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Long-term stability test of Mo/HZSM-5-N catalysts (HZSM-5-N stands for nano-sized HZSM-5) in methane dehydroaromatization (MDA) reaction has been performed with periodic CH 4 -H 2 switch at 1033–1073 K for more than 1000 h. During this test, methane conversion ranges from 13% to 16%, and mean yield to aromatics (i.e. benzene and naphthalene) exceeds 10%. N2-physisorption, XRD, NMR and TPO measurements were performed for the used Mo/HZSM-5 catalysts and coke deposition, and the results revealed that the periodic hydrogenation can effectively suppress coke deposition by removing the inert aromatic-type coke, thus ensuring Mo/HZSM-5 partly maintained its activity even in the presence of large amount of coke deposition. The effect of zeolite particle size on the catalytic activity was also explored, and the results showed that the nano-sized zeolite with low diffusion resistance performed better. It is recognized that the size effect was enhanced by reaction time, and it became more remarkable in a long-term MDA reaction even at a low space velocity.

Oxide nanostructures grown on noble metal surfaces are often highly active in many reactions, in which the oxide/metal interfaces play an important role. In the present work, we studied the surface structures of FeO x -on-Pt and NiO x -on-Pt catalysts and their activity to CO oxidation reactions using both model catalysts and supported nanocatalysts. Although the active FeO1−x structure is stabilized on the Pt surface in a reductive reaction atmosphere, it is prone to change to an FeO2−x structure in oxidative reaction gases and becomes deactivated. In contrast, a NiO1−x surface structure supported on Pt is stable in both reductive and oxidative CO oxidation atmospheres. Consequently, CO oxidation over the NiO1−x -on-Pt catalyst is further enhanced in the CO oxidation atmosphere with an excess of O2. The present results demonstrate that the stability of the active oxide surface phases depends on the stabilization effect of the substrate surface and is also related to whether the oxide exhibits a variable oxidation state.

A two-dimensional (2D) Si film can form between a graphene overlayer and a Ru(0001) substrate through an intercalation process. At the graphene/2D-Si/Ru(0001) surface, the topmost graphene layer is decoupled from the Ru substrate and becomes quasi-freestanding. The interfacial Si layers show high stability due to the protection from the graphene cover. Surface science measurements indicate that the surface Si atoms can penetrate through the graphene lattice, and density functional theory calculations suggest a Si-C exchange mechanism facilitates the penetration of Si at mild temperatures. The new mechanism may be involved for other elements on graphene, if they can bond strongly with carbon. This finding opens a new route to form 2D interfacial layers between graphene and substrates.

The structural evolution of a Pt surface with 0.4 monolayer (ML) subsurface Fe on annealing in 1.1 × 10−7 kPa O2 was studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. When the annealing temperature was 600 K, only dissociative adsorption of O2 occurred, which induced the local restructuring of surface regions. At 750 K, subsurface Fe atoms segregated onto the surface and were oxidized. When the annealing temperature was increased to 850 K, well defined monolayer FeO islands with periodic arrays of defects were formed. The defects were located at the hexagonal closest packed sites (fcc) of FeO Moire unit cells and were either single oxygen vacancies or multiple oxygen vacancies consisting of six missing O atoms. The formation of periodic defects on monolayer FeO islands may be a way to construct active sites on Pt-Fe model catalysts.

Adsorption of carbon tetrachloride (CCl 4 ) on Si(1 1 1)-7 × 7 at room temperature (RT) and low temperature (LT) was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). It was demonstrated that at RT CCl 4 dissociates on the Si(1 1 1)-7 × 7 surface leaving the surface extensively adsorbed with atomic Cl species. The dissociated Cl shows site preference to Si restatoms resulting in quick extinction of dangling bonds at the Si restatoms. At LT (around 120 K), both molecular and dissociative adsorption of CCl 4 occurs, which produces Cl, CCl x ( x ⩽ 3), and CCl 4 on the surface. The dangling bonds at the restatoms and adatoms were simultaneously quenched upon the LT CCl 4 adsorption. The site selectivity of restatoms to adatoms for molecule adsorption on the Si(1 1 1)-7 × 7 surface is discussed.

Ultrathin silica films with different thicknesses have been grown on a Pd(100) surface by depositing silicon in the presence of O 2 . The film composition and electronic properties were characterized by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and high-resolution electron energy loss spectroscopy (HREELS). Scanning tunneling microscopy was applied to investigate the film morphology and lattice structure. The results show that the obtained films are atomically flat and highly ordered in a long range. UPS and HREELS measurements indicate that the silica film has the same electronic and vibrational properties as bulk silica. A 2.8 nm thick film exhibits low defects in the film and high thermal stability up to 800 K, as evidenced by ion scattering spectroscopy and XPS.

