112,320 research outputs found
Pre-bonding prophylaxis and brackets detachment: an experimental comparison of different methods
No full textThe aim of the paper is to identify any loss of brackets adhesion to dental enamel after treatment with the most common methods of caries prophylaxis. Based on clinical experience gained in the Master of Science in Dental Hygiene at the University of Pavia (Lanteri C, Lanteri V, Segù M, Collesano V., 2007) the following protocols have been identified: 1) topical application of fluoride, 2) casein pastes, 3) application of ozone. The research was conducted in the lab by measuring the separation force of the brackets from bovine teeth, treated according to experimental protocols, inspected, and compared with an untreated control group. This experimental study produced interesting results, offering clinical insights for patients who have to undergo multibracket orthodontic treatment after anti-caries prophylaxis
Hydrogenation of CO2to methanol by the diphosphine-ruthenium(ii) cationic complex: a DFT investigation to shed light on the decisive role of carboxylic acids as promoters
No full textIn order to provide alternative paths for the CO2 conversion processes, we present a quantum-chemical investigation of the CO2 hydrogenation to methanol catalyzed by the recently proposed diphosphine-ruthenium(ii) cationic complex, named Ru2, in presence of carboxylic acids. Our results have shown that, in absence of carboxylate anion, CO2 molecule is not activated towards the hydride nucleophilic attack and, consequently, its hydrogenation does not occur. On the other hand, the addition of carboxylic acid 1) allows the replacement of the p-cymene ligand and, therefore, the formation of the active metal-carboxylate specie proposed experimentally, and 2) activates the CO2 molecule, thus confirming the experimental suggestion that carboxylic acid acts as promoter. Moreover, the influence of the size of the carbon chain in the carboxylate acid has been also investigated and rationalized. Eventually, DFT calculations have been carried out to explore the experimentally proposed catalytic cycle for the CO2 hydrogenation to methanol
In-depth DFT insights into the crucial role of hydrogen bonding network in CO2 fixation into propylene oxide promoted by Biomass-Derived deep eutectic solvents
No full textThe CO2 fixation into epoxide to produce cyclic carbonate is a promising route to realize the large-scale utilization of CO2. In this work, a detailed DFT investigation has been carried out to elucidate the mechanistic details of the CO2 conversion into propylene carbonate (PC) catalyzed by bio-mass derived deep eutectic solvents (bio-DESs), obtained by combining hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) molecules. Our calculations support the known beneficial effect of hydrogen bonds in activating the epoxide. On the other hand, DFT results also show that hydrogen bonds and transfers do also stabilize the negative charge of determining intermediates, with the consequent undesired effect of reducing the nucleophilicity of the oxygen atom involved in the final ring closure step. Moreover, hydrogen bonds are also established between the HBDs and the PC product, thus hampering its release. Our theoretical results offer an in-depth understanding of the CO2 fixation involving bio-DESs, which can guide towards the development of more efficient bio-catalysts
Cyclic Carbonate Formation from Epoxides and CO2Catalyzed by Sustainable Alkali Halide-Glycol Complexes: A DFT Study to Elucidate Reaction Mechanism and Catalytic Activity
No full textWe provide a comprehensive DFT investigation of the mechanistic details of CO2 fixation into styrene oxide to form styrene carbonate, catalyzed by potassium iodide-tetraethylene glycol complex. A detailed view on the intermediate steps of the overall reaction clarifies the role of hydroxyl substances as co-catalysts for the alkali halide-catalyzed cycloaddition. The increase of iodide nucleophilicity in presence of tetraethylene glycol is examined and rationalized by NBO and Hirshfeld charge analysis, and bond distances. We explore how different alkali metal salts and glycols affect the catalytic performance. Our results provide important hints on the synthesis of cyclic carbonates from CO2 and epoxides promoted by alkali halides and glycol complexes, allowing the development of more efficient catalysts
Photochemical CO2conversion on pristine and Mg-doped gallium nitride (GaN): A comprehensive DFT study based on a cluster model approach
No full textThe photochemical reduction of carbon dioxide (CO2) into methanol is very appealing since it requires sunlight as the only energy input. However, the development of highly selective and efficient photocatalysts is still very challenging. It has been reported that CO2 can be spontaneously activated on gallium nitride (GaN). Moreover, the photocatalytic activity for CO2 conversion into methanol can be drastically enhanced by incorporating a small amount of Mg dopant. In this work, density functional theory (DFT) based on a cluster model approach has been applied to further explore the photocatalytic activity of bare GaN towards CO2 adsorption and conversion. We extended the investigation of Mg-doping replacing one Ga atom with Mg on three different sites and evaluated the consequent effects on the band gaps and CO2 adsorption energies. Finally, we explore different routes leading to the production of methanol and evaluate the catalytic activity of bare GaN by applying the energetic span model (ESM) in order to identify the rate-determining states which are fundamental for suggesting modifications that can improve the photocatalytic activity of this promising material
ABC di corpi: alcuni alfabeti figurati del XVI secolo del Gabinetto Disegni e Stampe della Pinacoteca Nazionale di Bologna
