Core-shell Catalysts Research Articles

Rational construction of 3D MoNi/NiMoOx@NiFe LDH with rapid electron transfer for efficient overall water splitting

It is still a great challenge to develop efficient and stable monolithic electrocatalysts with effective cost and abundant natural resources. Herein, a 3D structured NiFe layered double hydroxide (NiFeLDH) nanosheets rationally decorated on MoNi/NiMoOx nanowires array supported on Ni foam (MoNi/NiMoOx@NiFe LDH) are fabricated by an in-situ growth process is firstly reported. The results show that MoNi alloys can build a fast electron transport channel, thus improving the efficiency of the electrocatalyst. And the ultrathin NiFe LDH nanosheets would provide abundant exposed defects and catalytic active sites. A low overpotential (278 mV at 100 mA cm−2) and Tafel slope (44.7 mV dec−1) for oxygen evolution reaction, and overpotential (134 mV at 100 mA cm−2) and Tafel slope of 81.0 mV dec−1 for hydrogen evolution reaction in 1 M KOH can achieved. The overall water splitting system composed with 3D core–shell catalyst (MoNi/NiMoOx@NiFe LDH) and MoNi/NiMoOx nanowire arrays, which can obtain 100 mA cm−2 at a voltage of 1.64V, even superior to the benchmark of IrO2 (+) // Pt/C (-).

Engineering heterostructured Ni@Ni(OH)2 core-shell nanomaterials for synergistically enhanced water electrolysis

Heterostructure engineering of electrocatalysts provides a fascinating platform to reasonably manipulate the physicochemical properties of nanomaterials and further improve their catalytic efficiency for water electrolysis. However, it still remains a huge challenge to construct well-designed core-shell heterostructured catalysts and identify the key role of components for synergistic catalysis. Herein, a melted polymeric salt tactics was innovatively developed to synthesize heterostructured Ni@Ni(OH)2 core-shell nanomaterials supported on porous carbon (named Ni@Ni(OH)2/PC), wherein well-defined Ni(OH)2-Ni heterostructure plays a pivotal role in improving electrocatalytic activity for water reduction and oxidation. Besides, the stable porous carbon support functions as a highway for continuous electron transfer between Ni and Ni(OH)2, and simultaneously enables the full exposure of accessible active sites. The fabricated Ni@Ni(OH)2/PC exhibits outstanding bifunctional electrocatalytic performance for overall water splitting (ƞ10 = 1.55 V) with a good long-time stability. This work sheds new light on the design of engineering heterostructure of active bifunctional electrocatalysts for efficient energy conversion system.

θ-Fe3C dominated Fe@C core–shell catalysts for Fischer-Tropsch synthesis: Roles of θ-Fe3C and carbon shell

θ-Fe3C formation is usually considered as a deactivation factor in Fischer-Tropsch synthesis (FTS). Herein, θ-Fe3C dominated Fe@C core–shell catalysts were fabricated by pyrolysis of MIL-101(Fe) in ethyne for studying the roles of θ-Fe3C and carbon shell in FTS. In the evolution of MIL-101(Fe) into Fe@C catalysts, ethyne decomposition promotes the formation of θ-Fe3C and carbon shells around nanoparticles. By modulating the pyrolysis temperature, θ-Fe3C and/or α-Fe nanoparticles coated with different carbon shells (amorphous carbon, graphene or graphite shells) are acquired. By exploring the relationship between phase composition, structure of catalysts and FTS performance, it is found that θ-Fe3C is confirmed to be FTS active, which exhibits superior CC chain growth ability, i.e., higher C5+ selectivity and lower CH4 selectivity. Meanwhile, graphene shell inhibits iron nanoparticles aggregation and stabilizes the catalysts.

