Probing the Positions of TeO Moieties in the Channels of the MoVNbTeO M1 Catalyst: A Density Functional Theory Model Study

Abstract

With hybrid DFT calculations applied to periodic models of the bulk MoVNbTeO M1 catalyst, we examined how [TeO]2+ species in the hexagonal channels of this material stabilize nearby reduced metal centers. In particular, an S2(Mo) site, with adjacent [TeO]2+ moieties at both sides, is calculated to be reduced to Mo5+. The modeling study presented offers insight into how the redox behavior of V and Mo centers, a crucial aspect of the M1 catalyst for the selective partial oxidation of small hydrocarbons, may be fine-tuned via TeO moieties at various distances from the metal centers. Graphic Abstract TeO moieties in hexagonal channels, adjacent on either side of an S2(Mo) center, stabilize a gap state at the Mo center, facilitating its reduction to Mo5+.

Detecting Framework Distortions Predicted by Hybrid-DFT: An Opportunity to Improve the M1 Catalyst

“…To conclude, our experimental work confirms the predictions of hybrid-DFT calculations which can correctly account for the presence of polarons in the M1 catalyst 25,26 and predict a symmetry breaking at the S2 site. To be able to observe these distortions, extreme attention has to be given to the aberration-corrected optical state of the microscope to ensure isotropic atom column projections, in our case S9, where no structural distortions are present.…”

“…27 HAADF images can be understood "intuitively" due to the Z 2 approximation, resulting in stronger contrast at atomic columns with higher Z, but caution should be used when using the virtual crystal approximation in describing atomic columns with mixed occupancies. 26 In bright-field images the Z 2 approximation no longer holds, and multislice frozen phonon simulations are needed to understand the contrast, which is reversed to the one observed in dark field and favors lighter atoms such as oxygen. 28 To probe possible elliptic projections of atom columns, we selected four particles from the same microscopy grid.…”

“…Standard DFT methods using a general gradient approximation of the exchange-correlation functional produce artifacts when describing simultaneously reduced and oxidized metal sites in oxides. , In two landmark studies based on hybrid-DFT calculations, Rösch et al , show how [TeO] 2+ defects in the M1 catalyst affect the V 4+ distribution and break the symmetry at the catalytic S2 site (see Figure ) and have important implications for catalysis: A polaron which was located at one of the S2 sites is no longer stable if it is in proximity to only one [TeO] 2+ entity, creating V 5+ sites in S4 and S7 as well as Mo 6+ on S2. The latter should be more active in the selective oxidation compared to S2 sites which are in the vicinity of two [TeO] 2+ entities.…”

“…The contrast in HAADF images scales approximately with Z 2 , Z being the atomic number of the element and is due to incoherent Rutherford scattering and channeling along certain directions of the atomic columns, resulting in an increase of probe intensity on the atomic columns and a decrease between them . HAADF images can be understood “intuitively” due to the Z 2 approximation, resulting in stronger contrast at atomic columns with higher Z , but caution should be used when using the virtual crystal approximation in describing atomic columns with mixed occupancies . In bright-field images the Z 2 approximation no longer holds, and multislice frozen phonon simulations are needed to understand the contrast, which is reversed to the one observed in dark field and favors lighter atoms such as oxygen. …”

Identification of the Intrinsic Active Site in Phase-Pure M1 Catalysts for Oxidation Dehydrogenation of Ethane by Density Functional Theory Calculations

“…Deshlahra et al. , provided the reaction network for ODHE and ODHP including all intermediates and products combined with experiments and DFT calculations. The confinement reaction was also illustrated by choosing light alkane (ethane and propane) as probe molecules. , Rösch et al − analyzed the electron distribution on the M1 (001) plane and established a mathematical model that can predict charge population. However, the active site on the M1 (001) plane is not precisely identified based on the electronic structure.…”