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The effects of low-valence dopants on the catalytic properties of samarium oxide xerogel catalysts were investigated in the oxidative coupling of methane (OCM). More specifically, very low concentrations (0.1 and 1.0 % by mol) of transition metal (Ag, Ni, and Cu) and traditional alkali metal (Li and K) dopants were investigated. At these low loadings, it was shown that transition metal dopants have potential to improve the activity and selectivity over an undoped Sm2O3 xerogel, but these dopants can only outperform alkali metal dopants under certain conditions. Even at a concentration of 0.1 mol %, the dopants significantly increased the number of basic sites compared with the pure Sm2O3 xerogel. However, no trend is evident between the number or strength of the basic sites and the activity or selectivity in the methane coupling reaction. The XRD data reveal a lattice expansion upon addition of the low valence dopants, which is consistent with substitutional doping and the formation of oxygen vacancies due to charge compensation. In most cases the majority of the dopant stayed in the lattice during reaction. The dopants were also shown to influence the Sm2O3 structure, and the dopants that were more effective in suppressing the transformation from cubic to monoclinic Sm2O3 in general resulted in the more active and selective catalysts. While the Ag- and Ni-dopants could outperform the alkali metal doped catalysts in narrow temperature ranges, the best performing catalysts were still the K-doped Sm2O3 catalysts, as the 1.0 % K catalyst exhibited the highest activity at the lowest temperature (500 °C) and the 0.1 % K-doped catalyst was the most stable during extended operation. These results indicate that transition metal dopants, at low concentrations, can positively affect the activity and selectivity of a methane coupling catalyst, such as Sm2O3, and suggests that there may be benefits to other OCM catalyst systems from traditionally non-selective dopants, as long as the concentrations are kept very low and stability issues are addressed.

The effects of high-valance dopants on the catalytic properties of samarium oxide xerogels were investigated at concentrations of 0.1 and 1.0 % (by mol) in the oxidative coupling of methane (OCM). Gd, Y, Zr, and V dopants were selected to examine the influence of oxidation states between +3 and +5 on the OCM performance. Even at these low loadings, the high-valance dopants were observed to have a significant impact on the OCM reaction. At the lowest loading, 0.1 mol %, and below 700 °C, all dopants improved the activity over that of the pure Sm2O3 xerogel, and most also improved the selectivity. In particular, the ethylene yield was significantly improved over these catalysts between 500 and 700 °C. However, the stability of the doped catalysts compared to the undoped Sm2O3 xerogel were inferior above 700 °C, and the higher concentration (1.0 mol %) resulted in catalysts with a lower stability. Time-on-stream experiments revealed that the 0.1 mol % high valence dopants improved the stability of the Sm2O3 xerogel at 700 °C. As a result of the higher stability, the doped catalysts retain more of the original specific surface area and appear to stabilize the more active cubic Sm2O3 phase compared with the undoped catalyst. Therefore, the doped catalysts have a higher number of available basic sites during reaction. This study reveals that high-valence dopants have potential to improve the low temperature (500–700 °C) activity of OCM catalysts. However, the concentrations must be kept very low, as dopants that increase the activity and selectivity at concentrations of 0.1 mol % can result in inferior catalysts at 1.0 mol %, and temperatures above 700 °C must be avoided or rapid deactivation can occur.

