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The
understanding and quantification of the CO adsorption modes
and strength on ultradispersed platinum catalysts supported on γ-Al2O3 is of prominent importance for analytic and
catalytic purposes. We report a multiscale experimental (AEIR, CO-TPD)
and theoretical approach to provide vibrational properties, adsorption
enthalpies, and desorption behaviors. First principles calculations
on Pt13(CO)
m
/γ-Al2O3 and Pt(111) surface models (using various exchange-correlation
functionals) provide a complementary view to experimental approches.
Adsorption enthalpies computed with the RPBE functional appear to
be the most compatible with the AEIR results. The occupation of top
sites by CO dominates the behavior of supported Pt clusters. CO coverage
reaches higher values in comparison to Pt(111) for similar operating
conditions, and considerable cluster reconstruction is observed at
high coverage. First principles calculations also confirm the IR assignment
related to the various adsorption modes on top and bridge sites and
demonstrate a particle size effect, lowering the frequency of linear
adsorption at top sites with respect to extended Pt(111) surfaces.
Finally, first principles-based microkinetic modeling of CO-TPD experiments
shows that the adsorption strengths predicted on the small-size cluster
by DFT are compatible with the experimental values. We discuss possible
reasons for the experimental desorption pattern to be much broader
than the computed pattern.
Toluene is an important compound in the chemical industry as well as an often chosen simple surrogate compound for aromatic components in transport fuels. As a result, an improved understanding of the liquid phase oxidation of toluene is of interest to both the chemical industry and the transportation sector. In this work, a detailed autoxidation mechanism for the liquid phase oxidation of toluene is developed using an automated mechanism generation tool. The resultant mechanism is significantly improved using quantum chemistry calculations to update the thermodynamic parameters of key species in solution. Comparisons are made between the predicted and experimentally measured induction period and the obtained mechanism. The agreement between both is found to be within 1 order of magnitude. Rate of production analysis and sensitivity analysis are carried out to explain and understand the reactions paths present in the mechanism. The behavior of the mechanism is commented upon qualitatively; however, no quantitative data could be obtained with the selected test method.
Performance of lean-burn gasoline spark-ignition engines can be enhanced through hydrogen supplementation. Thanks to its physicochemical properties, hydrogen supports the flame propagation and extends the dilution limits with improved combustion stability. These interesting features usually result in decreased emissions and improved efficiencies. This article aims at demonstrating how hydrogen can support the combustion process with a modern combustion system optimized for high dilution resistance and efficiency. To achieve this, chemical kinetics calculations are first performed in order to quantify the impacts of hydrogen addition on the laminar flame speed and on the auto-ignition delay times of air/gasoline mixtures. These data are then implemented in the extended coherent flame model and tabulated kinetics of ignition combustion models in a specifically updated version of the CONVERGE code. Three-dimensional computational fluid dynamics engine calculations are performed at λ = 2 with 3% v/v of hydrogen for two operating points. At low load, numerical investigations show that hydrogen enhances the maximal combustion speed and the flame growth just after the spark which is a critical aspect of combustion with diluted mixtures. The flame front propagation is also more isotropic when supported with hydrogen. At mid load, hydrogen improves the combustion speed and also extends the auto-ignition delay times resulting in a better knocking resistance. A maximal indicated efficiency of 48.5% can thus be reached at λ = 2 thanks to an optimal combustion timing.
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