The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH₃) on vanadium‐based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time‐resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH₃ coordinated to Lewis sites on vanadia on tungsta–titania (V₂O₅‐WO₃‐TiO₂), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono‐oxo vanadyl group that reduces only in the presence of both nitric oxide and NH₃. We were also able to verify the formation of the nitrosamide (NH₂NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.     

氨法选择性还原氮氧化物 V2O5/TiO2-PILC催化剂的抗硫性能

选择性催化还原(SCR)是目前去除氮氧化物最有效的方法之一. V2O5/TiO2催化剂被广泛应用于氨法选择性还原氮氧化物(NH3-SCR)反应,但该催化剂存在工作温度高(300–400oC)及 SO2氧化率高引起设备腐蚀和管路堵塞等问题,开发低温 SCR催化剂具有重要意义.过渡金属氧化物(如 Fe2O3, MnOx和 CuO等)催化剂用于低温SCR先后见诸文献报道,但这些催化剂在 SO2和 H2O存在下易失活.近年来柱撑黏土(PILC)引起科学家广泛关注, Yang等首次将 V2O5/TiO2-PILC催化剂应用于 NH3-SCR反应,发现其催化活性高于传统 V2O5/TiO2催化剂.柱撑黏土基催化剂在 NH3-SCR反应中也显示出良好抗硫性能,但 V2O5/TiO2-PILC催化剂的抗硫机理至今尚未见深入研究.因此我们制备了一系列 V2O5/TiO2-PILC催化剂,采用原位漫反射红外等方法详细研究了其抗硫性能较好的原因.
　　首先采用离子交换法制备出 TiO2-PILC载体,之后采用浸渍法制备了不同钒含量(质量分数x/%=0,3,4,5)的xV2O5/TiO2-PILC催化剂.同时,制备了传统 V2O5/TiO2和 V2O5-MoO3/TiO2催化剂作为对比.活性评价结果显示,4V/TiO2-PILC催化剂具有最高的催化活性,其催化性能与传统钒钛催化剂相当.在160oC时, NO转化率可达80%以上.同时,4V/TiO2-PILC催化剂还具有较宽的反应温度窗口,在260–500oC范围内, NO转化率保持在90%以上.向反应体系中加入0.05% SO2和10% H2O后,在低温(160oC以下)时所有催化剂的反应活性都有一定提高,可能是由于 SO2的加入提高了催化剂的表面酸性.继续升高温度,4V/TiO2和4V6Mo/TiO2催化剂活性均明显下降,而4V/TiO2-PILC催化剂的活性则未出现明显下降.原位漫反射红外光谱结果显示, SO2在三种催化剂表面的吸附以表面硫酸盐或亚硫酸盐物种以及离子态 SO42–物种形式存在,而在4V/TiO2-PILC催化剂表面离子态 SO42–物种的量最少. X射线光电子能谱及 O2程序升温脱附结果显示,在4V/TiO2-PILC催化剂上,表面吸附氧(Oads)的量最少.综合上述分析可以得出,在 SO2气氛下,离子态 SO42–物种在 SCR催化剂表面的累积可能是导致其失活的主要原因,而离子态 SO42–物种的形成可能与催化剂表面吸附氧的量有关.     

Redox transformations of adsorbed NO molecules on a Pt(100) electrode

The electrochemical behavior of adsorbed NO molecules on a Pt(100) electrode has been studied in perchloric acid solutions by means of cyclic voltammetry. According to the literature data, a saturated NO adlayer with a coverage of ～0.5 monolayers (MLs) is formed under open circuit conditions in an acidic nitrite solution as a result of a disproportionation reaction. The saturated adlayer is stable in the potential range of 0.4–0.9 V vs. a reversible hydrogen electrode in 0.1 M HClO₄. NO molecules are oxidized at 0.9–1.1 V with the formation of adsorbed nitrite anions, and they can be reduced to ammonia at potentials less than 0.4 V. In this paper it has been shown that the adlayer stability depends on the surface coverage and extent of ordering. An unsaturated NO adlayer demonstrates NO ? NH₃ redox transformations at 0.5–0.8 V.     

