Selective catalytic reduction of NO over supported silver catalysts—practical and mechanistic aspects

Abstract

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is one of the promising technologies for removal of NO in exhausts containing excess oxygen, such as diesel and lean burn gasoline engines. Supported Ag catalysts, especially Ag/Al2O3, are thought to be the promising candidates for use in diesel exhausts, as confirmed by several reports on engine bench tests. The HC-SCR performance of supported Ag catalysts is very sensitive to the reaction conditions, especially the type of hydrocarbons and the addition of H2. The control of reaction conditions would be key for practical use. The current research of supported Ag catalysts is reviewed from the viewpoints of practical use and the reaction mechanism, i.e., the reaction scheme, the role of surface adsorbed species, and the structure of active Ag species.

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

“…3,4,7,[13][14][15][16][17][18][19] Among the catalysts proposed for HC-SCR technology, zeolitic oxide catalysts are effective for NO x reduction, while their water tolerance has been little improved. 5 Noble metal catalysts such as Pt/Al 2 O 3 exhibit high deNO x activity in the low temperature range of 200-300 1C while exhibiting a narrow operation temperature window.…”

Review of Ag/Al2O3-Reductant System in the Selective Catalytic Reduction of NO x

“…Besides the SCR of NO x by NH 3 or urea as a reductant, the SCR of NO x by hydrocarbon (HC) as a reductant can be a possible method to the removal of NO x from diesel and lean-burn gasoline engines. In recent studies, relatively durable and inexpensive alumina-supported silver catalysts (Ag/Al 2 O 3 ) have been considered to be a promising candidate for practical usage in the HC-SCR of NO x [2][3][4][5][6][7][8]. In our research into the HC-SCR of NO x over Ag/ Al 2 O 3 , we found a novel enolic surface species on Ag/ Al 2 O 3 [9] and proposed a new mechanism to explain the high-level efficiency of the SCR of NO x by ethanol over Ag/Al 2 O 3 [7,9,10], as shown in Scheme 1.…”

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

“…Using C2H5OH as a reductant results in significantly higher NOx conversion when compared with C3H6, especially at low reaction temperatures [11]. Generally, the activation of the reductant is considered as the initial and key step in the HC-SCR process [3][4][5][6][19][20][21]. Ag sites are widely accepted as the activation spots for the reductant [22][23][24].…”

“…The mechanisms of NOx reduction by hydrocarbons have been extensively explored [3][4][5][6][19][20][21]; the general reaction pathway is shown in Scheme 1. First, NO is oxidized to NO2 by O2, which results in the formation of surface nitrates.…”

“…Currently, the removal of NOx from lean-burn exhausts remains a major challenge in environmental catalysis because, in the presence of excess oxygen, NOx cannot be efficiently removed by traditional three-way catalysts. Among the developed technologies to eliminate NOx emission from lean-burn exhausts, the selective catalytic reduction of NOx by hydrocarbons (HC-SCR) is a potential method, which has attracted much attention in the past few decades [3][4][5][6][7]. Al2O3-supported Ag catalyst Ag/Al2O3 is considered as one of the most effective materials for the HC-SCR process in the presence of excess oxygen [8][9][10].…”