Franklin Feng Tao

3D Honeycomb‐Like Structured Graphene and Its High Efficiency as a Counter‐Electrode Catalyst for Dye‐Sensitized Solar Cells

Graphene, a two-dimensional carbon sheet, has attracted great interest due to its unique properties. To explore its practical applications, large-scale synthesis with controllable integration of individual graphene sheets is essential. To date, numerous approaches have been developed for graphene synthesis, including mechanical cleavage, epitaxial growth, and chemical vapor deposition. All of those techniques are used to prepare flat graphene sheets on a substrate. Chemical exfoliation of graphite has been applied to prepare graphene oxide solutions and graphene-based composite materials. Recently, tuning graphene shapes is attracting much attention. Cheng and co-workers synthesized graphene foam using porous Ni foam as a template for the CVD growth of graphene, followed by etching away the Ni skeleton. The graphene foam consists of an interconnected flexible network of graphene as the fast transport channel of charge carriers for high electrical conductivity. Ruoff et al. prepared porous graphene paper from microwave exfoliated graphene oxide by KOH activation. The porous graphene, which has an ultra-high surface area and a high electrical conductivity, was exploited for supercapacitor cells, leading to high values of gravimetric capacitance and energy density. Feng, M llen, and co-workers synthesized hierarchical macroand mesoporous graphene frameworks (GFs). The GFs exhibited excellent performance for electrochemical capacitive energy storage. Yu et al. and Qu et al. fabricated graphene tubes that could be selectively functionalized for desirable applications. Choi et al. synthesized macroporous graphene using polystyrene colloidal particles as sacrificial templates in graphene oxide suspension, and the pore sizes can be tuned by controlling template particle size. These important results represent a significant topic—tuning the properties of graphene sheets by controlling their shapes. However, it is still a challenge to synthesize three-dimensional graphene (3D) with a desirable shape. Herein, we develop a novel strategy for the synthesis of a new type of graphene sheet with a 3D honeycomb-like structure by a simple reaction between Li2O and CO. Furthermore, these graphene sheets exhibited excellent catalytic performance as a counter electrode for dye-sensitized solar cells (DSSCs) with an energy conversion efficiency as high as 7.8%, which is comparable to that of an expensive platinum electrode. Li2O is widely exploited as a promoter in catalysts to inhibit carbon formation. However, this general principle is challenged by this work, in which Li2O is used to react with CO to form graphene-structured carbon [Eq. (1)]

Atomic-Scale Observations of Catalyst Structures under Reaction Conditions and during Catalysis

Heterogeneous catalysis is a chemical process performed at a solid-gas or solid-liquid interface. Direct participation of catalyst atoms in this chemical process determines the significance of the surface structure of a catalyst in a fundamental understanding of such a chemical process at a molecular level. High-pressure scanning tunneling microscopy (HP-STM) and environmental transmission electron microscopy (ETEM) have been used to observe catalyst structure in the last few decades. In this review, instrumentation for the two in situ/operando techniques and scientific findings on catalyst structures under reaction conditions and during catalysis are discussed with the following objectives: (1) to present the fundamental aspects of in situ/operando studies of catalysts; (2) to interpret the observed restructurings of catalyst and evolution of catalyst structures; (3) to explore how HP-STM and ETEM can be synergistically used to reveal structural details under reaction conditions and during catalysis; and (4) to discuss the future challenges and prospects of atomic-scale observation of catalysts in understanding of heterogeneous catalysis. This Review focuses on the development of HP-STM and ETEM, the in situ/operando characterizations of catalyst structures with them, and the integration of the two structural analytical techniques for fundamentally understanding catalysis.

Water-gas shift reaction on metal nanoclusters encapsulated in mesoporous ceria studied with ambient-pressure X-ray photoelectron spectroscopy.

