The enantioselective chemisorption of (S)- and (R)-propylene oxide is measured on a Pd(111) surface chirally modified using (S)- and (R)-2-butanol. Reflection-absorption infrared spectroscopic (RAIRS) data suggest that adsorbed 2-butanol forms 2-butoxide species when heated to approximately 150 K and converts to a ketone with a concomitant loss in chirality at 200 K. Methyl ethyl ketone, ethylene, methane, CO, and hydrogen are found as products in temperature-programmed desorption (TPD). Propylene oxide adsorbs reversibly on Pd(111) at 80 K without undergoing any thermal decomposition, thus providing an ideal probe of surface chirality. The coverage of (R)-propylene oxide adsorbing on an (R)-2-butoxide-covered surface, ratioed to that on one covered by (S)-2-butoxide, reaches a maximum value of approximately 2 at a relative 2-butoxide coverage of approximately 25% of saturation and decreases to unity at a coverage of approximately 50% of saturation. This implies that the enantioselectivity depends critically on coverage and arises due to chiral "pockets" formed on the surface.

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The pathways for the hydrogenation of adsorbed acetylene and vinylidene on Pd(111) are investigated using temperature-programmed desorption and infrared reflection-absorption spectroscopies. The chemistry of the vinyl intermediate formed by the hydrogenation of both species is investigated by adsorbing vinyl iodide on Pd(111) where it is found that vinyl species hydrogenate more rapidly than adsorbed acetylene, indicating that the rate-limiting step in acetylene hydrogenation is the addition of the first hydrogen to acetylene to form a vinyl species. Infrared spectroscopy also reveals that vinyl species convert to ethylidynes as Pd( 111) is heated above ∼160 K. The hydrogenation of vinylidene by up to ∼0.2 Torr of hydrogen involves intermediate ethylidyne species in accord with this observation. Surprisingly, the rate constant for the conversion of vinylidene into ethylidyne is identical to that for the titration of ethylidyne from the surface by hydrogen, an effect that may be explained in terms of the different saturation coverages of the two species.

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The nature of the bonding interactions between aryl isocyanides and gold and palladium surfaces was investigated using attenuated total refection infrared (ATR-IR) spectroscopy. The experiments were conducted by evaporating a film of either palladium or gold onto a ZnSe internal reflection element (IRE). The studies reveal that aryl isocyanides form only σ-bonded species when coordinated to gold and that these species are bonded relatively weakly to the gold surface, evidenced by their ready removal when subjected to ultrasound (sonication). In contrast, aryl isocyanides form at least two distinct types of species when bonded to a palladium surface:  one effectively σ-bonded, as with gold, but much more tenaciously, and the other species bonded strongly to the surface by a σ/π synergistic interaction. The presence of π back-donation from the palladium surface into the isocyanide π system provides a rationale for the observation that barriers to conduction are lower when diisocyanides bridge palladium electrodes than when diisocyanides or dithiols bridge gold electrodes.

A Perspective is offered on the lessons learned from surface-science studies on enantioselective chemistry on solid surfaces performed by the author's groups. Our emphasis is on studies on model systems, mainly metal single-crystal surfaces under controlled environments, but extension of such research to more realistic samples relevant to heterogeneous catalysis is also briefly discussed. Enantioselective chemistry on surfaces is here divided into three guiding modalities, depending on the underlying mechanism. First, enantioselective chemistry resulting from the use of intrinsically chiral surfaces, which can be made from achiral solids such as metals by exposing the appropriate planes, is discussed. Next, the imparting of enantioselectivity to achiral surfaces by modifying them with adsorbates is classified in terms of two operating mechanisms: first, via the formation of supramolecular surface ensembles with chiral adsorption sites, and second, by relying on the effect of the local chiral environment intrinsically provided by the chiral modifiers through a one-to-one interaction between the modifier and the reactant. A discussion is then provided on studies with more complex samples involving metal nanoparticles and highsurface-area porous oxides. Finally, the present state of our understanding of enantioselective surface chemistry and the prognosis for the future are provided.

Temperature programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (C0 2 ) on polycrystalline graphite are presented. CO adsorbs onto graphite 'with a very low sticking coefficient « 10-12 ).After CO chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K, and between 1000 and 1300 K, and CO 2 (mass 44 amu) des orbs below 950 K. The intensity of the CO 2 signal is less than one order of magnitude lower than the CO intensity. After CO 2 adsorption the major desorption product is CO at high temperatures (lOOO