Question

Problem 4: Horiuti-Polanyi Mechanism and Alkene Hydrogenation The hydrogenation of alkenes by heterogeneous catalysts has been studied for 80 years and has a mechanism that was proposed by Horiuti and Polanyi in 1934 which consists of 3 steps: i) alkene adsorption on the surface of the hydrogenated metal catalyst, ii) hydrogen migration to the beta- carbon of the alkene and finally ii) a reductive elimination of the free alkane Ki = k/k! k4 Assume that *, H*. C2H*, and C2Hs* are the MASI and all reversible reactions are QE, with capital K's representing equilibrium constants. a) Determine the overall rate law for this reaction. b) Derive an expression for the apparent activation energy assuming that *, H*, C2H4*, and C2Hs* are the MASI

Answer 1

Determine the overall rate law for this reaction

Ans: The products obtained are consistent with those expected from the Horiuti−Polanyi mechanism.

The second step is the Slow step, Hence the rate of the reaction will be depends upon this step i.e. hydrogen migration to the β-carbon of the alkene with formation of a σ-bond between the metal and α-C.

Therefore, rate = k [C₂H₄]