This study investigates the structural stability of small Pd@Pt core@shell octahedral nanoparticles (NPs) and their shell thickness dependent catalytic performance for p-chloronitrobenzene hydrogenation with H2. The 6–8 nm Pd@Pt octahedral NPs are prepared by a sequential reduction method, and the characterization results confirm that Pd@Pt octahedral NPs with one to four atomic Pt layers can be controllably synthesized. The Pd@Pt octahedral NPs with one atomic Pt layer demonstrate excellent structural stability with the maintenance of core–shell structures as well as high catalytic stability during cycle to cycle catalytic p-chloronitrobenzene hydrogenation reactions. The alumina-supported Pd@Pt octahedral NPs illustrate a superior catalytic performance relative to individual Pt and Pd and their physical mixtures. Theoretical calculations by density functional theory suggest that the unexpected structural stability for Pd@Pt octahedral NPs with thin Pt shells and their corresponding catalytic stability during hydrogenation reactions can be ascribed to the strong binding between Pt surfaces and reactants/products in catalytic reactions. The enhanced catalytic performance of Pd@Pt octahedral NPs possibly originates from the core–shell interaction, which adjusts the electronic state of surface Pt atoms to be suitable for selective p-chloronitrobenzene hydrogenation.Keywords: core−shell; hydrogenation; nanocatalysis; p-chloronitrobenzene; Pd@Pt

In this work, Pd–NiO@SiO2 core–shell mesoporous nanocatalysts with ~4 nm Pd–NiO heteroaggregate nanoparticle cores and ~17 nm mesoporous silica shells were successfully synthesized by a sol–gel method. The surfactant-capped PdNi alloy nanoparticles were coated with SiO2 through hydrolysis of tetraethylorthosilicate to obtain PdNi@SiO2 nanoparticles, and the mesoporous Pd–NiO@SiO2 core–shell nanocatalysts were formed after removal of surfactants by calcination at 500 °C and subsequent H2 reduction at 200 °C. The characterization results by XRD, TEM and BET revealed that Pd–NiO@SiO2 nanocatalysts were highly stable with the maintenance of intact core–shell structures under high-temperature thermal treatments. The Pd–NiO@SiO2 nanocatalysts illustrated a superior catalytic performance for p-chloronitrobenzene hydrogenation with H2 to the control Pd@SiO2 nanocatalysts. The catalytic performance enhancement of Pd–NiO@SiO2 nanocatalysts is ascribed to the strong interaction between Pd and NiO in the cores, where the interfaces may be beneficial for hydrogenation reactions.

Architecture controlled PtNi@mSiO2 and Pt–NiO@mSiO2 mesoporous core–shell nanocatalysts were synthesized for selective p-chloronitrobenzene hydrogenation to p-chloroaniline. Tetradecyl trimethyl ammonium bromide (TTAB) capped PtNi nanoparticles (NPs) were coated by SiO2 through the hydrolysis of tetraethylorthosilicate. The resultant PtNi@SiO2 core–shell NPs were calcined to remove TTAB to obtain mesoporous Pt–NiO@SiO2 core–shell nanocatalysts (Pt–NiO@mSiO2), which were subsequently reduced by hydrogen to form mesoporous PtNi@SiO2 core–shell nanocatalysts (PtNi@mSiO2). The relevant characterizations such as XRD, TEM, H2-TPR, and BET confirm that the PtNi@mSiO2 NPs consist of PtNi alloy nanoparticle cores and mesoporous SiO2 shells while the Pt–NiO@mSiO2 NPs contain Pt–NiO heteroaggregate nanoparticle cores and mesoporous SiO2 shells. The catalytic results for selective hydrogenation of p-chloronitrobenzene show that the selectivity of p-chloroaniline formation over the PtNi@mSiO2 and Pt–NiO@mSiO2 nanocatalysts is significantly improved relative to that of control Pt@mSiO2 nanocatalysts. Moreover, the PtNi@mSiO2 and Pt–NiO@mSiO2 nanocatalysts demonstrate high stability during multiple cycles of catalytic hydrogenation reactions. The enhanced catalytic performance is ascribed to the metal–metal interaction for the PtNi@mSiO2 catalysts and metal–oxide interaction for the Pt–NiO@mSiO2 catalysts.

