The present invention relates to a process of preparing a catalyst which supports highly dispersed metal particles utilized in the field of catalysts.
Catalysts which support metals are widely utilized in various chemical reactions. Especially in the case of utilizing such an expensive catalyst as a precious metal, in order to effectively utilize the catalytically active metal, the particle diameter of the metal particles are required to be reduced as much as possible and to be uniformly supported on a support. Many attempts have been made especially for supporting metal particles having the minimum diameters on a support.
As disclosed in U.S. Pat. No. 3,992,512 and U.S. Pat. No. 4,136,059, as a method of supporting metal particles having a diameter of below 20 .ANG. on a support, a method has been proposed which comprises reacting such a sulfur-containing compound as sodium sulfite and sodium dithionate and chloroplatinic acid, supporting resulting colloid-Like platinic acid complex compound containing the sulfur, and oxidatively treating it with such an oxidant as hydrogen peroxide to obtain the support supported with fine platinum particles. The method is effective as far as the supporting of the platinum particles is concerned.
In the platinum-supported catalyst obtained through the conventional process, liberated colloid-like sulfur and a sulfate formed by the reaction of the above compound are contained. The liberated sulfur which is a well-known catalytic poison against platinum strongly bonds the platinum to lower the catalytic performance. The sulfate is non-volatile so that its removal should be conducted by water-washing. The sulfate remained produces several inconveniences on the employment of the catalyst such as the existence as impurities and an undesired reaction with the support. Although the liberated sulfur remained can be removed by thermal treatment in a hydrogen flow or in an oxidative flow, the heating may create the increase of the particle diameter by means of the agglomeration of the metal particles so that the desired particles cannot be obtained.