Chemisorbed precursor state

The picture that emerges is that the bonding within the majority of thiophene molecules adsorbed on the catalyst surfaces is hardly perturbed, and this contrasts sharply with the situation in the thiophene complexes. The thiophene molecule parallel to the surface does not correspond to a metal f/ -bound thiophene. Rather, it is suggestive of a weakly chemisorbed precursor state of thiophene that lies parallel to the surface. In this state, the molecule interacts indiscriminately with the alumina, the basal or edge planes, or both. Moreover, the weakness of this binding enhances the surface mobility of thiophene and allows molecules to move across the surface to the catalytic site for reaction with hydrogen atoms. The few sulfur-bound thiophene molecules, no more than 5-10%, would then correspond to thiophene at the coordinatively unsaturated Mo (or Co) atoms. 

The presence of a precursor breaks the dynamical motion into tliree parts [34], First, there is the dynamics of trapping into the precursor state secondly, there is (at least partial) thennalization in the precursor state and, thirdly, the reaction to produce the desired species (possibly a more tightly bound chemisorbed molecule). 

In most cases surface reactions proceed according to well-established elementary steps, as schematized in Fig. 1. The first one comprises trapping, sticking, and adsorption. Gaseous reactants atoms and/or molecules are trapped by the potential well of the surface. This rather weak interaction is commonly considered as a physisorbed precursor state. Subsequently, species are promoted to the chemisorbed state, that is, a much stronger... 

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... 

It has been proposed that the precursor state [81, 82] for the adsorption-desorption reaction consists of weakly physisorbed CO. This can be CO sitting on an occupied site (COad-CO) or on an sterically unfavorable Pt site. According to Ertl [81], the desorption process occurs through a trapping mechanism on such sites if the surface is saturated by chemisorbed CO the desorption channel involves either a COad-CO potential well or a Pt-CO attractive well which is sterically weakened by the presence of pre-absorbed CO . 

STM experiments are in general agreement with the picture above. STM directly observes the two different molecular precursor states at low Ts and observes their dissociation by thermal annealing, photochemistry and viatunneling electrons [153,326]. The molecularly adsorbed states and dissociated show a strong Ts dependent clustering at low Ts, and this is evidence for a very mobile physisorbed precursor since the molecularly chemisorbed states are not significantly mobile at these Ts [326,327]. 

Fio. 16. Schematic representation of adsorption and desorption including a precursor state, ft is the probability for adsorption in the precursor statand i. 1a are the probabilities for diffusion and desorption from the precursor and chemisorbed states, respectively. f, and fa are the probabilities for the transition from the precursor state to the chemisorbed state and for the inverse process, respectively 101). 

The kinetic and dynamical aspects of the dissociative adsorption of 02 on the Pt(l 1 1), and surfaces vicinal to Pt(l 11), has been investigated in some detail. It provides a good example of precursor mediated dissociation, but is complicated by the fact that both physisorbed and chemisorbed molecular precursor states are involved, and access to the chemisorbed precursor is activated. It is also a good example of the role of step and defect sites in the overall conversion of the precursor states. The adsorption system has the advantage that the characterisation of a number of molecular and atomic states has also been the subject of considerable attention. 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

A low value for Kp means a big precursor effect since k 4 is low, that is the probability of desorption is low and the lifetime in the precursor state is high allowing a wide area of diffusion on the surface and thus a high probability for adsorption into the final chemisorbed state. Approximate diffusion circles are shown in fig. 13 based on the following simple Frenkel relationships,... 

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