Shuo Zhang

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.

Tuning the redox activity of encapsulated metal clusters via the metallic and semiconducting character of carbon nanotubes

Significance The unique property of carbon nanotube channels has triggered wide research interest in different fields. An increasing number of studies show that confinement of metal or metal oxide nanoparticles inside these channels often leads to significantly modified catalytic activity with respect to the same bare metal nanoparticles or those dispersed on the outer walls. We demonstrate here that reactions can be further modulated by the electronic nature (metallic vs. semiconducting character) of nanotubes by taking encapsulated rhenium nanocatalysts as a probe. Particularly, the chemical state of the encapsulated rhenium is tuned due to host–guest electronic interaction. This is of significance for catalytic reactions sensitive to the chemical state of active metals, because it may change the reaction pathways. We demonstrate that reactions confined within single-walled carbon nanotube (SWCNT) channels are modulated by the metallic and semiconducting character of the hosts. In situ Raman and X-ray absorption near-edge structure spectroscopies provide complementary information about the electronic state of carbon nanotubes and the encapsulated rhenium species, which reveal electronic interactions between encapsulated species and nanotubes. More electrons are transferred from metallic tubes (m-SWCNTs) to oxidic rhenium clusters, leading to a lower valence state rhenium oxide than that in semiconducting tubes (s-SWCNTs). Reduction in 3.5% (vol/vol) H2/Ar leads to weakened host–guest electronic interaction. The high valence state Re within s-SWCNTs is more readily reduced when raising the temperature, whereas only a sluggish change is observed for Re within m-SWCNTs. Only at 400 °C does Re reach a similar electronic state (mixture of Re0 and Re4+) in both types of tubes. Subsequent oxidation in 1% O2/Ar does not show changes for Re in s-SWCNTs up to 200 °C. In comparison, m-SWCNTs facilitate the oxidation of reduced rhenium (160 °C). This can be exploited for rational design of active catalysts with stable species as a desired valence state can be obtained by selecting specific-type SWCNTs and a controlled thermal treatment. These results also provide a chemical approach to modulate reversibly the electronic structure of SWCNTs without damaging the sidewalls of SWCNTs.

Ferrous Centers Confined on Core-Shell Nanostructures for Low-Temperature CO Oxidation

A noble metal (NM) can stabilize monolayer-dispersed surface oxide phases with metastable nature. The formed oxide-on-metal inverse catalyst presents better catalytic performance than the NM because of the introduction of coordinatively unsaturated cations at the oxide-metal boundaries. Here we demonstrate that an ultrathin NM layer grown on a non-NM core can impose the same constraint on the supported oxide as the bulk NM. Cu@Pt core-shell nanoparticles (NPs) decorated with FeO patches use much less Pt but exhibit performance similar to that of Pt NPs covered with surface FeO patches in the catalytic oxidation of CO. The oxide-on-core@shell inverse catalyst system may open a new avenue for the design of advanced nanocatalysts with decreased usage of noble metals.

Solvothermal Synthesis of Ternary Cu2MoS4 Nanosheets: Structural Characterization at the Atomic Level

Cu2 MoS4 nanosheets are synthesized by a solvothermal method in which the Cu2 O starting material acts as a sacrificial template. The microstructure of the Cu2 MoS4 nanosheets is characterized at the atomic level, and the growth mechanism is monitored at the nanoscale through systematic time-dependent experiments. As a result, the unprecedented observation of the allotropic phase change in Cu2 MoS4 that occurs during the solvothermal process is possible.

First principles study on the diffusion of alkali-metal ions on the armchair single-wall nanotubes.

In this paper we have performed density-functional study on the adsorption and diffusion of various alkali-metal ions on the surface of pristine and defective armchair single-wall carbon nanotubes. In the pristine SWNT system, the position above the hexagon is believed to be the most stable site for adsorption, while the adsorption is enhanced in the defective SWNT. In pristine SWNT all the ions prefer to diffuse along the axial direction, with low barriers less than 0.25 eV. In defective SWNT, the axial diffusion is also energetically most preferable, and the barriers increase only slightly and have little influence on the diffusion as compared to pristine SWNT.

