Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

Abstract

Abstract:A theoretical analysis was carried out on the mechanism of methane oxidation to methanol occurring on single site palladium oxide species [PdO] 2+ supported on a model of Al-MCM-41 silica. Both 6 and 8-membered ring structures were considered to represent the support.The energy profile for each elementary reaction was determined from density functional theory calculations with the OPBE functional. The calculated overall activation energies are close to the experimental values. Our calculations confirm that spin inversion can play a significant role in decreasing the barrier heights for the pathways. Indeed, in this type of reactions we could show a crossing between singlet and triplet reaction paths. We showed that the mechanism for the C-H bond cleavage and for the formation of methanol has a radical nature. According to our results, the [PdO] 2+ species located on a 8-membered ring of silica is more active than that deposited on a 6-membered ring. The calculated activation energies to cleave the methane C-H bond are 35 and 84 kJ/mol for the radical and ionic pathways, respectively. The activation barrier and the transition state geometry of this H-abstraction step are directly correlated with the optimal angle at which the substrate should approach the [Pd=O] 2+ moiety, with the elongation of the Pd-O oxo bond and finally with the energy of the π* acceptor orbital.2

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

“…Its additional charge transfer character is also evident based on the intense pink-purple color of the complex upon CO adsorption ( Fig S5). Previous studies claimed the formation of Pd(IV) cation on various solid supports and zeolites based on (1) the indirect evidence from stoichiometry from XRD refinement 27 (2) DFT-modeling 46 and (3) the presence of the high binding energy feature (relative to PdO with BE of Pd 3d5/2 at 336.5-337.0 eV) in the Pd 3d XPS spectra 24,28,29,47,48 . Interestingly, in some works a feature at 337.7-338.0 eV on ceria supports, for example, 49,50,51 [8][9][10] We also note that upon dehydration either in He or O2, the color of the sample changes from light yellow to light pink (Fig.…”

DFT investigations for the catalytic reaction mechanism of methane combustion occurring on Pd(ii)/Al-MCM-41

“…Furthermore we have also investigated the direct conversion of methane to ethanol by PdO/Al-MCM-41 and found that the [Pd=O] 2+ species supported on a 8-membered ring was more reactive than those supported on a 6-membered ring. 18 For this reason, we limit ourselves in the present work to location of the Pd 2+ cation on a 8-membered ring. We have represented the surface of acid silica-alumina using structures of silsesquioxane type that simulate the 8 membered rings in which two Si atoms were substituted by two Al atoms ((H 18 Al 2 Si 14 O 24 ), see Figure S1).…”

“…15 Those experimental works were also strongly supported by theoretical studies of CH 4 oxidation, mainly by means of Density Functional Theory (DFT). 15,18,19 In particular, the pivotal role of the acid sites was evidenced for the activation of CH 4…”

Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

“…Its additional charge transfer character is also evident based on the intense pink-purple color of the complex upon CO adsorption ( Fig S5). Previous studies claimed the formation of Pd(IV) cation on various solid supports and zeolites based on (1) the indirect evidence from stoichiometry from XRD refinement 27 (2) DFT-modeling 46 and (3) the presence of the high binding energy feature (relative to PdO with BE of Pd 3d5/2 at 336.5-337.0 eV) in the Pd 3d XPS spectra 24,28,29,47,48 . Interestingly, in some works a feature at 337.7-338.0 eV on ceria supports, for example, 49,50,51 was attributed to Pd(IV) ions or PdO2 .…”