Umicore (Germany)

Multiscale simulations of reactive flows are critical in many fields. However, their application is often hindered by the high computational cost of solving detailed chemical kinetics. Recent advances in surrogate models for reactive chemistry offer promising speedups, but ensuring physical consistency remains challenging. In particular, machine learning models for chemical kinetics must enforce atom balance and guarantee the positivity of predicted concentrations. Here, we introduce a positivity preserving projection and a correction by linear interpolation backtracking which simultaneously guarantee both constraints. We demonstrate this using two practical examples from atmospheric chemistry and heterogeneous catalysis, as well as for a large number of random, synthetically generated reaction systems. In all cases, our approach yields exclusively positive model predictions conforming to the atom balance, without reducing the overall accuracy of the model.

Liberale Profile. Freiheit und Verantwortung. Hrsg. v. Jürgen Morlok. Stuttgart: Seewald 1983. 270 S.

The proposed latent transformation approach allows building lightweight neural networks that accelerate reactor simulations significantly.

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH3-assisted selective catalytic reduction of NO (NH3-SCR) for the abatement of NOx emission from diesel vehicles. However, the presence of a small amount of SO2 in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO2 exposure, we have selectively prepared CuI and CuII species with different ligations, which are relevant for the NH3-SCR reaction. The highest reactivity toward SO2 was observed for CuII species coordinated to both NH3 and extraframework oxygen, in particular for [CuII2(NH3)4O2]2+ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO2 uptake was significantly lower. These results explain why SO2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [CuII2(NH3)4O2]2+ is a crucial intermediate in the low-temperature NH3-SCR catalytic cycle.

The majority of passenger cars and light-duty vehicles, especially in the United States and Japan, are propelled by spark-ignition engines operated under stoichiometric conditions. Different concepts of catalytic pollution control for stoichiometric spark-ignition engines have been used over time. An improvement in exhaust gas control technology for stoichiometric spark-ignition engines was achieved by the development of three-way catalysts that simultaneously convert all three regulated pollutants. Owing to the finite time response of the feedback control, the air/fuel ratio in the feedback-controlled mode performs regular λ variations around λ = 1. Three-way catalysts contain oxygen components that buffer the air/fuel variations. Regulations require the performance of the exhaust-gas converter system to be continuously monitored during vehicle operation. Much effort has been spent on replacing Pt group metals by less-expensive and more abundant materials. A combination of technical and price considerations led to the preferred use of different precious metals for different generations of three-way catalysts.

The widespread use of automobiles offers a degree of mobility to the individual that was unthinkable just a few centuries ago. With increasing car and truck population today, it is obvious that the massive use of combustion engine vehicles is not free of problems, one of the major problems being air pollution. Engine development in the past years has led to improved combustion processes under even more controlled operational conditions. These developments have contributed to new emission control solutions meeting the goals defined by the legislation. Carbon dioxide is an unavoidable combustion product of fossil fuels. The most effective type of catalyst exhaust gas system today is the three-way catalytic converter that simultaneously reduces concentrations of CO, NO, and hydrocarbons.

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