Heats of dissociative adsorption

Qm is the heat of adsorption in the molecular state. Qd is the heat of dissociative adsorption, and r... 

It is often observed that molecular adsorption prevails at lower temperatures and that dissociative adsorption occurs at higher temperature. This may be caused by kinetics the activation energy for dissociative adsorption is then too high for dissociation at lower temperature. It could also have a thermodynamic reason. If the number of surface sites where adsorption can take place is equal for molecular and dissociative adsorption, the surface can accommodate twice as many molecules in the molecular state than in the dissociative state. Hence, molecular adsorption will prevail if the heat of dissociative adsorption is not very much higher than the heat of molecular adsorption. The entropy change upon... 

The calculated heats of dissociative adsorption of water, ammonia, and methyl alcohol (Fig. 19) are also presented in Table VII. The heat of water adsorption was 61 kcal/mol (CNDO/BW) or 43.7 kcal/mol (STO-3G), and the respective value for ammonia was 38.7 kcal/mol (CNDO/BW). If the pseudo-atoms A model the lattice silicon atoms, structure (d) is somewhat... 

The decreased heat of dissociative adsorption of O2 to Pt by alloying with Au has this as an explanation. Also the weaker interaction energy of C atoms to such alloys compared to non-alloyed Pt is a consequence of this ensemble type effect. It is called the secondary ensemble effect. 

We calculated and ko (Table 2.6 the first values for x=2 and 1=2-4) according to Zhuravlev (2000), but obtained E values are lower than experimental heats of dissociative adsorption of water (2=100 - 280 kJ/mol) on different oxides. Therefore, we recalculated the E and ko values for bimolecular processes (Table 2.6 the second values for x=2 and 1=2-4). An increase of E leads to the growth of values, which corresponds better to the defined range for kg (Zhdanov 1991). 

Table 1.17 Heats of dissociative adsorption of N2 from observed and calculated ...

Estimation from the heat of dissociative adsorption of NO and O2. From Table 1.18. 

Table 3.2. Observed and calculated heats of dissociative adsorption of N2...

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

It has been learnt from [105] that nitric oxide is mostly adsorbed on metal oxides in a dimer form below 110 K. The heat of dissociation of the dimer adsorbate on the MgO surface (NO)2 = 2NO (AHdis = 3.2 kcal/mol) was measured in [104], The EPR signal of the dimer form in the system S0 2 /Zv02 + NO has been recorded, and parameters gx = gy = 1.993, gz = 1.940, D = 195 G were calculated [102]. The triplet-state NO-NO species were also observed after NO adsorption in Na-A zeolite gx = gy = 1.976, gz = 1.912, D =... 

In Fig. 4.52 the activity of H2 oxidation for different metals is plotted as a function of heat of oxygen adsorption. A volcano-type plot is formed. A maximum in rate is found for the metal that can dissociate 02, but does not bind CO or oxygen too strongly. Gold cannot dissociate 02, tungsten is inactive because it... 

Many catalysts adsorb hydrogen by a dissociative chemisorption mechanism. The heat of chemisorption (Qhz) being positive, such a mechanism involves a heat of adsorption of two hydrogen atoms (2Qh) which is larger than the heat of dissociation (DH2) of the hydrogen molecule (Fig. 1). 

---