A Fundamental Study of NO Reduction with Hydrogen over Ir(110)

Citation

Ibbotson, Dale Edward (1981) A Fundamental Study of NO Reduction with Hydrogen over Ir(110). Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xnsj-m676. https://resolver.caltech.edu/CaltechETD:etd-12142006-084027

Abstract

The heterogeneously catalyzed reduction of NO with hydrogen over Ir(110), as well as the chemisorption of the individual reactants and some of the products, has been studied at low pressures (<10^-5) torr). The experiments were performed with several surface sensitive probes-thermal desorption mass spectrometry (TDS), contact potential difference (CPD) measurements, LEED, X-ray and UV photoelectron spectroscopies and Auger electron spectroscopy.

Chapter 2 describes the chemisorption of hydrogen on Ir(110). The Ir(110)-(1x2) reconstructed surface is stable in hydrogen at pressures from 10^-9 to 10^-5 torr and surface temperatures from 130 to 1000 K, the conditions investigated. Absolute coverage measurements indicate the saturation density at 130 K on Ir(110) is (2.2±0.2)x10¹⁵ atoms cm^-2. Thermal desorption measurements indicate hydrogen obeys second order desorption kinetics and exhibits two features, β₁ and β₂ states, with intensities 2:1, respectively, which exchange isotopically with one another. However, β₂ hydrogen obeys first order adsorption kinetics with an initial probability of adsorption S_o equal to unity, while β₁ hydrogen has an S_o equal to 7x10^-3 and obeys second order kinetics. Rate parameters for hydrogen desorption from Ir(110) show a sympathetic increase up to at least half of saturation for the β₂ state where E_d and ν_d assume values of 23 kcal-mole^-1 and 1.5x10^-2cm²-s^-1, respectively. For the β₁ state, E_d = 17-100 kcal-mole^-1 from 0 equal to 0.4-0.7 and ν_d maintains an average value of 10^-7 cm²-s^-1. The CPD and UPS measurements are used to infer probable binding sites for the β₁ and β₂ states of hydrogen which are consistent with the absolute coverage determined from TDS.

Chapter 3 discusses the interaction of hydrogen and CO on Ir(110). The co-adsorption of hydrogen and CO was undertaken to understand the effects of a model poison, CO, for hydrogen chemisorption. The adsorption of hydrogen on adsorbed CO, or vice versa, causes less hydrogen to occupy the β₂ state and shifts the occupancy to the β₁ state preferentially. An apparent increase in the probability of adsorption of hydrogen in the β₁ state occurs for small CO coverages. At high CO coverages, the Ir(110) surface is poisoned to hydrogen adsorption. Exposing CO to preadsorbed hydrogen causes the binding energy of hydrogen to decrease with increasing CO exposure. Eventually, hydrogen is displaced from the surface for sufficient CO exposures. The induced dipole of hydrogen is unaffected by CO compared to the clean surface, as measured by the CPD. The results indicate CO poisons β₂ sites for hydrogen by a simple site blocking mechanism and may exclude β₁ sites at high CO coverages by a hydrogen-CO repulsive interaction.

Chapter 4 presents the results for the molecular chemisorption of N₂ and the coadsorption of N₂ with hydrogen on Ir(110) at low temperatures. Photoelectron spectroscopy shows molecular levels of N₂ at 8.0 (5σ + 1π) and 11.8 (4σ) eV in the valence band and at 399.2 eV with a satellite at 404.2 eV in the N(1s) region. The kinetics of adsorption and desorption of N₂ show that both precursor kinetics and interadsorbate interactions are important for this chemisorption system. Adsorption occurs with S_o equal to unity up to saturation coverage (4.8X10¹⁴ cm^-2) and thermal desorption gives rise to two peaks. The activation energy for desorption varies between 8.5 and 6.0 kcal-mole^-1 at low and high coverages, respectively. Results of the co-adsorption of N₂ and hydrogen indicate that adsorbed N₂ resides in the missing row troughs on Ir(110) - (1x2). Furthermore, N₂ is displaced by hydrogen, and the β₂ state of hydrogen blocks virtually all N₂ adsorption.

Chapter 5 considers the chemisorption of NO on Ir(110). Adsorption of NO on Ir(110) proceeds by presursor kinetics with S_o equal to unity independent of surface temperature. Saturation of Ir(110) is achieved in molecular form at 9.6x10¹⁴ cm^-2 below 300 K. Approximately 35% of a saturated overlayer desorbs as NO in two peaks with equal intensities. The balance desorbs as N₂ and O₂ where N₂ begins to desorb after the first peak of NO is nearly completed. Estimates were made of the activation energies for the various surface reactions that occur as the surface is heated. At low coverages of NO, N₂ desorbs with E equal to 36 kcal-mole^-1. The activation energy for the dissociation of NO is near 25 kcal-mole^-1 for a saturated overlayer, but varies for smaller coverages of NO. Desorption of NO at saturation is associated with energies of 23 and 33 kcal-mole^-1 for the two peaks. The first peak represents desorption of NO from an oxygen-free surface and the second peak represents, at least in part, the recombination of nitrogen and oxygen adatoms on a partially oxidized surface. Oxygen tends to stabilize NO to dissociation and desorption as N₂, as reflected in TDS. Moreover, UPS and CPD results indicate NO is stabilized on oxygen overlayers compared to the clean Ir(110) surface.

Chapter 6 discusses the reaction of NO and deuterium to form N₂, ND₃ and D₂O over Ir(110). In addition, the competitive co-adsorption of NO and deuterium and the thermal desorption of the resulting overlayer were performed to gain further insight into the observed steady state rates of reaction, via TDS and CPD measurements. Small precoverages of deuterium do not affect the adsorption kinetics of NO on Ir(110) but do cause more N₂ to desorb relative to NO at saturation on the clean surface. Deuterium will adsorb on a saturated overlayer of NO. However, deuterium is strongly blocked from adsorbing on an Ir(110) surface that has both NO and oxygen adsorbed, which is a condition that occurs for some steady state reaction conditions. Under steady state conditions, the reduction of NO shows a marked hysteresis as the surface temperature is cycled for a large enough value of R(P_D2/P_NO). A plateau in the reduction rate appears at some T that persists as T decreases until at lower values of T the rate falls irreversibly. For larger values of R, ND₃ is produced between 470 - 630 K and competes strongly with N₂ production. Otherwise, N₂ and D₂O are the only products of the reduction reaction. Tentative explanations of the empirical rate models derived from the steady state rate data are discussed in light of XPS, UPS and LEED results that are presented as well.

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