Direct activated adsorption

For direct activated adsorption, the dominant effect is that of the energy of the gas phase molecules [eq. (7)], though the distribution of this energy into the various degrees of freedom of the molecule can be crucial, as described in sect. 3.1 below. 

It can be the case that both adsorption channels are important for a particular system. Examples of this are given here for O2 adsorption on Ag and Cu and for N2 dissociation on Fe. In these cases we can generalise and say that the precursor mediated route tends to dominate at low substrate and gas temperatures, while direct activated adsorption dominates at high gas temperatures. Furthermore, in all these cases, molecular chemisorbed states of adsorption can exist which complicate the pathway of adsorption. A one dimensional potential energy profile is shown in fig. 8 for the case of O2 adsorption on Ag taken from the work of Dean and Bowker (1988/89, 1989) and of Campbell (1985), although this is likely to be a general representation for this type of adsorption system with other adsorbate/metal combinations. 

The results summarized in the previous sections show that, for the considered sample of a-TiCU, there are two types of adsorption of the triethyl-aluminum on a-titanium trichloride, one of them is connected with active centers which are directly active in the stereospecific polymerization of propylene. 

The importance of lattice coupling in direct molecular dissociation is at present poorly understood. However, there are at least two ways in which inclusion of the lattice can affect direct dissociative adsorption. First, conversion of Et to Eq competes with translational activation in dissociation. Second, thermal distortion of lattice atoms from their equilibrium positions may affect the PES, e.g., the barriers to dissociation V ( ). These two effects can be most simply thought of as a phonon induced modulation of the barrier along the translational coordinate and in amplitude, respectively. 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

Combinations of Bi203 and Mo03, promoted by P2Os at a constant P/Mo ratio (0.2) were studied over a full composition range by Ai and Ikawa [6], Acidity (and basicity) were measured directly by adsorption of compounds like ammonia, pyridine and acetic acid. The effect of the Bi/Mo ratio on the acidity (Fig. 14) parallels the effect on the overall butene oxidation activity [presented in Fig. 5, Sect. 2.3.2(a)(i)]. 

Results pointing in the same direction were obtained recently by Schuit and De Boer (17), who found that activated adsorption of hydrogen occurs only on a partially oxidized surface of nickel supported on silica (3 1) but not on a thoroughly reduced surface. According to Schuit and De Boer, very prolonged evacuation or heating of a reduced nickel catalyst in an inert atmosphere leads to a slow activated hydrogen adsorption. This effect, however, disappears on renewed careful reduction... 

Activated adsorption without a precursor. This is characterised by (a) exponential increase in rate with increasing temperature, (b) continuous fall in rate with increasing coverage, and (c) rate directly proportional to pressure. 

AQst decrease most rapidly as the coverage ratio increases indicating the presence of active adsorption sites that are occupied by the first molecules admitted on the talc surface. A direct proof of surface heterogeneity is also given by the deviation observed, at low partial pressures, between the BET model and the actual isotherm. 

More recently, Ustinov and coworkers [72, 73] developed a thermodynamic approach based on an equation of state to model the gas adsorption equilibrium over a wide range of pressure. Their model is based on the Bender equation of state, which is a virial-like equation with temperature dependent parameters based on the Benedict-Webb-Rubin equation of state [74]. They employed the model [75, 76] to describe supercritical gas adsorption on activated carbon (Norit Rl) at high temperature, and extended this treatment to subcritical fluid adsorption taking into account the phase transition in elements of the adsorption volume. They argued that parameters such as pore volume and skeleton density can be determined directly from adsorption measurements, while the conventional approach of He expansion at room temperature can lead to erroneous results due to the adsorption of He in narrow micropores of activated carbon. 

It must be noted that this is a schematic diagram where the abscissa is not a linear distance scale instead it represents the trajectory pathway of an incoming molecule to a surface. Dissociative adsorption can occur from a weakly held molecular state if the net barrier to adsorption is low (precursor mediated) but is of low probability if it is high. Then it is only the hot molecules of the Maxwell Boltzmann distribution of velocities (fig. 9) which can dissociate and they do this by direct passage over the energy barrier (direct activated). The rate of dissociation from a precursor state can be written as follows for the simple case in fig. 9,... 

Adsorption in such cases is regarded simply as an accumulation of the gas at the surface of the metal and the increase in catalytic activity is assumed to be due to an increase in the velocity of the reaction arising from an increase in the concentration of one of the components of the reacting system. If this assumption is time it follows, however, that catalytic activity should vary directly with adsorptive power. Catalysis would then be as Ostwald 88 defined it, only the acceleration of a chemical phenomenon which otherwise would take place slowly. This aspect of the problem has been made the subject of exhaustive investigation and au effort made to prove or disprove certain deductions which depend upon the veracity of the theory viz.,... 

---