Entwicklung fortschrittlicher Katalysatoren zur Speicherung erneuerbarer Energien durch Umsetzung von CO2

• Development of advanced catalysts to store renewable energy by converting CO2

Asmelash, Chalachew Mebrathu; Palkovits, Regina (Thesis advisor); Perathoner, Siglinda (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University und Dissertation, University of Messina, 2018

Abstract

Reduction of greenhouse gases such as CO2 requires enhancing the way how we store and use carbon sources. As the same time, the need of replacing non-renewable energy sources with renewables such as the wind and solar is growing. Therefore, due to the intermittent nature of renewable energy sources, a combined way to solve both issues is in demand. Among the possible solutions solar fuels production mainly methane by the Power to Gas (P2G) process (from renewable H2 and CO2) enables to solve both long-term as well as large-scale energy storage and transportation problems. Solar or renewable fuels are mainly synthetic hydrocarbons that derive from hydrogen (from water electrolysis using surplus renewable electricity) and the CO2 (from industries and captured). Possible examples of the fuels which can be produced using this process include methanol, methane, and liquid hydrocarbons. Compared amongst, those, methane is the most promising solution. Methane can be prepared with a single reaction (Sabatier reaction), has higher energy density and can be easily distributed using the existing natural gas network. The methanation of CO2 is an exothermic reaction favored at lower temperatures, but due to kinetic limitations, a catalyst needs to be utilized. For a catalyst to be of use in the industry it needs to meet certain cost, activity, selectivity, stability, recovery, reuse, and handling requirements. Nickel-based catalysts are the most commonly studied for CO2 methanation because of their high activity and low price. However, conventional Ni catalysts supported on alumina are easily deactivated as a result of sintering of Ni particles and coke deposition during the exothermic methanation reaction. Hence, Ni-based catalysts with improved properties are still in need for this reaction at the industrial level. This Ph.D. work, therefore, presents the synthesis and application of advanced Ni-based catalysts with better catalytic activity and stability than the conventional Ni/Al2O3 catalyst. In order to achieve the principal goal of the study, four types of catalysts were prepared using different methods. In the laboratory of catalysis for sustainable energy and productions (University of Messina) mixed oxide supported Ni-based catalysts and Ni catalysts with unique structure were prepared and their catalytic activities were investigated towards low-temperature CO2 methanation. As part of the project, during the mobility to RWTH Aachen, hydrotalcite derived Ni-Fe catalysts were synthesized in order to study the effect Fe as second metal and support basicity in the low-temperature CO2 methanation reaction. For the studies on the effect of mixed oxide supports, ternary and quaternary mixed oxide supports were prepared by an impregnation-precipitation method using commercial γ-Al2O3 powder as a host. The percentage of loading ZrO2, TiO2 and CeO2 promoters from their respective salt precursors were varied from 5 - 15%. As-prepared samples were characterized by BET, XRD, H2-TPR, CO-chemisorption, and CO-TPD analyses. The CO2 methanation performance was evaluated at 5 bar pressure, temperature range of 300-400oC and different Gas Hourly Space Velocities (GHSVs) by using a high throughput reactor. Experimental results showed that enhanced catalytic activity depends on both textural improvements (for the ternary mixed oxide supported Ni-based catalysts) and reducibility and metal dispersion (for the quaternary mixed oxide supported Ni-based catalysts). The comparison between both groups of catalysts revealed that addition of CeO2 to the ternary mixed oxide further improves the catalytic performance.In order to study the effect of Fe as second metal in hydrotalcite derived catalysts, (Mg, Al)Ox supported Ni-Fe bimetallic catalysts were prepared using Ni-Mg-Fe-Al hydrotalcite-like precursors by co-precipitation at pH=10+0.5. The catalytic performance of Ni-Fe/MgAlOx catalysts was investigated in the synthetic natural gas production from CO2 at 335°C, atmospheric pressure, and gas hourly space velocity of 12020 h-1. The catalysts were characterized by XRD, ICP-OES, BET specific surface area, TGA-DSC, STEM, H2-TPR, and irreversible acid adsorption. XRD analysis of the co-precipitated sample after drying confirmed the hydrotalcite-like structure of the precursors. STEM-EDS investigations proved that Ni-Fe alloys were obtained after the reduction pretreatment at 600oC. Among the investigated catalysts in the CO2 methanation reaction, Ni-Fe catalyst with a relatively lower content of Fe (Fe/Ni=0.1) showed better activity with a rate of 6.96 mmol CO2 conversion/mol metal/s, 99.3% of CH4 selectivity and excellent stability for 24 h at 335°C. Moving to the investigation on the effect of support basicity on the catalytic performance of hydrotalcite derived Ni-Fe, catalysts with various amount of MgO were prepared using the same procedure developed in the chapter described above. As-prepared materials were characterized by XRD, H2-TPR, CO2-TPD, XRF and SEM and STEM techniques. Reduction at 900oC led to the formation of metallic Ni-Fe alloyed particles supported on a spinel type (Mg, Al)Ox matrix. Catalytic measurements under differential conditions at 300oC revealed a reproducible CO2 conversion into CH4. Higher CO2 methanation activity was recorded over the Ni0.2Fe0.02Mg0.55Al0.23 with 0.251 mol CO2 conv./molNi+Fe/s CO2 conversion rate and 97% CH4 selectivity at 300oC. Catalytic CO2 methanation of this catalyst was found much better than the other catalysts with relatively higher active metal loadings. The Mg0.75Al0.25 (the Lewis base support) was inactive towards the CO2 methanation reaction under similar pretreatments and reaction conditions. The better activity of Ni0.2Fe0.02Mg0.55Al0.23 catalyst can be due to the optimal amount of basic sites, better metal dispersion and smaller average particle size obtained after reduction of the mixed oxide. Finally, the effect of catalyst morphology was studied by preparing nanosheet-like catalysts via a two-step hydrothermal method and characterized by several physicochemical analyses methods. Catalytic behavior in CO2 methanation was investigated in the 300-350°C temperature range and 5 bar pressure using a Microactivity Efficient equipment (Micromeritics) with two fixed bed continuous reactors. The catalytic performance was also compared with a commercial methanation catalyst. A higher activity at 300°C (about 860 molCH4 molNi -1 h-1) and 99% of selectivity to CH4 with stable performance for more than 50 h were observed for the nanosheet-like sample promoted by iron. With respect to commercial methanation catalysts, the Fe-promoted nanosheet-like samples show an activity almost similar (slightly improved) with a lower rate of deactivation at 300°C. The improved properties of as-prepared catalysts comprehend to the synergy between NS structure and Fe promotion.

Institutions

• Department of Chemistry [150000]
• Chair of Heterogeneous Catalysis and Technical Chemistry [155310]