Catalytic denitration by HCO2H of HNO3 concentrated media in the presence of Pt/SiO2 catalysts: mechanism and influence of Pt particle size

The denitration by HCO2H of highly concentrated HNO3 media (3–5 M) was studied on Pt/SiO2 catalysts with Pt particle size varying from 1 to 10 nm. The reaction was carried out in batch mode at 343 K. The actual active intermediate is HNO2, which is formed very slowly in the absence of Pt/SiO2. The chief function of Pt/SiO2 is the initial fast generation of HNO2 in order to reach a threshold concentration of 5×10−3 mol l−1. Above this threshold, the homogeneous process between HNO3 and HCO2H, with HNO2 as an autocatalytic species, prevails. The initial generation of HNO3 on Pt/SiO2 is regulated by a catalytic cycle involving “Pt-H” and Pt0 species. This was demonstrated by redox cycle experiments in which Pt-H was reacted with HNO3 to yield Pt0, which was back reactivated to Pt-H by HCO2H. Large Pt clusters were the most active, possibly due to a faster reaction rate between Pt-H and HNO3.