arXiv subjects

Mauritius C. M. van de Sanden

Publications and source records attributed to Mauritius C. M. van de Sanden.

4 recordsLinked to original sources

arXiv2026

Flow-thermochemistry coupling governs pressure-dependent CO$_2$ conversion in vortex-stabilized microwave plasma reactors: Insights from three-dimensional CFD modeling

In this work, a three-dimensional computational fluid dynamics model is developed for a vortex-stabilized microwave CO$_2$ plasma reactor operating over the pressure range of 100-400 mbar. The model combines experimentally constrained, emission-based plasma sizes and volumetric heat-source distributions with thermally dominated finite-rate heavy-particle chemistry for a multi-component mixture. Turbulent flow and transport are described using the SST k-omega model. The model reproduces the measured radial gas-temperature profiles in the plasma core and the non-monotonic pressure dependence of CO$_2$ conversion, including a maximum at 150 mbar and a pronounced decrease at 400 mbar. A vortex-driven recirculation region redistributes gas upstream. Turbulent mixing and cooling are strongest near the upper reactor boundary, but their contribution decreases as pressure increases. The pressure dependence of conversion is determined by the competition between CO$_2$ dissociation and CO recombination. CO$_2$ direct dissociation reaction dominates in the high-temperature plasma core, whereas O-assisted conversion reaction contributes near the plasma edges and in the surrounding hot region. At 150 mbar, enhanced CO$_2$ dissociation is accompanied by limited CO loss, resulting in the highest conversion. With pressure increasing to 400 mbar, slower cooling and more frequent three-body collisions promote CO recombination in the afterglow, causing more than 60% of the CO formed near the plasma to be lost downstream. Moreover, additional CO loss occurs in the upper region of the reactor at higher pressures because of the reduced cooling rate.

physics.plasm-ph↗

arXiv2018

Vibrational Kinetics in Plasma as a Functional Problem: a Flux-Matching Approach

A new approach to calculate the vibrational distribution function of molecules in a medium providing energy for vibrational excitation is proposed and demonstrated. The approach is an improvement of solution methods based on the drift-diffusion Fokker-Planck (FP) equation for a double differentiable function representing the vibrational populations on a continuum internal energy scale. A self-consistent numerical solution avoids approximations used in previous analytical solutions. The dissociation flux, a key parameter in the FP equation, is fixed using the kinetics of molecular dissociation from near-continuum levels, so that the vibrational kinetics becomes a functional problem. The approach is demonstrated for the kinetics of asymmetric stretching of CO2, showing that it represents an alternative, potentially much more efficient in computational terms, to the presently usual state-to-state approach which is based on the kinetics of the populations of individual levels, and gives complementary insight into the dissociation process.

physics.chem-ph↗

arXiv2017

Insight into CO2 dissociation in plasmas from numerical solution of a vibrational diffusion equation

The dissociation of CO2 molecules in plasmas is a subject of enormous importance for fundamental studies and the recent interest in carbon capture and carbon-neutral fuels. The vibrational excitation of the CO2 molecule plays an important role in the process. The complexity of the present state-to-state (STS) models makes it difficult to find out the key parameters. In this paper we propose as an alternative a numerical method based on the diffusion formalism developed in the past for analytical studies. The non-linear Fokker-Planck equation is solved by the time-dependent diffusion Monte Carlo method. Transport quantities are calculated from STS rate coefficients. The asymmetric stretching mode of CO2 is used as a test case. We show that the method reproduces the STS results or a Treanor distribution depending on the choice of the boundary conditions. A positive drift, whose energy onset is determined by the vibrational to translational temperature ratio, brings molecules from mid-energy range to dissociation. The high-energy fall of the distribution is observed even neglecting VT processes which are normally believed to be its cause. Our study explains several puzzling features of previous studies, provides new insights into the control of the dissociation rate and a much sought compression of the required data for modeling.

physics.chem-ph↗

arXiv2014

Chemical analysis of ligand-free silicon nanocrystal surfaces by surface enhanced Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) was used to probe the surface chemistry of chlorine-terminated silicon nanocrystal (Si-NC) surfaces in an air-free environment. SERS effect was observed from the thin films of Ag$_x$O using 514 nm laser wavelength. When a monolayer of Si-NCs were spin-coated on Ag$_x$O SERS substrates, a very clear signal of surface states, including Si-Cl$_x$, and Si-H$_x$ were observed. Upon air-exposure, we observed the temporal reduction of Si-Cl$_x$ peak intensity, and a development of oxidation-related peak intensities, like Si-O$_x$ and Si-O-H$_x$. In addition, first, second and third order transverse optical (TO) modes of Si-NCs were also observed at 519, 1000 and 1600 cm$^{-1}$, respectively. As a comparison, Raman analysis of a thick film (> 200 nm) of Si-NCs deposited on ordinary glass substrates were performed. This analysis only demonstrated the first TO mode of Si-NCs, and the all the other features originated from SERS enhancement did not appear in the spectrum. These results conclude that, SERS is not only capable of single-molecule detection, but also a powerful technique for monitoring the surface chemistry of nanoparticles.

cond-mat.mtrl-sci↗