Ordered tungsten and aluminum co-doped mesoporous KIT-6 catalysts (W-Al-KIT-6) with different Si/Al molar ratios were successfully synthesized by a one-pot synthesis method. The obtained W-Al-KIT-6 catalysts were tested for catalytic conversion of 1-butene and ethene to propene via isomerization of 1-butene to 2-butene and subsequent cross metathesis of 2-butene and ethene. Various characterization techniques, such as ICP-OES, XRD, BET, TEM, Raman, XPS and NH3-TPD, were used to characterize the catalysts. The introduction of Al did not change the mesoporous structure of KIT-6 when the nominal Si/Al was 10, 20 or 30. Moreover, the sample demonstrated a larger amount of acidic sites. The W-Al-KIT-6 catalysts with suitable Si/Al ratios illustrated a superior catalytic performance to W-KIT-6 catalyst. The origin of catalytic performance enhancement over W-Al-KIT-6 catalysts is preliminarily discussed and ascribed to the highly disperse W species and a large amount of acidic sites. The acidic sites were formed by the introduction of a suitable amount of Al in the W-KIT-6 framework, which accelerated the isomerization of 1-butene to 2-butene and promoted the cross metathesis of 2-butene and ethene to propene.

Tungsten substituted mesoporous FDU-12 (W-FDU-12) catalysts were synthesized by a one-pot hydrothermal process using F127 as the structure directing agent. The studies of TEM, SAXS and BET illustrated that the highly ordered mesoporous structure of FDU-12 was maintained in the doped W-FDU-12 samples. XPS studies revealed that a high concentration of W5+ species appeared in doped W-FDU-12 catalysts whereas supported WO3/FDU-12 and WO3/SiO2 catalysts only contained W6+ species. Tandem catalytic conversion of 1-butene and ethene to propene through isomerization of 1-butene to 2-butene and consecutive cross metathesis of 2-butene and ethene in a fixed-bed reactor at different temperatures and atmospheric pressure was used to evaluate the catalytic performance of the W-FDU-12 catalyst, combined with MgO. The catalytic results showed that the doped W-FDU-12 illustrated a superior catalytic performance relative to the supported WO3/FDU-12 and WO3/SiO2 catalysts. The higher metathesis activity of W-FDU-12 catalysts can be ascribed to the good dispersion of W species and the incorporation of W species into the framework of FDU-12, forming a substantial amount of W5+, which was beneficial for the cross metathesis of 2-butene and ethene to propene.

Mesoporous W-KIT-6 catalysts with various Si/W ratios were prepared by one-pot direct hydrothermal reactions and were studied in metathesis of 1-butene and ethene to propene. The catalysts were characterized by N2 physical adsorption, XRD, TEM, UV-DRS, FTIR and Raman. The characterization results showed that tungsten species could be finely dispersed in the framework of KIT-6 at Si/W ratios higher than 25, while a large amount of bulk WO3 was present on W-KIT-6 catalyst with a Si/W ratio of 15. The 1-butene conversion and propene selectivity increased with the decreasing Si/W ratio from 80 to 25 due to more active tungsten oxides species generated, and then decreased when the Si/W ratio further declined to 15 caused by the formation of inactive bulk WO3. The optimal Si/W ratio was 25 for obtaining highly dispersed active W species and excellent catalytic performance.

Bimetallic PdM (M = Ni, Co, Fe) nanoparticles were synthesized using butyllithium as a reductant, and were used to prepare Pd–MxOy/KIT-6 catalysts by appropriate oxidation and reduction treatments. These catalysts showed higher selectivity in the hydrogenation of p-chloronitrobenzene to p-chloroaniline than Pd/KIT-6. Relevant characterization was conducted using X-ray diffraction, transmission electron microscopy, N2 adsorption–desorption, X-ray photoelectron spectroscopy, inductive coupled plasma emission spectrometer, and H2 temperature-programmed reduction.

