Ag/alumina catalysts for the selective catalytic reduction of NO x using various reductants

Abstract

A series of Ag-based catalysts supported on unpromoted or Ce-promoted γ-alumina were prepared and studied for the selective catalytic reduction (SCR) of NO x 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 C 3H 6, CH 4 and CO used as reducing agents, C 3H 6 has the higher reducing activity. C 2H 4, C 2H 6, C 3H 8, C 4H 10, 1-C 4H 8 and 1,3-C 4H 6 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-C 4H 8) significantly improved the deNO x performance, achieving over 80% NO x reduction and an interestingly broad active temperature window (300–550 °C). NO x 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 x 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.