The effect of encapsulation in carbon nanotubes on properties of Fe-Ni nanoalloys with cubic and helical structures

The effect of encapsulation in carbon nanotubes on Fe-Ni nanoparticles (NPs), NP assemblies and nanowires (NWs) is studied by means of atomistic simulations with empirical potentials. The BCC Fe, L10, FeNi, L1 2 FeNi3 and FCC Ni stable phases of the bulk alloy are retrieved for both the freestanding (FS) and the encapsulated nanoalloys. As it requires large morphological changes, the BCC/L10 transition may be inhibited by the confinement in a nanotube. The results indicate that encapsulation has the effect to enhance Fe segregation at compositions intermediate between stable phases. When the nanotube is too narrow, encapsulated NWs do not support a cubic structure. They consist in coaxial layers with a central straight atomic row aligning with the tube axis. Each layer displays a helical structure which can be equivalently viewed as a folded atomic plane with low Miller indices. Such ultrathin helical Fe-Ni NWs, FS as well as encapsulated, behave as almost ideal solid solutions over the whole range of compositions. © 2012 Springer Science+Business Media, LLC.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Investigation of the Stability and Hydrogen Evolution Activity of Dual-Atom Catalysts on Nitrogen-Doped Graphene

Single atom catalysts (SACs) have received a lot of attention in recent years for their high catalytic activity, selectivity, and atomic utilization rates. Two-dimensional N-doped graphene has been widely used to stabilize transition metal (TM) SACs in many reactions. However, the anchored SAC could lose its activity because of the too strong metal-N interaction. Alternatively, we studied the stability and activity of dual-atom catalysts (DACs) for 24 TMs on N-doped graphene, which kept the dispersion state but had different electronic structures from SACs. Our results show that seven DACs can be formed directly compared to the SACs. The others can form stably when the number of TMs is slightly larger than the number of vacancies. We further show that some of the DACs present better catalytic activities in hydrogen evolution reaction (HER) than the corresponding SACs, which can be attributed to the optimal charge transfer that is tuned by the additional atom. After the screening, the DAC of Re is identified as the most promising catalyst for HER. This study provides useful information for designing atomically-dispersed catalysts on N−doped graphene beyond SACs

Single Metal Atoms Embedded in the Surface of Pt Nanocatalysts: The Effect of Temperature and Hydrogen Pressure

Embedding energetically stable single metal atoms in the surface of Pt nanocatalysts exposed to varied temperature (T) and hydrogen pressure (P) could open up new possibilities in selective and dynamical engineering of alloyed Pt catalysts, particularly interesting for hydrogenation reactions. In this work, an environmental segregation energy model is developed to predict the stability and the surface composition evolution of 24 Metal M-promoted Pt surfaces (with M: Cu, Ag, Au, Ni, Pd, Co, Rh and Ir) under varied T and P. Counterintuitive to expectations, the results show that the more reactive alloy component (i.e., the one forming the strongest chemical bond with the hydrogen) is not the one that segregates to the surface. Moreover, using DFT-based Multi-Scaled Reconstruction (MSR) method and by extrapolation of M-promoted Pt nanoparticles (NPs), the shape dynamics of M-Pt are investigated under the same ranges of T and P. The results show that under low hydrogen pressure and high temperature ranges, Ag and Au—single atoms (and Cu to a less extent) are energetically stable on the surface of truncated octahedral and/or cuboctahedral shaped NPs. This indicated that coinage single-atoms might be used to tune the catalytic properties of Pt surface under hydrogen media. In contrast, bulk stability within wide range of temperature and pressure is predicted for all other M-single atoms, which might act as bulk promoters. This work provides insightful guides and understandings of M-promoted Pt NPs by predicting both the evolution of the shape and the surface compositions under reaction gas condition

Investigation of finite-size effects in chemical bonding of AuPd nanoalloys

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Growth of carbon clusters on a Ni(1 1 1) surface

A Morse potential is designed to depict the interaction of carbon clusters with the perfect Ni(1 1 1) surface. This potential is fitted to a dataset obtained via DFT calculations and accounts for the carbon-carbon coordination dependence of the C-Ni interaction. The dynamics of carbon clusters on a rigid Ni(1 1 1) lattice is then studied in the presence of a homogeneous carbon source, simulating recent experiments on low-temperature clustering of carbon species by diffusion on an infinite and perfect Ni(1 1 1) surface. Carbon chains are predicted to grow with only limited cross-linking. It is suggested that an additional mechanism might be needed to convert carbon chains into a graphene sheet. © 2012 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Equilibrium Shapes of Ag, Ni, and Ir Nanoparticles under CO Conditions

Metal nanoparticles are widely used in catalysis by virtue of their excellent physicochemical properties, which are closely related to their morphology. In this work, we predict the reshaping of Ag, Ni, and Ir metal nanoparticles under a CO atmosphere using the recently proposed multiscale structure reconstruction model. In the low-pressure environment, temperature has little effect on the structures of Ag nanoparticles. However, the structures of Ag nanoparticles will change significantly in high- and low-temperature environments. Ni and Ir nanoparticles are greatly affected by the environment due to their stronger interactions with CO. This study demonstrates the structural changes of Ag, Ni, and Ir nanoparticles under different pressures and temperatures, providing theoretical guidance for in situ experiments and the rational design of nanocatalysts

Shape Evolution of Metal Nanoparticles in Binary Gas Environment

To control the shape and structure of a metal nanoparticle (NP) is a crucial strategy to improve its catalytic properties, but the understanding and quantitative description of the structure reconstruction of the catalysts under reaction conditions has not been achieved. Previous works are mostly focused on the single gas conditions, which is apparently not the case in the real catalytic reactions. In this work, a multiscale structure reconstruction model is established to describe the equilibrium structures of metal NPs in a mixed gas environment quantitatively. Taking NO and CO reaction as a model system, the structures of the Pd, Pt, and Rh NPs in a large range of temperature and pressure are fully presented. Moreover, we show the variation of <i>P</i>_<i>NO</i>:<i>P</i>_<i>CO</i> plays the critical role in determining the structures and therefore the number of active sites of the NPs at certain conditions. This work provides an efficient model to guide the future experiments in the real reactions