Real-Time Infrared Detection of Cyanide Flip on Silver-Alumina NO _x Removal Catalyst

Abstract

Spectroscopic studies of the mechanistic steps that occur on supported precious metal catalysts used in industrial and automotive applications are hampered by a dearth of suitable experimental methods. We used femtosecond laser excitation followed by nanosecond time-resolved in situ Fourier-transform infrared spectroscopy to initiate a catalytic reaction on alumina-supported silver catalysts, which are of interest in minimizing nitrogen oxide emissions from fuel-efficient lean-burn engines. We found that the key intermediate step in the reaction between carbon monoxide and nitric oxide is the flip of a cyanide group from a silver nanoparticle to the alumina support (with a lifetime of 2 microseconds), which indicates the central role played by the interface between the metal particle and the oxide support.

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

“…The characteristic IR peaks of -NCO located around 2230 and 2250-2260 cm À1 have been commonly observed on Ag/Al 2 O 3 during HC-SCR, while the assignments of their coordination sites are still controversial. Bion et al 191,192 and Thibault-Starzyk et al 196 observed the characteristic frequencies of -NCO on Ag/Al 2 O 3 located at 2255-2265 and 2228-2240 cm À1 , which were assigned to -NCO species coordinated with octahedral Al 3+ ions (Al VI -NCO) and tetrahedral Al 3+ sites (Al IV -NCO), respectively. During the reduction of NO x with ethanol and propene on , and ÀNCO (n) for the cases of (A), and (C), respectively, and (E) time dependence of N 2 and CO 2 concentration calculated from the mass signals for the cases of (C).…”

“…29,178,[191][192][193][194][195][196][197] With this in mind, the relationship between -NCO formation and the consumption of enolic species and acetate was investigated on Ag/Al 2 O 3 via the transient response of the DRIFTS method, and typical results are presented in Fig. 5.…”

Remarkable influence of reductant structure on the activity of alumina-supported silver catalyst for the selective catalytic reduction of NOx

“…Simultaneously, enolic species (1633 cm Fig. 3 (continued) [5,6,9,[32][33][34][35][36]. The peak at 1591 cm À1 can be assigned to formate species.…”

“…Switching the feed gas to NO + O 2 resulted in a sharp decrease in the intensity of enolic species and acetate. A new peak of ANCO appeared at 2229 cm À1 [5,6,9,[32][33][34][35][36], and its intensity increased with time on stream, reaching a maximum at 3 min and then decreasing gradually. After the flowing of NO + O 2 for 10 min, the enolic species decreased slowly and the intensity of the ANCO peak significantly decreased.…”

New Insights into the Active Surface Species of Silver Alumina Catalysts in the Selective Catalytic Reduction of NO

“…4 The next step in the reaction mechanism is the reaction between the adsorbed NO x and the partially oxidized carbonaceous species forming organic nitrites and nitro species (R-NO 2 ), amino species (R-NH 2 ), surface bound cyanide (-CN), and isocyanate species (-NCO). Nanosecond timeresolved IR studies have shown that silver-bound cyanide species flip to the alumina surface, forming isocyanate species in the presence of CO and NO on a silver alumina catalyst, 28 whereas other studies with propene have indicated that the cyanide and isocyanate species might belong to two parallel pathways, where cyanides belong to a minor reaction route and isocyanates are part of the major route formed from surface nitrates and carbonaceous species. 29 The organic nitrites, nitro, amino species, cyanide, and isocyanate species readily react with water-forming ammonia that can reduce surface-bound or gaseous NO x to N 2 .…”