High-Pressure Ammonia Adsorption and Dissociation on Clean Fe(111) and Oxygen-Precovered Fe(111) Studied by Sum Frequency Generation Vibrational Spectroscopy

摘要

The adsorption of gases N_2,H_2,O_2,and NH_3 that play a role in ammonia synthesis have been studied on the Fe(111) crystal surface by Sum Frequency Generation (SFG) vibrational spectroscopy using an integrated ultrahigh vacuum/high-pressure system.SFG spectra are presented for the dissociation intermediates,NH_2 (approx3325 cm~(-1)) and NH (approx3235 cm~(-1)) under high pressure of ammonia (200 Torr) on the clean Fe(111) surface.Addition of 0.5 Torr of oxygen to 200 Torr of ammonia does not significantly change the bonding of dissociation intermediates to the surface.However,it leads to a phase change of nearly 180 deg between the resonant and nonresonant second-order nonlinear susceptibility of the surface,demonstrated as a reversal of the SFG spectral features.Heating the surface in the presence of 200 Torr of ammonia and 0.5 Torr of oxygen reduces the oxygen coverage,which can be seen from the SFG spectra as another relative phase change of 180 deg.The reduction of the oxide is also supported by Auger electron spectroscopy.The result suggests that the phase change of the spectral features could serve as a sensitive indicator of the chemical environment of the adsorbates.Clean Fe(l11) is found to have a large SFG nonresonant signal.The magnitude of the nonresonant signal was dependent on the adsorption species;O_2 and N_2 decrease,while H_2 and NH_3 increase the SFG nonresonant signal.The change in nonresonant signal is correlated to the change in work function for Fe(lll) upon adsorption.Adsorption-induced changes in the SFG nonresonant signal was used as an indicator of surface conditions and to monitor surface reactions.