Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe₃O₄ film on Pt(1 1 1).It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface.Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.     

Oxygen adsorption and oxidation reactions on Au(2 1 1) surfaces: Exposures using O2 at high pressures and ozone (O3) in UHV

Nanosized gold particles supported on reducible metal oxides have been reported to show high catalytic activity toward CO oxidation at low temperature. This has generated great scientific and technological interest, and there have been many proposals to explain this unusual activity. One intriguing explanation that can be tested is that of Nørskov and coworkers [Catal. Lett. 64 (2000) 101] who suggested that the “unusually large catalytic activity of highly-dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au-support interface”. In particular, their calculations indicated that the Au(2 1 1) stepped surface would be much more reactive towards O₂ dissociative adsorption and CO adsorption than the Au(1 1 1) surface. We have now studied the adsorption of O₂ and O₃ (ozone) on an Au(2 1 1) stepped surface. We find that molecular oxygen (O₂) was not activated to dissociate and produce oxygen adatoms on the stepped Au(2 1 1) surface even under high-pressure (700 Torr) conditions with the sample at 300-450 K. Step sites do bind oxygen adatoms more tightly than do terrace sites, and this was probed by using temperature programmed desorption (TPD) of O₂ following ozone (O₃) exposures to produce oxygen adatoms up to a saturation coverage of θ_O = 0.90 ML. In the low-coverage regime (θ_O ? 0.15 ML), the O₂ TPD peak at 540 K, which does not shift with coverage, is attributed to oxygen adatoms that are bound at the steps on the Au(2 1 1) surface. At higher coverages, an additional lower temperature desorption peak that shifts from 515 to 530 K at saturation coverage is attributed to oxygen adsorbed on the (1 1 1) terrace sites of the Au(2 1 1) surface. Although the desorption kinetics are likely to be quite complex, a simple Redhead analysis gives an estimate of the desorption activation energy, E_d, for the step-adsorbed oxygen of 34 kcal/mol and that for oxygen at the terraces near saturation coverage of 33 kcal/mol, values that are similar to others reported on Au surfaces. Low Energy Electron Diffraction (LEED) indicates an oxygen-induced step doubling on the Au(2 1 1) surface at low-coverages (θ_O = 0.08-0.17 ML) and extensive disruption of the 2D ordering at the surface for saturation coverages of oxygen (θ_O ? 0.9 ML). Overall, our results indicate that unstrained step sites on Au(2 1 1) surfaces of dispersed Au nanoparticles do not account for the novel reactivity of supported Au catalysts for CO oxidation.     

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO2

RuO₂(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O₂ and high reaction temperatures. Subsequently the RuO₂ was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO₂ gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO₂ displays high reactivity at relatively low temperatures (?450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O₂ species. RuO₂ deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.     

Initial oxidation of Ni(1 1 1) observed by electron emission microscopy: PEEM and MEEM

The initial oxidation of Ni(1 1 1) in the temperature range of 550-700 K has been monitored by photoelectron emission microscopy (PEEM) and metastable-atom electron emission microscopy (MEEM). The PEEM and MEEM images show uniform patterns for the chemisorbed overlayer, reflecting the electronic homogeneity as seen at the μm scale. During the nucleation and lateral growth of oxide, however, the μm-scale pattern due to the formation of oxide domains appears and its evolution depends strongly on the substrate temperature and dose pressure of gaseous O₂. Our data indicate that the high-temperature oxidation is regarded as a successive multi-nucleation process in a reaction-diffusion field.     

Methanol adsorption on a CeO2(1 1 1)/Cu(1 1 1) thin film model catalyst

Adsorption and desorption of methanol on a CeO₂(1 1 1)/Cu(1 1 1) thin film surface was investigated by XPS and soft X-ray synchrotron radiation PES. Resonance PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Methanol adsorbed at 110 K formed adsorbate multilayers, which were partially desorbed at 140 K. Low temperature desorption was accompanied by formation of chemisorbed methoxy groups. Methanol strongly reduced cerium oxide by forming hydroxyl groups at first, which with increasing temperature was followed by creation of oxygen vacancies in the topmost cerium oxide layer due to water desorption. Dissociative methanol adsorption and creation of oxygen vacancies was observed as a Ce⁴⁺ → Ce³⁺ transition and an increase of the Ce 4f electronic state occupancy.     

