The complex role of bismuth as promoting element to inhibit deactivation of Pd-Bi/C catalysts in the selective oxidation of glucose to gluconic acid

Bismuth is a well-established promoter of Pd/C catalysts used for the partial oxidation of alcohols, aldehydes and, in particular, sugars. It inhibits deactivation due to overoxidation but, simultaneously, gets leached in the reacting solution, a fact that would suggest disappearance of its promoting effect and subsequent deactivation. The extent of leaching changes in a complex way. The present paper deals with complementary experiments carried out to understand how the soluble fraction of Bi could be involved in the overall mechanistic scheme of glucose oxidation into gluconate. Monometallic Pd/C and bimetallic Pd-Bi/C catalysts of various compositions were used. Whatever the initial catalyst composition, the XPS surface intensity ratio measured in used catalysts lies in the range 0.4-0.6, suggesting that the dynamic state of the catalyst involves the association of 1 Bi and 2 to 3 I'd atoms. The performances of a monometallic Pd/C catalyst are significantly improved in the presence of adequate amounts of soluble Bi. Considering that Bi-glucose and highly stable Bi-gluconate complexes can form, it is suggested that one of them, most likely the second one, can get very strongly bound to the surface of Id or Pd-Bi alloys and that this association constitutes the active site. A too large amount of soluble Bi complex inhibit's the reaction, probably by preventing access of glucose to the catalytic site. Long term deactivation can be due to either the action of oxygen or progressive loss of the surface complex.