Modified acid treated bentonites as catalysts in selective catalytic reduction of nitrogen oxides by ammonia (SCR-NH3)

Modified acid treated bentonites have been studied as catalysts for the removal of nitrogen oxides (NOx) from power plants off-gases in the selective catalytic reduction of NOx by ammonia (SCR-NH3). The main steps of this work have been the preparation of samples, characterization of samples (by XRD, FT-IR, UV-vis and TG) and the application of obtained catalysts in SCR-NH3. Clays and clay minerals are recognized as the materials of the 21st century for being abundant, inexpensive and environmentally friendly; therefore, they appear to be good candidates for their application in SCR to overcome the limits of commercial catalysts. Modified bentonites have been here investigated: acid treatment and pillaring resulted to be the best methods of modification of these clays. The first step for the preparation of an acid-saturated clay is its pre-saturation with Na+; for this reason, pure bentonite is treated with a solution of NaCl. All the samples pre-treated with this solution presented higher catalytic activities in the removal of NOx. Acid treatments have been conducted with different concentrations of acid solutions (0.4, 0.6, 0.8, 1M) to investigate the different attacks to the layered structure of clays. Treatments with HNO3, HCl and oxalic acid give good results. On the other hand, pillaring allows the creation of an additional porous structure and increases the specific surface area with the introduction of inorganic polymer hydroxycations, whose sizes are bigger than the original exchangeable cations. In addition to aluminum, also cobalt has been introduced as a pillaring agent. The addition of different fractions of another cation for pillaring improves the thermal, adsorptive and catalytic properties of the catalyst. As catalytically active metals, iron and cerium were added with incipient wetness impregnation to improve the removal of NOx. NaB0.6HNO3AlFeCe presents the highest catalytic performance among all the samples examined (93% of NO conversion). This work demonstrated that, with different modifications, pure bentonite and Na-Bentonite, which don’t present any catalytic activity (7% of NO removal as the maximum value), can be transformed into a good catalyst for the removal of NOx, reaching NO conversions above 90%.