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The research activity of the present thesis has focused on the development of carbon/inorganic hierarchical nanostructured hybrids to be employed as catalysts for two important energy processes: the water-gas shift reaction (WGSR) and the photocatalytic hydrogen production from renewable sources, such as biomass-derived oxygenated compounds (i.e. ethanol and glycerol). The design of the nanohybrids follows a specific hierarchy where a carbon component, consisting of properly functionalized multiwalled carbon nanotubes (MWCNTs) or carbon nanocones (CNCs), is used as scaffold for an inorganic phase that acts as the catalyst/cocatalyst. The inorganic phase in turns consist of noble metal nanoparticles (i.e. palladium) enclosed into a mesoporous metal oxide. The functionalization of the carbon components serves to equip the nanocarbon with anchor points for the metal phase and to enhance dispersion in liquid media. The as-prepared ternary hybrid is then subjected to specific thermal treatments, with the temperature chosen on the basis of thermogravimetric analysis, in order to crystallize the metal oxide phase and remove the organic ligands. The final catalyst package has shown remarkable catalytic features in both the investigated processes, confirming that the presence of the nanocarbon scaffold and the specific hierarchy result in a large improvement of the performance as compared to state-of-the art catalysts. Such improvement is related to the excellent electronic properties of the carbon nanostructures as well as to their ability to enhance robustness and stability of the inorganic phase. More specifically, CeO2-based catalysts display an increased activity and stability in the WGSR, while the TiO2-based photocatalysts were successfully used in the photocatalytic production of H2 with very high productivity. Characterization of the materials has been carried out through several techniques including HR-TEM, EDX mapping, XRD, microRaman, physi- and chemi-sorption analysis and TGA, which confirmed the obtainment of the desired assembly.

In this article we present the first comparative study of the transient decay dynamics of photo-generated charges for the three polymorphs of TiO2. To our knowledge, this is the first such study of the brookite phase of TiO2 over timescales relevant to the kinetics of water splitting. We find that the behavior of brookite, both in the dynamics of relaxation of photo-generated charges and in energetic distribution, is similar to the anatase phase of TiO2. Moreover, links between the rate of recombination of charge carriers, their energetic distribution and the mode of transport are made in light of our findings and used to account for the differences in water splitting efficiency observed across the three polymorphs.

In view of the sustainable H2 production, development of more efficient catalysts for photocatalytic reforming of oxygenated compounds is required. In this study, we report the preparation of TiO2 nanocomposite with anatase/brookite composition prepared by hydrothermal treatments of Na-titanate precursor. The anatase/brookite ratio can be modulated changing the synthetic parameters, i.e. precursor/water mass-to volume ratio and hydrothermal treatment duration. The obtained materials present well crystallized particles with polyhedral morphology. The anatase/brookite ratio in the nanocomposite affects the mean size of Pt nanoparticles grown by photodeposition and the photocatalytic activity in H2 production by photodehydrogenation of ethanol, with multiphasic materials presenting smaller Pt nanoparticles and higher H2 production. The present anatase/brookite nanocomposites show higher H2 production normalized to the surface area with respect a reference TiO2 prepared by conventional sol–gel synthesis, suggesting that the present materials might expose a higher fraction of highly reactive facets instead of the most thermodynamically stable.

Mechanically robust, chemically stable and electronically active carbon nanotubes (CNTs) are widely used as supports in catalysis. Synergistic effects between CNT and the active phase critically depend on the homogeneity of the carbon/inorganic interface, whose assembly is difficult to achieve without admixtures of free-standing inorganic matrix. Here we show that Fe-filled CNTs, employed as nanocatalyst supports, allow a facile preparation of highly pure and uniform CNT/nanocatalyst materials, by taking advantage of magnetic separation from poorly-defined components (e.g. aggregates of inorganic nanocatalysts). The higher homogeneity translates into higher catalytic activity in two industrially important processes: the photocatalytic hydrogen production and the water-gas shift reaction, WGSR (increase of ~48% activity for the former and up to ~45% for the latter as compared to catalysts isolated by standard filtration). In addition, the magnetic Fe core in the nanotubes enables effective separation and re-use of the nanocatalyst without loss of activity. This study demonstrates significant potential of magnetic CNTs as next generation of sustainable catalyst supports that can improve production of hydrogen and reduce the use of precious metals.

