Alumina supported model Pd-Ag catalysts: A combined STM, XPS, TPD and IRAS study

The bimetallic Pd-Ag model catalysts were prepared by physical vapor deposition on thin alumina films. The morphology and structure of the Pd-Ag particles were studied by STM, XPS, and by TPD and IRAS of CO. The results showed the formation of true alloy particles with Ag segregated at the surface. The addition of Ag first suppresses the most strongly bonded CO on threefold hollow sites of Pd. With further increasing Ag coverage, only isolated Pd atoms surrounded by Ag atoms are likely present on the surface. The results on CO adsorption suggest that the model Pd-Ag system mimics the structure of the real Pd-Ag catalysts.     

The influence of nitrogen vacancies on the magnetic and electronic properties of ruthenium mononitride: First-principles study

We present FLAPW-GGA calculations of the magnetic and electronic properties of cubic RuN_x (x=1.0, 0.75 and 0.50). We find that RuN exhibits a ferromagnetic ground state and the local ruthenium magnetic moment abruptly decreases with nitrogen vacancies. The relative positions of t_2g and e_g states and the Ru 4d–N 2p hybridization in RuN_x are discussed. We have also found that the magnetic behavior for RuN_x vs. FeN_x as dependent on the nitrogen vacancies content displays an opposite trend.     

Adsorption characteristics of atomic nitrogen on ruthenium surfaces

We have studied the adsorption, vibration, and diffusion of N atoms on Ru(0 0 0 1), , and surfaces by means of the 5-parameter Morse potential (5-MP) of interaction between atomic nitrogen and a metal surface. The adsorption sites, adsorption geometry, binding energy and eigenvibration of atomic nitrogen on the different ruthenium surfaces are calculated. It is shown that atomic nitrogen always preferably occupies the high coordination sites on Ru surfaces. The 4-fold site is the preferable adsorption site for atomic nitrogen on both open and surfaces while 3-fold site is the most stable adsorption site for atomic nitrogen on both Ru(0 0 0 1) and surfaces. Moreover, we find the lowest energy pathway of diffusion and diffusion barriers of atomic nitrogen on the surfaces.     

Electronic state of ruthenium deposited onto oxide supports: An XPS study taking into account the final state effects

The electronic state of ruthenium in the supported Ru/EO_x (EO_x = MgO, Al₂O₃ or SiO₂) catalysts prepared by with the use of Ru(OH)Cl₃ or Ru(acac)₃ (acac = acetylacetonate) and reduced with H₂ at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d_5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EO_x samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d_5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5-279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al₂O₃ and SiO₂ (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EO_x systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid-base properties of the support, the mean size of supported Ru crystallites (within the range of 2-10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.     

气相钌原子与氮分子作用机理计算研究

运用B3LYP/6-311+G*(LANL2DZ)详细地研究了Ru原子与N2分子相互作用的单端位直线型L-RuN2和单侧双配位S-RuN2及插入化合物NRuN弯曲型和直线型的不同自旋多重度下多个电子态的平衡几何结构、电子结构、轨道布居和振动光谱等性质,同时对构型转化和插入反应过渡态及反应的势能曲线进行了计算. 结果表明两种构型的复合物中一般低能态Ru对N2的活化都不大,直线型的3和3-相对基态反应物是能稳定存在的,轨道作用机制是授予与反馈. 化合物NRuN与实验对照,13B1和11A1态可能是实验所观察的,两种构型中众多电子态相对于Ru(a5F)+2N(4S)是稳定的. 势能曲线体现插入反应能垒很高,生成NRuN化合物在热力学和动力学上都是不利的.     

