A Template-Free, Combustion-Chemical Route to Macroporous Nickel Monoliths Displaying a Hierarchy of Pore Sizes

Abstract

Recently, we demonstrated a very general route to monolithic macroporous materials prepared without the use of templates (Rajamathi et al. J. Mater. Chem. 2001, 11, 2489). The route involves finding a precursor containing two metals, A and B, whose oxides are largely immiscible. Firing of the precursor followed by suitable sintering results in a monolith from which one of the oxide phases can be chemically leached out to yield a macroporous mass of the other oxide phase. The metals A and B that we employed in the demonstration were Ni and Zn. From the NiO−ZnO monolith that was obtained by decomposing the precursor, ZnO could be leached out at high pH to yield macroporous NiO. In the present work, we show that combustion-chemical (also called self-propagating) decomposition of a mixture of Ni and Zn nitrates with urea as a fuel yields an intimate mixture of the oxides that can be sintered and leached with alkali to form a macroporous NiO monolith. The new process that we present here thereby avoids the need for a crystalline single-source precursor. A novel and unanticipated aspect of the present work is that the combination of high temperatures and rapid quenching associated with combustion synthesis results in an intimate mixture of wurtzite ZnO and the metastable rock-salt Ni1 - x Zn x O where x is about 0.3. Leaching this monolith with alkali gives a macroporous mass of rock-salt Ni1 - x Zn x O, which upon reduction in H2/Ar forms macroporous Ni and ZnO. There are thus two stages in the process that lead to two modes of pore formation. The first is associated with leaching of ZnO by alkali. The second is associated with the reduction of porous Ni1 - x Zn x O to give porous Ni and ZnO.

Hierarchically porous materials: synthesis strategies and structure design

“…In the selective leaching technique, a significant volume loss usually occurs upon reduction of the metal, which expresses itself as smaller pores penetrating into the pore walls, although the mesopores are rather transient. 899 Seshadri et al have developed a novel strategy for forming hierarchically porous materials by using this volume loss. This process requires temperature control.…”

Spontaneous Transformations in the Solid State: Towards Porous and Biphasic Materials

“…[70] In a similar manner, combustion synthesis starting from ions mixed in solution can be used to give mixtures of NiO and ZnO. [71] As the starting precursor has the ions mixed on an atomic level, the material so formed has a random mixture of the two phases, thereby avoiding inhomogeneities that would arise due to powder mixing. The decomposed powder is quite fine, as the resulting biphasic materials have grains less than a micron in diameter.…”

Influence of chelating agent and reaction time on the swelling process for preparation of porous TiO2 particles

“…This way, it would be like the socalled "template-free" approach proposed by Seshadri and his co-workers. [14][15][16] Due to the strong reactivity of the alkoxide of titanium (IV) isopropoxide (TTIP) in the water, TTIP often results in an uncontrolled precipitation to form TiO 2 based materials. However, the major problem is to control hydrolysis rate that is generally too high, resulting in the formation of disordered precipitates 17 that make the swelling process difficult.…”