The influence of the support basicity, according to the Lewis and Bronsted definition, was investigated for the Ru catalyzed CO2 methanation in the temperature range from 200 °C to 400 °C. Due to the structure-sensitivity of the reaction, a novel building block approach was used to ensure a constant Ru particle size, while varying the support material. In this way, differences in the catalytic behaviour could be directly related to support effects. Eight oxides – the rare earth metal oxides Gd2O3, Sm2O3 and Y2O3 (REOs) as well as TiO2, ZrO2, Al2O3, MgO and SiO2 as a non-basic oxide – were chosen to cover different types and combinations of basic surface sites on the support, such as Bronsted basic hydroxyl groups, Lewis basic oxygen atoms and oxygen vacancies. Above 310 °C, the REO supported catalysts showed the highest methane formation rates. The consumption of carbonate species formed upon CO2 adsorption on all three types of basic sites indicated their catalytic involvement in the high temperature regime. Below 310 °C, TiO2 and – to a lesser extent – ZrO2 excelled the other supports. For ZrO2 the enhanced performance could be related to the sole presence of Lewis basic oxygen vacancies, acting as additional CO2 adsorption and activation sites on the support. On contrary, in case of TiO2 they seemed not to be directly but only indirectly involved by facilitating the conversion on the Ru particles on the basis of a favourable electronic metal–support interaction. The inferior catalytic results obtained with the other supports were in accord with the absence of basic sites or a spectator role of the carbonates formed – except for Al2O3 which stood out probably due to Bronsted basic OH-groups formed under reaction conditions. Overall, the study reveals that basic supports can noticeably contribute to the catalytic turnover by opening new support-related pathways in addition to the Ru-related pathway evidenced in all cases and/or by promoting the latter. Their impact is dependent on the type, density and strength of basic sites available and varies with temperature.

ChemCatChem (2022). doi:10.1002/cctc.202101878, Published by WILEY-VCH Verlag, Weinheim

COx hydrogenation reactions for hydrocarbon synthesis, such as methane, are becoming more and more important in terms of the energy transition. The formation of the byproduct water leads to a hydrothermal environment, which necessitates stable catalyst materials under harsh reaction conditions. Therefore, novel nanostructured core-shell catalysts are part of scientific discussion, since these materials offer an exceptional resistance against thermal sintering. Here we report on a core-shell catalyst - Co@mSiO2 - for the hydrogenation of CO/CO2 mixtures towards methane. CO methanation experiments reveal a rapid temperature-depended deactivation for temperatures above 350 °C caused by coking and possible blocking of the pores. In comparison to a Co/mSiO2 reference catalyst with the same Co particle size a significantly higher methane selectivity was found for CO2 hydrogenation, which we attribute to the confinement effect of the core-shell structure and therefore a higher probability of CO readsorption. Finally, the simultaneous CO/CO2 co-methanation experiments show a high flexibility of the catalyst materials on different gas feed compositions.

Nickel-containing hybrid ceramics were prepared by pyrolytic conversion from either methyl or methyl-phenyl polysiloxanes mixed with bistrimethoxysilylpropylamine (BisA) as a complexing agent and nickel salt. Materials with tailorable characteristics were generated by varying the pyrolysis temperature from 400 up to 600 °C in order to evaluate their applicability in the CO2 methanation. The materials were characterized by thermogravimetric analysis (TGA), N2 adsorption-desorption isotherms (BET-BJH), water and n-heptane adsorption, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In-situ X-ray diffraction analysis (in-situ XRD) was used to evaluate the Ni particle structure and size during a simulated catalytic reaction. Porous hybrid ceramics (ceramers) with high specific surface areas (100–550 m2 g−1), hydrophobic or hydrophilic surfaces and different Ni particle sizes (4–7 nm) were obtained by varying the pyrolysis temperature and polysiloxane composition. The pyrolytic conversion of polysiloxanes combined with the complexing amino-siloxane BisA not only permitted a good dispersion of the Ni nanoparticles but also enabled the formation of hierarchical porosity with micro-, meso- and macropores. Regarding the catalytic performance, ceramers prepared from methyl polysiloxane exhibited a more hydrophobic surface and improved catalytic performance compared to the ones prepared from methyl-phenyl polysiloxane. A negative effect on the catalytic performance of ceramers was observed with increasing pyrolysis temperatures, which led to an increase in Ni particle size (from 4 to 7 nm), and lower levels of conversion and selectivity. The ceramers pyrolyzed at 400 °C exhibited the best catalytic performance, showing selectivity up to ∼77% and good stability over a 10 h test, during which the Ni particle size was preserved.

