Luke Burkholder

Ethylene adsorption on Pd(111) studied using infrared reflection–absorption spectroscopy

The adsorption of ethylene has been studied on clean and hydrogen-covered Pd(1 1 1) using reflection–absorption infrared spectroscopy and molecular beam methods. Using a correlation diagram, in which vibrational frequencies are plotted versus a rp parameter proposed by Stuve and Madix, shows that ethylene is substantially rehybridized on Pd(1 1 1) having a rp parameter intermediate between those of ethylene on Ni(1 1 1) and Ru(0 0 1). In contrast, when ethylene adsorbs on hydrogen-covered Pd(1 1 1), only p-bonded species are detected. An additional species appears

Chemistry of glycine on Pd(111) : Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The surface chemistry of glycine is studied on clean Pd(111) using a combination of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Glycine adsorbs strongly into second and subsequent layers as well as on the first monolayer, where the first-layer coverage is measured by titrating the bare surface with carbon monoxide. A small portion of glycine adsorbed directly on the Pd(111) surface desorbs as intact molecules, whereas the majority thermally decomposes by C-C bond scission. The COO moiety desorbs as CO and CO 2 , whereas the nitrogen-containing fragment yields methylamine and HCN. XPS reveals that glycine adsorbs predominantly in its zwitterionic form on the clean surface, whereas the multilayer contains 70-80% zwitterionic glycine, the remainder adsorbing in the neutral form.

KINETIC AND REACTIVE PROPERTIES OF ETHYLENE ON CLEAN AND HYDROGEN-COVERED Pd(111)

Several molecular adsorption states are identified following ethylene adsorption on clean and hydrogen-covered Pd(111) using temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Di-σ-bonded ethylene forms on clean Pd(111) desorbing with an activation energy of 80 kJ/mol at low coverages. The strong intermolecular lateral interactions considerably reduce the desorption temperature at higher coverages. Π-bonded ethylene is formed on hydrogen-covered Pd(111), where the proportion of π-bonded species increases with hydrogen coverage. This species converts to the more stable di-σ-bonded species on heating. Ethane formation is detected in TPD from hydrogen-precovered Pd(111), which is predominantly formed by reaction with π-bonded ethylene.

The Surface Chemistry of Dimethyl Disulfide on Copper

The surface chemistry of dimethyl disulfide (DMDS) is studied on a Cu(111) single crystal and a polished copper foil in ultrahigh vacuum as a basis for understanding its tribological chemistry using a combination of temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). Low-energy electron diffraction reveals that the polished foil becomes ordered on heating in vacuo and displays identical surface chemistry to that found on the Cu(111) surface. Dimethyl disulfide reacts with the copper surface at 80 K to form thiolate species. Heating the surface to ∼230 K causes a small portion of the thiolate species to decompose to form methyl groups adsorbed on the surface. Further heating results in methane and C(2) hydrocarbon desorption at ∼426 K, due to a reaction of adsorbed methyl species, to completely remove carbon from the surface and to deposit atomic sulfur.

Enhanced hydrogenation activity and diastereomeric interactions of methyl pyruvate co-adsorbed with R -1-(1-naphthyl)ethylamine on Pd(111)

Unmodified racemic sites on heterogeneous chiral catalysts reduce their overall enantioselectivity, but this effect is mitigated in the Orito reaction (methyl pyruvate (MP) hydrogenation to methyl lactate) by an increased hydrogenation reactivity. Here, this effect is explored on a R-1-(1-naphthyl)ethylamine (NEA)-modified Pd(111) model catalyst where temperature-programmed desorption experiments reveal that NEA accelerates the rates of both MP hydrogenation and H/D exchange. NEA+MP docking complexes are imaged using scanning tunnelling microscopy supplemented by density functional theory calculations to allow the most stable docking complexes to be identified. The results show that diastereomeric interactions between NEA and MP occur predominantly by binding of the C=C of the enol tautomer of MP to the surface, while simultaneously optimizing C=O····H2N hydrogen-bonding interactions. The combination of chiral-NEA driven diastereomeric docking with a tautomeric preference enhances the hydrogenation activity since C=C bonds hydrogenate more easily than C=O bonds thus providing a rationale for the catalytic observations.

An Infrared Spectroscopic and Temperature-programmed Desorption Study of 1,1-Difluoroethylene on Clean and Hydrogen-covered Pd(111)

The surface chemistry of 1,1-difluoroethylene was studied on clean and hydrogen-covered Pd(111) using a combination of temperature-programmed desorption and reflection absorption infrared spectroscopy (RAIRS) to explore whether the larger infrared absorbance of 1,1-difluoroethylene than ethylene may be used to examine reactions under realistic catalytic conditions using RAIRS. It was found that the chemistry of 1,1-difluoroethylene on Pd(111) surfaces is similar to that of ethylene with bonding occurring in both the π- and di-σ-forms. However, due to the presence of C–F bonds in the molecule, the infrared absorbances for 1,1-difluoroethylene were much larger than those for ethylene. This provides the potential for using RAIRS for in situ studies of catalytic reactions that involve alkenes.