1,721,023 research outputs found
NANOSTRUCTURED MATERIALS FOR ENVIRONMENTAL AND ENERGY-RELATED APPLICATIONS
No full textThe world is facing an era of global environmental pollution, as a result of the tremendous population growth and the consequent massive fossil fuel-based energy consumption. A significant exploitation of renewable energies is needed to guarantee quality of human life and allow further sustainable growth, but this may take decades to happen. In order to mitigate the negative effect of human activities on the environment in the short- and mid-term, the development of more efficient technologies for emissions abatement and for renewable fuels production is imperative. Heterogeneous catalysis and photocatalysis are two key pillars of a multi-approach strategy to solve these issues.
During the last century, catalysts were explored by changing the formulation of multi-component systems in order to find the best performing material for a certain reaction. Since the late 90's, a new approach to catalytic systems improvement emerged: nano-catalysis. Exploiting the tools of nanotechnology, tailored nanostructured materials can now be produced, which show different properties in comparison to their bulky counterparts, often resulting in better catalytic performances. Furthermore, combining the elements of the periodic table in nano-alloys allows to expand the possibility of catalyst generation. Consistently with these approaches, the main focus of this thesis is the synthesis and characterization of well-defined nanostructured and hierarchical materials for environmental and energy-related applications, such as emissions control, biofuels synthesis and photocatalytic H2 production. We show that structural control at the nanoscale is a great instrument for understanding reaction pathways, for studying the nature of catalytic active sites, and for synthesizing more selective, active and stable catalysts.
Two synthetic strategies were followed to acquire nanostructural control: a self-assembly method was employed to prepare hierarchical materials starting from functional nanoparticles, and advanced solvothermal methods were used to prepare monodisperse nanocrystals having controlled size and composition. State-of-the-art hierarchical Pd-based catalysts embedded by metal oxide promoters were tested for methane catalytic oxidation in the presence of poisoning compounds typically found in real applications. Detailed surface studies allowed to propose deactivation mechanisms and strategies to improve catalysts resistance to deactivation. Well-controlled nanostructured Pt-based alloys and Ni-Cu alloys showed improved activity, stability and selectivity for hydrodeoxygenation reactions of biomass-derived feedstocks to produce biofuels. The control of nanostructure was pivotal to understand the reason for such enhanced performances. Finally, dye-sensitized photocatalysts were investigated in H2 photocatalytic production under visible light, and state-of-the-art stability and activities were demonstrated.
All these findings greatly contributed to the development of catalytic materials for energy-related applications
From metal to metal-free catalysts: Routes to sustainable chemistry
No full textCatalysts are making our world more sustainable day by day. But how sustainable are
catalysts themselves? In this contribution we will give a perspective overview of the progress
in dematerializing catalysts, i.e., in using less (critical) materials to deliver the same
(or better) level of functionality. This may be accomplished in many ways: improving
the catalyst performance and durability by gaining insights in reaction, activation,
and deactivation mechanisms; lowering the amount of critical or harmful catalytic
components, e.g., by finding cheaper, more abundant, and sustainable substitutes;
and making catalysts production and disposal processes more sustainable, e.g., by
recycling. Material science and nanotechnology are two essential actors in this process,
providing the tools to understand and optimize catalytic materials and processes, and
to assess the environmental and toxicological impact of nanomaterials
Brookite: Nothing new under the sun?
