Adsorption-surface diffusion

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

In the absence of transport limitations, the processes of adsorption, surface diffusion, surface reaction, and desorption can be treated via the transition state theory (Baetzold and Somorjai, 1976 Zhdanov et al, 1988). For example, the application of the TST to a single site adsorption process,... 

Figure 7-8 Elementary steps that must occur in a catalytic reaction on a surface. All catalytic processes involve transport through a boundary layer, adsorption, surface diffusion, reaction, and desorption.

With this information in mind, we can construct a model for the deposition rate. In the simplest case, the rate of flux of reactants to the surface (step 2) is equal to the rate at which the reactants are consumed at steady state (step 5). All other processes (decomposition, adsorption, surface diffusion, desorption, and transport away from the substrate) are assumed to be rapid. It is generally assumed that most CVD reactions are heterogeneous and first order with respect to the major reactant species, such that a general rate expression of the form of Eq. (3.2) would reduce to... 

The tools needed to analyze adsorption, surface diffusion, and surface reaction to form a product are the same as those used to analyze reactions on catalytic surfaces, the only difference being that in catalytic systems the product leaves the surface and desorbs into the fluid phase. In the processing of electronic materials, the product is the thin film that is formed on the surface. 

For most CVD reactions, the supersaturation is so high that calculated values of r are of atomic dimensions. For such reactions, the classical theory is not appropriate, and detailed atomic treatments must be considered. Because of the interesting fundamental questions underlying nucleation and the important applications of thin films, interest in modeling adsorption, surface diffusion, and nucleation has been considerable. These efforts are described in several, well-documented reviews (61, 75-78). 

Keywords Adsorption surface diffusion molecular rotors self-assembly metal surfaces Scanning Tunneling Microscopy molecular nanoscience. 

Fig. 3. Rates of adsorption, surface diffusion, and desorption for sites with differential heats of adsorption of 100 and 200 kJ mol . ...

The adsorption-surface diffusion-desorption mechanism of transport through the SSF membrane can simultaneously provide high separation selectivity between H2 and the impurities of the PSA waste gas and high flux for the impurities even when the gas pressure in the high-pressure side of the membrane is low to moderate (3-5 atm). 

Theoretical studies of the properties of the individual components of nanocat-alytic systems (including metal nanoclusters, finite or extended supporting substrates, and molecular reactants and products), and of their assemblies (that is, a metal cluster anchored to the surface of a solid support material with molecular reactants adsorbed on either the cluster, the support surface, or both), employ an arsenal of diverse theoretical methodologies and techniques for a recent perspective article about computations in materials science and condensed matter studies [254], These theoretical tools include quantum mechanical electronic structure calculations coupled with structural optimizations (that is, determination of equilibrium, ground state nuclear configurations), searches for reaction pathways and microscopic reaction mechanisms, ab initio investigations of the dynamics of adsorption and reactive processes, statistical mechanical techniques (quantum, semiclassical, and classical) for determination of reaction rates, and evaluation of probabilities for reactive encounters between adsorbed reactants using kinetic equation for multiparticle adsorption, surface diffusion, and collisions between mobile adsorbed species, as well as explorations of spatiotemporal distributions of reactants and products. 

The role of the adsorptive surface characteristics in many processes of practical importance is a topic of increasing interest in surfiice science. Adsorption, surface diffusion, and reactions on catalysts are some of the phenomena that are strongly dependent upon surface structure. Most materials have heterogeneous surfaces that, when interacting with gas molecules, present a complex spatial dependence of the adsorptive energy. This is specially the case for activated carbons, where many defects and impurity atoms and molecules are incorporated... 

There are various cases of particle-interface interactions, which require separate theoretical treatment. The simpler case is the hydrodynamic interaction of a solid particle with a solid interface. Other cases are the interactions of fluid particles (of tangentially mobile or immobile interfaces) with a solid surface in these cases, the hydrodynamic interaction is accompanied by deformation of the particle. On the other hand, the colloidal particles (both solid and fluid) may hydrodynamically interact with a fluid interface, which thereby undergoes a deformation. In the case of fluid interfaces, the effects of surfactant adsorption, surface diffusivity, and viscosity affect the hydrodynamic interactions. A special class of problems concerns particles attached to an interface, which are moving throughout the interface. Another class of problems is related to the case when colloidal particles are confined in a restricted space within a narrow cylindrical channel or between two parallel interfaces (solid and/or fluid) in the latter case, the particles interact simultaneously with both film surfaces. 

In this chapter we shall review the motion of atoms and molecules at surfaces. First we discuss how atoms vibrate about their equilibrium surface sites. Then, the elementary surface processes during the collisions of gas atoms and molecules with surfaces are described. We then discuss several elementary gas-surface interactions adsorption, surface diffusion, and desorption. 

In a surface catalytic process, the reaction occurs repeatedly by a sequence of elementary steps that includes adsorption, surface diffusion, the chemical rearrangements (bond breaking, bond forming, molecular rearrangement) of the adsorbed reaction intermediates and the desorption of the products. 

FIGURE 8.1 Molecular-level overview of a catalytic chemical reaction. Selected elementary-like steps of the CO oxidation reaction CO adsorption, surface diffusion of CO, the TS of the C0 +0 surface reaction, the COj desorption, and various adsorbed species. 

Integration of a H2 PSA process with an adsorbent membrane can meet this goal [23, 24]. A nano-porous carbon adsorbent membrane called Selective Surface Flow (SSF) membrane which selectively permeates CO2, CO and CH4 from their mixtures with H2 by an adsorption- surface diffusion-desorption transport mechanism may be employed for this purpose. The SSF membrane can produce an enriched H2 gas stream from a H2 PSA waste gas, which can then be recycled as feed to the PSA process for increasing the over-all H2 recovery. The membrane is prepared by controlled carbonization of poly-vinyledene chloride supported on a macro-porous alumina tube. The membrane pore diameters are between 6 -7 A, and its thickness is - 1-2 pm [25]. 

Transport of the released solvent molecules to the bulk solution. ThesestepsareveryslmilartothosedescribedSec.2.3. Steps 1 andSare controlled by the usual diffusion kinetics, while steps 2, 3 and 4 are controlled by the kinetics of surface adsorption, surface diffusion and of integration of growth units at kinks. 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

Once the vapor has been produced, the steps needed to grow the film are condensation of the vapor on the growing surface (adsorption), surface diffusion of the reactants, reaction to form the film, coalescence of the reacted material into islands of atoms, and desorption (re-evaporation) of any product or unused reactant from the surface. The following sections discuss these steps in more detail. 

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