Precursor state diffusion

The first step always occurs since, attractive forces between the undissociated molecule and the surface usually exist. This step may involve adsorption into a so-called precursor state where the molecule is mobile and diffuses across the... 

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 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. 

Historically, stabilized (and partially stabilized) zirconia ceramics were prepared from powders in which the component oxides are mechanically blended prior to forming and sintering. Because solid state diffusion is sluggish, firing temperatures in excess of 1800°C are normally required. Furthermore, the dopant was nonuniformly distributed, leading to inferior electrical properties. Trace impurities in the raw materials can also lead to enhancement of electronic conductivity in certain temperature ranges, which is also undesirable. To overcome these problems, several procedures have been developed to prepare reactive (small particle size) and chemically pure and homogeneous precursor powders for both fully stabilized and partially stabilized material. Two of these are alkoxide synthesis and hydroxide coprecipitation. 

Particle fonnation is thought to have proceeded as follows Metal oxide molecules formed as the aerosol vapor precursor reacted near the jet orifice. The oxide molecules collided to form particles that grow by the colli.sion-coalescence mechanism until the temperature fell to the point where coalescence was quenched. Particle coalescence was probably driven by solid-state diffusion and. perhap.s. surface diffusion. Metal oxides with higher diffusion coelTicients would be expected to fomt larger primary particles because they continue to coalesce at lower temperatures during the cooling period. 

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,... 

Fig. 27. Model Tor nitrogen adsorption on W(320) showing diffusion in a precursor state, dissociation at active (100) steps and diffusion of the atoms onto otherwise inactive (110) terraces. From Singh-Boparai et al. (1975)...

There are, however, three very important implicit assumptions in this model, apart from those of an ideal interface. Firstly, since desorption is only allowed to occur from a constant precursor state population (dn/dt = 0), it is effectively always a zeroth-order process. If a different order is observed, desorption is not the rate-limiting step. The second point is that this treatment is only appropriate for cases where the metal-metal bond energy (around the peripheries of the islands) is less than that for the metal- semiconductor, since for the opposite case the weaker adsorbate—surface bond will not prevent an atom desorbing once it has acquired sufficient energy to break the (stronger) metal—metal bond. Thirdly, no provision is made for possible diffusion of the adsorbate into the substrate during desorption. 

These diagrams are helpful in understanding how chemisorption of hydrogen occurs, but they have their limitations they imply for example that all hydrogen atoms are chemisorbed in the same way and with the same strength, but this as we shall see is rarely true. The initial collision may, for example, give atoms at places where they are uncomfortable and from which they may diffuse to more energetically favourable positions. This initial condition constitutes an atomic precursor stated... 

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