Rui Si

Shape and Crystal‐Plane Effects of Nanoscale Ceria on the Activity of Au‐CeO2 Catalysts for the Water–Gas Shift Reaction

The water–gas shift (WGS) reaction (CO+H2OQCO2+H2) plays an important role in fuel processing for polymer electrolyte membrane (PEM) fuel-cell applications. The hydrogen in the reformate gas is upgraded by removal of the carbon monoxide, which is a strong poison of the anode catalysts in current PEM fuel cells. Active shift catalysts that are also stable under the operating conditions of practical fuel-cell systems are under intense study, and nanostructured Au-CeO2, first reported by Fu et al. as a promising lowtemperature shift catalyst, holds a prominent position. This catalyst exploits the strong interaction of ceria with finely dispersed and stabilized gold atoms and clusters on the surface of ceria. Gold nanoparticles and clusters that interact strongly with oxide supports were first described by Haruta et al. to be extremely active CO oxidation catalysts. Numerous studies since then have reaffirmed the activity of well-dispersed gold for CO oxidation and many other reactions. While a full mechanism of this catalytic process still needs to be established, even for the simplest of these reactions (CO oxidation), a careful investigation of the reported strong metal–support interaction through structural studies may provide further mechanistic insights as well as rationalize the design of practical catalysts. For the WGS reaction on Au-CeO2, the importance of nanoscale ceria as a support that stabilizes active gold species has been demonstrated recently. Hydrolysis methods for the synthesis of ceria nanocrystals have proven to be powerful for controlling particle size and crystal shape. For example, Yan et al. have obtained single-crystalline CeO2 nanopolyhedra ({111} and {100}), nanorods ({110} and {100}), and nanocubes ({100}) by hydrolysis of cerium(III) salts, combined with a hydrothermal treatment, and have further found that oxygen storage takes place both at the surface and in the bulk for nanorods and nanocubes but is restricted to the surface for nanopolyhedra, just like its bulk ceria counterpart. Trovarelli et al. have studied the rearrangement of CeO2 crystallites under airaging and the exposure of more reactive {100} surfaces for CO oxidation. Very little is known for Au-CeO2 composite polycrystalline nanomaterials with respect to the shape/crystal plane effect of CeO2 on the gold species< activity/stabilization as highly active catalysts for the WGS reaction. Herein, we present activity correlations with the shape (rod, cube, polyhedron) and crystal plane of nanoscale ceria as a support for gold catalysts for this reaction. The interaction between deposited gold and different crystal orientations is investigated at ambient pressure andmonitored by several analytical techniques, including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction by hydrogen (H2-TPR). Figure 1 depicts our two-step preparation process, which includes hydrothermal synthesis of ceria nanorods, nano-

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.

Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions

Substituting Salt for Cerium Oxide The water-gas shift reaction converts carbon monoxide and water to hydrogen and carbon dioxide. Catalysts that operate at lower temperatures will be useful in fuel cells. Nanoparticles of platinum adsorbed on reducible oxides, such as ceria, can stabilize catalytically active Ptoxygen species. Zhai et al. (p. 1633) now show that, when alkali atoms are added, atomically dispersed Pt can be an active catalyst for the water-gas shift reaction at ∼100°C, even on simple oxides such as alumina and silica. The formation of hydrogen from carbon monoxide and water is catalyzed by the formation of oxidized platinum atoms. We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2. The alkali ion–associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-Ox(OH)y species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.

Raman Analysis of Mode Softening in Nanoparticle CeO2−δ and Au-CeO2−δ during CO Oxidation

Oxygen vacancy levels are monitored during the oxidation of CO by CeO(2-δ) nanorods and Au-CeO(2-δ) nanorods, nanocubes, and nanopolyhedra by using Raman scattering. The first-order CeO(2) F(2g) peak near 460 cm(-1) decreases when this reaction is fast (fast reduction and relatively slow reoxidation of the surface), because of the lattice expansion that occurs when Ce(3+) replaces Ce(4+) during oxygen vacancy creation. This shift correlates with reactivity for CO oxidation. Increases in the oxygen deficit δ as large as ~0.04 are measured relative to conditions when the ceria is not reduced.

A single iron site confined in a graphene matrix for the catalytic oxidation of benzene at room temperature.

A coordinatively unsaturated single iron site confined in a graphene matrix shows an ultrahigh activity for catalytic oxidation. Coordinatively unsaturated (CUS) iron sites are highly active in catalytic oxidation reactions; however, maintaining the CUS structure of iron during heterogeneous catalytic reactions is a great challenge. Here, we report a strategy to stabilize single-atom CUS iron sites by embedding highly dispersed FeN4 centers in the graphene matrix. The atomic structure of FeN4 centers in graphene was revealed for the first time by combining high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy with low-temperature scanning tunneling microscopy. These confined single-atom iron sites exhibit high performance in the direct catalytic oxidation of benzene to phenol at room temperature, with a conversion of 23.4% and a yield of 18.7%, and can even proceed efficiently at 0°C with a phenol yield of 8.3% after 24 hours. Both experimental measurements and density functional theory calculations indicate that the formation of the Fe═O intermediate structure is a key step to promoting the conversion of benzene to phenol. These findings could pave the way toward highly efficient nonprecious catalysts for low-temperature oxidation reactions in heterogeneous catalysis and electrocatalysis.

