Experimental and Computational Insights into the Catalytic Mechanism of Y1-xBaxCoO3-δ Perovskite Oxides with a Controlled Crystal Structure.

The crystal structure of Co-based perovskite oxides (ACoO3) can be controlled by adjusting the A-site elements. In this study, we synthesized Y1-xBaxCoO3-δ (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO3 (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y0.5Ba0.5CoO2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO3 (x = 1.0; hexagonal perovskite oxide; P63/mmc), which contains high-valent Co4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and 18O2. The CO oxidation reaction on YCoO3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO3 compared with that of the reference Pt/Al2O3, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t2g6eg0 states, which can translate to a magnetic t2g5eg1 configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO3 activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.

Ultralong Distance Hydrogen Spillover Enabled by Valence Changes in a Metal Oxide Surface.

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeOx (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeOx support extends over long distances (∼600 µm). Furthermore, the hydrogen-spillover-induced reduction of Fe4+ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports' surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties.

Local Structure and L1- and L3-Edge X-ray Absorption Near Edge Structures of Middle Lanthanoid Elements (Eu, Gd, Tb, and Dy) in Their Complex Oxides.

Relationship between the local structures of middle lanthanoid elements (Ln; Eu, Gd, Tb, and Dy) in their complex oxides and the characteristic features of the L1-edge and L3-edge X-ray absorption near edge structure (XANES) was investigated. There was a significant correlation between the pre-edge peak areas of the Ln L1-edge or the full widths at half maximum of the white line of the Ln L3-edge XANES spectra and the abstract physical indexes defined by bond angles formed by the middle Ln elements and the two adjacent oxygen atoms, which act as indicators of local configurational disorder of the target element. Theoretical simulation based on multiple scattering theory revealed that the pre-edge peak in the Ln L1-edge XANES spectra originates due to the p-d hybridization that occurs above the Fermi energy. This systematic survey demonstrated a universal method to estimate the local structure of the middle Ln elements by means of XANES spectroscopy.

A theoretical investigation into the role of catalyst support and regioselectivity of molecular adsorption on a metal oxide surface: NO reduction on Cu/γ-alumina.

The role of catalyst support and regioselectivity of molecular adsorption on a metal oxide surface is investigated for NO reduction on a Cu/γ-alumina heterogeneous catalyst. For the solid surface, computational models of the γ-alumina surface are constructed based on the Step-by-Step Hydrogen Termination (SSHT) approach. Dangling bonds, which appear upon cutting the crystal structure of a model, are terminated stepwise with H atoms until the model has an appropriate energy gap. The obtained SSHT models reflect the realistic infrared (IR) and ultraviolet-visible (UV/Vis) spectra. Vibronic coupling density (VCD), as a reactivity index, is employed to elucidate the regioselectivity of Cu adsorption on γ-alumina and that of NO adsorption on Cu/γ-alumina in place of the frontier orbital theory that could not provide clear results. We discovered that the highly dispersed Cu atoms are loaded on Lewis-basic O atoms, which is known as the anchoring effect, located in the tetrahedral sites of the γ-alumina surface. The role of the γ-alumina support is to raise the frontier orbital of the Cu catalyst, which in turn gives rise to the electron back-donation from Cu/γ-alumina to NO. In addition, the penetration of the VCD distribution of Cu/γ-alumina into the γ-alumina support indicates that the excessive reaction energy dissipates into the support after NO adsorption and reduction. In other words, the support plays the role of a heat bath. The NO reduction on Cu/γ-alumina proceeds even in an oxidative atmosphere because the Cu-NO bond is strong compared to the Cu-O2 bond.

Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ.

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NOx storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO3-δ (SFO) are reactive in the oxidation of NO to NO2 at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe0.8Mn0.2O3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NOx storage performance.

Local Structure Study of Lanthanide Elements by X-Ray Absorption Near Edge Structure Spectroscopy.

This paper describes a systematic study of the spectra and local structures of lanthanide (Ln) L-edge XANES. We found that Ln L1 and L3 -edge XANES spectra exhibit characteristic features correlated to their local symmetry through experimental and theoretical simulations. We also propose a simple local structure index criterion for a combination of XANES study and theoretical simulation. Possible solutions of intrinsic problems such as low resolution of characteristic features in the Ln L-edge XANES and site distributions are also discussed.

Self-regeneration of a Ni-Cu alloy catalyst during a three-way catalytic reaction.

Ni-Cu alloy supported on γ-Al2O3 catalysts prepared by high-temperature hydrogen reduction exhibit high catalytic activity and durability for a three-way catalytic reaction under both oxidative and reductive conditions because of their self-regenerating feature. DFT calculations showed that Ni-oxide was reduced to Ni metal by CO in the presence of Cu metal because of the Ni-Cu alloy effect but was not in the absence of Cu metal.

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.

AgFeOF2: A Fluorine-Rich Perovskite Oxyfluoride.

