Effect of Cu doping on the SCR activity of CeO2 catalyst prepared by citric acid method

CeO₂–CuO catalyst prepared by citric acid method was investigated for selective catalytic reduction of NO with NH₃. The activity of the CeO₂ catalyst was enhanced about 8–27% in the temperature range of 125–225 °C at a space velocity of 28,000 h⁻¹ by the addition of Cu. It was found that the state of Cu species had great impact on the SCR performance of CeO₂–CuO catalyst. Cu²⁺ can enhance the low temperature activity of SCR reaction, while CuO would promote NH₃ oxidation before SCR reaction at high temperature, which would cause the decrease of its high temperature SCR activity.     

Low-temperature SCR of NO with NH3 over noble metal promoted Fe-ZSM-5 catalysts

We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia at high temperatures (300–400 °C). In this work, we found that the reaction temperature could be decreased to 200–300 °C when a small amount of noble metal (Pt, Rh, or Pd) was added to the Fe-ZSM-5. The SCR activity follows the order Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5 at 250 °C. On the Pt promoted Fe-ZSM-5, 90% NO conversion was obtained at 250 °C at GHSV = 1.1 × 10⁵ h^–1. Moreover, the noble metal improved the resistance to H₂O and SO₂. The presence of H₂O and SO₂ decreased the SCR performance only very slightly.     

Unifying redox kinetics for standard and fast NH3‐SCR over a V2O5‐WO3/TiO2 catalyst

A dynamic Mars–van Krevelen kinetic model that unifies Standard and Fast SCR reactions into a single redox approach is herein proposed for V‐based catalysts for NO_x removal from Diesel exhausts. Such a mechanistic model is consistent with the detailed catalytic chemistry proposed for the NH₃‐NO/NO₂ reacting system in which NO₂ disproportionates to form nitrites and nitrates, nitrates are reduced by NO to nitrites in a key redox step, and nitrites react with NH₃ to form N₂ via decomposition of unstable ammonium nitrite. Intrinsic kinetic parameters were estimated by global multiresponse nonlinear regression of 42 transient runs. The model accounts for stoichiometry, selectivity, and kinetics of the global SCR process, reproducing successfully both the steady‐state and transient behaviors of the SCR reacting system over the full range (0–1) of NO₂/NO_x feed ratios in the 175–425°C temperature range. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Impacts of acid gases on mercury oxidation across SCR catalyst

A series of bench-scale experiments were completed to evaluate acid gases of HCl, SO₂, and SO₃ on mercury oxidation across a commercial selective catalytic reduction (SCR) catalyst. The SCR catalyst was placed in a simulated flue gas stream containing O₂, CO₂, H₂O, NO, NO₂, and NH₃, and N₂. HCl, SO₂, and SO₃ were added to the gas stream either separately or in combination to investigate their interactions with mercury over the SCR catalyst. The compositions of the simulated flue gas represent a medium-sulfur and low- to medium-chlorine coal that could represent either bituminous or subbituminous. The experimental data indicated that 5–50 ppm HCl in flue gas enhanced mercury oxidation within the SCR catalyst, possibly because of the reactive chlorine species formed through catalytic reactions. An addition of 5 ppm HCl in the simulated flue gas resulted in mercury oxidation of 45% across the SCR compared to only 4% mercury oxidation when 1 ppm HCl is in the flue gas. As HCl concentration increased to 50 ppm, 63% of Hg oxidation was reached. SO₂ and SO₃ showed a mitigating effect on mercury chlorination to some degree, depending on the concentrations of SO₂ and SO₃, by competing against HCl for SCR adsorption sites. High levels of acid gases of HCl (50 ppm), SO₂ (2000 ppm), and SO₃ (50 ppm) in the flue gas deteriorate mercury adsorption on the SCR catalyst.     

Bench-scale study of interactions between flue gas and cofired ash in an SCR

Samples of ash collected from a full-scale utility boiler cofiring 80% wood waste with 20% Powder River Basin (PRB) coal were mixed with ground selective catalytic reduction (SCR) catalyst and exposed to simulated flue gas. Changes in mass were recorded with time, and mass gains were found to be highest without SCR catalyst present. Ash samples were analyzed before and after testing to determine what mechanisms had led to mass gain. The ash had reacted with gas-phase SO₂ to form solid sulfates. Mass gain by sulfation would likely cause ash particles to grow and cover catalyst pores in the field, leading to catalyst deactivation and reduced NO_x control.     

