TiO2-pillared Clays Research Articles

Selective Catalytic Reduction of Nitrogen Oxides by Ammonia over Fe3+-Exchanged TiO2-Pillared Clay Catalysts

Fe-exchanged TiO2-pillared clay (PILC) catalysts were prepared and used for selective catalytic reduction (SCR) of NOx by ammonia. They were also characterized for surface area, pore size distribution, and by XRD, H2-TPR, and FT-IR methods. The Fe–TiO2–PILC catalysts showed high activities in the reduction of NOx by NH3 in the presence of excess oxygen. SO2 further increased the catalytic activities at above 350°C, whereas H2O decreased the activity slightly. The catalysts were about twice as active as commercial-type V2O5–WO3/TiO2 catalyst in the presence of H2O and SO2. Moreover, compared to the commercial catalyst, the Fe–TiO2–PILC catalysts had higher N2/N2O product selectivities (e.g., 0–1% vs 9% N2O at 400°C) and substantially lower activities (by 74–88%) for SO2 oxidation to SO3 under the same reaction conditions. The activity was further increased to over three times that of the vanadia-based catalyst when Ce was added. The high activity and low N2O selectivity for the Fe–TiO2–PILC catalysts were attributed to their low activity in the oxidation of ammonia, as compared with vanadia catalysts. XRD patterns of Fe–TiO2–PILC were similar to those of TiO2–PILC, showing no peaks due to iron oxide, even when the iron content reached 20.1%. The TPR results indicated that iron in the Fe–TiO2–PILC catalysts with lower iron contents existed in the form of isolated Fe3+ ions. The activities of Fe–TiO2–PILC catalysts were consistent with their surface acidities, which were identified by FT-IR of the NH3-adsorbed samples. The enhancement of activities by H2O+SO2 was attributed to the increase of surface acidity resulting from the formation of surface sulfate species of iron.

Acid- and base-treated Fe3+-TiO2-pillared clays for selective catalytic reduction of NO by NH3

Fe3+-ion-exchanged delaminated pillared clays (PILCs) have been found previously to be more active than the vanadia-based catalysts for selective catalytic reduction (SCR) of NO by NH3. The effects of acid treatment of the clay (before pillaring) and base treatments of the TiO2-PILC (before ion exchange) on the activities of the Fe–TiO2-PILC catalysts were studied. It was found that the acid treatment increased the activity (by 33%), but the base treatments decreased the activity (although they increased the cation exchange capacity of the pillared clay and, hence, the Fe content). The activities of the catalysts were directly related to their surface Brønsted acidities as identified by FT-IR of chemisorbed NH3.

Selective catalytic reduction of nitric oxide by ethylene in the presence of oxygen over Cu 2+ ion-exchanged pillared clays

Cu 2+ ion-exchanged pillared clays are substantially more active than Cu 2+-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H 2O (or SO 2) has only mild effects on their activities. First results on Cu 2+-exchanged TiO 2-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu 2+-ZSM-5. A delaminated pillared clay was subjected to Cu 2+ ion-exchange and studied for SCR by C 2H 4 in this work. The Cu 2+ ion-exchanged delaminated Al 2O 3-pillared clay yielded substantially higher SCR rates than both Cu 2+-exchanged TiO 2-pillared clay and Cu 2+-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h −1 (with O 2 = 2%). The peak temperature decreased as the concentration of O 2 was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C 2H 4, the NO conversion peaked at 2% O 2 for all temperatures. H 2O and SO 2 caused only mild deactivation, likely due to competitive adsorption (of SO 2 on Cu 2+ sites and H 2O on acid sites). The high activity of Cu 2+-exchanged Al 2O 3-pillared clay was due to a unique combination of the redox property of the Cu 2+ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O 2) on Cu 2+ O Al 3+-on the pillars, and C 2H 4 activation on the Lewis acid sites to form an oxygenated species.

Iron Oxide and Chromia Supported on Titania-Pillared Clay for Selective Catalytic Reduction of Nitric Oxide with Ammonia

TiO2-pillared clay (PILC) with high surface area, large pore volume, and large interlayer spacing was used as the support for mixed Fe2O3and Cr2O3as the catalyst for selective catalytic reduction (SCR) of NO with NH3. The Fe/Cr ratio was varied at a fixed total amount of oxide dopant of 10% (wt). The Fe–Cr/TiO2-PILC with Fe/Cr=3 showed the highest activity. Compared with commercial V2O5/TiO2catalysts, the activity (on a per gram basis) of the doped pillared clay was approximately twice as high under H2O- and SO2-free conditions and was approximately 40% higher under conditions with H2O and SO2. In addition, its activity for SO2oxidation was only 20%–25% of that of the V2O5-based catalysts. TPD of NH3on the Fe–Cr/TiO2-PILC catalyst showed that both M=O and M–OH (M=Fe or Cr) were necessary for the SCR reaction.In situIR spectra of NH3showed that there was a higher Brønsted acidity than the Lewis acidity on the surface under reaction conditions and that there existed a direct correlation between the SCR activity and the Brønsted acidity among pillared clays with different Fe/Cr ratios. These results, along with the transient response to O2, indicated that a similar mechanism to that on the V2O5catalyst was operative. The TiO2-pillared clay used as the support also contributed to the high activity of the Fe–Cr catalyst. The TiO2pillars combined with the tetrahedral SiO2surfaces of the clay apparently gave rise to a high dispersion of Fe2O3.

Adsorption and Acidic Properties of Clays Pillared With Oxide Sols

AbstractInterlayers of montmorillonite were pillared with TiO2, SiO2-TiO2and SiO2-Fe2O3sols. The pillared structures were thermally stable at least up to 500°C and retained unusually large basal spacings in the range of 24–45Å and surface areas as high as 300–500 m2/g. The TiO2pillared clay showed Type IV adsorption isotherm for nitrogen. Although SiO2-TiO2and SiO2-Fe2O3pillared clays had basal spacings much larger than that of TiO2pillared clay, these mixed oxide pillared clays had small pores and exhibited Type I isotherm. The acidic strength distributions were determined by a titration method using n-butylamine and Hammett indicators. All of the pillared clays had large acidities, but the acidic strength decreased in the following order: TiO2- SiO2» SiO2-Fe2O3pillared clays. Temperature-programmed desorption (TPD) spectra of ammonia were measured and interpreted in relation to the acidity distribution. Infrared spectra of pyridine adsorbed on TiO2pillared clay indicated that the acidity predominantly arises from Lewis acid sites.