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Galvanic replacement between metals has received notable research interest for the synthesis of heterometallic nanostructures. The growth pattern of the nanostructures depends on several factors such as extent of lattice mismatch, adhesive interaction between the metals, cohesive forces of the individual metals, etc. Due to the difficulties in probing ultrafast kinetics of the galvanic replacement reaction and particle growth in solution, real-time mechanistic investigations are often limited. As a result, the growth mechanism of one metal on the surface of another metal at the nanoscale is poorly understood so far. In the present work, we could successfully probe the galvanic replacement of silver ions with nickel nanoparticles, stabilized in a polymer membrane, using two complementary methods, namely, small-angle X-ray scattering (SAXS) and radiolabeling, and the results are supported by density functional theory (DFT) computations. The silver–nickel system has been chosen for the present investigation because of the high degree of bulk immiscibility caused by the large lattice mismatch (15.9%) and the weak adhesive interaction, which makes it a perfect model system for immiscible metal pairs. Membrane, as a host medium, plays a crucial role in retarding the kinetics of atomic and particle rearrangements (nucleation and growth) due to slower mobility of the atoms (monomers) and particles within the polymer network. This allowed us to examine the real-time concentration of silver monomers during galvanic replacement of silver ions with nickel nanoparticles and evolution of Ni/Ag nanoparticles. From combined experiment and DFT computations, it has been demonstrated, for the first time to the best of our knowledge, that the majority of silver atoms, which are produced on the nickel nanoparticle surface by galvanic reactions, do not form traditional core–shell nanostructures with nickel and undergo a self-governing sequential nucleation and growth of silver nanoparticles via formation of intermediate prenucleation silver clusters, leading to the formation of mixed metallic nanoparticles in the membrane. The surface of NiNPs has a heterogeneous effect on the silver nucleation pathway, which is evident from the reduced critical free energy barrier of nucleation (ΔGcrit). The present work establishes an original mechanistic pathway based on a sequential nucleation model for formation of mixed metallic nanoparticles by the galvanic replacement route, which opens up future possibilities for size-controlled synthesis in mixed systems.

Palladium-based alloys are commonly used in industry as a membrane material for the purification of hydrogen. In this work, we report a systematic theoretical study of all of the processes associated with permeation of H through a model PdCu membrane. The surface of the membrane is modeled using the most stable (110) surface. Our calculations show that the nuclear quantum effects due to the light mass of the H atom can significantly affect the stability and reaction rates. On the basis of a microkinetic model of the permeation process, we find that the permeation can be limited by diffusion of H in the membrane bulk or the reassociation of atomic H to form H2on the permeate side of the membrane depending on the operation temperature and membrane thickness. At typical operating conditions, for membranes thinner than 0.5 μm, the permeation at high temperature (T> 500 K) is limited by surface processes, whereas at lower temperatures it can be either diffusion-limited or reassociation-limited.

Modern Earth observation spacecrafts require high agility to maneuver with larger torque which eventually needs CMGs, as Reaction Wheels cannot meet the demand of such large torques. In order to obtain high-accuracy pointing control with CMG, extremely precise gimbal control is necessary. The current paper brings to light the degree of control precision achieved by gimbal control system with the high fidelity models of inaccuracies like Gimbal Friction, CMG Motor Ripple, structural compliance which hinder the level of accuracy obtainable from CMG attitude control. Proper compensation schemes based on the models help in facilitating an improved level of precision control with CMG. Simulations are carried out with periodic sinusoids as reference gimbal rate to 4 Single Gimbal Control Moment Gyros with roof-top configuration in MATLAB/SIMULINK.

The decomposition of SO3is energetically the most demanding step for producing hydrogen through the sulfur–iodine thermochemical cycle. Using first principles density functional theory, we report a comparative study of the SO3interaction with three tetramer clusters (Ag4, Pd4, and Ag2Pd2) deposited on the alumina surface. All calculations are performed using the plane wave based pseudopotential approach under the spin-polarized density functional theory. The results reveal that the supported Ag–Pd bimetallic clusters are more efficient to weaken the S–O bond in the SO3molecule. A comparison of the SO3interaction with Ag4, Pd4, and Ag2Pd2tetramers showed that while Ag4elongates the S–O bond by 7%, Ag2Pd2@Al2O3can stretch the S–O bond up to 14%. The higher efficiency of the Ag2Pd2@Al2O3system is attributed to both excess charge transfers from the oxide surface and the combination of acceptor–donor properties of Pd and Ag, respectively, which makes a better catalyst for weakening the S–O bond in the SO3molecule.

In this work, we report the growth behavior of small Pdnclusters (n= 1–7) on the α–Al2O3(0001) surface using a first principle approach based on the plane wave-pseudopotential method. The results reveal that in general the interaction of Pdnclusters with Al2O3surface deforms the equilibrium geometry of the isolated clusters. For Pd atom, the most preferred adsorption site is found to be on top of the oxygen atom with an interaction energy of 1.40 eV. For the dimer, binding between two Pd atoms is more favorable than atomic adsorption at a distance. The competition between Pd–Pd and Pd–surface interactions further governs the growth motif of larger clusters. As the size of the Pd cluster increases, it prefers an open structure to maximize the Pd–surface interaction. In case of Pd7, the compact pentagonal bipyramidal structure of the isolated cluster reorganizes into a hexagon with one central atom. Further, a comparison of chemical bonding analysis through electronic density of state (EDOS) between the gas phase and deposited clusters shows that the EDOS of the deposited Pdncluster is significantly broader, which has been ascribed to the enhanced spd hybridization.

Using state of the art spin-polarized density functional theory it is found that a chemically inert (BN) 36cluster can be activated by incorporating magnetic nanoparticles inside it. To illustrate this aspect we have calculated the geometries and electronic structure of Fe(BN) 36and Fe 4(BN) 36clusters, which showed the appearance of gap states localized on the impurity atoms. The reaction of O 2molecules with these clusters results in weak interaction and an elongation of the O−O bond. Further interaction of this complex species with an incoming CO molecule leads to the formation of CO 2. The reaction mechanism has been investigated viaLangmuir−Hinshelwood and Elay−Rideal routes, and the minimum energy path calculations are performed using the elastic band method. These results have implications in designing novel materials based on metal nanoparticles for potential applications as industrial catalyst.

Abstract: Multiphoton dissociation/ionization has been studied for CH3SCH3 at 355 and 532 nm using a time-of-flight mass spectrometer. The major ion signals observed at 355 nm are C+, CH3+, HCS+, CH2S+, CH3S+ and CH3SCH3+. Power dependence studies at 355 nm show a (2+1) REMPI process for the formation of parent ion. Peaks atm/e = 46, 47 and 61 show two-photon laser power dependence whereasm/e = 15 and 45 peaks show four-photon dependence. However, in 532 nm photo-ionisation, no parent ion signal is observed. A peak atm/e = 35 corresponding to SH3+ has been observed. SH3+ has been suggested to originate from CH3SCH2+ via a cyclic transition state. Photoionisation results of CH3SCH3 have been compared with those of CH3SSCH3, at these two wavelengths.