Creating and mastering nano-objects to design advanced catalytic materials

New developments in the synthesis of nano-materials have opened new possibilities for creating and mastering nano-objects in order to design novel advanced catalytic materials. This concise conceptual review will give a glimpse into this fast growing research area discussing some of the possibilities in this direction, the perspectives and the gap to reduce to develop selective catalysts for complex multistep reactions. Emphasis is given to the opportunities offered by a tailored nano-design of the catalysts, from exploiting nano-confinement effects and supramolecular active sites synergies in nano-reactors to the new possibilities offered by new concepts such as the reduction of the relaxation time between two consecutive turnover cycles on a single active site and forcing a vectorial active site sequence in complex, multistep reactions. Other aspects discussed include the development of hierarchic pore structure to maximize catalyst effectiveness, metal complexes confined with solid cavities and the concept of nano-reactors, nanostructured composites and ordered 1D-type metal oxides. It is shown how significant progress in nano-materials has still not corresponded with progress in understanding the relationship between nanostructure and catalytic performance and the development of a more general strategy on the design of next-generation nano-catalysts.     

Electrocatalytic conversion of CO2 to liquid fuels using nanocarbon-based electrodes

Recent advances on the use of nanocarbon-based electrodes for the electrocatalytic conversion of gaseous streams of CO₂ to liquid fuels are discussed in this perspective paper. A novel gas-phase electrocatalytic cell, different from the typical electrochemical systems working in liquid phase, was developed. There are several advantages to work in gas phase, e.g. no need to recover the products from a liquid phase and no problems of CO₂ solubility, etc. Operating under these conditions and using electrodes based on metal nanoparticles supported over carbon nanotube (CNT) type materials, long C-chain products (in particular isopropanol under optimized conditions, but also hydrocarbons up to C8–C9) were obtained from the reduction of CO₂. Pt-CNT are more stable and give in some cases a higher productivity, but Fe-CNT, particular using N-doped carbon nanotubes, give excellent properties and are preferable to noble-metal-based electrocatalysts for the lower cost. The control of the localization of metal particles at the inner or outer surface of CNT is an importact factor for the product distribution. The nature of the nanocarbon substrate also plays a relevant role in enhancing the productivity and tuning the selectivity towards long C-chain products. The electrodes for the electrocatalytic conversion of CO₂ are part of a photoelectrocatalytic (PEC) solar cell concept, aimed to develop knowledge for the new generation artificial leaf-type solar cells which can use sunlight and water to convert CO₂ to fuels and chemicals. The CO₂ reduction to liquid fuels by solar energy is a good attempt to introduce renewables into the existing energy and chemical infrastructures, having a higher energy density and easier transport/storage than other competing solutions (i.e. H₂).     

非晶碳纳米球的低温石墨化(英文)

The investigation by SEM/TEM, porosity, and X-ray diffraction measurements of the graphitization process starting from amorphous carbon nanospheres, prepared by glucose carbonization, is reported. Aspects studied are the annealing temperature in the 750–1000 °C range, the type of inert carrier gas, and time of treatment in the 2–6 h range. It is investigated how these parameters influence the structural and morphological characteristics of the carbon materials obtained as well as their nanostructure. It is shown that it is possible to maintain after graphitization the round-shaped macro morphology, a high surface area and porosity, and especially a large structural disorder in the graphitic layers stacking, with the presence of rather small ordered domains. These are characteristics interesting for various catalytic applications. The key in obtaining these characteristics is the thermal treatment in a flow of N2. It was demonstrated that the use of He rather than N2 does not allow obtaining the same results. The effect is attributed to the presence of traces of oxygen, enough to create the presence of oxygen functional groups on the surface temperatures higher than 750 °C, when graphitization occurs. These oxygen functional groups favor the graphitization process.     

