Synthesis, Characterization, and Thermochemical Redox Performance of Hf⁴⁺, Zr⁴⁺, and Sc³⁺ Doped Ceria for Splitting CO₂

We present results on the thermochemical redox performance and analytical characterization of Hf⁴⁺, Zr⁴⁺, and Sc³⁺ doped ceria solutions synthesized via a sol–gel technique, all of which have recently been shown to be promising for splitting CO₂. Dopant concentrations ranging from 5 to 15 mol % have been investigated and thermally cycled at reduction temperatures of 1773 K and oxidation temperatures ranging from 873 to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr doped materials is substantially higher than those of pure ceria and Sc doped ceria and increases with dopant concentration. Overall, 10 mol % Hf doped ceria results in the largest CO yields per mole of oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based on measured mass changes during oxidation. However, these yields were largely influenced by their rate of reoxidation, not necessarily thermodynamic limitations, as equilibrium was not achieved for either Hf or Zr doped samples after 45 min exposure to CO₂ at all oxidation temperatures. Additionally, sample preparation and grain size strongly affected the oxidation rates and subsequent yields, resulting in slightly decreasing yields as the samples were cycled up to 10 times. X-ray diffraction, Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX have been applied to characterize the elemental, crystalline, and morphological attributes before and after redox reactions

Computational Investigation and Design of Cobalt Aqua Complexes for Homogeneous Water Oxidation

We study the water oxidation mechanism of the cobalt aqua complex [Co­(H₂O)₆]²⁺ in a photocatalytic setup by means of density functional theory. Assuming a water-nucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an “ideal catalyst”. Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts

Synthesis and Characterization of 0D–3D Copper-Containing Tungstobismuthates Obtained from the Lacunary Precursor Na₉[B-α-BiW₉O₃₃]

The reaction of the lacunary polyoxometalate precursor Na₉[B-α-BiW₉­O₃₃]·19.5H₂O with Cu­(II) ions was explored in search of new economic ways to copper tungstobismuthates as interesting prototypes for water oxidation and reduction catalysts. The emerging series of new 0D–3D polyoxometalate architectures with distinct copper cores was structurally characterized. Na₆­Rb₆­[Cu₃­(H₂O)₃­(Bi­W₉­O₃₃)₂] (<b>Cu-4</b>) and 3D-K_6.56­Cu_0.43­H_2.20­[(Cu₃Cl)­(K_2.62­Cu_0.38­(H₂O)₃)­(B-α-BiW₉­O₃₃)₂]·13H₂O (<b>Cu-5</b>) display a Cu₃(H₂O)₃ core. The 2D representatives Na₁₂­[Cu₂­(H₂O)₄­Cl₂(BiW₁₀­O₃₅)₂] (<b>Cu-1a</b>), Na₁₀­[Cu₂­(H₂O)₆­(BiW₁₀­O₃₅)₂] (<b>Cu-1b</b>), 2D-Na₇K₃­Cu_0.5Cl­[Cu₂­(H₂O)₄­(BiW₁₀­O₃₅)₂] (<b>Cu-2</b>), and 2D-Na_5.5­K_2.5Cu­[Cu₂­(H₂O)₄­(BiW₁₀­O₃₅)₂] (<b>Cu-3</b>) contain Cu₂­(H₂O)_<i>n</i>­W₂O₄ cores. Interestingly, the bismuth-free 1D paratungstate B Na₄­K₄Cu­[H₂W₁₂­O₄₂] (<b>Cu-6</b>) is formed through reassembly of the precursor. <b>Cu-5</b> displays a disordered transition metal core, implying the presence of the polyanions [Cu₄­(H₂O)₄­(BiW₉­O₃₃)₂]^10– and [Cu₅­(H₂O)₅­(BiW₉­O₃₃)₂]^8–. The magnetic properties of <b>Cu-5</b> as well as its activity as visible-light-driven H₂ and O₂ evolution catalyst were evaluated

Closer to Photosystem II: A Co₄O₄ Cubane Catalyst with Flexible Ligand Architecture

We introduce the novel Co₄O₄ complex [Co^II₄(hmp)₄(μ-OAc)₂(μ₂-OAc)₂(H₂O)₂] (<b>1</b>) (hmp = 2-(hydroxymethyl)­pyridine) as the first Co­(II)-based cubane water oxidation catalyst. Monodentate acetate and aqua ligands lend the flexible environment of <b>1</b> closest resemblance to photosystem II among its tetranuclear mimics to date. Visible-light-driven catalytic activity of <b>1</b> increases with pH value through aqua ligand deprotonation. The Co­(II) core combines robustness and stability with flexibility through a new type of water-oxidation mechanism via mobile ligands

Homogeneous Photochemical Water Oxidation with Cobalt Chloride in Acidic Media

The precise mechanisms of four-electron-transfer water oxidation processes remain to be further understood. Oxide-based precipitation from molecular catalysts as a frequent observation during water oxidation has raised extensive debates on the differentiation between homogeneous and heterogeneous catalysis. Although soluble cobalt salts are known to be active in water oxidation, CoO_<i>x</i> species formed in situ were generally considered to be the true catalyst. Here we report on the possibility homogeneous water oxidation with cobalt chloride in acidic conditions, which prevent CoO_<i>x</i> precipitation. Interestingly, both the buffer media and counteranions were found to significantly influence the oxygen evolution activity, and their roles in the water oxidation process were analyzed with various techniques. This study sheds new light on Co²⁺ ions in key transformation processes of homogeneous water oxidation catalysts

Closer to Photosystem II: A Co₄O₄ Cubane Catalyst with Flexible Ligand Architecture

