Structure-insensitive deactivation

For a structure-insensitive deactivation Ctfp = 0)> when the reactant adsorption is also preferred on edges Ctf <0)> smaller particles deactivate less strongly, while large particles (15 nm) undergo a steep decline when edges were poisoned. On the contrary, when the reactant adsorption is preferred on terraces Ctf > 0)> deactivation profiles are independent on the cluster size. 

The deactivation of metal sites and acid sites on heterogeneous catalysts during structure insensitive reactions of hydrocarbons is illustrated. 

A little work on structure-insensitive reactions has been reported [18]. Both catalysts were very active for ethene hydrogenation, and rapid deactivation occurred even at 176 K. Ethyne and 1,3-butadiene react in a more controlled manner study of ethyne hydrogenation using both l4C-labeled ethyne and ethene showed that ethane formation took place directly from adsorbed ethyne, without the intervention of gas-phase ethene. 

When a catalyst is exposed to a stoichiometric gas mixture, whether oscillating or stationary, the resulting deactivation is very moderate when the temperature of the pretreatment is raised from 500°C to 900°C. This deactivation can only be proven by the values of the T50 recorded for the reaction between NO and HC and not for that between CO and O2. This confirms that the CO/O2 reaction is structure insensitive. Indeed, a moderated sintering of the metallic phase, as seen by microscopy, has no influence upon the reaction when compared to a catalyst aged from 500°C to 900°C. The implication is that the best way to observe an effect related to sintering is to consider the reactions involving HC or NO since they are usually structure sensitive [15,16]. 

Specific to the active metal, the exposed surface area and metal particle size most often decrease with deactivation. Specific adsorbents such as H2, CO, or H2-O2 are most often employed to estimate the metal surface area. It is not clear if VOC is a structure sensitive or structure insensitive reaction, i.e., whether the activity is proportional to exposed metal or not. In reducing environments the activity will be strictly proportional to exposed metal area for structure-insensitive reactions. In oxidizing environments, as in these reaction conditions, platinum may exist in a partially oxidized state. The techniques currently employed in catalyst characterization do not currently attempt to differentiate between exposed metal and exposed oxidized metal. 

The effect of changing active sites in deactivating catalysts is described by the structure-sensitive or structure-insensitive nature... 

Boudart et al. for supported metal catalysts to indicate whether the rate of reaction is nonlinearly or linearly proportional to the total surface area of the metal, but the concept can be extended to other types of catalysts as well. Thus, for acid catalysts, the rate of formation of a particular species may depend upon the total number of acid sites, or upon the number of a particular type, e.g., a particular range of acid strengths. In general, the deactivation process may diminish the number of a particular type of active sites, while the rate of formation of each product may depend upon other types of active sites (which may include part or all, or none, of the first type). The interrelation between these types denotes whether the various reactions are structure-sensitive or structure-insensitive, and determines how the selectivities change with... 

It has been seen that transport resistances and catalyst deactivation can have a significant effect on selectivity. It has also been seen that the effect of transport resistances is not necessarily detrimental, even though the selectivity is, in general, adversely affected by diffusion for consecutive reactions. The selectivity is independent of deactivation for structure-insensitive reactions when the same sites are involved for all reactions, provided that the concentration and temperature do not change with deactivation. This condition is rarely met in reactor operation and therefore, the selectivity changes with the extent of deactivation. 

The coke content due to multilayer coke or coke growth is contained in the second term in the bracket of Eq. 5.93 since Cc/Q = y when only monolayer coke is involved. For strictly monolayer coking, Kp approaches infinity, kp approaches zero, and Eq. 5.93 reduces to that for monolayer coking. For structure-insensitive reactions, the fractional catalytic activity remaining after deactivation, A. is equal to (1 — y). This can be used in Eq. 5.93 to obtain the following relationship ... 

The boundary conditions are the usual ones at the pellet center and surface. Here, the fraction of catalyst deactivated, y, is dependent on the pellet coordinate z. Without loss of generality, it may be assumed that the reaction is structure-insensitive, as indicated by the effective rate constant fc(l — y) in Eqs. 10.31 and 10.32. The rate of deactivation is much slower than the rate at which concentration and temperature reach their steady state values. This condition leads to the pseudosteady state assumption with respect to the reactor conservation equations. With this assumption, the fraction of catalyst deactivated can be expressed (Chapter 5) as ... 

Benzene is the sole product and no deactivation was observed. Both the dispersion and the rate decreased in the presence of a second metal however, the intrinsic activity (TOP) remained constant around 1 (s ), indicating that the rate is proportional to the surface active sites, and thus it is a structure-insensitive reaction. 

For a case when poisoning of edges is predominant (jp = —10) and the reaction is either structure-insensitive or reactant adsorption is preferred on terraces (x > 0). larger particles deactivate slower that smaller ones, as expected. When poisoning of edges is predominant (Xp = — i-0) and the reactant adsorption is also preferred on edges Cl " <0), the situation is less straightforward. The catalyst activity decay depends on the absolute values of these... 

The oxidation of CO by either 02 or NO was studied by Peden et al. and Oh et al. over Rh, Pd, Pt,and Ir single crystals (90-92). The CO + 02 reaction was relatively insensitive to the atomic structure of the surface, and the specific activities and kinetic parameters agreed for both crystal surfaces and for alumina-supported catalysts. The Rh surfaces deactivated at high 02 pressures due to the formation of a near-surface oxide (91, 92). On the other hand, the CO + NO reaction was very sensitive to... 

---