Effect of SO2 on a cordierite honeycomb supported CuO catalyst for NO reduction by NH3

Supporting CuO on a Al₂O₃-coated cordierite honeycomb yields a good catalyst (CuO/HC–Al) for selective catalytic reduction (SCR) of NO with NH₃ at 350–500 °C. SO₂ has complex effects on the catalysts activity. It significantly promotes the SCR activity through conversion of CuO to CuSO₄, however, when a certain amount of CuO is converted, it slightly decreases the SCR activity through competitive adsorption with NH₃. This competitive adsorption reduces the amount of NH₃ adsorbed on the catalyst surface, especially on the sites highly active to the SCR. It also prevents transformation of CuO to CuSO₄ and as a result, the catalysts subjected to pre-sulfation and in situ sulfation show different SCR behaviors.     

Manganese-enhanced porous phosphoric acid–based geopolymer templated by carbon for efficient NH3-SCR of NOx

Geopolymer-based ceramics as catalysts or catalyst supports have attracted tremendous interests in recent years owing to their low-cost and zeolite-like structure characteristics. However, most of the reported works focus on alkaline-based geopolymers, whereas the catalytic performance of acid based geopolymer has not yet been evaluated. This study aims to investigate the application potential of phosphoric acid–based geopolymer (PAG) for selective catalytic reduction (SCR) of NO_x with NH₃. To this end, the SCR reactivity of PAG and metal oxide (MnO_x)-loaded PAG were evaluated. Moreover, an activated carbon-based hard template route was proposed for further enhancing the SCR reactivity of the PAG-based catalyst. The as-prepared catalyst under the optimal condition exhibited a high NO conversion greater than 85% in a wide temperature range of 250–350°C, which is among the top literature-reported values, demonstrating its promising application prospect. A systematical X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, Brunauer–Emmet–Teller, NH₃-temperature-programed desorption, and H₂-temperature-programed reduction spectroscopic analyses were also conducted to better understand the structure evolution of PAG under elevated temperature and the SCR catalytic mechanism of the acid-based geopolymer catalysts. This study would provide valuable information on the potential application prospect of PAG and its modified form for efficient NO_x removal.     

Combined use of a mass-spectrometer and a UV analyzer in the dynamic study of NH3-SCR for diesel exhaust aftertreatment

A mass spectrometer (MS) and a novel UV analyzer were coupled in the experimental study of the transient Selective Catalytic Reduction (SCR) reactivity, aimed at the development of a dynamic numerical model of SCR converters for the selective reduction of NO _x in Diesel exhausts. Their parallel use revealed an effective method for the understanding of key elements in the SCR reaction mechanism under real operating conditions.     

Simulation of NOx and soot abatement with Cu‐Cha and Fe‐ZSM5 catalysts

The embodiment of the NO_x selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so‐called SCR‐on‐Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu‐Chabazite and Fe‐ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NO_x conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60–80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu‐Chabazite and Fe‐ZSM5 catalysts, as well as in‐series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238–248, 2017     

Effect of preparation methods on the performance of CeO2/Al2O3 catalysts for selective catalytic reduction of NO with NH3

CeO₂/Al₂O₃ catalysts prepared by three methods were investigated for selective reduction of NO with NH₃. It was found that the catalyst prepared by the single step sol–gel method had the best SCR activity and SO₂ resistance performance. From the results of BET, XRD, TPD and TPR, it can be concluded that large surface area, strong interaction, highly dispersed nano-crystalline ceria, high NH₃ adsorption capacity and good redox ability might be the main reasons for the excellent performance of CeO₂/Al₂O₃ catalyst prepared by the single step sol–gel method.     

A study of copper-exchanged mordenite natural and ZSM-5 zeolites as SCR–NOx catalysts for diesel road vehicles: Simulation by neural networks approach

Copper-catalysts, based on the ZSM-5 (CuZSM5) and Cuban natural Mordenite (CuMORD) zeolites have been prepared by a conventional ion-exchange method and their catalytic activity in the selective catalytic reduction (SCR) of NO was studied using ammonia in presence of H₂O and SO₂. A commercial catalyst SCR (CATCO) based on V₂O₅–WO₃–TiO₂, was also studied as reference. This paper presents experimental results using catalysts without the toxic vanadium and exploits a neural network based approach to predict NO_x conversion efficiency of three SCR catalysts. The derived mathematical functions are integrated in a numerical model for diesel road vehicle simulation to simulate diesel vehicles equipped with such SCR catalysts. The main results indicate that despite of toxic vanadium and N₂O formation, CATCO shows the better NO_x conversion efficiencies. However, CuMORD does not form N₂O and have better performance than the CuZSM5. The simulation results show lower level of NO_x for heavy-duty and light-duty diesel vehicles compared with homologation load cycles.     