PtSn bimetallic catalyst nanoparticles were grown on highly oriented pyrolytic graphite (HOPG) surfaces using the sequential deposition and co-deposition of Pt and Sn. The surface morphology and composition of the prepared PtSn catalysts were investigated by scanning tunneling microscopy and CO adsorption experiment. It is found that the structure of the PtSn catalysts can be controlled by the deposition process. The deposition sequence of 1st Pt and 2nd Sn and co-deposition of Pt and Sn produce Sn-shell and Pt-core structure, while the deposition sequence of 1st Sn and 2nd Pt leads to the growth of individual Pt and Sn particles. The formation of various PtSn bimetallic structures was attributed to the different surface energy of Pt and Sn, and the interaction of the metals with the HOPG surfaces.

A thin alumina film, the alumina model surfaces modified by the metallic molybdenum and by the MoO(3) are titrated by CO chemisorption. Two types of CO adsorption sites, namely octahedrally and tetrahedrally coordinated Al(3+) Sites, are present on the thin alumina film. The thin alumina film prepared under UHV conditions can be used to simulate the conventional high-surface-area alumina supports in real catalysis. The deposited metallic molybdenum preferentially occupies octahedrally coordinated Al(3+) site and suppresses CO chemisorption on this site, and oxidation of the surface molybdenum species enhances this suppression. (c) 2007 Published by Elsevier B.V.

The growth of Ag clusters on Si(111)‐7×7 surfaces was studied by scanning tunneling microscopy (STM), ultraviolet photoelectron spectroscopy, and x-ray photoelectron spectroscopy (XPS). A shift in the Ag 3d binding energy and a noticeable change in the valence-band structure reveal a significant modification of the electronic states of the Si(111)‐7×7 surface and the dispersed Ag clusters, which had a strong dependence on the coverage of Ag. Furthermore, these Ag clusters deposited on the Si surface alter the behavior of oxygen adsorption on the Si(111)‐7×7 surface. As evidenced by XPS, the presence of Ag inhibits the adsorbed surface oxygen species, the ins-ins and ad-ins oxygen, in which “ad” denotes an O atom bonding on top of the Si adatom and “ins” denotes an O atom inserted into a Si adatom back bond. The STM and high-resolution electron energy loss spectroscopy results show that the ins-ins oxygen species are more significantly suppressed by the Ag clusters compared to the ad-ins oxygen.

Highly dispersed Pt nanoparticles have been confined in the mesochannels of SBA-15 silica by a simple “one-pot” co-assembly method. Metallic Pt can be encapsulated in the mesochannels by changing the amount of Pt ions in the starting materials. The catalytic performances of Pt–SBA-15 were evaluated for methylcyclohexane dehydrogenation. Compared with conventional supported Pt–SiO2 catalysts, the Pt–SBA-15 catalysts show higher stability. TEM studies show that the confinement effect of ordered mesochannels of SBA-15 may restrict the further growth of Pt nanoparticles during the reaction.

Molybdenum deposition on the thin alumina film was investigated by HREELS, XPS and UPS. HREELS and XPS results show that a strong interaction occurs between the molybdenum and the oxide, and the electron transfers from the metallic nanoparticles to the underlying substrate. UPS measurements demonstrate that the Mo 4d valence band edge of the metallic nanoparticles locates at 1.36 eV, far below the Fermi level of the bulk molybdenum. This unique electronic structure of the metallic nanoparticles may provide a totally different property from the corresponding bulk materials, indicating a significant nanometer size effect.

The Pt(1 1 0) model surfaces modified by the metallic molybdenum and by the MoOx (molybdenum oxide species) were fabricated via thermal decomposition of Mo(CO)6 and subsequent oxidation on a clean Pt(1 1 0) substrate, by means of Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), as well as high-resolution electron-energy-loss spectroscopy (HREELS). CO chemisorption on these model surfaces was investigated using thermal desorption spectroscopy (TDS) and HREELS. The presence of the metallic molybdenum and MoOx species suppresses CO chemisorption on the Pt(1 1 0) substrate, indicating a physically site-blocking effect. The electron-deficiency of the Pt(1 1 0), due to the modification of metallic molybdenum, causes the low-temperature peak of CO desorption to shift downward in temperature by ∼30 K. The interaction between the MoOx and the underlying Pt(1 1 0) substrate has no influence on CO desorption temperature. © 2007 Published by Elsevier B.V.

