The macroporous supported Cu–V oxide prepared by a novel dissolution–reprecipitation process was found to be the first example of a promising substitute of Pt catalysts for sulfuric acid decomposition at moderate temperatures (∼600 °C), which is required in solar thermochemical hydrogen production. Stepwise impregnation of Cu­(NO3)2 and NH4VO3 onto 3-D ordered mesoporous SiO2, and subsequent heating at 650 °C yielded the deposition of copper pyrovanadate (Cu2V2O7, melting point: 780 °C) not only in mesopores but also on the external surface. Thermal aging at 800 °C caused the congruent melting of Cu2V2O7 followed by smooth penetration of the melt into mesopores and homogeneous covering of cavity walls. Because of the solubility of SiO2 into the molten vanadate, dissolution–reprecipitation should be equilibrated to allow substantial structural conversion from mesoporous to macroporous SiO2 frameworks. The resulting macroporous catalyst consisting of highly dispersed thin layers of active Cu2V2O7 is considered efficient for catalytic reactions and the mass transfer of reactants and products in the presence of high-concentration vapors.

The surface grafting of CeO2 onto Fe2O3 with a 1:5 molar ratio produced a thermally stable composite material with greater and faster oxygen storage/release than its separate constituents. In the composite, CeO2 and Fe2O3 were intimately contacted by interfacial Ce–O–Fe bonding, and no solid solutions or mixed Ce and Fe oxides were formed after heating at 900 °C. The oxygen storage capacity and initial rate of oxygen release/storage were both increased in the composite structure by virtue of the Fe2O3 and CeO2, respectively. The reduction–oxidation cycles in which Fe2O3 is reduced via Fe3O4 to Fe metal by CO or H2 and then reoxidized by O2 were stabilized by surface-grafting Fe2O3 with CeO2. In situ Raman spectra demonstrated that the surface-grafted CeO2 acts as an oxygen gateway, activating the dissociation of O2 into oxide ions or the recombination of oxide ions into O2 and transferring oxide ions to/from Fe2O3. Meanwhile, Fe2O3 acts as an oxygen reservoir that expands the O2 storage capacity. The composite material was tested in a simulated exhaust gas stream with lean/rich perturbations (which occur in automotive three-way catalysts). The synergistic effect of the surface grafting effectively buffered the system against air-to-fuel ratio fluctuations.

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SiO 2-supported nCuO-V 2 O 5 catalysts with different ratios (n=0, 1, 2, 3 and 5) were prepared to study their catalytic activity for SO 3 decomposition, which is a key reaction necessary for solar thermochemical H 2 production. Although four binary compounds, CuV 2 O 6 , Cu 2 V 2 O 7 , Cu 3 V 2 O 8 and Cu 5 V 2 O 10 , were formed on three-dimensional (3-D) mesoporous SiO 2 depending on the ratio (n), the thermal ageing caused their incongruent melting and precipitation of Cu 2 V 2 O 7. The highly corrosive molten vanadate phase resulted in mesopore-to-macropore conversion of SiO 2 , which was accompanied by significant decrease of BET surface area and pore volume. Nevertheless, the structural conversion yielded copper vanadate with a high surface coverage of SiO 2 cavity walls enabling efficient catalytic SO 3 decomposition at moderated reaction temperatures (~600 °C). Among nCuO-V 2 O 5 /SiO 2 catalysts, the highest catalytic activity was achieved for n=1, which corresponds to the phase with the lowest melting point (630 °C) of the present system.

SiO2-supported molten alkaline metal oxides (A–V–O/SiO2) were studied as SO3 decomposition catalysts for solar thermochemical water splitting. Their catalytic activities at moderate temperatures (≤600 °C), which were superior to those of Cu–V–O/SiO2 catalysts, were dependent on A, exhibiting the following sequence: Cs > Rb > K > Na. These activities increased with the A/V ratio. This result is in accordance with the basicity, which favors the adsorption of SO3 to form sulfate. Another important effect of A is to form molten liquid phases, which dissolve the sulfate and facilitate its decomposition to SO2/O2. However, the molten phase with high A/V ratios led to the collapse of the porous SiO2 structure by a corrosion effect. Consequently, the highest catalytic activity was achieved at a composition of A/V ≈ 1.0 for A = K and Cs. The long-term stability test of K–V–O/SiO2 at 550 °C demonstrated no indication of noticeable deactivation during the first 100 h, whereas 20% deactivation occurred during the following 400 h. The deactivation mechanism involves the vaporization loss of active components from the molten phase, which is accelerated in the presence of SO3.