Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

单空缺石墨烯负载的Pd单原子催化剂上NO还原的密度泛函理论研究

采用密度泛函理论（DFT）方法对单空缺石墨烯负载的Pd单原子（Pd/SVG）催化剂上H₂还原NO的反应进行了研究,探究了Pd/SVG上NO还原生成N₂和NH₃的路径。在Pd/SVG上NO容易加氢形成HNO,需要的活化能为67.0 kJ·mol^-1,显示了极高的催化活性。N₂生成的有利路径为NO活化生成HNO后,HNO继续加氢生成中间体NH₂O和NH₂OH,然后NH₂OH解离生成NH₂和OH,生成的NH₂中间体结合NO形成NH₂NO,然后NH₂NO异构化形成的NHNOH再经解离生成N₂与H₂O,这个过程中的决速步骤为NH₂NO分子内氢转移生成NHNOH,能垒为144.3 kJ·mol^-1。对于NH₃的生成,从NO的活化到中间体NH₂的形成与N₂的形成过程相同,最后NH₂加氢即可形成NH₃,这个过程中的决速步骤为NH₂O加氢生成NH₂OH,能垒为86.4 kJ·mol^-1。比较生成N₂和NH₃的决速步能垒可见,Pd/SVG催化剂上NO经H₂还原更容易形成NH₃。本研究为石墨烯负载型Pd基催化剂上H₂还原NO的实验及工业应用提供理论参考。     

A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/γ-Al2O3 under strongly oxidising conditions

The activity and excellent selectivity (>90%) of γ-Al₂O₃-supported Ni for the selective catalytic oxidation (SCO) of NH₃ to N₂ with excess O₂ has been shown by microreactor studies. Further studies of the mechanism involved in this reaction have been carried out using TPD, TPO, TPReaction as well as DRIFTS. N₂H₄ and NO have been used to model the intermediates of the SCO mechanism (direct formation of N₂ via the recombination of two NH_x species) and of the in situ SCR mechanism (two-step formation of N₂ via the reduction of an in situ produced NO species by a NH_x species), respectively. Two IR absorption bands appear during the TPO of NH₃ in the temperature range of N₂ formation and have been assigned to stable bidentate nitrate surface species. This represents strong evidence that under the present conditions, formation of N₂ occurs via the in situ SCR mechanism. This also explains the sudden “NO jump” observed on various systems once the temperature is high enough to activate 50% of the NH₃ molecules fed to the catalyst. The fact that NO and NH₃ are able to react to give N₂ at low temperature (from 100°C) confirms that activation of NH₃ is the limiting step. In contrast, no evidence has been found to support the possibility of the SCO mechanism.     

微波热解制备g-Fe2O3催化剂及其SCR脱硝性能

以FeSO₄·7H₂O[Fe(NO₃)₃·9H₂O]为铁源,采用新型微波热解法制备γ-Fe₂O₃[a-Fe₂O₃]催化剂样品,通过XRD、N₂等温吸附-脱附、压汞法等实验手段对催化剂样品晶相、微观孔结构等进行表征;考察两种催化剂样品的NH₃-SCR脱硝性能,通过归一化处理得到两种催化剂在不同温度下的本征脱硝反应速率,同时对比研究了γ-Fe₂O₃与钒系催化剂的脱硝活性;研究氨氮比、氧浓度等运行参数对γ-Fe₂O₃催化剂NH₃-SCR脱硝性能的影响规律,并对其抗硫抗水性能进行考察.结果表明:采用新型微波热解法可得到纯度较高的γ-Fe₂O₃催化剂,其介孔分布合理且大孔数量丰富;同时γ-Fe₂O₃催化剂表现出优于a-Fe₂O₃催化剂的脱硝性能,400℃时最大NO_x转化率达到96%,300、325、350℃下单位面积脱硝速率达到a-Fe₂O₃催化剂的3倍左右;γ-Fe₂O₃催化剂具备优良的抗硫抗水性能,其最佳氨氮比为1、最佳氧体积分数为3.5%.     

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.     

