Andrew J. Gellman

The oscillatory behavior of the CO oxidation reaction at atmospheric pressure over platinum single crystals: Surface analysis and pressure dependent mechanisms

The oscillatory behavior of the catalyzed oxidation of carbon monoxide has been studied over platinum single crystals of (111), (100), and (13,1,1) orientation. Surface properties were examined in ultra high vacuum, before and after the reaction, using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), and a Kelvin probe for work function measurements. The oxidation of carbon monoxide was carried out at both low pressure (10 −4 Torr) and atmospheric pressure. The mechanism for the oscillations was different at high and low pressures. A model is presented for the oscillations appearing in the high pressure reaction. This model invokes the formation of platinum oxide, the presence of which was determined experimentally. At atmospheric pressure, silicon was always present on the platinum surfaces and was necessary to detect oscillatory behavior.

Enantioselective Separation on Chiral Au Nanoparticles

The surfaces of chemically synthesized Au nanoparticles have been modified with d- or l-cysteine to render them chiral and enantioselective for adsorption of chiral molecules. Their enantioselective interaction with chiral compounds has been probed by optical rotation measurements during exposure to enantiomerically pure and racemic propylene oxide. The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by (R)- and (S)-propylene oxide is enhanced by interaction with Au nanoparticles. This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates. More importantly, chiral Au nanoparticles modified with either d- or l-cysteine selectively adsorb one enantiomer of propylene oxide from a solution of racemic propylene oxide, thus leaving an enantiomeric excess in the solution phase. Au nanoparticles modified with l-cysteine (d-cysteine) selectively adsorb the (R)-propylene oxide ((S)-propylene oxide). A simple model has been developed that allows extraction of the enantiospecific equilibrium constants for (R)- and (S)-propylene oxide adsorption on the chiral Au nanoparticles.

Chiral Surfaces: Accomplishments and Challenges

Chiral surfaces serve as media for enantioselective chemical processes. Their chirality is dictated by atomic- and molecular-level structure, and their enantioselectivity is determined by their enantiospecific interactions with chiral adsorbates. This Perspective describes three types of chiral metal surfaces: those modified by adsorption of chiral molecules, those templated by chiral lattices of adsorbed species, and those that are naturally chiral. A new paper in this issue of ACS Nano offers insight into the intermolecular interactions that govern chiral templating of surfaces. This Perspective then outlines three major challenges to the field of chiral surface science: development of methods for detection of enantiospecific interactions and enantioselective surface chemistry, preparation of high-area chiral metal surfaces, and the development of a fundamental, predictive-level understanding of the origin of enantioselectivity on chiral surfaces.

Effects of conformational isomerism on the desorption kinetics of n-alkanes from graphite

The dynamics of oligomer desorption from surfaces have been studied by measuring the desorption kinetics of a set of n-alkanes from the surface of single crystalline graphite. Desorption rates were measured using a set of 21 monodispersed n-alkanes (CNH2N+2,5⩽N⩽60) each adsorbed at coverages in the range 1 monolayers. Desorption is observed to be a first-order process with a desorption barrier (ΔEdes‡) that is independent of coverage. The pre-exponential of the desorption rate constant is independent of the oligomer chain length and has a value of ν=1019.6±0.5 s−1. We also find that ΔEdes‡ has a nonlinear dependence on chain length and takes the empirical form ΔEdes‡=a+bNγ, with the exponent having a value of γ=0.50±0.01. More interestingly, we have proposed a mechanism for the desorption process and a model for the energetics and the entropy of the oligomers on the surface that provide an extremely good quantitative fit to the observed chain length dependence of ΔEdes‡. ΔEdes‡ is given by the di...

Chiral single crystal surface chemistry

Several experiments have been performed to probe the enantiospecific properties of chiral single crystal surfaces. The surfaces chosen have been the (643) planes of Ag and Cu, both face centered cubic structures. The chirality of these surfaces arises from the handedness of their kinked step structures. These structures are such that the (643) and the .643/ surfaces are related by mirror symmetry but are non-superimposable. We denote them as (643) R and (643) S . As a consequence of this handedness it is expected that the interactions of these surfaces with the left- and right-handed enantiomers of a chiral molecule should be different. In other words the chemistry of chiral molecules on these surfaces should be enantiospecific. We have observed that the desorption energies of R-3-methyl-cyclohexanone differ by 0:22 0:05 kcal/mole on the Cu(643) R and the Cu(643) S surfaces. Similarly, on the Ag(643) R surface we have observed that the orientations of R- and S-2-butanoxy groups differ. This enantiospecific orientation is revealed by the intensities of the absorption bands in an infrared absorption spectra of these species on the Ag(643) R surface. These two results expand the small but growing set of observations of the enantiospecific properties of chiral single crystal surfaces.

Surface chemistry in tribology

Abstract Surface chemistry is key to the understanding of tribological phenomena in the absence of a thick lubricant film. Progress in the development of surface analytical techniques has opened a new window into tribochemical phenomena and holds the promise of a better understanding of many critically important tribological processes. In this review the areas in which surface chemistry has played an important role in enhancing tribological understanding are surveyed. These include boundary lubrication, surface-additive interactions, the anomalous tribological behaviour of quasicrystals and the lubrication of hard disks.

The CO coadsorption and reactions of sulfur, hydrogen and oxygen on clean and sulfided Mo(100) and on MoS2(0001) crystal faces

The chemisorption and reactivity of O2 and H2 with the sulfided Mo(100) surface and the basal (0001) plane of MoS2 have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10−8–10−5Torr) and high (1 atm) pressures of O2 and H2. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of 18O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S2 and O, but not SO2. A pressure of 1 × 10−7 Torr of O2 was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS2 was unreactive in the presence of 1 atm of O2 at temperatures of 520 K. Sputtering of the MoS2 produced a marked uptake of oxygen and the removal of sulfur under the same conditions.

Nanocatalysis: More than speed

The role of catalysts is greater than simply increasing the rate of a reaction. Modifying nanoparticles enhances two significant catalyst attributes: selectivity and thermal stability.

Catalytic hydrodesulfurization over the Mo(100) single crystal surface: II. The role of adsorbed sulfur and mechanism of the desulfurization step

The study of thiophene hydrodesulfurization (HDS) over initially clean Mo(100) surfaces has been extended to include sulfided surfaces. Low sulfur coverages (0 ≤ θs

Catalytic hydrodesulfurization over the Mo(100) single crystal surface: I. Kinetics and overall mechanism

0.67) inhibit HDS activity. Increasing the sulfur coverage in the range 0.67 ≤ θs ≤ 1.0 produces a surface with an HDS activity of about half that of the clean Mo(100) surface. Excessive exposure of the surface to a sulfur-containing environment results in the formation of a MoS2 layer which is, at least in part, responsible for complete catalytic deactivation. Radiotracer (35S) labeling techniques have been used to measure rates of hydrogenation of sulfur adsorbed on the Mo(100) surface. In ambient atmospheres of both hydrogen (1 atm) and the thiophene HDS reaction mixture (P(H2) = 1 atm, P(Th) = 2.5 Torr) the rate of hydrogenation of adsorbed sulfur is two orders of magnitude less than the HDS rate. This fact has been used to suggest that the desulfurization step of the reaction does not proceed via the formation of a tightly bound MoS species.