A reactor for the use in photocatalytic gas conversion comprising a solid rod having spaced thread profile that swirls helically along the rod, in which the rod is substantially wholly coated with photocatalyst; and a cylindrical sleeve coaxially enclosing the rod so that reacting gas flows helically in the space between the thread along the rod while contacting the photocatalyst coating thereon thence producing converted product upon illumination by a light source.