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

Abstract

AbstractGiven 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.