气相钌原子与氮分子作用机理计算研究

运用B3LYP/6-311+G*(LANL2DZ)详细地研究了Ru原子与N_2分子相互作用的单端位直线型L-Ru N_2和单侧双配位S-Ru N_2及插入化合物NRu N弯曲型和直线型的不同自旋多重度下多个电子态的平衡几何结构、电子结构、轨道布居和振动光谱等性质,同时对构型转化和插入反应过渡态及反应的势能曲线进行了计算.结果表明两种构型的复合物中一般低能态Ru对N_2的活化都不大,直线型的3Δ和3∑-相对基态反应物是能稳定存在的,轨道作用机制是σ/π授予与反馈.化合物NRu N与实验对照,13B1和11A1态可能是实验所观察的,两种构型中众多电子态相对于Ru(a~5F)+2N(~4S)是稳定的.势能曲线体现插入反应能垒很高,生成NRu N化合物在热力学和动力学上都是不利的.     

Influence of ultrasound on the preparation of ruthenium catalysts supported on alumina

Ultrasound waves are commonly used to clean or to erode the surface of solid materials thanks to the strength of the jets of solvent, hitting the surface itself, created by the collapse of the liquid phase. In the present work, ultrasound was used to obtain highly dispersed ruthenium on alumina catalysts. The samples were characterized with different techniques to study the influence of the sonication on both the metal crystallites and the support.     

Steady state models of stable isotopic distributions

This study reviews common calculations and mathematical models used in stable isotopic studies. Some approximations are adopted to simplify the algebra for use in steady state models, with more rigorous mathematics outlined in an Appendix. Review shows that steady-state isotopic models are easy to construct, provide good approximations of system behavior, and are very helpful in evaluating isotopic cycling in many kinds of systems.     

Steady state models of stable isotopic distributions

This study reviews common calculations and mathematical models used in stable isotopic studies. Some approximations are adopted to simplify the algebra for use in steady state models, with more rigorous mathematics outlined in an Appendix. Review shows that steady-state isotopic models are easy to construct, provide good approximations of system behavior, and are very helpful in evaluating isotopic cycling in many kinds of systems.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

Analysis of computed TPD curves involving a precursor state: Influence of the method on the parameters of the adsorbate

We have shown that, ignoring the existence of a precursor state, in the analysis of computed temperature programmed desorption curves, assuming an integral order for the desorption rate, leads to large errors in derived values of the heat of desorption and frequency factor. It may explain the very low values of frequency factors and the “compensation law” encountered in the literature. A method already known, based on variations of desorption rate with the heating rate at constant coverage, appears to be much better and gives the heat of desorption and its variations with coverage with reasonable accuracy. A careful check of the adsorption curves helps not only in determining the existence of the precursor state, but also in determining its properties.     

TPD and ITPD study of materials used as chemoresistive gas sensors

Temperature-programmed desorption (TPD) and a differential form of it, called intermittent temperature-programmed desorption (ITPD), turned out to be powerful characterising techniques for chemoresistive materials applied to gas sensing. We investigated samples of SnO₂, TiO₂ and solid solutions of them (Ti_xSn_1 ? xO₂). TPD and ITPD experiments were carried out in vacuum, with samples previously treated in pure O₂ (100 Torr, 500 °C, 30 min). Amounts of desorbed O₂ corresponded for all Ti-containing samples to less than 10% of a compact monolayer of ions O^2?. Corresponding values of the apparent activation energy of desorption (E_app) were calculated directly from the Arrhenius plots for each partial TPD and ranged from about 100 to 330 kJ mol^? 1 (1.16 to 3.82 eV).     

Adsorbed state and vibrational excitation of N2 on Pd(110)

High-resolution vibrational electron energy loss spectroscopy, low-energy electron diffraction and Auger electron spectroscopy have been used to study the interactions of nitrogen with the Pd(110) surface. At 120 K, N₂ is chemisorbed molecularly on the Pd(110) surface, and the (2 × 1)-N₂ structure is formed. Most probably, the N₂ molecules are chemisorbed in the on-top sites of the bulk-like Pd(110) surface in the upright-linear structure. The Pd---N₂ bond energy is estimated to be ˜ 6 kcal/mol. The Pd---N₂ and N---N stretching vibrations of N₂ admolecules on Pd(110) are observed at 30 and 278 meV, respectively. The primary-energy dependence and angle dependence of their excitation cross sections agree reasonably well with the prediction of the dipole theory. The electron beam-induced effects are briefly discussed.     

