Isocyanate formation and reactivity on a Ba-based LNT catalyst studied by DRIFTS

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS), coupled with the use of isotopically-labeled reactants (¹⁵N¹⁸O and ¹³CO), were employed to study the formation of isocyanate species during NOx reduction with CO, as well as isocyanate reactivity toward typical exhaust gas components. DRIFTS demonstrated that both Ba-NCO and Al-NCO were simultaneously formed during NOx reduction by CO under dry lean-rich cycling conditions. The Ba-NCO band was more intense than that of Al-NCO, and became comparatively stronger at high temperatures. During rich purging at 300 and 400°C, a near linear relationship was found between the increase in Ba-NCO band intensity and the decrease in Ba-NO₃ band intensity, suggesting that Ba-NCO is directly derived from the reaction of Ba nitrate with CO. Both temperature-programmed surface reaction (TPSR) and isothermal reaction modes (ISR) were utilized to study the reactivity of isocyanate species under lean conditions. Simultaneous DRIFTS and mass spectrometric measurements during TPSR indicated that isocyanate reaction with H₂O, O₂, NO and NO/O₂ took place almost immediately the temperature was raised above 100°C, and that all NCO species were removed below 300°C. The evolution of the NCO IR bands during ISR at 350°C demonstrated that the kinetics of NCO hydrolysis are fast, although a delay in N₂ formation indicated that N₂ is not the initial product of the reaction. In contrast, immediate N₂ evolution was observed during NCO reaction with O₂ and with NO+O₂. Overall, it can be inferred that under dry cycling conditions with CO as the sole reductant, N₂ is mainly generated via NCO reaction with NO/O₂ after the switch to lean conditions, rather than being evolved during the rich phase. However, in the presence of water, isocyanate undergoes rapid hydrolysis in the rich phase, N₂ generation proceeding via NH₃.