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Extrinsic precursor

To further demonstrate the power of the kinetic lattice gas approach we review briefly the work on precursor-mediated adsorption and desorption [60,61]. We consider an adsorbate in which, in addition to the most strongly bound chemisorbed (or physisorbed) adsorbed state, the adparticles can also be found in intrinsic or extrinsic precursor states. One introduces three sets of occupation numbers, , = 0 or 1, = 0 or 1, and /, = 0 or 1, depending... [Pg.470]

If we suppress the exchange of particles between the intrinsic and extrinsic precursors, ivie = 0, we get... [Pg.472]

VII Proconvertin Extrinsic Precursor of convertin (Vila) which activates X (extrinsic system)... [Pg.330]

Figure 14. Schematic representation of direct and precursor-mediated processes on a surface [129, 130]. Processes occurring along the surface normal are plotted along the abszissa. The processes are correlated with the potential energy diagram of Fig. 7(b) (ex = extrinsic precursor, in —intrinsic precursor, nc = number of impinging particles from the gas phase, a and a" are fractions of trapped molecules, p = probabilities. p"m = migration probability along the surface). Figure 14. Schematic representation of direct and precursor-mediated processes on a surface [129, 130]. Processes occurring along the surface normal are plotted along the abszissa. The processes are correlated with the potential energy diagram of Fig. 7(b) (ex = extrinsic precursor, in —intrinsic precursor, nc = number of impinging particles from the gas phase, a and a" are fractions of trapped molecules, p = probabilities. p"m = migration probability along the surface).
The Kislink factor, K, which takes values between 0 and 1, gives the degree of mobility of the precursor, with lower values associated with a highly mobile precursor. Values for the three CO/Pt systems are all around 0.4, indicating fairly mobile molecnlar extrinsic precursors. [Pg.180]

Even when the result of a gas—solid collision is the formation of a stable chemisorbed species, weakly bound precursor states can play a major role in the kinetic process. Evidence for such precursor states has recently been reviewed by Cassuto and King, [21] who draw a distinction between intrinsic precursor states, which exist at empty surface sites, and extrinsic precursor states, which exist over sites filled with chemisorbed species. The ability of colliding species to be trapped in these states and to be efficiently transported across the surface is an important mechanistic feature in adsorption. A confusion in nomenclature can arise when a metastable, or virgin , chemisorbed state can be formed on the surface as an intermediate between physisorbed and stable chemisorbed states for example, at low temperatures, a virgin, non-dissociatively chemisorbed state of CO is formed on tungsten which can be converted to a dissociatively bound state on heating [102]. In the few cases that have been investigated,... [Pg.62]

The striking kinetic consequence of the mobile extrinsic precursor — adsorption rates which may be effectively coverage-independent over a wide range of coverage — in fact constituted the first experimental evidence for its existence [2, 10]. It is not, however, the only evidence, as has recently been suggested [297]. [Pg.63]

Here, ka, kd and km are the rate coefficients for adsorption, desorption, and migration from the intrinsic precursor state, k m and are the rate coefficients for migration and desorption from the extrinsic precursor state, kv is the rate coefficient for transfer from the chemisorbed state to the intrinsic precursor state, and a and a are the trapping probabilities for molecules incident at intrinsic and extrinsic precursor sites, respectively. Direct transfer from gas phase to chemisorbed state or vice versa is included through the probability, sc, for adsorption and the rate coefficient, ftc, for desorption [427]. In order to generalise the rate expressions, we now introduce a group of terms F(0) which are only functions of the surface coverage 0. For a particular case, such as non-dissociative adsorption, these terms may be evaluated and inserted into the appropriate rate expression. [Pg.67]

Fm is the probability that an intrinsic precursor, in hopping, moves to a site configuration where an extrinsic precursor state can exist (=0 for non-dissociative adsorption). [Pg.67]

F is the probability that a collision takes place at an extrinsic precursor site (= 0 for non-dissociative adsorption). [Pg.67]

If the intrinsic and extrinsic precursors are energetically equivalent and each occupies only a single adsorption site, then the rates of non-dissociative and dissociative adsorption can be written as [37]... [Pg.337]


See other pages where Extrinsic precursor is mentioned: [Pg.470]    [Pg.470]    [Pg.115]    [Pg.194]    [Pg.196]    [Pg.199]    [Pg.285]    [Pg.286]    [Pg.245]    [Pg.221]    [Pg.63]    [Pg.63]    [Pg.66]    [Pg.68]    [Pg.337]    [Pg.269]   
See also in sourсe #XX -- [ Pg.11 ]




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