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The adsorption of nitrogen on iridium was studied with a field emission microscope equipped with a probe-hole assembly to enable emission experiments on individual emitter regions. The adsorption of nitrogen is markedly face-specific. Temperature programmed desorption reveals three binding states: γ 1 on the (100) face with a maximum heat of adsorption of 7–8 kcal/mole, γ 2 on the regions around (110) with a maximum heat of adsorption of 10–11 kcal/mole and γ 3 on the roughest tip regions (210), (320), (531) and (731) with a maximum heat of adsorption of 13–14 kcal/mole. Nitrogen adsorbed in the γ 1 and γ 3 states causes a decrease, but in the γ 2 state a small increase, in the work function. These results are discussed in relation with data on nickel, palladium, platinum and rhodium. While nitrogen is only weakly adsorbed on all these metals there is a marked difference in the nature of the pertinent adsorption complex.

The adsorption of hydrogen, ethylene, acetylene, cyclohexane and benzene was studied on both the (111) and stepped [6(111) × (100)] crystal surfaces of iridium. The techniques used were low energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry. At 30°C, acetylene, ethylene and benzene are adsorbed with a sticking probability near unity. The sticking probability of cyclohexane is less than 0.1 on both surfaces. Heating the (111) surface above 800°C, in the presence of the hydrocarbons, results in the formation of an ordered carbonaceous overlayer with a diffraction pattern corresponding to a (9 × 9) surface structure. No indication for ordering of the carbonaceous residue was found on the stepped iridium surface in these experimental conditions. The hydrocarbon molecules form only poorly ordered surface structures on both iridium surfaces when the adsorption is carried out at 30°C. Benzene is the only gas that can be desorbed from the surfaces in large amounts by heating. Ethylene remains largely on the surface, only a few percent is removed by heating while acetylene and cyclohexane cannot be desorbed at all. When adsorption is carried out at 30°C and the crystal is subsequently flashed to high temperature, hydrogen is liberated from the surface. The hydrogen desorption spectra from the iridium surfaces exposed to C2H4, C2H2, or C6H6 exhibit two hydrogen desorption peaks, one around 200°C and the second around 350°C. The temperatures where these peaks appear vary slightly with the type of hydrocarbon. The relative intensities of these two peaks depend strongly on the surface used. Arguments are presented that decomposition of the hydrocarbon molecules (C-H bond breaking nd possibly also C-C bond breaking) occurs easier on the stepped iridium surface than on the (111) surface. Hydrogen is desorbed at a higher temperature from an iridium surface possessing a high density of surface imperfections than from a perfect iridium (111) surface. The results are compared with those obtained previously on similar crystal surfaces of platinum. It appears that C-H bond breaking occurs more easily on iridium than on platinum.

The adsorption of CO, O2, and H2O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H2O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O2 was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O2 on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.

The adsorption of Xe on Pt was investigated using a field emission microscope, equipped with a probe-hole assembly. The initial heat of adsorption decreases in the order (321) > (111) ≈ (100) > (210) > (110). At low coverage Xe is adsorbed in surface sites providing a maximum coordination of the adatom with metal atoms. The heat of adsorption decreases with coverage, in particular on the smooth regions (111) and (100), where a closepacked Xe layer is apparently formed at high coverage. On the areas around (321) and (210) however, site adsorption is preferred to the formation of a close-packed Xe layer; by consequence the heat of adsorption decreases only little with coverage. The crystal face dependence of Xe adsorption on Pt is virtually identical to that found for Ir. When data reported for Xe on W are also included the general conclusion may be drawn that strong bonding of Xe on transition metals is favoured by 1. a large coordination of adatoms with metal atoms and 2. a high work function of the adsorbing crystal plane. Die Adsorption von Xe auf Pt wurde mit Hilfe eines Feldelektronenmikroskopes mit Lochblende untersucht. Die initiale Adsorptionswarme auf den verschiedenen Kristallflachen nimmt in der Reihenfolge (321) > (111) ≈ (100) > (210) > (110) ab. Bei geringer Bedekkung wird Xe auf Oberflachenplatzen mit maximaler Koordination des Adatoms durch Metallatome adsorbiert. Die Adsorptionswarme sinkt mit steigender Bedeckung insbesondere auf den glatten Gebieten (111) und (100), wo bei hoher Bedeckung anscheinend eine dichtgepackte Xe-Schicht ausgebildet wird. Auf den (321)- und (210)-Gebieten dagegen bleibt die Adsorption auf bestimmten Gitterplatzen auch bei hohem Bedeckungsgrad gunstiger als die Bildung einer dichtgepackten Schicht. Infolgedessen nimmt die Adsorptionswarme hier nur wenig ab mit zunehmender Bedeckung. Die Abhangigkeit der Xe-Adsorption von der Kristallflache ist fur Pt praktisch identisch mit der fur Ir gefundenen. Werden auch die fur W publizierten Werte mitberucksichtigt, so folgt, das allgemein eine starke Bindung des Xe mit einem Ubergangsmetall durch: 1. eine hohe Koordination des Adatoms mit Metallatomen; und 2. eine hohe Austrittsarbeit der adsorbierenden Kristallflache begunstigt wird.