This work investigates the viability and outlines the current challenges in electrochemical quartz crystal nanobalance (EQCN) experiments on supported Pt catalysts. EQCN experiments involving Pt supported on 2-D “surface-treated graphite sputtered onto quartz crystal” (Pt/MFG-H) catalysts were compared to standard polycrystalline Pt (Ptpoly), which showed similarities in frequency versus potential trends; however, the Pt/MFG-H catalysts obtained higher frequencies due to the support capacitance. The physical characterizations (XRD and XPS) and electrochemical responses, mainly cyclic voltammetry in acidic media and the ferri/ferrocyanide couple, of the 2-D Pt/MFG-H were compared to the representative 2-D Pt supported on treated highly orientated pyrolytic graphite (Pt/HOPG-H), in order to make assertions on the similarities between the two catalysts. The XRD diffraction patterns and the XPS valence band structure for the treated and untreated MFG (-H and -P, respectively) and HOPG (-H and -P, respectively) demonstrated similarities. Nevertheless, the cyclic voltammograms and peak positions of the ferri/ferrocyanide couple between the treated and untreated MFG and HOPG catalysts were dissimilar. However, EQCN may be used qualitatively between the two different 2-D catalysts since the same trends in electrochemical responses before and after treatment of the MFG and HOPG catalysts were seen. Hence, the EQCN technique can be used in future studies as an alternative method to study degradation mechanisms of Pt and carbon for PEFCs.
Abstract Metamagnetism occuring inside a ferromagnetic phase is peculiar. Therefore, Sr 4 Ru 3 O 10 , a T C = 105 K ferromagnet, has attracted much attention in recent years, because it develops a pronounced metamagnetic anomaly below T C for magnetic fields applied in the crystallographic ab -plane. The metamagnetic transition moves to higher fields for lower temperatures and splits into a double anomaly at critical fields H c1 = 2.3 T and H c2 = 2.8 T, respectively. Here, we report a detailed study of the different components of the magnetization vector as a function of temperature, applied magnetic field, and varying angle in Sr 4 Ru 3 O 10 . We discover for the first time a reduction of the magnetic moment in the plane of rotation at the metamagnetic transition. The anomaly shifts to higher fields by rotating the field from H ⊥ c to H || c . We compare our experimental findings with numerical simulations based on spin reorientation models taking into account magnetocrystalline anisotropy, Zeeman effect and antisymmetric exchange interactions. While Magnetocrystalline anisotropy combined with a Zeeman term are sufficient to explain a metamagnetic transition in Sr 4 Ru 3 O 10 , a Dzyaloshinskii-Moriya term is crucial to account for the reduction of the magnetic moment as observed in the experiments.
The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (ω) and momentum (k) and the probability to find them in a particular state with given ω and k is described by the spectral function A(k, ω). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample. In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.
Lithium-manganese-based compounds are promising intercalation host materials for aqueous battery systems due to their synergy with high ionic conductive aqueous electrolytes, safety, eco-friendliness, and low cost. Yet, due to poor electrical conductivity and trapping of diffused electrolyte cations within its crystal formation, achieving optimum cycle stability and rate capability remains a challenge. This unfortunately limits their use in modern day high-powered devices, which require quality output with high reliability. Here, the authors propose a facile method to produce LiMn2O4 and LiFe0.5Mn0.5PO4 and compare their structural stability and corresponding electrochemical performance by controlling the interfacial layer through multi-walled carbon nanotubes' (MWCNTs) infusion. High-resolution scanning electron microscopy results revealed that the active particles were connected by MWCNT via the formation of a three-dimensional wiring network, suggesting that stronger interfacial bonding exists within the composite. As a result, the conducting composite decreases the electron transport distance with an increased number of active sites, thus accelerating the lithium ion intercalation/de-intercalation process. Compared to C/LMO with a Rct of 226.3 Ω and change transfer (io) of 2.75 × 10-3, the C/LFMPO-composite has a reduced Rct of 138 Ω and enhanced rate of 1.86 × 10-4 A cm-2. The faster kinetics can be attributed to the unique synergy between the conductive MWCNTs and the contribution of both single-phase and two-phase regions in Li1-x(Fe,Mn)PO4 during Li+ extraction and insertion. The electrochemical features before and after modification correlate well with the interplanar distance of the expanded manganese and manganese phosphate layers shown by their unique surface features, as analyzed by advanced spectroscopy techniques. The results reveal that MWCNTs facilitate faster electron transmission whilst maintaining the stability of the host framework, which makes them favorable as next generation cathode materials.
