The evolutionary status of the bright high-latitude supergiant HD190390

Despite its mean apparent magnitude of m(v)=6.39, the evolutionary status of HD190390 (HR7671), a luminous F-type supergiant at high galactic latitude, is still not very clear, but in most papers a post-AGB classification is assumed. New observational material has been obtained with four different instruments and is presented here. An extensive abundance analysis based on high resolution, high signal-to-noise NTT+EMMI spectra confirms the metal deficiency of this object ([Fe/H]=-1.6), together with a high lithium content (log(Li)=1.9). A variability analysis based on Geneva photometry over seven years reveals beating with a period of +/-3000 days. It is, however, not clear whether this beating is caused by a stable triplet, or it is the consequence of small changes in the main frequency. More recent data obtained with the HIPPARCOS satellite and the Mercator telescope not only confirm the main period, but also support the presence of a second periodicity of 11 days, which was also found in the Geneva photometry. A conclusive evolutionary status of this object is not given, but alternative to the UU Her (i.e. post-AGB) status, a W Vir classification is discussed.Comment: 14 pages, 13 figures, accepted for publication in A&

HD172481: a super lithium-rich metal-deficient post-AGB binary with a red AGB companion

We present in this paper a study on the peculiar supergiant HD172481. Its spectral type (F2Ia), high galactic latitude (b=-10.37), circumstellar dust, high radial velocity and moderate metal deficiency ([Fe/H]=-0.55) confirm the post-AGB character of this object. A detailed chemical analysis shows slight but real s-process overabundances, however no CNO-enhancement was detected. Furthermore, the spectral energy distribution and the TiO bands in the red part of the spectrum reveal a red luminous companion. The luminosity ratio of the hot F type component and this cool M type companion L(F)/L(M) is derived for a reddening of E(B-V)=0.44 (L(F)/L(M)=1.8) and indicates that the companion must also be strongly evolved and probably evolving along the AGB. Neither our photometric data-set, nor our radial velocity monitoring show evidence for orbital variability which may indicate that the period is too large for direct binary interaction. Most interestingly, a strong lithium resonance line is detected, which yields an abundance of log(Li)=3.6. Several explanations for this large lithium content are explored.Comment: 11 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

First-principles kinetic modeling in heterogeneous catalysis: an industrial perspective on best-practice, gaps and needs

Electronic structure calculations have emerged as a key contributor in modern heterogeneous catalysis research, though their application in chemical reaction engineering remains largely limited to academia. This perspective aims at encouraging the judicious use of first-principles kinetic models in industrial settings based on a critical discussion of present-day best practices, identifying existing gaps, and defining where further progress is needed

Hydrogen radical additions to unsaturated hydrocarbons and the reverse β-scission reactions: modeling of activation energies and pre-exponential factors

The group additivity method for Arrhenius parameters is applied to. hydrogen-addition to alkenes and alkynes and the reverse beta-scission reactions, an important famliy of reactions in thermal processes based on radical chemistry. A consistent set of group additive values for 33 groups is derived to calculate the activation energy and pre-exponential factor for a broad range of hydrogen addition reactions. Thee;group additive values are determined from CBS-QB3 ab-initio-calculated rate coefficients. A mean factor of deviation of only two between CBS-QB3 and experimental rate coefficients for seven reactions in the range 300-1000 K is found. Tunneling. coefficients for these reactions were found to be significant;below 400 K and a correlation accounting for tunneling is presented. Application of the obtained group additive values to predict the kinetics for a set of 11 additions and beta-scissions yields rate coefficients within a factor of 3.5 of the CBS-QB3 results except for two beta-scissions with severe steric effects. The mean factor of deviation with respect to experimental rate coefficients of 2.0 shows that the group additive method with tunneling corrections can accurately predict the kinetics and is at least as accurate as the most commonly used density functional methods. The constructed group additive model can hence be applied to predict the kinetics of hydrogen radical additions for a broad range of unsaturated compounds

Quality control and correction method for air temperature data from a citizen science weather station network in Leuven, Belgium

