Restructuring Of Clusters Research Articles

Understanding the effects of cluster restructuring on water gas shift reaction over Cu/TM (TM = Cu, Ni and Pt) under high concentration of CO

Ambient pressure and resulting cluster restructuring have been proved to influence water gas shift reaction (WGSR), but the intrinsic role that high concentration of CO and cluster surface play during WGSR is still elusive. In this work, the rationale behind the dual effects of CO concentration and cluster condensation on WGSR over Cu-based bimetallic alloys is theoretically revealed using density functional theory (DFT) approach. The results demonstrate that the additional adsorption of CO molecules inhibits WGSR thermodynamically and kinetically. Specifically, the increase of concentration weakens the molecule adsorption (CO and water-related molecules) and hinders the development of intermediate evolutions in WGSR, while the cluster restructuring simultaneously raises the activation energy of water dissociation. The charge density difference and Bader distribution provide electronic support for the above conclusions. Furthermore, Cu/Ni is considered as the preferential catalyst under high concentration of CO, not only because of the remarkable adsorption capacity, but also the fine structure-catalytic stability. The selection of cluster shape and energy-temperature characteristics of intermediates are also introduced, enriching the understanding of group reactivity on copper based catalysts.

Understanding the Effects of Cluster Restructuring on Water Gas Shift Reaction Over Cu/Tm (Tm = Cu, Ni and Pt) Under High Concentration of Co

Understanding the Effects of Cluster Restructuring on Water Gas Shift Reaction Over Cu/Tm (Tm = Cu, Ni and Pt) Under High Concentration of Co

PBE Modeling of Flocculation of Microalgae: Investigating the Overshoot in Mean Size Profiles

Microalgae is considered as a viable feedstock to biomass gasification. After synthesis in water medium, microalgae are separated and dried to a suitable degree to be fed to the gasification process. In order to achieve an efficient separation, a flocculation process is employed, in which microalgae primary particles aggregate and form larger clusters. Although flocculation is a well-established process, there are still some unknown issues related to it, that are worth further research. Experiments show that the mean size of clusters during flocculation goes through a maximum and then decreases with time. We refer to this pattern in the mean size profile as the overshoot. Studying this phenomenon is crucial since the size of clusters has a significant effect on the overall efficiency of the separation of microalgae from water. In this work, we aim at investigating the mechanisms behind the overshoot.The flocculation process is modeled as an aggregation-breakup system by using population balance equations (PBEs). The primary results show that the aggregation and breakup alone cannot lead to the overshoot in the mean size profile. Thus, we suggested three mechanisms that can lead to the overshoot: deposition of large clusters (DLC), restructuring of clusters (RC), and primary particle aggregation (PPA). These mechanisms were examined with numerical simulations and it was revealed that all three lead to the overshoot.

Maximum hydrogen chemisorption on KL zeolite supported Pt clusters

Platinum clusters supported on KL zeolites were characterized by EPR, HRTEM, and EXAFS. Two kinds of hydrogen chemisorption experiments both result in a saturation value of 2.9 hydrogen atoms per platinum atom, significantly more than that reported so far. A hydrogen coverage-dependent cluster restructuring is suggested.

LIDAR: a protocol for stable and energy-efficient clustering of ad-hoc multihop networks

Clustering has been proposed as a promising method for simplifying the routing process in mobile ad hoc networks (MANETs). The main objective in clustering is to identify suitable node representatives, i.e. cluster heads (CHs) to store routing and topology information; CHs should be elected so as to maximize clusters stability, that is to prevent frequent cluster re-structuring. Since CHs are engaged on packet forwarding they are prone to rapidly drop their energy supplies, hence, another important objective of clustering is to prevent such node failures. Recently proposed clustering algorithms either suggest CH election based on node IDs (nodes with locally lowest ID value become CHs) or take into account additional metrics (such as energy and mobility) and optimize initial clustering. Yet, the former method is biased against nodes with low IDs (which are likely to serve as CHs for long periods and therefore run the risk of rapid battery exhaustion). Similarly, in the latter method, in many situations (e.g. in relatively static topologies) re-clustering procedure is hardly ever invoked; hence initially elected CHs soon suffer from energy drainage. Herein, we propose LIDAR, a novel clustering method which represents a major improvement over alternative clustering algorithms: node IDs are periodically re-assigned so that nodes with low mobility rate and high energy capacity are assigned low ID values and, therefore, are likely to serve as CHs. Therefore, LIDAR achieves stable cluster formations and balanced distribution of energy consumption over mobile nodes. Our protocol also greatly reduces control traffic volume of existing algorithms during clustering maintenance phase, while not risking the energy availability of CHs. Simulation results demonstrate the efficiency, scalability and stability of our protocol against alternative approaches.

