Ultra-small Co nanoparticles embedded in hierarchically porous carbon were made in situ from metal–organic frameworks and used as catalysts in the Fischer–Tropsch synthesis.
Exchange-coupled nanocomposites of Nd2Fe14B/α-Fe are successfully synthesized by thermal decomposition and reductive annealing process. The phase size, composition, as well as magnetic properties, can be readily tuned by changing the ratio between Nd-Fe-B-oxide and α-Fe. The optimum properties shows coercivity of 12 000 Gs and an enhanced remanence of Mr/M3T = 0.63.
Because of the structural complexity and inhomogeneity, the effect of the coordination environment on the catalytic properties is underexplored in heterogeneous catalytic systems. To address this challenge, the atomically dispersed Pt is anchored on two Mo-based supports with similar morphology and particle size, that is, face-centered cubic-structured α-MoC and MoN. Spectroscopic and computational investigations demonstrate that the Pt atoms are coordinated with N atoms in Pt/MoN but with Mo atoms in Pt/α-MoC, leading to an entirely different catalytic performance in the oxygen reduction reaction (ORR). The Pt mass activity for Pt/MoN reaches 0.71 A/mgPt at 0.9 V [vs reversible hydrogen electrode (RHE)], which is 15 times higher Pt mass activity than that of Pt/α-MoC. Density functional theory calculations correlate the better ORR performance of Pt/MoN with the weaker adsorption of OH* because of the modulation of electronic properties of Pt by the coordination with N atoms. This study highlights the importance of controlling the coordination environment of metal atoms in heterogeneous (electro)catalysis and suggests that tuning the coordination environment could be an effective strategy in catalyst development.
A precise understanding of the catalytic surface of nanoparticles is critical for relating their structure to activity. For silica-supported Pt–Cr bimetallic catalysts containing nominal Cr/Pt molar ratios of 0, 1.9, and 5.6, a fundamental difference in selectivity was observed as a function of composition for propane dehydrogenation, suggesting different surface structures. The formation of bimetallic catalysts and the phases present were confirmed by synchrotron in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) of the nanoparticle as a function of reduction temperature. With the increasing reduction temperature, there is a systematic increase in the Pt LIII edge X-ray absorption near edge structure (XANES) energy, which is consistent with the incorporation of more metallic Cr into the nanoparticles. Pt LIII edge extended X-ray absorption fine structure (EXAFS) shows that the nanoparticles are Pt rich regardless of the reduction temperature, and XRD shows the presence of both Pt and Pt3Cr phases at temperatures below about 700 °C. For the latter, a full Pt3Cr intermetallic alloy forms after reduction at 800 °C. This work also presents a method for the characterization of the catalytic surface by the analysis of XAS difference spectra and XRD difference patterns of the (reduced and oxidized) catalysts. The surface analysis suggests that Pt3Cr formation begins at the surface, and at low reduction temperatures, a core–shell morphology is formed containing a Pt core with a Pt3Cr surface. By combining the XAS and XRD analyses with transmission electron microscopy (TEM) particle sizes, the thickness of the shell can be approximated. All evidence indicates that the shell thickness increases with the increasing reduction temperature until a full alloy is formed after reduction at about 800 °C but only if there is enough Cr2O3 available near Pt nanoparticles to form Pt3Cr. Catalysts containing a full monolayer coverage of Pt3Cr have higher olefin selectivity (>97%) compared with partially covered Pt surfaces (88%).
