On the mechanism of the selective catalytic reduction of NO with higher hydrocarbons over a silver/alumina catalyst

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

“…loading was between 5 and 7 wt % in contrast to literature, 2,8,15,17,20,21,33,34,42,43 where 2 wt % is typically reported as the standard catalyst. It is believed that these high loadings of silver can be obtained by using boehmite as the precursor for γ-alumina, as this ensures a high surface area for the final catalyst (see Table 1) and has been reported 35 to maintain a strong interaction between silver and alumina, preventing silver from sintering during calcination.…”

“…29 In addition to the above-discussed surface reactions, it has been shown that homogeneous gas-phase reactions occur under SCR conditions 15 and that the presence of water enhances the formation of gaseous ammonia. 21 The addition of small amounts of hydrogen (<1%) significantly improves the performance of silver alumina and silverexchanged ZSM-5 catalysts, 2,10,30,31 whereas this effect is not observed for ZrO 2 -, TiO 2 -, Ga 2 O 3 -, or SiO 2 -supported silver catalysts nor for Co or Pt on alumina catalysts. 7 The role of hydrogen in the SCR reaction has been debated in the literature; this effect has been attributed to changes in the active silver sites or to a chemical effect of hydrogen on the reaction intermediates.…”

“…1,9,[11][12][13][14] The lack of certainty on the type of the active site is due to the complex nature of the SCR reaction, 1,15 the mobility of silver species under reaction condition, 3,16 and the influence of temperature and gas-phase composition on the formed surface species. 17 Hydrocarbons are used as reducing agents for the SCR reaction (HC-SCR) with silver alumina catalysts, and in the past decades the use of several different hydrocarbons, ranging from short-chain hydrocarbons, such as propane 2,7,12,18 and propene, 11,19 to long-chain hydrocarbons such as octane, 15,20,21 decane, 17 and real diesel fuel, 22 has been studied. The structure of the hydrocarbon only influences the activity of the catalysts; increasing the number of carbons and the linearity of the carbon chain increases the activity of the catalyst but does not affect its selectivity.…”

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

“…However, this may be a difficult task because these gas phase intermediates possess very high reactivity and a short lifetime. Eränen et al 215 proposed that gas phase reaction behind the silver catalyst may be involved in the reaction of activated NO x species with amines and ammonia. Organic N-containing species such as R-NO 2 , R-NCO, and R-CN are intermediates in the formation of amines and ammonia, which may also take part in this gas reaction.…”

“…Using a conventional electron-impact (EI) ionization mass spectrometer and liquid nitrogen trap, unfortunately, only R-CN species were detected during the NO x reduction by octane over Ag/Al 2 O 3 , together with nitrogen-free species due to the partial oxidation of the reductant. 215 During the NO x reduction by ethanol or octane over Ag/Al 2 O 3 , large amounts of N-containing species such as NH 3 and HCN were detected by a conventional MS technique. 24 Considering that the two gas phase species are not so active as to produce N 2 in a catalyst-free system, this may not be the true story.…”