The transition from surface to bulk oxide growth on Pt(1 0 0): Precursor-mediated kinetics

We investigated the kinetics governing the transition from surface (2D) to bulk (3D) oxide growth on Pt(1 0 0) in ultrahigh vacuum as a function of the surface temperature and the incident flux of an oxygen atom beam. For the incident fluxes examined, the bulk oxide formation rate increases linearly with incident flux (Φ_O) as the oxygen coverage increases to about 1.7 ML (monolayer) and depends only weakly on the surface temperature in the limit of low surface temperature (T_S < 475 K). In contrast, in the high temperature limit (T_S > 525 K), the bulk oxide formation rate increases with for oxygen coverages as high as 1.6 ML, and decreases with increasing surface temperature. We show that the measured kinetics is quantitatively reproduced by a model which assumes that O atoms adsorb on top of the 2D oxide, and that this species acts as a precursor that can either associatively desorb or react with the 2D oxide to form a 3D oxide particle. According to the model, the observed change in the flux and surface temperature dependence of the oxidation rate is due to a change in the rate-controlling steps for bulk oxide formation from reaction at low temperature to precursor desorption at high temperature. From analysis of flux-dependent uptake data, we estimate that the formation rate of a bulk oxide nucleus has a fourth-order dependence on the precursor coverage, which implies a critical configuration for oxide nucleus formation requiring four precursor O atoms. Considering the similarities in the development of surface oxides on various transition metals, the precursor-mediated transition to bulk oxide growth reported here may be a general feature in the oxidation of late transition metal surfaces.     

Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm^-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm^-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm^-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.     

Self-limited growth of a thin oxide layer on Rh(111)

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.     

Initial stages of oxidation of binary alloys: The case of the stepped Pt3Ti(510) single crystal surface

The oxidation of the (510) oriented surface of the ordered fcc Pt₃Ti alloy was studied by low energy electron diffraction and other surface sensitive techniques. The clean Pt₃Ti(510) surface showed an ordered array of surface steps. Exposing the surface to oxygen at pressures in the range from 1×10⁻⁷ to 1×10⁻⁵ Torr at temperatures of 800 K or higher caused the formation of titanium oxide islands on the surface. This process caused the progressive disappearance of the ordered array of steps, which was replaced by large facets oriented along the (100) and (210) planes.     

Spatio-temporal pattern formation during CO oxidation on Pt(100) at low and intermediate pressures: A comparative study

Experimental studies of CO oxidation on Pt(100) over two different ranges of reactant pressures will be reviewed. Using photoemission electron microscopy (PEEM), spatio-temporal pattern formation was observed at temperatures between 420 and 540 K in the 10(-5) mbar pressure range. In an attempt to bridge the "pressure-gap," ellipsomicroscopy for surface imaging was used to follow pattern formation at temperatures around 600 K in the 10(-2) mbar pressure range. The features of the nonlinear phenomena, observed in these two different pressure regimes, are markedly different. This is shown by comparison of various qualitative and quantitative features of spatio-temporal pattern formation as well as the dynamics of the macroscopic reaction rate. Subsurface oxygen is proposed as a tentative alternative to the surface phase transition for oscillations in the reaction rate at higher temperatures and intermediate pressures. (c) 2002 American Institute of Physics.     

In situ XPS study of Pd(1 1 1) oxidation at elevated pressure, Part 2: Palladium oxidation in the 10 mbar range

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 0.4 mbar O₂. The in situ XPS data were complemented by ex situ TPD results. A number of oxygen species and oxidation states of palladium were observed in situ and ex situ. At 430 K, the Pd(1 1 1) surface was covered by a 2D oxide and by a supersaturated O_ads layer. The supersaturated O_ads layer transforms into the Pd₅O₄ phase upon heating and disappears completely at approximately 470 K. Simultaneously, small clusters of PdO, PdO seeds, are formed. Above 655 K, the bulk PdO phase appears and this phase decomposes completely at 815 K. Decomposition of the bulk oxide is followed by oxygen dissolution in the near-surface region and in the bulk. The oxygen species dissolved in the bulk is more favoured at high temperatures because oxygen cannot accumulate in the near-surface region and diffusion shifts the equilibrium towards the bulk species. The saturation of the bulk “reservoir” with oxygen leads to increasing the uptake of the near-surface region species. Surprisingly, the bulk PdO phase does not form during cooling in 0.4 mbar O₂, but the Pd₅O₄ phase appears below 745 K. This is proposed to be due to a kinetic limitation of PdO formation because at high temperature the rate of PdO seed formation is compatible with the rate of decomposition.     

Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.     

Low-temperature measurements of the fraction of re-emitted positronium from a Cu(111)+S surface

A positron beam has been utilized to measure the positronium (Ps) fraction re-emitted from a Cu(111)+S sample from 40 K to 350 K for incident energies ranging from 0.5 to 5 keV. Our results at 525 eV incident energy are compared with two recent theories from 40 K to 850 K. The Ps fraction shows only a slight positive temperature dependence below room temperature and we conclude that the positron trapping rate and probability of Ps formation at the surface are largely independent of sample temperature for Cu(111)+S. In addition we present evidence that positrons in the bulk material are not localized in shallow traps at low temperatures for well-annealed high-purity single crystals of Cu and Al.     