We synthesized a silver iron oxyfluoride AgFeOF2 by using a high-pressure reaction. Synchrotron X-ray and neutron diffraction, X-ray absorption, and 57Fe Mössbauer spectroscopy indicate that AgFeOF2 crystallizes in the ideal perovskite structure with iron in a trivalent state, although electron microscopy revealed weak super-reflections. A possible partial ordering in the FeO2F4 octahedron is inferred from Mössbauer spectroscopy. The synthesis of the fluorine-rich sample offers an opportunity to study a composition-property relation in AFeIIIO3- nF n ( n = 0, 1, and 2). AgFeOF2 exhibits a G-type antiferromagnetic ordering below TN ≈ 480 K, which is much lower than the n = 0 and 1 cases, suggesting a weaker superexchange interaction between Fe moments via F 2p orbitals (vs O 2p orbitals).

A detailed insight into the catalytic reduction of NO operated by Cr-Cu nanostructures embedded in a CeO2 surface.

Replacing rare and expensive elements, such as Pt, Pd, and Rh, commonly used in catalytic devices with more abundant and less expensive ones is mandatory to realize efficient, sustainable and economically appealing three-way catalysts. In this context, the surface of a Cr-Cu/CeO2 system represents a versatile catalyst for the conversion of toxic NO into harmless N2. Yet, a clear picture of the underlying mechanism is still missing. We provide here a detailed insight into such a reaction mechanism by means of a combined experimental and theoretical study. Fourier-transform infrared spectroscopy is used to detect all the products resulting from catalytic reactions of NO and CO on the surface of a Cr-Cu/CeO2 nanocatalyst. CO pulsing experiments unveil that reactions of CO with O atoms at the Cr-Cu/CeO2 surface are the major factors responsible for the formation of surface vacancies. On these grounds, a comprehensive picture of the NO reduction and the role of both Cu and Cr dopants and vacancies is rationalized by first-principles modeling. Our findings provide a general route for the realization of ceria-based cost-effective catalysts.

Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis.

The problem of activating N2 and its subsequent hydrogenation to form NH3 has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N2, but without catalytic formation of NH3. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH2 and BaTiO2.5H0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH3 under H2/N2 flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·-1·h-1) almost comparable to conventional supported Ru catalysts such as Cs-Ru/MgO or Ru/BaTiO3 that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activity for Ti, making this a previously overlooked element, but our results show that by introducing hydride, the repertoire of heterogeneous catalysts can be expanded to include formerly unexamined compositions without resorting to precious metals.

Enhancement of CO Evolution by Modification of Ga2O3 with Rare-Earth Elements for the Photocatalytic Conversion of CO2 by H2O.

Modification of the surface of Ga2O3 with rare-earth elements enhanced the evolution of CO as a reduction product in the photocatalytic conversion of CO2 using H2O as an electron donor under UV irradiation in aqueous NaHCO3 as a pH buffer, with the rare-earth species functioning as a CO2 capture and storage material. Isotope experiments using 13CO2 as a substrate clearly revealed that CO was generated from the introduced gaseous CO2. In the presence of the NaHCO3 additive, the rare-earth (RE) species on the Ga2O3 surface are transformed into carbonate hydrates (RE2(CO3)3·nH2O) and/or hydroxycarbonates (RE2(OH)2(3-x)(CO3)x) which are decomposed upon photoirradiation. Consequently, Ag-loaded Yb-modified Ga2O3 exhibits much higher activity (209 µmol h-1 of CO) than the pristine Ag-loaded Ga2O3. The further modification of the surface of the Yb-modified Ga2O3 with Zn afforded a selectivity toward CO evolution of 80%. Thus, we successfully achieved an efficient Ag-loaded Yb- and Zn-modified Ga2O3 photocatalyst with high activity and controllable selectivity, suitable for use in artificial photosynthesis.

Enhanced oxygen-release/storage properties of Pd-loaded Sr3Fe2O7-δ.

This study proves that a small amount of Pd loading (1 wt%) on Sr3Fe2O7-δ can dramatically enhance the oxygen-storage properties of Sr3Fe2O7-δ. The topotactic oxygen intake and release between Sr3Fe2O6.75 and Sr3Fe2O6 takes place in response to gas switching between an O2 flow and H2 flow, regardless of the presence or absence of Pd loading. The effect of Pd loading is significant for the oxygen-release process under H2 atmosphere; that is, highly dispersed Pd metal nanoparticles sized less than 1 nm formed on Pd/Sr3Fe2O7-δ to promote H2 dissociation, resulting in the improvement of the oxygen-release temperature and rate. Pd/Sr3Fe2O7-δ with a layered perovskite structure has a higher oxygen-release property at lower temperature than Pd/SrFeO3-δ with a perovskite phase without the layered structure. These facts indicate that the surface reaction as well as the crystal structure are responsible for the oxide ion mobility in perovskite structure, and also provide guidelines for designing novel oxygen-storage materials.

Investigation of the electrochemical and photoelectrochemical properties of Ni-Al LDH photocatalysts.

