Evaluation of density functionals for elementary steps of selective oxidation reactions

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+.

Reactivity trends of the MoVOx mixed metal oxide catalyst from density functional modeling

We studied how polarons affect the oxidative dehydrogenation of hydrocarbons in a MoVO_x surface model and the related surface hydrolysis.

DFT Variants for Mixed-Metal Oxides. Benchmarks Using Multi-Center Cluster Models

Mixed-metal oxides, e.g., V-Mo and Bi-Mo, are promising selective oxidation catalysts. Yet, their intricate chemical composition and electronic structure often confound DFT methods. This study addresses problems arising from the simultaneous presence of two kinds of transition metals, by probing eight functionals-five hybrid functionals (MN15, M06, PBE0-D3, B3LYP-D3, and TPSSh-D3), the meta-GGA functional M06-L-D3, the range-separated functional ωB97XD, and the GGA functional PBE-D3. We examine the ability of these functionals to localize reducing electrons, and to reproduce reaction energies from CCSD(T) calculations. Accordingly, hybrid functionals containing 20% or more exact exchange perform considerably better in both tests. The B3LYP-D3 approach exhibits the lowest overall mean absolute deviation of reaction energies (OMAD), 21 kJ mol^-1, and gave electron distributions as expected from the local lattice structure according to the pseudo-Jahn-Teller effect. MN15 and PBE0-D3 reproduced the electron distributions, but bore slightly higher OMAD values, at 31 and 32 kJ mol^-1. Despite acceptable OMAD values, M06 (28 kJ mol^-1) and TPSSh (23 kJ mol^-1) in some cases did not yield the expected electron distributions. The range-separated functional ωB97XD experienced the opposite problem, yielding correct electron distributions but a poor OMAD of 41 kJ mol^-1. M06-L-D3 and PBE-D3 performed relatively poorly, regarding the electron distribution and the OMAD values, 39 and 65 kJ mol^-1, respectively.

How the distribution of reduced vanadium centers affects structure and stability of the MoVOx material

The energy ordering of configurations, with 6 among the 10 V centers reduced, is predicted using 3 easily accessible variables.