Influence of the terminal group on the thermal decomposition reactions of carboxylic acids on copper: nature of the carbonaceous film

Mechanistic and Kinetic Measurements of Elementary Surface Reactions Using Temperature-Programmed X-ray Photoelectron Spectroscopy

This paper describes a method by which surface-reaction kinetics can be measured by slowly and precisely ramping up the surface temperature at a constant rate while simultaneously collecting X-ray photoelectron spectra (XPS). This approach results in the collection of a large amount of data over relatively small temperature steps to produce quasi-continuous kinetic data. The method is illustrated for the desorption and reaction of diethyl disulfide (DEDS) on a Au(111) substrate in ultrahigh vacuum, where the results can be compared with previous conventional temperature-programmed desorption (TPD) data from Au(111). Experiments were carried out using a double-pass cylindrical-mirror analyzer with a channeltron detector to demonstrate how this approach can be implemented in a routine, multitechnique vacuum chamber. The approach will be even more effective in a more modern, specialized XPS apparatus with high-transmission hemispherical analyzers with multichannel array detectors, which will enable the spectra of several elements to be measured simultaneously. The results yielded an activation energy for multilayer desorption of DEDS of 41 ± 1 kJ/mol, with a pre-exponential factor of 8 ± 7 × 1012 s-1, an activation energy of 53 ± 6 kJ/mol and pre-exponential factor of 9 ± 8 × 1013 s-1 for monolayer desorption and an activation energy of 90 ± 6 kJ/mol with a prefactor of 1.0 ± 0.3 × 1015 s-1 for the reaction of adsorbed ethyl thiolate species to adsorbed DEDS. While these results were collected for a system for which the kinetic data could have been obtained using conventional TPD, this method can be more usefully applied to those surface reaction processes that do not rely on the formation of desorption products. This system, having been previously studied by TPD, facilitates a comparison with results obtained by conventional methods.

Surface chemistry at the solid-solid interface: mechanically induced reaction pathways of C8 carboxylic acid monolayers on copper

Mechano- or tribochemical processes are often induced by the large pressures, of the order of 1 GPa, exerted at contacting asperities at the solid-solid interface. These tribochemical process are not very well understood because of the difficulties of probing surface-chemical reaction pathways occurring at buried interfaces. Here, strategies for following surface reaction pathways in detail are illustrated for the tribochemical decomposition of 7-octenoic and octanoic acid adsorbed on copper. The chemistry was measured in ultrahigh vacuum by sliding either a tungsten carbide ball or a silicon atomic force microscope (AFM) tip over the surface to test a previous proposal that the nature of the terminal group in the carboxylic acid, vinyl versus alkyl, could influence its binding to the counterface, and therefore the reaction rate. The carboxylic acids bind strongly to the copper substrate as carboxylates to expose the hydrocarbon terminus. The tribochemical reaction rate was found to be independent of the nature of the hydrocarbon terminus, although the pull-off and friction forces measured by the AFM were different. The tribochemical reaction is initiated in the same way as the thermal reaction, by the carboxylate group tilting to eliminate carbon dioxide and deposit alkyl species onto the surface. This reaction occurs thermally at ∼640 K, but tribochemically at room temperature, producing significant differences in the rates and selectivities of the subsequent decomposition pathways of the adsorbed products.