Chemical denitration aims to reduce nuclear fuel reprocessing wastes volume and concentration by adding formic acid. The presence of Pt/SiO2 in these heavy nitric acid concentrated wastes suppresses the induction period of the chemical denitration. This is due to the fast initial catalytic generation of HNO2 from HNO3 on Pt/SiO2, which become further the active species in the homogeneous phase. There is a leaching of Pt from the catalyst to the solution. This leaching is much Limited for large Pt aggregates (>10 nm), but becomes very significant for nano-sized particles (1-2 nm). It is proposed that HNO2 generation passivated the metal phase, which is in turn reactivated by formic acid.