(177e) Colloids in Combustion: A Scalable Method to Synthesize Highly Crystalline Inorganic Nanomaterials with Tailored Porosity

Colloids in Combustion: A
Scalable Method to Synthesize Highly Crystalline Inorganic Nanomaterials with
Tailored Porosity

Since
its discovery more than 30 years ago, solution combustion synthesis has been extensively
utilized to produce various nanoscale materials (e.g., oxides, metals, alloys,
etc.) in an energy and time efficient manner. However, materials synthesized by
this method generally possess irregular structures with large crystal sizes,
relatively low specific surface areas and small pore volumes. Achieving
structural controllability of synthesized materials has been a daunting task due
to the complex nature of chemistry and physics of combustion processes, which
take place at very short timescale.

      A novel method, colloidal solution combustion synthesis (CSCS) has
been invented to produce nanomaterials with tailored
porosity via combustion of a colloidal solution. CSCS is a facile
bottom-up synthesis method to mass produce crystalline nanostructures with
controlled composition and tunable pore structure, using colloids in combustion
synthesis. Importantly, mobile colloidal particles play
a significant role beyond that of a template affecting rheology as well as heat
and mass transport of the system. Furthermore, combustion takes place in a
confined nanospace between colloids, leading to a highly uniform porous
structure after removal of colloids. In contrast to the other
energy-intensive wet-chemical methods in which a continuous heat supply and
long reaction times are necessary for crystal nucleation and growth, a mild
exothermic reaction between homogeneously mixed precursors in CSCS leads to a
product formation within seconds without additional energy consumption. The
combustion is mild because the silica colloids are chemically inert and act as
heat sinks, absorbing generated heat from the combustion reaction. Also, hard
silica colloids serve as physical barriers, preventing formed crystals from
further growth.

    
 Depending on the size of colloids used, uniform porous oxides with adjustable
pore sizes, large surface areas, high crystallinity, high thermal stability and
large pore volumes can be obtained on a large scale. We synthesized crystalline
mesoporous CeO₂ with ultralarge 22 nm mesopores (Fig. 1A), high
specific surface areas (130 m²/g) and large pore volumes (0.6 ml/g)
by CSCS. Mesoporous NiO with extremely high surface
area 223 m²/g, 0.39 ml/g pore volume,
and uniform 22 nm pores (Fig. 1B) is also produced.  Both
oxides demon­strated excellent catalytic activities for carbon monoxide and
soot oxidation reactions. Successful use of this method has also been
demonstrated in the synthesis of highly active Ir-modified Ni-CeO₂
and Co-CeO₂ catalysts for N₂H₄∙H₂O
decomposition and reverse water-gas shift reaction, respectively. CSCS has well
proven to be a powerful synthetic method for mass producing high-quality porous
inorganic nanomaterials for various state-of-the-art applications.

Figure 1. TEM images of (A) CeO₂ and (B)
NiO catalysts synthesized via CSCS method.

 [1]
A. A. Voskanyan, K.-Y. Chan, C.-Y. V. Li, Colloidal Solution Combustion
Synthesis: Toward mass production of a crystalline uniform mesoporous CeO₂
catalyst with tunable porosity. Chem. Mater. 2016, 28,
2768-2775.

[2]
A. A. Voskanyan, K.-Y. Chan, Scalable Synthesis of Three-Dimensional
Meso-Macroporous NiO with Uniform Ultralarge Randomly Packed Mesopores and High
Catalytic Activity for Soot Oxidation, ACS Appl. Nano Materials, 2018,
1, 556-563.

[3]
Y.-P. Qiu, H. Yin, H. Dai, L.-Y. Gan, H.-B. Dai, P. Wang, Tuning the Surface
Composition of Ni/meso-CeO₂ with Iridium as an Efficient Catalyst
for Hydrogen Generation from Hydrous Hydrazine, Chem. Eur. J., 2018,
24, 4902-4908.

[4]
L. Wang, H. Lui, Mesoporous Co-CeO₂ Catalyst Prepared by Colloidal
Solution Combustion Method for Reverse Water-Gas Shift Reaction, Catal.
Today, 2018, doi.org/10.1016/j.cattod.2018.04.015