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

Abstract

The adsorption of gases N2, H2, O2, and NH3 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, NH2 ( approximately 3325 cm-1) and NH ( approximately 3235 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 degrees 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 degrees . 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(111) is found to have a large SFG nonresonant signal. The magnitude of the nonresonant signal was dependent on the adsorption species; O2 and N2 decrease, while H2 and NH3 increase the SFG nonresonant signal. The change in nonresonant signal is correlated to the change in work function for Fe(111) upon adsorption. Adsorption-induced changes in the SFG nonresonant signal was used as an indicator of surface conditions and to monitor surface reactions.

Heat of adsorption of CO on Pt(111) obtained by sum frequency generation vibrational spectroscopy—a new technique to measure adsorption isotherms

“…The non-resonant background can also change the shape of the resonant signal depending on the relative phase between the two contributions according to Eq. (2) [21,22]. The amplitude and phase of the background in the present study was determined by fitting the spectra from the CO adsorption to Eq.…”

The impact of surface science on the commercialization of chemical processes

“…Potassium is an electron donor (even in its oxide form) to the metal surfaces and ammonia, the product molecule, is also an electron donor. The donor-donor repulsive interaction lowers the heat of adsorption of ammonia by about 2.5 kcal/mol [28]. In this circumstance, at high ammonia pressures near equilibrium, the presence of potassium reduces the surface concentration of ammonia and thereby product poisoning is minimized, giving rise to a much higher concentration of ammonia in the gas phase [25,27,29].…”

Single Crystal Surfaces

“…However, the addition of K to Fe(111) does not change the activation energy of reaction and thus does not change the reaction mechanism. Recent high-pressure SFG results have indicated that, in the presence of 200 torr of ammonia, NH 2 and NH species exist on the Fe(111) surface at room temperature [46]. As the surface temperature is increased to 373 K, dehydrogenation occurs [47].…”