Cooperative effect induced by the mixing of Na-ZSM-5 and Pd/H3PW12O40/SiO2 in the selective catalytic reduction of NO with aromatic hydrocarbons

Pd was loaded on the dispersed H₃PW₁₂O₄₀ (HPW) over the SiO₂ surface, and the catalyst was applied to the selective reduction of NO with aromatic hydrocarbons. The catalyst exhibited high activity in the NO reduction when branched aromatic hydrocarbons, such as toluene and xylene, were used as reductants. The catalytic activity of Pd/HPW/SiO₂ was improved remarkably by physically mixing it with Na-ZSM-5. From the temperature programmed desorption (TPD) of toluene and the analysis of the products, it was inferred that the activity was enhanced when Pd/HPW/SiO₂ and Na-ZSM-5 were mixed. In other words, aromatic hydrocarbons were partially oxidized to yield oxygenated hydrocarbons, e.g., benzaldehyde and phthalic anhydride, over Pd/Na-ZSM-5; in this reaction, a part of Pd migrated from Pd/HPW/SiO₂ to Na-ZSM-5 during the course of the physical mixing procedure. Subsequently, the oxygenated hydrocarbons reacted with NO entrapped with HPW over Pd to yield N₂.     

On the quantitative aspects of hydrolysis of isocyanic acid on TiO2

The selective catalytic reduction with aqueous solutions of urea is currently seen having the highest potential to reduce NO_x and particulate emissions for commercial diesel powered vehicles. Ammonia as the actual reduction medium is formed from urea in two consecutive reactions, i.e. via the thermolysis of urea to isocyanic acid and NH₃ and the catalyzed hydrolysis of HNCO over TiO₂ to NH₃ and CO₂. A kinetic model for the hydrolysis reaction was derived for a reaction scheme comprising a set of elementary steps. To minimize the number of unknown variables in the kinetic model for the overall rate, the equilibrium constants for both reactants (HNCO and H₂O) and products (NH₃ and CO₂) were determined from adsorption isotherms using Langmuir and multilayer adsorption models. A data set consisting of 49 data points for the rate determined at varying reactant concentrations was fitted with the kinetic model using a non-linear least mean squares regression analysis.     

Selective catalytic reduction of NO and NO2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO₂ is a powerful means to enhance the NO_x conversion over a given SCR catalyst. NO₂ fractions in excess of 50% of total NO_x should be avoided because the reaction with NO₂ only is slower than the standard SCR reaction.

At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH₃ and NO₂ react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH₃ and NO₂ form ammonium nitrate in close analogy to a NO_x storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation.

The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO₂/NO_x ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO₂.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

Support effects in Pt/TiO2–ZrO2 catalysts for NO reduction with CH4

ZrO₂–TiO₂ mixed oxides, prepared using the sol–gel method, were used as supports for platinum catalysts. The effects of catalyst pre-reduction and surface acidity on the performance of Pt/ZT catalysts for the reduction of NO with CH₄ were studied. The diffuse reflectance infrared Fourier transformed (DRIFT) spectra of CO adsorbed on the Pt/ZT catalysts, and also on the Pt/T and Pt/Z references, pre-reduced at 773 K in hydrogen, revealed that an SMSI state is developed in the Ti-rich oxide-supported platinum catalysts. However, no shift in the binding energy of Pt 4f_7/2 level for Pt/T and Pt deposited on Ti-rich support counterparts pre-reduced at 773 K was found by photoelectron spectroscopy. The DRIFT spectra of the catalysts under the NO+O₂ co-adsorption revealed the appearance of nitrite/nitrate species on the surface of the Zr-containing catalysts, which displayed acidic properties, but were almost absent in the Pt/T catalyst. The intensity of these bands reached a maximum for the Pt/ZT(1:1) catalyst, which in turn exhibited a larger specific area. In the absence of oxygen in the feed stream, the NO+CH₄ reaction showed DRIFT spectra assigned to surface isocyano species. Since the intensity of this band is higher for the Pt/ZT (9:1) catalyst, it seems that such species are developed at the Pt–support interface.     

Co–ZSM-5 catalysts in the decomposition of N2O and the SCR of NO with CH4: Influence of preparation method and cobalt loading

Co–ZSM-5 prepared via different methods with Co/Al ratios ranging from 0.03 to 0.83 are investigated in both the direct N₂O decomposition and the selective catalytic reduction (SCR) of NO with CH₄. UV–vis and H₂-TPR are used to get an insight in the active species in these reactions. It is observed that in catalysts with low Co loadings (Co/Al < 0.3) Co is predominantly present as mono-atomic Co species, located at ion exchange positions in ZSM-5. Higher Co loadings result in the formation of different kinds of Co-oxides, which constitute the majority of species in the over-exchanged catalysts (Co/Al > 0.5). The mono-atomic species show the highest activity in the direct decomposition of N₂O, whereas the oxidic Co species do not seem to contribute much to the overall decomposition. In the SCR, the Co-oxide species catalyze the combustion of CH₄ whereas the selectivity towards NO reduction is much increased at low Co loadings. Therefore, over-exchange of Co–ZSM-5 does not seem to be favorable for both the direct N₂O decomposition and the SCR of NO with CH₄. Co/Al ratios <0.3 give the best results both in terms of conversion and activity per Co atom in both reactions.     