The growing trend toward urbanisation and the increasingly frequent occurrence of extreme weather events emphasise the need for further monitoring and understanding of weather in cities. In order to gain information on these intra-urban weather patterns, dense high-quality atmospheric measurements are needed. Crowdsourced weather stations (CWSs) could be a promising solution to realise such monitoring networks in a cost-efficient way. However, due to their nontraditional measuring equipment and installation settings, the quality of datasets from these networks remains an issue. This paper presents crowdsourced data from the “Leuven.cool” network, a citizen science network of around 100 low-cost weather stations (Fine Offset WH2600) distributed across Leuven, Belgium (50∘52′ N, 4∘42′ E). The dataset is accompanied by a newly developed station-specific temperature quality control (QC) and correction procedure. The procedure consists of three levels that remove implausible measurements while also correcting for inter-station (between-station) and intra-station (station-specific) temperature biases by means of a random forest approach. The QC method is evaluated using data from four WH2600 stations installed next to official weather stations belonging to the Royal Meteorological Institute of Belgium (RMI). A positive temperature bias with a strong relation to the incoming solar radiation was found between the CWS data and the official data. The QC method is able to reduce this bias from 0.15 ± 0.56 to 0.00 ± 0.28 K. After evaluation, the QC method is applied to the data of the Leuven.cool network, making it a very suitable dataset to study local weather phenomena, such as the urban heat island (UHI) effect, in detail. (https://doi.org/10.48804/SSRN3F, Beele et al., 2022).</p

Kinetics of alpha hydrogen abstractions in thiols, sulfides and thiocarbonyl compounds

Hydrogen abstraction reactions involving organosulfur compounds play an important role in many industrial, biological and atmospheric processes. Despite their chemical relevance, little is known about their kinetics. In this work a group additivity model is developed that allows predicting the Arrhenius parameters for abstraction reactions of alpha hydrogen atoms from thiols, alkyl sulfides, alkyl disulfides and thiocarbonyl compounds by carbon-centered radicals at temperatures ranging from 300 to 1500 K. Rate coefficients for 102 hydrogen abstractions were obtained using conventional transition state theory within the high-pressure limit. Electronic barriers were calculated using the CBS-QB3 method and the rate coefficients were corrected for tunneling and hindered rotation about the transitional bond. Group additivity values for 46 groups are determined. To account for resonance and hyperconjugative stabilization in the transition state, 8 resonance corrections were fitted to a set of 32 reactions. The developed group additivity scheme was validated using a test set containing an additional 30 reactions. The group additivity scheme succeeds in reproducing the rate coefficients on average within a factor of 2.4 at 300 K and 1.4 at 1000 K. Mean absolute deviations of the Arrhenius parameters amount to, respectively, 2.5 kJ mol(-1) for E-a and 0.13 for log A, both at 300 and 1000 K. This work hence illustrates that the recently developed group additivity methods for Arrhenius parameters extrapolate successfully to hetero-element containing compounds

Study of n-butanol conversion to butenes : effect of Si/Al ratio on activity, selectivity and kinetics

As bio-butanol is gaining more and more interest as a commercially available bioresource, the dehydration of this alcohol towards butenes and higher carbons gains more of interest. In general HZSM-5 has shown to be the most promising catalyst for this conversion. The role of the zeolite’s Si/Al ratio in the butanol dehydration reaction is still not fully understood. Experimental data obtained for a series of HZSM-5 with decreasing Si/Al ratio revealed an increase in activity of the catalyst per active site without affecting the selectivity profile. To understand the underlying effects, a microkinetic model was constructed for H-ZSM-5 with a Si/Al ratio of 25, based on literature DFT calculations, and the model was further modified by fitting the key parameters to the measured data at the four different temperatures studied in this work. This resulted in an adequate model for the dehydration of butanol across the evaluated temperature range of 503K to 533K. Investigation of the occurring mechanisms indicated a inhibiting effect due to the strong adsorption of di-n-butylether. This ‘poisoning’ of the catalyst surface resulted in a peculiar S-like curve for the conversion site time relation, which was also experimentally observed. This newly fitted base model was used to obtain more insight in the effect of the Si/Al ratio by implementing an additional H parameter, which is related to the adsorption strength of n-butanol in the base model. H varies between -4.8 to +11.3 kJ/mol and provides a good fit for Si/Al ratios ranging from 15 to 140. The higher dehydration rates observed with decreasing Si/Al can be traced back to an increase in adsorption strength resulting in an overall increase in surface coverage. The constant selectivity-conversion profile can be explained by a similar dependency of all elementary steps on the adsorption strength. The model developed in this study enables to simulate and understand the experimentally observed effects of temperature and Si/Al ratio on the n-butanol dehydration