Dario Stacchiola

Highly active copper-ceria and copper-ceria-titania catalysts for methanol synthesis from CO2

Converting CO2 into methanol by catalysis By hydrogenating CO2, scientists can transform a greenhouse gas into methanol, a desirable fuel. Graciani et al. cast copper in the role of the highly active catalyst for this reaction by putting copper particles on cerium oxide. The interface between the cerium oxide and the copper enables the reverse water-gas shift reaction that converts CO2 into CO, which reacts more readily with hydrogen to make methanol. This result takes a step forward in innovating catalysts for this environmentally friendly process. Science, this issue p. 546 Synergy at a metal-oxide interface generates highly active catalysts for carbon dioxide hydrogenation to methanol. The transformation of CO2 into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO2 by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO2→CH3OH conversion.

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)]

Gold, Copper, and Platinum Nanoparticles Dispersed on CeOx/TiO2(110) Surfaces: High Water-Gas Shift Activity and the Nature of the Mixed-Metal Oxide at the Nanometer Level

At small coverages of ceria on TiO(2)(110), the CeO(x) nanoparticles have an unusual coordination mode. Scanning tunneling microscopy and density-functional calculations point to the presence of Ce(2)O(3) dimers, which form diagonal arrays that have specific orientations of 0, 24, and 42 degrees with respect to the [1 -1 0] direction of the titania substrate. At high coverages of ceria on TiO(2)(110), the surface exhibits two types of terraces. In one type, the morphology is not very different from that observed at low ceria coverage. However, in the second type of terrace, there is a compact array of ceria particles with structures that do not match the structures of CeO(2)(111) or CeO(2)(110). The titania substrate imposes on the ceria nanoparticles nontypical coordination modes, enhancing their chemical reactivity. This phenomenon leads to a larger dispersion of supported metal nanoparticles (M = Au, Cu, Pt) and makes possible the direct participation of the oxide in catalytic reactions. The M/CeO(x)/TiO(2)(110) surfaces display an extremely high catalytic activity for the water-gas shift reaction that follows the sequence Au/CeO(x)/TiO(2)(110) < Cu/CeO(x)/TiO(2)(110) < Pt/CeO(x)/TiO(2)(110). For low coverages of Cu and CeO(x), Cu/CeO(x)/TiO(2)(110) is 8-12 times more active than Cu(111) or Cu/ZnO industrial catalysts. In the M/CeO(x)/TiO(2)(110) systems, there is a strong coupling of the chemical properties of the admetal and the mixed-metal oxide: The adsorption and dissociation of water probably take place on the oxide, CO adsorbs on the admetal nanoparticles, and all subsequent reaction steps occur at the oxide-admetal interface. The high catalytic activity of the M/CeO(x)/TiO(2)(110) surfaces reflects the unique properties of the mixed-metal oxide at the nanometer level.

High catalytic activity of Au/CeOx/TiO2(110) controlled by the nature of the mixed-metal oxide at the nanometer level

Mixed-metal oxides play a very important role in many areas of chemistry, physics, materials science, and geochemistry. Recently, there has been a strong interest in understanding phenomena associated with the deposition of oxide nanoparticles on the surface of a second (host) oxide. Here, scanning tunneling microscopy, photoemission, and density-functional calculations are used to study the behavior of ceria nanoparticles deposited on a TiO2(110) surface. The titania substrate imposes nontypical coordination modes on the ceria nanoparticles. In the CeOx/TiO2(110) systems, the Ce cations adopt an structural geometry and an oxidation state (+3) that are quite different from those seen in bulk ceria or for ceria nanoparticles deposited on metal substrates. The increase in the stability of the Ce3+ oxidation state leads to an enhancement in the chemical and catalytic activity of the ceria nanoparticles. The codeposition of ceria and gold nanoparticles on a TiO2(110) substrate generates catalysts with an extremely high activity for the production of hydrogen through the water–gas shift reaction (H2O + CO → H2 + CO2) or for the oxidation of carbon monoxide (2CO + O2 → 2CO2). The enhanced stability of the Ce3+ state is an example of structural promotion in catalysis described here on the atomic level. The exploration of mixed-metal oxides at the nanometer level may open avenues for optimizing catalysts through stabilization of unconventional surface structures with special chemical activity.

