NO adsorption/desorption pathways over Pd/SSZ-13 low temperature NOx adsorbers investigated by operando FT-IR spectroscopy and microreactor study

Modification to Enhance Surface Acidity and Reactive Species of Coal Fly Ash: A Study of Efficient NOX Removal

“…To investigate the adsorption state and species changes of NH 3 on the surface of AB-CFA-9, in situ DRIFTS was utilized to analyze its adsorption behavior and identify acid sites on the surface through the adsorbed species of NH 3 . , As shown in Figure b, the peaks observed at 1650–1690 and 1436–1380 cm –1 are attributed to the asymmetric bending vibration and symmetric bending vibration of the N–H bond in *NH 4 + species formed on Brønsted acid sites, which explains the appearance of a broad band at 503 °C on the NH 3 -TPD. ,, Similarly, peaks at 1605–1592 and 1282–1120 cm –1 are attributed to the symmetric bending vibration and asymmetric bending vibration of the N–H bond in *NH 3 adsorbed on Lewis acid sites, which corresponds to the sharp band at 586 °C on NH 3 -TPD. − The peak at 1547 cm –1 is attributed to −NH 2 species. , Additionally, vibrational peaks at 1060–960 cm –1 are attributed to the adsorbed unstable gaseous NH 3 on the sample surface …”

An Assessment of Zeolite Framework Effect for Low-Temperature NOX Adsorbers

“…With this in mind and on the base of the literature data, the bands at 2211 and 2192 cm −1 are assigned to di-carbonyls (νasym(CO) and νsym(CO) modes, In the spectral region 2220-2170 cm −1 (where very strong bands are present in the case of Pd/SSZ-13 sample, Figure 3 trace a), it has to be considered that, due to the σ-donation contribution in the Pd 2+ -CO bond, the higher the band frequency, the more coordinatively unsaturated the site. With this in mind and on the base of the literature data, the bands at 2211 and 2192 cm −1 are assigned to di-carbonyls (ν asym (CO) and ν sym (CO) modes, respectively) of isolated Pd 2+ ions formed by ion exchange with the Brønsted acid sites of the zeolite [24,28]; the shoulder at 2187 cm −1 is instead assigned to CO adsorbed on Pd 2+ sites of PdO x particles [23].…”

“…CO adsorption-In Figure 3, the FT-IR spectra of CO (20 mbar) adsorbed at Room Temperature (RT) on the activated samples are displayed. The spectra are characterized by complex envelopes of bands, which are related to Pd carbonyl species in different oxidation states and coordination, as discussed in our previous paper [23], where the precise assignment of each component in the spectrum of Pd/SSZ-13 was made. As discussed in ref.…”

“…As discussed in ref. [23], with reference to the Pd/SSZ-13 sample (trace a), it is possible to divide the bands into different spectral regions, namely 2220-2170 cm −1 and 2170-2110 cm −1 , associated with linear carbonyls of Pd 2+ and Pd + , respectively [13,[24][25][26]. The absorption near 2228 cm −1 is assigned to carbonyls of extra-framework Al 3+ [27].…”

Effect of H2O and CO on the NO Operating Cycle in Pd/CHA Passive NOx Adsorbers

“…The existence of monomeric (Pd +2 , Pd +1 , [PdOH] + ) and dimeric Pd sites ([PdOH–PdOH] +2 , [Pd–O–Pd] +2 ) has been proposed in Pd/CHA, which enables NO x adsorption at low temperatures and its desorption in the targeted temperature window of the downstream NO x reduction catalysts. Bell and co-workers ascertained the presence of Pd +1 sites based on their theoretical as well as experimental studies. , NO adsorption–desorption experiments carried out by Harold and co-workers and Tronconi and co-workers on Pd/CHA also suggest the presence of Pd +1 sites. , Various in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies suggested that monomeric Pd­(II) species (Pd +2 , [PdOH] + ) are suitable for NO adsorption. − However, Crocker and co-workers cautioned that it may not be possible to confirm the presence of [PdOH] + sites through DRIFTS spectra due to the overlapping vibrational frequencies of Al–OH bond and Pd–OH bond . Nevertheless, theoretical studies suggest that the [PdOH] + site is stable in CHA and its formation energy is close to that of the cationic Pd +2 site. , Moreover, the hydroxylation of Pd +2 results in the formation of the [PdOH] + site .…”