Search

MnO2 is a cheap and versatile material, being able to act as a catalyst for different electrochemical reactions. The addition of small amounts of Au nanoparticles (NPs) enables the possibility to explore the localized surface plasmon resonance (LSPR) effect to improve the photocatalytic properties of MnO2. The LSPR effect can accelerate several reactions under visible light irradiation, improving the charge separation and light harvesting properties of semiconductors. Here, by employing MnO2 nanowires decorated with Au NPs (MnO2–Au), we demonstrated that MnO2 catalytic activity can be greatly improved by the LSPR in a hybrid system. The catalytic properties of the semiconductor were demonstrated for the H2O2 oxidation as a model reaction. A clear increase in the current of H2O2 oxidation was verified for gold decorated nanowires under visible light irradiation, with an increase of 116% in the sensitivity compared with dark conditions. Mechanistic analysis indicates that the increased performance was related to a more efficient charge separation by plasmonic generated hot electrons and holes. Moreover, we demonstrated that the thermal effect had no significant contribution to the increase in activity observed, as it was verified no expressive increase in temperature under light irradiation with a thermal camera. We believe the results reported herein provide an approach to achieve improved light harvesting and performance in the visible range of the spectrum for semiconductors, inspiring the use of plasmonic heterostructures toward electrocatalysis. [ABSTRACT FROM AUTHOR]

The conversion of CO2 into CO is an important step in CO2 utilization to achieve clean fuels and value‐added chemicals. Herein, we explored the pyrolysis of zeolitic imidazolate framework‐8 (ZIF‐8) loaded with different amounts of Ni2+ to obtain Ni−Zn carbide (Ni3ZnC) embedded in N‐doped carbon. Ni is present in the intermetallic compound, while Zn excess remains on the N‐doped carbon. The Ni3ZnC phase catalyzes the selective hydrogenation of CO2 into CO via the reverse water gas shift reaction, reaching 100 % CO selectivity at ∼30 % CO2 conversion at 450 °C and atmosphere pressure (CO2 : H2=1 : 4, GHSV=30000 mL gcat−1 h−1). The methanation reaction of CO2/CO, which is usually favored over Ni catalysts, is suppressed. The selectivity to CO at the expense of CH4 is related to the stability of chemisorbed CO in the Ni3ZnC surface, which is lower compared to Ni surfaces. The Ni3ZnC@NC catalyst is selective towards CO over a wide range of conditions, including high pressure, that is usually required for the conversion of CO to hydrocarbons and alcohols via the Fisher‐Tropsch synthesis (FTS) process. Contrarily, a classical Ni/SiO2 catalyst prepared by impregnation produces CH4 under high pressure. [ABSTRACT FROM AUTHOR]

Bimetallic Ni−Pd and monometallic reference catalysts were prepared by decomposing organometallic precursors, Ni(cod)2 and Pd2(dba)3, leading to nanoparticles with sizes ranging from 3 to 6 nm. Two different synthesis procedures were followed: i) solution synthesis using capping ligand (hexadecylamine) followed by impregnation of pre‐formed nanoparticles on SiO2, called Sol‐immobilization (SI); and 2) direct precursor decomposition onto SiO2, without stabilizer, called Direct Decomposition (DD). Samples prepared by SI procedure are alloyed bimetallic nanoparticles, whereas samples obtained by DD one show phase segregation. Interestingly, DD samples show better activity for CO2 hydrogenation into CO (reverse water‐gas shift reaction ‐ RWGS) than SI ones. The best compromise between activity for CO2 activation (at lower temperature) and CO selectivity was achieved with Ni DD and NiPd DD catalysts. Moreover, the addition of palladium increased the concentration of surface undercoordinated sites, which chemisorb CO weakly, thus improving activity and selectivity, in opposition to other samples that chemisorb CO strongly, in multibond configuration. In the presence of Pd, different decomposition rates drives the formation of smaller and more active Ni clusters. The knowledge acquired here on the influence of synthesis conditions on the catalytic properties of Ni−Pd catalysts should guide us to better catalysts for CO2 transformations into valuable products. [ABSTRACT FROM AUTHOR]

Luminescent nanoparticles of Y2O3 doped with europium (Eu) and/or titanium (Ti) were synthesized using modified sol–gel routes. The crystalline cubic phase was confirmed using X‐ray powder diffraction (XRD). Particle morphology and size were evaluated using scanning and transmission electron microscopy. High‐resolution transmission electron microscopy showed that the synthesis method affected the average particle size and the Fourier transform of the images showed the lattice plane distances, indicating that the samples presented high crystallinity degree in accordance with the XRD pattern. The Ti valence was investigated using X‐ray absorption near edge spectroscopy and the tetravalent form was the dominant oxidizing state in the samples, mainly in Eu and Ti co‐doped Y2O3. Optical behaviour was investigated through X‐ray excited optical luminescence and photoluminescence under ultraviolet–visible (UV–vis) and vacuum ultraviolet (VUV) excitation. Results indicated that Eu3+ is the emitting centre in samples doped with only Eu and with both Eu and Ti with the 5D0→7F2 transition as the most intense, indicating Eu3+ in a noncentrosymmetric site. Finally, in the Eu,Ti‐doped Y2O3 system, Ti3+ (or TiIV) excitation was observed but no Ti emission was present, indicating a very efficient energy transfer process from Ti to Eu3+. These results can aid the development of efficient nanomaterials, activated using UV, VUV, or X‐rays. [ABSTRACT FROM AUTHOR]