Mechanism of interaction of NO with NH3 on vanadium-containing catalysts based on IR spectroscopic data

Adsorption of ammonia and nitrogen oxide on V₂O₅/Al₂O₃ samples of different degrees of reduction has been studied by IR spectroscopy. On an oxidized surface, ammonia is coordinated by V³⁺ and V⁴⁺ ions to form ammonium ions; NO is not adsorbed. On a reduced surface, the coordination of NO by V³⁺, V⁴⁺, and V⁵⁺ ions is observed, which results in the formation of nitrosyl complexes. A strong mutual influence between NO and NH₃ occurs during coadsorption or consecutive interaction on a reduced catalyst surface.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 236–241, February, 1994.     

Structural study of titanium doped vanadia and vanadium doped titania catalysts

The morphologies and structures of nanostructurally assembled V₂O₅ doped with Ti as well as of the inverse system, V-doped TiO₂, have been studied using transmission electron microscopy and Raman spectroscopy. The bulk structure of the Ti-doped vanadia particles was found to be crystallized in a rod-like shape and to have the phase composition of V₂O₅ with titanium atoms nonuniformly distributed over the surface. A coherent interface between supported V₂O₅ and TiO₂ particles was found to be the main structural peculiarity of the inverse system, V-doped TiO₂ (anatase). The vanadium atoms are partially exchanged for titanium atoms at the interface, which leads to a change in the bond lengths of V=O and V-O-Ti in comparison with those observed in the monolayer supported vanadia catalysts. Both materials showed good catalytic behavior in the reaction of selective reduction of NO by NH₃. This revised version was published online in June 2006 with corrections to the Cover Date.     

Density-functional theory calculation on the nitrogen oxide reduction reaction with carbon monoxide on a titanium dioxide nanocluster

The density functional theory was used for simulation of the NO reduction reaction with carbon monoxide on a reduced Ti₈O₁₅ nanocluster. The reaction proceeds on oxygen vacancies formed via the removal of terminal or bridging O atoms. In the case of adsorption of two NO molecules of such sites, a stable adsorption complex with the bidentate ligand ‥ONNO is produced. When a CO molecule approaches one of the O atoms of this cycle, the following exothermic reaction yielding N₂ and CO₂ adsorbed on the Ti₈O₁₆ cluster takes place: 2NO+ CO + Ti₈O₁₅ → N₂+ Ti₈O₁₆ · CO₂. The proposed model of the reaction agrees well with experimental data.     

Layered structure of the silver coordination polymer with nonrigid aromatic diamine {Ag[CH2(C6H4NH2)2]2(CH3C6H4NH2)}NO3

The complex [Ag(DDM)₂(CH₃C₆H₄NH₂)]NO₃, where DDM is 4,4-diaminodiphenylmethane [CH₂(C₆H₄NH₂)₂], was synthesized and its structure was determined. The crystals are monoclinic, space group P2₁/n, a = 9.543(2) ?, b = 18.056(4) ?, c = 1.901(2) ?, β = 106.94(3)°, V = 1796.8(6) ?³, ρ_calcd = 1.443 g/cm³, Z = 4. The Ag atom (at the inversion center) is coordinated at the vertices of an almost undistorted octahedron by six nitrogen atoms of the primary amino groups from four bridging DDM molecules and two terminal p-toluidine molecules (Ag-N, 2.546(3) ?; NAgN, 89.7–90.3°). Wavelike layers composed of conjugate multiunit metal rings, each containing four Ag⁺ ions and four bridging DDM ligands, are formed in the structure in the [101] direction (a 2D polymer). Uncoordinated NO ₃ ⁻ anions are arranged in the cavities between the layers and link them by N-H⋯O hydrogen bonds. Original Russian Text ? Yu.V. Kokunov, V.V. Kovalev, Yu.E. Gorbunova, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 12, pp. 1992–1998.     

