Journal of Catalysis最新文献

In metal-containing zeolites, sintering and redispersion processes exercise control over the identity of metal site structures, but the thermodynamic and kinetic factors that influence these molecular-level processes are not completely understood. Here, we assess the ability of first principles informed free energy models (for supported and unsupported nanoparticles) and kinetic models integrating Ostwald ripening with atom trapping to describe the interconversion between Pt cations and nanoparticles encapsulated in chabazite (CHA) zeolites. Density functional theory-derived thermodynamic phase diagrams show that the interconversion between cations, favored in oxidizing environments, and particles, favored in reducing environments, is fully reversible within a wide range of their respective conditions (temperatures and pressures) for CHA and several other zeolite topologies. Kinetic Monte Carlo simulations of Pt redispersion are consistent with experimentally observed redispersion kinetics of encapsulated Pt nanoparticles in CHA zeolites, and model results suggest the zeolite host imparts additional stability for Pt nanoparticles. We envision our thermodynamic and kinetic models for Pt-CHA are also capable of describing nanoparticle and cation interconversions for other zeolite frameworks under similar conditions.

A spectrokinetic methodology was developed to determine reactive surface species by combining operando ultraviolet–visible spectroscopy and charge transfer kinetic models. The methodology consisted of three general steps: 1) concomitant measurement of reaction rates and charge transfer via ultraviolet–visible spectroscopy; 2) development of rate expressions from kinetic models involving the adsorbed species of interest. These rate expressions relate reaction rates, charge transfer, and partial pressures of gas phase species; and 3) evaluation of the goodness of fit of the rate expressions to the experimental data. The species whose rate expressions show the best fit are the more likely reactive surface species for the studied reaction. An example is presented for the determination of reactive oxygen species during ethanol oxidation over Au/TiO₂. The charge transfer spectrokinetic analysis showed that surface hydroperoxyl, hydroxyl, and atomic oxygen species were reactive surface intermediates, whereas surface molecular oxygen was not.

The first applying SrTiO₃ semiconductor as a photocatalyst in organic synthesis and using NPh₃ as a redox catalyst in photocatalysis was reported. With SrTiO₃ as the heterogeneous photocatalyst, NPh₃ as the homogeneous redox catalyst, H-bond as the promoter, paraformaldehyde as the cheap and atom economic methylene source, various 1,2,3,9b-tetrahydrobenzo[d]imidazo[1,5-b]isothiazole 5,5-dioxides can be efficiently synthesized through the visible-light-induced cascade decarboxylative coupling/annulation reaction.

The trifluoromethyl group (-CF₃) containing motifs are valuable leads for the small molecule drug discovery process and have multiple applications in medicine, agriculture, and material science. Not many CF₃-containing motifs are commercially accessible; therefore, it is critical to design more advanced building blocks. In this work, we demonstrate an oxidant-free, scalable, and user-friendly protocol for the direct C(sp²)-H trifluoromethylation of functionalized arenes using Iron(II) triflate as a robust photocatalyst and bench-stable CF₃SO₂Na (1.5 equiv.) as a CF₃ source under blue light irradiation. This method was extrapolated to >30 examples and compatible with potential functional groups (-NO₂, -NH₂, OH, -CHO, -CO₂H, halogens, etc.). Notably, the utility of this approach is showcased for the synthesis of novel CF₃-containing synthetic building blocks and a few bioactive compounds.

In recent decades, there has been notable interest in understanding the influence of the support composition on the reactivity of gold species in oxidation processes. This study fits in with this scientific trend and investigates the effects of incorporating phosphate ions into Au catalysts supported on mixed iron-niobium oxides in methanol oxidation. All materials were thoroughly characterized using XRD, ICP-OES, DR-UV-Vis, N₂ physisorption, HRTEM, HAADF-STEM, SEM-EDX, XPS, TPD-NH₃, TPD-CO₂, and in situ FTIR combined with adsorption of NO. Activity of the catalysts was evaluated using a fixed-bed flow reactor combined with gas chromatography and operando FTIR-MS system. It was observed that the phosphate-doped catalyst supported on mixed Fe-Nb oxide exhibited significantly higher activity than phosphate-free sample. This improvement resulted from increased electron mobility, enhanced acidity, and optimized distribution of gold nanoparticles on the former catalyst. Knowledge resulting from this work can lead to the development of more efficient gold catalysts.

The direct sp³ C–H functionalization of alcohol with N-heterocycles is the most atom-economical pathway to produce hydroxyalkylated N-heterocycles. However, the targeted C–H activation to enable the direct C–C coupling still remains of great challenge due to the concomitant activation of alcoholic O–H. This work puts forward a H-bonding protection for alcoholic O–H for the targeted activation of alcoholic C_α-H bonds. Atomic Fe-N sites anchored in graphitic carbon nitride with uncoordinated nitrogen sites has thus been proposed for this strategy, in which atomic Fe(II)-N sites are supposed to be responsible for the targeted activation of alcoholic C_α-H and the surface uncoordinated nitrogen to for the H-bonding with alcoholic O–H. To demonstrate the strategy, the precise modulation on the atomic Fe-N coordination in the 6-fold cavity from three heptazines have been elaborated. Atomic Fe with four N atoms from two heptazines assisted with the uncoordinated N from the free one heptazine has presented a synergistic H-bonding protection of alcoholic O–H bonds. As a result, the targeted generation and excellent stabilization of hydroxyethyl radicals have been achieved under ambient temperature. The strategy has been successfully applied for 1-propanol, 2-propanol, 1-butanol, and 1-pentanol, thus achieving efficient C–C formation in the hydroxyalkylation of varied N-heterocycles.

