Surface active sites present in the orthorhombic M1 phases: low energy ion scattering study of methanol and allyl alcohol chemisorption over Mo–V–Te–Nb–O and Mo–V–O catalysts

Abstract

The outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo-V-O and Mo-V-Te-Nb-O phases were determined employing methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS). These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo-V-Te-Nb-O catalyst indicated surface depletion for V ()23%) and Mo ()27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400°C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo-V-Te-Nb-O structure is terminated by a unique active and selective surface layer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO x sites in the orthorhombic Mo-V-Te-Nb-O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO x and NbO x sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VO x and MoO x sites. These findings suggested that vastly different catalytic behavior exhibited by the Mo-V-O and Mo-V-Te-Nb-O phases is related to different surface locations of V 5+ ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts. Although the proposed isolated V 5+ pentagonal bipyramidal sites in the orthorhombic Mo-V-O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO x redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo-V-Te-Nb-O phase. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.

Active Sites for Propane Oxidation: Some Generic Considerations

“…It is not clear as yet [42][43][44] that the active sites of the M1 phase are exclusively located on the (001) basal faces (''a-b plane'') of the needle morphology [42] but there is strong and general belief that the (001) face contains the motif of the active site that may exist in the termination structure of the whole surface of the material. For this reason the exact crystallographic analysis [40,45] of the basal plane structure not only in terms of its connectivity but also in terms of its cation ordering statistics [45][46][47] is of particular relevance.…”

On the nature of active sites in MoVTeO and MoVTeNbO catalysts: The influence of catalyst activation temperature

“…procedure, composition, etc [3,4,8,9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. However, the catalytic behavior of these catalysts is strongly related to the presence of M1 phase [18,22,23], although the presence of M2, in some case, could have a synergic effect [17][18][19].…”

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

“…EPR was then employed to confirm the presence of V +4 and Mo +5 in M1 and V +4 in M2. He Guliants et al employed the surface sensitive low-energy ion scattering (LEIS) after methanol and allyl alcohol adsorption and reached the conclusion that surface TeO x and NbO x sites are unable to chemisorb these probe molecules to the same extent as the MoO x and VO x sites [55]. The different catalytic behaviors of these sites in Mo-V-O and Mo-V-Te-Nb-O systems can be attributed to the different locations of V +5 sites [55].…”

“…He Guliants et al employed the surface sensitive low-energy ion scattering (LEIS) after methanol and allyl alcohol adsorption and reached the conclusion that surface TeO x and NbO x sites are unable to chemisorb these probe molecules to the same extent as the MoO x and VO x sites [55]. The different catalytic behaviors of these sites in Mo-V-O and Mo-V-Te-Nb-O systems can be attributed to the different locations of V +5 sites [55]. DFT studies, however, indicate that that the reduced Mo is the most favorable site for ammonia activation [56], whereas Te is more active than V for the H abstraction from propane [57].…”