Most operando studies in heterogeneous catalysis are carried out under steady-state reaction conditions at well-defined reactant concentrations, catalyst bed temperatures and process pressures. In a number of processes, however, the concentration of reactants varies over a wide range during practical operation. For example, in an automotive after-treatment system, concentration gradients develop regularly as the engine operation changes in response to the driving conditions, such as frequent start–stop for city driving and frequent acceleration–idling for highway driving. This is quite challenging for exhaust catalytic converters that usually work under optimized operation conditions.

Highly fuel-efficient combustion engines operate with a strongly oxygen-rich fuel mixture. Therefore, the demanding task of reducing NO x in a highly oxidizing exhaust gas stream becomes even more challenging. To reduce NO x from the exhaust gas mixture, a sacrificial reducing agent (for example, NH3) must be added to the gas stream before it reaches the catalyst bed. This technology, called the selective catalytic reduction (SCR) of NO x by NH3, has become practically viable due to the development of the highly active and hydrothermally stable small-pore zeolite-based catalyst, Cu-SSZ-13. Copper ions in different coordination environments have been identified as active catalytic sites in NO x reduction with NH3. So far, the process has been implemented under a constant supply of ammonia to the catalyst and the effect of strong NH3 coordination to both Cu(i) and Cu(ii) centres in the reaction mechanism has been clearly established, albeit only under steady-state reaction conditions, through kinetics and in situ/operando spectroscopy studies1,2,3,4,5,6. The nature of the active site changes with reaction temperature: in the low-temperature operation regime (<250 °C) the active copper centre is always coordinated to NH3, and removed from its well-defined crystallographic position in the zeolite pores. At high temperatures, however, most of the NH3 is removed from the coordination sphere of the copper ions, and the copper ions now occupy cationic positions in the zeolite structure1,2,3,4. While it is known that the low- and high-temperature regimes follow distinct mechanisms and differ in reaction intermediates6, many aspects of the mechanism have remained elusive.

Now, writing in Nature Catalysis, a team lead by Maarten Nachtegaal and Davide Ferri report that they have designed transient experiments to gain further insights into the standard NH3 SCR (4NO + 4NH3 + O2 = 4N2 + 6H2O) process with Cu-SSZ-137. With the aid of time-resolved X-ray absorption near-edge structure (XANES) spectroscopy, they looked at the evolution of crucial species in the process during ammonia cut-off experiments. The authors were able to confirm the reoxidation of Cu(i) intermediates as the rate-limiting step at low temperatures, as recently proposed1,2. Moreover, they discovered an NH3 inhibiting effect on its cut-off at low temperatures, and identified a fourfold coordinated Cu(ii) species as a reactive intermediate during relaxation of this inhibition. Furthermore, evidences are provided that copper nitrates are not intermediates in the presence of NH3. These new findings are significant for our understanding of SCR mechanisms, and are critical for designing improved SCR catalysts.

Linear combination fit (LCF) analysis of the XANES data — which has been established as a powerful tool for the quantitative speciation of Cu centres in in situ/operando studies for Cu/zeolite SCR catalysts3,4 — provided a central set of data in the present study. As shown in Fig. 1, four reference compounds, that is, Cui(NH3)2, Cuii(NH3)4, Cuii(NO x ) y and Cuii-Z, were used for the LCF analysis.

Fig. 1: Changes under transient conditions.
figure 1

a–i, Schematic of the experimental procedure used by Nachtegaal and co-workers7 (a) and time-resolved Cu speciation by linear combination of quick extended X-ray absorption fine structure (QEXAFS) spectroscopy data (b–e) and mass spectrometer signals (f–i) of NO and NH3 following NH3 cut-off from the equilibrated SCR mixture. Figure adapted from ref. 7, Macmillan Publishers Ltd.

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The transient NO x conversion increase observed on NH3 shut-off at 190 ºC and 225 ºC is fully consistent with the critical role of Cui(NH3)2 reoxidation as the rate-limiting step under low-temperature operation at low Cu loadings. One way to overcome this limitation is to increase the Cu loading to facilitate O2 activation, as has been proposed and demonstrated previously1,2,3. With increasing reaction temperature, the NH3-containing copper complexes can lose their NH3 ligands, resulting in immobilized Cu(ii) ions bound to the zeolite framework to become the active catalytic sites.

While the Cu species in Fig. 1 are properly chosen as the most representative reaction intermediates, they should not be regarded as an exhaustive list. Cu(ii) species that interact with both NH3 and the framework should not be ruled out. But at the current state of the art, they are probably difficult to distinguish from Cuii(NH3)4 via XANES, and are difficult to replicate synthetically3. The mobile Cuii(NH3)4 complex may be better described as a species in dynamic equilibrium with the framework via Cuii(NH3)4 + nOL ⇌ Cuii(OL) n (NH3)4−n + nNH3, where OL represents lattice oxygen. The involvement of spectroscopically similar Cuii(OL) n (NH3)4−n, for example, could explain why the concentration of mobile Cu(ii) species temporarily increases on NH3 cut-off at low reaction temperatures. In contrast, the concentration of Cuii(NH3)4 cannot further increase without continuous NH3 supply. This may lead to a slightly modified picture for NH3 ligated Cu(ii) species — including, but not limited to Cuii(NH3)4 — serving as low-temperature active species.

Results from the current study have important implications for the optimization of the catalyst’s performance (that is, maximum NO x conversion) under varying transient conditions. Besides catalyst design, urea dosing optimized on actual conditions via fast and accurate NH3 sensing is critical, as clearly shown.

This study demonstrates how application of time-resolved operando X-ray absorption spectroscopy can open new avenues towards the understanding of structure–function relationships in heterogeneous catalysis under transient, non-steady-state conditions.