Deactivation mechanism of activated carbon supported copper oxide SCR catalysts in C2H4 reductant

Current state‐of‐the‐art NH₃‐SCR technology based on vanadium catalysts suffers problems associated with NH₃ slip and poisoning of the catalyst and blockage of heat recovery steam generators (HRSG). If environmentally‐friendly catalysts capable of efficient operation at lower temperatures could be developed that used a reductant other than NH₃, the issues with current state‐of‐the‐art SCR could be significantly lessened. Hence, in this study, activated carbon (AC) supported copper oxide‐based catalysts for SCR while using C₂H₄ as a reductant was discussed. Reaction testing of catalysts demonstrated high initial NO conversion with steeply declining activity over 2 h of testing when C₂H₄ was used as the reductant; in comparison, with the same catalyst and NH₃ as the reductant, stable, long‐term NO conversion was achieved, but at a lower rate than the initial reactivity with C₂H₄. As a consequence, catalyst characterization studies were performed to assess deactivation mechanisms when C₂H₄ was the reductant. These studies included x‐ray diffraction, BET surface area and porosity, temperature programmed reduction, scanning electron microscopy, Raman spectroscopy and x‐ray photoelectron spectroscopy of both fresh and deactivated catalysts. The analytical results showed the surface area and porosity of the catalyst remained unchanged and the initially highly‐dispersed Cu species became agglomerated and more crystalline during reaction testing. Furthermore, carbon black was also detected on the catalyst surface after testing, presumably formed during the decomposition of C₂H₄. Both agglomeration of the active Cu species and blockage by carbon deposits would decrease the availability of active sites and lead to decreased catalytic activity.     

Enhanced catalytic activity for selective catalytic reduction of NO over titanium nanotube-confined CeO2 catalyst

Titanium nanotubes (TNTs)-confined ceria were for the first time prepared in this paper and used for selective catalytic reduction (SCR) of NO with ammonia. In comparison with the catalysts supported by TiO₂ nanoparticles, the confined ceria showed a superiority in SCR of NO due to the improved redox potential and special adsorption of NH₃, where its NO conversion could exceed 95% at reaction temperature of 270–500 °C, which was much higher than that of TiO₂ nanoparticle supported catalysts.     

Simulation of SCR equipped vehicles using iron-zeolite catalysts

Iron-catalysts, based on ZSM-5 (FeZSM5) and Cuban natural Mordenite (FeMORD) zeolites have been prepared by a conventional ion-exchange method and their catalytic activity in the selective catalytic reduction (SCR) of NO with ammonia was studied in the presence of H₂O and SO₂. A commercial SCR catalyst (CATCO) based on V₂O₅–WO₃–TiO₂, was also studied as a reference. This paper presents the experimental results of using these catalysts without toxic vanadium and also exploits a neural network-based approach to predict NO_x conversion efficiency of three SCR catalysts. The mathematical functions derived have been integrated into a numerical model to simulate diesel road vehicles equipped with SCR catalysts such as those studied here. The main results indicate that despite toxic vanadium and N₂O formation, CATCO shows better NO_x conversion efficiencies. However, FeMORD does not produce N₂O and performs better than the FeZSM5. The simulation results on real cycles show lower level of NO_x for heavy-duty and light-duty diesel vehicles compared with homologation load cycles.     

Transient TPSR, DRIFTS-MS and TGA studies of a Pd/ceria-zirconia catalyst in CH4 and NO2 atmospheres

In this study, the parameters governing the activity of Pd/ceria-zirconia catalysts in the selective catalytic reduction (SCR) of NO_x assisted by methane are investigated using a combination of temperature-programmed spectroscopic and thermogravimetric techniques and transient SCR conditions. By DRIFTS of adsorbed CO, it is established that Pd species on Ce_0.2Zr_0.8O₂ are mainly present in cationic form but exhibit high reducibility. As found by temperature-programmed surface reaction (TPSR) in CH₄ + NO₂ atmosphere, the CH₄-SCR reaction is initiated at 280 °C on Pd/Ce_0.2Zr_0.8O₂ and yields almost 100% N₂ above 500 °C. DRIFTS-MS and TGA experiments performed under transient SCR conditions show that DeNO_x activity is due to a surface reaction between some methane oxidation products on reduced Pd sites with ad-N_xO_y species presumably located on the support. The detrimental effect of O₂ on DeNO_x is explained by the promotion of the total combustion of methane assisted by the ceria-zirconia component at the expense of the SCR reaction above 320 °C.     