Water‐Gas Shift Reaction on a Highly Active Inverse CeOx/Cu(111) Catalyst: Unique Role of Ceria Nanoparticles

=0.03 nA. Theheight image at the bottom right, showing the inside of a ceria island,was taken at imaging conditions of 2.7 V, 0.05 nA. The scheme (bot-tom left) was composed using the line profile indicated by the greenline shown near the middle of the top right image.[*] Dr. J. A. Rodriguez, Dr. J. Graciani, Dr. J. B. Park, Dr. F. Yang,Dr. D. Stacchiola, Dr. S. D. Senanayake, Dr. S. Ma, Dr. P. Liu,Dr. J. HrbekChemistry Department, Brookhaven National LaboratoryUpton, NY 11973 (USA)Fax: ( +1)631-344-5815E-mail: rodrigez@bnl.govProf. J. Evans, Prof. M. PrezFacultad de Ciencias, Universidad Central de VenezuelaCaracas 1020A (Venezuela)Prof. J. F. SanzDepartamento de Qumica Fsica, Universidad de Sevilla41012-Seville (Spain)[**] TheworkperformedatBNLwassupportedbytheUSDepartmentofEnergy, Office of Basic Energy Sciences, under contract DE-AC02-98CH10886. J.E. and M.P. are grateful to INTEVEP for partialsupport of the work carried out at the UCV. The work done at Sevillewas funded by MICINN, grant no MAT2008-04918 and theBarcelona Supercomputing Center—Centro Nacional de Super-computacin (Spain).Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.200903918.

Importance of the Metal–Oxide Interface in Catalysis: In Situ Studies of the Water–Gas Shift Reaction by Ambient-Pressure X-ray Photoelectron Spectroscopy†

The traditional approach to the optimization of metal/oxide catalysts has focused on the properties of the metal and the selection of the proper oxide for its dispersion. The importance of metal–oxide interfaces has long been recognized, [1] but the molecular determination of their properties and role is only now emerging. [2] Atoms with properties ranging from metallic to ionic are available at the interface and create unique reaction sites. We show herein how sites associated with a metal–ceria interface can dramatically change the reaction mechanism of the water–gas shift reaction (WGSR; CO + H2O!H2 + CO2). The WGSR is critical in the production of hydrogen. Multiple reaction mechanisms have been proposed. [3] In the redox mechanism, CO reacts with oxygen derived from the dissociation of H2O. In the associative process, the formation of a carbonaceous COxHy intermediate must precede the production of H2 and CO2. In situ studies are essential for the detection of surface species and active phases only present under the reaction conditions. [4] We present a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP XPS), infrared reflection absorption spectroscopy (IRRAS), and density functional theory (DFT) calculations used to study the WGSR on CeOx nanoparticles deposited on Cu(111) and Au(111). Under WGSR conditions, adsorbed bent carboxylate (CO2 d� ) species were identified over both CeOx/Cu(111) and CeOx/ Au(111), with the ceria in a highly reduced state. By combining in situ experimental results with calculations, we

Role of ceria in oxidative dehydrogenation on supported vanadia catalysts

The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

Unique Properties of Ceria Nanoparticles Supported on Metals: Novel Inverse Ceria/Copper Catalysts for CO Oxidation and the Water-Gas Shift Reaction

