Michele Melchionna

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.

Carbon nanotubes and catalysis: the many facets of a successful marriage

Carbon nanotubes have emerged as unique carbon allotropes that bear very interesting prospects in catalysis. Their use is mostly related to that of supports for inorganic metal catalysts, including molecular catalysts, metal nanoparticles, metal oxides or even more complex hierarchical hybrids. However, several reports have shown that they can intriguingly act as metal-free catalysts, with performance often superior to that of other carbon materials, in particular when ad hoc organic functional groups are attached prior to catalytic screening. The range of catalytic reactions is quite wide, and it includes standard organic synthesis, electrocatalysis, photocatalysis as well as other important industrial processes. In the last few years, the energy sector has acquired a dominant role as one of the most sought-after fields of application, given its ever-increasing importance in society.

Wire Up on Carbon Nanostructures! How To Play a Winning Game

Carbon nanotubes and graphene possess a unique extended π-system that makes them stand out among carbon nanostructures. The resulting electronic properties enable electron or charge flow along one or two directions, respectively, thus offering the opportunity to connect electronically different entities that come into contact, be they living cells or catalytic systems. Using these carbon nanostructures thus holds great promise in providing innovative solutions to address key challenges in the fields of medicine and energy. Here, we discuss how chemical functionalization of these carbon nanostructures is a crucial tool to master their properties and deliver innovation.

Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution

Considering the depletion of fossil-fuel reserves and their negative environmental impact, new energy schemes must point towards alternative ecological processes. Efficient hydrogen evolution from water is one promising route towards a renewable energy economy and sustainable development. Here we show a tridimensional electrocatalytic interface, featuring a hierarchical, co-axial arrangement of a palladium/titanium dioxide layer on functionalized multi-walled carbon nanotubes. The resulting morphology leads to a merging of the conductive nanocarbon core with the active inorganic phase. A mechanistic synergy is envisioned by a cascade of catalytic events promoting water dissociation, hydride formation and hydrogen evolution. The nanohybrid exhibits a performance exceeding that of state-of-the-art electrocatalysts (turnover frequency of 15000 H2 per hour at 50 mV overpotential). The Tafel slope of ∼130 mV per decade points to a rate-determining step comprised of water dissociation and formation of hydride. Comparative activities of the isolated components or their physical mixtures demonstrate that the good performance evolves from the synergistic hierarchical structure.

The Unexpected Advantages of Using D-Amino Acids for Peptide Self- Assembly into Nanostructured Hydrogels for Medicine

Self-assembled peptide hydrogels have brought innovation to the medicinal field, not only as responsive biomaterials but also as nanostructured therapeutic agents or as smart drug delivery systems. D-amino acids are typically introduced to increase the peptide enzymatic stability. However, there are several reports of unexpected effects on peptide conformation, self-assembly behavior, cytotoxicity and even therapeutic activity. This mini-review discusses all the surprising twists of heterochiral self-assembled peptide hydrogels, and delineates emerging key findings to exploit all the benefits of D-amino acids in this novel medicinal area.

Solar and visible light photocatalytic enhancement of halloysite nanotubes/g-C3N4 heteroarchitectures

Novel heteroarchitectures made of graphitic carbon nitride (g-C3N4) and halloysite nanotubes (HNTs) were prepared by a facile and soft self-assembly strategy. The morphological, structural and electronic properties of the HNTs/g-C3N4 nanocomposites were determined by means of a plethora of techniques including N2 physisorption, XRD, FT-IR, TEM, UV-Vis absorption, XPS, ζ potential and photoluminescence (PL). Their photocatalytic activity was evaluated under both simulated solar light and pure visible light irradiation against the photodegradation of neutral, positively and negatively charged pollutants, namely phenol, methylene blue (MB) and methyl orange (MO), respectively. The prepared HNTs/g-C3N4 nanocomposites were proven to be durable and significantly more efficient than the pure g-C3N4 reference for the degradation of positively charged and neutral organics. The nanocomposites presented increased charge carrier formation and reduced recombination rates due to the tight contact between the two components. The enhanced photoactivity was attributed to the dual function of HNTs enhancing (a) the abundance and stability of the photogenerated e− and h+ pairs and (b) the adsorption of positively charged organics on the nanocomposite. Both functions originate from the charged surface of HNTs.

Carbon Nanostructures for Nanomedicine: Opportunities and Challenges

Carbon nanostructures (e.g., fullerenes, CNTs, graphene, nanohorns, nanodiamonds) play a key role in the development of nanomedicines. Here we analyze opportunities and barriers for their application in clinical settings. In particular, we discuss the unique properties of certain carbon nanostructures that allow for innovative solutions in theranostics, as well as current pitfalls around clinical applications of the EPR effect, the formation of the protein corona, and how functionalization of CNTs is a key tool to modulate their biodistribution and toxicity.

Catalysis-Material Crosstalk at Tailored Nano-Carbon Interfaces

The use of carbon nanomaterials as supports for molecular and nanostructured catalysts is becoming a more and more popular strategy to improve heterogeneous catalysis. Their outstanding electronic and optical properties together with high surface area and thermal and mechanical stabilities make them ideal elements to provide catalysts with additional or improved characteristics. The role of the carbon nanostructures in the different types of catalysis is more intricate and often involves active and strong interactions between the support and the catalytic active species, creating a synergistic effect that in many cases leads to performance enhancement and an expanded range of possible applications. In particular, photocatalysis and electrocatalysis seem to benefit from the features of these types of carbon support, although applicability can be extended to more classic transformations of organic substrates.

Carboxylated, Fe‐Filled Multiwalled Carbon Nanotubes as Versatile Catalysts for O2 Reduction and H2 Evolution Reactions at Physiological pH

The development of new electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) at physiological pH is critical for several fields, including fuel cells and biological applications. Herein, the assembly of an electrode based on carboxyl-functionalised hydrophilic multiwalled carbon nanotubes (MWCNTs) filled with Fe phases and their excellent performance as electrocatalysts for ORR and HER at physiological pH are reported. The encapsulated Fe dramatically enhances the catalytic activity, and the graphitic shells play a double role of efficiently mediating the electron transfer to O2 and H2 O reactants and providing a cocoon that prevents uncontrolled Fe oxidation or leaching.

Making H2 from light and biomass-derived alcohols: the outstanding activity of newly designed hierarchical MWCNT/Pd@TiO2 hybrid catalysts

Hydrogen evolution is among the most investigated catalytic processes given the importance of H2 from an industrial and an energy perspective. Achieving H2 production through green routes, such as water splitting or more realistically photoreforming of alcohols, is particularly desirable. In this work, we achieve a remarkable H2 productivity through photoreforming of either ethanol or glycerol as a sacrificial electron donor by employing a hybrid nanocatalyst where the properties of multi-walled carbon nanotubes (MWCNTs), Pd nanoparticles and crystalline TiO2 are optimally merged through appropriate engineering of the three components and an optimised synthetic protocol. Catalysts were very active both under UV (highest activity 25 mmol g−1 h−1) and simulated solar light (1.5 mmol h−1 g−1), as well as very stable. Critical to such high performance is the intimate contact of the three phases, each fulfilling a specific task synergistically with the other components.