Active Sites, Deactivation and Stabilization of Fe-ZSM-5 for the Selective Catalytic Reduction (SCR) of NO with NH3

Abstract

Fe-ZSM-5 has been systematically investigated as catalyst for the selective catalytic reduction (SCR) of NO with NH(3), concentrating on the active sites, the deactivation mechanism during hydrothermal aging and the chemical possibilities to stabilize this type of SCR catalyst. Regarding the active SCR sites, it could be shown that monomeric species start to become active at the lowest temperatures (E(a,app) ≈ 36.3 ± 0.2 kJ/mol), followed by dimeric species at intermediate temperatures (E(a,app) ≈ 77 ± 16 kJ/mol) and oligomeric species at high temperatures. Experiments with Fe-ZSM-5 samples, in which the Brønsted acidity was specifically removed, proved that Brønsted acidity is not required for high SCR activity and that NH(3) can also be adsorbed on other acidic sites on the zeolite surface. The hydrothermal deactivation of Fe-ZSM-5 could be explained by the migration of active iron ions from the exchange sites. Parallel to the iron migration dealumination of the zeolite framework occurs, which has to be regarded as an independent process. The migration of iron can be reduced by the targeted reaction of the aluminum hydroxide groups in the lattice with trimethylaluminium followed by calcination. With respect to the application of iron zeolites in the SCR process in diesel vehicles, the most efficient stabilization method would be to switch from the ZSM-5 to the BEA structure type. The addition of NO(2) to the feed gas is another effective measure to increase the activity of even strongly deactivated iron zeolites tremendously.

Environmental Reactions of Air-Quality Protection on Eco-Friendly Iron-Based Catalysts

“…This evidence is congruent with most of previous literature on iron-based SCR catalysts, and in particular on Fe-zeolite catalysts. On Fe-zeolites high de-NO x activity at low temperature has been associated with mononuclear iron species involved as redox sites in the catalytic cycle [44], while dimeric, oligomeric and partially uncoordinated sites at the surface of FeO x particles have been hypothesized to be active species responsible for SCR activity of Fe-zeolites at high temperatures [45,46]. Therefore, the co-presence of isolated and aggregated iron phases in our samples could justify the observed catalytic behavior.…”

Progress of selective catalytic reduction denitrification catalysts at wide temperature in carbon neutralization

“…On the basis of the medium temperature catalyst, the temperature window is widened, in order to meet the situation that the flue gas temperature cannot reach the design temperature due to the reduced load operation in the actual operation of the power plant. The active components of the medium and low temperature SCR catalysts are usually ( Kröcher and Brandenberger, 2012 ) metal manganese oxides, copper-based catalysts, iron-based catalysts, chromium oxide and nickel oxide and other metal oxides, which are composed of one or more single-metal or bi-metal catalysts. The supports selected in the research process of medium and low temperature SCR denitration catalysts include ( Li, 2009 ; Qiu et al, 2015 ; Zhang M. et al, 2019 ) molecular sieve, TiO 2 , Al 2 O 3 , carbon materials, etc.…”

Mössbauer spectroscopy as a tool to predict the catalytic activity of the Fe3+ sites in an exchanged Fe/hydroxyapatite system

“…For example, good activity in the NOx decomposition at low temperature using Fe/zeolites has been related with isolated iron species which would be involved as redox sites in the catalytic cycle [5]. Instead, isolated, dimeric and oligomeric iron species would be responsible for selective catalytic reduction of NO by NH3, but each of these sites has significant differences in its TOF values [6,7]. Differently, iron catalysts with high activity in selective catalytic oxidation of NH3 (NH3-SCO) at low temperature, have a great number of isolated Fe 3+ sites, which are adequate for ammonia adsorption (Lewis sites) and for generating active oxygen adsorbed species.…”