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Use of Oxide Ligands in Designing Catalytic Active Sites

“…The weak shoulder band at 1070 cm À1 is characteristic of the silica network TO mode and has been assigned to perturbed Si(ÀO À ) 2 and SiÀO À silica vibrations that are also indicative of the formation of bridging VÀOÀSi bonds [40]. The partial reduction of the 1038, 905, and 340 cm À1 Raman bands at 600 C under the reducing H 2 environment, shown in curve B of Figure 1.3 (left), confirms the assigned V¼O, VÀOÀSi and bending VÀO vibrations to surface VÀO vibrations [43]. Figure 1.4 (left) and give rise to a single sharp band at $1035 cm À1 from the n s (V¼O) stretching mode of the dehydrated surface VO x species.…”

“…The preferential anchoring of the surface M 1 O x species at the surface M 2 O x species over the exposed SiO 2 support sites is a general phenomenon for anchoring of group 5-7 transition metal oxides on the surface-modified SiO 2 support. The surface vanadium oxide species in the dehydrated V 2 O 5 /SiO 2 catalyst exhibit a sharp and intense Raman vibration at 1038 cm À1 , as shown in Figure 1.3 (left), curve labeled A, and has been assigned to the terminal n s (V¼O) stretch of isolated surface VO 4 species [39][40][41][42][43][44][45]. This Raman band position is also independent of exposure to D 2 O at elevated temperatures indicating that there are no VÀOH functionalities (not shown for brevity).…”

“…Therefore, monolayer coverage or maximum dispersion limit is a critical parameter that allows distinguishing between two-dimensional molecularly dispersed surface metal oxide species and the three-dimensional NPs present in supported metal oxide materials. For the purposes of simplifying and focusing the discussion, only SiO 2 -supported metal oxide catalysts at submonolayer coverages will be discussed in this chapter since the catalysis literature has demonstrated that only isolated surface metal oxide species are present at low coverage for supported MO x /SiO 2 catalysts [14,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56].…”

Catalysis by Supported Metal Oxides

“…Related to this difference in metal oxide dispersion is the change in the dispersion of supported metal oxide during a reduction treatment. An illustrating example is the V205/Si02 catalysts, as shown in Figure 18 [58]. Reduction results in the removal of oxygen atoms from the supported vanadium oxide species, and at high V205 loadings the reduced species have the tendency to interact with a neigboring surface vanadium site by sharing oxygen atoms.…”

“…This may result in a change in the di persion of the active site at the catalyst surface. An examp 1 is the clustering of an isolated active site into a new clu: tered metal oxide phase at the support surface and has alread been discussed for the reduction process of V205/Si02 ca alysts with a high V205 loading [49,58]. The metal oxd clusters formed, in casu microcrystalline V205 oxides, most] have a lower activity or may catalyze undesirable side reac tions.…”