Oxides play a central role in important industrial processes, including applications such as the production of renewable energy, remediation of environmental pollutants, and the synthesis of fine chemicals. They were originally used as catalyst supports and were thought to be chemically inert, but now they are used to build catalysts tailored toward improved selectivity and activity in chemical reactions. Many studies have compared the morphological, electronic, and chemical properties of oxide materials with those of unoxidized metals. Researchers know much less about the properties of oxides at the nanoscale, which display distinct behavior from their bulk counterparts. More is known about metal nanoparticles. Inverse-model catalysts, composed of oxide nanoparticles supported on metal or oxide substrates instead of the reverse (oxides supporting metal nanoparticles), are excellent tools for systematically testing the properties of novel catalytic oxide materials. Inverse models are prepared in situ and can be studied with a variety of surface science tools (e.g. scanning tunneling microscopy, X-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, low-energy electron microscopy) and theoretical tools (e.g. density functional theory). Meanwhile, their catalytic activity can be tested simultaneously in a reactor. This approach makes it possible to identify specific functions or structures that affect catalyst performance or reaction selectivity. Insights gained from these tests help to tailor powder systems, with the primary objective of rational design (experimental and theoretical) of catalysts for specific chemical reactions. This Account describes the properties of inverse catalysts composed of CeOx nanoparticles supported on Cu(111) or CuOx/Cu(111) as determined through the methods described above. Ceria is an important material for redox chemistry because of its interchangeable oxidation states (Ce⁴⁺ and Ce³⁺). Cu(111), meanwhile, is a standard catalyst for reactions such as CO oxidation and the water-gas shift (WGS). This metal serves as an ideal replacement for other noble metals that are neither abundant nor cost effective. To prepare the inverse system we deposited nanoparticles (2-20 nm) of cerium oxide onto the Cu(111) surface. During this process, the Cu(111) surface grows an oxide layer that is characteristic of Cu₂O (Cu¹⁺). This oxide can influence the growth of ceria nanoparticles. Evidence suggests triangular-shaped CeO₂(111) grows on Cu₂O(111) surfaces while rectangular CeO₂(100) grows on Cu₄O₃(111) surfaces. We used the CeOx/Cu₂O/Cu(111) inverse system to study two catalytic processes: the WGS (CO + H₂O → CO₂ + H₂) and CO oxidation (2CO + O₂ → 2CO₂). We discovered that the addition of small amounts of ceria nanoparticles can activate the Cu(111) surface and achieve remarkable enhancement of catalytic activity in the investigated reactions. In the case of the WGS, the CeOx nanoparticle facilitated this process by acting at the interface with Cu to dissociate water. In the CO oxidation case, an enhancement in the dissociation of O₂ by the nanoparticles was a key factor. The strong interaction between CeOx nanoparticles and Cu(111) when preoxidized and reduced in CO resulted in a massive surface reconstruction of the copper substrate with the introduction of microterraces that covered 25-35% of the surface. This constitutes a new mechanism for surface reconstruction not observed before. These microterraces helped to facilitate a further enhancement of activity towards the WGS by opening an additional channel for the dissociation of water. In summary, inverse catalysts of CeOx/Cu(111) and CeO₂/Cu₂O/Cu(111) demonstrate the versatility of a model system to obtain insightful knowledge of catalytic processes. These systems will continue to offer a unique opportunity to probe key catalytic components and elucidate the relationship between structure and reactivity of novel materials and reactions in the future.

Cloud point extraction, preconcentration and spectrophotometric determination of erbium(III)-2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol

The extraction and preconcentration of metal chelates via surfactant-mediated phase separation was studied. The effects on extraction parameters of several experimental variables were evaluated. A successive approximation method, using a least-squares computer program, was employed for the calculation of partition and acid dissociation constants of the chelating reagent. A new, high-sensitive and low-cost methodology for the determination of Er(III)-2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol — with the nonionic surfactant, polyethyleneglycolmono-p-nonylphenylether (PONPE 7.5), with a cloud point extraction step prior to absorptiometric determination — was developed and optimized. Under the optimal experimental conditions, the molar absorptivity was 1.27 × 105 1 mol−1cm−1. Calibration plot of absorbance (584 nm) vs. concentration was linear within the range 0.02–2 mg l−1 Er(III). The lower limit of detection (LOD) was 1.48 × 10−7 mol l−1. The proposed procedure was successfully applied to the determination of Er(III) in synthetic samples, reproducing superconducting materials and permanent magnets.

