Using a new structured multilayer reactor (MLR) that houses Ni based thin layer catalysts, catalytic decomposition of natural gas (CDNG) for making "COx-free" hydrogen in a T-R range of 773-873 K was investigated. The influence of Ni loading (5-50 wt%), Ni particle size, T-R, and GHSV (h(-1)) on the reaction pattern was evaluated. A volcano shape relationship between H-2 space time yield (STYH2, st p dm(H2)(3)/kgN(i)/h) and Ni loading with a maximum range of 20-25 wt% was found. H-2 productivity and catalyst lifetime are controlled by the nature of the coke formed on Ni active species. As such, T-R and GHSV significantly affect both the kinetics and the mechanism of coking. At T-R < 873 K and low GHSV (<2000 h(-1)), the prevailing formation of filamentous carbon (whisker-like) prolongs the catalyst lifetime, while at T-R = 873 K and high GHSV (>2000 h(-1)) encapsulating carbon is formed, causing a sudden decline in catalytic activity. A preliminary attempt to find optimal operating conditions ensuring the highest values of H-2 productivity is presented. The features of the MLR that allowed CDNG reaction under a quasi-isothermal regime without any pressure drop, irrespective of operating conditions and the extent of carbon formation, are discussed. (C) 2009 Elsevier B.V. All rights reserved.