Recent experiments have demonstrated an intriguing phenomenon in which adsorption of a nonracemic mixture of aspartic acid (Asp) enantiomers onto an achiral Cu(111) metal surface leads to autoamplification of surface enantiomeric excess, ee s , to values well above those of the impinging gas mixtures, ee g . This is particularly interesting because it demonstrates that a slightly nonracemic mixture of enantiomers can be further purified simply by adsorption onto an achiral surface. In this work, we seek a deeper understanding of this phenomena and apply scanning tunneling microscopy to image the overlayer structures formed by mixed monolayers of d - and l -Asp on Cu(111) over the full range of surface enantiomeric excess; ee s = −1 (pure l -Asp) through ee s = 0 (racemic dl -Asp) to ee s = 1 (pure d -Asp). Both enantiomers of three chiral monolayer structures are observed. One is a conglomerate (enantiomerically pure), another is a racemate (equimolar mixture of d - and l -Asp); however, the third structure accommodates both enantiomers in a 2:1 ratio. Such solid phases of enantiomer mixtures with nonracemic composition are rare in 3D crystals of enantiomers. We argue that, in 2D, the formation of chiral defects in a lattice of one enantiomer is easier than in 3D, simply because the stress associated with the chiral defect in a 2D monolayer of the opposite enantiomer can be dissipated by strain into the space above the surface.

Bimetallic AgCu catalysts have gained considerable interest, as both metals have demonstrated ability to perform selective oxidation reactions. Many of these studies have shown increased selectivity arising from the combination of Ag and Cu, but the mode of selectivity enhancement for Cu in Ag remains unclear. The AgCu near-surface alloy provides a well-defined model system with which to study selective oxidation reactions. By using a combination of high-resolution scanning tunneling microscope imaging and temperature-programmed reaction studies, we demonstrate that the addition of a single monolayer of Ag to Cu increases the overall selectivity for the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene from ∼40% on Cu(111) to 100% on the complete monolayer. Specifically, the near-surface alloy undergoes dynamic restructuring that brings Cu atoms into the surface layer, which enhances oxygen dissociation on the Ag surface, but the Ag overlayer inhibits the formation of extended Cu oxide domains that also catalyze the combustion pathway. Together, these results indicate that high-surface-area catalysts comprised mostly of Ag with very small amounts of Cu may exhibit promising selective epoxidation chemistry.