Molybdenum-doped mesoporous SBA-15, mesoporous SBA-15-supported MoO3/SBA-15, and traditional silica-supported MoO3/SiO2 were successfully synthesized. Various techniques, such as XRD, TEM, BET, UV-DRS, Raman, XPS and IR, were used to characterize the above obtained materials. The studies of TEM, XRD and BET confirmed that the highly ordered mesoporous structure of SBA-15 was maintained in the doped Mo-SBA-15 whereas supported MoO3/SBA-15 showed a significant reduction in surface area due to the deposition of MoO3 nanoparticles into the SBA-15 channels. XPS studies revealed that a high concentration of Mo5+ species appeared in doped Mo-SBA-15 whereas supported MoO3/SBA-15 and MoO3/SiO2 only contained Mo6+ species. The metathesis reaction of 1-butene and ethene to propene was used to evaluate the catalytic performance of Mo-containing materials. The doped Mo-SBA-15 illustrated a superior catalytic performance over the supported MoO3/SBA-15 and MoO3/SiO2 catalysts. The enhancement of catalytic performance for doped Mo-SBA-15 was assigned to the incorporation of Mo species into the SBA-15 framework. Due to the doping method, Mo-SBA-15 exhibited a well-ordered mesoporous structure, a high surface area, and a high concentration of Mo5+ species, which is beneficial to the catalytic performance for metathesis reactions.

Bimetallic Au3M (M = Ni, Co, Fe) nanoparticles (NPs) were synthesized by reducing a mixture of Au3+ and metal cations (Ni2+, Co2+, Fe3+) by butyllithium in the presence of oleylamine. Au3M NPs were then deposited onto a commercial SiO2 support. The Au3M/SiO2 samples were not particularly active in the catalytic reduction of p-nitrophenol unless they were converted into Au–MOx/SiO2 after appropriate thermal treatment in air. The Au–MOx/SiO2 catalysts showed good thermal stability and significantly higher p-nitrophenol conversions than Au/SiO2. Relevant characterization was conducted employing X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and ultraviolet–visible spectroscopy.

Tungsten-doped mesoporous KIT-6 (W-KIT-6), mesoporous silica supported WO3/KIT-6, and traditional silica supported WO3/SiO2 catalysts have been successfully synthesized and tested for catalytic metathesis of 1-butene and ethene to propene. The resultant materials were comprehensively characterized by XRD, BET, TEM, UV–DRS, IR, XPS, H2-TPR, and TGA. For W-KIT-6 catalysts, high concentration of W5+ species by XPS and the difficulty of reduction of W species by TPR suggested the incorporation of W species into the KIT-6 framework. The studies of small-angle XRD, BET, and TEM illustrated that the 3D ordered mesoporous structure and their high surface area of KIT-6 were maintained in W-KIT-6. The doped W-KIT-6 illustrated superior catalytic performance to the supported WO3/KIT-6 and WO3/SiO2 catalysts. The origin of catalytic performance enhancement for W-KIT-6 was preliminarily discussed and was assigned to the incorporation of W species into KIT-6 framework. This study demonstrated the influence of neighboring environment of active components on catalytic performance and was helpful to design metathesis catalysts.

PtM (M = Ni, Fe, Co) alloy nanoparticles were synthesized by a liquid phase reduction method employing butyllithium as a reducing agent. The alumina-supported PtM materials were then used as precursors to obtain the Pt-MxOy/Al2O3 catalysts through calcination. The influence of synthesis conditions of PtM alloy nanoparticles and the catalytic performance of the Pt-MxOy/Al2O3 catalysts for p-chloronitrobenzene hydrogenation reaction were investigated. The relevant characterizations such as XRD, XPS, and TEM were conducted for PtM alloy nanoparticles and Pt-MxOy/Al2O3 catalysts, and the result showed that the PtM nanoparticles are uniform alloy. Moreover, compared to PtM alloy nanoparticles, the Pt particle size of Pt-MxOy/Al2O3 using PtM alloy nanoparticle precursors did not increase by calcination, indicating good thermal stability. The catalytic activities of Pt-MxOy/Al2O3 for p-chloronitrobenzene hydrogenation reaction were significantly higher than that of control Pt/Al2O3 catalysts due to its strong Pt-MxOy interaction.