The effect of Pt on Ni3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni₃Al single crystals, with and without Pt addition, at 300-900 K under oxygen pressures of ca. 10⁻⁷ Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al_xO_y. At high-temperature (i.e., 700-800 K), NiO is replaced by a thick layer of Al_xO_y. By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al_xO_y; and (2) shift the establishment of the thick Al_xO_y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.     

Oxidation of Pt(1 0 0)-hex-R0.7° by gas-phase oxygen atoms

We utilized temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), and low energy electron diffraction (LEED) to investigate the oxidation of Pt(1 0 0)-hex-R0.7° at 450 K. Using an oxygen atom beam, we generated atomic oxygen coverages as high as 3.6 ML (monolayers) on Pt(1 0 0) in ultrahigh vacuum (UHV), almost 6 times the maximum coverage obtainable by dissociatively adsorbing O₂. The results show that oxidation occurs through the development of several chemisorbed phases prior to oxide growth above about 1 ML. A weakly bound oxygen state that populates as the coverage increases from approximately 0.50 ML to 1 ML appears to serve as a necessary precursor to Pt oxide growth. We find that increasing the coverage above about 1 ML causes Pt oxide particle growth and significant surface disordering. Decomposition of the Pt oxide particles produces explosive O₂ desorption characterized by a shift of the primary TPD feature to higher temperatures and a dramatic increase in the maximum desorption rate with increasing coverage. Based on thermodynamic considerations, we show that the thermal stability of the surface Pt oxide on Pt single crystal surfaces significantly exceeds that of bulk PtO₂. Furthermore, we attribute the high stability and the acceleratory decomposition rates of the surface oxide to large kinetic barriers that must be overcome during oxide formation and decomposition. Lastly, we present evidence that structurally similar oxides develop on both Pt(1 1 1) and Pt(1 0 0), therefore concluding that the properties of the surface Pt oxide are largely insensitive to the initial structure of the Pt single crystal surface.     

Temperature dependent water adsorption on Bi2Sr2CaCu2O8 single crystal

Temperature dependent infrared study of water adsorption on the bc-plane of a Bi₂Sr₂CaCu₂O₈ (Bi2212) single crystal reveals that water is molecularly physisorbed, and forms hydrogen-bonded clusters in the temperature range of 85-150 K. Although dissociative adsorption is expected on surfaces terminated with Ca-O and Cu-O groups, no dissociated species is detected. The main absorption bands occur between 3200 and 3500 cm⁻¹ and are assigned to the O-H stretching and the overtone of O-H bending. For exposures higher than 1.0 L, the sticking coefficients are similar for different temperatures. The features in the O-H stretch region for water clusters red-shift as a function of temperature between 85 and 140 K. Water clusters have an amorphous structure between 85 and 140 K, and form a crystalline structure at 150 K.     

Scanning tunneling microscopy and spectroscopy investigations of copper phthalocyanine adsorbed on Al2O3/Ni3Al(1 1 1)

Low temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) have been used to investigate adsorbed copper phthalocyanine (C₃₂H₁₆N₈Cu) molecules on an ordered ultrathin Al₂O₃ film on the Ni₃Al(1 1 1) surface as a function of coverage and annealing temperature. For sub-monolayer coverage and a deposition temperature of 140 K two different planar molecular adsorption configurations rotated by 30° with respect to each other were observed with submolecular resolution in the STM images. The template effect of the underlying oxide film on the CuPc orientation, however, is only weak and negligible at higher coverages. For θ_CuPc ≈ 1 ML, before completion of the first layer, the growth of a second layer was already observed. The measured spacing of 3.5 Å between first and second layer corresponds to the distance between the layers in the α-modification of crystalline CuPc. The molecules deposited at 140 K are thermally stable upon prolonged annealing to temperatures up to 250 K. By the use of STS the lowest unoccupied molecular orbital (LUMO) of the adsorbed copper phthalocyanine molecules has been identified at an energy of 1.2 eV above E_F. The lateral distribution of the electronic states of the CuPc has been analyzed and mapped by STS.     

