Significant catalytic effects induced by the electronic interactions between carboxyl and hydroxyl group modified carbon nanotube supports and vanadium species for NO reduction with NH3 at low temperature

摘要

The relative content of carboxyl and hydroxyl groups on carbon nanotubes (CNTs) was tuned through a controlled reduction process. It was found that the CNTs decorated by varying ratio of carboxyl and hydroxyl groups can obviously enhance the catalytic activity of supported vanadium oxide catalysts for selective catalytic reduction (SCR) of NO with NH3. The improvement effects induced by carboxyl and hydroxyl groups depend significantly on the reacting feedstock with and without SO2 and H2O. In the absence of SO2 and H2O, hydroxyl-modified CNT supported vanadium oxide (V_y~(x+)O_z~(2-) (x ≤ 4)/ CNTs-OH) shows more significant improvement on SCR activity, while more obviously promoting effect was observed over carboxyl-modified CNT supported vanadium oxide (V_y~(x+)O_z~(2-) (x ≤ 5)/CNTs-COOH) catalyst in the presence of SO2. The variation in oxidation states of active vanadium species (V~(5+), V~(4+) and V~(3+)) originated from the electronic interactions between carboxyl and hydroxyl modified CNTs and vanadium precursors are indicated to be the main reason for the very different SCR performances over the two V_y~(x+)O_z~(2-)/CNTs catalysts. Low valence V~(4+) and V~(3+) species show much higher SCR activity in the absence of SO2 and H2O, while the high valence V~(5+) species are very beneficial to NO reduction in the presence of SO2, The V~(5+) species have an excellent catalytic ability of oxidizing SO2 to SO3, leading to the in situ formation of ammonium sulfate, and are capable of catalyzing the reaction between the sulfate salts and NO. Furthermore, both of the V_y~(x+)O_z~(2-)/CNTs catalysts are very stable for SCR of NO even in the presence of SO2 and H2O. These findings give some insight into the mechanisms behind the SCR reaction over CNT supported vanadium oxides, which would be beneficial to the design and synthesis of more efficient and much stable SCR catalysts.