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Study of total oxidation of light hydrocarbons on perovskitic materials La1-xSrxMnO3

Georgakopoulou-Georgiou Theodora

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Summary

Climate change, air pollution and the degradation of the natural environment in general is one of humanity's most pressing issues. For this reason, the replacement of traditional fossil fuels with sustainable and cleaner energy sources in recent years is a one-way street. During the global transition to renewable energy sources, there is an ever-increasing reliance on natural gas (with methane typically >90%) as it is considered a reliable and affordable alternative source that simultaneously promotes the development of "green" energy sources. The main disadvantage of using CH4 concerns the ever-increasing emissions of unburned methane (of the order of 0.1-1.0%). It is known that both CO2 and CH4 are the main gases associated with the greenhouse effect, however CH4 has a 25 times greater potential contribution to the phenomenon compared to CO2. Moreover, in the global air pollution, volatile organic compounds are considered to be very important pollutants, which are high-risk substances with serious impact on the atmosphere and human health. Therefore, the development of highly active and thermally stable catalysts for their complete oxidation and therefore the control of their emissions, is of high environmental importance.On this thesis, the performance of perovskites La1-xSrxMnO3 and the Ir/La1-xSrxMnO3 catalysts was studied towards deep methane oxidation. LSxM supports were synthesized by co-precipitation method and the addition of 2 wt% Ir to LSxM supports was incorporated by the wet impregnation method. This study was carried out in a temperature range between 400-900οC and under conditions of oxygen excess (1% CH4 + 5% O2 in balance with He, 1 bar) with wGHSV=90000 mL/g*h (FΤ=75 mL/min). Additionally, the performance of La1-xSrxMnO3 perovskites was studied for the complete oxidation of C3H8 and C3H6. The temperature range was between 200oC-800oC and 100 oC-700oC respectively and the reactions were carried out under conditions of oxygen excess (0.33% C3H8 + 5% O2 in equilibrium with He, 1 bar) and (0.5% C3H6 + 5% O2 in balance with He, at 1 bar) with wGHSV=90000 mL/g*h (FΤ=75 mL/min). The catalytic activity and thermal stability of the catalysts were studied based on different catalyst processing protocols (i.e., reduction, oxidation, and aging). The thesis aims to investigate the effect of gradual replacement of the A-site of perovskite (La) from Sr, on its properties and on its catalytic behavior in the reactions in question. In addition, to establish a correlation between the structure of perovskites and their activity, the physicochemical and structural properties of the materials were evaluated by various catalyst characterization techniques (XRD, BET-BJH, H2-TPR, H2- Chem).According to the results of the experiments on the catalytic oxidation of methane, it was found that the main parameter affecting the activity of the studied catalytic materials was the degree (X) of substitution of the A=La position by Sr and not so much the addition of Ir, the latter caused a small inhibition of activity, rather than an enhancement, when in the oxidized state (IrO2 ) while a noticeable but not large enhancement was observed when it was in its metallic state (Ir0). The main factors, through which the parameter X affected the reactivity of the materials, were the changes induced in the specific surface area of the materials and the reducibility, parameters that act cooperatively. The activity of the materials followed an inverse volcanic behavior as a function of X with the most active catalysts being the pair LS00M-Ir/LS00M and the least active being the pair LS50M-Ir/LS50M. Large differences in T50 (about 300oC) were recorded between the highest and lowest activity catalysts. The inhibitory effect caused by the addition of Ir on the activity of LSxM perovskite was attributed to a partial blocking of the pores and consequently a reduction in the number of active sites, combined with the low activity of the IrO2 particles. For the pre-reduced catalysts, the reactive metallic state of iridium (Ir0) enhanced the activity, especially at low temperatures. In the case of the catalytic oxidation of VOCs, C3H8 and C3H6 in LSxM perovskites, it was found that both the composition of the LSXM perovskite (i.e., the rate of substitution of La by Sr, x) and the oxidized or reduced state are parameters that cause variations in catalytic activity of the materials, causing shifts in T50 up to ~100oC. The effect of the x parameter on reactivity had an inverted volcanic behavior, with the most active in all cases being the perovskite with X = 0% (i.e. LS00M = LaMnO3) and the least active being the perovskite with X = 50% (i.e. LS50M. Phenomena hysteresis during the heating/cooling cycles - as in the case of the catalytic oxidation of CH4 - appeared only in the case of the pre-reduced catalysts, while in the pre-oxidized materials the light-off and light-out curves closely coincided with each other.Finally, for all three gaseous pollutants, it was found that the stability of the materials after 12 hours of continuous operation was generally good as up to 10% reduction in their activity was observed.

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