Vanadium oxide surfaces

Fig. 7. Diagrams of the relaxed IDM (Part a) and the relaxed AIM FF indices (Part b) of toluene in the surface complex M with the bipyramidal cluster of the vanadium oxide surface. The modes are arranged in accordance with increasing hardnesses h the hardness tensor reflects the isolated reactant AIM charges from the MNDO and scaled-INDO calculations on toluene and cluster, respectively. Numbers in parentheses report the w values. The three diagrams in Part b display the AIM FF distribution of toluene in M, calculated from the toluene block of i/ 1, and its resolution into the CT and P components, respectively...

Fig. 8. Diagrams of the PNM (Part a) and the relaxed 1DM (Part c) of the bipyramidal cluster of the vanadium oxide surface in complex M of Fig. 7a. The corresponding AIM FF plots are shown in Parts b and d, together with their resolutions into P and CT components. Modes are ordered in accordance with increasing h (h,M) values (a.u.) reported at the bottom of the mode contour. Numbers in parentheses are the corresponding w, (w ) values...

So far, theoretical studies on vanadium oxide surfaces have focused exclusively on single crystal surfaces which are described by low Miller indices and are believed to be energetically favorable. These surfaces are most easily accessible by theory due to their relatively simple geometry although their relevance as to catalytic activity has been doubted. 

Theoretical studies on various physical and chemical parameters of vanadium oxide surfaces have been performed using both repeated slab as well as local cluster models. However, the vast majority of studies has focused on the (010) surface of the pentoxide, V2O5. This is a result of rather little experimental information on details of V Oy surface systems other than V205(010) that has attracted great interest due to its possible importance in catalytic applications. Further, most of the theoretical work has been based on cluster type studies where local surface behavior, in particular near surface oxygen sites, is discussed in detail. This will be reflected in the following discussion. 

Atomic and molecular adsorption at vanadium oxide surfaces have been studied theoretically using both periodic slab and cluster models where so far studies are restricted to the pentoxide, V2O5, as a substrate due to its possible importance in catalytic applications as mentioned before. Further, adsorbate species include in all cases atoms (H [122-123, 126, 136-142], O (see below)) or rather small molecules (O2 (see below), H2O [143-144], NH3 [145-147], NO [146, 148], C2H4 [149], propene (CsHg) [140], toluene (CeHsCHj) [140]) that are of catalytic interest but also small enough to make meaningful calculations feasible. 

Surnev S, Ramsey MG, Netzer FP (2003). Vanadium oxide surface studies. Prog SurfSci, 73, 117... 

The link between structure and reactivity is again demonstrated by the complicated succession of vanadium oxide surface phases predicted by FP [77]. At certain O2 partial pressures, the metal substrate is computed to stabilise thin film phases that are not known in equivalent bulk form. The impliaation is that STM studies of thin insulator fihm on conducting substrates may have to contend with the complex, and sometimes novel, chemistry of thin films [2]. A phase diagram of non-stoichiometric surfaces is also generated by FP in Ref. [53], this time for silver oxidation. The aim is to bri(%e the pressure gap between ultra-high-vacuum research and the industrial reality of high-pressure reactors. 

Theory has been used predominantly to probe the nature of the sites on vanadium clusters and model vanadium oxide surfaces. Cluster and p>eriodic DFT calculations [68,69] have been carried out in order to imderstand the electronic and structural properties of the exposed (100) surface of (VO)2P207. Both cluster and slab calculations reveal that surface vanadium sites can act as both local acid and base sites, thus enhancing the selective activation of n-butane as well as the adsorption of 1-butene. Vanadium accepts electron density from methylene carbon atoms and, thus aids in the subsequent activation of other C-H bonds. Calculations reveal that that the terminal P=0 bonds lie close to the Fermi level and thus present the most nucleophihc oxygen species present at the surface for both the stoichiometric as well as phosphate-terminated surfaces. These sites may be involved in the nucleophilic activation of subsequent CCH bonds necessary in the selective oxidative conversion of butane into maleic anhydride. Full relaxation of the surface, however, tends to lead to a contraction of the terminal P=0 bonds and a lengthening of the P V bonds. This pushes the P V states, initially centered on the oxygen atoms, higher in energy and thus increases their tendency to be involved in nucleophilic attack . 

High resolution transmission electron microscopy (TEM) was further used to confirm the results described above. Figure 12.14 shows the TEM images of the catalyst after stable operation and after the aging stress test. While the catalyst after stable operation only shows the lattice fringes of anatase covered by a layer of amorphous vanadium oxide surface species, the stressed catalyst shows a more complex structure. The core of the support oxide particle still shows the lattice fringes of anatase, but lattice distances of rutile can be seen in a surface-near layer around... 

Eckert, H. and Wachs, I.E. Solid-state vanadium-51 NMR structural studies on supported vanadium(V) oxide catalysts vanadium oxide surface layers on alumina and titania supports. J. Phys. Chem. 1989, 93, 6796-6805. 

Figure 3.3 ReaxFF reactive force fields are developed to reproduce DFT data for complex chemical reactions, such as heterogeneous catalysis. In the top panel, a comparison of a calculated ReaxFF reaction energy profile (dashed line) is compared to the corresponding DFT results (solid line) for a model catalytic reaction, CH4 + O2 ->CH20 + H2O on a small V4O10 cluster. The method allows modelling of complex chemical reactions on picosecond or longer time-scales to be carried out for large simulation cells involving thousands of atoms or more under realistic temperature and pressure conditions. The lower panel shows a snapshot from a simulation of VO c catalysis of complex hydrocarbons using a force field for hydrocarbon catalysis on vanadium oxide surfaces. ...

Multi-metal-Oxide Catalysis. The ReaxFF potential has also been utilized to study the catalytic properties of complex metal oxides. Che-noweth et al. developed and implemented a V/O/C/H force-field that, when combined with the existing hydrocarbon force-field, can model the interaction between gas-phase hydrocarbons and the vanadium oxide surface. For motivation, the authors cite numerous examples in which V2O5 is used to catalyze industrial processes that selectively oxidize both... 

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