Mechanism of NO decomposition over Cu-ZSM-5

In the preceding Letter Shelef [1] has proposed a mechanism for NO decomposition involving coordinatively unsaturated Cu²⁺ sites on which NO molecules are chemisorbed in the gem-dinitrosyl form. At reaction temperature this complex is supposed to decompose into N₂ and O₂ without involving a redox process. That such a process is feasible has been pointed out by Moser [2]. Shelef cited several reasons in support of this view and others that have led him to think that a cyclic redox mechanism is not operative. These arguments are countered herein and some new data are presented showing the infrared spectra of surface species recorded under in situ reaction conditions.On leave from Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, Hungary.     

On the mechanism of NO decomposition on Cu-ZSM-5 catalysts

Decomposition of NO was studied on Cu-ZSM-5 catalysts prepared by solid state ion exchange using CuCl₂ (I), CuO (II) and by conventional liquid phase ion exchange with copper acetate (III). There was no difference in the catalytic activity among samples (I), (II) and (III) using the same copper loading. Treatment of the samples in argon, in air or in NO/Ar mixture at 700°C was necessary to develop optimum catalytic activity. Transient kinetic experiments using NO carried out under isothermal conditions, showed overshoots in the N₂ and O₂ concentration at the front and tail edge, respectively. Fourier transform-infrared studies indicated the formation of oxidized copper sites and adsorbed NO₂ species during the NO decomposition. In a proposed mechanism Cu²⁺(O)(NO)(NO₂) intermediate was suggested to play a key role in the NO decomposition.     

A refined model for the active site within the NO decomposition catalyst, Cu-ZSM-5

Static atomistic simulation techniques have been employed to identify a model for the active site configuration and its location within the NO decomposition catalyst, Cu-ZSM-5. We propose that the active site comprises a copper pair, bridged by OH and forming a six membered ring, specifically, {-O-Cu(II)-OH-Cu(I)-O-Al-}, within the zeolite framework. The six-membered ring arises from the strong association of both of the copper species with a single aluminium in the zeolite framework and consequently the ring is strained, reflected in the low (3.1 Å) intercopper distance in the cluster. Indeed, this Cu-Cu distance compares well with the experimentally determined value of 3.0. In addition, it is expected that the strain in the cluster influences the activity of the cluster, which we suggest may be responsible for its unique activity for NO decomposition.     

Thermodynamic features of the Cu-ZSM-5 catalyzed NO decomposition reaction

Over a Cu-ZSM-5 catalyst with a quantified amount of the active Cu²⁺-dimers (Cu²⁺-O^2--Cu²⁺), the kinetics of the catalytic NO decomposition to N₂ and O₂ was derived on the basis of the proposed reaction mechanism, and such thermodynamic data as adsorption enthalpies of NO and O₂ onto the Cu ion dimer sites were evaluated. It was revealed that the enthalpy of the adsorption of NO (δH=-34.1 kcal/mol) onto a reduced Cu⁺-dimer, as the initiating step of NO decomposition catalysis, was higher than that (δH=-27.8 kcal/mol) onto an oxidized Cu²⁺-dimer, or that (δH=-27.4 kcal/mol) of the dissociative adsorption of O₂ onto the two reduced Cu⁺-dimers in neighbor. The strong inhibition effect of gas phase oxygen on the kinetic rate of NO decomposition at 400–600 ‡C could be explained by the thermodynamic predominance of the oxidized Cu²⁺-dimers against the active reduced Cu⁺-dimers on the catalyst even at high temperature and under the low partial pressure of oxygen. It was also found that the maximum catalytic activity at temperatures around 500 ‡C, which was commonly observed in the Cu-ZSM-5 catalyzed NO decomposition reaction, was attributed to the relatively large enthalpy of NO adsorption onto the reduced Cu⁺-dimers as compared to that of the reaction activation energy (=19.5 kcal/mol), resulting in less favored NO adsorption at the higher temperatures than 500 ‡C.     

Preparation effects on the activity of Cu-ZSM-5 catalysts for NO decomposition

Effects of Cu-ZSM-5 catalyst preparation on the activity of over-exchanged copper for NO decomposition are reported. The Cu-ZSM-5 catalysts were prepared by incorporating Cu²⁺ cations into ZSM-5 zeolites from an aqueous cupric acetate solution adjusted to different pH values by adding either acetic anhydride or aqueous ammonia in the solution. The Cu²⁺ exchange levels increased with increasing pH level. STEM/EDX analysis identified CuO particles (5–6 nm) on the zeolite surface for the materials exchanged at pH>6. Conversion and kinetics measurements of NO decomposition to N₂ over these catalysts showed that the over-exchanged copper was not active. Short-time wash with aqueous ammonia removed this copper. The catalyst activity correlated very well with the amount of copper remaining in the ZSM-5 channels.     

