Ir/H-ZSM-5 Catalysts in the Selective Reduction of NOx with Hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir⁰ and IrO₂. The presence of Ir⁰ was essential for high DeNOx activity. The ratio of Ir⁰/Ir⁴⁺ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir⁰. Crystallite size and oxidation state of Ir have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.     

The role of acid sites in cobalt zeolite catalysts for selective catalytic reduction of NOx

The role of the acidic support in ion-exchanged cobalt-zeolite, lean NO_x catalysts has been determined by studying the individual steps in the selective reduction pathway. At a GHSV of 10,000 and reaction temperatures below 400°C, NO oxidation is not sufficiently rapid to obtain equilibrium over, for example, 1–4 wt% Co-mordenite catalysts. The NO oxidation rate increases in the order H⁺Co²⁺ Co oxide, and neither the number, nor the strength of the acid sites affects the specific rate of the Co²⁺ ions. For reduction of NO₂ by propylene at 300°C and methane at 400°C, the formation of N₂ is suggested to occur at support protons sites. In addition, the rate of N₂ formation increases linearly with an increase in the number of acid sites, and the specific activity increases with an increase in acid strength. Cobalt (2+) ions do not contribute significantly to the formation of N₂, but do non-selectively reduce NO₂ to NO. It is proposed that the formation of N₂ occurs by protonation of the reducing agent followed by attack of the carbocation by gas phase NO₂. Thus, the selective reduction of NO requires two catalytic functions, metal and acid sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectroscopic Evidence for a Nitrite Intermediate in the Catalytic Reduction of NOx with Ammonia on Fe/MFI

The mechanism of selective catalytic reduction (SCR) of NO_x with NH₃ over Fe/MFI was studied using in situ FTIR spectroscopy. Exposing Fe/MFI first to NH₃ then to flowing NO + O₂ or using the reversed sequence, invariably leads to the formation of ammonium nitrite, NH₄NO₂. In situ FTIR results in flowing NO + NH₃ + O₂ at different temperatures show that NH₃ is strongly adsorbed and reacts with impinging NO_x. The intensity of the NH₄NO₂ bands initially increases with temperature, but passes through a maximum at 120 °C because the nitrite decomposes to N₂ + H₂O. The mechanistic model rationalizes that the consumption ratio of NO and NH₃ is close to unity and that the effect of water vapor depends on the reaction temperature. At high temperature H_2O enhances the rate because it is needed to form NH₄NO₂. At low temperature, when adsorbed H₂O is abundant it lowers the rate because it competes with NO_x for adsorption sites.     

CO含量对烟气选择性非催化还原反应的影响

分别以氨水和尿素溶液作为还原剂,在管式反应器上详细研究了CO含量对烟气SNCR（选择性非催化还原）脱硝性能的影响。结果表明,CO含量增加导致：（1）SNCR脱硝温度窗口和最佳脱硝温度向低温方向移动,同时脱硝温度窗口宽度变窄、最大脱硝效率降低,尤其是以氨水作为还原剂时更为突出;（2）N₂O排放的峰值升高;当以尿素溶液作还原剂时,N₂O排放温度窗口主要向低温方向扩展;当以氨水作还原剂时,N₂O排放温度窗口向低温和高温两个方向扩展;（3）NH₃及HNCO残留量曲线向低温方向移动。维持相同的φ（CO）/φ（urea）,氨氮比增加将导致最佳脱硝温度略微降低。氨氮比或氧含量的变化对NO_x、N₂O、NH₃及HNCO的排放趋势没有影响。     

Model Studies of NOx Storage and Sulphur Deactivation of NOx Storage Catalysts

The storage of NO _x under lean conditions in model NO _x storage catalysts as well as the deactivation by sulphur have been studied. We find that NO₂ plays an important role in the storage mechanism as an oxidising agent. Two different mechanisms for this are discussed: the formation of surface peroxides and the oxidation of nitrites to nitrates. FTIR studies show that NO _x is stored as surface nitrates. The sulphur deactivation is found to be more severe when SO₂ is added during the rich phase than when SO₂ is added during the lean period. FTIR shows the formation of bulk sulphates both under lean and rich conditions.     

Novel Sn–Ce/Al2O3 Catalyst for the Selective Catalytic Reduction of NOx Under Lean Conditions

Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al₂O₃ catalyst prepared by sol–gel method for the selective reduction of NO_x with propene under lean conditions was studied. Sn–Ce/Al₂O₃ catalysts exhibited higher activity than Sn/Al₂O₃ catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO₂ and facilitate the activation of propene, both of which are important steps for the NO_x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect.     

