Noble metal nanoparticles, a process for preparing these and their use

Abstract

Nanoparticles which contain noble metals alone or noble metals in combination with base metals. The nanoparticles are embedded in an aqueous solution of a temporary stabilizer based on a polysaccharide.

1 . Nanoparticles containing noble metals only or noble metals in combination with base metals, said nanoparticles being embedded in a aqueous solution of a temporary stabilizer which is a polysaccharide.  
     
     
         2 . Nanoparticles according to  claim 1 , wherein temporary stabilizer can be removed by pyrolysis at temperatures up to 250° C.  
     
     
         3 . Nanoparticles according to  claim 1 , wherein temporary stabilizer can be removed by breaking glyosidic bonds of said polysaccharide in the presence of acids or alkalis.  
     
     
         4 .Nanoparticles according to  claim 1 , wherein the aqueous solution has a pH-value in the range of from 5 to 8.  
     
     
         5 . Nanoparticles according to  claim 1 , wherein the aqueous solution has a total chlorine concentration of less than 100 ppm.  
     
     
         6 . Nanoparticles according to  claim 1  containing at least one noble metal selected from the group consisting of platinum, palladium, rhodium, iridium, ruthenium, osmium, gold and silver and optionally at least one base metal selected from the group consisting of iron, cobalt, nickel, copper, titanium, vanadium, chromium, manganese, molybdenum, tungsten and rhenium.  
     
     
         7 . Nanoparticles according to  claim 6 , haveing a particle size from 0.1 nm to 100 nm.  
     
     
         8 . Nanoparticles according to  claim 7 , wherein the polysaccharide is gum arabic, xanthan gum, tragacanth or mixtures thereof.  
     
     
         9 . Nanoparticles according to  claim 8 , wherein the concentration of particles in the aqueous solution is from 0.01 to 500 grams per liter.  
     
     
         10 . Nanoparticles according to  claim 9 , wherein the ratio by weight of nanoparticles to stabilizer is from 10:1 to 1:10.  
     
     
         11 . A process for preparing nanoparticles according to  claim 1 , comprising reducing precursor compounds for the desired noble metal and optionally the base metal with a total chlorine concentration of less than 500 ppm, using a reducing agent, in an aqueous solution in the presence of the polysaccharide.  
     
     
         12 . A process for preparing nanoparticles according to  claim 11 , wherein said reducing agent is selected from the group consisting of hydrogen, hydrazine, formic acid, formaldehyde, lower aliphatic alcohols and compounds which decompose to give gaseous constituents during the reduction reaction.  
     
     
         13 . A catalyzed inomer membrane made with the nanoparticles according to  claim 1 .  
     
     
         14 . A gas diffusion electrode made with the nanoparticles according to  claim 1 .  
     
     
         15 . A membrane electrode assembly made with the nanoparticles according to claim  
     
     
         16 . A supported electrocatalyst on a conductive carbon support having been made using the nanoparticles according to  claim 1 , and having a noble metal content of 10 to 80% by weight with respect to the total weight of the suggested electrocatalyst.  
     
     
         17 . A catalyst ink for catalysing ionomer membranes, gas distribution structures and membrane electrode assemblies for fuel cells comprising the nanoparticles according to  claim 1  in a carrier therefor.  
     
     
         18 . A process for the catalytic coating of an ionomer membrane comprising applying the noble metal nanoparticles according to  claim 1 , to said membrane, after application of the noble metal nanoparticles, removing the polysaccharide by breaking the glycosidic bonds in the presence of an acid or alkali.  
     
     
         19 . A process for the catalytic coating of a gas diffusion electrode comprising applying the noble metal nanoparticles according to  claim 1  to said electrode, after application of the noble metal nanoparticles, removing the polysaccharide by pyrolysis at temperatures up to 250° C.  
     
     
         20 . A process for the preparation of a carbon-supported electrocatalyst comprising depositing the noble metal nanoparticles according to  claim 1 , onto said carbon, after deposition of the noble metal nanoparticles, removing polysaccharide by breaking the glycosidic bonds in the presence of an acid or alkali.  
     
     
         21 . A process for the preparation of a carbon-supported electrocatalyst comprising depositing the noble metal nanoparticles according to  claim 1 , onto said carbon after deposition of the noble metal nanoparticles, removing the polysaccharide by pyrolysis at temperatures up to 250° C.  
     
     
         22 . A membrane electrode assembly for a fuel cell, containing nanoparticles obtained by using nanoparticles in accordance with  claim 1  and having a total platinum loading of less than 0.5 mg Pt/cm 2 .  
     
     
         23 . A fuel cell stack, comprising a plurality of the membrane electrode assembly according to claim  22 .