Using Transient FTIR Spectroscopy to Probe Active Sites and Reaction Intermediates for Selective Catalytic Reduction of NO on Cu/SSZ-13 Catalysts

Abstract

A Cu/SSZ-13 catalyst containing predominately Z2Cu sites is prepared. Using FTIR spectroscopy, two nonsteady-state measurements, (1) continuous NO titration of an NH3 saturated catalyst and (2) intermittent NO on/off cycles during quasi-steady state (i.e., NO perturbation), are conducted to shed light on active sites, reaction intermediates, and possible reaction mechanisms. During continuous NO titration, a strong NH3 inhibition effect is found, where Cu active sites containing NH3 ligands are less active than Cu sites depleted of NH3 ligands. In the NO perturbation experiments, it is demonstrated that NO+NH3 interactions lead to formation of Brønsted acid sites, which further interact with NH3 to form NH4 +. Perturbation measurements, using ND3 to replace NH3 in order to distinguish ammonia and nitrate vibrations, further show that NH4NO3 and other surface nitrates are not involved in the SCR process under typical low-temperature quasi-steady-state conditions.

Mechanistic insights into the oxidation of copper(i) species during NH3-SCR over Cu-CHA zeolites: a DFT study

“…species, and thus they seem more likely to act as off-cycle resting states, which play a minor role in the standard SCR reaction. This conclusion is consistent with recent experimental studies using transient FTIR spectroscopy 43 and time-resolved X-ray absorption spectroscopy (XAS), 48 which suggest that nitrates are not involved in the SCR process under typical low-temperature conditions. Based on the experimental 21,43,48 and current theoretical results, we propose that in the redox reaction of the SCR process, the main contribution to the oxidation of Cu(I) to Cu(II) is from the exclusive O2 oxidation rather than the NOassisted oxidation process.…”

“…Gao et al 14 [Cu(NO3)] + ), and the computed activation barrier of 78 kJ/mol is higher than the situation when the nitrate species is formed on a pair of complexes. The role of such nitrate species in the SCR process is still unclear and controversial; [43][44][45] they possibly act as off-cycle resting states or even show a poisoning effect if NH4NO3 acts as a major side product. 46 Understanding the further conversion of the nitrate species to recover the original Cu species is key to explain their role in SCR reaction, but the corresponding elementary steps have not been fully explored yet.…”

Formation and Reactions of NH4NO3 during Transient and Steady-State NH3-SCR of NOx over H-AFX Zeolites: Spectroscopic and Theoretical Studies

“…The role of such Cu nitrates in the SCR reaction is not yet fully understood, especially the question as to whether these are involved in the reaction cycle. 39,40 Nevertheless, recent timeresolved IR 40 and X-ray absorption spectroscopy (XAS) 41…”

Promoting Effect of Mn on In Situ Synthesized Cu-SSZ-13 for NH3-SCR

“…The reaction between nitrate species and NH 3 was studied. First, catalysts were exposed to NO and O 2 for 1 h until saturated, and NH 3 was introduced from 0 min after purging by N 2 for 1 h. As can be seen in Figure 7a, the peaks between 1650-1500 cm −1 were assigned to Cu-NO 3 − species [49,55,56]. After NH 3 was introduced, the intensity of these peaks decreased, indicating that nitrate species were consumed by NH 3 , which further proved that the L-H mechanism existed during NH 3 -SCR for the catalysts.…”