Fe Promoted NO_x Storage Materials: Structural Properties and NO_x Uptake

Abstract

Fe promoted NO x storage materials were synthesized in the form of FeO x /BaO/Al 2 O 3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NO x storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results were compared with the conventional BaO/Al 2 O 3 NO x storage system. Our results suggest that the introduction of Fe in the BaO/Al 2 O 3 system leads to the formation of additional NO x storage sites which store NO x mostly in the form of bidentate nitrates. NO 2 adsorption experiments at 323 K via FTIR indicate that, particularly in the early stages of the NO x uptake, the NO x storage mechanism is significantly altered in the presence of Fe sites where a set of new surface nitrosyl and nitrite groups were detected on the Fe sites and the surface oxidation of nitrites to nitrates is significantly hindered with respect to the BaO/Al 2 O 3 system. Evidence for the existence of both Fe 3+ as well as reduced Fe 2+/(3-x)+ sites on the freshly pretreated materials was detected via EELS, FTIR, Raman, and XRD experiments. The influence of the Fe sites on the structural properties of the synthesized materials was also studied by performing ex situ annealing protocols within 323-1273 K followed by XRD and Raman experiments where the temperature dependent changes in the morphology and the composition of the surface domains were analyzed in detail. On the basis of the TPD data, it was found that the relative stability of the stored NO x species is influenced by the morphology of the Ba and Fe containing NO x-storage domains. The relative stabilities of the investigated NO x species were found to increase in the following order: N 2 O 3 /NO + < nitrates on γ-Al 2 O 3 < surface nitrates on BaO < bidentate nitrates on FeO x sites < bulk nitrates on BaO.

Fine-Tuning the Dispersion and the Mobility of BaO Domains on NOx Storage Materials via TiO2 Anchoring Sites

“…Subsequently, TPD experiments were carried out. Evolved NO x species during the desorption process were monitored by recording the QMS signals with mass to charge ratios (m/z) equal to 18,28,30,32,44, and 46 in pressure vs time mode. A linear temperature ramp within 323-1023 K was used during the TPD experiments where the ramp rate was 12 K/min.…”

“…Thermal Behavior of the Adsorbed NO x Species on Ba/Ti/Al via TPD. TPD profiles of the 8Ba/Al and 20Ba/Al samples with different Ba loadings upon NO 2 (g) adsorption at 323 K (Figures 9 and 10) can be readily analyzed on the basis of one of our former reports 28 as well as other reports in the literature. 34,36,[49][50][51] It is visible in Figures 9 and 10 that the first major desorption feature for the 8(20)Ba/Al samples appears at 389 K due to the decomposition of the N 2 O 3 /NO + species, which is followed by a desorption feature associated with the decomposition of the nitrates that reside on the exposed γ-Al 2 O 3 sites (600-650 K).…”

“…28,[34][35][36][37] The most prominent band after the first few initial NO 2 doses in Figure 3a at 1230 cm -1 and the shoulder at about 1320 cm -1 are attributed to the nitrites on the alumina surface. Concomitant with the formation of nitrites, further NO 2 doses lead to the oxidation of the surface nitrite species to nitrates.…”

Palladium doped perovskite-based NO oxidation catalysts: The role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy

“…All of the FTIR spectroscopic experiments were conducted in transmission mode using a Bruker Tensor 27 spectrometer coupled to a batch-type catalytic reactor whose details have been described elsewhere [20,21]. All of the FTIR spectra were collected at 323 K. All of the adsorption/pre-treatment steps were also performed in batch mode.…”

“…Instrumental details regarding the other utilized experimental techniques (i.e. XRD, TEM, BET, TPD and Raman spectroscopy) were described elsewhere [20,22].…”