Diagnostic for the characterization of nanometric structures in high temperature reactive systems

It is now well know that the fraction of particulate mater in the ambient air defined as ultrafine particles can be considered the most critical for adverse human health effects because of their chemical composition and the ability of these particles to penetrate deeply into the respiratory tract. Moreover combustion has been recognised as the major source of harmful fine and ultrafine particles. The aim of the present thesis work is to investigate carbonaceous nanoparticles formation by combustion processes. An experimental procedure based on the use of the fifth harmonic of a Nd:YAG laser at 213 nm as exiting source and on an accurate signals acquisition has been realized. In-situ spectral optical measurements based on a combination of: Laser Induced Fluorescence (LIF), Laser Induced Incandescence (LII), Light Extinction (Kext) and Laser Light Scattering (Qvv) techniques have allowed to follow particles formation and their evolution directly in combustion environments with high spatial and temporal resolution. Laminar premixed and laminar and turbulent diffusion flames have been investigated burning ethylene, methane and benzene as fuels. Optical results are then compared with Particle Size Distribution Function (PSDF) obtained by Scanning Mobility Particles Sizer (SMPS) measurements in same flame conditions. An experimental investigation of the particulate emissions from commercial burners for home appliances fueled with natural gas has been also included. The experimental evidences, in according to literature in laminar premixed conditions, allow to conclude that two classes of nanoparticles are formed in flame:Nanoparticles of Organic Carbon (NOC) with sizes smaller than three nanometers and “primary” soot particles with sizes larger than ten nanometers that lead to the formation of soot aggregates. Moreover, the thesis work shows that these combustion-generated nanoparticles strongly depend on the type of fuel, type of combustion system and eventual exhaust treatment systems

Variable Temperature Synthesis of Tunable Flame-Generated Carbon Nanoparticles

In this study, flame-formed carbon nanoparticles of different nanostructures have been produced by changing the flame temperature. Raman spectroscopy has been used for the characterization of the carbon nanoparticles, while the particle size has been obtained by online measurements made by electrical mobility analysis. The results show that, in agreement with recent literature data, a large variety of carbon nanoparticles, with a different degree of graphitization, can be produced by changing the flame temperature. This methodology allows for the synthesis of very small carbon nanoparticles with a size of about 3–4 nm and with different graphitic orders. Under the perspective of the material synthesis process, the variable-temperature flame-synthesis of carbon nanoparticles appears as an attractive procedure for a cost-effective and easily scalable production of highly tunable carbon nanoparticles

Steam reforming of tar in hot syngas cleaning by different catalysts: Removal efficiency and coke layer characterization

Syngas produced by biomass and waste gasification processes must be ade-quately clean of tar compounds before being utilized in value-added applica-tions. Syngas cleaning by tar cracking at high temperatures is a promisingtechnique that can utilize different kinds of catalysts. However, their use islimited by the deposition of coke layers, which induces a masking phenome-non on the active surface, and, consequently, the rapid deactivation of the cat-alyst. This study addresses how the temperature (750 and 800 C) and thesteam concentration (0% and 7.5%) can affect the extent of water–gas andreforming reactions between steam and coke deposits. Two catalysts wereused: a market-available activated carbon and an iron-based alumina catalyst.The tests showed better performance of the Fe/γ-Al2O3catalyst. A massincrease of the bed was measured in tests with both the catalysts, which con-firms the deposition of the coke layer produced by tar dehydrogenation andcarbonization. Scanning electronic microscopy-energy-dispersive X-ray analy-sis (SEM-EDX) and Raman spectroscopy were utilized to investigate the natureof coke layers over the catalyst surface, with the aim of acquiring informationabout their reactivity towards the water gas reaction. SEM-EDX observationsindicate that the thickness of these carbon layers is less than 2μm. Ramanspectra suggest a negligible effect of the reaction temperature in the testedrange and, in particular, that the amorphous nature of coke layers deposited inthe presence of steam is relatively more graphitic than that obtained withoutsteam

On the Formation and Accumulation of Solid Carbon Particles in High-Enthalpy Flows Mimicking Re-Entry in the Titan Atmosphere

The problem relating to the formation of solid particles enabled by hypersonic re-entry in methane-containing atmospheres (such as that of Titan) has been tackled in the framework of a combined experimental-numerical approach implemented via a three-level analysis hierarchy. First experimental tests have been conducted using a wind tunnel driven by an industrial arc-heated facility operating with nitrogen as working gas (the SPES, i.e., the Small Planetary Entry Simulator). The formation of solid phases as a result of the complex chemical reactions established in such conditions has been detected and quantitatively measured with high accuracy. In a second stage of the study, insights into the related formation process have been obtained by using multispecies models relying on the NASA CEA code and the Direct Simulation Monte Carlo (DSMC) method. Through this approach the range of flow enthalpies in which carbonaceous deposits can be formed has been identified, obtaining good agreement with the experimental findings. Finally, the deposited substance has been analyzed by means of a set of complementary diagnostic techniques, i.e., SEM, spectroscopy (Raman, FTIR, UV-visible absorption and fluorescence), GC-MS and TGA. It has been found that carbon produced by the interaction of the simulated Titan atmosphere with a solid probe at very high temperatures can be separated into two chemically different fractions, which also include "tholins"

