Enhanced Low-Temperature NOx Conversion by High-Frequency Hydrocarbon Pulsing on a Dual Layer LNT-SCR Catalyst

摘要

Current NOx emission reduction systems, selective catalytic reduction (SCR) and NOx storage and reduction (NSR), function well after achieving their operation temperature (typically ca. 250℃) but have unsatisfactory NOx conversion at lower exhaust temperatures encountered during cold start and low load operation. The reduced exhaust temperature of advanced diesel engines with higher fuel efficiency challenges the low-T NOx reduction. We report here a new concept of high low-T deNOx efficiency of up to 80% at a feed temperature of ca. 200℃ at relevant space velocities (70k h~(-1)). It utilizes high-frequency hydrocarbon pulsing on a dual-layer LNT-SCR monolithic catalyst under lean conditions. This system has the potential to expand the operating temperature window of the conventional deNOx devices. Inspired by Toyota's Di-Air ("Diesel NO_x aftertreatment by Adsorbed Intermediate Reductants) process [1], the proposed dual-layer LNT-SCR operation utilizes rapid short-cycle injection of hydrocarbons to increase the NOx conversion at low temperatures (< 250℃). The new operation exploits the reported Di-Air mechanism which involves generation of short-lived intermediates in the LNT base layer that can be captured and utilized by a top layer SCR-zeolite catalyst for incremental NOx conversion. Our experiments consisted of rapid short cyclic injection of propene into a lean NOx stream fed to an aged LNT-SCR dual-layer catalyst. High frequency HC pulsing on the dual-layer reduced the light-off temperature by ca. 50℃ below that of an LNT catalyst subjected to conventional NSR cycling. The enhanced low-T performance can be attributed to both chemical and thermal effects. Under high frequency operation the deposited SCR layer enables utilization of intermediate species (C_xH_yO_z and C_xH_yO_zN_t) and appreciable storage of heat both generated from oxidation of the hydrocarbon by O_2/NOx in the underlying LNT layer. Under fast cycling the low-T NOx conversion of the dual-layer catalyst exceeds that of either the LNT alone or of LNT-SCR dual-brick catalysts. Increased PGM loading of the dual-layer catalyst to 120 g/ft~3 decreases the light-off temperature to a feed temperature of around 175℃.