Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the ...OER activity in a La1–x Sr x CoO3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoO x H y ) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity–stability factor for CoO x H y /LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.
Binding configurations of CO2 and CO2 − on perfect and oxygen-deficient anatase (101) surfaces were explored using first-principles calculations on both cluster and periodic models. The solvent ...effect was taken into account via the polarizable continuum model. Analysis of molecular orbitals, charge, and spin density distributions was used to help identify the radical anion CO2 − adsorbed on the surface. On defect-free surfaces, it is found to bind as a bridging bidentate configuration with both oxygens coordinating to the 5-fold Ti ions. Analysis of vibrational frequencies provides a specific signature of the CO2 anion to distinguish it from other species in experiments. The reduction potential of adsorbed CO2 on a (101) surface is lower by 0.24 V than the reduction potential of a CO2 molecule, both in aqueous solution, due to the formation of hybridized orbitals, which facilitates charge transfer to CO2. The reduced (101) surface of TiO2 is much more favorable for CO2 binding with accompanying charge transfer to CO2.
Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania ...nanoparticles. The formation of H atoms (reduction product) and •OH radicals (oxidation product) from water, and CO3 − radical anions (oxidation product) from carbonates, was detected in CO2-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, •OCH3, and methyl, •CH3, radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO2 on the surface of TiO2 is supported by the results of first-principles calculations.
Photoirradiated metal oxide semiconductors are known to reduce carbon dioxide to methane. This multistep reaction is commonly represented as a sequence of proton-coupled two-electron reactions ...leading from carbon dioxide to formate to formaldehyde to methanol and to methane. We suggest that the actual reaction mechanism is more complex, as it involves two-carbon molecules and radicals in addition to these one-carbon species. The ″stepping stone″ of this mechanism for carbon dioxide fixation could be glyoxal, which is the product of recombination of two formyl radicals, or glycolaldehyde, which is its reduced form. We demonstrate the main steps of this reduction chain and suggest a catalytic cycle integrating these steps and the radical chemistry. In addition to methane, this cycle generates complex organic molecules, such as glycolaldehyde, acetaldehyde, and methylformate, which were observed in product analyses. This cycle can be regarded as one of the simplest realizations of multistep, photosynthetic fixation of atmospheric carbon in prebiotic nature.
Traditional transparent conducting oxides (TCOs) have been widely used for various optoelectronic applications, but have the trade-off between conductivity and transmittance. Recently, perovskite ...oxides, with structural and chemical stability, have exhibited excellent physical properties as new TCOs. We focus on SrVO
3
-based perovskites with a high carrier concentration and BaSnO
3
-based perovskites with a high mobility for n-type TCOs. In addition, p-type perovskites are discussed, which can serve as potential future options to couple with n-type perovskites to design full perovskite based devices.
Temperature-dependent frequencies and linewidths of transverse-optical (TO) and longitudinal-optical (LO) in γ–CuI were fitted according to the anharmonic phonon coupling theory. The results indicate ...that the asymmetric three-phonon and symmetric four-phonon processes are both responsible for the decay of LO and TO phonons in γ–CuI. Additionally, the possible decay channels are also suggested as: TO (135 cm-1) →LA (∼100 cm-1) +TA (35 cm-1) at L point and three TA (45cm-1) at X point; LO (152 cm-1) → LA (∼102 cm-1) +TA (∼50 cm-1) or three TA (50cm-1) at K point in Brillouin zone. These results are meaningful for the future applications of γ–CuI in optoelectronic and electronic fields.
Five particle sizes and six dosages of polymerized aluminum chloride (PAC) residue were taken to prepare polymerized aluminum chloride residue cement mortar (PACRM). The compressive and flexural ...strengths were tested, and SEM analysis was performed. The results showed that the change pattern of strength of PAC waste residue mixed into cement mortar was similar to that of fly ash at the same substitution rate. However, the strength of PACRM decreased with the increase of substitution rate. The flexural and compressive strengths of PACRM reached their peak at all ages after replacing cement with PAC waste residue with particle size of 0-0.075 mm and 30% doping. Their strength ratios were greater than 62%. It does not contribute much to the early strength development of PACRM, but contributes more to the later strength development after the cement is replaced by PAC waste residue. PAC waste residue mainly acts as microaggregate and fills into the mortar pores at low substitution rate. The cracks of the matrix structure are filled by C-S-H, so the mortar structure is more dense and the material strength is enhanced. When the substitution rate increases, the water requirement of mortar increases, resulting in an uncompact structure. The internal pores increase after hardening, and the mortar strength decreases rapidly.
Photoactive metal oxides, such as hydrated TiO2, are known to reduce carbon dioxide to methane, but the mechanism for this photoreaction is insufficiently understood. In particular, it is not known ...whether the reduction of crucial reaction intermediates, including the formate anion, involves one- or two-electron reactions. In this study, we demonstrate that formic acid and its derivatives can be reduced to the formyl radical via a concerted reaction in which the electron transfer is coupled to oxygen transfer to a Ti3+ center on the oxide surface. Several other examples of such heteroatom-transfer reactions are demonstrated, suggesting a general pattern. The implications of these reactions for photocatalytic methanogenesis, perchlorate diagenesis, and planetary chemistry on Mars are discussed.
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