Adsorption of NO and CO over transition-metal-incorporated mesoporous catalytic materials

Novel high-surface-area mesoporous catalysts of type Al-MCM-41 containing transition metals such as iron, nickel, cobalt, zinc, copper, and cobalt were prepared and characterized using techniques such as BET, FTIR, ICP-MS, XPS, and XRD. The XPS measurements indicated that the transition-metal particles are located in the bulk or pore channels of the Al-MCM-41 materials. A detailed in situ FTIR investigation undertaken on the adsorption and disproportionation of NO and CO over the transition-metal-Al-MCM-41 mesoporous catalysts indicated the formation of various NO/CO species or complexes with active metal sites. The structure and dynamics of the metal activated complex and reactive species formed during the CO/NO reaction together with advantages of these catalysts and the influence of reaction temperature and pressure have been studied. NO adsorption at room temperature leads to the formation of adsorbed N(2)O, NO(2), MNO(2), MNO, and [M(NO)(2)] complexes. CO adsorption at room temperature leads to the formation of physisorbed carbon dioxide and cationic Lewis acid carbonyl moieties as well as transition-metal carbonyl complexes. The copper mesoporous catalysts prepared by different procedures (ion exchanged and as-synthesized) were compared for their interactions with CO and NO probe molecules.     

活性炭纤维吸附脱除NO过程中NO氧化路径分析

在小型固定床吸附实验台上开展了黏胶基活性炭纤维吸附脱除NO的实验研究。采用H₂O₂溶液浸渍以及热处理方法对活性炭纤维表面进行修饰,以获得表面孔隙结构接近而含氧官能团含量不同的样品;考察样品在惰性氮气气氛、含氧气氛下吸附脱除NO的效果,以及表面含氧含氮官能团的变化规律。探讨了含氧官能团在NO催化氧化过程中的作用及含氧气氛下O₂对于NO转化为NO₂的影响,分析了活性炭纤维表面吸附的NO向NO₂的主要转化途径。结果表明,在氮气气氛下活性炭纤维表面C-O官能团对吸附态的NO起到氧化作用,吸附态NO被C-O官能团氧化生成-NO₂官能团;在含氧气氛下活性炭纤维吸附NO后表面出现-NO₂、-NO₃官能团,通过长时间实验测定三种样品在含氧气氛下对NO吸附的效果,发现三种样品稳定时催化氧化效果一致,表明含氧官能团对初始NO的物理吸附影响较大,而对整个吸附过程影响较小。吸附在活性炭纤维表面上的NO与环境气氛中的游离态O₂发生氧化反应是NO转变为NO₂的主要途径。     

FTIR spectroscopic study of low temperature NO adsorption and NO + O2 coadsorption on H-ZSM-5

Adsorption of NO and coadsorption of NO and O2 on H-ZSM-5 have been studied at low and room temperature by means of FTIR spectroscopy. For better interpretation of the spectra, experiments involving isotopic labeled molecules have been performed. Low temperature adsorption of NO on H-ZSM-5 results initially in formation of NO which is H-bonded to the zeolite acidic hydroxyls. A second NO molecule is inserted into the OH-NO species at higher coverages, thus forming OH(NO)2 complexes. Different kinds of NO dimers are also formed. Negligible amounts of oxygenated compounds have been detected. In the presence of oxygen, the (di)nitrosyl species are oxidized very fast even at 100 K to N2O3, NO+, NO2, and N2O4. Different kinds of adsorbed N2O3 species have been evidenced. With increasing temperature, NO+ migrates and occupies cationic positions. The latter species interacts with NO at low temperature to give an [ONNO]+ complex. This reaction is used to prove that the different bands in the 2206-2180 cm(-1) region are also due to NO+ species.     

NO在Cu(111)表面吸附和分解的XPS和TPD研究：不同氧物种的影响

利用X射线光电子能谱和程序升温脱附谱研究了NO在清洁和预吸附氧的Cu(111)表面上的吸附和反应.通过改变NO的暴露量和退火温度,在Cu(111)表面可以制备出不同种类的化学吸附氧物种,其O 1s的结合能分别位于531.0 eV (O₅₃₁)和529.7 eV (O₅₂₉).表面O₅₃₁物种的存在对NO的不同吸附状态有着显著影响,同时使得大部分NO吸附分子(NO(a))在加热过程中发生分解并以N₂O和N₂形式脱附; 而表面O₅₂₉物种对NO(a)的解离脱附有着明显的抑制作用.相对于O₅₃₁物种来说,O₅₂₉物种对NO吸附表现出更强的位阻效应.上述结果表明,NO在Cu(111) 表面的吸附和分解行为与预吸附氧物种的种类和覆盖度密切相关.     

