The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane

Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 gcat-1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat-1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed

Berechnungen zur Laserlicht-Übertragung in Laser-Fusionsreaktoren bei verschiedenen Dichten und Dichteprofilen

In this thesis the laserlight-propagation in a Laser-Fusionreactor at various densities and density-profiles shall be examined in order to determine the optimum density under various conditions. For this purpose the computer code RAMSES has been written, which simulates in twc dimensions the photon-motion through a gas or plasma. The method of simulation is essentially a PIC- (particle-in-cell) model. The programme takes into account refraction-effects and variations of the group velocity caused by the local electron density, which may be supplied either explicitly or calculated from the ionisation rates. A parameter study has been carried out in oder to calculate the laser-transmission-rates for different density profiles and under various conditions. With the restraining condition of a transmission of 95 % of the original pulse energy into the target region the optimum density in Bach case is determined. Finally as an example for application - these optimum densities lead to restrictions an the wall-temperature of the reactor-vessel

Perovskites as Precursors for Ni/La₂O₃ Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co-Precipitation, Reduction Mechanism and Effect of Ru-Doping

LaNiO3 perovskite is an interesting precursor for Ni/La2O3 catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co-precipitation without, with 2.5 at%, and with 5 at% Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden-Popper mixed oxides Lan+1NinO3n+1, which was observed for the Ru-free sample upon calcination at 1000 °C (n=3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO2.5-like intermediate. The second major step is the reduction to Ni metal supported on La2O3 independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni-Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5% Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru

LDH-derived Ni-catalysts in dry reforming of methane at high temperatures for an efficient CO₂-conversion into syngas

The efficient conversion of CO2 into chemicals and fuels is a prospective building block for a more sustainable usage of our global resources. Among the various strategies to convert CO2 into higher-energy intermediates, heterogeneously catalyzed processes are of special interest, because they are scalable, based on a mature and flexible technology, which is already applied in chemical industries, and can be integrated into existing value chains. The dry reforming of methane (DRM) with carbon dioxide is an interesting option to convert these two greenhouse gases into CO/H2 mixtures (eq. 1). The resulting gas mixture can then be used in the well-established downstream syngas chemistry. (1) CO2 + CH4 ⟶ 2 CO + 2 H2 DH298 = 247 kJ mol-1 It is well known that Ru, Rh or Pt catalysts are very active in this reaction. Active base metals, in particular Ni, suffer from fast deactivation by coking. However, from an economical point of view Ni-based catalysts are more suitable for commercial application than noble metal ones. Thus, a current challenge is to find a noble metal-free catalyst that is resistant against coking. We have found that mitigation of the coking problem of noble metal-free Ni catalyst for DRM is possible by elevating the operation temperature towards 900 °C. Compared to lower reaction temperatures, the formation of fibrous carbon was substantially lowered. This favorable operational window can be exploited only if nanostructured catalysts with sufficient thermal stability are available to survive these harsh conditions. We present the synthesis, characterization, and catalytic testing of a highly active and stable Ni/MgAlOx catalyst that is characterized by small Ni particles (10 nm), which are partially embedded in an oxide matrix with a high specific Ni and total BET surface area. Despite the high Ni loading of 55 wt.-%, this catalyst shows only minor sintering at 900 °C and performs stably in DRM over 100 hours with an outstanding high rate of syngas formation. The stability of the nanostructure is ascribed to the embedding nature of the oxide matrix as a result of the uniform elemental distribution within the layered double hydroxide (LDH) catalyst precursor

Redox dynamics of Ni catalysts in CO₂ reforming of methane

The influence of redox dynamics of a Ni/MgAl oxide catalyst for dry reforming of methane (DRM) at high temperature was studied to correlate structural stability with catalytic activity and coking propensity. Structural aging of the catalyst was simulated by repeated temperature-programmed reduction/oxidation (TPR/TPO) cycles. Despite a very high Ni loading of 55.4 wt.%, small Ni nanoparticles of 11 nm were obtained from a hydrotalcite-like precursor with a homogeneous distribution. Redox cycling gradually changed the interaction of the active Ni phase with the oxide support resulting in a crystalline Ni/MgAl2O4-type catalyst. After cycling the average particle size increased from 11 to 21 nm – while still a large fraction of small particles was present – bringing about a decrease in Ni surface area of 72%. Interestingly, the redox dynamics and its strong structural and chemical consequences were found to have only a moderate influence on the activity in DRM at 900 °C, but lead to a stable attenuation of carbon formation due to a lower fraction of graphitic carbon after DRM in a fixed-bed reactor. Supplementary DRM experiments in a thermobalance revealed that coke formation as a continuous process until a carbon limit is reached and confirmed a higher coking rate for the cycled catalyst

Correction: The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane

An incorrect equation has been erroneously inserted into the introduction

Investigation of Coking During Dry Reforming of Methane by Means of Thermogravimetry

Coking dynamics of Ni-based and Ni-free catalysts were studied in a magnetic suspension thermobalance under methane dry reforming conditions. Ni-rich catalysts undergo strong coking featured with a surface saturation point where the coking rate is drastically reduced. Catalyst resistance towards coking may be enhanced by using noble-metal-based Ni-free precursors or decreasing the Ni content in the catalytic system. The post reaction performed temperature-programmed oxidation experiment of the coked catalyst is diffusion-limited due to large amounts of formed carbon