Reversible transformation of sub-nanometer Ga-based clusters to isolated ^[4]Ga_(4Si) sites creates active centers for propane dehydrogenation

Ga-based propane dehydrogenation (PDH) catalysts are explored in industry as an alternative to PtSn and CrOx-based catalysts. Yet, at present, there is only limited understanding of the structural dynamics of surface sites in Ga-based PDH catalysts. Here, we employ atomic layer deposition (ALD) to engineer a sub-monolayer of Ga species on dehydroxylated silica, which serves as a model PDH catalyst. While the ALD-grown shell contains, after calcination at 500 degrees C, tetra- and pentacoordinate Ga3+ sites with both Si and Ga atoms in the second coordination sphere (i.e., Ga-[4]((Si/Ga)) and Ga-[5]((Si/Ga)) sites), its exposure to ambient air leads to sub-nanometer GaxOy(OH)(z) clusters with Ga-[4]((Ga)) and Ga-[6]((Ga)) sites, due to the hydrolysis of the Ga-O-Si linkages by the moisture of ambient air. When calcining the material at 650 degrees C, the Ga-[4]((Ga)) and Ga-[6]((Ga)) sites evolve leading to a silica surface dominated by isolated tetracoordinate Ga-[4]((4Si)) sites, that is, [equivalent to( SiO)(3)Ga(XOSi)equivalent to] sites, where X is H or Si. Exposure of the dehydroxylated material with Ga-[4]((4Si)) sites to ambient air reforms the sub-nanometer GaxOy(OH)(z) clusters, indicating the reversibility of the Ga dispersion and agglomeration as a function of the extent of silica (de)hydroxylation. The presence of Ga-[4]((4Si)) sites coincides with a high performance in PDH, achieving an initial turnover frequency of ca. 12 h(-1) and propene selectivity of ca. 85%, while deactivating by only 34% over 20 h of time on stream. Overall, our results highlight the dynamic nature of the dispersion and agglomeration of Ga3+ sites during the dehydroxylation (by calcination) and rehydration (ambient air exposure).