Desorption study of NO and O2 on Cu-ZSM-5

A detailed temperature-programmed desorption (TPD) study on NO and O₂ saturated Cu-ZSM-5 at different temperatures (300–723 K) has been performed. In the temperature range 373–723 K, the evolution of O₂ and NO₂ accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O₂ absorbed was very much lower than that of NO. The desorption profile of O₂ after contacting Cu-ZSM-5 with O₂ at 623 K showed a low temperature peak (369K) confirming the spontaneous ability of O₂ desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O₂ and vice versa have been performed at various temperatures (298–623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O₂ are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O₂ and other NO_x species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O₂ for the adsorption sites, therefore the adsorption of NO at 623 K on O₂ saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed     

Kinetics of nitrous oxide decomposition over Cu-ZSM-5

The kinetics of nitrous oxide decomposition on an overexchanged Cu-ZSM-5 catalyst were measured using a gradientless reactor. Isothermal oscillations of nitrous oxide and oxygen concentrations can be observed in a broad range of experimental conditions. A transition of the catalytic activity during oscillations is accompanied by a change in the oxygen content of the catalyst and by the formation of traces of nitric oxide. The presence of excess oxygen does not significantly alter the behaviour of the catalyst whereas NO concentrations as low as 10 ppm quench the oscillations in the whole temperature range studied (375 to 450°C), maintaining steady-state operation at maximum catalytic activity. Reaction rates in this ‘ignited’ state are first order with respect to nitrous oxide concentration and not affected by either oxygen or nitric oxide. At temperatures above 400°C, the observed reaction rates are influenced by pore diffusion effects. In the region of intrinsic kinetics, the temperature dependence of the first order rate constant can be described by an activation energy of ca. 100 kJ/mol.     

The catalytic activity of Cu-ZSM-5 and Cu-Y zeolites in NO decomposition: dependence on copper concentration

NO decomposition was studied on Cu-ZSM-5 (Cu exchange extent from 23 to 210%) and Cu-Y (Cu exchange extent from 5 to 105%) catalysts at 773 K. The results show that the activity (NO molecules decomposed per gram of catalyst per second) increases by roughly 100-fold when the extent of exchange with copper in the ZSM-5 framework increases from 80 to 100%. This behaviour shows that not all Cu sites are equivalent in their decomposition activity. Cu-ZSM-5 samples prepared with either H-ZSM-5 or Na-ZSM-5 show the same activity pattern.     

Turnover frequency for NO decomposition over Cu-ZSM-5 catalysts: insight into the reaction mechanism

In this letter we present a simple model useful to understand the relationship between the turnover frequency for NO decomposition over Cu-ZSM-5 catalysts (N _Cu), the number of Al atoms per unit cell of the ZSM-5 zeolite (p),and the copper loading expressed as percent of exchange (E). Our simple model is able to explain the literature data. We show that: (1) on catalysts with the highest activity (Cu exchange levelsE>90%),N _Cu increases withp (i.e. decreasing the Si/A1 ratio) indicating that the most active sites may contain two close copper ions; (2) at low Cu exchange levels (E<80%) the catalysts have lower activity and, moreover,N _Cu decreases withP, according to previous results of Iwamoto et al. (1986). The present results are also in agreement with the evidence that the redox couple Cu^2–/Cu^– play a key role in the reaction mechanism.Some of the ideas discussed in this letter were presented at the 2nd Italian national meeting on Science and Technology of Zeolites held in Modena, Italy, 6–8 October 1993.     

In situ XANES characterization of the Cu oxidation state in Cu-ZSM-5 during NO decomposition catalysis

The oxidation state of Cu in Cu-ZSM-5 has been investigated by the X-ray absorption near-edge structure (XANES) spectroscopic method during NO decomposition catalysis. We designed an in situ reactor system with which we can measure the relative NO decomposition rate while taking XANES spectra. We observed that the 1s4p electronic transition of Cu(I) in Cu-ZSM-5 appears as a narrow, intense peak which is an effective measure of changes in the population of copper oxidation states. This transition is quite intense after Cu-ZSM-5 is activated in inert gas flow. However, its intensity decreases but by no means disappears after the admission of a NO/N₂ gas mixture. We conducted the reaction in a temperature cycle around the optimum conversion temperature of 773 K and recorded the XANES at each temperature. We observed that the integrated intensity of the Cu(I) 1s4p transition, which is proportional to the cuprous ion concentration in Cu-ZSM-5, was well correlated with the NO decomposition rate. This finding supports the conjecture that Cu(I) participates in a redox mechanism during catalyzed NO decomposition in Cu-ZSM-5 at elevated temperature.     

