Supported Manganese Oxide Catalysts for NO Oxidation in Nitric Acid Production
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Nitric acid is an important bulk chemical in the petrochemical industry, and the main product is ammonium nitrate for fertilizers. It is produced by the Ostwald process, where one of three chemical steps is the oxidation of NO. Today, NO oxidation is a homogeneous gas phase reaction driven by maintaining high pressure and low temperature by removal of the reaction heat. Catalytic oxidation of NO is sought out to maximize NO2 throughput, decrease the capital costs, and increase heat recovery. Thus, the overall aim of this project was to investigate new cost-efficient catalysts for NO oxidation at industrial conditions. Manganese oxide (MnOx ) catalysts are known to catalyze oxidation reactions, and research has shown that MnOx catalysts exhibit activity for NO oxidation at ppm levels. Supported MnOx catalysts were thus of interest for application in NO oxidation at in- dustrial conditions; T = 350°C, P = 1 atm, 10% NO, 6% O2, 15% H2O, and balance Ar. As support, γ-alumina, silica, and zirconia (Mn/Al2O3, Mn/SiO2, and Mn/ZrO2) were chosen. Of the synthesized catalysts, Mn/ZrO2 catalysts had the highest conversion of ≈41% at 350°C. A total of ten catalysts were prepared by incipient wetness impregnation, three weight percentages i.e. 5-, 10-, and 20 wt% Mn on alumina, silica, and zirconia in addition to 1 wt%Pt−10 wt% Mn/Al2O3. Manganese nitrate was used as the metal precursor with a calcination temperate of 500°C. The catalysts were characterized by O2 and CO chemisorption, N2 adsorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX). Activity testing of the catalysts was performed in a vertical tubular reactor at atmospheric pressure and tested for temperature dependency (150−450°C) and stability at 350°C in dry and wet conditions. For alumina- and silica-supported catalysts the activity increased with increasing loading. This gave the highest conversion for 20 Mn/Al2O3 with a rate of reaction of 8.3 μmol·s−1·g−1 and 8.9 μmol·s−1·g−1 for 20 Mn/SiO2 in dry conditions. In wet conditions, there was observed an inhibiting effect of water as the conversion decreased, giving a rate of reaction for 20 Mn/Al2O3 of 5.0 μmol·s−1·g−1 and 5.8 μmol·s−1·g−1 for 20 Mn/SiO2. The conversion of NO on zirconia-supported catalysts was independent of Mn loading and had the overall highest activity with a rate of reaction of ∼11.4 μmol·s−1·g−1 and ∼8.4 μmol·s−1·g−1 in dry and wet conditions. Pt promotion of 10 Mn/Al2O3 resulted in a higher dispersion and activity. The results showed thatsupported-zirconia catalysts with low Mn loading are promising cost-efficient catalysts for NO oxidation in nitric acid production.