Ag/alumina catalysts for the selective catalytic reduction of NOx using various reductants

A series of Ag-based catalysts supported on unpromoted or Ce-promoted gamma-alumina were prepared and studied for the selective catalytic reduction (SCR) of NOx with various reductants. The activity for NO reduction increased as the oxygen-content in the reaction mixture increased from 2% to 10%. The influence of Ag loading, the catalytic support used and the type of the reductant on NO reduction was examined. Moderate silver loadings (3%) exhibit the most efficient deNO(x) activity, while increasing the metal loading or using a Ce-promoted alumina support inhibits the catalytic activity. Among C3H6, CH4 and CO used as reducing agents, C3H6 has the higher reducing activity. C2H4, C2H6, C3H8, C4H10, 1-C4H8 and 1,3-C4H6 were also tested as reducing agents, suggesting that use of higher and less saturated hydrocarbons results in an enhanced deNO(x) performance. Higher concentrations of the optimum reductant (1-C4H8) significantly improved the deNO(x) performance, achieving over 80% NOx reduction and an interestingly broad active temperature window (300-550 degreesC). NOx is reduced by reacting with intermediates generated from partial oxidation of hydrocarbons used as reductants. Increased availability of these species either by more C atoms in higher hydrocarbons, their easier formation from less saturated hydrocarbons or higher reductant concentration explains the improvement of NO, reduction. The performance of Ag catalysts was linked with the formation of different crystal phases, which are stabilized through a strong interaction with the alumina support. Existence of Ag in oxidation state +1 seems to be the active phase that favors most the NO reduction. (C) 2004 Elsevier B.V. All rights reserved.