Tiziano Montini

Exceptional Activity for Methane Combustion over Modular Pd@CeO2 Subunits on Functionalized Al2O3

Addressing a Burning Issue Complete combustion of methane is required in order to avoid the unproductive emission of this greenhouse gas into the atmosphere. Palladium catalysts can help to promote complete combustion, but high-temperature operating conditions also promote aggregation of catalyst particles (“sintering”) that lowers their surface area and overall activity. Cargnello et al. (p. 713; see the Perspective by Farrauto) report that cerium oxide–coated Pd catalyst particles could be fully dispersed on an alumina surface prepared with a hydrophobic coating. This treatment resisted Pd sintering up to temperatures of 800°C, and also enabled complete combustion of methane to occur at temperatures as low as 400°C. A catalyst allows complete combustion of methane, a more powerful greenhouse gas than carbon dioxide, to occur at lower temperatures. There is a critical need for improved methane-oxidation catalysts to both reduce emissions of methane, a greenhouse gas, and improve the performance of gas turbines. However, materials that are currently available either have low activity below 400°C or are unstable at higher temperatures. Here, we describe a supramolecular approach in which single units composed of a palladium (Pd) core and a ceria (CeO2) shell are preorganized in solution and then homogeneously deposited onto a modified hydrophobic alumina. Electron microscopy and other structural methods revealed that the Pd cores remained isolated even after heating the catalyst to 850°C. Enhanced metal-support interactions led to exceptionally high methane oxidation, with complete conversion below 400°C and outstanding thermal stability under demanding conditions.

Fundamentals and Catalytic Applications of CeO2-Based Materials

Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40(th) anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are deepening our understanding of these materials, helping us to predict their behavior and application potential. This review has a wide view on all those aspects related to ceria which promise to produce an important impact on our life, encompassing fundamental knowledge of CeO2 and its properties, characterization toolbox, emerging features, theoretical studies, and all the catalytic applications, organized by their degree of establishment on the market.

Surface Phases and Photocatalytic Activity Correlation of Bi2O3/Bi2O4-x Nanocomposite

UV-visible irradiation induces surface alteration of Bi2O3 leading to Bi2O3/Bi2O4-x nanocomposites with excellent photocatalytic activity.

Embedded Phases: A Way to Active and Stable Catalysts

Industrial catalysts are typically made of nanosized metal particles, carried by a solid support. The extremely small size of the particles maximizes the surface area exposed to the reactant, leading to higher reactivity. Moreover, the higher the number of metal atoms in contact with the support, the better the catalyst performance. In addition, peculiar properties have been observed for some metal/metal oxide particles of critical sizes. However, thermal stability of these nanostructures is limited by their size; smaller the particle size, the lower the thermal stability. The ability to fabricate and control the structure of nanoparticles allows to influence the resulting properties and, ultimately, to design stable catalysts with the desired characteristics. Tuning particle sizes provides the possibility to modulate the catalytic activity. Unique and unexpected properties have been observed by confining/embedding metal nanoparticles in inorganic channels or cavities, which indeed offers new opportunities for the design of advanced catalytic systems. Innovation in catalyst design is a powerful tool in realizing the goals of more green, efficient and sustainable industrial processes. The present Review focuses on the catalytic performance of noble metal- and non precious metal-based embedded catalysts with respect to traditional impregnated systems. Emphasis is dedicated to the improved thermal stability of these nanostructures compared to conventional systems.

The Potential of Supported Cu2O and CuO Nanosystems in Photocatalytic H2 Production

Hy wire: Supported Cu(2)O nanosystems and CuO nanowires obtained by chemical vapor deposition were used in the photocatalytic splitting of methanol/water solutions to produce hydrogen. The results obtained with these systems open appealing perspectives for the clean conversion of sunlight into storable chemical energy.

CuOx−TiO2 Photocatalysts for H2 Production from Ethanol and Glycerol Solutions†

Hydrogen production by photocatalytic reforming of aqueous solutions of ethanol and glycerol was studied with the use of impregnated and embedded CuO(x)/TiO(2) photocatalysts. Embedded CuO(x)@TiO(2) was prepared by a water-in-oil microemulsion method, which consists in the formation of Cu nanoparticles in the microemulsion followed by controlled hydrolysis and condensation of tetraisopropyl orthotitanate with the aim of covering the protected metal particles with a surrounding layer of porous titanium oxyhydroxide. Mild calcination leads to the complete removal of the organic residues, the crystallization of TiO(2), and an unavoidable oxidation of copper. Two reference samples were prepared by classical wet impregnation of preformed TiO(2) with different ratios of anatase, rutile, and brookite polymorphs. The two supports were prepared by sol-gel (TiO(2)-SG) and microemulsion (TiO(2)-ME) methods. Superior performances have been observed for the embedded system, which shows higher hydrogen production rates with respect to the impregnated systems using either ethanol or glycerol as sacrificial molecules. Deep structural characterization of the materials has been performed by coupling high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray absorption fine structure (XAFS), and electron paramagnetic resonance (EPR) techniques. Correlation between copper oxidation state and its dispersion and reactivity has been attempted. Finally, the stability of the CuO(x)/TiO(2) catalysts was also studied with respect to carbonaceous deposits and copper leaching.