Temperature programmed reduction of unsupported molybdena and vanadia catalysts by ammonia and hydrogen

Temperature-programmed reduction (TPR) is a valuable tool for the characterisation of catalysts. The reductant mostly used is hydrogen. Hydrogen reduces the catalyst by producing water. The amount of hydrogen reacted is measured by menas of a thermal conductivity detector. In this paper the temperature-programmed reduction of vanadia and molybdena catalysts by ammonia is emphasized. During the temperature-programmed reduction ammonia is consumed. The compounds formed in this process are detected by means of a mass spectrometer. It was found that almost exclusively N₂, H₂O and H₂ were formed. Hydrogen formed by the decomposition of ammonia over vanadia and molybdena above 600° produces hydroxyl groups, which are a source for the formation of water at temperatures above 600°.NH₃-TPR gives more relevant information than H₂-TPR for processes in which ammonia is used as a reactant.     

Synthesis and study of acid ammonium metaphenyleneamine dodecatungstenphosphate (C6H4(NH2)(NH3))2H[PW12O40] · 8H2O

Acid ammonium metaphenyleneamine dodecatungstenphosphate of the composition (C₆H₄(NH₂)(NH₃))₂H[PW₁₂O₄₀] · 8H₂O is synthesized and studied by mass spectrometry, X-ray diffraction, IR spectroscopy, and thermogravimetry. The crystals are rhombic, space group Pnma, a = 18.421(2), b = 19.569(3), c = 13.443(3) ?, Z = 4, ρ(calc) = 4.431 g/cm³, V = 4845.9(2) ?³. Original Russian Text ? S. Holguin Quinones, G.Z. Kaziev, A. de Ita, A.M. Koroteev, V.E. Zavodnik, O.A. Kutanova, 2007, published in Koordinatsionnaya Khimiya, 2007, Vol. 33, No. 9, pp. 660–664.     

The Role of Lewis and Brønsted Acid Sites in NO Reduction with NH<Subscript>3</Subscript> on Sulfur Modified TiO<Subscript>2</Subscript>-Supported V<Subscript>2</Subscript>O<Subscript>5</Subscript> Catalyst

V₂O₅/S-doped TiO₂ was prepared by the sol-gel and impregnation methods. The adsorption of NO, NH₃, and O₂ over the catalyst was studied by in situ DRIFTS spectroscopy to elucidate the reaction mechanism of the low-temperature selective catalytic reduction of NO with NH₃. Exposing the catalyst to O₂ and NO, three types of nitrates species appeared on the surface. The introduction of S to TiO₂ could generate large amounts of acid sites for ammonia adsorption on the catalyst, which was believed to be an important role in the SCR reaction and hereby improved the catalytic activity. The results indicated two possible SCR reaction pathways for catalyst. One was that NO was absorbed to form nitrite species, which could react with NH₃ on Lewis acid sites, producing N₂ and H₂O. Another way was that NH₃ was adsorbed, then reacted with gas phase NO (E–R) and nitrite intermediates on the surface (L–H).     

负载型V2O5/TiO2催化剂表面分散状态和性质对氨选择性催化还原NO性能的影响（英文）

采用多种物理化学手段研究了不同负载量V2O5/TiO2催化剂的VOx物种分散状态、表面酸性、可还原性及其选择性催化还原(SCR)NO性能.结果表明,V2O5在锐钛矿TiO2表面的实测单层分散容量约为1.14mmol V/100m2TiO2,与"嵌入模型"的估算值相符,表明分散态的钒离子应键合在TiO2表面的八面体空位上.随着V2O5负载量的增加,V2O5/TiO2催化剂上NO转化频率(TOF)先急剧增加,至0.70mmol V/100m2TiO2(略超过分散容量的一半)时达到极大(约8.3×10-3s-1),然后又急剧下降;同时,孤立VOx物种可能倾向于分散在相邻的八面体空位上,且通过V-O-V化学键相连形成聚合的VOx物种,V-O-V键所占比例增加而V-O-Ti键所占比例减小,催化剂表面单位钒离子的Brnsted酸中心量增加,故催化剂的TOF急剧增加.随着负载量进一步增加,虽然催化剂表面单位钒离子的Brnsted酸中心量仍缓慢增加,但V-O-Ti键所占比例减少,导致钒离子的可还原性下降,另外,分散容量以上时晶相V2O5的形成也导致钒离子表面利用率下降,从而导致催化剂的TOF下降.桥式Brnsted酸位(V-O(H)-V)也是SCR反应活性中心之一,不同负载量V2O5/TiO2催化剂上SCR活性与表面VOx物种的分散状态、表面酸性和钒离子可还原性密切相关.     

