The Cu-CHA deNO_x Catalyst in Action: Temperature-Dependent NH₃-Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES

Abstract

The small-pore Cu-CHA zeolite is today the object of intensive research efforts to rationalize its outstanding performance in the NH3-assisted selective catalytic reduction (SCR) of harmful nitrogen oxides and to unveil the SCR mechanism. Herein we exploit operando X-ray spectroscopies to monitor the Cu-CHA catalyst in action during NH3-SCR in the 150-400 °C range, targeting Cu oxidation state, mobility, and preferential N or O ligation as a function of reaction temperature. By combining operando XANES, EXAFS, and vtc-XES, we unambiguously identify two distinct regimes for the atomic-scale behavior of Cu active-sites. Low-temperature SCR, up to ∼200 °C, is characterized by balanced populations of Cu(I)/Cu(II) sites and dominated by mobile NH3-solvated Cu-species. From 250 °C upward, in correspondence to the steep increase in catalytic activity, the largely dominant Cu-species are framework-coordinated Cu(II) sites, likely representing the active sites for high-temperature SCR.

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

“…Importantly, the NO x conversion decrease with increasing temperature between~250 and 350 • C indicates a temperature-dependent change in the nature of active Cu sites, which has now been repeatedly demonstrated via in situ/operando XAS. In particular, below this temperature range, Cu ions are fully solvated by NH 3 as mobile mono-Cu complexes in SSZ-13 cages, while above 350 • C, Cu ions anchor at (complex to) cationic sites of the zeolite framework [15].…”

“…In comparison to NO x and H 2 O, interactions between Cu ions and NH 3 are much stronger; this renders a generally agreed scenario that, under low-temperature NH 3 -SCR reaction conditions, Cu-NH 3 complexes are abundant and some of these are key intermediates for reaction [15,17,20]. Therefore, it is of considerable relevance to study species formed by adsorption of NH 3 on Cu/SSZ-13 and identify their mechanistic involvement in NH 3 3 adsorption and purging at 150 • C, the apparent discrepancy can be understood from the gas-phase NH 3 pressure differences, where the presence of gas phase NH 3 allows for higher coordination with NH 3 ligands.…”

“…Secondly, apparent reaction activation energies of~140 kJ/mol are much higher than those typically found in the low-temperature regime (60-90 kJ/mol), suggesting rather demanding reaction rate-limiting steps. Recent operando XAS studies by Lomachenko et al [15] demonstrate that, above 300 • C, mobile Cu components quickly diminish and four coordinated Cu(II) complexes convert to 3 coordinated ones that spectroscopically resemble a [Cu(II)-NO 3 − ]-Z species. This finding may be used to justify an SCR mechanism proposed by Janssens et al [13], who postulated that Cu(I) oxidation to Cu(II) is achieved by nitrate intermediate formation from NO+O 2 as the rate-limiting step:…”

“…Note that Reactions (15)(16)(17) are, again, only hypothetical and likely incomplete. Other processes, for example direct NH 3 + N 2 O 3 reactions, are also possible:…”

“…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.…”