A kinetic model of the hydrogen assisted selective catalytic reduction of NO with ammonia over Ag/Al2O3

A global kinetic model which describes H₂‐assisted NH₃‐SCR over an Ag/Al₂O₃ monolith catalyst has been developed. The intention is that the model can be applied for dosing NH₃ and H₂ to an Ag/Al₂O₃ catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH₃‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO_x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH₃ and H₂ of 1:1:2. Further reactions included in the model are the oxidation of NH₃ to N₂ and NO, oxidation of H₂, and the adsorption and desorption of NH₃. The model was fitted to the results of an NH₃‐TPD experiment, an NH₃ oxidation experiment, and a series of H₂‐assisted NH₃‐SCR steady‐state experiments. The model predicts the conversion of NO_x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013     

Entfernung von Stickstoffoxiden aus Sauerstoff enthaltenden Automobil‐Abgasen

The most important systems for downstream reduction of NO_x under lean conditions of exhaust gases are discussed. The systems operate either continuously under direct chemical reduction of NO_x to nitrogen, or discontinuously with prior adsorption. The selective catalytic reduction (SCR) of nitrogen oxides using ammonia (processed in an NH₃ generator) or ammonia‐forming substances (urea as solution or solid) as reducing agents is almost at the application stage in trucks, while hydrocarbons as reducing agents (so‐called HC‐SCR) are not sufficiently selective for NO_x reduction because the hydrocarbons are themselves predominantly oxidized. The principle of NO_x storage as nitrates followed by regeneration and reduction to nitrogen requires no separate or external reducing agents and claims therefore to meet future emission standards as long as problematic sulfur incorporation is avoided. The common characteristics of all these NO_x abatement systems is that they are all based on the principle »oxidation before reduction« and involve the key molecule NO₂.     

燃煤电厂真实烟气条件下SCR催化剂脱硝性能

制备了具有工业应用价值的V₂O₅/TiO₂负载WO₃与MoO₃的蜂窝状SCR催化剂,并在某燃煤电厂烟气环境下,对其催化性能进行了测试,结果表明:增大催化剂体积能够提高脱硝效率,但活性增量逐步下降,当脱硝效率达到80%以上时,继续增大催化剂体积其活性提高不显著。催化剂的脱硝活性随着积灰时间的延长而降低,脱硝系统运行4 h后,催化剂脱硝活性减少9.4%。在燃煤电厂尾气飞灰浓度约为0.031 kg·m^-3环境下,为满足尾气中NO_x浓度排放要求(<200 mg·m^-3),每连续运行6 h需进行一次吹灰。催化剂的脱硝性能随着运行时间的延长,先急剧下降,后缓慢降低。在连续运行48~168 h范围内,脱硝活性下降值小于1%,SO₂氧化率下降值小于0.1%。在12个月内,平均每连续运行700 h,脱硝活性降低1%。     

A new synthesis method for supported composite oxides: preparation of Ce-Cu/TiO2 catalysts by ice-melting method

In SCR denitrification technology, the conventional coprecipitation method has the disadvantages of high temperature and difficulty in controlling the precipitation rate. Various Ce_xCuTiO₂ catalysts were synthesized using ice-melting (Ce_xCuTi-Ice) and conventional coprecipitation (Ce_xCuTi-Con) methods for the selective catalytic reduction (SCR) of NO with NH₃. Ce_0.4CuTi-Ice catalyst exhibited excellent catalytic activity among the Ce_xCuTiO₂ catalysts, and 80% NO_x conversion was achieved within a temperature range of 250–375 °C, exceeding that of Ce_0.4CuTi-Con catalyst by 20%; in addition, N₂ selectivity was nearly 100%. To elucidate the release characteristics of the precursor solution during ice-melting synthesis, the changes of Cu and Ce concentrations in the solution were investigated by ICP-OES. The precursor solution was released at a slow rate via the ice-melting method, resulting in a large surface area, small crystallite sizes, and effective uniform nanoparticles with abundant active species and increased surface acidity. The promoted mechanism could be attributed to the enhanced oxidation of NO to NO₂ at low temperatures and the rapid reaction between NO species and coordinated NH₃ at high temperatures. © 2022 Society of Chemical Industry (SCI).     

Kinetic modeling of Fe‐BEA as NH3‐SCR catalyst—effect of phosphorous

The focus of this work is to investigate whether a previously developed microkinetic deactivation model for hydrothermally treated Fe‐BEA as NH₃‐SCR catalyst can be applied to describe chemical deactivation of Fe‐BEA due to phosphorous exposure. The model describes the experiments well for Fe‐BEA before and after phosphorous exposure by decreasing the site density, representing deactivation of sites due to formation of metaphosphates blocking the active iron sites, while the kinetic parameters are kept constant. Furthermore, the results show that the activity for low‐temperature selective catalytic reduction (SCR) is very sensitive to loss of active monomeric iron species due to phosphorous poisoning compared to high‐temperature SCR. Finally, the ammonia inhibition simulations show that exposure to phosphorous may affect the internal transport of ammonia between ammonia storage sites buffering the active iron sites, which results in a lower SCR performance during transient conditions. © 2014 American Institute of Chemical Engineers AIChE J, 61: 215–223, 2015