Model Catalysis of Ammonia Synthesis ad Iron-Water Interfaces - ASum Frequency Generation Vibrational Spectroscopic Study of Solid-GasInterfaces and Anion Photoelectron Spectroscopic Study of Selected Anionclusters

Abstract

The ammonia synthesis reaction has been studied using single crystal model catalysis combined with sum frequency generation (SFG) vibrational spectroscopy. The adsorption of gases N{sub 2}, H{sub 2}, O{sub 2} and NH{sub 3} that play a role in ammonia synthesis have been studied on the Fe(111) crystal surface by sum frequency generation vibrational spectroscopy using an integrated Ultra-High Vacuum (UHV)/high-pressure system. SFG spectra are presented for the dissociation intermediates, NH{sub 2} ({approx}3325 cm{sup -1}) and NH ({approx}3235 cm{sup -1}) under high pressure of ammonia or equilibrium concentrations of reactants and products on Fe(111) surfaces. Special attention was paid to understand how potassium promotion of the iron catalyst affects the intermediates of ammonia synthesis. An Fe(111) surface promoted with 0.2 monolayers of potassium red shifts the vibrational frequencies of the reactive surface intermediates, NH and NH{sub 2}, providing evidence for weakened the nitrogen-hydrogen bonds relative to clean Fe(111). Spectral features of these surface intermediates persisted to higher temperatures for promoted iron surfaces than for clean Fe(111) surfaces implying that nitrogen-iron bonds are stronger for the promoted surface. The ratio of the NH to NH{sub 2} signal changed for promoted surfaces in the presence of equilibrium concentrations of reactants and products. Themore » order of adding oxygen and potassium to promoted surfaces does not alter the spectra indicating that ammonia induces surface reconstruction of the catalyst to produce the same surface morphology. When oxygen is co-adsorbed with nitrogen, hydrogen, ammonia or potassium on Fe(111), a relative phase shift of the spectra occurs as compared to the presence of adsorbates on clean iron surfaces. Water adsorption on iron was also probed using SFG vibrational spectroscopy. For both H{sub 2}O and D{sub 2}O, the only spectral feature was in the range of the free OH or free OD. From the absence of SFG spectra of ice-like structure we conclude that surface hydroxides are formed and no liquid water is present on the surface. Other than model catalysis, gas phase anion photoelectron spectroscopy of the Cl + H{sub 2} van der Waals well, silicon clusters, germanium clusters, aluminum oxide clusters and indium phosphide clusters were studied. The spectra help to map out the neutral potential energy surfaces of the clusters. For aluminum oxide, the structures of the anions and neutrals were explored and for silicon, germanium and indium phosphide the electronic structure of larger clusters was mapped out.« less