Isabella Nova

SCR of NO by NH3 over TiO2-supported V2O5–MoO3 catalysts: reactivity and redox behavior

Abstract The reactivity in the selective catalytic reduction (SCR) reaction and the redox behavior of V2O5–MoO3/TiO2 catalysts was investigated by means of the temperature programmed reduction (TPR)/reaction technique, and compared with that of binary V2O5/TiO2 and MoO3/TiO2 catalysts having the same metal oxide loading. It was found that the ternary catalysts are more active in the SCR reaction at low temperatures compared to the corresponding binary samples: hence the ‘temperature window’ of the reaction is widened and shifted towards lower temperatures. Transient reactivity data provide clear evidence in favor of the hypothesis of a redox mechanism for the SCR reaction and point out that the ternary catalysts are more easily reduced and reoxidized than the corresponding binary samples: this indicates that the simultaneous presence of V and Mo enhances the catalyst redox properties, and thus its reactivity. Such conclusions are also in line with the results of the characterization studies pointing out the existence of electronic interactions involving the V and Mo surface oxide species. The overall picture closely resembles the one obtained in the case of the analogous V2O5–WO3/TiO2 system and indicates that the effects of the addition of WO3 and MoO3 to V2O5/TiO2 are similar, both oxides acting as ‘chemical’ promoters besides playing a ‘structural’ function as well.

On the dynamic behavior of “NOx-storage/reduction” Pt–Ba/Al2O3 catalyst

The NO x storage and reduction functions of a Pt-Ba/Al 2 O 3 NO x storage-reduction catalyst has been investigated in the present work by applying the transient response and the temperature programmed reaction methods, by using propylene as the reducing agent. It is found that: (i) the storage of NO x occurs first at BaO and then at BaCO 3 , which are the most abundant sites following regeneration of catalyst with propylene; (ii) the overall storage process at BaCO 3 is slower than at BaO; (iii) CO 2 inhibits the NO x storage at low temperatures; (iv) the amount of NO x stored up to catalyst saturation at 350 °C corresponds to 17.6% of Ba; (v) the reduction of stored NO x groups is fast and is limited by the concentration of propylene in the investigated T range (250-400°C); (vi) selectivity to N 2 is almost complete at 400 °C but is significantly lower at 300 °C due to the formation of NO which can be tentatively ascribed to the presence of unselective Pt-O species.

Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts

Urea-SCR technology for deNOx after treatment of diesel exhausts , Urea-SCR technology for deNOx after treatment of diesel exhausts , کتابخانه مرکزی دانشگاه علوم پزشکی ایران

Dynamics of SCR reaction over a TiO2-supported vanadia–tungsta commercial catalyst

The dynamics of NH 3 adsorption-desorption and of selective catalytic reduction (SCR) of NO with NH 3 are investigated in this work over a commercial V 2 O 5 -WO 36/TiO2 catalyst by transient response techniques. Linear variations in the inlet concentration of one or more reactants are imposed while analyzing the system outlet response. The transient experiments could be successfully described by a kinetic model based on: (i) a negligible NO adsorption on the catalyst surface; (ii) a non-activated NH 3 adsorption; (iii) a Temkin-type NH 3 coverage dependence of the desorption energy; and (iv) an Eley-Rideal mechanism for the SCR reaction with a non-linear dependence of the reaction rate on the NH 3 surface coverage. The presence of low concentrations of water in the feed stream does not seem to affect the ammonia adsorption-desorption process on the surface but significantly inhibits the SCR reaction; an inhibiting effect of adsorbed NH3 on the SCR reaction is also pointed out.

A “Nitrate Route” for the low temperature “Fast SCR” reaction over a V2O5–WO3/TiO2 commercial catalyst

A novel mechanism is proposed for the Fast SCR reaction of NH(3), NO and NO(2) at low temperature involving the formation of ammonium nitrate as intermediate and its subsequent reaction with NO as the rate determining step.

Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5–WO3 and V2O5–MoO3 catalysts

A comparison between commercial and model WO3–V2O5/TiO2 and MoO3–V2O5/TiO2 SCR catalysts is considered in this study. The data indicate that WO3 and MoO3 behave as “structural” and “chemical” promoters for the catalysts. MoO3-based catalysts are more active but less selective than WO3–V2O5/TiO2 catalysts in the SCR reaction, although in the presence of water the catalytic performances of the investigated samples are comparable.

Transient kinetic study of the SCR-DeNOx reaction

The unsteady-state kinetics of the selective catalytic reduction (SCR) of NO with NH3 is studied over V2O5–WO3/TiO2 model catalysts by means of the transient response method. NH3 strongly adsorbs onto the catalyst surface whereas NO does not adsorb appreciably. A dynamic mathematical model based on a Temkin-type desorption process for NH3 and a SCR reaction rate with a complex dependence on the ammonia surface coverage is well suited to represent the data.

Current status of modeling lean exhaust gas aftertreatment catalysts

Decreasing emission limits lead to the development of combined aftertreatment systems, consisting of combinations of different catalyst technologies and particulate filters. Modeling such systems can contribute considerably in reducing development time and cost. The methodology for developing catalyst models is reviewed and models for the diesel oxidation catalyst (DOC) with hydrocarbon (HC) adsorption, the NOx storage and reduction catalyst (NSRC) and the urea–selective catalytic reduction system (urea–SCR) are developed. Applications for exhaust aftertreatment system modeling are shown.

In situ FT-IR and reactivity study of NOx storage over Pt-Ba/Al2O3 catalysts

The mechanism of NOx storage on Pt–Ba/Al2O3 and, for comparison, on Ba/Al2O3 catalysts was investigated. In situ FT-IR spectroscopy was used to follow temporal changes in the types and relative amounts of surface species formed at three different temperatures (423, 523, 623 K) during the reaction with NO, NO/O2, NO2 or NO2/O2 atmospheres. At 623 K the NOx storage capability was also investigated by a mass-spectrometry transient response method (TRM) as a complementary technique. The results show that in absence of oxygen appreciable amounts of NO are adsorbed only on the Pt–Ba/Al2O3 system mainly as ionic nitrites. When oxygen is present, NO is stored according to two different mechanisms, both promoted by Pt. The first occurs through the formation of surface nitrites that evolve to nitrates; the second occurs through the NO oxidation to NO2 over Pt, followed by NO2 migration to the Ba phase and nitrate formation. When NO2 is used with or without O2, an extensive storage in the form of ionic nitrates occurs both on Ba/Al2O3 and Pt–Ba/Al2O3 catalysts at all the temperatures studied, without the formation of nitrites. NO2 storage occurs initially through a very fast mechanism, giving nitrates without NO release. In parallel, it proceeds via a disproportionation reaction of NO2 to nitrate and NO. Both mechanisms are not promoted by Pt.

Experimental Study of the NO Oxidation to NO2 Over Metal Promoted Zeolites Aimed at the Identification of the Standard SCR Rate Determining Step

Steady state and transient kinetic runs devoted to the comparative analysis of NO oxidation and standard SCR reactions over commercial Cu- and Fe-promoted zeolite catalysts are herein presented with the aim to clarify whether NO oxidation to NO2 is the rate determining step (rds) of the standard SCR reaction. It is found that this statement seems questionable in the light both of the herein collected experimental results and of scattered evidence from the literature.