Die Ammonolyse von Halogenokomplexen des vierwertigen Platins

Ammonolysis of Halogeno Complexes of Tetravalent Platinum Reactions of liquid ammonia and ammonium hexahalogenoplatinates(IV) at ?40°C yield mixtures of halogenoammine complexes [Pt(NH₃)_6?nX_n]X_4?n (X = Cl, Br, I; n = 3, 2, 1, 0). Hexaammine platinum(IV) salts, [Pt(NH₃)₆]X₄, may be isolated as main product only after several weeks of reaction. Interactions at room temperature of liquid ammonia and hexachloro or hexabromo complexes produce quantitatively the novel dinuclear di-m?-amido-bis[tetraammineplatinum(IV)] complex, [(H₃N)₄Pt(NH₂)₂Pt(NH₃)₄]X₆. By interaction of gaseous or liquid ammonia and subsequent addition of potassium amide solution in excess potassium hexaamido platinate(IV), K₂[Pt(NH₂)₆], is formed in good yield.     

Crystal structure of new ammonium fluoroindate(III) (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>[InF<Subscript>5</Subscript>]

A new indium(III) fluoride complex with the ammonium cation (NH₄)₂[InF₅] is synthesized and its crystal structure is studied. The structure of (NH₄)₂[InF₅] is formed of NH₄ ⁺ cations and complex [InF₅]^2– anions. The In atom in the complex anion surrounded by four terminal and two bridging F atoms forms an almost regular octahedral coordination polyhedron (CN 6) with two terminal F atoms in the axial positions and two terminal and two bridging F atoms in the equatorial plane. Through bridging F atoms, the InF6 polyhedra are arranged in polymer trans- vertex-connected corrugated anion chains (InF₅) _n ^2n? directed along the c axis. The N–H?F hydrogen bonds organize the chains in a three-dimensional framework.     

Structure and synthesis of nitrosoruthenium trans -diammines [Ru(NO)(NH3)2Cl3] and [Ru(NO)(NH3)2(H2O)Cl2]Cl·H2O

A DTA study of thermal decomposition of (NH₄)₂[Ru(NO)Cl₅] in helium atmosphere has been carried out, a synthetic procedure for preparation of the trans-diammine complex mer-[Ru(NO)(NH₃)₂Cl₃] (I) with yield ∼70% has been developed. On re-crystallization of I from aqueous solution a trans-aquanitroso complex [Ru(NO)(NH₃)₂Cl₂(H2O)]Cl·H₂O (II) has been isolated. The structures of the compounds have been determined by single crystal X-ray diffraction: space group Pbcn, a = 6.607(1) ? b = 11.255(2) ? c = 9.878(2) ? (I) and space group Ima2, a = 8.3032(3) ?, b = 8.0890(2) ?, c = 15.9192(5) ? II). Original Russian Text Copyright ? 2008 by M. A. Il’in, V. A. Emel’yanov, and I. A. Baidina __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 49, No. 6, pp. 1128–1136, November–December, 2008.     

In situ study of electrochromic properties of self-assembled TiO2 nanotubes

Electrochromical properties of anodic self-assembled nanotubes were investigated. It was found that amorphous titania nanotubes were able to insert H⁺ ions in a highly reversible manner. Coloration of the TiO₂ nanotubes occurred at potentials below ?0.5 V vs. Ag/AgCl in 1M (NH₄)₂SO₄ aqueous solution. The proton insertion reaction probably leads to the formation of a Ti³⁺/Ti⁴⁺ solid solution in the amorphous titania electrode, as was shown by the analysis of the derivative curve. The nanotubular titania electrode shows reasonable color efficiency when compared with other electrochromic materials and it is a promising candidate for the fabrication of low-cost interdigitated electrochromic devices.     