Deoxygenation of IrO2(1 1 0) surface: Core-level spectroscopy and density functional theory calculation

Deoxygenation of the IrO₂(1 1 0) surface is investigated at 403-493 K, using the core-level spectroscopy and density functional theory (DFT) calculation. The Ir-4f_7/2 signals of 1f-cus-Ir with and without on-top oxygen (O_top) emerge as surface features of the baked-out surface, whose positive and negative shifts in binding energy are in line with the DFT computation results. Progressively increasing the reduction temperature, the 1f-cus-Ir feature quickly disappears and the signal of 2f-cus-Ir emerges at 403 K. Meanwhile the feature of 1f-cus-Ir + O_top diminishes but persists when the Ir metal signal is evident. The intriguing coexistence of 1f-cus-Ir + O_top and Ir metal at 433-443 K is elucidated in the theoretical pathway study. DFT calculation reveals that O₂ desorption via pairing two neighboring O_top atoms is the rate-determining step of surface deoxygenation. Under the UHV conditions, O_top is replenished via migration of the surface oxygen species, including the threefold coordinated oxygen (O_3f) of a reduced surface. Hence the O_top atom is an active and long-lived surface species, which does not vanish until O_3f is consumed and surface Ir begins to cluster. Under the realistic pressure conditions, O_top can also be refreshed via the dissociative adsorption of gas-phase oxygen. In either pathway, O_top is a critical intermediary of IrO₂(1 1 0) oxidation catalysis.     

The interactions of thiophene with polycrystalline UO2

We have studied the adsorption and desorption of thiophene on polycrystalline UO₂ as function of coverage, over the temperature range 100-640 K, using X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron stimulated desorption (ESD). Thiophene is found to adsorb molecularly on stoichiometric UO₂. C 1s and S 2p XPS spectra are measured at different thiophene exposures and at different temperatures; they show no evidence for the presence of dissociation fragments, confirming that thiophene adsorbs and desorbs molecularly on a polycrystalline stoichiometric UO₂ surface. The variation of the S 2p and C 1s intensity as function of exposure, together with ESD measurements of O⁺ as function of exposure, can be connected to the growth mode of a thiophene film on UO₂; the thiophene film converts from a flat-lying configuration to an inclined structure as coverage increases. The effects of X-rays, UV, and electron irradiation on thiophene films have been studied in two different coverage regimes, monolayer and multilayer. Irradiation leads to a modification of thiophene films, and appreciable concentrations of species stable to 640 K are present on the surface for both regimes. The XPS results suggest that irradiation induces polymerization and oligomerization, as well as formation of thiolates and dissociation fragments of thiophene. The adsorption and reactivity of thiophene on defective UO₂ surfaces have also been studied. The O vacancies and defects in the oxide surface cause cleavage of C-H and C-S bonds leading to the dissociation of thiophene at temperatures as low as 100 K. These results illustrate the important role played by O vacancies in the chemistry of thiophene over an oxide surface.     

Growth and decay of the Pd(1 1 1)-Pd5O4 surface oxide: Pressure-dependent kinetics and structural aspects

Growth and decomposition of the Pd₅O₄ surface oxide on Pd(1 1 1) were studied at sample temperatures between 573 and 683 K and O₂ gas pressures between 10⁻⁷ and 6 × 10⁻⁵ mbar, by means of an effusive O₂ beam from a capillary array doser, scanning tunnelling microscopy (STM) and thermal desorption spectrometry (TDS). Exposures beyond the p(2 × 2)O adlayer (saturation coverage 0.25) at 683 K (near thermodynamic equilibrium with respect to Pd₅O₄ surface oxide formation) lead to incorporation of additional oxygen into the surface. To initiate the incorporation, a critical pressure beyond the thermodynamic stability limit of the surface oxide is required. This thermodynamic stability limit is near 8.9 × 10⁻⁶ mbar at 683 K, in good agreement with calculations by density functional theory. A controlled kinetic study was feasible by generating nuclei by only a short O₂ pressure pulse and then following further growth kinetics in the lower (10⁻⁶ mbar) pressure range.Growth of the surface oxide layer at a lower temperature (573 K) studied by STM is characterized by a high degree of heterogeneity. Among various metastable local structures, a seam of disordered oxide formed at the step edges is a common structural feature characteristic of initial oxide growth. Further oxide nucleation appears to be favoured along the interface between the p(2 × 2)O structure and these disordered seams. Among the intermediate phases one specifically stable phase was detected both during growth and decomposition of the Pd₅O₄ layer. It is hexagonal with a distance of about 0.62 nm between the protrusions. Its well-ordered form is a superstructure.Isothermal decay of the Pd₅O₄ oxide layer at 693 K involves at first a rearrangement into the structure, indicating its high-temperature stability. This structure can break up into small clusters of uniform size and leaves a free metal surface area covered by a p(2 × 2)O adlayer. The rate of desorption increases autocatalytically with increasing phase boundary metal-oxide. We propose that at close-to-equilibrium conditions (693 K) surface oxide growth and decay occur via this intermediate structure.     

