Electrodeposition of Rh/Mg/Al hydroxides with different Mg-contents on metallic foams as catalyst precursors

This work investigated the electrodeposition of Rh/Mg/Al mixed hydroxides with different Mg-contents on open-cell foams through the electro-base generation method using nitrate electrolytes. The influence of Mg2+ on both the electrodeposition process and properties of the coating (after electrodeposition, calcination, and catalytic partial oxidation (CPO) of CH4 tests) was studied. The nature of cations being precipitated (Mg2+ or Al3+) influenced significantly the production of OH– through reduction of nitrate and water, which in turn determined the quality of the coating in terms of composition and adhesion. In reference samples, an absence of either Mg2+ (Rh5Al95) or Al3+ (Rh5Mg95) declined the coating quality, namely, more cracks developed due to the prominent reduction of H2O (in case of Rh5Al95) or reduction of Rh3+ to Rh0 occurred (Rh5Mg95 ). On the other hand, the presence of both Mg2+ and Al3+ in Rh/Mg/Al hydrotalcite-type (HT) samples improved significantly the coating loading and its adhesion, even after thermal treatment at 900 °C, as well as the dispersion of Rh in the reduced catalyst. Moreover, increasing the amount of Mg2+ to a given Mg/Al ratio at which a pure Rh/Mg/Al HT precursor was formed (e.g. Rh5Mg70Al25) resulted in significant increment of Rh dispersion compared to those with lower Mg content (i.e. Rh5Mg50Al45). Such modifications on the properties of the coating (loading, adhesion, and Rh dispersion) subsequently influenced the performance in the CPO of CH4. Finally, temperature profiles along the catalytic bed were reported in correlation with CH4 conversion and syngas selectivity.