Influence of the metal substrate on the adsorption properties of thin oxide layers: Au atoms on a thin alumina film on NiAl(110)

Thin oxide films grown on metal substrates are widely used in surface science to model bulk oxides, assuming their chemical and electronic properties to be similar. In some cases, however, this might not be justified as the present scanning tunneling microscopy studies demonstrate for Au atoms on a thin alumina film on NiAl(110). Au atoms were evaporated onto the oxide film at a sample temperature of approximately 10 K. At low coverage, this leads to the formation of one-dimensional clusters with unusually large Au-Au distances of 5.6-6.0 A. A direct interaction between the Au atoms can be excluded, and a substrate-mediated mechanism is supposed instead. This assumption is strengthened by the finding that the Au chains exhibit a preferential orientation: They are almost aligned with the [001] direction of the NiAl(110) substrate, clearly indicating that the metal substrate participates in the binding of the Au atoms.     

A LEED-AES study of the oxidation of Fe (110) and Fe (100)

Simultaneous LEED and AES observations have been used to study the initial stages of oxidation of the Fe(110) and Fe(100) single crystal surfaces at 300 K and 400 K and of a clean Fe polycrystal at 300 K. Accurate surface lattice spacings of the precursory oxide structures have been measured and attempts have been made to quantitatively evaluate the corresponding surface oxygen density.On the (110) single crystal surface the final structure is FeO-like with a lattice spacing 4% larger than that of bulk FeO. The transition to the FeO-like structure starts with a surface lattice expansion in the [11&#x0304;0] direction followed by an expansion in the [001] direction in order to accommodate the (111) face of the FeO-like structure. On the (100) single crystal face the oxygen and iron form an fcc (100) face which initially contracts and then expands with increasing oxygen doses. The structure formed at 300 K is spinel-like but heat treatment causes a transition to FeO(100).The changes of the surface unit cell dimensions are interpreted as the result of an interaction between adsorbate and substrate. This interaction is strongest in a direction parallel to the close packed rows of the substrate, making the corresponding periodicity of the adsorbate more resistant to lattice changes.In the case of the polycrystal a hexagonal structure was observed after oxygen adsorption with no simple relation to the oxide structures observed on the single crystals. The initial sticking coefficients in the interval 0–10^?5 torr sec ranged from 0.07 to 0.36 depending on temperature and crystal face observed. The latter dependence is explained in terms of the surface structure.     

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.     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

The adsorption and decomposition of NO on Pd(110)

The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N₂) and nitrous oxide (N₂O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal.     

First-principles statistical mechanics study of the stability of a subnanometer thin surface oxide in reactive environments: CO oxidation at Pd(100)

We employ a multiscale modeling approach to study the surface structure and composition of a Pd(100) model catalyst in reactive environments. Under gas phase conditions representative of technological CO oxidation (approximately 1 atm, 300-600 K) we find the system on the verge of either stabilizing subnanometer thin oxide structures or CO adlayers at the surface. Under steady-state operation this suggests the presence or continuous formation and reduction of oxidic patches at the surface, which could be key to understand the observable catalytic function.     

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.     

Morphology and composition of nanoscale surface oxides on Fe–20Cr–18Ni{1 1 1} austenitic stainless steel

Surface oxidation ranging from initial stages to the onset of passive oxide layer formation have been investigated on Fe–20Cr–18Ni{1 1 1} single crystal surface by X-ray photoelectron spectroscopy (XPS). Surface segregation of the alloying elements and the morphology of the surface oxide nanostructure were characterized quantitatively by inelastic electron background analysis. Our results demonstrate that by increasing the oxidation temperature the relative concentrations of Fe²⁺ and Fe³⁺ cations increase due to their enhanced mobility. Higher temperature also improves the mobility of chromium, thus enhancing its segregation to the oxygen-rich surface and thereby reinforcing the passive layer on the alloy. This is in agreement with the results showing the sudden decrease in oxide film thickness at the oxidation temperatures exceeding 600 K. Additionally, a pronounced segregation of metallic nickel is found in the interface between the surface oxide layer and the bulk alloy.     

Zur Oxydation von Metallen bei sehr tiefen Temperaturen

The formation of oxide films on various metals has been investigated in the temperature range between 1·5 and 100°K. The metal films (50 to 150Å thick) were produced by condensation of the vapor on a quartz plate at about 100°K. After cooling down the films to helium temperatures, oxygen molecules were condensed to the fresh metal surface. The electrical resistance was measured before, during and after this O₂ condensation at temperatures between 1·5 and 300°K. The formation of an oxide layer causes an increase of the electrical resistance, since the conductivity of the oxide is comparatively low. The resistance behaviour of the metals investigated indicates two different steps of oxidation each one starting at a certain “characteristic” temperature below 25°K. Below the lower characteristic temperature, no reaction takes place. Increasing the temperature slowly, the oxide layer grows up to a final thickness. No further growing is detectable between 50 and 150°K for most of the metals. The results are discussed with regard to theoretical considerations ofMott, Cabrera andHauffe.     

Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112?2}, but at coverages near saturation, facets were produced at {101?0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO₃ from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10^?4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.     

Structural analysis of oxygen segregated Nb(110) surface by photoelectron diffraction

The atomic structure of the Nb(110) surface with oxygen segregated from bulk by annealing at 1500K in UHV has been analyzed by electron and photoelectron diffraction techniques. The observed LEED patterns indicate the modulation of topmost Nb layer in [11ˉ0] direction. Relation of unit cells of substrate and overlayer was determined. The surface component in the Nb 3d photoelectron spectra has a chemical shift of 1.6eV corresponding to that of NbO. The thickness of Nb oxide overlayer is estimated to be about one or two atomic layers. The Nb 3d spectra collected at 4945 different directions are fitted with bulk and surface components. Photoelectron diffraction patterns from Nb atoms within overlayer as well as bulk are successfully extracted. Topmost oxygen atoms are found to be located at about 1.2 Å above the Nb plane. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.