Small Ni Clusters Research Articles

Glucose electrooxidation on carbon supported NiAu electrocatalysts

NiAu/C nanomaterials are synthesised using a wet chemistry method with targeted Au atomic ratios of 10 %, 20 % and 30 %. Physicochemical characterisations indicate that the materials have mean compositions close to the nominal ones but ca. 20 at% Au richer in average than expected (Au ratios of 13.6 at%, 23.1 at% and 35.9 at%, respectively). The NiAu/C materials are composed of Au-rich spherical-like Janus particles of several tenths nm and of a phase of very small Ni-rich nanoparticles and Ni(OH2) clusters. The electrochemical measurements in a 0.1 M NaOH/0.1 M glucose electrolyte indicate that the NiAu20/C catalyst is the most active for the glucose oxidation reaction, leading to a mass activity at +0.6 V vs RHE >1.5 times higher than that with a pure Au/C catalyst, although the Au content is almost 5 times lower. The chronoamperometry measurements for 900 s at +0.6 V vs RHE confirm the activity gain with the NiAu20/C catalyst. The electrolysis measurement at a cell voltage of + 0.6 V for 6 h shows that the NiAu20/C catalyst is selective towards the production of gluconic acid, with a faradaic efficiency higher than 100 %, indicating the occurrence of a 1-electron reaction with anodic hydrogen coproduction. At +0.8 V, the faradaic efficiency is lower than 100 %, indicating the formation of other products than gluconic acid, but at a very low extent (not detectable by HPLC) guarantying a very high selectivity towards gluconic acid.

Multiscale modelling of CO2 hydrogenation of TiO2-supported Ni8 clusters: on the influence of anatase and rutile polymorphs

The selection of TiO2 phase, whether anatase or rutile, for supporting small Ni clusters significantly influences the activity and selectivity in CO2 hydrogenation to methane.

Revealing the effect of metal-support interactions at the Ni/In2O3(111) interface on the selective CO2 hydrogenation

In2O3-supported Ni catalysts exhibit remarkable catalytic activity and selectivity in CO2 hydrogenation to methanol, but the underlying mechanisms and metal-oxide interactions during the reaction remain elusive. Herein, we investigate the Ni-In2O3 interaction by physical vapor deposition of Ni onto well-defined In2O3(111) thin films. In-situ near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to probe the CO2 hydrogenation processes on these Ni/In2O3(111) model systems. Our results reveal that the small Ni clusters supported on the In2O3(111) surface at low Ni coverages exhibit cationic states. The chemical bonding and associated electron transfer at the Ni/In2O3(111) interface play crucial roles in the activation of H2 and CO2. Importantly, reaction intermediates (CO3*, OH, and HCOO*) are readily formed and desorbed under CO2 hydrogenation conditions. Our study highlights the significance of metal-support interactions on the selectivity of CO2 hydrogenation. These findings provide valuable insights into the rational design of advanced In2O3-based catalysts for CO2 hydrogenation.

Structure and size-dependent vibrational and thermal properties of Ni clusters: A systematic ab initio approach

There is scarce information on the vibrational and thermal properties of small Ni clusters. Here, the outcomes of ab initio spin-polarized density functional theory calculations on the size and geometry effects upon the vibrational and thermal properties of Nin (n = 13 and 55) clusters, are discussed. For theses clusters a comparison is presented between the closed shell symmetric octahedral (Oh) and the icosahedral (Ih) geometries. The results indicate that the Ih isomers are lower in energy. Besides, ab initio molecular dynamics runs at T = 300K show that Ni13 and Ni55 clusters transform from their initial Oh geometries towards the corresponding Ih ones. For Ni13, we also consider the lowest energy less symmetric layered 1-3-6-3 structure, and the cuboid, recently observed experimentally for Pt13, which is competitive in energy but is unstable, as phonon analysis reveals. We calculate their vibrational density of states (νDOS) and heat capacity, and compare with the Ni FCC bulk counterpart. The characteristic features of the νDOS curves of these clusters are interpreted in terms of the clusters’ sizes, the interatomic distance contractions, the bond order values as well as the internal pressure and strains of the clusters. We find that the softest possible frequency of the clusters is size and structure-dependent, being the smallest for the Oh ones. We identify mostly shear, tangential type displacements involving mainly surface atoms for the lowest frequency of the spectra of both Ih and Oh isomers. For the maximum frequencies of these clusters the central atom shows anti-phase movements against groups of nearest neighbor atoms.An excess of heat capacity at low temperatures with respect to the bulk is found, while at high temperatures a constant limiting value, close but lower to the Dulong and Petit value, is determined.