In situ preparation of palladium nanoclusters in cerium metal-organic frameworks Ce-MOF-808, Ce-UiO-66 and Ce-BTC as nanoreactors for room temperature Suzuki cross-coupling reaction

Abstract Ultrasmall thiolated palladium nanoclusters (Pdn(l-Cys)m) were immobilized inside the cavities of three kinds of cerium-organic frameworks (Ce-MOFs), with different structures and coordinate environments Ce-MOF-808, Ce-UiO-66 and Ce-BTC as sacrificial templates, donated as core-shell catalysts (Pdn@Ce-MOFs). The Pdn(l-Cys)m clusters have an ultrasmall size with narrow distribution (1 ± 0.2 nm), amounting to a cluster size of ca. 40 Pd atoms. For comparison, the palladium nanoclusters were loaded over the three Ce-MOFs with the well-known impregnation method, donated as supported material catalysts (Pdn/Ce-MOFs). Pdn@Ce-MOFs and Pdn/Ce-MOFs catalysts were utilized for the Suzuki cross-coupling reaction in an environmentally friendly solvent system (ethanol/water) under mild reaction conditions. Pdn@Ce-MOFs catalysts showed a remarkably high catalytic activity, due to the unique atomic packing structure and electronic properties of the palladium nanoclusters and the porosity and high specific surface area of the Ce-MOFs that have a great influence on the Suzuki cross-coupling reaction. Pdn@Ce-MOF-808 showed the highest turnover frequency (132,530.1 h−1) observed in the microwave-assisted Suzuki cross-coupling reaction with a very low Pdn loading (0.007 mol%, 0.0224 μmol Pd). Moreover, the Pdn@Ce-MOF-808 catalyst was readily recovered and recycled in at least five cycles without significant leaching or loss of catalytic activity. The particle size of the palladium nanoclusters inside the Ce-MOFs cavities is still around 1 ± 0.2 nm as confirmed by high resolution-transmission electron microscopy (HR-TEM) analysis. The crystallinity of the Pdn@Ce-MOFs and Pdn/Ce-MOFs catalysts was characterized by X-ray diffraction (XRD). The charge of unloaded palladium clusters (Pdn(l-Cys)m) and loaded palladium clusters in Ce-MOFs (Pdn@Ce-MOFs) and on Ce-MOFs (Pdn/Ce-MOFs) was investigated by X-ray photoelectron spectroscopy (XPS). Liquid nitrogen sorption isotherms at _196 °C for the prepared catalysts were measured to investigate their texture properties.

Robust charge carrier by Fe3O4 in Fe3O4/WO3 core-shell photocatalyst loaded on UiO-66(Ti) for urea photo-oxidation

In this study, a facile four-step hydrothermal method was utilized to deposit a core-shell structure on UiO-66(Zr/Ti) nanoflake (NFs) as a visible-light-driven photocatalyst. The core was magnetic Fe3O4 which served as a charge carrier coated with WO3 shell. The as-prepared photocatalyst was characterized by XRD, VSM, BET, FTIR, FE-SEM, UV–Vis-DRS, and PL techniques which proved successful deposition of Fe3O4@WO3 core/shell particle on UiO-66(Zr/Ti)-NFs. The obtained photocatalyst was subsequently applied for urea photo-oxidation. This magnetically recoverable photocatalyst exhibited superior activity due to its desirable band alignment, high stability, and generation of the photo-induced charge carriers, as well as providing a high surface area with low mass transfer resistance. Fe3O4 core acted as charge-carrier to transport the photogenerated charges of UiO-66(Zr/Ti)-NFs (electron-donor) to WO3 charge-collectors for effective photoconversion. The central composite design was applied to design the experiments matrix in which flow rate, pH, irradiation time, catalyst mass, and initial urea concentration were considered as operational factors. The optimized condition was found by defining the desirability function. 90% degradation percentage was achieved at 550 mL/min solution flowrate, pH = 7, 120 min irradiation time, 0.22 g UiO-66(Zr)-NFs-Fe3O4@WO3, and 40 mg/L of the initial concentration of urea with the desirability value of 0.89. Such a superior photocatalytic activity of UiO-66-Fe3O4@WO3 can be ascribed to the reclamation of Fe3O4 as a low bandgap carrier, which accelerated the conveyance of electrons and followed surpassing charge separation. Our present findings open a new strategy to produce a wide range of core-shell heterogeneous catalysts to be applied in photoreactors scale-up.

