NOVELTY - Method (M1) to form catalyst having nanoparticles, involves: (A) providing solution of precursor in solvent at ambient conditions; (B) providing suspension of support material in solvent at ambient conditions; (C) optionally sonicating support; (D) mixing solution of precursor and suspension of support material; (E) providing reactive solvent in supercritical or subcritical state; (F) mixing mixture of precursor and support in reactive solvent; (G) injecting reaction solution in reactor tube for reaction to form catalyst nanoparticles on support; and (H) withdrawing catalytic structure.    USE - For preparing catalytic structure having catalyst nanoparticles, useful for fuel cell (preferably direct alcohol fuel cell) (all claimed), and for industrial conversion of compounds.    ADVANTAGE - The method provides improved method for providing catalytic structures with catalytic nanoparticles. By the method catalyst nanoparticles are synthesized continuously in supercritical or subcritical solvent, which gives excellent control of morphology, crystallinity, size and uniformity of particles which are all important characteristics for catalytic properties of nanoparticles. The presence of support material in synthesis allows that catalyst nanoparticles to be formed directly on support material in continuous process without agglomeration or precipitation of formed particles. This further provides that catalyst nanoparticles can be distributed evenly on support material, and that spacing of nanoparticles can be controlled. The size of the catalyst nanoparticles is (1-50 nm, preferably 1-10 nm, especially 1-5 nm); and catalyst nanoparticles are monodispersed and have diameters with standard deviation of up to 50%. The catalytic structure can furthermore be prepared directly in one-step reaction in flow synthesis reactor so that final catalytic structure can be withdrawn from reactor requiring only minimum of additional processing steps e.g. to purify catalytic structure. The method enables use of environmentally friendly solvents in continuous flow production of catalyst nanoparticles, and offers laboratory like control while providing high throughput for larger productions and scalability for industrial application. The advantages of avoiding agglomeration also allow more efficient process with increased yield from expensive starting materials.    DETAILED DESCRIPTION - Method (M1) for preparing a catalytic structure having catalyst nanoparticles, involves:    (A) providing a solution of a precursor compound in a solvent at ambient conditions;    (B) providing a suspension of a support material having a specific surface area of greater than or equal to 1 m2/g in a solvent at ambient conditions;    (C) optionally sonicating the suspension of the support material;    (D) mixing the solution of the precursor compound and the suspension of the support material;    (E) providing a reactive solvent in a supercritical or subcritical state;    (F) mixing the mixture of the solution of the precursor compound and the suspension of the support material in the supercritical or subcritical reactive solvent to form a reaction solution;    (G) injecting the reaction solution into a reactor tube via an inlet allowing a reaction of the precursor compound in the supercritical or subcritical reactive solvent in the reactor tube to form the catalyst nanoparticles on the support material to provide the catalytic structure; and    (H) withdrawing the catalytic structure from the reactor tube via an outlet.    INDEPENDENT CLAIMS are included for the following:    (1) a catalytic structure obtainable in the method (M1); and    (2) a fuel cell comprising the catalytic structure.