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

Abstract

Ga-based propane dehydrogenation (PDH) catalysts are explored in industry as an alternative to PtSn and CrO_x-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 °C, tetra- and pentacoordinate Ga³⁺ sites with both Si and Ga atoms in the second coordination sphere (i.e., ^[4]Ga_(Si/Ga) and ^[5]Ga_(Si/Ga) sites), its exposure to ambient air leads to sub-nanometer Ga_xO_y(OH)_z clusters with ^[4]Ga_(Ga) and ^[6]Ga_(Ga) sites, due to the hydrolysis of the Ga–O–Si linkages by the moisture of ambient air. When calcining the material at 650 °C, the ^[4]Ga_(Ga) and ^[6]Ga_(Ga) sites evolve leading to a silica surface dominated by isolated tetracoordinate ^[4]Ga_(4Si) sites, that is, [(SiO)₃Ga(XOSi)] sites, where X is H or Si. Exposure of the dehydroxylated material with ^[4]Ga_(4Si) sites to ambient air reforms the sub-nanometer Ga_xO_y(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 ^[4]Ga_(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).