Nanospace-Confined High-Temperature Solid-State Reactions: Versatile Synthetic Route for High-Diversity Pool of Catalytic Nanocrystals

Abstract

The present study proposes a methodology to extend the utility of solid-state reactions to a synthetic route for producing a high-diversity pool of nanocrystals (NCs) by circumventing the problematic sintering of nanoparticles at high temperatures. For this purpose, nanometer-scale-confined NC formations/transformations were investigated using specifically designed SiO 2 nanospheres with a radially differentiated core@shell structure as a reaction medium. The core of the SiO 2 medium was modified by aminosilanes, thus providing binding sites for the metal ions and enabling pore creation through thermochemical treatment. This hindered the outward diffusion of fast-moving tiny species initially generated within the tens of nanometers-sized medium. The entire evolution process of Pd NCs during the high temperature reaction was confined within the SiO 2 nanosphere, which transforms from the aminecontaining nanosphere, Pd 2+ /SiO 2 (NH 2 ), to a hollow nanosphere, Pd@h-SiO 2 . This nanoscale confinement strategy was applicable to the first-row transition metals with high diffusivity in SiO 2 medium. Accordingly, M 2+ /SiO 2 (NH 2 ) 2 (M = Co, Ni, or Cu) and (Pd 2+ /M 2+ )/SiO 2 (NH 2 ) 2 were thermally developed into

Nanosilica‐Confined Synthesis of Orthogonally Active Catalytic Metal Nanocrystals in the Compartmentalized Carbon Framework

“…The Raman spectroscopy revealed the high D/G band ratio ≈3.51 (Figure h), conferring that the carbonization generates graphitic domains in Pd@Com‐CF, which was also confirmed by deconvoluted C1s XPS peaks at 284.5, 285.5, and 286.5 eV attributed to CC, C–C, and C–O respectively in the ratio of 1:0.1:0.06 (Figure S2, Supporting Information) . To verify the NC‐confinement ability of NH 2 ‐SiO 2 NPs during carbonization process, Pd 2+ @SiO 2 NPs (devoid of amines) as a mixture with maltodextrin were reductively annealed, resulting Pd NCs to diffuse out of the SiO 2 NP and to be coalesced and sintered in to bulk particle (Figure S3, Supporting Information), which was clearly due to the absence of pore generation ability and inefficient confinement by NH 2 ‐devoid SiO 2 NPs, affirming the indispensable role of NH 2 ‐SiO 2 as primary confiners which provides the Lewis‐basic binding sites to the metal precursors and creates the pores through thermal or chemically induced decomposition, circumventing the outward NC‐diffusion . Though annealing of Pd 2+ @SiO 2 NPs at lower temperature might result in confinement of Pd NCs which would be insufficient for the required high‐degree of carbonization.…”

“…To achieve the metal NCs compartmentalized in carbon framework (M@Com‐CF), first, Pd 2+ ‐embedded SiO 2 NPs (Pd 2+ @NH 2 ‐SiO 2 ) of diameter [ d ] = 35 ± 3 nm were synthesized following our previous method which comprise amine‐rich silica at the interior and normal silica at the exterior of the NP (Figure S1a, Supporting Information) . Next, a mixture of preformed Pd 2+ @NH 2 ‐SiO 2 and sucrose (as carbon source), was subjected to reductive annealing (4% H 2 in Ar) at 800 °C for 3 h, to carry out simultaneous solid‐state carbonization and porous‐nanosilica‐confined Pd NCs formation resulting Pd@h‐SiO 2 occluded in carbonized‐framework (Pd@SiO 2 @CF); unexpectedly, annealed product contained hollow‐SiO 2 NPs enclosed in carbon with agglomerated and nonconfined Pd NCs, because of the small size, sucrose can pass through the porous medium of silica nanospheres and can easily access the metal NCs and binds to the metal NCs through free hydroxyl groups which causes the instability and leaching out of metal NCs out of silica nanospheres during carbonization process (Figure S1, Supporting Information).…”

Yolk@Shell Nanoreactors Carrying a Cluster of Metal Nanocrystals Stabilized Inside the Hollow Carbon Shell

“…However, the annealing with relatively thinner PDA‐shell (~5 nm) resulted the sintering of escaped Au NCs into larger crystals patched at the outside due to lack of chemical interaction (Supporting Information Figure S3). In addition, such a high temperature annealing has induced the loss of organic moieties in NH 2 SiO 2 ; thus generating small pores in silica, which are beneficial to provide the stability to tiny Au NCs in a confined environment against any kind of diffusion of NCs or merging into larger particles even at high annealing temperature 20 . Control reaction involving the reductive annealing at a higher temperature (700 ° C) led to an increase in Au NCs size ( d = 6 ± 2 nm) due to the promoted metal migration followed by sintering at elevated temperature (Supporting Information Figure S4).…”

Cucurbit[7]uril as Nanoreactor for the Fixation of CO2 with Oxirane: A Density Functional Theory Investigation

“…Moreover, confinements have been considered a powerful tool among researchers to modify the structure, electronic properties and efficiency of the catalyst. Also, confinement (or confined) catalysis has gained vast interest in heterogeneous catalysis as it influence the reactivity [25–47] . Presently, confined catalysis has been widely used for the catalytic conversion of CO 2 into useful chemicals [48–51] .…”