In Situ Spectroscopic Studies on the Redox Cycle of NH₃−SCR over Cu−CHA Zeolites

Abstract

The selective catalytic reduction of NO with ammonia (NH3−SCR) catalyzed by Cu−CHA zeolites is thoroughly investigated using in situ spectroscopic experiments combined with on‐line mass spectroscopy (MS) under steady‐state NH3−SCR conditions and transient conditions for Cu(II)/Cu(I) redox cycles. Quantitative analysis of the in situ XANES spectra of Cu−CHA under steady‐state conditions of NH3−SCR show that NH3‐coordinated Cu(II) species is the dominant Cu species at low temperatures (100–150 °C). At higher temperatures, Cu(II) species and [Cu(NH3)2]+ complex coexist, possibly because the rate of the Cu(II)→Cu(I) reduction step is comparable to that of the Cu(I)→Cu(II) oxidation step. In situ XANES, IR/MS, and UV‐vis/MS experiments on the reduction half cycle demonstrate that the reduction of Cu(II) species occurs via the reaction of NH3‐liganded Cu(II) with NO to yield N2 and H2O. For the oxidation half cycle, in situ XANES experiments of Cu(I) oxidation in 10 % O2 at 200 °C indicate that an increased density in CHA zeolite exhibits a higher oxidation rate. In situ UV‐vis experiments of Cu(I) reoxidation using different mixtures of oxidant feed gas demonstrate the key role of O2 in the oxidation cycle. It is suggested that the reoxidation of Cu(I) to Cu(II) species occurs with only O2 as the oxidant, and a high Cu density in CHA zeolite promotes SCR activity by enhancing the oxidative activation of Cu(I) to Cu(II) during the catalytic cycle.

On the Redox Mechanism of Low‐Temperature NH3‐SCR over Cu‐CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

“…Cu II reduction by NO only is rather limited on Cu‐CHA (Figure 3 a, ≈3 %), consistent with the above DFT derivation and with literature spectroscopic observations [6, 12] . Such conclusions were further consolidated by a TRM test (Figure 3 b, ≈9 % of Cu II reduction by NO; calculation details on how to derive this value are provided in SI‐S8), in which Cu‐CHA was sequentially exposed to NO first and then to NO+NH 3 , with the latter step resulting in titration of residual Cu II after NO exposure [9, 10, 13] . The presence of BaO/Al 2 O 3 , as discussed above, would promote Cu II reduction via a scavenging effect, which was experimentally confirmed by an identical TRM test over the Cu‐CHA+BaO/Al 2 O 3 mixture (Figure 3 c, ≈41 % of Cu II reduction by NO; negligible effect of Ba 2+ on NH 3 adsorption; see details in SI‐S8).…”

“…By deconvolution of the TPD profile (Figure 4 b), we estimated 0.278 mmol g cat −1 for the Lewis‐NH 3 and 0.780 mmol g cat −1 for the Brønsted‐NH 4 + . These values, divided by the Cu‐loading in the tested sample, give NH 3 /Cu ratios of 1.00 and 2.80, respectively; adding weakly‐desorbed NH 3 (0.613 mmol g cat −1 ; negligible rig contribution by a blank test, Figure S6) to the Lewis‐adsorbates, the former of which actually represents additional NH 3 molecules coordinated with Cu‐ions when gaseous NH 3 exists and solvates Cu, [6, 9] leads to an NH 3 /Cu ratio of ≈3.20, that is in line with a Cu II (OH)(NH 3 ) 3 configuration widely documented in literature [4b, 6, 9, 13] . Further, as revealed by using transient CO oxidation as a probe reaction in a recent work from some of us, [16] enhanced mobility of Cu II OH conferred by NH 3 ‐ligands significantly promotes CO oxidation by favoring the formation of dual‐site Cu II ‐NH 3 complexes, experimentally confirming the ability of Cu II (OH)(NH 3 ) 3 , a single‐charge cluster, to diffuse between CHA cages.…”

“…Such arguments also apply to the discussion of the LT‐SCR redox cycle. As aforementioned, the LT‐OHC is now accepted to occur over dimeric Cu‐oxo species [3a, 4, 13] ; if dual‐site structures can directly launch the RHC, there seems no need to send back one of the Cu II ‐NH 3 complexes to reestablish their original isolated state, especially since the latter one is thermodynamically less favorable. Further, given that the Two‐P consists of two unperturbed Cu II OH, thereby NO oxidative activation may also occur on it.…”

On the Redox Mechanism of Low‐Temperature NH3‐SCR over Cu‐CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

“…first and then to NO + NH 3 ,w ith the latter step resulting in titration of residual Cu II after NO exposure. [9,10,13] The presence of BaO/Al 2 O 3 ,a sd iscussed above,w ould promote Cu II reduction via ascavenging effect, which was experimentally confirmed by an identical TRM test over the Cu-CHA + BaO/Al 2 O 3 mixture ( Figure 3c, % 41 %o fC u II reduction by NO;n egligible effect of Ba 2+ on NH 3 adsorption;s ee details in SI-S8). Since NH 3 is also able to consume nitrite species [3a,4c, 5, 7, 8] and is in much more intimate contact than the mechanically-mixed BaO/Al 2 O 3 powders,such ascavenging effect is expected to be even more effective when NH 3 is present, as demonstrated here by the complete reduction of Cu II when exposed to NO + NH 3 ( Figure 3a).…”

“…where k app is the apparent rate constant for the LT-RHC scheme in Figure 7a nd [Cu II (OH)(NH 3 ) 3 ]i st he amount of Cu II (OH)(NH 3 ) 3 at ac ertain time.O bviously,E quation (13) has the same form as Equation (4) on regarding Cu II OH as the majority Cu II -species,suggesting that this Tw o-P based mechanism aligns with the observed 2 nd -order kinetics in LT-RHC.B esides,t his mechanism invokes am obile nitrite-precursor intermediate,w hich is able to react with different types of NH 3 (e.g.L ewis-NH 3 here and Brønsted-NH 4 + in literature [7,10] )a tl ow temperatures;t his is essentially compatible with recent findings that Brønsted-NH 4…”