Recently, in situ studies using nuclear magnetic resonance (NMR) have shown the possibility to monitor local transport phenomena of gas-phase reactions inside opaque structures. Their application to heterogeneously catalyzed reactions remains challenging due to inherent temperature and pressure constraints. In this work, an NMR-compatible reactor was designed, manufactured, and tested, which can endure high temperatures and increased pressure. In temperature and pressure tests, the reactor withstood pressures up to 28 bars at room temperature and temperatures over 400 °C and exhibited only little magnetic shielding. Its applicability was demonstrated by performing the CO2 methanation reaction, which was measured operando for the first time by using a 3D magnetic resonance spectroscopic imaging sequence. The reactor design is described in detail, allowing its easy adaptation for different chemical reactions and other NMR measurements under challenging conditions.

The reaction kinetics for the CO2 methanation and reverse water gas shift reaction over an ordered-mesoporous Ni/Al2O3 catalyst were determined. For the parameter estimation and model discrimination, the kinetic data were obtained by means of spatially-resolved measurement in a catalytic plate reactor. In detail, ~21,000 high-resolution gas composition data were gathered along the reactor axis using a movable sampling capillary connected to a mass spectrometer. Additionally, the catalyst surface temperature was determined via infrared thermography. The influence of reaction temperature (320–420 °C), total pressure (1.2–7.3 barabs), and GHSV, as well as possible inhibition of products such as CH4 and H2O, were investigated. A one-dimensional model of the reactor was developed describing the conservation of mass in the bulk gas and catalyst phase. The Bayesian approach was used to estimate the kinetic parameters of 20 proposed Langmuir-Hinshelwood rate expressions for the CO2 methanation that were derived based on three different mechanisms (i.e., direct dissociation, hydrogen assisted dissociation, and hybrid mechanism). Two kinetic models reflected the measured data very well. The most probable models suggest that the rate determining step includes the reaction of an oxygenated complex (COH* or HCOO*) with an active site (*) or an adsorbed hydrogen (H*). Furthermore, water was assumed to be adsorbed as a hydroxyl species (OH*), while methane did not influence the reaction. Temperature- and time-resolved Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements confirmed the presence of both adsorbed surface intermediates.

The vulnerability of groundwater from chemical leaching through soil is a concern at some locations. Because measurements are laborious, time-consuming, and expensive, simulation models are frequently used to assess leaching risks. But the significance of simulated solute movement through a layered soil is questionable if vertical homogeneity of physical soil properties has been assumed. In the present study, a semi-analytical model for solute leaching in soils is presented. The model is relatively simple, but it does account for soil layers having different physical properties. The model includes the mobile-immobile model (MIM) to describe one-dimensional (1-D) nonequilibrium, transient solute transport under steady-state flow conditions. The MIM is rewritten as a second-order differential equation and solved by a numerical scheme. Differing from fully analytical or fully numerical solutions, the new approach solves the differential equation numerically with respect to time and analytically with respect to distance. Numerical experiments for a single layered soil profile show that the semi-analytical solution (SA-MIM) is numerically stable for a wide range of parameter values. The accuracy of SA-MIM predictions is comparable to that of analytical solutions. Numerical experiments for a multilayered profile indicate that the model correctly predicts effluent curves from finite layered soil profiles under steady-state flow conditions. The SA-MIM simulations with typical parameter values suggest that neglecting vertical heterogeneity of flow paths in a layered soil can lead to inaccurate prediction of soil-solute leaching. The quality of predictions is generally improved if parameter estimates for the different soil layers are considered. However, the mobile-immobile-parameter estimates obtained in a number of previous studies may not be transferable to a field situation that is characterized by a slow and steady flow of water. Further field experiments to determine mobile-immobile parameters under such conditions are desirable.