Aggregation kinetics and structure of cryoimmunoglobulins clusters

Cryoimmunoglobulins are pathological antibodies characterized by a temperature-dependent reversible insolubility. Rheumatoid factors (RF) are immunoglobulins possessing anti-immunoglobulin activity and usually consist of an IgM antibody that recognizes IgG as antigen. These proteins are present in sera of patients affected by a large variety of different pathologies, such as HCV infection, neoplastic and autoimmune diseases. Aggregation and precipitation of cryoimmunoglobulins, leading to vasculiti, are physical phenomena behind such pathologies. A deep knowledge of the physico-chemical mechanisms regulating such phenomena plays a fundamental role in biological and clinical applications. In this work, a preliminary investigation of the aggregation kinetics and of the final macromolecular structure of the aggregates is presented. Through static light scattering techniques, the gyration radius R g and the fractal dimension D m of the growing clusters have been determined. However, while the initial aggregation mechanism could be described using the universal reaction-limited cluster–cluster aggregation (RLCCA) theory, at longest times from the beginning of the process, the RLCCA theory fails and a restructuring of clusters is observed together with an increase of the cluster fractal dimension D m up to a value D m∼3. The time t n, at which the restructuring takes place, and the final cluster size can be modulated by varying the quenching temperature.

Flocculation Kinetics and Cluster Morphology in Illite/NaCl Suspensions

Using static and dynamic light-scattering techniques, we investigated the flocculation of geologic specimen illite and soil illite colloids suspended in NaCl solution at pH 8. Our results show that the two universality classes proposed by Lin and co-workers to characterize the diffusion and reaction-limited régimes of cluster–cluster flocculation do not apply to illite colloids, which have nonspherical morphology. Static light-scattering measurements yielded mass fractal dimensions for the soil illite clusters that were significantly above those required by universality. Those for specimen illite were influenced by the initial colloid concentration, suggesting that cluster restructuring had occurred during flocculation. Dynamic light-scattering measurements revealed a significant departure from linear growth of the mean cluster size with time, as required by universality in the diffusion-limited régime, and, for specimen illite, an oscillatory dependence of the mean cluster radius on time was observed in the transition from diffusion-limited to reaction-limited flocculation. Overall, the morphology, interactions, and restructuring of primary particles are believed to be responsible for the departure from universality in the flocculation behavior of the illite colloids.

Continuum percolation and depletion flocculation

Using Brownian dynamics simulations, we investigate continuum percolation in a system of colloid particles aggregating due to depletion flocculation. For all values of aggregating potential, there is a lower threshold volume fraction of colloid particles at which a percolating cluster appears. This threshold defines a percolation “phase” boundary that crosses the phase boundary between the colloid fluid phase and colloid fluid+crystal coexistence. In the two-phase coexistence regime, the lifetime of percolated clusters increases dramatically, and therefore the percolation threshold may be related to a transient gel threshold reported from experiment. In the phase coexistence regime, there is evidence of aging due to the thermal restructuring of clusters, implying a finite lifetime for percolating structures.

Light-scattering study on the fractal aggregates of polystyrene spheres: Kinetic and structural approaches

Static and dynamic light-scattering measurements on the fractal aggregates of polystyrene spheres (diameter=40 nm), induced by the addition of salt (0.2–1.5 mol·dm −3NaCl) at a constant pH of 9.2, are presented. The kinetic approach with the use of dynamic light scattering enables us to distinguish between fast and slow aggregation and the structural approach by using static light scattering provides us an appropriate way to determine the fractal dimension. By using the DLVO colloid stability theory to calculate the potential energy curves between polystyrene spheres at various salt concentrations we have concluded that the interparticle interactions would not only govern the kinetic growth of clusters, but also determine the fractal structure and the degree of cluster restructuring. In the category of fast aggregation, the smaller the interparticle attraction in the primary minimum, the larger the fractal dimension resulted from restructuring.

The structure of fractal colloidal aggregates of finite extent

The structure of fractal colloid aggregates formed in both the diffusion- and the reaction-limited regimes is studied by static light scattering experiments. The crossover region of the structure factor of the clusters is measured, and the effects of the finite extent of the fractal structure on the scattering are investigated. The polydispersity of the cluster mass distribution markedly changes the shape of the measured scattering intensity. A form for the structure factor obtained from computer-generated clusters is found to describe the colloidal aggregates very well, for both regimes. Other available models for the crossover region are also discussed. In addition, the effects of the optical plasma resonance in the case of metallic colloids and the effects of cluster restructuring on the static scattering are discussed.