Application of Ion Chromatography for the Reliable Quantification of Ammonium in Electrochemical Ammonia Synthesis Experiments: A Practical Guide

Assessing novel electrocatalysts for the electrochemical ammonia synthesis (EAS) requires reliable quantitative trace analysis of electrochemically produced ammonia to infer activity and selectivity. This study concerns the development of an ion chromatography (IC) method for quantitative trace analysis of ammonium in 0.1 M sulfuric acid electrolyte, which is applied to EAS gas-diffusion electrode (GDE) experiments with commercial chromium nitride as electrocatalyst. The developed IC method is highly sensitive, versatile, and reliable, achieving a limit of quantification (LOQ) of 6 μg l^-1 (6 ppb_mol ) ammonium. The impacts of the sample matrix, dilution, and neutralization, as well as contamination, on the quantitative analysis by IC are analyzed. Experimental constraints result in an effective LOQ including dilution of 60 μg l^-1 for the determination of ammonium in 0.1 M sulfuric acid electrolyte, owing to necessary sample dilution. The practical guide presented herein is intended to be very relevant for the field of EAS as a guideline and applicable to a broad range of catalyst systems and ion chromatography devices.

Discrete Arsonate-Grafted Inverted-Keggin 12-Molybdate Ion [Mo12O32(OH)2(4-N3C2H2-C6H4AsO3)4]2- and Formation of a Copper(II)-Mediated Metal-Organic Framework

The discrete inverted-Keggin ion [Mo₁₂O₃₂(OH)₂(4-N₃C₂H₂-C₆H₄AsO₃)₄]^2- (1) has been prepared in an aqueous acidic (pH 0.8) medium by the reaction of MoO₃ with the (4-triazolylphenyl)arsonic acid 4-N₃C₂H₂-C₆H₄AsO₃H₂ under hydrothermal conditions and was isolated as a sodium salt in 21% yield. The exact same reaction in the presence of Cu²⁺ ions resulted in the neutral metal-organic framework (MOF) Cu₂[Mo₁₂O₃₄(4-N₃C₂H₂-C₆H₄AsO₃)₄] (Cu-1) in 68% yield. The inverted-Keggin ion 1 comprises a metal-oxo core, which is capped by four organoarsonate groups, and in Cu-1, individual polyanions are linked in the solid state by coordination of the Cu²⁺ ions with the triazolyl groups. The discrete ion 1 was characterized by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and atomic absorption (AA) spectroscopy, as well as thermogravimetric analysis (TGA), and the POM-MOF Cu-1 was characterized by single-crystal and powder XRD, FT-IR, TGA, and gas sorption. Cu-1 has channels with a diameter of around ∼0.9 nm and exhibits a water-vapor adsorption capacity of 89.7 cm³ g^-1 (p/p₀ = 0.95).

Organically Functionalized Mixed-Valent Polyoxo-30-molybdate Wheel and Neutral Tetramolybdenum(V) Oxo Cluster

The first organofunctionalized mixed-valent polyoxo-30-molybdate wheel, [Mo^VI₁₈Mo^V₁₂O₈₄{AsO₂(CH₃)₂}₁₈]^18- (Mo₃₀), was synthesized in aqueous, mildly acidic conditions, and upon further acidification, the neutral tetramolybdenum(V) oxo cluster [Mo^V₄O₈{AsO₂(CH₃)₂}₄] (Mo₄) was obtained. Single-crystal X-ray diffraction (XRD) revealed that Mo₃₀ comprises 18 Mo^VI and 12 Mo^V ions arranged in a cyclic fashion with alternating {Mo^V₂} and {Mo^VI₃} groups, which are capped by 18 dimethylarsinate ligands, resulting in a novel polyoxo-30-molybdate wheel with a central cavity of ∼1.5 nm. On the other hand, Mo₄ has a distorted-cubic structure, with the corners of the cube being occupied by alternating Mo^V ions and oxo ligands. The compounds were characterized in solution by ¹H and ¹³C NMR and UV-vis spectroscopy and in the solid state by X-ray photoelectron spectroscopy and powder XRD. Mo₃₀ represents a novel type of polyanionic cycle with manyfold possibilities regarding host-guest chemistry.

