Oxidation of Zircaloy-4 by H2O followed by molecular desorption

The interaction of H₂O with Zircaloy-4 (Zry-4) is investigated using Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) methods. Following adsorption of H₂O at 150 K the Zr(MNV) and Zr(MNN) Auger features shift by ∼6.5 and 4.5 eV, respectively, indicating surface oxidation. Heating H₂O/Zry-4 results in molecular desorption of water at both low and high temperatures. The low-temperature desorption is attributed to ice multilayers, whereas, three overlapping high-temperature features are presumably due to recombinative desorption. This high-temperature desorption begins before the surface oxide is dissolved, continues upon its removal, and is atypical for water/metal systems. Unexpectedly, no significant desorption of hydrogen is observed near 400 K, as is typically observed following O₂ adsorption on Zr-based materials. However, we do observe that H₂O adsorption on Zry-4 surfaces roughened by argon ion sputtering results in H₂ desorption.     

CO2 adsorption on the bimetallic Zn-on-Cu(1 1 0) system

Adsorption probability measurements (molecular beam scattering) have been conducted to examine the adsorption dynamics (i.e. the gas-surface energy transfer processes) of CO₂ adsorption on the Zn-on-Cu(1 1 0) bimetallic system. The results indicate surface alloy formation, which is in agreement with prior studies. Depositing Zn at 300 K on Cu(1 1 0), above the condensation temperature of CO₂, leads to a “blocking” of CO₂ adsorption sites by Zn which is incorporated in the Cu(1 1 0) surface. This apparent site blocking effect indicates a lowering of the CO₂ binding energy on the alloyed surface as compared with the clean Cu(1 1 0) support. The Zn coverage has been calibrated by Auger electron spectroscopy and thermal desorption spectroscopy.     

Electronic decoupling of surface layers from bulk and its influence in oxidation catalysis: A molecular beam study

Interactions between oxygen and Pd-surfaces have important implications, especially towards oxidation reactions, and influence of subsurface oxygen to oxidation reactions is the focus of the present study. In our efforts to understand the above aspects, CO oxidation reactions have been carried out with mixed molecular beam (MB), consisting CO and O₂, on Pd(1 1 1) surfaces under a wide variety of conditions (T = 400-900 K, CO:O₂ = 7:1 to 1:10). A new aspect of the above reaction observed in the transient kinetics regime is the evidence for oxygen diffusion into Pd subsurface layers, and its significant influence towards CO oxidation at high temperatures (≥600 K). Interesting information derived from the above studies is the necessity to fill up the subsurface layers with oxygen atoms to a threshold coverage (θ_O-sub), above which the reactive CO adsorption occurs on the surface and simultaneous CO₂ production begins. There is also a significant time delay (Γ) observed between the onset of oxygen adsorption and CO adsorption (and CO₂ production). Above studies suggest an electronic decoupling of oxygen covered surface and subsurface layers, which is slightly oxidized, from the metallic bulk, which induces CO adsorption at high temperatures and simultaneous oxidation to CO₂.     

Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.     

Modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O3 thin films induced by CO2 adsorption

We report on the modification of ferroelectric hysteresis in Pb(Nb,Zr,Ti)O₃ thin films under the adsorption of CO₂ gas. The samples were exposed to the gas in ultra high vacuum while different voltages between the top (Ag) and bottom (Pt) electrodes were applied. After dosing, the samples were heated from room temperature at 1.8 K/s, and a mass spectrometer was used to detect the desorbed molecules. The effective molecular sticking was demonstrated by the appearance of the carbon Auger peak in the surface of the sample and by the presence of CO₂ in the desorption curves. After +(−) 9.9 V were applied to the bottom electrode during dosing, we found a shift of −0.52 (+0.58) μC/cm² in the remnant polarization. These evidences suggest the existence of a depolarizing field induced by molecular adsorption at the surface of the top electrode, and contribute to highlight the potential use of ferroelectric thin films as gas sensors.     

Adsorption of glycine on a NiAl(1 1 0) alloy surface

The adsorption and desorption of glycine (NH₂CH₂COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH₂CH₂COO⁻). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al-O layer.     

Highly active surfaces for CO oxidation on Rh, Pd, and Pt

Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O₂/CO ratio above 0.5. The catalytic surfaces, formed at discrete O₂/CO ratios >0.5, exhibit rates 2-3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O₂/CO ratios required to achieve these so-called “hyperactive” states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O₂/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation.     

Adsorption dynamics of CO2 on Cu(1 1 0): A molecular beam study

Molecular beam scattering measurements have been conducted to examine the adsorption dynamics of CO₂ on Cu(1 1 0). The initial adsorption probability, S₀, decreases exponentially from 0.43 ± 0.03 to a value close to the detection limit (∼0.03) within the impact energy range of E_i = (0.12-1.30) eV. S₀ is independent of the adsorption temperature, T_s, and the impact angle, α_i, i.e., the adsorption is non-activated and total energy scaling is obeyed. The coverage, Θ, dependent adsorption probability, S(Θ), agrees with precursor-assisted adsorption dynamics (Kisliuk type) above T_s ∼ 91 K. However, below that temperature adsorbate-assisted adsorption (S increases with Θ) has been observed. That effect is most distinct at large E_i and low T_s. The S(Θ) data have been modeled by Monte Carlo simulations. No indications of CO₂ dissociation were obtained from Auger Electron Spectroscopy or the molecular beam scattering data.     

