Characterization of molybdenum carbide nanoparticles formed on Au(111) using reactive-layer assisted deposition

Temperature programmed desorption (TPD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) have been used to characterize molybdenum carbide nanoparticles prepared on a Au(111) substrate. The MoC(x) nanoparticles were formed by Mo metal deposition onto a reactive multilayer of ethylene, which was physisorbed on a Au(111) substrate at low temperatures (<100 K). The resulting clusters have an average diameter of approximately 1.5 nm and aggregate in the fcc troughs located on either side of the elbows of the reconstructed Au(111) surface. Core level XPS shows that the electronic environment of the Mo and C atoms in the nanoparticles is similar to that found in Mo(2)C(0001) single crystals and carburized Mo metal surfaces. Peak intensities in XPS and AES spectra were used to estimate an average Mo/C atomic ratio of 1.2 +/- 0.3 for nanoparticles annealed above 600 K.     

Epitaxial growth of single-crystalline Al(2)O(3) films on Cr(2)O(3)(0001)

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.     

Density-functional study of oxygen adsorption on Mo(112)

Atomic oxygen adsorption on the Mo(112) surface has been investigated by means of first-principles total energy calculations. Among the variety of possible adsorption sites it was found that the bridge sites between two Mo atoms of the topmost row are favored for O adsorption at low and medium coverages. At about one monolayer coverage oxygen atoms prefer to adsorb in a quasithreefold hollow sites coordinated by two first-layer Mo atoms and one second layer atom. The stability of a structural model for an oxygen-induced p(2 x 3) reconstruction of the missing-row type is examined.     

Oxygen, carbon, and sulfur segregation in annealed and unannealed zerovalent iron substrates

Finely ground and pretreated iron substrates known as "zerovalent iron" or "Fe0" are used as reductants in the environmental remediation of halogenated hydrocarbons, and the composition of their surfaces significantly affects their reactivity. Samples of unannealed and annealed (heat-treated under H2/N2) zerovalent iron were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Surface concentration of the iron and of the impurities observed by XPS and AES, carbon, chlorine, sulfur, and oxygen, were measured before and after soaking in trichloroethylene (TCE) and in water saturated with TCE (H2O/TCE) to simulate chlorocarbon remediation conditions. Samples pretreated by annealing at high temperature under H2 contained less iron carbide. The carbide contaminant was evident in both iron and carbon XPS spectra, with binding energies of 709.0 and 283.3 eV for the Fe 2p3/2 and C 1s, respectively. The annealed Fe0 surface also contained more sulfur. The carbide concentration was essentially unchanged by TCE and H2O/TCE exposure, whereas the sulfur decreased in proportion to chlorine adsorption following the dechlorination reaction. While oxygen concentration is initially lower on the annealed substrate surface, it rapidly increased during the model TCE remediative treatment process and thus does not represent a significant effect of the annealing process on surface reactivity.     

Carbon-nitrogen place exchange on NO exposed beta-Mo2C

Atomic nitrogen and oxygen were deposited on beta-Mo(2)C through dissociative adsorption of NO. Reflectance absorbance infrared spectroscopy (RAIRS), thermal desorption, and synchrotron X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the interplay between atomic nitrogen, carbon, and oxygen in the 400-1250 K region. The combination of the high resolution and high surface sensitivity offered by the synchrotron XPS technique was used to show that atomic nitrogen displaces interstitial carbon onto the carbide surface. Thermal desorption measurements show that the burnoff of the displaced carbon occurs at approximately 890 K. The incorporation of nitrogen into interstitial sites inhibits oxygen dissolution into the bulk. RAIRS spectroscopy was used to identify surface oxo, terminal oxygen, species formed from O(2) and NO on beta-Mo(2)C.     

The efficient synthesis of a molybdenum carbide catalyst via H2-thermal treatment of a Mo(VI)-hexamethylenetetramine complex

An efficient method for preparation of Mo(2)C catalyst is described, where Mo(2)C is obtained by the heat treatment of a single solid precursor containing (NH(4))(6)Mo(7)O(24) and hexamethylenetetramine (HMT) at 923 K in H(2) flow without conventional prolonged carbonization. The catalysts are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface area measurement, and transmission electron microscopy (TEM). Furthermore, these catalysts are evaluated in the dibenzothiophene (DBT) hydrodesulfurization (HDS) reaction, and proved to be superior to those prepared by a temperature-programmed reduction (TPRe) method. The better catalytic performance is ascribed to higher dispersion of Mo(2)C on the support and a lower surface polymeric carbon content. This hydrogen thermal treatment (HTT) method provided a new strategy for the preparation of a highly active molybdenum carbide catalyst.     

