Role of Coating-Metallic Support Interaction in the Properties of Electrosynthesized Rh-Based Structured Catalysts

Rh-structured catalysts for the catalytic partial oxidation of CH₄ to syngas were prepared by electrosynthesis of Rh-containing hydrotalcite-type (HT) compounds on FeCrAlloy foams followed by calcination at 900 °C. During the calcination the simultaneous decomposition of the layered HT structure and formation of the protective FeCrAlloy outer shell in alumina occurred. Here, we studied the role of the coating-metallic support interaction in the properties of the catalysts after calcination, H₂ reduction, and catalytic tests, by a combination of electron (FEG-SEM/EDS) and synchrotron X-ray (XRF/XRPD and XRF/XANES) microscopic techniques. The characterization of crystalline phases in the metallic support and coating and distribution of Rh active species was carried out on several samples prepared by modifying the Rh content in the electrolytic solution (Rh/Mg/Al = 11.0/70.0/19.0, 5.0/70.0/25.0, 0/70.0/30.0 atomic ratio). A sample was also prepared with no aluminum in the electrolytic solution (Rh/Mg/Al = 13.6/86.4/0.0 atomic ratio) and calcined at 550 and 900 °C. The interaction between the elements of the metallic support and the catalytic coating increased the film adhesion during the thermal treatment and catalytic tests and modified the catalyst crystalline phases. A chemical reaction between Al coming from the foam and Mg in the coating occurred during calcination at high temperature leading to the formation of spinel phases in which rhodium is solved, together with some Rh₂O₃ and Rh⁰. The metallic support was oxidized forming the corundum scale and chromium oxides, moreover ι-Al₂O₃ was identified. For the Rh_11.0Mg_70.0Al_19.0 catalyst the inclusion of Rh in the spinel phase decreased its reducibility in the H₂ pretreatment. The reduction continued during catalytic tests by feeding diluted CH₄/O₂/He gas mixtures, evidenced by the catalyst activation. While under concentrated gas mixtures the deactivation occurred, probably by oxidation