Dali Tan

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen

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. Methyl radicals that form at isolated iron sites in a silica matrix form gas-phase products and do not deposit solid carbon. The efficient use of natural gas will require catalysts that can activate the first C–H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Synergetic Effect of Surface and Subsurface Ni Species at Pt-Ni Bimetallic Catalysts for CO Oxidation

Various well-defined Ni-Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO(1-x)/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt-Ni nanoparticles via activation in H(2) at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt-Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O(2), producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.

Visualizing Chemical Reactions Confined under Graphene

An undercover agent: graphene has been used as an imaging agent to visualize interfacial reactions under its cover, and exhibits a strong confinement effect on the chemistry of molecules underneath. In a CO atmosphere, CO penetrates into the graphene/Pt(111) interface and reacts with O(2) therein, whereas intercalated CO desorbs from the Pt surface.

Freestanding Graphene by Thermal Splitting of Silicon Carbide Granules

Mass production of high-quality graphene, necessary for the further development of graphene-based technologies such as fuel-cell electrocatalysts, is expected to be possible by the novel synthetic approach reported here. Freestanding single-layer graphene nanosheets containing few defects and with good oxidation stability are produced from commercial polycrystalline silicon carbide granules using a non-liquid-phase method.

Effect of aluminum on the formation of zeolite MCM-22 and kenyaite

The synthesis of zeolite MCM-22 and kenyaite under static hydrothermal conditions using hexamethyleneimide (HMI) as a directing agent was investigated. Kenyaite was the favored product instead of the purr silica analogue of MCM-22 for a pure silica synthesis mixture. With increasing SiO2/Al2O3 ratio, the crystallized products varied from purr MCM-22 to a mixture of MCM-22 and kenyaite, and finally pure layered silicate kenyaite. The fraction of MCM22 crystallites increases, whilst that of kenyaite decreases with the aluminum content. All the products occlude HMI molecules, and the amount of HMI increases with the aluminum content. MCM-22 and kenyaite have different layer structures, but the Si atoms in the HMI-included kenyaite have similar NMR chemical shifts as those in MCM-22. Aluminum atoms, which are present as tetrahedral framework species, are most possibly incorporated into the framework of MCM-22 only. The presence of aluminum seems to be critical for the formation of MCM-22 zeolite

An exchange intercalation mechanism for the formation of a two-dimensional Si structure underneath graphene

AbstractA 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.

Preparation of novel Raney-Ni catalysts and characterization by XRD, SEM and XPS

In the present paper, we report on a novel procedure for preparing high performance Raney-Ni catalysts, which includes preparation of NI-AI ribbons by melt-quenching. pretreatment with hydrogen and activation through Al leaching with sodium hydroxide solution. The resultant catalyst exhibits about five times high activity in the catalytic hydrogenation of cyclohexanone than that of the Raney-Ni catalyst obtained by a conventional preparation method. The structural variations of the catalyst caused by the different pretreatments were characterized by XRD. SEM and XPS. As demonstrated, melt-quenching and H-2-pretreatment play a crucial role for forming the certain structure being composed of a high density of Ni2Al3 phase. which, in the subsequent leaching process. benefit to form ultra-uniform Raney-Ni catalyst with large surface areas. Metallic Al in hydrogen-pretreated alloys appears to be more easily leached, remaining aluminum as the dominant Al-related species on the surface, which acts as a matrix stabilizing the highly dispersed Ni and consequently improving the activity of cyclohexanone hydrogenation

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite.

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

Unique Reactivity of Confined Metal Atoms on a Silicon Substrate

When small metal assembles such as single atoms or small clusters consisting of a few atoms are supported on the semiconducting or oxide substrates, the sp electrons in the metal can be confined by the metal-substrate interface, which may be critical to surface reactivity. To shed lights on the electron confinement effect, in particular the sp electron confinement, in supported metal atoms and its effect on the catalytic activity, we report here a comparative reactivity study of bulk Ag(111) surface and a Ag monolayer film on Si(111) surface by means of scanning tunneling microscopy (STM), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), photoemission electron microscopy (PEEM), and density functional theory (DFT) calculations. Ag deposited on Si(111) can form a stable periodic array of Ag atoms confined in a 2D Ag monolayer film on Si(111) with the (√3×√3) symmetry. The most simple halogen methane, CCl4, was chosen as the probe molecule to study the surface chemistry of the Ag surfaces due to the high activity of halogen towards Ag. Our study shows that these two surfaces present distinct reactivity towards CCl4 dissociation. Monolayer Ag is inert toward dissociation of CCl4 compared to bulk Ag. Specifically, it is found that confinement of 5sp electron of Ag atoms in the √3×√3-Ag-Si surface, which is delocalized in the bulk Ag(111) surface, is decisive to the different reactivity.

Decomposition of NO2 on Pt(110): formation of a new oxygen adsorption state

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.