Insights into coated NiCrAl open-cell foams for the catalytic partial oxidation of CH4

Abstract

High resistance alloys are the basis of structured catalysts that outperform pelletized materials in mass and/or heat transfer limited processes and allow movement from large- to small-scale and more efficient processes for energy and environmental applications. Herein, open-cell NiCrAl foams coated with catalysts are investigated as an alternative to the FeCrAl, tracking the change of both the foam and coating materials and their interaction during life-time (as-prepared, calcined and after catalytic tests). Bare open-cell foams are activated by electrodeposition of Rh/Mg/Al hydrotalcite type compounds, followed by calcination at 900 °C, and tested under harsh conditions in the catalytic partial oxidation of methane (CPO) for H₂ production; the axial temperature profiles along the centerline of the catalytic bed were also evaluated. The combination of spectroscopic, microscopic and imaging diffraction techniques reveals insights into the structure of the materials from the macro- to nano-scale (e.g. nano XRD/XRF synchrotron tomography). The foam surface is rapidly and evenly coated by hydrotalcite-type compounds through electrodeposition; a pretreatment of the bare foam is, however, mandatory. Activated foams, after calcination, develop a scale made of Cr₂O₃, NiCr₂O₄, and α-Al₂O₃, which is stable under reaction conditions and does not alter the coating, either its structure (MgO and MgAl₂O₄) or composition. Although NiCrAl foams are not thermally treated to develop the protective Al₂O₃ scale, catalytic coatings are stable and performances are unaltered. Under reaction conditions, a decrease of the hot spots in the CPO of methane is measured in comparison to FeCrAl foams.