2</sub>O<sub>3</sub> of marked thermochemical stability permits the first significant
application of MOF membranes for catalytic membrane reactor (MR) in
severe and practical process conditions, performing water–gas
shift reaction (CO + H<sub>2</sub>O ↔ CO<sub>2</sub> + H<sub>2</sub>) at considerably high temperatures (473–673 K) and
steam concentrations (20–40%). The findings and results provide
significant new insights on the property and stability of ZIF membranes
with extensive opportunities for thermochemical processes that have
been permitted only for the inorganic membranes such as zeolites,
palladium, and metal oxides