Low-temperature reduction of NO(2) on oxidized Mo(110)

The reactions of nitrogen dioxide (NO(2)) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO(2) reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N(2), and trace amounts of N(2)O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO(2) to yield N(a) and O(a). N(2) forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO(2) reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO(2). The selectivity for N(2) formation over N(2)O is greater for NO(2) as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO(2) reaction than by O(2) reaction, indicating that NO(2) is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N(2)O(4), NO(2), and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N(2)O(4) desorbs by 200 K, leaving only NO(2) and NO on the surface. Infrared spectroscopy of NO(2) on (18)O-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.     

First principles study of CO oxidation on TiO2(110): the role of surface oxygen vacancies

The reactivities of the stoichiometric and partially reduced rutile TiO2(110) surfaces towards oxygen adsorption and carbon monoxide oxidation have been studied by means of periodic density functional theory calculations within the Car-Parrinello approach. O2 adsorption as well as CO oxidation are found to take place only in the presence of surface oxygen vacancies (partially reduced surface). The oxidation of CO by molecularly adsorbed O2 at the O-vacancy site is found to have an activation energy of about 0.4 eV. When the adsorbed O2 is dissociated, the resulting adatoms can oxidize incoming gas-phase CO molecules with no barrier. In all studied cases, once CO is oxidized to form CO2, the resulting surface is defect-free and no catalytic cycle can be established.     

Carbon-nitrogen place exchange on NO exposed beta-Mo2C

Atomic nitrogen and oxygen were deposited on beta-Mo(2)C through dissociative adsorption of NO. Reflectance absorbance infrared spectroscopy (RAIRS), thermal desorption, and synchrotron X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the interplay between atomic nitrogen, carbon, and oxygen in the 400-1250 K region. The combination of the high resolution and high surface sensitivity offered by the synchrotron XPS technique was used to show that atomic nitrogen displaces interstitial carbon onto the carbide surface. Thermal desorption measurements show that the burnoff of the displaced carbon occurs at approximately 890 K. The incorporation of nitrogen into interstitial sites inhibits oxygen dissolution into the bulk. RAIRS spectroscopy was used to identify surface oxo, terminal oxygen, species formed from O(2) and NO on beta-Mo(2)C.     

SO2和NO在Na-γ-Al2O3上吸附行为的研究

利用吸附曲线和漫反射红外光谱（DRIFTS）研究了150 ℃SO2和NO在Na-γ-Al2O3上的吸附行为。吸附实验在固定床反应器内进行,原料气体积组成为NO (0.1％)、 SO2 (0.51％)、O2 (4.5％)、Ar平衡。研究表明,150 ℃不论气相中是否有氧,Na-γ-Al2O3均能单独吸附SO2或NO,Na-γ-Al2O3单独吸附SO2时,观察不到有SO42-生成,Na-γ- Al2O3单独吸附NO时,NO吸附量较少,有微量NO2（或表面硝酸盐）生成;Na-γ- Al2O3均能同时吸附SO2和NO,SO2和NO在Na-γ- Al2O3上吸附时相互作用,NO促进SO2氧化生成SO42-,SO2吸附量增加,SO2促进NO氧化转化,SO2也促使NO2脱附。气相氧促进了SO2和NO在Na-γ- Al2O3上的吸附以及SO2和NO的相互作用。     

Understanding the reactivity and basicity of zeolites: a periodic DFT study of the disproportionation of N(2)O(4) on alkali-cation-exchanged zeolite Y

The disproportionation of N(2)O(4) into NO(3)(-) and NO(+) on Y zeolites has been studied through periodic DFT calculations to unravel 1) the role of metal cations and the framework oxygen atoms and 2) the relationship between the NO(+) stretching frequency and the basicity of zeolites. We have considered three situations: adsorption on site II cations with and without a cation at site III and adsorption on a site III cation. We observed that cations at sites II and III cooperate to stabilize N(2)O(4) and that the presence of a cation at site III is necessary to allow the disproportionation reaction. The strength of the stabilization is due to the number of stabilizing interactions increasing with the size of the cation and to the Lewis acidity of the alkali cations, which increases as the size of the cations decreases. In the product, NO(3)(-) interacts mainly with the cations and NO(+) with the basic oxygen atoms of the tetrahedral aluminium through its nitrogen atom. As the cation size increases, the NO(3)(-)...cation interaction increases. As a result, the negative charge of the framework is less well screened by the larger cations and the interaction between NO(+) and the basic oxygen atoms becomes stronger. NO(+) appears to be a good probe of zeolite basicity, in agreement with experimental observations.     