Layered double hydroxide (LDH) photocatalysts, including Ni-Al LDH, are active for the photocatalytic conversion of CO2 in water under UV light irradiation. In this study, we found that a series of LDHs exhibited anodic photocurrent which is a characteristic feature corresponding to n-type materials. Also, we estimated the potentials of photogenerated electrons and holes for LDHs, which are responsible for the photocatalytic reactions, using electrochemical techniques. The flat band potential of the Ni-Al LDH photocatalyst was estimated to be -0.40 V vs. NHE (pH = 0), indicating that the potential of the photogenerated electron is sufficient to reduce CO2 to CO. Moreover, we revealed that the flat band potentials of M(2+)-M(3+) LDH are clearly influenced by the type of trivalent metal (M(3+)) components.

Effect of the chloride ion as a hole scavenger on the photocatalytic conversion of CO2 in an aqueous solution over Ni-Al layered double hydroxides.

The photocatalytic conversion of CO2 into useful chemical compounds in water without using organic sacrificial reagents is a promising method to overcome environmental and energy problems. Various synthesized layered double hydroxides (LDHs) are capable of reducing CO2 to CO in an aqueous solution under UV light irradiation. However, it is difficult to oxidize H2O to O2 in a photocatalytic system using LDHs as photocatalysts. In this study, we investigated the photocatalytic conversion of CO2 using a Ni-Al LDH in an aqueous solution of NaCl. Hypochlorous acid (HClO) was produced as an oxidation product of Cl(-) with the formation of reduction products such as CO and H2 under photoirradiation. We propose the inclusion of Cl(-) in the reaction solution to be one of the most promising ways for obtaining a hole scavenger, an approach that would enable the construction of an artificial photosynthesis system for the conversion of CO2.

Effects of SO2 on selective catalytic reduction of NO with NH3 over a TiO2 photocatalyst.

The effect of SO2 gas was investigated on the activity of the photo-assisted selective catalytic reduction of nitrogen monoxide (NO) with ammonia (NH3) over a TiO2 photocatalyst in the presence of excess oxygen (photo-SCR). The introduction of SO2 (300 ppm) greatly decreased the activity of the photo-SCR at 373 K. The increment of the reaction temperature enhanced the resistance to SO2 gas, and at 553 K the conversion of NO was stable for at least 300 min of the reaction. X-ray diffraction, FTIR spectroscopy, thermogravimetry and differential thermal analysis, x-ray photoelectron spectroscopy (XPS), elemental analysis and N2 adsorption measurement revealed that the ammonium sulfate species were generated after the reaction. There was a strong negative correlation between the deposition amount of the ammonium sulfate species and the specific surface area. Based on the above relationship, we concluded that the deposition of the ammonium sulfate species decreased the specific surface area by plugging the pore structure of the catalyst, and the decrease of the specific surface area resulted in the deactivation of the catalyst.

A doping technique that suppresses undesirable H2 evolution derived from overall water splitting in the highly selective photocatalytic conversion of CO2 in and by water.

Photocatalytic conversion of CO2 to reduction products, such as CO, HCOOH, HCHO, CH3OH, and CH4, is one of the most attractive propositions for producing green energy by artificial photosynthesis. Herein, we found that Ga2O3 photocatalysts exhibit high conversion of CO2. Doping of Zn species into Ga2O3 suppresses the H2 evolution derived from overall water splitting and, consequently, Zn-doped, Ag-modified Ga2O3 exhibits higher selectivity toward CO evolution than bare, Ag-modified Ga2O3. We observed stoichiometric amounts of evolved O2 together with CO. Mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. Doping of the photocatalyst with Zn is expected to ease the adsorption of CO2 on the catalyst surface.

An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity.

In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

Ruthenium-catalyzed intermolecular hydroacylation of internal alkynes: the use of ceria-supported catalyst facilitates the catalyst recycling.

Versatile and practical: Intermolecular hydroacylation of internal alkynes takes place in the presence of Ru catalysts together with HCO(2)Na and Xantphos to give the corresponding conjugated enones. Aromatic aldehydes with or without coordinating groups could be used in the present catalytic system. The solid Ru/CeO(2) catalysts can be recycled for several times without significant decreases in yield (see scheme).

Reactivity of Lattice Oxygen in Ti-Site-Substituted SrTiO3 Perovskite Catalysts.

An environmental catalyst in which a transition metal (Mn, Fe, or Co) was substituted into the Ti site of the host material, SrTiO3, was synthesized, and the reactivity of lattice oxygen was evaluated. For CO oxidation, Mn- and Co-doped SrTiO3 catalysts, which provided high thermal stabilities, exhibited higher activities than Pt/Al2O3 catalysts despite their low surface areas. Temperature-programmed reduction experiments using X-ray absorption fine structure (XAFS) measurements showed that the lattice oxygen of Co-doped catalyst was released at the lowest temperature. Isotopic experiments with CO and 18O2 revealed that the lattice oxygen was involved in CO oxidation on Fe- and Co-doped catalysts; that is, CO oxidation on these catalysts proceeded via the Mars-van Krevelen mechanism. On the other hand, for Mn-doped catalyst, the contribution of lattice oxygen to CO oxidation was relatively negligible, indicating that the reaction proceeded according to the Langmuir-Hinshelwood mechanism. This paper clearly demonstrates that the catalytic mechanism can be adjusted by substituting transition metals into SrTiO3.