Preferential flow in soil can enhance the leaching of agricultural chemicals. In a number of studies it has been shown that the mobile-immobile solute transport model (MIM) is a useful tool to characterize preferential flow. In the present study, a new laboratory method for determining the MIM parameters θm and θim (mobile and immobile water content), as well as α (mass transfer coefficient), is developed. The computations are uncomplicated and the method requires only simple equipment. It is applied to short, undisturbed soil columns. Measured values ranged from 0.11 to 0.27 for θim θ—1 and from 0.015 h—1 to 0.034 h—1 for α for an Iowan soil (Nicollet silt loam). For two sandy Eutric Gleysols from Germany, low values for θim θ—1 from 0.04 to 0.07 and from 0.001 h—1 to 0.008 h—1 for α were determined. Although the new method is a flow-interruption technique, values for the Nicollet silt loam compare well with those from conventional leaching experiments. Values for the Eutric Gleysols agree with the observation that these soils were poorly structured. Because the new method does not assume negligible dispersion, it is applicable to a wider range of soils and boundary conditions than comparable approaches. We conclude that the new method provides parameter values that are suited to describe non-equilibrium solute transport. Labormethode zur Bestimmung des immobilen Anteils am Bodenwasser und des Massenaustauschkoeffizienten Praferenzielle Wasserbewegung in landwirtschaftlich genutzten Boden kann die Auswaschung von Agrarchemikalien beschleunigen. Eine Reihe von Studien hat gezeigt, dass sich das 2-Regionen-Modell (MIM) zur Beschreibung des Losungstransports in Boden mit Makroporen gut eignet. In der vorliegenden Arbeit wird eine neue Methode zur Bestimmung der MIM-Parameter θm und θim (mobiler und immobiler Porenwasseranteil), sowie α (Massenaustauschkoeffizient) vorgestellt. Die Methode ist rechnerisch unkompliziert, erfordert nur einfache Laborausrustung und wird an ungestorten kurzen Stechzylinderproben durchgefuhrt. Fur einen schluffigen Lehm (Nicollet silt loam) wurden Werte fur θim θ—1 von 0,11 bis 0,27 und fur α von 0,015 h—1 bis 0,034 h—1 ermittelt. Fur zwei Sandboden aus Norddeutschland lagen die Werte zwischen 0,04 und 0,07 fur θim θ—1 und zwischen 0,001 h—1 und 0,008 h—1 fur α. Obwohl die neue Methode auf einer Unterbrechung des Losungsflusses basiert, sind die fur den schluffigen Lehm ermittelten Werte vergleichbar mit Ergebnissen vorangegangener Perkolationsstudien. Die Werte fur die Sandboden sind aufgrund der schwachen Strukturierung dieser Boden ebenfalls schlussig. Da die Methode nicht von vernachlassigbarer Dispersion ausgeht, ist sie uber einen weiteren Bereich von Bodenarten und Randbedingungen anwendbar als vergleichbare Verfahren. Die Ergebnisse zeigen, dass die mit der neuen Methode ermittelten Parameterwerte sich zur Beschreibung von Makroporenfluss eignen.

A three-year long lysimeter experiment with a fine-grained aluminum (Al) recycling by-product and a mixture of this by-product and a coal combustion waste was conducted. The wastes were proposed as possible soil substitutes in an engineered surface barrier covering a potash mining residue mount. To evaluate the suitability of the wastes as surface barrier material, their hydrological behavior under field conditions must be known. Lysimeter experiments provide one means to study the hydrological behavior of soils or soil-like materials. However, it is difficult to estimate the long-term hydrological behavior from short-term lysimeter studies. The present study was conducted therefore to derive from short-term lysimeter observations the long-term hydrological behavior of the two waste materials. The lysimeter data were used to calibrate the one-dimensional soil water flow model HYDRUS-1D. With the calibrated model, hydrological simulations for the site of the residue mount were carried out for a period of 31 yr. Calculated long-term annual seepage from the lysimeters was 237 mm for the pure Al waste and 186 mm for the mixture, or 39% and 24% of the average annual precipitation (764 mm). The average discharge of the bare mount is 482 mm or 63%. We conclude that a soil cover could considerably reduce the discharge and that the mixture is better suited as surface barrier than the pure Al waste. Modellrechnungen zur Wasserdynamik in mit feinkornigen industriellen Abfallprodukten gefullten Lysimetern In einer dreijahrigen Lysimeterstudie wurde die Eignung eines feinkornigen Abfallprodukts der Wiederaufarbeitung von Aluminiumschrott und einer Mischung aus diesem Abfall und einem Ruckstand der Kohleverbrennung als Bodenersatzmaterialien in einer Deckschicht uber einer Ruckstandshalde des Kalibergbaus getestet. Zur Bewertung der Eignung als Deckschichtmaterialien mussen die hydrologischen Eigenschaften der Materialien unter Feldbedingungen bekannt sein. Allerdings ist es schwierig, das langfristige Sickerverhalten allein aus den Ergebnissen kurzfristiger Lysimeterstudien abzuschatzen. Die vorliegende Studie hatte zum Ziel, das langfristige hydrologische Verhalten der untersuchten Abfallstoffe mithilfe von Modellrechnungen aus kurzfristigen Lysimeterbeobachtungen zu schatzen. Die (kurzfristigen) Lysimeterdaten wurden zur Kalibrierung des eindimensionalen Bodenwassertransportmodells HYDRUS-1D verwendet. Mithilfe des kalibrierten Modells wurden Simulationsrechnungen des Wasserhaushalts uber eine Periode von 31 Jahren ausgefuhrt. Die berechnete mittlere jahrliche Sickerung aus den mit reinem Al-Ruckstand gefullten Lysimetern betrug 237 mm, die aus den mit der Mischung gefullten Lysimetern 186 mm. Die Werte entsprechen 39% bzw. 24% des mittleren langjahrigen Niederschlags (764 mm). Der Abfluss von der nicht abgedeckten Halde betragt 482 mm oder 63%. Hieraus schliesen wir, dass eine Abdeckung den Abfluss erheblich reduzieren wurde und dass die Mischung aus Al-Ruckstand und dem Ruckstand aus der Kohleverstromung besser als Deckschichtmaterial geeignet ist als unvermischter Al-Ruckstand.