Adsorption and desorption of ammonia,hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH₃ molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ? 2 × 10^?4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.     

A TPD and IR study of co-adsorption of NO and oxygen on Ru(001)

The adsorption of NO on Ru(001), and its co-adsorption with oxygen, has been investigated using temperature programmed desorption and infra-red reflection-absorption spectroscopy (IRAS). Only one band due to the NO stretch of adsorbed NO could be observed and was assigned to the linearly bound v₂ species. Pre-adsorption of oxygen caused surprisingly little effect on the total desorbed flux of NO and its dissociation products in the TPD spectrum. The main effect of oxygen was to shift intensity from the v₁ peak to a new desorption peak v₁(O), which is accompanied by a splitting in the IRAS spectrum of the v₂ species. The absence of the band observed in the “bridging region” in EELS experiments and the small influence of the pre-adsorbed oxygen on the v₂ species is discussed in the light of current models for this adsorption system.     

The characterisation of model Cu/Ru(001) bimetallic catalysts by static SIMS with XPS and TPD

The deposition of Cu onto Ru(001) at 540 and 1080 K has been studied by SSIMS supplemented by XPS and TPD. The SSIMS mass spectrum of the Cu/Ru surfaces showed the presence of Cu⁺ and Ru⁺ ions as well as the cluster ions RuCu⁺, Ru⁺₂, Cu⁺₂ and lower intensities of Cu⁺₃, RuCu⁺₂ and Ru₂Cu⁺. Copper coverage was independently monitored by XPS and TPD. It was shown that Cu coverage in the sub-monolayer region was directly proportional to $\text{RuCu}{}^{\mathrm{+}}\text{Ru}{}^{\mathrm{+}}$. Secondary ion emission was strongly influenced by work function changes consequent on Cu adsorption.Studies of the variation of the elemental and cluster ion intensities as a function of Cu coverage, at the two temperatures, has provided detailed information on the dispersion of Cu in the sub-monolayer region, and on the physical form of the ad-layer above a monolayer coverage.     

Adsorption and isotopic exchange of nitrogen on polycrystalline iron films between 273 and 600 K

The adsorption of nitrogen on evaporated iron films under uhv conditions has been studied between 273 and 600 K by means of direct volumetric and pressure measurements. The dissociative adsorption occurs with sticking probabilities in the range of 10^?8 to 10^?7 and apparent activation energies between ?1 and 5 kJ mol^?1 depending on the structure of the film. A mechanism for the dissociative adsorption of nitrogen is proposed taking into account molecularly adsorbed nitrogen as a precursor. After the adsorption of nitrogen enriched in ¹⁵N₂ the establishing of the iso topic equilibrium was observed using isothermal desorption.     

Real-space study of the growth of magnesium on ruthenium

The growth of magnesium on ruthenium has been studied by low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). In LEEM, a layer-by-layer growth is observed except in the first monolayer, where the completion of the first layer in inferred by a clear peak in electron reflectivity. Desorption from the films is readily observable at 400 K. Real-space STM and low-energy electron diffraction confirm that sub-monolayer coverage presents a moiré pattern with a 12 Å periodicity, which evolves with further Mg deposition by compressing the Mg layer to a 22 Å periodicity. Layer-by-layer growth is followed in LEEM up to 10 ML. On films several ML thick a substantial density of stacking faults are observed by dark-field imaging on large terraces of the substrate, while screw dislocations appear in the stepped areas. The latter are suggested to result from the mismatch in heights of the Mg and Ru steps. Quantum size effect oscillations in the reflected LEEM intensity are observed as a function of thickness, indicating an abrupt Mg/Ru interface.