A thin and homogeneous alumina film was prepared by deposition and oxidation of aluminum on a refractory Re(0 0 0 1) substrate under ultrahigh vacuum conditions. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron-energy-loss spectroscopy (HREELS) demonstrate that the oxide film is long-range ordered, essentially stoichiometric and free from surface hydroxyl groups. The chemisorption and thermal decomposition of Mo(CO)6 on the Al2O3 film were investigated by means of XPS and UPS. Mo(CO)6 adsorbs molecularly on the oxide film at 100 K; however, thermal decomposition of the adsorbate occurs upon annealing at high temperatures. Consequently the metallic molybdenum clusters are deposited on the thin alumina film via complete decarbonylation of Mo(CO)6.

CO chemisorption on the metallic molybdenum nanoparticles supported on the thin alumina film was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). A binary compound of molybdenum and CO is found to be formed on the surface upon CO dose, accompanied with a positive binding energy shift of the Mo 3d doublet and a localized Mo 4d valence band. A loose packing of the metallic molybdenum favors the formation of this intermediate Mox(CO)y species. The formation of the Mox(CO)y species implies that the property of the metallic molybdenum nanoparticles on the thin alumina film is much different from that of the bulk molybdenum, indicating a significant nanometer size effect.

The surface chemistry of NO and NO, on clean and oxygen-precovered Pt(110)-(1 x 2) surfaces were investigated by means of high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At room temperature, NO molecularly adsorbs on Pt(110), forming linear NO(a) and bridged NO(a). Coverage-dependent repulsive interactions within NO(a) drive the reversible transformation between linear and bridged NO(a). Some NO(a) decomposes upon heating, producing both N-2 and N2O. For NO adsorption on the oxygen-precovered surface, repulsive interactions exist between pre-covered oxygen adatoms and NO(a), resulting in more NO(a) desorbing from the surface in the form of linear NO(a). Bridged NO(a) experiences stronger repulsive interactions with precovered oxygen than linear NO(a). The desorption activation energy of bridged NO(a) from oxygen-precovered Pt(110) is lower than that from clean Pt(110), but the desorption activation energy of linear NO(a) is not affected by the precovered oxygen. NO2 decomposes on Pt(110)-(1 x 2) surface at room temperature. The resulted NO(a) (both linear NO(a) and bridged NO(a)) and O(a) repulsively interact each other. Comparing with NO/Pt(110), more NO(a) desorbs from NO2/Pt(110) as linear NO(a), and both linear NO(a) and bridged NO(a) exhibit lower desorption activation energies. The reaction pathways of NO(a) on Pt(110), desorption or decomposition, are affected by their repulsive interactions with coexisting oxygen adatoms. (c) 2006 Elsevier B.V. All rights reserved.

The desulfurization of thiophene on Raney Ni and rapidly quenched skeletal Ni (RQ Ni) has been studied in ultrahigh vacuum (UHV) by X-ray photoelectron spectroscopy (XPS). The Raney Ni or RQ Ni can be approximated as a hydrogen-preadsorbed polycrystalline Ni-alumina composite. It is found that thiophene molecularly adsorbs on Raney Ni or RQ Ni at 103 K. At 173 K, thiophene on alumina is desorbed, while thiophene in direct contact with the metallic Ni in Raney Ni undergoes C-S bond scission, leading to carbonaceous species most probably in the metallocycle-like configuration and atomic sulfur. On RQ Ni, the temperature for thiophene dissociation is about 100 K higher than that on Raney Ni. The lower reactivity of RQ Ni toward thiophene is tentatively attributed to lattice expansion of Ni crystallites in RQ Ni due to rapid quenching. The existence of alumina and hydrogen may block the further cracking of the metallocycle-like species on Raney Ni and RQ Ni at higher temperatures, which has been the dominant reaction pathway on Ni single crystals. By 473 K, the C 1s peak has disappeared, leaving nickel sulfide on the surface.

SmOx modified Rh(1 0 0) surfaces have been in-situ prepared by depositing metallic Sm and subsequently oxidizing under controlled conditions, and the interaction between the lanthanide oxide and transition metal has been characterized by means of X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy (HREELS) as well as thermal desorption spectroscopy (TDS). As evidenced, the adsorption of CO on the modified surfaces shows some different features to the original surface of Rh(1 0 0). The covering of SmOx blocks some sites on the surface and consequently suppresses adsorption of the typical CO species with an uptake at about 500 K, while a novel desorption peak centered at 260 K emerges in the CO TDS. Correspondingly, the XP spectrum exhibits a new C 1s peak at 287.9 eV and O 1s peak at 532.6 eV. The intensity of the low temperature peak varies with the coverage of SmOx, which shows an actual correlation to the perimeter sites of SmOx particles on the surface.