The Effect of Carbon Monoxide and Hydrocarbons on NOx Storage at Low Temperature

One possible way to reduce NO _x in lean exhausts is by using NO _x trap catalysts. This paper addresses storage of NO _x on such catalysts at temperatures below the catalyst light-off. Experiments carried out on commercial samples in synthetic exhausts revealed a large capacity for storage of NO _x when NO₂ was added at temperatures below 150°C. In contrast, when NO was added instead, no storage took place. CO was found to decrease the storage by reacting with NO₂ and forming NO and CO₂. Propene inhibited the reaction between NO₂ and CO and therefore gave rise to larger NO _x storage when CO was present. The paper concludes with a discussion of a possible mechanism for the storage of NO _x at low temperatures.     

Effects of Lean/Rich Timing and Nature of Reductant on the Performance of a NOx Trap Catalyst

A NO _x trap catalyst was studied in a laboratory reactor under simulated diesel passenger car conditions. The effects of lean/rich duration and the nature of reductant are investigated. At 300°C, the average NO _x conversion decreases with increasing lean duration; conversely the NO _x conversion increases with increasing rich duration. The NO _x conversion at this temperature was found to be a direct function of reaction stoichiometry. That is, the quantity of trapped NO _x under lean conditions must be balanced by the quantity of reductant during the rich trap regeneration step. At extreme temperatures, other factors, reaction kinetics (at lower temperatures) and NO _x storage capacity (at higher temperatures), dominate the NO _x conversion process. Overall, carbon monoxide was found to be the most effective reductant. Hydrocarbon, e.g., C₃H₆, is effective at higher temperatures (T>350°C), while H₂ is more efficient than other reductants at low temperatures (T<200°C). The individual steps of the NO _x conversion process are discussed.     

In situ high temperature FTIR studies of NOx reduction with propylene over Cu/ZSM-5 catalysts

High temperature in situ FTIR has been used to investigate the surface species present on Cu/ZSM-5 during the reduction of NO_x with propylene in a lean environment. Parallels have been observed between adsorbed surface species and catalytic activity for this reaction. Species detected at low temperatures are not representative of those detected at high temperatures where the catalyst is active. An oxidized nitrogen-containing species has been observed at 2580 cm^–1 on Cu during reaction conditions (400°C). In contrast, at low temperatures, where the catalyst is less active, coke and Cu⁺-CO predominated. The effects of Cu weight loading, C/NO ratio, reaction temperature, and catalyst deactivation by steaming have been investigated with IR.     

Long-term stability of the V2O5/Al2O3 catalyst for the selective reduction of nitrogen oxides

Changes of the V₂O₅/Al₃O₃ catalyst aged for up to 10 years under real conditions of the selective catalytic reduction of NO _x by ammonia (SCR) at the tail gases of the nitric acid plant were characterized by⁵¹V NMR spectroscopy, porosimetry, temperature programmed reduction (TPR) and catalytic activity measurements. The catalytic activity and the redox properties of the catalyst were found intact. Only small variations of the ratio of the octahedral and tetrahedral vanadia species were documented by⁵¹V NMR on aged catalyst.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

Use of Double Wash-Coatings of Platinum and Zeolite Catalysts to Improve Selective Reduction of NOx by Hydrocarbons

The selective catalytic reduction by hydrocarbons (HC-SCR) of NO _x under lean conditions has been improved by the use of double-layered catalysts with a lower layer of Pt/SiO₂ and an upper layer of a zeolite such as H-, Ce-, and Cu-ferrierite (-FER). H-FER wash-coated over Pt/SiO₂ (H-FER//Pt/SiO₂) performed best among the samples examined. The promotional effect was attributed to the synergy of the oxidation catalyst (Pt/SiO₂) in converting NO into NO₂, which is more reactive to C₃H₆, and the HC-SCR catalyst (H-FER). Cu-FER//Pt/SiO₂ was also effective at widening the temperature window, but with this combination the performance was attributed to a simple summation of the activity of two HC-SCR catalysts that were active at different temperatures.     

新型铬钴复合氧化物中低温选择性催化NOx还原及原位机理研究

采用固相法合成系列铬钴复合氧化物催化剂,该催化体系在中低温[(180~300)℃]下具有优异的氨选择性催化氮氧化物还原活性,其中,Cr(0.5)-Cr Ox催化剂在空速50 000 h-1、反应温度200℃和220℃条件下,NOx转化率达100%。采用原位DRIFIS研究催化剂表面吸附物种以及催化机理,在反应温度220℃考察Cr(0.5)-Co Ox催化剂表面NH3与NO的吸附态形式和NH3-SCR反应过程中中间态及其反应机理。结果表明,Cr(0.5)-Cr Ox催化剂上NH3吸附在L酸位,也能吸附在B酸位,但只与气态的NOx反应,生成中间体NH2NO,再进一步反应,最终生成N2与H2O。吸附态的NOx不参与SCR反应,反应遵循Eley-Rideal机理。     

