On the Formation of Cd–Zn Sulfide Photocatalysts from Insoluble Hydroxide Precursors

The formation of Cd–Zn sulfide solid solutions from mixed hydroxides under hydrothermal conditions is investigated in detail. The work specifically aims to understand the formation and the role of nanotwinned mixed sulfide particles that have been reported to show excellent performance in photocatalytic water splitting (Liu, M.; et al. <i>Energy Environ. Sci.</i> <b>2011</b>, <i>4</i>, 1372). The influence of additives, pH, autoclave tumbling, and the state of the mixed hydroxide precursor on the mixed sulfides was studied by XRD, XPS, TEM, DR UV–vis, and N₂ physisorption. Cd–Zn sulfides are formed via a dissolution–precipitation mechanism. Agitation of the synthetic medium and the formation of soluble intermediate complexes during hydrothermal treatment suppress the formation of a hexagonal wurtzite crystal phase and improve the photocatalytic activity of the mixed sulfides. The role of additives can be understood in terms of complex formation, pH maintenance, and adsorption on the facets of growing crystallites. All Cd–Zn sulfide samples exhibit compositional inhomogeneities, resulting in XRD line broadening and decreased bandgaps as compared with the values predicted by Vegard’s law. Detailed TEM analysis revealed that the samples with higher amounts of nanotwinned particles were significantly less active in water reduction. The influence of nanotwinned particles is discussed in terms of extended crystal defects and charge carrier recombination

Evaluating the Stability of Co₂P Electrocatalysts in the Hydrogen Evolution Reaction for Both Acidic and Alkaline Electrolytes

The evaluation of the stability of emerging earth-abundant metal phosphide electrocatalysts by solely electrochemical current–potential sweeps is often not conclusive. In this study, we investigated Co₂P to evaluate its stability under both acidic (0.5 M H₂SO₄) and alkaline (1.0 M KOH) hydrogen evolution (HER) conditions. We found that the electrochemical surface area (ECSA) of Co₂P only slightly increased in acidic conditions but almost doubled after electrolysis in alkaline electrolyte. The surface composition of the electrode remained almost unchanged in acid but was significantly altered in alkaline during current–potential sweeps. Analysis of the electrolytes after the stability test shows almost stoichiometric composition of Co and P in acid, but a preferential dissolution of P over Co could be observed in alkaline electrolyte. Applying comprehensive postcatalysis analysis of both the electrode and electrolyte, we conclude that Co₂P, prepared by thermal phosphidization, dissolves stoichiometrically in acid and degrades to hydroxides under alkaline stability testing

Promoted Iron Nanocrystals Obtained via Ligand Exchange as Active and Selective Catalysts for Synthesis Gas Conversion

Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na₂S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process. The catalyst composition was optimized for the Fischer–Tropsch direct conversion of synthesis gas into lower olefins. At industrially relevant conditions, these nanocrystal-based catalysts with controlled composition were more active, selective, and stable than catalysts with similar composition but synthesized using conventional methods, possibly due to their homogeneity of properties and synergic interaction of iron and promoters

Temperature-Dependent Kinetic Studies of the Chlorine Evolution Reaction over RuO₂(110) Model Electrodes

Ultrathin single-crystalline RuO₂(110) films supported on Ru(0001) are employed as model electrodes to extract kinetic information about the industrially important chlorine evolution reaction (CER) in a 5M concentrated NaCl solution under well-defined electrochemical conditions and variable temperatures. A combination of chronoamperometry (CA) and online electrochemical mass spectrometry (OLEMS) experiments provides insight into the selectivity issue: At pH = 0.9, the CER dominates over oxygen evolution, whereas at pH = 3.5, oxygen evolution and other parasitic side reactions contribute mostly to the total current density. From temperature-dependent CA data for pH = 0.9, we determine the apparent free activation energy of the CER over RuO₂(110) to be 0.91 eV, which compares reasonably well with the theoretical value of 0.79 eV derived from first-principles microkinetics. The experimentally determined apparent free activation energy of 0.91 eV is considered as a benchmark for assessing future improved theoretical modeling from first principles

