The Mechanism of Methane Combustion

There is no single mechanism that describes adequately the Idnetics of oxidation of various hydrocarbons such as alkanes, alkenes, aromatics, and so on over perovskites, but various reaction mechanisms and reaction orders applied for different molecules and catalysts. Nevertheless, there are some general rules. 

One of them is that the oxidation of organics over perovsldte oxides proceed by two reaction pathways the first suprafacial and the second intrafacial. In the suprafacial process, the reaction rate is correlated with the electronic configurations of the surfeice transition metal ions and takes place between the adsorbed species on the surface at relatively low temperatures. Conversely, the intrafacial mechanism takes over at high temperatures and the reaction rate appears to be correlated primarily with the thermodynamic stability of oxygen vacancies adjacent to a transition metal ion. However, the role of these oxygen species in complete and selective oxidation and their participation in combustion over various ceramic perovskite phases is still an open question. 

A second rule is that the oxidation of higher hydrocarbons, especially on perovskites doped with noble metals for increasing activity, may be described by Langmuir-Hinshelwood kinetics, indicating that both oxygen and CH species are adsorbed on the surface sites. On the contrary, for methane, the oxidation proceeds more or less via Rideal-Eley (RD) mechanism, which is in line with the notorious difficulty of this molecule to be activated. Let us review some cases from the literature. 

Ladavos and Pomonis [23] studied the catalytic combustion of CH4 with stoichiometric amounts of oxygen on two perovskite series Lai tSr cNi04 x prepared via the nitrate and citrate methods. They suggested that there is a twin 

Stojanovic et al. [25] found that the catalytic activity for methane oxidation over LaCri, Ni t04 increases monotonically with the value of x and described well by a MVK model proposing Ni-O-Ni ensembles as the key surface reagents. 

---