Carbonized core-shell diatomite for efficient catalytic furfural production from corn cob

A novel core-shell sulfonated tin-loaded diatomite catalyst, namely SO42−/CX-DMSn, was synthesized by hybridizing sulfuric carbon layer on DM-Sn intermedium. Lewis and Brønsted acid sites displayed on the surface of DM core and on the coated carbon layer of the SO42−/CX-DMSn, respectively. The catalyst exhibited excellent catalytic ability, high water tolerance and good stability. The existence of acid sites and carbon layer on the diatomite were characterized by SEM, EDS, XRD, XPS, NH3-TPD and Py-IR. The unique core-shell structure of the carbonized diatomite catalyst significantly improved furfural production from xylose and lignocellulosic matrix even in organic solvent-free process. Results showed 66.24% of furfural yield and 79.92% of selectivity were obtained from xylose, whilst 65.74% of furfural yield and 66.83% of selectivity from corn cob, implying the side reactions for furfural production were well-eliminated. This strategy shows a promising application for novel SO42−/CX-DMSn material toward effective and green catalytic production of furfural from biomass materials.

ZSM-5(8T)@γ-Al2O3 supported AlCl3 core-shell catalyst: Mechanism research on disproportionation reaction of methylchlorosilanes

Organosilanes are important raw materials in today’s technology-intensive industries, and dichlorodimethylsilane is the most widely used and the largest consumed organosilicon monomer. In this article, B3LYP/6–311++G(3df, 2pd) basis set was used to calculate the disproportionating mechanism for the synthesis of the dichlorodimethylsilane by core-shell catalysts ZSM-5(8T)@γ-Al2O3 (active sites 1–5) and AlCl3/ZSM-5(8T)@γ-Al2O3 (active sites 6 and 7). The theory analysis of results showed that the activation energies of the main reactions’ rate determine steps in ZSM-5(8T)@γ-Al2O3 (active sites 1–5) were: 144.37, 145.40, 149.82, 152.98, 167.39 kJ·mol−1, respectively. ZSM-5(8T)@γ-Al2O3 is the catalyst with ZSM-5 as the core layer and γ-Al2O3 as the shell layer, which were catalyzed by Bronsted acids in essence. The activation energies (AlCl3/ZSM-5(8T)@γ-Al2O3′ active sites 6 and 7) of the rate determine steps in the main reactions were: 96.51 and 127.98 kJ·mol−1, respectively. It can be seen from the energy data that the catalysts exhibit strong Lewis acid activity after loading AlCl3, which was more conducive to the formation of target products. By comparing the calculation of energies, vibration frequency analysis, discussion of frontier molecular orbital theory with the Electron Localization Function (ELF) and the Localized Orbital Locator (LOL) figures, the results were quite consistent with the proposed mechanism.

Pressure Dependency of the Hydrogen Oxidation and Evolution Reaction Kinetics on Carbon Supported Pt Catalysts Using a PEMFC Based Hydrogen Pump Approach

The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated in electrocatalysis due to its importance for a broad variety of applications especially in electrolysis and fuel cells (FC). While the HOR/HER reaction rates on platinum and platinum alloy catalysts in alkaline environment can be determined precisely with rotating disk electrode (RDE) measurements,1-3 this is not possible at low pH due to the two orders of magnitude higher reaction rates that far exceed the slow mass transport limited rates afforded by the RDE configuration.4 As a result, meaningful measurements are limited to setups with much faster mass transport rates, such as floating electrodes or proton exchange membrane (PEM) fuel cell based electrochemical H2-pumps,4, 5 but the effect of hydrogen partial pressure and kinetic limitations at higher overpotentials are not yet fully understood.Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC based H2-pump approach. The measurements were performed on assymmetric membrane electrode assemblies (MEAs), where the working electrode has an ultra-low loading of only 1-2 µgPt cmMEA -2 (resulting in roughness factors of ≈1.5 cmPt 2 cmMEA -2), while the counter electrode has a much higher loading of 0.4 mgPt cmMEA -2. Using a fuel cell test station that allows for the feed of humidified hydrogen on both the anode and the cathode, we were able to show that the apparent activation energy of the HOR/HER on platinum increases with increasing partial pressure of hydrogen, which we ascribe to a decreasing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to H2 partial pressure is also depending on the temperature. We further observed that the HOR quickly reaches a limiting current with increasing HOR overpotential, ηHOR (orange area in Fig. 1), which we ascribe to a change in the rate-determining step (rds) from the Volmer reaction (i.e., the electrochemical oxidation of adsorbed H) at low ηHOR to the Tafel reaction (i.e., the dissociative adsorption of H2 on the Pt surface) at higher ηHOR. The Tafel-limited current shows a pressure-independent activation energy and a direct proportionality to the partial pressure of hydrogen, which is exactly what one would expect for the dissociative Langmuir adsorption as the rds. Finally, the HOR/HER reaction rates on a Pt3Co alloy and a Ru@Pt core-shell catalyst will be compared to that on a Pt/C catalyst.