Resolving the atomic structure of vanadia monolayer catalysts: monomers, trimers, and oligomers on ceria

Supported vanadium oxide catalysts have received considerable attention owing to their high activity for selective oxidation reactions. The reactivity has been shown to depend strongly on the oxide support, with reducible oxides (e.g., ceria, titania, and zirconia) exhibiting much higher turnover frequencies for oxidative dehydrogenation (ODH) reactions than irreducible oxides (e.g., silica and alumina). Structural characterization of the catalysts has been performed primarily using Raman and UV/Vis spectroscopy (see Ref. [4, 6,7] and references therein), as well as X-ray absorption spectroscopy. These results have been used to postulate that vanadia catalysts consist of isolated and polymer structures that wet the supporting oxide (so-called “monolayer catalysts”). To elucidate the surface chemistry of vanadia, different model systems, such as vanadia single crystals and thin films as well as vanadia clusters supported on planar metal oxide substrates, have been studied experimentally by surface-science techniques and computational means. To rationalize structure–reactivity relationships, welldefined systems are required. Of the reducible metal oxide supports that are known to be highly active in ODH reactions, ceria is particularly suited, because well-ordered thin films can be grown with a known surface termination. Previously, the structure and reactivity of vanadia supported on CeO2(111) has been studied using photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD). 15] However, the atomic structure of ceria-supported vanadia monolayer catalysts has not been resolved. Herein, using a combination of high-resolution scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and PES with synchrotron radiation, we unambiguously demonstrate the formation of monomeric O=VO3 species on the CeO2(111) surface at low vanadia loadings. For the first time, we show a direct relationship between the nuclearity of vanadia species (monomeric vs. polymeric) as observed by STM and their vibrational properties. We show that ceria stabilizes the vanadium + 5 oxidation state, leading to partially reduced ceria upon vanadium deposition. These experimental results are fully supported by density functional theory (DFT) calculations. The results indicate that ceria surfaces stabilize small vanadia species, such as monomers and trimers, that sinter into two-dimensional, monolayer islands. Such stabilization probably plays a crucial role in the enhanced activity observed for ceriasupported vanadia in ODH reactions. Indeed, low-nuclearity species revealed reactivities at much lower temperatures than those with higher nuclearity, which in turn show strong similarities to the reactivity of vanadia clusters supported on alumina and silica. 13] Figure 1 presents compelling evidence for the presence of vanadia monomers on ceria at low coverage (ca. 0.3 V atoms nm ). The STM image in Figure 1 a shows that highly dispersed and randomly distributed species are formed upon deposition of vanadium in an ambient oxygen atmosphere onto a CeO2(111) thin film (see the Experimental Section). The absence of preferential nucleation sites indicates a strong interaction between vanadia species and the underlying ceria support. In the atomically resolved image (inset of Figure 1a), the two protruding spots (ca. 3 in diameter and 1.2 in height) appear to be monomers positioned atop protrusions in the ceria substrate. The apparent height of these vanadia species depends on the tunneling bias and monotonically decreases from approximately 1.8 at 2.2 V to approximately 0.9 at 3 V. At certain voltages, a dark “halo” is visible around a few of the monomeric species (see Figure 1a), which may be related to defect structures of the ceria film. The IR spectrum corresponding to the sample shown in Figure 1a is depicted in Figure 1d with an absorption feature at 1006 cm . This peak is assigned to the vanadyl (V=O) stretching vibration on the basis of comparison with other vanadia systems and reference compounds in which the V=O [*] M. Baron, Dr. H. Abbott, Dr. O. Bondarchuk, Dr. D. Stacchiola, Dr. A. Uhl, Dr. S. Shaikhutdinov, Prof. Dr. H.-J. Freund Fritz Haber Institute of the Max Planck Society Chemical Physics Department Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49)30-8413-4105 E-mail: shamil@fhi-berlin.mpg.de