An ultra-thin and homogeneous alumina film is prepared by deposition and oxidation of aluminum on a platinum substrate under ultrahigh vacuum conditions. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy (HREELS) demonstrate that this film is completely stoichiometric and characteristic of Al2O3. Temperature-programmed desorption (TPD) and XPS characterization show that the Mo(CO)6 decomposes and leaves molybdenum with C and O contaminants on the Al2O3 film at 700 K. Subsequent heat treatment at 1200 K results in the formation of a model surface of metallic-molybdenum-modified alumina, which could be applied to simulate the activation of CH4.

no2 decomposition on pt(1 1 0) was studied by xps and tds at various temperatures. at room temperature, no, decomposes to adsorbed no and oxygen adatoms on pt(1 1 0). strong repulsive interactions exist between the adsorbed no and the oxygen adatoms. above 450 k, noads desorbs completely from the pt(i 10) surface and only adsorbed oxygen is left on the surface. o-2 thermal desorption spectra results reveal that, comparing with oxygen adsorption on pt(1 1 0), no, exposure above 350 k gives rise to an additional oxygen adsorption state on pt(l 10) that desorbs at a lower temperature. the new oxygen adsorption state has an o1s binding energy of 529.2 ev. the formation of the new oxygen adsorption state may be associated with the local phase transition of the pt(l 10) surface from (i x 2) structure to (1 x 1) structure induced by the noads that is formed from no2 decomposition. () 2002 published by elsevie(c) science b.v.

Upgrading Methane Sans Oxygen Direct routes to converting methane to higher hydrocarbons can allow natural gas to be used to provide chemical feedstocks. However, the reaction conditions needed to activate the strong C-H bond tend to overoxidize the products. Guo et al. (p. 616 ) report a high-temperature nonoxidative route that exposes methane to isolated iron sites on a silica catalyst. Methyl radicals were generated and coupled in the gas phase to form ethylene and aromatics along with hydrogen. The isolation of the active sites avoided surface reactions between the radicals that would deposit solid carbon.

The growth of silver thin film on the Pt(1 1 0)-(1×2) surface was studied by AES and CO thermal desorption spectroscopy. At 300 K, the silver growth follows the Stranski–Krastanov mode, i.e., layer-by-layer growth up to 1.5 monolayer (ML) followed by 3D island growth. During the course of the initial 1.5 ML layer-by-layer growth, the first 0.5 ML silver atoms fill in the missing-row sites of the Pt(1 1 0)-(1×2) surface and form a pseudo Pt(1 1 0)-(1×1)–0.5 ML Pt–0.5 ML Ag surface structure, then the subsequent 0.5 ML and the last 0.5 ML silver atoms are located on the silver sites and the platinum sites of the pseudo (1×1) surface, respectively. The initial 0.5 ML silver atoms not only block the sites of Pt(1 1 0) for CO adsorption, but also inhibit the local surface restructuring of the Pt(1 1 0)-(1×2) to the Pt(1 1 0)-(1×1) induced by CO adsorption.

tetrapropylammonium bromide (tpabr) has been used as a template for the synthesis of titanium silicalite-l (ts-l), and the involved crystallization mechanism and the characteristics of the incorporated ti in the zeolites were investigated by applying a variety of methods, e.g. solid-state nmr, ir, uv-vis, xrd and sem, as well as ph-meter techniques. it is revealed that the crystallization process performed with tpabr follows the well-known solution-mediated transformation mechanism, and that the ti atoms were introduced into the frameworks synchronously with the formation of the zeolite structures. in addition, the formation of less amount of inactive tio2 crystals with an anatase structure is not in the initial stage of the crystallization as reported by some authors, but through the whole process of the zeolite synthesis, which suggests that a careful control of the synthesis parameters throughout the whole crystallization process is quite important for obtaining the anatase-free titanium-silicalite zeolite. (c) 2000 elsevier science s.a. all rights reserved.

The reactivity of a bulk Ag surface, an Ag monolayer film on Si(111)- 7 × 7 (denoted as the [Formula: see text]-Ag-Si surface), and Si(111)-7 × 7 to CCl(4) was investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission electron microscopy (UV-PEEM). In situ UV-PEEM was used to monitor simultaneously the CCl(4) dissociation on different surface domains, including the bulk Ag, [Formula: see text]-Ag-Si, and Si(111). The PEEM results combined with XPS data show that CCl(4) adsorbs dissociatively on bulk Ag(111) and Si(111) but adsorbs molecularly on the [Formula: see text]-Ag-Si surface, and the surface reactivity follows the order of [Formula: see text]-Ag-Si.