Photocatalytic reactions could enhance the share of chemicals produced through renewable sources. The efficiency of photocatalysts drastically depends on light absorption, on the surface energy of the crystals, and on the properties of the nanobuilding blocks assembled in devices. Here, we show that photoelectrochemical water oxidation on brookite TiO2 nanorods is greatly enhanced by engineering the location of Au nanoparticles deposition. Brookite photoanodes show a very low onset potential for water oxidation to H2O2 of -0.2 VRHE due to energetics of exposed crystal facets. By combining electrochemical measurements and ultrafast optical spectroscopy, we link the water oxidation activity with electron-hole recombination phenomena. The preferential Au decoration at the electrode/water interface produces highly enhanced photocurrent, while when Au is distributed along the whole film thickness, the activity is depressed with respect to pure brookite. In the latter case, Au nanoparticles act as recombination centers with plasmonic carriers recombining on the same time scale of their generation (fs). Conversely, Au surface decoration enables a hot electrons lifetime 4 orders of magnitude longer (ns) due to efficient hopping on brookite lateral facets, thus providing an efficient path for plasmon-enhanced solar water oxidation. (Graph Presented).

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.

Clean and efficient hydrogen production is of great interest because hydrogen is envisioned as the fuel of the future. In particular, hydrogen production from biomass-derived alcohols has attracted great attention because of the potential application in fuel cells. In this short review, the major results obtained in the last years by the Material, Environment and Energy (MEE) research group at the University of Trieste (Italy) in the photocatalytic production of hydrogen are summarized. Our attention has been devoted to the use of biomass-derived oxygenated compounds (mainly ethanol and glycerol) as sacrificial agents to improve hydrogen production. Various synthetic techniques (sol-gel, hydrothermal synthesis etc.) have been adopted to prepare nanostructured TiO2-based photocatalysts with different phase composition and/or morphology in the form of powders. Different strategies have been adopted to improve the performances of TiO2-based materials, especially favoring the photocatalytic activity under simulated sunlight. Metal nanoparticles (Cu, Pt, Au, Pd), self-doping of TiO2 and hierarchically organized nanocomposite with carbon nanotubes strongly improve the hydrogen production. The results will highlight the role of different parameters (phase composition, morphology, doping and nanocomposite formulation) in the improvement of photocatalytic hydrogen production.

Photocatalytic reforming of renewable raw materials in aqueous solutions represents a valid possibility for the valorization of wastes from agricultural and industrial processes. In this study, we prepared TiO2 materials with different phase compositions by changing the urea/Ti molar ratio employed during the hydrothermal preparation method. After deposition of Pt nanoparticles, the photocatalytic H-2 production was investigated using ethanol and glycerol as sustainable sacrificial agent. While anatase/rutile nanocomposites showed poor catalytic activity, the anatase/brookite nanocomposites showed more promising performances. Specifically, the anatase-rich materials showed the best performances on mass bases. When normalized with respect to the surface area of the photocatalysts, the activity continuously increased with the brookite content, indicating that exposed facets of brookite possess an intrinsic higher activity than that of the other polymorphs.

Production of molecular hydrogen (H-2) is becoming an increasingly prominent process, due to high expectations as a new green energy carrier and key reagent for many industrial processes. Herein we report the high efficiency of H-2 production via photoreforming of ethanol using a catalyst based on hierarchical carbon nanocones hybridised with an inorganic layer of nanocrystalline TiO2 containing Pd nanoparticles.

An efficient method is presented for preparing hierarchical catalysts composed of multi-walled carbon nanotubes (MWCNTs) and Pd@CeO2 core–shell nanoparticles. These materials were then examined for the water gas shift reaction (WGSR), which demonstrated intimate contact between the constituent parts. The integration of the carbonaceous support improves the stability of the nanoparticles by ordering the dispersion of the inorganic phase and increases the activity by suppressing the deactivation of the active phase that is commonly observed in conventional Pd–CeO2 under reducing conditions, e.g. WGSR conditions. An optimum MWCNTs:Pd@CeO2 ratio exists that affords totally homogeneous structures and provides the best catalytic properties.