The introduction of active transition metal sites (TMSs) in carbon enables the synthesis of noble-metal-free electrocatalysts for clean energy conversion applications; however, there are often multiple existing forms of TMSs, which are of different natures and catalytic models. Regulating the evolution of distinctive TMSs is highly desirable but remains challenging to date. Anions, as essential elements involved in the synthesis, have been totally neglected previously in the construction of TMSs. Herein, the effects of anions on the creation of different types of TMSs are investigated for the first time. It is found that the active cobalt-nitrogen sites tend to be selectively constructed on the surface of N-doped carbon by using chloride, while metallic cobalt nanoparticles encased in protective graphite layers are the dominant forms of cobalt species with nitrate ions. The obtained catalysts demonstrate cobalt-sites-dependent activity for oxygen reduction reaction and hydrogen evolution reaction in acidic media. The remarkably enhanced catalytic activities approaching that of benchmark Pt/C in an acidic medium have been obtained on the catalyst dominated with cobalt-nitrogen sites, confirmed by the advanced spectroscopic characterization. This finding demonstrates a general paradigm of anion-regulated evolution of distinctive TMSs, providing a new pathway for enhancing performances of various targeted reactions related with TMSs.
Abstract A one‐step ligand‐free method based on an adsorption–precipitation process was developed to fabricate iridium/cerium oxide (Ir/CeO 2 ) nanocatalysts. Ir species demonstrated a strong metal–support interaction (SMSI) with the CeO 2 substrate. The chemical state of Ir could be finely tuned by altering the loading of the metal. In the carbon dioxide (CO 2 ) hydrogenation reaction it was shown that the chemical state of Ir species—induced by a SMSI—has a major impact on the reaction selectivity. Direct evidence is provided indicating that a single‐site catalyst is not a prerequisite for inhibition of methanation and sole production of carbon monoxide (CO) in CO 2 hydrogenation. Instead, modulation of the chemical state of metal species by a strong metal–support interaction is more important for regulation of the observed selectivity (metallic Ir particles select for methane while partially oxidized Ir species select for CO production). The study provides insight into heterogeneous catalysts at nano, sub‐nano, and atomic scales.
Abstract The great interest in fuel cells inspires a substantial amount of research on nonprecious metal catalysts as alternatives to Pt‐based oxygen reduction reaction (ORR) electrocatalysts. In this work, bimodal template‐based synthesis strategies are proposed for the scalable preparation of hierarchically porous M–N–C (M = Fe or Co) single‐atom electrocatalysts featured with active and robust MN 2 active moieties. Multiscale tuning of M–N–C catalysts regarding increasing the number of active sites and boosting the intrinsic activity of each active site is realized simultaneously at a single‐atom scale. In addition to the antipoisoning power and high affinity for O 2 , the optimized Fe–N–C catalysts with FeN 2 active site presents a superior electrocatalytic activity for ORR with a half‐wave potential of 0.927 V (vs reversible hydrogen electrode (RHE)) in an alkaline medium, which is 49 and 55 mV higher than those of the Co–N–C counterpart and commercial Pt/C, respectively. Density functional theory calculations reveal that the FeN 2 site is more active than the CoN 2 site for ORR due to the lower energy barriers of the intermediates and products involved. The present work may help rational design of more robust ORR electrocatalysts at the atomic level, realizing the significant advances in electrochemical conversion and storage devices.
Large carbon networks featuring hierarchical pores and atomically dispersed metal sites (ADMSs) are ideal materials for energy storage and conversion due to the spatially continuous conductive networks and highly active ADMSs. However, it is a challenge to synthesize such ADMS-decorated carbon networks. Here, an innovative fusion-foaming methodology is presented in which energetic metal-organic framework (EMOF) nanoparticles are puffed up to submillimeter-scaled ADMS-decorated carbon networks via a one-step pyrolysis. Their extraordinary catalytic performance towards oxygen reduction reaction verifies the practicability of this synthetic approach. Moreover, this approach can be readily applicable to a wide range of unexplored EMOFs, expanding scopes for future materials design.
An ancient material for magnetic resonance (MR) imaging: For the first time, Fe5C2 is prepared as colloidal stable nanoparticles with good aqueous stability. The nanoparticles boast strong magnetization, excellent chemical inertness, low toxicity, and one of the highest r2 relaxivities reported to date. These nanoparticles hold great potential in MR imaging as well as in other biomedical areas. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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