Ag-ZSM-5催化剂上CH4选择催化还原NOx的研究

摘要研究Ag-ZSM-5催化剂上CH₄选择性催化还原NO_x的反应性能,采用TPD和TPSR技术研究NO和O₂共吸附于Ag-ZSM-5催化剂表面形成的吸附物种及其和CH₄之间的反应。结果表明,Ag-ZSM-5催化剂上CH₄选择性还原NO_x活性和选择性较高。NO和O₂共吸附在Ag-ZSM-5催化剂上形成的NO₃(s)吸附物种能被CH₄还原生成N₂.在NO₃(s)和O₂共存的体系中,CH₄能优先并选择性还原NO₃(s)生成N₂.     

NO在氧化铝负载的Pd催化剂上吸附的TPD-MS研究

消除汽车尾气中的氮氧化物(NOx)对保护大气环境有着重要意义.为了除去NOx,已经进行了许多卓有成效的研究,例如NOx在分子筛上的直接分解和催化还原,在贵金属三效催化剂上的还原等.     

Role of Zr4+ cations in NO2 adsorption on Ce(1-x)Zr(x)O2 mixed oxides at ambient conditions

Mixed oxides Ce(1-x)Zr(x)O(2) prepared by slow coprecipitation in NaOH were tested for NO(2) adsorption in dynamic conditions at room temperature. The samples were characterized before and after exposure to NO(2) by XRD, N(2)-adsorption, thermal analysis, potentiometric titration, and FT-IR. Mixed oxides show a better NO(2) adsorption capacity than the parent materials (CeO(2) and Zr(OH)(4)). This effect is linked to the presence of reduced cerium and oxygen vacancies induced by the addition of Zr(4+) cations to the structure. The results indicate that NO(2) reacts with Ce(3+) to form nitrite and nitrate species on the surface. The NO retention increases with an increase in the Zr(OH)(4) content. A decrease in the density of -OH groups on the surface after the exposure to NO(2), suggests their involvement in reactive adsorption of NO and/or NO(2). From the structural point of view, no real difference was observed on the Ce(1-x)Zr(x)O(2) materials before and after exposure to NO(2).     

Interaction of oxygen with TiN(001): N<-->O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450 K. At the lowest temperature, there was chemisorption of oxygen (O(2,gas)-->2O(ads)) without significant surface oxidation. In contrast, at 450 K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (DeltaE=2.84 eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2 -->TiOx + NO reaction is an important exit channel for nitrogen in the oxidation process.     

EPR spectroscopy of Cu(I)-NO adsorption complexes formed over Cu-ZSM-5 and Cu-MCM-22 zeolites

The Cu(I)-NO adsorption complexes were formed over copper exchanged and autoreduced high siliceous Cu-ZSM-5 and Cu-MCM-22 zeolites and studied by EPR spectroscopy at X-, Q-, and W-band frequencies. The spin Hamiltonian parameters of the Cu(I)-NO species are indicative of a nitrogen-centered radical complex with a bent geometry and a significant contribution of the Cu(I) 4s atomic orbital to the wave function of the unpaired electron. Two different Cu(I)-NO species were found in both zeolites. It has been confirmed by comparing the experimental data with the results of previous theoretical studies that the presence of two different species is due to the formation of Cu(I)-NO adsorption complexes from two different Cu(I) sites in the zeolite matrix with different numbers of oxygen coligands. The structure of the two sites in the Cu-ZSM-5 and Cu-MCM-22 zeolites must be similar as the spin Hamiltonian parameters are found to be almost independent of the zeolite matrix, where the Cu(I)-NO complex is formed. The EPR signal intensity of the Cu(I)-NO species was studied as a function of the NO loading, and the formation of diamagnetic Cu(I)-(NO)(2) species with rising NO pressure at the expense of paramagnetic Cu(I)-NO monomers could be demonstrated for both systems at low temperatures.     