Effect of Pt Solution Concentration during Direct Preparation of Core–Shell Pt Cathode in MEA Catalyst Layer for PEFC

Core-shell catalyst, which consists of thin Pt shell on non-Pt metal core, has been attracting attention as a low-Pt catalyst for polymer electrolyte fuel cell (PEFC). Typically, it is synthesized by using Cu-underpotential deposition (Cu-UPD) followed by surface limited redox replacement (SLRR). However, since this method is normally performed on a rotating disk electrode, it can only be prepared on a microgram scale. We have been developing a novel method to overcome this problem by directly performing Cu-UPD and SLRR to the catalyst layer made from the core material. Using this method, the reaction area is increased since the reaction occurs on particles in the three-dimensional network. In our previous study, we have reported that a Pd@Pt/C catalyst layer prepared by this method shows higher mass activity than conventional Pt/C. However, deposition at the surface of the catalyst layer appeared to be larger than in the interior of the layer. In this study, the effect of Pt ion solution concentration on the Pt deposition behavior was studied to develop highly active low-Pt catalyst layer by this novel method.Pd/C was used to fabricate a catalyst layer made of core material. Cu-UPD and SLRR was performed twice on this catalyst layer. In the SLRR process, Pt ion solution with various concentration was used. When a high concentration solution was used, large deposition at the surface was apparent. On the other hand, when a low concentration solution was used, the deposition at the surface was suppressed but the amount of Pt deposition was insufficient. By using both the low and high concentration solution, metallic luster caused by the large depositon of Pt was not observed, and a sufficient amount of Pt was deposited. The performance of a MEA prepared by this catalyst layer showed higher performance than that prepared by the previously reported method.

Insights into Electrochemical Behavior of Manganese Oxides in Catalyzing the Oxygen Reduction and Evolution Reactions and the Effect of Operation Conditions

