Oxygen-covered Surface Research Articles

Transient behavior and reaction mechanism of CO catalytic ignition over a CuO–CeO2 mixed oxide

This study focuses on the variation in activity-controlling factors during CO catalytic ignition over a CuO–CeO2 catalyst. The activity for CO combustion follows the decreasing order of CuO–CeO2 > CuO > CeO2. Except for inactive CeO2, increasing temperature induces CO ignition to achieve self-sustained combustion over CuO and CuO–CeO2. However, CuO provides enough copper sites to adsorb CO, and abundant active lattice oxygen, thus obtaining a higher hot zone temperature (208.3°C) than that of CuO–CeO2 (197.3 °C). Catalytic ignition triggers a kinetic transition from the low-rate steady-state regime to a high-rate steady-state regime. During the induction process, Raman, X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD) and infrared (IR) spectroscopy results suggested that CO is preferentially adsorbed on oxygen vacancies (Cu+-[Ov]-Ce3+) to yield Cu+-[CO]-Ce3+ complexes. Because of the self-poisoning of CO, the adsorbed CO and traces of adsorbed oxygen react at a relative rate, which is entirely governed by the kinetics on the CO-covered surface and the heat transport until the pre-ignition regime. Nonetheless, the Cu+-[CO]-Ce3+ complex is a major contributor to CO ignition. The step-response runs and kinetic models testified that after ignition, a kinetic phase transition occurs from a CO-covered surface to an active lattice oxygen-covered surface. During CO self-sustained combustion, the rapid gas diffusivity and mass transfer is beneficial for handling the low coverage of CO. The active lattice oxygen of CuO takes part in CO oxidation.

Ethanol sensing characteristics of (La,Ba)(Fe,Ti)O3 nanoparticles with impurity phases of BaTiO3 and BaCO3

To further improve the ethanol sensing performance of LaFeO3 (LFO) nanoparticles, the co-doping of acceptor and donor at La and Fe sites at equal mole amount was designed to increase the oxygen adsorbing ability of the resultant nanoparticles. A series of (La,Ba)(Fe,Ti)O3 nanoparticles, with the designed mole ratios of Ba/La (Ti/Fe) as 0, 0.25, 0.33, 0.50, and 1.0, were prepared by a citric sol–gel method. XRD confirmed the main phase of orthorhombic LFO, and the impurity phases of cubic BaTiO3 (BTO) and orthorhombic BaCO3. The incorporation of Ba and Ti into the LFO lattice was verified by the increased unit cell volume of LFO upon more doping. TEM, BET, and XPS measurements indicated that the appropriate co-doping of Ba and Ti into the LFO lattice and the simultaneous existence of BTO and BaCO3 resulted in smaller grain size, larger BET surface area, and thus higher concentration of adsorbed oxygen species than those in the pristine LFO nanoparticle. Among all (La,Ba)(Fe,Ti)O3 sensors, the sensor based on the (La,Ba)(Fe,Ti)O3 nanoparticles with the designed Ba/La = 0.50 exhibited the highest gas response and sensitivity toward ethanol at the prime working temperature of 132 °C, which was ascribed to the higher surface oxygen coverage contributed by the synergistic effect of co-doping and impurity phases on the number of surface active sites and oxygen adsorbing ability.

Unexpected kinetic behavior of structured Pd/CeO2–ZrO2 toward undesired ammonia formation and consumption during nitrites reduction: Role of the reactivity of oxygen from ceria

This study deals with the kinetics of successive ammonia production and unexpected consumption in the course of the catalytic water denitrification over Pd/CexZr1-xO2 catalysts. A fast conversion of nitrites to ammonium ions was found weakly dependent of the particle size of palladium likely due to the juxtaposition of electronic effects stabilizing electron-rich Pd active sites more prone to produce ammonia instead of nitrogen. Subsequent slow consumption of ammonia in anaerobic conditions occurred on Pd/CexZr1-xO2 whereas Pd/ZrO2 was found inactive for this sequential reaction. Kinetic measurements from semi batch experiments led to the estimation of rate constants. Particular attention was paid to ammonia oxidation reaction with a pseudo rate constant depending on the surface coverage of reactive oxygen from ceria then revealing the importance of the synthesis route of CexZr1-xO2 and peculiar interaction with Pd.

