Rethinking the molecular structures of W^VIO_x sites dispersed on titania: distinct mono-oxo configurations at 430 °C and temperature-dependent transformations

Abstract

The structural properties of the (WOx)n phase dispersed on TiO2 (P25, anatase) at surface densities of 0.5–4.5 W/nm2 (i.e. up to approximate monolayer) were explored using in situ Raman and...

Heterogeneity of the vanadia phase dispersed on titania. Co-existence of distinct mono-oxo VOx sites

“…Four bands due to surface hydroxyls are observed at 3710, 3675, 3640 and 3540 cm −1 for TiO 2 (P25), in agreement with a recent report. 31 The spectra in Fig. 8 confirm that a progressive surface hydroxylation takes place upon lowering the temperature in the 430 → 250 → 175 °C sequence, thereby corroborating the aforementioned dissociative adsorption action of water.…”

“…5, the terminal V 16 O mode is already red-shifted from the first 18 O/ 16 O cycle, thereby indicating that the V– 16 O–Ti → V– 18 O–Ti substitution precedes the V 16 O → V 18 O substitution. 31,57,59…”

“…The last is affirmed by the slight red shift of the V 16 O mode during the first 18 O/ 16 O exchange cycles, which indicates that the initiation of the isotope exchange takes place on the V–O–Ti linkages followed by a subsequent 18 O substitution on the V 16 O sites, according to the next-nearest-neighbor vibrational isotope effect. 31,57,59…”

“…Outstanding paradigms of spectroscopically evidenced heterogeneity of oxo-metallic phases dispersed on TiO 2 include the existence of distinct mono-oxo and di-oxo ReO x sites, 28 reversible temperature-dependent transformations between MoO x species, 29,30 and the existence of distinct mono-oxo WO x species. 31 Additional examples of dispersed oxo-metallic phase heterogeneity on other supports ( e.g. SiO 2 , Al 2 O 3 , CeO 2 ) can also be found in the literature on supported transition metal oxide catalysts.…”

“…19,20,39,40 As a paradigm, water molecules formed as a result of the titration of surface hydroxyls by the V precursor species 41 during the deposition preparation step can be retained on the titania surface by H-bonds. 31 Therefore, the state of a catalyst under dehydrated feed conditions should rather be described as hydroxylated, since the dissociative adsorption of extant water molecules would result in OH formation, e.g. by protonation of O atoms along V-O-Ti anchoring bonds.…”