To address the needs for clean energy, the regenerative fuel cell (RFC) and rechargeable metal-air batteries are considered as very promising approaches. Notwithstanding the abundant investigation in the past decades, their viability is still limited by the sluggish oxygen reactions on the oxygen electrode and the scarcity of conventional noble metal electrocatalysts (e.g. Pt, Ir, Ru, and their oxides) for the oxygen reduction (ORR) and oxygen evolution reactions (OER). For enhancing the bifunctional catalytic activity on the oxygen electrode, manganese oxides-based materials are under the spotlight due to their bifunctional electrocatalysis performance for both fuel cell (FC) and water electrolysis (WE) working modes in alkaline media at a low cost. However, the multivalent and diverse polymorphism of MnOx can lead to a series of complex electrochemical reactions that present different oxygen catalytic activities towards ORR and OER. To date, only a limited amount of study has been done on ultrafine manganese-based core-shell bifunctional oxygen catalysts and there remains the insufficient understanding of the ORR/OER catalysis mechanism as well as the redox pathway of the Mn sites. To address the knowledge gap in the literature, we have investigated the oxygen catalysis bifunctionality and the Mn site electrochemical behavior of Mn/Mn3O4 core-shell structural nanomaterial, and compared this to the commonly used β-MnO2, γ-MnO2 commercial catalysts in 5 M potassium hydroxide electrolyte.The crystalline structures of these commercial samples were characterized by X-ray diffraction (XRD). It should be noted that in the case of the nano core-shell structural sample, besides the diffraction from the Mn (core) and the Mn3O4 (shell) planes, the diffraction pattern also possesses intensive ramsdellite diffraction peaks. Its surface defects (oxygen vacancies), amorphous shell structure and hybrid Mn oxidation states lead to a facilitated potassium uptake in the MnOx polyhedrons1, suppression of Mn dissolution by the K-MnOx bonding structure reinforcement2, and higher oxygen adsorption capacity3 with an enlarged surface adsorption energy4.With the help of cyclic voltammetry of different types of manganese oxide, we have uncovered a series of electrochemical and chemical reactions involved with the change of Mn oxidation states. The difference in the crystallographic structure between different samples is revealed in their electrochemical response (Fig. 1 and Fig. 2). In terms of potassium uptake, it is favoured in ramsdellite crystalline through their wide 1 X 2 tunnels and in the Mn3O4 amorphous area by absorption instead of the narrow 1 X 1 tunnels in pyrolusite crystalline. This crystallographic dissimilarity results in different Mn site oxidation and reduction pathways. After initial cycling, some parallel behaviors among these oxides can be observed.In addition, we also investigated the effect of the operating voltage range. In comparison to the ORR or OER operating mode, when the voltage range was extended to cycle between the ORR and OER potential range, the Mn/Mn3O4 electrodes delivered enhanced O2 and HO2 - reduction activities along with boosted OER performance.With respect to O2 in the electrolyte, the dissolved O2 not only acts as a reactant for the ORR reaction but also influences the OER activity. Notwithstanding the oxygen molecule may block the active sites on the bulk catalysts (e.g., β-MnO2 and γ-MnO2) and thereby diminish their OER capability by half, the Mn/Mn3O4 shows double the OER current density in O2 saturated 5 M KOH electrolyte.These interesting results require a better understanding of the Mn site oxidation and reduction pathways along with oxygen reaction catalysis. The influence of operational conditions should also be considered in the protocol for bifunctional catalytic activity assessment. Among the investigated samples, the structural crystalline ramsdellite shows better electrochemical behavior and performance in comparison to the pyrolusite material. In particular, the core-shell structural Mn/Mn3O4 offers a potential approach to meet the needs of the practical reversible oxygen reactions in the RFC without losing OER catalytic activity or the need to purge the electrolyte between charge and discharge.References P. H. Benhangi, A. Alfantazi, and E. Gyenge, Electrochim. Acta, 123, 42–50 (2014).G. Fang et al., Adv. Funct. Mater., 29, 1808375 (2019).S. Yan, Y. Xue, S. Li, G. Shao, and Z. Liu, ACS Appl. Mater. Interfaces, 11, 25870–25881 (2019).B. C. Han, C. R. Miranda, and G. Ceder, Phys. Rev. B - Condens. Matter Mater. Phys., 77, 075410 (2008). Figure 1

Ag-Pd core–shell electrocatalysts for ethanol oxidation and oxygen reduction reactions in alkaline medium

Ag-Pd core–shell nanoparticles with different thickness of Pd shell are successfully prepared applying polyol methods with 1,2-propanediol as a solvent and reducing agent. A strongly electronic interaction between the sublayer Ag and surface Pd atoms are measured for the Ag-Pd core–shell particles with lower Pd/Ag atom ratios (Pd:Ag ⩽ 3:10), which derives from the near-surface alloying effect. Upon increasing the thickness of Pd shell, the effect of Ag core on surface Pd atoms become weaker, correspondingly the surface Pd atoms present the features closed to pure Pd. All Ag10@Pdx/C core–shell catalysts exhibit improved ethanol oxidation reaction (EOR) activities compared to pure Pd/C. Here, the EOR activities of Ag10@Pdx/C catalysts decrease when adding the thickness of the Pd shell, in which the Ag10@Pdx/C with the lowest Pd/Ag ratios (Pd/Ag = 1:10) shows the highest i p value ten times that of Pd/C. The participation of Ag as a core with low electronegativity and larger lattice constant can increase the electron densities and bring the lattice expansion of surface Pd atoms, due to the electron effect and the geometric effect, which effectively strengthen the bonding energy of reaction species on Pd surface and benefit the OH– adsorption for EOR, especially in lower Pd/Ag ratios than forming a monolayer Pd. For oxygen reduction reaction (ORR), the Ag10@Pdx/C samples also exhibit enhanced activities than pure Pd/C. Differently, there is a volcanic relation between the mass activities and the Pd/Ag atom ratios, where the Ag10@Pd3/C with about a monolayer Pd shell exhibits highest ORR activity. The improved ORR activities may be relay on the appropriate OH– adsorption, the exposed surface Ag atoms and the quick transformation of OH from Pd atoms.

