Surface reaction probability

Selected surface reaction probabilities (7) for irreversible reactive uptake of trace gases on condensed surfaces.a 

Reaction Surface Type Temp. (K) 7 Un- certainty Factor 

Note 1 Values of 7 are strongly dependent on the H2SO4 concentration of the droplet, increasing with decreasing [H2SO4]. 

---

The plasma-wall interaction of the neutral particles is described by a so-called sticking model [136, 137]. In this model only the radicals react with the surface, while nonradical neutrals (H2, SiHa, and Si H2 +2) are reflected into the discharge. The surface reaction and sticking probability of each radical must be specified. The nature (material, roughness) and the temperature of the surface will influence the surface reaction probabilities. Perrin et al. [136] and Matsuda et al. [137] have shown that the surface reaction coefficient of SiH3 is temperature-independent at a value of = 0.26 0.05 at a growing a-Si H surface in a... 

Heterogeneous uptake on surfaces has also been documented for various free radicals (DeMore et al., 1994). Table 3 shows values of the gas/surface reaction probabilities (y) of the species assumed to undergo loss to aerosol surface in the model. Only the species where a reaction probability has been measured at a reasonable boundary layer temperature (i.e. >273 K) and on a suitable surface for the marine boundary layer (NaCl(s) or liquid water) have been included. Unless stated otherwise, values for uptake onto NaCl(s), the most likely aerosol surface in the MBL (Gras and Ayers, 1983), have been used. Where reaction probabilities are unavailable mass accommodation coefficients (a) have been used instead. The experimental values of the reaction probability are expected to be smaller than or equal to the mass accommodation coefficients because a is just the probability that a molecule is taken up on the particle surface, while y takes into account the uptake, the gas phase diffusion and the reaction with other species in the particle (Ravishankara, 1997). 

Surface reaction probability (ylxn) is the net fraction of gas-condensed phase collisions that leads to the irreversible uptake of the gas due to chemical reaction. The symbol yrxn (or sometimes commonly used for reaction probabilities. 

Similarly, there is some experimental evidence for surface reactions of organics. For example, Schweitzer et al. (1998) report evidence for a surface reaction (probably protonation) of glyoxal, (CHO)2, on acid surfaces at temperatures below 273 K. 

D. A. Doughty, J.R. Doyle, G.H. Lin, A. Gallagher Surface reaction probability of film-producing radicals in silane glow discharge. J. Appl. Phys. 6T, 6220 (1990)... 

Form these two reactions the reactant fraction is given by f = I / G + 1) provided that the reactant flow rates are sufficiently large that product gases do not significantly accumulate inside the furnace. Under this restriction, the reactant fraction is f = 0.33 for a relative flow rate of G = 2. From the surface reaction we see that two moles of gas-phase product are produced for each mole of reactant, giving / = 1 and a normalized reactant yield of = /. = 0. 33. The parameters and b for the surface reaction probability are approximately 147 kJ/mol and 446, respectively. ... 

In the recollision method the probability of multiple collisions with the reaction site are considered. The reaction probability (]3 ) is expressed in terms of the recollision probability ), which is the probability that a molecule starting at r = f collides with the reaction surface rather than escaping to r=b, and the first collision probability (1 ), which is the probability that a molecule starting at the initiation surface collides with the reaction surface rather than escaping to r = oo. Both these probabilities are obtained from Brownian dynamics. In addition, the surface reaction probability ([Pg.812]

The conclusion is that for relatively small molecules (H2, CO2, etc.), permeation in microporous (silica) membranes is not limited by surface reactions and direct penetration in the pores is the dominant mechanism in a wide range of temperature and pressure conditions [63]. This conclusion does not hold for large non-spherical molecules. Here sorption is necessary, the sticking coefficient becomes very important and surface reactions probably will limit the permeation as soon as bulk permeation becomes appreciable. To the knowledge of the present author, no investigations of this phenomenon in microporous membranes have yet been reported. 

Nuruddin, A., Doyle, J. R., and Abelson, J. R., Surface reaction probability in hydrogenated amorphous silicon growth. 7. Appl. Phys. 76, 3123-3129 (1994). 

The mechanism of the Grignard reaction has not been specified precisely. The reaction takes place at the metal surface. Reaction probably commences with an... 

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. 

Figure B3.4.10. Schematic figure of a ID double-well potential surface. The reaction probabilities exliibit peaks whenever the collision energy matches the energy of the resonances, which are here the quasi-bound states in the well (with their energy indicated). Note that the peaks become wider for the higher energy resonances—the high-energy resonance here is less bound and Teaks more toward the asymptote than do the low-energy ones.

The lime is mixed with water and volcanic ash and used to bond stone, brick, or even wood. The water reacts with lime, turning it into Ca(OH)2 but in doing so, a surface reaction occurs with the ash (which contains SiOj) probably giving a small mount of (Ca0)3(Si02)2(H20)3 and forming a strong bond. Only certain volcanic ashes have an active surface which will bond in this way but they are widespread enough to be readily accessible. 

Let us thus consider a model in which the association energy depth changes when two reacting particles are approaching the surface see Refs. 86,90. If in the vicinity of the surface the binding energy is lower than it is far from the surface, the probability of the chemical reaction to occur in the surface zone decreases. Similarly to the previous case, we consider an equimolar mixture of associating hard spheres of equal diameters. The interaction between the species a and (3 is assumed in the form... 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Figure 6. Initial rovibrational state specified reaction probabilities. Solid line exact quantum mechanical numerical solution. Solid line with solid square generalized TSH with use of the nonadiabatic coupling vector. Solid line with open circle generalized TSH with use of Hessian. Sur= 1(2) means the ground (excited) potential energy surface. Taken from Ref. [51].

Note that the rates of product formation and reactant conversion indeed have the dimensions of mol per unit of time, and that these rates are proportional to the number of sites, or, in fact, the amount of catalyst present in the reactor. Also, in the case of a second order reaction, e.g. betv een adsorbed species A and B, we write the rate in the form r = Nk0j 0 by applying the mean-field approximation. Here the rate is proportional to both the total number of sites on the surface and the probability of finding a species A adjacent to a species B on the surface, the latter being proportional to the coverages of A and B. In the mean-field approximation A and B are distributed randomly over the N available sites this only tends to be valid when the adsorbents repel each other. Thus the rate is not r= k(N0/ )(N02,) since the reactants need to be on adjacent sites. Another important consideration is that we want the rate to be linearly proportional to the amount of catalyst in the reactor, in accordance with r = Nk0A0B for a second order surface reaction. 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

The mechanism Implied by these expressions Is that of a submonolayer of surface carbon which blocks n sites for NH adsorption. This suppresses NH decomposition which would occur on the clean surface (Figure 3(a)) and allows NH fragments to react with carbon to form HCN. Selectlvltles of HCN production exceed 90% at 1 Torr, and reaction probabilities (fraction of CH flux yielding HCN) approach 0.01. 

---