Interplay between Surface Chemistry, Precursor Reactivity, and Temperature Determines Outcome of ZnS Shelling Reactions on CuInS₂ Nanocrystals

ZnS shelling of I–III–VI₂ nanocrystals (NCs) invariably leads to blue-shifts in both the absorption and photoluminescence spectra. These observations imply that the outcome of ZnS shelling reactions on I–III–VI₂ colloidal NCs results from a complex interplay between several processes taking place in solution, at the surface of, and within the seed NC. However, a fundamental understanding of the factors determining the balance between these different processes is still lacking. In this work, we address this need by investigating the impact of precursor reactivity, reaction temperature, and surface chemistry (due to the washing procedure) on the outcome of ZnS shelling reactions on CuInS₂ NCs using a seeded growth approach. We demonstrate that low reaction temperatures (150 °C) favor etching, cation exchange, and alloying regardless of the precursors used. Heteroepitaxial shell overgrowth becomes the dominant process only if reactive S- and Zn-precursors (S-ODE/OLAM and ZnI₂) and high reaction temperatures (210 °C) are used, although a certain degree of heterointerfacial alloying still occurs. Remarkably, the presence of residual acetate at the surface of CIS seed NCs washed with ethanol is shown to facilitate heteroepitaxial shell overgrowth, yielding for the first time CIS/ZnS core/shell NCs displaying red-shifted absorption spectra, in agreement with the spectral shifts expected for a type-I band alignment. The insights provided by this work pave the way toward the design of improved synthesis strategies to CIS/ZnS core/shell and alloy NCs with tailored elemental distribution profiles, allowing precise tuning of the optoelectronic properties of the resulting materials

Stability of Pt/γ-Al₂O₃ Catalysts in Lignin and Lignin Model Compound Solutions under Liquid Phase Reforming Reaction Conditions

The stability of a 1 wt % Pt/γ-Al₂O₃ catalyst was tested in an ethanol/water mixture at 225 °C and autogenic pressure, conditions at which it is possible to dissolve and depolymerize various kinds of lignin, and structural changes to the catalysts were studied by means of X-ray diffraction (XRD), ²⁷Al MAS NMR, N₂ physisorption, transmission electron microscopy (TEM), H₂ chemisorption, elemental analysis, thermogravimetric analysis-mass spectrometry (TGA-MS), and IR. In the absence of reactants the alumina support is found to transform into boehmite within 4 h, leading to a reduction in support surface area, sintering of the supported Pt nanoparticles, and a reduction of active metal surface area. Addition of aromatic oxygenates to mimic the compounds typically obtained by lignin depolymerization leads to a slower transformation of the support oxide. These compounds, however, were not able to slow down the decrease in dispersion of the Pt nanoparticles. Vanillin and guaiacol stabilize the aluminum oxide more than phenol, anisole, and benzaldehyde because of the larger number of oxygen functionalities that can interact with the alumina. Interestingly, catalyst samples treated in the presence of lignin showed almost no formation of boehmite, no reduction in support or active metal surface area, and no Pt nanoparticle sintering. Furthermore, in the absence of lignin-derived aromatic oxygenates, ethanol forms a coke-like layer on the catalyst, while oxygenates prevent this by adsorption on the support by coordination via the oxygen functionalities

Structure and Basicity of Microporous Titanosilicate ETS-10 and Vanadium-Containing ETS-10

ETS-10 has attracted considerable attention as a base catalyst. It is desirable to confirm the location of basic sites. Vanadium-substituted ETS-10 also attracts much attention for the interesting feature that the Ti can be fully replaced by V without changing its topology. It is important to characterize the local environment upon V substitution for understanding the property and reactivity of ETVS-10. The structural and acid–base properties of pure titanosilicate ETS-10 and a series of vanadium-substituted ETVS-10 with different framework V content were studied by a combination of Raman spectroscopy and FTIR of absorbed acetylene and carbon monoxide as molecular probes. The substitution of up to 70% of Ti atoms with V in the structure of ETS-10 results in ETVS-10 materials with a homogeneous distribution of Ti and V species. At higher V concentrations, a distinct phase separation between the vanadium-rich domains is observed. The intrinsic basicity of ETVS-10 as revealed by FTIR spectroscopy of adsorbed C₂H₂ gradually increases with the increasing V content. It is shown that the specific basicity of the ETS-10 lattice is mainly associated with the presence of highly basic oxygen centers adjacent to the lattice defects. Liquid phase Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was used as a test reaction to investigate the catalytic reactivity of different basic sites in the synthesized materials. The reactivity of the materials considered in the base-catalyzed Knoevenagel condensation is determined not only by the strength of the basic sites but also by their density. The optimum combination of both factors is achieved for the ETVS-10 material with V/(Ti+V) ratio of 70%