Discovery of Polythioplatinate(II) [Pt3S2(SO3)6]10- and Study of Its Solution and Catalytic Properties

We have discovered the first polythioplatinate(II), [Pt^II₃S₂(SO₃)₆]^10- (1), which was synthesized in aqueous basic medium (pH 11) by hydrothermal heating at 150 °C. Polyanion 1 comprises a discrete, triangular assembly of three Pt²⁺ ions linked by two μ₃-sulfido ligands, and their square-planar coordination geometry is completed by two terminal S-bound sulfito ligands. Polyanion 1 was isolated as a hydrated sodium salt, Na₁₀[Pt^II₃(μ₃-S)₂(SO₃)₆]·22H₂O (Na-1), which was characterized in the solid state by single-crystal X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectra, and elemental analysis, in solution by ¹⁹⁵Pt NMR and atomic absorption spectroscopy, and in the gas phase by electrospray ionization mass spectrometry. Density functional theory calculations were performed, and the ¹⁹⁵Pt NMR chemical shifts of 1 were computed theoretically and shown to match well with the experimental data. Furthermore, the discrete title polyanion 1 was immobilized on mesoporous SBA-15 support and used as a precatalyst for the hydrogenation of o-xylene.

Combinatorial Screening of Cu-W Oxide-Based Photoanodes for Photoelectrochemical Water Splitting

Metal oxide libraries for photoanodes for the oxygen evolution reaction (OER) were generated by printing a metal salt solution in an array layout, followed by calcination to yield 22 ternary metal oxide systems. The libraries included a ternary metal cation system based on CuWO₄ with one out of eight transition or posttransition metal ions Cr, Mn, Fe, Co, Ni, Zn, Bi, and Ga in different overall atomic ratios. The photocatalyst libraries were screened by scanning photoelectrochemical microscopy for the highest anodic photocurrents. Array elements that showed promising performance were printed in another set of eight libraries with smaller increments of overall composition. Improved performance with respect to CuWO₄ was found for Ga, Co, and Ni as the third element. A comparison of the most active composition of those arrays within one library showed the highest activity for Cu₄₈Ga₃W₄₉O_x. Printing spots of identical composition (Cu₄₈Ga₃W₄₉O_x, Cu₄₄Ni₉W₄₇O_x, and Cu₄₄Co₉W₄₇O_x) over a larger area facilitated further characterization by X-ray photoelectron spectroscopy ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction, scanning electron microscopy, chopped light voltammetry, and scanning electrochemical microscopy for the OER. High and stable steady-state photocurrents were generated in a photoelectrochemical cell for all three electrodes even at a low constant bias voltage. The best overall photoanode composition Cu₄₈Ga₃W₄₉O_x showed currents that were 36 times higher than the currents of the binary Cu₅₀W₅₀O_x system. Significant n-doping was found by UPS valence band spectra for Ga-containing materials.

Discovery of a Neutral 40-PdII-Oxo Molecular Disk, [Pd40O24(OH)16{(CH3)2AsO2}16]: Synthesis, Structural Characterization, and Catalytic Studies

We report on the synthesis and structural characterization of a giant, discrete, and neutral molecular disk, [Pd₄₀O₂₄(OH)₁₆{(CH₃)₂AsO₂}₁₆] (Pd₄₀), comprising a 40-palladium-oxo core that is capped by 16 dimethylarsinate moieties, resulting in a palladium-oxo cluster (POC) with a diameter of ∼2 nm. Pd₄₀, which is the largest known neutral Pd-based oxo cluster, can be isolated either as a discrete species or constituting a 3D H-bonded organic-inorganic framework (HOIF) with a 12-tungstate Keggin ion, [SiW₁₂O₄₀]^4- or [GeW₁₂O₄₀]^4-. ¹H and ¹³C NMR as well as ¹H-DOSY NMR studies indicate that Pd₄₀ is stable in aqueous solution, which is also confirmed by ESI-MS studies. Pd₄₀ was also immobilized on a mesoporous support (SBA15) followed by the generation of size-controlled Pd nanoparticles (diameter ∼2-6 nm, as based on HR-TEM), leading to an effective heterogeneous hydrogenation catalyst for the transformation of various arenes to saturated carbocycles.

Discovery and Supramolecular Interactions of Neutral Palladium-Oxo Clusters Pd16 and Pd24

We report on the synthesis, structure, and physicochemical characterization of the first three examples of neutral palladium-oxo clusters (POCs). The 16-palladium(II)-oxo cluster [Pd16 O24 (OH)8 ((CH3 )2 As)8 ] (Pd16 ) comprises a cyclic palladium-oxo unit capped by eight dimethylarsinate groups. The chloro-derivative [Pd16 Na2 O26 (OH)3  Cl3  ((CH3 )2  As)8 ] (Pd16 Cl) was also prepared, which forms a highly stable 3D supramolecular lattice via strong intermolecular interactions. The 24-palladium(II)-oxo cluster [Pd24 O44 (OH)8 ((CH3 )2 As)16 ] (Pd24 ) can be considered as a bicapped derivative of Pd16 with a tetra-palladium-oxo unit grafted on either side. The three compounds were fully characterized 1) in the solid state by single-crystal and powder XRD, IR, TGA, and solid-state 1 H and 13 C NMR spectroscopy, 2) in solution by 1 H, 13 C NMR and 1 H DOSY spectroscopic methods, and 3) in the gas phase by electrospray ionization mass spectrometry (ESI-MS).

