Dissociative Chemisorption Of O2 Research Articles

Dissociative chemisorption of O2 on Al(111): dynamics on a potential energy surface computed with a non-self-consistent screened hybrid density functional approach.

Density functional theory (DFT) at the generalized gradient approximation (GGA) level is often considered the best compromise between feasibility and accuracy for reactions of molecules on metal surfaces. Recent work, however, strongly suggests that density functionals (DFs) based on GGA exchange are not able to describe molecule-metal surface reactions for which the work function of the metal surface minus the electron affinity of the molecule is less than 7 eV. Systems for which this is true exhibit an increased charge transfer from the metal to the molecule at the transition state, increasing the delocalisation of the electron density. This enlarged delocalisation can cause GGA-DFT to underestimate energy values relative to the gas-phase and thus underestimate the barrier height, similar to what has been observed for several gas-phase reactions. An example of such a molecule-metal surface system is O2 + Al(111). Following a similar strategy as for gas-phase reactions, previous work showed results of increased accuracy when using a screened hybrid DF for O2 + Al(111). However, even screened hybrid DFs are computationally expensive to use for metal surfaces. To resolve this, we test a non-self-consistent field (NSCF) screened hybrid DF approach. This approach computes screened hybrid DFT energies based on self-consistent-field (SCF) GGA electronic densities. Here, we explore the accuracy of the NSCF screened hybrid DF approach by implementing the NSCF HSE03-1/3x@RPBE DF for O2 + Al(111). We compute and analyse molecular beam sticking probabilities as well as a set of sticking probabilities for rotationally aligned O2. Our results show that the NSCF approach results in reaction probability curves that reproduce SCF results with near-chemical accuracy, suggesting that the NSCF approach can be used advantageously for exploratory purposes. An analysis of the potential energy surface and the barriers gives insight into the cause of the disagreement between the SCF and NSCF reaction probabilities and into the changes needed in theoretical modelling to further improve the description of the O2 + Al(111) system. Finally, the hole model yields fair agreement with dynamics results for the reaction probability curve, but results in an increased slope of the reaction probability curve compared to the molecular dynamics, with a shift to lower or higher energies depending on whether the vibrational energy of the molecule is included in the initial energy of the molecule or not.

Interaction of Water and Oxygen Molecules with Phosphorene: An Ab Initio Study.

Phosphorene, the 2D form of black phosphorus, has recently attracted interest for optoelectronic and tribological applications. However, its promising properties are affected by the strong tendency of the layers to oxidize in ambient conditions. A significant effort has been made to identify the role of oxygen and water in the oxidation process. In this work, we introduce a first-principles study of the phosphorene phase diagram and provide a quantitative estimate of the interaction of pristine and fully oxidized phosphorene layers with oxygen and water molecules. Specifically, we study oxidized layers with oxygen coverages of 25% and 50% that keep the typical anisotropic structure of the layers. We found that hydroxilated and hydrogenated phosphorene layers are both energetically unfavorable, leading to structural distortions. We also studied the water physisorption on both pristine and oxidized layers, finding that the adsorption energy gain doubled on the oxidized layers, whereas dissociative chemisorption was always energetically unfavorable. At the same time, further oxidation (i.e., the dissociative chemisorption of O2) was always favorable, even on oxidized layers. Ab initio molecular dynamics simulations of water intercalated between sliding phosphorene layers showed that even under harsh tribological conditions water dissociation was not activated, thus further strengthening the results obtained from our static calculations. Overall, our results provide a quantitative description of the interaction of phosphorene with chemical species that are commonly found in ambient conditions at different concentrations. The phase diagram that we introduced confirms the tendency of phosphorene layers to fully oxidize due to the presence of O2, resulting in a material with improved hydrophilicity, a piece of information that is relevant for the application of phosphorene, e.g., as a solid lubricant. At the same time, the structural deformations found for the H- and OH- terminated layers undermine their electrical, mechanical, and tribological anisotropic properties and, therefore, the usage of phosphorene.