Atomically Dispersed Pd–O Species on CeO₂(111) as Highly Active Sites for Low-Temperature CO Oxidation

Ceria-supported Pd is a promising heterogeneous catalyst for CO oxidation relevant to environmental cleanup reactions. Pd loaded onto a nanorod form of ceria exposing predominantly (111) facets is already active at 50 °C. Here we report a combination of CO-FTIR spectroscopy and theoretical calculations that allows assigning different forms of Pd on the CeO₂(111) surface during reaction conditions. Single Pd atoms stabilized in the form of PdO and PdO₂ in a CO/O₂ atmosphere participate in a catalytic cycle involving very low activation barriers for CO oxidation. The presence of single Pd atoms on the Pd/CeO₂-nanorod, corroborated by aberration-corrected TEM and CO-FTIR spectroscopy, is considered pivotal to its high CO oxidation activity

Heterovalent Tin Alloying in Layered MA₃Sb₂I₉ Thin Films: Assessing the Origin of Enhanced Absorption and Self-Stabilizing Charge States

Heteroatom alloying of lead-free perovskite derivatives is a highly promising route to tailor their optoelectronic properties and stability for multiple applications. Here, we demonstrate the facile solution-based synthesis of Sn-alloyed layered MA3Sb2I9 thin films by precursor engineering, combining acetate and halide salts. An increasing concentration of tin halides in different oxidation states leads to a strong boost in absorption over the whole visible spectrum. We demonstrate phase-pure synthesis and elucidate the heterovalent incorporation of Sn into the MA3Sb2I9 lattice, proving the formation of additional electronic states in the bandgap by theoretical calculations. On this basis, we dissect the strong absorption increase into three components that we attribute to intervalence and heteroatom-induced interband absorption. Finally, we show the charge-stabilizing effect of the system through robustness toward precursors in mixed oxidation states and trace the improved ambient stability of this material back to this feature

Ex Situ and Operando Studies on the Role of Copper in Cu-Promoted SiO₂–MgO Catalysts for the Lebedev Ethanol-to-Butadiene Process

Dehydrogenation promoters greatly enhance the performance of SiO₂–MgO catalysts in the Lebedev process. Here, the effect of preparation method and order of addition of Cu on the structure and performance of Cu-promoted SiO₂–MgO materials is detailed. Addition of Cu to MgO via incipient wetness impregnation (IWI) or coprecipitation (CP) prior to wet-kneading with SiO₂ gave similar butadiene yields (∼40%) as when Cu was added to the already wet-kneaded catalyst. In contrast, the catalyst prepared by impregnation of Cu on SiO₂ first proved to be the worst catalyst of the series. TEM, XRD, and XPS analyses suggested that, for all catalyst materials, Cu²⁺ forms a solid solution with MgO. This was confirmed by UV–vis, XANES, and EXAFS data, with Cu being found in a distorted octahedral geometry. As a result, the acid–base properties, as determined by Pyridine- and CDCl₃–IR as well as NH₃-TPD, are modified, contributing to the improved performance. Operando XANES and EXAFS studies of the evolution of the copper species showed that Cu²⁺, the only species initially present, is extensively reduced to a mixture of Cu⁰ and Cu⁺, leaving only a limited amount of unreduced Cu²⁺. This formation of Cu⁰ is the result of the reducing environment of the Lebedev process and is thought to be mainly responsible for the improved performance of the Cu-promoted catalysts