Effect of the support upon the behavior of Cu in NO decomposition exemplified on Cu-ZSM-5 containing Zr

ZSM-5 containing Zr in the lattice was prepared following the procedure for ZSM-5 using ZrCl₄ as zirconium precursor. Zr was substituted for Si in the ZSM-5 lattice and then this zeolite was loaded with Cu. Loading these catalysts with Cu was carried out following Iwamoto’s procedure. An increase in Zr results in a decrease in Cu loading, although the exchange solutions were the same. The increased Zr loading and, hence, decreased Cu loading resulted in a decreased NO decomposition activity relative to the Zr-free ZSM-5 loaded with Cu. NO conversions, obtained on Cu-ZSM-5 zeolites in which zirconium replaced Si, were below 40%, which is about 30% lower than those recorded on Cu-exchanged pure zeolites. The amount of NO₂ detected in the reaction products proved that, for total conversions lower than 20%, the reaction of NO takes place almost exclusively without decomposition to N₂ and O₂. The catalysts were characterized by DRIFTS and NO-DRIFTS, Raman spectroscopy, O₂ — TPD and XPS. The OH region observed in DRIFTS changed with an increase in Zr loading and, hence, Cu(II)–O–Cu(I) species cannot be formed. This may play a role in the decreased activity. Raman spectra showed that increased Zr also results in increased CuO which may lead to the decreased activity. Although the exposure of the samples with high Zr content to NO results in the apparent redispersion of CuO to isolated sites within the ZSM-5, the Cu moves to locations which are inactive and/or inaccessible to NO and, hence, the activity does not increase even though the Cu is dispersed. The formation of NO₂ follows the mechanism proposed by Shelef.     

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO₂ intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO₂ itself is reduced only back to NO. Thus, for the selective reduction of NO₂ to N₂ (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O₂ reaction to form NO₂ parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al₂O₃ or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO₂ back to NO and O₂ while the other catalysts are much less active. The inhibiting effect of water on the NO + O₂ catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO₂ intermediate.     

Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N₂, increased as Cu loading increased. N₂ selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H₂ (H₂-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu²⁺ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H₂-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu⁺ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO₂ suggested that CuO could form N₂, but, independent of the CuO dispersion, nitrogen oxides (NO_x) were formed above 350 °C. Cu⁺ and the square-pyramidal and distorted square-pyramidal forms of Cu²⁺ showed low activity for AN decomposition. Temperature-programmed desorption of NH₃ suggested that N₂ formation from NH₃ proceeded on Cu²⁺, resulting in the formation of Cu⁺. The Cu⁺ ions were oxidized to Cu²⁺ at around 300 °C. Thus, high N₂ selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu²⁺, which can be reversibly reduced and oxidized.     

On the mechanism of nitric oxide decomposition over Cu-ZSM-5

Cu-ZSM-5, a copper-containing zeolite, catalytically decomposes NO at temperatures below those of other catalysts. A mechanism is proposed which is based on active sites consisting of coordinatively unsaturated cupric (Cu²⁺) ions in a square planar configuration. These sites are posited to chemisorb NO molecules in the gem-dinitrosyl form. The pair of adsorbed NO molecules desorbs as N₂ and O₂. This mechanism accounts for the experimental behavior in chemisorption and decomposition without invoking a cyclical oxyreduction of the surface sites.     

钐掺杂对锰基催化剂的NO分解性能影响

采用共沉淀法,以乙酸锰为前体,pH为10,焙烧温度为700℃时,制备得到不同含量钐元素（Sm）掺杂锰基催化剂,研究Sm掺杂对催化剂物相、形貌、比表面积及NO催化转化能力的影响。结果表明：Sm掺杂有利于活性物质MnO₂的生成,Sm/Mn摩尔比为0.05时,锰基催化剂在100~700℃的整个温度窗口内,NO的转化率都较高,且在温度高于150℃时NO转化率超过80%。钐嵌入锰晶格形成钐锰固溶体,三价钐取代四价锰,在催化剂中形成阴离子空位,产生缺陷,表面缺陷位增多使得活性位点增加,促进了催化反应。     