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.     

The adsorption of CCl4 on Ag(1 1 1): Carbene and CC bond formation

The adsorption of CCl₄ on Ag(1 1 1) has been investigated from 100 K to 300 K using absolute sticking probability measurement, temperature programmed desorption, Auger electron spectroscopy, low energy electron diffraction, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At 100 K, CCl₄ adsorbs molecularly with a sticking probability of 1.0, forming a (3 × 3) adsorption structure. At 300 K the following overall reaction occurs,

2CCl_4(g)→4Cl_(chem)+C₂Cl_4(g),     

CO adsorption and dissociation on iron oxide supported Pt particles

We studied CO adsorption on Pt particles deposited on well-ordered Fe₃O₄(1 1 1) thin films grown on Pt(1 1 1) by temperature programmed desorption (TPD). A highly stepped Pt(1 1 1) surface produced by ion sputtering and annealing at 600 K was studied for comparison. Structural characterization was performed by scanning tunneling microscopy and Auger electron spectroscopy. The TPD spectra revealed that in addition to the desorption peaks at ∼400 and 480 K, assigned to CO adsorbed on Pt(1 1 1) facets and low-coordination sites respectively, the Pt nanoparticles annealed at 600 K exhibit a desorption state at ∼270 K. This state is assigned to initial stages of strong metal support interaction resulting in partial Fe-Pt intermixing. On both Pt/Fe₃O₄(1 1 1) and stepped Pt(1 1 1) surfaces CO is found to dissociate at 500 K. The results suggest that CO dissociation and carbon accumulation occur on the low-coordinated Pt sites.     

Oxygen-induced nano-faceting of Ir(2 1 0)

The adsorption of oxygen and the nanometer-scale faceting induced by oxygen have been studied on Ir(2 1 0). Oxygen is found to chemisorb dissociatively on Ir(2 1 0) at room temperature. The molecular desorption process is complex, as revealed by a detailed kinetic analysis of desorption spectra. Pyramid-shaped facets with {3 1 1} and (1 1 0) orientations are formed on the oxygen-covered Ir(2 1 0) surface when annealed to T?600 K. The surface remains faceted for substrate temperatures T<850 K. For T>850 K, the substrate structure reverts to the oxygen-covered (2 1 0) planar state and does so reversibly, provided that oxygen is not lost due to desorption or via chemical reactions upon which the planar (2 1 0) structure remains. A clean faceted surface was prepared through the use of low temperature surface cleaning methods: using CO oxidation, or reaction of H₂ to form H₂O, oxygen can be removed from the surface while preserving (“freezing”) the faceted structure. The resulting clean faceted surface remains stable for T<600 K. For temperatures above this value, the surface irreversibly relaxes to the planar state.     

Adsorption of oxygen and oxidation of CO on the ruthenium (001) surface

The adsorption of oxygen on the ruthenium (001) surface has been studied using a combination of techniques: LEED/Auger, Kelvin probe contact potential changes, and flash desorption mass spectrometry. Oxygen is rapidly adsorbed at 300 K, forming an ordered LEED structure having apparent (2 × 2) symmetry. Two binding states of oxygen are inferred from the abrupt change in surface work function as a function of oxygen coverage. LEED intensity measurements indicate that the oxygen layer undergoes an order-disorder transition at temperatures several hundred degrees below the onset of desorption. The order-disorder transition temperature is a function of the oxygen coverage, consistent with two binding states. A model involving the adsorption of atomic oxygen at θ < 0.5 and the formation of complexes with higher oxygen content at θ > 0.5 is proposed. The oxidation of CO to form CO₂ was found to have the maximum rate of production at a ruthenium temperature of 950 K.     

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.     

Atomic and molecular oxygen adsorption on Ag(111)

The adsorption of O₂ on Ag(111) between 150 and 650 K has been studied with thermal desorption spectroscopy, Auger and photoelectron spectroscopies, and low-energy electron diffraction. A molecularly adsorbed O₂ species is populated with extremely low sticking probability (~ 5 X 10^?6) at 150 K. This species desorbs, with little dissociation, at 217 K. An atomically adsorbed species, with an O(1s) BE of 528.2 eV, is populated at 490 K with a sticking probability near 10^?6. This species exists in islands of local coverage θ_O ? 0.41, displaying a p(4 X 4)-O LEED pattern. It associatively desorbs at 579 K as O₂, and can be titrated at room temperature with CO to produce CO₂. There is also evidence for a subsurface oxygen species which reactivates below 600 K. Surface carbonate (CO_3,a) can be produced from O_a and CO₂ gas. These results are compared with similar species on Ag(110). A kinetic model is developed which describes the interaction of O₂ with these surfaces over a broad range of temperatures, and provides energetic values for the O₂/Ag interaction potential.     

