Nitrogen fixation through MoO3-based catalysts embedded in a microwave plasma post-discharge

Application of plasmas, as an inherently green technology, in gas conversion processes is rising rapidly. Nitrogen fixation (NF) is an important gas conversion process aiming to form Nitrogen products that are associated with many applications such as food and pharmaceutical industry or chemical products. The development of plasma technology for NF is progressing but is still far from the benchmark. The synergy between plasma and catalysis is a promising approach in the roadmap of plasma-based processes, therefore, many research projects are in progress to optimize NF process based on this approach1. We studied NOx formation in a catalyst-assisted microwave plasma (MWP) in the post-discharge zone. It was confirmed by A. Fridman that high-frequency plasmas i.e. MW and RF are fitting better with gas conversion process compared to others. Since this sort of plasmas generates a considerable amount of vibrationally excited species that are necessary for gas conversion2. In addition, to produce NOx in an RF plasma reactor, MoO3 and WO3 catalysts positioned in the discharge zone gave the best performance up to now in terms of yield and energy efficiency3. Accordingly, heterogeneous MoO3 catalysts supported on γAl2O3 were prepared via the “conventional wet impregnation” method then characterized and used in the MWP with N2/O2 mixes. The FTIR study of the products reveals that there is an enhancement of NOx production mostly due to the synthesis of NO2 whereas NO is the only species detected without catalyst. According to our results, the major catalytic effect comes from γ-Al2O3 however, with different MoO3 loading, a minor effect can be identified. The highest NOx formation was obtained with γ-Al2O3-MoO3 10 wt%. At this loading, the γ-Al2O3 surface is covered by a Molybdenum oxide monolayer. Surface characterization of samples via XPS revealed two oxidation states of Mo on γ-Al2O3. Comparing Mo 3d5/2 peak position and FWHM value of different samples suggest that by increasing MoO3 loading, these values are approaching the pure reference MoO3, which agrees with our XRD and TEM studies. Results for different calcination temperatures of Al2O3 show the highest NOx formation for a substrate treated at 800°C and it was unexpected since at this temperature phase transition from γ to δ takes place and is accompanied by a surface area reduction. Furthermore, we evaluated the influence of the gas residence time (τr) inside the discharge zone on the NOx formation. Results show with increasing τr, the vibrational temperature is decreasing, and rotational temperature is increasing which corresponds to a reduction of NOx production. Finally, the efficiency of the catalysts in terms of NOx formation was improved by modifying their physicochemical properties via the addition of CoO as a promoter. Higher NOx production rate was achieved with MoO3-CoO (5 & 0.5 wt% respectively) on γ-Al2O3. Nevertheless, considering the position of catalysts in our experiment and the lifetime of the reactive species generated by the plasma, one of our challenges is to move toward the discharge zone to enhance plasma-catalyst interactions.