微波离子交换法制备Cu-ZSM-11及微波辐照MeOx/Cu-ZSM-11催化分解NO

用微波辐照离子交换法制备了Cu-ZSM-11,制备的Cu-ZSM-11和金属氧化物（MeO_x）机械混合制备了微波催化剂MeO_x/Cu-ZSM-11。考察了MeO_x在微波辐照下的升温行为,筛选出吸波性能好的MeO_x（MnO₂ >CuO >Ni₂O₃）为吸波组分。分别考察了微波辐照下金属氧化物、Cu-ZSM-11和MeO_x/Cu-ZSM-11直接分解NO性能,并与常规加热条件下比较。结果表明：微波辐照显著提高了NO的转化率,微波催化剂Ni₂O₃/Cu-ZSM-11在350℃时,NO转化率达到99.27%,N₂选择性达到99.9%;相同条件下,微波辐照MeO_x/Cu-ZSM-11直接分解NO转化率高于对应金属氧化物或Cu-ZSM-11分解NO的转化率,表明微波具有催化作用。微波辐照下氧气浓度对Ni₂O₃/Cu-ZSM-11直接分解NO性能几乎无影响,微波辐照消除了氧气阻抑作用,表现出微波选择效应。水汽存在对转化率有较小影响。反应进口气体不需要预热,进出口气体温度基本不变。     

Isothermal oscillations in the catalytic decomposition of nitrous oxide over Cu-ZSM-5

The catalytic decomposition of nitrous oxide was studied on a copper-exchanged ZSM-5 catalyst in the temperature range 648–723 K. Using a mixture of 1000 ppm N₂O in nitrogen, isothermal oscillations both in nitrous oxide and oxygen concentrations occurred, accompanied by formation of small amounts of NO. While the addition of excess oxygen did not significantly change frequency and amplitude of the oscillations, even the presence of small amounts of NO immediately quenched the oscillations. The reacting system then remained in the ignited state at high nitrous oxide conversions.     

Infrared study of acetone and nitrogen oxides on Cu-ZSM-5

The oxidation of bulky thioethers has been carried out on Ti-Beta and Ti-MCM-41 catalysts, using H₂O₂ andt-butyl hydroperoxide (TBHP) as oxidants. The intrinsic activity of Ti-Beta was higher than that of Ti-MCM-41 for the oxidation of methyl phenyl sulfide which can penetrate in the pores of Beta, while in the case of the larger isopentyl phenyl sulfide, which diffuses more slowly in Ti-Beta zeolite, Ti-MCM-41 gives a larger activity. Ti-Beta is able to perform better for the more demanding oxidation of sulfoxides to sulfones giving, therefore, higher selectivities to sulfones than Ti-MCM-41. Similar results were obtained when using either H₂O₂ or TBHP as oxidants. However, the sterical effects were enhanced when TBHP was used as oxidant.     

直接法合成杂原子Cu-ZSM-5用于直接催化分解NO_x的研究

以硅溶胶为硅源,硫酸铝为铝源,采用水热合成法直接合成杂原子Cu-ZSM-5催化剂,通过改变合成条件制备一系列催化剂,并用模拟烟气考察直接催化分解NO的活性。实验结果表明,当硅铝物质的量的比为50,Cu的负载量为6%,晶化温度为165℃,晶化时间为24 h时,合成的催化剂效果最好,温度过低或晶化时间过短则不能合成分子筛,温度过高或晶化时间过长则容易产生杂晶,影响催化效果;同时发现,当采用Na OH作为碱源时,效果优于其他强碱或弱碱,硫酸作为酸源时,效果优于其他强酸和中强酸。XRD的表征结果表明,高结晶度、无杂晶、适当存在晶型缺陷的Cu-ZSM-5沸石分子筛最有利于提高催化脱硝性能。     

NO adsorption on Cu-ZSM-5: assignment of IR band at 2133 cm−1

The adsorption of nitrogen oxides on Cu-ZSM-5 was studied by infrared spectroscopy to elucidate the species associated with the band at 2133 cm^–1. The band was found for both NO and NO₂ adsorption. Labeling experiments with¹⁵NO revealed that the associated surface species contained nitrogen and, most likely, an N-O bond. Co-adsorption experiments of NO and oxygen produced adsorbed nitronium, NO ₂ ⁺ , as the principal, associated species. Adsorption of nitrogen oxides on dispersed CuO and the HZSM-5 support demonstrated that the 2133 cm^–1 band was not necessarily associated with copper ions. A relatively strong correlation between the bands at 2133 and 3615 cm^–1 indicates that the primary adsorption sites of NO ₂ ⁺ are the strongly protic, bridging Si(OH)Al framework hydroxyls. Once these were filled, other, weaker acid sites began to adsorb NO ₂ ^O .