Theoretical study of oxygen adsorption on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for M = Ti, Fe and n = 3-7

A comparative study of the adsorption of an O2 molecule on pure Au(n+1)+ and doped MAu(n)+ cationic gold clusters for n = 3-7 and M = Ti, Fe is presented. The simultaneous adsorption of two oxygen atoms also was studied. This work was performed by means of first principles calculations based on norm-conserving pseudo-potentials and numerical basis sets. For pure Au4 +, Au6+, and Au7+ clusters, the O2 molecule is adsorbed preferably on top of low coordinated Au atoms, with an adsorption energy smaller than 0.5 eV. Instead, for Au5+ and Au8+, bridge adsorption sites are preferred with adsorption energies of 0.56 and 0.69 eV, respectively. The ground-state geometry of Au(n)+ is almost unperturbed after O2 adsorption. The electronic charge flows towards O2 when the molecule is adsorbed in bridge positions and towards the gold cluster when O2 is adsorbed on top of Au atoms, and both the adsorption energy and the O-O bond length of adsorbed oxygen increase when the amount of electronic charge on O2 increases. On the other hand, we studied the adsorption of an O2 molecule on doped MAu(n)+ clusters, leading to the formation of (MAu(n)O2+) ad complexes with different equilibrium configurations. The highest adsorption energy was obtained when both atoms of O2 bind on top of the M impurity, and it is larger for Ti doped clusters than for Fe doped clusters, showing an odd-even effect trend with size n, which is opposite for Ti as compared to Fe complexes. For those adsorption configurations of (MAu(n)O2+) ad involving only Au sites, the adsorption energy is similar to or smaller than that for similar configurations of Au(n)+1O2 + complexes. However, the highest adsorption energy of (MAu(n)O2+) ad is higher than that for (Au(n)+1O2+) ad by a factor of approximately 4.0 (1.2) for M = Ti (M = Fe). The trends with size n are rationalized in terms of O-O and O-M bond distances, as well as charge transfer between oxygen and cluster substrates. The spin multiplicity of those (MAu(n)O2+) ad complexes with the highest O2 adsorption energy is a maximum (minimum) for M = Fe (Ti), corresponding to parallel (anti-parallel) spin coupling of MAu(n)+ clusters and O2 molecules. Finally, we obtained the minimum energy equilibrium structure of complexes (Au(n)O2+) dis and (MAu(n)O2+) dis containing two separated O atoms bonded at different sites of Au(n)+ and MAu(n)+ clusters, respectively. For (MAu(n)O2 (+)) dis, the equilibrium configuration with the highest adsorption energy is stable against separation in MAu(n)+ and O2 fragments, respectively. Instead, for (Au(n)O2+) dis, only the complex n = 6 is stable against separation in Au(n)+ and O2 fragments. The maximum separation energy of (MAu(n)O2+) dis is higher than the O2 adsorption energy of (MAu(n)O2+) ad complexes by factors of approximately 1.6 (2.5), 1.6 (1.7), 1.5 (2.4), 1.5 (1.3), and 1.6 (1.8) for M = Ti (Fe) complexes in the range n = 3-7, respectively.     

小分子配位对血红素活性影响的原子-键电负性均衡方法研究

应用原子-键电负性均衡方法, 计算了血红素与小分子的配位络合物的电荷分布和Fukui函数. 血红素与氧、水、一氧化碳和一氧化氮结合时, 铁离子电荷转移到配体原子上. 活性中心铁离子的Fukui函数均大于氧和水配体中的配位氧原子, 而小于一氧化碳和一氧化氮配体中的配位碳和配位氮原子的Fukui函数. 从Fukui函数可以得出, 一氧化碳和一氧化氮很难从它们与活性中心血红素结合的配位络合物中解离出来, 而氧和水易于从它们与血红素结合的配位络合物中解离出来, 进而, 血红素可以再与其它配体结合. 血红素与KCN和NaN3抑制剂作用时, 铁离子的Fukui函数均小于与其配位的碳和氮原子, 表明在过氧化氢酶中血红素的活性作用减弱或被抑制.     