Preparation and ozone-surface modification of activated carbon. Thermal stability of oxygen surface groups

The control of the surface chemistry of activated carbon by ozone and heat treatment is investigated. Using cherry stones, activated carbons were prepared by carbonization at 900 °C and activation in CO₂ or steam at 850 °C. The obtained products were ozone-treated at room temperature. After their thermogravimetric analysis, the samples were heat-treated to 300, 500, 700 or 900 °C. The textural characterization was carried out by N₂ adsorption at 77 K, mercury porosimetry, and density measurements. The surface analysis was performed by the Bohem method and pH of the point of zero charge (pH_pzc). It has been found that the treatment of activated carbon with ozone combined with heat treatment enables one to control the acidic-basic character and strength of the carbon surface. Whereas the treatment with ozone yields acidic carbons, carbon dioxide and steam activations of the carbonized product and the heat treatment of the ozone-treated products result in basic carbons; the strength of a base which increases with the increasing heat treatment temperature. pH_pzc ranges between 3.6 and 10.3.     

Interaction of ethylene with palladium clusters supported on oxidised tungsten foil

The reactivity with ethylene of palladium clusters supported on oxidised tungsten foil has been investigated by synchrotron radiation-induced photoelectron spectroscopy and temperature programmed desorption. The effect of the heat pre-treatment of the sample on the interaction strength with ethylene is demonstrated. Already at room temperature, adsorption of ethylene causes breaking of both the C-H and C-C bonds and the appearance of a highly reactive C₁ phase with unsaturated bonds. A part of this phase is oxidised to carbon monoxide by oxygen supplied by the support immediately after ethylene adsorption. Another part of ethylene is probably adsorbed in the form of ethylidyne. Heating at temperatures between 400 K and 500 K brings about the dissolution of the C₁ phase in the shallow subsurface region of the Pd clusters. Further oxidation of the C₁ phase by oxygen from the support proceeds at ∼600 K. Substantial reduction of the concentration of C₁ phase at room temperature is observed after heat pre-treatment of the sample at 500 K, while complete suppression of the room temperature ethylene chemisorption proceeds upon heat pre-treatment at 800 K. This effect is related to thermally induced encapsulation of palladium clusters in surface tungsten oxide.     

CO2 motion in carbon dioxide deuterohydrate determined by applying maximum entropy method to neutron powder diffraction data

Crystal structures of carbon dioxide deuterohydrate were studied by neutron powder diffraction at temperatures from 10 to 200 K. Maps of scattering length density distribution were obtained using a maximum entropy method (MEM), which clarified the motion of CO₂ molecules in the hydrate. In small cages, the carbon atom of the CO₂ molecule is at the center of the cage, and the oxygen atoms of CO₂ revolve freely around the carbon atom. In large cages, the carbon atom also is at the center of the cage, but the oxygen atoms tend to revolve around the carbon atom along the plane parallel to the hexagonal facets of the cage.     

Adsorption of CO2 on PbO at ambient temperature

The adsorption of CO₂ on metal oxides at ambient temperature received less study largely due to the small adsorption amount. However, the adsorption is of interest in refreshing the atmosphere of isolated spaces. It was shown in the present work that P_bO was sensitive to low concentration CO₂ in the presence of water. An XPS examination indicated that P_bO changed to P_bCO₃ after the adsorption of CO₂; therefore, the adsorption is chemical in nature. In order to enlarge the CO₂ capacity, P_bO was dispersed on the surface of a silica gel with large surface area (710 m²/g). Both CO₂ capacity and adsorption rate indicated that the optimal dispersion manner of P_bO is the mono-molecular layer surface coverage. Breakthrough experiments showed that the prepared adsorbent could effectively capture low-concentration CO₂ at ambient temperature and pressure yielding a CO₂ capacity of 59.1 mg g⁻¹. The saturated adsorbent was regenerated on heating at 380 °C and the CO₂ capability was recovered.     

Interaction of C2N2 with clean and oxygen dosed Cu(111) surface studied by AES,ELS and TDS measurements

The adsorption and surface reaction of cyanogen on clean and oxygen covered Cu(111) have been investigated. From electron energy loss measurements, thermal desorption spectroscopy and electron beam effects in Auger spectroscopy, it is proposed that cyanogen adsorbs dissociatively on Cu(111) at 300 K. The activation energy for the desorption was calculated to be 180 kJ/mol. Cyanogen adsorption onto oxygen predosed Cu(111) is inferred to produce the NCO surface species. This interpretation was aided by data of electron energy loss measurements and from HNCO adsorption onto Cu(111) at 300 K. A reaction began in the co-adsorbed layer above 400 K, yielding CO₂ and N₂.