First-principles investigation of methanol synthesis from CO2 hydrogenation on Cu@Pd core–shell surface

In this paper, the density functional theory calculation is used to investigate the reaction mechanisms of CO2 hydrogenation to CH3OH on the Cu@Pd core–shell surface. In particular, the possible adsorption sites, structural parameters, and adsorption energies of all intermediates are determined. All the reaction energies and the activation barriers of elementary steps involved in the possible hydrogenation mechanisms, including the HCOO pathway, the COOH pathway, and the RWGS + CO-Hydro pathway, are studied in detail. The calculated activation barrier of the rate-determining step is 1.84 eV for the HCOO pathway, 1.45 eV for the COOH pathway, and 1.17 eV for the RWGS + CO-Hydro pathway. Therefore, the most favorable hydrogenation mechanism on the Cu@Pd core–shell surface is following the reverse water–gas shift reaction followed by CO hydrogenation and will be completed via the route of CO2* → trans-COOH* → cis-COOH* → CO* → HCO* → HCOH* → H2COH* → CH3OH*. Compared with Cu-based catalysts such as Cu(111), Cu29 cluster, Cu19 cluster, and Cu(211), the Cu@Pd core–shell catalyst is demonstrated to have higher catalytic activity toward CO2 hydrogenation to CH3OH.

Carbon quantum dots decorated heteroatom co-doped core-shell Fe0@POCN for degradation of tetracycline via multiply synergistic mechanisms

In this study, novel core-shell catalyst with a new ternary heterostructure was synthesized (Fe0@POCN/CQDs) for the degradation of tetracycline (TC). The TEM results showed that the Fe0 particles were wrapped in POCN material and many nano CQDs were uniformly dispersed in the material. The new ternary nanocomposite exhibits excellent photocatalytic activity for the removal of TC, which was approximately 4.76 times higher than that of GCN. The enhancement of photocatalytic activity was attributed to the effective heterojunction as well as the multiply synergistic effects of POCN combined with Fe0 and CQDs, which was beneficial for retardation of recombination rate of photogenerated electron–hole pairs and generation of more free radicals for the oxidation of TC. Besides, the reactive oxygen species (ROS) of h+, •O2− and •OH played pivotal roles in the degradation of TC by Fe0@POCN/CQDs during the photocatalytic reaction. At the same times, sulfate radical (SO4•-) and hydroxyl radical (•OH) highlighted the dominant role in the degradation process compared with other free radicals under persulfate hybrid mixture system (PS system), which was further confirmed by radical scavenger experiments and electron spin resonance (ESR) analysis. The response surface methodology (RSM) study indicated that the optimal removal parameters of tetracycline could reach 97.57% within 30 min under PS system. In addition, the possible degradation pathway intermediates of TC were studied by HPLC-MS and the reaction catalytic activity mechanism of Fe0@POCN/CQDs/persulfate system was discussed.

Local Coordination and Ordering Engineering to Design Efficient Core-Shell Oxygen Reduction Catalysts

For most core–shell nanostructured Pt-based alloy electrocatalysts, the local environment and configuration of surface Pt layers are randomly generated, which would hinder the further improvement of the utilization, activity and stability of Pt atoms. Herein, we have selected AuCu alloys with different structural ordering as substrates and deposited a Pt shell on their surface via the precise control of replacing Cu with Pt atoms. Various physical and electrochemical characterizations indicate that the structural ordering of an AuCu core could influence the distribution of surface Pt atoms due to the rearrangement of surface atoms. After optimization, the obtained catalysts displayed a high mass activity (0.75 A·mgPt−1) and specific activity (0.491 mA·cmPt−2) for the oxygen reduction reaction (ORR), which were nearly 7.5 times and 3.6 times those of commercial Pt/C, respectively. In addition, the catalysts could also exhibit a high stability with a negligible activity decay after 10,000 cycles of accelerated durability tests (ADTs). The density functional theory (DFT) calculation reveals that the precisely controlled Pt local environment could lower the Gibbs free energy barrier for the rate-determining step of ORR more than a random distribution could, thus enhancing the catalytic performance of the prepared catalysts.

