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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.

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.

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.