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

摘要

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 NO,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 (300-550degC).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.