We report the synthesis and application of a material composed of Fe3O4 nanoparticles embedded in N‐doped graphene sheets (Fe3O4@N‐doped graphene) as a negative electrode for Li batteries. We study the influence of N‐doped graphene on the storage capacity of Fe3O4 using different electrochemical techniques. The as‐prepared Fe3O4 materials presented high‐quality crystalline nanostructures. The N‐doped graphene sheets improve the conductivity between the Fe3O4 nanoparticles, allowing a faster charge transfer process than that for pure magnetite, as well as the presence of porous particles in the hybrid composite. The Fe3O4@N‐doped graphene material show the best Li storage capacity maintaining specific capacity values of 910 mA h g−1 during 150 cycles performed at 0.05 A g−1 and 850 mA h g−1 at 0.1 A g−1 during the following 50 cycles. The N‐doped graphene sheets resist the volume changes that occur during cycling processes in rate capability experiments. We provide a simple and novel method to obtain a material with a higher superficial area and conductivity between particles, allowing great performance as a negative electrode for Li batteries application. [ABSTRACT FROM AUTHOR]

Keywords: bimetallic nanoparticles; CO2 hydrogenation; Segregation; alloy; activity EN bimetallic nanoparticles CO2 hydrogenation Segregation alloy activity 2887 2887 1 06/08/20 20200605 NES 200605 B The Cover Feature b shows two types of bimetallic catalysts obtained by different methods and used in the CO SB 2 sb hydrogenation reaction. Bimetallic nanoparticles, CO2 hydrogenation, Segregation, alloy, activity. [Extracted from the article]

Single-atom catalysts (SACs) often exhibit dynamic responses to the reaction and pretreatment environment that affect their activity. The lack of understanding of these behaviors hinders the development of effective, stable SACs, and makes their investigations rather difficult. Here we report a reduction-oxidation cycle that induces nearly 5-fold activity enhancement on Pt/TiO 2 SACs for the reverse water-gas shift (rWGS) reaction. We combine microscopy (STEM) and spectroscopy (XAS and IR) studies with kinetic measurements, to convincingly show that the low activity on the fresh SAC is a result of limited accessibility of Pt single atoms (Pt 1 ) due to high Pt-O coordination. The reduction step mobilizes Pt 1 , forming small, amorphous, and unstable Pt aggregates. The reoxidation step redisperses Pt into Pt 1 , but in a new, less O-coordinated chemical environment that makes the single metal atoms more accessible and, consequently, more active. After the cycle, the SAC exhibits superior rWGS activity to nonatomically dispersed Pt/TiO 2 . During the rWGS, the activated Pt 1 experience slow deactivation, but can be reactivated by mild oxidation. This work demonstrates a clear picture of how the structural evolution of Pt/TiO 2 SACs leads to ultimate catalytic efficiency, offering desired understanding on the rarely explored dynamic chemical environment of supported single metal atoms and its catalytic consequences., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Controlling the selectivity of CO 2 hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO 2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH 4 formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH 4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO 2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO 2 hydrogenation to CO processes toward high-added-value products.

Luminescent nanoparticles of Y 2 O 3 doped with europium (Eu) and/or titanium (Ti) were synthesized using modified sol-gel routes. The crystalline cubic phase was confirmed using X-ray powder diffraction (XRD). Particle morphology and size were evaluated using scanning and transmission electron microscopy. High-resolution transmission electron microscopy showed that the synthesis method affected the average particle size and the Fourier transform of the images showed the lattice plane distances, indicating that the samples presented high crystallinity degree in accordance with the XRD pattern. The Ti valence was investigated using X-ray absorption near edge spectroscopy and the tetravalent form was the dominant oxidizing state in the samples, mainly in Eu and Ti co-doped Y 2 O 3 . Optical behaviour was investigated through X-ray excited optical luminescence and photoluminescence under ultraviolet-visible (UV-vis) and vacuum ultraviolet (VUV) excitation. Results indicated that Eu 3+ is the emitting centre in samples doped with only Eu and with both Eu and Ti with the 5 D 0 → 7 F 2 transition as the most intense, indicating Eu 3+ in a noncentrosymmetric site. Finally, in the Eu,Ti-doped Y 2 O 3 system, Ti 3+ (or Ti IV ) excitation was observed but no Ti emission was present, indicating a very efficient energy transfer process from Ti to Eu 3+ . These results can aid the development of efficient nanomaterials, activated using UV, VUV, or X-rays., (© 2019 John Wiley & Sons, Ltd.)