Dissociation of Methane on La₂O₃ Surfaces Doped with Cu, Mg, or Zn

Abstract

We use density functional theory to examine methane dissociative adsorption on La 2 O 3 substitutionally doped with Cu, Mg, or Zn. We find that these dopants activate the surface oxygen atoms and make methane dissociation exothermic. Cu-doped lanthana is very active, but it loses oxygen too easily and the surface is likely to be reduced at the temperatures at which methane dissociates. The reduced surface has lower activity. The effect of Mg doping on methane activation is less than that of Zn. We focus, therefore, on Zndoped lanthana and calculate the activation barrier for methane dissociation. We suggest that, for dissociative adsorption, the BrønstedÀEvansÀPolanyi rule needs to be modified to take into account not only the binding energy of the products but also the distance between the fragments formed by dissociation. We also show that an oxygen vacancy on the Zn-doped lanthana surface adsorbs oxygen from the gas and converts it into a species that should be described as O 2 À . This reacts with methane and dissociates it. Our main conclusion is that Zn-doped lanthana is a much better catalyst than lanthana, for methane activation, if one can prepare it so that the Zn dopants are in the surface layer. Unfortunately, the calculations show that the Zn atom prefers to be located in the bulk (in the absence of gases) and its influence on the chemistry of the surface, when this happens, is substantially diminished.

Conjugation-Driven “Reverse Mars–van Krevelen”-Type Radical Mechanism for Low-Temperature C–O Bond Activation

“…Radical intermediates on oxide surfaces are ubiquitous in catalytic oxidation of alkanes and alkenes [55][56][57][58][59] ; yet this chemistry frequently involves C-H scission barriers >1 eV and thus requires high temperatures (>300 o C) in order to achieve appreciable reaction rates. The low barrier in the C-OH scission signifies that radicals can also mediate heterogeneous catalytic processes involving reduction, potentially at much lower temperatures (<200 o C).…”

Dual‐function catalysis in propane dehydrogenation over Pt1–Ga2O3 catalyst: Insights from a microkinetic analysis

“…Actually, despite the superior catalytic performance of single‐atom catalysts in many reactions of commercial importance, 13,21‐32 the nature of the active phase is not always clear, which is due primarily to the increased complexity in the electronic structure that arises from the change in surface composition 33 . For instance, it has been demonstrated that coadsorption of a pair of amphoteric species (e.g., propyl and H in PDH) could be significantly more stable than the adsorption of the isolated species on many oxide surfaces due to Lewis acid–base interaction, 34‐36 which makes it possible to tailor the catalytic properties of oxides to a particular chemical reaction by doping the surfaces with single metal atoms. In this way, either single metal atoms or their immediate surroundings could make a major contribution to the kinetics of the reaction, 37,38 and sometimes they could even function cooperatively, showing a bifunctional character 39 …”

Ethylene formation by methane dehydrogenation and C–C coupling reaction on a stoichiometric IrO2 (110) surface – a density functional theory investigation

“…Among them, Li-doped MgO 10,[15][16][17][18] and La 2 O 3 -based [19][20][21][22][23][24][25] catalysts have been most studied for oxidative coupling of methane (OCM) using O 2 as an oxidant. Because of the chemical inertness of the CH 4 molecule, these catalysts generally require high operating temperatures (ca.…”