Selective co-oligomerizations between α-olefins and ethylene are of high interest since they allow to extend the α-olefin scope employing inexpensive and abundantly available ethylene. Here we report on an easily accessible zirconium catalyst that permits the co-oligomerization of 1-hexene and ethylene with an activity of 2200 kg/mol h bar and a co-oligomer selectivity of 65 mol%. In more detail, we report synthesis and structure of six novel Zr precatalysts, their ethylene homo-oligomerization behavior and the co-oligomerization of the most promising of the six precatalysts. Criteria were activity and selectivity (α-value). Some of the Zr catalyst systems introduced here show an extremely high activity in ethylene homo-oligomerization up to 96,600 kg/mol h bar.

Introducing transition metals into CuAl₂O₄ spinel enhances catalyst stability and Cu sintering resistance in methanol steam reforming. Yet, the influence of doping on vacancy formation and the adsorption behaviors of CO₂ (the primary product) and CO (the notorious byproduct) remains unclear. Herein, we employed DFT + U to investigate CO and CO₂ adsorption on perfect, M-doped (Fe, Co, and Ni), and M-doped oxygen-deficient CuAl₂O₄ spinel (1 0 0) and (1 1 0) surfaces. We find that stronger CO adsorption on (1 0 0) than (1 1 0) surfaces across all M-doped surfaces, while CO₂ adsorbs more stronger on (1 1 0) surfaces. The weakened CO adsorptions are observed on Fe and Ni-doped surfaces, demonstrating that doping plays a significant role in improving the resistance to CO poisoning. Co-doping promotes CO adsorption via a CO₃-like structure on CuAl₂O₄(1 1 0) surface and boosts the CO oxidation. Furthermore, infrared spectroscopy simulation indicates that the vibrational frequencies for CO linear adsorption, formation of bent CO₂- and CO₃-like structures are within the ranges of 2042–2078, 1463–1566, and 1497–1816 cm⁻¹, respectively. In addition, Ov on Ni-doped surfaces can significantly strengthen the CO₂ adsorption by 0.6–1.3 eV, highlighting the doping and oxygen-defect engineering in enhancing the CO₂ capture. This research uncovers the critical impact of metal doping and oxygen vacancies on CO and CO₂ adsorptions over CuAl₂O₄ spinel catalyst, providing insights for developing catalysts with improved resistance to CO poisoning and enhanced CO oxidation which is vital for methanol steam reforming.

Metal nanocrystals, such as Ag and Pt nanocrystals, are widely used as catalysts for various chemical reactions due to their high activity. The activity of metal nanocrystals can be further enhanced by forming multiple composite nanocrystal (synergistic effect) or increasing the heterogeneity of the nanocrystal morphology (high surface area to volume ratio). Here, we fabricated Pt-Ag nanocrystals with highly heterogeneous morphologies, mushroom-like/flower-like/half-cage-like, through a “seed-mediated” synthesis method in a hydrophobic layer, which can be used to suppress the nanocrystals aggregation issue for catalytic applications. When these Pt-Ag nanocrystals are used as 4-nitrophenol reduction catalysts, the catalytic performance of these nanocrystals largely depends on their composition and morphology. Compared with mushroom-like Pt-Ag nanocrystals, flower-like and half-cage-like Pt-Ag nanocrystals exhibit higher catalytic activity, which is attributed their morphological effect (high surface area to volume ratio) and composition effect (synergistic effect), respectively. And compared with other Pt/Ag/Pt-Ag nanocrystals as 4-nitrophenol (4-NP) reduction catalysts reported in the literature, these highly heterogeneous Pt-Ag nanocrystals on the hydrophobic layer showed significant catalytic performance enhancement effects, which can be attributed to their composite and morphological effects and suppresses nanocrystal aggregation. These Pt-Ag nanocrystals can also be used as photocatalysts under visible light irradiation to further enhance their catalytic performance, due to the excellent plasmonic response of Ag nanocrystals under visible light.

Covalent organic frameworks (COFs) are promising crystalline materials for photocatalytic organic transformations. It has been reported that the donor–acceptor (D-A) structure is favorable for the separation of photogenerated carriers, while the impact of flexible monomers on the optoelectronic performance and photocatalytic property of D-A COF remains unclear. Accordingly, a novel flexible D-A COF was synthesized by the Schiff-base condensation reaction of thieno [3,2-b] thiophene-2,5-dicarboxaldehyde (TTDC) with 2,4,6-tri (4-aminophenoxy)-1,3,5-triazine (TPT), named as TPT-COF. While the rigid D-A COF was constructed by the condensation reaction of TTDC with 2,4,6-tris (4-aminophenyl)-1,3,5-triazine (TAPT), named as TTT-COF. Compared with TTT-COF, TPT-COF showed much better catalytic performance for the photosynthesis of benzimidazole. It might be due to its larger interlayer distance and more efficient photogenerated carrier separation and migration. Notably, this newly designed COF also exhibited outstanding stability and tolerance for diverse substrates. This study might provide a rational inspiration for developing efficient COF photocatalysts through the molecular design strategy.