Comparative Evaluation of Dimethylsulfoxide and Dimethylsulfone Adsorption on a Smooth Platinum Electrode in Acidic Environment

a comparative assessment of dimethylsulfoxide and dimethylsulfone adsorption on a smooth platinum electrode in acidic medium in the region of anode potentials is presented. The adsorption isotherms in the “hydrogen” potential range, depending on the dimethylsulfoxide and dimethylsulfone concentration, are S-shaped. At the adsorbate concentrations in the solution exceeding 0.1 M, the surface coverage with dimethylsulfoxide and dimethylsulfone is nearly the same and changes but slightly. A significant difference in the adsorption characteristics of dimethylsulfoxide and dimethylsulfone in the potential region of oxygen adsorption is found. According to the Pt-electrode surface coverage with oxygen in the presence of the test substances, it was concluded that the value of the dimethylsulfoxide and dimethylsulfone adsorption is maximal only when their bulk concentration is higher than 0.1 M, here the oxygen surface coverage in the presence of dimethylsulfone exceeds that in the background solution.

Effect of Surface Oxygen on the Wettability and Electrochemical Properties of Boron-Doped Nanocrystalline Diamond Electrodes in Room-Temperature Ionic Liquids.

This paper reports on how the surface chemistry of boron-doped nanocrystalline diamond (BDD) thin-film electrodes (H vs O) affects the wettability and electrochemical properties in two room-temperature ionic liquids (RTILs): [BMIM][PF6] and [HMIM][PF6]. Comparative measurements were made in 0.5 mol L-1 H2SO4. The BDD electrodes were modified by microwave or radio-frequency (RF) plasma treatment in H2 (H-BDD), Ar (Ar-BDD), or O2 (O-BDD). These modifications produced low-, medium-, and high-oxygen surface coverages. Atomic O/C ratios, as determined by X-ray photoelectron spectroscopy (XPS), were 0.01 for H-BDD, 0.08 for Ar-BDD, and 0.17 for O-BDD. The static contact angle of ultrapure water on the modified electrodes decreased from 110° (H-BDD) to 41° (O-BDD) with increasing surface oxygen coverage, as expected as the surface becomes more hydrophilic. Interestingly, the opposite trend was seen for both RTILs as the contact angle increased from 20° (H-BDD) to 50° (O-BDD) with increasing surface oxygen coverage. The cyclic voltammetric background current and potential-dependent capacitance in both RTILs were largest for BDD electrodes with the lowest O/C ratio (H-BDD) and smallest contact angle. Slightly larger voltammetric background currents and capacitance were observed in [HMIM][PF6] than in [BMIM][PF6]. Capacitance values ranged from 8 to 16 μF cm-2 over the potential range for H-BDD and from 4 to 6 μF cm-2 for O-BDD. The opposite trend was observed in H2SO4 as the voltammetric background current and capacitance were largest for BDD electrodes with the highest O/C ratio (O-BDD) and smallest contact angle. In summary, reducing the surface oxygen on BDD electrodes increases the wettability to two RTILs and this increases the voltammetric background current and capacitance.

Distribution of O atoms on partially oxidized metal targets, and the consequences for reactive sputtering of individual metal oxides

We investigate the oxidation of a wide range of metal surfaces by ab-initio calculations. We go through a wide range of metals (Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Cu, Ag, Au, Zn, Cd, Al) and surface oxygen coverages (ΘO). Calculations of the adsorption energy per O atom (Eads_min) are followed by characterizing the preferred distribution of O atoms on a partially oxidized surface (considering up to 329 distributions per metal). We find that individual metals exhibit qualitatively different Eads_min(ΘO) dependencies and thermodynamical preferences: (i) decreasing Eads_min(ΘO) and heterogeneous distribution of O atoms (stoichiometric oxide + metal; e.g. Al or La), (ii) increasing Eads_min(ΘO) and homogeneous distribution of O atoms (substoichiometric oxide; e.g. Ti or Zr), (iii) concave Eads_min(ΘO) and homogeneous distribution of O atoms at low ΘO and heterogeneous at high ΘO (e.g. Sc or Y), etc. This is of crucial importance for the quantities such as secondary electron emission yield, which are guaranteed to correspond to a weighted average of those of stoichiometric oxide and metal only in the case of heterogeneous O atom distribution. The results are correlated with formation enthalpies of oxides of various compositions, surface atomic densities of individual metals, and experimental results such as the time dependence of the magnetron voltage during sputter cleaning of oxidized metal targets. Furthermore, we demonstrate use of these results by presenting static and dynamic Monte Carlo simulations of sputtering.

Initial oxidation and surface stability diagram of Ge(100) as a function of the temperature and oxygen partial pressure through ab initio thermodynamics

Density functional theory calculations in conjunction with thermodynamic modeling were performed to examine the oxygen adsorption on a Ge(100) c(4 × 2) surface and the subsequent initial oxidation. For several possible adsorption sites, the adsorption energy of atomic oxygen as well as the atomic configuration and electronic properties of the adsorbed structure were examined. Then the effects of the surface coverage of oxygen from 1/64 to 1/4 monolayers on the adsorption energy were considered. Through the surface Gibbs free energy as a function of the temperature (T) and oxygen partial pressure (), the (T, ) surface stability diagram was predicted for the O/Ge(100) c(4 × 2) surface. The theoretical prediction well reproduced previous experimental observations and provides an insight to control the initial oxidation process of Ge surface with tuned T

New insights into oxygen surface coverage and the resulting two-component structure of graphene oxide

Abstract Elucidating the essential details of the structure of graphene oxide (GO) is still a challenge. There is no consensus in the increasingly abundant literature, especially relating to the epoxy groups as the main surface complexes in the basal plane, as well as the simultaneous presence of GO sheets and oxidative debris (OD), with a large difference in their oxygen content. In the present work we characterized the base-washed GO (bwGO) sheets, the OD and the humic fraction of the OD obtained by base digestion, when the parent GO was dispersed by applying sonication, a routine procedure when starting from dried graphite oxide. When sonication is not applied, the amount of OD detected is considerably lower, indicative of its formation before base digestion. The presence of lactols and carboxylic anhydrides as the dominant surface complexes at graphene edges is consistent with all the characterization results, as well as with the general knowledge of surface chemistry of carbon materials ranging from coal to graphite. These findings suggest that the Hummers-Offeman reaction produces a chemical scissor effect during the water/hydrogen peroxide quenching step, yielding a broad size distribution of GO sheets, with little in-plane oxidation and the vast majority of edges being oxidized to form oxepinone-type functionalities.

Enabling a new method of dynamic field-effect gas sensor operation through lithium-doped tungsten oxide

Abstract. To fulfil today's requirements, gas sensors have to become more and more sensitive and selective. Temperature-cycled operation has long been used to enhance the sensitivity and selectivity of metal-oxide semiconductor gas sensors and, more recently, silicon-carbide-based, gas-sensitive field-effect transistors (SiC-FETs). In this work, we present a novel method to significantly enhance the effect of gate bias on a SiC-FET's response, giving rise to new possibilities for static and transient signal generation and, thus, increased sensitivity and selectivity. A tungsten trioxide (WO3) layer is deposited via pulsed laser deposition as an oxide layer beneath a porous iridium gate, and is doped with 0.1 AT % of lithium cations. Tests with ammonia as a well-characterized model gas show a relaxation effect with a time constant between 20 and 30 s after a gate bias step as well as significantly increased response and sensitivity at +2 V compared to 0 V. We propose an electric field-mediated change in oxygen surface coverage as the cause of this novel effect.

DFT + U Study of the Adsorption and Dissociation of Water on Clean, Defective, and Oxygen-Covered U3Si2{001}, {110}, and {111} Surfaces.

The interfacial interaction of U3Si2 with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U3Si2 surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H2 formation mechanisms. The adsorption strength decreases in the order U3Si2{001} > U3Si2{110} > U3Si2{111} for both molecular and dissociative H2O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H2O on the oxygen-covered U3Si2 surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H2O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces.

Ambient Temperature NO Adsorber Derived from Pyrolysis of Co-MOF(ZIF-67).

Co-, Ni-, and Zn-containing MOFs are prepared and then pyrolyzed to generate materials for ambient temperature NO adsorption. Materials containing Co are much more efficient for NO adsorption than those containing Ni and Zn; therefore, Co is identified as the active phase. The best performing material studied here achieves 100% low concentration (10 ppm) NO adsorption for more than 15 h under a weight hourly space velocity of 120 000 mL g–1 h–1. Powder X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared, and Raman spectroscopies, along with scanning electron microscopy and TEM, are used to probe the physicochemical properties of the materials, particularly the Co active phase, and chemistries involved in NO adsorption–desorption. NO adsorbs on oxygen-covered Co nanoparticle surfaces in the form of nitrates and desorbs as NO at higher temperatures as a result of surface nitrate decomposition. NO storage capacity decreases gradually upon repeated NO adsorption–desorption cycles, likely because of Co3O4 formation during these processes.

Theoretical Insights of Ni2P (0001) Surface toward Its Potential Applicability in CO2 Conversion via Dry Reforming of Methane

This study reports the potential application of Ni2P as highly effective catalyst for chemical CO2 recycling via dry reforming of methane (DRM). Our DFT calculations reveal that the Ni2P (0001) surface is active towards adsorption of the DRM species, with the Ni hollow site being the most energetically stable site and Ni-P and P contributes as co-adsorption sites in DRM reaction steps. Free energy analysis at 1000 K found CH-O to be the main pathway for CO formation. The competition of DRM and reverse water gas shift (RWGS) is also evidenced with the latter becoming important at relatively low reforming temperatures. Very interestingly oxygen seems to play a key role in making this surface resistant towards coking. From microkinetic analysis we have found greater oxygen surface coverage than that of carbon at high temperatures which may help to oxidize carbon deposits in continuous runs. The tolerance of Ni2P towards carbon deposition was further corroborated by DFT and micro kinetic analysis. Along with the higher probability of C oxidation we identify poor capacity of carbon diffusion on the Ni2P (0001) surface with very limited availability of C nucleation sites. Overall, this study opens new avenues for research in metal-phosphide catalysis and expands the application of these materials to CO2 conversion reactions.

Generation of positively and negatively charged molecular ions during sputtering of a copper target by low-energy Ar+ ion bombardment in a dilute argon/oxygen gas mixture

Generation of positively and negatively charged ions during 500 eV Ar+ ion bombardment of a copper surface in a dilute argon/oxygen atmosphere was investigated by means of energy-resolved mass spectrometry. Ar+, Ar2+, Ar2+, , and ions are produced by gas phase reactions. Cun+, Cun− (n = 1−3), ArCu+, O+, and O− ions are produced by interaction with the oxygen-covered Cu surface. Interaction with reactive oxygen species gives rise to the formation of positively or negatively charged copper oxide ions.

Influence of Carbon and Oxygen Chemical Potentials on the Hydrogen Donor Identity During Methanation on Ni, Co, and Ni‐Co Clusters

AbstractRate measurements in the kinetically controlled regime and equilibrium carbon and oxygen chemical titrations show two distinct mechanistic paths during COx methanation reactions on first‐row transition metal clusters. On Ni and, for a limiting set of conditions, Ni−Co clusters, the reaction occurs via the addition of a hydrogen adatom into CH3* intermediates on clusters partially covered with carbon. On Co and, in a subset of conditions, Ni−Co clusters, it occurs via the donation of hydrogen from OH* to CH3* on clusters partially covered with reactive oxygen adatoms (O*). The [CO]2‐to‐[CO2] and [CO2]‐to‐[CO] operating ratios are the surrogates of carbon and oxygen chemical potentials, respectively, as a consequence of water‐gas shift equilibration. These ratios, together with the carbon and oxygen binding energies, determine the relative surface coverages of carbon and oxygen, the involvement of H* vs. OH* in the kinetically‐relevant step, and in turn, the rate dependencies. Stronger carbon and oxygen binding energies lead to more stabilized transition states of the kinetically relevant steps and larger methanation rates.

Crossing the great divide between single-crystal reactivity and actual catalyst selectivity with pressure transients

The quantitative prediction of catalyst selectivity is essential to the design of efficient catalytic processes and requires a detailed knowledge of the reaction mechanism and rate constants. Here we present a study that accurately predicts, using the kinetics and a mechanism derived from fundamental studies on single-crystal gold, the product distribution resulting from the complex reaction network that governs the oxidative coupling of methanol, catalysed by nanoporous gold between 360 and 425 K and for a vast range of pressures. Analysis of the transient product responses to micropulses of methanol over nanoporous gold yields a precise understanding of the marked dependence of selectivity on pressure, surface oxygen coverage and temperature. The key to a high selectivity for methyl formate is the surface lifetime and abundance of the methoxy. This successful microkinetic modelling of catalytic reactions across a wide set of reaction conditions is broadly applicable to predicting catalytic selectivity and provides a pathway to designing more efficient catalytic processes. Catalytic studies on single crystals are very insightful, but it is often difficult to extend their conclusions to an actual catalytic process due to gaps in the experimental conditions. Now, Madix and co-workers report a method to bridge these gaps using the oxidative coupling of methanol on gold as an example.

A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract.

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

Selective Activation of Methyl C–H Bonds of Toluene by Oxygen on Metallic Gold

In accord with its gas phase acidity, toluene is activated by atomic oxygen adsorbed on Au(111) at the methyl C–H bonds. Titration of this species from the surface with HCOOD yields C6H5CHD2, indicating that much of the toluene has been doubly deprotonated at the methyl carbon. Even at low surface coverages of atomic oxygen the thusly-activated species yields only CO, CO2 and water as gas phase products when heated above room temperature, indicating the facile formation of an adsorbed benzoate, not simple oxygen insertion into the carbene to form benzyl benzyloxy. At higher O pre-coverages benzoic acid forms between 400 and 500 K.

The Role of Oxides in Catalytic CO Oxidation over Rhodium and Palladium

Catalytic CO oxidation is a seemingly simple reaction between CO and O2 molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxide films are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures.

Atmospheric interaction with nanosatellites from observed orbital decay

Nanosatellites have gained considerable presence in low Earth orbits wherein the atmospheric interaction with exposed surfaces plays a fundamental role in the evolution of motion. These aspects become relevant with the increasing applicability of nanosatellites to a broader range of missions objectives. This investigation sets out to determine distinctive drag coefficient development and attributes of atmospheric gas-surface interactions in nanosatellites in the common form of standard 3U CubeSats from observed orbital decay. As orbital decay can be measured with relative accuracy, and its mechanism broken down into its constituent sources, the value of drag-related coefficients can be inferred by fitting modelled orbit predictions to observed data wherein the coefficient of interest is the adjusted parameter. The analysis uses the data of ten historical missions with documented passive attitude stabilisation strategies to reduce uncertainties. Findings indicate that it is possible to estimate fitted drag coefficients in CubeSats with physical representativeness. Assessment of atomic oxygen surface coverage derived from the fitted drag coefficients is broadly consistent with theoretical trends. The proposed methodology opens the possibility to assess atmospheric interaction characteristics by using the unprecedented opportunity arising from the numerous observed orbital decay of nanosatellites.

N‐butane partial oxidation in a fixed bed: A resolved particle computational fluid dynamics simulation

AbstractMaleic anhydride (MA) is an important chemical intermediate, which is mainly produced by the highly exothermic partial oxidation of n‐butane in fixed bed reactors. Production of maleic anhydride is limited by problems of temperature control and loss of selectivity. Here we present three‐dimensional resolved‐particle computational fluid dynamics (CFD) simulations of n‐butane partial oxidation in a randomly packed bed of 807 spherical catalyst particles. We used a semi‐empirical two‐site surface mechanism from the literature and obtained distributions of temperature, gas phase species, and surface species in the bed. The local selectivity patterns are strongly controlled by the changes in the gas phase flow field, the temperature in the fixed bed, and the weakly adsorbed oxygen surface coverage fraction (λO) profiles inside the particles. Most of the selectivity loss due to product combustion happens in the interior of the catalyst particles, as opposed to the production of MA, which is mainly in the near‐surface layers. We observed loss of selectivity and yield in the centre of the bed at increased bed depths, because of the temperature increase and decrease in gas phase oxygen there. The simulations show sharp λO gradients inside the particles. Near the cooled tube wall λO is strongly affected by the temperature, while at the bed centre gas phase oxygen diffusion limitations contribute to a depleted oxygen surface resulting in lower selectivity.