Interactions of water vapor with polycrystalline uranium surfaces - The low temperature regime

The initial interaction of water vapor with polycrystalline uranium surfaces at low temperatures (LT, 200 K), was studied by combined measurements utilizing Direct Recoil Spectrometry (DRS), Auger electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Three stages of water dissociation and adsorption can be observed: Stage (1) 0-0.6 oxygen monolayer coverage: full (H₂O → O + 2H) dissociation is dominant, coexisting with partial dissociation (H₂O → OH + H). In contrast to room temperature, where the adsorption is of a Langmuir type, in the present low temperature case it is a precursor-state type - the oxygen accumulation is linear, indicating that a constant fraction of the water molecules impinging on the surface diffuses to a dissociation and adsorption site. Only minor oxidation of the uranium occurs. Stage (2) 0.6-full oxygen coverage: only partial dissociation occurs. Still only minor oxidation of uranium takes place. Stage (3) buildup of a second hydroxyl layer, concurrent with slow continuous oxidation of uranium. Subsequent heating of the sample after the described exposure was accompanied by additional continuous oxidation. Above ∼230 K, the main process seems to be OH decomposition and desorption. A comparison is made to the dissociation and adsorption processes at room temperature.     

Oxidation of ultrathin indium layers on W(1 1 0)--a core-level photoemission study

We have studied the reaction of ultrathin In overlayers on W(1 1 0) with molecular oxygen at 300 K. At a coverage of 0.25 monolayers (ML) oxygen first chemisorbs dissociatively at free tungsten sites and oxidation of In occurs with some delay. At an In coverage of 1.2 ML complete oxidation of the closed overlayer is observed. Layers of 3 ML thickness first show rapid transformation from In to an In₂O₃-like species until an oxide monolayer is formed. Further oxidation occurs at much reduced rate. No oxygen-induced restructuring is observed for In at 300 K, in contrast to the response of Ag monolayers deposited on W(1 1 0).     

Oxidation of CO on Fe2O3 model surfaces

Using first principles calculations based on a gradient corrected density functional formalism we show that Fe₂O₃ nano-particles with (1 0 0) and (0 0 0 1) surface orientations can oxidize CO to form CO₂ with or without the presence of O₂. However, depending on the surface orientation, the oxidation occurs through differing sequences. On the (1 0 0) surface, in the absence of O₂, two CO molecules are required for one CO oxidation in a concerted reaction while on a oxygen terminated (0 0 0 1) surface, a single CO molecule itself, without the aid of a second CO, can react with the lattice oxygen atoms to form CO₂. In the presence of O₂, the O vacancies created by an initial oxidation through lattice oxygen act as the favored sites for O₂ adsorption which can subsequently oxidize the incoming CO. Detailed reaction paths and the corresponding energetics for the proposed mechanisms are also studied.     

