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Written by Karel and Josef Čapek.Mode of access: Internet

Effect of Calcination Temperature on the Structure and Catalytic Performance of the Ni/Al₂O₃ and Ni–Ce/Al₂O₃ Catalysts in Oxidative Dehydrogenation of Ethane

We studied the effect of calcination temperature of Ni/Al₂O₃ and Ni–Ce/Al₂O₃ catalysts on the specific surface area, acidity, ratio of pore volume and specific surface area, reducibility reflecting the changing population of Ni­(T_<i>d</i>) and Ni­(O_<i>h</i>) species, and activity/selectivity in oxidative dehydrogenation of ethane. It should be stressed that the role of Ce as a promoter on the catalytic activity of Ni–Ce/Al₂O₃ catalysts decreased with an increasing value of the calcination temperature. At a reaction temperature of 500 °C, the highest productivity to ethene was observed for the Ni–Ce/Al₂O₃ catalyst calcined at 500 °C. Ni–Ce/Al₂O₃ catalysts calcined at 500–750 °C showed the same selectivity to ethene. The Ni–Ce/Al₂O₃ catalysts calcined at 900 and 1000 °C showed a sharp decrease in the selectivity and the activity, which was probably associated with the formation of NiAl₂O₄ spinel

Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, <i>i</i>.<i>e</i>., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed <i>in situ</i> out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions

Surface Properties of Hydrotalcite-Based Zn(Mg)Al Oxides and Their Catalytic Activity in Aldol Condensation of Furfural with Acetone

Basic mixed oxides MgAl, ZnMgAl, and ZnAl were successfully prepared from hydrotalcite precursors synthesized by urea method. Materials with the same molar ratio (M²⁺/Al³⁺) = 2 were studied to describe the influence of Mg/Zn ratio on their physicochemical properties. Materials were tested as catalysts of the aldol condensation of furfural with acetone. For samples with similar particle sizes and surface BET areas, the varying catalytic activity was related to the different acidobasic properties. Higher furfural conversion and selectivity to longer carbon chain F₂Ac product was observed for samples with higher total amount of basic sites. More specifically, it correlated with the population of Me²⁺–O^2– pairs that represented dominant type of basic sites in all studied catalysts. At the same Al loading, Mg₂Al mixed oxide exhibited higher specific surface area, higher total amount of basic sites and higher amount of acid sites than Zn₂Al oxide

Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, <i>i</i>.<i>e</i>., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed <i>in situ</i> out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions

Influence of Mg–Al Mixed Oxide Compositions on Their Properties and Performance in Aldol Condensation

The influence of chemical composition of Mg–Al mixed oxides on their properties and catalytic performance in aldol condensation of furfural and acetone was studied. Pure alumina, Mg–Al layered double hydroxides with a wide range of Mg/Al molar ratios from 0.5 to 15, and pure magnesia were prepared by the same synthesis method, and corresponding oxides were produced by calcination of the as-prepared samples at T = 450 °C. The physicochemical properties of the samples were investigated by XRD, N₂ physisorption, SEM, CO₂-TPD, and NH₃-TPD, and their catalytic performance was evaluated in aldol condensation of furfural and acetone. SEM images revealed that the Al content in the samples influenced the size and the morphology of both large agglomerates and plate-like crystals. The CO₂-TPD measurements proved that the concentration of basic sites and the distribution of their strengths was determined by the Mg/Al molar ratio in the Mg–Al mixed oxides. However, over the whole range of Mg/Al compositions studied here, no clear correlation between the chemical composition and the number and strength of basic sites was found. Nonetheless, the change in the strength and in the density of basic sites affected the catalytic performance of calcined Mg–Al mixed oxides. Additionally, morphology and crystal size also influenced the performance of the catalysts. Finally, the composition of reaction products was affected by acid–base and textural properties of the prepared Mg–Al-mixed oxides

Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals

Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solution-phase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl₂, the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn²⁺ by Cd²⁺ at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd²⁺ ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs