Changes in surface composition of Fe-Si-Cr-K oxide alkylation catalyst: the marked role of potassium ferrites

The influence of gas-phase methylation of phenol on the state of the Fe-Si-Cr-K oxide catalyst surface was investigated by thermodesorption of K atoms and ions. Changes in potassium desorption energies, as determined from Arrhenius-like plots, varied from 3.07 eV to 1.21 eV for the atoms and from 2.59 eV to 2.89 eV for ion desorption from the active and deactivated samples, respectively. The results were discussed in terms of transformation of the catalyst surface and formation of β-ferrite.     

Atom desorption energies for sodium clusters

The desorption energies of supported sodium clusters have been determined as a function of cluster size. Na _n clusters were formed by surface diffusion of sodium atoms adsorbed from a thermal atomic beam on a LiF(100) single crystal. Measurements have been performed by temperature programmed thermal desorption. The signals reflect fractional order desorption kinetics. The average cluster size could be controlled by varying the total number of sodium atoms on the surface. It was determined from scattering experiments. We find that the binding energies vary between 0.55 and 0.8 eV and only approach a constant value for clusters with diameters as large as 1,000 Å.     

纳米二氧化钛气相光催化降解三氯乙烯

采用气相色谱－质谱联用方法,Ｘ射线光电子能谱和程序升温脱附方法研究了纳米二氧化钛表面三氯乙烯的气相光催化降解反应。检测到四种新的含三个碳原子的中间体,说明三氯乙烯在反应过程中发生了Ｃ＝Ｃ双键的裂解及加成反应。研究表明,水蒸气对降解反应的影响不公与水蒸气的浓度有关,还与催化剂对水的吸收能力有关。反应副产物在催化剂表面的积累是导致催化剂活性降低的主要原因。催化剂的Ｘ射线光电子能谱显示,反应后催化剂表面     

Dehydrogenation of isopropyl alcohol on modified cobalt catalyst

The effects of plasmochemical processing and of Ce, K, and Hf additives on the rate of dehydrogenation for isopropyl alcohol on a 5 wt % Co/SiO₂ catalyst is studied under static and flow conditions. Glow discharge plasma in O₂ and Ar and high-frequency electrodeless plasma in H₂ (HF-H₂) are used. Except for one sample containing Hf, an increase in catalytic activity is observed due to the formation of new active centers. The change in the composition of the initial catalyst’s surface after treatment with Ce and with oxygen, argon, and HF-H₂ plasmas is determined by means of X-ray photoelectron spectroscopy. The change in the size and shape of Co particles after treating the catalyst with HF-H₂ plasma and Ce is determined via X-ray phase analysis. It is suggested that the new catalytic centers formed after treatment in O₂ and Ar plasma contain carbon atoms with C1s bond energies of 282.1 eV; after treatment with HF-H₂ plasma, active centers contain hydrogen and carbon atoms with C1s bond energies of 282.5 eV; with cerium, the C1s bond energy is 297.7 eV.     

Kinetics of photo-induced dissociation of Na clusters deposited on Mica

Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE _kin of the photodesorbed atoms at the surface temperatureT _S =300 K was found to beE _kin=18±5 meV, independent of laser irradiance (3 µJ/cm²...20 mJ/cm²) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E _kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT _S =30 K andT _S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos²-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption.     

On the use of single-photon ionization for inorganic surface analysis

Summary A general surface analysis method has been developed based on non-selective photoionization of sputtered or desorbed neutral atoms and molecules above the surface, followed by time-of-flight mass spectrometry. The approach, currently utilizes two main types of ionizing radiation and a variety of desorption probes. For photoionization, coherent untuned sources are used; an intense focused pulsed UV laser beam is used for non-resonant multiphoton ionization to give elemental and limited chemical information, usually used for inorganic analysis; a coherent VUV source is used for single-photon ionization at 118 nm (10.5 eV) produced by frequency tripling of 355 nm light from a Nd:YAG laser. This paper focuses on single-photon ionization for inorganic systems. The desorption probes used are ion, electron, and laser beams as well as thermal desorption. For depth profiling, ion beams are specifically used. Any focused desorption probe beam can provide lateral spatial resolution.     

Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

Pinning mononuclear Au on the surface of titania

We have deposited Au atoms on the surface of titania without sintering or surface damage. Mass-selected Au+ atoms were deposited from the gas phase at room temperature with kinetic energies from <3 to 190+/-3.5 eV. Scanning tunneling microscopy reveals island formation following deposition at <3 eV, while mainly atomic features are observed for energies between approximately 35 and approximately 190 eV. A mixture of islands and atomic features is observed at a landing energy of 20+/-3.5 eV, suggesting a critical energy above which pinning occurs. Cluster size is also probed as a function of coverage in the deposition of Au+ with 100 eV of energy, revealing that sintering begins at a coverage of only 0.06 ML. These observations suggest a mechanism in which high-energy collision leads to the annealing of any impact-created surface damage and the pinning of Au atoms to the surface. We provide a new method of preparing isolated Au atoms on an oxide surface, which can serve as a platform for catalytic studies.     

Mechanism of the decomposition of the surface oxide film on polycrystalline palladium

The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O₂ desorption from the chemisorbed oxygen layer (O_ads) and the passage of oxygen inserted under the surface layer of the metal (O_abs) and oxygen dissolved in metal subsurface layers (O_dis) to the surface. O₂ desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent O_ads atoms (ε_aa). At ε_aa = 10 kJ/mol and when the activation energy of O₂ desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E_des⁰ = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of O_abs and O_dis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E₂ and E₃, respectively). As the oxide film is heated, O₂ desorption is accompanied by the passage of O_abs and then O_dis to the surface, which leads to an increase in the O_ads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between O_ads atoms, the bonds between O_ads and the surface weaken and E_des decreases. As a consequence, the O₂ desorption rate increases and a low-temperature peak with T_max ≈ 710 K, which is due to the passage of O_abs atoms to the surface, and then a high-temperature peak with T_max ≈ 770 K, which is due to the passage of O_dis atoms to the surface, appear in the TPD spectrum. At ε_aa = 10 kJ/mol, E_des⁰ = 230 kJ/mol, E₂ = 145 kJ/mol, and E₃ = 160 kJ/mol and when the number of inserted oxygen monolayers is θ_abs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θ_dis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O₂ TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.     

First-Principles Study of Pd Single-Atom Catalysis to Hydrogen Desorption Reactions on MgH2(110) Surface

MgH₂ is a promising and popular hydrogen storage material. In this work, the hydrogen desorption reactions of a single Pd atom adsorbed MgH₂(110) surface are investigated by using first-principles density functional theory calculations. We find that a single Pd atom adsorbed on the MgH₂(110) surface can signi cantly lower the energy barrier of the hydrogen desorption reactions from 1.802 eV for pure MgH₂(110) surface to 1.154 eV for Pd adsorbed MgH₂(110) surface, indicating a strong Pd single-atom catalytic effect on the hydrogen desorption reactions. Furthermore, the Pd single-atom catalysis significantly reduces the hydrogen desorption temperature from 573 K to 367 K, which makes the hydrogen desorption reactions occur more easily and quickly on the MgH₂(110) surface. We also discuss the microscopic process of the hydrogen desorption reactions through the reverse process of hydrogen spillover mechanism on the MgH₂(110) surface. This study shows that Pd/MgH₂ thin films can be used as good hydrogen storage materials in future experiments.     

The interaction of hyperthermal argon atoms with CO-covered Ru(0001): scattering and collision-induced desorption

Hyperthermal Ar atoms were scattered under grazing incidence (θ(i) = 60°) from a CO-saturated Ru(0001) surface held at 180 K. Collision-induced desorption involving the ejection of fast CO (～1 eV) occurs. The angularly resolved in-plane CO desorption distribution has a peak along the surface normal. However, the angular distribution varies with the fractional coverage of the surface. As the total CO coverage decreases, the instantaneous desorption maximum shifts to larger outgoing angles. The results are consistent with a CO desorption process that involves lateral interaction with neighboring molecules. Furthermore, the data indicate that the incident Ar cannot readily penetrate the saturated CO overlayer. Time-of-flight measurements of scattered Ar exhibit two components-fast and slow. The slow component is most evident when scattering from the fully covered surface. The ratio and origin of these components vary with the CO coverage.     

Adsorption of formic acid on Si(111)7 × 7 at room temperature: a valence band photoemission and Si2p photodesorption study

Formic acid is the simplest of the carboxylic acids and a model adsorption system for several surfaces. In spite of the simple structure, formic acid reactivity and photoreactivity may be quite complex. In this paper, a study is presented on the deuterated formic acid adsorption on Si(111)7 × 7 at room temperature. The study is performed both by valence band photoemission and by photon‐stimulated desorption as a function of time and of photon energy in the 90–120 eV range. A primarily adsorption on rest atoms is found. This is verified by monitoring rest atoms and adatom intensity as a function of formic acid exposure. Further checks were made to control that surface adatoms were still free to react after the adsorption of formic acid. The photon stimulated desorption produces 5 single positively charged fragments: D⁺, O⁺, OD⁺ CO⁺ and CDO⁺. Possible fragmentation mechanisms are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Single atom catalytic oxidation mechanism of formaldehyde on Al doped graphene at room temperature

Formaldehyde (HCHO) is one kind of common indoor toxic pollutant, the catalytic oxidation degradation of formaldehyde at room temperature is desired. In this work, a new single atomic catalyst (SAC), Al doped graphene, for the catalytic oxidation of HCHO molecules was proposed through density function theory (DFT) calculations. It is found that Al atoms can be adsorbed on graphene stably without aggression. Then HCHO can be effectively oxidized into CO₂ and H₂O in the presence of O₂ molecules on Al doped graphene with a low energy barrier of 0.82 eV and releasing energy of 2.29 eV with the pathway of HCHO → HCOOH → CO → CO₂. The oxidation reaction can happen promptly with reaction time τ = 56.9 s at the speed control step at room temperature. Therefore, this work proposed a high-performance catalyst Al-doped graphene without any noble metal for HCHO oxidation at ambient temperature, and corresponding oxidation pathway and mechanism are also deeply understood.     

Catalytic decomposition of azomethane and reactions of adsorbed methyl radicals on the molybdenum surface

The methods of temperature-programmed reaction/desorption (TPR/TPD) are used to study azomethane (CH³N=NCH³) decomposition and the reactions of the products of its pyrolysis (CH ₃ ^* radicals and N₂) on the polycrystalline molybdenum surface. A TPR spectrum of adsorbed azomethane decomposition shows mainly N₂, H₂, and unreacted azomethane. Upon preliminary adsorption of azomethane pyrolysis products on a catalyst sample, a TPR spectrum shows N₂, H₂, and CH₄ in comparable amounts. The difference in the composition of desorption products found for these two types of experiments shows that, in the decomposition of adsorbed azomethane, surface methyl moieties are not formed. The rate constants were calculated for the dissociation of adsorbed CH₃, CH₂, and CH, recombination of hydrogen atoms with each other and with CH₃ and CH₂, and the recombinative desorption of nitrogen atoms. Deceased.     

X-ray photoelectron spectroscopic characterization of the acetylene cyclotrimerization catalyst NbCl2(CnHn) (n = 10–12)

The elemental and ionic compositions of the surface of NbCl₂(C _n H _n ) (n = 10–12), an active catalyst for acetylene cyclotrimerization into benzene, have been determined by X-ray photoelectron spectroscopy. Binding energy data for the sample sputtered with argon ions E _b (Nb3d _5/2) = 203.8–204.2 eV) suggest that the oxidation state of niobium in the active catalyst is +2 or +3. The narrow C1s line indicates the equivalence of all carbon atoms, and the corresponding binding energy, E b(C1s) = 284.0 eV, is close to the BE value for cyclic unsaturated hydrocarbons with conjugate double bonds. Interacting with atmospheric oxygen and moisture during sample preparation, niobium ions on the catalyst surface oxidize to their highest oxidation state, +5, characterized by E _b (Nb3d _5/2) = 207.3–207.7 eV. These data suggest that niobium oxychlorides or oxides form ion the sample surface. The catalyst is stable in a high vacuum and undergoes slight charging under the action of an X-ray beam, showing poor dielectric properties.     