Relevant Properties of Carbon Support Materials in Successful Fe-N-C Synthesis for the Oxygen Reduction Reaction: Study of Carbon Blacks and Biomass-Based Carbons

Fe-N-C materials are promising non-precious metal catalysts for the oxygen reduction reaction in fuel cells and batteries. However, during the synthesis of these materials less active Fe-containing nanoparticles are formed in many cases which lead to a decrease in electrochemical activity and stability. In this study, we reveal the significant properties of the carbon support required for the successful incorporation of Fe-N-related active sites. The impact of two carbon blacks and two activated biomass-based carbons on the Fe-N-C synthesis is investigated and crucial support properties are identified. Carbon supports having low portions of amorphous carbon, moderate surface areas (>800 m2/g) and mesopores result in the successful incorporation of Fe and N on an atomic level and improved oxygen reduction reaction (ORR) activity. A low surface area and especially amorphous parts of the carbon promote the formation of metallic iron species covered by a graphitic layer. In contrast, highly microporous systems with amorphous carbon provoke the formation of less active iron carbides and carbon nanotubes. Overall, a phosphoric acid activated biomass is revealed as novel and sustainable carbon support for the formation of Fe-Nx sites. Overall, this study provides valuable and significant information for the future development of novel and sustainable carbon supports for Fe-N-C catalysts.

Tuning Coordination Geometry of Nickel Ketoiminates and Its Influence on Thermal Characteristics for Chemical Vapor Deposition of Nanostructured NiO Electrocatalysts

Nickel-based nanostructured materials have gained widespread attention, particularly for energy-related applications. Employing chemical vapor deposition (CVD) for NiO necessitates suitable nickel precursors that are volatile and stable. Herein, we report the synthesis and characterization of a series of new nickel β-ketoiminato complexes with different aliphatic and etheric side chain substitutions, namely, bis(4-(isopropylamino)-pent-3-en-2-onato)nickel(II) ([Ni(ipki)₂], 1), bis(4-(2-methoxyethylamino)pent-3-en-2-onato)nickel(II) ([Ni(meki)₂], 2), bis(4-(2-ethoxyethylamino)pent-3-en-2-onato)nickel(II) ([Ni(eeki)₂], 3), bis(4-(3-methoxy-propylamino)-pent-3-en-2-onato)nickel(II) ([Ni(mpki)₂], 4), and bis(4-(3-ethoxypropylamino)pent-3-en-2-onato)nickel(II) ([Ni(epki)₂], 5). These compounds have been thoroughly characterized with regard to their purity and identity by means of nuclear magnetic resonance spectroscopy (NMR) and electron impact mass spectrometry (EI-MS). Contrary to other transition metal β-ketoiminates, the imino side chain strongly influences the structural geometry of the complexes, which was ascertained via single-crystal X-ray diffraction (XRD). As a result, the magnetic momenta of the molecules also differ significantly as evidenced by the magnetic susceptibility measurements employing Evan's NMR method in solution. Thermal analysis revealed the suitability of these compounds as new class of precursors for CVD of Ni containing materials. As a representative precursor, compound 2 was evaluated for the CVD of NiO thin films on Si(100) and conductive glass substrates. The as-deposited nanostructured layers were stoichiometric and phase pure NiO as confirmed by XRD, Rutherford backscattering spectrometry (RBS), and nuclear reaction analysis (NRA). X-ray photoelectron spectroscopy (XPS) indicated the formation of slightly oxygen-rich surfaces. The assessment of NiO films in electrocatalysis revealed promising activity for the oxygen evolution reactions (OER). The current densities of 10 mA cm^-2 achieved at overpotentials ranging between 0.48 and 0.52 V highlight the suitability of the new Ni complexes in CVD processes for the fabrication of thin film electrocatalysts.