Synthesis of Dispersible Pd@CeO2 Core−Shell Nanostructures by Self-Assembly

A methodology is described for the preparation of Pd@CeO(2) core-shell nanostructures that are easily dispersible in common organic solvents. The method involves the synthesis of Pd nanoparticles protected by a monolayer of 11-mercaptoundecanoic acid (MUA). The carboxylic groups on the nanoparticle surfaces are used to direct the self-assembly of a cerium(IV) alkoxide around the metal particles, followed by the controlled hydrolysis to form CeO(2). The characterization of the nanostructures by means of different techniques, in particular by electron microscopy, allowed us to demonstrate the nature of core-shell systems, with CeO(2) nanocrystals forming a shell around the MUA-protected Pd core. Finally, an example of the use of these nanostructures as flexible precursors for the preparation of heterogeneous catalysts is reported by investigating the reactivity of Pd@CeO(2)/Al(2)O(3) nanocomposites toward CO oxidation, water-gas shift (WGS), and methanol steam reforming reactions. Together with CO adsorption data, these observations suggest the accessibility of the Pd phase in the nanocomposites.

Nanostructured Cu/TiO2 Photocatalysts for H2 Production from Ethanol and Glycerol Aqueous Solutions.

The sustainable development of the human society requires an increasing use of renewable raw materials and energy sources. In this context, photocatalysis represents a promising and necessary way to produce solar fuels and chemicals. The photocatalytic hydrogen production of renewable oxygenated compounds from aqueous solutions could represent an important alternative 6] to the more complex water splitting, or to conventional thermal reforming processes. Noble and base metals, including Pt, Au, Pd, Ni, Cu, and Ag, have been reported to be very efficient at increasing the production of H2 in TiO2 photocatalysis. [11] Increasing attention has been devoted to Cu2O and CuO as photocatalysts. [12, 13] Both copper oxides are abundant natural p-type semiconductors and are attractive owing to their virtual non-toxicity. Their bandgaps (2.1 eV for Cu2O and 1.2 eV for CuO) are suitable for photosplitting water to produce hydrogen by using visible light. However, these materials have been shown to be unstable in electrolytic solutions owing to their facile photooxidation. Siripala et al. prepared a Cu2O/TiO2 heterojunction by electrodeposition of copper oxide; the device was shown to perform photoelectrolysis of water, the TiO2 layer providing protection against photocorrosion. Herein, a cheap and active photocatalysts based on Cu nanoparticles dispersed on TiO2 supports, which are capable of operating under solar radiation, are investigated for the production of hydrogen from ethanol and glycerol. Second generation ethanol and sugars, extracted from lignocellulosic parts of vegetables, and glycerol, produced as a by-product of bio-diesel, are attractive and largely available sacrificial agents. 16] Two different TiO2 supports were prepared, the former from titanium isopropoxide (TiO2-SG) [17] by a sol–gel method, the latter from titanyl sulphate (TiO2-PS) by precipitation, [18] followed in both cases by calcination at 450 8C for 6 h. TiO2-SG (surface area 69 mg ) was composed of a mixture of polymorphs (Figure S1 in the Supporting Information). The analysis of its XRD pattern, following the work of Zhang et al. , evidenced the presence of anatase (64 wt %), rutile (8 wt %) and brookite (28 wt %). Mean crystallite sizes of 11, 32, and 11 nm were calculated for anatase, rutile, and brookite, respectively. TiO2-PS possessed a slightly higher surface area (104 m 2 g ). Its powder XRD pattern showed the presence of a pure anatase phase (Figure S1 in the Supporting Information), with a mean crystallite size of 8 nm. The photodeposition of Cu on the surface of both TiO2 supports was performed by using UV/Vis irradiation (2 h) in the presence of copper nitrate and CH3OH as a hole scavenger (Figure S2). XRD analysis of the Cu/TiO2 nanocomposites did not allow the identification of Cu-related phases probably because of their low amount and/or high dispersion. Therefore, the Cu phases photodeposited on TiO2 supports were characterized by using X-ray absorption near-edge structure (XANES) or extended X-ray absorption fine structure (EXAFS) spectroscopy. Immediately after photodeposition, XANES spectra at the Cu K edge was in good agreement with that of Cu foil used as reference standard (Figure 1), thus indicating that copper is deposited in the form of zero-valent copper. The results of the analysis of the EXAFS spectra are summarized in Table 1.

F-Doped Co3O4 Photocatalysts for Sustainable H2 Generation from Water/Ethanol

p-Type Co(3)O(4) nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H(2) from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co(3)O(4) results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co(3)O(4) films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes.

Identification of the Structural Phases of CexZr1−xO2 by Eu(III) Luminescence Studies

Despite the wide application of ceria-zirconia based materials in Three Way Catalysts (TWCs), Solid Oxides Fuel Cells (SOFCs), and H(2) production and purification reactions, an active debate is still open on the correlation between their structure and redox/catalytic performances. Existing reports support the need of either (i) a homogeneous solid solution or (ii) materials with nanoscale heterogeneity to obtain high activity and stability. Here we report on a simple and inexpensive approach to solve this problem taking advantage of the luminescence properties of Eu(III), used as a structural probe introduced either in the bulk or on the surface of the samples. In this way, the real structure of ceria-zirconia materials can be revealed even for amorphous high surface area samples. Formation of small domains is observed in catalytically important metastable samples which appear homogeneous by conventional XRD.