Distribution and thickness of the surface contaminations on STM tungsten tips,studied by AES/SEM and ARXPS

The combination of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and angle resolved X-ray photoelectron spectroscopy (ARXPS) has been applied to the analysis of the distribution of elements at the surface region of electrochemically etched tungsten tips and the determination of the thickness of a layer with oxygen and carbon contamination. Auger line profiling revealed a homogeneous distribution of oxygen and significant enrichment of carbon on the W tip between 0 and 1.5 m from the top. The thickness of the contamination layer on various W materials, electrochemically etched, was found to be 1.35±0.15 nm as measured using ARXPS, and was estimated to be about 1–3 nm as measured by AES.     

Low-temperature reduction of NO(2) on oxidized Mo(110)

The reactions of nitrogen dioxide (NO(2)) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO(2) reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N(2), and trace amounts of N(2)O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO(2) to yield N(a) and O(a). N(2) forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO(2) reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO(2). The selectivity for N(2) formation over N(2)O is greater for NO(2) as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO(2) reaction than by O(2) reaction, indicating that NO(2) is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N(2)O(4), NO(2), and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N(2)O(4) desorbs by 200 K, leaving only NO(2) and NO on the surface. Infrared spectroscopy of NO(2) on (18)O-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.     

固相配位化学反应研究LXVII．金属铜表面M－S（M＝Mo，W）簇合物膜的组成

采用ＦＴＩＲ、ＸＰＳ和ＡＥＳ研究了金属铜表面Ｍ－Ｓ（Ｍ＝Ｍｏ，Ｗ）簇合物膜。结果表明，Ｍｏ（Ｗ）与铜表面的Ｃｕ＿２Ｏ反应，形成了Ｍｏ（Ｗ）－Ｓ－Ｃｕ键；簇合物膜由Ｍｏ（Ｗ）、Ｓ、Ｃｕ、Ｏ元素组成，分别呈＋６、－２、＋１、－２价，膜为多分子层结构并保持ＭｏＳ＿４，或ＷＳ＿２单元，膜表面只有Ｃｕ、Ｏ而不存在Ｍｏ（Ｗ）、Ｓ．膜层厚度与反应时间有关，时间越长，膜越厚。膜为多组分的复杂体系，其颜色是各组分统计分布的结果。     

A simple synthesis route and characterisation of Co(3)Mo(3)C

A simple, one-step thermal decomposition method for the preparation of Co(3)Mo(3)C is reported in this paper. In this novel synthesis route, a mixed-salt precursor, containing Co(CH(3)COO)(2) x 4H(2)O, (HMT)(2)(NH(4))(4)Mo(7)O(24) x 2H(2)O (HMT = hexamethylenetetramine), and excess HMT is directly decomposed to the bimetallic carbide under flowing argon at 1023 K. The role of HMT in the preparation process has been investigated and a detailed reaction mechanism is proposed based on the experimental results. The bimetallic carbide is characterised by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, BET surface area measurement and X-ray photoelectron spectroscopy. Furthermore, the activity of the as-prepared Co(3)Mo(3)C is evaluated by a 3-methylpyridine hydrodenitrogenation (HDN) reaction. The catalyst produced from this method provides better reactivity compared to the Co(3)Mo(3)C catalyst prepared by the conventional temperature-programmed reduction method.     

Surface characterisation of laser irradiated SiC ceramics by AES and XPS

Samples of sintered silicon carbide (SSiC) were irradiated with a KrF excimer laser (λ = 248 nm) at energy densities of 10, 15 and 25 J/cm² in He atmosphere. The composition of the near surface region was investigated by Auger electron spectroscopy (AES) and photoelectron spectroscopy (XPS) after lapping, laser irradiation and tribological treatment, respectively. By laser irradiation a surface layer is formed which contains about 30% oxygen. The existence of different bonding states of Si, C and O was established by factor analysis of the AES depth profiles and by XPS. By laser irradiation SiC is decomposed and a siliconoxycarbide with the average composition SiC_3.5O_1.5 is formed. Beneath the oxidised surface layer the nominal elemental composition SiC is found but the sample represents a mixture of Si, graphite and siliconoxycarbide with a small amount of SiC only. Obviously, the decomposition zone exceeds in a depth > 300 nm.     