程序升温脱附研究CoH-FBZ上NOx的吸附

富氧条件下具有FAU和BEA两种拓扑结构的CoH-FBZ选择催化CH4还原NO，显示出较CoH Y和CoH Beta机械混合催化剂更好的催化活性。应用吸附和程序升温脱附(TPD)方法研究了NO和NO+O2与催化剂表面间的相互作用。结果表明，载体的拓扑结构直接影响N、O物种在催化剂表面的稳定性。NO与O2在CoH-FBZ表面形成的吸附态 NOy及NO在CoH-FBZ表面形成的吸附态相对更稳定。CoH-FBZ的NO+O2-TPD脱附曲线在630K和660K形成两个NO2脱附峰，表明在CoH-FBZ表面形成了新的 NOy吸附中心，即可能有新的Co位产生，该新Co位与沸石催化剂CoH-FBZ中新强酸位协同作用，使CoH-FBZ表现出新的CH4-SCR催化特性。     

Effect of reduction treatment on copper modified activated carbons on NO(x) adsorption at room temperature

Activated carbon was impregnated with copper salt and then exposed to reductive environment using hydrazine hydrate or heat treatment under nitrogen at 925 °C. On the obtained samples, adsorption of NO(2) was carried out at dynamic conditions at ambient temperature. The adsorbents before and after exposure to nitrogen dioxide were characterized by X-ray diffraction (XRD), thermal analysis, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), N(2)-sorption at -196 °C, and potentiometric titration. Copper loading improved the adsorption capacity of NO(2) as well as the retention of NO formed in the process of NO(2) reduction on the carbon surface. That improvement is linked to the presence of copper metal and its high dispersion on the surface. Even though both reduction methods lead to the reduction of copper, different reactions with the carbon surface take place. Heat treatment results in a significant percentage of metallic copper and a reduction of oxygen functional groups of the carbon matrix, whereas hydrazine, besides reduction of copper, leads to an incorporation of nitrogen. The results suggest that NO(2) mainly is converted to copper nitrates although the possibility to its reduction to N(2) is not ruled out. A high capacity on hydrazine treated samples is linked to the high dispersion of metallic copper on the surface of this carbon.     

Effect of O2 on the adsorption of SO2 on carbon-supported Pt electrocatalysts

Adsorption of SO(2) in the presence of O(2) on Pt/C catalysts often used as electrocatalysts has been investigated by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The amounts of SO(2) adsorption on Pt/C in the presence of O(2) were much higher than those in the absence of O(2) (SO(2)-N(2)) and from the carbon support (Vulcan XC-72) alone. Adsorption is dependent on oxygen concentration over the range 0-20% but reaches saturation at 20% O(2). The spillover of SO(2) from Pt to the carbon support has been proposed for 10, 20, and 40% Pt loadings, characterized by desorption temperatures of approximately 150 and 260 °C for SO(2) adsorbed on Pt and carbon, respectively. Adsorbed Pt-S, C-S, C-SO(x), and Pt-SO(4) species were identified by XPS as S-containing species on both Pt and carbon. Both TPD and XPS indicate that the carbon support plays a major role in SO(2) adsorption, primarily as SO(x) (x = 3, 4). The bonding of S and SO(x) on the carbon support was strong enough that back diffusion to the Pt surface did not occur.     

Nickel and cobalt hexamethylentetramine complexes (NO3)2Me(H2O)6(HMTA)2.4H2O (Me = Co2+, Ni2+): new molecular precursors for the preparation of metal dispersions

Nickel and cobalt hexamethylenetetramine (HMTA) complexes (NO3)2Me(H2O)6(HMTA)2.4H2O were prepared and characterized structurally by single-crystal X-ray diffraction. Both compounds crystallize in the triclinic P1 space group with the same structure. The structures are three-dimensional hydrogen-bonded supramolecular frameworks containing two-dimensional cationic assemblies connected with proton acceptors, which are noncoordinated anionic species (nitrate) and neutral HMTA molecules. Thermal decomposition of these compounds under an inert atmosphere leads to the high-surface-area metal-carbon foams containing nickel and cobalt nanoparticles embedded within onionlike carbon shells. The decomposition process as studied by thermal analysis and in situ X-ray absorption spectroscopy (XAS) measurements occurs as a rapid loss of oxygen in the form of CO, beginning at temperatures as low as 323 K. As the in situ XAS study demonstrated, the evolution of nickel and cobalt coordination spheres occurs with intermediate formation of amorphous metal carbides, which subsequently decompose to the metal nanoparticles covered with carbon shells.