Promotion effect of hydrogen on surface steps in SCR of NO by propane over alumina-based silver catalyst as examined by transient FT-IR

Abstract

The mechanistic cause of enhancement of C 3 H 8 -SCR activity by addition of H 2 over Ag/Al 2 O 3 was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C 3 H 8 + O 2 , nitrates were mainly formed on Ag/Al 2 O 3 . An addition of H 2 into a C 3 H 8 -SCR atmosphere increased the concentration of surface acetate significantly, but decreased the concentration of surface nitrates. Formation and consumption rates of acetate and nitrates were estimated with transient in-situ IR measurement. By the addition of H 2 , both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C 3 H 8 + O 2 and acetate in a flow of NO + O 2 were also increased by the addition of H 2 . From a comparison between the evolutions of adsorbed species (nitrate and acetate) and gaseous species (NO and C 3 H 8 ), it was clarified that the NO reduction activity is controlled by partial oxidation of C 3 H 8 to mainly surface acetate. The addition of H 2 results in remarkable promotion of partial oxidation of C 3 H 8 to mainly surface acetate, which is the rate-determining step of C 3 H 8 -SCR in the absence of H 2 .

Hydrogen promotes the selective catalytic reduction of NO by ethanol over Ag/Al2O3

“…623 K using a series of lower alkanes (alkenes) as reductants. Using in situ FTIR spectroscopy, it was discovered that the formation of some intermediates, such as acetate, nitrite, nitrate species, and isocyanate (ANCO), was promoted by the addition of H 2 over Ag/ Al 2 O 3 surface in the SCR of NO x with several hydrocarbons [7][8][9][10]. However, the SCR of NO x by oxygenates in the presence of H 2 has not been reported.…”

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

“…12 Therefore, it seems likely that, on active silver alumina catalysts that do not contain large metallic silver clusters, NO 2 is not formed by direct oxidation. Other studies have shown that different partially oxidized carbonaceous species can form depending on the reaction conditions, 6 and it has been suggested that enolic species, 4,27 acetates 13 or acrylates, 11 are the key species in SCR, while formates and carboxylates would not be involved in the reaction. 13 The formation of acetates has been proposed to be the rate-determining step of propane SCR.…”

“…Other studies have shown that different partially oxidized carbonaceous species can form depending on the reaction conditions, 6 and it has been suggested that enolic species, 4,27 acetates 13 or acrylates, 11 are the key species in SCR, while formates and carboxylates would not be involved in the reaction. 13 The formation of acetates has been proposed to be the rate-determining step of propane SCR. 13 The presence of surface-bound NO x species enhances the activation of the reducing agent, 13,15 and it has been suggested that acrylates are formed via a nucleophilic activation that occurs in the presence of NO, while acetates are formed via an electrophilic activation that takes place mainly in the absence of NO.…”

“…13 The formation of acetates has been proposed to be the rate-determining step of propane SCR. 13 The presence of surface-bound NO x species enhances the activation of the reducing agent, 13,15 and it has been suggested that acrylates are formed via a nucleophilic activation that occurs in the presence of NO, while acetates are formed via an electrophilic activation that takes place mainly in the absence of NO. 6 Enolic species are mainly observed when alcohols are used as reducing agents.…”

“…1,[4][5][6][7][8][9][10][11][12] The large Ag 0 clusters cause unselective combustion of the reducing agent and the formation of N 2 O, a harmful greenhouse gas, whereas all of the other silver species have been proposed to be the active sites for SCR. 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.…”