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

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 °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 GaxOy(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, [([triple bond, length as m-dash]SiO)3Ga(XOSi[triple bond, length as m-dash])] sites, where X is H or [triple bond, length as m-dash]Si. Exposure of the dehydroxylated material with [4]Ga(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 [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).

 

(15) Chen, C.; Serykh, A.; Kierzkowska, A.; Gajan, D.; Docherty, S.; Yakimov, A.; Abdala, P.; Copéret, C.; Florian, P.; Fedoroc, A.; Müller, C. "Reversible transformation of sub-nanometer Ga-based clusters to isolated Ga sites creates active centers for propane dehydrogenation". Catal. Sci. Technol., 2024, 14, 379-390. 

DOI: 10.1039/D3CY01446K

Link to the open access publication: https://pubs.rsc.org/en/content/articlelanding/2024/cy/d3cy01446k

Link to raw data: https://doi.org/10.1039/D3CY01446K