Effect of the particle size and surface area of tungstated zirconia on the WO x nuclearity and n-heptane isomerization over Pt/WO 3–ZrO 2

Abstract

Four series of tungstated zirconia of different surface area ranging from 100 to 20 m 2 g −1 and of micro- and nano-meter crystal size were investigated with respect to nuclearity and reducibility of WO x species, concentration of strong and weak acid sites, and n-heptane isomerization over Pt/WO 3–ZrO 2. The nuclearity of WO x species was analyzed by diffuse reflectance UV–Vis spectroscopy and their reducibility by TPR-H 2. TPD of ammonia distinguished acid sites of different strengths. n-Heptane isomerization proceeds over Pt/WO 3–ZrO 2 in hydrogen/nitrogen mixture at 250 °C. At least four types of WO x species differing in nuclearity were identified in tungstated zirconia by a maxima of absorption at 39 000, 35 000, 30 000 and 27 000 cm −1, intensity of which depended on the surface area, crystal size and WO 3 loading. From the changes in the relative intensities of the characteristic individual bands of WO x species with tungsten loading, at different surface area and crystal size, it follows that at the lowest loading mononuclear W–O–Zr species (39 000 cm −1) and two-dimensional poly-tungstate like WO x species (35 000 cm −1) are formed, which are at loading above a nominal monolayer started to transform into a larger defined WO x , likely tri-dimensional clusters (30 000 cm −1). The absorption maximum at 27 000 cm −1 corresponds to bulk-like WO 3. The nuclearity of WO x species was reflected in their reducibility, which increased with increased nuclearity, and it was higher for the nano-size tungstated zirconia. The isomerization activity followed the relative population of poly-tungstate WO x species (35 000 cm −1), but was not in strict agreement with the concentration of strong acid sites. This might be connected with the rate determining step of the isomerization process or formation of radical cations depending on reaction conditions. The nano-size tungstated zirconia exhibited high isomerization activity advantageously at low W loading (6–12 wt.%) compared to the micro-meter size ones. This finding corresponds to formation of poly-tungstate WO x species already at low loadings, likely at the defective sites of the nano-size zirconia.