Existence and Properties of Isolated Catalytic Sites on the Surface of β-Cristobalite-Supported, Doped Tungsten Oxide Catalysts (WOx/β-SiO2, Na-WOx/β-SiO2, Mn-WOx/β-SiO2) for Oxidative Coupling of Methane (OCM): A Combined Periodic DFT and Experimental Study

The nature of isolated tungsten oxide (WOx) sites in the dispersed phase on the surface of a beta-cristobalite (beta-SiO2) support in undoped and Na- or Mn-doped WOx/SiO2 model catalysts used for the oxidative coupling of methane (OCM) has not been explored previously. This work provides a computational model for isolated surface WOx sites (doped and undoped) supported on beta-cristobalite (beta-SiO2) and computationally explores their molecular structure, degree of hydration, and energetics over a wide range of OCM-relevant temperatures from 500 to 1300 K. Ab initio thermodynamic analysis showed that the most stable molecular configuration of the surface sites in all cases (WOx, Na-WOx, Mn-WOx) was the digrafted, dioxo pseudotetrahedral WO4. The thermal stability of the surface WO4 sites was in the order Na-WO4 >> WO4 > Mn-WO4 in the OCM-relevant temperature range of 850-1300 K. A spin analysis of an Mn-WO4 isolated surface site indicates a paramagnetic high-spin Mn2+-O-W6+ electronic state, in line with literature reports. The computed frequencies for isolated surface WO4 sites agree well with the experimental in situ Raman spectra of the corresponding model catalysts, proving their existence. Finally, steady-state OCM studies showed that the C-2 selectivity was highest for Na-WO4 sites, followed by Mn-WO4 and WO4, a trend mimicking the degree of distortion in the molecular geometry of each site. Na-WO4 exhibited the lowest reactivity toward CH4 and the highest degree of W=O bond elongation.