溶胶-凝胶原位合成宽活性温度V2O5/TiO2脱硝催化剂

利用溶胶-凝胶技术原位合成一系列不同V₂O₅担载量的V₂O₅/TiO₂催化剂,通过BET、XRD、NH₃-TPD及紫外-可见光等手段对催化剂进行表征。结果表明：制备的催化剂均具有介孔结构,V₂O₅在TiO₂表面高度分散,且存在3种典型的酸性位。通过选择性催化还原反应对V₂O₅/TiO₂催化剂进行活性评价,结果显示随着V₂O₅含量的增加,NO转化率大于75%的温度窗口向低温方向偏移,含10% （质量分数）V₂O₅的催化剂的NO转化率为80%的温度窗口最宽为200～450℃,240℃时20 h连续实验表现出稳定的抗硫抗水性能。结合紫外-可见光谱分析,揭示了钒掺杂所形成的单聚和低聚钒酸盐为催化剂的活性组分。     

Zeolite-mediated removal of NOx by NH3 from exhaust streams at low temperatures

NH₃ stored on zeolites in the form of NH₄⁺ ions easily reacts with NO to N₂ in the presence of O₂ at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO₂ deposited on the external surface of zeolite Y by precipitation considerably enhances the NO_x conversion by zeolite fixed NH₄⁺ ions in the presence of water at 400–430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO₂ phase structure. The MnO₂ is a non-stoichiometric, amorphous phase that contains minor amounts of Mn²⁺ ions. It loses O₂ upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H₂ in two stages via intermediate formation of Mn₃O₄. The manufacture of extrudates preserving stored NH₄⁺ ions for NO_x reduction is described. It was found that MnO₂ can oxidize NO by bulk oxygen. This enables the reduction of NO to N₂ by the zeolitic NH₄⁺ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH₄⁺ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O₂ or additional reductants. The catalyst is likewise suitable for steady-state DeNO_x operation at higher space velocities if gaseous NH₃ is permanently supplied.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

钇掺杂三氧化二铁催化剂的脱硝性能研究

为提升三氧化二铁(Fe₂O₃)催化剂的脱硝性能,扩展催化剂的活性温度窗口,采用共沉淀法引入助剂钇(Y)元素对Fe₂O₃催化剂进行改性。通过X射线衍射(XRD)、氮气等温吸-脱附(N₂-BET)、X射线光电子能谱(XPS)、氨气程序升温脱附(NH₃-TPD)、氢气程序升温还原(H₂-TPR)等表征方法对样品进行了表征分析。XRD和N₂-BET结果表明,Y的掺杂使催化剂结构发生变化,比表面积增加、孔径减小。XPS和NH₃-TPD结果证明,Y掺杂Fe₂O₃具有更多的表面吸附氧(O_Ⅰ)、Fe²⁺以及更多的酸量。H₂-TPR结果表明,Y的掺杂使催化剂的氧化还原能力略有下降。测试了不同含量Y掺杂的Fe₂O₃催化剂在150～400℃的脱硝性能,其中Fe<...     

Fe-SAPO-44分子筛制备及催化脱硝性能

运用N2保护下的离子交换法制备Fe-SAPO-44分子筛催化剂,通过X射线粉末衍射(XRD)、N2吸附-脱附、电感耦合等离子体发射光谱仪(ICP-AES)、扫描电子显微镜(SEM)和氨气程序升温脱附(NH3-TPD)等研究Fe-SAPO-44催化剂的物化性质。对Fe-SAPO-44催化剂进行脱硝活性测试,考察其抗水热老化性能。结果表明,制备的Fe-SAPO-44催化剂的脱硝活性良好,并具有较好的抗水热老化能力。     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

NOX removal in excess oxygen by plasma-enhanced selective catalytic reduction

In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO₂. In combination with appropriate catalysts and ammonia (NH₃-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO₂) synergistically to molecular nitrogen.

The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NO_X at low temperatures. Using NH₃ as a reducing agent, NO_X was converted to N₂ on zeolites or NH₃-SCR catalysts like V₂O₅–WO₃/TiO₂ at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH₃ stored by ion exchange on the zeolite and for continuous NH₃ supply.

