Reduction of stored NOx on Pt/Al2O3 and Pt/BaO/Al2O3 catalysts with H2 and CO

In situ Fourier transform infrared spectroscopy, coupled with mass spectrometry and time-resolved X-ray diffraction, were used to study the efficiency of nitrate reduction with CO and H2 on Pt/Al2O3 and Pt/BaO/Al2O3 NOx storage reduction (NSR) catalysts. Surface nitrates were generated by NO2 adsorption, and their reduction efficiencies were examined on the catalysts together with the analysis of the gas-phase composition in the presence of the two different reductants. H2 was found to be a more effective reducing agent than CO. In particular, the reduction of surface nitrates proceeds very efficiently with H2 even at low temperatures (∼420 K). During reduction with CO, isocyanates form and adsorb on the oxide components of the catalyst; however, these surface isocyanates readily react with water to form CO2 and ammonia. The NH3 thus formed in turn reacts with stored NOx at higher temperatures (>473 K) to produce N2. In the absence of H2O, the NCO species are stable to high temperatures and are removed from the catalyst only when they react with NOx thermal decomposition products to form N2 and CO2. The results of this study point to a complex reaction mechanism involving the removal of surface oxygen atoms from Pt particles by either H2 or CO, the direct reduction of stored NOx with H2 (low-temperature NOx reduction), the formation and subsequent hydrolysis of NCO species, and the direct reaction of NCO with decomposing NOx (high-temperature NOx reduction).