In Germany, field-average soil NO3- measurements are used to identify agricultural risks of groundwater pollution and to evaluate the compliance of land users with environmental regulations. In the present study, it was tested at three typical agricultural sites if common practices of soil NO3- sampling were appropriate to obtain reliable estimates of the mean field NO3- content in fall. Three sites of 1 ha area were extensively sampled for NO3-. Ordinary statisticial analyses were conducted on the NO3- data in order to calculate how many samples per ha were required to predict the mean field NO3- with a given accuracy. Variograms were derived to assess applicability and relevance of ordinary statistical methods for heterogeneous fields. Results from statistical analysis suggest that at two sites common practices of soil NO3- sampling would have been adequate to obtain estimates of the actual mean field NO3- content with a sampling error less than 10 kg NO3--N ha—1 at a 95% probability level. At the other site, common practices obviously would have failed because NO3- contents varied much more spatially. It remains a problem of soil sampling for NO3- analysis that information on field heterogeneity is frequently not available a priori. Raumliche Variabilitat herbstlicher Nitratgehalte in landwirtschaftlich genutzten Boden In Deutschland dienen spatherbstliche Bodennitratmessungen dazu, unsachgerechte Flachenbewirtschaftung und potenzielle Grundwassergefahrdungen aufzuzeigen. Fraglich ist, welchen Einfluss die raumliche Heterogenitat von Boden auf die Aussagekraft von gemittelten Nitratmesswerten hat. Dazu wurden im Spatherbst drei typische Ackerstandorte intensiv auf Bodennitrat beprobt. Durch Konfidenzschatzung wurde berechnet, wie viele Proben erforderlich sind, um den tatsachlichen mittleren Nitratgehalt einer Flache mit einer gegebenen Genauigkeit abzuschatzen. Anhand von Semivariogrammen wurde untersucht, welche Relevanz die Konfidenzschatzung fur heterogene Boden besitzt. Die Ergebnisse der statistischen Analyse zeigen, dass mit gangigen Methoden der Nitratbeprobung auf zwei der Flachen eine Abschatzung des tatsachlichen mittleren Nitratgehaltes mit einer Unsicherheit von weniger als 10 kg NO3--N ha—1 bei einem Wahrscheinlichkeitsniveau von 95% moglich war, wahrend auf der dritten Flache wegen der deutlich ausgepragteren raumlichen Variabiliat der Nitratgehalte weit mehr Proben als ublich erforderlich waren, um dieselbe Genauigkeit zu erzielen. Das Problem bei Beprobungskampagnen wird sein, dass es im Regelfall nicht vorhersehbar ist, welche Flachen besonders heterogen sind und daher intensiver beprobt werden mussten, um zuverlassige Schatzwerte zu erzielen.