High-temperature Selective Catalytic Reduction Research Articles

TiO2-coated CeCoOx-PVP catalysts derived from Prussian blue analogue for synergistic elimination of NOx and o-DCB: The coupling of redox and acidity

Achieving efficient selective catalytic reduction (SCR) of NOx and catalytic oxidation of dioxins (o-dichlorobenzene as a model) on one catalyst is still a big challenge. In order to balance the N2 selectivity of SCR and the catalytic oxidation of dioxins, the catalysts with suitable redox properties and acidity need to be designed. Herein, we synthesized Prussian blue analogue (PBA)-derived CeCoOx oxides for the simultaneous removal of NOx and o-DCB by a co-precipitation method. The crystallization rate of PBA was slowed down by the addition of Polyvinylpyrrolidone (PVP), which increased the oxides size, enhanced the crystallinity of oxides and the uniformity of the surface elements distribution. These structural effects enhanced the low-temperature redox performance of the catalysts, and thus improved o-DCB conversion and CO2 selectivity. Furthermore, the PBA structure was coated by introducing TiO2, which improved the amounts of medium-acids and the mobility of oxygen species, thus increasing the mid-and high-temperature SCR performance and N2 selectivity. In addition, the H2O resistance and stability of the catalysts were enhanced due to the protective effect of the TiO2 coating on the PBA core. The optimized CeCoOx-PVP@TiO2 catalyst exhibited stable bifunctional removal efficiencies for NOx and o-DCB, with 90% NOx conversion and 92% o-DCB conversion at 300 ℃. The possible reaction mechanisms for NH3-SCR and the catalytic oxidation of o-DCB over the CeCoOx-PVP@TiO2 catalyst were proposed based on in situ DRIFTS experiments. This work provides an efficient strategy to design advanced catalysts for the elimination of dioxins and NOx.

Preparation of coal fly ash supported high-temperature SCR catalyst by a novel alkali-acid combined activation

Coal fly ash (CFA) is an alternative carrier of selective catalytic reduction (SCR) catalyst, but its reactivity is usually poor. Herein, the alkali-acid combined activation was proposed to prepare the CFA-supported high-temperature SCR catalyst. The results revealed that 1.2 g/gCFA of NaOH and 0.6 g/gCFA of H2SO4 was the optimal CFA activation condition. The NaOH first destroyed the quartz crystal over the CFA, generating the Si-OH groups. Subsequently, H2SO4 enhanced the extraction of aluminates and enriched the acid sites, which promoted the standard SCR. Besides, the generated porous fragments facilitated the dispersion of Fe3+ species and improved the fast SCR.

Kinetic modeling of NH3-selective catalytic reduction (SCR) over a mildly hydrothermally aged commercial Cu-SSZ-13 catalyst

Cu-SSZ-13 is often studied as a selective catalytic reduction (SCR) of NOx catalyst for diesel after-treatment systems. Mild hydrothermal aging (HTA) is commonly known to cause changes in catalyst performance. In this study, a kinetic model of SCR over a Cu-SSZ-13 catalyst that includes mild HTA and its impact on the SCR reaction was developed. The migration of Cu species during HTA, and how this change in Cu distribution affects low-temperature and high-temperature NH3 oxidation and SCR reaction mechanisms, were considered. Flow reactor test results were used for parameter estimation and verification. Different characterization techniques, including H2 temperature-programmed reduction (H2-TPR), NO + NH3-TPR, NH3 temperature-programmed desorption (NH3-TPD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were used to characterize and quantify catalyst site densities. The model can simulate the migration of Cu species and the NOx reduction performance changes during mild hydrothermal aging as a function of aging temperature and time.

Improved hydrothermal durability of Cu-SSZ-13 NH3-SCR catalyst by surface Al modification: Affinity and passivation

To improve the hydrothermal durability of Cu-SSZ-13 selective catalytic reduction (SCR) catalyst, a thin Al2O3 modification layer was coated on the external surface of Cu-SSZ-13 by a chemical liquid deposition (CLD) method. The high-temperature SCR activity of the Al-modified Cu-SSZ-13 can be maintained upon 750–800 °C hydrothermal aging, and over-oxidation of NH3 is largely inhibited. Based on H2-TPR and EPR results, it is revealed that CuAl2O4-like species forms upon Al modification and converts further to CuAlO2-like species during aging, while the accumulation of CuO-like species is inhibited. The high affinity of Al-containing species (i.e., Al2O3 and CuAl2O4-like species) with Cu(OH)2 is suggested by DFT calculations, indicating that Cu(OH)2 detached from ion-exchange sites are captured by Al to form CuAlO2-like species instead of CuO-like species. The passivation of detached Cu species by coated Al leads to somewhat weakened low-temperature SCR activity, but successfully preserves the high-temperature SCR performance upon aging, which can be applied to high-temperature SCR scenarios.