Electrodeposition of Ni on MWNTs as a promising catalyst for CO2RR

AbstractNovel methods for the preparation of metal‐supported materials are desired for better catalytic performance and selectivity. In this article, Ni supported on multiwall nanotubes (Ni/MWNTs) is synthesized with electrodeposition method for electrocatalytic CO2 reduction reaction (CO2RR). The sample drying time after electrodeposition is found critical to oxidize Ni for better activity, and higher Ni oxidation state is required for CO2RR. The loading effect study proves that higher current density is achieved from small Ni clusters than that from large nano particles. This work provides a pioneer approach to synthesize small Ni clusters using electrodeposition method for electrocatalytic CO2 conversion.

The stability of γ′ precipitates in a multi-component FeCoNiCrTi0.2 alloy under elevated-temperature irradiation

Precipitation hardening by γ′ precipitates is an effective strengthening mechanism for designing novel high-entropy alloys (HEAs). The stability of these γ′ precipitates under elevated temperature irradiation is a major concern for their application in nuclear industry. In the present study, a γ′-strengthened FeCoNiCrTi0.2 HEA is irradiated using 6.4 MeV Fe3+ ions at 400 °C, 500 °C and 600 °C, respectively. After irradiation, transmission electron microscopy (TEM) and atom probe tomography (APT) has characterized the structural and compositional stability of the γ′ precipitates. The results show that the ordered L12 structure of γ′ precipitate is susceptible to the radiation damage, whereas their basic compositional features can sustain up to ∼13 dpa at these temperatures. APT reveals that the dissolution of γ′ precipitates start from the surface of the precipitates at the early stage of irradiation. Dissolution and disordering of the precipitates are dependent on the irradiation dose and temperature. APT also shows that small Ni and Ti-rich clusters form after irradiation at elevated temperatures. The behavior of γ′ precipitates under irradiation is believed to be controlled by two competing factors, displacement damage and radiation enhanced diffusion. Both can influence the radiation induced disordering/dissolution and reprecipitation.

The role of Ti and TiC nanoprecipitates in radiation resistant austenitic steel: A nanoscale study

This work encompasses an in-depth transmission electron microscopy and atom probe tomography study of Ti-stabilized austenitic steel irradiated with Fe-ions. The focus is on radiation induced segregation and precipitation, and in particular on how Ti and TiC affect these processes. A 15-15Ti steel (grade: DIN 1.4970) in two thermo-mechanical states (cold-worked and aged) was irradiated at different temperatures up to a dose of 40 dpa. At low irradiation temperatures, the cold-worked and aged materials evolved to a similar microstructure dominated by small Si and Ni clusters, corresponding to segregation to small point defect clusters. TiC precipitates, initially present in the aged material, were found to be unstable under these irradiation conditions. Elevated irradiation temperatures resulted in the nucleation of nanometer sized Cr enriched TiC precipitates surrounded by Si and Ni enriched shells. In addition, nanometer sized Ti- and Mn-enriched G-phase (M6Ni16Si7) precipitates formed, often attached to TiC precipitates. Post irradiation, larger number densities of TiC were observed in the cold-worked material compared to the aged material. This was correlated with a lower volume fraction of G-phase. The findings suggest that at elevated irradiation temperatures, the precipitate-matrix interface is an important point defect sink and contributes to the improved radiation resistance of this material. The study is a first of its kind on stabilized steel and demonstrates the significance of the small Ti addition to the evolution of the microstructure under irradiation.

