Nanomaterial kinetics

In studies of reactions in nanomaterials, biochemical reactions within the cell, and other systems with small length scales, it is necessary to deal with reactive dynamics on a mesoscale level that incorporates the effects of molecular fluctuations. In such systems mean field kinetic approaches may lose their validity. In this section we show how hybrid MPC-MD schemes can be generalized to treat chemical reactions. 

The resulting activation energies Eh as well as the collision factors mCOO are displayed in Table 2.2. The most active material among the nanomaterials is the Co/MW catalyst, with the highest values for both kinetic parameters (Ek and f m co.o)- The lowest activation energy and collision factor, in contrast, is seen with the herringbone material. 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

If one accepts the premise that self-assembly will be an important component of the formation of nanomaterials, it is clearly important to understand it as a process (or, better, class of processes). The fundamental thermodynamics, kinetics, and mechanisms of self-assembly are surprisingly poorly understood. The basic thermodynamic principles derived for molecules may be significantly different for those that apply (or do not apply) to nanostructures the numbers of particles involved may be small the relative influence of thermal motion, gravity, and capillary interactions may be different the time required to reach equilibrium may be sufficiently long that equilibrium is not easily achieved (or never reached) the processes that determine the rates of processes influencing many nanosystems are not defined. 

Not mentioned in this review but certainly important to multiscale modeling related to solid mechanics are topics, such as self-assemblies, thin films, thermal barrier coatings, patterning, phase transformations, nanomaterials design, and semiconductors, all of which have an economic motivation for study. Studies related to these types of materials and structures require multiphysics formulations to understand the appropriate thermodynamics, kinetics, and kinematics. 

Abstract. It has been revealed that carbonic nanomaterials (fullerene, single- and multiwall nanotubes, nanofibers) display high activity at low temperatures (77K) in reactions of chain halogenation (F2, Cl2) with kinetic chain length up to 104 -105. The ESR spectra of active free- radical intermediates were recorded. The presence of vibration bands of C-Cl bonds in products has been indicated by IR method. 

The hydrocarbon chlorination reactions usually display the radical-chained character of their mechanisms. Such a mechanism can be suggested as well for low temperature photoinduced chlorination of carbon nanomaterials. The kinetic chain length in this process is as high 3xl04-106 (for chlorides of C60CI2 and CgoCh compounds correspondingly). 

Tatarenko, V.A., Radchenko, T.M., and Molodkin, V.B. (2004) Kinetics of the hydrogen-isotope short-range ordering in interstitial solid solutions h.c.p.-Zw-H(D, T), in T.N. Veziroglu et al. (eds.), Hydrogen Materials Science and Chemistry of Carbon Nanomaterials NATO Science Series, Series II Mathematics, Physics and Chemistry 172, Kluwer Academic Publishers, Dordrecht, pp. 59-66. 

The underlying concept of this method for the synthesis of filamentous carbonaceous nanomaterials is fairly simple. As the temperature rises above a certain limit, which depends on the thermodynamic and kinetic para meters of carbon containing compounds, such as hydrocarbons, such compounds tend to pyrolyze in the air free conditions to form free carbon. For example, the noncatalytic pyrolysis of methane can be achieved at ambient pressure and at temperatures above 900—1000 K to produce soot (near spherical nanosized carbon particles) and hydrogen ... 

In situ Raman spectroscopy during heating in a controlled environment allows for a time-resolved investigation of the oxidation kinetics of carbon nanomaterials and can identify changes in material structure and composition during oxidation. In this chapter, we describe the application of in situ Raman spectroscopy to determine conditions for selective oxidation and purification of carbon nanotubes (CNT) and nanodiamond (ND). 

The stmctural diversity of carbon at the nanoscale exceeds that of all other materials [1]. Detailed information on the nature of the material and the structure-dependency of the oxidation kinetics is thus crucial for providing the required selectivity. While some nanomaterials, such as carbon nanotubes, have been studied extensively and are generally well understood, other nanostructures such as nanodiamond (ND) have received much less attention. However, in order to study their properties and open avenues for new applications, one has to provide a material of high purity and defined composition. 

Although much progress has been made in both synthesis and purification of carbon nanomaterials, commercial samples still contain nanostrucmres of different size, shape, and composition. As-produced carbon nanomaterials are frequently composed of mixtures of CNTs, fullerenes, carbon onions, amorphous carbon and graphite, which are structurally different and possess different reactivity. Since the oxidation kinetics are closely related to structural features, reaction rates and activation energies are expected to differ for the distinct carbon forms, which is an important issue for oxidation-based purification or surface functionalization. In analogy to graphite [3-6], oxidation of a carbon nanostmcture [7-9] can be described by a first-order reaction, with respect to the carbon component. 

In order to successfully apply oxidation methods to carbon nanomaterials, one has to systematically study their interactions with gases and liquids, monitor changes in structure and composition, and simultaneously follow the reaction kinetics of the different carbon nanostructures. This has been partially achieved for carbon nanotubes [25-27], which have been thoroughly studied, but remains a major challenge for other forms of carbon, including ND or carbon onions. 

In the processing of nanoparticles, coarsening is common, and may be accompanied by phase transformation to the macroscopic stable structure. Here we will focus on the kinetics of phase transformations and crystal growth in nanocrystalline particles. We will show later that conventional kinetic models that are widely employed for analysis of macroscopic materials behavior may have to be modified prior to their application to nanomaterials. 

Although the formal JMAK theory has been used to model some nanomaterials systems, serious limitations are imposed due to the fundamental assumptions. In certain instances, the parameters may only be fitting constants with limited or no physical meaning. In many cases, the JMAK theory may not be optimal for analysis of kinetics of reactions involving nanoparticles. 

Much of the work to date on particle size effects on phase transformation kinetics has involved materials of technological interest (e.g., CdS and related materials, see Jacobs and Alivisatos, this volume) or other model compounds with characteristics that make them amenable to experimental studies. Jacobs and Alivisatos (this volume) tackle the question of pressure driven phase transformations where crystal size is largely invariant. In some ways, analysis of the kinetics of temperature-motivated phase transformations in nanoscale materials is more complex because crystal growth occurs simultaneously with polymorphic reactions. However, temperature is an important geological reality and is also a relevant parameter in design of materials for higher temperature applications. Thus, we consider the complicated problem of temperature-driven reaction kinetics in nanomaterials. 

The hulk of a nanomaterial will refer to that portion that is not clearly defined as part of the surface, i.e., not within several atomic layers of the surface. This implies that 1-nm nanomaterials consist only of surface. Surface volume will refer to the volume of a nanocrystal or nanoparticle that is assumed to be part of the surface. The habit of a crystal is the particular external form that is presented within the options allowed by the point group symmetry. Common descriptive terms are tabular (tablet-like), cubic, acicular, and so forth. Occasionally, habits inconsistent with point group symmetry can occur from kinetic phenomena. 

Nanomaterials are expected to have a huge impact on active material design, due in part to their better resistance to structural strains and improved kinetics whatever the electrochemical system used (lead-acid, Li-ion or Ni-MH). 

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