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.     

NO reduction with NH3 over chromia–vanadia catalysts supported on TiO2: an in situ Raman spectroscopic study

In situ Raman spectroscopy was used for studying the ternary 2% CrO₃–6% V₂O₅/TiO₂ catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH₃. The structural properties of this catalyst were studied under NH₃/NO/O₂/N₂/SO₂/H₂O atmospheres at temperatures up to 400 °C and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed.     

Pt/K–βAl2O3 solid electrolyte cell as a “smart electrochemical catalyst” for the effective removal of NOx under wet reaction conditions

This study has shown that the phenomenon of electrochemical promotion can be used to activate a metal catalyst for the selective catalytic reduction of nitrogen oxides (NO_x) in the presence of water in the feed. The application of different potentials optimized the catalytic performance of the Pt catalyst-working electrode at each reaction temperature range. In addition, the measurement of the open circuit voltage in the cell gave useful information on the competitive adsorption between the reactants. Thus, very interestingly, the combined use of the Pt/K–βAl₂O₃ cell as a sensor and as an electrochemical catalyst allows anticipating and optimizing the catalytic behaviour of the system under changing reaction conditions, such as those found in the exhaust of an engine. Finally, characterization of the cell by Cyclic Voltammetry along with NO_x analysis and XRD provided useful information about the nature of the promoter species under wet reaction conditions.     

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

The influence of NO₂ on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO₂ in the feed drastically enhanced the NO_x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO₂/NO_x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH₃:NO:NO₂ ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO₂/NO_x = 100%. The addition of NO₂ to the feed gas was always accompanied by the production of N₂O at lower and intermediate temperatures. The absence of N₂O at the high temperature end is explained by the N₂O decomposition and N₂O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO₂ and water suppresses the oxidation of NO to NO₂, which is an essential preceding step of the actual SCR reaction for NO₂/NO_x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N₂ is formed with NO and the side-product N₂O by reaction with gas phase NO₂.     

Formation of isocyanic acid during the reaction of mixtures of NO, CO and H2 over supported platinum catalysts

On-line infrared spectroscopy has been used to show that isocyanic acid, HNCO, is a substantial primary product of the reaction between NO, CO and H₂ over a Pt/SiO₂ catalyst. At 230°C it accounts for more than 40% of the CO converted. No isocyanic acid is seen when Al₂O₃ is placed downstream of Pt/SiO₂, or when using a Pt/Al₂O₃ catalyst under the same conditions, due to rapid hydrolysis of HNCO on the alumina. Isocyanic acid may exist as a trace intermediate in automobile catalytic converters during their warm-up phase but it is unlikely to emerge from the pore system of aluminabased catalysts.     

A practical scale evaluation of sulfated V2O5/TiO2 catalyst from metatitanic acid for selective catalytic reduction of NO by NH3

The characteristics of sulfated V₂O₅/TiO₂ honeycomb catalyst from metatitanic acid (MTA) were studied in the practical conditions of pilot plant using high dust flue gas from coal fired utility boiler. The effects of reaction temperature, NH₃/NO mole ratio, space velocity and operation time on the reduction of nitric oxide (NO) were mainly investigated for engineering application. The catalyst showed high NO reduction of about 90% at a space velocity of 4000 h⁻¹, NH₃/NO mole ratio of 1.0 and reaction temperature of 300–400 °C. The efficiency of this catalyst remained constant during the present experiment of 2400 h and the erosion by fly ash was lower than that of the commercial catalysts. These results clearly demonstrate the high potential for this catalyst to be applied commercially for the control of NO_x emissions from coal fired utility boiler.     

Formation of active sites on Ir/WO3–SiO2 for selective catalytic reduction of NO by CO

Pretreatment conditions for the activation of Ir/WO₃–SiO₂ for the selective catalytic reduction of NO by CO in the presence of excess O₂ were studied. Sequential treatment involving calcination in the presence of O₂ and H₂O followed by reduction and then re-oxidation under mild conditions was found to effectively activate Ir/WO₃–SiO₂. Temperature-programmed desorption during calcination, X-ray diffraction, and temperature-programmed reduction by H₂ revealed that calcination was necessary for oxidative removal of the NH₃ ligands from the iridium precursor, that reduction produced metallic iridium and partially reduced tungsten oxide, and that re-oxidation produced tungsten oxide with low reducibility. Transmission electron microscopy revealed that Ir was supported on finely dispersed tungsten oxide and that the iridium particle size after the sequential activation was 1–1.5 nm.     

