Promoting Photochemical Water Oxidation with Metallic Band Structures

Operando Soft X-ray Absorption of LaMn1-x Co x O3 Perovskites for CO Oxidation

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1-x Co x O3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Facile Deficiency Engineering in a Cobalt-Free Perovskite Air Electrode to Achieve Enhanced Performance for Protonic Ceramic Fuel Cells

As a crucial component responsible for the oxygen reduction reaction (ORR), cobalt-rich perovskite-type cathode materials have been extensively investigated in protonic ceramic fuel cell (PCFC). However, their widespread application at a commercial scale is considerably hindered by the high cost and inadequate stability. In response to these weaknesses, the study presents a novel cobalt-free perovskite oxide, Ba0.95La0.05(Fe0.8Zn0.2)0.95O3-δ (BLFZ0.95), with the triple-conducting (H+|O2-|e-) property as an active and robust air electrode for PCFC. The B-site deficiency state contributes significantly to the optimization of crystal and electronic structure, as well as the increase in oxygen vacancy concentration, thus in turn favoring the catalytic capacity. As a result, the as-obtained BLFZ0.95 electrode demonstrates exceptional electrochemical performance at 700 °C, representing extremely low area-specific resistance of 0.04 Ω cm2 in humid air (3 vol.% H2O), extraordinarily high peak power density of 1114 mW cm-2, and improved resistance against CO2 poisoning. Furthermore, the outstanding long-term durability is achieved without visible deterioration in both symmetrical and single cell modes. This study presents a simple but crucial case for rational design of cobalt-free perovskite cathode materials with appreciable performance via B-site deficiency regulation.

Promotion of Acid-Water Oxidation by Lattice Distortion and Orbital Hybridization Induced by Ionic Dopant in Pyrochlore Y2Ru2O7

For acid-water oxidation, pyrochloric ruthenates are thought to be extremely effective electrocatalysts. In this work, through partial B-site replacement with larger M2+ cations, the electronic states of Y2Ru2O7 with strong electron correlations are reasonably managed, by which the inherent performance is tremendously promoted. Based on this, the improved Y2Ru1.9Sr0.1O7 electrocatalyst exhibits an outstanding durability and presents a highly inherent mass activity of 1915.1 A gRu-1 (at 1.53 V vs RHE). The enhanced oxygen-evolving reaction (OER) activity by ionic dopant in YRO pyrochlore can be attributed to two aspects, i.e., the lattice distortion induced inhibition of the grain coarsening, which results in a large surface area for YRO-M and increases the OER active sites, and the weakening of electron correlation via broadening of the Ru 4d bandwidths due to the increase of the average radius of B-site ions, which gives rise to an enhancement of conductivity and a strengthened hybridization between Ru 4d and O 2p orbitals and improves the reaction kinetics. The synergistic effects of lattice distortion and orbital hybridization promote the enhanced OER activity. The results would provide fresh concepts for the design of improved electrocatalysts and underscore the significance of managing the intrinsic performance through the dual modification of microstructure morphology and electronic structure.

Antiferromagnetically Coupled Double Perovskite as an Efficient and Robust Catalyst for Visible Light Driven Water Splitting at Neutral pH

Green and sustainable energy production through renewable sources is enormously an exciting field of research. Herein, we report A-site lanthanum doped oxygen excess ruthenate (predominantly Ru5+-ions) double perovskite system, CaLaScRuO6+δ...

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y₂Ru₂O₇ alternative through partial A-site substitution of Sr²⁺ for Y³⁺, efficiently improving its intrinsic OER activity. The optimized Y_1.7Sr_0.3Ru₂O₇ candidate observes a highly intrinsic mass activity of 1018 A g_Ru^-1 at an overpotential of 300 mV with excellent durability in 0.5 M H₂SO₄ electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr²⁺ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Solvent and additive-free efficient aerobic oxidation of alcohols by a perovskite oxide-based heterogeneous catalyst

A perovskite oxide has been utilized for the solvent and additive-free heterogeneous oxidation of various alcohols.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

Doping low-cost LDHs with noble metal atoms represents a promising approach to develop effective heterogeneous Water Oxidation Catalysts.

O-O Bond Formation and Liberation of Dioxygen Mediated by N5 -Coordinate Non-Heme Iron(IV) Complexes

Formation of the O-O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O-O bond in solution, from non-heme, N5 -coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O-O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [FeIV (O)] and [FeV (O)] being the resting and active principles of the O-O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O-O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Atomic-scale perturbation of oxygen octahedra via surface ion exchange in perovskite nickelates boosts water oxidation

A substantial amount of interest has been focused on ABO₃-type perovskite oxides over the past decade as oxygen electrocatalysts. Despite many studies on various compositions, the correlation between the structure of the oxygen octahedra and electrocatalytic property has been overlooked, and there accordingly have been a very limited number of attempts regarding control of atomistic structure. Utilizing epitaxial LnNiO₃ (Ln = La, Pr, Nd) thin films, here we demonstrate that simple electrochemical exchange of Fe in the surface region with several-unit-cell thickness is notably effective to boost the catalytic activity for the oxygen evolution reaction by different orders of magnitude. Furthermore, we directly establish that strong distortion of oxygen octahedra at the angstrom scale is readily induced during the Fe exchange, and that this structural perturbation permits easier charge transfer. The findings suggest that structural alteration can be an efficient approach to achieve exceptional electrocatalysis in crystalline oxides.

