Amedeo Andreini

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Activity of supported tungsten oxide catalysts for the metathesis of propene

Abstract A high-temperature activation under inert gas of a WO 3 SiO 2 catalyst greatly increases the break-in rate and the catalytic activity for the metathesis of propene. During activation oxygen atoms and hydroxyl groups are eliminated from a tungsten oxide surface compound. As a result, tungsten ions become more exposed; some of these also undergo a change in oxidation state. The results suggest that the level of catalytic activity as well as the break-in rate depend on the number of these ions and on the rate of their formation. For a given catalyst type and structure, it depends primarily on the conditions of the activation whether a break-in appears or not. WO 3 Al 2 O 3 catalysts are characterized by a maximum in the reaction rate as a function of the reaction temperature. They attain, under the conditions of the proposed activation, high activity in a very short time and, under certain circumstances, are affected by product hindrance.

Selective catalytic reduction of NO with NH3 over Nb2O5-promoted V2O5/TiO2 catalysts

In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH3 only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst: calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings.

Evidence for a compensation effect during the temperature-programmed reduction of copper oxide in hydrogen

The reduction of copper oxide derived from basic Cu-carbonate in hydrogen has been studied under temperature-programmed conditions (TPR) and the TPR patterns were analyzed by means of Arrhenius plots at constant conversion (Friedman plots). These plots indicate that the reduction process cannot be described on the basis of constant kinetic parameters and reveal the presence of isokinetic temperatures. These suggest the presence of a compensation effect requiring a modification of the rate equation.

Promotion and deactivation of V2O5/TiO2 SCR catalysts by SO2 at low temperatures

The effect of SO 2 on the activity for the selective catalytic reduction (SCR) of NO with NH 3 over V 2 O 5 /TIO 2 catalysts, both unpromoted and promoted by Nb 2 O 5 , has been studied at temperatures below 473 K Enhanced Bronsted acidity due to the formation of surface sulphates promotes the SCR activity, whereas deposition of ammonium (bi)sulphates may cause severe deactivation. The combination of activity tests, TPD and FTIR shows that both effects occur simultaneously at these low temperatures. The support morphology, the presence of Nb 2 O 5 and especially the V 2 O 5 loading determine the net effect of SO 2 in terms of activity. The new insights can be used to design a catalyst on which promotion largely overrules deactivation, even at temperatures below 473 K.

Selective catalytic reduction of nitric oxide with ammonia on vanadia/alumina catalysts.

A series of V2O5/Al2O3 catalysts have been examined for the selective catalytic reduction (SCR) of NO with NH3 as a function of vanadia loading, metal oxide additives (Mo, W, Ni, Co), and reaction temperature. Increasing the vanadia loading, or surface vanadia coverage, increased the Bronsted acidity and reactivity of the V2O5/Al2O3 catalysts. Introducing additives that increased the Bronsted acidity, Mo oxide and W oxide, also increased the reactivity. Introducing additives that did not influence the Bronsted acidity, Ni oxide and Co oxide, did not affect the reactivity. However, the addition of Ni oxide and Co oxide increased the ratio of polymerized to isolated surface vanadium oxide species on alumina, which reveals that the SCR reaction is not sensitive to the surface vanadia structure on alumina. A model that explains these observations consists of a dual site: a surface redox site and an adjacent surface Bronsted acid site. Consequently, increasing the surface vanadium oxide coverage and introducing additives that increase the surface concentration of Bronsted acid sites is beneficial for the SCR of NO with NH3.