Open-cell foams as support for catalysts: analysis of the mass transfer limitations

ALEJO AGUIRRE; HANS J.A.M. KUIPERS; FRANK PETERS; VISHAK ARALAGUPPE DEEPAK CHANDRA; FERNANDA NEIRA D'ANGELO

NH Conference Centre Leeuwenhorst in Noordwijkerhout

Conferencia; NCCC, The Netherlands' Catalysis and Chemistry Conference.; 2019

NCCC

Solid foams are a novel type of structure that have gained significant attention in the field of catalysis over the last years. For example, they have proved beneficial during CH4 combustion, NH3 oxidation, VOC abatement and Fischer-Tropsch synthesis. A key advantage of solid foams is their high external surface area per unit volume, which makes them excellent substrates to deposit a porous catalytic wash-coat layer with high metal dispersion. The thickness of the wash-coated layer can be easily tuned to prevent internal mass transfer limitations, leading to high catalytic activity with excellent transport properties and resulting in optimal catalyst utilization. However, the analysis of the external mass transfer limitations using foam catalyst has been briefly addressed in the literature [1]. In this work, the analysis of the external mass transfer phenomena around catalytic foams is carried out through the combination of computational fluid dynamics (CFD) simulations and experiments, using the CO oxidation on Pt(1%)/γ-Al2O3/foam as a model reaction.First, the influence of the local hydrodynamic effects on the diffusion-reaction phenomena occurring across the fluid-solid interface of the wash-coated open-cell solid foams were investigated by Direct Numerical Simulations [2] assuming a first order reaction at the foam surface. Through a parametric study based on dimensionless numbers, correlations for the Sherwood number as function of Reynolds, Schmidt and Damköhler numbers for practical applications were proposed. To validate this experimentally, aluminum foams (Duocel®) of 10, 20 and 40 pores per linear inches (ppi) and 6-8% of relative density were coated with layers of Pt(1%)/γ-Al2O3 of thickness from 15 to 50 µm, following a wash-coating methodology earlier developed in our group [3]. The obtained foams were characterized by SBET, SEM and TEM, and eventually tested at different reaction conditions for the CO oxidation: 1 bar, 135-185ºC and CO/O2 = 1, 1/2, 1/4.The results of this work show that the synthesized wash-coated catalyst foams have a high surface area, are mechanically stable and active for CO oxidation. The experimental values of the apparent activation energy (Figure 1) for the CO oxidation suggest the presence of external mass transfer limitations for low flow rates and low external surface area, in agreement with the predictions of the numerical simulations. Finally, this work provides a crucial insight on the external mass transport around foam catalysts, and the importance of mass transfer limitations in the reactor performance is highlighted.