Surface desorption

Lykke K R and Kay B D 1990 State-to-state inelastic and reactive molecular beam scattering from surfaces Laser Photoionization and Desorption Surface Analysis Techniquesvo 1208, ed N S Nogar (Bellingham, WA SPIE) p 1218... 

Kinetic theories of adsorption, desorption, surface diffusion, and surface reactions can be grouped into three categories. (/) At the macroscopic level one proceeds to write down kinetic equations for macroscopic variables, in particular rate equations for the (local) coverage or for partial coverages. This can be done in a heuristic manner, much akin to procedures in gas-phase kinetics or, in a rigorous approach, using the framework of nonequihbrium thermodynamics. Such an approach can be used as long as... 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Now possibilities of the MC simulation allow to consider complex surface processes that include various stages with adsorption and desorption, surface reaction and diffusion, surface reconstruction, and new phase formation, etc. Such investigations become today as natural analysis of the experimental studying. The following papers [282-285] can be referred to as corresponding examples. Authors consider the application of the lattice models to the analysis of oscillatory and autowave processes in the reaction of carbon monoxide oxidation over platinum and palladium surfaces, the turbulent and stripes wave patterns caused by limited COads diffusion during CO oxidation over Pd(110) surface, catalytic processes over supported nanoparticles as well as crystallization during catalytic processes. 

Under closed-circuit conditions, the electrochemical reactions involve a number of sequential steps, including adsorption/desorption, surface diffusion of reactants or products, and the charge transfer to or from the electrode. Charge transfer is restricted to a narrow (almost one-dimensional) three-phase boundary (tpb) among the gaseous reactants, the electrolyte, and the electrode-catalyst. 

Consider an example from nucleation and growth of thin films. At least three length scales can be identified, namely, (a) the fluid phase where the continuum approximation is often valid (that may not be the case in micro- and nanodevices), (b) the intermediate scale of the fluid/film interface where a discrete, particle model may be needed, and (c) the atomistic/QM scale of relevance to surface processes. Surface processes may include adsorption, desorption, surface reaction, and surface diffusion. Aside from the disparity of length scales, the time scales of various processes differ dramatically, ranging from picosecond chemistry to seconds or hours for slow growth processes (Raimondeau and Vlachos, 2002a, b). 

P. Joos, Dynamic Surface Phenoma, VSPO (The Netherlands). 1999. (Dynamic surface tension, rates of adsorption/desorption, surface rheology, emphasis on Gibbs monolayers. The team of editors consisted of V.P. Fainerman, G. Loglio, E.H. Lucassen-Re3mders, P. Petrov and R.H. Miller.)... 

Reversed-flow gas chromatography methodologies have been utilized for the investigation of various catalytic processes, and a large number of physicochemical quantities related to the kinetics of the elementary steps (adsorption, desorption, surface reaction) and the nature of the active sites have been determined. These parameters are summarized as follows ... 

Figure 4.5 The set of transition probabilities from a specific configuration cto a large number of alternative configurations involving a single reaction step (surface diffusion, adsorption, desorption, surface reaction) during the KMC simulation of the electrodeposition of copper.

Adsorption/desorption +/- Surface activity of roles or tools Acid washed hard glass containers. PTFE or HOPE pre-rinsed surfaces... 

Apart from desorption, surface reaction with adsorbate can be stimulated by the laser irradiation. In this chapter we will demonstrate the formation of new surface species by the CO2 laser induced reaction of CDF3 with the surface of SIO2 (17,18). In order to elucidate the mechanism of the reaction especially to determine the surface species, ir spectroscopy was used. A systematic investigation was performed Including the determination of reaction yields as a function of the laser frequency, the laser intensity and the gas pressure as well as the reaction products, and the determination of the correlation between the excited species and the reaction path. 

According to this formalism, the details of film growth (see Section 5.1.3) are described by the relative rates of processes such as the impingement and sticking rate of the adsorbate onto the substrate, desorption, surface diffusion, nucleation, attachment to an island, and coalescence. These individual rates are dependent on the deposition conditions such as the substrate temperature and the pressure of the adsorbate in the vapor phase relative to its equilibrium value, p. The dependence of the morphology on these parameters will be discussed in Section 5.1.3.2. 

The chemical steps involved in heterogeneous reactions (adsorption, desorption, surface reaction) are generally treated as elementary reactions. In many cases, one reaction is the slow step and the remaining steps are at equilibrium. For heterogeneous reactions, we add a site balance to account for the vacant and occupied sites that take part in the reaction steps. 

PSD Photon Stimulated Desorption Surfaces with adsorbed species Far u.v. light E>10eV Ions — analyzed with mass spectrometer X-RAYS 0.1-2 nm Structure and desorption kinetics of adsorbed atoms and molecules 22... 

These studies concern 1) activity/selectivity measurements 2) the determination of kinetic rate constants (adsorption, desorption, surface bonding) and 3) investigation of the surface topography. Important questions answered in the last study are 1) What amount of CO molecules is adsorbed on the catalyst surface 2) Where are the molecules on the siuface (e.g., Au particles or support) 3) What is the nature of the siuface chemical bonds ... 

Boundary conditions differ but the same information can be obtained from the two kinds of experiment. During a permeation experiment, the sum of the absorption resistance (on the feed side) and the desorption resistance (on the permeate side) is measured. During an absorption experiment, the sum of the absorption-surface resistances of each side is measured. During a desorption experiment, the sum of the desorption-surface resistance of each surface is measured. There is no possibility to separately measure one surface resistance at a time. 

Statistical mechanical Monte Carlo as well as classical molecular dynamic methods can be used to simulate structure, sorption, and, in some cases, even diffusion in heterogeneous systems. Kinetic Monte Carlo simulation is characteristically different in that the simulations follow elementary kinetic surface processes which include adsorption, desorption, surface diffusion, and reactivity . The elementary rate constants for each of the elementary steps can be calculated from ab initio methods. Simulations then proceed event by event. The surface structure as well as the time are updated after each event. As such, the simulations map out the temporal changes in the atomic structure that occur over time or with respect to processing conditions. 

The simulation of surfaces typically requires defining an appropriate lattice. These can either be a simple lattice model or off-lattice simulations which attempt to treat sites more exphcitly. The simulation proceeds in essentially the same maimer as described with the one exception that we explicitly follow the surface of the lattice. At any given instant in time the entire surface is surveyed in order to construct a detailed fist of all possible surface events that can occur, including adsorption, desorption, surface reaction, and diffusion. Each possible event is assigned a rate (or rate constant) based on the nature of the event and the explicit molecular environment around each species. The rates (rate constants) for each of these possible events are added together to determine the cumulative probability for that particular event. The computer draws a random number which is then used in Eq. (C5) [a modified version of Elq. (C3)] in order to establish the time step of the next event. 

Figure 3. Three dimensional representation of the primary desorption surface.

Long R Q, Yang R T (2001) Temperature-programmed desorption/surface reaction (TPD/ TPSR) study of Fe-exchanged ZSM-5 for selective catalytic reduction of nitric oxide by ammonia. J. Catal. 198 20-28. 

Surface reaction mechanisms include adsorption, desorption, surface nucleation, polynucleation, mononucleation and ion exchange reaction. The dependencies of amounts of precipitate and solution composition on time are different for each mechanism. For example, linear, exponential and logarithmic rate equations are established for volume diffusion, polynuclear growth and spiral growth, respectively. 

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