TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Component: The reformer (30) of a fuel cell system (100) comprises a reforming reaction portion generating hydrogen gas from a fuel and a carbon monoxide reducing portion reducing a carbon monoxide concentration in hydrogen gas by an oxidizing reaction of carbon monoxide with an oxidant. The carbon monoxide reducing portion comprises the carbon monoxide oxidizing catalyst. The fuel cell system comprises a reformer comprising a reforming reaction portion generating hydrogen gas from a fuel through a catalyst reforming reaction using heat energy, a carbon monoxide reducing portion reducing a carbon monoxide concentration in hydrogen gas by an oxidizing reaction of carbon monoxide with an oxidant, an electricity generating element (11) to generate electrical energy by electrochemical reaction of hydrogen gas and oxidant, a fuel supplier (50) for supplying fuel to reforming reaction portion and an oxidant supplier (70) to supply oxidant to the carbon monoxide reducing portion and the electricity generating element.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Properties: The gold-silver alloy nanoparticles have average diameter of 0.5-2 nm, preferably 0.9-1.1 nm. The atomic ratio of gold with respect to silver in the gold-silver alloy nanoparticles is 0.5-2, preferably 0.9-1.1. Preferred Composition: The gold-silver alloy nanoparticles further comprises potassium and/or calcium. Preferred Process: The gold and silver precursors are added to obtain atomic ratio of gold with respect to silver in the gold-silver alloy nanoparticles of 0.5-2. The calcination is performed at 500-600 degrees C for 1-5 hours. Preferred Compound: The carrier is alumina, silica, titanium dioxide, zirconium oxide and/or magnesium oxide. The carrier is a mesoporous alumina. The gold precursor is tetrachloroaurate (III) hydrate and/or tetracyanoaurate (III) hydrate. The silver precursor is silver nitrate, silver acetate and/or silver percholorate hydrate. The ionic surfactant is hexadecyl trimethyl ammonium bromide. The reducing agent is sodium borohydride, potassium borohydride, rubidium borohydride or cesium borohydride. EXAMPLE - Tetrachloroaurate (III) hydrate (in g) (1.080) and silver nitrate (0.233) were added to aqueous solution (0.1 ml) of hexadecyltrimethyl ammonium bromide (3) and water (1000 ml) to obtain a yellow precursor solution. To the yellow precursor solution, sodium borohydride (0.3632) and mesoporous alumina (14.8) were added. The precursor solution was dried and calcinated at 550 degrees C for 1 hour to obtain gold-sliver alloy nano-particle carbon monoxide oxidizing catalyst supported on mesoporous alumina. The gold-sliver alloy nano-particle carbon monoxide oxidizing catalyst had average diameter of 1 nm. The carbon monoxide oxidizing catalyst on filling in a carbon monoxide reducing portion at 150 degrees C had carbon monoxide oxidation selectivity of 65.32%, carbon monoxide conversion rate of 37.45%, released carbon monoxide concentration of 3013 ppm and released hydrogen amount of 479 ml/minute.