Metal nanoclusters (Au, Pt, Pd, Cu) encapsulated in channels of mesoporous ceria (mp-CeO(2)) were synthesized. The activation energies of water-gas shift (WGS) reaction performed at oxide-metal interfaces of metal nanoclusters encapsulated in mp-CeO(2) (M@mp-CeO(2)) are lower than those of metal nanoclusters impregnated on ceria nanorods (M/rod-CeO(2)). In situ studies using ambient-pressure XPS (AP-XPS) suggested that the surface chemistry of the internal concave surface of CeO(2) pores of M@mp-CeO(2) is different from that of external surfaces of CeO(2) of M/rod-CeO(2) under reaction conditions. AP-XPS identified the metallic state of the metal nanoclusters of these WGS catalysts (M@mp-CeO(2) and M/rod-CeO(2)) under a WGS reaction condition. The lower activation energy of M@mp-CeO(2) in contrast to M/rod-CeO(2) is related to the different surface chemistry of the two types of CeO(2) under the same reaction condition.

Catalysis on singly dispersed bimetallic sites

A catalytic site typically consists of one or more atoms of a catalyst surface that arrange into a configuration offering a specific electronic structure for adsorbing or dissociating reactant molecules. The catalytic activity of adjacent bimetallic sites of metallic nanoparticles has been studied previously. An isolated bimetallic site supported on a non-metallic surface could exhibit a distinctly different catalytic performance owing to the cationic state of the singly dispersed bimetallic site and the minimized choices of binding configurations of a reactant molecule compared with continuously packed bimetallic sites. Here we report that isolated Rh1Co3 bimetallic sites exhibit a distinctly different catalytic performance in reduction of nitric oxide with carbon monoxide at low temperature, resulting from strong adsorption of two nitric oxide molecules and a nitrous oxide intermediate on Rh1Co3 sites and following a low-barrier pathway dissociation to dinitrogen and an oxygen atom. This observation suggests a method to develop catalysts with high selectivity.

Synthesis, catalysis, surface chemistry and structure of bimetallic nanocatalysts

Studies of bimetallic catalysts can be tracked back to the early 1960s in the Exxon Research and Engineering Company. The term ‘‘bimetallic clusters’’ was introduced by John H. Sinfelt in the early 1980s to refer to highly dispersed bimetallic entities present on the surface of a support such as silica or alumina. The main bimetallic catalysts at that time included Ni–Cu, Ru–Cu, Os–Cu, Pt–Ir, and Pt–Ru. The library of bimetallic catalysts has been significantly enriched in the past decades. Bimetallic catalysts have been one of the major categories of catalysts in heterogeneous catalysis. Fundamental studies of the synthesis, catalysis, surface chemistry and structure of bimetallic catalysts have been a fast-growing and exciting field of heterogeneous catalysis for energy conversion and chemical transformations. Adding a second metal (here called the guest metal) to the first metal (called the host metal) can tune catalytic performances (activity, selectivity, durability, etc.) through modification of electronic and/or structural factors. A bimetallic catalyst is widely defined as a catalyst crystallite which consists of two metal components. The constituent metals can form an alloy, intermetallic, or nanocomposited structure. Alloy catalysts include bulk alloy, surface alloy, and near surface alloy. Nanocomposited structures include core–shell structured bimetallic nanoparticles, nanodendrites, and others. Due to the diversity of bimetallic catalysts, tuning catalytic performance of a host metal could be performed through (a) an ensemble or geometric effect, in which the coordination of atoms of a guest metal to an atom of the host metal on the surface provides new geometries of active sites, (b) the electronic or ligand effect, wherein the addition of a guest metal alters the electron properties of the active sites of the host metal by electron transfer between guest and host metals. A concept to evaluate the influence of a guest metal on the host metal is the d-band center of the host metal. In most cases, the difference in catalytic performance between a bimetallic catalyst and a monometallic catalyst of the host metal can be rationalized through an electronic and/or geometric effect. Unfortunately, it is quite challenging to distinguish the two effects if both of them have a role. From the catalytic point of view, there could be another effect of a formed bimetallic catalyst which can be understood as a synergetic effect or bi-functional effect. In this case, atoms of the two metals are necessary parts of a catalytic site and thus play a unique role such as adsorption for different reactants or different intermediates. Formation of a bimetallic catalyst exhibits the feature of continual tuning through changes in composition of the host and guest metals and flexible modification of the electronic and/or geometric structure of bimetallic catalysts through synthesis. This largely enhances the capability in tuning the catalytic performance. Thus, bimetallic catalysts have the capability of improving catalytic activity, enhancing catalytic selectivity, increasing catalytic stability, and cutting the cost of catalysts. The spectacular advances in the synthesis of bimetallic nanocatalysts with wet chemistry in the past decade have offered numerous methods and protocols Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN, USA 46556. E-mail: ftao@nd.edu w Part of the bimetallic nanocatalysts themed issue.