ANTENNA EFFECT IN LUMINESCENT LANTHANIDE CRYPTATES: A PHOTOPHYSICAL STUDY

Excited state emission and absorption decay measurements have been made on the cage-type cryptate complexes [M bpy.bpy.bpy]ⁿ⁺, where Mⁿ⁺= Na⁺, La³⁺, Eu³⁺, Gd³⁺ or Tb³⁺ and [bpy.bpy.bpy] is a tris-bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity ¹π-π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H₂O or D₂O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mⁿ⁺= Na⁺, La³⁺ and Gd³⁺ the lowest excited state of the cryptate is a ³ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H₂O solution and in the second-millisecond time scale at 77 K in MeOH-EtOH. For Mⁿ⁺= Eu³⁺, the lowest excited state is the luminescent ⁵D₀ Eu³⁺ level which in H₂O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 10³ s^_1 at room temperature and 1.2 × 10³ s^?′ at 77 K. The relatively low efficiency of ⁵D₀ population upon ¹ππ* excitation is attributed to the presence of a ligand-to-metal charge transfer level through which ¹ππ* decays directly to the ground state. For Mⁿ⁺= Tb³⁺ the lowest excited state is the luminescent ⁵D₄ Tb³⁺ level. The process of ⁵D₄ population upon ¹ππ* excitation is ?100% efficient, but at room temperature it is followed by a high-efficiency, activated back energy transfer from the ⁵D₄ Tb³⁺ level to the ³ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm^_1) and the intrinsically long lifetime of ⁵D₄. At 77 K back energy transfer cannot take place and the ⁵D₄ Tb^3* level deactivates to the ground state with rate constant 5.9 × 10² s^-′ (H₂O solution). The relevance of these results toward the optimization of Eu³⁺ and Tb³⁺ cryptates as luminescent probes is discussed.     

Nanocarbons: Opening New Possibilities for Nano-engineered Novel Catalysts and Catalytic Electrodes

Nanocarbons find increasing relevance for the development of advanced, sometimes radically new, catalysts and catalytic electrodes. This perspective contribution discusses the potential of nanocarbons as a new class of catalytic materials, even if carbons (in the form mainly of active carbon materials) are already extensively applied as supports for catalysts. The control of nano-dimension and the improved understanding in tailoring the surface reactivity open new possibilities for their nano-engineering and the development of novel catalytic materials. With focus on the nature of the active sites in nanocarbon catalysts, we discuss here some of the novel possibilities opened by these materials to address the new challenges for catalysis deriving from moving to a more sustainable chemical and energy production.     

碳基催化剂:为开发下一代纳米工程催化材料开辟新方法(英文)

This essay analyses some of the recent development in nanocarbons (carbon materials having a defined and controlled nano-scale dimension and functional properties which strongly depend on their nano-scale features and architecture), with reference to their use as advanced catalytic materials. It is remarked how their features open new possibilities for catalysis and that they represent a new class of catalytic materials. Although carbon is used from long time in catalysis as support and electrocatalytic applications, nanocarbons offer unconventional ways for their utilization and to address some of the new challenges deriving from moving to a more sustainable future. This essay comments how nanocarbons are a key element to develop next-generation catalytic materials, but remarking that this goal requires overcoming some of the actual limits in current research. Some aspects are discussed to give a glimpse on new directions and needs for RD to progress in this direction.     

Specific activity of copper species in decomposition of NO on Cu-ZSM-5

The kinetids of NO decomposition to N₂ over Cu-ion exchanged ZSM-5 with different Si/Al ratios and the change of the specific activity per copper site as a function of the Cu/Al ratio suggest that the copper species formed at a Cu/Al ratio of 0.5 or higher and probably containing extra-lattice oxygen have higher turnover frequency in the reaction.     

Oxide thin films based on ordered arrays of 1D nanostructure. A possible approach toward bridging material gap in catalysis

TiO(2) thin films based on ordered arrays of 1D nanostructures (nanorods, nanotubes) are proposed as suitable model materials in studies for bridging material and complexity gap in catalysis. The samples were prepared by anodic oxidation of Ti foils. By changing the preparation conditions (pH, procedure of application of the potential), different types of 1D nanostructure and different characteristics of the ordered array of these 1D nanostructures could be obtained. This allows studying the effect of nanodimension and 3D nanoarchitecture on the characteristics and reactivity of these catalysts. It is also shown that TiO(2) thin films characterized by a well-ordered array of titania nanorod behave as photonic materials, thus showing unique properties of light harvesting efficiency.