Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13

Abstract

Practical catalysts often operate under dynamic conditions of temperature variations and sudden changes of feed composition that call for understanding of operation and catalyst structure under analogous experimental conditions. For instance, the copper exchanged small pore SSZ-13 catalyst used currently in the selective catalytic reduction of harmful nitrogen oxides from the exhaust gas of diesel fuelled vehicles operates under recurrent ammonia dosage. Here we report the design of unsteady state experiments that mimic such dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation we were able to capture an ammonia inhibition effect on the rate-limiting Cu re-oxidation at low temperature. The practical relevance of this observation was demonstrated by the optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and the increase of NO conversion at low temperature.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…According to a very recent operando XAS study by Marberger et al [26], Cu nitrate species appear only when NH 3 is not readily available; that is, away from SCR conditions. However, at elevated (>300 • C) reaction temperatures, there are currently no experimental findings that disprove this proposal.…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”

Selective Catalytic Reduction of NO with NH3 on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

“…The colour assignments of species denote the spectral components observed in time‐resolved XAS measurements at each step in the reaction scheme for the standard SCR mode in the work of Marberger et al. . Green indicates the additional Cu I −Z species observed through MCR analysis in this work.…”

“…Owing to the development of time‐resolved operando methods such as X‐ray absorption spectroscopy (XAS), the study of catalysts under the dynamic conditions at which they operate is now possible . The selective catalytic reduction (SCR) of NO x using small pore size and low metal‐loaded Cu−SSZ‐13 is such a reaction that takes place under dynamic conditions and has gained significant attention recently due to the high low‐temperature conversion rate of nitrogen oxides in the presence of NH 3 . The SCR reaction proceeds according to Equation . …”

“…Whilst initial studies suggested Cu I and Cu II cations were occupying sites in the 6‐member ring, numerous studies have since identified that under SCR conditions the Cu I and Cu II ions can be solvated into the pores of the SSZ‐13 structure . Recent work following the auto‐reduction of Cu loaded chabazite systems by Martini et al .…”