Rh/CeO₂ thin catalytic layer deposition on alumina foams: Catalytic performance and controlling regimes in biogas reforming processes

The application of ceramic foams as structured catalyst supports is clearly expanding due to faster mass/heat transfer and higher contact efficiency than honeycomb monoliths and, mainly, packed beds. In this paper, alumina open-cell foams (OCFs) with different pore density (20, 30 and 40 ppi) were coated with Rh/CeO₂ catalyst via a two steps synthesis method involving: (i) the solution combustion synthesis (SCS) to in-situ deposit the CeO₂ carrier and (ii) the wet impregnation (WI) of the Rh active phase. The catalytic coatings were characterized in terms of morphology and adhesion properties by SEM/EDX analysis and ultrasounds test. Permeability and form coefficient were derived from pressure drop data. Catalytic performance was evaluated towards biogas Steam Reforming (SR) and Oxy-Steam Reforming (OSR) processes at atmospheric pressure by varying temperature (800–900 °C) and space velocity (35,000–140,000 NmL·g⁻¹·h⁻¹). Characteristics time analysis and dimensionless numbers were calculated to identify the controlling regime. Stability tests were performed for both SR and OSR over 200 h of time-on-stream (TOS) through consecutive start-up and shut-down cycles. As a result, homogenous, thin and high-resistance catalytic layers were in situ deposited on foam struts. All structured catalysts showed high activity, following the order 20 ppi < 30 ppi ≈ 40 ppi. External interphase (gas-solid) and external diffusion can be improved by reducing the pore diameter of the OCF structures. Anderson criterion revealed the absence of internal heat transfer resistances, as well as Damköhler and Weisz-Prater numbers excluded any internal mass transfer controlling regime, mainly due to thin coating thickness provided by the SCS method. Good stability was observed over 200 h of TOS for both SR and OSR processes.