Structure and properties of Ni(110) surface coadsorbing chlorine and oxygen

The coadsorption of chlorine with oxygen on Ni(110) surface has been investigated by XPS, UPS, AES and work function measurements. The chlorine preadsorption drastically inhibits the further uptake of oxygen. On the contrary, precovered oxygen has hardly any influence on the additional adsorption of chlorine due to the incorporation of precovered oxygen into the subsurface driven by the chlorine coadsorption. ARXPS measurements provide the evidence for this coadsorption model. The thermal desorption of chlorine and oxygen from the coadsorption surface is very similar to that of both individual adsorbates under the same heating conditions, but the desorption temperature of both the adsorbates apparently decreases on the coadsorption surface. The coadsorption and thermodesorption mechanisms are also discussed in detail.     

Interactions of O(2) with Pd nanoparticles on alpha-Al(2)O(3)(0001) at low and high O(2) pressures

The interaction of O(2) with small Pd particles (2-10 nm) supported on an alpha-Al(2)O(3)(0001) single crystal under both ultrahigh vacuum (UHV) and high-pressure conditions has been studied by temperature-programmed desorption (TPD), temperature-programmed low-energy ion scattering (TP-LEIS), and X-ray photoelectron spectroscopy (XPS). A low O(2) exposure (30 L) at 500 K leads to surface oxygen adatoms on the Pd nanoparticles, which desorb in TPD as O(2) in a peak at approximately 880 K. Surface O adatoms on the smallest Pd particles move to subsurface sites starting at 400 K, and they almost all move subsurface by approximately 750 K, desorbing mainly at considerably higher temperature. The dominant oxygen species above 700 K is subsurface, implying that it is more stable than oxygen adatoms on Pd. Exposures of the Pd nanoparticles to 25 Torr O(2) at 373-473 K readily convert the Pd to a species whose Pd XPS peak shifts by the same amount as the binding energy difference between bulk Pd and bulk PdO. We attribute this to PdO nanoparticles (or a thin film of PdO on or under the Pd for the larger particles). The decomposition of the PdO on these nanoparticles to Pd in an equilibrium O(2) pressure of 10-7 Torr does not occur until approximately 750 K, or approximately 200 K higher than the equilibrium decomposition of bulk PdO. This is attributed to the higher energy of Pd nanoparticles compared to bulk Pd and, for the larger particles, to the adhesion energy of the PdO film to the Pd, both of which stabilize the PdO on these Pd nanoparticles relative to bulk PdO. This PdO-like film on the larger particles may be similar to the ordered oxide thin film previously reported to form on Pd(111) but may also reside at the alpha-Al(2)O(3) interface and be partially stabilized by adhesion to this interface.     

Identification of subsurface oxygen species created during oxidation of Ru(0001)

The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d(5/2) core levels of the two topmost Ru layers. The evolution of the Ru 3d(5/2) spectra with oxygen exposure at 475 K and the corresponding O2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d(5/2) images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.     

Catalytic functions of Mo/Ni/MgO in the synthesis of thin carbon nanotubes

The functions and structures of Mo/Ni/MgO catalysts in the synthesis of carbon nanotubes (CNTs) have been investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Thin 2-5-walled CNTs with high purities (over 90%) have been successfully synthesized by catalytic decomposition of CH(4) over Mo/Ni/MgO catalysts at 1073 K. It has been found that the yield of CNTs as well as the outer diameter or thickness correlates well with the contents of these three elements. The three components Mo, Ni, and MgO are all necessary to synthesize the thin CNTs at high yields since no catalytic activity was observed for CNT synthesis when one of these components was not present. The outer diameter of the CNTs increases from 4 to 13 nm and the thickness of graphene layers also increases with increasing Mo content at a fixed Ni content, while the inner diameter stays at 2-3 nm regardless of their contents. Furthermore, the average outer diameter is in good agreement with the average particle size of metal catalyst. That is, the thickness or the outer diameter can be controlled by selecting the composition of the Mo/Ni/MgO catalysts. XRD analyses have shown that Mo and Ni form a Mo-Ni alloy before CNT synthesis, while the Mo-Ni alloy phase is separated into Mo carbide and Ni. These alloy particles are supported on MgO cubic particles 15-20 nm in width. It has been found that only small Mo-Ni alloy particles 2-16 nm in size catalyze CNT synthesis, with larger particles over 15 nm exhibiting no activity. Mo carbide and Ni should play different roles in the synthesis of the thin CNTs, in which Ni is responsible for the dissociation of CH(4) into carbon and Mo(2)C works as a carbon reservoir.     

