Selective Catalytic Reduction

Monolayer catalysts containing binary active phases (VOx-CrOx, VOx-MoOx) were prepared by simultaneous deposition of the corresponding transition metal-oxo species on the TiO2 (anatase) surface using the equilibrium deposition filtration technique. The prepared samples contained various amounts of each transition metal but almost the same total metal loading. They were characterized by atomic absorption spectroscopy, N2 adsorption, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and tested for the selective catalytic reduction of NO by NH3 in the temperature range 250-450 degrees C. It was found that the transition-metal ionic species used for the preparation of these catalysts compete for the same surface sites of the TiO2 carrier upon co-deposition. Small amounts of the second phase (Mo- or Cr-oxo phase) are sufficient in order to promote the catalytic activity at relatively high temperatures, in contrast to what happens in the corresponding industrial catalysts prepared by conventional methods. An electronic interaction between V- and Cr-oxo species favored at a V/Cr atomic ratio around 3 is probably responsible for the relatively high catalytic performance of the corresponding TiCrV catalyst. The activity of the studied catalysts is well correlated with the intensity of a DRS absorption band that appeared at ca. 400 nm, which is considered as a measure of the magnitude of interactions exerted between the monolayer transition metal-oxo species and the TiO2 carrier. This correlation is independent of the transition metals combination used and follows the same linear relationship found previously for single-active-phase catalysts.

The kinetics of a catalyst for hydrocarbon-selective catalytic reduction (HC-SCR) exhaust aftertreatment has been examined by means of transient experiments on a heavy-duty diesel engine rig. The influences of temperature, NO2 concentration, and the transient injection of hydrocarbon on the conversion of NOx, CO, and hydrocarbon were studied in a systematic manner. Hydrocarbon conversion was high and NOx conversion was related to the amount of injected hydrocarbon at high temperatures. At lower temperatures hydrocarbon conversion was low and NOx conversion was not directly related to hydrocarbon injection rate. Increased exhaust NO2/NO ratio resulted in NOx conversion at lower temperatures and also in accumulation of NOx on the catalyst surface. The findings are in agreement with results from recent studies of the selective catalytic reduction of NO by propene. A catalyst model was designed in accordance to these studies and fitted to results from tailored and standard European transient cycles (ETC). The model shows reasonable agreement with experimental CO, NOx and NO2 data. Experimental hydrocarbon data are not as well reproduced, presumably due to the model approximation of hydrocarbons to one species. The full catalyst model used in the study is presented, including reaction kinetics and equations for mass and heat transfer. Mechanistic aspects are discussed and related to other studies.

In this research reduction of contaminant NOx by selective catalytic reduction was investigated. Catalysts were made of a mixture of carrier materials, such as titania or γ-alumina, and active components such as V2O5 and promoters such as La2O3, Ce2O3, ...

ABSTRACT Physical mixtures of CuFER and HZSM-5, CuFER and AgMOR, and CoFER and AgMOR zeolites were investigated as potential dual pore system catalysts for continuous lean NOx reduction with propene or isooctane as reducing agents. The effects of variations in temperature and oxygen concentrations were studied using steady-state experiments. Despite lower rates of external and internal mass transfer, the mixture of CuFER and HZSM-5 was found to have the highest NOx reduction activity with isooctane as the reducing agent. A maximum NOx reduction of about 40% was found for this catalyst at 350 °C and 12% oxygen. In this case, the CuFER extrudate acted as an NO oxidation catalyst, and the NO2 formed was reduced by the isooctane in the HZSM-5 extrudate. The better performance of the CuFER and HZSM-5 mixture compared with the other mixtures was attributed to a higher selectivity for NO2 to form nitrogen in the reduction reaction. When propene was used, the two mixtures containing CuFER showed similar reduction activity with a maximum conversion of about 65% at 370 °C and 2% oxygen. In contrast with the isooctane, the NOx reduction with propene occurred primarily in the extrudate containing ferrierite. Again, the selectivity of NO2 to form nitrogen governed the overall NOx reduction.

Fe-ZSM-5 catalysts, prepared by different methods, have been characterized by TPR and XRD and tested in the NO-SCR by NH3. The sublimation method leads to the most active catalysts. Nevertheless the preparation starting from Fe(acac)3, which is a preparation easy to implement from an industrial point of view, seems to be a very attractive alternative way.On the most active catalyst,