Investigation of annealing-induced oxygen vacancies in the Co-doped ZnO system by Co K-edge XANES spectroscopy

To clarify the mechanism of the observed room-temperature ferromagnetism (RTF), many studies have been focused on dilute magnetic semiconductor systems. Several investigations have demonstrated that oxygen vacancies play a significant role in mediating the RTF behavior so that much effort has been devoted to confirm their presence. In this investigation, X-ray absorption spectroscopy was combined with ab initio calculations of the electronic structure of Co and Zn in the Zn(0.9)Co(0.1)O system before and after annealing, which has been recognized as an effective method of originating oxygen vacancies. A feature at about 20 eV after the rising edge of the Co K-edge XANES that disappears after annealing has been associated with the presence of an oxygen vacancy located in the second shell surrounding the Co atom. Moreover, Zn K-edge XANES spectra point out that this oxygen vacancy affects the electronic structure near the Fermi level, in agreement with density functional theory calculations.

Initial Reaction Mechanism of Platinum Nanoparticle in Methanol–Water System and the Anomalous Catalytic Effect of Water

Understanding the detailed reaction mechanism in the early stage of noble metal nanoparticles is very critical for controlling the final crystals size, morphology, and properties. Here, we report a systematic study on the initial reaction mechanism of Pt nanoparticles in methanol-water system and demonstrate an anomalous catalytic effect of H2O on the reduction of H2PtCl6 to Pt nanoparticles using a combination of UV-vis, X-ray absorption spectroscopy (XAS), liquid chromatography mass spectrometry (LCMS), and first-principles calculation methods. The observations reveal the transformation route [PtCl6](2-) → [PtCl5(CH3O)](2-) → [PtCl4](2-) → [PtCl3(CH3O)](2-) → [PtCl2](2-) and finally to form Pt nanoparticles in a pure CH3OH solution. With 10 vol % water adding in the CH3OH solution, a new and distinct chemical reduction pathway is found in which the precursors change from [PtCl6](2-) to [PtCl5(CH3O)(H2O)](2-) to [PtCl4](2-) to [PtCl3(CH3O)(H2O)](2-) to [PtCl2](2-) and to Pt nanoparticles. Notably, the supernumerary water molecular can significantly accelerate the rate of chemical reduction and greatly shorten the reaction time. This work not only elucidates the initial reaction mechanism of Pt nanoparticles but also highlights the pronounced influence of H2O on the reaction pathway, which will provide useful insights for understanding the formation mechanism of noble metal nanoparticles and open up a high efficient way to synthesize new functional nanomaterial.

Highly efficient NOx purification in alternating lean/rich atmospheres over non-platinic mesoporous perovskite-based catalyst K/LaCoO3

A mesoporous perovskite, LaCoO3, with a high specific surface area of 75 m2 g−1 was synthesized by a nano-casting method. Its related NOx storage/reduction catalyst K/LaCoO3, which contains no noble metals, exhibits excellent de-NOx performance under alternating lean/rich conditions, showing a high NOx reduction efficiency of 97.0% and a high NOx to N2 selectivity of 97.3%.

Lattice distortion and its role in the magnetic behavior of the Mn-doped ZnO system

Puzzling magnetic data on the Zn1-xMnxO system such as a small magnetization values or a large negative values of the Curie-Weiss temperature have been obtained in many experimental investigations. Here we report element-specific structural and magnetic investigations on a high-quality Zn0.95Mn0.05O nanocrystalline sample. Combining low-temperature x-ray absorption spectroscopy and theoretical simulations, we show that the formation of substitutional spin-antiparallel pairs induces a large local distortion involving a contraction of the Mn-Mn distance and a reduced Mn-O-Mn bond angle. The first-principles calculation considering hole-doping reveals that such a distortion can result in a localized hole around a dopant atom, generating a ferrimagnetic ordering with a magnetization of 0.45 mu(B)/Mn. This result may give a new insight for a better understanding of the reported magnetic data.