Revisiting γ-alumina surface models through the topotactic transformation of boehmite surfaces

Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al2O3 with Varying Hydroxyl Contents during NO Reduction by CO

“…Calculations of OH* and H 2 O* on γ-Al 2 O 3 (010) b confirm previous results, , indicating that the surfaces corresponding to γ-Al 2 O 3 (110) cds are partly covered with OH* even above 500 K and with water pressures below 10 –4 bar when studied at 373–623 K and 10 –5 to 10 0 bar (Figure ). We examined concentrations of 0–15.27 and 0–9.05 H 2 O nm –2 on (010) b with and without HO–Rh­(CO) 2 , respectively, where the model has a surface area of 1.77 nm 2 .…”

“…Surface formation energies for the dehydrated (100) b and (010) b surfaces are 13.2 and 12.2 kJ mol –1 Å –2 , both higher than that for the (001) b facet (7.3 kJ mol –1 Å –2 ). While this indicates that the (001) b surface is more stable than the (100) b or (010) b surfaces, prior work has shown that these (010) b and (100) b surfaces remain hydroxylated up to temperatures >700 K. ,,, …”

“…The facet supporting HO–Rh­(CO) 2 also affects CO frequency, both directly through Rh-support interactions and indirectly by equilibrium θ W . For instance, HO–Rh­(CO) 2 on dehydrated (001) b has frequencies of 2090 and 2020 cm –1 , and the (001) b facet retains fewer OH than the (010) b or (100) b facets. , The facet dependence of θ W and the likeliness of OH*–OH* interactions may lead to distinct patches of high and low θ W on γ-Al 2 O 3 rather than a continuous distribution, which would lead to the appearance of the two relatively distinct sets of Rh­(CO) 2 observed in FTIR.…”

“…The γ-Al 2 O 3 model employed here was developed from a bulk structure constructed by modeling the dehydration of boehmite [γ-AlO­(OH)] to form γ-Al 2 O 3 ( a = 5.587 Å, b = 8.413 Å, c = 8.068 Å; α = γ = 90.00°, β = 90.59°). ,− This approach is necessary because previous cubic defect spinel-like models were less stable in DFT calculations and as-synthesized γ-Al 2 O 3 has low crystallinity, rendering periodic models for DFT difficult to construct from characterization data alone . We illustrate the instability of cubic defect spinel models during DFT calculations and compare the boehmite-derived model and a cubic defect spinel model ( a = b = c = 7.887 Å; α = β = γ = 90.00°) in Section S2 of the Supporting Information.…”

“…Theoretical studies have suggested the existence of an OH ligand on a Rh +1 (CO) 2 species supported on γ-Al 2 O 3 ; however, such studies did not account for a range of possible OH concentrations on the surface or the effects of changing Rh oxidation states through interactions with other ligands or compare calculations with experimental characterization data. OH groups on the surface of γ-Al 2 O 3 vary in concentration and structure as conditions change according to density functional theory (DFT) calculations of a γ-Al 2 O 3 model derived from dehydrated boehmite, − and thermogravimetric and surface area analyses suggest that average coverages can range from 8 to 12 OH nm –2 at the temperatures relevant to CO desorption (573–673 K). , Neighboring OH may also form hydrogen bonding networks: at high coverage, densely packed OH can form ice-like regions on the support, and OH gathered in surface depressions can form nest-like structures where water can accumulate, , similar to H-bond networks near silanol nests on zeolites. − Furthermore, IR bands associated with hydroxyl groups at 3679 and 3735 cm –1 decrease when CO is introduced to a 2.2 wt % Rh/γ-Al 2 O 3 sample while symmetric and asymmetric Rh­(CO) 2 peaks grow, indicating that particular OH groups are displaced or consumed during the dissolution and oxidation of Rh clusters to Rh +1 (CO) 2 . , Interestingly, exposing the Rh/γ-Al 2 O 3 to NH 3 mitigates this Rh dispersal process by bonding to γ-Al 2 O 3 hydroxyl groups . A better understanding of the role of OH groups in the behavior and structure of these Rh 1 species is critical to studying these active sites for NO x reduction.…”