Role of Spin in the Catalytic Oxidation of CO by N2O Enabled by Co+: New Insights from Temperature-Dependent Kinetics and Statistical Modeling.

Abstract

The catalytic oxidation of CO by N2O promoted by Co+ was studied as a function of temperature in a variable-ion source temperature-adjustable selected-ion flow tube (VISTA-SIFT). Each step of the cycle, Co+ + N2O and CoO+ + CO was studied individually for unambiguous interpretation of the results. The rate constant of CoO+ + CO is (1.5 ± 0.4) × 10-10 × (T/300 K)-0.7±0.2 cm3 s-1 is in disagreement with a previously reported upper limit of 10-13 cm3 s-1, with the discrepancy likely due to the earlier report having studied the reactions in tandem. The reaction of Co+ + N2O produces CoO+ with a much smaller rate constant of 1.4 ± 0.4 × 10-12 cm3 s-1 at 300 K. The association product, Co(N2O)+, was also produced with a rate constant of 1.6 × 10-28 cm6 s-1. While the rate constant for termolecular association decreased with temperature in accordance with a decreasing time scale for stabilization, the production of CoO+ increased with temperature in a manner that is not well described by simple functional forms. Statistical modeling of calculated reaction coordinates was employed and the experimental data reproduced only by assuming an intersystem crossing to yield ground state CoO+ occurring competitively with the spin-allowed formation of excited state CoO+.