Semiconductive microporous hydrogen-bonded organophosphonic acid frameworks

Herein, we report a semiconductive, proton-conductive, microporous hydrogen-bonded organic framework (HOF) derived from phenylphosphonic acid and 5,10,15,20-tetrakis[p-phenylphosphonic acid] porphyrin (GTUB5). The structure of GTUB5 was characterized using single crystal X-ray diffraction. A narrow band gap of 1.56 eV was extracted from a UV-Vis spectrum of pure GTUB5 crystals, in excellent agreement with the 1.65 eV band gap obtained from DFT calculations. The same band gap was also measured for GTUB5 in DMSO. The proton conductivity of GTUB5 was measured to be 3.00 × 10-6 S cm-1 at 75 °C and 75% relative humidity. The surface area was estimated to be 422 m2 g-1 from grand canonical Monte Carlo simulations. XRD showed that GTUB5 is thermally stable under relative humidities of up to 90% at 90 °C. These findings pave the way for a new family of organic, microporous, and semiconducting materials with high surface areas and high thermal stabilities.

Correction: Micro-spectroscopy of HKUST-1 metal–organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host–guest chemistry and electrical conductivity

Correction for ‘Micro-spectroscopy of HKUST-1 metal–organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host–guest chemistry and electrical conductivity’ by Miguel Rivera-Torrente et al., Phys. Chem. Chem. Phys., 2019, 21, 25678–25689.

Photoelectrochemistry of Ferrites: Theoretical Predictions vs. Experimental Results

This paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferrites MFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites with M = Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange of M and Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.

Sol-gel chemistry in molten Brønsted acids towards "activated" carbons and beyond

"Chemical activation" using Brønsted acids as chemical agents is widely used to generate activated carbons for various sorption applications. Commercially relevant is especially a process using phosphoric acid as activating agent applied to abundant and inexpensive biomass such as wood or coconut shells. In this manuscript, we revisit the porogenesis mechanism based on experiments involving molecular model compounds and oxygen-free polymer precursors, as well as different molten acids as activating agents. Describing acid activation with principles of sol-gel chemistry results in a more general understanding and uncovers a versatile synthetic tool for materials nanochemistry.

Photocatalytic activity of ZrO2 composites with graphitic carbon nitride for hydrogen production under visible light

The synthesized ZrO₂/g-C₃N₄ composites exhibit superior performance in water splitting for hydrogen production due to the effective electron–hole separation at the composite interface.

Micro-spectroscopy of HKUST-1 metal–organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host–guest chemistry and electrical conductivity

Guest@MOF materials have potential in next-generation materials for electroconductive devices. Micro-spectroscopy studies of TCNQ@HKUST-1 electroconductive composites revealed the effects of spatial distribution and water vapor on this material.

Graphitic carbon nitride synthesized by simple pyrolysis: role of precursor in photocatalytic hydrogen production

g-C₃N₄ with structural defects and low polymerization synthesized by urea as the precursor for photocatalytic H₂ production under visible light.

Photocatalytic degradation of the herbicide chloridazon on mesoporous titania/zirconia nanopowders

Advanced oxidation processes using semiconducting photocatalysts for the degradation of organic pollutants are a promising approach for the remediation of pesticide-contaminated wastewater. High photodegradation efficiency and stability of the photocatalyst are of key importance for practical application of the semiconductor. In this study, mesoporous TiO₂/ZrO₂ nanopowders were synthesized via two techniques; evaporation-induced self-assembly (EISA) and sol-gel using triblock copolymers Pluronic P123 and F127. The photodegradation activities of the composites were determined by employing the herbicide chloridazon as a model compound. Due to well-developed mesoporosity, the TiO₂/ZrO₂ nanocomposite synthesized by EISA displays high surface area and small crystallite sizes leading to higher photocatalytic activity than pristine TiO₂ prepared under similar condition and commercial Degussa P25 nanopowder. The optimum amount of zirconium required for the highest activities was identified and found to be 0.14 and 0.05 mol% for the EISA and sol-gel-prepared samples, respectively. Systematic studies of the post-thermal treatment step for both samples show that Zr inhibits an anatase-to-rutile phase transition only up to 600 °C, at higher temperature phase separation occurs. Samples synthesized by EISA method showed enhanced degradation activity than sol-gel-synthesized samples.