Hydroxylation-induced modifications of the Al2O3/NiAl(0 0 1) surface at low water vapour pressure

STM, STS, LEED and XPS data for crystalline θ-Al₂O₃ and non-crystalline Al₂O₃ ultra-thin films grown on NiAl(0 0 1) at 1025 K and exposed to water vapour at low pressure (1 × 10⁻⁷-1 × 10⁻⁵ mbar) and room temperature are reported. Water dissociation is observed at low pressure. This reactivity is assigned to the presence of a high density of coordinatively unsaturated cationic sites at the surface of the oxide film. The hydroxyl/hydroxide groups cannot be directly identify by their XPS binding energy, which is interpreted as resulting from the high BE positions of the oxide anions (O_1s signal at 532.5-532.8 eV). However the XPS intensities give evidence of an uptake of oxygen accompanied by an increase of the surface coverage by Al³⁺ cations, and a decrease of the concentration in metallic Al at the alloy interface. A value of ∼2 for the oxygen to aluminium ions surface concentration ratio indicates the formation of an oxy-hydroxide (AlO_xOH_y with x + y ∼ 2) hydroxylation product. STM and LEED show the amorphisation and roughening of the oxide film. At P(H₂O) = 1 × 10⁻⁷ mbar, only the surface of the oxide film is modified, with formation of nodules of ∼2 nm lateral size covering homogeneously the surface. STS shows that essentially the valence band is modified with an increase of the density of states at the band edge. With increasing pressure, hydroxylation is amplified, leading to an increased coverage of the alloy by oxy-hydroxide products and to the formation of larger nodules (∼7 nm) of amorphous oxy-hydroxide. Roughening and loss of the nanostructure indicate a propagation of the reaction that modifies the bulk structure of the oxide film. Amorphisation can be reverted to crystallization by annealing under UHV at 1025 K when the surface of the oxide film has been modified, but not when the bulk structure has been modified.     

Characterization of LaMnAl11O19 by FT-IR spectroscopy of adsorbed NO and NO/O2

The nature of the NO_x species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on LaMnAl₁₁O₁₉ sample with magnetoplumbite structure obtained by a sol-gel process has been investigated by means of in situ FT-IR spectroscopy. The adsorption of NO leads to formation of anionic nitrosyls and/or cis-hyponitrite ions and reveals the presence of coordinatively unsaturated Mn³⁺ ions. Upon NO/O₂ adsorption at room temperature various nitro-nitrato structures are observed. The nitro-nitrato species produced with the participation of electrophilic oxygen species decompose at 350 °C directly to N₂ and O₂. No NO decomposition is observed in absence of molecular oxygen. The adsorbed nitro-nitrato species are inert towards the interaction with methane and block the active sites (Mn³⁺ ions) for its oxidation. Noticeable oxidation of the methane on the NOx⁻-precovered sample is observed at temperatures higher than 350 °C due to the liberation of the active sites as a result of decomposition of the surface nitro-nitrato species. Mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed.     

Combined experimental and theoretical investigation of the CO2 adsorption on LaMnO3+y perovskite oxide

The surface interaction of CO₂ with the perovskite-type oxide LaMnO_3+y has been investigated by means of density functional theory calculations and experimental measurements of adsorption isotherms in the temperature range 298-473 K. A (1 0 0) oriented slab of the cubic structure was used for modeling CO₂ adsorption. The reference unit cell contains alternating LaO⁺ layers and layers; one slab is LaO⁺-terminated and the opposite surface is terminated. A Freundlich isotherm fitted the experimental data satisfactorily. Analysis of the isosteric heat revealed an energetically heterogeneous character for the lanthanum manganite oxide surface, mainly due to the degree of heterogeneity of the adsorption center and due to the adsorbate-adsorbate lateral interactions. Considering theoretical calculations and thermodynamical approaches, two types of active sites were found to be responsible for irreversible and reversible adsorption of CO₂ as a function of surface coverage and O₂ treatment. Strong adsorption takes place on the surface containing La cations. The strongest adsorption is associated with surface oxygen vacancies, center. The next strongest adsorption, a flat adaptation of CO₂ molecules with respect to the surface sites, with a strong binding to a surface oxygen, leads to chemisorbed carbonate species. These adsorption modes are chiefly indicative of a high basic character of the lanthanum manganite oxide surface. Several cationic sites formed by lanthanum and manganese cations are able to weakly adsorb CO₂ molecules in perpendicular or bridged forms. The latter adsorption modes suggest a weak acidic character of the manganite adsorbent.     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.