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Two types of Ti−β zeolites synthesized by a hydrothermal synthesis method under different conditions using OH- and F- ion as anions of the structure-directing agents (SDA) exhibited photocatalytic reactivity for the reduction of CO2 with H2O at 323 K to produce CH4 and CH3OH. In situ photoluminescence, diffuse reflectance absorption, and XAFS (XANES and FT-EXAFS) investigations of these Ti−β zeolites indicate that the titanium oxide species are highly dispersed in their frameworks and exist in a tetrahedral coordination state. From the H2O adsorption isotherm on these Ti−β zeolites at 300 K, it was found that the Ti−β zeolites synthesized using OH- ions (Ti−β(OH)) exhibited hydrophilic properties and the Ti−β zeolites synthesized using F- ions showed hydrophobic properties. With the addition of H2O, Ti−β(OH) exhibited a more efficient quenching of the photoluminescence of the highly dispersed tetrahedrally coordinated titanium oxide species and a more remarkable decrease in the preedge intensity of the XANES spectra of the Ti K-edge by the addition of H2O as compared with that of Ti−β(F) having hydrophobic properties. These results indicated that the H2O molecules added were easily able to gain access to the tetrahedrally coordinated titanium oxide species in the Ti−β(OH) zeolite. The differences in the H2O affinity to the zeolite surface led to a strong influence on the reactivity and selectivity for the photocatalytic reduction of CO2 with H2O. Therefore, the properties of the zeolite cavities were important factors controlling the reactivity and selectivity in the photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH on these Ti−β zeolite catalysts.

The present work has demonstrated the reasons why CeO 2 becomes an active catalyst for diesel particulate (soot) abatement, which attracts recent worldwide attention in the development of clean diesel automobiles. Four typical fluorite-type oxides, CeO 2 , ZrO 2 , Pr 6 O 11 , and a CeO 2 -ZrO 2 solid solution have been studied as model catalysts for soot oxidation in conjunction with the redox property and the reactivity of solid oxygen species. It was found that the redox property measured in terms of oxygen storage/release capacity was not the sole determining factor for the observed catalytic activity decreasing in the order of CeO 2 . Pr 6 O 11 ≈ CeO 2 -ZrO 2 > ZrO 2 . The reactivity of oxygen species involved in the redox cycles would rather be important. The ESR measurement showed that admission of O 2 to the pre-reduced CeO 2 surface generated superoxide ions (O 2 -). Such reactive oxygen species were less abundant on CeO 2 -ZrO 2 and were not detected on ZrO 2 and Pr 6 O 11 . The labeled and unlabeled O 2 pulse experiments demonstrated that reactive oxygen species on pre-reduced CeO 2 caused a temporal oxidation of soot even at quite a low temperature of 150 °C, compared to more than 350 °C required for successive catalytic soot oxidation. The reactive oxygen is formed from gaseous O 2 adsorbed at the three-phase boundary between soot, reduced CeO 2 , and the gas phase, but another active oxygen species, which is formed from the lattice oxygen at the CeO 2 /soot interface, contributes much more to the total soot oxidation. Silver loading onto CeO 2 enhanced further the generation of superoxide and thus the catalytic activity for soot oxidation.

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