We introduce the novel Co₄O₄ complex [Co^II₄(hmp)₄(μ-OAc)₂(μ₂-OAc)₂(H₂O)₂] (<b>1</b>) (hmp = 2-(hydroxymethyl)­pyridine) as the first Co­(II)-based cubane water oxidation catalyst. Monodentate acetate and aqua ligands lend the flexible environment of <b>1</b> closest resemblance to photosystem II among its tetranuclear mimics to date. Visible-light-driven catalytic activity of <b>1</b> increases with pH value through aqua ligand deprotonation. The Co­(II) core combines robustness and stability with flexibility through a new type of water-oxidation mechanism via mobile ligands

Chitosan-Thioglycolic Acid as a Versatile Antimicrobial Agent

As functionalized chitosans hold great potential for the development of effective and broad-spectrum antibiotics, representative chitosan derivatives were tested for antimicrobial activity in neutral media: trimethyl chitosan (TMC), carboxy-methyl chitosan (CMC), and chitosan-thioglycolic acid (TGA; medium molecular weight: MMW-TGA; low molecular weight: LMW-TGA). Colony forming assays indicated that LMW-TGA displayed superior antimicrobial activity over the other derivatives tested: a 30 min incubation killed 100% Streptococcus sobrinus (Gram-positive bacteria) and reduced colony counts by 99.99% in Neisseria subflava (Gram-negative bacteria) and 99.97% in Candida albicans (fungi). To elucidate LMW-TGA effects at the cellular level, microscopic studies were performed. Use of fluorescein isothiocyanate (FITC)-labeled chitosan derivates in confocal microscopy showed that LMW-TGA attaches to microbial cell walls, while transmission electron microscopy indicated that this derivative severely affects cell wall integrity and intracellular ultrastructure in all species tested. We therefore propose LMW-TGA as a promising and effective broad-band antimicrobial compound

Promoting Photochemical Water Oxidation with Metallic Band Structures

The development of economic water oxidation catalysts is a key step toward large-scale water splitting. However, their current exploration remains empirical to a large extent. Elucidating the correlations between electronic properties and catalytic activity is crucial for deriving general and straightforward catalyst design principles. Herein, strongly correlated electronic systems with abundant and easily tunable electronic properties, namely La_1–<i>x</i>Sr_<i>x</i>BO₃ perovskites and La_2‑xSr_<i>x</i>BO₄ layered perovskites (B = Fe, Co, Ni, or Mn), were employed as model systems to identify favorable electronic structures for water oxidation. We established a direct correlation between the enhancement of catalytic activity and the insulator to metal transition through tuning the electronic properties of the target perovskite families via the La³⁺/Sr²⁺ ratio. Their improved photochemical water oxidation performance was clearly linked to the increasingly metallic character. These electronic structure–activity relations provide a promising guideline for constructing efficient water oxidation catalysts

Bi₂O₂CO₃ Growth at Room Temperature: In Situ X‑ray Diffraction Monitoring and Thermal Behavior

The room-temperature formation of bismuth oxycarbonate (Bi₂O₂CO₃) from Bi₂O₃ in sodium carbonate buffer was investigated with in situ powder X-ray diffraction (PXRD) in combination with electron microscopy and vibrational spectroscopy. Time-resolved PXRD measurements indicate a pronounced and rather complex pH dependence of the reaction mechanism. Bi₂O₂CO₃ formation proceeds within a narrow window between pH 8 and 10 via different mechanisms. Although a zero-dimensional nucleation model prevails around pH 8, higher pH values induce a change toward a diffusion-controlled model, followed by a transition to regular nucleation kinetics. Ex situ synthetic and spectroscopic studies confirm these trends and demonstrate that in situ monitoring affords vital parameter information for the controlled fabrication of Bi₂O₂CO₃ materials. Furthermore, the β → α bismuth oxide transformation temperatures of Bi₂O₂CO₃ precursors obtained from different synthetic routes differ notably (by min 50 °C) from commercially available bismuth oxide. Parameter studies suggest a stabilizing role of surface carbonate ions in the as-synthesized bismuth oxide sources. Our results reveal the crucial role of multiple preparative history parameters, especially of pH value and source materials, for the controlled access to bismuth oxide-based catalysts and related functional compounds

{Co₄O₄} and {Co_<i>x</i>Ni_4–<i>x</i>O₄} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides

The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and heterogeneous catalyst design. We introduce the active and stable {Co^II₄O₄} cubane [Co^II₄(dpy­{OH}­O)₄­(OAc)₂(H₂O)₂]­(ClO₄)₂ (<b>Co</b>_<b>4</b><b>O</b>_<b>4</b><b>-dpk</b>) as the first molecular WOC with the characteristic {H₂O-Co₂(OR)₂-OH₂} edge-site motif representing the <i>sine qua non</i> moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of <b>Co</b>_<b>4</b><b>O</b>_<b>4</b><b>-dpk</b> under photocatalytic conditions ([Ru­(bpy)₃]²⁺/S₂O₈^2–) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with ¹H/²H exchange experiments highlight a favorable intramolecular base functionality of the dpy­{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {Co^II₄O₄} core undergoes oxidation to Co­(III) or higher valent states, which recover rather slowly to Co­(II). Complementary ex situ chemical oxidation experiments with [Ru­(bpy)₃]³⁺ furthermore indicate that the oxidation of all Co­(II) centers of <b>Co</b>_<b>4</b><b>O</b>_<b>4</b><b>-dpk</b> to Co­(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [Co^II_<i>x</i>Ni_4–<i>x</i>­(dpy­{OH}­O)₄­(OAc)₂(H₂O)₂]­(ClO₄)₂ (<b>Co</b>_<b>x</b><b>Ni</b>_{<b>4–<i>x</i></b>}<b>O</b>_<b>4</b><b>-dpk</b>) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers