Total pressure dissociative adsorption

The interesting point is that only the ratio of the partial pressures of the reactants enters this equation, but not the total pressure. The physical reason is obvious. Both reactants are competing for free adsorption sites on the surface and since CO inhibits the dissociative adsorption of oxygen the partial pressure of the latter compound enters into the denominator of the rate expression. This result indicates that the actual surface composition and, in addition, the reaction mechanism will not be affected by the total pressure. The only effect of increasing the total pressure (at a fixed p0l pco ratio) will be that rmax will be shifted towards higher temperatures since reduction of... 

Using activation energies for CO dissociation and carbon hydrogenation reported in the literature (i.e., 230 and 145 kJ/mol, respectively), calculations showed that changes in heats of adsorption of the reactants H, and CO on Ni significantly alter methanation behavior. Turnover frequencies for the Ni and TiOx/Ni surfaces are plotted in an Arrhenius fashion in Figure 9 for a total pressure of 120 Torr and an H,/C0 ratio of 4. The plots show that... 

Figure 15-1 Total pressure dependence of the best pseudo-first-order kinetic rate constant when a first-order rate law approximates a Hougen-Watson model for dissociative adsorption of diatomic A2 on active catalytic sites. Irreversible triple-site chemical reaction between atomic A and reactant B (i.e., 2Acr - - Bcr -> products) on the catalytic surface is the rate-limiting step. The adsorption/desorption equilibrium constant for each adsorbed species is 0.25 atm. ...

Upon switching, although the gas-phase isotopic label is changed from 16 to 18, the overall steady state between dissociative adsorption and recombinative desorption is maintained — the total pressure of O2 and the total coverage of surface oxygen remain constant. Using Eqs. 6 and 7, and taking F(t) as a step function rather than an exponential decay, we have ... 

Larsson et al. (1998) performed DHP kinetic experiments on a Pt-Sn/ Y-AI2O3 catalyst (0.54 wt% Pt and 1.53 wt% Sn) under these experimental conditions temperature 507-547 °C, total pressure 1.5 bar, and flow rate 60ml/min (propane 18.1—27.2 ml/min, propylene 2.7—4.5 ml/min, and H2 4.1-6.7 ml/min). A power-law model (see Table 2.4), and two categories of L-H models with dissociative adsorption of propane and propylene desorption as the rate-determining step (RDS) respectively were proposed. But the authors had difficulties in obtaining some important parameters mainly due to the fact that the experiments were conducted in a relatively limited range of operating condition. Finally, they adopted a power-law equation for the kinetic model. 

Using the notation that is the activity-based equilibrium con.stant for the dissociation reaction, develop expressions for- the amount of molecular hydrogen adsorbed as a function of the equilibrium constants and the hydrogen partial pressure for the two cases (adsorption without dissociation and adsorption with dissociation). How would you discern which process was occurring if you had experimental data on the total hydrogen adsorption as a function of its partial pressure ... 

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