Adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface

The authors have studied the adsorption and reaction of oxygen and CO on a stepped Pt surface with varying amounts of Au, using temperature-programmed desorption and reaction (TPD and TPR), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy, and steady-state reaction measurements. When the surface is fully covered with Au it is inert to oxygen adsorption and to CO oxidation, and supports only a single weakly bound CO adsorption state. The surface covered with 0.7 ML Au, however, exhibits properties different from either bare Pt or bare Au. Their TPD and LEED results suggest the coexistence of completely Au-covered regions and regions with Au on the step edges but not on the terraces. Dissociative oxygen adsorption is reduced by 90%, and the remaining oxygen is confined to Pt sites near the Au/Pt boundaries. The Au-covered regions support weakly bound CO adsorption states with desorption temperatures of 120, 190, and 240 K. CO in these states can diffuse rapidly and react efficiently with adsorbed atomic oxygen at temperatures as low as 150 K. In low-temperature TPR experiments the reaction is limited by the availability of adsorbed oxygen under almost all conditions. Under steady-state conditions, however, it is limited by themore » availability of CO even at low temperatures and CO partial pressures up to 10{sup {minus}6} Torr. Adding CO partial pressure does not inhibit the reaction. Consequently, adsorbed CO does not completely block all the sites at which oxygen dissociates on this surface, unlike on bare platinum.« less

The effects of water coadsorption on the adsorption of oxygen over metal oxides: I. Temperature-programmed desorption study of Co 3O 4

The adsorption and decomposition reactions of carbon dioxide and methanol have been studied on clean Ag(100), O/Ag(100) and ZnO/Ag(100) surfaces. The clean Ag(100) surface itself is unreactive in so far as no low-pressure adsorption of either molecule is observed at ambient temperatures. At lower temperatures a weakly chemisorbed molecular state of methanol may be distinguished. In the presence of pre-adsorbed surface oxygen, however, uptake of both molecules is seen at 300 K to give carbonate and methoxy species. The methoxy species may undergo dehydrogenation to yield formaldehyde or oxidation to surface formate which then decomposes at ca. 390–400 K. The growth of ZnO on the Ag(100) surface leads to the gradual suppression of the chemistry of the silver surface and the appearance of desorption features which relate closely to those seen from polycrystalline ZnO samples or the non-polar faces of ZnO crystals. The uptake of CO2 is strongly inhibited by annealing the oxide films; this is attributed to the ordering of the oxide surface to generate a polar surface. In the case of methanol adsorption, desorption features also reveal the formation of carbonate-like intermediates on the ZnO surface.

Temperature-programmed desorption (TPD) and Density Functional Theory (DFT) were used to investigate the reactions of oxametallacycles derived from ethylene oxide on clean and oxygen-covered Ag(110) surfaces. Ethylene oxide ring-opens following adsorption at 250 K on both clean and O-covered Ag(110) to form a stable oxametallacycle. On the clean Ag(110) surface, the oxametallacycle reacts to reform the parent epoxide at 280 K during TPD, while the aldehyde isomer, acetaldehyde, is observed at higher oxametallacycle coverages. In the presence of coadsorbed oxygen atoms, a portion of the oxametallacycles dissociate to release ethylene. However, of those that react to form oxygen-containing products, the fraction forming ethylene oxide is similar to that on the clean surface. The acetaldehyde product of oxametallacycle reactions combusts via formation of acetate species; the acetates react to form CO2 at temperatures as low as 360 K on the O-covered surface. No evidence was observed for other combustion channels. This work provides experimental evidence for the connection of oxametallacycles to combustion via acetaldehyde formation as well as to ring-closure to form ethylene oxide.

Various forms of atomic and molecular adsorption of oxygen on the Ag(110) surface are studied using a four- layer cluster Ag32, which has a structure of an orthorhombic prism. It is shown that a qualitatively correct description of molecular and dissociative adsorption of oxygen allows for metallic properties of silver. A simple and effective method for taking into account metallic properties of a single crystal are suggested. This method involves self- consistent shifts of the one- electron matrix elements of silver atoms by the same value for the purpose of leveling the HOMOs of the cluster against the experimental Fermi level. This procedure reproduces the experimental geometry and the binding energy of atomic adsorption in the bridging position on the long chain of the Ag(110) surface. Additional positions of atomic adsorption on and below the surface are predicted. Molecular chemisorption of oxygen on the Ag(110) surface is investigated. Four different molecular forms of oxygen, whose O- O axis is parallel to the surface, are obtained. According to their electronic structure, all of these forms correspond to peroxide. The most probable structure is the bridged structure over the long chain at an angle of 30‡ to the [001] direction with the binding energy of 9.5 kcal/mole (experimental estimate 9.3 kcal/mole).

The adsorption of oxygen, nitrogen and carbon dioxide onto a carbon fibre composite was investigated using static and dynamic techniques. Molecular-sieving effects in the composite were highlighted by the adsorption of carbon dioxide, a more sensitive probe molecule for the presence of micro-porosity in adsorbents. The kinetic studies revealed that oxygen was more rapidly adsorbed on the composite than nitrogen and with a higher uptake under equilibrium conditions. Preliminary experiments indicated that the carbon fibre composite was capable of separating oxygen and nitrogen from air on the basis of the different diffusion rates of the two molecules in the micropore network of the composite. It is proposed that the relatively high electrical conductivity of the carbon fibre composite material could be exploited for air separation by facilitating the production of O2 and N2 through electrical swing adsorption rather than the depressurization of adsorber beds.

A field ion-scanning tunneling microscope (FI-STM), a combination of a scanning tunneling microscope (STM) and a field ion microscope (FIM), was constructed with successful performance. It was applied to the investigation of the oxygen adsorption on the Ag(110) surface. The FI-STM results show that atomic oxygen is mobile on the Ag surface at room temperature until it finds two free mobile Ag atoms to form an Ag-O-Ag linear chain along the direction. Our FI-STM data suggest that, in addition to this type of atomic oxygen adsorption, there is another type of oxygen adsorption over the Ag-O-Ag linear chains with much weaker bonding to the Ag substrate

Biogas is mainly consisted of methane and carbon dioxide in the presence of other contaminants. The biogas purification by adsorption using metal-organic frameworks is getting attention due to the low-cost operation and high-efficiency process. Co-gallate was predicted to give a promising performance in CO2 and CH4 adsorption. However, the behaviours of CO2 and CH4 adsorption on Co-gallate are not well-explained. Therefore, this work is to synthesize Co-gallate and its performance was discussed in terms of adsorbed amount of CO2 and CH4. The experimental CO2 and CH4 pure adsorption isotherms were then fitted with equilibrium isotherm and kinetic models to describe the adsorption behaviours. Co-gallate offered a greater CO2 adsorption capacity than CH4 due to a stronger adsorbent-adsorbate interaction. The experimental pure adsorption isotherms were best fitted with Toth model compared to Langmuir, Freundlich and Sips models according to the values. Toth model described the CO2 adsorption was multilayer and heterogeneous. Thermodynamic property suggested the CO2 and CH4 adsorption were classified as exothermic process and physisorption. For kinetic models, pseudo-first order model brought the highest goodness-of-fit in terms of rate of adsorption compared to pseudo-second order and Elovich models. Pseudo-first order model reflects the adsorption rate is proportional to the number of vacant sites. It confirmed CO2 adsorption was more favourable than CH4, at lower temperature condition. In this work, the equilibrium isotherm and kinetic models were employed to select the best-fitted model in explaining the adsorption behaviours. Therefore, these behaviours of CO2 and CH4 adsorption on Co-gallate are useful in designing the future practical operation of CO2/CH4 gas adsorption.

A study on oxygen adsorption and coadsorption with carbonmonoxide on a stepped nickel surface

The kinetics of interaction of gas-phase oxygen with oxygen of perovskite-like manganites La1 - x A x MnO3 ± δ (A = Ca, Sr, Ba) at a temperature of 850°C and an oxygen pressure of 1 kPa was studied by isotope exchange of oxygen with analysis of the gas phase. The rates of the interphase exchange, dissociative adsorption, and incorporation of oxygen in the crystal lattice of perovskites La0.7A0.3MnO3 ± δ (A = Ca, Sr, Ba) and La1–x Sr x MnO3 ± δ (x = 0, 0.3, 0.4) were calculated. The dissociative adsorption of oxygen was found to be the limiting stage of the exchange. The influence of the defective structure of oxide La1 - x A x MnO3 ± δ on the mechanism of exchange with oxygen of the gas phase was discussed.

Adsorption and methanation of carbon dioxide on a nickel/silica catalyst

Polyacrylonitrile-based activated carbon fibers (ACFs), modified using potassium hydroxide (KOH) or tetraethylenepentamine (TEPA), were investigated for carbon dioxide (CO2) adsorption, which is one of the promising alleviation approaches for global warming. The CO2 adsorption isotherms were measured, and the values of isosteric heat of adsorption were calculated. The results showed that the KOH-modified ACFs exhibited a great deal of pore volume, and a specific surface area of 1565 m2/g was obtained. KOH activation made nitrogen atoms easily able to escape from the surface of ACFs. On the other hand, the surface area and pore volume of ACFs modified with TEPA were significantly reduced, which can be attributed to the closing or blocking of micropores by the N-groups. The CO2 adsorption on the ACF samples was via exothermic reactions and was a type of physical adsorption, where the CO2 adsorption occurred on heterogeneous surfaces. The CO2 uptakes at 1 atm and 25 °C on KOH-activated ACFs reached 2.74 mmole/g. This study observed that microporosity and surface oxygen functionalities were highly associated with the CO2 uptake, implying the existence of O-C coordination, accompanied with physical adsorption. Well cyclability of the adsorbents for CO2 adsorption was observed, with a performance decay of less than 5% over up to ten adsorption-desorption cycles.

The interaction of CO with structurally well-defined PdAg/Pd(111) surface alloys was investigated by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) to unravel and understand contributions from electronic strain, electronic ligand and geometric ensemble effects. TPD measurements indicate that CO adsorption is not possible on the Ag sites of the surface alloys (at 120 K) and that the CO binding strength on Pd sites decreases significantly with increasing Ag concentration. Comparison with previous scanning tunneling microscopy (STM) data on the distribution of Pd and Ag atoms in the surface alloy shows that this modification is mainly due to geometric ensemble effects, since Pd(3) ensembles, which are the preferred ensembles for CO adsorption on non-modified Pd(111), are no longer available on Ag-rich surfaces. Consequently, the preferred CO adsorption site changes with increasing Ag content from a Pd(3) trimer via a Pd(2) dimer to a Pd monomer, going along with a successive weakening of CO adsorption. Additionally, the CO adsorption properties of the surface alloys are also influenced by electronic ligand and strain effects, but on a lower scale. The results are discussed in comparison with previous findings on PdAg bulk alloys, supported PdAg catalysts and PdAu/Pd(111) model systems.

Adsorption of oxygen on Ni, Cu, Pd, Ag, and Au surfaces has been investigated by employing UV and X-ray photoelectron spectrscopy as well as electron energy loss spectroscopy (EELS). Molecularly chemisorbed (singlet) oxygen is found on Ni, Cu, Ag, and Au surfaces showing features such as stabilization of the rB orbital, destabilization of the .nu orbital, higher O(1s) binding energy than the atomic species, and a band 2-3 eV below the Fermi level due to metal d-O(2p)u interaction. 0-0 and metal-oxygen stretching frequencies have been observed in EELS. Physical adsorption of O 2 is found to occur on Pd and Ni surfaces, only at high exposures in the latter case. Physical adsorption and multilayer condensation of CO, on metal surfaces are distinguished by characteristic relaxation shifts in UPS as well as O(1s) binding energies. Adsorption of CO on a Ni surface covered with presorbed atomic oxygen gives rise to CO 2 .

Combining scanning tunneling microscopy (STM), IR reflection absorption spectroscopy (IRAS) and molecular beam (MB) techniques, we have investigated particle size effects on a Pd/Fe(3)O(4) model catalyst. We focus on the particle size dependence of (i) CO adsorption, (ii) oxygen adsorption and (iii) Pd nanoparticle oxidation/reduction. The model system, which is based on Pd nanoparticles supported on an ordered Fe(3)O(4) film on Pt(111), is characterized in detail with respect to particle morphology, nucleation, growth and coalescence behavior of the Pd particles. Morphological changes upon stabilization by thermal treatment in oxygen atmosphere are also considered. The size of the Pd particles can be varied roughly between 1 and 100 nm. The growth and morphology of the Pd particles on the Fe(3)O(4)/Pt(111) film were characterized by STM and IRAS of adsorbed CO as a probe molecule. It was found that very small Pd particles on Fe(3)O(4) show a strongly modified adsorption behavior, characterized by atypically weak CO adsorption and a characteristic CO stretching frequency around 2130 cm(-1). This modification is attributed to a strong interaction with the support. Additionally, the kinetics of CO adsorption was studied by sticking coefficient experiments as a function of particle size. For small particles it is shown that the CO adsorption rate is significantly enhanced by the capture zone effect. The absolute size of the capture zone was quantified on the basis of the STM and sticking coefficient data. Finally, oxygen adsorption was studied by means of MB CO titration experiments. Pure chemisorption of oxygen is observed at 400 K, whereas at 500 K partial oxidation of the particles occurs. The oxidation behavior reveals strong kinetic hindrances to oxidation for larger particles, whereas facile oxidation and reduction are observed for smaller particles. For the latter, estimates point to the formation of oxide layers which, on average, are thicker than the surface oxides on corresponding single crystal surfaces.