New Insight into the In Situ SO2 Poisoning Mechanism over Cu-SSZ-13 for the Selective Catalytic Reduction of NOx with NH3

To reveal the nature of SO2 poisoning over Cu-SSZ-13 catalyst under actual exhaust conditions, the catalyst was pretreated at 200 and 500 °C in a flow containing NH3, NO, O2, SO2, and H2O. Brunner−Emmet−Teller (BET), X-ray diffraction(XRD), thermo gravimetric analyzer (TGA), ultraviolet Raman spectroscopy (UV Raman), temperature-programmed reduction with H2 (H2-TPR), temperature-programmed desorption of NO+O2 (NO+O2-TPD), NH3-TPD, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and an activity test were utilized to monitor the changes of Cu-SSZ-13 before and after in situ SO2 poisoning. According to the characterization results, the types and generated amount of sulfated species were directly related to poisoning temperature. Three sulfate species, including (NH4)2SO4, CuSO4, and Al2(SO4)3, were found to form on CZ-S-200, while only the latter two sulfate species were observed over CZ-S-500. Furthermore, SO2 poisoning had a negative effect on low-temperature selective catalytic reduction (SCR) activity, which was mainly due to the sulfation of active sites, including Z2Cu, ZCuOH, and Si-O(H)-Al. In contrast, SO2 poisoning had a positive effect on high-temperature SCR activity, owing to the inhibition of the NH3 oxidation reaction. The above findings may be a useful guideline to design excellent SO2-resistant Cu-based zeolite catalysts.

Unraveling the Unexpected Offset Effects of Cd and SO2 Deactivation over CeO2-WO3/TiO2 Catalysts for NOx Reduction.

It is challenging for selective catalytic reduction (SCR) of NOx by NH3 due to the coexistence of heavy metal and SO2 in the flue gas. A thorough probe into deactivation mechanisms is imperative but still lacking. This study unravels unexpected offset effects of Cd and SO2 deactivation over CeO2-WO3/TiO2 catalysts, potential candidates for commercial SCR catalysts. Cd- and SO2-copoisoned catalysts demonstrated higher activity for NOx reduction than a Cd-poisoned catalyst but lower than that for an SO2-poisoned catalyst. In comparison to SO2, Cd had more severe effects on acidic and redox properties, distinctly decreasing the SCR activity. After sulfation of Cd-poisoned catalysts, SO42- preferentially bonded with the surface CdO and released CeO2 active sites poisoned by CdO, thus reserving the highly active CeO2-WO3 sites and maintaining a high activity. The sulfation of Cd-poisoned catalysts also provided more strong acidic sites, and the synergistic effects between the formed cerium sulfate and CeO2 contributed to the high-temperature SCR performance. This work sheds light on the deactivation mechanism of heavy metals and SO2 over CeO2-WO3/TiO2 catalysts and provides an innovative pathway for inventing high-performance SCR catalysts, which have great resistance to heavy metals and SO2 simultaneously. This will be favorable to academic and practical applications in the future.

Re-using of coal-fired fly ash for arsenic vapors in-situ retention before SCR catalyst: Experiments and mechanisms

Arsenic is easily evaporated with coal combustion, which not only causes serious environmental issues but also leads to the deactivation of selective catalytic reduction (SCR) catalyst. This study focused on the re-using of coal-fired fly ash for arsenic vapors in-situ retention before SCR catalyst in the furnace. Experiments were carried out to estimate the effects of typical fly ash compounds (Ca-, Fe-, and Al-bearing components) as well as acid gases (SO2/NO) on arsenic capture at temperatures of high-temperature furnace stage (900 °C) or SCR system entry stage (450 °C). The results demonstrated that, regardless of collection plants, all the ash samples showed certain capacity in arsenic vapors retention and the capture performance was enhanced at 900 °C than at 450 °C. Both physical and chemical adsorptions occurred for arsenic capture at low temperature, while chemical adsorption by effective mineral components dominated at high temperature. The role of Ca-compounds was more remarkable than Fe/Al-compounds and CaSO4/calcium silicates were identified as the key calcium compounds that acted on arsenic adsorption by fly ash. Insignificant effects were found regarding the acid gases (SO2 and NO) on arsenic retention by fly ash owing to the high resistance of CaSO4 and calcium silicates to acid gases. These findings provided reference for the in-situ retention of arsenic by reusing fly ash that enriched in specific compositions.