While perovskite oxides are well-studied for their oxygen electrocatalysis performances, the impact of oxygen octahedra is less studied. Here, authors show electrochemical exchange of lanthanide nickelate surfaces to distort oxygen octahedra, facilitating charge transfer and improving catalysis.

eg occupancy as an effective descriptor for the catalytic activity of perovskite oxide-based peroxidase mimics

A peroxidase catalyzes the oxidation of a substrate with a peroxide. The search for peroxidase-like and other enzyme-like nanomaterials (called nanozymes) mainly relies on trial-and-error strategies, due to the lack of predictive descriptors. To fill this gap, here we investigate the occupancy of e_g orbitals as a possible descriptor for the peroxidase-like activity of transition metal oxide (including perovskite oxide) nanozymes. Both experimental measurements and density functional theory calculations reveal a volcano relationship between the e_g occupancy and nanozymes’ activity, with the highest peroxidase-like activities corresponding to e_g occupancies of ~1.2. LaNiO_3-δ, optimized based on the e_g occupancy, exhibits an activity one to two orders of magnitude higher than that of other representative peroxidase-like nanozymes. This study shows that the e_g occupancy is a predictive descriptor to guide the design of peroxidase-like nanozymes; in addition, it provides detailed insight into the catalytic mechanism of peroxidase-like nanozymes.

The search for peroxidase-like as well as other enzyme-like nanozymes mainly relies on trial-and-error strategies, due to the lack of predictive descriptors. Here, the authors fill this gap by investigating the occupancy of e_g orbitals as a possible descriptor for the peroxidase-like activity of transition metal oxide nanozymes

Promoting Photocatalytic Overall Water Splitting by Controlled Magnesium Incorporation in SrTiO3 Photocatalysts

SrTiO₃ is a well-known photocatalyst inducing overall water splitting when exposed to UV irradiation of wavelengths <370 nm. However, the apparent quantum efficiency of SrTiO₃ is typically low, even when functionalized with nanoparticles of Pt or Ni@NiO. Here, we introduce a simple solid-state preparation method to control the incorporation of magnesium into the perovskite structure of SrTiO₃ . After deposition of Pt or Ni@NiO, the photocatalytic water-splitting efficiency of the Mg:SrTiO_x composites is up to 20 times higher compared to SrTiO₃ containing similar catalytic nanoparticles, and an apparent quantum yield (AQY) of 10 % can be obtained in the wavelength range of 300-400 nm. Detailed characterization of the Mg:SrTiO_x composites revealed that Mg is likely substituting the tetravalent Ti ion, leading to a favorable surface-space-charge layer. This originates from tuning of the donor density in the cubic SrTiO₃ structure by Mg incorporation and enables high oxygen-evolution rates. Nevertheless, interfacing with an appropriate hydrogen evolution catalyst is mandatory and non-trivial to obtain high-performance in water splitting.

Boosting the Peroxidase-Like Activity of Nanostructured Nickel by Inducing Its 3+ Oxidation State in LaNiO3 Perovskite and Its Application for Biomedical Assays

Catalytic nanomaterials with intrinsic enzyme-like activities, called nanozymes, have recently attracted significant research interest due to their unique advantages relative to natural enzymes and conventional artificial enzymes. Among the nanozymes developed, particular interests have been devoted to nanozymes with peroxidase mimicking activities because of their promising applications in biosensing, bioimaging, biomedicine, etc. Till now, lots of functional nanomaterials have been used to mimic peroxidase. However, few studies have focused on the Ni-based nanomaterials for peroxidase mimics. In this work, we obtained the porous LaNiO₃ nanocubes with high peroxidase-like activity by inducing its 3+ oxidation state in LaNiO₃ perovskite and optimizing the morphology of LaNiO₃ perovskite. The peroxidase mimicking activity of the porous LaNiO₃ nanocubes with Ni³⁺ was about 58~fold and 22~fold higher than that of NiO with Ni²⁺ and Ni nanoparticles with Ni⁰. More, the porous LaNiO₃ nanocubes exhibited about 2-fold higher activity when compared with LaNiO₃ nanoparticles. Based on the superior peroxidase-like activity of porous LaNiO₃ nanocubes, facile colorimetric assays for H₂O₂, glucose, and sarcosine detection were developed. Our present work not only demonstrates a useful strategy for modulating nanozymes' activities but also provides promising bioassays for clinical diagnostics.

Promoting photocatalytic H2 evolution by tuning cation deficiency in La and Cr co-doped SrTiO3

A moderate cation deficiency in La and Cr co-doped SrTiO₃ can increase the charge carrier mobility and promote the photocatalytic water reduction.

Earth-Abundant Heterogeneous Water Oxidation Catalysts

Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces.