Adsorption States Of Oxygen Research Articles

Unveiling Oxygen Adsorption States on One-Dimensional Pt-Induced Nanowires on Ge(001)

Oxygen adsorption on Pt-induced nanowires on Ge(001) (NWs/Ge(001)) was investigated on the atomic scale. At 77 K, we observed two disparate features based on scanning tunneling microscopy images an...

Oxygen adsorption and dissociation on yttria stabilized zirconia surfaces

We have modelled the nature of oxygen species on the surfaces of yttrium stabilized zirconia (YSZ) in relation to its application as a catalyst for partial oxidation of methane. Quantum mechanical DFT-GGA calculations have been carried out to characterize the interaction between oxygen molecules and the perfect ZrO2 and defective YSZ (111) planar and stepped surfaces. In general, the results suggest that the creation of oxygen vacancies by yttrium doping provides an active site for oxygen adsorption. Depending on the topography and defect dispersion, various oxygen intermediates have been identified and analyzed by their density of states. The low-coordinated Zr cations on the YSZ surfaces can attract strongly reduced oxygen species and the most stable adsorption state of oxygen achieves a higher bond saturation of the neighbouring Zr site. In addition, the probable reaction pathways have been predicted for oxygen dissociation on the plane and stepped (111) surfaces of YSZ.

Direct decomposition of NO into N2 and O2 on BaMnO3-based perovskite oxides

Effects of dopant in BaMnO3 perovskite oxide on the NO direct decomposition activity were investigated. NO direct decomposition activity was greatly elevated by doping La and Mg for Ba and Mn site in BaMnO3, respectively. The highest N2 yield was achieved on Ba0.8La0.2Mn0.8Mg0.2O3. The NO decomposition rate increased with increasing NO partial pressure with PNO1.19. Coexistence of oxygen lowered the N2 yield with PO2−0.18; however, N2 yield of 40% was sustained even under coexisting of 5% O2 at 1123K. Adsorption state of oxygen was also studied with temperature programmed desorption (TPD) method and the desorption temperature of oxygen was lowered by doping Mg for Mn site in BaMnO3.

Interaction of Oxygen with the Polar HfC(111) Surface: Angle-Resolved Photoemission Study

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the adsorption state of oxygen on a HfC(111) surface. Oxygen adsorbs dissociatively on the HfC(111) surface at room temperature forming a (1×1) overlayer at the saturation coverage. The two-dimensional band structure of the chemisorption bonding states formed through the hybridization of O 2p and Hf 5d orbitals (O 2p-derived states) is measured for the HfC(111)(1×1)-O surface. It is found that the 2pz-derived band is formed in slightly higher binding energy region relative to the 2px- and 2py-derived bands, indicating that the vertical interaction involving the 2pz orbital is slightly more important in the chemisorption bond formation relative to the lateral interaction involving the 2px and 2py orbitals. Hf 4f spectra are measured for the clean and oxygen-covered HfC(111) surfaces, and it is found that the oxidation of the surface Hf atoms to form a Hf oxide does not proceed at room temperature.

Oxygen adsorption states on Mo( [formula omitted]) surface studied by HREELS

Oxygen adsorption on a Mo(1 1 2) has been studied using HREELS, LEED and TPD. Oxygen molecules dissociatively adsorb to occupy quasi-threefold hollow site, long bridge site and atop site on a clean Mo(1 1 2) at 100 K. After annealing this surface to 300 K, a loss peak corresponding to the oxygen atoms at atop sites disappears, changing the adsorption sites into quasi-threefold and long bridge sites. Further annealing to 600 K resulted in the change of the location of oxygen atoms from long bridge site to quasi-threefold hollow site, where the surface shows p(1×2) LEED subspots, indicating that the oxygen atoms in the Mo(1 1 2)–p(1×2)-O surface occupy quasi-threefold hollow sites. Oxygen adsorption on the Mo(1 1 2)–p(1×2)-O surface was also investigated and it was found that oxygen molecules associatively adsorb on the surface to give bridged peroxo species, which dissociates at 200 K to yield oxygen atoms at atop sites. The atop oxygen atoms remain on the surface after annealing to higher temperatures in contrast to the adsorption on clean Mo(1 1 2). Further annealing to 800 K resulted in a disordered surface associated with a molybdenum oxide (Mo x O y ) layer.

Low temperature oxidation of methane over Pd-based catalysts—effect of support oxide on the combustion activity

Pd catalysts supported on metal oxides were investigated for low temperature catalytic combustion of methane. Of the catalysts examined, Pd/Al 2O 3-36NiO and Pd/SnO 2 catalysts demonstrated excellent activities, despite its low specific surface area as compared with the conventional Pd/γ-Al 2O 3 catalyst. It was observed by a transmission electron microscope and an X-ray line broadening analysis that the PdO phase was well-dispersed on the surface of the SnO 2 and Al 2O 3-36NiO supports, respectively, despite their low surface areas. Temperature-programmed desorption (TPD) of oxygen has revealed that the catalytic activity depends on the adsorption state of oxygen on palladium. It was suggested from TPD of O 2, in situ XPS, and in situ XRD that the dissociation of oxygen from the PdO bulk, as well as from its surface proceeded at higher temperatures for the Pd/Al 2O 3-36NiO and Pd/SnO 2 catalysts than for Pd/Al 2O 3 because of the strong palladium-support interaction. The catalytic activity on Pd/SnO 2 was zero-order with respect to the oxygen concentration, whereas an increase in water concentration significantly deteriorated the catalytic activity. This suggests that adsorption of both oxygen and water is strong on the Pd surface. The support oxides appear to modify relative adsorption strength oxygen and water, and then combustion activity of palladium.

Investigation of electrochemical promotion using temperature-programmed desorption and work function measurements

The origin of electrochemical promotion was investigated via temperature-programmed desorption (TPD) of oxygen from polycrystalline Pt and Ag films deposited on YSZ under high-vacuum conditions and via measuring the work function of a Pt film deposited on YSZ under catalytic reaction conditions at atmospheric pressure. It was found that electrochemical O 2− pumping to Pt and Ag catalysts in the presence of pre-adsorbed oxygen causes backspillover of large amounts of oxygen on the catalyst surface and leads to the formation of two oxygen adsorption states, i.e. strongly bonded anionic oxygen along with weakly bound atomic oxygen. Furthermore, the desorption activation energy of oxygen adsorbed on Pt and Ag catalyst films was found to decrease linearly with increasing catalyst potential. Finally, by increasing the Pt-catalyst potential from −0.9 to +1.3 V during C 2H 4 oxidation on Pt, i.e. under electrochemical promotion conditions, a 1.42 eV increase in catalyst work function was observed. These results provide a straightforward explanation of the effect of electrochemical promotion on Pt and Ag deposited on O 2− conducting solid electrolytes.

Oxygen adsorption on a ZrC(111) surface: angle-resolved photoemission study

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the adsorption state of oxygen on a ZrC(111) surface. Oxygen adsorbs dissociatively at room temperature forming a ( 3 × 3 )R30° overlayer at 0.4–0.8 L and a (1×1) overlayer for further exposure. The two-dimensional band structure of the O 2p-derived states is measured for the (1×1)-O surface in the two symmetry directions; 〈2 1 ̄ 1 ̄ 〉 (Γ̄M̄) and 〈01 1 ̄ 〉 (Γ̄K̄) directions of the ZrC(111) surface. It is found that the O 2p x , 2p y and 2p z -derived bands are formed at nearly the same binding energy region, indicating that the O 2p x , 2p y and 2p z orbitals should have an almost equal contribution to the chemisorption bonding on the ZrC(111) surface.

Temperature Programmed Desorption of Oxygen from Ag Films Interfaced with Y2O3-Doped ZrO2

The origin of the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion was investigated via temperature-programmed desorption of oxygen from polycrystalline Ag films deposited on 8 mol% Y2O3-stabilized ZrO2 (YSZ), an O2− conductor, under high-vacuum conditions and temperatures of 200 to 400°C. Oxygen was adsorbed both via the gas phase and electrochemically, as O2−, via electrical current application between the Ag catalyst film and a Au counterelectrode. Gaseous oxygen adsorption gives a single adsorption state (Tp≈290–335°C) at low adsorption temperatures with subsurface oxygen (Tp≈500°C) also forming for higher adsorption temperatures (T>350°C). Electrochemical or mixed gaseous–electrochemical adsorption was found to cause significant oxygen backspillover from the YSZ solid electrolyte onto the Ag surface and the creation of two distinct oxygen adsorption states, i.e., a strongly bonded ionic state and a weakly bonded state. The strongly bonded state (Tp≈380–400°C) is identical to the one formed via electrochemical adsorption. The weakly bonded state (Tp≈320–340°C) desorbs at temperatures similar with the gaseous adsorption state. The creation of these two states is also manifest by high temperature cyclic voltammetry. Mixed adsorption leads to much higher oxygen coverages than gaseous adsorption. The binding strength of the weakly bonded oxygen state was investigated as a function of applied potential. It was found that the binding energy of adsorbed oxygen decreases linearly with increasing catalyst potential and work function. The results suggest that, similarly to the case of Pt, the non-Faradaic rate enhancement observed in NEMCA studies of catalytic oxidations on Ag/YSZ is due to the creation of two adsorption states, of which the weakly bonded one is highly reactive while the strongly bonded one acts as a sacrificial promoter. The fact that, unlike Pt, the Tp of the weakly bonded state is not lower than that of oxygen formed via gaseous adsorption can account for the less pronounced NEMCA behavior of Ag vs Pt catalysts.

Interaction of Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface: Molecular and Dissociative Channels

We have examined the interaction of molecular oxygen with the TiO2(110) surface using temperature-programmed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO2(110) surface in the temperature range between 100 and 300 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water TPD. Adsorption of O2 at 120 K on a TiO2(110) surface with 8% oxygen vacancies (about 4 × 1013 sites/cm2) occurs with an initial sticking probability of 0.5−0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no O2 desorption below a coverage of 4 × 1013 molecules/cm2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 × 1013 molecules/cm2, a first-order O2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16O2 and 18O2 at 120 K indicate that each adsorbed O2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O2 TPD state observed at high O2 exposures. ELS measurements suggest charge transfer from the surface to the O2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O2- species based on comparisons with Ti−O2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces.

Low-Temperature Oxidation of Methane over Pd Supported on SnO2-Based Oxides

Abstract Pd catalysts supported on various metal oxides were investigated for the low-temperature catalytic combustion of methane. Of the catalysts examined, a Pd/SnO2 catalyst demonstrated excellent activity despite its low specific surface area. The activity of the Pd/SnO2 catalyst was lowered with the addition of M ( = Al, Ce, Fe, Mn, Ni, and Zr) oxide to the support, Pd/SnO2–MOx. It was observed by a transmission electron-microscope observation that the PdO phase was well-dispersed on the outer surface of SnO2support uniformly, observed as an intimately contacted layer on fine SnO2 particles. The temperature-programmed desorption of oxygen has revealed that the catalytic activity depends on the adsorption state of oxygen on palladium. An X-ray photoelectron spectroscopic analysis suggested that reduction of the palladium surface proceeded slowly under a strong palladium-support interaction for Pd/SnO2. In situ X-ray diffraction indicated that the PdO phase was stabilized at higher temperature on SnO2 than on the Al2O3 support. It is considered that the catalytic activity is strongly influenced by the interaction between palladium and SnO2.

Oxidation of methane over Pd/mixed oxides for catalytic combustion

Palladium catalysts supported on mixed oxides (Pd/Al 2O 3–MO x ; M=Co, Cr, Cu, Fe, Mn, and Ni) were investigated for the low-temperature catalytic combustion of methane. Although the surface area decreased with increasing NiO in Pd/ mAl 2O 3– nNiO, Pd/Al 2O 3–36NiO demonstrated an excellent activity due to the small particle size of palladium. Also, the catalytic activity strongly depended on the composition of the support. Temperature-programmed desorption of oxygen revealed that the catalytic activity in the low-temperature region depends on the adsorption state of oxygen on palladium. The activity was enhanced when the amount of adsorbed oxygen increased. In-situ XRD analysis indicated that the PdO phase was thermally stabilized on Pd/Al 2O 3–36NiO.

Temperature-Programmed Desorption of Oxygen from Pt Films Interfaced with Y2O3-Doped ZrO2

The origin of the effect of nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion was investigated via temperature-programmed desorption (TPD) of oxygen from polycrystalline Pt films deposited on 8 mol% Y2O3-stabilized ZrO2(YSZ), an O2−conductor, under high-vacuum conditions and temperatures of 600 to 900 K. Oxygen was adsorbed both via the gas phase and electrochemically, as O2−, via electrical current application between the Pt catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives a single adsorption state (Tp≈720–730 K) but electrochemical or mixed gaseous-electrochemical adsorption was found to cause significant oxygen backspillover from the YSZ solid electrolyte onto the Pt surface and the creation of two distinct oxygen adsorption states, i.e., a strongly bonded ionic state (Tp≈750–780 K) and a weakly bonded state (Tp≈675–685 K). The creation of these two states is also manifest by potentiometric work function measurements and high temperature cyclic voltammetry. These results provide a straightforward explanation of the effect of electrochemical promotion on Pt deposited on O2−conducting solid electrolytes. The observed pronounced catalytic rate enhancement in electrochemical promotion studies is due to the high reactivity of the weakly bonded oxygen state, while strongly bonded ionic oxygen acts as a sacrificial promoter. The binding strength and average dipole moment of the weakly bonded oxygen state was investigated as a function of applied potential. It was found that the binding energy of adsorbed oxygen decreases linearly with increasing catalyst potential and work function in agreement with recent ab initio quantum mechanical calculations.

Catalytic Combustion of Methane over Pd Supported on Metal Oxides

Abstract A Pd/SnO2 catalyst demonstrated excellent activity for low temperature catalytic combustion of methane inspite of its low surface area. Palladium clusters were observed as an intimately contacted layer on fine SnO2 particles. Temperature programmed desorption of oxygen revealed that methane combustion was found to be dependent on the adsorption state of oxygen on palladium. It is considered that catalytic activity is strongly influenced by the existence of interaction between palladium and tin oxide.

Characterisation of the oxygen adsorption states on clean and oxidized Ir(110) surfaces

The adsorption of O 2 and the reaction of CO with O 2 have been investigated on a clean as well as on two differently oxidized Ir(110) surfaces in the pressure range 10 −6−10 −1 mbar. X-ray photoelectron spectroscopy (XPS), work-function (WF) measurements via a Kelvin probe, LEED and rate measurements were employed as experimental methods. Oxygen adsorption causes a total WF change up to 1.2 eV on the clean surface and up to 1.6 eV on the weakly oxidized surface using the clean surface as a reference level in both cases. On the strongly oxidized surface, the maximum WF change due to chemisorbed oxygen is about 1.8 eV with respect to the WF level of the oxidized surface. XPS measurements of the strongly oxidized, oxygen-saturated surface show two oxygen states characterized by O 1s binding energies of 530.6 and 528.9 eV, with the first state corresponding to the oxide species and chemisorbed oxygen while the second state correlates with the chemisorbed state which is responsible for the large WF increase. Rate measurements during catalytic CO oxidation, which were conducted by varying p CO and p O 2 and keeping T fixed, showed the usual clockwise rate hysteresis up to 10 −1 mbar with no indication of oscillatory behavior.

Molecular and atomic adsorption states of oxygen on Cu(111) at 100–300 K

Adsorption states of oxygen on Cu(111) at 100–300 K were investigated by means of HREELS. Two molecular species were characterized by different OO stretching frequencies ( v(OO)) at 610 cm −1 and 820–870 cm −1, which are assigned to the peroxo-like species (O 2− 2) adsorbed in a bridged form and the one in a bidentate form bound on an atop site, respectively. The bridged peroxo species is preferred at the low coverage and the atop peroxo species becomes dominant at the higher coverage. In addition to the peaks due to the molecular oxygen, a peak assigned to v(CuO) of atomic oxygen was observed at 370 cm −1 at the high coverage. The frequency of this mode was higher than the frequency reported for Cu(111) exposed to oxygen above 300 K, indicating that the adsorption state of atomic oxygen formed at 100 K is different from that above 300 K. The v(OO) modes became faint after annealing to 170 K because of O 2 dissociation. The v(CuO) mode of the atomic oxygen formed at 100 K remained up to 230 K and disappeared after annealing to 300 K. No desorption of O 2 was detected on annealing to 300 K. It was also found that vibrational spectra for adsorbed NH 3 are influenced by the adsorption states of atomic oxygen on Cu(111).

The reactivity of molecular and atomic oxygen in oxygen-exchange reaction between NO and O2 coadsorbed on a Pt(111) surface

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature-programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen-exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen-exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long-range order. On the contrary, the oxygen-exchange reaction is greatly suppressed when NO is coadsorbed on the surface with well-ordered p(2×2) oxygen atoms.

Oxygen Adsorption Study on Rh(100) Surface by Electron-Stimulated Desorption Spectroscopy

Adsorption states of oxygen on a Rh (100) surface have been studied by electron-stimulated desorption (ESD) spectroscopy. The ESD measurement was performed time sequentially during exposure of the surface to oxygen and heating of the surface. At least three adsorption states were distinguished for oxygen by ESD. One state gave no ESD ions at low temperature. The other two states gave ESD oxygen ions of different kinetic energies. The yield of ESD oxygen ions changed drastically at 350 and 500 K. Decrease of oxygen on the surface was clearly observed above 600 K in hydrogen gas.

The formation of new oxygen adsorption states on Pt(100) by facetting induced by catalytic reaction

Catalytic oxidation of CO on a Pt(100) surface may lead to the formation of microfacets as monitored by LEED as well as of atomic disorder type sites. Both structural elements are associated with an enhanced oxygen sticking coefficient and are hence responsible for the associated increase of catalytic activity. While the facets are stabilized by the presence of adsorbed oxygen, they are annealed during thermal desorption of O 2 from the (110) terrace sites. The second type of sites is thermally more stable and manifests itself in two high-temperature O 2 thermal desorption states.

The adsorption of oxygen and the oxidation of methanol on silver-platinum alloys

AgPt alloy is prepared by hydrazine reduction in solution. The adsorption states of oxygen on AgPt alloys containing Pt < 9.7 at.% are studied using XPS and TDS. The catalytic activity and selectivity of AgPt alloys for oxidizing methanol into aldehyde are measured in a microreactor. Two kinds of adsorption states of oxygen are found on the surface of AgPt alloy. It is shown that silver and platinum preserve their own characteristics of adsorbing oxygen after alloying. The Pt atoms alloyed with Ag promote the decomposition of formaldehyde. The rate equation of methanol oxidation on AgPt alloy deduced on the assumption that the surface reaction is the rate-determining step fits the experimental results.