In situ Raman and XPS characterization of MnCoCeOx catalysts active for the purification of H2 rich streams

LETICIA GOMEZ; EDUARDO MIRO; BRENDA SOLLIER; ALICIA BOIX; JOHN MÚNERA

Santa Fe

Congreso; VI San Luis Conference; 2018

INTEC

The CO Preferential Oxidation (COPrOx) reaction is used to purify H2-rich stream suitable for fuel cells1. A series of MnCoCe oxides with 10 wt. % of Co and Mn/Co molar ratio 1/8, 1/1, 1/4 and 4/1, active in COPrOx were studied by in situ Raman spectroscopy under reaction conditions. This work aimed to analyze the active centers in the catalyst at different temperatures, following the phase changes during the reaction. The use of Raman cell, in which reactant concentration, flow and temperature are controlled, allowed obtaining the spectra of the solid under real catalytic conditions. In addition, the nature of the species on the catalyst surface was analyzed by XPS.Before being carried out the in situ Raman experiment, the catalysts were evaluated in the COPrOx reaction in a fixed-bed reactor under the following reaction conditions: 1% CO, 1% O2, 40% H2 and He (W/F = 2.1 mg∙min∙cm-3). The most active catalyst was that with Mn/Co: 1/4. This catalyst reached 100 % of CO conversion between 150-180 °C, which indicates a broad temperature window of CO maximum conversion for this reaction.Raman spectra were recorded between room temperature and 500 °C under similar reaction conditions. The spectrum corresponding to MnCoCe(1/4) catalyst, showed at room temperature, the main CeO2 signal at 462 cm-1 and the bands corresponding to Co3O4 at 193, 475, 520, 615 and 680 cm-1 2. Next spectra were measured during 3 h at 150°C, where the CO conversion reached 100 %. The catalytic stability test had also showed that the maximum CO conversion was stable for 80 h. The slight shift of the CeO2 main signal was the only change detected during 3 hours at 150 °C. A broad signal between 500-600 °C was associated to oxygen vacancies of CeO2, or CoO species dispersed over the support surface3,4. At 300 °C, the bands related to CeO2 and Co3O4 were still present, although the intensity of the Co3O4 signals showed a decrease in comparison to the spectra recorded at lower temperature. When the temperature reached 350°C, the bands belonging to Co3O4 were almost undetected due to Co reduction by the H2 presence. However, the bands at 500-600 cm-1 were still distinguished and the main signal of CeO2 was detected at 446 cm-1 (=16 cm-1). The other samples with Mn/Co ratio 1/8 and 1/1 which had showed a good performance as catalysts, also exhibited the Raman signals belonging to CeO2 and Co3O4 species. Moreover, the spectra corresponding to the active species (Co3O4) remained stable during the temperature interval at which they showed the highest activity, and the less active catalyst (Mn/Co:4/1) mainly exhibited signals corresponding to manganese oxides. In these catalysts, cobalt species are involved in a redox process where Co2+ species are oxidized by oxygen from Mn oxides or CeO2 support, and then they are oxidized by oxygen from gas phase. The redox couple Co2+/Co3+- Ce3+/Ce4+ is improved by MnOx presence.The XPS results of Co 2p spectra indicated that cobalt species were present in the surface of catalysts with Mn/Co ratios of 1/8, 1/4 and 1/1 as Co2+ and Co3+, and the satellite intensity indicated that they were under Co3O4 structure, while in MnCoCe(4/1) it was found a higher Co2+ concentration, which is agree with the Raman results. On the other hand, the concentration of Co catalyst surface is lower than the bulk composition, while Mn enrichment was observed