Core-shell structure effect on CeO2 and TiO2 supported WO3 for the NH3-SCR process

A series of CeTi samples (CeO2/TiO2, TiO2@CeO2, TiO2/CeO2, and CeO2@TiO2) was prepared and modified by tungsten. The 5 %WO3/TiO2@CeO2 (W/Ti@Ce) catalyst exhibited the best NH3-SCR performance (NO conversion and N2 yield above 95 % at 250–500 ºC) due to the synergistic effect of redox ability and acidity. The techniques of XPS, H2-TPR, and NH3-TPD were introduced to reveal the effect of Ce3+ atom contents, redox properties and acidity on catalytic performance. W/Ti@Ce shows good redox ability, which is conducive to the low-temperature selective catalytic reduction (SCR) reaction. NH3-TPD results show that W/Ti@Ce have a large amount of medium- and strong-strength acids, which are conducive to high-temperature SCR reaction. Therefore, the core-shell catalyst has the optimal synergistic effect of redox property and acidity, which is in favor of the excellent NH3-SCR performance. Furthermore, the W/Ti@Ce with a core-shell structure displayed better SO2 resistance due to the stronger interaction between Ce and Ti.

One-step synthesized SO42–-TiO2 with exposed (001) facets and its application in selective catalytic reduction of NO by NH3

A sample of sulfated anatase TiO2 with high-energy (001) facets (TiO2-001) was prepared by a simple one-step hydrothermal route using SO42– as a morphology-controlling agent. After doping ceria, Ce/TiO2-001 was used as the catalyst for selective catalytic reduction (SCR) of NO with NH3. Compared with Ce/P25 (Degussa P25 TiO2) and Ce/P25-S (sulfated P25) catalysts, Ce/TiO2-001 was more suitable for medium- and high-temperature SCR of NO due to the high surface area, sulfation, and the excellent properties of the active-energy (001) facets. All of these facilitated the generation of abundant acidity, chemisorbed oxygen, and activated NOx-adsorption species, which were the important factors for the SCR reaction.

High Temperature and High Concentration SCR of NO with NH3 for the Oxyfuel Combustion Process: Fitting of Kinetics to Data from a Laboratory Reactor Experiment

This study shows the result of a simulation of a proposed kinetic reaction scheme of high-concentration (5000 ppm NO) and high-temperature (570–800 K) selective catalytic reduction of NO with NH3 on a commercial vanadia based catalyst at a system pressure of 2.2 bar. The simulations are performed with COMSOL Multiphysics ver. 5.1. Experimental data from a catalytic flow reactor can be nicely fitted to a system of kinetic expressions for the selective catalytic reduction (SCR) reaction and its side reactions. Especial attention is given to the formation of N2O. All reaction rate expressions contain an ammonia adsorption term. The value for its pre-exponential factor is 9e−9 and its activation energy is −139 kJ/mol. The fastest reaction is the standard SCR reaction with a pre-exponential factor of 5.28e7 s−1 and an activation energy of 85 kJ/mol. This reaction is affected by internal diffusion limitations. The oxidation of NH3 to NO is the second fastest reaction with parameters 5e12 mol/m3s and 200 kJ/mol. The NSCR of NH3 to N2O is the third fastest reaction with values of 7.5e5 s−1 and 110 kJ/mol. The oxidation of NH3 to N2O has the parameters 8e11 mol/m3/s and 190 kJ/mol. At 640 K inlet temperature the rate of the SCR reaction is about 1.3 mol/m3/s. The oxidation of NH3 to NO is 4.7e−4 mol/m3/s. The rate of the NSCR producing N2O is 1.63e−4 mol/m3/s. The rate of the direct oxidation of NH3 to N2O is 3.7e−5 mol/m3/s. Thus the rate of the SCR reaction is about 35,000 faster than the high temperature production of N2O at this temperature.

アンモニア燃焼ガスタービン発電用の低NOx燃焼器開発のための試験設備

Ammonia is focused as a hydrogen energy carrier and carbon free fuel. National Institute of Advanced Industrial Science and Technology (AIST) collaborated with Tohoku University successfully performed an ammonia combustion gas turbine power generation in 2015. However, the weakness of the facilities was that they required large-size Selective Catalytic Reduction (SCR) equipment in order to suppress a high concentration of NOx. Although NOx reduction in the combustion process is desirable, low NOx combustion technology is difficult because ammonia had been thought of as a source of fuel-NO. AIST prepared a combustor test rig to test combustor alone without a compressor or turbine. Sharing with a 1-ton ammonia cylinder, a vaporizer and a gas compressor, combustor test rig was added two engine compressors, air surge tanks, and a high temperature SCR. A regenerator was used to preheat combustion air. Combustor test rig can achieve the same combustion condition as the gas turbine that was successfully demonstrated.