No full textFra le diverse tipologie di motivi ornamentali, gli alfabeti figurati costituiscono un vero e proprio genere a se stante, che attinge al repertorio iconografico del proprio tempo e lo riflette in composizioni dotate di grande fascino e originalità. Per la quantità, la qualità e l’eterogeneità degli esemplari custoditi, il Gabinetto Disegni e Stampe della Pinacoteca Nazionale di Bologna costituisce un caso più unico che raro in Italia, vantando alfabeti figurati che abbracciano ampi confini geografici e cronologici, spaziando dal tardo gotico al XIX secolo. Dopo una breve introduzione sulle vicende collezionistiche di un nucleo grafico così insolito e particolare, l’articolo si concentra su alcuni esemplari di alfabeti figurati del Cinquecento di area transalpina. Partendo dall’alfabeto dei bambini di Hans Weiditz (1521), costruito rispettando le più rigorose regole geometriche, codificate nei coevi trattati rinascimentali, ma anche riflesso del rinato gusto per i fregi all’antica con putti, si passa agli alfabeti di Peter Flötner (1534) e di Jost Amman (1567), dove le lettere sono integralmente costituite da corpi umani, forzati in complicate posture, talvolta persino licenziose. Questi esemplari rappresentano degli interessanti esperimenti calligrafici, dove l’uomo diviene concretamente l’unità di misura dei caratteri dell’alfabeto
Electronic Properties of Pure and Fe-Doped β-Ni(OH)2: New Insights Using Density Functional Theory with a Cluster Approach
No full textNiOx has recently emerged as a robust catalyst with high catalytic activity for water oxidation reaction. Despite extensive studies, the origin of the high oxygen evolution reaction activity upon Fe doping is not fully solved, even for one of its simplest phases, β-Ni(OH)2. We present here density functional theory calculations using for the first time a cluster approach to revisit the electronic structure of pure and Fe-doped β-Ni(OH)2. First, our findings agree with a recent hypothesis that the band gap of the pure case reduces upon Fe doping. Second, in agreement with earlier calculations, we find that the highest occupied state consists of O and Ni states in pure and Fe-doped β-Ni(OH)2. However, the lowest unoccupied orbitals are Ni and O for pure β-Ni(OH)2 and mainly Fe for Fe-doped β-Ni(OH)2. We argue that the two different states for the highest occupied state and for the lowest unoccupied state of Fe-doped β-Ni(OH)2 may lead to low electron-hole recombination. Third, the delocalized nature of the band edge states that may be associated with high mobility is not damaged after Fe doping. These electronic effects could be some of the reasons experiments show that doping β-Ni(OH)2 with Fe enhances overall efficiency
Practical cluster models for a layered β-NiOOH material
No full textDue to the high oxygen evolution reaction (OER) activity, stability, and abundance of NiOx materials, they are found to be promising catalysts, competitive with expensive metal oxides such as IrO2 and RuO2. From a theoretical point of view, studies reported in the literature so far are mostly based on density functional theory using periodic slab models for the bulk and surface of βNiOOH, one of the active NiOx phases. However, cluster models are a valid method to investigate many aspects about structure, charge carrier transport properties, and OER activity of β-NiOOH. Hence, here we present new cluster models for the surface of β-NiOOH, where the oxygen atoms are bonded to Mg effective core potentials (ECPs) mimicking neighboring atom cores. This cluster embedding procedure is superior to saturating the cluster with hydrogen atoms, and to using other atomic ECPs for β-NiOOH.We find that layered materials such as β-NiOOH are more vulnerable to geometrical rupture and therefore a cluster approach requires additional care in choosing the embedding approach. We evaluated the models by using them to calculate the energy required for water adsorption and deprotonation, which are essential ingredients for OER. Specifically, our results agree with previous slab models that the first deprotonation reaction step requires a large amount of energy. In addition, we find that water and hydroxyl groups have high adsorption energy and therefore the first deprotonation step is limiting the reaction efficiency
L'impiego delle strutture colonnari negli scaloni dei palazzi nobiliari del Settecento: la scala di palazzo Butera a Palermo (1760-1765c). Il contesto italiano
No full textThis paper focuses on the 18th-century Sicilian red marble staircase
in the palace of the princes of Butera in the Kalsa district of Palermo.
It consists in an in-depth study of the construction phases of palaz
zo Butera, a survey with a laser scanner and the construction of a 3D
model of the staircase. This investigation brings to light the remark
able boldness of the staircase in palazzo Butera, which is compared
to staircase in Genoese buildings
DFT Study of GaN Clusters Decorated with Rh and Pt Nanoparticles for the Photochemical Reduction of CO2
No full textObtaining chemicals and fuels from the reduction of carbon dioxide (CO2) represents a promising strategy to mitigate the growing greenhouse gas emissions. Because of the high thermodynamic stability of CO2, the real challenge is the development of efficient and selective catalysts. In this regard, photocatalysis is receiving much attention because it exclusively relies on energy input from sunlight. Gallium nitride (GaN) semiconductors can effectively promote the CO2reduction. Moreover, the addition on the semiconductor surfaces of transition metal nanoparticles, such as Rh and Pt, can further improve the efficiency and selectivity toward CH4rather than CO, along with improving the optical absorptions in the visible spectral region by decreasing the wide band gap of the pristine GaN. Water is commonly used as an atomic hydrogen donor for CO2reduction. In this regard, GaN was previously reported as an excellent photocatalyst for water oxidation. Here, we present a density functional theory investigation based on a cluster model approach to shed light on the effective role of the metal nanoparticles on the CO2reduction in the presence of water. Our calculations have underlined a more favored dissociative adsorption of H2O with respect to CO2. Moreover, while the dissociative H2O adsorption on the GaN surface occurs without the involvement of the Rh metal, the role of the metal center in activating the CO2molecule is found to be crucial. Highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and calculated absorption spectra have shown that the presence of the adsorbed nanoparticles not only intensifies the absorption next to the UV region but also extends it to all visible regions. Particularly, while the presence of Rh exhibits a stronger light absorption property in the visible region, enhanced in the blue-green region, Pt nanoparticles have a clear red-shift effect
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