Joanna Gryboś

Morphology and dispersion of nanostructured manganese–cobalt spinel on various carbon supports: the effect on the oxygen reduction reaction in alkaline media

In this work a model nanometric manganese–cobalt spinel was deposited on five selected carbon carriers (Vulcan XC-72, Printex85, multiwall carbon nanotubes (MWCNTs), mesoporous carbon CMK-1, and amorphous carbon C-am.) to examine their role in modifying the electrocatalytic properties of the supported active phase for the oxygen reduction reaction (ORR) in alkaline media. The synthesized materials were thoroughly characterized by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). The results confirmed the formation of a highly crystalline nanometric manganese–cobalt spinel and allowed for assessment of an amorphous phase content in the carbon supports used. The composition of the obtained catalysts was investigated by thermogravimetric analysis (TGA) and X-ray fluorescence (XRF) measurements. The electrocatalytic properties of the supported spinels were determined by the rotating disk electrode (RDE) and the rotating ring disk electrode (RRDE) methods and compared with those of a commercial platinum catalyst (20 wt% Pt/Vulcan XC-72). STEM analysis revealed that the carbon support governs both the dispersion and the morphology of the deposited spinel. In the case of mesoporous and amorphous carbon supports, the spinel nanocrystals exhibit a polyhedral shape with almost equal abundance of the (111) and (100) facets. The shape of the spinel nanocrystals deposited on the Vulcan XC-72 and Printex85 supports is dominated by the (111) termination, whereas for the MWCNT support the (100) facet is the most abundant accompanied by the highest dispersion of the nanoparticles. Electrochemical studies of the ORR revealed that the amorphous phase fraction in the carbon support promotes 2e− reduction, leading to production of HO2−. This undesired pathway is inhibited by preferential exposition of the (100) facets. The superior performance of the MWCNT support in the 4e− reduction process results from three factors: lowest content of the amorphous component, best dispersion of the spinel active phase, and its ability to promote preferential (100) faceting of the nanocrystals.

On the selection of the best polymorph of Al2O3 carriers for supported cobalt nano-spinel catalysts for N2O abatement: an interplay between preferable surface spreading and damaging active phase–support interaction

A series of cobalt spinel catalysts dispersed over various alumina supports (with varying alpha-, gamma-, delta- and theta-Al2O3 phase contents) were prepared and thoroughly characterized (XRF, XRD, μRS, SEM/TEM/EDX/SAED, TPR) and their reactivity was evaluated in low temperature N2O decomposition as a probe reaction. The observed significant differences in catalytic activities were discussed in terms of the supports nature, active phase dispersion, nanocrystal morphology and the alumina support–Co3O4 interactions. The more reactive amorphous phase as well as the gamma-, delta- and theta-Al2O3 phases gives rise to high dispersion of the spinel nanocrystals (∼10 nm) which is, however, sacrificed by the formation of mixed Co3−xAlxO4 spinels of much less activity. It was revealed that the best support for the deN2O catalyst is an α-Al2O3 phase of micrometric grain size and the optimal size of the Co3O4 nanocrystals is in the range of 20–30 nm. This catalyst was also found to be more resistant to H2O, NO and O2 contaminants in comparison to the catalysts based on γ-Al2O3.