Insights into incipient soot formation by atomic force microscopy

Abstract Combustion-generated soot particles can have significant impact on climate, environment and human health. Thus, understanding the processes governing the formation of soot particles in combustion is a topic of ongoing research. In this study, high-resolution atomic force microscopy (AFM) was used for direct imaging of the building blocks forming the particles in the early stages of soot formation. Incipient soot particles were collected right after the particle nucleation zone of a slightly sooting ethylene/air laminar premixed flame at atmospheric pressure and analyzed by AFM after a rapid sublimation procedure. Our data shed light on one of the most complex and still debated aspect on soot formation, i.e., the nucleation process. The molecular constituents of the initial particles have been individually analyzed in detail in their chemical/structural characteristics. Our data demonstrate the large complexity/variety of the aromatic compounds which are the building blocks of the initial soot particles. Nevertheless, some fundamental and specific characteristics have been clearly ascertained. These include a significant presence of penta-rings as opposed to the purely benzenoid aromatic compounds and the noticeable presence of aliphatic side-chains. In addition, there were indications for the presence of persistent π radicals. Incipient soot was also investigated by Raman spectroscopy, the results of which agreed in terms of chemical and structural composition of the particles with those obtained by AFM

Soot inception: A DFT study of σ and π dimerization of resonantly stabilized aromatic radicals

Recent advances in the soot studies have shown experimental evidences of π-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. π-radicals could have an important role in particle nucleation by increasing the binding energy between polycyclic aromatic hydrocarbons with respect to pure van der Waals interactions. In this work we use density functional theory by Grimme D3 dispersion correction (DFT-D3) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify the clustering behaviors of two aromatic radicals visualized experimentally by atomic force microscopy (Commodo et al. Combust. Flame 205: 154–164, 2019). These aromatic radicals have different topological structures and delocalization of the unpaired electron. The binding energy and energy bandgap characteristics of the clusters are calculated. The theoretical results show a different clustering behavior for the two aromatic radicals. The one with a partial localization of the unpaired electron tends to form a σ-dimer; conversely, the radical with a greater delocalization of the unpaired electron leads to π-stacking formation with a slight overbinding of few kcal mol−1 with respect to pure van der Waals interactions and a marked lowering of the energy bandgap. The formation of π-stacking induced by delocalized π-radicals could in part explain some spectroscopic evidences observed during soot nucleation. © 2020 Elsevier Lt

π-Diradical Aromatic Soot Precursors in Flames.

Soot emitted from incomplete combustion of hydrocarbon fuels contributes to global warming and causes human disease. The mechanism by which soot nanoparticles form within hydrocarbon flames is still an unsolved problem in combustion science. Mechanisms proposed to date involving purely chemical growth are limited by slow reaction rates, whereas mechanisms relying on solely physical interactions between molecules are limited by weak intermolecular interactions that are unstable at flame temperatures. Here, we show evidence for a reactive π-diradical aromatic soot precursor imaged using non-contact atomic force microscopy. Localization of π-electrons on non-hexagonal rings was found to allow for Kekulé aromatic soot precursors to possess a triplet diradical ground state. Barrierless chain reactions are shown between these reactive sites, which provide thermally stable aromatic rim-linked hydrocarbons under flame conditions. Quantum molecular dynamics simulations demonstrate physical condensation of aromatics that survive for tens of picoseconds. Bound internal rotors then enable the reactive sites to find each other and become chemically cross-linked before dissociation. These species provide a rapid, thermally stable chain reaction toward soot nanoparticle formation and could provide molecular targets for limiting the emission of these toxic combustion products

Solid carbon produced during the simulation of re-entry in the Titan atmosphere by means of an arc-driven flow facility

Spacecraft entry into Titan’s atmosphere has been investigated using a dedicated (Small Planetary Entry Simulator) facility (SPES). While in earlier works much attention was paid to the joint numerical-experimental simulation of typical entry physical parameters (namely, heat flux and total enthalpy); in the present analysis we focus on some unexpected results recently obtained at the University of Naples, in collaboration with CNR, in the framework of a new test campaign dedicated to various planetary atmospheres (including Titan itself). Such findings concern the presence of a carbon-like substance on the surface of the measuring probes used for the experiments, which seem to align with the results yielded by other authors with other strategies (an inductive plasma torch). We have confirmed the carbonaceous nature of such particulate matter via various diagnostic techniques such as SEM, Raman, FT-IR, UV-visible absorption and fluorescence spectroscopy, GC-MS and TGA. The present work is devoted to the presentation of such results together with a critical discussion of the novelty relating to the present experimental approach (arc plasma versus inductive torch) and an analysis of the chemical-physical differences pertaining to the carbon obtained with the two different methods

Resistive Switching Phenomenon Observed in Self-Assembled Films of Flame-Formed Carbon-TiO2 Nanoparticles

Nanostructured films of carbon and TiO2 nanoparticles have been produced by means of a simple two-step procedure based on flame synthesis and thermophoretic deposition. At first, a granular carbon film is produced on silicon substrates by the self-assembling of thermophoretically sampled carbon nanoparticles (CNPs) with diameters of the order of 15 nm. Then, the composite film is obtained by the subsequent thermophoretic deposition of smaller TiO2 nanoparticles (diameters of the order of 2.5 nm), which deposit on the surface and intercalate between the carbon grains by diffusion within the pores. A bipolar resistive switching behavior is observed in the composite film of CNP-TiO2. A pinched hysteresis loop is measured with SET and RESET between low resistance and high resistance states occurring for the electric field of 1.35 × 104 V/cm and 1.5 × 104 V/cm, respectively. CNP-TiO2 film produced by flame synthesis is initially in the low resistive state and it does not require an electroforming step. The resistance switching phenomenon is attributed to the formation/rupture of conductive filaments through space charge mechanism in the TiO2 nanoparticles, which facilitate/hinder the electrical conduction between carbon grains. Our findings demonstrate that films made of flame-formed CNP-TiO2 nanoparticles are promising candidates for resistive switching components