No full textAdvances in the synthesis of pure brookite and brookite-based TiO2 materials have opened the way to fundamental and applicative studies of the once least known TiO2 polymorph. Brookite is now recognized as an active phase, in some cases showing enhanced performance with respect to anatase, rutile or their mixture. The peculiar structure of brookite determines its distinct electronic properties, such as band gap, charge–carrier lifetime and mobility, trapping sites, surface energetics, surface atom arrangements and adsorption sites. Understanding the relationship between these properties and the photocatalytic performances of brookite compared to other TiO2 polymorphs is still a formidable challenge, because of the interplay of many factors contributing to the observed efficiency of a given photocatalyst. Here, the most recent advances in brookite TiO2 material synthesis and applications are summarized, focusing on structure/activity relation studies of phase and morphology-controlled materials. Many questions remain unanswered regarding brookite, but one answer is clear: Is it still worth studying such a hard-to-synthesize, elusive TiO2 polymorph? Yes
Fundamentals and Catalytic Applications of CeO2-Based Materials
Full text linkCerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th 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
Catalytic Oxidation of Methane: Pd and Beyond
No full textThis brief review is focused on recent advancements in methane catalytic oxidation, an important reaction for environmental remediation and clean power generation. Particular attention is given to Pd‐based catalysts and novel strategies to gain a fundamental understanding of the reaction mechanism and to improve the catalytic activity and stability of Pd catalysts. The power of combined approaches exploiting nanostructured catalysts, operando spectroscopic and microscopic measurements and computational methods is demonstrated
H2 production by photocatalytic reforming of oxygenated compounds using TiO2-based materials
No full textClean 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 productio
Deciphering Size and Shape Effects on the Structure Sensitivity of the CO2 Methanation Reaction on Nickel
No full textThis study advances the understanding of structure sensitivity in CO2 methanation over nickel-based catalysts by highlighting the combined influence of the metal nanoparticle (NP) size and shape on catalytic performance. Density functional theory (DFT) calculations of the metal nanoparticle structure and activity provide the theoretical underpinnings of the experimentally observed structure sensitivity of CO2 methanation over nickel-based catalysts. This is achieved by taking into account the diversity of shapes of metal nanoparticles (NPs) under the reaction conditions and the corresponding distribution of active sites at different metal NP sizes. We built a large ensemble of Ni metal NPs with different shapes and sizes in the range of 0.5-10 nm and quantified the distribution of the potential active sites for each NP. We then computed the reaction rate over each of these active sites on the metal surface to evaluate the activity as a function of the metal NP diameter. Our calculations reveal that the activity at the active sites located at the edge between the Ni(100) and Ni(111) facets largely dominates the overall observed activity. Furthermore, metal NPs can be categorized into families based on their shape, specifically the fraction of exposed Ni(100) facets. The observed maximum in turnover frequency (TOF) for 2-3 nm metal NPs is linked to the dominance of NP families with high Ni(100) fractions. Conversely, experimental conditions favoring NP families with higher Ni(111) fractions result in a hockey stick trend in the TOF. These findings resolve key debates on structure sensitivity in CO2 methanation and offer broader applicability to other structure-sensitive reactions, such as ammonia synthesis, decomposition, and Fischer-Tropsch synthesis, where similar sensitivities have been widely debated
Highly efficient hydrogen production through ethanol photoreforming by a carbon nanocone/Pd@TiO2 hybrid catalyst
No full textProduction of molecular hydrogen (H2) 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 H2 production via photoreforming of ethanol using a catalyst based on hierarchical carbon nanocones hybridised with an inorganic layer of nanocrystalline TiO2 containing Pd nanoparticles
The H2 Pressure Dependence of Hydrodeoxygenation Selectivities for Furfural over Pt/C Catalysts
No full textHydrodeoxygenation of furfural was studied over a 10-wt% Pt/C catalyst at 453 K, under both low- and high-pressure conditions. With vapor-phase furfural as the feed and H-2 pressures below 1 bar, decarbonylation to furan is a major product, with the selectivity to furfuryl alcohol and dimethylfuran increasing with increasing H-2 pressure. When the reaction is performed at 33 bar, using 1-wt % furfural in 1-propanol solvent and high-pressure H-2, no evidence for decarbonylation was observed. At high pressures, the reaction is sequential, with all the furfural proceeding to methylfuran, which in turn reacts to over-hydrogenated products, including 2-methyltetrahydrofuran and 2-pentanone. It is suggested that the hydrogen surface coverage is responsible for the apparent differences in the reaction network at high and low pressures
Enhanced photocatalytic hydrogen generation using carbazole-based sensitizers
No full textPhenothiazine-, phenoxazine- and carbazole-based dyes have been synthesized and used as photosensitizers in Pt/TiO2 films for photocatalytic hydrogen generation. Compared to commonly used phenothiazine dyes, planar and sulphur-free carbazole derivatives showed different molecular and supramolecular features which in turn yielded greatly enhanced (one order of magnitude) H2 production performances
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