High-capacity hydrogen storage of magnesium-decorated boron fullerene

By theoretical analysis, we have explored the feasibility of functionalizing boron fullerene (B₈₀) by adsorbing Mg atoms for the application as hydrogen storage nanomaterials. Our results show that due to the charge transfer from Mg to B atoms Mg atoms reside above the pentagonal faces of the B₈₀ cage. The electric field induced around the positive charged Mg atoms polarizes H₂ molecules, and the resulting binding is strong enough to adsorb H₂ without dissociation. Further calculations indicated that the 12Mg-decorated-B₈₀ has a high hydrogen storage capacity storing up to 96 H₂ molecules with an ideal binding energy of 0.20 eV/H₂ according to the approximation of GGA and 0.5 eV/H₂ according to LDA, corresponding to a hydrogen uptake of 14.2%. This suggested a possible method of engineering new structure for high-capacity hydrogen storage materials with the reversible adsorption and desorption of hydrogen molecules.     

The effect of the disperse composition of a titanium catalyst on the kinetic heterogeneity of isoprene polymerization centers

The kinetic heterogeneity of centers of isoprene polymerization on fractions of titanium catalyst particles is studied. It is found that the isoprene polymerization with a catalyst consisted of particles 0.03–0.14 μm in diameter involves centers of one type with low reactivity. On catalyst particles 0.15–4.50 μm in diameter, the active centers of polymerization of two types with high reactivity may be formed. The addition of modifiers, a reduced temperature of catalyst formation, and the hydrodynamic effect result in the appearance of a narrow fraction of particles 0.15–0.18 μm in diameter with one type of surface active center that generates high-molecular-mass cis-1,4-polyisoprene. The obtained results are in accordance with the concept of particles 0.15–4.50 μm in diameter as aggregates of the elementary crystallites of β-TiCl₃ connected via additional Al-Cl bonds to surface titanium atoms. At the same time, catalyst particles 0.03–0.14 μm in diameter are formed by the minimum number of elementary crystallites, where titanium atoms are bound to a smaller number of chlorine atoms.     

A small paramagnetic platinum cluster in an NaY zeolite: characterization and hydrogen adsorption and desorption

A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt13Hm, possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m=30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.     

Oxidation of the polycrystalline gold foil surface and XPS study of oxygen states in oxide layers

Gold oxide films obtained on the surface of polycrystalline gold foil upon oxidation by oxygen activated by a high-frequency discharge have been studied by X-ray photoelectron spectroscopy. High-frequency O₂ activation affords oxide films more than 3–5 nm thick. As follows from Au4f spectra, the surface gold atoms are oxidized to the oxidation state +3. The O1s spectra have a composite shape and are decomposed into four components that characterize nonequivalent states of oxygen in the resulting oxide films. It is assumed that the two major oxygen states (E _b(O1s) = 529.0 and 530.0 eV) correspond to the oxygen atoms in two-and three-dimensional gold oxide Au₂O₃, respectively. The oxygen states characterized by the higher binding energies (E _b(O1s) = 531.8 and 535.2 eV) likely correspond to molecular oxygen in peroxide and superoxide groups, respectively.     

A density-functional-theory study of atomic nitrogen abstraction from Si(100)-(2 x 1) by a gaseous O(3P) atom

We have employed density-functional theory (DFT) to investigate the abstraction of a nitrogen atom from the Si(100)-(2 x 1) surface by a gas-phase O(3P) atom for different initial bonding configurations of nitrogen at the surface. For the N-Si(100) structures investigated, nitrogen abstraction by an O(3P) atom is predicted to be exothermic by at least 1.9 eV. Abstraction in a single elementary step is found only for the interaction of an O(3P) atom with nitrogen bound in a coordinatively saturated configuration, and an energy barrier of 0.20 eV is computed for this reaction. For nitrogen bound in coordinatively unsaturated configurations, abstraction is predicted to occur by precursor-mediated pathways in which the initial O-surface collision results in the formation of a N-O bond and the concomitant release of between 2.7 and 4.8 eV of energy into the surface, depending on the initial N-Si(100) structure. This initial step produces different surface structures containing an adsorbed NO species, which can then undergo a series of elementary steps leading to NO desorption. Since the barriers for these steps are found to be less than 1 eV in all cases, a significant excess of energy is available from initial N-O bond formation that could activate NO desorption within no more than a few vibrational periods after the initial gas-surface collision. Nitrogen abstraction by such a pathway is essentially an Eley-Rideal process since NO desorption occurs rapidly after the initial gas-surface collision, without the reactants thermally accommodating with the surface. These computational results indicate that nitrogen abstraction by gaseous O(3P) atoms should be facile, even at low surface temperatures, if nitrogen is bound to the Si(100) surface in coordinatively unsaturated configurations.