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

Given its role as a primary side product and a potential soft oxidant in the oxidative coupling of methane (OCM), understanding the effect of CO 2 co‐feeding on OCM emerges as a key milestone to optimize the process. To grasp the molecular impact of CO 2 , 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 O 2 and CO 2 co‐feeding in order to assess eventual structural changes induced in the catalyst by the presence of CO 2 in the feed. Global significance was achieved in both regressions and experimental trends were successfully reproduced by the specifically determined catalyst descriptors. CO 2 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 (D 16 ) from 10 × 10 −5 mol/m 2 to 7 × 10 −5 mol/m 2 . In the presence of the CO 2 ‐induced phase, the catalyst exhibits higher attraction for unsaturated hydrocarbons as indicated by the higher initial sticking probabilities of CH 3 • (D 11 ) and C 2 H 4 (D 15 ), 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 CH 4 (D 1 ) from 14 to 6 kJ/mol. The operating conditions, in particular the O 2 content, are critical in distinguishing the effect of CO 2 co‐feeding. While at typical operating conditions, CO 2 promotes the total oxidation of methane, in the prerequisite of reduced amount of O 2 , it may also act as an additional oxygen donor. This work provides molecular details on the CO 2 induced changes in catalyst properties but also provides unprecedent quantified insights of the reaction mechanism underlying experimental observations.