Templating with colloidal crystals composed of monodisperse spheres is a convenient chemical method to obtain porous materials with well-ordered periodicity and interconnected pore systems. The three-dimensionally ordered macroporous (3DOM) products or inverse opals are of interest for numerous applications, both for the optical properties related to structural color of these photonic crystal materials and because of their bicontinuous nanostructure, i.e., a continuous nanostructured skeleton with large interfacial area and a three-dimensionally interconnected pore system with low tortuosity. This review outlines various synthetic methods used to control the morphology of 3DOM materials with different compositions. It highlights aspects of the choice of colloidal particles, assembly of the colloidal crystal template, infiltration and processing, template removal, and other necessary modifications to enhance the functionality of the materials. It also considers syntheses within the confinement of 3DOM materials and summarizes characterization methods that are particularly useful in the analysis of 3DOM materials. The review then discusses chemical applications of 3DOM materials, namely sorption and controlled release, optical and electrochemical sensors, solar cells, lithium ion batteries, supercapacitors, fuel cells, and environmental and chemical fuel catalysis. A focus is on structural features and materials properties that enable these applications.

Finding pathways to renewable generation of fuels is a crucial step toward mitigating the ecological impacts of fossil fuel combustion. A renewable fuel requires a sustainable energy input and abundant feedstocks. One promising route is through the use of concentrated solar energy to drive the thermochemical splitting of H 2 O and CO 2 . The splitting of H 2 O generates hydrogen fuel and oxygen. Additionally, splitting both H 2 O and CO 2 generates syngas (CO and H 2 ) that can be converted into hydrocarbon fuels through the FischerÀTropsch process. Direct splitting of H 2 O and CO 2 in a single step is extremely endergonic (ΔG H 2 > 0 at T < 4700 K; ΔG CO > 0 at T < 3200 K), and it is difficult to separate the product gases at the required temperature. 1,2 Therefore, direct "one-step" splitting remains impractical.Metal oxides can be used to circumvent the challenges of one step thermochemical fuel production by breaking the process into two steps. 3 First, the solid metal oxide is thermally reduced at high temperatures (>1200 °C), releasing O 2 . The reactive material is then reoxidized by H 2 O or CO 2 at lower temperatures, producing H 2 or CO. This cyclic process allows for lower requisite temperatures for splitting, permits recycling of the metal oxide, and provides intrinsic separation of product gases (O 2 in one step and H 2 or CO in the other). 4,5 The most commonly investigated two-step metal oxide cycles are the zinc (ZnOÀZn) and ferrite (FeOÀFe 3 O 4 ) cycles. However, problems are encountered in both systems. Notably, facile recombination of zinc vapor with O 2 during thermal reduction occurs in the zinc cycle, 6,7 and an inert zirconium oxide phase is needed to stabilize the active materials in the ferrite cycle. 8,9 Cerium oxide is an attractive alternative for solar thermochemical fuel production (eqs 1À3) from metal oxides. Both water splitting and CO 2 splitting with CeO 2 have been investigated, first in catalytic systems 10,11 and then as a solar thermochemical process. 5,12À17 CeO 2 has found use in automotive three-way catalysis and other catalytic systems, due to its ability to reversibly store and release lattice oxygen. 18À20 This mechanism occurs due to the partial reduction of the Ce 4+ cations in CeO 2 to Ce 3+ , and it results in the formation of nonstoichiometric, cubic phases via the formation of oxygen vacancies without significant reorganization of the lattice. 20,21 When compared to other metal oxide materials for fuel production, CeO 2 has a higher melting point (2400 °C), improved thermal stability, and lack of crystal reordering phase transitions in the operating temperature range. 16 CeO 2 h CeO 2Àδ þ 0:5δO 2

The oxidation of three-dimensionally ordered macroporous (3DOM) CeO2 (ceria) by H2O and CO2 at 1100 K is presented in comparison to the oxidation of nonordered mesoporous and sintered, low porosity ceria. 3DOM ceria, which features interconnected and ordered pores, increases the maximum H2 and CO production rates over the low porosity ceria by 125% and 260%, respectively, and increases the maximum H2 and CO production rates over the nonordered mesoporous cerium oxide by 75% and 175%, respectively. The increase in the kinetics of H2O and CO2 splitting with 3DOM ceria is attributed to its enhanced specific surface area and to its interconnected pore system that facilitates the transport of reacting species to and from oxidation sites.

Two-step thermochemical cycling was achieved using CeO2 with sub-micrometer sized macropores, allowing for substantially improved CO production at fast cycle rates when compared to nonporous CeO2. The effects of porosity, pore order, and packing density were probed by synthesizing ceria materials with different morphologies. Polymeric colloidal spheres were used as templates for the synthesis of three-dimensionally ordered macroporous (3DOM) CeO2 and nonordered macroporous (NOM) CeO2. Aggregated CeO2 nanoparticles with feature sizes similar to those in 3DOM CeO2 were prepared by fragmenting 3DOM CeO2 into its building blocks using ultrasonication. The three templated materials and nonporous, commercial CeO2 were tested in thermochemical cycles using an infrared furnace. CeO2 was reduced at ∼1200 °C, and the reduced CeO2−δ materials were reoxidized under CO2 at ∼850 °C. The high temperatures required for cycling induced changes in the morphology of the porous materials, which were characterized by electron microscopy, X-ray diffraction, and nitrogen sorption measurements. In spite of sintering, the macroporous materials retained an interconnected pore network during 55 cycles, providing a 10-fold enhancement in CO productivity and production rate when compared to nonporous CeO2. Additionally, 3DOM CeO2 provided the fastest rate of CO production of all tested materials and also retained the smallest solid feature sizes. This boost in reaction kinetics allowed for extremely rapid cycling with less than a minute required for complete reduction or oxidation. Characterization of the porous materials also provided some insight into thermal gradients that developed in the sample bed as a result of rapid heating and cooling.

Nanoporous and nanostructured materials are becoming increasingly important for advanced applications involving, for example, bioactive materials, catalytic materials, energy storage and conversion materials, photonic crystals, membranes, and more. As such, they are exposed to a variety of harsh environments and often experience detrimental morphological changes as a result. This article highlights material limitations and recent advances in porous materials--three-dimensionally ordered macroporous (3DOM) materials in particular--under reactive or high-temperature conditions. Examples include systems where morphological changes are desired and systems that require an increased retention of structure, surface area, and overall material integrity during synthesis and processing. Structural modifications, changes in composition, and alternate synthesis routes are explored and discussed. Improvements in thermal or structural stability have been achieved by the isolation of nanoparticles in porous structures through spatial separation, by confinement in a more thermally stable host, by the application of a protective surface or an adhesive interlayer, by alloy or solid solution formation, and by doping to induce solute drag.