Pd@SiO2 core–shell nanoparticles were successfully synthesized by a sol–gel method. Tetradecyl trimethyl ammonium bromide capped Pd nanoparticles were coated with SiO2 through the hydrolysis of tetraethylorthosilicate. The as-synthesized Pd@SiO2 particles consist of Pd cores with a particle size of 3.7 nm and SiO2 shells with a thickness from 10 to 30 nm at different synthetic conditions. The Pd@SiO2 nanocatalysts with 1.9–2.4 nm mesoporous SiO2 shells were generated after removal of tetradecyl trimethyl ammonium bromide from Pd@SiO2 core–shell particles by calcination and following H2 reduction. The relevant characterizations such as XRD, TEM, FT-IR, and BET were carried out for Pd@SiO2 particles, and the results showed that the Pd@SiO2 nanocatalysts were highly stable with the protection of silica shells, and the Pd cores did not increase during thermal treatment and H2 reduction. The studies of catalytic CO oxidation at high temperatures and hydrogenation of nitrobenzene with H2 were tested for Pd@SiO2 nanocatalysts, and the results indicated that Pd@SiO2 nanocatalysts were stable at high temperatures and highly active and stable for hydrogenation of nitrobenzene even after long time use.

Au–Pt bimetallic nanoparticles (NPs) were synthesized by a seeded growth method. Au NPs with different sizes were obtained by reducing HAuCl4 with butyllithium, and AuPt bimetallic NPs were synthesized by reducing H2PtCl6 with oleylamine using the pre-synthesized Au NPs as seeds. The size of Au seeds was found to be a key factor on the structure of Au–Pt bimetallic NPs. Using big Au NP seeds (8 nm or 12 nm) resulted in the formation of Au–Pt dendritic structures. While relatively small Au NPs (3 nm) were used as seeds, the fast atomic diffusion inside relatively small bimetallic NPs will result in an Au–Pt alloy formation.

In this work, Pd–NiO@SiO2 core–shell mesoporous nanocatalysts with ~4 nm Pd–NiO heteroaggregate nanoparticle cores and ~17 nm mesoporous silica shells were successfully synthesized by a sol–gel method. The surfactant-capped PdNi alloy nanoparticles were coated with SiO2 through hydrolysis of tetraethylorthosilicate to obtain PdNi@SiO2 nanoparticles, and the mesoporous Pd–NiO@SiO2 core–shell nanocatalysts were formed after removal of surfactants by calcination at 500 °C and subsequent H2 reduction at 200 °C. The characterization results by XRD, TEM and BET revealed that Pd–NiO@SiO2 nanocatalysts were highly stable with the maintenance of intact core–shell structures under high-temperature thermal treatments. The Pd–NiO@SiO2 nanocatalysts illustrated a superior catalytic performance for p-chloronitrobenzene hydrogenation with H2 to the control Pd@SiO2 nanocatalysts. The catalytic performance enhancement of Pd–NiO@SiO2 nanocatalysts is ascribed to the strong interaction between Pd and NiO in the cores, where the interfaces may be beneficial for hydrogenation reactions.

Molybdenum-doped mesoporous SBA-15, mesoporous SBA-15-supported MoO3/SBA-15, and traditional silica-supported MoO3/SiO2 were successfully synthesized. Various techniques, such as XRD, TEM, BET, UV-DRS, Raman, XPS and IR, were used to characterize the above obtained materials. The studies of TEM, XRD and BET confirmed that the highly ordered mesoporous structure of SBA-15 was maintained in the doped Mo-SBA-15 whereas supported MoO3/SBA-15 showed a significant reduction in surface area due to the deposition of MoO3 nanoparticles into the SBA-15 channels. XPS studies revealed that a high concentration of Mo5+ species appeared in doped Mo-SBA-15 whereas supported MoO3/SBA-15 and MoO3/SiO2 only contained Mo6+ species. The metathesis reaction of 1-butene and ethene to propene was used to evaluate the catalytic performance of Mo-containing materials. The doped Mo-SBA-15 illustrated a superior catalytic performance over the supported MoO3/SBA-15 and MoO3/SiO2 catalysts. The enhancement of catalytic performance for doped Mo-SBA-15 was assigned to the incorporation of Mo species into the SBA-15 framework. Due to the doping method, Mo-SBA-15 exhibited a well-ordered mesoporous structure, a high surface area, and a high concentration of Mo5+ species, which is beneficial to the catalytic performance for metathesis reactions.