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

Abstract

DFT calculations suggest that Cu(i) oxidation with O2 as the sole oxidant plays a major role in the oxidation half cycle of standard NH3-SCR over Cu-CHA zeolites.

Theoretical Studies on the Mechanism of deNOx Process in Cu–Zn Bimetallic System—Comparison of FAU and MFI Zeolites

“…Based also on previous detailed mechanism studies by Bendrich et al, systems with different ways of partial hydratized dimer were proposed [ 44 ]. The re-oxidation step for copper centers, presented in detail in the literature [ 45 ], is not discussed here; instead, we concentrated on reactions barriers for deNOx in the Cu–O–Zn dimer.…”

Selective catalytic reduction of NO with NH3 over Cu-exchanged CHA, GME, and AFX zeolites: a density functional theory study

“…† Note that previous studies only considered the per-site to place the two [Cu I (NH 3 ) 2 ] + species. 35,58 It is found that the INT1 structures with the persite are over 50 kJ mol −1 higher in energy compared to those with the para-site. The following INT2 and INT4 structures also have the same preference for the para-site.…”

Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

“…The reoxidation step, requiring activation of O 2 by Cu I ions, is crucial in the NH 3 -SCR reaction cycle, and is vigorously debated in the literature. [21][22][23][24][25][26][27][28][29] The current understanding is that Cu-pair formation, facilitated by mobile Cu I (NH 3 ) 2 complexes, makes the O 2 activation possible, 25,26,30 with the formation of side-on m-Z 2 ,Z 2 -peroxo diamino dicopper(II) complexes. 16,31 This mechanism is proposed for the reaction at low temperatures (r200 1C), where the NH 3 molecule is able to solvate and mobilize Cu II and Cu I ions.…”

“…8,25,[32][33][34][35][36] NH 3 -Solvated Cu pairs have also been proposed to be catalytically relevant in the reduction half-cycle. 19,20 Some aspects are still under debate in the oxidation half cycle regarding the role of NO in mediating the activation of O 2 , [22][23][24]29 and which is the structure of the formed intermediates relevant for the catalytic cycle. 18,19,27,28,[37][38][39][40][41] Cu-(NQO)-NH 2 (copper nitrosamine) species, postulated for both the reduction and oxidation half cycles, 18,39 have been recently detected using transient modulation excitation spectroscopic experiments.…”

“…40,[46][47][48] Cu-nitrates have been proposed to act as off-cycle resting states on the basis of time-resolved operando XAS measurements and DFT calculations. 22,41 On the other hand, a nitrate/nitrite route has recently been proposed to be relevant for NH 3 -SCR on Cu-CHA below 350 1C, on the basis of a combined EPR/FTIR spectroscopic and DFT study. 27 Whereas Zhang et al used modulated FTIR measurements to conclude that no significant amount of nitrate is formed during the NH 3 -SCR cycle, 49 Greenaway et al showed some nitrate formation using a similar approach.…”