Catalytic Oxidation of CO by N₂O Enabled by Al₂O_2/3⁺: Temperature Dependent Kinetics and Statistical Modeling

Abstract

The reactions of Al2O2 + + N2O and Al2O3 + + CO, forming a catalytic cycle oxidizing CO by N2O, have been investigated from 300 to 600 K in a variable ion source, temperature adjustable, selected-ion flow tube (VISTA-SIFT). Reaction coordinates have been calculated using density functional theory and statistical modeling of those surfaces compared to experimental kinetics data for mechanistic insight. The reaction of Al2O2 + + N2O proceeds at the Su–Chesnavich collisional limit at all temperatures studied, yielding only Al2O3 +, with the exception of a small (<5%) amount of association product, Al2O2(N2O)+ at 300 K. The reaction of Al2O3 + with CO produces Al2O2 + with a rate constant of 4.7 ± 1.2 × 10–10 cm3 s–1 at 300 K, decreasing with temperature as T –0.5±0.2. In addition, a significant amount of association product, Al2O3(CO)+, was observed with rate constants for formation ranging from 10–11 to 10–10 cm3 s–1 dependent upon He buffer gas concentration and temperature. Implications of these kinetic measurements with regard to the reactive surface are discussed.

Old School Techniques with Modern Capabilities: Kinetics Determination of Dynamical Information Such as Barriers, Multiple Entrance Channel Complexes, Product States, Spin Crossings, and Size Effects in Metallic Ion–Molecule Reactions

“…The magnitude and temperature dependence of the bimolecular reaction forming Al 2 O 2 + + CO 2 (21a) was very well characterized by the statistical modeling, as well as the magnitude, temperature dependence, and helium concentration dependence of the association reaction forming Al 2 O 3 (CO) + (Figure ). The minor energetic adjustments made to the calculated surface (<0.25 eV) are not unique due to the sheer complexity of the multiple pathways; however, several mechanistic insights are clear. The modeling employed was only successful including contributions from each of the five entrance complexes and assuming symmetric formation of each complex.…”

“…This is not always true. In the reactions of AlVO 3 + and Al 2 O 2 + with N 2 O, modeling of the data was only successful with asymmetric well formation. , In both cases, the reactive site was a bicoordinate Al and was found to be formed >65% of the time. This complex quickly led to products.…”

Effect of Intersystem Crossings on the Kinetics of Thermal Ion–Molecule Reactions: Ti+ + O2, CO2, and N2O

“…The key variables for this reaction are therefore that crossing lifetime, the isomerization TS1a-1b energy, and the relative amount of entrance channel formation. In previous work, we have generally assumed equivalent formation of entrance complexes; however, several systems have shown better fits to the data with asymmetric well formation. , Figure shows the least-squares parameter as a function of crossing lifetime for three values of the isomerization transition-state energy, and for each energy, several values of the amount of initial complex formation ratio are displayed. Figure displays the best fit to the data.…”

Temperature and energy dependences of ion–molecule reactions: Studies inspired by Diethard Böhme

“…A number of these, such as the Al 2 O 4 + + CO reaction which had five different entrance complexes, are well modeled with equivalent formation of each complex. However, several systems have displayed a clear asymmetry (Ard et al, 2020; Shuman et al, 2021; Sweeny et al, 2018, 2020). Details of the long range potential of these reactions are needed to understand the reasons for these asymmetries, and future work in our group will try to better understand when and why multiple entrance complexes play a significant kinetic role.…”