The role of Lewis
and Brønsted sites in the dehydration of
glycerol on niobium oxide and Na+-exchanged niobium oxide
is investigated using FTIR spectroscopy supported by DFT calculations.
Glycerol is impregnated on the catalysts at room temperature using
an ex-situ method. Under high vacuum conditions,
glycerol forms a stable multidentate alkoxy species through its primary
hydroxyl groups with the Lewis sites. When coordinated this way, the
primary C–O bonds are polarized, favoring dehydration in this
position to form hydroxyacetone. In contrast, dehydration of the secondary
alcohol group is kinetically favored over Brønsted acid sites
in the absence of steric constraints. The primary product of this
reaction, 1,3-propenediol, is further dehydrated to acrolein. When
more than a monolayer of glycerol is impregnated on niobia, monoaromatic
compounds are also formed on the surface upon heating.
Equilibrium adsorption of gas phase mixtures of D-and Lalanine (Ala) onto the naturally chiral Cu{3,1,17} R&S surfaces has been studied by both experiment and DFT-based modeling. Isotopically labeled *L-Ala (HO 2 13 CCH(NH 2 )CH 3 ) and unlabeled D-Ala allow mass spectrometric enantiodifferentiation of the adsorbed species during temperature-programmed decomposition, following equilibrium adsorption. Measurements of the relative equilibrium coverages of D-and *L-Ala on the Cu{3,1,17} R&S surfaces, θ D/R /θ *L/R = θ *L/S /θ D/S , at gas phase partial pressure ratios of P *L /P D = 1/2, 1, and 2 indicate that the D-Ala and *L-Ala conglomerate phases are more energetically stable than a D*L-Ala racemate phase, but that their adsorption energies are not measurably enantiospecific, ΔΔE DL ≈ 0. Although the DFT simulations provide a self-consistent structure of Ala overlayers on Cu{3,1,17} R&S they overestimate the enantiospecificity of the adsorption energetics.
Restructuring of metals by chiral molecules represents an important route to inducing and controlling enantioselective surface chemistry. Tartaric acid adsorption on Cu(110) has served as a useful system for understanding many aspects of chiral molecule adsorption and ordering on a metal surface, and a number of chiral and achiral unit cells have been reported. Herein, we show that given the appropriate annealing treatment, singly deprotonated tartaric acid monolayers can restructure the Cu metal itself, and that the resulting structure is both highly ordered and chiral. Molecular resolution scanning tunneling microscopy reveals that singly deprotonated tartaric acid extracts Cu atoms from the Cu(110) surface layer and incorporates them into highly ordered, chiral adatom arrays capped by a continuous molecular layer. Further evidence for surface restructuring comes from images of atom-deep trenches formed in the Cu(110) surface during the process. These trenches also run in low symmetry directions and are themselves chiral. Simulated scanning tunneling microscopy images are consistent with the appearance of the added atom rows and etched trenches. The chiral imprinting results in a long-range, highly ordered − ( ) 2 1 6 7 unit cell covering the whole surface as confirmed by low energy electron diffraction. Details of the restructuring mechanism were further investigated via time-lapse imaging at elevated temperature. This work reveals the stages of nanoscale surface restructuring and offers an interesting method for chiral modification of an achiral metal surface.
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