Certain modifications of Al₂O₃ and ZrO₂ have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h⁻¹ at the catalyst. The synergistic combinations of NTP and both NH₃- and HC-SCR have been verified under real diesel engine exhaust conditions.     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

NH3-TPD and XPS studies of Ru/Al2O3 catalyst and HDS activity

The effects of pretreatment of catalyst on its surface properties and the HDS activity of a 0.49% Ru/Al₂O₃ catalyst were studied in a single-pass, differential microreactor. The surface properties of the catalyst were measured by NH₃-TPD and XPS analysis. The Ru/Al₂O₃ catalyst was pretreated in three ways: reduced in H₂ (Ru-R catalyst), oxidized in air and subsequently reduced in H₂ (Ru-OR catalyst), or sulfided in H₂S/H₂ (Ru-S catalyst). Three types of peaks (low, middle, and high temperatures) were observed in the NH₃-TPD study. The predominant high-temperature peak was observed for both the Ru-OR and Ru-S catalyst, pretreated at 300°C. Mass spectrometry showed that the high-temperature peak in NH₃-TPD consisted of N₂ and H₂ formed from the decomposition of NH₃ on the ruthenium sites. NO adsorption of unsaturated Ru species was related to the low-temperature peak in the NH₃-TPD. The XPS analysis showed that the peaks at 279.9 eV, 280.6 eV, and 282.5 eV were ascribed to metallic ruthenium, RuO₂, and RuO₃, respectively. The low-, middle-, and high-temperature peaks were assigned to RuO₂, acid sites on alumina, and metallic Ru, respectively. Metallic ruthenium was effective in the HDS of thiophene and the decomposition of NH₃.     

KINETIC STUDY OF REMOVAL OF NO IN MOLTEN SALT SYSTEM

The kinetics of the reduction of NO by NH₃ in the presence of O₂ in molten salts of 50mol% NH₄HSO₄ and 50mol% NaHS04 with a V₂O₅ as catalyst was investigated by chemical absorption method using a bubble column reactor at temperatures ranging 150 to 180°C. The rate of the reduction of NO could be expressed as first-order with respect to the concentration of NO. The first-order reaction rate constants with V₂O₅ and V₂O₅-NH₄Br-TiO₂-SiO₂ as catalyst were determined. The Henry's law constants of NO in the molten salts were determined in the same range of temperature.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

Catalytic selective reduction of NO with propylene over Cu-Al2O3 catalysts: influence of catalyst preparation method

NO reduction to N₂ by C₃H₆ was investigated and compared over Cu-Al₂O₃ catalysts prepared by four different methods, namely, the conventional impregnation, co-precipitation, evaporation of a mixed aqueous solution, and xerogel methods. It was found that the catalyst preparation method as well as the Cu content exerts a significant influence on catalyst activity. For the catalysts prepared by the first three preparation methods, with the increase of Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased slightly, but the temperature for the maximum NO reduction also decreased. For the xerogel Cu-Al₂O₃, there was a significant decrease in NO reduction conversion with the increase of Cu content from 5 to 10 wt%. In the absence of water vapour, the Cu-Al₂O₃ catalyst prepared by the impregnation method exhibited the highest activity toward NO reduction. The purity of alumina support was found to be a crucial factor to the activity of the Cu-Al₂O₃ catalyst prepared by impregnation. In the presence of water vapour, a substantial decrease in NO conversion was observed for the Cu-Al₂O₃ catalysts prepared by the first three methods, especially for the impregnated Cu-Al₂O₃ catalyst. In contrast, the presence of water vapour showed only a minor influence on the xerogel 5 wt% Cu-Al₂O₃ and it showed the highest activity for NO reduction in the presence of 20% water vapour. The xerogel 5 wt% Cu-Al₂O₃ catalyst was also found to be less affected by a 5 wt% sulfate deposition than the Cu-Al₂O₃ catalysts prepared by other methods.     

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.     

Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO₂, NH₃, H₂O). NO₂ adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu²⁺NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N₂O₃ → N₂O₄ → NO₂ → NO₃. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO₂, N₂, O₂, and are only slightly affected by H₂O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO_x into highly active surface nitrates.