Activity of Ir(100) and IrO2(110) for the Catalytic Oxidation of Methane

Identifying and characterizing active surface phases are critical for understanding the behavior of oxidation catalysts. In this study, we investigated the catalytic activity of Ir(100) and an IrO2(110) film for the complete oxidation of methane at pressures near 1 Torr and characterized the surface activity using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) measurements and density functional theory (DFT) calculations. Our results show that IrO2(110) is highly active for catalytic CH4 oxidation in CH4-rich mixtures (∼90% CH4) and moderate temperature (650 K), whereas the Ir(100) surface is inactive at the same conditions. AP-XPS shows that the Ir(100) surface remains metallic when immersed in the CH4-rich mixture at temperatures up to 650 K but becomes nearly saturated with chemisorbed O atoms. According to DFT, CH4 activation is facile on clean Ir(100), but activation via H transfer to a chemisorbed O atom has a significantly higher energy barrier. The DFT calculations further predict that the dissociative chemisorption of O2 is more efficient than CH4 dissociation on Ir(100) and generates high O coverages even in CH4-enriched reactant mixtures. As a result, the preferential adsorption of oxygen effectively poisons Ir(100) toward initial CH4 activation, rendering this surface inactive for catalytic CH4 oxidation. Our results clarify the fundamental origins of the catalytic activity of Ir(100) and IrO2(110) toward CH4 oxidation and provide insights about reaction conditions that can enhance the activity of Ir catalysts for this chemistry.

Structural and Electronic Rearrangements in Fe2S2, Fe3S4, and Fe4S4 Atomic Clusters under the Attack of NO, CO, and O2.

We report results, based on density functional theory-generalized gradient approximation calculations, that shed light on how NO, CO, and O2 interact with Fe2S2, Fe3S4, and Fe4S4 clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O2 is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe4S4 that splits into two Fe2S2 subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe-Fe interaction inducing antiparallel couplings that, in the case of Fe4S4, clearly arise from indirect Fe-Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron-sulfur clusters. This increase is noticeably more pronounced for Fe2S2 than for the other iron-sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with FemSm (m = 1-4), the Fe2S2CO product results as a prominent one.

Dissociative Chemisorption of O2 on Al(111): Dynamics on a Correlated Wave-Function-Based Potential Energy Surface.

Dissociative chemisorption of O2 on the Al(111) surface represents an extensively studied prototype for understanding the interaction between O2 and metal surfaces. It is well known that the experimentally observed activation barrier for O2 dissociation is not captured by conventional density functional theory. The interpretation of this barrier as a result of spin transitions along the reaction path has been challenged by recent embedded correlated wave function (ECW) calculations that naturally yield an adiabatic barrier. However, the ECW calculations have been limited to a static analysis of the reaction pathways and have not yet been tested by dynamics simulations. We present a global six-dimensional potential energy surface (PES) for this system parametrized with ECW data points. This new PES provides a reasonable description of the site-specific and orientation-dependent activation barriers. Quasi-classical trajectory calculations on this PES semiquantitatively reproduce both the observed translational energy dependence of the sticking probability and steric effects with aligned O2 molecules.

Efficient aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran on manganese oxide catalysts

A cryptomelane-type manganese oxide octahedral molecular sieve with a (2×2, 4.6Å×4.6Å) tunnel size (OMS-2) efficiently catalyzed aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with a high yield of 97.2% at 383K and 0.5MPa O2 in N,N-dimethylformamide. OMS-2 was superior to other MnO2 catalysts with different morphologies, including OMS-1, OMS-6, and OMS-7 with various tunnel sizes, amorphous MnO2 and birnessite-type MnO2, apparently due to its (2×2) tunnel structure and consequently high reducibility and oxidizability. Kinetic and isotopic studies on OMS-2 showed near half-order dependence of the activities on HMF and O2 concentrations and marked kinetic isotope effects for deuterated HMF at its methylene group. These results, together with the similar initial rates under aerobic and anaerobic conditions, suggest that HMF oxidation to DFF on OMS-2 proceeds via a redox mechanism involving kinetically-relevant steps of C–H bond cleavage in adsorbed alcoholate intermediate, derived from quasi-equilibrated dissociation of HMF, using lattice oxygen and reoxidation of Mn3+ to Mn4+ by dissociative chemisorption of O2.

Experimental data and kinetic modeling of the catalytic and electrochemically promoted CH4 oxidation over Pd catalyst-electrodes

This aim of this work is to model the kinetics of the electrochemically promoted methane oxidation over impregnated Pd catalyst-electrodes in the temperature range between 300 and 400°C. Six different kinetic models based on the Eley–Rideal, Langmuir–Hinshelwood and Mars van Krevelen mechanisms were fitted to the experimental data. It was found that Langmuir–Hinshelwood mechanism, considering dissociative chemisorption of O2 and only one oxygen surface species consumed in the rate determining step, successfully predicted the kinetics of the methane consumption rate under both open circuit state and electrochemical promotion conditions. In addition, the influence of the polarizations on the components adsorptions was analyzed. An increase in the adsorption constant and the partial charge transfer parameter of the methane was obtained with increasing values of the applied anodic potential.

Nondissociative Adsorption of O2on Ge(100)

Surface scattering experiments indicate that the dissociative chemisorption of O2 on Ge(100) and Si(100) both proceed through precursor mediate mechanisms. The theoretical studies of O2/Ge(100), unlike those of O2/Si(100) that are extensive and comprehensive, have not yet been able to account for such experimental data because of the omission of the unusual triplet spin nature of the ground state of O2. In this work we have tracked by first principles calculations the reaction paths of both triplet and singlet O2/Ge(100) and identified several nondissociative chemisorption states that “mediate” the subsequent dissociative chemisorption of ground state O2. In addition to explaining the experimentally observed precursor mediate routes, our computational comparison of O2/Ge(100) and O2/Si(100) also predicts, consistent with known experimental data, a smaller sticking coefficient for O2/Ge(100).

First-Principles Study on Water and Oxygen Adsorption on Surfaces of Indium Oxide and Indium Tin Oxide Nanoparticles

We present a first-principles study on surface structures of In2O3 and indium tin oxide (ITO) nanoparticles upon exposure to O2 and H2O molecules. It was found that dissociative chemisorption of O2 and H2O is facile on the metal-terminated oxide surfaces. O2 chemisorption is energetically more favorable than H2O dissociative chemisorption until the metal-terminated surfaces are saturated by chemisorbed O2 dimers, at which point the surfaces become virtually identical to the O-terminated surfaces and the dissociative chemisorption of H2O becomes the energetically preferred process. The calculated surface structures and the surface states as represented by the density of states spectra are in good agreement with experimental surface characterizations and the UPS photoelectron spectra of In2O3 and ITO thin films.

Role of Rotational Alignment in Dissociative Chemisorption and Oxidation: O2 on Bare and CO‐Precovered Pd(100)

The adsorption of gas-phase molecules that approach solid surfaces in well-defined quantum states represents a fundamental step for the understanding of heterogeneous chemical reactions and for better control over the growth of selfassembled layers. This objective has been achieved only in a few cases to date (NO, H2, [5] CH4 ). Vibrational excitations and conversion of translational to vibrational energy were thereby found to account for strong enhancements in the dissociation probability. Less attention has been devoted to rotations because they involve less energy, and are therefore important only for physisorption. We show here that rotations as well as the alignment of the rotational axis play a role in the dissociative chemisorption of O2 onto a CO Pd(100) surface. The effect, which arises from the requirements needed to pass through the CO adlayer, leads to different sticking probabilities and average O CO distances, and has possible general applications for controlling reactions and film growth. The alignment of rotational angular momentum can be exploited to investigate stereodynamical effects in molecular adsorption processes. 11–13] Such an alignment, which consists of a propensity of molecules to populate specific helicity states (defined by the quantum number M, the projection of the rotational angular momentum along the propagation direction) can be naturally induced by collisions in seeded supersonic molecular beams (MBs). Diatomic, linear, and linear-like molecules flying in the MB with M= 0 behave as cartwheels when impinging on the surface at normal incidence, while when M is maximum they move in a helicopterlike manner. In particular, it has been shown that for the interaction of C2H4 with an O2-precovered Ag(100) surface, 11] and for C3H6/Ag(100), [12] molecules with helicopterlike motion adsorb more efficiently than cartwheeling ones only at intermediate hydrocarbon coverage. The stereoinsensitivity of the initial sticking probability S0 and its coverage dependence indicated that the effect is due to the collision between trapped molecules, which still remember about their original alignment, and preadsorbed ones, which lie flat on the surface. Rotational alignment was found to be ineffective for more strongly bound systems such as s-bonded C2H4/Pd(100) [13] and O2/Ag(100) (interactions mediated via a chemisorbed molecular precursor). This result was attributed to steering forces turning the incoming molecule into the most favorable configuration and causing it to lose memory of its initial state. For O2 on CO-precovered Pd(100), however, we find that molecules moving as cartwheels stick better than those with helicopter-like motion and that molecular alignment also affects the initial sticking probability. The system under investigation is important because of the effective rate of CO oxidation on Pd(100) when exposed to molecular oxygen. Herein we focus on stereodynamical effects of O2 on the S0 value and on the production of CO2. According to Ref. [14], rotational alignment of O2 is generated by collisions during a supersonic seeded expansion of O2 diluted in lighter and faster carrier atoms, typically He and Ne. Under these conditions most of the oxygen molecules in the MB are relaxed in the ground rotational level (K= 1) and cartwheeling motion prevails on average over helicopterlike motion. In addition, as demonstrated by the use of a high-resolution mechanical velocity selector (consisting of eight rotating slotted disks) the angular momentum of the O2 molecules belonging to the slow tail (ST) of the velocity distribution are nearly randomly oriented (ca. 33% inM= 0), while those in the fast tail (FT) fly mostly with cartwheeling motion (> 80%). In the present experiment (see the Experimental Section and the Supporting Information) the fraction moving as cartwheels are approximately 0.4 and 0.7 for the ST and FT regions, respectively. Figure 1 shows the partial pressure of O2 in the chamber, measured by a quadrupole mass spectrometer (QMS), during exposure of the Pd(100) sample at 394 K to the STor FTof the O2 supersonic MB seeded in He at normal incidence. The sample is initially shielded by an inert cover (flag), so that no direct adsorption occurs. The background pressure typically increases by 2 C 10 10 mbar, mostly because of He, so that adsorption from the background over the time scale of an experiment (< 100 s) is also negligible. When the flag is removed (t1) the beam strikes the reactive surface of the sample. The relative decrease in the QMS signal corresponds to the O2 sticking probability S. [16] The lower traces refer to the interaction with bare Pd(100). No difference is observed between the FT and ST experiments, thus indicating either that steering is efficient in turning the molecule into the most favorable configuration for dissociation or that the interaction [*] Dr. A. Gerbi, Dr. L. Savio, Dr. L. Vattuone Dipartimento di Fisica CNISM Unit) di Genova Universit) di Genova Via Dodecaneso 33, 16146 Genova (Italy) Fax: (+39)010-314-218 E-mail: vattuone@fisica.unige.it Homepage: http://www.fisica.unige.it/~ vattuone

Influence of catalyst morphology on the performance of electrolytic silver catalysts for the partial oxidation of methanol to formaldehyde

Scanning electron microscopy (SEM), specific surface area measurements, in situ Raman spectroscopy and steady-state microreactor experiments were used to explore the relationship between silver catalyst morphology and performance in the titled reaction. Two electrolytic silver catalysts, obtained from different commercial suppliers, were examined in relation to their physico-chemical and catalytic properties. The work focused on the activation behaviour of the catalysts under conditions of industrial CH2O synthesis (ca. 923–953 K, 1 atm). The results show that the silver catalysts, denoted A and B, differed enormously in their surface morphology, which in turn impacted on their oxygen chemisorption behaviour and initial catalytic properties. Silver catalyst A possessed a high grain boundary density in its surface, and exhibited enhanced initial activity and selectivity to CH2O during CH3OH oxidation at 598–973 K compared to catalyst B, which possessed significantly lower surface area and roughness. Performance differences correlated strongly with the speciation of oxygen on the catalysts during CH3OH oxidation, which is discussed in terms of a framework comprising three chemically distinct atomic oxygen species: two surface species (denoted Oα and Oγ) and a bulk-dissolved species (denoted Oβ). The population of each state was found to be both temperature and structure-sensitive. The weakly bound Oα species was dominant at low temperatures and opened reaction pathways towards CH2O, HCOOCH3, CO2 and HCOOH. At temperatures of industrial CH2O manufacture (ca. 923 K), the strongly bound Oγ species was the dominant oxygen state and selectively activated the catalysts for the oxidative-dehydrogenation of CH3OH to CH2O+H2O. The dissociative chemisorption of O2 and the surface segregation of Oβ served to replenish the Oα and Oγ consumed by reaction. The CH2O yield on both catalysts increased with temperature up to 923 K, reflecting accompanying increases in the surface Oγ/Oα ratio. Grain boundaries facilitated the formation of Oγ and Oβ, which explained why catalyst A exhibited the higher initial activity and selectivity to CH2O. The performance of both catalysts, in particular catalyst B, improved with time-on-stream at 923 K before stabilising after approximately 48 h of operation. These improvements were due to reaction-induced changes in catalyst morphology, which created structures that promoted Oβ and Oγ formation. Results were used to develop reaction schemes for CH3OH oxidation on fresh and aged silver catalysts under industrial conditions. The critical role of defects in the silver catalyst structure, such as grain boundaries in the efficient ‘start up’ operation of electrolytic silver catalysts for CH2O production is demonstrated.

Effects of steps and defects on O2 dissociation on clean and modified Cu(1 0 0)

Dissociative chemisorption of O2 on Cu(100), S/Cu(100) and Ag/Cu(100) surface alloy has been investigated by Auger electron spectroscopy (AES). A strong reduction in the initial O2 chemisorption probability (S0) from 0.05 to 7.4×10−3 is observed already at an Ag coverage of 0.02 ML. Further Ag deposition results only in a moderate decrease in S0. Similar inhibition of O2 dissociation is observed on S/Cu(100). It is concluded that at very low Ag coverages, the reduced reactivity of Ag/Cu(100) towards O2 dissociation is primarily due to the steric blocking of the surface defects and that any electronic effects are only secondary and present only at higher Ag coverages.

Oxidation of CO on a Pt/Al2O3 Catalyst: From the Surface Elementary Steps to Lighting-Off Tests: II. Kinetic Study of the Oxidation of Adsorbed CO Species Using Mass Spectroscopy

Experiments in the transient regime using a mass spectrometer as a detector are performed at T=300 K on a 2.9% Pt/Al2O3 catalyst to study the oxidation of the linearly adsorbed CO species (denoted L) with several x% O2/z% Ar/He mixtures (x and z in the range 0.5–4). CO and O2 adsorption measurements show that the L CO species and a strongly adsorbed oxygen species (denoted Osads) formed by the dissociative chemisorption of O2 are adsorbed on the same sites of the freshly reduced Pt particles. L CO and Osads species have high heats of adsorption: 115 and 175 kJ/mol, respectively, at full coverage of the Pt surface and do not desorb in helium at a temperature lower than 350 K. Moreover, it is shown that at 300 K a preadsorbed L CO species is not displaced by oxygen adsorption but is converted into CO2 by a weakly adsorbed oxygen species (denoted Owads) according to the elementary step (denoted S3): L+Owads→CO2 (rate constant, k3). This confirms the conclusion of a previous study performed using FTIR spectroscopy. O and C mass balances during the transient regime reveal (a) that during the first seconds of the transient the oxygen consumption is mainly due to the formation of the Owads species and that its adsorption equilibrium is rapidly attained and (b) that an Osads species is adsorbed for each L CO species removed by oxidation. This leads to a Pt surface where the coverage of the LCO species decreases while that of the Osads species increases with the duration of the oxidation. However, it is shown that the rate of the reaction (denoted S3a), L+Osads species, is significantly lower than that of step S3 and does not contribute to the CO2 formation in the presence of O2. A comparison is presented with the literature data on Pt single crystals (UHV studies) and supported Pt catalysts.

Oxidation of CO on a Pt/Al2O3 Catalyst: From the Surface Elementary Steps to Lighting-Off Tests: III. Experimental and Kinetic Model for Lights-Off Tests in Excess CO

The adsorption of CO (1% CO/He mixture) at 300 K on a 2.9% Pt/Al2O3 catalyst leads to the detection of a strong IR band at 2075 cm−1 associated with weak and broad IR bands in the range 1900–1700 cm−1 ascribed to linear (denoted L) and multibound (bridged and threefold coordinated) CO species, respectively. In parts I and II of the present study it was shown that the L CO species is oxidized at T<350 K according to the surface elementary step denoted S3, L+Owads→CO2, where Owads is a weakly adsorbed oxygen species formed without competition with the L CO species by the dissociative chemisorption of O2. In the present study the coverage of the L CO species as well as the conversion of CO into CO2 are determined during the increase in the reaction temperature Tr from 300 to 740 K (lighting-off tests) using several 1% CO/x% O2/He mixtures, with x≤0.5. In an excess of CO (x<0.5), it is shown that the experimental curves θL=f(Tr) can be fitted by a kinetic model by considering that θL is determined by the equilibrium between the rates of adsorption, oxidation, and desorption of the L CO species. The parameters used in the kinetic model are those previously determined by studying (a) the adsorption equilibrium of the L CO species in the temperature range 300–740 K and (b) its oxidation by O2 at T<350 K (in the absence of CO). The evolution of the experimental turnover frequency (TOFex) during the increase in Tr is determined and compared to the theoretical TOFth for low CO conversions. It is shown that TOFex=f(Tr) is in very good agreement with a Langmuir–Hinshelwood mechanism via step S3, with the kinetic parameters used to fit the curves θL=f(Tr).

On the viability of single atom abstraction in the dissociative chemisorption of O2 on the Al(111) surface

The dissociative chemisorption of O2 on the Al(111) surface is investigated by means of a Monte Carlo simulation that incorporates two mechanisms that have been proposed for this reaction in the literature: single atom abstraction and two-atom adsorption that generates translationally hot atoms on the surface. A comparison is made to the much-debated STM results of Brune et al. [J. Chem. Phys. 99, 2128 (1993)], in which the oxygen island density (number of islands per binding site) was determined as a function of coverage. Since the two-atom channel has been discussed heavily in the literature, we focus primarily on the abstraction mechanism. We show that atom abstraction in its basic form is incompatible with the STM results; however, we propose two simple modifications that enable atom abstraction to reproduce the STM results. In the first modification, the probability of dissociation is higher at sites next to preexisting O adatoms. In essence, we are proposing that the increased Al–O bond strength at sites next to preexisting O adatoms [Jacobsen et al., Phys. Rev. B 52, 14954 (1995)] stabilizes the transition state for dissociation. If atom abstraction is assumed to be the only operative mechanism, and if its probability increases by a factor of ∼10 next to a site that is occupied versus unoccupied, the STM island density data can be approximately reproduced. In the second modification, the abstracted atom is permitted to make a single hop in the direction of a preexisting, nearby O adatom. The allowance of merely a single, directed hop has a dramatic effect on the coverage dependence of the island density.