Ni clusters embedded in multivacancy graphene substrates

In the present study, we performed density functional theory (DFT) calculations in order to study the structure and stability of small Ni clusters embedded in graphene multivacancy systems. The Bader charges, band structure, total density of states, partial density of states (PDOS), and spin density magnetization were obtained to understand the effects of the cluster size on the electronic and magnetic properties, and thus to determine their potential applications in spintronic devices. Ni cluster adsorption, modified graphene multivacancy substrate behavior, and conversion from a conductor to a semiconductor could be achieved when the cluster size was changed. A linear dispersion relationship around the reciprocal point K was conserved for most of the systems with the inclusion of a band gap between the Dirac cones, which is important for obtaining semiconductors with massless fermions. Comparisons with Ni clusters adsorbed in pristine graphene showed that combining vacancy defects with Ni allows higher band gaps to be obtained. We also analyzed the interactions between Ni clusters and vacancy defects based on the PDOS results. Ni clusters with different sizes could generate ferromagnetic and ferrimagnetic couplings with graphene that exhibited three, four, and six (D2h and D6h symmetry) vacancies. The generation of ferromagnetism when Ni3 and Ni4 absorbed in graphene (with four and six vacancies, respectively) demonstrates both ferromagnetic and semiconductor behaviors can be obtained, thereby making them good candidates for use as dilute magnetic semiconductors. Ni clusters did not magnetize or exhibit slight magnetization on pristine graphene, and the ferromagnetic coupling was only achieved after the introduction of vacancy defects.

Giant Enhancement in the Supercapacitance of NiFe-Graphene Nanocomposites Induced by a Magnetic Field.

The rapid rise in energy demand in the past years has prompted a search for low-cost alternatives for energy storage, supercapacitors being one of the most important devices. It is shown that a dramatic enhancement (≈1100%, from 155 to 1850 F g-1 ) of the specific capacitance of a hybrid stimuli-responsive FeNi3 -graphene electrode material can be achieved when the charge/discharge cycling is performed in the presence of an applied magnetic field of 4000 G. This result is related to an unprecedented magnetic-field-induced metal segregation of the FeNi3 nanoparticles during the cycling, which results in the appearance of small Ni clusters (<5 nm) and, consequently, in an increase in pseudocapacitive sites. The results open the door to a systematic improvement of the capacitance values of hybrid supercapacitors, while moving the research in this area towards the development of magnetically addressable energy-storage devices.

Room Temperature Methane Capture and Activation by Ni Clusters Supported on TiC(001): Effects of Metal-Carbide Interactions on the Cleavage of the C-H Bond.

Methane is an extremely stable molecule, a major component of natural gas, and also one of the most potent greenhouse gases contributing to global warming. Consequently, the capture and activation of methane is a challenging and intensively studied topic. A major research goal is to find systems that can activate methane, even at low temperatures. Here, combining ultrahigh vacuum catalytic experiments, X-ray photoemission spectra, and accurate density functional theory (DFT) based calculations, we show that small Ni clusters dispersed on the (001) surface of TiC are able to capture and dissociate methane at room temperature. Our DFT calculations reveal that two-dimensional Ni clusters are responsible for this chemical transformation, confirming that the lability of the supported clusters appears to be a critical aspect in the strong adsorption of methane. A small energy barrier of 0.18 eV is predicted for CH4 dissociation into adsorbed methyl and atomic hydrogen species. In addition, the calculated reaction free energy profile at 300 K and 1 atm of CH4 shows no effective energy barriers in the system. Comparison with other reported systems which activate methane at room temperature, including oxide and zeolite-based materials, indicates that a different chemistry takes place on our metal/carbide system. The discovery of a carbide-based surface able to activate methane at low temperatures paves the road for the design of new types of catalysts which can efficiently convert this hydrocarbon into other added-value chemicals, with implications in climate change mitigation.

