FTIR, TPD and DFT studies of intermediates on Ag/Al2O3 during the selective catalytic reduction of NO by C2H5OH

Abstract

The intermediates on Ag/Al 2 O 3 during the C 2 H 5 OH-SCR of NO were studied using in situ DRIFTS, TPD, and DFT calculations. A ring structure enolic species was found on the surface of Ag/Al 2 O 3 during the C 2 H 5 OH-SCR of NO by in situ DRIFTS, and this finding was supported by DFT calculations. The adsorbed enolic species is more prone to react with NO + O 2 on Ag/Al 2 O 3 than acetate is, and plays a crucial role in the formation of NCO which is a key intermediate during the SCR of NO. TPD results suggest that the enolic species is derived from the aldol condensation reaction of a partial oxidation product of C 2 H 5 OH (such as CH 3 CHO) on the surface of Ag/Al 2 O 3 . The results of DFT calculations are in good agreement with the DRIFTS spectra, and support our conjecture that the enolic species has a ring structure.

Environmentally-benign catalysts for the selective catalytic reduction of NOxfrom diesel engines: structure–activity relationship and reaction mechanism aspects

“…This indicates that a condensation reaction occurs synchronously during the partial oxidation of ethanol over Ag/Al 2 O 3 , which also agrees with our TPD-MS experiment. 173 The condensation reactions of aldehydes, as well as ketones, are widely used in organic synthesis and are commonly catalyzed by zeolites, Al 2 O 3 , and TiO 2 . [175][176][177] On a detailed mechanistic level, the role of silver species in the HC-SCR of NO x over Ag/Al 2 O 3 was mainly attributed to enhancement of the partial oxidation of reductants to form active intermediates, and their further reaction to produce N 2 .…”

“…162,165,166,173,179 Two kinds of surface enolic species, adsorbed only on Al sites (CH 2 QCH-CHQCH-O-Al 6 O 5 (OH) 8 ) and bound on or close to Ag (CH 2 QCH-O-Ag, CH 2 QCH-CHQCH-O-Al-Ag) were suggested by DFT calculations over Ag/Al 2 O 3 and Al 2 O 3 , with the simulation molecular structure model and corresponding FTIR spectrum shown in Fig. 3D-F, respectively.…”

“…164 Surface enolic species, acetate, and -NCO were identified by DRIFTS and DFT calculations. 165,173 (Reproduced with permission from Elsevier Inc.) 165 principally reacts with oxygen to form acetaldehyde, which is followed by isomerization to ethenol. Subsequently, a C 2 enolic anion (CH 2 QCH-O À )-M + is formed by hydrogen extraction when ethenol is adsorbed on the surface of Ag/Al 2 O 3 .…”

“…Meanwhile, the occurrence of aldol condensation of acetaldehyde, which has been confirmed by PIMS and TPD-MS measurement, may lead to the formation of C 4 enolic species (CH 2 QCH-CHQCH-O À )-M + as shown in Scheme 5. 164,173 Obviously, if the formation of surface enolic species follows the hypothesis described above, two prerequisites must be met. First, the selected reductant must contain at least one C-C bond.…”

Adsorption states of typical intermediates on Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol

“…Two typical processes should be addressed in this mechanism: (1) the partial oxidation step of hydrocarbons and (2) the surface reaction between nitrates and oxygenates. Among the oxygenated species possible, a novel surface enolic species has been suggested as an important intermediate that is related to the high efficiency observed for the SCR of NOx by alcohols [11,[22][23][24]. Considering Al2O3-based catalysts, the rate-determining step for HC-SCR is thought to be the surface reaction between nitrates and oxygenates [3,20,25], during which the product -NCO species is considered as the most important intermediate [26][27][28].…”