Role of zeolite structure on NO reduction with diesel fuel over Pt supported zeolite catalysts

A highly desirable selective catalytic reduction (SCR) of NO with real life diesel fuel over Pt supported zeolites with different topologies (Pt-ZSM-5, Pt-FER, Pt-MOR and Pt-BEA) is studied under simulated exhaust conditions. The catalysts are characterized by CO chemisorption, NH₃-TPD and TGA. The NO conversion ability of these catalysts has been correlated with zeolite structure and acidity. Pt-MOR is found to be the most active catalyst, 90% NO conversion at 300 °C, however Pt-FER showed highly desirable low temperature window, 77% NO conversion below 260 °C. Over ZSM-5, BEA and Y with three dimensional pore structures extensive carbonaceous deposits are observed by TGA which are detrimental to NO conversion. On the other hand, FER zeolite having one dimensional pore structure did not allow extensive coke formation resulting in a highly desired low temperature NO conversion. The results suggest that, NO reduction mainly take place near the zeolite pore opening, which is in reasonable agreement considering the long and bulky molecules in diesel fuel.     

Electrochemical promotion of NO reduction by C2H4 in 10% O2 using a monolithic electropromoted reactor with Rh/YSZ/Pt elements

The reduction of NO by C₂H₄ in high excess of O₂ and temperatures 200−300 °C was investigated using a monolithic electropromoted reactor (MEPR) with twenty-two Rh/YSZ/Pt parallel plate elements. It was found that at 220–240 °C and 10% O₂ the selective catalytic reduction (SCR) of NO can be electropromoted by 450% with near 100% selectivity to N₂ and Λ_NO values up to 2.4. The corresponding rate enhancement ratio of complete C₂H₄ oxidation is up to 900% with Faradaic efficiency, , values up to 350. The system appears promising for practical applications.     

制备条件对Cu/Al2O3 选择性催化还原NO的影响研究

考察了制备方法、活性组分负载量和焙烧温度对Cu/Al₂O₃ 选择性催化还原NO的影响。结果表明，采用溶胶-凝胶+浸渍法制备的Cu/Al₂O₃催化剂活性最好；负载Cu质量分数为15%时，催化剂的活性温域最宽，最大活性温度最低，催化活性最好；最佳焙烧温度为750 ℃。     

堇青石蜂窝陶瓷载CuO选择催化还原NO的研究

以堇青石蜂窝陶瓷(CC)为载体、CuO和不同助剂为活性组分，用浸渍法制备CuO/CC、CuO-NiO/CC和CuO-NiO-CeO₂/CC催化剂, 采用TPR、XRD和XPS等测试方法对催化剂进行表征。TPR结果表明，催化剂主要以Cu²⁺形式存在。采用程序升温和恒温法在固定床反应器常压条件下研究了以尿素作还原剂还原模拟汽车尾气中的NO。结果显示，CuO-NiO-CeO₂/CC催化剂在(150～350) ℃具有较高的活性，250 ℃时具有较高的转化率。     

Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5

The reaction kinetics of selective catalytic reduction (SCR) by NH₃ on NO (standard SCR) and on NO + NO₂ (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N₂ production rate was transiently promoted in the absence of gaseous NH₃; this enhancement can be attributed to the negative reaction order of NH₃ (between −0.21 and −0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NO_ad and O_ad to form NO₂. In the fast SCR, however, the promotion behavior in the absence of gaseous NH₃ was not observed and the apparent NH₃ order changed from positive to negative with NH₃ concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO₂ and NH₃ was occurring due to strong NO₂ adsorption; this must be the main reason for the absence of the promotion effect.     

Selective reduction of NO with CO in the presence of O2 with Ir/WO3 catalysts: Influence of preparation variables on the catalytic performance

Several Ir/WO₃ catalysts were prepared, which were different in the loading of Ir (up to 10 wt.%) and in the pretreatment conditions, and used for the reduction of NO (500 ppm) with CO (3000 ppm) in the presence of excess O₂ (5%), SO₂ (2 ppm), and H₂O (1%). The rate of NO conversion per unit mole of Ir was found to be maximal at a small Ir loading of 0.5–1.0 wt.%. The good catalytic performance was achieved when the Ir precursors (IrO_x) supported on WO₃ were reduced under mild conditions by He at 400 °C or H₂/He at a lower temperature of 130 °C. The catalysts were characterized by XRD, FTIR after CO adsorption, and H₂-TPR. The characterization results show that the active sites are exposed zero-valent Ir species, in accordance with previous works using Ir on WO₃ and other supports, and that the fraction of these active species is larger for a higher degree of Ir dispersion (smaller Ir particles). The preparation conditions for highly active Ir/WO₃ catalysts were confirmed and possible reaction mechanisms were discussed.     

Development of carbon supported metal catalysts for the simultaneous reduction of NO and N2O

The development of an activated carbon supported bimetallic catalyst for the simultaneous reduction of NO and N₂O is described. Base metal catalysts were found to deactivate due to the oxidation of the metallic phase, suggesting the use of a noble metal. Platinum catalysts were very active for NO reduction, but they lacked selectivity towards N₂, since N₂O was produced. The association of Pt and K, a good N₂O reduction catalyst, was capable of achieving the complete conversion of both gases at 350 °C, the catalyst being stable over extended periods of time (up to 17 h). A synergistic effect between Pt and K was observed, similar to the effect previously reported for Ni and K.