Catalytic wet air oxidation of ammonia in water over manganese-based oxide catalysts: Impact of the redox state of manganese

SSCI-VIDE+CARE+HAY:CDSInternational audienceCatalytic Wet Air Oxidation of ammonia in water over manganese-based oxide catalysts: Impact of the redox state of manganeseHana AYADIa, Laurence BOISb, Claude DESCORMEa,*aUniversity of Lyon, CNRS, IRCELYON, 2 avenue Albert Einstein, Villeurbanne, F-69626, FrancebUniversity of Lyon, CNRS, LMI, 43 boulevard du 11 novembre 1918, Villeurbanne, F-69622, France*Corresponding author: claude.descorme@ircelyon.univ-lyon1.frIn order to achieve a possible rational design of the manganese-based catalysts for the Catalytic Wet Air Oxidation of ammonia, a detailed evaluation and characterization of four different unsupported manganese oxide catalysts (MnO2, Mn2O3, Mn3O4 and MnO) was initiated, before and after reaction. The objectives of such studies were (i) to get better insights into the impact of the initial redox properties of the oxides on their catalytic performances and (ii) to evidence any possible changes upon reaction. The initial oxidation state of manganese was shown to have a significant impact on the catalytic activity and MnO2 was the best. Moreover, in some cases, both the crystalline structure and the redox properties of the oxides were observed to be significantly modified upon reaction. It was suggested that the balance between MnIV and MnIII is of crucial importance for the activation of ammonia and its subsequent oxidation.1. ScopeAmmonia in water is a top environmental issue. It is frequently used in the industry and often produced as an intermediate upon the decomposition of (larger) nitrogen-containing organic compounds. Ammonia can be removed from water via biological processes but they are quite limited to low ammonia concentrations and they require long periods of treatment. As a possible alternative, the Catalytic Wet Air Oxidation (CWAO) presents several advantages such as the possibility to treat concentrated effluents with a high selectivity toward molecular nitrogen. Although supported noble metal catalysts were demonstrated to be highly efficient in the CWAO of ammonia and highly selective towards molecular nitrogen1,2, alternative catalysts based on manganese have been developed. Among the different oxides which were evaluated to support manganese oxides, ceria was shown to be the best in the CWAO of methylamine3, a clear synergetic effect was evidenced between the two oxides and a combined redox cycle was suggested. To confirm this hypothesis and get deeper insights into the reaction mechanism, a detailed analysis of the redox properties of the manganese phase and their evolution upon completion of the reaction was initiated. In order to isolate the different phenomena and benefit from pure phase manganese oxides, the preliminary studies were carried out over commercial unsupported manganese oxides with different manganese oxidation states.2. Results and discussionDifferent unsupported manganese oxides (MnO2, Mn2O3, Mn3O4 and MnO) were purchased from Sigma-Aldrich and used as received, without any further treatment. The catalysts’ texture, structure and redox properties were studied by specific surface area measurement, X-ray diffraction (XRD), temperature programmed reduction (H2-TPR) as well as X-ray photoelectron spectroscopy (XPS). The CWAO experiments were performed in a batch reactor using an ammonium hydroxide solution (100 mmol L-1). The operating conditions were fixed at T= 230°C and PO2= 4 bar. The evolution of the reaction mixture composition was monitored by measuring the total nitrogen content (TOC-VCSH TNM-1, Shimadzu) and using ion chromatography (881 Compact IC Pro, Metrohm) to determine the ammonium, nitrite and nitrate concentrations. The nitrogen balance was checked by comparing the TN and IC results. The evolution of the pH was also monitored.BET(m2 g-1)XRDpHPZCMnO20.9Pyrolusite4.4Mn2O32.2Bixbyite5.9Mn3O40.8Hausmannite6.9MnO2.2Manganosite8.8Table 1. Main characteristics of the fresh manganese oxidesThe main characteristics of the fresh catalysts are summarized in Table 1. It was confirmed from the XRD patterns that the four different commercial manganese oxides existed as pure phases: the -MnO2 pyrolusite phase, the -Mn2O3 bixbyite phase, the Mn3O4 hausmannite phase and the MnO manganosite phase, respectively. Moreover, the specific surface area of the unsupported manganese oxides was evidenced to be extremely low (a few m2 g-1). In addition, the pH at the point of zero charge (pHZPC) was measured4 to vary from slightly acidic in the case of MnO2 to slightly basic in the case of MnO. Finally, no manganese leaching was ever detected in the aqueous phase at the end of the reaction, confirming the manganese oxides stability under the applied reaction conditions. Figure 1. XRD patterns of the unsupported manganese oxide catalysts before and after reactionAll the unsupported MnxOy catalysts were shown to be moderately active in the CWAO of ammonia at 230°C under 50 bar total pressure. On the overall, the reactivity order varied as follows: MnO2 > Mn3O4 > Mn2O3 > MnO (not shown), indicating that the initial oxidation state of manganese had an impact on the catalytic performances. More importantly, drastic changes were observed in some cases in the crystalline structure, the redox state of manganese and the reducibility of the oxide upon completion of the reaction. These later observations were confirmed by combining XRD (Figure 1), H2-TPR and XPS data. All these results will be thoroughly analyzed, quantified and discussed during the presentation. It was hypothesized that manganese would balance between MnIV and MnIII upon activation and reaction of ammonia. Moreover, it is expected for the ceria-supported manganese dioxide catalyst that cerium would simultaneously balance between CeIII and CeIV upon activation of oxygen and regeneration of the manganese active site.3. ConclusionsUnsupported manganese oxides (MnO2, Mn2O3, Mn3O4 and MnO) were shown to be active in the catalytic wet air oxidation of ammonia, even though they all developed a very limited specific surface area. The initial oxidation state of manganese was evidenced to have a clear impact on the catalytic performances of those oxides. Moreover, major changes were observed in the crystalline structure of the bulk oxide, the redox state of manganese and the reducibility of the oxide at the end of the reaction. MnIV/MnIII was proposed to be the key component of the active phase for the oxidation of ammonia.References 1. L. Oliviero, J. Barbier Jr., D. Duprez, Appl. Catal. B: Environ. 2003, 40(3), 163–184.2. C. Lousteau, M. Besson, C. Descorme, Catal. Today 2015, 241, 80-85.3. F. Schmit, L. Bois, F. Chassagneux, C. Descorme, Catal. Today 2015, 258, 570-575.4. T. Preočanin, N. Kallay, Croatica Сhemica Аcta 1998, 71(4) 1117-1125