Formation of the rhodium oxides Rh2O3 and RhO2 in Rh/NaY

The oxidation states of Rh in NaY supported catalysts have been studied by temperature programmed reduction (TPR). After calcination of the exchanged catalyst to 380°C, both RhO₂ and Rh₂O₃ are identified, besides small amounts of RhO⁺ and Rh³⁺. Quantitative reduction is possible for samples calcined at temperatures not exceeding 500°C. Re-oxidation of the reduced samples leads to formation of RhO₂ and Rh₂O₃, with negligible protonolysis to Rh³⁺. The dioxide prevails after re-oxidation at 320°C, but the sesquioxide after oxidation at 500°C. In the temperature regime where both oxides coexist the reduction of NO with propane is catalyzed even at an O₂/C₃H₈ ratio of 10. Total oxidation of propane reaches 80% at 350°C.     

Synergic effect of Pd/γ-alumina and Cu/ZSM-5 on the performance of NO x storage reduction catalyst

NO _x reduction with a combination of catalysts, Pd catalyst, NO _x storage reduction (NSR) catalyst and Cu/ZSM-5 in turn, was investigated to elucidate for the high NO _x reduction activity of this catalyst combination under oxidative atmosphere with periodic deep rich operation. The catalytic activity was evaluated using the simulated exhaust gases with periodically fluctuation between oxidative and reductive atmospheres, and it was found that the NO _x reduction activity with this catalyst combination was apparently higher than that of the solely accumulation of these individual activities, which was caused by the additional synergic effect by this combination. The Pd catalyst upstream of the NSR catalyst improved NO _x storage ability by NO₂ formation under oxidative atmosphere. The stored NO _x was reduced to NH₃ on the NSR catalyst, and the generated NH₃ was adsorbed on Cu/ZSM-5 downstream of the NSR catalyst under the reductive atmosphere, and subsequently reacted with NO _x on the Cu/ZSM-5 under the oxidative atmosphere.     

Selective Catalytic Reduction of NOx over H-ZSM-5 Under Lean Conditions Using Transient NH3 Supply

The selective catalytic reduction (SCR) of NO _x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO _x , O₂ and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO₂. With transient ammonia supply the total NO reduction increased and the selectivity to N₂O formation decreased compared to continuous supply. The FTIR experiments revealed that NO _x reacts with ammonia adsorbed on Brønsted acid sites as NH₄ ⁺ ions. These experiments further indicated that adsorbed -NO₂ ^– is formed during the SCR reaction over H-ZSM-5.     

Promotional effect of Ce on the activity of In/W–ZrO2 for selective reduction of NO x with methane

The Ce modified In/W–ZrO₂ catalysts were prepared by impregnation and mechanical mix method. Their activities for SCR of NO _x with methane were investigated. The activity of the In/W–ZrO₂ catalyst was enhanced by addition of Ce with both methods, while the promotional effect was more pronounced for catalyst prepared by mechanical mix method compared to impregnation method. The function of Ce was to improve the oxidation of NO to NO₂. The maximum NO _x conversion over the mechanical mixed catalyst can be stabilized at 74% at 450 °C in a dry gas flow and 37% at 500 °C in wet flow (24,000 h⁻¹). For the impregnated catalysts, Ce was found to compete with In to adsorb on strong acid site over W–ZrO₂ support and inhibited the formation of InO⁺, which resulted in the lower activity of these catalysts than mechanical mixed catalysts.     

Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

Effect of primary amine hydrocarbon chain length for the selective catalytic reduction of NOx from diesel engine exhaust

The effect of increasing primary amine hydrocarbon chain length on the SCR of NO_x from diesel engine exhaust was investigated and compared to ammonia. Methylamine (CH₃NH₂), ethylamine (CH₃CH₂NH₂), propylamine (CH₃CH₂CH₂NH₂) and butylamine (CH₃CH₂CH₂CH₂NH₂) were tested using a 12 cell mini core NH₃ - SCR catalyst cut from a 400 cpsi block. There is a steady decrease in NO_x conversion as the length of the hydrocarbon chain increases (from 50% for methylamine to 26% for butylamine). For the same number of carbons in the amine, primary amines are more active reductants than methyl substituted secondary or tertiary amines. For example, ethylamine (NO_x conversion of 45%) is more active than dimethylamine (NO_x conversion of 34%).Since the amines are reactive in the gas phase in the temperature range of diesel engine exhaust, gas phase conversions were estimated by replacing the mini core SCR catalyst with an equivalent length of quartz beads. There was no smooth transition in gas phase NO and NO_x conversions with increasing hydrocarbon chain length. The results suggest a different mechanism for gas phase reactions depending on the nature of the amine.