Solid nickel powder is used industrially to catalyze the hydrogenation of ethene to ethane at 150 °C and 3.0 atm total pressure. Assume that the only adsorbing species of consequence are H2 and C2H4 and that the catalytic surface possesses a fixed, large number of identical sites.
(a) Write a logically ordered sequence of elementary surface events (adsorption, surface reaction, and desorption) that converts gaseous C2H4 and H2 into gaseous C2H6 while regenerating a free nickel site after each catalytic cycle.
(b) Describe, in terms of molecular orientation and activation-energy changes, two distinct ways in which the nickel surface increases the rate of this reaction compared with the uncatalyzed gas-phase process.
(c) Contrast the role played by the nickel surface in this heterogeneous reaction with the role played by a homogeneous acid catalyst in an electrophilic addition to an alkene. Focus your answer on (i) how each catalyst binds reactants, (ii) how intermediates are stabilized, and (iii) how the catalyst is regenerated.
(d) The measurable amount of nickel in the reactor does not decrease during the run even though the metal participates in every catalytic cycle. Explain why the net concentration of the catalyst remains constant and clarify how this observation is consistent with the sequence you proposed in part (a).