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Fundamental Understanding of Nanoparticle chemistry: towards the prediction of Particulate emissions and Material synthesis

Periodic Reporting for period 3 - FUN-PM (Fundamental Understanding of Nanoparticle chemistry: towards the prediction of Particulate emissions and Material synthesis)

Reporting period: 2021-02-01 to 2022-07-31

The capability of modern societies to reduce harmful particulate emissions from combustion devices and at the same time to take full advantage of flame-based technologies for production of nanomaterials is limited by the inaccuracy of the industrial simulation tools for the design and implementation of practical and efficient solutions. In particular, our fundamental understanding of the nanoparticle chemistry is not only far from being comprehensive but also not sufficient to allow such tools to be accurate and predictive. Nanoparticle formation is a very complex, multi-step process driven by the chemistry of unburned hydrocarbons, including formation and growth of polycyclic aromatic hydrocarbons (PAHs, considered the particle building blocks), nanoparticle nucleation, and subsequent particle growth and oxidation. Unfortunately, many of these steps are still poorly understood, mainly due to their intrinsic complexity and the limitations in the experimental and theoretical laboratory-based techniques.
With the goal of providing answers to the existing long-stand unresolved questions related to the nanoparticle chemistry, a new multi-disciplinary, multi-step approach is considered where the particle formation processes are isolated and studied singularly using a combination of complementary advanced and innovative experimental, theoretical, and modelling techniques. The following objectives will be achieved during the research program:
• experimentally investigate the chemistry of key fuel components and mixtures at conditions relevant to combustion systems using advanced conventional laboratory-based techniques for studies on i) formation of small PAHs, and ii) particle growth and oxidation;
• develop innovative Synchrotron-based methodologies to overcome the limitations in the conventional techniques and perform investigations on growth to large PAHs and nanoparticle nucleation;
• develop and validate a comprehensive and detailed chemical kinetic model for nanoparticle chemistry based on the newly obtained experimental results with the support of theoretical calculations and analyses.
The model will constitute a greatly enhanced component of industrial codes for predictive simulations of particulate emissions from modern combustion devices and nanomaterial synthesis in industrial processes, with considerable benefits to the standards of living of European citizens, the environment, and the European Union economy, towards the future of clean energy conversion and novel nanomaterials.
The activities related to FUN-PM started with the development of a new experimental facility for the measurement of species profiles in shock tube experiments. A conventional shock tube at the laboratory ICARE - CNRS was converted for use with a new gas chromatographic system specifically designed for detection of relatively small polycyclic aromatic hydrocarbons (PAHs).
The new facility, called high-purity shock tube (HPST), was tested and used to perform experiments on several aromatic fuels (benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, phenylacetylene, propyne, propylene, o-/m-/p-xylene, n-heptane, cyclopentene, styrene) and their mixtures with small aliphatic components (acetylene/ethylene, propylene/propyne) with the goal of probing various reaction pathways leading to the formation of PAH molecules (aromatic-aromatic radical recombination and aromatic + aliphatic pathways). The experiments were used for the development and validation of a comprehensive and detailed chemical kinetic model for PAH formation chemistry.
Besides the work on the relatively small PAHs, a new miniature high-repetition-rate, fully-automated shock tube (ICARE-HRRST) for use at Synchrotron facilities was designed and built. The ICARE-HRRST was coupled to the double imaging photoelectron/photoion coincidence spectroscopy technique (i2PEPICO) at the SOLEIL Synchrotron facility, France, for the measurement of species profiles, including PAHs. The first ever experiments with the ICARE-HRRST/i2PEPICO system were performed on the pyrolysis of ethanol while other experimental campaigns were performed at SOLEIL (on pyrolytic chemistry of toluene, ethylbenzene, and styrene) and at the Swiss Light Source (preliminary tests).
Finally, the development of a new double-laser particle detection system coupled to the heated shock tube (HST) facility at ICARE started. At the moment, the system is capable to measure various kinetic parameters for particle appearance and growth, i.e. induction delay times, growth rates, yields. The collected particles were also used to analyze primary particle size and morphology by microscopy. Experiments were performed on several aromatic fuels and mixtures similar to the ones investigated in the HPST. The development of a code for generating detailed chemical kinetic models for solid-phase soot chemistry was finally initiated.
All the experimental techniques developed during the FUN-PM program present aspects which progress beyond the state-of-the-art techniques in the chemical kinetic field. The newly developed high purity shock tube (HPST) facility at ICARE has some unique characteristics which makes it a very performant set-up for measuring profiles of polycyclic aromatic hydrocarbon (PAH) products (improved sensitivities, below 0.1 parts per million, and species separation capabilities). The experimental results obtained with the HPST allowed to analyze the PAH formation chemistry with remarkable accuracy and constituted the base for the development and validation of a new detailed chemical kinetic model for PAH chemistry with improved performance compared to previous models in the literature.
Concerning the miniature high-repetition-rate shock tube (HRRST), it is in itself a technology that encompass the capabilities of the conventional techniques with the goal of accessing the most advanced diagnostics at Synchrotron facilities. The ICARE-HRRST is a totally-automated shock tube of one-meter length (versus 10 meters for regular shock tubes) that can operate at rates up to 2 experiments per second (versus typical rates of 1-2 experiments per hour) with reproducible operational conditions. The coupling between the HRRST and the double imaging photoelectron/photoion coincidence spectroscopy (i2PEPICO) at the SOLEIL Synchrotron facility, France, provides a unique and novel methodology for obtaining fundamental information on the high-temperature high-pressure fuel kinetics.
Results from future research activities include investigations on the PAH chemistry from reactions involving other important fuels (HPST facility and HRRST/i2PEPICO) which will serve as a benchmark for the extension of the gas-phase kinetic model for PAH chemistry. Collaborations started to carry out future experiments with the ICARE-HRRST at other Synchrotron and XFEL facilities with the goal of investigating the nucleation process. Finally, experiments on particle growth and oxidation has started with similar fuel components and mixtures as the ones studied in the PAH investigations. The extensive datasets on particle kinetics will be used to develop and validate a comprehensive chemical kinetic model for detailed nanoparticle chemistry applied to real fuels. The model will constitute an extension of the improved PAH chemistry model with coupling to the solid-phase chemistry, for use in industrial codes for nanoparticle simulations.
FUN-PM overview