The hydrogenation reaction: A + H2 B occurs over a catalyst. Two mechanisms have been postulated for this reaction, mechanism I that involves a single type of catalyst site, and mechanism II that involves two different sites. The mechanisms are shown below: KH2,1 KA,2 Mechanism 1, single site: H2 + 0 + H2O A+OHA.O k3 H2:0+A:0 Boto k-3 B to Bio K3,4 (1) (2) (3) (4) Mechanism 2, two sites: H2 + 01 + H2:01 K42,5 (5) A + 02 A02 KA,6 (6) H2:01 +A:02 B:02 +01 (7) K-7 B + 02 + B:02 KB,8 (8) kz Initial rate data in experiments where the H2 concentration was held constant, indicated that at low partial pressures of A the rate increased as A partial pressure increased but then it approached an asymptotic limit. Similarly, if the partial pressure of A was held constant and the partial pressure of H2 increased, the rate increased with increasing hydrogen partial pressure at low partial pressures and then approached an asymptote. Also, it was observed that the initial rate decreased with increasing concentration of product B. a. Assuming that the surface reactions (3 and 7) are reversible, and that they are the limiting steps for mechanisms I and II , respectively, derive the Hougen-Watson expressions for each of these mechanisms. Also indicate the initial rate form for these equations. b. Is either of these mechanisms consistent with the experimental observations? Assume that the initial rate data cover a wide range of partial pressures so that all predicted trends will be observed. Explain. Assume dissociative adsorption of H2 on two sites of the same type; is this assumption a possible alternative explanation to either of the two mechanisms? For this case the limiting step will be the termolecular surface reaction between two adsorbed H atoms and the adsorbed A molecule. Explain. c. The hydrogenation reaction: A + H2 B occurs over a catalyst. Two mechanisms have been postulated for this reaction, mechanism I that involves a single type of catalyst site, and mechanism II that involves two different sites. The mechanisms are shown below: KH2,1 KA,2 Mechanism 1, single site: H2 + 0 + H2O A+OHA.O k3 H2:0+A:0 Boto k-3 B to Bio K3,4 (1) (2) (3) (4) Mechanism 2, two sites: H2 + 01 + H2:01 K42,5 (5) A + 02 A02 KA,6 (6) H2:01 +A:02 B:02 +01 (7) K-7 B + 02 + B:02 KB,8 (8) kz Initial rate data in experiments where the H2 concentration was held constant, indicated that at low partial pressures of A the rate increased as A partial pressure increased but then it approached an asymptotic limit. Similarly, if the partial pressure of A was held constant and the partial pressure of H2 increased, the rate increased with increasing hydrogen partial pressure at low partial pressures and then approached an asymptote. Also, it was observed that the initial rate decreased with increasing concentration of product B. a. Assuming that the surface reactions (3 and 7) are reversible, and that they are the limiting steps for mechanisms I and II , respectively, derive the Hougen-Watson expressions for each of these mechanisms. Also indicate the initial rate form for these equations. b. Is either of these mechanisms consistent with the experimental observations? Assume that the initial rate data cover a wide range of partial pressures so that all predicted trends will be observed. Explain. Assume dissociative adsorption of H2 on two sites of the same type; is this assumption a possible alternative explanation to either of the two mechanisms? For this case the limiting step will be the termolecular surface reaction between two adsorbed H atoms and the adsorbed A molecule. Explain. c
