The steam reforming of methane was studied over Rh and Ni-based catalysts exposed to naphthalene, which was used as a representative of polyaromatic tars found in biomass-derived biogas. In particular, two Ni-based samples in which part of the metal was encapsulated within silicalite-1 nanoboxes were tested. The reforming reaction was carried at 700, 800 and 900 degrees C using a model feed and high space velocities to limit methane full conversion and better evidence any deactivation. A strong decrease of methane conversion was observed in the presence of 1400 ppm of naphthalene, stressing the marked deleterious effect of this molecule in the present conditions. The effect of naphthalene was partly reversible, especially at higher temperatures. The silicalite-1 membrane could not prevent the deactivation of embedded nickel particles, probably because naphthalene (kinetic diameter = 0.62 nm) could diffuse throughout the MFI-type (pore diameter ca. 0.56 nm) porous layer at the high reaction temperatures used. The effect of 5 ppm of the bulkier pyrene (kinetic diameter = 0.74 nm) was investigated at 700 degrees C and also led to a rapid deactivation of the Ni@silicalite-1, likely because pyrene was cracked into naphthalene, which could then enter the silicalite-1 nanoboxes. The poisoning effect of toluene on the Ni-based catalysts was minor in comparison to that induced by the polyaromatics. A marked sintering of the embedded Ni was also observed. (C) 2016 Elsevier B.V. All rights reserved.