Dynamic Behavior of Rh Species in Rh/Al2O3 Model Catalyst during Three-Way Catalytic Reaction: An Operando X-ray Absorption Spectroscopy Study.

The dynamic behavior of Rh species in 1 wt% Rh/Al2O3 catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NOx meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, operando. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful operando XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts.

Low-Temperature Oxygen Storage of CrIV-CrV Mixed-Valence YCr1-xPxO4-δ Driven by Local Condensation around Oxygen-Deficient Orthochromite.

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-xPxO4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating-cooling cycles in the temperature range from 50 to 600 °C. YCr1-xPxO4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV, while the reduced YCr1-xPxO4-δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-xPxO4-δ phases were stabilized by forming linking polyhedral CrIV2O76- via corner sharing between oxygen-deficient CrIVO32- and adjacent CrIVO44-. YCr1-xPxO4 was found to have an extremely low reduction enthalpy of about 20 kJ mol-1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.

N2O decomposition properties of Ru catalysts supported on various oxide materials and SnO2.

Nitrous oxide (N2O) is a stratospheric ozone depleting greenhouse gas that has global warming potential. As the catalytic decomposition of N2O is one of the most promising techniques for N2O emissions abatement, in this study, for this purpose the properties of Ru supported on various oxide materials were investigated under excess O2 conditions, and the identities of the N2O adsorption species on the catalysts were confirmed. To clarify the correlation between the catalytic properties and N2O decomposition activity, the supported Ru catalysts were characterised by means of powder X-ray diffraction, X-ray fluorescence measurements, energy-dispersive X-ray mapping and several gas sorption techniques. The results showed that the redox properties for Ru (RuO2) at low temperature are closely associated with N2O decomposition activity. The local structures, optimal Ru loading and N2O adsorption species of the novel Ru/SnO2 catalysts were studied and they showed high activity for N2O decomposition.

Photocatalytic hydrogen peroxide splitting on metal-free powders assisted by phosphoric acid as a stabilizer.

Hydrogen peroxide (H2O2) has received increasing attention as an energy carrier. To achieve a sustainable energy society, photocatalytic H2O2 splitting (H2O2 (l) → H2 (g) + O2 (g); ΔG° = + 131 kJ mol-1) is a desirable reaction for on-site H2 generation. However, this reaction has not been reported because conventional photocatalysis decomposes H2O2 by disproportionation (H2O2 (l) → H2O (l) + 1/2O2 (g); ΔG° = -117 kJ mol-1) and by promoting H2O2 reduction instead of H+ reduction. Here we report the successful example of H2O2 splitting. Visible light irradiation of a graphitic carbon nitride loaded with graphene quantum dots as co-catalysts (GQDs/g-C3N4) in a H2O2 solution containing phosphoric acid (H3PO4) produces H2. H3PO4 associates with H2O2 via hydrogen bonding, and this stabilization of H2O2 suppresses its reduction, thus promoting H+ reduction. The all-organic photosystem with H3PO4 as a stabilizer may provide a basis of photocatalytic H2O2 splitting.

Skewed electronic band structure induced by electric polarization in ferroelectric BaTiO3.

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

Supported binary CuO x -Pt catalysts with high activity and thermal stability for the combustion of NH3 as a carbon-free energy source.

Recently, NH3 has been thought to be a renewable and carbon-free energy source. The use of NH3 fuel, however, is hindered by its high ignition temperature and N2O/NO production. To overcome these issues, in this study, the combustion of NH3 over copper oxide (CuO x ) and platinum (Pt) catalysts supported on aluminium silicates (3Al2O3·2SiO2), aluminium oxides (Al2O3), and silicon oxides (SiO2) were compared. To achieve high catalytic activity for the combustion of NH3 and high selectivity for N2 (or low selectively for N2O/NO), conditions for the preparation of impregnated binary catalysts were optimised. With respect to the binary catalysts, sequentially impregnated CuO x /Pt/Al2O3 exhibited relatively higher activity, N2 selectivity, and thermal stability. From XRD and XAFS analyses, CuO x and Pt in CuO x /Pt/Al2O3 were present as CuAl2O4 and metallic Pt, respectively. Given that the combustion activity was closely associated with the Pt nanoparticle size, which was estimated from the Scherrer equation and the pulsed CO technique, highly dispersed Pt nanoparticles were crucial for the low-temperature light-off of NH3. For single and binary catalysts, although NH (imide) deformation modes as a key species for N2O production were detected by in situ FTIR spectral analysis, the band intensity of CuO x /Pt/Al2O3 was less than those of CuO x /Al2O3 and Pt/Al2O3. Therefore, CuO x /Pt/Al2O3 exhibits high selectivity for N2 in NH3 combustion.

Catalytic SO3 Decomposition Activity and Stability of Pt Supported on Anatase TiO2 for Solar Thermochemical Water-Splitting Cycles.

Pt-loaded anatase TiO2 (Pt/TiO2-A) was found to be a highly active and stable catalyst for SO3 decomposition at moderate temperatures (∼600 °C), which will prove to be the key for solar thermochemical water-splitting processes used to produce H2. The catalytic activity of Pt/TiO2-A was found to be markedly superior to that of a Pt catalyst supported on rutile TiO2 (Pt/TiO2-R), which has been extensively studied at a higher reaction temperature range (≥800 °C); this superior activity was found despite the two being tested with similar surface areas and metal dispersions after the catalytic reactions. The higher activity of Pt on anatase is in accordance with the abundance of metallic Pt (Pt0) found for this catalyst, which favors the dissociative adsorption of SO3 and the fast removal of the products (SO2 and O2) from the surface. Conversely, Pt was easily oxidized to the much less active PtO2 (Pt4+), with the strong interactions between the oxide and rutile TiO2 forming a fully coherent interface that limited the active sites. A long-term stability test of Pt/TiO2-A conducted for 1000 h at 600 °C demonstrated that there was no indication of noticeable deactivation (activity loss ≤ 4%) over the time period; this was because the phase transformation from anatase to rutile was completely prevented. The small amount of deactivation that occurred was due to the sintering of Pt and TiO2 and the loss of Pt under the harsh reaction atmosphere.

A nanometric Rh overlayer on a metal foil surface as a highly efficient three-way catalyst.

Pulsed arc-plasma (AP) deposition of an Rh overlayer on an Fe-Cr-Al stainless steel foil produced a composite material that exhibited high activity for automotive three-way catalysis (TWC). The AP pulses deposited metallic Rh nanoparticles 1-3 nm in size, whose density on the surface increased with the number of pulses. This led to coalescence and grain growth on the foil surface and the eventual formation of a uniform two-dimensional Rh overlayer. Full coverage of the 51 µm-thick flat foil by a 3.2 nm-thick Rh overlayer was achieved after 1,000 pulses. A simulated TWC reaction using a miniature honeycomb fabricated using flat and corrugated foils with the Rh overlayers exhibited successful light-off at a practical gaseous hourly space velocity of 1.2 × 10(5) h(-1). The turnover frequency for the NO-CO reaction over the metallic honeycomb catalyst was ca. 80-fold greater than that achieved with a reference Rh/ZrO2-coated cordierite honeycomb prepared using a conventional wet impregnation and slurry coating procedure. Despite the nonporosity and low surface area of the foil-supported Rh overlayer compared with conventional powder catalysts (Rh/ZrO2), it is a promising alternative design for more efficient automotive catalysts that use less Rh loading.