Integrated fast-mass transfer and high Ti-sites utilization into hybrid amphiphilic TS@PMO catalyst towards efficient solvent-free methyl oleate epoxidation

For solvent-free catalytic oxidations, low efficiency resulted from poor mass transfer and insufficient utilization of active centers remains a tough problem. Herein, we demonstrate a novel hybrid core-shell catalyst (TS@PMO) with an amphiphilic shell and a Ti-surface-enriched mesoporous TiO2-SiO2 (TS) core to address this challenge. Such TS@PMO realizes its amphiphilicity via an ex situ formed periodic mesoporous organosilica (PMO) shell. Simultaneously, by a unique etching effect induced by organic precursor growth on [SiO4] tetrahedra in TS core, active Ti sites are facilely enriched in near-surface layer of core and extra mesoporous cavities are introduced for substrate reservation. When applied for solvent-free epoxidation of methyl oleate (MO) with H2O2, TS@PMO exhibits remarkably boosted catalytic activity (X = 90.2%) and epoxide selectivity (S = 70.2%), overwhelming the unmodified titanosilicate (X = 63.7%, S = 49.2%) and Ti-containing organosilica (X = 39.8%, S = 25.0%). Such result benefits from an evidently enhanced interphase mass transfer and sufficiently accessible active Ti sites in TS@PMO. On the one hand, amphiphilic PMO shell can efficiently collect hydrophobic substrate and H2O2, while abundant mesopores in the shell offer open-path for them to access active sites in the core; on the other hand, an increased framework Ti (IV) density and their surface-enrichment in TS core greatly improve the utilization of active Ti sites. This study effectively makes up for the deficiencies of slow mass transfer and insufficient utilization of conventional titanosilicates in biphasic reactions, which paves a new avenue to exploit other hybrid catalysts for high-efficiency solvent-free catalysis.

CeO2-coated Pd core-shell catalyst supported by mesoporous γ-Al2O3 for methane oxidation: The effect of high temperature pretreatment on the active Pd species

CeO2-coated Pd core-shell units with thin shell was synthesized and supported on mesoporous alumina via electrostatic method. The effect of high temperature pretreatment on the transformation of Pd species was investigated. The results revealed that CeO2-coated Pd nanoparticles were highly dispersed on mesoporous alumina, resulting in a 90% CH4 conversion at 392 °C. The catalyst pretreated at 600 ℃ was found to possess the best sintering resistance performance for CH4 oxidation and the final combustion temperature was as low as 570 °C. During the aging process of catalyst, the formation of Ce1-XPdXO2-δ solid-solution interface could effectively retard the decomposition of PdO and agglomeration of Pd species, consequently enhancing the catalytic performance of the material.

Efficient Co@Co3O4 core-shell catalysts for photocatalytic water oxidation and its behaviors in two different photocatalytic systems

Co@Co 3 O 4 photocatalyst was prepared in situ to investigate its water splitting performance and mechanism in two photocatalytic systems. • Photocatalytic activity of catalyst 1 in two systems were compared for the first time. • Catalyst 1 exhibited excellent photocatalytic performances. • Core-shell structure of Catalyst 1 can promote light capture and electron transmission. The photocatalytic performances of water oxidation were usually carried out in two different systems, photosensitizer and non-photosensitizer systems. There is few report about the same catalyst used in two systems and therefore it is of great significant to compare its role of the same catalyst in two systems and explore its different reaction mechanisms. In this work, first 4 kinds of metallic Co microparticles were obtained by different reduction methods through hydrothermal processes, and Co@Co 3 O 4 core-shell microparticles (1–4) were obtained from these metallic Co microparticles oxidized in air or in the reacting solution in situ. The core-shell structure of 1 was characterized by a series of analytical techniques. 1–4 exhibited excellent activities and stabilities in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system when they were used as catalysts for the photocatalytic water oxidation. The maximum O 2 evolution of 1 after 20 min’s illumination was 98.2 μmol, the O 2 yield was 65.5%, the initial turnover frequency was 6.6 × 10 −3 , the initial quantum efficiency ( Φ QY initial ) was 15.0% higher than Co (8.3%), CoO (11.7%), Co 2 O 3 (1.2%), Co 3 O 4 (2.8%) and 5 (2.2%). Even after the sixth run, the catalytic activity of recovered 1 still remained 85.1% of initial activity. In addition, the photocatalytic performances of 1 in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system and S 2 O 8 2− /light system were compared for the first time. In the non-photosensitizer system, 1 shows bifunctional roles and acts as optical absorption center and active catalytic site, and oxygen evolution rate is lower and it takes longer time. In the photosensitizer system, 1 only acts as a catalyst, the photosensitizer enhances the light absorption and promotes water oxidation reaction with higher O 2 yield and QE, meanwhile the photosensitizer brings the defect of high cost and instability into the system. Based on the results the two different reaction mechanisms were deeply discussed.