Understanding complete oxidation of methane on spinel oxides at a molecular level

It is crucial to develop a catalyst made of earth-abundant elements highly active for a complete oxidation of methane at a relatively low temperature. NiCo2O4 consisting of earth-abundant elements which can completely oxidize methane in the temperature range of 350-550 °C. Being a cost-effective catalyst, NiCo2O4 exhibits activity higher than precious-metal-based catalysts. Here we report that the higher catalytic activity at the relatively low temperature results from the integration of nickel cations, cobalt cations and surface lattice oxygen atoms/oxygen vacancies at the atomic scale. In situ studies of complete oxidation of methane on NiCo2O4 and theoretical simulations show that methane dissociates to methyl on nickel cations and then couple with surface lattice oxygen atoms to form -CH3O with a following dehydrogenation to -CH2O; a following oxidative dehydrogenation forms CHO; CHO is transformed to product molecules through two different sub-pathways including dehydrogenation of OCHO and CO oxidation.

Formation of Nanometer-Sized Surface Platinum Oxide Clusters on a Stepped Pt(557) Single Crystal Surface Induced by Oxygen: A High-Pressure STM and Ambient-Pressure XPS Study

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O(2) pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O(2) and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O(2) to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O(2) gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.

A New Scanning Tunneling Microscope Reactor Used for High Pressure and High Temperature Catalysis Studies

We present the design and performance of a homebuilt high-pressure and high-temperature reactor equipped with a high-resolution scanning tunneling microscope (STM) for catalytic studies. In this design, the STM body, sample, and tip are placed in a small high pressure reactor ( approximately 19 cm(3)) located within an ultrahigh vacuum (UHV) chamber. A sealable port on the wall of the reactor separates the high pressure environment in the reactor from the vacuum environment of the STM chamber and permits sample transfer and tip change in UHV. A combination of a sample transfer arm, wobble stick, and sample load-lock system allows fast transfer of samples and tips between the preparation chamber, high pressure reactor, and ambient environment. This STM reactor can work as a batch or flowing reactor at a pressure range of 10(-13) to several bars and a temperature range of 300-700 K. Experiments performed on two samples both in vacuum and in high pressure conditions demonstrate the capability of in situ investigations of heterogeneous catalysis and surface chemistry at atomic resolution at a wide pressure range from UHV to a pressure higher than 1 atm.

Nanoscale-Phase-Separated Pd–Rh Boxes Synthesized via Metal Migration: An Archetype for Studying Lattice Strain and Composition Effects in Electrocatalysis

Developing syntheses of more sophisticated nanostructures comprising late transition metals broadens the tools to rationally design suitable heterogeneous catalysts for chemical transformations. Herein, we report a synthesis of Pd-Rh nanoboxes by controlling the migration of metals in a core-shell nanoparticle. The Pd-Rh nanobox structure is a grid-like arrangement of two distinct metal phases, and the surfaces of these boxes are {100} dominant Pd and Rh. The catalytic behaviors of the particles were examined in electrochemistry to investigate strain effects arising from this structure. It was found that the trends in activity of model fuel cell reactions cannot be explained solely by the surface composition. The lattice strain emerging from the nanoscale separation of metal phases at the surface also plays an important role.

Preparation and Catalysis of Carbon‐Supported Iron Catalysts for Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis (FTS) is essential for the transformation of natural gas, coal, and biomass to clean transportation fuels and value‐added chemicals. Traditionally, iron catalysts for FTS are predominantly fused iron catalysts and precipitated iron catalysts using silica as the support. Owing to an intense surge in interest in carbon materials during recent years, along with the unique properties of these materials, such as high surface area, high porosity, and ample structures, carbon‐supported iron‐based FTS catalysts have attracted increasing attention. In this detailed review of the progress of the Fe/C catalysts for FTS in the last three decades, particular emphasis is put on their preparation, characterization, and catalytic performance relevant to the characteristics of carbon materials. This review is intended to be a valuable resource to researchers interested in this exciting field of catalysis, as well as the foundation for those investigating applications of novel carbon materials. A brief discussion is also devoted to the challenges and opportunities regarding the future development of Fe/C FTS catalysts.