The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH3-Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES.

The small-pore Cu-CHA zeolite is today the object of intensive research efforts to rationalize its outstanding performance in the NH3-assisted selective catalytic reduction (SCR) of harmful nitrogen oxides and to unveil the SCR mechanism. Herein we exploit operando X-ray spectroscopies to monitor the Cu-CHA catalyst in action during NH3-SCR in the 150-400 °C range, targeting Cu oxidation state, mobility, and preferential N or O ligation as a function of reaction temperature. By combining operando XANES, EXAFS, and vtc-XES, we unambiguously identify two distinct regimes for the atomic-scale behavior of Cu active-sites. Low-temperature SCR, up to ∼200 °C, is characterized by balanced populations of Cu(I)/Cu(II) sites and dominated by mobile NH3-solvated Cu-species. From 250 °C upward, in correspondence to the steep increase in catalytic activity, the largely dominant Cu-species are framework-coordinated Cu(II) sites, likely representing the active sites for high-temperature SCR.

Systematic mechanism study of the high temperature SCR of NOX by NH3 over a W/TiO2 catalyst

In this study, the reaction mechanism for the high temperature selective catalytic reduction of NO by NH3 over a W/TiO2 catalyst was investigated. A systematic mechanistic pathway for the reaction was proposed based on the results of transient response analysis (TRA), infrared spectroscopy (IR), temperature programmed reaction (TPR) and O2 shut-off tests. These experimental results showed that during the initial SCR reaction step, ammonia was adsorbed first and activated at Lewis acid sites (W=O), and then was decomposed into N2 and H2O by gaseous NO+O2. During the redox reaction, gaseous oxygen was involved in the reaction, whereas the effect of catalytic lattice oxygen was very slight (WO3→WO2.9X). Therefore, these findings provide important information to better understand the mechanism responsible for high temperature SCR.

High-temperature and high-concentration SCR of NO with NH3: Application in a CCS process for removal of carbon dioxide

This study investigates several commercial selective catalytic reduction (SCR) catalysts (A–E) for application in a high-temperature (approximately 525°C) and high-concentration (5000ppm NO) system in combination with CO2 capture. The suggested process for removing high concentrations of NOx seems plausible and autothermal operation is possible for very high NO concentrations. A key property of the catalyst in this system is its thermal stability. This was tested and modelled with the general power law model using second-order decay of the BET surface area with time. Most of the materials did not have very high thermal stability. The zeolite-based materials could likely be used, but they too need improved stability. The SCR activity and the possible formation of the by-product N2O were determined by measurement in a fixed-bed reactor at 300–525°C. All materials displayed sufficiently high activity for a designed 96% conversion in the twin-bed SCR reactor system proposed. The amount of catalyst needed varied considerably and was much higher for the zeolithic materials. The formation of N2O increased with temperature for almost all materials except the zeolithic ones. The selectivity to N2 production at 525°C was 98.6% for the best material and 95.7% for the worst with 1000ppm NOx in the inlet; at 5000ppm NOx, the values were much better, i.e., 98.3 and 99.9%, respectively.

Deactivation of a Fe-ZSM-5 catalyst during the selective catalytic reduction of NO by n-decane: An operando DRIFT study

The selective catalytic reduction (SCR) of NO by n-decane was investigated on a Fe-ZSM-5 prepared by the FeCl3 sublimation method. NO conversion profiles versus temperature were followed in both temperature programmed surface reaction (TPSR, 10°Cmin−1) and steady state experiments. A higher NO conversion with a maximum of ca. 80% at 400°C is observed in the course of the TPSR tests. This phenomenon has been attributed to strong adsorption of n-decane which protects the active sites against the poisoning. Indeed, in steady state experiments at 390°C the strong decrease of activity as a function of time on stream is due to the polymerisation of conjugated nitriles. This study indicates that long chain alkanes are not the most adequate reductants of NO for high temperature SCR applications. Moreover, due to an easier polymerization of conjugated nitriles on iron zeolites (stronger Fe Lewis sites), this type of catalyst seems less attractive than Cu-zeolite catalysts for the SCR of NO by hydrocarbons in this respect.