丙烷氧化脱氢不同硅基载体负载钒氧化物催化剂的TPSR表征

研究了钒负载不同氧化硅载体(Silica-gel,SBA-15,MCM-41,fumed-SiO2,Nano-SiO2)的丙烷氧化脱氢(ODH)催化剂的结构特征和催化性能,结合催化剂的程序升温表面反应(TPSR)的差热热重质谱(TG-DSC-MS)和原位紫外漫反射光谱(UV-vis DRS)等技术,研究钒在载体上的分散度和晶格氧的反应性。结果表明:负载型钒氧化物催化剂的活性取决于钒在不同硅基载体上的分散度,高度分散的隔离的四配位V5+是丙烷氧化脱氢的活性位。C3H6选择性主要与催化剂的平均孔径相关联,平均孔径越小,产物C3H6越易发生深度氧化。另外,不同氧化硅载体晶格氧与钒的结合强度对C3H6的选择性也产生影响,结合力较弱的V-O-Si中的晶格氧是丙烷氧化脱氢的燃烧位,且燃烧温度随晶格氧与钒、硅结合强度的减小而降低。而与钒结合力较强的V=O和V-O-V中的晶格氧是丙烷氧化脱氢的选择氧化位。硅基载体形貌和结构的不同导致负载型钒氧化物催化剂丙烷氧化脱氢活性和选择性发生差异。     

A DFT study on the hydrated V2O5-TiO2-anatase catalyst: stability of monomeric species

A combined cluster and periodic study has been carried out to elucidate the stability of hydrated species on the vanadia/titania catalyst. First, the hydration of a V₂O₅ cluster was analyzed for the successive adsorption of one to four water molecules. The dimeric skeleton is found to be preserved at a low water concentration. However, after the adsorption of four water molecules on the dimer, it is found to break to generate stable monomeric OV(OH)₃ units. The two moieties are related by the equation: Such OV(OH)₃ units have been taken as a monomer model for the periodic representation of the vanadia/titania catalyst. On the (100) surface, the OV(OH)₃ moieties are anchored by three V–O–Ti bonds to the support surface in a pyramidal arrangement. The vanadyl V=O bond is found to be very stable.     

Probing Surface Sites of TiO2: Reactions with [HRe(CO)5] and [CH3Re(CO)5]

Two carbonyl complexes of rhenium, [HRe(CO)₅] and [CH₃Re(CO)₅], were used to probe surface sites of TiO₂ (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O₂ or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti⁺³? OH and Ti⁺⁴? OH. IR and extended X‐ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface‐bound rhenium tricarbonyls, when [HRe(CO)₅] was adsorbed, or rhenium tetracarbonyls, when [CH₃Re(CO)₅] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti⁺³ and Ti⁺⁴ ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO)₅] or [CH₃Re(CO)₅] determined a ranking of the reactivity of the surface OH sites, with the Ti⁺³? OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites.     

钼钒磷杂多酸铵蓝的合成和性质研究

本实验采用分步加原料的方法，以不同的磷钼钒比，直接合成了三种１：１２系列的钼钒磷杂多酸铵蓝，即（ＮＨ４）４Ｈ２·１５Ｈ２Ｏ（ｂｌｕｃ—１），（ＮＨ４）５Ｈ２·和（ＮＨ４）６Ｈ２·３０Ｈ２Ｏ（ｂｌｕｃ－３），并研究了反应条件对这些钼钒磷杂多酸铵蓝形成的影响．通过元素分析、电位滴定、电子光谱和Ｘ射线衍射等测试手段研究了配合物的性质，并确定了配合物的组成．     

First example of ruthenium nitroso monoammine complex. Crystal structure of [Ru(NO)(NH3)3(H2O)Cl] × [Ru(no)(NH3)3(OH)Cl][Ru(NO)(NH3)Cl4]2Cl·2H2O

The structure of the interaction products of (NH₄)₂[Ru(NO)Cl₅] solution with ammonium acetate on heating is studied. The crystal structure of the [Ru(NO)(NH₃)₃(H₂O)Cl][Ru(NO)(NH₃)₃(OH)Cl] × [Ru(NO)(NH₃)Cl₄]₂Cl-2H₂O compound (compound I) containing a previously unknown anion of the nitrosomonoammine series is determined: Cc space group; a = 33.530(7) ?, b = 8.202(2) ?, c = 11.844(2) ?; β= 101.54(3)°.