Ba deposition and oxidation on theta-Al2O3/NiAl(100) ultrathin films. Part I: Anaerobic deposition conditions

Room-temperature Ba deposition on an oxygen-terminated theta-Al(2)O(3)/NiAl(100) ultrathin film substrate under ultrahigh vacuum (UHV) conditions is studied using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. In addition, Ba oxidation by the ions of the alumina substrate at 300 K < T < 1200 K in the absence of a gas-phase oxidizing agent is investigated. Our results indicate that at room temperature Ba grows in a layer-by-layer fashion for the first two layers, and Ba is partially oxidized. Annealing at T < 700 K results in further oxidation of the Ba species, whereas annealing at higher temperatures leads to loss of Ba from the surface via desorption and subsurface diffusion.     

Structure and properties of the Mo3Nb2O14 oxide

Mixed Nb-Mo oxides were prepared by solid-state reaction of Nb(V) and Mo(VI) oxides at 973-1123 K. Optimal conditions were determined for the formation of the Mo3Nb2O14 compound. As established by Rietveld refinement of the powder X-ray diffraction patterns, the Mo3Nb2O14 oxide has the tetragonal cell with a = 23.150(6) A and c = 3.998(4) A and a tunnel structure similar to that of the Mo5O14 oxide. The solids were characterized by several physical techniques, including scanning and transmission electron microscopy, FT-IR, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance spectroscopy. It has been shown that the Mo3Nb2O14 solid prepared in air at 973-1073 K after cooling to room temperature contains high amounts of Mo(V) species (ca. 1% of total molybdenum). The presence of paramagnetic species correlates with the intense green color of the solids and the strong d-d transition band in the UV-visible spectra, typical for the d1 species. The amount of paramagnetic species does not depend on the solid annealing and/or on the small variations of its composition. Neither is it related to the oxygen release upon the solid heating, being therefore an intrinsic property of the Mo3Nb2O14 oxide. The unusual stabilization of reduced Mo species in the highly oxidizing conditions was explained by the substitution disorder between Nb and Mo atoms. It is supposed that a configuration containing mu3 oxygen bonded to three Mo(VI) atoms is unstable and decomposes, leading to a Mo(V) center and a hole in the valence band.     

The growth of silver on an ordered alumina surface

The growth of Ag on an ordered Al2O3 surface was studied by low energy ion scattering spectroscopy (LEIS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Three-dimensional (3D) growth of Ag clusters was observed with STM and LEIS, with the cluster size increasing with Ag coverage. The XPS core level binding energies and the Auger parameters indicate a weak interaction between the Ag clusters and the Al2O3 support. Final state effects are determined to be the primary contribution to the Ag core level binding energy shift. Nonzero order kinetics was observed for Ag desorption in TPD with the Ag sublimation energy decreasing with decreasing cluster size.     

Oxidation of a depleted uranium-5 wt% molybdenum (U-5Mo) alloy in UHV by AES and XPS

Early stage oxidation of a dilute-depleted uranium-molybdenum alloy was analysed in situ under ultra-high vacuum conditions by AES and XPS. At the equivalent of less than 300 ns at 1-atm O₂, U-5Mo oxidizes to form stoichiometric UO₂. No molybdenum oxidation is observed. After an oxygen dose of approximately 39 L, the oxide layer approached a limiting thickness of approximately 2.4 nm. The oxidation kinetics followed a logarithmic rate law, with the best fit to the experimental data for the oxide thickness, d, being given by d = 1.26 log(0.12t + 0.56). Changes in oxygen KLL and 1s peak positions associated with transformation from chemisorbed oxygen to metal oxide were observed at similar oxygen doses of 2.3 and 2.6 L O₂ by AES and XPS, respectively, which opens up the possibility of using well-characterized XPS chemical information to inform Auger peak shifts.     

不锈钢上黑色钼盐转化膜的研究

用阴极电沉积法从钼酸盐和磷酸盐温和溶液中获得了黑色的不锈钢转化膜,该膜具有良好的热稳定性。电子能谱（ＸＰＳ和ＡＥＳ）分析表明,膜厚约为８２０ｎｍ,膜的表面铝以Ｍｏ（Ⅵ）存在,而在膜内则以Ｍｏ（Ⅵ）与Ｍｏ（Ⅳ）共存。从ＡＥＳ深度剥蚀曲线的组成恒定区求得膜的组成为：Ｏ５０．９％,Ｍｏ２９．４％,Ｐ１２．６％和Ｆｅ７．１％。循环伏安的氧化峰也证明膜内存在Ｍｏ（Ⅳ）。