The influence of high electric fields on water and methanol surface electrochemistry.

摘要

Understanding the behavior of molecules in high electric fields, particularly near catalyst surfaces, is of fundamental importance to electrochemistry and high field surface chemistry. This study focuses on the behavior of water and methanol molecules adsorbed on a Pt emitter tip in high fields (∼1 V/Å) and provides further understanding of dissociation and ion cluster emission. Mass resolved ramped field desorption (RFD) experiments from field adsorbed water layers on Pt have provided field dependencies of specific ion cluster masses for temperatures ranging from 170 to 300 K. Isothermal RFDs showed that as the field increased, water ion clusters H+(H ₂O)_n were emitted, beginning with large n clusters (up to 7) and proceeding through each lower n cluster. The onset field depended on the value of n (lower for higher n), but was relatively independent of temperature; however, ion signal intensity was temperature dependent. Constant field, cyclical temperature experiments provided Arrhenius graphs showing thermal deactivation energies for ion emission from the tip. The observed deactivation corresponds to the thermal desorption of the nth solvating water molecule of the ion cluster with energies of 0.85 eV, 0.76 eV and 0.55 eV for n = 3, 4 and 5, respectively. For RFD experiments with ice, all observed H+(H₂O)_n ions appeared simultaneously in a flash at a critical onset field, demonstrating a common limiting step. The onset of ionization occurred at a higher field for 109 K RFDs (0.44 V/Å) than for 145 K RFDs (0.32 V/Å). However, clusters n = 2 and 3, which were dominant at 109 K, were not observed at 145 K. Results suggest a two step dissociative ionization/desorption mechanism in which the strongly temperature dependent dissociative ionization event determines emission onset at low temperatures, while the field dependent ion emission event determines emission onset at higher temperatures. Pulsed time of flight mass spectrometry of field adsorbed water/methanol layers at 165 K demonstrated analogous behavior to the pure water system, but with richer spectra, particularly near mass 33. H+(CH₃OH) _m clusters were observed for m up to 4. Mixed H+(CH₃OH)_m(H₂O) _n clusters were also observed.