Size-Tunable Ni Nanoparticles Supported on Surface-Modified, Cage-Type Mesoporous Silica as Highly Active Catalysts for CO₂ Hydrogenation

In this study, ultrasmall Ni nanoparticles (Ni NPs) were controllably supported in the cage-type mesopores of −COOH-functionalized mesoporous silica SBA-16 (denoted as Ni­(<i>x</i>)@S16C, where <i>x</i> is the Ni loading) via wet impregnation under alkaline conditions, followed by thermal reduction. The particle sizes of the Ni NPs ranged from 2.7 to 4.7 nm, depending on the Ni loading. Under the appropriate alkaline conditions (i.e., pH 9) deprotonation of the carboxylic acid groups on the cage-type mesopore surfaces endowed the effective incorporation of Ni²⁺ precursors via favorable electrostatic interactions, and thus well-dispersed Ni NPs confined in the cage-type mesopores of SBA-16 were achieved. The combination of the cage-type mesopores and the surface functionality provided dual beneficial features to confine the immobilized Ni NPs and to tune their particle sizes. The remarkably enhanced catalytic activities of the Ni­(<i>x</i>)@S16C materials for CO₂ hydrogenation and CH₄ formation were demonstrated. The cage-type SBA-16 support provided a positive effect for the Ni NPs to enrich the surface sites, which can strongly adsorb CO and CO₂, thus leading to high catalytic rates for CO₂ and CO hydrogenation. The reaction mechanism, catalytic kinetics, and active sites were investigated to correlate to the high reaction rate for CO₂ hydrogenation to form CH₄

Comparative Study on the Morphology-Dependent Performance of Various CuO Nanostructures as Anode Materials for Sodium-Ion Batteries

In this work, CuO samples with three different nanostructures, i.e., nanoflakes, nanoellipsoids, and nanorods, are successfully synthesized by a facile and environmentally friendly hydrothermal approach based on the use of different structure directing agents. The morphological influence on the anodic electrochemical performances, such as capacity, cycling stability, rate capability, and diffusion coefficient measurements of these different CuO nanostructures is comparatively investigated for sodium-ion batteries. The capacity and cycling stability are higher for the CuO nanorods (CuO-NRs) based electrode as compared to the cases of CuO nanoellipsoids (CuO-NEs) and CuO nanoflakes (CuO-NFs). At a low current density of 25 mA g^–1, the CuO-NRs based electrode exhibits an excellent reversible capacity of 600 mA h g^–1. It also exhibits a capacity of 206 mA h g^–1 after 150 cycles with a capacity retention of 73% even at a higher current density of 1000 mA g^–1. The exceptional performance of CuO-NRs is attributable to its slim nanorod morphology with a smaller particle size that provides a short diffusion path and the maximized surface area facilitating good diffusion in electrolytes, ensuring good electronic conductivity and cycling stability. The comparative analysis of these materials can provide valuable insights to design hierarchical nanostructures with distinct morphology to achieve better materials designed for sodium-ion batteries

Probing the Nature and Local Structure of Phosphonic Acid Groups Functionalized in Mesoporous Silica SBA-15

Well-ordered mesoporous silicas SBA-15 functionalized with variable contents of phosphonic acid groups (up to 25 mol % based on silica) have successfully synthesized via cocondensation of tetraethoxysilane (TEOS) and diethylphosphatoethyltriethoxysilane (PETES) using triblock copolymer Pluronic P123 as the structure-directing agent under acidic conditions. The status and local structures of the phosphonic functional groups are investigated by extensive multinuclear solid-state NMR studies. Solid-state ¹³C and ³¹P NMR results reveal that phosphonic ester moieties are obtained for the as-synthesized samples and for the samples subjected to template removal by concentrated H₂SO₄. The generation of phosphonic acid groups can be accomplished by dealkylation reaction via treating the template-extracted samples with concentrated HCl. Two distinct local environments for the phosphorus sites of phosphonic acid groups have been observed at 32 and 22 ppm in the ³¹P magic angle spinning (MAS) NMR spectra. The relative ratio between these two species is not sensitive to the loading of phosphonic acid groups incorporated, but it strongly depends on the moisture present in the materials. The PO₃H₂ groups forming the hydrogen bonds with the nearby Q³ Si–OH are the major species responsible for the 22 ppm peak based on the results of ¹H → ³¹P → ²⁹Si double cross-polarization NMR experiments and density functional theory calculations (DFT). Of particular interest is that ²⁹Si{³¹P} rotational echo double resonance (REDOR) NMR experiments are utilized to measure ³¹P–²⁹Si distances between the phosphorus site in the functional groups and the silicon sites in the silica framework. A ²⁹Si–³¹P distance of 5.0 Å is obtained for the phosphorus site in the functional groups to the silicon site of the Q³ species for the as-synthesized sample. A reasonable fitting to the REDOR data for the acidified sample can also be achievable by assuming the presence of different structural units, whose ³¹P–²⁹Si distance information is referred from the DFT results. The combination of REDOR and ¹H → ³¹P → ²⁹Si double cross-polarization NMR measurements and the DFT calculations allow one to gain deeper insights into the local environments of the organic groups functionalized in mesoporous silica materials