Lydia Singoredjo

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.

Alumina supported manganese oxides for the low-temperature selective catalytic reduction of nitric oxide with ammonia

Abstract Alumina supported manganese oxides exhibit a high and selective activity for the catalytic reduction of nitric oxide with ammonia (SCR) between 385 and 575 K. Samples with 3–15 wt.-% manganese were studied at space velocities between 22 000–116 000 h −1 and at standard conditions of 500 ppm NO, 550 ppm NH 3 and 2% O 2 . Manganese acetate results in a better dispersion of the manganese oxide on the support and a higher specific catalyst activity than manganese nitrate as precursor, for which crystalline structures could be detected. Temperature-programmed reduction revealed that acetate yields Mn 2 O 3 and nitrate mainly MnO 2 on the γ-alumina support. The nitric oxide conversion per amount of manganese is fairly independent of the loading for the catalysts prepared from each precursor. The use of 15 NH 3 reveals that it reacts in a 1:1 molar ratio with nitric oxide towards 15 NN and/or 15 NNO. The SCR activity (to nitrogen) is strongly dependent on the oxygen partial pressure, whereas water inhibits reversibly. Lattice oxygen of the catalyst is not able to maintain the SCR reaction in the absence of oxygen. The nitrous oxide formation is independent of the oxygen partial pressure, but increases with increasing manganese loading and with temperature, resulting in lower selectivities for nitrogen formation. The nitrogen and nitrous oxide formation probably occur at different sites. Above 525 K 15 NH 3 oxidation occurs, yielding mainly 15 N 2 O and 15 NO, depending on the temperature. The nitrous oxide is not further reduced by ammonia over this type of catalyst. The addition of tungsten to the catalyst increases the selectivity for nitrogen considerably. The stability of the ex-acetate catalyst is good, for at least 600 h the activity remained constant. The catalysts are sensitive towards sulphur dioxide, the ex-acetate catalysts the least, due to the strong interaction with the alumina support, as is revealed by TPR.

Selective catalytic reduction of NO with NH3 over carbon supported copper catalysts.

The catalytic activity for the selective catalytic reduction of NO of carbon supported copper salts was studied in order to develop catalysts, active at temperatures below 473 K. At equal copper to carbon loadings and a space velocity of 60.000 h−1 the activity order was CuO > CuBr2⋍ CuSO4 > CuCl2. The carbon support showed only a low activity. In the lower temperature region (<473 K) CuO was even more active than titania supported vanadium- and vanadium-/tungsten- oxides. The NO-conversion curves of the carbon supported catalysts showed a maximum, which could be explained by both a change in selectivity and in the structure of the catalysts. The CuO/C and the CuBr2/C exhibited a constant activity at 423 K for 110 h. Above 450 K catalyzed gasification of the carbon support will occur.

Modified activated carbons for the selective catalytic reduction of NO with NH3

Abstract Activated carbons, modified with nitrogen- and oxygen-containing organic compounds by wet impregnation, followed by pyrolysis and CO2 activation, have been used for the low temperature selective catalytic reduction (SCR) of NO with NH3 (385–550 K). Of all the additives studied, only glucosamine results in an outstanding increase of the activity, which might be ascribed to the creation of stable surface oxygen complexes. These complexes can only be removed at relatively high temperatures and probably affect the carbon surface in such a way that adsorption of the reactants is improved, resulting in higher NO conversions compared to the original carbon. The explicit role of the incorporated nitrogen is not unequivocally clear. Apparently, the overall SCR activity is the result of a combination of the following factors: interaction of oxygen with the carbon, the presence of stable oxygen groups, the nitrogen content and the accessibility of the pores. For the modified carbons the SCR reaction is zeroth order in NH3 and first order in NO. Furthermore, the oxygen dependency can be modelled by a Langmuir type of adsorption. Over the unmodified carbons a second type of selective NO reduction can be distinguished in which the carbon only acts as an adsorbents and for which a negative apparent activation energy is observed.

Selective catalytic reduction of NO with NH3 over activated carbons. I: Effect of origin and activation procedure on activity

The activity for the selective catalytic reduction of NO with NH3 (SCR) of a suite of different activated carbons has been studied in the temperature range 373–550 K. The effect of origin (bituminous coal, peat, plum stones, and olive pits), activation agent (steam, CO2), and degree of activation was investigated by activity measurements. In general, the SCR activity, which is relatively high at 373 K, decreases with increasing temperature, and for some carbons increases again above 450 K. Two types of SCR processes are operative. In the lower temperature range, the narrow microporosity affects the rate by increased adsorption of NO, resulting in higher local concentrations. In the higher temperature range, oxygen functional groups determine the SCR activity level. Precursors yielding narrow microporous carbons result in better activities below 450 K. Steam-activated carbons gave higher SCR activity above 450 K than CO2-activated carbons.

Alumina Supported Manganese Catalysts for Low Temperature Selective Catalytic Reduction of no with NH3

Abstract Alumina supported manganese oxides exhibit high and selective catalytic reduction (SCR) activities between 385 K and 575 K, stable for at least 600 h. The use of 15 NH 3 reveals that the SCR reaction occurs between one molecule of NO and one of 15 NH 3 to form 15 NN or 15 N 2 O. The N 2 O formation increases with both increasing manganese loading and temperature, resulting in lower selectivity towards N 2 . The addition of tungsten improves the selectivity. The reaction is also inhibited reversibly by H 2 O. Above 525 K 15 NH 3 oxidation occurs, mainly under formation of 15 N 2 O.