Microkinetic analysis of the CO2 effect on OCM over a La-Sr/CaO catalyst

Abstract

Given its role as a primary side product and a potential soft oxidant in the oxidative coupling of methane (OCM), understanding the effect of CO₂ co-feeding on OCM emerges as a key milestone to optimize the process. To grasp the molecular impact of CO₂, a mechanistic investigation over a La-Sr/CaO catalyst was carried out via microkinetic modeling. Seven catalyst descriptors with a precise physico-chemical meaning were regressed for both pure O₂ and CO₂ co-feeding in order to assess eventual structural changes induced in the catalyst by the presence of CO₂ in the feed. Global significance was achieved in both regressions and experimental trends were successfully reproduced by the specifically determined catalyst descriptors. CO₂ co-feeding is deemed responsible for generating a new active phase, for example, by converting metal oxides into (oxy-)carbonates, among others, resulting in a decrease in active site density (D₁₆) from 10 × 10⁻⁵ mol/m² to 7 × 10⁻⁵ mol/m². In the presence of the CO₂-induced phase, the catalyst exhibits higher attraction for unsaturated hydrocarbons as indicated by the higher initial sticking probabilities of CH₃• (D₁₁) and C₂H₄ (D₁₅), which increase from 4.9 × 10⁻⁴ to 8 × 10⁻² and from 2.1 × 10⁻² to 3 × 10⁻², respectively. Additionally, there are also lower the overall energy barriers for the activation of hydrocarbons on the catalyst, stemming from the decrease in the H abstraction enthalpy from CH₄ (D₁) from 14 to 6 kJ/mol. The operating conditions, in particular the O₂ content, are critical in distinguishing the effect of CO₂ co-feeding. While at typical operating conditions, CO₂ promotes the total oxidation of methane, in the prerequisite of reduced amount of O₂, it may also act as an additional oxygen donor. This work provides molecular details on the CO₂ induced changes in catalyst properties but also provides unprecedent quantified insights of the reaction mechanism underlying experimental observations.