Fe based core\u2013shell model catalysts for the reaction of CO2 with H2

Fe@SiO2 core–shell model catalysts were investigated for the conversion of CO2 and H2 into CH4, CO and H2O. For evaluation of the effect of core size on the catalytic activity, samples with Fe particle sizes of 4, 6 and 8 nm were prepared. Fresh and spent catalysts were thoroughly characterized by X-ray diffraction, 57Fe Mössbauer spectroscopy, transmission electron microscopy, temperature programmed hydrogenation and X-ray photoelectron spectroscopy. As a result, the yield of the major product CO as well as CH4 was increased with Fe core size. Additionally, growing Fe cores led to stronger carburization and higher amount of reactive carbide entities, which drive the CH4 formation. Finally, formation of inactive bulk carbon deposition is strongly suppressed for the core–shell catalysts in comparison to bulk iron oxide catalysts used for CO2 hydrogenation.

Improved ethanol electro-oxidation at Ni@Pd/C and Ni@PdRh/C core–shell catalysts

This paper describes the synthesis and catalytic activity of carbon-supported Pd/C, PdNi/C, and PdNiRh/C catalysts with core–shell morphologies towards the oxidation of ethanol in alkaline media. The catalysts are synthesised by a chemical reduction method using sodium borohydride as the reducing agent. Electrochemical characterisations show that the addition of Ni and Rh enhances remarkably the electrocatalytic activity of the materials due to the bifunctional and electronic effects in PdNi/C and PdNiRh/C catalysts, and the synergetic effect of the bifunctional and core–shell morphology on the Ni@Pd/C and Ni@PdRh/C catalysts. Electrochemical impedance spectra reveal lower values of charge transfer resistance for Pd60Ni20Rh20/C and Ni20@Pd60Rh20/C. Chronoamperometric tests also reveal the Ni20@Pd60Rh20/C catalyst displays current densities 9.3 times higher than the Pd/C catalyst. The superior catalytic performance of the trimetallic core–shell Ni20@Pd60Rh20/C catalyst can result from the synergy between the metals and the presence of core–shell morphology.

Enhanced reduction reaction by Cu–Ag core-shell nanowire catalyst

The synthesis of well-defined copper core-silver shell nanowire by a modified hydrothermal method followed by immersion coating process is reported here. The HRTEM image of the individual Cu–Ag core-shell nanowire successfully demonstrates that the silver nanocrystals were effectively deposited over the surface of Cu nanowires, resulting in a core-shell structure by the immersion coating process. The catalytic performance of the prepared copper core-silver shell nanowires with different ratios have been investigated by taking the reduction of para nitrophenol to para aminophenol in presence of sodium borohydride as the model reaction. The Cu–Ag core-shell nanowires show remarkable catalytic activity toward the reduction of p-nitrophenol at room temperature with a maximum apparent rate constant value of 0.216 min−1. In comparison with the monometallic copper nanowires, the Cu–Ag core-shell nanowires exhibit 3.5 times enhancement in the catalytic performance and good recyclability for at least three cycles of the reduction reaction. Copper core-silver shell nanowire: a potential catalyst for the reduction of paranitrophenol was developed by a modified hydrothermal method followed by immersion coating process. Remarkable catalytic activity with a maximum apparent rate constant value of 0.216 min−1 was obtained. The use of eco-friendly reducing agent and the high purity synthetic approach makes this work significant.