Identifying the Active Site for the V/P Mixed Oxides

Identifying the active site is one of the central questions for the multicomponent catalysts. Usually, different components in the multicomponent systems can be divided into active centers and promoters (or linkers). However, recent theoretical works indicated that the boundary between the active and inactive region would be blurred. Taking mixed oxides for example, Guliants et al.[1] suggested that H atom is preferentially adsorbed on Te=O rather than V=O or Mo=O on the M1 phase of MoVTeNbO_x catalysts, while Goddard et al. [2] and our group [3] pointed out that in the binary vanadium phosphorus oxides (VPO) model catalysts, the activity of P=O is favored over V=O in the oxidation of n-butane. In above cases, the activity can be dispersed from the seemingly active sites to the nearest neighbour atoms/sites, which are chemically inert in their pristine form. Accordingly, fundamental questions are emerged as: (i) whether the activities are able to disperse to those sites outside the nearest ones? (ii) If so, what are the factors to determine their activities.

To simplify the question, here we concentrate on the binary VPO system[4]. It was found experimentally that the outermost layer of the VPO is phosphorus rich with the ratio of P/V up to 4, and the valence of V should be +5 rather than +4. However, the surface phosphate species are structurally ill-defined such that there is not a straightforward way to build an exact model to describe the surface structure. Alternatively, crystalline P₂O₅ is of layer structure, composed of a two-dimension network of PO₄. When some P⁵⁺ ions in P₂O₅ are displaced by V⁵⁺, conceptual models for the P rich VPO catalysts can be obtained. Density functional theory (DFT) calculations are carried out to explore the initial activation of n-butane by V⁵⁺=O and P⁵⁺=O as well. We show that the activation of n-butane by the VPO can be viewed as a PCET process, in which V⁵⁺ serves as electron acceptor, while both the V=O and the P=O can act as proton acceptor. For P=O abstraction, the activation barrier (ΔE_a) correlate well with the reaction energies (ΔE), which are gradually decay with increasing the separated -OP- chain(s) from the V⁵⁺. We find that both the basicity of O and the degree of charge seperation contribute to the reation barrier. For the H abstraction by V=O, the weak basicity of O could be compensated by the negligible charge seperation; whereas when the reaction happens on P=O, the benefit gained from the strong basicity of the P=O could counteract the cost of bond reorganization along the -(OP)- chain(s) and charge seperation as well.

According to our calculations, the center of O lone-pair band (ε_lp) is a good descriptor, which can quantitatively describe the tendency of activity. The good linear correlation between ε_lp and ΔE can be nicely explained through surface bonding interaction. It is clear that not only the occupied P=O lone-pair electron states but also the empty V 3d_xy state plays crucial roles. The lone-pair band of P=O would directly interact with H 1s, resulting in one bonding and one antibonding orbitals. The low-lying bonding orbital is doubly occupied, which makes the combined system more stable. On the contrary, the remainder electron could be promoted into the unoccupied V 3d_xy, destabilizing the whole system. Hence, the ΔE would be closely related with the net energy gain/loss of orbital interaction. We expect that such a descriptor is amenable to fast screening the potential active sites over mixed oxide catalysts.

References

1. Govindasamy, A.; Muthukumar, K.; Yu, J.; Xu, Y.; Guliants, V. V. Phys. Chem. C 2010, 114, 4544-4549.
2. Cheng, M.-J.; Goddard, W. A. Am. Chem. Soc. 2013, 135, 4600-4603.
3. Fu, G.; Yuan, R.; Wang, P.; Wan, H. Chinese J. Catal. 2015, 36, 1528-1534.
4. Fu, G.; Wang, P.; Wan, H. ACS. Catal. 2017, 7, 5544−5548.