Thermal stability and repartition of potassium promoter between the support and active phase in the K-Co2.6Zn0.4O4|α-Al2O3 catalyst for N2O decomposition: Crucial role of activation temperature on catalytic performance

Abstract

The effect of calcination temperature of potassium promoted cobalt spinel catalyst supported on α-alumina extrudates was investigated for understanding its diverge role in N2O decomposition. The catalyst were thoroughly characterized by XRD, μRS, IR, XPS, X-ray μ-tomography, SEM, STEM-EDX techniques. Potassium stability and surface location were evaluated by species resolved thermal alkali desorption (SR-TAD). It was found that the potassium promoter is unevenly distributed between the spinel active phase and the support, due to different energetic stability of 2.4-2.6eV and 1.7eV, respectively. Strong beneficial influence of the K-dopant on the catalyst activity, observed for the catalysts calcined at 500°C, substantially attenuated upon increasing the calcination temperature above 600°C. It was accounted for by the migration of potassium from the support to the spinel nanocrystals (below 500°C) and subsequent formation of potassium cobaltate overlayer (above 600°C). Thus while the support is acting as a potassium reservoir, the spinel active phase acts as a potassium sink. The presence of the cobaltate shell around the spinel nanocrystallites was revealed by FTIR and SR-TAD techniques, and corroborated by STEM-EDX elemental mapping. The results point out that in the case of the supported cobalt spinel catalysts promoted by alkali careful selection of the proper activation conditions is crucial for their optimal performance.