Experimental and modeling-based analysis of reaction pathways on catalysts for natural gas engines under periodic lean/rich oscillations

Methane (CH4) has the prospective of becoming one of the major energy carriers for heavy-duty mobility, but its catalytic abatement remains a challenge. CH4 abatement reaction pathways were analyzed using targeted periodic lean/rich oscillations, which resulted in higher CH4 conversion than under static conditions. At both the lean-to-rich and rich-to-lean transitions of such oscillations, local maxima of CH4 concentration were followed by local minima during each lean and rich phase. The CH4 concentration profile during one oscillation displayed a 'W' shape. Four reaction regimes were identified corresponding to the four sections of the characteristic concentration shape. Entering the rich phase, Steam reforming (SR) started, which was then hampered by the formation of carbonaceous species and CO interaction with the catalytic surface. By entering the lean phase, SR is again enhanced as more active sites became available due to oxygen related reactions. However, increasing chemisorbed oxygen slowed down the oxidation of CH4. The transition between the consecutive regimes was linked to changes of the active Ce3+ coverage ratio, which plays an important role in SR reaction. Similar 'W' shapes were observed under various oscillation conditions and at different measurement points along the catalyst axis. The CH4 concentration profile was found to shift downwards with decreasing oscillation frequencies until a frequency threshold, which is marked by the equivalence of oscillation rich/lean phase length and the time needed to completely deplete/replenish the Oxygen storage capacity (OSC). The conversion characteristics were reproduced by numerical simulations, which revealed the spatial distribution of reactions in the catalyst under these dynamic conditions. Optimal overall CH4 conversion was reached for an even distribution of SR along the catalyst.