Highly active and noble-metal-alternative hydrogenation catalysts prepared by dealloying Ni-Si intermetallic compounds.

Noble-metal-alternative Ni-Si catalysts more active than Pd in the hydrogen storage reaction were developed using a unique procedure, i.e., surface dealloying with hydrofluoric acid treatment. The combination of the structural analysis and the DFT calculation revealed a specific active site structure, a Ni cluster embedded in a SiO2 matrix, and its unprecedented role in the molecular conversion.

Unveiling the effects of doping small nickel clusters with a sulfur impurity

Small free-standing Ni clusters have been widely investigated during the last decade, but not many of their derived chalcogenides, despite their interest in technology and the new prospects that the nanoscale may open. The present work uncovers the effects of the S-doping on the structural, electronic, and magnetic properties of $$\hbox {Ni}_n$$ , n = 1–10 clusters. Density functional theoretical calculations within the generalized gradient approximation for the exchange and correlation were conducted to explore the structural, electronic, and magnetic properties of the resulting $$\hbox {Ni}_n\hbox {S}$$ chalcogenide nanoparticles. The sulfur impurity is always adsorbed on the threefold hollow sites available on the nickel host, in qualitative agreement with recent results of S adsorption on Ni(111) surfaces. S-doping tends to enlarge the average Ni–Ni inter-atomic distance but enhances the thermodynamical stability of Ni clusters. It also increases the vertical ionization energy and electron affinity. However, S-doping has a small effect on the magnetism of small Ni clusters. According to the spin-dependent HOMO–LUMO gap, most of these clusters are good candidates as molecular junctions for spin filtering at low bias voltage.

Density functional theory study of uni- and bi-metallic small Pt and Ni clusters for fuel conversion applications

In this work, we employed density functional theory calculations to probe the binding characteristics of C and OH on small uni-metallic (Pt3 and Ni3) and bi-metallic (Pt2Re and Ni2Fe) clusters for energy conversion applications. Both uni-metallic and bi-metallic clusters were found to possess relatively good HOMO-LUMO gaps, indicating that the clusters are electronically stable against excitations and high energy environment. It is found that the introduction of foreign atoms on uni-metallic clusters impacts C and OH binding characteristics, which could be useful in the design and fine-tuning of novel nanocatalysts for energy conversion applications such as CO/CO2 methanation and hydrogenation reactions. It is further inferred that Pt3 and Pt2Re clusters bind C and OH moderately, and could be utilized in further studies concerning energy conversion.

Back Cover: Functionalization of CeO2(1 1 1) by Deposition of Small Ni Clusters: Effects on CO2 Adsorption and O Vacancy Formation (ChemCatChem 4/2015)

Back Cover: Functionalization of CeO₂(1 1 1) by Deposition of Small Ni Clusters: Effects on CO₂ Adsorption and O Vacancy Formation (ChemCatChem 4/2015)

Functionalization of CeO2(1 1 1) by Deposition of Small Ni Clusters: Effects on CO2 Adsorption and O Vacancy Formation

AbstractIn this study we have used density functional theory calculations to investigate the stability, structure and catalytic properties of Ni clusters supported on CeO2. We have found Ni clusters to be stabilized with increasing cluster size up to ten atoms both as isolated clusters and adsorbed on CeO2(1 1 1). Analysis of the structural properties showed an opening of the Ni particles when deposited on CeO2(1 1 1) indicating facilitated accessibility for reacting molecules. The reactivity of the functionalized surface has been examined on the example of CO2 adsorption and activation and compared to the one on pristine CeO2(1 1 1) and isolated Ni clusters. Furthermore, the effect of Ni cluster deposition on the formation and characteristics of surface and subsurface oxygen vacancies in CeO2 has been investigated. The oxygen vacancy position is found to significantly affect the stability and electronic structure of the Ni/CeO2 system.