The temperature dependence of the exchange bias field and coercive field was studied in a polycrystalline NiFe layer coupled with a diluted NiO layer. The temperature behavior of both fields is modified by cooling the bilayer below the Curie, Neel, and/or blocking temperatures. Below these temperatures, the presence of double hysteresis loops demonstrates the key role of the NiFe multidomain state during the cooling procedure.
The identification of electronic states and the analysis of their evolution with $n$ is key to understanding $n$-layered ruthenates. To this end, we combine polarization-dependent O 1$s$ x-ray absorption spectroscopy, high-purity Sr${}_{n+1}$Ru${}_{n}$O${}_{3n+1}$ ($n=1,2,3$) single crystals, and ab initio and many-body calculations. We find that the energy splitting between the empty ${x}^{2}\ensuremath{-}{y}^{2}$ and $3{z}^{2}\ensuremath{-}1$ state is considerably smaller than previously suggested and that, remarkably, Sr bands are essential to understanding the spectra. At low energy, we identify the main difference among the materials with a substantial rearrangement of ${t}_{2g}$ orbital occupations with increasing $n$. This rearrangement is controlled by the interplay of Coulomb repulsion, dimensionality, and changes in the ${t}_{2g}$ crystal field.
Mn substituted Mn x Zn1-x Co2O4 (x = 0, 0.3, 0.5, 0.7, 1) oxides were synthesized by a facile co-precipitation method followed by calcination at 600 °C. The presence of manganese ions causes appreciable changes in the structural and magnetic properties of the Mn-substituted ZnCo2O4. The morphologies, structures, and electronic properties of Mn-Zn-Co oxide microspheres were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The X-ray diffraction and Fourier transform infrared spectroscopy results confirmed the formation of spinel Mn x Zn1-x Co2O4. It was shown that the Mn-Zn-Co oxide microspheres increase in size and become regular in shape with increasing Mn concentration with the crystal size lying in the range from 19.1 nm to 51.3 nm. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature and 10 K. The saturation magnetization is observed to increase with increasing Mn concentration from x = 0 to x = 1.
Abstract A series of Pd nanoparticles supported on V 2 O 5 immobilized on functionalized carbon, % Pd (1, 3, and 5) and % V 2 O 5 (10, 20, and 30), were prepared by sodium borohydride‐assisted microwave polyol synthesis for glycerol oxidation reaction (GlyOR) in an alkaline medium. Electrocatalysts loading, temperature, V 2 O 5 immobilization, and their synergistic effect on the electrocatalytic performance are systematically studied. The electrocatalysts′ morphology and electronic properties were investigated using X‐ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, Transmission electron microscopy, and X‐ray photoelectron spectroscopy. A significantly improved GlyOR is observed with increased V 2 O 5 content and Pd percentage. The 5 %Pd/30 %V 2 O 5 ‐fAC showed the highest mass activity of 2157.3 mA . mg Pd −1 , a more negative onset potential of 0.62 V RHE , versus the commercial equivalent, and possessed high stability and durability. The increase in electrocatalytic activity is attributed to the effective immobilization of V 2 O 5 on fAC efficient synergism between Pd and V 2 O 5 , strong metal support interaction (SMSI), and great exposure of the electroactive sites. The results herein contribute significantly to the understanding of the physicochemical and electrochemical effects of metal oxide immobilization, microwave irradiation, % Pd/% Metal oxide optimization, and SMSI on metal oxide‐carbon hybrid electrocatalysts for GlyOR, opening new avenues for fabricating high‐performance direct alkaline glycerol fuel cells.
The transformation of various organic molecules into value-added chemicals has been driven by the success in development of highly active catalytic systems. Heterogeneous catalysts have found use in many industrial processes by virtue of their ease of separation and high activities in various reactions. However, many processes employing heterogeneous catalysts in the transformation of organic molecules suffer significantly when it comes to product selectivity. Herein, we report on the synthesis of highly selective palladium nanoparticle (Pd NP)-containing catalysts. The heterogeneous catalysts reported herein consist of active mixed-metal oxides, in the form of perovskites as catalysts, and as catalytic supports for Pd NPs. The activity of pure perovskites when applied as catalysts in the hydrogenation of cinnamaldehyde is 3 factors lower compared with Pd NPs immobilized on them. However, considering the fact that perovskites achieved percentage conversions between 18 and 25% in a short period of time makes them perfect candidates to replace platinum group metals in the future. In addition to being earmarked as the future of catalysis, perovskites induced a synergistic effect on the conversion of the substrate compared to when Pd NPs are immobilized on the silica support. Furthermore, these catalysts are 100% selective to hydrocinnamaldehyde and stable for up to five catalytic cycles. With regard to reusability of the catalysts, Pd/LaFeO3 was used as a benchmark catalyst and revealed the need for surface restructuring of the catalyst for optimum activity.
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