Ordered bilayer ruthenium–platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts

Fabricating subnanometre-thick core-shell nanocatalysts is effective for obtaining high surface area of an active metal with tunable properties. The key to fully realize the potential of this approach is a reliable synthesis method to produce atomically ordered core-shell nanoparticles. Here we report new insights on eliminating lattice defects in core-shell syntheses and opportunities opened for achieving superior catalytic performance. Ordered structural transition from ruthenium hcp to platinum fcc stacking sequence at the core-shell interface is achieved via a green synthesis method, and is verified by X-ray diffraction and electron microscopic techniques coupled with density functional theory calculations. The single crystalline Ru cores with well-defined Pt bilayer shells resolve the dilemma in using a dissolution-prone metal, such as ruthenium, for alleviating the deactivating effect of carbon monoxide, opening the door for commercialization of low-temperature fuel cells that can use inexpensive reformates (H2 with CO impurity) as the fuel.

Active gold species on cerium oxide nanoshapes for methanol steam reforming and the water gas shift reactions

We report that a small amount (<1 at%) of gold on ceria single crystals prepared as nanorods (10 ± 2.8 by 50–200 nm) of {110} and {100} crystal surfaces shows excellent catalytic activity in both the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions at low temperatures (<250 °C). The ceria nanorods bind and stabilize gold as atoms and clusters (<1 nm, TEM invisible). On the other hand, gold nanoparticles (∼3 nm) are found on the {100} surfaces of ceria nanocubes. Very low rates of SRM and WGS were measured on the Au–ceria {100} cubes, while the rates on Au–ceria {110} rods were at least an order of magnitude higher. However, the apparent activation energies did not depend on the shape of ceria. Strong bonded Aun–O–Ce species are the active sites and these are present only in negligible concentrations on the {100} surfaces. Thus, both reactions are structure-insensitive on Au–ceria. SRM proceeds through the methyl formate route. The Au–ceria {110} catalyst shows both high SRM activity and high selectivity to CO2 at temperatures below 250 °C.

Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in NaxREF3+x Nanocrystals

Efficient tailoring of upconversion emissions in lanthanide-doped nanocrystals is of great significance for extended optical applications. Here, we present a facile and highly effective method to tailor the upconversion selectivity by engineering the local structure of lanthanides in Na(x)REF(3+x) nanocrystals. The local structure engineering was achieved through precisely tuning the composition of nanocrystals, with different [Na]/[RE] ([F]/[RE]) ratio. It was found that the lattice parameter as well as the coordination number and local symmetry of lanthanides changed with the composition. A significant difference in the red to green emission ratio, which varied from 1.9 to 71 and 1.6 to 116, was observed for Na(x)YF(3+x):Yb,Er and Na(x)GdF(3+x):Yb,Er nanocrystals, respectively. Moreover, the local structure-dependent upconversion selectivity has been verified for Na(x)YF(3+x):Yb,Tm nanocrystals. In addition, the local structure induced upconversion emission from Er(3+) enhanced 9 times, and the CaF2 shell grown epitaxially over the nanocrystals further promoted the red emission by 450 times, which makes it superior as biomarkers for in vivo bioimaging. These exciting findings in the local structure-dependent upconversion selectivity not only offer a general approach to tailoring lanthanide related upconversion emissions but also benefit multicolor displays and imaging.

Platinum-Modulated Cobalt Nanocatalysts for Low-Temperature Aqueous-Phase Fischer−Tropsch Synthesis

Fischer-Tropsch synthesis (FTS) is an important catalytic process for liquid fuel generation, which converts coal/shale gas/biomass-derived syngas (a mixture of CO and H2) to oil. While FTS is thermodynamically favored at low temperature, it is desirable to develop a new catalytic system that could allow working at a relatively low reaction temperature. In this article, we present a one-step hydrogenation-reduction route for the synthesis of Pt-Co nanoparticles (NPs) which were found to be excellent catalysts for aqueous-phase FTS at 433 K. Coupling with atomic-resolution scanning transmission electron microscopy (STEM) and theoretical calculations, the outstanding activity is rationalized by the formation of Co overlayer structures on Pt NPs or Pt-Co alloy NPs. The improved energetics and kinetics from the change of the transition states imposed by the lattice mismatch between the two metals are concluded to be the key factors responsible for the dramatically improved FTS performance.

A Graphene Composite Material with Single Cobalt Active Sites: A Highly Efficient Counter Electrode for Dye‐Sensitized Solar Cells

The design of catalysts that are both highly active and stable is always challenging. Herein, we report that the incorporation of single metal active sites attached to the nitrogen atoms in the basal plane of graphene leads to composite materials with superior activity and stability when used as counter electrodes in dye-sensitized solar cells (DSSCs). A series of composite materials based on different metals (Mn, Fe, Co, Ni, and Cu) were synthesized and characterized. Electrochemical measurements revealed that CoN4 /GN is a highly active and stable counter electrode for the interconversion of the redox couple I(-) /I3 (-) . DFT calculations revealed that the superior properties of CoN4 /GN are due to the appropriate adsorption energy of iodine on the confined Co sites, leading to a good balance between adsorption and desorption processes. Its superior electrochemical performance was further confirmed by fabricating DSSCs with CoN4  /GN electrodes, which displayed a better power conversion efficiency than the Pt counterpart.