The Relation between Nature and Stability of H 2 ‐Dissociating Sites and Propene Selectivity in Silica‐Supported (Ga,Al) 2 O 3 Mixed Oxide Propane Dehydrogenation Catalysts

Colloidal solutions of gallia‐alumina (Ga,Al) 2 O 3( x : y ) solid‐solution nanoparticles with nominal atomic Ga : Al ( x:y ) ratios of 1 : 6, 1 : 3, 3 : 1, and 1:0 were used to prepare silica‐supported catalysts for propane dehydrogenation (PDH). A comparison of the unsupported and silica‐supported catalysts reveals that the dispersion on silica increases the Ga‐normalized PDH rates for all catalysts, albeit with a notably lower propene selectivity for (Ga,Al) 2 O 3(1:6) /SiO 2 . Fourier transform infrared (FTIR) spectroscopy allows contrasting the H 2 dissociation sites in the calcined and H 2 ‐treated (Ga,Al) 2 O 3( x : y ) /SiO 2 , indicating a transformation of Ga 3+ surface sites with Al (mainly) and Ga atoms in the second coordination sphere (Ga (Al/Ga) sites) in the calcined (Ga,Al) 2 O 3(1:6) /SiO 2 to predominantly Ga (Ga/Si) surface sites in the H 2 ‐treated material. The resulting sites are similarly unselective as in amorphous gallia on silica. H 2 produced during the PDH reaction can cause a similar transformation as H 2 pretreatment in (Ga,Al) 2 O 3(1:6) /SiO 2 , rapidly resulting in a notably lowered selectivity. The stable and selective Ga (Al/Ga) surface sites in (Ga,Al) 2 O 3(1:3) /SiO 2 yield a Ga−H band at ca. 1990 cm −1 under H 2 dissociation conditions while the less selective surface sites, observed for the other Ga : Al ratios, give Ga−H bands at ca. 2040 and 2060 cm −1 .