Effective Conversion of CO2 to Carbonate in Surface Oxidation Processes at Si(100)

It is found that effective carbonate formation occurs at a clean Si(100) surface between CO2 and surface oxidants: N2O and O2. The reactions of CO2 with N2O or O2 are studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. CO2 physisorbed on Si(100) desorbs exclusively at ∼75 K. However, when CO2 is coadsorbed with N2O or O2, CO2 is converted very effectively into surface carbonate species below 140 K. The carbonate species are dissociated to produce desorbed species of CO or CO2 in the temperature range 200−600 K. By using C18O2, it is confirmed that the carbonate is composed of the adsorbate CO2 and one oxygen atom from the surface oxidant. In the case of N2O/CO2 coadsorption, the carbonate formation is mainly induced by thermal dissociation of N2O adsorbates. In the case of O2/CO2 coadsorption, the carbonate formation is not only induced by dissociative chemisorption of O2, but also proceeded with chemisorbed dioxygen that likely adsorbs in a peroxy bridging configuration.

Deposition of Palladium Overlayers on Oxygen-Precovered Ruthenium(100)

We have studied the deposition of Pd on oxygen-precovered Ru(100) at 300 K as a function of both oxygen and Pd coverage. A layered growth of Pd similar to that observed on clean Ru is maintained in the presence of preadsorbed oxygen. This results in complete removal of oxygen from contact with the Ru surface with efficient diffusion to the Pd−vacuum interface. This was confirmed with the complete reactive removal of preadsorbed oxygen by CO at 300 K for θPd > 6 ML. The out-diffused oxygen overlayer is unusually dense and cannot be made by dissociative chemisorption of O2 on Pd overlayers on Ru(100). This additional oxygen is desorbed from a new thermal desorption state at 700 K with the remaining oxygen desorbing at 1037 K for θPd > 2 ML.

Palladium Catalyzed Oxidation of Amorphous Carbon: A Study by in Situ Transmission Electron Microscopy

Palladium clusters, 10-30 nm in size, were grown in UHV on amorphous carbon in a transmission electron microscope (TEM). The Pd catalyzed oxidation of carbon was then followed in situ in the TEM between 720 and 800 K at a pressure p of O2 with 0.25 < p (10-4 mbar) < 1.0). The behavior of hundreds of individual clusters was recorded on videotape in real time. Clusters moved as they dug irregular channels throughout the carbon film. They exhibited liquid-like behavior and frequent change of contrast (flashing). These active clusters started to move after an induction period. Some clusters remained inactive. Active clusters coalesced with inactive clusters to form active clusters. After some time, active clusters became and remained inactive. The site time yield of catalyzed oxidation, defined as the number of C atoms reacting per exposed Pd atom per second was almost independent of temperature and proportional to pressure of O2. Under the conditions of this work, it appears that the rate determining step of the oxidation of carbons is the dissociative chemisorption of O2 on Pd, with every O2 molecule sticking to the Pd surface reacting to C with formation of CO2.

Isotope effect in dissociative chemisorption of O2 on Ag(111)

Les experiences adsorption dissociative/desorption de recombinaison sont effectuees pour les isotopes 16 O 2 et 18 O 2 . L'effet isotopique est modelise par un calcul preliminaire de la probabilite de transmission pour l'effet de tunnel de la molecule a travers de la barriere pendant la formation ou dissociation de O 2 −

Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy

We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co-adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditions but contrasts with previous assignment of a direct (or quasidirect) dissociation process at high energies. We have also reexamined a number of previous observations in terms of this new picture, including the initial decline in dissociation probability with increasing kinetic energy. This falloff is attributed to a decrease in the trapping probability into a physisorption state, as recently suggested by Luntz et al. Considering the present results in the light of other recent studies, it now seems clear that the physisorption state is then a precursor to the molecular chemisorption state which can also be accessed directly at high kinetic energy. In this picture the molecular chemisorption state is then a precursor to dissociation even at high kinetic energy, and the dissociation probability depends on the (temperature-dependent) branching ratio between the dissociation and ultimate desorption of these species.