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

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 VOx 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 TeOx and NbOx sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VOx and MoOx 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 V5+ ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts. Although the proposed isolated V5+ 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 VOx 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