Identifying active iron sites for NOx pollution control in porous matrices

Related publication: Beale A. M., Greenaway A. G., Kroner A. B., Lezcano-González I., Agote-Arán M., Hayama S. & Díaz-Moreno S. Operando HERFD-XANES/XES studies reveal differences in the activity of Fe-species in MFI and CHA structures for the standard selective catalytic reduction of NO with NH3. Appl. Catal. A Gen. 570, 283–291 (2018). DOI: 10.1016/j.apcata.2018.11.026

Publication keywords: NH3-SCR; NO; Fe-containing zeolites; HERFD-XANES; XES 

Diesel is the fuel of choice for heavy goods vehicles, but diesel combustion generates nitrogen oxides (NOx) that are known to be harmful to human health; NOx has been estimated to lead to 38,000 premature deaths globally each year. Selective catalytic reduction with ammonia (NH3) is a widely-applied technology for converting NOx emissions from diesel engines into harmless nitrogen gas and water.

Iron-based microporous materials are known to be active catalysts for NOx removal, but it is difficult to know which combination of iron species and porous structure give the best performance – once this is known, it is possible to develop a more efficient catalyst or process.
 
A team of researchers from University College London, the Research Complex at Harwell, and Diamond Light Source investigated iron-containing zeolites, an important class of microporous materials known to catalyse the NOx reduction when the NOx is combined with NH3 and a small amount of heat is applied. The NOx reduction with NH3 results in the formation of harmless nitrogen and oxygen gases.
 
They used the X-ray Spectroscopy beamline (I20-Scanning) to collect high-resolution X-ray Absorption Near Edge Structure (XANES) and X-ray Emission Spectroscopy (XES) spectra, which allowed them to extract detailed chemical information from the features of the X-ray absorbing element.
 
Their results suggested that isolated octahedral iron (Fe3+) species on H-ZSM-5 are highly active under the conditions studied. In contrast, isolated tetrahedral Fe3+ sites present in Silicalite-1 exhibited lower redox properties, leading to a reduced NO conversion. Clusters and large FexOy particles on H-SSZ-13 exhibited low selective catalytic reduction activity. 
 
Figure 1: (a) UV-vis absorption spectra of Fe/zeolites (0.5 wt. %) synthesised using different zeolite topologies and Fe precursors, and (b) NO conversion of Fe/zeolites after 1 h of NH<sub>3</sub>-SCR under
5000 ppm NO, 5000 ppm NH<sub>3</sub> and 5 % O<sub>2</sub> flow at 300 °C, GHSV = 35000 h<sup>-1<sup>.
Figure 1: (a) UV-vis absorption spectra of Fe/zeolites (0.5 wt. %) synthesised using different zeolite topologies and Fe precursors, and (b) NO conversion of Fe/zeolites after 1 h of NH3-SCR under 5000 ppm NO, 5000 ppm NH3 and 5 % O2 flow at 300 °C, GHSV = 35000 h-1.
Nitrogen oxides (NOx) are one of the major sources of air pollution produced from engines during fuel combustion processes. Today, selective catalytic reduction (SCR) of NO with ammonia (NH3-SCR) is a widely applied technology for converting NOx emissions from diesel engines into harmless N2 and H2O1. Fe-containing microporous materials are among the most active catalysts for NH3-SCR. Much research has been devoted to investigating the relationship between structure and activity of these materials. This is, however, challenging, mostly due to the presence of many different types of Fe species (i.e. isolated species, oligomers, or large particles)2. Catalyst improvement for the control of NOx emissions relies on a better understanding of the effect of Fe speciation on the catalytic activity, as well as further insight into the NH3-SCR reaction mechanism. To this end, a set of Fe/zeolites (~ 0.5 wt. %) were prepared using different microporous materials as support. The supports studied were H-ZSM-5 (MFI structure, Si/Al = 15), H-SSZ-13 (CHA structure, Si/Al = 15), and Silicalite-1 (MFI structure, Si/Al = ∞). Ultraviolet–visible (UV-Vis) spectroscopy (Fig. 1a) revealed different Fe speciation for each microporous support: Fe/H-ZSM-5 comprised isolated Fe3+ species with both octahedral (Oh) and tetrahedral (Td) geometries, Fe/Silicalite-1 prepared with Fe citrate (Fe/S1-T-citr) contained highly dispersed Fe species, mainly in tetrahedral coordination, while Fe/ Silicalite-1 prepared with ferric nitrate (Fe/S1-T-citr) presented increased amounts of FexOy clusters and Fe2O3 particles. Fe/H-SSZ-13 catalyst meanwhile contained mainly large Fe2O3 particles. Comparison of the catalytic activity of these materials (Fig. 1b) revealed Fe/H-ZSM-5 to be the most active at these temperatures. The structure and behaviour of Fe species under real reaction conditions, particularly in Fe-ZSM-5, was then probed by High Energy Resolution Fluorescence Detected X-ray Absorption Near Edge Structure (HERFD-XANES), and X-ray Emission Spectroscopy (XES) recorded on beamline I20-Scanning3
 