Methanation of CO2 and reverse water gas shift reactions on Ni/SiO2 catalysts: the influence of particle size on selectivity and reaction pathway

The consecutive and parallel reaction pathways show preferences for small Ni clusters and large Ni particles, respectively.

Self-diffusion of small Ni clusters on the Ni(111) surface: A self-learning kinetic Monte Carlo study

We studied self-diffusion of small 2D Ni islands (consisting of up to 10 atoms) on Ni (111) surface using a self-learning kinetic Monte Carlo (SLKMC-II) method with an improved pattern-recognition scheme that allows inclusion of both fcc and hcp sites in the simulations. In an SLKMC simulation, a database holds information about the local neighborhood of an atom and associated processes that is accumulated on-the-fly as the simulation proceeds. In this study, these diffusion processes were identified using the drag method, and their activation barriers calculated using a semi-empirical interaction potential based on the embedded-atom method. Although a variety of concerted, multi-atom and single-atom processes were automatically revealed in our simulations, we found that these small islands diffuse primarily via concerted diffusion processes. We report diffusion coefficients for each island size at various tepmratures, the effective energy barrier for islands of each size and the processes most responsible for diffusion of islands of various sizes, including concerted and multi-atom processes that are not accessible under SLKMC-I or in short time-scale MD simulations.

Ni nanocrystals on HOPG(0001): A scanning tunnelling microscope study.

The growth mode of small Ni clusters evaporated in UHV on HOPG has been investigated by scanning tunnelling microscopy. The size, the size distribution, and the shape of the clusters have been evaluated for different evaporation conditions and annealing temperatures. The total coverage of the surface strongly depends on the evaporation rate and time, whereas the influence of these parameters is low on the cluster size. Subsequent stepwise annealing has been performed. This results in a reduction of the total amount of the Ni clusters accompanied by a decreasing in the overall coverage of the surface. The diameter of the clusters appears to be less influenced by the annealing than is their height. Besides this, the cluster shape is strongly influenced, changing to a quasi-hexagonal geometry after the first annealing step, indicating single-crystal formation. Finally, a reproducible methodology for picking up individual clusters is reported [1].

Study of Nickel Segregation at the TiNi-Titanium Oxide Interface

Ab-intio investigations of atomic and molecular oxygen on TiNi(110) surface are performed by using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. Our results confirm the formation of a Ni-rich interface TiO2(100)/TiNi(110), for which the formation energies (Hf) of point defects at the interfacial layers were estimated. It is shown that Hf of swap Ti-Ni defect has a lower energy than that for the Ni antisites at the interfacial layers. The formation energies of point defects in bulk TiNi, monoclinic TiO, and rutile TiO2 are also calculated. Our results demonstrate that Hf of Ni-antisites in TiO is twice less than that in TiO2. The formation of small Ni clusters is also discussed.

Trends in the magnetic properties of Fe, Co, and Ni clusters and monolayers on Ir(111), Pt(111), and Au(111)

We present a detailed theoretical investigation on the magnetic properties of small single-layered Fe, Co and Ni clusters deposited on Ir(111), Pt(111) and Au(111). For this a fully relativistic {\em ab-initio} scheme based on density functional theory has been used. We analyse the element, size and geometry specific variations of the atomic magnetic moments and their mutual exchange interactions as well as the magnetic anisotropy energy in these systems. Our results show that the atomic spin magnetic moments in the Fe and Co clusters decrease almost linearly with coordination on all three substrates, while the corresponding orbital magnetic moments appear to be much more sensitive to the local atomic environment. The isotropic exchange interaction among the cluster atoms is always very strong for Fe and Co exceeding the values for bulk bcc Fe and hcp Co, whereas the anisotropic Dzyaloshinski-Moriya interaction is in general one or two orders of magnitude smaller when compared to the isotropic one. For the magnetic properties of Ni clusters the magnetic properties can show quite a different behaviour and we find in this case a strong tendency towards noncollinear magnetism.