Platinum supported on alumina catalyzes the oxidation of carbon monoxide in automobile exhaust according to 2CO(g)+O2(g)→2CO2(g). At 500K, a catalyst pellet contains 1.8×10−6mol of surface sites uniformly accessible to the gas. Under steady flow, the following gas composition is maintained over the catalyst: PCO=30mbar, PO2=20mbar, balance inert gases. The material balance shows that CO is consumed at a rate of 1.1×10−4mols−1.
(a) Calculate the turnover frequency (TOF) in s−1 for CO conversion under the stated conditions. Express TOF as the number of CO molecules reacted per surface site per second.
(b) When PO2 is increased to 100mbar while PCO remains 30mbar, the CO consumption rate drops to 4.0×10−5mols−1. Provide a mechanistic explanation for why a higher O2 pressure can suppress the reaction rate, even though O2 is a reactant.
(c) Using the rate given in part (b), determine the fraction of surface sites that remain free (not occupied by any adsorbate) if the intrinsic surface reaction step requires two adjacent free sites—one bearing CO∗ and one bearing O∗—to proceed. State clearly any approximations you employ.
(d) Propose one operational strategy that could restore or enhance the catalytic rate when high O2 concentrations are unavoidable, and justify how your strategy addresses the surface phenomenon discussed in part (b).