Formation of hybrid ABX3 perovskite compounds for solar cell application: first-principles calculations of effective ionic radii and determination of tolerance factors

The development of hybrid organic-inorganic perovskite solar cells is one of the most rapidly growing fields in the photovoltaic community and is on its way to challenge polycrystalline silicon and thin film technologies. High power conversion efficiencies can be achieved by simple processing with low cost. However, due to the limited long-term stability and environmental toxicity of lead in the prototypic CH₃NH₃PbI₃, there is a need to find alternative ABX₃ constitutional combinations in order to promote commercialization. The Goldschmidt tolerance factor and the octahedral factor were found to be necessary geometrical concepts to evaluate which perovskite compounds can be formed. It was figured out that the main challenge lies in estimating an effective ionic radius for the molecular cation. We calculated tolerance factors and octahedral factors for 486 ABX₃ monoammonium-metal-halide combinations, where the steric size of the molecular cation in the A-position was estimated concerning the total charge density. A thorough inquiry about existing mixed organic-inorganic perovskites was undertaken. Our results are in excellent agreement with the reported hybrid compounds and indicate the potential existence of 106 ABX₃ combinations hitherto not discussed in the literature, giving hints for more intense research on prospective individual candidates.

Crystalline and permanently porous porphyrin-based metal tetraphosphonates

The first porous MOF containing a porphyrin-based phosphonic acid was synthesized and characterized regarding its sorption properties and proton conductivity.

Construction of strontium tantalate homo-semiconductor composite photocatalysts with a tunable type II junction structure for overall water splitting

The presence of at least two different strontium tantalates with a type II junction structure for efficient photocatalytic overall water splitting.

Rational fabrication of a graphitic-C3N4/Sr2KNb5O15 nanorod composite with enhanced visible-light photoactivity for degradation of methylene blue and hydrogen production

Formation of proper interfaces between g-C₃N₄ and Sr₂KNb₅O₁₅ by direct growth approach greatly enhanced the photodegradation and hydrogen production activity.

Proton Conduction in Sulfonated Organic-Inorganic Hybrid Monoliths with Hierarchical Pore Structure

Porous organic-inorganic hybrid monoliths with hierarchical porosity exhibiting macro- and mesopores are prepared via sol-gel process under variation of the mesopore size. Organic moieties in the pore walls are incorporated by substituting up to 10% of the silicon precursor tetramethylorthosilicate with bisilylated benzene molecules. After functionalization with sulfonic acid groups, the resulting sulfonated hybrid monoliths featuring a bimodal pore structure are investigated regarding proton conduction depending on temperature and relative humidity. The hierarchical pore system and controlled mesopore design turn out to be crucial for sulfonation and proton conduction. These sulfonated hybrid hierarchical monoliths containing only 10% organic precursor exhibit higher proton conduction at different relative humidities than sulfonated periodic mesoporous organosilica made of 100% bisilylated precursors exhibiting solely mesopores, even with a lower concentration of sulfonic acid groups.

Hydrothermal carbonization of biomass from landscape management - Influence of process parameters on soil properties of hydrochars

Besides pyrolysis the technology of hydrothermal carbonization (HTC) is tested to produce hydrochars for soil improvement. The chemical and physical properties of the hydrochars mainly depend on the feedstock and the process parameters reaction time and process temperature. Systematic investigations on the influences of these process parameters on soil properties of hydrochars like water holding capacity (WHC) and cation exchange capacity (CEC) are missing. In this study, a rush-rich biomass was carbonized within defined HTC process conditions under variation of reaction time and process temperature to produce hydrochars. Analysis of WHC, CEC, the elemental composition and Fourier-transform infrared spectroscopy (FT-IR) were performed to evaluate the influence of HTC process conditions on the pedological hydrochar properties. The results indicated that at increasing reaction severity (reaction time and process temperature) WHC and CEC decreased as well as the elemental O/C ratio. The decrease of WHC and CEC is based on the decrease of the hydrochar surface polarity. However, even the lowest WHC and CEC of investigated hydrochars still exceeded those of pure quartz sand by factors of 5-10. An application of hydrochars produced at severe HTC conditions could improve WHC and CEC of sandy soils. This has to be investigated in further studies.

Structure property relationships affecting the proton conductivity in imidazole loaded Al-MOFs

A systematic investigation on the influence of steric effects and the possibility to form host-guest interactions on the proton conductivity was carried out using imidazole loaded derivatives of Al-MIL-53 [Al(OH)(1,4-BDC-(CH₃)_x)] (x = 0, 1, 2) and CAU-11.

Improved charge carrier separation in barium tantalate composites investigated by laser flash photolysis

Charge carrier dynamics in pure Ba₅Ta₄O₁₅ and in a Ba₅Ta₄O₁₅–Ba₃Ta₅O₁₅ composite have been studied by means of diffuse reflectance laser flash photolysis spectroscopy in the presence and absence of methanol.

Electrochemical deposition of Fe2O3 in the presence of organic additives: a route to enhanced photoactivity

The photoelectrochemical activity of hematite films prepared by electrochemical deposition (ED) in the presence of organic additives is discussed.