Figure 2: Fe K-edge HERFD-XANES spectra collected for Fe/H-ZSM-5 after activation in 20 % O<sub>2</sub>/He (500 °C), under 0.1 % NO/He and 1 % NH3/He (200 °C), and under SCR conditions (5 % O<sub>2</sub>, 5000 ppm
NO, 5000 ppm NH3 in He, 300 °C); (a) General features for the catalyst spectra, and (b) comparison of the pre-edge features for the Fe reference model compounds indicated in the figure and the catalyst.
Figure 2: Fe K-edge HERFD-XANES spectra collected for Fe/H-ZSM-5 after activation in 20 % O2/He (500 °C), under 0.1 % NO/He and 1 % NH3/He (200 °C), and under SCR conditions (5 % O2, 5000 ppm NO, 5000 ppm NH3 in He, 300 °C); (a) General features for the catalyst spectra, and (b) comparison of the pre-edge features for the Fe reference model compounds indicated in the figure and the catalyst.
In order to obtain an in-depth understanding of the nature and behaviour of the different types of Fe species, HERFD-XANES and XES spectra were recorded under a number of gas compositions (chosen to mimic the conditions that the catalyst experiences in the field): 1) 20 % O2 in He flow at 500 °C after activation, 2) 0.1 % NO in He at 200 °C, 3) 1 % NH3 in He at 200 °C and, 4) SCR conditions (5000 ppm NO, 5000 ppm NH3 and 5 % O2 in He) at 300 °C; the last condition being nowadays more commonly described as under operando. Fig. 2a shows the HERFD-XANES spectra collected for the most active and responsive catalyst (Fe/H-ZSM-5), while Fig. 2b compares the pre-edge features with those for reference compounds with known Fe speciation; this comparison allows to evaluate the local geometry and the chemical state of Fe species in the samples4.
 

The HERFD-XANES data reveal a dynamic chemical state of Fe in Fe/H-ZSM-5 that changes with gas atmosphere. The pre-edge features of the catalyst before reaction suggests the presence of mainly isolated Fe3+ species with both Oh and Td species. Small changes in the XANES spectrum can be observed when flowing NO, which has previously been attributed to NO adsorption onto Fe3+ centres, leading to a partial Fe reduction (i.e. oxidative addition of NO). In the presence of reductive gases, however, bigger changes are observed; the pre-edge is seen to shift to lower energies under NH3 (centroid position goes from 7113.41 to 7112.95 eV), indicating reduction to Fe2+ - probably due to ammonia coordination to the metal, and donation of the free electron pair of the nitrogen, resulting in the formation of Fe2+-NH2 complexes. A similar shift is observed under NH3-SCR conditions, and is consistent with reoxidation of Fe2+ to Fe3+ being a slow step in the reaction process. Note that during reaction, a mass spectrometer was used to verify that the catalyst was actively reducing NOx

Figure 3: Kβ XES mainlines for Fe/zeolites acquired at room temperature after the activation
(20 % O<sub>2</sub>/He flow, 2 h at 500 °C).
Figure 3: Kβ XES mainlines for Fe/zeolites acquired at room temperature after the activation (20 % O2/He flow, 2 h at 500 °C).

In addition to the HERFD-XANES data discussed above, Kβ XES spectra were also acquired. Fig. 3 shows the Kβ’ and Kβ1,3 mainlines (3p→1s transitions of the absorbing atom) for the Fe/zeolites (recorded at room temperature after calcination). All the spectra present a well-defined Kβ’ feature, indicating they constitute high-spin complexes. For metal complexes with the same spin-state, the centroid of the Kβ1,3 feature can be correlated with the covalent (vs. ionic) character of the metal-ligand bond; it has been reported that Kβ1,3 emission shifts to higher energies with increasing ionic character. In Fig. 3, it appears that all the spectra seem identical although the Kβ1,3 peak in the Fe/H-ZSM-5 sample is slightly shifted to higher energies (i.e. Kβ1,3 maxima at 7059.15 eV while for the rest of the references is at 7058.78 eV). Such a shift may be indicative of a different, more ionic, metal-ligand bond character with respect to the other samples. This could be a consequence of the fact that Fe is providing charge compensation of the framework AlO4- charge. This is not the case of Fe/ Silicalite-1 catalysts as these materials do not contain framework Al, while for Fe/H-SSZ-13 the majority of the species present are Fe2O3 particles.

Fe/H-ZSM-5 gives higher NH3-SCR activity, this can be attributed to the isolated Oh Fe+3 species with enhanced redox behaviour. The presence of framework Al appears to promote the formation of such species at ion exchange sites, probably providing charge compensation facilitating Fe redox activity. The absence of spectral changes in Fe/Silicalite-1 catalysts points that Td Fe3+ species barely interact with the reactants, showing no reduction. The activity in these samples is attributed to the presence of Fe clusters/particles. These observations suggest that the reducibility of Fe, and its capacity to coordinate with reactant gases, is important for realising low-temperature activity, which is essential during cold-start/idling of vehicles. Copper (Cu) based catalysts are usually the choice for low temperature SCR; they show 100 % NO conversion already at 200 °C, while Fe-based catalysts require at least 300 °C. Nonetheless, since Fe is cheaper than Cu, there is now an opportunity to understand or effect the reducibility of Fe to realising low(er) temperature activity of these catalysts for NOx removal.
 
References:
  1. Beale A. M. et al. Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials. Chem. Soc. Rev. 44, 7371–7405 (2015). DOI: 10.1039/C5CS00108K
  2. Brandenberger S. et al. The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catal. Rev. - Sci. Eng. 50, 492–531 (2008). DOI: 10.1080/01614940802480122
  3. Diaz-Moreno S. et al. I20; The Versatile X-ray Absorption spectroscopy beamline at Diamond Light Source. J. Phys. Conf. Ser. 190, 12038 (2009). DOI: 10.1088/1742-6596/190/1/012038
  4. Boubnov A. et al. Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis. J. Synchrotron Radiat. 22, 410–426 (2015). DOI: 10.1107/ s1600577514025880
Funding acknowledgement:
EPSRC (EP/K007467/1); Diamond Light Source; Johnson Matthey Plc.
 
Corresponding author:
Prof Andrew M. Beale, University College London, andrew.beale@ucl.ac.uk