Low-Temperature Selective Catalytic Reduction (SCR) of NO_x with n-Octane Using Solvent-Free Mechanochemically Prepared Ag/Al₂O₃ Catalysts

Abstract

Low-temperature (<200 °C) hydrocarbon selective catalytic reduction of NO x has been achieved for the first time in the absence of hydrogen using a solvent-free mechanochemically prepared Ag/Al2O3 catalyst. Catalysts prepared by this ball-milling method show a remarkable increase in activity for the reduction of nitrogen oxides with octane by lowering the light-off temperature by up to 150 °C compared with a state-of-the-art 2 wt % Ag/Al2O3 catalyst prepared by wet impregnation. The best catalyst prepared from silver oxide showed 50% NO x conversion at 240 °C and 99% at 302 °C. The increased activity is not due to an increased surface area of the support, but may be associated with a change in the defect structure of the alumina surface, leading to the formation of the small silver clusters necessary for the activation of the octane without leading to total combustion. On the other hand, since one possible role of hydrogen is to remove inhibiting species from the silver, we cannot exclude some change in the chemical properties of the silver as a result of the ball-milling treatment.

Water Effect on Preparation of Ag/Al2O3 Catalyst for Reduction of NOx by Ethanol

“…The wet impregnation method, which has always been considered as the best way to prepare Ag/ Al 2 O 3 catalysts, 17,26 was also tested in this study. It was confirmed that wet impregnation produced the best catalysts among all samples in the reduction of NOx.…”

Mechanochemical preparation of Ag catalysts for the n-octane-SCR de-NOx reaction: Structural and reactivity effects

“…A number of studies have shown that the HC-SCR reaction to be greatly enhanced by the addition of hydrogen to the feed [29][30][31][32][33][34][35][36]46] and this has also been shown to be the case for the ball milled catalysts [9]. In fact, the ball milled catalyst in the presence of hydrogen has resulted in the lowest light off temperature reported, to date, under thermally activated conditions (165 • C) although lower temperature activity has been observed under non-thermal plasma conditions [47].…”

“…[9] For the purposes of comparison we use the light off temperature, which we define as the temperature for 50% NO x conversion. The silver catalyst prepared by wet-impregnation has a light off temperature of 390 • C whereas the ball milled catalyst has a light off temperature of only about 240 • C. In an attempt to gain greater understanding on how the ball milling affects the activity of the catalyst the effect of the ball milling parameters was studied.…”

“…Furthermore, in some cases, mechanochemically-prepared catalysts are actually superior to those prepared by conventional methods [8]. A recent example is the mechanochemical preparation of Ag/Al 2 O 3 catalysts for selective catalytic reduction of NO x [9]. However, despite the fact that, in some cases, mechanochemical methods can lead to superior catalysts/products than the more conventional methods, there is currently little understanding of the ball milling process.…”

“…In addition, the effect of the addition of hydrogen has been extensively studied [29][30][31][32][33][34][35][36][37][38][39]. A large increase in the activity of the catalyst was observed on addition of hydrogen, for example Burch and co-workers [29,30] observed a decrease in the light off temperature for a wet impregnated Ag/Al 2 O 3 catalyst from 410 to 195 • C. In the previous reports of the mechanochemical preparation of Ag/Al 2 O 3 catalysts was carried out by simply ball milling the alumina and Ag precursor, for example Ag 2 O or Ag metal [9]. These materials were highly active for the HC-SCR of NO x even in the low temperature range, showing light-off temperatures below 300 • C with the catalyst prepared using silver oxide resulting in a light-off temperature of 240 • C. In these systems, the ball milling did not significantly increase the BET surface area of the catalyst.…”