NOVELTY - A fuel cell reformate cleanup substrate comprises porous honeycomb ceramic substrate (502) placed in a stream of reformed hydrogen fuel from fuel cell reformer, where substrate has porosity of 60-85% and a structure formed of bonded ceramic fibers, where substrate is formed by extrusion process involving mixing ceramic-material fiber with additives and fluid to form extrudable mixture, extruding mixture into green substrate, curing it into porous ceramic substrate, where substrate has washcoat adapted to adsorb components such as hydrogen sulfide (506) from reformed hydrogen fuel stream.    USE - As fuel cell reformate cleanup substrate for altering composition of a gas; as a fuel cell reformate cleanup filter for reforming hydrogen fuel (claimed); and in fuel cells for producing electricity from fuel.    ADVANTAGE - The substrate enables an efficient fuel cell reformate cleanup filtration using a highly porous and permeable honeycomb substrate having a washcoat that adsorbs impurities in the reformate stream upstream of fuel cell. The substrate has high porosity and an associated high permeability so that the reformate stream can be effectively cleaned in a compact unit without high back-pressure. The substrate is cost-effective to manufacture and can be manufactured with flexible physical, chemical and reaction properties. The penetration of washcoat into porous substrate provides improvement in efficiency and performance of the porous substrate as fuel cell reformate cleanup filter. The substrate has high porosity; and has open pore network that enables an associated high permeability; and has high strength according to application needs. The substrate has sufficient cost effectiveness to enable widespread use of resulting filters and catalytic converters. The extrusion system is easily scalable to mass production, and allows for flexible chemistries and constructions to support multitudes of applications. The fibrous extrudable mixture enables extrusion of substrates with very high porosities, at a scalable production, and in a cost-effective manner.    DETAILED DESCRIPTION - A fuel cell reformate cleanup substrate comprises a porous honeycomb ceramic substrate (502) placed in a stream of reformed hydrogen fuel from a fuel cell reformer, where the porous ceramic substrate has a porosity of 60-85% and has a structure formed of bonded ceramic fibers, where the substrate is produced by an extrusion process involving mixing ceramic-material fiber with additives and a fluid to form extrudable mixture, extruding the extrudable mixture into a green substrate, curing the green substrate into the porous ceramic substrate, where the substrate has a washcoat applied to at least a portion of substrate and washcoat is adapted to adsorb components from reformed hydrogen fuel stream. The washcoat disposed within the ceramic substrate has affinity for the adsorption of hydrogen sulfide (506). An INDEPENDENT CLAIM is included for altering a composition of a gas involving: providing a porous ceramic substrate, where the substrate has a porosity of 60-85% and the substrate has a structure formed of bonded ceramic fibers, where the substrate is produced by extrusion process involving: mixing ceramic-material fiber with additives and a fluid to form an extrudable mixture; extruding the extrudable mixture into a green substrate; curing the green substrate into the porous ceramic substrate, where the substrate has a washcoat applied to at least a portion of the porous ceramic substrate and the washcoat is adapted to reversibly adsorb components in the gas; and exposing the porous ceramic substrate to a gas stream.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of reformate stream filter.    Reformate stream filter (501)    Porous substrate (502)    Inlet line (503)    Output line (504)    Hydrogen sulfide (506)    Input fuel (507)    Clean fuel. (508)